V₂O₅-Based nanomaterials: synthesis and their applications

Abstract

As an important part of lithium-ion batteries, the cathode material can directly affect the performance of lithium-ion batteries. However, with the increasing demand for high-energy and high-power devices, the energy and power density of electrode materials need to be further improved. Among all metal oxides, vanadium pentoxide (V₂O₅) is regarded as a promising candidate to serve as a cathode material for LIBs due to its high theoretical capacity. Herein, a thorough survey of the synthesis of V₂O₅-based nanomaterials with various structures and chemical compositions and their application as positive electrodes for LIBs is provided. This review covers V₂O₅ with different morphologies ranging from 1D nanorods/nanowires/nanotubes/nanofibers/nanobelts, to 2D nanosheets, and to 3D hollow and porous nanostructures. Nanocomposites consisting of V₂O₅ and different carbonaceous supports, e.g., amorphous carbon, carbon nanotubes, and graphene, are also investigated. The cation-doped V₂O₅ samples as the cathode material for LIBs are briefly discussed as well. The aim of this review is to provide an in-depth and rational understanding of how the electrochemical properties of V₂O₅-based cathodes can be effectively enhanced by achieving proper nanostructures with optimized chemical composition.

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1. Introduction

Recently, with the shortage of traditional energy sources and fear of greenhouse gas-induced global warming, scientists and engineers are looking for renewable, highly efficient methods of creating and storing energy without disruption to the environment.^1,2 With the increasing demand for green energies in the 21st century, efforts have been devoted to replace the non-renewable fossil fuels by other sustainable energies, like solar, wind, nuclear and hydroelectric power.³ Therefore, long-life, environmentally friendly, and low-cost reliable batteries are greatly needed to meet the crucial energy storage demands of modern society. Since lithium-ion batteries were first commercialized in 1991 by Sony, they have been widely used in many fields such as consumer electronics because of their high energy density and high safety.^4,5 Nowadays, lithium-ion batteries (LIBs) and supercapacitors (SCs) represent two major practically applied energy storage systems; nevertheless, both have some shortcomings originating from their charge storage mechanisms.⁶ In this regard, rechargeable LIBs play a significant role due to their high gravimetric and volumetric energy, high power density, long cycle life and low self-discharge property.^7–9 Furthermore, they have proved to be the most efficient energy storage strategy for a wide range of portable devices like cellular phones, laptops and digital electronics.^10–12 Current LIB technology is well developed for portable electronic devices and has been widely used for the past twenty years. However, the employment of Li-ion batteries in hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV) and pure electric vehicles (PEV) needs two to five times more energy density than the existing lithium batteries can offer (150 W h kg⁻¹).¹³ Therefore, further LIB material and system developments are necessary.

Fig. 1 shows a typical lithium-ion battery.²⁵ Obviously, there are four key components in an LIB system: cathode, anode, separator and electrolyte.¹⁴ The electrolyte allows lithium ions to travel between the electrodes, and a separator keeps the anode and the cathode from making direct contact.^18–21 First-generation LIBs employ the graphite as the anode, layered LiCoO₂ as the cathode, and the organic liquid LiPF₆/ethylene carbonate (EC)/dimethylene carbonate (DMC) as the electrolyte.^15–17 The LIB performance (e.g., cell potential, capacity and energy density) is largely dependent on the intrinsic chemistry of the negative and positive electrode materials. The cathode usually consists of a metal oxide, and the anode tends to be a carbon material. At present, cathode materials for commercial LIBs are mainly transition metals oxides or active phosphates, such as LiCoO₂,^26,27 LiNiO₂,^28,29 LiMn₂O₄,^30,31 and LiFePO₄.^32,33 Comparison of several kinds of cathode materials for lithium ion batteries is listed in Table 1, while graphite is commonly used as the anode active material. Lithium-ion batteries charge and discharge through a process of lithiation (lithium insertion) and de-lithiation (lithium extraction) by means of electrochemical reactions. In this process, lithium ions diffuse back and forth through the electrolyte between the anode and the cathode. During lithiation (discharging), the lithium bonded with the anode material breaks apart, producing lithium ions and electrons, and these lithium ions travel and bond with metal oxides on the cathode side and the electrons produce the electrical energy that powers devices. During de-lithiation (charging), the lithium metal oxides from the cathode break apart, producing lithium ions that pass through the electrolyte and bond with the material on the anode side with the addition of electrons.^22–24

