Silicon oxides: a promising family of anode materials for lithium-ion batteries

Abstract

Silicon oxides have been recognized as a promising family of anode materials for high-energy lithium-ion batteries (LIBs) owing to their abundant reserve, low cost, environmental friendliness, easy synthesis, and high theoretical capacity. However, the extended application of silicon oxides is severely hampered by the intrinsically low conductivity, large volume change, and low initial coulombic efficiency. Significant efforts have been dedicated to tackling these challenges towards practical applications. This Review focuses on the recent advances in the synthesis and lithium storage properties of silicon oxide-based anode materials. To present the progress in a systematic manner, this review is categorized as follows: (i) SiO-based anode materials, (ii) SiO₂-based anode materials, (iii) non-stoichiometric SiO_x-based anode materials, and (iv) Si–O–C-based anode materials. Finally, future outlook and our personal perspectives on silicon oxide-based anode materials are presented.

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Zhenhui Liu

Zhenhui Liu received his BS degree from the Department of Applied Chemistry, Wuhan University of Technology in 2014 and he is currently working toward his PhD degree in Materials Science at Wuhan University of Technology. His current research involves nanomaterials and devices for energy storage.

Qiang Yu

Qiang Yu obtained his Bachelor's degree in 2015 from the School of Materials Science and Engineering, Liaocheng University. He is now a PhD candidate at Wuhan University of Technology with Prof. Liqiang Mai and Prof. Dongyuan Zhao. His research interests focus on the development of nanostructured carbon materials for application in supercapacitors and lithium-ion batteries.

Liang Zhou

Liang Zhou received his BS (2006) and PhD (2011) from Fudan University (with Prof. Dongyuan Zhao and Prof. Chengzhong Yu). He then carried out postdoctoral research in Prof. Xiong Wen (David) Lou's group at Nanyang Technological University and Prof. Chengzhong Yu's group at The University of Queensland. He is currently a Professor at Wuhan University of Technology. His research interests focus on functional nanomaterials for energy storage and conversion applications.

Liqiang Mai

Liqiang Mai received his PhD degree from Wuhan University of Technology in 2004. He then carried out postdoctoral research in Prof. Zhonglin Wang's group at Georgia Institute of Technology (2006–2007) and worked as an advanced research scholar in Prof. Charles M. Lieber's group at Harvard University (2008–2011). He is currently a Chair Professor at Wuhan University of Technology. His research interests include nanowires, micro-/nano-energy-storage devices, and energy-based nano-bio interfaces.

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1. Introduction

With high energy density, long lifespan, and environmental friendliness, lithium-ion batteries (LIBs) represent one of the most attractive energy storage devices and are playing more and more important roles in modern society.^1–9 They have already conquered the markets of portable electronics, such as cell phones, laptops, and digital cameras. They have also been identified as the power sources of choice for electric vehicles and stationary energy storage. However, the current state-of-the-art LIBs cannot satisfy the ever-increasing demands of electric vehicles and large-scale energy storage.

Up to now, most commercialized LIBs have adopted intercalation reaction-based anode materials, such as graphite and Li₄Ti₅O₁₂. Such anode materials share the common features of outstanding cycle life but limited capacity. The theoretical capacities for graphite and Li₄Ti₅O₁₂ are 372 and 175 mA h g⁻¹, respectively.^10,11 The relatively low capacity severely limits the energy density of the batteries. Although anode materials based on alloying/de-alloying and conversion reactions, such as P (2596 mA h g⁻¹),^12,13 Sn (960 mA h g⁻¹),^7,14,15 Sb (660 mA h g⁻¹),^16,17 Ge (1600 mA h g⁻¹),^18–22 and transition metal oxides (TMOs)^23–25 are considered as promising alternative high-capacity candidates, none of them reached large-scale commercialization.

Among the emerging anode materials proposed for LIBs, Si has been considered as the most promising candidate to replace graphite. It is the second most abundant element in the earth's crust, environmentally friendly, and possesses an ultrahigh theoretical capacity (4200 mA h g⁻¹).^26–28 However, the drastic volume fluctuation of Si (∼400%) during lithiation/de-lithiation and the high production cost of nanostructured Si seriously hinder the widespread application.^6,29–32 Recently, silicon oxides have been considered as promising substitutes for elemental Si because of their extremely abundant reserves, low cost, and easy synthesis. In addition, silicon oxides show a small volume change during cycling when compared to elemental Si. The in situ generated Li₂O and lithium silicates during the first lithiation may buffer the large volume change and lead to improved cycling stability. Despite these merits, silicon oxides also suffer from several drawbacks, hindering their extensive application. (I) Silicon oxides are insulators with low intrinsic electrical conductivity, which lowers the electrochemical activity. (II) Although the volume change issue is not as serious as that in elemental Si, it is non-negligible. (III) The initial coulombic efficiency (ICE) of silicon oxides is relatively low, which is associated with the irreversible formation of Li₂O and lithium silicates in the first cycle.

Recently, significant efforts have been devoted to tackling the above-mentioned issues and pushing the silicon oxide-based anode materials towards practical applications in LIBs.³³ Actually, silicon oxides have been blended with graphite (usually with a content of less than 10%) and used in commercialized LIBs.^2,34 Up to now, there have been several excellent reviews on Si-based anode materials;^35–37 however, a comprehensive review exclusively focusing on silicon oxide-based anode materials is lacking. This Review summarizes the most recent advances in the microstructure, lithium storage mechanisms, rational synthesis, and electrochemical properties of silicon oxide-based anode materials. To present the results in a systematic manner, this review is categorized as follows: (i) SiO-based anode materials, (ii) SiO₂-based anode materials, (iii) non-stoichiometric SiO_x-based anode materials, and (iv) Si–O–C-based anode materials. At the end of this Review, our personal perspectives on silicon oxide-based anode materials are presented.

2. SiO-based anode materials

Among the various silicon oxides (SiO, SiO₂, SiO_x, and Si–O–C), SiO has attracted the most attention as the anode materials for LIBs. It was first reported in 1887 by Charles F. Mabery.³⁸ Commercially available SiO is usually produced by heating SiO₂ and Si mixtures at elevated temperatures to generate gaseous SiO (SiO₂ + Si = 2SiO) and then condensing the SiO vapor.³⁹

2.1 The atomic structure of amorphous SiO

The atomic structure of SiO has been a subject of controversy since its discovery.^40,41 Philipp proposed a random-bonding model, which described the SiO as a homogeneous single-phase material with a continuous Si–(O_xSi_4−x) network composed of randomly distributed Si–Si and Si–O bonds.^42,43 Brady⁴⁴ and Temkin⁴⁵ suggested a random-mixture model, in which the SiO was described as a mixture of nanosized amorphous Si and SiO₂. More realistic models suggested that silicon suboxide may exist in the interfacial regions between Si and SiO₂ domains.^46–48 The interface boundary layer accounts for 20–25% of the total number of atoms in SiO.⁴⁸

Due to the limitation in spatial resolution of conventional characterization techniques, the direct experimental observation of the local atomic structure of SiO is very challenging. Very recently, Chen and co-workers provided experimental evidence on the atomic-scale disproportion of amorphous SiO. Using angstrom-beam electron diffraction (ABED), they detected silicon suboxide-type tetrahedral coordinates at the Si/SiO₂ boundaries.⁴⁹ Based on the ABED and synchrotron X-ray scattering results as well as computer simulations, the model of disproportionated SiO was proposed (Fig. 1).

Fig. 1 Atomic models of amorphous Si (a), interfacial silicon suboxide (b), and amorphous SiO₂ (c). The model of amorphous SiO (d). Fractions of the five atomic coordinates existing in amorphous SiO (e). The Si–Si₄ is from the Si clusters; the Si–O₄ is from the SiO₂ matrix; the Si–(Si₃O), Si–(Si₂O₂), and Si–(SiO₃) are from the Si/SiO₂ interfaces. Reproduced with permission from ref. 49. Copyright 2016, Nature Publishing Group.

Due to the inhomogeneous nature (disproportion) on the atomic-scale, SiO is thermodynamically unstable and it tends to disproportionate to Si and SiO₂ (2SiO → Si + SiO₂).^50–52 Considering this fact, the Si/SiO₂-based composites derived from disproportionated SiO are denoted as d-SiO in this Review.

