Recent advances in graphene-based hybrid nanostructures for electrochemical energy storage

Pan Xiong; Junwu Zhu; Lili Zhang; Xin Wang

doi:10.1039/C5NH00134J

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DOI: 10.1039/C5NH00134J (Review Article) Nanoscale Horiz., 2016, 1, 340-374

Recent advances in graphene-based hybrid nanostructures for electrochemical energy storage

Pan Xiong ^a, Junwu Zhu *^a, Lili Zhang *^b and Xin Wang *^a
^aKey Laboratory for Soft Chemistry and Functional Materials of Ministry Education, Nanjing University of Science and Technology, Nanjing 210094, China. E-mail: wangx@njust.edu.cn; zhujw@njust.edu.cn; Fax: +86-25-8431-5054; Tel: +86-25-8430-5667
^bJiangsu Key Laboratory for Chemistry of Low-Dimensional Materials, Huaiyin Normal University, Huai’an, Jiangsu 223300, P. R. China. E-mail: zll@hytc.edu.cn

Received 30th December 2015 , Accepted 16th March 2016

First published on 16th March 2016

Abstract

In recent years, graphene has emerged as a promising candidate for electrochemical energy storage applications due to its large specific surface area, high electrical conductivity, good chemical stability, and strong mechanical flexibility. Moreover, its unique two-dimensional (2D) nanostructure can be used as an ideal building block for controllable functionalization with other active components and the resulting graphene-based hybrids exhibit desirable properties for improved energy storage capability. This review summarizes the most recent progress on graphene and graphene-based hybrid nanostructures for three frontier electrochemical energy storage device applications, i.e., lithium-ion batteries, lithium–sulfur batteries and supercapacitors. Finally, we outline the future perspectives and trends in this research field including several challenges and opportunities.

Pan Xiong

Pan Xiong received his PhD degree in materials chemistry from the Nanjing University of Science and Technology in 2005. From December 2013 to December 2014, he worked in Professor Guihua Yu's group at the University of Texas at Austin as an exchange visiting scholar. His research interests focus on the synthesis of hybrid nanostructures for use in photocatalysis and energy storage applications.

Junwu Zhu

Junwu Zhu received his PhD degree in materials chemistry from the Nanjing University of Science and Technology in 2005. He is presently working as professor in the same university. His main research interests are focused on the preparation and application of functional materials based on carbon and nanostructured composites.

Lili Zhang

Lili Zhang is a professor of chemistry at Huaiyin Normal University. She received her PhD degree in materials chemistry from the Nanjing University of Science and Technology in 2006, followed by two year postdoctoral research with Professor Xin Wang at the same university. Her research interests are focused on nanostructured material systems for advanced energy and environmental technologies.

Xin Wang

Xin Wang received his PhD degree in inorganic chemistry at the Department of Chemistry, Nanjing University, in 1985. After that, he joined the Nanjing University of Science and Technology and was promoted to full professor in 1988. His research interests are focused on the design and synthesis of nanostructured materials for use in energy storage and conversion.

1. Introduction

Due to the ever-increasing energy demand and consumption, along with the resulting global environmental problems and air pollution, the development of energy storage systems, especially high-performance, low-cost, and environmentally friendly electrochemical energy storage devices, has attracted tremendous attention around the world during the past several years.^1–6 Generally, for electrochemical energy storage devices, the nature of electrode materials plays a vital role in improving the key electrochemical parameters, such as energy and power density (both gravimetric and volumetric capacities), cyclability, rate capability, safety and temperature dependence. For example, if the graphite anode having a theoretical capacity of 372 mA h g⁻¹ in a standard Sony 18650 battery is replaced with an anode material having a capacity greater than 1000 mA h g⁻¹, then more than a 20% increase in total capacity can be obtained.⁷ Therefore, for the purpose of pursuing higher performances for energy storage devices, rational design and facile synthesis of advanced electrode materials with higher energy storage capability, longer cycle life, better safety, and lower cost are of great importance.

Since the two-dimensional (2D) single-atom-thick layer of carbon was isolated by Novoselov and co-workers in 2004⁸ graphene and graphene-based composites have attracted increasing research interest in the past decade. The unique characteristics of graphene, such as large theoretical surface area (2630 m² g⁻¹), high charge carrier mobility (20 m² V⁻¹ s⁻¹), mechanical flexibility, chemical stability, thermal conductivity (5000 W m⁻¹ K⁻¹), and optical transmittance (97.7%), make graphene an ideal alternative or a versatile additive for energy storage applications.^1,2,4,9–11 For example, the first graphene-based supercapacitor was developed in 2008 with specific capacitances of 135 and 99 F g⁻¹ in aqueous and organic electrolytes, respectively.¹² The best graphene anode with a specific capacity of ∼1200 mA h g⁻¹ at 100 mA g⁻¹ was reported in 2010.¹³ A graphene-wrapped sulfur cathode for lithium–sulfur batteries with high and stable specific capacities of up to ∼600 mA h g⁻¹ over more than 100 cycles was explored in 2011.¹⁴ Moreover, with regard to its unique 2D structural features, graphene has also been used as an ideal building block for controllable functionalization with other electroactive components, such as metal oxides/hydroxides and conducting polymers, and the resulting graphene-based hybrid nanostructures exhibit desirable properties for energy storage applications.^{9–11,15–18}

In such a constantly developing area, a large number of fascinating research results related to graphene and graphene-based hybrids for energy storage applications have been presented in recent years. Therefore, it is of great importance to highlight the latest discoveries and achievements timely, and outline a roadmap for developing potential candidate materials. In this review, the recent advances in the applications of graphene and graphene-based hybrid nanostructures for three frontier electrochemical energy storage devices, namely lithium-ion batteries, lithium–sulfur batteries and supercapacitors, are summarized with a particular focus on the progress made over the last three years. The remaining challenges and future prospects are also discussed.

2. Graphene-based nanocomposites for lithium-ion batteries

Since first being introduced to the market in 1991 by Sony, lithium-ion batteries have become one of the main power sources for portable electronic devices, such as cell phones, laptop computers, and digital cameras, due to their high energy density (120–170 W h kg⁻¹) and durable cycling life.^19–22 However, lithium-ion batteries suffer from low power density, e.g., the complete charge time is usually about 6 h for a lithium-ion battery equipped mobile phone or laptop in the market at present. In principle, the charge/discharge process of lithium-ion batteries is realized through the migration/transport of electrons and Li⁺ between the anode and the cathode. Therefore, the key to developing a fast charge/discharge process (high power capability) is to design advanced electrode materials with high electrical conductivity for effective electron transport, large surface areas and well-organized nanostructures with a short diffusion length for Li⁺ transport. In this regard, 2D graphene with superior electrical conductivity (106 S cm⁻¹) and a high surface area (2630 m² g⁻¹) is expected to be a promising candidate for high-performance lithium-ion batteries.^{1,2,9,10,16,23} Recently, various strategies based on graphene-based electrode materials have been developed, which will be summarized in the following sections.

2.1 Graphene-based anode materials

A commercially used graphite anode has a theoretical capacity of 372 mA h g⁻¹ by forming LiC₆ upon Li-intercalation between stacked layers.^19,24 While single-layer graphene sheets can theoretically host two times as many Li⁺ ions as conventional graphite through Li-adsorption on both sides.²⁵ Yoo et al. first explored the Li storage capacity of graphene nanosheets prepared by a controlled exfoliation and reassembly process. The specific capacity of graphene nanosheets was found to be ∼540 mA h g⁻¹, which is much larger than that of graphite.²⁶ After that, a number of studies on graphene-based anodes have also been reported (Table 1).

Table 1 Summary of graphene-based anodes for Li-ion batteries

Electrode materials	Capacity	Rate capability	Cycle life	Ref.
N-doped graphene	740 mA h g⁻¹ (100 mA g⁻¹)	300 mA h g⁻¹ (3 A g⁻¹)	100 (96.5%)	39
S-doped graphene	870 mA h g⁻¹ (374 mA g⁻¹)	285 mA h g⁻¹ (11.22 A g⁻¹)	100 (96.5%)	40
P-doped graphene	460 mA h g⁻¹ (100 mA g⁻¹)		80 (100%)	41
Edge-fluorinated graphene	807.4 mA h g⁻¹ (50 mA g⁻¹)	650.3 mA h g⁻¹ (500 mA g⁻¹)	500 (76.6%)	46
Full fluorinated graphene	843 mA h g⁻¹ (50 mA g⁻¹)	336 mA h g⁻¹ (500 mA g⁻¹)	50 (92.9%)	48
N,S-codoped graphene	1090 mA h g⁻¹ (200 mA g⁻¹)	297 mA h g⁻¹ (5 A g⁻¹)	500 (66.6%)	49
Solvated graphene frameworks	1158 mA h g⁻¹ (100 mA g⁻¹)	472 mA h g⁻¹ (5 A g⁻¹)	500 (93%)	54
N,S-codoped porous graphene	860 mA h g⁻¹ (0.5 A g⁻¹)	220 mA h g⁻¹ (80 A g⁻¹)	3000 (100%)	55
N-doped graphene frameworks	640 mA h g⁻¹ (0.5 A g⁻¹)	210 mA h g⁻¹ (10 A g⁻¹)	300 (100%)	57
MnO/graphene	2014.1 mA h g⁻¹ (200 mA g⁻¹)	625.8 mA h g⁻¹ (3000 mA g⁻¹)	400 (96%)	63
MnO₂/3D porous graphene	836 mA h g⁻¹ (100 mA g⁻¹)	433 mA h g⁻¹ (1600 mA g⁻¹)	200 (75.6%)	65
MnO₂/3D N-doped graphene	909 mA h g⁻¹ (400 mA g⁻¹)	636 mA h g⁻¹ (1500 mA g⁻¹)	200 (>100%)	66
MnO₂/N-doped graphene	607.5 mA h g⁻¹ (100 mA g⁻¹)	90 mA h g⁻¹ (12000 mA g⁻¹)	3000 (100%)	67
Mn₃O₄/graphene	500 mA h g⁻¹ (60 mA g⁻¹)	200 mA h g⁻¹ (1500 mA g⁻¹)		68
Mn₃O₄/N-doped graphene	800 mA h g⁻¹ (200 mA g⁻¹)	400 mA h g⁻¹ (2000 mA g⁻¹)	40 (∼100%)	69
Fe₂O₃/graphene aerogel	995 mA h g⁻¹ (100 mA g⁻¹)	372 mA h g⁻¹ (5000 mA g⁻¹)	50 (95.2%)	71
Fe₂O₃/graphene sheet-on-sheet	1074.9 mA h g⁻¹ (100 mA g⁻¹)	622.4 mA h g⁻¹ (1000 mA g⁻¹)		72
Hollow Fe₃O₄/graphene	940 mA h g⁻¹ (200 mA g⁻¹)	420 mA h g⁻¹ (2000 mA g⁻¹)	50 (100%)	73
Fe₃O₄/graphene foam	785 mA h g⁻¹ (1C)	190 mA h g⁻¹ (60C)		74
Fe₃O₄/graphene foam	1200 mA h g⁻¹ (924 mA g⁻¹)	300 mA h g⁻¹ (18.48 A g⁻¹)	500 (>100%)	75
Fe₃O₄/graphene foam	1200 mA h g⁻¹ (200 mA g⁻¹)	444 mA h g⁻¹ (5 A g⁻¹)		77
CoO/graphene	640 mA h g⁻¹ (100 mA g⁻¹)	170 mA h g⁻¹ (20 A g⁻¹)		78
Co₃O₄ fiber/graphene	1005.7 mA h g⁻¹ (100 mA g⁻¹)	295 mA h g⁻¹ (1000 mA g⁻¹)	40 (83.5%)	82
Co₃O₄ nanowall/graphene	800 mA h g⁻¹ (100 mA g⁻¹)	320 mA h g⁻¹ (5000 mA g⁻¹)	500 (100%)	83
Porous Co₃O₄/graphene	1236 mA h g⁻¹ (89 mA g⁻¹)	371 mA h g⁻¹ (4450 mA g⁻¹)		85
MoO₂/3D graphene	975.4 mA h g⁻¹ (50 mA g⁻¹)	537.3 mA h g⁻¹ (1000 mA g⁻¹)		86
Porous MoO₂/graphene	1300 mA h g⁻¹ (83.8 mA g⁻¹)	390 mA h g⁻¹ (4190 mA g⁻¹)		87
MoO₂/N-doped graphene	1138.5 mA h g⁻¹ (100 mA g⁻¹)	873.7 mA h g⁻¹ (1000 mA g⁻¹)		88
MoO₃/graphene film	291 mA h g⁻¹ (100 mA g⁻¹)	151 mA h g⁻¹ (2000 mA g⁻¹)	100 (59.1%)	89
MoO₃/graphene	1176 mA h g⁻¹ (200 mA g⁻¹)	894 mA h g⁻¹ (3000 mA g⁻¹)	100 (92%)	90
ZnFe₂O₄/graphene	1181 mA h g⁻¹ (100 mA g⁻¹)	713 mA h g⁻¹ (2000 mA g⁻¹)	30 (101.6%)	93
ZnFe₂O₄/graphene	1090 mA h g⁻¹ (100 mA g⁻¹)	650 mA h g⁻¹ (2000 mA g⁻¹)	200 (99%)	95
ZnMn₂O₄ nanorod/graphene	707 mA h g⁻¹ (100 mA g⁻¹)	440 mA h g⁻¹ (2000 mA g⁻¹)	50 (73.6%)	97
Porous ZnMn₂O₄/graphene	926.4 mA h g⁻¹ (200 mA g⁻¹)	560.8 mA h g⁻¹ (1200 mA g⁻¹)		98
ZnMn₂O₄@C/graphene	705 mA h g⁻¹ (232 mA g⁻¹)	403.5 mA h g⁻¹ (4640 mA g⁻¹)	180 (99.4%)	94
ZnMn₂O₄/graphene	∼860 mA h g⁻¹ (200 mA g⁻¹)	∼568 mA h g⁻¹ (3200 mA g⁻¹)	1500 (∼81%)	96
CoFe₂O₄@C/graphene	925.6 mA h g⁻¹ (100 mA g⁻¹)	305.3 mA h g⁻¹ (1600 mA g⁻¹)		102
CoFe₂O₄/graphene	916.8 mA h g⁻¹ (500 mA g⁻¹)	556 mA h g⁻¹ (3200 mA g⁻¹)	200 (99.2%)	103
CoFe₂O₄/graphene	1300.7 mA h g⁻¹ (200 mA g⁻¹)	574.9 mA h g⁻¹ (1600 mA g⁻¹)	50 (69.5%)	104
CoFe₂O₄/graphene	1091 mA h g⁻¹ (100 mA g⁻¹)	636 mA h g⁻¹ (3000 mA g⁻¹)	100 (102%)	106
NiFe₂O₄/graphene/C	1150 mA h g⁻¹ (100 mA g⁻¹)	520 mA h g⁻¹ (1000 mA g⁻¹)		109
Porous NiCo₂O₄/graphene	974 mA h g⁻¹ (100 mA g⁻¹)	396 mA h g⁻¹ (800 mA g⁻¹)	70 (80.1%)	112
Porous NiCo₂O₄/graphene	1850 mA h g⁻¹ (200 mA g⁻¹)	404 mA h g⁻¹ (2000 mA g⁻¹)		113
TiO₂/graphene	189 mA h g⁻¹ (100 mA g⁻¹)	94 mA h g⁻¹ (10 A g⁻¹)		121
TiO₂/graphene	215 mA h g⁻¹ (200 mA g⁻¹)	100 mA h g⁻¹ (1600 mA g⁻¹)	100 (74%)	123
TiO₂ nanosheet/graphene	275 mA h g⁻¹ (335 mA g⁻¹)	200 mA h g⁻¹ (13400 mA g⁻¹)	1000 (80%)	125
Porous TiO₂/graphene aerogel	200 mA h g⁻¹ (100 mA g⁻¹)	99 mA h g⁻¹ (5000 mA g⁻¹)		126
TiO₂/3D N-doped graphene	165 mA h g⁻¹ (168 mA g⁻¹)	96 mA h g⁻¹ (3360 mA g⁻¹)	200 (83.3%)	122
Porous TiO₂/graphene	206 mA h g⁻¹ (100 mA g⁻¹)	128 mA h g⁻¹ (5 A g⁻¹)		127
Li₄Ti₅O₁₂/graphene	162 mA h g⁻¹ (1750 mA g⁻¹)	126 mA h g⁻¹ (17.5 A g⁻¹)	500 (96.2%)	130
Li₄Ti₅O₁₂/graphene	187 mA h g⁻¹ (175 mA g⁻¹)	128 mA h g⁻¹ (14 A g⁻¹)	2000 (63.2%)	132
Li₄Ti₅O₁₂/graphene oxide	201 mA h g⁻¹ (87.5 mA g⁻¹)	162 mA h g⁻¹ (5.25 A g⁻¹)	300 (97%)	131
MoS₂/graphene	912 mA h g⁻¹ (100 mA g⁻¹)	571 mA h g⁻¹ (1000 mA g⁻¹)	100 (88.6%)	134
MoS₂/graphene	1400 mA h g⁻¹ (100 mA g⁻¹)	591 mA h g⁻¹ (1000 mA g⁻¹)	200 (96.5%)	136
MoS₂/3D graphene	1020 mA h g⁻¹ (100 mA g⁻¹)	466 mA h g⁻¹ (4 A g⁻¹)	50 (86%)	135
MoS₂/graphene paper	1681 mA h g⁻¹ (100 mA g⁻¹)	864 mA h g⁻¹ (1000 mA g⁻¹)	100 (65.6%)	137
MoS₂/graphene@carbon fiber	1225 mA h g⁻¹ (200 mA g⁻¹)	780 mA h g⁻¹ (2.5 A g⁻¹)	250 (57.7%)	138
Si NW/graphene/RGO	1600 mA h g⁻¹ (2.1 A g⁻¹)	500 mA h g⁻¹ (8.4 A g⁻¹)	100 (80%)	156
Si/graphene	2250 mA h g⁻¹ (100 mA g⁻¹)	1000 mA h g⁻¹ (10 A g⁻¹)	120 (85%)	151
Si/graphene	2699 mA h g⁻¹ (0.56 A g⁻¹)	1148 mA h g⁻¹ (28 A g⁻¹)	1000 (100%)	152
Si NW/graphene	705 mA h g⁻¹ (0.1 A g⁻¹)	550 mA h g⁻¹ (6.8 A g⁻¹)		157
Porous Si/graphene	1703 mA h g⁻¹ (200 mA g⁻¹)	390 mA h g⁻¹ (5 A g⁻¹)	160 (∼100%)	153
Multilayered Si/graphene	2198 mA h g⁻¹ (120 mA g⁻¹)	700 mA h g⁻¹ (24 A g⁻¹)	150 (87%)	154
Sn@graphene/graphene	1022 mA h g⁻¹ (200 mA g⁻¹)	270 mA h g⁻¹ (10 A g⁻¹)	1000 (96.3%)	163
SnO₂/graphene	1090 mA h g⁻¹ (200 mA g⁻¹)	491 mA h g⁻¹ (5 A g⁻¹)	200 (∼90%)	164
SnO₂/graphene	776 mA h g⁻¹ (100 mA g⁻¹)	147 mA h g⁻¹ (5 A g⁻¹)	1000 (∼100%)	160
SnO₂/N-doped graphene	1021 mA h g⁻¹ (500 mA g⁻¹)	417 mA h g⁻¹ (20 A g⁻¹)	500 (>100%)	162
SnO₂/N-doped graphene	1326 mA h g⁻¹ (200 mA g⁻¹)	460 mA h g⁻¹ (5 A g⁻¹)	100 (87.9%)	165
Ge/graphene	915 mA h g⁻¹ (100 mA g⁻¹)	273 mA h g⁻¹ (5 A g⁻¹)	400 (84.9%)	167
Ge NW@graphene	1059 mA h g⁻¹ (4.8 A g⁻¹)	363 mA h g⁻¹ (24 A g⁻¹)	200 (90%)	168
Ge NW@graphene	463 mA h g⁻¹ (1.6 A g⁻¹)	170 mA h g⁻¹ (32 A g⁻¹)	50 (∼94.5%)	169
Ge/graphene/CNT	1181.7 mA h g⁻¹ (100 mA g⁻¹)	644.8 mA h g⁻¹ (3200 mA g⁻¹)	100 (∼73%)	170
CNT/graphene	900 mA h g⁻¹ (100 mA g⁻¹)	370 mA h g⁻¹ (1500 mA g⁻¹)	250 (98.92%)	173
CNT/graphene paper	375 mA h g⁻¹ (100 mA g⁻¹)		100 (88%)	174
CNT/graphene paper	303 mA h g⁻¹ (30 mA g⁻¹)		50 (100%)	175
CNT/graphene hybrid foam	2640 mA h g⁻¹ (186 mA g⁻¹)	236 mA h g⁻¹ (27.9 A g⁻¹)	100 (100%)	176
MWCNT/graphene nanoribbon	880 mA h g⁻¹ (100 mA g⁻¹)	157 mA h g⁻¹ (10 A g⁻¹)		177
Porous carbon/graphene	988 mA h g⁻¹ (0.1 A g⁻¹)	280 mA h g⁻¹ (2 A g⁻¹)	100 (68.8%)	182
g-C₃N₄/graphene	1705 mA h g⁻¹ (100 mA g⁻¹)	934 mA h g⁻¹ (1000 mA g⁻¹)	50 (89.4%)	185

