Recent progress in nanostructured transition metal nitrides for advanced electrochemical energy storage

Abstract

On the heels of the rapid development of portable electronics, electric vehicles, and renewable energy, electrochemical energy storage (EES) devices have become more prevalent. Electrode materials are key components for EES devices and largely determine their energy storage performance. Transition metal nitrides (TMNs) are promising electrode materials suitable for a wide range of EES devices including supercapacitors and rechargeable batteries due to their unique electronic structure, high conductivity, and large volumetric energy density, as well as good electrocatalytic activity. However, the practical implementation of TMNs in EES systems is hampered by their limited electrochemically active sites, unsatisfactory capacitance (capacity), and poor durability and flexibility. In comparison, TMN-based nanocomposites have been demonstrated to possess improved EES properties because they boast potential synergistic effects that ameliorate the electron and ion conductivity, prevent agglomeration, enhance the active sites, and improve the electrochemical stability. In this paper, recent advances pertaining to TMN-based hybrid materials are reviewed from the perspective of advanced EES systems with the focus on advanced supercapacitors and lithium–sulfur batteries. The challenge and future opportunities confronting TMN-based electrode materials in high-performance EES systems are also discussed.

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Biao Gao

Biao Gao received his MS and PhD in Materials Science from Wuhan University of Science and Technology in 2012 and 2016. He is currently a lecturer in the State Key Laboratory of Refractories and Metallurgy at Wuhan University of Science and Technology. His main research activity includes nanostructured electrode materials for electrochemical energy storage.

Xingxing Li

Xingxing Li is a PhD student under the supervision of Prof. Kaifu Huo in the State Key Laboratory of Refractories and Metallurgy at Wuhan University of Science and Technology. His research interest focuses on the design and synthesis of nanostructured metal nitrides for electrochemical energy storage.

Paul K Chu

Paul K. Chu received his PhD in chemistry from Cornell University. He is Chair Professor of Materials Engineering in the Department of Physics and Department of Materials Science and Engineering at City University of Hong Kong. He is a Fellow of the American Physical Society (APS), American Vacuum Society (AVS), Institute of Electrical and Electronics Engineers (IEEE), Materials Research Society (MRS), and Hong Kong Institution of Engineers (HKIE). He is also a Fellow of the Hong Kong Academy of Engineering Sciences (HKAES). His research interests are quite diverse encompassing plasma surface engineering, materials science and engineering, surface science, and functional materials. He is a highly cited researcher in materials science according to Clarivate Analytics (Web of Science).

Kaifu Huo

Kaifu Huo received his BS in Applied Chemistry from China University of Petroleum in 1997 and PhD in Physical Chemistry from Nanjing University in 2004. He is currently a Professor in the National Laboratory for Optoelectronics at Huazhong University of Science and Technology. He is an associate editor of Nanoscience and Nanotechnology Letters (NNL). He has authored/co-authored more than 100 papers in international refereed journals, which are cited more than 4000 times (current H-index: 34). His main research activities encompass bioactive nanomaterials and nanostructured electrode materials for electrochemical biosensors and energy storage devices.

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1. Introduction

Owing to environmental pollution and depletion of fossil fuels, clean and renewable energy such as solar and wind energy is playing a more important role.¹ However, the deployment of these sustainable energy technologies requires more efficient and reliable electrochemical energy storage (EES) systems that could offset this intermittency by storing the energy generated and making it available upon demand. Moreover, the rapid development of portable electronic devices and electric vehicles has spurred a remarkably increased interest in EES devices. Therefore, high-efficiency EES devices have been explored extensively in recent years and attracted increasing interest spanning from fundamental research to industrial applications.^2,3

Current EES systems include mainly supercapacitors (SCs), Li-ion batteries (LIBs), and Li–sulfur (Li–S) batteries, as well as Li-ion hybrid capacitors (LiHCs).⁴ In spite of different definitions, they have common features in the configurations. Generally, EES devices are composed of three indispensables: a negative electrode (anode), a positive electrode (cathode), and an aqueous/organic electrolyte.^4,5 Since charges are stored on/in the electrodes in EES systems, the energy storage performance has been determined primarily by the electrode materials,^6,7 and therefore, the development of high-efficiency electrode materials is vital to obtaining high energy and power densities as well as long cycle life for EES devices.

Various electrode materials have been developed for EES devices, for instance, carbon materials,^5,8–11 metal oxides,^12–15 carbides,^16,17 nitrides,^17–20 sulfides,^21–23 phosphides²⁴ and alloy elements²⁵ and impressive advances have been achieved. Among them, group IVB–VIB early-transition-metal nitrides (early-TMNs) are remarkable electrode materials suitable for a wide range of EES devices including SCs, LIBs, Li–S batteries, and LiHCs because of their unique electronic structure, high conductivity, large volumetric energy density, and excellent catalytic activity.^17,18 Different TMNs with various morphologies including nanoparticles (NPs), nanowires (NWs), nanorods (NRs), nanotubes (NTs), nanosheets (NSs) and nanohybrids of titanium nitride (TiN), vanadium nitride (VN), molybdenum nitride (MoN or Mo₂N) and niobium nitride (Nb₄N₅ or NbN) have been fabricated and their EES characteristics have been studied.^{17,18,26–30} Nevertheless, the practical implementation of TMNs in EES systems has been stifled by the limited active sites, small capacitance (capacity), unsatisfactory durability, and brittleness. TMN-based nanocomposites with potential synergistic effects can prevent agglomeration, enhance the active sites, improve the electronic conductivity, and facilitate ion transport to boost the EES performance. Recently, several review papers have summarized advances pertaining to the synthesis and potential applications of TMNs in LIBs, SCs, fuel cells and electrocatalytic water splitting.^{17,18,20,31,32} However, the progress of TMN-based hybrid nanostructures in EES systems has not been discussed extensively. Moreover, the past few years have witnessed the rapid development of TMNs for EES devices including Li–S batteries and LiHCs and the progress has not been reviewed.

In this review, the structure and physical properties of TMNs are described and recent advances in TMN-based electrode materials in SCs, Li–S batteries, LIBs, and LiHCs are systematically summarized with the focus on high volumetric capacitance SCs and high-energy density Li–S batteries (Fig. 1). Finally, the challenges and future opportunities of TMN-based materials in high-performance EES systems are discussed.

Fig. 1 Nanostructured TMNs for typical energy storage devices such as SCs, LIBs, LiHCs and Li–S batteries. Copyright permission has been received for all the images in figures.

2. Structure and physical properties of TMNs

Group IVB–VIB TMNs such as TiN, VN, NbN, MoN, Mo₂N, and WN are interstitial compounds in which the nitrogen atoms are integrated into the interstitial sites of the parent metals.³³Table 1 shows the structure and physical properties of common Group IVB–VIB TMNs including TiN, VN, NbN, Nb₄N₅, MoN, Mo₂N and WN. TMNs typically have cubic, hexagonal closed packed and simple hexagonal structures (Fig. 2).¹⁷ Generally, TMNs possess an unusual combination of metallic, covalent, and ionic properties.^19,33,34 The metallic and covalent characteristics endow early-TMNs with high conductivity (for example, the conductivity of VN is 1.44 × 10⁵ S m⁻¹),²⁸ large hardness, high melting point, and Young's modulus (above 360 GPa).¹⁷ Therefore, TMNs are widely used as protective coatings on industrial components and diffusion-barrier layers in microelectronic devices.^35,36 Moreover, the M–N bonding in TMNs causes expansion of the parent metal lattice and constriction of the metal d-band.^17,37 The smaller deficiency in the d-band occupation and higher density of states of the metal near the Fermi level endow TMNs with noble metal-like catalytic activity. The appealing properties of TMNs with high electronic conductivity, catalytic activity and stable chemical properties play irreplaceable roles in advanced EES devices. Furthermore, TMNs have larger densities (for example, VN: 6.13 g cm⁻³) than carbon-based materials (generally <1.0 g cm⁻³) thus giving rise to a larger volumetric energy density in EES. TMNs with these unique properties are potential electrode materials in EES devices. In view of the thriving of TMNs, we will discuss the latest progress in the synthesis and applications of TMN-based electrode materials in high-performance EES devices such as SCs, Li–S batteries, LIBs, and LiHCs.

Table 1 Physical properties of some selected TMNs^{17–19,37,73–76}

Parent	Nitride	Crystal structure	Density (g cm^−3)	Melting point (°C)	Young's modulus (GPa)	Electrical conductivity (S m^−1)
a d-decompose.
Ti	TiN	Cubic	5.40	3050	420	3.70 × 10^6
V	VN	Cubic	6.13	2390	380	1.49 × 10^6
Nb	NbN	Cubic	8.47	2630d^a	360	1.60 × 10^6
	Nb4N5	Tetragonal	7.51	—	—	2.31 × 10^3
Mo	Mo2N	Cubic	9.46	790d^a(0.7 MPa)	∼365	2.97 × 10^4
	MoN	Hexagonal	7.06	—	—	2.31 × 10^3
	MoN2	Rhombohedral	5.49	—	—	—
W	WN	Hexagonal	17.7	—	∼430	6.97 × 10^3

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Fig. 2 Common crystal structures in TMN compounds: (a) cubic; (b) hexagonal closed packed; and (c) simple hexagonal. The blue points represent transition metal atoms and the brown points represent nitrogen atoms.¹⁷ Reproduced with permission from ref. 17. Copyright 2016, Wiley-VCH.

3. Synthesis of nanostructured TMNs and TMN based composites

Nanostructured TMNs are generally produced from oxide precursors via post NH₃ reduction annealing. This conversion process maintains the morphology of the oxides but produces abundant mesopores due to volume shrinkage from oxide to nitride as a result of the density difference between the oxide precursors and nitride products. For example, Gao et al. prepared mesoporous VN NWs by thermally nitriding V₂O₅ NWs³⁸ and Lee et al. prepared mesoporous Mo₃N₂ NWs by nitridation of MoO₃ NWs.³⁹ Very recently, Bi et al. prepared (001) exposed VN mesocrystal NSs via nitriding a layered Na₂V₆O₁₆ self-sacrificing template²⁸ and Xiao et al. fabricated two-dimensional (2D) MoN NSs with subnanometer thickness and sheet size exceeding 20 μm via nitriding 2D h-MoO₃.⁴⁰ The topochemical conversion from transition metal oxides (TMOs) to TMNs could produce various TMN nanostructures such as VN, TiN, and Nb₄N₅ with different morphologies from zero dimension (0D) to three dimension (3D) because of the enriched morphologies of the TMOs.^{17,18,26,29,41}

TMNs/C hybrids were fabricated by mixing the existing TMNs with CNTs or coating carbon (C) on TMNs. For example, Xiao et al. prepared a VN NWs/CNT hybrid electrode by simple physical mixing of VN NWs and CNTs.⁴² Lu et al. obtained carbon coated TiN NWs by hydrothermal treatment of TiN NWs in glucose solution, followed by annealing at 800 °C in nitrogen.⁴³ TMNs could be deposited on carbon to form TMNs/C hybrids. For example, Zhang et al. deposited a VN layer on a CNT array anchored on glassy carbon or Inconel substrates via magnetron sputtering of the vanadium target in mixed argon/nitrogen plasma.⁴⁴ Yang et al. fabricated corn-like TiN nanoarrays on carbon cloth (CC) by atomic layer deposition (ALD) of TiO₂ on the cobaltous dihydroxycarbonate (Co₂(OH)₂CO₃) NW precursor followed by thermal nitridation and removal of the Co₂(OH)₂CO₃ precursor in a diluted acid.⁴⁵ Haldorai et al. prepared TiN nanopillars on graphene by refluxing the Ti(OBu)₄ precursor and GO (graphene oxide) followed by thermal nitridation.⁴⁶

Other TMN-based hybrid materials such as TMN supported TMOs (TMOs/TMNs), TMN supported conducting polymers (CPs/TMNs), and TMN supported nitrides (TMNs/TMNs) were fabricated by using TMNs as supporting materials to deposit or load other materials.^47–49 For example, Peng et al. electrochemically polymerized polyaniline (PANI) on the outer/inner surface as well as wall nanoholes of TiN NT arrays (NTAs) to form a coaxial PANI/TiN/PANI hybrid nanostructure.⁴⁸ In recent years, although tremendous progress has been achieved, there is still a great challenge to produce TMNs and their hybrid nanostructures with desired dimensions, morphologies and pore structures via low-cost and scalable methods.