Fig. 1 Schematic of a lithium-ion battery during the charge/discharge process.²⁵

Table 1 Comparison of several kinds of cathode materials for lithium-ion batteries

Material category	LiCoO2	LiNiO2	LiMn2O4	LiFePO4
Operating voltage/V	2.5–4.2	2.5–4.2	3–4	About 3.4
Theoretical capacity/mA h g^−1	274	275	148	170
Actual capacity/mA h g^−1	130–150	150–220	110–130	135–153
Electrical conductivity/S cm^−1	10^−3	10^−1	10^−5	10^−9
Safety	Poor	Poor	Good	Good
Cycle life	>500	Poor	>500	>500
High rate discharge performance	Good	—	Good	Good

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In the past decade, because of the significant advances in nanotechnology and materials science, numerous nanostructured materials have been applied as electrode materials for LIBs. Among the potential cathode materials, layered vanadium pentoxide (V₂O₅) is one of the most attractive cathode materials and has been extensively studied because of its low cost, abundance, ease of synthesis, good safety and relatively high theoretical capacity of ∼437 mA h g⁻¹, which is much higher than that of the currently used LiCoO₂-based anodes (∼274 mA h g⁻¹).^34,35 In a Li/V₂O₅ half-cell, the Li⁺ intercalation and de-intercalation process can be expressed by the following equation:³⁶

V₂O₅ + xLi⁺ + xe⁻ → Li_xV₂O₅

As Li⁺ are inserted into the layers of V₂O₅, phase transformation occurs. Theoretically, the process of Li⁺ intercalation into V₂O₅ can be divided into several stages and different phases of Li_xV₂O₅ can be formed in each stage. Fig. 2 shows the electrochemical lithium intercalation into V₂O₅, showing the evolution of phases with the degree of lithium intercalation.⁴⁰ The structural behaviour of vanadium oxide with increased Li insertion is quite complex and it can split into several different phases. According to the amount of lithium, x, there are five phases, α-V₂O₅ (x < 0.01), ε-Li_xV₂O₅ (0.35 < x < 0.7), δ-Li_xV₂O₅ (x = 1), γ-Li_xV₂O₅ (x < 2) and ω-Li_xV₂O₅ (x > 2).^37,38 The phases of the lithiated vanadium oxide Li_xV₂O₅ and the corresponding electrochemical reactions are listed in Table 2. Among these phases of Li_xV₂O₅, the first phase α-V₂O₅ has little effect on the V₂O₅ structure. This phase is followed by the ε-phase in which the vanadium oxide layers become more puckered. The δ-phase occurs when x = 1, and as can be seen from Fig. 2, at this phase, there is a sudden decrease in cell potential. Due to the intercalation of Li⁺, slight structural modifications occur, such as puckering of the layers and increasing of interlayer spacing; however, the basic layered structure is maintained. If more than one Li⁺ are intercalated, the δ-phase is transformed into the γ-Li_xV₂O₅ phase via an irreversible reconstruction mechanism. The γ-Li_xV₂O₅ phase can itself be reversibly cycled in the range of 0 < x < 2 while maintaining the γ-type structure. When even more Li⁺ are intercalated, a rock salt structure of ω-Li_xV₂O₅ is irreversibly formed. It has been reported that discharging V₂O₅ to the ω-Li_xV₂O₅ phase would result in a rapid capacity loss with increased cycling, which is not desirable for the cathode material. Although the theoretical capacity of V₂O₅ is pretty high, the practical capacity of unmodified V₂O₅ is far from satisfactory because of low electrical conduction, slow Li⁺ diffusion and irreversible phase transitions upon deep discharge.³⁹ This would induce a huge influence in the electrode material, which leads to disintegration and loss of electric contact and eventually results in quick capacity fading upon prolonged cycling.