2.1.1 Graphene and its derivatives. Among these reports, a variety of reduced graphene oxides (RGOs) synthesized using different reduction processes, including chemical reduction, thermal reduction, and photothermal reduction, etc., are the most widely studied electrode materials.^13,27–30 Interestingly, these RGOs can exhibit exceptionally high reversible capacities of 800–1300 mA h g⁻¹, which are higher than the theoretical capacity of single-layer graphene nanosheets.^{13,28,29,31,32} This phenomenon is mainly caused by Li storage on defects such as edges and/or oxygen- and hydrogen-containing groups.^28,31–33 However, it is essential to control the number of such defects, because they are also partly associated with the low first-cycle Coulombic efficiency and rate capability as well as notable capacity fading.^29,34 High surface areas of RGO with the defects increase the contact area between the anode and the electrolyte and result in more solid electrolyte interphase (SEI) formation in the first discharge. The progressive reduction of oxygen-containing groups will inevitably lead to re-stacking of RGO, which lowers the storage capacity over repeated cycling. Besides, the released oxygen can partly oxidize the electrolyte and consequently induce electrochemical instability.³⁴

Doped graphenes, such as nitrogen (N)-doped,^35–39 boron (B)-doped,³⁶ sulfur (S)-doped,⁴⁰ and phosphorus (P)-doped graphenes,⁴¹ have attracted great interest due to their improved electrochemical performance and electron transport ability, as shown both theoretically and experimentally.^42–44 As the electronegativity of heteroatoms such as N (3.5) is higher than that of C (2.55), N-doped graphene is more favorable for the insertion of lithium. For example, Wu et al. reported that N-doped graphene and B-doped graphene could show a high reversible capacity of 1043 and 1540 mA h g⁻¹ at a low rate of 50 mA g⁻¹, respectively. Moreover, a high capacity of ∼199 and ∼235 mA h g⁻¹ was still obtained for the N-doped graphene and B-doped graphene at a high rate of 25 A g⁻¹ (about 30 s to fully charge), respectively. The high-rate performance of N- and B-doped graphenes is due to their disordered surface morphology, heteroatomic defects, enhanced surface wettability, increased interlayer distance, improved electrical conductivity, and thermal stability, which are beneficial for rapid surface Li⁺ absorption, ultrafast Li⁺ diffusion and electron transport.³⁶ Cao's group used three different defect models (graphitic, pyridinic, and pyrrolic graphenes) to theoretically investigate the effects of the electron-deficiency of N-doped graphene on its Li storage ability based on first-principles calculations. The results showed that the pyridinic graphene is the most suitable for Li storage with a high storage capacity.⁴⁵ Halogen atoms (F, Cl, Br, and I) with high electronegativity can also be incorporated into graphene for enhancing the electrochemical performance.^46–48 Jeon et al. reported the synthesis of edge-selectively fluorinated graphene nanoplatelets by a mechanochemically driven reaction between active carbon species at the edges of graphene and fluorine gas (20 vol% in argon). Due to the large difference in electronegativity between C (χ = 2.55) and F (χ = 3.98), the edge-fluorinated graphene can provide enhanced chemical stability (a charge retention of 76.6% after 500 cycles).⁴⁶ Feng's group reported a full fluorinated graphene nanosheet by liquid exfoliation of fluorinated graphite in 2-isopropanol (IPA) solvent. The resulting 2D amorphous-like microstructure with high fluorine content rendered such F-graphene nanosheets abundant F active sites for lithium storage and facilitated the rapid diffusion of lithium ions during charging/discharging processes, leading to a high reversible capacity of 780 mA h g⁻¹ at 50 mA g⁻¹ and excellent cycle performance.⁴⁸ In addition to single heteroatom-doped graphene materials, co-doped graphene materials have also been developed to enhance the electrochemical performance. Recently, An et al. reported a N and S co-doped graphene through thermal treatment of covalently functionalized GO with 2-aminothiophenol (ATP) as a source of both N and S. Benefiting from the synergistic effects of N and S co-doping, the resulting N,S co-doped graphene anode showed super-high reversible capacity, excellent rate capability as well as long-term cycling performance for lithium-ion batteries (Fig. 1).⁴⁹


	Fig. 1 (a) Schematic illustration of the structure of N and S co-doped graphene. (b) SEM image of N and S co-doped graphene. Inset: Higher-magnification SEM image of N and S co-doped graphene. (c) TEM image of N and S co-doped graphene. Inset: Corresponding SAED pattern of N and S co-doped graphene. (d) Rate and cycling capability of the N and S co-doped graphene electrode at different rates. Reprinted with permission from ref. 49, copyright (2014), Wiley-VCH.

Besides, three-dimensional (3D) graphene nanostructure-based anodes, such as graphene papers^50,51 and graphene frameworks,^52–55 have attracted great interest for lithium-ion batteries due to their large specific surface area, multidimensional continuous electron-transport pathways, and rapid ion-diffusion characteristics as well as excellent mechanical flexibility. In particular, Duan's group prepared solvated graphene frameworks (SGFs) through a convenient solvent-exchange approach. The as-obtained SGFs could be used directly as electrodes without adding any other binders or conductive additives and delivered a high reversible capacity of ∼1158 mA h g⁻¹ at a charge/discharge rate of 0.1 A g⁻¹, which was ∼2.6 times higher than that of unsolvated graphene frameworks (439 mA h g⁻¹). Moreover, the SGFs showed excellent rate capability with a significantly large capacity of 472 mA h g⁻¹ at a high charge/discharge rate of 5.0 A g⁻¹ and superior cycling stabilities with a capacity retention of 93% over 500 charge/discharge cycles at 5.0 A g⁻¹ (Fig. 2).⁵⁴


	Fig. 2 (a) Schematic representation of solvent exchange for the preparation of SGFs. (b) Preparation of a binder-free SGF electrode. A piece of SGF (left), its pressed-film electrode on copper foil placed in a coin cell case (middle), and SEM image of the cross-section of the pressed SGF film (right). Cycle performance and Coulombic efficiency of (c) SGF and (d) unsolvated graphene framework electrodes at a low current density of 0.1 A g⁻¹. (e) Rate capabilities and cycle performance of SGF and unsolvated graphene framework electrodes over a wide range of current densities from 0.2 to 5.0 A g⁻¹. (f) Cycling stability of the SGF electrode at a high current density of 5.0 A g⁻¹ after a rate performance test. Reprinted with permission from ref. 54, copyright (2015), Wiley-VCH.

Construction of a graphene nanostructure that combines a 3D highly conductive network, a hierarchically porous structure, and heteroatom doping together seems to be an ideal approach for achieving a high-performance electrode.^55–59 Wang et al. reported the in situ fabrication of 3D doped hierarchically porous graphene. Due to the synergistic effect of the special 3D nanostructure and heteroatom doping, the as-prepared graphene electrode exhibited a high-power density of 116 kW kg⁻¹, while the energy density remained as high as 322 W h kg⁻¹ at 80 A g⁻¹ (only 10 s to fully charge).⁵⁵

2.1.2 Graphene–transition metal oxide nanocomposites. Transition metal oxides have been extensively studied as anode materials for lithium ion batteries due to their potentially higher specific capacities than that of commercial graphite. However, the low electronic/ionic conductivity and detrimental structural collapse upon lithiation/delithiation are two major drawbacks of transition metal oxide-based anodes. Moreover, transition metal oxide nanoparticles spontaneously tend to agglomerate and grow due to their high surface energy, therefore it is very difficult to uniformly disperse or mix them just with carbon additives and binders to make an electrode. With the help of graphene, which can act as a perfect matrix to uniformly support transition metal oxide nanoparticles and provide interconnected electron-transport pathways for effective charge transport, graphene–transition metal oxide anodes exhibited much improved performances compared to conventional carbon-based anodes.^17,18,60,61

Recently, graphene sheets combined with a number of transition metal oxides, such as MnO,^62,63 MnO₂,^64–67 Mn₃O₄,^68,69 Fe₂O₃,^70–72 Fe₃O₄,^73–77 CoO,^78–80 Co₃O₄,^81–85 MoO₂,^86–88 and MoO₃,^89,90 have been explored. For instance, Kan et al. prepared a Fe₂O₃/graphene sheet-on-sheet sandwich-like nanocomposite by a solvothermal method. Since Fe₂O₃ nanosheets have a better structure affinity with graphene nanosheets compared to Fe₂O₃ nanoparticles, which can more effectively prevent the agglomeration of graphene, larger intimate contact areas exist in sheet-on-sheet structure as compared to particle-on-sheet structure. The as-prepared Fe₂O₃/graphene sheet-on-sheet nanocomposite exhibited better specific capacities and cycling performances (a high capacity of 662.4 mA h g⁻¹ was preserved after 100 cycles at 1000 mA g⁻¹) compared with the conventional Fe₂O₃/graphene particle-on-sheet composite.⁷² A novel conversion mechanism between lithium and these transition metal oxides has been identified. The transition metal oxides are reduced to metal nanoparticles homogeneously embedded in a Li₂O matrix. Simultaneously, Li₂O can be reversibly decomposed in the presence of a transition metal as a catalyst.⁹¹ Sun and co-workers observed a reconstruction phenomenon of MnO/graphene electrodes due to the conversion reactions. During the lithiation and delithiation process, the initial MnO nanosheets were first reduced to Mn nanograins, which were then oxidized to Mn²⁺ and Mn⁴⁺, resulting in a thinner film composed of ultrafine MnO_x nanoparticles that were uniformly loaded on or sandwiched between the graphene layers. Despite the initial morphology being lost, the close electrochemical connection between MnO_x nanoparticles and the graphene matrix formed by the reconstruction process facilitated effective and rapid charge transfer, leading to high specific capacity and rate capability.⁶³ Su et al. reported the direct observation of the conversion mechanism of Fe₂O₃/graphene anode by in situ transmission electron microscopy (TEM) technology. Upon first lithiation, single-crystalline Fe₂O₃ nanoparticles were transformed into multicrystalline Fe nanoparticles embedded in a Li₂O matrix, which almost disappeared after delithiation. During the following delithiation process, FeO instead of Fe₂O₃ was detected. Therefore the eventual electrochemical processes of Fe₂O₃ anodes were revealed to be reversible conversion reactions between Fe nanograins and FeO nanograins.⁹²

Recently, mixed transition metal oxides, such as ZnFe₂O₄,^93–95 ZnMn₂O₄,^96–98 CoFe₂O₄,^99–106 NiFe₂O₄,^107–109 NiCo₂O₄,^110–113 and MnFe₂O₄,^114–116 have been widely explored as upgraded anode materials owing to their higher electronic conductivity and larger specific capacities than those of simple transition metal oxides.¹¹⁷ One interesting example of these mixed transition metal oxides is ZnMn₂O₄, which can store Li⁺ through not only the conversion reaction, but also the alloying reaction between Zn and Li, leading to a high theoretical capacity of 784 mA h g⁻¹. In order to resolve the intrinsically poor electronic/ionic conductivities of ZnMn₂O₄, Yu's group and our group designed a facile two-step synthesis approach for uniform growth of ultrafine ZnMn₂O₄ nanocrystals (4–5 nm) on RGO sheets to form a chemically integrated 2D ZnMn₂O₄/graphene nanostructure. A large specific capacity of ∼806 and 568 mA h g⁻¹ was obtained at a low current density of 200 mA g⁻¹ and a high current density of 3200 mA g⁻¹, respectively, resulting in high rate capability with ∼71% capacity retention. A high long-term cyclability with a reversible capacity of ∼650 mA h g⁻¹ over 1500 cycles at a current density of 2000 mA g⁻¹ was obtained as well. The outstanding electrochemical performances of this 2D ZnMn₂O₄/graphene hybrid nanosheet can be largely attributed to the well-designed integrated 2D nanostructure, which not only provides a high surface area for Li storage, but also shortens diffusion pathways for Li⁺ shuttling in/out. Moreover, the possible covalent interaction between ZnMn₂O₄ and the graphene substrate not only contributes to the effective and rapid charge transfer between ZnMn₂O₄ and the current collector through the highly conductive graphene, but also plays an important role in maintaining the structural integrity upon cycling (Fig. 3).⁹⁶ Spinel ferrites (MFe₂O₄, M = Co, Ni, Cu, Mn, and Zn), formed by replacement of one Fe in Fe₃O₄ by another metal cation, have been extensively regarded as promising anodes for lithium-ion batteries. Our group has explored a general one-step hydrothermal strategy for facile synthesis of a series of graphene–ferrite composite anodes, including ZnFe₂O₄/graphene,¹¹⁸ CoFe₂O₄/graphene,¹⁰⁰ NiFe₂O₄/graphene,¹⁰⁸ CuFe₂O₄/graphene,¹¹⁹ and MnFe₂O₄/graphene.¹²⁰ By suitable tuning of the mass ratio between graphene and ferrite species for best synergetic effects, high reversible specific capacities of up to ∼950–1200 mA h g⁻¹ as well as excellent cycling stability and rate capability can be achieved.


	Fig. 3 (a) Schematic illustration of the 2D hybrid ZnMn₂O₄/graphene nanostructure and synergistic electrochemical characteristics of the 2D hybrid nanosheets. (b) SEM image of 2D ZnMn₂O₄/graphene hybrid nanosheets. (c) Rate performances of three different electrodes at various current densities of 200, 400, 800, 1600, and 3200 mA g⁻¹. (d) Cycling performance and Coulombic efficiency of a 2D ZnMn₂O₄/graphene hybrid electrode at a current rate of 2000 mA g⁻¹ for more than 1500 cycles. Reprinted with permission from ref. 96, copyright (2014), American Chemical Society.

Different from the above transition metal oxides, TiO₂ follows a lithium ion insertion/extraction mechanism, which possesses an advantage of negligible structure variation and consequently high cycling stability.^121–127 Zhao's group reported a simple sol–gel strategy towards controllable manipulation of nucleation, growth, anchor, and crystallization of TiO₂ nanoparticles on graphene, resulting in a TiO₂ nanocrystal/RGO sheet hybrid with ultra-dispersed TiO₂ nanoparticles (∼5 nm), ultrathin graphene sheets (≤3 layers), and a high surface area of ∼229 m² g⁻¹. Owing to the potential of strongly synergistic coupling effects, the resulting TiO₂ nanocrystal/RGO sheet hybrid anode exhibited a high specific capacity of ∼94 mA h g⁻¹ at ∼59C (10 A g⁻¹), which was twice that of mechanically mixed composites (∼41 mA h g⁻¹).¹²¹ Furthermore, Qiu and co-workers reported a simple one-step hydrothermal method toward the in situ growth of ultra-dispersed mesoporous TiO₂ nanocrystals with (001) facets on 3D graphene aerogels (GAs), which could provide multidimensional electron transport pathways for Li storage. Due to the strong interaction between TiO₂ and GAs, facet characteristics, high electrical conductivity, and the 3D hierarchically porous structure of the as-prepared TiO₂/3D GA composites, a reversible capacity of 99 mA h g⁻¹ could still be delivered at a very high rate of 5000 mA g⁻¹.¹²⁶

Similarly, spinel Li₄Ti₅O₁₂ possessing reversible Li⁺ insertion/extraction properties together with negligible structural changes during the charge/discharge process has attracted considerable attention as an anode material for lithium-ion batteries.^128–132 Kong et al. synthesized a 3D graphene-wrapped Li₄Ti₅O₁₂ dandelion-like microsphere electrode by using a facile and scalable solution route. Due to the introduction of the graphene network which provides a fast and conductive electron transport path, Li₄Ti₅O₁₂/graphene hybrid anodes displayed excellent rate performance and high cycling stability. An initial capacity of 261 mA h g⁻¹ and a stable capability of 206 mA h g⁻¹ after 500 cycles at 0.12 A g⁻¹ were obtained.¹²⁹

2.1.3 Graphene–metal sulfide hybrids. Metal sulfides, such as MoS₂,^133–138 SnS₂,^139–142 and WS₂,¹⁴³ with a graphite-like layered structure have shown great potential as alternative anode materials of lithium-ion batteries. The large spacing between neighboring layers in principle can act as a host to accommodate the Li⁺ guest. However, their cycling stability and rate performance are still unsatisfactory because of the relatively low electron conductivity. A widely used approach to overcome this problem is to design and optimize graphene–metal sulfide nanocomposites with synergetic interaction for improved electrochemical performances.