4. Nanostructured TMN-based electrode materials for high-performance SCs

SCs, also referred to as electrochemical capacitors, have attracted increasing scientific and technological attention that bridges the gap between conventional electrostatic capacitors and batteries.^50,51 SCs possess a high charge/discharge rate, long cycle life (more than 10 000 cycles), and high power density, and are widely used in portable electronics, regenerative braking of trams, hybrid electric vehicles and memory back-up devices.^52–55 According to their charge–discharge mechanisms, SCs are classified as electrical double-layer capacitors (EDLCs) and pseudocapacitors.^50,53,55 The former store charges via fast ion adsorption/desorption on large-surface-area carbon electrode materials such as carbon nanotubes (CNTs), activated carbon, or graphene,^52,54 while the latter store charge mainly by fast and reversible redox reactions at or near the surface of the electrode materials, which have higher specific capacitances than carbon materials.^56,57

Since the capacitance of SCs is highly associated with the ion-accessible surface area of the electrode materials, nanostructured materials with a large effective area should possess improved capacitive storage performance. Nanostructured carbon materials such as CNTs, activated carbon, graphene, and three-dimensional carbon foams are employed as electrode materials in SCs. However, the low volumetric capacitance as a result of the small density (<1.0 g cm⁻³) and moderate gravimetric capacitance (generally <300 F g⁻¹) has limited their application in portable and miniaturized capacitive storage.⁵² Pseudocapacitive materials including TMOs such as MnO₂, WO₃, and MoO₃ as well as CPs like PANI and polypyrrole (PPy) exhibit higher specific capacitances (300–2000 F g⁻¹).^56,58 However, TMO or CP electrodes generally suffer from low electrical conductivity (e.g. conductivity of MnO₂ is 10⁻⁵ to 10⁻⁶ S cm⁻¹) or poor electrochemical stability.^59–61

Nanostructured TMNs have emerged as promising electrode materials for SCs due to their intrinsic metallic conductivity, high pseudocapacitance and large density. Studies have demonstrated the use of TMNs in SCs and the remarkable capacitive properties of various nanostructured TMNs such as NPs, NWs or NTs of TiN,^41,48 VN,^62–64 Mo₂N,^65,66 MoN,⁴⁰ Mo₃N₂,³⁹ Nb₄N₅,^27,29,67 WN,⁶⁸ GaN,⁶⁹ TiVN,⁷⁰ and LaN⁷¹ have been reported. For example, Choi et al. reported that VN NPs exhibited a large specific capacitance of 1340 F g⁻¹ in an aqueous KOH electrolyte.⁶² Lee et al. prepared mesoporous Mo₃N₂ NWs by nitridation of MoO₃ NWs whose capacity was over 200 F g⁻¹ even at a large charging/discharging rate of 200 mV s⁻¹, better than that of MoO₃ NWs.³⁹ Recently, 2D structures such as MoN, VN, and TiN have been reported for SCs.²⁸ For example, Bi et al. fabricated mesoporous VN NSs with the highest electrical conductivity of 1.44 × 10⁵ S m⁻¹ and flexible SCs based on VN NSs and polyvinyl alcohol (PVA)/LiCl gel with a superior volumetric capacitance of 1937 mF cm⁻³ at a current density of 5 mA cm⁻².²⁸ Xiao et al. fabricated 2D MoN NSs via nitriding MoO₃ grown by the assistance of a NaCl crystal template.^40,72 The MoN film electrodes obtained by vacuum filtration of the 2D MoN exhibited a high volumetric capacitance of 928 F cm⁻³ at 2 mV s⁻¹ in H₂SO₄ electrolyte and excellent rate performance with a high capacitance of 200 F cm⁻³ even at an ultra-high scan rate of 20 V s⁻¹. Moreover, this method can also be extended to fabricate other 2D nitrides such as W₂N and V₂N NSs. Because of their superior electrical conductivity and mechanical stability, nanostructured TMNs are also excellent conducting frameworks to load other electroactive materials for high-performance SCs. For example, highly conductive TiN NTs were used as backbones and supportive materials to load MnO₂ (ref. 77) or PANI⁴⁸ to achieve enhanced capacitive storage properties.

Although nanostructured TMNs have been demonstrated to be promising electrodes for SCs with high energy and power density, there are still some obstacles needed to be overcome to achieve high-performance capacitive storage, especially for flexible SCs. Firstly, nanostructured TMNs generally have small specific surface areas (generally <40 m² g⁻¹), resulting in limited electroactive sites for capacitive storage and compromised capacitive performance. Secondly, TMNs commonly suffer from severe capacitance decay during the long-term process in an aqueous acidic or alkaline electrolyte due to irreversible electrochemical oxidation.^38,43 Thirdly, TMNs are inorganic nonmetallic ceramic materials which are brittle and have poor flexibility, and therefore they could not be directly applied in bendable and flexible SCs.^43,78 To improve the specific capacitance, and cycle life, as well as mechanical flexibility, much effort has been devoted to the design and fabrication of TMN-based nanocomposites. In the following subsections, the latest progress associated with the development of TMN-based nanohybrids (mainly Ti, V, Nb, and Mo based nitrides) is described.

According to recent developments, TMN-based nanohybrids for SCs can be divided into three categories: (i) active TMNs deposited on metal or carbon substrates, (ii) freestanding or self-standing films of TMNs for flexible SCs, and (iii) TMNs as supporting frameworks for loading other electroactive materials.

4.1 Depositing active TMNs on metal or carbon substrates for SCs

Direct growth or deposition of TMNs on conducting substrates can prevent agglomeration and provide channels for electron and ion transport to enhance the capacitive properties. These include coating and growing active materials of TMNs on metal substrates or carbon-based electrodes CC, carbon nanofibers (CNFs)/CNTs, and graphene.

4.1.1 Metal foil-supported nanostructured TMN electrodes. Metals have been extensively used as electrode substrates or current collectors in SCs because of their high electrical conductivity and good mechanical strength. Direct growth of porous TMN arrays on metal current collectors avoids the use of conductive additives and binders compared to conventional powder-form materials.^41,79 Moreover electrically connected metal substrates and TMN arrays offer fast electron transfer pathways and porous arrays facilitating quick ion transport and leading to larger SC energy and power densities. Furthermore, TMNs in situ grown on thin and bendable metal foils can act as flexible electrodes for flexible SCs.

Xie et al. fabricated TiN nanopore arrays with a diameter of 80–100 nm and a thickness of 10.7 μm on a Ti substrate via electrochemical anodization of a Ti foil and further thermal nitridation. The areal specific capacitance of TiN nanopore arrays was 99.7 mF cm⁻² at a current density of 0.20 mA cm⁻² in 1 M H₂SO₄ solution, which is much higher than that of TiO₂ nanoarrays (11.8 mF cm⁻²).⁷⁹ Recently, we reported hierarchical TiN NP assembled (H-TiN) nanopillars grown on a Ti foil via nitriding hierarchical TiO₂ nanopillar nanoarrays produced via hydrothermal treatment of anodic TiO₂ NTs in pure water (Fig. 3a–d).⁴¹ The H-TiN nanopillar electrode exhibited a large volumetric capacitance of 120 F cm⁻³ at a current density of 0.83 A cm⁻³, which is almost 1.5-fold higher than that of TiN NT arrays. The improved volumetric capacitance of H-TiN nanopillars should be attributed to the higher number of active sites and larger surface area of H-TiN nanopillars as well as favourable electron/ion transport nanochannels. Such a structural design provides a promising method to improve specific capacitance via changing the morphology without increasing the volume of the electrode film. A symmetrical all-solid-state SC was assembled with two H-TiN nanopillars grown on Ti foil as electrodes and H₃PO₄/PVA as a gel-like electrolyte. The H-TiN nanopillar based device showed a high volumetric capacitance of 5.9 F cm⁻³ (based on the volume of the electrodes and electrolyte) at 0.02 A cm⁻³ and outstanding rate performance with a capacitance retention of 81.4% when the current density increased 10 times from 0.02 to 0.2 A cm⁻³. Recently, Cui et al. fabricated highly ordered Nb₄N₅ nanostructured arrays by anodizing the Nb foil and performing thermal nitridation (Fig. 3e).²⁹ The Nb₄N₅ nanochannel electrode exhibited prominent electrical conductivity and electrochemical activity and a high areal capacitance of 225.8 mF cm⁻² at a current density of 0.5 mA cm⁻² in 1 M H₂SO₄ and also high rate capability with 60.8% retention from 0.5 to 10 mA cm⁻² (Fig. 3f). Gao et al. synthesized mesoporous Nb₄N₅ quasi-aligned nanobelt (NB) arrays using the Nb foil as both the substrate and Nb precursor via hydrothermal growth of KNb₃O₈ arrays followed by protonation and thermal nitridation.⁶⁷ The as-obtained Nb₄N₅ NBs showed an areal capacitance of 37.4 mF cm⁻² at a current density of 0.2 mA cm⁻² and robust rate performance with only 38% capacitance decay when the current density increased from 0.1 to 5 mA cm⁻².

Fig. 3 (a) Schematic diagram of the fabrication process of TiN NTs and H-TiN nanopillars; TEM images of the TiN NTs (b) and H-TiN nanopillars (c); (d) rate performance;⁴¹ (e and f) schematic illustration of the synthesis, rate performance and coulombic efficiency of Nb₄N₅.²⁹ (a–d) Reproduced with permission from ref. 41. Copyright 2018, The Royal Society of Chemistry. (e and f) Reproduced with permission from ref. 29. Copyright 2016, Wiley-VCH.

Despite these recent advances (Table 2), the conducting substrates (Nb and Ti) are usually much thicker and heavier than the coated TMN active materials. Even if the gravimetric specific capacitance based only on the active materials of TMNs is increased by the nanostructure design, the heavy metallic current collector gives rise to low volumetric and gravimetric capacitance of the whole SC device. Furthermore, the lower corrosive resistance of the metal substrate limits its implementation in acidic or alkaline electrolytes.

Table 2 Metal substrate-supported nanostructured TMN electrodes for SCs

Electrodes	Specific capacitance	Rate performance	Capacitance retention (%) after cycles	Ref.
TiN/Ti foil	120 F cm^−3 at 0.83 A cm^−3	67 F cm^−3 at 83 A cm^−3	94.3% after 3000 cycles	41
TiN/Ti foil	99.7 mF cm^−2 at 0.20 mA cm^−2	68.8 mF cm^−2 at 4.00 mA cm^−2	98.5% after 100 cycles	79
TiN/Ti fiber	2.4 mF cm^−1 at 10 mV s^−1	0.83 mF cm^−1 at 5 V s^−1	80% after 10 000 cycles	80
Nb4N5/Nb foil	225.8 mF cm^−2 at 0.5 mA cm^−2	137.2 mF cm^−2 at 10 mA cm^−2	70.9% after 2000 cycles	29
Nb4N5/Nb foil	37 mF cm^−2 at 0.2 mA cm^−2	23 mF cm^−2 at 5 mA cm^−2	80% after 1000 cycles	67
Ni–Mo–N/Ni foam	2446 mC cm^−2 at 2 mA cm^−2	708 mC cm^−2 at 100 mA cm^−2	80.1% after 6000 cycles	81

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4.1.2 Carbon-supported nanostructured TMN electrodes. Compared to metal substrates, carbon-based supporting substrates such as carbon paper, carbon foam, CNTs, CNFs, and graphene are lighter and have larger corrosion resistance. Therefore, they are alternative frameworks to deposit or incorporate TMN active materials for SCs.
(1) Carbon cloth or carbon paper supported TMN electrodes for SCs. Carbon cloth (CC) is a flexible fabric consisting of carbon microfibers with good flexibility, high electronic conductivity, and excellent corrosion resistance, and is much lighter than metal foil thus acting as a good substrate to deposit or load TMNs for flexible SCs.^82,83 Yang et al. fabricated corn-like TiN nanoarrays on CC via thermal nitridation of TiO₂ nano-arrays on CC.⁴⁵ A coin-like symmetrical capacitor comprising an organic electrolyte (1 mol L⁻¹ LiClO₄ in acetonitrile solution) and corn-like TiN electrodes was constructed. The TiN-based SCs showed a high power density of 150 W cm⁻³ at an energy density of 0.30 mW h cm⁻³ and had great potential in the instantaneous delivery of high power and energy.