2.2 Lithiation behavior of SiO

To the best of our knowledge, the utilization of SiO in LIBs was first reported in the 1990s.^53–55 In 2001, Tatsumisago et al. reported the lithium storage performances of amorphous SiO/SnO (50/50 mol%) prepared by mechanical milling of SiO and SnO.⁵⁶ A high reversible capacity of ∼800 mA h g⁻¹ was obtained for the SiO/SnO composite. In 2002, Yang and co-workers examined a series of SiO_x with different O contents (SiO, SiO_0.8, SiO_1.1) and found that the capacity dropped with the increase of O content.⁵⁷ On the other hand, the increased O content (decreased capacity) alleviated the volume change and thus improved the cycling performance. SiO_0.8 demonstrated a reversible capacity of ∼1600 mA h g⁻¹. These pioneering works inspire scientists’ interests in SiO-based anode materials.^58–66

Understanding the lithiation/de-lithiation mechanisms is of vital importance to further improve the electrochemical performance of SiO.^67–77 It is generally accepted that Li_xSi alloys, lithium silicates (Li₄SiO₄, Li₂Si₂O₅, Li₆Si₂O₇, Li₂SiO₃), and Li₂O are formed during the first lithiation of SiO. The extraction/insertion of Li⁺ in Li_xSi alloys is reversible. Assuming all the Si in SiO can be converted into Li_4.4Si during lithiation, the theoretical reversible capacity of SiO reaches 2680 mA h g⁻¹. The lithium silicates and Li₂O are generally irreversible phases. On the one hand, they serve as a buffering matrix against the volume change during lithiation/de-lithiation, enhancing the cycling performance of SiO.^68,78 On the other hand, the formation of irreversible lithium silicates and Li₂O lowers the ICE of SiO.^68,70,75

The composition of the lithiated products of SiO is relatively complex. Various lithium silicates have been observed in the lithiated SiO and they behave quite differently depending on the compositions. Sohn et al. identified three lithium silicate species (Li₂Si₂O₅, Li₄SiO₄ and Li₆Si₂O₇) in the discharge products of SiO. Among these phases, only the Li₂Si₂O₅ phase was demonstrated to be reversible, whereas the Li₄SiO₄ and Li₆Si₂O₇ were irreversible during cycling.^74,76 To understand the kinetics of the lithiated products of SiO, Yoon et al. controlled the amount of Li⁺ insertion (“mole control”) at various C-rates.⁷⁵ It is found that the irreversible capacity increased with mole rate but decreased with C-rates. This study suggests that the composition of the discharged products is related to the depth of discharge and C-rate.

The lithiation process of SiO was studied by Han et al. through first-principles molecular dynamics simulations. The calculations revealed that lithium silicates were dominant over Li₂O as irreversible matrix components of the lithiated SiO (Li_4.4SiO).⁷⁷ Although Li₂O had a minor portion in the lithiated SiO, the Li⁺ diffusivity in Li₂O was at least two orders of magnitude higher than those in lithium silicates. As a result, it could act as the main Li⁺ transport channel to access the Li_xSi core, maximizing the capacity and rate capability of SiO. This study provides the possibility of boosting the electrochemical performances of SiO-based anode materials via controlling the inactive matrix components.

Based on the above studies, the lithium storage mechanism of SiO can be proposed as follows:

SiO + 2Li⁺ + 2e⁻ → Si + Li₂O (1)

4SiO + 4Li⁺ + 4e⁻ → 3Si + Li₄SiO₄ (2)

5SiO + 2Li⁺ + 2e⁻ → 3Si + Li₂Si₂O₅ (3)

7SiO + 6Li⁺ + 6e⁻ → 5Si + Li₆Si₂O₇ (4)

3SiO + 2Li⁺ + 2e⁻ → 2Si + Li₂SiO₃ (5)

Si + xLi⁺ + xe⁻ ↔ Li_xSi (6)

2.3 Electrochemical performances of SiO

Due to the existence of lithium silicates and Li₂O, the volume variation of SiO during lithiation/de-lithiation is significantly reduced and the cycling stability is enhanced when compared to Si. However, complete elimination of the volume change issue is difficult. According to recent literature studies, the volume expansion of SiO is around 200% upon lithiation.^68,79,80 Such a drastic volume expansion may induce stress in the active materials, resulting in active material pulverization and isolation, electrode disintegration, high surface side reactions, as well as repetitive solid electrolyte interface (SEI) film formation and rupture.⁸¹ Therefore, the large volume change of SiO is considered to be the main reason for its capacity fading upon cycling. Besides, SiO also suffers from low ICE and low rate capability. The former originates from the formation of irreversible lithium silicates and Li₂O during the first cycle, whereas the latter is caused by low intrinsic electrical conductivity. For practical applications, there is still considerable room for improving the cyclability, ICE, and rate capability of SiO.

2.3.1 Bare SiO anode materials. Various strategies, such as downsizing,^72,74 constructing porous structures,⁷⁶ nanocompositing,^62,82,83 and employing new binders,^79,80,84,85 have been dedicated to tackling the volume change issue of SiO-based anodes. The commercially available SiO is usually micrometer-sized and shows a poor cycling performance. To reduce the particle size and broke the microstructure of SiO, Sohn et al. employed a high-energy mechanical milling (HEMM) process (Fig. 2a).⁷² Combining the disproportionation reaction of SiO (2SiO → Si + SiO₂) and HEMM, a d-SiO-based anode composed of nanosized Si embedded in a SiO_x matrix was obtained. The bare HEMM-treated d-SiO delivered a reversible capacity of ∼1000 mA h g⁻¹ with good retention (Fig. 2b). By HEMM SiO followed by surface etching with NaOH, SiO with better electrochemical performances could be obtained.⁷⁴ The synthesized SiO provided a high capacity of 1260 mA h g⁻¹ after 50 cycles and the capacity retention is 86.5% against the second cycle. The enhanced capacity and cyclability were ascribed to the reduced particle size and the removal of O-rich SiO_x on the surface, which would slow the Li⁺ diffusion. Both studies indicate that even bare SiO may show good lithium storage performances provided that its microstructure and size are properly engineered.

Fig. 2 Schematic illustration for the preparation of d-SiO (a). The synthesis of d-SiO includes two steps: disproportionation of SiO at 1200 °C and HEMM. Cycling performances of SiO (b), HEMM-treated d-SiO (3 h) and HEMM-treated d-SiO (10 h). Reproduced with permission from ref. 72. Copyright 2013, Elsevier Ltd.

Constructing porous structures has also been employed to boost the cycling stability of SiO. The porous structures provide not only free space for volume expansion accommodation but also efficient channels for Li⁺ diffusion. As mentioned previously, the disproportionated SiO (d-SiO) is a composite material composed of nanocrystalline Si and an amorphous SiO₂ matrix. When subjected to NaOH solution, the nanocrystalline Si was attached by NaOH vigorously, while the SiO₂ matrix was etched away slowly. In this sense, the nanocrystalline Si acted as a pore generating agent, and porous SiO_x was obtained after NaOH etching (Fig. 3a–c).⁷⁶ The porous SiO_x without any carbon coating demonstrated a high capacity of ∼1240 mA h g⁻¹ after 100 cycles (Fig. 3d). Due to the fast Li⁺ diffusion in the pores, the porous SiO_x manifested excellent rate performance as well (Fig. 3e).

Fig. 3 Schematic illustration for the preparation of porous SiO_x (a); TEM (b) and HRTEM (c) images of porous SiO_x; cycling performances of SiO, d-SiO, HEMM-treated d-SiO, and porous SiO_x (d); rate performances of HEMM-treated d-SiO and porous SiO_x (e). 1C = 1500 mA h g⁻¹. The inset in e shows the SAED pattern of porous SiO_x. Reproduced with permission from ref. 76. Copyright 2014, Elsevier Ltd.

2.3.2 SiO/C hybrid anode materials. Among the various electrochemical performance optimization strategies, compositing SiO with conductive carbon has been most extensively studied. Various SiO/C composites have been constructed and assessed.^86–92 The carbon not only alleviates the overall volume change, but also improves the conductivity of SiO, leading to enhanced cycling stability and rate capability. Ohzuku's group reported a SiO/C composite composed of 50 wt% SiO, 21 wt% graphite, 9 wt% carbon fiber, and 20 wt% deposited carbon, which manifested a discharge capacity of 700 mA h g⁻¹ after 100 cycles in laminate-type cells.⁸⁷ By coupling the SiO/C composite anode with a LiCo_1/3Ni_1/3Mn_1/3O₂/LiCoO₂ cathode, prototype 14 500 batteries were fabricated. The prototype 14 500 battery delivered a capacity of 1 A h at 200 mA with 70% capacity retention after 300 cycles.⁹³ Using chemical vapor deposition (CVD), Oh and co-workers prepared a micrometer-sized d-SiO/C composite (Fig. 4a).^89,90 The d-SiO/C composite exhibited a high capacity fading attributed to the electric disconnection, limited buffering effect of lithium silicates, and crumbling of d-SiO/C. In another study, Choi and co-workers fabricated N-doped carbon coated d-SiO (d-SiO/NC) using N-containing ionic liquid as the carbon precursor.⁹¹ The micrometer-sized NC–d-SiO manifested enhanced capacity and rate capability compared with the carbon coated d-SiO and bare d-SiO counterparts.

Fig. 4 SEM image of d-SiO/C (a) and cycling performance of d-SiO/C. (b) Reproduced with permission from ref. 90. Copyright 2013, Elsevier Ltd.