Among these metal sulfides, MoS₂ is the most studied anode for Li storage. So far, a number of MoS₂/graphene hybrids have been reported with significantly improved cycle life and rate performance. For example, Chen's group prepared MoS₂/graphene composites by the L-cysteine-assisted hydrothermal method. The layered MoS₂ crystals were anchored and grown on the surfaces of the RGO substrate. The incorporation of graphene can greatly enhance the whole conductivity of the MoS₂/graphene composite electrode and provide rapid electron transport, resulting in a high specific capacity of ∼1100 mA h g⁻¹ at a current density of 100 mA g⁻¹, as well as excellent cycling stability and high-rate capability.¹³³ Later, Zhang and co-workers deposited MoS₂ on the surfaces of 3D graphene networks (3DGNs) by a facile chemical vapor deposition (CVD) method. Since the 3DGNs can provide highly efficient pathways for electronic and Li⁺ ion exchange during the charge/discharge cycles, the resulting MoS₂/3DGN composites could be used as binder-free, conductive additive-free anodes for Li-ion batteries, exhibiting a high reversible capacity of 877 mA h g⁻¹ at 100 mA g⁻¹ and an excellent rate capability of 466 mA h g⁻¹ at a high current density of 4 A g⁻¹.¹³⁵ Due to the strong covalent bonds within layers and weak van der Waals forces between layers, the layered MoS₂ can be exfoliated to single-layer or few-layer sheets, showing different physical and chemical properties compared with its bulk counterparts.^144–147 Recently, the exfoliated MoS₂ nanosheets synthesized by the hydrolysis of lithiated MoS₂ have been supported on the surfaces of the graphene substrate, resulting in a graphene-like MoS₂/graphene nanocomposite. Due to the synergetic effect between highly conductive graphene nanosheets and graphene-like MoS₂, the MoS₂/graphene anodes exhibited a high reversible capacity of 1300–1400 mA h g⁻¹, good cycling stability at 100 mA g⁻¹, and excellent rate capability (Fig. 4).¹³⁶


	Fig. 4 (a) Schematic formation process of the MoS₂/graphene nanocomposite. (b) Cycling behaviors of the synthesized samples: bulk MoS₂, restacked MoS₂, MoS₂/graphene-15 and MoS₂/graphene-30. (c) Rate capability and coulombic efficiency of the MoS₂/graphene-15 electrode. Reprinted with permission from ref. 136, copyright (2014), Royal Society of Chemistry.

2.1.4 Graphene–Si/Sn/Ge-based nanocomposites. Silicon/tin/germanium (Si/Sn/Ge)-based materials, such as metallic Si/Sn/Ge and Si/Sn/Ge-based oxides, are another class of promising anodes for Li-ion batteries, due to their ultrahigh theoretical specific capacities.^18,148 For example, Si possesses a theoretical capacity of ∼4200 mA h g⁻¹, which is the highest value among currently reported anode materials.¹⁴⁹ However, the large volume change of Si/Sn/Ge-based anodes during the charge/discharge process (∼297%, ∼257%, and ∼270% volume expansion for Si, Sn, and Ge, respectively) is the most crucial problem that should be solved before moving on to practical applications. Graphene has been demonstrated as an effective support material for accommodating the large volume change of Si/Sn/Ge due to its strong mechanical flexibility, superior electrical conductivity, and high surface area.¹⁸

Some graphene–Si nanocomposites with zero-dimensional (0D) Si nanoparticles^150–155 or one-dimensional (1D) Si nanowires (NW)^156,157 have been explored with enhanced lithium storage properties. For example, Cho's group reported a graphene–Si nanocomposite where island-shaped amorphous Si nanoparticles in the size range of 5–10 nm were strongly anchored on the graphene backbone. The as-prepared electrodes benefited from the synergetic effect between highly flexible graphene and critically sized Si nanoparticles for self-compacting behavior. The thickness of these loosely interconnected nanostructure electrodes decreased after cycling until reaching appropriate agglomeration without any cracks. Thus an excellent average charge capacity of ∼1103 mA h g⁻¹ over 1000 cycles without capacity fading was achieved (Fig. 5a and b).¹⁵² Furthermore, a Si NW-based architecture via encapsulation of Si NWs with overlapped graphene sheets and RGO overcoats was explored by Wang and co-workers. On one hand, the overlapped graphene sheaths adapted to the volume change of embedded Si NWs by synergistic transformation. On the other hand, the RGO overcoats acted as mechanically robust and flexible matrices to accommodate the volume change of embedded Si NW/graphene nanocables. Thus a high reversible specific capacity of 1600 mA h g⁻¹ at 2.1 A g⁻¹ and high cycling stability of 80% capacity retention after 100 cycles were obtained due to the well-maintained structural and electrical integrity of the electrodes.¹⁵⁶ Recently, Chen's group fabricated a 3D multilayered Si/RGO hybrid anode by assembly of alternating Si/RGO layers on porous Ni foams. The multilayered structures offering ample space for Si expansion were able to effectively mitigate detrimental volume expansion upon lithiation. As a result, high reversible specific capacity (2300 mA h g⁻¹ at 0.05C), good rate capability (700 mA h g⁻¹ at 10C), and superior cycling capability (87% capacity retention at 10C after 152 cycles) were obtained.¹⁵⁴ Compared with silicon, silicon oxides (SiO_x, 0 < x ≤ 2) with less volume change during lithiation usually exhibit better cycling stability. In order to improve the poor electronic conductivity of silicon oxides, Li et al. prepared a graphene nanoplatelet-supported SiO_x-disordered carbon composite by self-assembly between graphene nanoplatelets and C₂H₅Si(OC₂H₅)₃, followed by high-temperature treatment. A stable reversible capacity of about 630 mA h g⁻¹ at a current density of 100 mA g⁻¹ was achieved without obvious capacity fading after 250 cycles.¹⁵⁸


	Fig. 5 (a) Schematic view of elastic Si/graphene nanocomposites before and after electrochemical cycling. (b) Cycling performance of Si/graphene at a charge current density of 14 A g⁻¹ and a discharge density of 2.8 A g⁻¹ over 1000 cycles. Reprinted with permission from ref. 152, copyright (2014), American Chemical Society. (c) Schematic illustration of 3D Sn@graphene/graphene. (d) Cycle performance of the 3D Sn@graphene/graphene electrode at current densities of 0.2, 0.5, and 1 A g⁻¹ for the initial six cycles and then 2 A g⁻¹ for the subsequent 1000 cycles. Reprinted with permission from ref. 163, copyright (2014), American Chemical Society. TEM images of (e) as-grown Ge NWs, (f) a single layer of graphene on a Ge NW, and (g) a bilayer of graphene grown on a Ge NW. (h) Cycle performance of a graphene/Ge NW at each C-rate from 0.5 to 4.0C. Reprinted with permission from ref. 168, copyright (2013), Wiley-VCH.

For Sn–graphene hybrid anodes, the incorporation of graphene^159,160 and modified graphene (such as N-doped graphene,^161,162 3D graphene,^163,164 and 3D N-doped graphene¹⁶⁵) also provides impressive advantages. Yang et al. described a facile solvothermal strategy for the in situ growth of mesoporous SnO₂ on the graphene surface. The formation of mesoporous SnO₂ crystals on the surface of graphene can avoid the aggregation of graphene, while the graphene substrate can restrain the collapse of the mesoporous structure. Therefore, the as-synthesized SnO₂/graphene with large surface area, good electrical conductivity and stable porous structure exhibited greatly improved electrochemical performances as an anode material for Li-ion batteries.¹⁵⁹ Later, SnO₂ nanocrystals were hybridized with N-doped graphene sheets via Sn–N bonding as reported by Guo's group. The N-doped graphene sheets not only act as a flexible buffer to accommodate the huge volume change of SnO₂ nanocrystals, but also provide enhanced lithium electrochemical activity compared with undoped graphene. The as-obtained SnO₂/N-doped graphene hybrid showed a stable capacity of 1021 mA h g⁻¹ under a current density of 0.5 A g⁻¹. Moreover, a reversible charge capacity as high as 1346 mA h g⁻¹ could be still achieved after 500 cycles.¹⁶² In another work, novel 3D porous graphene networks anchored with Sn nanoparticles (5–30 nm) encapsulated with graphene shells (∼1 nm) were synthesized using an in situ CVD technique using metal precursors as a catalyst and 3D self-assembled NaCl particles as a template. As a result, long-term cycling stability at high rates (approximately 96.3% capacity retention was maintained at 2 A g⁻¹ after 1000 cycles) and superior rate capacity (a reversible capacity of 270 mA h g⁻¹ at 10 A g⁻¹) were achieved. The superior performances of this 3D hybrid anode can mainly be ascribed to the dual graphene encapsulation: flexible graphene shells suppress the aggregation of Sn nanoparticles and accommodate the volume expansion, leading to structural and interfacial stabilization of Sn nanoparticles; 3D porous graphene networks with superior electrical conductivity, high mechanical flexibility, and large surface area facilitate diffusion and transport of electrons and ions, leading to remarkably enhanced structural and electrical integrity (Fig. 5c and d).¹⁶³

Ge also possesses a high theoretical capacity of 1624 mA h g⁻¹. Recently, significant research efforts have been devoted to the graphene-supported Ge-based hybrid anodes.^166–170 Lee's group used a low-pressure thermal evaporation approach to uniformly deposit crystalline Ge particles on graphene surfaces or embed into graphene sheets. A high initial coulombic efficiency of 80.4% in the first cycle and a capacity retention of 84.9% after 400 cycles were achieved.¹⁶⁷ However, for the above deposit-type composite, it is difficult to provide a high-quality integrated protection layer for volume expansion, which frequently results in poor capacity retention during long cycling. An ideal approach to mitigate the volume expansion issue of Ge is to directly grow single to a few layers of graphene on Ge nanostructure surfaces. Cho and co-workers reported the direct growth of a single to a few (less than four) layers of graphene on Ge NWs. The Ge NW/graphene hybrid anode exhibited a high reversible specific capacity of 1059 mA h g⁻¹ even after 200 cycles at a rate of 4.8 A g⁻¹ (Fig. 5e–h).¹⁶⁸ Then, Wang et al. reported a similar but completely catalyst-free route to prepare uniform, highly crystalline in situ graphene encapsulated Ge nanowires via the arc-discharge method. The graphene encapsulated Ge NW composites provided a reversible capacity of 430 mA h g⁻¹ at a current density of 1600 mA g⁻¹.¹⁶⁹ The above two cases demonstrated that such a high-quality single- or few-layered graphene coating on Ge NWs not only would guarantee mechanically tight holding of the Ge NWs, but also would be suitable for Li ions to diffuse into and out of the Ge NWs throughout long cell cycles.

2.1.5 Graphene–other carbon hybrid nanomaterials. In spite of the high theoretical capacitance, relatively low specific capacity values of graphene anodes were obtained, which may be attributed to the re-stacking of RGO to form a graphite-like analogue, subsequently limiting Li storage capacity. Recently, it has been reported that the combination of graphene with other carbon nanomaterials, such as carbon nanotubes (CNTs),^171–177 carbon nanofibers,^178,179 carbon nanospheres,¹⁸⁰ and porous carbon,^181,182 can deliver higher Li storage capacity. For example, the introduction of CNTs can provide multiple benefits to increase the storage capacity, including (1) suppression of the restacking of graphene nanosheets, (2) increase of interlayer space as CNT can be intercalated between graphene layers, (3) improvement of cross-plane conductivity between two isolated graphene sheets, and (4) additional Li storage capacity.

Honma's group first reported a graphene/CNT composite anode for Li-ion batteries. The interlayer distance of graphene was enhanced through CNT interaction. The graphene/CNT anode showed an increased specific capacity of 730 mA h g⁻¹, which is much larger than that of graphite (540 mA h g⁻¹).²⁶ However, this graphene/CNT composite was prepared by a simple physical mixing of pre-fabricated CNTs and graphene, which may lead to poor interaction, and consequently unfavorable charge transport. To this point, Vinayan et al. reported the incorporation of multi-walled carbon nanotubes (MWCNTs) along with graphene sheets by strengthened electrostatic interaction between cationic polyelectrolyte modified graphene and acid treated MWCNTs. A discharge capacity of 768 mA h g⁻¹ was obtained at a current density of 90 mA g⁻¹ after 100 cycles.¹⁷¹ Wang's group prepared graphene/CNT composites by an in situ chemical vapor reduction and deposition process. CNTs with various lengths were uniformly grown on the surfaces of neighboring graphene to prevent restacking. Besides, the electrical conductivity of the graphene/CNT composite along the vertical direction was also improved compared with pure graphene. Thus, high reversible capacities of 573 mA h g⁻¹ at a small current density of 74 mA g⁻¹, and 520 mA h g⁻¹ at a large current density of 744 mA g⁻¹ were obtained, respectively.¹⁷² Similarly, Wang et al. reported a 3D graphene/CNT structure based on the optimized growth of hybrid CNT and graphene nanostructures on copper foil by a two-step CVD process. The seamlessly connected graphene/CNT hybrids provided a high reversible capacity of 900 mA h g⁻¹ at a current density of 100 mA g⁻¹ and a capacity retention of 98.92% at a current density of 600 mA g⁻¹ for 250 cycles.¹⁷³

2.1.6 Graphene–graphitic carbon nitride composites. As mentioned above, N doping is an efficient method to improve reversible capacity, initial Coulombic efficiency, or cycle performance.^36,42–44 Graphitic carbon nitride (g-C₃N₄), regarded as N-substituted graphite with the highest N-doping level, may have a considerable potential as an anode material for high-rate Li-ion batteries.^183,184 However, the poor conductivity of g-C₃N₄ restricts its use as an electrode material. Recently, our group designed and synthesized a covalently coupled g-C₃N₄/graphene hybrid by functionalization of GO with diacyanamide via the post-thermal treatment. The combination of g-C₃N₄ with highly-conductive graphene via covalent interactions resulted in a significantly enhanced electrochemical performance. The g-C₃N₄/graphene hybrid exhibited a high reversible capacity of 1525 mA h g⁻¹ at a current density of 100 mA g⁻¹ after 50 cycles. A reversible capacity of 943 mA h g⁻¹ could still be retained even at a large current density of 1000 mA g⁻¹.¹⁸⁵

2.2 Graphene-based materials for cathodes

The most common cathode materials for Li-ion batteries are lithium transition-metal oxides (such as LiCoO₂ and LiMn₂O₄), lithium transition-metal phosphates LiMPO₄ (M = Fe, Mn, Co, or Ni), and vanadium oxide (V₂O₅).^9,10,186,187 Unfortunately, the inherently low ionic and electrical conductivities of these materials seriously limit Li⁺ insertion/extraction and charge transport rates. For example, the electronic conductivities of LiCoO₂, LiMn₂O₄, and LiFePO₄ are 10⁻⁴, 10⁻⁶, and 10⁻⁹ S cm⁻¹, respectively, which are fairly low and can impair rate capability.¹⁰ Over the past decade, graphene and its derivatives have been extensively employed in the cathode system to enhance the electrochemical properties (Table 2).^9,10

Table 2 Summary of graphene-based cathodes for Li-ion batteries

Electrode materials	Capacity	Rate capability	Cycle life	Ref.
LiMn₂O₄/graphene	137 mA h g⁻¹ (148 mA g⁻¹)	101 mA h g⁻¹ (14.8 A g⁻¹)	100 (96%)	188
LiFePO₄/graphene	147.1 mA h g⁻¹ (20 mA g⁻¹)	63 mA h g⁻¹ (5000 mA g⁻¹)	200 (94.2%)	195
LiFePO₄/graphene	120 mA h g⁻¹ (1C)	85.1 mA h g⁻¹ (50C)	950 (91.4%)	196
C@LiFePO₄/graphene	164 mA h g⁻¹ (0.2C)	124 mA h g⁻¹ (20C)	1000 (95%)	197
LiFePO₄/graphene	168 mA h g⁻¹ (0.5C)	115 mA h g⁻¹ (10C)		198
LiFePO₄/graphene	208 mA h g⁻¹ (0.1C)	125 mA h g⁻¹ (10C)		199
LiFePO₄/N-doped graphene	166 mA h g⁻¹ (0.5C)	127 mA h g⁻¹ (20C)		200
Li₃V₂(PO₄)₃/graphene	125.3 mA h g⁻¹ (0.1C)	105.7 mA h g⁻¹ (20C)	100 (96.7%)	203
Li₃V₂(PO₄)₃/graphene	186 mA h g⁻¹ (0.1C)	135 mA h g⁻¹ (10C)		202
V₂O₅/graphene	278 mA h g⁻¹ (0.1C)	165 mA h g⁻¹ (2C)	100 (78%)	208
V₂O₅ nanosheet/graphene	223 mA h g⁻¹ (0.2C)	76 mA h g⁻¹ (50C)	160 (52%)	207

The spinel LiMn₂O₄ cathode is being widely used due to its low production cost, environmental friendliness and good safety. In addition to its poor ionic and electronic conductivities, the dissolution of Mn²⁺ in the electrolyte is another severe obstacle that should be solved. With introduction of graphene which can serve as protection layer and provide large surface areas and improved Li diffusion kinetics, the cycling stability and rate capability of LiMn₂O₄/graphene hybrid cathodes have been remarkably improved compared to the pure LiMn₂O₄ cathode.^188,189 Partially substituting Mn of LiMn₂O₄ with other metal ions such as Ni²⁺ afforded spinel LiNi_0.5Mn_1.5O₄, which possesses fast 3D Li⁺ diffusion channels, a theoretical capacity of 146.7 mA h g⁻¹, and a high operational voltage of ∼4.7 V. Similarly, in order to solve the poor conductivity of LiNi_0.5Mn_1.5O₄, the GO-coated LiNi_0.5Mn_1.5O₄ cathode¹⁹⁰ and self-assembled LiNi_0.5Mn_1.5O₄/graphene composites¹⁹¹ have been successfully developed for improving cycling stability and rate performance.