Considering the flexibility and bendability of the CC and carbon paper substrates, growth of NFs or NRs of TMNs on the flexible CC or carbon paper is promising for flexible SCs. Lu et al. fabricated VN NWs on CC by nitriding hydrothermally formed TiO₂ NWs.⁸⁴ The CC supported VN NW electrode exhibited remarkable flexibility and a high specific capacitance of 298.5 F g⁻¹ at a scan rate of 10 mV s⁻¹ (based on the mass loading of VN NWs). Moreover, the VN NWs delivered outstanding rate performance with a capacitance retention of 71.5% when the scan rate was increased from 10 to 100 mV s⁻¹ (Fig. 4a). The flexible asymmetric SC device based on CC supported VO_x NWs (cathode) and VN NWs (anode) delivered a high volumetric capacitance of 1.6 F cm⁻³ (Fig. 4b) and retained excellent flexibility under repeated bending states (Fig. 4c). In addition to VN NWs, Lu et al. prepared TiN NWs on CC which exhibited a highest capacitance of 123 F g⁻¹ at a scan rate of 10 mV s⁻¹, which was almost 10 times higher than that of the corresponding TiO₂ counterpart. The flexible SCs based on TiN NW electrodes grown on CC and LiCl/PVA gel-like electrolyte showed a volumetric capacitance of 0.33 F cm⁻³ at 2.5 mA cm⁻³ and maintained a good energy storage performance under different bending conditions (Fig. 4d–f).⁷⁸

Fig. 4 (a) Rate performance of the CC supported VN NW electrode. The insets in (a) are digital pictures of VO_x and VN NWs grown on CC; (b) rate performance of the VO_x//VN device; (c) cycling performance of the solid state device and the insets in (c) show the capacitance retention of the devices at different bending angles;⁸⁴ (d) schematic diagram showing the process for preparing TiN NWs on CC and scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the TiN NW; (e) rate performance of TiO₂ and TiN NW electrodes; (f) schematic diagram of an all solid-state TiN based SC. The inset is the digital picture of the device.⁷⁸ (a–c) Reproduced with permission from ref. 84. Copyright 2013, American Chemical Society. (d–f) Reproduced with permission from ref. 78. Copyright 2012, American Chemical Society.

Recently, He et al. prepared CC supported carbon coated W₂N core–shell NWs (W₂N@C/CC) by aerosol-assisted chemical vapor deposition followed by NH₃ annealing.⁸⁵ The W₂N@C/CC electrodes exhibited a large areal specific capacitance of 693.0 mF cm⁻² at 5 mV s⁻¹ and outstanding rate capability with 59% capacitance retention when the current was increased from 5 to 50 mV s⁻¹. The asymmetrical coin-cell type SC based on W₂N@C CSA/CC (negative electrode) and nitrogen-doped graphene fabricated on commercial carbon paper (positive electrode) with an ionic liquid as the electrolyte exhibited a high capacitance of 170 mF cm⁻² at 1 mA cm⁻² with 85% capacitance retention when the current density increased to 10 mA cm⁻².

The capacitive properties of CC supported Ni₃N NSs and gallium nitride NW (GaN NW)/graphite paper (GP) nanocomposites were reported.^69,87 The 3D Ni₃N/carbon composite electrode showed large capacitances of 842 F g⁻¹ at a scan rate of 10 mV s⁻¹ and 252.6 F g⁻¹ at 100 mV s⁻¹. The composite electrode of GaN NW/GP had a large areal capacitance of 23.7 mF cm⁻² at 10 mV s⁻¹ and extraordinary cycling stability with 99% capacitance retention after 50 000 cycles.⁶⁹ The flexible symmetrical SC based on GaN NW/GP electrodes and H₂SO₄/PVA gel-like electrolyte showed high energy/power densities (0.65 μW h cm⁻²/45 mW cm⁻²).

Although the CC supported nanostructured TMN electrodes deliver promising flexible energy storage performance (Table 3), noncapacitive CC increases the device volume and weight, resulting in a lower specific capacitance and energy/power density considering the entire electrode and devices.

Table 3 Carbon cloth or carbon paper supported TMN electrodes for SCs

Electrodes	Specific capacitance	Rate performance	Capacitance retention (%) after cycles	Ref.
TiN/CC	159 F g^−1 at 0.25 A g^−1	124.5 F g^−1 at 5 A g^−1	91.7% after 15 000 cycles	43
TiN/CC	123 F g^−1 at 10 mVs^−1	79 F g^−1 at 400 mVs^−1	83.0% after 15 000 cycles	78
TiON/CC	38.3 F g^−1 at 0.1 A g^−1	16.0 F g^−1 at 1 A g^−1	83.0% after 8000 cycles	88
VN/CC	298.5 F g^−1 at 10 mV s^−1	213.4 F g^−1 at 100 mV s^−1	87.5% after 10 000 cycles	84
W2N/CC	693.0 mF cm^−2 at 5 mV s^−1	540.5 mF cm^−2 at 50 mV s^−1	91% after 20 000 cycles	85
Ni3N/CC	990 F g^−1 at 10 mA cm^−2	800 F g^−1 at 40 mA cm^−2	81% after 200 cycles	87
Fe2N/CC	169 F g^−1 at 10 mV s^−1	130 F g^−1 at 10 mV s^−1	—	89
GaN/GP	237 mF cm^−2 at 0.1 mA cm^−2	180.5 mF cm^−2 at 5 mA cm^−2	98% after 10 000 cycles	69

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(2) Nano-carbon/TMN hybrid materials for SCs. Nano-carbon materials including 1D CNTs, 2D graphene, and 3D mesoporous carbon have large surface areas, excellent electrical properties, and high electrochemical stability and also provide abundant sites for loading active materials.^90–92 Therefore, they are good substrates or supporters to introduce pseudocapacitive TMN materials for high performance SCs (Table 4).

Table 4 Nano-carbon/TMN hybrid materials for SCs

Electrodes	Specific capacitance	Rate performance	Capacitance retention (%) after cycles	Ref.
TiN/CNTs	6.4 mF cm^−2 at 100 mV s^−1	4.3 mF cm^−2 at 1000 mV s^−1	86.5% after 20 000 cycles	86
VN/CNT	289 F g^−1 at 20 mV s^−1	230 F g^−1 at 1000 mV s^−1	64% after 600 cycles	44
VN/carbon fiber	406.5 F g^−1 at 0.5 A g^−1	305.3 F g^−1 at 5 A g^−1	65.3% after 5000 cycles	93
VN/graphite foam	177 C g^−1 at 5 mV s^−1	124 C g^−1 at 5 mV s^−1	—	98
VN/carbon NS	280 F g^−1 at 1 A g^−1	166 F g^−1 at 10 A g^−1	60% after 5000 cycles	103
VN@porous carbon nanospheres	229.7 F g^−1 at 1 A g^−1	128.9 F g^−1 at 16 A g^−1	65.5% after 5000 cycles	104
VN@carbon fiber	240.5 F g^−1 at 0.5 A g^−1	173.4 F g^−1 at 5 A g^−1	78.2% after 10 000 cycles	105
VN/porous carbon	255 F g^−1 at 1 A g^−1	175 F g^−1 at 5 A g^−1	66% after 1000 cycles	107
VTiN/carbon	430.7 F g^−1 at 0.5 A g^−1	141.7 F g^−1 at 10 A g^−1	63% after 600 cycles	106
MoxN/carbon	251 F g^−1 at 0.5 A g^−1	187 F g^−1 at 5 A g^−1	78.6% after 15 000 cycles	94
Mo2N@C-rGO	514.54 F g^−1 at 0.25 A g^−1	304.32 F g^−1 at 10 A g^−1	96% after 3000 cycles	97
FeN/carbon	507 F g^−1 at 0.5 A g^−1	366 F g^−1 at 20 A g^−1	—	108

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Zhang et al. deposited nanocrystalline VN on CNTs anchored on glassy carbon or Inconel substrates via magnetron sputtering of V in N₂/Ar plasma (Fig. 5a and b).⁴⁴ The VN functionalized CNT showed a respectable specific capacitance of 289 F g⁻¹ in 1 M KOH electrolyte at a scan rate of 20 mV s⁻¹. The well connected and highly electrically conductive VN/CNT arrays had an excellent rate capability with 80% capacitance retention at a high scan rate of 1000 mV s⁻¹. Achour et al. designed and fabricated hierarchical composite electrodes consisting of porous nanostructured TiN grown on vertically aligned CNTs for micro-SC devices by reactive direct current sputtering.⁸⁶ The electrodes deposited on silicon substrates had an areal capacitance of 18.3 mF cm⁻² at 1 V s⁻¹. Moreover, the areal capacitance of the TiN/CNT composite increased linearly with the thickness of the TiN layer. The porous TiN layer in the TiN/CNT composite with 1200 nm thickness exhibited a 360-fold increase in the areal capacitance compared to the pristine CNT electrode (Fig. 5c and d).

Fig. 5 (a) Schematic illustration of the fabrication process and morphology of the 3D VN/CNT arrays; (b) rate performance of the VN/CNT arrays;⁴⁴ (c) side-view SEM image of the TiN coated CNTs; (d) representative cyclic voltammogram (CV) curve of the TiN coated CNTs.⁸⁶ (a and b) reproduced with permission from ref. 44. Copyright 2011, American Chemical Society. (c and d) Reproduced with permission from ref. 86. Copyright 2014, Elsevier.

In addition to depositing TMNs on the surface of CNTs and CNFs, TMN NPs also were encapsulated into CNFs to improve the electrochemical properties. Wu et al. fabricated VN quantum dot/nitrogen-doped microporous carbon CNFs (VNQD/CNFs) by electrostatic spinning followed by thermal nitridation. 8–10 nm VNQDs with a content of 48 wt% were dispersed in the CNFs.⁹³ The composite structure prevented the aggregation of VN NPs, improved the conductivity, and enhanced ion migration. Thus, the VNQD/CNF composite had a large specific capacitance of 406.5 F g⁻¹ at 0.5 A g⁻¹ in 6.0 M KOH solution and good rate capability with 75.1% capacitance retention when the current density was increased from 0.5 to 5 A g⁻¹. Tan et al. fabricated Mo_xN dispersed CNFs (C/Mo_xN) with a diameter of 200 nm via electrostatic spinning and annealing in N₂/NH₃.⁹⁴ The C/Mo_xN electrode showed a capacitance of 251 F g⁻¹ at 0.5 A g⁻¹ and a capacity retention of 70.9% when the current density was increased 10 times from 0.5 to 5 A g⁻¹. The symmetrical SCs of C/Mo_xN//C/Mo_xN devices had an energy density of 4.51 W h kg⁻¹ and a power density of 250 W kg⁻¹.

Compared to 1D CNTs or CNFs, 2D graphene has a larger theoretical surface area of 2630 m² g⁻¹, higher conductivity, better mechanical strength, and flexibility,⁹⁵ and so graphene is a promising supporting framework for TMNs to enhance the capacitive properties. Moreover, the mass loading of TMNs in TMNs/graphene is higher than that of the TMNs/CNT because 2D graphene provides a larger surface area to incorporate TMNs.⁹⁶ The GO obtained by “Hummers'” method is rich in functional groups on its surface, which is beneficial for in situ coupling with kinds of TMO nanostructures to form various TMO/rGO composites. The TiN nanopillars/reduced graphene oxide (TiN/rGO) obtained from refluxing the Ti(OBu)₄ precursor and GO followed by thermal nitridation showed a capacitance of 415 F g⁻¹ at 0.5 A g⁻¹ and a good rate performance of 275 F g⁻¹ at 5 A g⁻¹.⁴⁶ Vadahanambi et al. fabricated a 3D architecture of a carbon sheathed Mo₂N-reduced graphene oxide (Mo₂N@C-rGO) hybrid as shown in the TEM image (Fig. 6a).⁹⁷ The Mo₂N@C-rGO electrodes showed high capacitances of 514.54 F g⁻¹ (0.25 A g⁻¹) and 304.32 F g⁻¹ (10 A g⁻¹) (Fig. 6b) as well as remarkable cyclability with a capacitance retention of 96% after 3000 cycles.⁹⁷ Wang et al. prepared porous VN hierarchical nanostructures by NH₃ annealing of 3D graphite foam supported V₂O₅ nanostructures. The graphene supported VN had capacitances of 177, 90, and 154 C g⁻¹ in 1 M KOH, 1 M LiCl, and 1 M LiPF₆, respectively.⁹⁸ Recently, Zhu et al. reported all TMN-based solid-sate asymmetrical SCs with the TiN cathode, Fe₂N anode and PVA/LiCl solid electrolyte (Fig. 6c and d).⁸⁹ The porous TiN and Fe₂N NPs were both grown on vertically aligned graphene nanosheet (GNS) substrates. The asymmetrical SCs had a capacitance of 58 F g⁻¹ after 20 000 cycles at 4 A g⁻¹ and a high volumetric energy density of 0.51 mW h cm⁻³ and a power density of 211.4 mW cm⁻³ at a large current density of 8 A g⁻¹.