Fig. 2 Various phases associated with the electrochemical intercalation of lithium into V₂O₅.⁴⁰

Table 2 Phases of the lithiated vanadium oxide Li_xV₂O₅ and the corresponding electrochemical reactions

Value of x in LixV2O5	Phase	Electrochemical reactions
x < 0.01	α-V2O5	V2O5 + xLi^+ + xe^− → α-LixV2O5
0.35 < x < 0.7	ε-LixV2O5	V2O5 + xLi^+ + xe^− → ε-LixV2O5
x = 1	δ-LixV2O5	V2O5 + xLi^+ + xe^− → δ-LixV2O5
x < 2	γ-LixV2O5	V2O5 + xLi^+ + xe^− → γ-LixV2O5
x > 2	ω-LixV2O5	V2O5 + xLi^+ + xe^− → ω-LixV2O5

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In this review, we propose a systematic description of V₂O₅-based nanomaterials ranging from synthesis, to modification, structure and electrochemical application. In the first section, we will provide an overview of the formation of V₂O₅ materials through dimensional structure design. It includes the survey of phase-pure V₂O₅ with unique 1D, 2D, and 3D nanostructures. Then, we will illustrate the development of carbonaceous materials into V₂O₅ electrodes for LIBs. In the last section, cation-doped V₂O₅ samples as the cathode material for LIBs will be briefly discussed. We thus believe that this review article could serve as a good reference for V₂O₅-based nanomaterials.

2. Phase-pure V₂O₅

As a cathode material for LIBs, vanadium-based oxides have attracted worldwide attention because of their high capacity, low cost and abundance.^41,42 Moreover, V₂O₅ has also been widely used in photocatalysts,⁴³ sensors^44,45 and electrochromic devices.⁴⁶ To synthesize nanostructured V₂O₅, a variety of methods, including sol–gel processing, template-based methods, thermal evaporation, hydrothermal/solvothermal synthesis, reverse micelle techniques, and electrochemical deposition, have been developed. Among these methods, hydrothermal synthesis is considered the easiest and most effective way. With the assistance of surfactants, different morphologies of V₂O₅ are prepared by the hydrothermal method.^47–50 Table 3 summarizes the electrochemical properties of vanadium pentoxide as a cathode material for various 1D, 2D and 3D nanostructures.

Table 3 Electrochemical performance of one-dimensional nanostructured vanadium pentoxide for Li-ion battery cathode materials