Olivine-structured LiFePO₄ is one of the most popular commercial cathode materials for Li-ion batteries owing to its several advantageous features for Li storage such as low cost, environmental benignity, relatively large capacity, thermal stability, and excellent cycle life. However, the inherently low conductivity has hindered its practical application. Several reports have demonstrated the enhanced capacity of graphene/LiFePO₄ composite cathodes, which were usually prepared by pyrolyzing GO or RGO together with either LiFePO₄ precursors or LiFePO₄ particles.^192–198 Hu and co-workers reported the modification of LiFePO₄ with few-layer graphene obtained by a scalable electrochemical exfoliation method. The as-obtained graphene-wrapped LiFePO₄ hybrid cathode was able to deliver a capacity of 208 mA h g⁻¹, which is beyond the theoretical capacity of LiFePO₄ (170 mA h g⁻¹). The excess capacity can be attributed to the reversible redox reaction between the Li ions of the electrolyte and the exfoliated graphene flakes, where the graphene flakes exhibited an ultrahigh capacity (>2000 mA h g⁻¹).¹⁹⁹ Recently, N-doped graphene prepared by using urea as a nitrogen source has also been employed to combine with LiFePO₄. The resulting LiFePO₄/N-doped graphene nanocomposite shows enhanced electrochemical performance in terms of high discharge capacities and voltages at high rates including sub-zero temperature conditions.²⁰⁰ Besides, considerable efforts have been devoted to the integration of graphene with other lithium transition-metal phosphate cathodes such as Li₃V₂(PO₄)₃, due to their higher discharge potential (e.g. 3.6–4.55 V) and higher theoretical capacity (e.g. 197 mA h g⁻¹).^201–204

V₂O₅ is another promising cathode candidate for Li-ion batteries because of its higher theoretical capacity (440 mA h g⁻¹) as compared with those of the above cathodes. Besides the inferior electronic conductivity, the particle agglomeration and resultant short cycle life (typically less than a couple of hundred cycles) are more challenging issues for practical applications. Various graphene–V₂O₅ composites have been developed to resolve the sluggish electronic conductivity of V₂O₅ and increase Li⁺ transportation kinetics.^205–208 However, the short cycle life of these V₂O₅/graphene hybrid cathodes has yet to be well resolved. Recently, Lee et al reported a hybrid structure composed of ultrathin V₂O₅ NWs homogeneously entangled with graphene sheets, where the agglomeration of the V₂O₅ NWs could be efficiently suppressed by the robust graphene sheets. As a result, a substantially improved cycle life over 100 [thin space (1/6-em)] 000 cycles at a high current density of 10000 mA g⁻¹ was achieved.²⁰⁹

2.3 Graphene-based materials for lithium ion full batteries

The most common Li-ion batteries composed of a commercial Li-intercalated compound cathode (such as LiCoO₂ or LiFePO₄) and a graphitic anode have high energy densities of ∼120–150 W h kg⁻¹, but suffer from low power densities compared to electrochemical capacitors.²¹ The direct method of increasing the rate capability is to replace the electrode materials with other nanomaterials having larger Li-storage capacity, a shortened Li⁺ diffusion length and better charge transport kinetics. To this end, a large fraction of current research is concentrated on assembly of Li-ion full batteries based on graphene^210,211 and graphene-based hybrid^212,213 electrodes, owing to their large specific capacity, high rate capability, and superior mechanical flexibility (Table 3).^1,214

Table 3 Summary of graphene-based materials for Li-ion full batteries

Anode materials	Cathode materials	Capacity	Rate capability	Cycle life	Ref.
a The discharge capacity of this full battery is based on the mass loading of anode materials. The discharge capacities of other full batteries shown in this table are based on the mass loading of cathode materials.
Graphene	LiFePO₄	165 mA h g⁻¹ (1C)		80 (87%)	210
Graphene film	LiCoO₂	127 mA h g⁻¹ (0.2C)			211
Si/graphene	LiNi_1/3Mn_1/3Co_1/3O₂	137 mA h g⁻¹ (C/15)		15 (70%)	212
Ge/graphene/C	LiCoO₂	900 mA h g⁻¹^a (1C)		100 (73%)	213
Li₄Ti₅O₁₂/3D graphene foam	LiFePO₄/3D graphene foam	143 mA h g⁻¹ (0.2C)	117 mA h g⁻¹ (10C)	100 (96%)	215
2D ZnMn₂O₄/graphene	LiFePO₄ nanosheets	132 mA h g⁻¹ (0.2C)	58 mA h g⁻¹ (10C)	100 (94%)	216

Hassoun and co-workers reported an advanced Li-ion full battery based on a graphene ink-based anode and a commercial LiFePO₄ cathode. Due to the use of a high-performance graphene anode produced via drop casting graphene ink on a Cu substrate, the as-prepared full cell was operated for over 80 charge/discharge cycles at a 1C (170 mA g⁻¹) rate with a reversible capacity of 165 mA h g⁻¹, and displayed an estimated energy density of about 190 W h kg⁻¹, exceeding ∼25–60% over the current lithium ion battery technology.²¹⁰ Tuan's group assembled a lithium ion full battery by using a carbon-coated Ge nanoparticle/RGO anode and a commercial LiCoO₂ cathode. The full cell displayed a stable capacity of ∼1000 mA h g⁻¹ in 100 cycles and was able to power a wide range of electronic devices, including an light-emitting-diode (LED) array consisting of over 150 bulbs, blue LED arrays, a scrolling LED marquee, and an electric fan.²¹³

Although the above full batteries using graphene or graphene hybrid anodes instead of conventional graphite exhibited much enhanced performances, the use of commercial micropowder cathodes had hindered the total charge transfer. In order to achieve higher performances, it is necessary to further replace the commercial micropowder cathodes with nanostructured alternatives for fabrication of full batteries, so that both the anode and the cathode belong to the nanostructure. For example, an all-3D graphene foam-based Li-ion full battery composed of Li₄Ti₅O₁₂-loaded graphene foam anodes and LiFePO₄-modified graphene foam cathodes was explored by Chen's group. By employing graphene foam, a 3D, flexible, and conductive interconnected network, as a current collector, the all-3D graphene foam-based lithium ion full battery showed promising rate capability, delivering a high specific capacity of 117 mA h g⁻¹ at 10C with reserving 88% of the capacity at 1C. Moreover, a thin, lightweight, and flexible lithium ion full battery was also fabricated with a high-rate performance and energy density under repeated bending.²¹⁵ Recently, Yu's group and our group reported an all-2D nanosheet-based Li-ion full battery with the 2D ZnMn₂O₄/graphene anode and the LiFePO₄ nanosheet cathode. By taking advantage of these two unique 2D nanostructured electrodes, which can provide excellent electronic conductivity, more electrochemically active surfaces, and a reduced diffusion path for Li ion extraction/insertion, the as-assembled Li-ion full battery exhibited superior cycling performance and rate capability compared with those of control full batteries composed of the conventional graphite anode and the commercial LiFePO₄ powder cathode. Further, a flexible pouch cell based on these two 2D nanostructures was also assembled. Even under various bending states including bent, folded and rolled, a stable reversible capacity could still be delivered (Fig. 6).²¹⁶


	Fig. 6 (a) Schematic of an all-nanosheet-based Li-ion full battery using 2D ZnMn₂O₄/graphene hybrid nanosheets and LiFePO₄ nanosheets as the anode and the cathode, respectively. (b) Comparison of rate capabilities of an all-nanosheet-based full battery and a control full battery. (c) A flexible pouch cell based on the nanosheet anode and cathode can light a red LED logo (UT) at flat, 90° bent, 180° bent, and rolled states. (d) Cyclic performance of the pouch cell at flat, 90° bent, 180° bent, and rolled states for 20 cycles, respectively. Reprinted with permission from ref. 216, copyright (2015), Elsevier Ltd.

3. Graphene-based nanocomposites for lithium–sulfur batteries

During the pursuing of even higher energy density for rechargeable batteries, alternative electrodes that possess different mechanisms other than the intercalation of Li ions have been extensively investigated. A lithium–sulfur (Li–S) battery that utilizes a Li anode and a S cathode can provide a high theoretical capacity of ∼1675 mA h g_sulfur⁻¹ upon the complete redox reaction (S₈ + 16Li = 8Li₂S), which corresponds to a high theoretical specific energy and volumetric energy density of ∼2572 W h kg⁻¹ and ∼2835 W h L⁻¹, respectively.^6,217 Although tremendous progress in Li–S batteries has been made so far, some of the key challenges and critical issues still need to be addressed.^5,218–223 Firstly, element S and its various lithiation products (Li₂S_x, x = 1–8) are electrical and ionic insulating materials. For example, the electrical conductivity of S is 5 × 10⁻³⁰ S cm⁻¹ at 25 °C, which is so low that it will create a large internal resistance and block the charge transport. Thus, development of sulfur-based composites in which nanosized S particles are well embedded into conductive matrices such as conducting polymers or carbon materials has been a compelling research topic.^218,224 Secondly, a large volume expansion of ∼80% will occur during lithiation due to the conversion of sulfur (∼2.03 g cm⁻³) to lithium sulfide (∼1.63 g cm⁻³). Repeated volume expansion/contraction during charge/discharge cycles may induce severe collapse of material's structure, resulting in rapidly declining capacity. A hollow structure or a buffering layer should be employed to provide sufficient space for accommodating volume changes.^220,225,226 Thirdly, various soluble polysulfide intermediates (Li₂S_x, x = 3–6) shuttle between the cathode and the anode, which lead to a low Coulombic efficiency and parasitic self-discharging. Finally, continuous deposition of the insulting Li₂S layer onto the surface of the anode inhibits ion and electron transport, leading to degraded performances.^11,227,228

A number of approaches have been proposed to overcome the above obstacles.^{5,222,229,230} Among these strategies, introduction of carbon nanomaterials, especially graphene in Li–S batteries, has been widely investigated for the following reasons: (1) highly conductive graphene can provide a conductive framework for improved electrical properties and effective charge transfer; (2) flexible graphene can act as a 2D buffering layer and/or build a 3D porous network to accommodate the volume change of sulfur during repeated charge/discharge cycles; (3) functionalized graphene networks immobilize sulfur and lithium polysulfides, suppressing the shuttle phenomenon. The recent advances in graphene-based Li–S batteries are summarized in the following sections.

3.1 Graphene-based sulfur cathodes

Integrating sulfur with 2D conductive, flexible graphene is expected to afford improved performances for Li–S batteries. In this regard, various methods including ball-milling,^231,232 melt-diffusion,^233–237 self-assembly,^14,238 chemical reaction approach,^226,239 solution precipitation,²⁴⁰ electrodeposition,²⁴¹ and electrochemical assembly²⁴² have been explored to effectively combine sulfur with graphene-based or GO-based nanostructures for graphene-based sulfur cathodes (Table 4).

Table 4 Summary of the graphene-based sulfur cathode for Li–S batteries

Cathode materials	S mass loading	Capacity^a	Cycle life	Ref.
a The capacity is calculated based on the mass of sulfur.
S/GO	66 wt%	1000 mA h g⁻¹ (0.1C)	50 (95.4%)	226
S/GO core–shell structure	50 wt%	900 mA h g⁻¹ (1 A g⁻¹)	1000 (88.9%)	240
S/graphene	70 wt%	966.1 mA h g⁻¹ (2C)	500 (53%)	232
S/graphene	70.2 wt%	934 mA h g⁻¹ (0.1 A g⁻¹)	60 (95.4%)	241
S/porous graphene	67 wt%	927 mA h g⁻¹ (1C)	100 (74%)	234
S/graphene nanoshell	62 wt%	1098 mA h g⁻¹ (1C)	1000 (38.2%)	235
S/templated graphene	64 wt%	734 mA h g⁻¹ (10C)	200 (85.5%)	236
S/graphene core–shell structure	83.3 wt%	514 mA h g⁻¹ (3C)	500 (94.2%)	239
S/graphene nanowalls	66 wt%	1261 mA h g⁻¹ (209 mA g⁻¹)	120 (96%)	242
S/EDA–RGO	60 wt%	820 mA h g⁻¹ (0.5C)	350 (80%)	243
S/N-doped graphene	60 wt%	967 mA h g⁻¹ (1C)	500 (52.4%)	245
S/N-doped graphene	80 wt%	1356.8 mA h g⁻¹ (0.1C)	100 (62.5%)	246
S/3D graphene	73 wt%	1260 mA h g⁻¹ (0.1C)	100 (55.6%)	237
S/3D graphene	63 wt%	700 mA h g⁻¹ (0.75 A g⁻¹)	100 (77.3%)	238
S/3D graphene	80 wt%/12 mg cm⁻²	6.0 mA h cm⁻²/625 mA h g⁻¹ (0.1C)	300 (75.5%)	248
S/3D graphene foam	52 wt%	402.8 mA h g⁻¹ (3.2 A g⁻¹)	400 (74.5%)	249
S/3D N-doped graphene	87.6 wt%	743 mA h g⁻¹ (1.5 A g⁻¹)	200 (85.6%)	250
S/graphene-porous carbon	68 wt%/0.74 mg cm⁻²	885.5 mA h g⁻¹ (0.5C)	100 (70%)	252
S-activated carbon/graphene	1.2–1.5 mg cm⁻²	1113 mA h g⁻¹ (0.2C)	300 (88.9%)	259
S-MWCNT/graphene	70 wt%	1396 mA h g⁻¹ (0.2C)	100 (60.5%)	254
S-MWCNT/graphene	68.93 wt%	727.2 mA h g⁻¹ (5C)	200 (85.3%)	255
S/graphene/CNT	50 wt%	1048 mA h g⁻¹ (1C)	1000 (56.7%)	256
S/3D graphene-CNT	1.0 mg cm⁻²	1179.6 mA h g⁻¹ (0.2C)	200 (83%)	257
S/3D graphene-CNT	70 wt%	946.2 mA h g⁻¹ (0.3C)	100 (74.4%)	258
S/graphene-SWCNT	60 wt%	822 mA h g⁻¹ (5C)	100 (80%)	260
S/graphene-CNT@porous carbon	77 wt%	914 mA h g⁻¹ (1C)	150 (72%)	261
S/N-doped CNT-graphene	1.0 mg cm⁻²	1152 mA h g⁻¹ (1C)	80 (76.4%)	262
S/GO@PANI	60.1 wt%/1.0 mg cm⁻²	1248 mA h g⁻¹ (0.5C)	100 (80.6%)	263
S/curved graphene@PANI	55 wt%	851 mA h g⁻¹ (0.2C)	100 (90%)	264
S/porous graphene@PEDOT:PSS	60.1 wt%/1.0 mg cm⁻²	1198 mA h g⁻¹ (0.1C)	200 (70.5%)	267
S/N-doped graphene@PANI	52.5 wt%	1277.3 mA h g⁻¹ (0.5C)	100 (54.3%)	265
S/porous carbon/graphene@PPy	64 wt%	470 mA h g⁻¹ (3C)	400 (80%)	266

3.1.1 Graphene–sulfur composites. GO with functional groups, such as epoxy and hydroxyl groups, provide adequate anchor spots for sulfur deposition. For example, Zhang's group used a chemical reaction approach to immobilize sulfur on GO to prepare GO/S nanocomposite cathodes for Li–S batteries. The oxygen functional groups of GO can strongly adsorb S atoms and effectively prevent the dissolution of subsequently formed polysulfides into the electrolyte during cycling. Due to the strong interaction between GO and sulfur or polysulfides, the GO/S nanocomposites can maintain a high capacity of ∼950 mA h g⁻¹ over 50 cycles at 0.1C (1C = 1675 mA g⁻¹).²²⁶ Dai and co-workers synthesized a GO/S composite via a simple assembly process between polyethylene glycol (PEG)-coated S particles and carbon black decorated GO sheets. The resulting GO/S composite showed high and stable specific capacities of up to ∼600 mA h g⁻¹ over 100 cycles.¹⁴ Zhou and co-workers reported a facile method to uniformly coat GO on S particles by solution ionic strength engineering. GO sheets tend to crumple, form wrinkles and restack to minimize their surface energy when dispersed in ionic solution. While in the presence of S particles, GO is prone to coat and precipitate on the particle surface, rendering a core–shell GO/S structure. The spacing between stacked GO layers can not only act as a channel for lithium ion transportation, but also impede polysulphide dissolution. Thus a specific capacity of ∼800 mA h g⁻¹ was retained after 1000 cycles at a 1 A g⁻¹ current rate with a capacity decay rate of less than 0.02% per cycle (Fig. 7).²⁴⁰ However, it should be noted that the use of low conductivity GO suppresses the performances to some extent.


	Fig. 7 (a) Digital camera images of GO dispersed in different solutions at the beginning, after 12 h and after adding sulfur particles. (b) Galvanic charge/discharge performance and Coulombic efficiency of sulfur/GO at 1 A g⁻¹ for 1000 cycles. Discharge specific capacities calculated based on the weight of sulfur only (green dotted line) and the total weight of sulfur/GO are plotted. Reprinted with permission from ref. 240, copyright (2014), American Chemical Society.

Graphene with high conductivity and large surface areas is in favour of more effective charge transfer compared to GO. Wang et al. synthesized sulfur/graphene composites by heating a mixture of graphene nanosheets and elemental sulfur and the later was incorporated into the graphene aggregates. The sulfur/graphene composite showed improved electrical conductivity, while the cycling performance was not improved remarkably compared to pure sulfur.²³³ Li et al. explored an electrochemical assembly strategy to prepare a vertically aligned sulfur/graphene nanowall structure, in which sulfur nanoparticles were homogeneously embedded in the interlayers of graphene and the ordered graphene arrays were arranged perpendicularly to the electrically conductive substrates. Such a unique nanowall structure is favourable for fast lithium and electron transport in the electrode, leading to a high-rate performance (over 400 mA h g⁻¹ at 8C, 13.36 A g⁻¹) (Fig. 8). However, upon increasing sulfur content to 80.8%, the cycle performance decayed obviously.²⁴²


	Fig. 8 (a) Schematic diagrams of vertically aligned sulfur–graphene nanowalls as a cathode for a Li–S battery, facilitating the fast diffusion of both lithium and electrons. (b and c) TEM images of the sectional slices of sulfur–graphene nanowalls with different magnifications, revealing that sulfur nanoparticles are anchored in between graphene layers and ordered graphene arrays in each sulfur–graphene nanowall. (d) Rate performance of sulfur–graphene nanowalls cycled at various current rates and long cycle performance at a constant current rate of 4C. Reprinted with permission from ref. 242, copyright (2015), American Chemical Society.