Fig. 6 (a) Low-resolution TEM picture of Mo₂N/rGO; (b) rate performance of the Mo₂N/rGO electrode;⁹⁷ (c) schematic diagram of the preparation process of the TMN cathode and anode materials; (d) rate performance of the electrode and all-nitride-based solid-sate asymmetrical device.⁸⁹ (a and d) Reproduced with permission from ref. 97. Copyright 2018, The Royal Society of Chemistry. (c and d) Reproduced with permission from ref. 89. Copyright 2015, Wiley-VCH.

With regard to nanostructured TMN electrodes in SCs, cycling stability is one of the main concerns due to the irreversible electrochemical oxidation of TMNs during cycling resulting in severe capacitance decay in the long-term process. A carbon coating can improve the cycling performance of TMN electrodes.^38,43 Our group reported nitrogen-doped carbon encapsulated mesoporous VN (MVN@NC) NWs for long-life SC electrodes, which are shown in Fig. 7a and b. The mesoporous VN NWs had abundant active sites for charge storage and the NC shell suppressed the electrochemical dissolution of the inner MVN NWs in an alkaline electrolyte, leading to excellent capacitive properties. The MVN NWs had a high areal capacitance of 282 mF cm⁻² and excellent long-term stability with 91.8% capacitance retention after 12 000 cycles in a KOH electrolyte (Fig. 7c), which was superior to that of pure MVN NWs (6.7%).³⁸ Lu et al. demonstrated that carbon shell coated TiN NWs boosted the stability with a high capacitance retention of 91.7% after 15 000 cycles, which was much better than that of uncoated TiN NWs (9.1%) (Fig. 7d and e).⁴³ Grote et al. also showed that a carbon coating on TiN NT arrays led to better cycle stability compared to the bare TiN NT arrays.⁹⁹ Yuan et al. fabricated 2D-layered carbon wrapped TiN (C@TiN) via one-step nitridation of transition metal carbides (MXenes).¹⁰⁰ The carbon atoms generated during nitridation were deposited on the surface of TiN NSs producing a thin carbon coating (<5 nm). The in situ grown carbon layer not only prevented the electrochemical oxidation of TiN by the electrolyte but also worked as the cushion to accommodate the structural deformation. The C@TiN showed a high capacitance retention of 93.1% after 10 000 cycles at 2 A g⁻¹, which was better than that of pure TiN (29.2%).¹⁰⁰ Zhang et al. directly grew carbon coated well-aligned VN NWs on CNT fibers which showed a high areal capacitance of 715 mF cm⁻² at 1 mA cm⁻² in a three electrode system. Moreover, the carbon-coated VN NWs exhibited improved cycling stability with a capacitance retention of about 90% after 10 000 cycles compared to pure VN NW arrays (the capacitance retention was below 20%) since the carbon layer remitted electrochemical oxidation and delamination from the inner VN cores.¹⁰¹ Very recently, we reported carbon-encapsulated VN nanodot intercalated graphene nanosheets (VN@C/G), which were prepared by one-step thermal nitridation of polyaniline intercalated V₂O₅ NSs in NH₃. The sandwiched V₂O₅ layer was converted into 3–5 nm VN nanodots which were encapsulated and intercalated into the in situ carbonized graphene layer forming pillared lamellar NSs. The VN@C/G NSs showed an ultrahigh volumetric capacitance of 1048.8 F cm⁻³ at 1.1 A cm⁻³ and a rate capability of 546 F cm⁻³ at 210 A cm⁻³ surpassing those of carbon, TMOs and other metal nitrides. Moreover, the VN@C/G NSs exhibited outstanding cycling stability with 90% capacitance retention after 10 000 cycles.¹⁰²

Fig. 7 (a) Schematic of the fabrication process of MVN@NC NWs;³⁸ (b) TEM images of MVN@NC NWs; (c) cycling performance of the MVN NW and MVN@NC NW film electrodes in a 6 M KOH electrolyte at a scan rate of 200 mV s⁻¹; (d) schematic of the process of carbon coating on the TiN NWs; (e) cycling performance of the TiN (squares) and TiN@C (circles) NW electrodes in the CV test for 15 000 cycles (100 mV s⁻¹).⁴³ (a–c) Reproduced with permission from ref. 38. Copyright 2015, Wiley-VCH. (d and e) Reproduced with permission from ref. 43. Copyright 2015, Wiley-VCH.

Although graphene and TMN hybrids show obvious synergetic effects in increasing the capacitance and improving the cycling stability of TMNs, graphene is easily restacked during the hybridization process and TMNs are easily aggregated at a high mass loading giving rise to decreased electrochemical charge storage properties.

4.2 Self-supported TMN-based composite film electrodes

TMNs are inorganic nonmetallic ceramic materials which are brittle and have poor flexibility and thus they could not be directly applied in bendable and flexible SCs. Recently, binder-free films composed of 1D TMNs active materials and highly flexible 1D CNTs or 2D graphene have been reported to demonstrate potential in high volumetric electrochemical performance SCs. Binder-free film electrodes which avoid current collectors, conductive additives, and binders reduce the device weight, resulting in high volumetric energy and power density.⁵⁵

Our group fabricated a flexible freestanding mesoporous VN NWs (MVN NWs)/CNT hybrid electrode by simply vacuum-filtering the VN NW and CNT mixture (Fig. 8a–d).⁴² The binder-free MVN/CNT film exploits the synergistic effects of the electrochemical performance of MVN NWs and the high conductivity and mechanical consolidation of CNTs. The CV curves indicated that the capacitance of the MVN/CNT film electrode was larger than those of the CNT and MVN NW electrodes. The areal capacitance of the freestanding MVN/CNT hybrid film was 178 mF cm⁻² at a current density of 1.1 mA cm⁻² (Fig. 8e). The interpenetrating MVN/CNT and the mesoporous structure of VN enable effective electrolyte transport and active-site accessibility, giving rise to better utilization and larger specific capacitance. The all solid-state symmetrical SC devices composed of the H₃PO₄/PVA gel as the electrolyte showed a volumetric capacitance of 7.9 F cm⁻³ at a current density of 0.025 A cm⁻³ and a considerable rate performance with a capacitance of 4 F cm⁻³ when the current was increased 20 times from 0.025 to 0.5 A cm⁻³ (Fig. 8f and g).

Fig. 8 (a) Schematic showing the synthesis of mesoporous VN NWs; (b) digital pictures of the flexible electrode film; (c) low and (d) high-resolution cross-view SEM images of the film; (e) rate performance of the MVN/CNT based electrode; (f) galvanostatic charging/discharging (GCD) curves and (g) rate performance of the MVN/CNT based devices.⁴² (a–g) Reproduced with permission from ref. 42. Copyright 2013, Wiley-VCH.

Compared with 1D CNTs, 2D graphene has a larger specific surface area, is more flexible, and can more easily form bendable electrodes.⁵ Recently, we developed an electrostatic self-assembly method combined with NH₃ annealing to fabricate alternately stacked mesoporous Mo₂N NB and rGO NS (MMNNBs/rGO) composite film electrodes.¹⁰⁹ The detailed assembly process to assemble the flexible SCs is described in Fig. 9a. The as-prepared α-MoO₃ NBs were positively charged by decoration with poly diallyldimethylammonium chloride under vigorous stirring and the positively charged MoO₃ NB dispersion was electrostatically assembled with negatively charged-GO to form the MoO₃/GO composite. The composite film was fabricated by vacuum filtration and peeling from the filter membrane. The MMNNBs/rGO films were prepared by annealing the MoO₃/GO composite film in NH₃ (Fig. 9b). Owing to the highly uniform composite and layer-by-layer structure, the MMNNBs/rGO film had outstanding flexibility. The as-prepared MMNNBs/rGO hybrid electrode had a high Mo₂N mass loading of 95.6% and areal capacitances of 142 and 98 mF cm⁻² at 1 and 150 mA cm⁻² in 1 M H₂SO₄, respectively (Fig. 9c). The outstanding electrochemical performance was attributed to the synergetic effects of the mesoporous Mo₂N NBs and alternately stacked and interconnected rGO networks. The flexible all-solid-state SCs produced by sandwiching two film electrodes of the MMNNBs/rGO hybrid in conjunction with the H₃PO₄/PVA gel electrolyte showed a high volumetric capacitance of 15.4 F cm⁻³ at 0.1 mA cm⁻³ and a capacitance retention of 64% when the current density was increased to 1 mA cm⁻³ (Fig. 9d). The high rate performance of the Mo₂N based all-solid-state SCs was attributed to the small resistance (7.6 Ω cm⁻²) as shown in Fig. 9e. The freestanding and flexible Nb₄N₅/rGO composite film with a high Nb₄N₅ content of 91% was also fabricated by our group,^27,110 The Nb₄N₅/rGO hybrid film showed a high areal capacitance of 141 mF cm⁻² at 1 mA cm⁻² and good rate performance with a capacitance of 60 mF cm⁻² when the current density was increased 100 times from 1 to 100 mA cm⁻². The symmetrical all-solid-state SCs based on a paper-like Nb₄N₅/rGO film electrode and the PVA/H₂SO₄ electrolyte showed a high volumetric capacitance (based on the whole device) of 19 F cm⁻³ at 0.1 mA cm⁻³ and favorable rate performance with a high volumetric capacitance of 5 F cm⁻³ at 5 mA cm⁻³. The devices exhibit high mechanical flexibility with the capacity unchanged after repeated bending.

Fig. 9 (a) Schematic illustration of the fabrication procedures for the MMNNBs/rGO flexible electrodes and SCs; (b) SEM images of the MMNNBs/rGO composites; rate performance of the electrode (c) and device (d); (e) Nyquist plot.¹⁰⁹ (a–e) Reproduced with permission from ref. 109. Copyright 2015, The Royal Society of Chemistry.

Although TMNs/CNT or TMNs/rGO composite films have promising applications in flexible SCs, they generally suffer from limitations such as inherent aggregation of CNTs/rGO, nonuniform mixture, and unstable junctions between the VN NWs and CNTs thus hampering further development to attain higher capacity, durability, and flexibility in practice. In this respect, our group fabricated paper-like self-supported MVN@NC NW electrodes consisting of 3D intertwined ultralong MVN@NC NWs by vacuum filtration of polydopamine coated V₂O₅ NWs followed by NH₃ annealing.³⁸ The VN content in the freestanding MVN@NC NWs film was as high as 84.5 wt% and the NC shell suppressed the electrochemical oxidation and dissolution of the inner MVN alkaline electrolyte thereby giving long-term cycling stability. The freestanding MVN@NC NW film electrode showed a high areal capacitance of 282 mF cm⁻² and excellent long-term stability with 91.8% capacitance retention after 12 000 cycles in a KOH electrolyte. The all-solid-state flexible SCs assembled by sandwiching two flexible MVN@NC NW film electrodes with the PVA/KOH gel-like electrolyte boasted a large volumetric capacitance of 10.9 F cm⁻³, energy density of 0.97 mW h cm⁻³, and power density of 2.72 W cm⁻³ at a current density of 0.051 A cm⁻³ based on the entire cell. By virtue of the excellent mechanical flexibility, high capacitance and large energy/power density, the self-supported MVN@NC NW paper-like electrodes have large potential in portable and wearable flexible electronics.

The abovementioned TMN-based freestanding and binder-free film electrodes have promising applications in flexible SCs (Table 5). However, it is difficult to fabricate a paper-like electrode by a low-cost and scalable method. Developing some advanced technologies to produce freestanding electrode films with high volumetric and areal capacitance is highly desirable.

Table 5 Self-supported TMN-based composite film electrodes for SCs

Electrodes	Specific capacitance	Rate performance	Capacitance retention (%) after cycles	Ref.
VN@N doped carbon	196 F g^−1 at 1.44 mA cm^−2	123.48 F g^−1 at 144 mA cm^−2	91.8% after 12 000 cycles	38
VN/CNT	178 mF cm^−2 at 1.1 mA cm^−2	53 mF cm^−2 at 11 mA cm^−2	82% after 10 000 cycles	42
Mo2N/graphene	142 mF cm^−2 at 1 mA cm^−2	98 mF cm^−2 at 150 mA cm^−2	85.7% after 4000 cycles	109
Nb4N5/rGO	175 F g^−1 at 1 A g^−1	131 F g^−1 at 10 A g^−1	90% after 6000 cycles	27
TiN@carbon	167 F g^−1 at 1 A g^−1	95 F g^−1 at 50 A g^−1	70% after 6000 cycles	99

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4.3 TMNs as supportive materials for high-performance electrode materials

Owing to the superior electrical conductivity and mechanical stability, nanostructured TMNs constitute a promising electronic conducting framework to load TMOs, CPs, and even other TMNs to achieve enhanced capacitive properties for high-performance SCs. In the hybrid nanostructures, TMNs offer mechanical support and fast electron transfer according to the sorts of pseudo-capacitive materials. TMN hybrids can be classified as TMOs/TMNs, CPs/TMNs, and TMNs/TMNs.