More recently, a fascinating vertical graphene coated micrometer-sized d-SiO (d-SiO@vG) anode was prepared through a CVD method by Liu and co-workers.⁹⁴ Due to the unique structure with stable electrical contacts, fluent Li⁺ diffusion channels, and elastic buffering layer (Fig. 5a–c), the d-SiO@vG demonstrated a high specific capacity of 1600 mA h g⁻¹ with excellent capacity retention (Fig. 5d). Full cells based on the d-SiO@vG/graphite (5% : 95%) anode and NCA cathode (LiNi_0.8Co_0.15Al_0.05O₂) were also assembled. With the addition of only 5 wt% d-SiO@vG, the energy density of the full cell was increased by 15% (Fig. 5e and f).

Fig. 5 Schematic illustration of the d-SiO@vG during charge/discharge (a); schematic structure (b) and TEM image (c) of d-SiO/vG; cycling performances of d-SiO and d-SiO@vG at 320 mA g⁻¹ (d); digital photos of the assembled d-SiO@vG/graphite//NCA 18 650 full cells (e); cycling performance of the full cell (f). Reproduced with permission from ref. 94. Copyright 2017, American Chemical Society.

In most of the reports on SiO/C composite anodes, ball milling is combined with carbon coating to further enhance the performance.^{62,82,83,95–97} For example, Morita and Takami synthesized a Si/SiO_x/C nanocomposite (d-SiO/C) with a Si size of 5–10 nm by ball milling, furfuryl alcohol polymerization, and disproportionation of SiO.⁶² The obtained Si/SiO_x/C nanocomposite (d-SiO/C) delivered a high reversible capacity of ∼700 mA h g⁻¹ for 200 cycles. Sohn et al. prepared a d-SiO/C composite by ball-milling and pyrolysis.⁸² Polyvinyl alcohol (PVA) was used as the carbon precursor and the SiO was disproportionated into nanocrystalline Si and amorphous SiO₂.

The resultant d-SiO/C exhibited an ICE value of 76% and a capacity of 710 mA h g⁻¹ after 100 cycles at 100 mA g⁻¹. A d-SiO/graphite composite was prepared by the same group; it demonstrated a first discharge capacity of 1002 mA h g⁻¹, retaining 710 mA h g⁻¹ after 100 cycles.⁹⁵ A SiO/graphite composite anode was synthesized by Doh and co-workers through HEMM.⁸³ The SiO/graphite manifested a capacity of 688 mA h g⁻¹ after 30 cycles. Takeda et al. fabricated a SiO/carbon nanofiber (SiO/CNF) anode using HEMM; the as-synthesized SiO/CNF provided a specific capacity of 724 mA h g⁻¹ with excellent cycling performance for 200 cycles.⁹⁶

The fabrication of porous structures with conductive carbon coating on the surface integrates the advantages of both strategies.^98–100 Lee and Park developed a metal-assisted chemical etching process for the fabrication of a three-dimensional (3D) porous SiO anode from bulk SiO.⁹⁹ Ag nanoparticles were first deposited on the surface of SiO through galvanic displacement (Fig. 6a). When the Ag/SiO was immersed in HF/H₂O₂, the SiO was continuously etched off through localized electrochemical processes, forming a 3D porous structure. After removal of the Ag nanoparticles with HNO₃, 3D porous SiO could be obtained. Coating the porous SiO with acetylene-derived carbon by CVD led to the formation of porous SiO/C (Fig. 6b). The porous SiO/C delivered a high capacity of 1490 mA h g⁻¹ after 50 cycles at 0.1C (Fig. 6c). In addition, a capacity of 1130 mA h g⁻¹ could be achieved at 3C.

Fig. 6 Schematic illustration of the synthesis of porous SiO via metal-assisted etching (a); SEM image of porous SiO/C (b); discharge–charge profiles of porous SiO/C (c); HRTEM image of porous d-SiO showing the nanocrystalline Si/amorphous SiO_x composite structure (d); cycling performances of porous SiO/C and d-SiO/C (e). (a–c) Reproduced with permission from ref. 99. Copyright 2013, Elsevier Ltd. (d and e) Reproduced with permission from ref. 100. Copyright 2012, Wiley-VCH.

The disproportionation of SiO into Si/SiO₂ usually takes place at relatively high temperatures (∼1000 °C). The introduction of catalyst (NaOH) lowers the disproportionation temperature to ∼700 °C (the so-called chemical-assisted thermal disproportionation). By combining metal-assisted chemical etching and chemical-assisted thermal disproportionation, Park and co-workers prepared a porous multi-component Si/crystalline SiO₂/amorphous SiO_x (d-SiO) composite (Fig. 6d).¹⁰⁰ After coating with carbon, the porous d-SiO/C manifested excellent electrochemical performance, even better than that of porous SiO/C. A reversible capacity of ∼1600 mA h g⁻¹ could be obtained at 0.1C, retaining 91.3% of the capacity after 100 cycles (Fig. 6e).

2.3.3 SiO/metal and SiO/metal oxide hybrid anode materials. Besides carbon, metals and metal oxides have also been used to composite with SiO to improve its lithium storage performance.^{56,65,101–103} Early in 2001, Tatsumisago reported the synthesis of SiO/SnO composites by mechanical milling SiO and SnO in an inert atmosphere (Table 1).⁵⁶ By HEMM of SiO and Al, Kim et al. prepared a nanostructured SiAl_0.2O anode composed of Si nanocrystals (<10 nm) embedded in amorphous aluminosilica.⁶⁵ The SiAl_0.2O provided a high capacity of 800 mA h g⁻¹ over 100 cycles. Jeong et al. reported a TiO₂ coated SiO composite anode by a sol–gel process. The TiO₂ coated SiO demonstrated higher specific capacity, higher ICE, better rate capability, as well as better cycling stability than the pristine SiO.¹⁰¹ A Fe₂O₃ nanoparticle decorated SiO composite was prepared by mechanical milling by Wang et al.¹⁰³ The SiO/Fe₂O₃ composite exhibited enhanced lithium storage performances in terms of ICE (68%), rate capability (600 mA h g⁻¹ at 4.8 A g⁻¹), and cyclability (71% capacity retention after 50 cycles) compared to bare SiO.

Table 1 Electrochemical performances of SiO-based anode materials

Year	Sample	Initial discharge/charge capacity (mA h g^−1)	ICE (%)	Cycling performance (mA h g^−1)	Ref.
2001	SiO/SnO	∼590/∼293	43	340 (20 cycles)	56
2002	SiO	2600/1600	62	∼810 (17)	57
2006	d-SiO/C	∼1000/∼700	∼68	<700 (200 cycles)	62
2007	d-SiO/C	1050/800	76	710 (100 cycles)	82
2007	Li/SiO derived composite	951/770	81	762 (50 cycles)	63
2007	Ni doped SiO	1850/1556	84	—	78
2008	SiO/graphite	1556/693	45	688 (30 cycles)	86
2009	Li/SiO	1364/994	72.8	∼860 (20 cycles)	109
2010	Li/SiO	989/1000	101.07	710 (20 cycles)	110
2010	d-SiO/graphite	1516/1002	66	710 (100 cycles)	95
2010	SiAl0.2O	2240/1510	67.4	800 (100 cycles)	65
2011	SiO/C	1200/870	72.5	700 (100 cycles)	87
2011	SiO/CNF	2027/724	35.7	675 (200 cycles)	96
2012	d-SiO/C	1380/1307	94.7	578 (90 cycles)	89
2012	d-SiO/C	1930/1598	82.8	1459 (100 cycles)	100
2012	SiO/TiO2	1757/1265	72	—	101
2012	SiO/Sn30Co30C40	1480/1030	69.6	1040 (50 cycles)	104
2013	SiO/C	1990/1520	76.4	1490 (50 cycles)	99
2013	d-SiO/C	1958/1413	72.2	∼600 (100 cycles)	90
2013	SiO	1779/1002	56.3	∼1000 (50 cycles)	72
2013	SiO	2366/1467	62	1260 (50 cycles)	74
2013	d-SiO/NC	2024/1496	73.9	955 (200 cycles)	91
2013	SiO/Fe2O3	2773/1893	68	1335 (50 cycles)	103
2013	SiO/Fe2SiO4	2095/1900	89.3	∼1000 (25 cycles)	113
2013	SiO/Sn30Fe30C40	1427/961	67	538 (100 cycles)	105
2014	Porous SiO	2653/1709	64.4	1242 (100 cycles)	76
2016	Li/SiO	1220/1001	82.12	∼750 (15 cycles)	111
2016	Pre-lithiated SiO	—	94.9	906 (100 cycles)	112
2017	d-SiO@vG	—	—	1600 (100 cycles)	94

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Amine and co-workers reported a novel family of composite anode materials, SiO/Sn_xCo_yC_z, by ultrahigh energy ball milling. The anode material composed of 50 wt% SiO and 50 wt% Sn₃₀Co₃₀C₄₀ delivered stable specific capacities of 1040 and 900 mA h g⁻¹ at 100 and 300 mA g⁻¹, respectively.¹⁰⁴ A family of SiO/Sn_xFe_yC_z materials was prepared by the same group as well. Fe was chosen to replace Co in the composite anode materials due to its low cost and environmental friendliness. The SiO/Sn₃₀Fe₃₀C₄₀ anode delivered specific capacities of 900 and 700 mA h g⁻¹ at 300 and 800 mA g⁻¹, respectively.¹⁰⁵