In the above reports, the active sulfur was just impregnated between the interlayers of graphene nanosheets. Consequently, most of the active sulfur was exposed to the electrolyte and the formed polysulfides could still readily diffuse out of the graphene structure, which significantly undermined the cycling stability. To this point, Zhang's group confined active sulfur in the nanopores of highly porous activated graphene nanosheets (AGNs). The resultant AGN/S nanocomposites exhibited capacity retentions of 67% and 74% after 100 cycles at rates of 0.5C and 1C, respectively. The nanopores of AGNs could not only act as “microreactors” for the electrochemical reactions of sulfur, minimizing the dissolution and shuttling of polysulfide, but also ensure good electrolyte penetration for fast transport of lithium ions.²³⁴ Recently, Zhao et al. synthesized an intrinsically unstacked double-layer templated graphene (DTG) via a template-directed CVD method. The as-obtained DTG was composed of two unstacked graphene layers separated by a large quantity of protuberances. After incorporation of sulfur, this DTG/S composite exhibited high reversible capacities of ∼530 mA h g⁻¹ and ∼380 mA h g⁻¹ after 1000 cycles at 5C and 10C, respectively. The high performances of the DTG/S cathode were attributed to its excellent electrical conductivity, as well as to its unique porous structure, which allowed the effective storage of sulfur in the mesosized lamellar interlayer space, leading to an efficient connection between sulfur and graphene, which prevented the diffusion of polysulphides into the electrolyte.²³⁶ Besides, in order to better confine the sulfur nanocrystals in the electrode and impede polysulfide diffusion more effectively, graphene-wrapped sulfur core–shell nanocomposites have been explored.^235,239,241 For example, Peng et al. employed hollow graphene nanoshells (HGNs) with a diameter of ca. 10–30 nm and a pore volume of 1.98 cm⁻³ g⁻¹ as hosts to accommodate sulfur for Li–S batteries. The use of HGNs offered free space for sulfur expansion during discharge and prevented the diffusion of lithium polysulfide, resulting in an ultraslow decay rate of 0.06% per cycle during 1000 cycles. Besides, interconnected HGNs provided intercrossed ion channels for rapid diffusion of ions and excellent electron pathways, leading to a high retention of 70% when the current density increased from 0.1C to 2.0C (Fig. 9).²³⁵


	Fig. 9 (a) Schematic illustration of the in situ catalytic self-limited assembly of HGNs. (b) SEM, (c) TEM, and (d) HR-TEM images of HGNs. The scale bars in b, inset of b, c, and d are 1 μm, 100 nm, 20 nm, and 5 nm, respectively. (e) Cycling performance at a current density of 1C and schematic illustration of HGN-S during discharge and charge (inset). Reprinted with permission from ref. 235, copyright (2014), American Chemical Society.

3.1.2 Functionalized graphene–sulfur hybrids. To further immobilize sulfur, graphene is modified with functional groups or doped with heteroatoms. Because of the relatively weak interactions between non-polar carbon materials and polar lithium polysulfides, controllable surface modification of graphene with target functional groups is essential to provide intimate combination of graphene and polysulfides, which effectively confines polysulfides in the electrode and increases cycling stability. Zhang's group has proved that both epoxy and hydroxyl groups of partially reduced GO can not only anchor S atoms, but also enhance the coupling of S to the C–C bonds, thus reducing the loss of soluble polysulfides.²²⁶ Recently, Wang et al. used amino-functionalized RGO, such as ethylenediamine (EDA)-modified RGO, as a scaffold for the strongly covalent stabilization of sulfur and its discharge products. Based on the density-functional theory (DFT) calculations, the strong affinity of lithium sulphides for EDA-functionalized RGO (EFG) was further verified by a high binding energy of 1.13–1.38 eV. Consequently, the EFG/S cathodes exhibited a high capacity retention of 80% for 350 cycles with high capacities of ∼650 mA h g⁻¹ and excellent high-rate response up to 4C for 200 cycles.²⁴³

As mentioned before, heteroatom doping can endow graphene with various new or improved properties, such as conductivity and lithium storage capacity.^42,244–246 Zhang's group reported an N-doped graphene-wrapped sulfur cathode for Li–S batteries. The effects of N-doping are manifested not only in the significant improvement of the electronic conductivity of the overall S cathode, but also in the enhancement of ionic attractions between N and Li for effective lithium polysulfide trapping. Thus, the cycle life was prolonged to more than 2000 cycles with an extremely low capacity-decay rate (0.028% per cycle).²⁴⁵

3.1.3 3D graphene–sulfur composites. 3D graphene with interconnected porous structures has been extensively investigated as a unique platform for energy storage applications, including Li–S batteries.^{237,238,247–251} An S/3D graphene composite with 73 wt% sulfur showed a significantly enhanced cycling performance (>700 mA h g⁻¹ after 100 cycles at a 0.1C rate with a Coulombic efficiency >96%) as well as high rate capability. Compared with 2D graphene, 3D graphene provides a unique 3D conductive matrix for fast electron and ion transfer. In spite of this, 3D graphene offers a highly porous inner space to restrain dissolution of polysulfides and suppress the “shuttle effect”.²³⁷ Furthermore, Zhou et al. prepared a 3D graphene/S hybrid by a one-pot hydrothermal assembly method. Ultrafine sulfur nanocrystals (5–10 nm) were incorporated into a highly porous network structure constructed by fibrous graphene. The graphene/S cathode maintained a capacity as high as 541 mA h g⁻¹ at 0.75 A g⁻¹ over 100 cycles (Fig. 10). The high performances can be attributed to two reasons. First, the combination of highly conductive fibrous graphene and small sulfur nanocrystals can greatly improve the kinetics of charge and ion transfer. Second, the strong interactions between sulfur/polysulfides and oxygen-containing functional groups on graphene can effectively reduce the dissolution of polysulfide intermediates into the electrolyte.²³⁸ Similarly, Lu et al. reported a 3D sulfur/graphene hybrid of sulfur embedded into porous graphene sponges. This 3D electrode with large areal mass loading of sulfur (12 mg cm⁻²) was directly used as the cathode and delivered an extremely high areal specific capacitance of 6.0 mA h cm⁻² together with a low capacity decay rate (0.08% per cycle after 300 cycles).²⁴⁸ Recently, Huang and co-workers explored a porous 3D N-doped graphene/sulfur composite (3D-NGS) as the sulfur cathode for Li–S batteries. Due to the flexible porous 3D structure and N-doping, the 3D-NGS with a high sulfur content of 87.6 wt% exhibited excellent rate capability, especially, a good cycle performance with a capacity of 671 mA h g⁻¹ after 200 cycles was still maintained even at a high rate of 1500 mA g⁻¹ (Fig. 11).²⁵⁰


	Fig. 10 (a) Illustration of the formation process of the 3D graphene/S hybrid and schematic of fabrication of a self-supporting electrode. (b) SEM image of the interconnected fibrous microstructure of the graphene/S hybrid. (c) TEM and (d) HRTEM images of the graphene/S hybrid. (e) Capacity at different current densities of the graphene/S cathode. (f) Cycling performance and Coulombic efficiency of the graphene/S cathode at 0.75 A g⁻¹ for 100 cycles after the high current density test. Reprinted with permission from ref. 238, copyright (2013), American Chemical Society.


	Fig. 11 (a) Schematic illustration for the synthesis process of the 3D-NGS composite: (a) GO suspension, (b) 3D-NG obtained from GO through a solvothermal process, and (c) the final 3D-NGS composite. (b) SEM images of the 3D-NG framework. (c) SEM image of the microstructure of the 3D-NGS composite. (d) Rate capability of 3D-NGS with current densities ranging from 100 mA g⁻¹ to 1500 mA g⁻¹ and the first discharge–charge profiles at different current densities. (e) Cycling stability and the corresponding coulombic efficiency of 3D-NGS at a current density of 100 mA g⁻¹ for the first two cycles and 1500 mA g⁻¹ for the following cycles. Reprinted with permission from ref. 250, copyright (2014), Royal Society of Chemistry.

3.1.4 Graphene-based hybrids for sulfur cathodes. A well-designed cathode in Li–S batteries should possess high conductivity, large lithium storage and effective polysulfide trapping ability. Obviously, single component graphene can hardly meet all the requirements for high performances. A promising approach is the rational integration of graphene with other components to form graphene-based hybrids which combine the synergetic properties of individual components.

Chen's group improved the confinement of sulfur and polysulfides by uniformly coating a thin layer of porous carbon on both faces of graphene. In this architecture, graphene behaves as the electronic conductive channel, while the porous carbon layer provides high sulfur loading by impregnating sulfur in its pores and acts as a reservoir to alleviate the shuttle effect of polysulfide.²⁵² Liu's group prepared a coaxial graphene–wrapped sulfur–carbon nanofiber (G–S–CNFs) nanocomposite for the cathode of Li–S batteries. However, it should be noted that graphene was wrapped around the S–CNF composites via electrostatic attraction. The obvious capacity decay in the first few cycles revealed that the polysulfides still partially diffused probably due to the loose cohesion between graphene and carbon nanofibers (Fig. 12).²⁵³ Compared with carbon fibers, carbon nanotubes have a much smaller diameter and a higher surface area, which is more beneficial for sulfur loading.^254–256 Chen et al. sandwiched sulfur coated MWCNT composites in the interlayer of graphene sheets (GS). Due to the synergistic effects of GS and MWCNTs, much improved cycling stability and rate capability were achieved for the GS–MWCNT@S composite cathode compared with the composite lacking GS or MWCNTs.²⁵⁴ Besides, a higher sulfur loading of 49% was obtained compared with 33% in the case of carbon nanofibers.²⁵³ Furthermore, 3D graphene/CNT nanostructures with higher conductivity and better mesoporous structure were created as scaffolds for higher sulfur loading and superior electrochemical properties.^257,258 For instance, He and co-workers synthesized a 3D CNT/graphene/sulfur (3DCGS) sponge with a high sulfur loading of 80.1%. The reversible discharge capacity for sulfur and the whole electrode was 1217 and 877.4 mA h g⁻¹, respectively. A low capacity decay of 0.08% per cycle and a high-rate capacity of 653.4 mA h g⁻¹ up to 4C were achieved as well (Fig. 13).²⁵⁷ Apart from the above carbon scaffolds, biomass-derived carbon was also composited with graphene to form a promising carbon hybrid substrate for loading sulfur in Li–S batteries.²⁵⁹


	Fig. 12 (a) Schematic illustration of the assembled G–S–CNF multi-layered coaxial nanocomposites. TEM images of (b) pristine CNFs, (c) CNFs with sulfur coating, and (d) G–S–CNFs nanocomposite. (e) Capacity as a function of cycle numbers (50 cycles) of the S–CNF nanocomposite with (G–S–CNFs) and without (S–CNFs) graphene wrapping. (f) Cycling performances of the G–S–CNFs electrode at a high rate of 1C for 1500 cycles. Reprinted with permission from ref. 253, copyright (2013), American Chemical Society.


	Fig. 13 (a) Schematic of the synthesis procedure of the 3DCGS composite. (b) Cycling performance of the 3DGS and 3DCGS cathode at 0.2C for 200 cycles. (c) Rate performance at various C-rates of the 3DGS and 3DCGS cathode. Reprinted with permission from ref. 257, copyright (2015), Royal Society of Chemistry.

The interactions between the components of most of the above-mentioned graphene/carbon hybrids are based on noncovalent effects such as electrostatic attraction, which may be unfavourable for polysulfide trapping and may increase contact resistance.^253,255 Accordingly, covalently integrated graphene/CNT hierarchical architecture is highly desired with full inherited advantages of the component materials or even with unexpected properties.^260–262 A graphene/single-walled carbon nanotube (G/SWCNT) hybrid was successfully fabricated by Wei's group via a facile template-assisted CVD process. The CNTs covalently anchored on graphene sheets enlarged internal spaces between the two stacked graphene layers for sulfur storage. A high electrical conductivity of 3130 S cm⁻¹ enabled the as-obtained G/SWCNT–S cathode to exhibit a capacity as high as 650 mA h g⁻¹ after 100 cycles even at a high current rate of 5C.²⁶⁰ Later, they explored an N-doped aligned CNT/graphene hybrid as a scaffold for sulfur. N-doping introduced more defects and active sites into the carbon framework, improving the electrochemical behaviours because of the enhanced affinity between sulfur and the scaffold. A high reversible capacity of ca. 880 mA h g⁻¹ after 80 cycles could be achieved at 1.0C and a reversible capacity of ca. 770 mA h g⁻¹ could be achieved even at a high current density of 5.0C.²⁶² Furthermore, the same group coated a meso-/microporous carbon layer onto the sp² interlinked graphene/CNT networks. In this graphene/CNT@porous carbon hybrid, the seamlessly linked graphene/CNT scaffold provides efficient pathways for electron transport, while the introduced porous carbon accommodates sulfur and polysulfides. An ultrahigh specific capacity of 1121 mA h g⁻¹ at 0.5C and a favourable high-rate capability of 809 mA h g⁻¹ at 10C were obtained. Moreover, the gravimetric energy density of the packaged cell was expected to be 400 W h kg⁻¹ at a power density of 10 [thin space (1/6-em)] 000 W kg⁻¹, approaching the level of engine driven systems (Fig. 14).²⁶¹


	Fig. 14 (a) Schematic illustration showing the nanoarchitectured graphene/CNT@porous carbon hybrid formation. (b) Rate performance compared with GSH–S, GSH@PC–S cathode materials in which the contents of S were ca. 50 wt%. (c) Ragone plot of the Li–S battery based on diverse sp² nanocarbon cathode materials as well as other energy storage devices (all based on total mass of packaged device). Reprinted with permission from ref. 261, copyright (2014), Wiley-VCH.

Some polymers have also been used to wrap the graphene/sulfur composites to prevent the diffusion of polysulfides. Conducting polymers, such as polyaniline (PANI),^263–265 polypyrrole (PPy),²⁶⁶ and poly(3,4-ethylenedioxythiophene)–poly(styrene sulfonate) (PEDOT–PSS),²⁶⁷ with good compatibility with sulfur, are suitable for encapsulating sulfur. Dong et al. reported a rationally integrated sandwich-type hybrid nanosheet by conductive coating a PPy layer on sulfur-infiltrated graphene-backboned mesoporous carbon nanosheets. In this system, the 2D graphene backbone facilitates electron transport and ionic diffusion, the mesoporous carbon nanosheet reserves sulfur in its pores, and the outermost conducting PPy nanocoating prevents polysulfides from escaping and strengthens the entire structural integrity. The resulting sandwich-type hybrid nanosheet exhibited a high reversible capacity and a long lifespan of 400 cycles with an ultra-slow decay rate of 0.05% per cycle at the high rate of 1–3C (Fig. 15).²⁶⁶ Similarly, a conductive PEDOT-PSS layer was coated onto a sulfur impregnated porous graphene composite to facilitate charge transportation and prevent the dissolution of polysulfide. The sulfur composite cathode showed a reversible capacity of 718 mA h g⁻¹ at 2C and a reversible capacity of 845 mA h g⁻¹ at 0.1C after 200 cycles.²⁶⁷ Apart from the above conductive polymers, some natural polymers, such as amylopectin with hydroxyl groups, can also interact with GO to form a cross-linked 3D structure, helping to accommodate the volumetric expansion of sulfur. However, the relatively low Coulombic efficiency just stabilized around 90% and the capacity gradually faded during cycling, indicating that there was still some polysulfide dissolution and shuttling.²⁶⁸


	Fig. 15 (a) Schematic illustration of the synthesis procedure of GCS@PPy hybrid nanosheets. (b) Schematic illustration of structural merits of the GCS@PPy hybrid nanosheets towards lithium storage. (c) Long-term cycling stability of the GCS@PPy electrodes at current rates of 0.5C, 1C and 3C. Cycling curve of the GS electrode at 0.5C is also displayed for comparison. Reprinted with permission from ref. 266, copyright (2015), Royal Society of Chemistry.