4.3.1 TMN supported TMO hybrids. TMOs such as MnO₂, MoO₃ and NiCo₂O₄ are widely investigated pseudocapacitive materials which exhibit higher capacitance (even up to 2000 F g⁻¹) than EDLC electrodes but suffer from low rate performance as a result of their low conductivity.^50,91 One solution to improve the capacitive properties of TMOs is to deposit pseudocapacitive TMOs on highly conductive TMN substrates. Among the commonly available TMNs, TiN is primarily employed as the support for other SC materials. We fabricated porous dual-layered MoO_x/TiN/MoO_x NT arrays via electrochemical deposition of MoO_x on highly conductive TiN NTAs (Fig. 10a–c).⁴⁷ The conductive TiN facilitated electron transfer and the porous MoO_x wall allowed the electrolyte to permeate the NTs readily. The as-obtained MoO_x/TiN/MoO_x NTA electrode displayed a high specific capacitance of 97 mF cm⁻² (323 F g⁻¹ based on MoO_x mass loading) and an excellent rate capability with 60% specific capacitance retention when the current density was increased 20 times from 1 to 20 mA cm⁻² (Fig. 10d). The symmetrical SC assembled with two MoO_x/TiN/MoO_x NTA electrodes exhibited a high volumetric capacitance of 24 F cm⁻³ without obvious fading after 10 000 cycles (Fig. 10e). Recently, Wang et al. deposited ultrathin TiN shells on NiCo₂O₄ nanofiber arrays grown on carbon fiber cloth substrates by ALD.¹¹¹ The conformal ultrathin TiN coating facilitated transportation of electrons at the electrode/electrolyte interface and acted as a mechanical buffer to prevent the structural deformation of the underlying NiCo₂O₄ nanofiber arrays during cycling. Additionally, the TiN shell contributed pseudocapacitively through the faradaic reactions. The core–shell NiCo₂O₄@TiN composite arrays displayed a high specific capacitance of 998 mF cm⁻² at 2 mA cm⁻² and a capacitance retention of 59% when the current density was increased from 2 to 20 mA cm⁻². The symmetrical all-solid-state SCs based on the hybrid NiCo₂O₄@TiN electrodes had a power density of 58.2 mW cm⁻³ at an energy density of 0.061 mW h cm⁻³, which was the highest values of any NiCo₂O₄-based all-solid-state SC reported. In addition to TMOs, other hydroxide/TMNs including Ni(OH)₂/TiN and Ni_xCo_2x(OH)_6x/TiN as well as TMN/MoS₂ hybrid electrodes were reported as high-performance electrodes materials for SCs.^112–114 Some of the recently reported TMN supported TMO electrodes and their capacitive properties are summarized in Table 6.

Fig. 10 (a) Schematics of the fabrication procedures of MoO_x/TiN/MoO_x NTAs; (b) TEM and (c) HR-TEM images of MoO_x/TiN-10s; (d) rate capability (based on areal capacitance) of the TiN, MoO_x/TiN-10s, and MoO_x/TiN-30s electrodes (deposition time of 10 s and 30 s); (d) cycling properties of the MoO_x/TiN-10s electrode at a current density of 2 mA cm⁻² in 1 M Na₂SO₄ solution.⁴⁷ (a–e) Reproduced with permission from ref. 47. Copyright 2014, Wiley-VCH.

Table 6 TMNs as supportive materials for high-performance SC electrode materials

Electrodes	Specific capacitance	Rate performance	Capacitance retention (%) after cycles	Ref.
TMO/TMN
MoOx/TiN	323 F g^−1 at 1 mA cm^−2	193.8 F g^−1 at 20 mA cm^−2	100% after 10 000 cycles	47
MnO2/TiN	681.0 F g^−1 at 2 A g^−1	390.2 F g^−1 at 400 A g^−1	97% after 1000 cycles	77
MnO2–TiN	853.3 F g^−1 at 1.0 A g^−1	225 F g^−1 at 15 A g^−1	91.7% after 1000 cycles	122
TiNxOy/MnO2	95.5 mF cm^−2 at 0.5 mA cm^−2	77 mF cm^−2 at 20 mA cm^−2	93.88% after 10 000 cycles	123
Co(OH)2/VN	62.4 F g^−1 at 0.2 A g^−1	34.4 F g^−1 at 10 A g^−1	80% after 4000 cycles	124
NiCo2O4@TiN	998 mF cm^−2 at 2 mA cm^−2	582 mF cm^−2 at 20 mA cm^−2	70% after 20 000 cycles	111
NixCo2x(OH)(6x)/TiN	2543 F g^−1 at 5 mV s^−1	660 F g^−1 at 500 mV s^−1	93.75% after 5000 cycles	113
TiN@NiCo2O4	1200 F g^−1 at 2.0 A g^−1	763.2 F g^−1 at 100 A g^−1	92.2% after 5000 cycles	125
CP/TMN
PANI/TiN/PANI	242 mF cm^−2 at 0.2 mA cm^−2	167 mF cm^−2 at 10 mA cm^−2	83% after 3000 cycles	48
PPy–TiN	1265 F g^−1 at 0.6 A g^−1	459 F g^−1 at 15 A g^−1	72.6% after 2000 cycles	117
PANI/C/TiN	1093 F g^−1 at 1.0 A g^−1	708 F g^−1 at 5.0 A g^−1	98% after 2000 cycles	118
PPy/TiN/PANI	1471.9 F g^−1 at 0.5 A g^−1	1077.4 F g^−1 at 10 A g^−1	38.8% after 1000 cycles	119
PANI/TiN	1064 F g^−1 at 1 A g^−1	787.5 F g^−1 at 5 A g^−1	95% after 200 cycles	126
TMN/TMN
TiN@VN	198.3 F g^−1 at 5 A g^−1	0.52 F cm^−2 at 10 mA cm^−2	95% after 2000 cycles	49
TiN/VN	247.5 F g^−1 at 2 mV s^−1	160.8 F g^−1 at 50 mV s^−1	88% after 500 cycles	121
Ti@TiN/MON	736.6 mF cm^−2 at 10 mV s^−1	552.45 mF cm^−2 at 100 mV s^−1	—	127
MoNx/TiN	121.5 mF cm^−2 at 0.3 mA cm^−2	112.6 F g^−1 at 15 A g^−1	93.8% after 1000 cycles	128
γ-Mo2N/Co3Mo3N	109.9 F g^−1 at 10 mVs^−1	—	67% after 4000 cycles	129
Others
TiN@MoS2	662.2 mF cm^−2 at 1 mA cm^−2	525 mF cm^−2 at 10 mA cm^−2	100% after 50 000 cycles	114
PMo12/PANI/TiN	469 F g^−1 at 1.0 A g^−1	317 F g^−1 at 10 A g^−1	64% after 1000 cycles	120

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4.3.2 TMN supported CP hybrids. CPs such as PANI and PPy have attracted increasing attention due to the high theoretical pseudocapacitance of SCs. However, CPs generally suffer from structural and volume changes during cycling as a result of ion doping and dedoping, resulting in poor cycle stability.^115,116 Du et al. fabricated PPy coated TiO₂ and TiN NTs by an electrochemical deposition method. The specific capacitance of the PPy–TiN NT hybrid was 1265 F g⁻¹ at a current density of 0.6 A g⁻¹, which was higher than that of the PPy–TiO₂ NT hybrid (382 F g⁻¹). The highly conductive TiN substrate promoted the electrochemical capacitance of PPy more significantly than the TiO₂ semiconductor, leading to better supercapacitance performance of PPy–TiN.¹¹⁷ We polymerized PANI on the outer and inner surfaces as well as wall nanoholes of TiN NTAs to form PANI/TiN/PANI hybrid nanostructures (Fig. 11a).⁴⁸ The coaxial and interpenetrating PANI/TiN/PANI NTAs provided abundant space between the NTs and inside each tube so that the electrolyte could readily access the pseudocapacitive PANI (Fig. 11b and c). The nanocomposite electrode exhibited a high specific capacitance of 242 mF cm⁻² at a current density of 0.2 mA cm⁻² and good rate capability with the specific capacitance retained at 69% when the current density was increased 50 fold from 0.2 to 10 mA cm⁻². After charging/discharging for 3000 cycles, 83% of the initial capacitance was preserved (Fig. 11d and e). Xie et al. fabricated PANI/C/TiN ternary hybrid electrodes for SCs by thermal nitridation of CC supported TiO₂ NW arrays (obtained by a seed-assisted hydrothermal reaction), carbon coating, and PANI electrochemical deposition.¹¹⁸ The inner TiN core provided fast electron transport channels, whereas the space between the NWs provided rapid ion diffusion channels. The PANI/C/TiN electrode showed a high capacitance of 1093 F g⁻¹ at 1.0 A g⁻¹ and good rate performance with a specific capacitance retention of 65% (708 F g⁻¹) when the current density was increased from 1 to 5 A g⁻¹. Other CPs/TMN hybrid electrodes such as PPy/TiN/PANI¹¹⁹ and phosphomolybdic acid/PPy/TiN¹²⁰ were reported to have promising capacitive properties as summarized in Table 6.

Fig. 11 (a) Schematic of the fabrication process of coaxial PANI/TiN/PANI-NTAs NTAs; (b) TEM and (c) high-resolution TEM images of the coaxial PANI/TiN/PANI NT; (d) rate performance of the PANI/TiN/PANI-NTA electrode; (e) cycling performance of the PANI/TiN/PANI-NTA electrode at a current density of 0.2 mA cm⁻².⁴⁸ (a–e) Reproduced with permission from ref. 48. Copyright 2013, The Royal Society of Chemistry.

4.3.3 TMNs/TMN hybrids. TMNs/TMN hybrids are emerging electrode materials for SCs synergistically exploiting the advantages of different TMNs to achieve enhanced capacitive properties. Zhou et al. fabricated porous VN coated TiN core–shell CNFs, which combined the higher specific capacitance of VN with the better rate capability of TiN and exhibited a high specific capacitance of 247.5 F g⁻¹ at 2 mV s⁻¹ and a rate capability of 160.8 F g⁻¹ at 50 mV s⁻¹.¹²¹ Pang et al. prepared core–shell TiN@VN NW arrays on 3D carbon for high-performance SCs (Fig. 12a).⁴⁹ The VN offered a large pseudo-capacitance and TiN NWs served as the highly conductive electron collectors and contributed to the total capacitance. The 3D carbon@TiN@VN electrode displayed a large areal capacitance of 0.64 F cm⁻² (corresponding to a volumetric capacitance of 6 F cm⁻³) at a current density of 1 mA cm⁻², which was larger than that of the 3D carbon@TiN (0.58 F cm⁻²) in 1 M KOH, as shown in Fig. 12b. Moreover, the capacitance retention of the 3D carbon@TiN@VN was 81% after 4000 cycles (Fig. 12c), suggesting good cycling stability. Ruan et al. prepared 3D porous micropillar molybdenum oxynitride (MON) on Ti metal wire supported TiN NTAs (Ti@TiN/MON) by nitridation of TiO₂ NTA supported MoO₃ micropillars (Ti@TiO₂/MoO₃).¹²⁷ In the Ti@TiN/MON electrode, the TiN nanoarrays mainly offered high conductivity and the 3D MON porous micropillars provided active sites. Thus, the Ti@TiN/MON wire electrode exhibited both high linear and areal capacitances of 64.8 mF cm⁻¹ and 736.6 mF cm⁻² at a scan rate of 10 mV s⁻¹. When the scan rate was increased 10 times from 10 to 100 mV s⁻¹, 71% of the capacitance was retained. The asymmetrical devices assembled with Ti@TiN/MON as the anode, Ti@TiN/MnO₂ as the cathode, and PVA/KOH gel as the electrolyte had a capacitance of 12.7 mF cm⁻¹/75.1 mF cm⁻² (based on the length and area of the device) at a scan rate of 10 mV s⁻¹ and good energy and power densities with maximum values of 23.7 μW h cm⁻² and 8.84 mW cm⁻², respectively. The various TMNs/TMN hybrid SC electrodes reported so far are summarized in Table 6. Although TMN supported electrodes such as TMOs/TMNs, CPs/TMNs and TMNs/TMNs showed promising capacitive properties, most of the reported TMN nano-arrays were grown on other substrates such as metal foil, CC, or CNT arrays leading to the increase of the weight and the volume of the electrodes. Moreover, TMN supporters only act as the scaffolds to load TMOs, CPs or TMNs, and the capacitive properties of TMNs are not fully utilized to synergistically achieve enhanced capacitive properties.