2.3.4 Optimizing the binder for SiO-based anode materials. Polymer binders have a great influence on the cyclability of SiO-based anodes as well. The high capacity of SiO is accompanied by a large volume expansion of 200%, which causes the notorious problems of active material pulverization at the particle level and disintegration at the electrode level. Suitable binders can provide persistent contacts among the active materials, conductive carbons, and current collector, improving the electrode integrity effectively. Komaba et al. examined four types of polymer binders, including poly(acrylic acid) (PAA), PVA, sodium carboxymethyl cellulose (CMCNa), and conventional poly(vinylidene fluoride) (PVDF). They found PAA was the most suitable binder for the SiO anode (Fig. 7a).⁷⁹ The SiO-based electrode with PAA binder delivered a capacity over 700 mA h g⁻¹ with excellent cycling stability (Fig. 7b). The improved cyclability was attributed to the strong interactions between –COOH groups and surface Si–OH groups as well as the uniform coating of amorphous PAA on SiO, which effectively held the fractured SiO particles and suppressed the loss of electrical isolation (Fig. 7c and d). By screening environmentally friendly aqueous binders for SiO/C, Yang et al. found that PAA, styrene butadiene rubber/sodium carboxymethyl cellulose (SCMC), and especially sodium alginate (Alg) led to much better lithium storage performance than PVA.⁸⁰ The SiO/C electrodes with Alg and SCMC binders led to a high specific capacity of 940 mA h g⁻¹ after 100 cycles.

Fig. 7 Chemical structures of PVDF, PVA, CMCNa, and PAA binders (a). Cycling performances of SiO-based electrodes with different binders (b). Schematic illustration of the SiO-based electrodes with PVDF (c) and PAA (d) binders during charge/discharge. Reproduced with permission from ref. 79. Copyright 2011, American Chemical Society.

To achieve satisfactory cycling performances for high-capacity anode materials, larger amounts of binders and conductive carbons than those used in traditional electrodes are usually required. These inactive additives inevitably sacrifice the energy density of the batteries, making them less competitive. By employing a functional conductive polymer binder, poly(9,9-dioctylfluorene-co-fluorenone-co-methylbenzonic ester) (PFM), Liu et al. fabricated highly durable SiO-based electrodes with 98% micro-sized SiO/C, 2% PFM binder without any conductive carbons (Fig. 8a).⁸⁴ Such a novel binder enabled molecular-level electronic connections between active materials and the conductive polymer matrix by integrating adhesion and electrical conduction (Fig. 8b and c). The SiO/C electrode with PFM binder demonstrated a stable high reversible capacity (>1000 mA h g⁻¹) for 500 cycles with a capacity retention of over 90% (Fig. 8d). The areal capacity reached close to 2 mA h cm⁻² (Fig. 8e). By calendering the electrode to optimize the porosity, the area capacity of the SiO-based electrode with PFM binder could be improved to ∼3.5 mA h cm⁻².⁸⁵ By introducing a sacrificial agent, NaCl, in the electrode, the porosity of the electrode could be well controlled and the areal capacity of the electrode could be further increased to ∼4 mA h cm⁻².¹⁰⁶

Fig. 8 Schematic illustration of a SiO-based electrode with low-concentration functional conductive polymer binder and high-concentration SiO/C (a). Chemical structure of the PFM functional conductive polymer binder with both strong adhesion and electrical conduction functionalities (b). Trans-esterification reaction between the ester functional group of the PFM binder and the surface Si–OH group, providing strong adhesion (c). Cycling performances of SiO anodes with 2, 5, 10 wt% PFM binder at 200 mA g⁻¹ (d and e). Reproduced with permission from ref. 84. Copyright 2014, American Chemical Society.

2.3.5 ICE improvement for SiO-based anode materials. The large irreversible capacity during the initial cycle significantly hinders the practical application of SiO in LIBs. It necessitates an excess amount of cathode materials solely for the initial cycle, sacrificing the energy density of the full cell. Therefore, many efforts have been devoted to improving its ICE. For example, by HEMM SiO with Li, Wen et al. prepared a composite anode material composed of nanocrystalline Si, Li₄SiO₄, and other Li-rich components (Li₂O).⁶³ The obtained Li/SiO derived composite exhibited a first discharge capacity of 951 mA h g⁻¹ with a high ICE of 81%. This method could be easily generalized to the construction of nanosized Si/Sn-based composite and Si-based glassy composite anodes.^107,108 Yoon et al. synthesized a pre-lithiated SiO/C anode, which demonstrated reduced initial capacity loss and enhanced ICE in comparison with the pristine SiO/C.¹⁰⁹ A SiO and Li powder double layer anode technique was also developed by the same group, and an ICE value of over 100% was achieved.¹¹⁰ By heating the SiO and Li mixture at high temperatures (600 °C), the ICE of SiO could also be increased.¹¹¹ The heated SiO and Li mixture with a weight ratio of 7 : 1 demonstrated an ICE value of 82.12%, much higher than that of pristine SiO (58.52%). It was found that irreversible phases, such as lithium silicates and Li₂O, were formed during the solid state reaction of SiO and Li, and the preformed irreversible phases prevented the consumption of Li⁺ during the first cycle.

More recently, a scalable roll-to-roll pre-lithiation technology based on electrical shorting with Li was developed by Choi et al. (Fig. 9).¹¹² The degree of pre-lithiation could be finely tuned via controlling the accurate shorting time and voltage monitoring. With an optimized shorting time, the pre-lithiated SiO showed a high ICE value of 94.9%. When coupled with a NCA cathode, the full cell led to an energy density 1.5 times that of traditional graphite//LiCoO₂ counterparts based on the weight of active materials.

Fig. 9 Schematic illustration of the pre-lithiation process (a) and a scalable roll-to-roll pre-lithiation process (b). Cycling performances of pristine and pre-lithiated SiO in half-cells (c). 1C = 1500 mA g⁻¹. Cycling performances of pristine and pre-lithiated SiO in SiO/NCA full cells (d). Reproduced with permission from ref. 112. Copyright 2016, American Chemical Society.

Interestingly, compositing SiO with metals or metal oxides could also improve the ICE of SiO to some extent. For example, Miyachi et al. synthesized Fe, Ni, and Ti doped SiO anodes, among which the Ni doped SiO (25 wt% Ni + 75 wt% SiO) demonstrated an ICE value of 84%. With the addition of Li in Ni doped SiO, the ICE value could be further increased to a nearly 100%.⁷⁸ By reacting SiO with Fe₂O₃ at 800 °C for 3 h in Ar, Yamamura and Iba prepared a Fe₂SiO₄ uniformly coated SiO anode, which exhibited a high ICE value of 89.3%.¹¹³

3. SiO₂-based anode materials

Silicon dioxide (SiO₂), also known as silica, is one of the most abundant materials on the earth. Both amorphous and various crystalline polymorphs of SiO₂ have been commonly found in nature. For example, quartz, a crystalline polymorph of SiO₂, is the major constituent of sand, while amorphous SiO₂ has been widely found in various living organisms, such as diatoms, rice husks, and reed leaves.^114–122

3.1 Lithiation behavior of SiO₂

In early studies, SiO₂ was believed to be electrochemically inactive towards Li.¹²³ Actually, it was regarded as an inert matrix, buffering the volume expansion of active components upon lithiation. In 2001, Zhou et al. found that commercially available SiO₂ nanoparticles (∼7 nm) could react with Li, providing a specific capacity of ∼400 mA h g⁻¹ (Table 2).¹²⁴ In 2008, Wang and co-workers provided direct experimental evidence on the electrochemical reduction of SiO₂ through solid-state nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED).¹²⁵ They prepared a nano-SiO₂/hard carbon composite by a hydrothermal reaction and high-temperature carbonization; the resultant composite demonstrated a reversible capacity of 630 mA h g⁻¹. Using XPS, HRTEM, and SAED, Fu et al. also demonstrated the electrochemical activity of SiO₂ towards Li.¹²⁶ They prepared a SiO₂ thin-film electrode by reactive radio frequency sputtering, and the thin-film electrode exhibited discharge capacities of 416–500 mA h g⁻¹ for 100 cycles.