3.1.5 Graphene–Li₂S composite cathodes. Recently, lithium sulfide (Li₂S) has attracted particular interest as an alternative cathode material with high capacity (theoretically 1166 mA h g⁻¹) for Li–S batteries because it can be paired with non-lithium-metal anodes (e.g., Si, Sn, C) to avoid potential safety issues. However, similar to the sulfur cathode, the Li₂S cathode suffers from poor electronic conductivity and polysulfide shuttling. Among various strategies that have been explored to combat these problems, incorporating lithium sulfides into graphene has recently been proved to be an effective method.^269–275

Yushin's group reported a simple, low-cost, and scalable solution-based method to prepare nanostructured Li₂S/graphene composites. The nanoparticles of Li₂S precipitated heterogeneously on the defective graphene surface. There was a stable performance of Li₂S/graphene composites without any Li SEI stabilizing electrolyte additive (such as LiNO₃ or polysulfides) for over 200 cycles.²⁷¹ Chen and co-workers reported in situ synthesis of a thermally exfoliated graphene/Li₂S (TG/Li₂S) nanocomposite via chemical reduction of TG/S composites by lithium triethylborohydride (LiEt₃BH). A discharge capacity of 791 mA h g⁻¹ based on the mass of Li₂S after 100 cycles at 0.1C was retained. Moreover, a Li₂S/Si full cell was produced by coupling the TG/Li₂S cathode with the Si thin film anode. A high specific capacity of ∼900 mA h g⁻¹ based on the mass of Li₂S was delivered.²⁷² Li et al. reported an in situ formed Li₂S/graphene composite via one-pot pyrolysis of a mixture of graphene nanoplatelet aggregates (GNAs) and lithium sulfate (Li₂SO₄). The GNAs not only act as a reductant to reduce Li₂SO₄ and create Li₂S at elevated temperature (Li₂SO₄ + 2C → 2CO₂↑ + Li₂S), but also are etched to thin graphene sheets, which are highly conductive 2D hosts for immobilization of Li₂S.²⁷⁵ Recently, Wang et al. reported a Li₂S/RGO cathode paper by simple drop-coating of the RGO paper with the Li₂S anhydrous ethanol solution. This free-standing and binder-free Li₂S/RGO paper could be directly used as a cathode without a metal substrate which showed a reversible discharge capacity of 816.1 mA h g⁻¹ after 150 cycles at 0.1C, and 462.2 mA h g⁻¹ after 200 cycles at 5C (Fig. 16).²⁷³ Furthermore, Hwa et al. reported the Li₂S/GO nanospheres by coating a conformal carbon layer, which can prevent polysulfide dissolution and reduce the whole electrode resistance. Accordingly, the resulting core–shell Li₂S/GO@C cathodes exhibited a high initial discharge capacity of 650 mA h g⁻¹ of Li₂S and a very low capacity decay rate of only 0.046% per cycle for 1500 cycles at 2C.²⁷⁴ Recently, Zhou et al. embedded dissolved lithium polysulphides (Li₂S₆) in a 3D N,S-codoped 3D graphene sponge. The N,S-codoped graphene conductive framework with high electrical conductivity and strong adsorption ability for polysulphides can provide Li ion rapid transport channels. As a result, a high-rate capacity of 430 mA h g⁻¹ at 2C and excellent cycling stability with ∼0.078% capacity decay per cycle for 500 cycles were achieved (Fig. 17).²⁷⁶


	Fig. 16 (a) Schematic illustration of the material preparation processes of the nano-Li₂S/rGO paper and structural changes during cycling of nano-Li₂S/RGO paper: (a) RGO paper. (b) Drop coating of the RGO paper by the solution of Li₂S in anhydrous ethanol in the glovebox. (c) The final product, nano-Li₂S/RGO paper. (d) The possible charge situation of the nano-Li₂S/RGO paper. (b) Rate performance of the nano-Li₂S/RGO paper electrode with a current rate ranging from 0.05 to 7C. (c) Cycling performance of the nano-Li₂S/RGO paper electrode at a high current rate of 5C. Reprinted with permission from ref. 273, copyright (2015), American Chemical Society.


	Fig. 17 (a) A lightweight N,S-codoped graphene sponge standing on a dandelion. (b) Illustration of the formation process of the N,S-codoped graphene electrode and schematic of the fabrication of a Li/dissolved polysulphide cell with the N,S-codoped graphene electrode after adding the polysulphide catholyte. (c) SEM image of the N,S-codoped graphene sponge. (d) Rate performance of the N,S-codoped graphene electrode with a graphene-coated separator at different current densities. (e) Cycling performance and Coulombic efficiency of the Li polysulphide batteries with the N,S-codoped graphene electrodes and the graphene-coated separator at a 0.5C rate for 200 cycles. Reprinted with permission from ref. 276, copyright (2015), Nature Publishing Group.

3.2 Graphene-based interlayers

As discussed above, much attention has been focused on the ‘inside’ modification of the cathodes, namely employing sulfur–graphene composites to enhance the electrical conductivity and suppress the loss of soluble polysulphide intermediates for high capacity and long cycle life. Modification of the ‘outside’ of the cathodes, such as cell configuration, may be a new strategy for improving the performance of Li–S batteries. Recent studies illustrate that adding a suitable interlayer between the sulfur cathode and separator has shown great promise in preventing the diffusion of soluble polysulphide intermediates for enhanced cyclability.²³⁰ An ideal interlayer is expected to selectively control the shuttling of soluble polysulphides anions via strong interactions, while not disturbing the rapid charge transport at the same time. Therefore, various carbon-based interlayers including microporous carbon,²⁷⁷ CNTs,²⁷⁸ GO,²⁷⁹ RGO,²⁸⁰ and graphene^227,281 have been developed. Especially, Chen's group designed a unique sandwich structure with pure sulfur between two graphene membranes. One sulfur coated graphene membrane was used as a current collector to provide excellent electric conductivity and the other graphene membrane coated on a commercial polymer separator acted as an interlayer to preserve lithium polysulfides localized in the cathode side. Besides, the sandwich structure could accommodate the large volumetric expansion of sulfur during lithiation. These effects could improve the cyclic stability and rate capability of the Li–S battery.²²⁷ Later, the same group used a graphene coated commercial polypropylene (PP) separator as an internal current collector to support the sulfur cathode, resulting in a flexible integrated structure of sulfur and graphene on a PP separator for Li–S batteries. A high capacity of 663 mA h g⁻¹ after cycling over 500 cycles at 1.5 A g⁻¹ and a very small capacity decay of 0.064% per cycle were achieved. A high capacity of 522 mA h g⁻¹ was still maintained at 3 A g⁻¹ corresponding to a capacity retention of 71.1% of its initial value at 1.5 A g⁻¹. Moreover, a flexible Li–S battery with a retained capacity of 722 mA h g⁻¹ and a Coulombic efficiency of ∼98% at 0.75 A g⁻¹ after 30 cycles was obtained (Fig. 18).²⁸¹


	Fig. 18 (a) Schematic of the electrode configuration using an integrated structure of sulfur and the G@PP separator and the corresponding battery assembly. (b) Cycling stability of the S–G@PP separator cell at 1.5 and 3 A g⁻¹ for 500 cycles after the high-current-density test. (c) The optical images show a red LED logo lighted by a bent Li–S battery. (d) Cycling performance of the battery under the bent state for 30 cycles at 0.75 A g⁻¹. Reprinted with permission from ref. 281, copyright (2015), Wiley-VCH.

As discussed above, the nonpolar graphene shows weak interaction with polar polysulfide anions, thus functionalized graphene with controllable surface modification has been developed for effective confinement of polysulfides. For example, Zhang and co-workers reported a permselective GO membrane for high-performance Li–S batteries. The GO membrane with negatively charged oxygen groups was permeable for positively charged lithium ions but rejected the transportation of negatively charged polysulfide ions (S_n²⁻) due to the electrostatic interactions. Consequently, an improved Coulombic efficiency from 67–75% to over 95–98% at 0.1C, and a reduced cyclic capacity decay rate from 0.49% per cycle to 0.23% per cycle were obtained.²⁷⁹ Vizintin et al. explored a fluorinated reduced graphene oxide (F-RGO) deposited on the glass fiber sheets as a separator for Li–S batteries. Better capacity retention upon cycling proved the superior polysulfide confinement efficiency of this F-RGO separator interlayer compared to the pure glass fiber separator.²⁸² Recently, Huang et al. applied a TiO₂/graphene hybrid film as a promising interlayer for Li–S batteries. The interconnected graphene provided rapid electron transfer paths and physically entrapped S and polysulfides. Meanwhile, TiO₂ further chemically suppressed the dissolution of polysulfides based on electrostatic attraction (S–Ti–O). The sulfur composite cathode modified with the TiO₂/graphene interlayer exhibited a high reversible capacity of 1040 mA h g⁻¹ over 300 cycles at 0.5C (Fig. 19).²⁸³


	Fig. 19 (a) Schematic of electrode configuration for the Li–S battery with a graphene/TiO₂ coating film. (b) Typical photographs of the cathode coated with graphene/TiO₂ film. Typical (c) cross-sectional SEM image and (d) front-view SEM image of the cathode with graphene/TiO₂ coating film. (e) Cycling stability of PCNTs–S, PCNTs–S@G, PCNTs–S@G/3%T cathodes at 0.5C. Reprinted with permission from ref. 283, copyright (2015), Wiley-VCH.

3.3 Graphene-based anodes

A Li–S battery usually contains a metallic lithium-based anode. Although lithium metal has an extremely high specific capacity (∼3860 mA h g⁻¹), its drawbacks, especially safety issues, restrict its practical application. Uncontrolled dendrite lithium arising from unstable SEI formed on the surfaces of the Li anode may pierce the separator and cause short circuit. In addition, shuttle effects may gradually thicken the Li₂S₂/Li₂S passivation layer on lithium anode surfaces, increasing the resistance and decreasing the Coulombic efficiency.^5,11 Therefore, developing attractive anode candidates to replace metallic lithium in Li–S batteries is necessary. In addition to employing the lithium sulfide (Li₂S) cathode, which can be combined with safer Li-free anodes (e.g., Si, Sn, C) to avoid potential safety vulnerabilities as mentioned above,²⁷² recent studies demonstrate that the lithiated graphene anode may be a promising candidate for safe Li–S batteries.^284–286

Jang's group reported a safe graphene-supported Li metal anode to solve the above issues. Graphene sheets with a high specific surface area could significantly reduce the anode current density to prolong the dendrite initiation time and restrain the growth of dendrites.²⁸⁴ Mukherjee et al. reported a defect-induced plating of metallic lithium into the interior of a porous graphene network. The network structure could act as a cage to effectively entrap lithium metal and suppress the lithium dendritic growth.²⁸⁵ Recently, Zhang and co-workers proposed an in situ formed SEI coated graphene framework with Li deposition as a high-efficiency and high-stability Li metal anode for Li–S batteries. The graphene framework could not only inhibit the growth of Li dendrites, but also sustain the rapid Li ion diffusion. Consequently, a stable Li metal anode with a cycling Coulombic efficiency of ∼97% and dendrite-free morphology after more than 100 cycles at 1.0C was obtained.²⁸⁶

4. Graphene-based nanocomposites for supercapacitors

As another promising class of alternative energy storage systems for future sustainable energy supply, supercapacitors, also known as electrochemical capacitors, have attracted significant interest during the past few decades due to their long cycle life (>100 [thin space (1/6-em)]

000 cycles), rapid charging/discharging rate, especially ultrahigh power density (at least 10 kW kg⁻¹).^287–290 However, the commercialized supercapacitors can only deliver a limited energy density of 5–10 W h kg⁻¹, which is much lower than that of batteries (such as 120–170 W h kg⁻¹ for Li-ion batteries).

Generally, supercapacitors can be divided into two classes: porous carbon nanomaterial-based electrochemical double-layer capacitors (EDLCs) and conducting polymer- or transition metal oxide/hydroxide-based pseudocapacitors.

4.1 Graphene-based composites for electrochemical double-layer capacitors

In EDLCs, charge can be stored by reversible adsorption of electrolyte ions on electrode material surfaces, without involving Faradaic processes during charging/discharging processes. Theoretically, the amount of stored charge is directly proportional to the specific surface areas (SSAs) of electrode materials.¹² In this regard, graphene and graphene-based nanocomposites with large SSAs and high electrical conductivity would be a perfect candidate for boosting the capacity of EDLCs (Table 5).^291,292

Table 5 Summary of graphene-based nanostructures for EDLCs

Electrode materials	SSA (m² g⁻¹)	Capacity^a	Energy density	Cycle life	Ref.
a The numbers 2 and 3 refer to two- and three-electrode test systems, respectively.
Activated graphene	3100	165 F g⁻¹ (2)	70 W h kg⁻¹	10000 (∼97%)	296
Activated RGO	2400	120 F g⁻¹ (2)	26 W h kg⁻¹	2000 (∼95%)	298
Activated graphene	3290	174 F g⁻¹ (2)	74 W h kg⁻¹	1000 (∼94%)	299
Activated graphene	3290	100 F cm⁻³ (2)	44 W h L⁻¹	1000 (∼94%)	299
Graphene hydrogel		175 F g⁻¹ (2)			302
Graphene hydrogel	951	220 F g⁻¹ (2)	5.7 W h kg⁻¹	2000 (∼92%)	303
3D porous graphene film		71.0 mF cm⁻² (2)	9.8 μW h cm⁻²	5000 (∼98.3%)	304
3D graphene	1005	250 F g⁻¹ (2)			305
Graphene films		215 F g⁻¹ (2)	150.9 W h kg⁻¹	10000 (∼97%)	306
Graphene films		273.1 F g⁻¹ (2)	150.9 W h kg⁻¹	10000 (∼97%)	306
3D porous graphene	835	169 F g⁻¹ (2)	3.76 W h kg⁻¹	5000 (∼92%)	307
Compact graphene films		255.5 F cm⁻³ (2)	59.9 W h L⁻¹		309
Holy graphene framework	1560	298 F g⁻¹ (2)	35 W h kg⁻¹	10000 (∼91%)	310
Holy graphene framework	1560	212 F cm⁻³ (2)	49 W h L⁻¹	10000 (∼91%)	310
N-doped graphene		280 F g⁻¹ (2)		10000 (∼99.8%)	315
N-doped graphene	465.0	248.4 F g⁻¹ (2)		5000 (∼96.1%)	314
3D N-doped graphene	280	484 F g⁻¹ (3)		1000 (∼100%)	317
N-doped graphene hydrogels	1521	326 F g⁻¹ (3)		1200 (∼92%)	316
3D N,B-doped graphene	249.0	239 F g⁻¹ (3)	8.65 W h kg⁻¹	1000 (∼100%)	318
3D N,B-doped graphene	249.0	62 F g⁻¹ (2)	8.65 W h kg⁻¹	1000 (∼100%)	318
Graphene/carbon black		112 F g⁻¹ (2)		3000 (∼94%)	322
Graphene/CNT		331 F g⁻¹ (3)		2000 (∼80%)	323
3D graphene/CNT	903	413 F g⁻¹ (3)		5000 (∼115%)	321
3D graphene/CNT	237	318 F g⁻¹ (2)	11.1 W h kg⁻¹	1000 (∼95%)	320
3D graphene/CNT	2285		60 W h kg⁻¹	5000 (∼100%)	325
N-doped RGO/SWCNT	396	305 F cm⁻³ (3)	6.3 mW h cm⁻³	10000 (∼93%)	324
N-doped RGO/SWCNT	396	300 F cm⁻³ (2)	6.3 mW h cm⁻³	10000 (∼93%)	324

4.1.1 Graphene and its derivatives. The theoretical SSA of single-layer graphene is reported to be as high as ∼2630 m² g⁻¹²⁹³ and a capacitance of 550 F g⁻¹ would be obtained if the surface area could be completely utilized²⁹⁴ However, the capacitances of graphene-based supercapacitors obtained so far are far below the theoretical value. Ruoff et al initiated the exploration of graphene for supercapacitors.^12,295–298 A chemically modified graphene (CMG) with an SSA of ∼705 m² g⁻¹ was synthesized by reduction of GO aqueous dispersion with hydrazine hydrate and delivered specific capacitances of 135 and 99 F g⁻¹ in aqueous and organic electrolytes, respectively.¹² It should be noted that the relatively small SSA may result in the decrease of capacity, which is mainly attributed to the aggregation and restacking of graphene sheets during electrode preparation.

In order to avoid the restacking and increase the SSA of graphene, several strategies have been developed. A chemical activation method was explored to create a 3D meso- and microporous graphene network with an ultrahigh SSA of up to ∼3290 m² g⁻¹.^296,299 An enhanced capacitance of 165 or 200 F g⁻¹ was obtained in organic or ionic liquid electrolytes, respectively. Although a higher SSA was obtained, the accessible surface area to electrolytes was still limited. Besides, the continuous charge transport pathway was suppressed due to the poor connections between isolated graphene sheets. To solve these problems, an effective strategy is to construct 3D graphene-based frameworks with continuously interconnected macroporous structure for the integration of perfect sheet-to-sheet connectivity, large surface area, and high electrical conductivity towards high performance.^300,301 Shi's group reported the synthesis of a self-assembled graphene hydrogel (GH) via a simple hydrothermal reduction.^302,303 The GH has a well-defined and crosslinked 3D porous structure, with a large SSA of ∼964 m² g⁻¹, exhibiting a specific capacitance as high as 220 F g⁻¹.³⁰³ Recently, Xiong and co-workers fabricated a flexible large-area hierarchical porous graphene film by blade-casting of GO hydrogel and post-casting reduction. Such films possess interpenetrating 3D hierarchical porous structures, high strength and modulus, large specific area, and good electrical conductivity. A flexible supercapacitor fabricated with these porous graphene films exhibited high areal specific capacitance (71.0 mF cm⁻² at 1 mA cm⁻²), excellent rate performance (79.0% retention at 100 mA cm⁻²), long cycle life (98.3% retention after 5000 cycles), as well as almost negligible capacity loss at different bending states (Fig. 20).³⁰⁴ Wang et al. developed a 3D strutted graphene with a large SSA of 1005 m² g⁻¹ and a high conductivity of 20 [thin space (1/6-em)] 000 S m⁻¹ by using a sugar-blowing approach. The as-obtained 3D graphene nanostructures exhibited a high capacitance of 250 F g⁻¹ at 1 A g⁻¹.³⁰⁵ Li's group developed a flow-directed self-assembly method to prepare self-stacked solvated graphene films. A large amount of water trapped in the solvated graphene film could act as a “spacer” to prevent the restacking of CMG sheets. The obtained solvated graphene film displayed greatly improved performance compared to its freeze-dried or thermal-annealed forms. The maximum capacitance reached 215 F g⁻¹ in an aqueous electrolyte and a capacitance of 156.5 F g⁻¹ could be retained even at an ultrahigh current density of 1080 A g⁻¹.³⁰⁶ Recently, Zou and co-workers reported a mild and environmentally friendly method to fabricate a 3D interconnected graphene framework, which has hierarchical macropores and in-plane nanopores. Different from the well-known KOH-activation process which can produce in-plane nanopores on graphene sheets but possibly destroy the original 2D continuity, such a 3D graphene framework was prepared by a novel NaI-activation process. As a result, the 3D hierarchically porous graphene framework with original continuity, a shortened mass diffusion length, and suppressed interlayer restacking delivered a specific capacitance of up to 169 F g⁻¹ and surface-normalized capacitance close to 21 μF cm⁻².³⁰⁷ Although the above reported 3D graphene nanostructures could give a high gravimetric capacitance of over 200 F g⁻¹, a large number of void spaces (macropores) decreased the packing density (∼0.069 g cm⁻³) of final devices, thus leading to a low volumetric capacitance (∼18 F cm⁻³).^2,306,308 To this point, Li's group further prepared a highly compact graphene film with a packing density of up to ∼1.33 g cm⁻³. This liquid-mediated densely compacted graphene film exhibited both a high gravimetric capacitance of over 200 F g⁻¹ and volumetric capacitance exceeding 200 F cm⁻³. An ultrahigh energy density of 59.9 W h L⁻¹, comparable to lead–acid batteries (50–90 W h L⁻¹), was also obtained.³⁰⁹ Duan et al. explored a highly dense holey graphene framework (HGF) through a mild defect-etching reaction.^310,311 The in-plane nanopores can function as the ion diffusion shortcuts between different layers of graphene to greatly speed up the ion transport across the entire film and facilitate ion access to the entire surface area, which is impossible to be realized in non-holey GF.³¹¹ In this way, both high gravimetric capacitance (298 F g⁻¹) and volumetric capacitance (212 F cm⁻³) could be achieved simultaneously.³¹⁰


	Fig. 20 (a) Schematic illustration of the fabrication process of the porous graphene film. (b) Photographs of the graphene film. (c) SEM and (d) TEM images and electron diffraction patterns of the porous graphene film. (e) Photographs of the corresponding flexible supercapacitor. (f) Areal specific capacitance and (g) cyclic life at the normal and 180° bending states. Reprinted with permission from ref. 304, copyright (2015), Wiley-VCH.