Fig. 12 (a) Fabrication of the 3D carbon@TiN@VN electrode; (b) GCD plots of TiO₂, TiN or TiN@VN NWs at 1 mA cm⁻²; (c) cycling performance of 3D carbon@TiN@VN and 3D carbon@TiN at 5 mA cm⁻².⁴⁹ (a–c) Reproduced with permission from ref. 49. Copyright 2014, Wiley-VCH.

5. TMNs for Li–S batteries

Li–S batteries have been considered as one of the most promising next-generation energy storage systems due to their high theoretical energy density of 2600 W h kg⁻¹.^130–134 Moreover, S is naturally abundant, inexpensive, and environmentally friendly. However, the practical implementation of Li–S batteries is plagued by several challenges. Firstly, S and the discharge product Li₂S are electronically and ionically insulating resulting in slow reaction kinetics. Secondly, S suffers from an 80% volume change during charging and discharging involving phase transitions of solid S–liquid polysulfide (LiPS) intermediates–solid sulfides. Thirdly, the long-chain soluble LiPSs (Li₂S_n, 3 ≤ n ≤ 8) dissolve into the electrolyte and shuttle from the cathode to the anode during cycling causing loss of the S cathode and passivation of the lithium–metal surface with the insoluble solid Li₂S₂/Li₂S.^14,135–137

To overcome these hurdles, one of the promising approaches is encapsulation of S within conductive supporters to form S-based composite cathodes. Incorporation of S into porous carbon materials forming S/C composites has been widely investigated to improve the electrical conductivity of cathodes and alleviate LiPS shuttling through physical confinement.^131,132 However, the weak interaction between nonpolar carbon and polar LiPS species inevitably causes effusion and irreversible loss of LiPSs from the cathodes (Fig. 13a) resulting in rapid capacity fading during cycling. Polar metal oxides such as SiO₂, TiO₂, MnO₂, and some metal sulfides have recently been explored as host materials for S cathodes in Li–S batteries and the S/metal oxide (or sulfide) composites have demonstrated enhanced cycle stability via chemical anchoring of LiPSs on the cathodes.^{14,131,138–141} However, if the chemically anchored LiPSs are not quickly converted into insoluble Li₂S during the reduction process, the adsorbed LiPSs will accumulate around the host surface decreasing the LiPS adsorption/trapping efficiency and irreversible loss of LiPSs, especially for the S cathode with a high S mass loading. Moreover, the low conductivity of most metal oxides/sulfides causes a low S utilization ratio and poor rate performance.^14,138 To improve the electrochemical properties of S cathodes and restrain the shuttling of LiPSs, a promising way is to design highly conductive host materials with abundant polar active sites to adsorb LiPSs and high catalytic activity to catalytically promote LiPS conversion to solid Li₂S₂/Li₂S, and vice versa.

Fig. 13 Schematic illustration of the advantages of TMNs as cathode materials in Li–S batteries.

TMNs are desirable host materials for S cathodes in Li–S batteries because of their high conductivity, good mechanical stability, strong chemical polarity, and noble metal-like catalytic activity.¹⁴⁴ The high conductivity of TMNs offers fast electron transfer and the strong chemical interaction between TMNs and LiPSs to trap the LiPSs. The high electrocatalytic activity of TMNs catalytically accelerates the LiPSs–Li₂S₂/Li₂S conversion to enhance the redox kinetics and mitigate LiPS loss (Fig. 13b). In addition to acting as the host materials in S cathodes, TMN-based films are also utilized as interlayers in Li–S batteries to restrain the shuttling of LiPSs and improve the battery performance.^145,146 The recent progress in TMNs in Li–S batteries is summarized in Table 7.

Table 7 Recent progress in TMNs as host/interlayer materials for Li–S batteries (1C = 1650 mA g⁻¹)

Electrode	Morphology	S content (%) (areal loading, mg cm^−2)	Reversible capacity (mA h g^−1)	Capacity retention	Large rate capability (mA h g^−1)	Ref.
Host
TiN	Mesoporous particles	58.8(1.0)	644 after 500 cycles at 0.5C	65.2%	776 at 1C	142
TiN	Particles	75(1.2)	780 after 50 cycles at 1C	70.0%	700 at 5C	143
TiN	Nanopowder	58.8(1.0)	660 after 200 cycles at 0.5C	59.0%	555 at 5C	147
TiN	Hollow porous tubes	73.8(1)	840 after 450 cycles at 0.5C	93.3%	694 at 2C	148
VN	NBs	78.2(1.2)	837 after 1000 cycles at 1C	76.0%	812 at 5C	149
VN/C	N-carbon coated mesoporous NWs	57.2(2.8)	636 after 200 cycles at 1C	61.2%	543 at 10C	152
VN	Nanoribbon on graphene	−(3.0)	1252 after 100 cycles at 0.2C	85.0%	701 at 3C	153
VN	Nanoarray on porous carbon fiber	−(9.5)	1052 after 250 cycles at 0.1C	80.3%	591 at 5C	154
Co4N	Mesoporous spheres	72.3(1.5–2)	930 after 500 cycles at 1.0C	84.5%	882 at 2C	145
WN	Nano-plates	−(8)	980 after 100 cycles at 0.1C	—	665 at 1C	151
Mo2N	Mesoporous particles	48.2(1.1)	771 after 200 cycles at 0.5C	77.5%	—	150
Interlayers
TiN	TiO2–TiN nanopillars	88.0(1.2)	927 after 300 cycles at 0.3C	91.9%	682 at 2C	146
TiN	NPs on graphene	80.0(2.0)	550 after 1000 cycles at 2.0C	85.0%	578 at 5C	155

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5.1 Nanostructured TMNs for high-performance S cathodes in Li–S batteries

5.1.1 TMNs as hosts for Li–S battery cathodes. TMNs have high conductivity, strong chemical polarity, and noble metal-like catalytic activity, and are promising host materials for S cathodes in Li–S batteries. Cui et al. reported mesoporous TiN as the S host material for Li–S batteries.¹⁴² Mesoporous TiN with a large surface area was synthesized by a solid–solid phase separation method with zinc titanate as the starting material. Benefiting from the high electrical conductivity, large surface area and strong chemical adsorption of LIPSs (Fig. 14a), the TiN–S porous framework composite cathode exhibited a high specific discharge capacity of 1121 mA h g⁻¹ at 0.1C (1C = 1650 mA g⁻¹) (Fig. 14b) with a S content of 51 wt% and an areal S mass loading of 1 mg cm⁻². Compared with mesoporous TiO₂–S and Vulcan C–S composite cathodes, the TiN–S composite cathode displayed the best cycling stability, with a capacity decay of only 0.07% per cycle over 500 charge/discharge cycles (Fig. 14c). Hao et al. employed TiN nanopillars as a highly efficient immobilizer to chemically trap LiPSs.¹⁴⁷ The S/TiN composite cathode with 56 wt% S showed a high reversible capacity of 1012 mA h g⁻¹ and cycling stability for 200 cycles at 0.5C. Deng et al. reported hollow TiN tubes as the S host material with a high S mass loading of 73.8 wt%.¹⁴⁸ The strong interaction between TiN tubes with LiPSs reduced the shuttling effect of LiPSs during cycling and improved the stability. The initial discharge capacity at a high current density of 5.0C was as high as 1026 mA h g⁻¹ and the battery showed a large reversible capacity of over 840 mA h g⁻¹ at 0.5C after 450 cycles. Jeong et al. investigated the electrocatalytic effects of TiN for the redox reaction of LiPSs.¹⁴³ Compared to oxides of Ti, such as TiO₂ or Ti₄O₇, TiN showed strong adsorption and heterogeneous catalysis to promote LIPS fragmentation and fast conversion to short-chain solid Li₂S resulting in a longer second plateau and smaller polarization (Fig. 14d–f).

Fig. 14 (a) N 1s XPS spectra of TiN and TiN–S; (b) rate performance and (c) cycling stability of the TiN/S, TiO₂/S and C/S;¹⁴² (d) rate capability of the bare cathode and C/TiN cathode compared to the carbon interlayer (ABM) and C/TiN cathode with ABM (C/TiN-ABM); (e) GCD profiles at 0.2C; (f) polarization potential of the second plateau of the bare and C/TiN cathodes at 0.2C.¹⁴³ Reproduced with permission from ref. 142. Copyright 2016, Wiley-VCH. (d–f) Reproduced with permission from ref. 143. Copyright 2017, American Chemical Society.

Very recently, Ma et al. showed that porous-shell VN nanobubbles (VN-NBs) could serve as an efficient S host in Li–S batteries (Fig. 15a).¹⁴⁹ The highly conductive polar VN not only promoted fast transfer of electrons and chemical adsorption of soluble LiPSs, but also catalytically accelerated fast conversion of LiPSs. S was encapsulated in VN-NBs (S@VN-NBs) with a high mass loading of 5.4 mg cm⁻² and the materials showed a high areal/specific capacity of 5.81 mA h cm⁻²/1076 mA h g⁻¹ (Fig. 15b), superior rate capability of 632 mA h g⁻¹ at 5.0C, and long cycling stability with a capacity decay of 0.024% per cycle after 1000 cycles.

Fig. 15 (a) Schematics of the fabrication of S@VN-NB composites; (b) cycling capability of the S@VN-NB cathodes with a S areal loading of 3.3, 5.4, and 6.8 mg cm⁻², respectively;¹⁴⁹ (c) possible mechanism of using mesoporous-Mo₂N as the S host during charging–discharging; (d) cycling performance and coulombic efficiency of the mesoporous-Mo₂N/S, nonporous-Mo₂N/S and mesoporous-MoO₃/S cathodes at 0.5C for 200 cycles;¹⁵⁰ (e) fine XPS spectra of Co 2p before and after adsorbing Li₂S₆; (f) CV curves of Co₃O₄/S, Co₄N/S, Co/S, and Sup P/S; (g) schematic illustration of a Co₄N sphere and its interaction with LiPSs during the GCD process; (h) cycling performance of the Co₄N/S at 2C and 4C.¹⁴⁵ (a and b) Reproduced with permission from ref. 149. Copyright 2017, American Chemical Society. (c and d) Reproduced with permission from ref. 150. Copyright 2018, Elsevier. (e–h) Reproduced with permission from ref. 145. Copyright 2017, American Chemical Society.

Other TMNs such as Co₄N spheres, WN nanoplates, mesoporous Mo₂N, and Mo₂N nanorods have been investigated as host materials for S cathodes.^145,150,151 For example, Jiang et al. reported a mesoporous, conductive Mo₂N cathode with a polar surface as the host material for Li–S batteries.¹⁵⁰ The mesoporous-Mo₂N/S cathode showed a high capacity of 995 mA h g⁻¹, and cycling stability with 91.9% capacity retention after 100 cycles, which are superior to those of nonporous-Mo₂N/S and mesoporous-MoO₃/S (Fig. 15c and d). Deng et al. fabricated mesoporous Co₄N spheres as effective cathode hosts for S cathodes in Li–S batteries and X-ray photoelectron spectroscopy (XPS) confirmed the presence of Co–S bonding after adsorbing Li₂S₆ (ref. 145) (Fig. 15e). Co₄N served as a conductive Lewis base “catalyst” matrix to facilitate oxidization and phase transition reactions from short-chain to long-chain LiPSs and to S₈ (Fig. 15f and g). With a S content of 72.3 wt% in the composite, the Co₄N@S showed a high specific discharge capacity of 1659 mA h g⁻¹ at 0.1C which was close to the theoretical capacity. Moreover, at high rates of 2 and 5C, the discharge capacity stabilized at 805 and 585 mA h g⁻¹ after 300 cycles (Fig. 15h).

Despite the great promise, the small surface area and low pore volume of TMN host materials cannot efficiently suppress the outward diffusion of Li₂S_n, especially with a large S concentration, thus leading to unsatisfactory capacity retention and cycle life.