Table 2 Electrochemical performances of SiO₂-based anode materials

Year	Sample	Initial discharge/charge capacity (mA h g^−1)	ICE (%)	Cycling performance	Ref.
2001	SiO2	∼940/∼415	∼44	—	124
2008	SiO2/C	∼630/∼1010	∼62	—	125
2008	SiO2	539/388	72	∼500 (100 cycles)	126
2011	SiO2/C	891/536	60	∼500 (55 cycles)	148
2011	SiO2	980/323	33	334 (500 cycles)	143
2011	PMO	970/276	28	242 (300 cycles)	144
2012	SiO2	2243/827	37	∼660 (200 cycles)	139
2013	SiO2	3084/1457	47	919 (30 cycles)	146
2013	SiO2/C	835/505	60.5	600 (100 cycles)	171
2014	SiO2	2404/1040	43.3	1247 (100 cycles)	147
2014	SiO2	1134.5/622.1	54.8	876.7 (500 cycles)	137
2014	SiO2/C	1380/841	61	780 (350 cycles)	152
2014	SiO2/C	712/418	59	560 (30 cycles)	149
2015	SiO2/C	∼2900/∼1200	42	800 (100 cycles)	150
2015	SiO2/C	—	—	636 (200 cycles)	153
2015	SiO2/C	2547/1438	56.5	1055 (150 cycles)	155
2015	SiO2/C	1071/629	58.7	658 (100 cycles)	163
2015	SiO2/C	1050/697	66.4	620 (300 cycles)	173
2017	SiO2/C	880/530	68	441 (500 cycles)	157
2017	SiO2/Ni	1195/676	56.5	672 (50 cycles)	174

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To date, the reaction mechanism between Li and SiO₂ has not been well understood; there is still debate on the reaction pathway and final products. Theoretical computations and in situ TEM were used to reveal the lithiation of SiO₂.^127–130 Using density functional theory (DFT) and molecular dynamics (MD) simulations, Zhao et al. concluded that neither interstitial Li, nor the formation of Li₂O and Si–Si bonds (full reduction) were energetically favorable. Instead, the lithiation could proceed through partial reduction, namely, two Li attacked and broke a Si–O bond.¹²⁷ Through first-principles calculations, Balbuena et al. revealed that Li was incorporated via breaking Si–O bonds and partial reduction of the Si atoms.¹²⁹ The breaking of Si–O bonds became less favorable at high lithiation degrees and was accompanied by Si–Si bond formation and nucleation of Li₆O complexes. Leivia's group calculated the free energy of reaction for the lithiation of SiO₂ for different products using DFT. They found the formation of Li₂Si₂O₅ is energetically most favorable.¹³¹ In another study, they extended the analysis to the free energy of reactions involving the lithiation of SiO₂ as a function of cell potential. It was found that the most probable lithiation product was cell-potential-dependent.¹³²

Zhao et al. integrated a single SiC@SiO₂ core@shell nanowire into a nanobattery setup (Fig. 10a) and monitored the structural evolution of the nanowire during in situ electrochemical lithiation/de-lithiation by TEM and SAED.¹²⁸ The pristine SiC@SiO₂ nanowire consisted of a crystalline SiC core and a uniform amorphous SiO₂ shell with a thickness of 5–10 nm. The SiC core was inactive towards Li and did not change upon lithiation. While for the SiO₂ shell, its thickness increased and its surface roughened during lithiation, forming a wavy structure after the first lithiation (Fig. 10b). Through SAED characterization, it was found that crystalline Li₂O was formed at the early stage of lithiation, and further lithiation led to the formation of a mixture of Li₄SiO₄ and Li₂O, the so-called Li–Si–O glass (Fig. 10c). Theoretical studies revealed that the electrical conductivity of SiO₂ was significantly enhanced during lithiation/de-lithiation, while the Li⁺ diffusivity decreased.

Fig. 10 Schematic illustration of a nanobattery consisting of a SiC@SiO₂ single-nanowire working electrode and Li/Li₂O counter electrode/electrolyte (a). TEM images of a SiC@SiO₂ nanowire from the pristine state to the end of first lithiation (b). The corresponding SAED patterns at given lithiation states (c). Reproduced with permission from ref. 128. Copyright 2014, American Chemical Society.

3.2 Electrochemical performance of SiO₂

3.2.1 Bare SiO₂ anode materials. Despite its high theoretical capacity (1965 mA h g⁻¹), bulk SiO₂ shows little electrochemical activity towards Li due to the sluggish Li⁺ diffusivity and poor intrinsic electrical conductivity. Reducing the particle size has been demonstrated to be an effective strategy to shorten the Li⁺ diffusion length and enhance the electrochemical activity of certain electrode materials.^133–136 Therefore, efforts have been dedicated to preparing fine-sized SiO₂. For example, Jiao et al. synthesized Stöber SiO₂ spheres with an average size of ∼400 nm through a sol–gel process, which delivered a first charge capacity of 622 mA h g⁻¹ with an ICE value of 54.8%.¹³⁷ After 500 discharge/charge cycles, the charge capacity increased to 877 mA h g⁻¹. Mechanical milling is an effective strategy to reduce the particle size of SiO₂. Zheng and co-workers synthesized submicron SiO₂ by planetary ball milling bulk SiO₂, and the optimized sample provided a specific capacity of ∼600 mA h g⁻¹ for 150 cycles.¹³⁸ By HEMM quartz, a common crystalline polymorph of SiO₂, a sample (HEMM-treated SiO₂) consisting of nanosized Si embedded in amorphous SiO₂ was obtained.¹³⁹ The HEMM-treated SiO₂ exhibited an initial charge capacity of 827 mA h g⁻¹, retaining 80% of the capacity after 200 cycles. Hollow structures with hollow cavities for volume variation accommodation and thin shells for Li⁺ diffusion represent a promising family of electrode architecture.^140–142 Various hollow structured SiO₂, such as hollow spheres,^143–145 hollow cubes,¹⁴⁶ hollow nanobelts, and nanotubes,¹⁴⁷ have been explored in lithium storage. Nakashima reported the synthesis of SiO₂ hollow nanospheres (Fig. 11a and b) with a uniform size of ∼30 nm using poly(styrene-b-2-vinyl pyridine-b-ethylene oxide) (PS-PVP-PEO) triblock copolymer as the structure directing agent.¹⁴³ The resultant SiO₂ hollow nanospheres delivered a capacity of 242 mA h g⁻¹ after 300 cycles.¹⁴⁴ Chen's group constructed hollow SiO₂ nanocubes (Fig. 11c and d) through a hard-templating method.¹⁴⁶ The nanospheres demonstrated a stable discharge capacity of 334 mA h g⁻¹ after 500 cycles. Using a similar approach, the same group also prepared periodic organosilica (PMO) hollow SiO₂ nanocubes which exhibited a first discharge capacity of 3084 mA h g⁻¹ and an ICE of 47%. The capacity decreased quickly to 919 mA h g⁻¹ after 30 cycles. Ozkan et al. fabricated SiO₂ nanotubes (Fig. 11e and f) using commercial anodic aluminum oxide (AAO) as the hard template and polydimethylsiloxane as the precursor.¹⁴⁵ When applied as the anode in LIBs, the SiO₂ nanotubes displayed first discharge and charge capacities of 2404 and 1040 mA h g⁻¹, respectively. Besides high specific capacity, the SiO₂ nanotubes also demonstrated impressive rate capability and cyclic performance.

Fig. 11 Schematic illustrations and TEM images of SiO₂ hollow nanospheres (a and b), hollow porous SiO₂ nanocubes (c and d), and SiO₂ nanotubes (e and f). (a and b) Reproduced with permission from ref. 143. Copyright 2011, Royal Society of Chemistry. (c and d) Reproduced with permission ref. 146. Copyright 2013, Nature Publishing Group. (e and f) Reproduced with permission from 147. Copyright 2014, Nature Publishing Group.

3.2.2 SiO₂/C hybrid anode materials. Among the silicon oxide-based materials, SiO₂ possesses the lowest conductivity. Many efforts have been dedicated to improving the conductivity of SiO₂ through compositing with carbon.^125,148 Besides enhancing the electrical conductivity, coupling with carbon can also buffer the volume fluctuation of SiO₂. Thus, the SiO₂/C composites usually manifested superior cyclability.

Among the various SiO₂/C composites, porous, especially mesoporous SiO₂/C hybrids have attracted much attention.^149–156 The porous structure not only provides free space for the volume expansion of SiO₂ upon lithiation but also enables rapid Li⁺ diffusion. As a result, both the cycling stability and rate capability can be enhanced. For example, Wu and co-workers fabricated a porous bicontinuous SiO₂/C nanocomposite using tetraethoxysilane (TEOS) as the SiO₂ precursor and sucrose as the carbon source.¹⁴⁹ In the constructed composite, the continuous SiO₂ framework provided abundant active sites for lithium storage; the carbon not only wired the whole composite, but also enhanced the structural integrity; the nanopores effectively accommodated the volume expansion of SiO₂ and enabled the rapid diffusion of Li⁺. The porous SiO₂/C composite manifested a capacity of 560 mA h g⁻¹ after 30 cycles. Through a multi-constituent co-assembly approach, the same group fabricated a N-doped ordered mesoporous carbon/SiO₂ nanocomposite with unique interpenetrating SiO₂ and carbon frameworks (Fig. 12a and b).¹⁵⁰ The mesoporous SiO₂/C composite delivered a capacity of ∼800 mA h g⁻¹ after 100 cycles. Using a similar approach, Wang and co-workers prepared a mesoporous SiO₂/C composite with a reversible capacity of 670 mA h g⁻¹.¹⁵¹

Fig. 12 Schematic illustrations and TEM images of a mesoporous SiO₂/C composite (a and b), plum-pudding like mesoporous SiO₂ nanosphere/flake graphite (c and d), and carbon core/mesoporous SiO₂/carbon shell three-layer structure (e and f). (a and b) Reproduced with permission from ref. 150. Copyright 2015, Royal Society of Chemistry. (c and d) Reproduced with permission from ref. 154. Copyright 2015, Royal Society of Chemistry. (e and f) Reproduced with permission from ref. 155. Copyright 2015, Royal Society of Chemistry.