Precursor material	Nanostructures	Initial capacity/mA h g^−1	Current density/mA g^−1	Cycles (times)/capacity (mA h g^−1)	References
V2O5 powder	Nanorod	335	10	50/260	53
NH4VO3	Nanorod	418.8	50	50/180.5	54
NH4VO3	Nanorod	265	29.4	∼	55
V2O5 powder	Nanorod	260	10	30/240	56
LiV3O8 powder	Nanorod	388	58.8	70/325	58
V2O5 powder	Nanorod	292	100	20/286	59
V2O5 powder	Nanorod	338	101	50/326	60
Ammonium metavanadate	Nanowire	390	30	50/201	65
Cassava starch	Nanowire	209	60	30/198	69
Vanadyl acetylacetonate	Nanowire	225	58.8	60/125	144
VOSO4·nH2O	Nanotube	300	∼	10/160	72
V2O5 powder	Nanotube	457	30	10/270	73
V2O5 powder	Nanotube	284.7	60	10/218.5	74
V2O5 powder	Nanotube	204	20	4/87.7	75
V2O5 powder	Nanotube	275.2	58.8	50/204	76
V2O5·nH2O	Nanofiber	370	800	40/347	77
Vanadium acetylacetonate	Nanofiber	310	29.4	50/229	78
V2O5 powder	Nanofiber	139	800	100/133.9	79
Vanadium hydroxylamido complex	Nanofiber	440	64	30/200	80
V2O5 powder	Nanobelt	142	50	100/141	81
NH4VO3	Nanobelt	288	50	50/246	82
V2O5 powder	Nanobelt	127.4	60	200/114.7	83
V2O5 powder	Nanobelt	281	58.8	50/242	84
Ammonium metavanadate	Nanorod	287	100	50/207	140
NH4VO3	Nanosheet	185.6	294	50/179.5	85
V2O5 crystal	Nanosheet	290	59	50/274	86
V2O5 powder	Nanosheet	108	2940	200/104	87
V2O5 powder	Nanosheet	264(3)	50	50/237	88
NH4VO3	Nanosheet	147	100	100/144	89
V2O5 powder	Nanosheet	135	300	200/126.6	91
V2O5 powder	Nanosheet	251	500	100/206	92
Vanadium oxytriisopropoxide	Nanosheet	277	300	100/211	93
NH4VO3	Nanosheet	310	29.4	50/234	94
NH4VO3	Hollow porous microspheres	283	100	60/217	95
NH4VO3	Hollow spheres	273	58.8	50/189	96
NH4VO3	Hollow microspheres	241	300	60/190	97
[V(acac)3]	Hollow microspheres	286.4	58.8	∼	98
V2O5 powder	Hollow sphere arrays	About 292	147	300/285	99
V2O5 powder	Porous microspheres	390	40	50/200	100
NH4VO3	Hierarchical and porous microspheres	141	147	100/102	101
NH4VO3	Porous hierarchical octahedrons	135	100	60/141	102
Vanadium oxytriisopropoxide	Porous microspheres	146.3	75	100/130	103
V2O5 powder	Hierarchical microspheres	275	294	200/243	104
NH4VO3	Hierarchical 3D microspheres	275	58.8	50/243	105
V2O5 powder	Hierarchical 3D microstructures	274	300	50/219	106
Vanadium oxytriisopropoxide	Hollow microflowers	277	300	100/211	107
VO(acac)2	Hierarchical microflowers	285	29.4	50/249	108
NH4VO3	Hierarchical microflowers	145	200	150/128	109
Vanadium(IV) acetylacetone	Yolk–shell microspheres	280	58.8	30/220	110
Vanadium(IV) acetylacetone	Hierarchical microspheres	270.5(2)	300	80/166.8	111
Vanadium oxytriisopropoxide	Hollow microspheres	291	100	6/254	112
Vanadium oxytriisopropoxide	Box	119	1500	400/111	113
VOC2O4	Hollow microspheres	256	300	50/227	114
NH4VO3	Multi-shelled	447.9	1000	100/402.4	115
V2O5	Hollow microspheres	137(2)	300	50/128	116

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2.1 1D nanostructures

Among the many vanadium oxide nanomaterials, one-dimensional nanostructured vanadium oxide is the most studied because its preparation method is relatively simple, it has a large specific surface area compared with traditional cathode materials, and it shows good cycling stability and high specific capacity. Among the various types of nanostructures, 1D V₂O₅ nanostructures with nanoscale radial dimensions, including nanorods,^53–60 nanowires,^65–69 nanotubes,^72–76 nanofibers^77–80 and nanobelts,^81–84 have exhibited distinct advantages in LIBs.^51,52