5.1.2 TMN hybrids as hosts for S cathodes in Li–S batteries. Compositing TMNs with nano-carbon is a promising strategy to improve the sulfur loading and enhance the electrochemical properties because of the increased surface area and further physical trapping of carbon. Our group reported a novel S cathode for Li–S batteries comprising S nanodot impregnated microporous carbon encapsulated conductive mesoporous VN NWs (S/MVN@C NWs) (Fig. 16a–d).¹⁵² S nanodots with a size of 2–5 nm were impregnated into the mesopores of MVN@C NWs and then encapsulated with microporous carbon. The MVN@C NWs exhibited several advantages as a S host material for Li–S batteries. Firstly, the inner mesopores of the MVN core offered nano-scale containers for active S and the S nanodots were mainly stitched into the mesopores of VN providing short electron/ion transfer paths. Secondly, the soluble LiPS intermediates were chemically tethered by mesoporous VN and further physically trapped by the microporous C shell within the cathode, thereby effectively inhibiting the outward diffusion and the shuttle effect of soluble LiPSs. Thirdly, the mesoporous space of the inner MVN buffered the stress originating from the volume changes of S during the charging/discharging processes, resulting in high structural stability of the electrode. Fourthly, the long S/MVN@C NWs formed freestanding/binder-free film electrodes which afforded more fast, efficient and continuous charge transport for the redox process being beneficial for the assembly of bendable Li–S batteries. The freestanding and binder-free S/MVN@C NW cathode delivered a large and stable areal capacity of 7.1 mA h cm⁻² at a large mass loading of 9.7 mg cm⁻², as shown in Fig. 16e. With a large S content of 57.2 wt% and areal mass loading of 2.8 mg cm⁻², the cathode showed a higher rate capacity of 543 mA h g⁻¹ at 10C (Fig. 16f) and stable long cycle life over 200 cycles with a capacity of 636 mA h g⁻¹ (Fig. 16g).

Fig. 16 (a) Schematic diagram illustrating the preparation procedure and cycling process of S/MVN@C NW cathodes; (b) TEM and (c) HR-TEM images of S/MVN@C NWs (S NPs marked by the white dotted line); (d) STEM images of S/MVN@C NWs and corresponding elemental maps of S, V and C; (e) cycling performance of S/MVN@C NWs with different S areal mass loadings; rate performances (f) and (g) cycling performance of S/CNT, S/VN and S/MVN@C NWs;¹⁵² (h) ultraviolet/visible absorption spectra and adsorption performance of RGO and VN/G; (i) side view of a Li₂S₆ molecule on a nitrogen-doped graphene surface and VN (200) surface; (j) GCD curves of the VN/G and RGO cathodes at 0.2C; (k) rate performance of the RGO and VN/G cathodes.¹⁵³ (a–g) Reproduced with permission from ref. 152. Copyright 2017, Elsevier. (h–k) Reproduced with permission from ref. 153. Copyright 2017, Nature Publishing Group.

Sun et al. synthesized a conductive porous VN nanoribbon/graphene (VN/G) hybrid as the S host.¹⁵³ VN provided a strong chemical affinity for LiPSs (Fig. 16h and i) and the conductive network facilitated the transportation of electrons and Li ions. The hybrid cathode showed a high specific capacity of 1461 mA h g⁻¹ after 100 cycles at 0.2C as well as a high rate performance of 956 mA h g⁻¹ at 2C (Fig. 16j and k). Very recently, Zhong et al. reported a porous carbon fibers/VN array (PCF/VN) composite as a S scaffold,¹⁵⁴ in which the PCF with a highly porous structure provided a large space to accommodate active S and possessed cross-linked maze channels to physically immobilize the LiPS species. The VN NB arrays demonstrated the strong ability to chemically anchor LiPSs to retard the shuttle effect. The PCF/VN/S electrode displayed a high reversible capacity of 1310.8 mA h g⁻¹ at 0.1C, and a good cyclability of 1052.5 mA h g⁻¹ after 250 cycles with a capacity retention of 80.3%.

5.2 TMNs as functional interlayers for Li–S batteries

TMNs could act as functional interlayers coated on the separator or inserted between the separator and the sulfur cathode to improve the electrochemical properties of Li–S batteries. Deng et al. designed rGO-supported TiN-nanoparticle (TiN/rGO) cover layers on the separator by an in situ synthesis method.¹⁵⁵ The multifunctional TiN-based layer created an airtight chamber for the S conversion reactions at a macroscopic level to effectively block the shuttling effect during discharging/charging (Fig. 17a). When pure S powders were directly used as the active materials with an areal loading of 2.8 mg cm⁻². The discharge specific capacity of the first cycle was 1580 mA h g⁻¹ at 0.05C. The S cathode could be operated at 2C for over 1000 cycles with a low capacity decay rate of 0.015% per cycle. Recently, Zhou et al. reported twinborn TiO₂–TiN heterostructure-modified separators enabling long life Li–S batteries. The TiO₂–TiN heterostructure combined the merits of highly adsorptive TiO₂ and conducting TiN and achieved smooth trapping–diffusion–conversion of LiPSs across the interface.¹⁴⁶ With the catalytic conversion effect of TiN, the LiPSs were rapidly converted to insoluble Li₂S₂ and Li₂S avoiding the accumulation of LiPSs in the electrolyte and further inhibiting the shuttle effect (Fig. 17b). Fast diffusion of LiPSs from TiO₂ to TiN helped to achieve both high trapping efficiency and fast conversion. The cell with this interlayer exhibited a capacity of 927 mA h g⁻¹ after 300 cycles at 0.3C (Fig. 17c), and a capacity retention of 73% and 67% over 2000 cycles at 1C for S areal mass loadings of 3.1 and 4.3 mg cm⁻², respectively.

Fig. 17 (a) The schematic of the multifunctional TiN/rGO cover layer to create a macro-chamber for S redox reactions for lithium–sulfur batteries;¹⁵⁵ (b) schematic illustration of the LiPS conversion procedure on TiN, TiO₂ and the TiO₂–TiN heterostructure surface; (c) cycling performance samples with different TiN contents at 0.3C over 300 cycles.¹⁴⁶ (a) Reproduced with permission from ref. 155. Copyright 2017, The Royal Society of Chemistry. (b–c) Reproduced with permission from ref. 146. Copyright 2017, The Royal Society of Chemistry.

TMN-based host materials show enhanced electrochemical characteristics for S cathodes and a brief summary of TMN-based materials for Li–S batteries is shown in Table 7. However, the pore volume and active surface area of TMN host materials need to be optimized to improve the S mass loading, especially areal mass loading, and enhance the catalytic efficiency and redox kinetics. Moreover, the process in which TMNs catalytically accelerate the LiPSs–Li₂S₂/Li₂S conversion is not well understood and a better understanding is vital to optimize the S cathodes and battery performance.

6. TMNs for LIBs and LiHCs

Different from SCs which store charge via surface ion adsorption/desorption or surface redox reactions, LIBs store energy in the bulk of active materials through the Li⁺ intercalation/de-intercalation reactions.⁹² Currently, LIBs dominate EES systems and are the mainstream power supply devices for mobile electronics and electric vehicles due to their high energy density, long-term lifespan (>500 cycles), and no/little memory effect.^156–158 The electrochemical performance of LIBs depends strongly on the electrode materials.^158,159 Generally, anode materials react with Li by three mechanisms, namely (i) insertion, (ii) alloying, and (iii) conversion.^160,161 Graphite is widely used as a commercial anode in LIBs via the insertion/desertion mechanism. Nevertheless, the small theoretical capacity of 372 mA g⁻¹ cannot satisfy the requirement for a high energy density. TMNs are promising electrode materials for Li storage due to their high conductivities, large theoretical capacities, and good Li ion diffusion capabilities.^{18,19,162,163} Moreover, the conversion reaction of TMNs results in the formation of metal NPs embedded in the Li₃N matrix, which is a lithium superionic conductor (6 × 10³ S cm⁻¹ at ambient temperature).¹⁸ In this subsection, we will mainly discuss the latest progress pertaining to TMN-based composites in LIBs and LiHCs.

Fu et al. confirmed the electrochemical reaction mechanism of VN with Li forming V and Li₃N by ex situ X-ray diffraction, high-resolution transmission electron microscopy and selected area electron diffraction as well as in situ spectroelectrochemical measurements.¹⁶⁴ However, similar to the conversion reaction of TMOs with Li, TMNs suffer from large volume changes (for example, 240% for VN calculated based on the density of VN, Li₃N, and V) during cycling, resulting in particle pulverization, severe particle–particle electronic contact loss, and poor cycle performance.^20,165 Combining TMNs with a carbon matrix is a promising strategy to improve the Li storage performance. Wang et al. prepared hybrid 2D–0D graphene-VN quantum dots (G-VNQDs) by a hydrothermal reaction of GO with NH₄VO₃ and subsequent treatment in NH₃ (Fig. 18a and b).¹⁵⁸ The VN quantum dots with sizes of 2–5 nm were homogeneously dispersed on graphene and the 2D–0D G-VNQD hybrid prevented the aggregation of VN during cycling, accommodated the volume changes of VN effectively, and enhanced electron transfer and conversion reaction kinetics with Li. Electrochemical results showed that G-VNQDs had a higher specific capacity of 715 mA h g⁻¹ at 0.2C (1C = 372 mA h g⁻¹) than the single component of either VN (365 mA h g ⁻¹) or NG (433 mA h g⁻¹). Moreover, the G-VNQDs delivered a good cycle performance with a large capacity of 280 mA h g⁻¹ at 5C after 800 cycles (Fig. 18c and d).

Fig. 18 (a) Schematic illustration of the fabrication of hybrid 2D–0D graphene-VN quantum dots; (b) HR-TEM images of G-VNQD-500 (nitriding temperature is 500 °C); (c) rate performance of VN, NG, and G-VNQD-500; (d) cycling stability of G-VNQDs at a rate of 5C for 800 cycles;¹⁵⁸ (e) schematic illustration of the experimental procedures of the synthesis for the TiN/TiO₂ NWs on Ti; (f) HR-TEM image and (g) cycling performance of the TiN/TiO₂ NWs at a current density of 0.11 mA cm⁻²;¹⁶⁹ (h) schematic illustration of the formation of Mo₂N nanolayer coated MoO₂ hollow nanostructures by a controlled stepwise method; (i) HR-TEM characterization of the Mo₂N nanolayer coated MoO₂ hollow nanostructures; (j) cycling stability of the MoO₂ hollow nanostructures before and after Mo₂N nanolayer coating at 100 mA g⁻¹.¹⁷¹ (a–d) Reproduced with permission from ref. 158. Copyright 2016, Wiley-VCH. (e–g) Reproduced with permission from ref. 169. Copyright 2017, Elsevier. (h–j) Reproduced with permission from ref. 171. Copyright 2013, The Royal Society of Chemistry.

In addition to being the active materials of electrodes in LIBs, TMNs have been widely applied in LIBs as supporting materials to ameliorate the structural stability and improve the electrical conductivity of other electrode materials because they possess good electrical conductivity, high hardness, and high chemical and thermal stability.^166–168 For example, Kim et al. reported that TiN acted as an inactive material to deposit Si to maintain the electrochemical stability of the nanocomposites.¹⁶⁶ Tong et al. improved the first reversible capacity and Li storage properties of SnS₂ by growing SnS₂ NSs on porous VN substrates grown on CC. VN provided mechanical stability and shortened the Li-ion and electron pathways. The 3D porous VN/SnS₂ NSs yielded a high specific capacity of 819 mA h g⁻¹ at a current density of 0.65 A g⁻¹ and an excellent capacity retention of 97% after 100 cycles.¹⁶⁷ Wen et al. reported that core/shell TiN/TiO₂ NW arrays exhibited pseudocapacitance-dominated charge storage of Li ions (Fig. 18e and f).¹⁶⁹ The single-crystal TiN NW cores served as the highly conductive nanostructured current collectors and the branched shell consisting of nanoporous anatase TiO₂ mesocrystals was the active material. The gravimetric specific capacity of TiO₂ in the TiN/TiO₂ NW arrays was 188 mA h g⁻¹ (0.146 mA h cm⁻²) at 0.11 mA cm⁻² (corresponding to 1C on the basis of the mass of TiO₂). At a large current density of 1.0 mA cm⁻² and after 1000 cycles, the discharge capacity was maintained at 0.081 mA h cm⁻² (Fig. 18g).

Apart from utilizing TMNs such as TiN as the conducting supporters and current collectors to improve the electrochemical properties of active materials in LIBs, TMNs are also used as the inactive and conductive protective coatings for electrode materials to improve the Li storage properties.¹⁶⁸ For example, Zhang et al. prepared a TiN/Li₄Ti₅O₁₂ composite via high-energy ball-milling of a Li₄Ti₅O₁₂ and TiN powder mixture.¹⁷⁰ In the TiN/Li₄Ti₅O₁₂ composite, an amorphous TiN layer with a thickness of several manometers was coated on the Li₄Ti₅O₁₂ particles. Although the capacity of the TiN/Li₄Ti₅O₁₂ was similar to that of the pristine Li₄Ti₅O₁₂, the TiN/Li₄Ti₅O₁₂ showed a higher capacity of 130 mA h g⁻¹ at a high current density of 20C (1C = 175 mA g⁻¹) compared to pure Li₄Ti₅O₁₂ (<13 mA h g⁻¹). The excellent rate performance of TiN/Li₄Ti₅O₁₂ can be attributed to the high electron conductivity of the TiN and good Li ion conducting ability because of the formation of the Li₃N phase during the conversion reaction of the TiN coating. Recently, Liu et al. reported the fabrication of a hollow MoO₂ nanostructure coated with a Mo₂N nanolayer, showing a capacity of 815 mA h g⁻¹ after 100 cycles (Fig. 18h–j). The Mo₂N layer not only provided the electrode with better electrical conductivity, but also mitigated structural deterioration, thus ensuring the superior rate capability and cycle life of the batteries, respectively.¹⁷¹Table 8 summarizes the specific capacity and rate performance of different TMN electrode materials in LIBs.