Doped graphene sheets are also promising electrode materials for supercapacitors because of their better conductivity and stability as well as improved wettability and the existence of pseudocapacitance.^42,312,313 Chen's group developed an efficient and facile strategy to fabricate highly crumpled N-doped graphene nanosheets by thermal treatment of cyanamide modified GO. The resulting crumpled N-doped graphene nanosheets with a pore volume as high as 3.42 cm³ g⁻¹ exhibited a high specific capacity of 248 F g⁻¹.³¹⁴ Jeong et al. developed N-doped graphene using a simple plasma doping process. The as-prepared N-doped graphene exhibited 4 times larger specific capacitance compared to that of pristine graphene.³¹⁵ Guo et al. synthesized N-doped graphene hydrogels through a one-pot hydrothermal route with GO as raw material and urea as reducing-doping agent, which had a large specific surface area of ∼1500 m² g⁻¹ and yielded a high specific capacitance of 308 F g⁻¹.³¹⁶ The effects of N doping content and doping configurations on the capacitive performance of N-doped graphene were further studied. The results showed that graphitic N can facilitate electron transfer, thereby improving the capacitive behaviour by lowering the charge-transfer resistance of the electrode at a high current density,³¹⁵ while pyridinic and pyrrolic N can offer high pseudocapacitance in an alkaline aqueous solution, consequently leading to large specific capacitance.³¹⁶ Besides, a 3D N-doped graphene framework (NGF) was developed by a hydrothermal method of GO aqueous suspension mixed with pyrrole. Based on the synergetic effect of 3D open-pore structure and N doping, the 3D NGF-based supercapacitor displayed a high specific capacitance of 484 and 415 F g⁻¹ at 1 and 100 A g⁻¹, respectively (Fig. 21).³¹⁷ A 3D B, N co-doped graphene was explored by hydrothermal reaction and freeze-drying processes using GO and ammonia boron trifluoride (NH₃BF₃) as precursors. The 3D B, N co-doped graphene electrodes exhibited a high capacitance of 239 F g⁻¹ because of the synergetic effects between the two co-dopants, outperforming the control samples without doping or doped with only B or N.³¹⁸


	Fig. 21 (a and b) Photographs of an as-prepared superlight NGF and one with a piece of NGF size of 1.8 cm × 1.1 cm × 1.2 cm standing on a dandelion. (c and d) SEM images of the NGF. (e) The specific capacitances calculated from the discharge curves under different current densities. (f) Cyclic stability of the NGF-based capacitor with a current density of 100 A g⁻¹. Reprinted with permission from ref. 317, copyright (2012), Wiley-VCH.

4.1.2 Graphene–carbon nanomaterial composites. Carbon materials, such as activated carbon, carbon spheres or CNTs, can be used as spacers to form graphene–carbon material hybrid nanostructures, in which carbon materials can not only suppress agglomeration and restacking of graphene, but also provide an additional capacitance.^319–324 Recently, Yu et al. developed a scalable strategy to synthesize a hierarchically structured CNT/graphene microfiber. The resultant fibers showed a specific volumetric capacity as high as 305 F cm⁻³ in sulfuric acid and 300 F cm⁻³ in polyvinyl alcohol (PVA)/H₃PO₄ electrolyte, respectively. A full micro-supercapacitor fabricated with these fibers exhibited a volumetric energy density of ∼6.3 mW h cm⁻³ comparable to that of thin-film lithium batteries. Moreover, the volumetric capacitance of the micro-supercapacitors was ∼45.0 F cm⁻³ at ∼26.7 mA cm⁻³ and ∼25.1 F cm⁻³ at ∼800 mA cm⁻³, outperforming most previously reported carbon-based micro-supercapacitors with capacitances in the range of 1–18 F cm⁻³.³²⁴ For most reported graphene/carbon hybrids, the nature of interaction between these two components is still unclear. Recently, Tour's group observed a covalent transformation of sp² carbon between the planar graphene and the single-walled carbon nanotubes (SWCNTs) during the formation of a 3D graphene/CNT hybrid. Due to the covalent interaction inside the 3D graphene/CNT hybrid, the supercapacitor showed an energy density of >60 W h kg⁻¹, which is the highest value among the reported carbon-based supercapacitors.³²⁵ Furthermore, a micro-supercapacitor based on this covalently connected 3D graphene/CNT hybrid was fabricated by the same group. A high volumetric energy density of 2.42 mW h cm⁻³ in the ionic liquid was obtained.³²⁶

4.2 Graphene-based composites for pseudocapacitors

Typical active pseudocapacitive materials such as conducting polymers or transition metal oxides/hydroxides possessing larger specific capacitance enable higher energy densities compared with carbon nanomaterials in EDLCs.^289,327,328 However, they usually suffer from relatively poor conductivity and limited stability, inferior to EDLCs in terms of power density and cycling life.¹⁵ In this regard, multi-component electrode materials composed of graphene and pseudocapacitive electrode materials could be viable candidates for improving performances of pseudocapacitors (Table 6). Usually, in these multi-component electrode materials, metal oxides/hydroxides or conducting polymers can enhance the capacitance remarkably, while graphene, as a support, can not only increase the effective utilization of active materials, but also improve the electrical conductivity and structural integrity of the composite electrodes.³²⁹

Table 6 Summary of graphene-based nanocomposites for pseudocapacitors

Electrode materials	Capacity^a	Energy density	Cycle life	Ref.
a The numbers 2 and 3 refer to two- and three-electrode test systems, respectively.
MnO₂/graphene	280 F g⁻¹ (3)		1000 (93%)	330
MnO₂/graphene	411 F g⁻¹ (3)		10000 (85%)	331
Co₃O₄/graphene	472 F g⁻¹ (2)	39.0 W h kg⁻¹	1000 (95.6%)	332
Co₃O₄/graphene	580 F g⁻¹ (2)	80 W h kg⁻¹	20000 (86.2%)	333
Ni(OH)₂/N-doped graphene	1151 F g⁻¹ (3)		2500 (∼100%)	334
Ni(OH)₂/graphene	1672 F g⁻¹ (3)		2000 (81%)	335
NiO dot/graphene	1181.1 F g⁻¹ (3)		1000 (83.5%)	336
Fe₃O₄/graphene	368 F g⁻¹ (3)		1000 (∼100%)	337
V₂O₅/graphene	218 F g⁻¹ (3)	22 W h kg⁻¹		338
TiO₂/graphene	225 F g⁻¹ (2)	20.1 W h kg⁻¹	2000 (86.5%)	340
TiO₂/N-doped graphene	385.2 F g⁻¹ (3)		50000 (98.8%)	341
NiCo₂O₄/graphene	618 F g⁻¹ (3)			342
ZnMn₂O₄/graphene	618 F g⁻¹ (2)	37 W h kg⁻¹	4000 (91%)	343
Ni₃S₂/graphene@Ni	987.8 F g⁻¹ (3)		3000 (97.9%)	344
Co₃S₄/graphene@Ni	1369 F g⁻¹ (3)		3000 (96.6%)	344
CoNi₂S₄/graphene	2009.1 F g⁻¹ (3)			345
Ni(OH)₂/MnO₂/graphene	1985 F g⁻¹ (3)	54 W h kg⁻¹		347
Ni(OH)₂/low-defect graphene	1162.7 F g⁻¹ (3)		2000 (>90%)	348
3D MnO₂/graphene	29.8 F g⁻¹ (2)	6.8 W h kg⁻¹	500 (93.3%)	350
3D MnO₂/graphene	389 F g⁻¹ (3)			351
3D Ni(OH)₂/graphene	532.5 F g⁻¹ (3)	14.9 W h kg⁻¹	2000 (∼100%)	352
3D Co₃O₄/graphene	757.5 F g⁻¹ (3)		500 (94.5%)	353
3D Co₃O₄/graphene	1390 F g⁻¹ (3)	35.14 W h kg⁻¹	800 (90%)	357
3D Co₃O₄/graphene	1100 F g⁻¹ (3)			354
3D NiO/graphene	425 F g⁻¹ (3)		2000 (79%)	356
3D CoMoO₄/graphene	2741 F g⁻¹ (3)		100000 (96.36%)	355
3D NiCo₂O₄/graphene	1402 F g⁻¹ (3)		5000 (∼76.6%)	358
Graphene/PANI	863.2 F g⁻¹ (3)	11.3 W h kg⁻¹	1000 (85.6%)	360
Graphene/PANI	1170 F g⁻¹ (3)		1000 (85.7%)	361
Graphene/PANI	23 mF cm⁻² (2)		2000 (100%)	362
Graphene/PANI	286 F g⁻¹ (3)	19.02 W h kg⁻¹	2000 (94%)	363
Graphene/PANI	431 F g⁻¹ (3)		500 (74%)	371
Graphene/PPy	350 F g⁻¹ (2)		1000 (∼100%)	364
Graphene/PPy	351 F g⁻¹ (3)	82.4 W h kg⁻¹	1000 (82%)	365
Graphene/PANI/graphene	581 F g⁻¹ (3)	10.79 W h kg⁻¹	10000 (85%)	377
3D graphene/PANI	1024 F g⁻¹ (3)	120 W h kg⁻¹	5000 (86.5%)	372
3D graphene/PANI	751.3 F g⁻¹ (3)		1000 (93.2%)	373
3D graphene/PANI	790 F g⁻¹ (2)	17.6 W h kg⁻¹	5000 (80%)	374
3D graphene/PANI	530 F g⁻¹ (2)		10000 (93%)	375
3D graphene/PANI	380 F g⁻¹ (2)		5000 (90%)	376
Coupled graphene/PANI	590 F g⁻¹ (3)		1000 (91%)	370
Coupled graphene/PANI	422 F g⁻¹ (3)		1000 (76%)	369
Graphene/MnO₂/PEDOT:PSS	380 F g⁻¹ (3)		3000 (95%)	378
Graphene/MnO₂/PANI	636.5 F g⁻¹ (3)		10000 (85%)	385
Graphene/Fe₂O₃/PANI	638 F g⁻¹ (3)	107 W h kg⁻¹	5000 (92%)	379
Graphene/SnO₂/PPy	616 F g⁻¹ (3)	19.4 W h kg⁻¹	1000 (100%)	380
Graphene/SnO₂/PEDOT	184 F g⁻¹ (3)	22 W h kg⁻¹	5000 (100%)	381
Graphene/CoFe₂O₄/PANI	767.7 F g⁻¹ (3)	79.7 W h kg⁻¹	5000 (96%)	382
Graphene/NiFe₂O₄/PANI	667 F g⁻¹ (3)	92.7 W h kg⁻¹	10000 (90%)	383
Graphene/MnFe₂O₄/PANI	307.2 F g⁻¹ (2)	13.5 W h kg⁻¹	2000 (74%)	384

4.2.1 Graphene–metal oxide/hydroxide nanostructures. A large number of graphene–metal oxide/hydroxide composites have been explored by anchoring various simple metal oxides/hydroxides, such as MnO₂,^330,331 Co₃O₄,^332,333 NiO/Ni(OH)₂,^334–336 Fe₃O₄,³³⁷ V₂O₅,^338,339 and TiO₂,^340,341 as well as mixed metal oxides, such as NiCo₂O₄³⁴² and ZnMn₂O₄,³⁴³ onto graphene substrates. Similar to metal oxides, metal sulfides such as Ni₃S₂,³⁴⁴ Co₃S₄,³⁴⁴ and CoNi₂S₄³⁴⁵ have also been combined with graphene for high performances.³⁴⁶ Usually, in these graphene–metal oxide/hydroxide composites, graphene substrates can induce the nucleation, growth and formation of uniform metal oxides/hydroxides with well-defined sizes, shapes and crystallinity, moreover, they can improve the electrical conductivity and structural integrity. In this way, metal oxides/hydroxides anchored or dispersed on graphene sheets suppress the restacking of graphene sheets, providing large surface areas for high electrochemical activity. Typically, Chen et al. reported a Ni(OH)₂/MnO₂/RGO ternary hybrid through a one-step hydrothermal co-deposition of GO and the precursors of Ni(OH)₂ and MnO₂. Due to the abundantly porous nanostructure, relatively high specific surface area, well-defined spherical morphology, and the synergetic effect of Ni(OH)₂, MnO₂, and RGO, the as-obtained Ni(OH)₂/MnO₂/RGO ternary hybrid exhibited significantly enhanced specific capacitance (1985 F g⁻¹) and energy density (54.0 W h kg⁻¹), higher than those of most previously reported MnO₂/Ni(OH)₂, MnO₂/graphene, and Ni(OH)₂/graphene composites (Fig. 22).³⁴⁷ Unlike most reported graphene–metal oxide/hydroxide hybrids using GO as the starting material which may inevitably result in a significant amount of defects, our group synthesized low-defect Ni(OH)₂/graphene and Ni(OH)₂/CNT via hydrolysis of nickel nitrate in a graphene or CNT dispersed water/N-methyl-pyrrolidone (NMP) suspension. During the hydrolysis, the as-produced H⁺ and NO₃⁻ species can react with carbon atoms of graphene or CNTs to form a small amount of oxygen-containing functional groups for the deposition of Ni(OH)₂ nanoparticles. As a result, the low-defect Ni(OH)₂/graphene hybrid exhibited a high specific capacity of 1087.9 F g⁻¹ at a current density of 1.5 A g⁻¹ together with no noticeable capacity decay after 2000 cycles at a current density of 10 A g⁻¹.³⁴⁸


	Fig. 22 (a) Possible formation mechanism of Ni(OH)₂/MnO₂/RGO hybrid spheres. (b) Typical SEM image of the Ni(OH)₂/MnO₂/RGO hybrid spheres. (c) Typical TEM images of the hybrid sphere (the inset is the SAED pattern of the whole hybrid sphere). (d) Comparison of specific capacitances of Ni(OH)₂/MnO₂/RGO, Ni(OH)₂/MnO₂/G12 (C₁₂H₂₅N(CH₃)₂(CH₂)₆(CH₃)₂NC₁₂H₂₅Br₂), and Ni(OH)₂/RGO electrodes. Reprinted with permission from ref. 347, copyright (2014), American Chemical Society.

It should be noted that, for the above graphene–metal oxide/hydroxide hybrids, the noncovalent interaction between metal oxide/hydroxide and graphene limits the charge transfer at the interfaces and it is incapable of sustaining structural integrity during long-term cycling. To this end, Zhou and co-workers reported the self-assembly of MoO₃ nanocrystal on graphene nanosheets by oxygen-bonding interactions at the interface. The resulting MoO₃/graphene hybrids possessing abundant exposed active sites, rapid ion diffusion and electron transport exhibited high specific capacitances and excellent cycling stability in both aqueous (527 F g⁻¹ at the current density of 1.0 A g⁻¹, 100% retention after 10 [thin space (1/6-em)] 000 cycles) and solid electrolytes (373 F g⁻¹ at 1.0 A g⁻¹, 100% retention after 5000 cycles).³⁴⁹

Besides, 3D graphene nanostructures such as hydrogels, foams, and films have also been employed to prepare the graphene–metal oxide/hydroxide 3D hybrid nanostructures.^350–358 For example, Choi et al. reported a 3D macroporous nanohybrid by depositing a thin layer of MnO₂ onto the CMG framework. The 3D porous graphene with a large surface area allowed fast ionic and electronic transport, and thus endowed 3D MnO₂/CMG hybrid electrodes with high electrochemical performances, such as a high specific capacitance of 389 F g⁻¹ at 1 A g⁻¹ and 97.7% capacitance retention upon a current increase to 35 A g⁻¹.³⁵¹ Our group reported a facile heterogeneous chemical deposition method to confine Ni(OH)₂ particles in a graphene film, resulting in a sandwich-like 3D graphene/Ni(OH)₂ film. Due to the simultaneously ordered ion diffusion channels and high electrical conductivity, the 3D hybrid film showed a high capacitance of 532.5 F g⁻¹ and no capacitance loss after 2000 cycles.³⁵² Recently, Yu et al. prepared a novel honeycomb-like strongly coupled CoMoO₄/3D graphene hybrid by hydrothermal deposition of CoMoO₄ on 3D graphene foam. This 3D CoMoO₄/graphene hybrid electrode exhibited excellent specific capacitance as high as 2741 and 1101 F g⁻¹ at a current density of 1.43 and 85.71 A g⁻¹, respectively. Moreover, an outstanding cycling stability with 96.36% retention of its initial specific capacitance was observed after 100 [thin space (1/6-em)] 000 cycles at a current density of 400 A g⁻¹ (Fig. 23).³⁵⁵


	Fig. 23 (a) The typical synthesis procedure of honeycomb-like CoMoO₄/3D graphene hybrid electrodes. (b) Cross-sectional SEM image of a honeycomb-like CoMoO₄/3D graphene hybrid. (c) Typical TEM images of honeycomb-like CoMoO₄. (d) Cycling performance of a honeycomb-like CoMoO₄/3D graphene hybrid electrode for 100000 cycles at a current density of 400 A g⁻¹. (bottom) The initial profiles for the first ten charge/discharge cycles, ten charge/discharges profiles after 50000 cycles, and ten charge/discharge profiles after 100000 cycles, respectively. Reprinted with permission from ref. 355, copyright (2014), Wiley-VCH.