By coating ordered mesoporous SiO₂ with carbon, Zhao et al. designed a SiO₂/C composite anode, which demonstrated a high specific capacity of 780 mA h g⁻¹ after 350 cycles.¹⁵² Through a hydrothermal method, Zhang and co-workers fabricated a plum-pudding like mesoporous SiO₂ nanosphere/flake graphite composite (Fig. 12c and d).¹⁵⁴ The obtained SiO₂/FG composite exhibited a high capacity of 702 mA h g⁻¹ after 100 cycles. A carbon core@mesoporous silica@carbon shell three-layer structure (Fig. 12e and f) was designed by Cao et al.¹⁵⁵ The product delivered a capacity of 1055 mA h g⁻¹ after 150 cycles.

To integrate the advantages of both hollow structures and carbon coating, hollow structured SiO₂/C composites have also been explored.^157–161 Zhang and co-workers prepared SiO₂/C mesoporous hollow spheres by combining a sol–gel process and carbon coating.¹⁵⁷ The SiO₂/C mesoporous hollow spheres demonstrated a capacity of 441 mA h g⁻¹ after 500 cycles at 500 mA g⁻¹. At a lower current density of 100 mA g⁻¹, the specific capacity could reach 695 mA h g⁻¹. Combining an aerosol spray drying process with polyacrylonitrile (PAN) coating and carbonization, Wang and co-workers prepared a hollow core–shell structured SiO₂/C composite, which possessed a high specific capacity of 670 mA h g⁻¹ after 100 cycles.¹⁶¹

Besides porous/hollow structures, various other nanostructured SiO₂/C composites have also been extensively studied in lithium storage.^162–171 These nanostructures include SiO₂/C nanofibers,^162,163,172 SiO₂/C spheres,¹⁷³ SiO₂/graphene,¹⁶⁵ and SiO₂/C/graphene spheres.¹⁶⁶

3.2.3 SiO₂/metal hybrid anode materials. Coupling SiO₂ with conductive metal has also been demonstrated to be effective in electrical conductivity improvement and electrochemical activity enhancement.^174–176 For example, Zhou, Mai, and co-workers constructed a novel SiO₂/Ni nanocomposite with hierarchical hollow spherical structure (Fig. 13a–c) through the reduction of nickel silicate in H₂/Ar (5%/95%).¹⁷⁴ The obtained SiO₂/Ni nanocomposite has the following structural merits for lithium storage: (I) hollow spherical structure for volume fluctuation accommodation and strain relaxation; (II) ultrathin nanosheet building blocks for efficient Li⁺ diffusion; and (III) ultrafine Ni nanoparticle decoration for electron transfer enhancement. Benefited from these unique structural features, the SiO₂/Ni hierarchical hollow spheres demonstrated high specific capacities of 672 mA h g⁻¹ after 50 cycles at 100 mA g⁻¹ and 337 mA h g⁻¹ after 1000 cycles at 10 A g⁻¹ (Fig. 13d and e). It is anticipated that this approach could be generalized for the preparation of other SiO₂/metal nanocomposites.

Fig. 13 Schematic illustration of SiO₂/Ni hierarchical hollow microspheres constructed by ultrafine Ni nanoparticle decorated SiO₂ nanosheets (a); SEM (b) and TEM (c) images of SiO₂/Ni hierarchical hollow spheres; cycling performances of SiO₂/Ni hierarchical hollow spheres at 100 mA g⁻¹ (d) and 10 A g⁻¹ (e). Reproduced with permission from ref. 174. Copyright 2017, Wiley-VCH.

4. SiO_x-based anode materials

Just like SiO and SiO₂, nonstoichiometric silicon suboxides (SiO_x) have also been identified as an attractive high-capacity anode material for next-generation LIBs, especially when the O content is lower than 1.0 (Si-rich SiO_x).¹⁷⁷

4.1 Lithiation behavior of SiO_x

The lithiation behavior of SiO_1/3 was investigated by Chou and Hwang by DFT calculations.¹⁷⁷ It was found that the SiO_1/3 host matrix gradually disintegrated into smaller fragments with lithiation. Meanwhile, the Si–O–Si units ruptured and the O atoms tended to be surrounded by Li atoms (Fig. 14). With increasing Li contents, the Si–Li coordination number increased monotonically to around 10 in amorphous Li₄SiO_1/3, whereas the O–Li coordination number tended to saturate far before full lithiation at around 6.

Fig. 14 Structural evolution of amorphous Li_xSiO_1/3 with increasing Li contents. Reproduced with permission from ref. 177. Copyright 2013, American Chemical Society.

4.2 Electrochemical performances of SiO_x

4.2.1 Bare SiO_x anode materials. The research on bare SiO_x-based anodes has mainly focused on studying the effect of O contents in SiO_x on their electrochemical performances.^178–185 Early in 2002, Yang et al. performed a comparative study on SiO_0.8, SiO, and SiO_1.1.⁵⁷ It was found that the decrease in O content in SiO_x enhanced the reversible capacity as well as the ICE,¹⁸⁶ but sacrificed the cycling stability. This conclusion was further confirmed by several groups independently.^181–185

It is generally accepted that the Si-rich SiO_x possesses high capacity but suffers from poor cycling performance, while the O-rich SiO_x is beneficial for volume change accommodation. To integrate the advantages of both components, Yan and co-workers designed a SiO_x/SiO_y bi-layer nano-membrane anode (Fig. 15a).¹⁸⁷ The inner Si-rich SiO_y layer (y = 0.5) contributed a high Li storage capacity to the composite; meanwhile, the outer O-rich SiO_x layer (x = 1.85) acted as a buffering layer for volume change accommodation. Due to the smart design, the SiO_x/SiO_y bi-layer nano-membrane delivered a specific capacity of ∼1300 mA h g⁻¹ after 100 cycles.

Fig. 15 Schematic structure of the rolled-up SiO_x/SiO_y bi-layer nano-membrane (a); cycling performance of the SiO_x/SiO_y bi-layer nano-membrane at a current density of 100 mA g⁻¹ (b). Reproduced with permission from ref. 187. Copyright 2014, Wiley-VCH.

4.2.2 SiO_x/C hybrid anode materials. Compared to unmodified SiO_x, the SiO_x/C composites demonstrated better cyclability and rate capability due to the excellent electrical conductivity and volume change accommodation ability of carbon. The sol–gel process of silane has been demonstrated to be an effective approach for the construction of SiO_x/C composite anode materials. For example, a series of SiO_x/C composites, such as SiO_x/C spheres,¹⁸⁸ SiO_x/graphene/carbon¹⁸⁹ SiO_x/C nanorods,¹⁹⁰ and SiO_x/multi-walled carbon nanotube/carbon (SiO_x/MWCNT/C), have been fabricated by Li et al. through the hydrolysis and condensation of ethyltriethoxysilane (CH₃CH₂Si(OCH₂CH₃)₃). All these materials demonstrated high specific capacities (600–800 mA h g⁻¹) and excellent cycling performances (150–450 cycles). Using the sol–gel process of TEOS, Zhao et al. synthesized a series of SiO_x/C composite anode materials, including core@shell nanoparticles (Table 3),^191,192 mesostructured architectures,¹⁵² and dual-phase glass.¹⁹³ These materials delivered high reversible capacities of 600–1000 mA h g⁻¹ with stable cycling (50–350 cycles). Using the sol–gel reaction of triethoxysilane, Kim et al. prepared hydrogen silsesquioxane (HSiO_1.5) nanospheres.¹⁹⁴ After high-temperature annealing and carbon coating, Si/SiO_x/C nanospheres could be obtained. The resultant Si/SiO_x/C nanospheres manifested a reversible capacity of ∼950 mA h g⁻¹ at 200 mA g⁻¹, retaining 740 mA h g⁻¹ after 100 cycles. Wang et al. designed a SiO_x/C/rGO ternary nanocomposite using TEOS as the silicon precursor.¹⁹⁵ The SiO_x/C/rGO ternary nanocomposite exhibited a very high capacity of 1284 mA h g⁻¹ after 100 cycles. Recently, Zhou, Mai, and co-workers fabricated monodisperse and homogeneous SiO_x/C (Fig. 16a–c) composite microspheres through co-condensation of vinyltriethoxysilane and resorcinol/formaldehyde.¹⁹⁵ With the uniform dispersion of sub-nanosized SiO_x in amorphous carbon (Fig. 16d), high electrical conductivity and outstanding cycling stability could be achieved. When used as the anode materials for LIBs, the SiO_x/C composite microspheres demonstrated a high specific capacity of 689 mA h g⁻¹ at 500 mA g⁻¹ and a capacity retention of 91.0% after 400 cycles. Mai's group also prepared unique yolk@shell structured SiO_x/C composite microspheres through sol–gel synthesis, selective etching, and carbonization. To further improve the structural stability and electrical conductivity, they adopted a CVD process to coat the SiO_x/C with semi-graphitic carbon on both the exterior and interior surfaces (SiO_x/C-CVD) (Fig. 17a and b). The as-prepared SiO_x/C-CVD demonstrated a high reversible capacity (1165 mA h g⁻¹ at 100 mA g⁻¹) as well as outstanding durability (972 mA h g⁻¹ after 500 cycles at 500 mA g⁻¹) (Fig. 17c and d). The excellent electrochemical performance can be ascribed to the following two aspects: (I) the yolk@shell structure provides enough space for volume change accommodation. (II) The semi-graphitic carbon coating layers on the exterior and interior surfaces promote the electrical conductivity and structural integrity of the whole composite.¹⁹⁷