Table 8 Typical TMN electrode materials for LIBs

Types of materials	Morphology	Specific capacity (mA h g^−1)	Rate performance (mA h g^−1)	Capacitance retention (mA h g^−1) after cycles	Ref.
a mA h cm^−2. b mA h cm^−3.
TiN
Si/TiN	Particles	300 at 0.25 mA cm^−2	—	—	166
TiN/TiO2	Core–shell NWs	0.146^a at 0.11 mA cm^−2	0.055^a at 11.0 mA cm^−2	0.08^a after 1000 cycles at 1.0 mA cm^−2	169
TiN	NWs on CC	567 at 0.335 A g^−1	288 at 1.675 A g^−1	455 after 100 cycles at 0.335 A g^−1	177
TiO2@TiN	NWs on CC	537 at 335 mA g^−1	136 at 10 A g^−1	443 after 200 cycles at 335 mA g^−1	178
VN
VN	Hollow spheres	720 at 0.05 A g^−1	460 at 0.05 A g^−1	460 after 70 cycles at 4 A g^−1	26
VN	NPs on graphene	715 at 240 mA g^−1	201 at 24 A g^−1	280 after 800 cycles at 6 A g^−1	158
VN	Thin films	1156 at 28 μA h cm^−2	—	800 after 50 cycles at 28 μA h cm^−2	164
SnS2/VN	NSs on porous VN	819 at 0.65 A g^−1	349 at 4 A g^−1	231 after 650 cycles at 13 A g^−1	167
Others
MoNx	Thin films	0.118^a at 100 μA cm^−2	—	0.069^a after 100 cycles at 0.1 mA cm^−2	179
Mo2N coated MoO2	Hollow structures	815 at 0.1 A g^−1	415 at 5 A g^−1	815 after 100 cycles at 0.1 A g^−1	171
Fe2N/C	Carbon coated nanocubes	1295^b/0.1 A g^−1	356/10.0 A g^−1	365 after 2500 cycles at 10.0 A g^−1	180
Fe3N/Fe3O4	NPs	1250/0.05 A g^−1	404/1.6 A g^−1	739 after 60 cycles at 0.05 A g^−1	168
GaN@Cu	Nanorods	980 mA h g^−1/0.25 A g^−1	—	509 after 3000 cycles at 10 A g^−1	181

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Considering the large Li storage capacity, high conductivity, and excellent Li ion conductivity, TMNs are desirable anode materials for LiHCs to provide both higher energy and power densities.^172,173 LiHCs consisting of battery-type anodes and SC-like cathodes have emerged as a new type of hybrid energy storage device that bridges the gap between LIBs and SCs.^160,161 In LiHCs, the large capacity offered by the battery-type anodes and capacitor-like cathodes yields a fast charge–discharge rate and high power density. Therefore, LiHCs can deliver a larger energy density than SCs and higher power density than LIBs and have potential applications in the EES of hybrid electric vehicles.

Wang et al. reported LiHCs employing a 3D VN–RGO composite anode and a porous carbon NR cathode. The 3D VN–RGO composites were prepared by combining VN NWs with rGO (Fig. 19a). The VN NWs provided large Li ion storage pseudocapacitance while the connected rGO network with high conductivity accelerated electron transfer. The 3D VN–RGO composite exhibited a large Li-ion storage capacity of 640 mA h g ⁻¹ at 0.1 A g⁻¹ and a fast charge/discharge rate of 270 mA h g⁻¹ at 5 A g⁻¹ (Fig. 19b). The LiHCs displayed a high capacity of 73 and 28.9 F g⁻¹ (based on the total mass of cathodic and anodic active materials) at 0.1 and 5 A g⁻¹ (Fig. 19c) and an ultrahigh energy density of 162 and 64 Wh kg⁻¹ at a power density of 0.2 and 10 kW kg⁻¹, respectively.¹⁷⁴ Cui et al. reported NbN/nitrogen-doped graphene nanocomposites as anode materials for LiHCs. The NbN/NG hybrid electrode delivered a good rate performance of 116 mA h g⁻¹ at a current density of 2000 mA g⁻¹.¹⁷⁵ The LiHCs comprising the NbN/NG hybrid anode and activated carbon cathode had high energy densities of 122.7–98.4 W h kg⁻¹ at power densities of 100–2000 W kg⁻¹, respectively. Yan et al. also utilized porous NbN as the anode and activated carbon as the cathode to assemble the device of LiHCs, which exhibited a high energy density of 149 W h kg⁻¹ and a high power density of 45 kW kg⁻¹ as well as an excellent stability with 95% capacitance retention after 15 000 cycles at 1.0 A g⁻¹.¹⁷⁶Table 9 summarizes the latest progress in LiHCs based on TMNs. In order to meet the practical applications, much efforts should be devoted to design and optimize TMNs anodes and carbon cathodes to improve the energy/power density of LiHCs.

Fig. 19 (a) Schematic showing the fabrication of the 3D porous VN–RGO hybrid by the hydrothermal treatment and annealing process in NH₃; (b) rate capability of VN NWs and 3D VN–RGO composite electrodes at various current densities ranging from 0.1 to 5 A g⁻¹; (c) specific capacitance values of 3D VN–RGO//APDC (activated polyaniline-derived carbon) LiHCs at different current densities.¹⁷⁴ (a–c) Reproduced with permission from ref. 174. Copyright 2015, Wiley-VCH.

Table 9 Recent progress in TMN electrode materials for LiHCs

Anode	Specific capacitance	Rate performance	Capacitance retention after cycles	Energy density (W h kg ^−1) vs. power density (W kg ^−1)	Ref.
VN NW/rGO	73 F g^−1 at 0.1 A g^−1	28.9 F g^−1 at 5 A g^−1	83% after 1000 cycles at 2 A g^−1	162 vs. 200	174
NbN NPs/N-graphene	260 mA h g^−1 at 0.1 A g^−1	116 mA h g^−1 at 2 A g^−1	81.7% after 1000 cycles at 0.5 A g^−1	122.7–98.4 vs. 100–2000	175
Porous NbN	67.2 F g^−1 at 0.1 A g^−1	20.7 F g^−1 at 5 A g^−1	95% after 15 000 cycles at 1.0 A g^−1	149 vs. 200	176

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7. Conclusion and outlook

Electrical energy storage is a key technology for a clean energy economy. It enables society-changing advances, especially in portable electronic devices such as smart phones and tablet computers, electric vehicles, and renewable energy grid storage. Electrodes are the key components in EES devices and determine their energy storage performance. TMNs are emerging candidates for high-performance EES devices including SCs, Li–S batteries, LIBs, and LiHCs due to their interesting morphology, high conductivity, mechanical stability, high volumetric energy density, and good catalytic activity. Compared to their bulk counterparts, nanostructured TMNs deliver better electrochemical performance such as higher capacity and better electrocatalytic activity due to their smaller particle size and larger surface area thus enabling shorter diffusion paths for electrons and ions and providing sufficient contact between the active materials and electrolytes or reactants. In this review, we discuss the recent progress in TMN-based nanohybrids of Ti, V, Nb, Mo and W in SCs, Li–S batteries, LIBs and LiHCs.

For SCs, we discuss TMN-based hybrids as the active materials to synergistically improve the capacitive storage performance. Combining TMNs with nano-carbons including CNTs, CNFs and graphene enhances the electrochemical utilization, improves the specific capacitance, and ameliorates the cycle stability and mechanical flexibility. Thus, high areal or volumetric SCs with high rate capability and good mechanical flexibility can be constructed with TMN-based hybrid electrode materials. Highly conductive and mechanically stable TMNs are appealing supporters or backbones for other active SC materials to enhance the capacitive storage performance. For Li–S batteries, the slow conversion between soluble LiPSs and insoluble Li₂S₂/Li₂S is the main cause for the shuttling of LiPSs in the electrolyte. Nanostructured TMNs are the ideal S host materials because they have high conductivity, a strong affinity for LiPSs, and good mechanical stability. More importantly, they can catalytically propel the LiPSs–Li₂S₂/Li₂S conversion due to their high catalytic activity. With regard to LIBs and LiHCs, nanostructured TMNs show large capacity and high rate capability. Moreover, TMNs act as supportive materials to load electrochemically active materials to enhance the battery performance. The promising Li storage properties and rapid Li ion storage kinetics of TMN-based electrode materials render them suitable for application in LiHCs with both high energy and power densities and have great potential in EES devices.

Although considerable advances have been made in TMN-based electrode materials, there is still substantial room for the improvement and development of advanced TMNs with better electrochemical performance. Several fundamental issues need to be resolved in order that TMNs can be widely implemented in next-generation high-performance EES devices.

(1) The electrochemical performances of TMNs are associated with their structure and surface areas. However, current TMNs and TMN-based composites generally have small surface areas and insufficient active sites. The development of novel TMN nanostructures such as 2D TMNs can improve the electrochemical properties and promote the applications of TMNs in EES devices, especially high volumetric capacitance SCs. Moreover, ternary or mixed phase TMNs show improved capacitance (capacity) and excellent rate capability compared to single nitrides, but few have been reported for LIBs and SCs. Compared to TMOs and carbon materials, the detailed electrochemical reactions of TMNs in EES devices are poorly understood from the perspective of thermodynamics and kinetics. The development of data mining and high-throughput methodologies to predict and design high-performance multi-component TMNs for EES should be actively pursued.

(2) Although there have been many studies on TMNs as capacitive electrodes in SCs, the origin and intrinsic mechanisms responsible for the large pseudocapacitive properties of TMNs are not fully understood. Generally, TMNs are prepared by nitridation of metal oxide precursors via a topochemical reaction but this process unavoidably results in the presence of remaining oxygen in the bulk of TMNs, resulting in composition differences in the product. However, the influence of the oxygen concentrations and microstructures of metal oxynitrides in TMNs on the capacitive properties remains unclear. On the other hand, most TMNs have larger specific capacitance in an aqueous acidic or basic electrolyte but they have poor cycling stability in these electrolytes and so TMNs must be modified or coated with other protective layers to improve the cyclability. However, the capacitance decay mechanism of TMNs is not well understood. Advanced characterization techniques such as in situ Raman scattering, X-ray absorption near edge structure and infrared spectroscopy, as well as theoretical simulation can provide some insights. Additionally, the development of flexible devices based on TMN electrodes is highly desirable and needs to be the prospective research.

(3) TMNs are promising S host materials which not only offer a strong affinity for LiPSs, but also catalytically propel the LiPSs–Li₂S₂/Li₂S conversion. Designing TMNs with a large surface area and high catalytic activity is crucial to enhancing the redox kinetics between the captured LiPSs and insoluble Li₂S₂/Li₂S. This is expected to effectively suppress the shuttling of LiPSs and improve the battery performance. However, the detailed electrocatalytic mechanism is unclear in this stage. A better understanding of the catalytic mechanism is expected to provide a design principle for catalytic S host materials. In situ characterization and theoretical calculation are needed to fathom the catalytic process of TMNs from LiPSs and Li₂S₂/Li₂S.

(4) TMNs deliver good Li storage performance, suggesting their potential in high-rate LIBs and LiHCs due to their high conductivity, high ion conductivity and good mechanical stability. However, data reported so far for TMNs in Li ion storage are mainly based on a half-cell using Li foil as the counter electrode. Important parameters such as the tap density of electrode materials and the mass loading of active materials as well as the electrode thickness are not given in detail, thereby making it difficult to compare the Li storage properties with the actual energy and power densities of these devices. Although the capacity fading of some TMN anodes are less remarkable in a half cell, achieving stable cycle life in a high-energy full cell is a tough challenge. Efforts should also be devoted to exploring their potential applications in new energy storage systems such as Na/K ion batteries taking advantage of the merits of TMNs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 51572100, 21875080, 51504171 and 61434001), Major Project of Technology Innovation of Hubei Province (2018AAA011), HUST Key Interdisciplinary Team Project (2016JCTD101), Fundamental Research Funds for the Central Universities (HUST: 2015QN071), Wuhan Yellow Crane Talents Program, and City University of Hong Kong Applied Research Grant (ARG) No. 9667122 and Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No. CityU 11205617.

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Footnote

† These authors contributed equally to this work.

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