4.2.2 Graphene–conducting polymer nanocomposites. Conducting polymers have been regarded as one of most promising electrode materials for electrochemical energy storage due to their low cost, facile synthesis, especially, large specific capacitance (∼500–3400 F g⁻¹). Unfortunately, they often suffer from large volumetric swelling and shrinking during charging/discharging processes, which usually leads to structural break down and fast capacitance decay.^289,359 A promising approach is incorporating conducting polymers with some robust carbon backbones like graphene for graphene-conducting polymer nanocomposites. In this regard, a number of graphene-conducting polymer nanocomposites, such as graphene/PANI,^360–363 graphene/PPy,^364,365 and graphene/PEDOT,^366,367 have been developed via chemical oxidative polymerization or electropolymerization and applied as hybrid electrode materials for supercapacitors.

For these graphene-conducting polymer composites, taking graphene/PANI as an example, graphene and PANI may interact via electrostatic attraction, hydrogen bonding and π–π stacking, and the resulting synergistic effects can lead to higher electrochemical capacitance and better stability.³⁶⁸ However, it is still worth pointing out that the noncovalent interaction between graphene and PANI is helpless for the charge transfer at the graphene/PANI interface, and incapable of accommodating the volumetric changes of PANI during repeated charging/discharging cycles. Intimately coupled interaction, such as covalent bonding, is expected to decrease interfacial resistance and endure volumetric changes upon long-term cycling. To this end, a covalently grafted graphene/PANI composite was synthesized by Gao and co-workers. The graphene substrates were first modified with p-aniline groups, which further served as initiation sites for the chemical grafting and polymerization of aniline. The as-prepared chemically grafted graphene/PANI composite delivered a high electrochemical capacitance of 422 F g⁻¹ at a discharge rate of 1 A g⁻¹, which was dramatically higher than that of graphene or pure PANI.³⁶⁹ Similarly, Wang et al. chemically grafted the PANI chains onto surfaces of graphene modified with the aminophenyl group, which acted as initiation sites for aniline polymerization.³⁷⁰ Recently, Kim and co-workers reported a novel fabrication method of highly flexible and scalable graphene/PANI hybrid film by the solution process. The as-prepared redoped graphene/PANI m-cresol/chloroform suspension was cast onto a large-sized glass substrate (23 cm × 23 cm, 13 inch) and then placed on a heat source to evaporate any residual solvent. The large-scale graphene/PANI film (23 cm × 23 cm, 13 inch) was obtained after the annealing process. Additionally, due to the great advantage of the solution process, scalable films with different sizes were fabricated easily by changing the size of the glass substrate. As electrode materials for supercapacitors, these scalable and flexible films exhibited a high capacitance of 431 F g⁻¹ and stable performances even under 180° bending (Fig. 24).³⁷¹


	Fig. 24 (a) Schematic illustration of the sequential steps for fabricating a large-scale graphene/PANI film. (b) Digital photographs of a flexible graphene/PANI film. The top image shows the folding characteristics of the film. The bottom images illustrate the high flexibility (bending and twisting) of the film. (c) CV curves of graphene/PANI film electrodes with two different bending angles of 0° and 180° at a scan rate of 5 mV s⁻¹. Reprinted with permission from ref. 371, copyright (2014), Wiley-VCH.

Moreover, 3D graphene nanostructures such as 3D graphene frameworks^372–374 and 3D porous graphene films^375,376 have also been used as substrates to produce 3D graphene/PANI composites. Especially, Xiao et al. reported a scalable and modular method to prepare a unique sandwich-structured RGO/PANI/RGO nanohybrid paper. The freestanding and flexible RGO paper was firstly fabricated by a facile and scalable printing technique and the bubbling delamination method. Then, a PANI layer was uniformly coated on the RGO paper by in situ electropolymerization, followed by wrapping an ultrathin RGO layer on its surface. A symmetric supercapacitor based on this RGO/PANI/RGO nanohybrid paper and the polymer gel electrolyte exhibited a large areal capacitance of 107 mF cm⁻² at a current density of 2 mA cm⁻², a high energy density of 5.4 mW h cm⁻³ (10.79 W h kg⁻¹), and excellent cycling stability (Fig. 25).³⁷⁷


	Fig. 25 (a) Schematic image illustrating the formation of a freestanding RGO paper. (b) Fabrication process of a GO film by the printing method. (c and d) Peeling GO film from the printing paper using the bubbling delamination method in 1 M HCl (since HI is dark colored, HCl was used instead to illustrate this process). (e) Photograph of a large-scale RGO paper. (f) Photograph of the RGO paper with excellent flexibility. (g) Photograph of RGO, PANI/RGO and RGO/PANI/RGO papers. (h) Areal capacitance versus discharge current for the solid-state symmetric device. The inset shows the photograph of a red LED lit by in-series solid-state SCs with three units. (i) Ragone plot of energy density versus power density for different solid-state device. Reprinted with permission from ref. 377, copyright (2015), Nature Publishing Group.

4.2.3 Graphene–metal oxide–conducting polymer ternary hybrids. It has been reported that ternary hybrid nanostructures composed of graphene, conducting polymers and metal oxides are able to exhibit much higher performances than the individual components or the binary composites. The advancements in this field have been summarized in a recent review.³²⁹

The most popular strategy to prepare the ternary graphene–metal oxide-conducting polymer ternary composites includes two steps: synthesis of graphene/metal oxide binary hybrids first and then coating the conducting polymers onto the surfaces of binary hybrids. Based on this strategy, Yu et al. developed a ternary graphene/MnO₂/PEDOT:PSS composite through 3D conductive wrapping of graphene/MnO₂ nanostructures with PEDOT:PSS. As a result, the specific capacitance of the electrode had substantially increased to ∼380 F g⁻¹, outperforming the graphene/MnO₂ nanostructures (∼260 F g⁻¹) without conductive wrapping. Additionally, more than 95% capacitance retention was obtained after 3000 cycles.³⁷⁸ Our group reported series work on graphene–metal oxide-conducting polymer ternary composites, such as graphene/Fe₂O₃/PANI,³⁷⁹ graphene/SnO₂/PPy,³⁸⁰ and graphene/SnO₂/PEDOT,³⁸¹ and demonstrated their improved electrochemical performances. Further considering that mixed transition metal oxides such as ferrites exhibit unique electrochemical attributes such as different redox states and good electrochemical stability, a series of graphene–ferrite-conducting polymer ternary hybrids, including graphene/CoFe₂O₄/PANI,³⁸² graphene/NiFe₂O₄/PANI³⁸³ and graphene/MnFe₂O₄/PANI³⁸⁴ have also been explored as upgraded electrode materials for supercapacitors by our group. These ternary graphene–metal oxide-conducting polymer hybrids exhibited large specific capacitance, good rate capability and high cycling stability, superior to those of individual components and corresponding binary hybrids. Recently, Li et al. reported a facile two-step strategy to fabricate a free-standing MnO₂ nanoflake/PANI nanorod/RGO ternary composite paper. MnO₂ nanoflakes were first electrodeposited on RGO paper, followed by assembly of PANI nanorods between MnO₂ nanoflakes by in situ polymerization. The flexible MnO₂/PANI/RGO ternary hybrid paper was directly used as the working electrode and showed a large specific capacitance (636.5 F g⁻¹ at 1.0 A g⁻¹), high rate capacity (53.8% capacitance retention at 50 A g⁻¹), and excellent cycling stability (85% capacitance retention after 104 cycles) (Fig. 26).³⁸⁵


	Fig. 26 (a) Schematic illustration of the preparation process of the RGO/MnO₂/PANI nanocomposite. (b) Digital images of the RGO paper (the inset shows the digital image of RGO/MnO₂/PANI). (c) SEM image of the cross-sectional view of RGO/MnO₂/PANI. (d) Specific capacitances of RGO/PANI, RGO/MnO₂ and RGO/MnO₂/PANI at different current densities. (e) Cycling stability of RGO/MnO₂/PANI at a current density of 20 A g⁻¹, the inset shows the last 10 cycles of the charge/discharge curves. Reprinted with permission from ref. 385, copyright (2015), Royal Society of Chemistry.

4.3 Graphene-based composites for asymmetric supercapacitors

In order to achieve high energy density without sacrificing power density, asymmetric supercapacitors (ASCs) that integrate the Faradic electrode (as an energy source) and the capacitive electrode (as a power source) in one configuration have been extensively explored. Usually, carbon-based materials, such as graphene with high surface area and conductivity, which facilitates fast charge/discharge, can be utilized for the negative electrode (capacitive electrode), while transition metal oxides and conducting polymers, which provide reversible redox reactions, are suitable for the positive electrode (Faradic electrode). Accordingly, there is no doubt that graphene/transition metal oxide composites and graphene/conducting polymer hybrids with improved specific capacitance and cycling stability can also be used as positive electrodes. In other words, graphene-based nanostructures can be employed in both negative and positive electrodes for ASCs with high power and energy density (Table 7).^386–395

Table 7 Summary of graphene-based nanocomposites for asymmetric supercapacitors

Electrode materials	Capacity^a	Energy density^b	Power density^c	Cycle life	Ref.
a Specific capacitance of asymmetric supercapacitors. b Maximum energy density of asymmetric supercapacitors. c Maximum power density of asymmetric supercapacitors.
IL–CMG//RuO₂/IL–CMG	175 F g⁻¹	19.7 W h kg⁻¹	6.8 kW g⁻¹	2000 (∼95%)	386
Porous graphene//Ni(OH)₂/graphene	218.4 F g⁻¹	77.8 W h kg⁻¹	15.2 kW kg⁻¹	3000 (∼94.3%)	387
Activated graphene//PANI/graphene/CNT	107 F g⁻¹	41.5 W h kg⁻¹	25.0 kW kg⁻¹	5000 (∼91.4%)	389
RGO paper//MnO₂/RGO paper	113 mF cm⁻²	35.1 μW h cm⁻²	3.8 mW cm⁻²	3600 (∼84%)	388
Fe₃O₄/graphene//NiOOH/Ni₃S₂/3D graphene	233 F g⁻¹	82.5 W h kg⁻¹	930 W kg⁻¹		392
RGO//Ni(OH)₂/RGO		75 W h kg⁻¹	40000 W kg⁻¹	10000 (∼89%)	394
CNT/graphene//Mn₃O₄/graphene	72.6 F g⁻¹	32.7 W h kg⁻¹	9.0 kW kg⁻¹	10000 (∼86%)	396

Yan et al. fabricated a high-voltage ASC using porous graphene as a negative electrode and Ni(OH)₂/graphene as a positive electrode, respectively. The optimized ASC could be cycled reversibly in a high-voltage region of 0–1.6 V and displayed a maximum specific capacitance of 218.4 F g⁻¹ and a high energy density of 77.8 W h kg⁻¹.³⁸⁷ Sharma's group reported an ASC using graphene nanoribbons (GNRs) as a negative electrode and metal oxide modified GNRs as a positive electrode, respectively.^393,395 For example, an ASC using GNR and CoMn₂O₄/GNR composites as negative and positive electrodes, respectively, demonstrated a high energy density of 84.69 W h kg⁻¹ and long term stable cycling performance with ∼96% capacitance retention and ∼97.5% columbic efficiency.³⁹³ In addition, metal oxide or carbon nanomaterial modified graphene can also be used as negative electrode for ASCs.^390–392 For example, an all-solid-state ASC with MnO₂/graphene foam as the positive electrode and CNT/graphene as the negative electrode exhibited high energy density, remarkable rate capability, and reasonable cycling performance in a high-voltage region of 0 to 1.8 V.³⁹¹ In some cases, graphene-based ternary hybrids have been exploited as negative or positive electrodes for ASCs. Liu et al. deposited MnO₂ or PPy onto 3D graphene foam/carbon nanotube (GF/CNT) films and then fabricated an ASC based on the resulting GF/CNT/MnO₂ and GF/CNT/PPy ternary hybrid films. The ASC was able to provide an output voltage of 1.6 V and deliver a high energy density of 22.8 W h kg⁻¹.³⁹⁰

The development of smart wearable or rolling-up electronic devices has prompted the demand for flexible energy storage devices, including flexible ASCs that can function under considerable physical deformation. Carbonaceous nanomaterials, especially graphene, has been widely used to fabricate flexible ASCs because of its unprecedented combination of large surface area, superior electrical conductivity, and excellent mechanical flexibility.^{386,390,391,396,397} Choi et al. fabricated a solid-state flexible ASC based on an ionic liquid functionalized-chemically modified graphene (IL–CMG) film as the negative electrode and a hydrous RuO₂/IL–CMG composite film as the positive electrode. The as-prepared all-graphene film-based ASCs not only showed a high energy density (19.7 W h kg⁻¹) and power density (6.8 kW g⁻¹), but also could operate even under harsh mechanical conditions including twisted and bent states (Fig. 27).³⁸⁶ Gao et al. reported an all-solid-state ASC based on free-standing CNT/graphene paper and Mn₃O₄/graphene paper electrodes. The resultant all-graphene paper-based ASC demonstrated an increased cell voltage of 1.8 V, stable cycling performance (capacitance retention of 86.0% after 10 [thin space (1/6-em)] 000 cycles), an improved energy density of 32.7 W h kg⁻¹, and more importantly, a distinguished mechanical flexibility (Fig. 28).³⁹⁶


	Fig. 27 (a) Experimental procedure of the synthesis for water-soluble IL–CMG and RuO₂/IL–CMG hybrids. (b) Long-term cycling stability of ASC devices at normal, twisted, and bent states with a constant current density of 1 A g⁻¹ over 2000 cycles. The inset shows the schematic diagram of an all-solid-state flexible thin ASC. Reprinted with permission from ref. 386, copyright (2012), Royal Society of Chemistry.


	Fig. 28 (a) Illustration of the fabrication process of a flexible all-solid-state ASC based on a polymer gel electrolyte and free-standing CNT/graphene and Mn₃O₄/graphene paper electrodes. (b) Cross-sectional SEM image of the CNT/graphene paper. (c) Photograph shows that the free-standing CNT/graphene paper is flexible enough to be folded up. (d) Cross-section SEM image of the Mn₃O₄/graphene paper. (e) Photograph shows that the free-standing Mn₃O₄/graphene paper is flexible enough to be folded up. (f) Ragone plots of ASC (line a) and two corresponding symmetric supercapacitors (line b and c). (g) Cycling behavior of ASC with a cell voltage of 1.8 V at a scan rate of 50 mV s⁻¹. (h) A green light-emitting diode (LED) lighted by two ASCs connected in series. (i) Specific capacitance retention ratio of the ASC at normal (n), bending (b), and twisting (t) states and after being bent repeatedly (number of times indicated). The inset shows the photograph of the ASC at normal, bending, and twisting states. Reprinted with permission from ref. 396, copyright (2012), American Chemical Society.

5. Conclusions, challenges and perspectives

Owing to its unique properties such as 2D structure, large specific surface area, excellent electrical conductivity, together with strong mechanical flexibility, graphene has shown great potential not only in its direct use as an electrode material, but also in acting as a versatile building block for synthesis of graphene-based hybrid nanostructures with improved energy storage capability. This review summarized the recent progress on graphene and graphene-based hybrid nanostructures for three frontier energy storage systems, that is, Li-ion batteries, Li–S batteries and supercapacitors. Graphene and its derivatives, such as doped graphene and 3D graphene frameworks, as well as graphene-based multicomponent hybrids with tunable pore structure and morphology, large surface area, and an interconnected charge transport pathway, have shown improved performances in terms of specific capacity, rate capability, cycling stability, and power/energy densities. These exciting developments have opened up new pathways for high-performance graphene-based materials for energy storage.

Despite the advances, great challenges urgently need to be addressed before future commercialization. Firstly, similar to many other novel materials in the past, the main important thing is to shorten the distance between laboratory-scale research and practical applications of graphene-based hybrids for energy storage. Nowadays, most graphene-based materials are produced by the reduction of GO to RGO, which has the potential for scalability but may introduce intrinsic and extrinsic defects. These defects can strongly affect electrochemical properties. For example, more defects on graphene are propitious to enhance the Li storage capability, but obviously reduce the conductivity of graphene. As a result, RGO-based anodes exhibit an attractive initial capacity but often suffer a low first-cycle Coulombic efficiency and fast capacity fading. Therefore, how to manufacture large-scale and high-quality graphene and graphene-based materials in a cost-effective way is the most essential issue. Secondly, the mechanisms for the improved performances of graphene and graphene-based hybrids are still not completely clear. A better understanding of the charge transfer and storage mechanisms on single-layer graphene and graphene-based hybrids, as well as the relationship between charge storage and defects, layer number, size, and surface state of graphene could provide a direction for the nanostructure design and property optimization. To this end, in situ characterization techniques combined with theoretical calculations are required to reveal the complicated mechanisms. Thirdly, the highly porous nature of graphene offers a large surface area and favourable ion diffusion for high gravimetric capacitance but results in low volumetric energy density due to the low packing density. Graphene-based composites containing a large portion of graphene usually show a low packing density. While a high loading of nanoparticles on graphene usually results in poor cycle stability because of their weak interaction and the severe aggregation of nanoparticles on graphene sheets. Therefore, to improve the volumetric energy density and maintain high gravimetric capacitance simultaneously, the mass ratio of graphene to nanoparticles should be appropriately controlled. Finally, it is crucial to achieve the desirable controllability of interface interactions among individual components inside of graphene-based hybrids, especially for energy storage applications that involve charge transfer processes. A well-integrated hybrid with intimate interactions such as chemical covalent bonding will be able to offer effective charge transport for high rate capability as well as well-sustained structural integrity for long cycle life. Unfortunately, most graphene-based composites reported so far are synthesized simply by mixing or dispersing all components, which leads to a relatively poor interfacial interaction. There is little reason to believe that a breakthrough in energy storage can be achieved by such cursory blend.

In addition to the above mentioned systems, graphene and graphene-based composites have also shown outstanding performances in sodium-ion batteries,^398–402 lithium–air batteries,^403–406 and sodium–air batteries.^407,408 More encouragingly, increasing efforts has been devoted to the development of graphene-based energy storage devices with small, thin, lightweight and even flexible properties for advanced wearable electronics. Although many challenges still need to be overcome, the past few years have witnessed the exciting advances in the research on graphene and graphene-based hybrids for energy storage. In the future, it may be possible that low-cost graphene-based supercapacitors exhibit a power density of ∼100 kW kg⁻¹, batteries show an energy density of at least 250 W h kg⁻¹ and stable cyclability up to 5000 cycles. Overall, with the continuous efforts of many scientists and engineers around the world, it is believed that these interesting targets will be realized in the future.

Acknowledgements

This work was supported by National High Technology Research and Development Program of China (863 Program, No. 2013AA050905), NSF of China (No. 51572125, 51472101, 21206075) and PAPD of Jiangsu.

Notes and references

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