Table 3 Electrochemical performances of SiO_x-based anode materials

Year	Sample	Initial discharge/charge capacity (mA h g^−1)	ICE (%)	Cycling performance	Ref.
2011	SiOx/C	2224/1052	47.3	∼800 (50 cycles)	191
2013	NiSix@SiOx	4058/1737	42.8	∼800 (100 cycles)	201
2014	SiOx/C	1380/841	61	780 (350 cycles)	185
2014	SiOx/SiOy	2595/1372	53	∼1300 (100 cycles)	187
2014	Si/SiOx/C	1629/951	58.4	740 (100 cycles)	194
2014	SiOx/C/rGO	2403/1226	51	1284 (100 cycles)	195
2015	SiOx/graphene/C	896/607	67.7	630 (250 cycles)	189
2015	SiOx/C	1160/820	70.7	630 (150 cycles)	188
2015	SiOx/C	∼1150/644	56	840 (100 cycles)	193
2015	SiOx/C	1475/1015	68.8	817 (100 cycles)	192
2016	SiOx/C	3097/1394	45	1230 (200 cycles)	119
2016	SiOx/C	1324/906	68.4	720 (350 cycles)	190
2016	SiOx/MWCNT/C	1093/720	66	621 (450 cycles)	203
2016	SiOx/C	999/466	46.6	582 (100 cycles)	120
2017	FeSi@Si/SiOx	1550/803	51.8	727 (600 cycles)	202
2018	SiOx/C	1460/999	66.1	853 (150 cycles)	196
2018	SiOx/C	1734/1165	67.4	972 (500 cycles)	197
2018	SiOx/C	785/645	82.2	580 (500 cycles)	198
2018	SiOx@GO	1510/1042	69	740 (1000 cycles)	199

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Fig. 16 SEM (a), TEM (b) and EDS elemental mapping (c) images of SiO_x/C composite microspheres; schematic illustration (d) of the homogeneous SiO_x/C composite microspheres with; cycling performances of SiO_x/C composite microspheres at 500 mA g⁻¹ (e). Reproduced with permission from ref. 196. Copyright 2018, Elsevier Ltd.

Fig. 17 Schematic illustration of yolk@shell SiO_x/C microspheres with semi-graphitic carbon coating on the exterior and interior surfaces (SiO_x/C-CVD) (a); TEM (b); discharge–charge profiles (c) of SiO_x/C-CVD; cycling performances (d) of SiO_x/C and SiO_x/C-CVD at 500 mA g⁻¹. Reproduced with permission from ref. 197. Copyright 2018, Elsevier Ltd.

Guo et al. prepared graphite-like SiO_x/C composites with SiO_x particles anchored in the carbon matrix uniformly (Fig. 18a).¹⁹⁸ Such a unique structure effectively addressed the disadvantages of SiO_x such as low mass loading, large volume change, and poor electrical conductivity. The resultant SiO_x/C demonstrated high reversible capacity (645 mA h g⁻¹), excellent cycling stability (≈90% capacity retention for 500 cycles), and impressive rate performance (Fig. 18b and c).

Fig. 18 Schematic illustration showing the synthetic process of SiO_x/C hydrogels (a); rate capabilities of SiO_x/C anodes at a mass loading of 3.5 mg cm⁻² (b); cycling performance of SiO_x/G and SiO_x/C anodes at a pressing density of 1.3 g cm⁻³ (c). Reproduced with permission from ref. 198. Copyright 2018, Wiley-VCH.

Guo's group also prepared flake-graphite-like SiO_x@G with SiO_x encapsulated in a graphene bubble film through a facile protonated chitosan assisted self-assembly process (Fig. 19a–c).¹⁹⁹ The unique structure enabled efficient electrical contact of SiO_x with amorphous carbon, and a high tap density with low particle agglomeration/exposure; mechanical strength can also be realized. The obtained SiO_x@G exhibited a capacity far above the theoretical capacity of graphite and the retention after 1000 cycles at 1 A g⁻¹ is 80%.

Fig. 19 Schematic illustration (a) and TEM images (b) of flake-graphite-like SiO_x@G; cycling performances of SiO_x/G and SiO_x@G under a current density of 200 mA g⁻¹ for the first 3 cycles and 1000 mA g⁻¹ for later cycles (c). Reproduced with permission from ref. 199. Copyright 2018, Wiley-VCH.

4.2.3 SiO_x/metal hybrid anode materials. Besides SiO_x/C composites, SiO_x/metal hybrids are also appealing for lithium storage.^200–202 For example, NiSi_x@SiO_x heterostructured nanowires (Fig. 20a and b) prepared by CVD exhibited a reversible capacity of 1737 mA h g⁻¹ (based on the mass of SiO_x).²⁰¹ The capacity decreased slowly upon cycling, remaining at 800 mA h g⁻¹ after 100 cycles. In another study, core@shell structured FeSi alloy@Si/SiO_x was prepared through controlled oxidation of FeSi alloy (containing 73.3 wt% Si and 21.4 wt% Fe) in air.²⁰² Impressively, the core@shell structured FeSi alloy@Si/SiO_x delivered capacities of 967 mA h g⁻¹ after 50 cycles at 50 mA g⁻¹ and 727 mA h g⁻¹ after 600 cycles at 200 mA g⁻¹. At a current density of 1000 mA g⁻¹, 86% of the capacity (compared to the capacity at the third cycle) could be retained after 1000 cycles.

Fig. 20 TEM and SEM (inset) images of NiSi_x@SiO_x heterostructured nanowires (a); STEM HAADF image and EDX elemental mappings of an individual NiSi_x@SiO_x heterostructured nanowire (b); SEM image of core@shell structured FeSi alloy@Si/SiO_x (c); cycling performances of the core@shell structured FeSi_x@Si/SiO_x at current densities of 50–1000 mA g⁻¹ (d). (a and b) Reproduced with permission from ref. 201. Copyright 2013, Elsevier Ltd. (c and d) Reproduced with permission from ref. 202. Copyright 2017, Elsevier Ltd.

5. SiOC-based anode materials

Silicon oxycarbide (SiOC) refers to a family of carbon-containing silicate ceramics wherein O and C atoms share bonds with Si in the amorphous network.²⁰⁴ In the 1990s, Dahn et al. first demonstrated the lithium storage capability of SiOC glass (Table 4).^205–209 Since then, significant efforts have been dedicated to the preparation of SiOC-based anode materials.^210,211

Table 4 Electrochemical performances of SiOC-based anode materials

Year	Sample	Initial discharge/charge capacity (mA h g^−1)	ICE (%)	Cycling performance (mA h g^−1)	Ref.
1994	SiOC	∼900/∼550	61–67	∼400 (25)	205
2011	SiOC thin film	∼1500/∼1100	73	∼1000 (45)	211
2012	SiOC thin film (based on the mass of Si)	1403/4731	30	842 (7200)	230
2014	SiOC/Sn	1022/651	64	562 (110)	243
2016	SiOC/rGO	∼1030/∼700	∼68	588 (1020)	213
2016	SiOC	∼1200/676	∼56	804 (250)	225

---

5.1 Microstructure and lithium storage mechanism of SiOC

5.1.1 Microstructure of SiOC. It is necessary to mention the microstructure of SiOC before discussing its lithium storage mechanism and performance. SiOC is a high-temperature ceramic with an open polymer-like network structure consisting of an amorphous SiOC glass phase and a free carbon phase.^202,213 Its detailed microstructure can be represented by the nanodomain model.²¹⁴ In such a model, the free carbon forms a continuous matrix, encapsulating the SiOC nanodomains. SiO₄ tetrahedra locate at the center of the SiOC nanodomains and SiO_nC_4−n (0 ≤ n < 4) tetrahedra sit at the interface of SiOC glass and free carbon phases.

5.1.2 Lithium storage mechanism of SiOC. The lithium storage mechanism of SiOC is an interesting topic. However, the origin of lithium storage activity of SiOC is still a controversial issue.^{212,215–220} Kanamura et al. observed three types of electrochemically active sites by employing 7Li NMR: interstitial spaces or edges of the graphene layers, the amorphous SiOC glass phase, and the micropores.²¹⁵ Among the three types of active sites, the interstitial spaces or edges of the graphene layers were believed to be the major active site for lithium storage. Using ²⁹Si NMR and XPS, however, Liu et al. found that the major share of the reversible capacity of SiOC was obtained from the alloy reaction between Li and the SiOC glass phase.^212,217 Among the different SiOnC_4−n units, the SiO₂C₂, SiO₃C, and SiO₄ units were electrochemically active and contributed to the reversible capacity; however, the SiOC₃ units only contributed to the irreversible capacity and were transformed into SiC₄ units without electrochemical activity. Using first-principles calculations, Liao, Zheng, and co-workers investigated the lithiation behavior of SiOC with different carbon contents.²¹⁸ It was found that the Li atoms preferred to bond with O atoms and stayed in the voids near the carbon atoms.

5.2 Electrochemical performance optimization of SiOC

5.2.1 Bare SiOC anode materials. Although the polymer-derived ceramic SiOC contains a free carbon phase, we consider it as pristine SiOC herein unless otherwise mentioned.

Pristine SiOC are commonly synthesized via the pyrolysis of Si-containing polymer precursors, such as polysiloxane and polysilsesquioxane.^{205,208,221–226} For example, Dahn's group reported the synthesis of SiOC anodes from polymethylphenylsiloxane (PMPS) and polyphenylsesquisiloxane (PPSS).²⁰⁵ Lee et al. reported the simple and scalable production of SiOC from a commercial phenyl-rich silicone oil precursor.²²⁵ Impressively, the obtained SiOC demonstrated a stable specific capacity of ∼800 mA h g⁻¹ for up to 250 cycles. To tailor the chemical composition and microstructure of SiOC, extra organic species, such as pitch, citric acid, and acenaphthylene, can also be introduced during the synthesis.^{206,209,227,228}

Besides SiOC powdered samples, SiOC-based thin films have also been constructed for LIB applications.^{211,229–234} Raj et al. prepared SiOC thin films by spraying a Si-containing polymer precursor on Cu with subsequent annealing.²¹¹ The SiOC thin-film anode with a thickness of 0.5 μm demonstrated a reversible capacity of ∼1100 mA h g⁻¹ and an ICE value of 73%. Osaka's group developed an electrodeposition method for the fabrication of SiOC-based thin films using SiCl₄ as the precursor, LiClO₄ in propylene carbonate /ethylene carbonate as the electrolyte.^229–234 It is believed that the electrodeposition of Si is accompanied by the decomposition of the organic solvent, resulting in the formation of SiOC-based films.²³² Based on the mass of Si, the specific capacity of the as-synthesized SiOC thin films reached 1045 mA h g⁻¹ at the 2000th cycle and 842 mA h g⁻¹ at the 7200th cycle.²³⁰

5.2.2 SiOC/C hybrid anode materials. Despite its high specific capacity, the lithium storage performances of pristine SiOC are still far from practical applications. Thus, efforts have been devoted to constructing SiOC/C hybrids for further enhancement of electrochemical performance.^{213,235–242} More than a decade ago, exfoliated graphite was used to couple with SiOC for electrical conductivity improvement.^235,236

Recently, CNTs,^237,238 carbon nanofibers,²³⁹ graphene,^213,240 and carbon paper²⁴¹ were also employed for electrochemical performance enhancement. Interestingly, when these one-dimensional/two-dimensional carbon materials were introduced, free-standing SiOC-based electrodes could be obtained. As an excellent example, Singh et al. constructed a novel free-standing SiOC/rGO composite paper electrode through vacuum filtration (Fig. 21a).²¹³ The obtained free-standing electrode (60SiOC with a SiOC content of 65–78 wt%) afforded a specific capacity of 702 mA h g⁻¹ at 100 mA g⁻¹, retaining 543 mA h g⁻¹ at 2400 mA g⁻¹ (Fig. 21b). In addition, after cycling at 1600 mA g⁻¹ for 1010 cycles, a capacity of 588 mA h g⁻¹ can be recovered after reducing the current density to 100 mA g⁻¹. In the composite electrode, the SiOC particles acted as the major active component for lithium storage, while the rGO served as the electron conductor as well as the current collector (Fig. 20c).

Fig. 21 Digital photo, schematic illustration of the free-standing SiOC/rGO composite, and the atomic structure of SiOC (a); rate performance of SiOC/rGO composites with different SiOC contents (b), and schematic illustration showing the lithiation/de-lithiation mechanism of SiOC/rGO composites (c). Reproduced with permission from ref. 213. Copyright 2016, Nature Publishing Group.

5.2.3 SiOC/metal hybrid anode materials. Introducing metal into SiOC to prepare SiOC/metal hybrid anodes has also been regarded as an effective approach to realize further improvement in the electrochemical performance. Kaspar et al. prepared SiOC/Sn nanocomposites through chemical modification of polysiloxane Polyramic RD-684a (SiOC) with tin(II) acetate, and then annealing at 1000 °C. The obtained SiOC/Sn exhibited a capacity of 562 mA h g⁻¹ at 74 mA g⁻¹ with little fading after 20 cycles. Even at a high current density of 744 mA g⁻¹, a significant electrochemical activity (133 mA h g⁻¹) can still be detected (Fig. 22a–c).²⁴³

Fig. 22 TEM image of SiOC/Sn (a); high-resolution image with EDS pattern of the amorphous SiOC matrix (b); rate performance and coulombic efficiency (η) of SiOC/Sn (c). Reproduced with permission from ref. 243. Copyright 2014, Wiley-VCH.

6. Conclusions

Silicon oxide-based materials, including SiO, SiO₂, SiO_x, and SiOC, represent an attractive family of high-capacity anode materials for next-generation LIBs. The issues facing such materials include the low intrinsic electrical conductivity, drastic volume fluctuation during lithiation/de-lithiation, and low ICE. This Review presents a comprehensive summary on the most important progress in the microstructure, lithium storage mechanism, synthesis, and electrochemical properties of silicon oxide-based anode materials. Particular emphasis has been placed on tackling the remaining issues of silicon oxide-based anode materials through rational structural design, such as constructing unique nanostructures, fabricating silicon oxide/carbon hybrids, and designing silicon oxide/metal composites. Other strategies for electrochemical performance enhancement, such as controlled pre-lithiation and polymer binder optimization has also been covered.

Despite the tremendous efforts, the development of silicon oxide-based anode materials is still in its infancy. There is a long way to go before large-scale practical application. Among the various silicon oxide-based anode materials, the SiO and Si-rich SiO_x are relatively close to commercialization. Their volume change can be alleviated through delicate structure optimization and a stable cycling of over hundreds of cycles can be achieved. However, the ICE and average coulombic efficiency during cycling remain relatively low. For SiO₂, O-rich SiO_x, and SiOC, they display excellent cycling stability after proper structure design; however, they also suffer from the coulombic efficiency issue. In addition, the large voltage hysteresis (the difference in charge and discharge voltages) hinders their practical applications.

As for the future research on silicon oxide-based anode materials, we believe that it should be devoted to the following aspects:

(i) Feasible, scalable, and controlled pre-lithiation technologies should be developed to improve the ICE of silicon oxide-based anode materials. The ICE of bare silicon oxides is only 50–60%. Even for carbon coated silicon oxides, it is challenging to achieve an ICE value of over 80%. For practical applications in full cells, an ICE value of above 85% is required.

(ii) The O content of silicon oxides should be optimized to achieve high specific capacity, extended cycle life, and low voltage hysteresis (high energy efficiency). The O content plays a significant role in the lithium storage performances of silicon oxides. The difficulty in achieving extended cycle life and low voltage hysteresis simultaneously is that they require contradictory material features. The extended cycle life requires a high O content (more Li₂O and lithium silicates) for volume change accommodation. However, a high O content usually leads to high voltage hysteresis.

(iii) Effective and controllable reduction methods for converting SiO₂ to SiO_x should be developed. Generally speaking, SiO_x with a relatively low O content (including SiO) is required in LIB technology. However, Si mainly exists as Si(IV), that is to say, SiO₂ and silicates, in nature. This situation necessitates cost-effective reduction methods.

(iv) The reaction mechanism between Li and silicon oxides should be better understood. There is still debate on the reaction pathways and final products. Deepening the basic understanding of the reaction mechanism would help in designing better silicon oxide-based anode materials.

(v) Novel binders and electrolyte additives suitable for high-capacity silicon oxide-based anode materials should be explored. The large volume variation of silicon oxides during cycling would lead to electrode disintegration and SEI film rupture. Novel binders and electrolyte additives which can improve electrode integrity and SEI film stability are worth exploring.

With continuous and intensive worldwide efforts, we believe that breakthroughs will no doubt be made in silicon oxide-based anode materials in the near future. And we are confident that the silicon oxide-based anode materials will play an increasingly important and active role in next-generation high-energy density LIBs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key R&D Program of China (2018YFB0104200).

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Footnote

† These authors contributed equally to this work.

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