2.1.1 V₂O₅ nanorods. In 2008, Glushenkov's group⁵³ reported a novel solid-state process capable of producing large quantities of belt-like V₂O₅ nanorods. In this approach, V₂O₅ powder was mechanically milled in a ball mill first to produce a nanocrystalline precursor and then annealed in air. Nanorods were formed via a controlled recrystallization process during annealing. The growth process, morphology and structure of the nanorods were presented. The authors discovered that the annealing time was advantageous for the nanorod formation. Starting from the second cycle, the nanorods maintained a stable capacity of around 260 mA h g⁻¹ with no sign of significant degradation. The V₂O₅ nanorods in our study demonstrated a good cycling stability, which could be correlated to their small size and beneficial crystal orientation. Zhu et al. discovered an electrospinning method to produce V₂O₅ nanorods (Fig. 3). The method added H₂C₂O₄ to increase the solubility of cheap NH₄VO₃ in water at room temperature and took mixed solvent of deionized water and N,N-dimethylformamide (DMF) to make the spinning process easier. In the end, V₂O₅ micro/nanorods were successfully prepared. Further, the V₂O₅ micro/nanorod cathodes showed a high lithium storage performance with the first discharge capacity of 418.8 mA h g⁻¹ and the 50th discharge capacity of 180.5 mA h g⁻¹ when the half-cell was cycled between 2.0 and 4.0 V at a current density of 50 mA g⁻¹.

Fig. 3 Electrospinning reaction process for V₂O₅ micro/nanorods.⁵⁴

Subsequently, Glushenkov et al.⁵⁶ prepared nanorods of V₂O₅ via a two-stage procedure of ball milling and annealing in air. Commercially purchased V₂O₅ powder was milled in a ball mill as the first step of the synthesis. The as-milled precursor was subsequently annealed in air to produce nanorods via solid-state recrystallization. Takahashi et al.⁵⁷ synthesized single-crystal V₂O₅ nanorod arrays using template-based electrodeposition and the electrochemical results demonstrated that the nanorod array electrodes had significantly higher current density and energy storage density than sol–gel-derived V₂O₅ films. Uniformly sized vanadium oxide nanorods with a length of about 10 μm and with diameters ranging from 100 to 200 nm were grown over a large area with near unidirectional alignment. This electrode exhibited high rate discharge capacity and good cycling stability.

2.1.2 V₂O₅ nanowires. Since oxide nanorods were first produced by Yang and Lieber in 1996, oxide nanorods and nanowires have received great interest.⁶¹ Recently, oxide nanowire electrodes have been found to exhibit excellent cycling performance because of their high surface area, short Li-ion diffusion distance, and facile strain relaxation on electrochemical cycling.^62–64 Particularly, among all nanomaterials, nanowires not only reduce the volume strain, but also provide effective channels for the transmission of electrons along the length. Mai et al.⁶⁵ prepared ultralong hierarchical vanadium oxide nanowires by electrospinning combined with annealing. They used cheap ammonium metavanadate as the raw material; after the addition of polyvinyl alcohol (PVA) in water and ammonium metavanadate, a precursor aqueous solution was prepared. Then the NH₄VO₃/PVA composite nanowires were prepared by electrospinning, and the surface of the nanowires was bonded with nanorods with a diameter of about 50 nm and length of about 100 nm. The electrospun composite nanowires were then annealed at 480 °C in air for 3 h to obtain vanadium oxide nanowires. After annealing, the nanowires could retain their length as continuous structures, while the diameter decreased to 100–200 nm. Notably, it was found that the ultralong vanadium oxide nanowires were constructed from attached nanorods of diameter of approximately 50 nm and length of 100 nm. In the voltage window of 1.75–4.0 V, the cathode obtained an initial discharge capacity of 390 mA h g⁻¹ and 201 mA h g⁻¹ during the first and 50th cycles at a current density of 30 mA g⁻¹, respectively. Moreover, the as-prepared ultralong hierarchical vanadium oxide nanowires were found to offer high charge/discharge capacities and improved cycling stability.

Velazquez et al.⁶⁶ prepared V₂O₅ nanowire arrays on silicon substrates by the thermal evaporation method. The nanowires obtained were single crystalline and highly oriented with their lengths and substrate coverage controlled by the duration of the reaction, reaction temperature, and flow velocity. The growth of these nanowire arrays has great value in the fabrication of novel battery architectures based on individual nanowires. Ramasami et al.⁶⁹ synthesised a V₂O₅ NWC by the combustion method using cassava starch. The noteworthy features of the synthesis are that it is a simple and time-saving process, gives high yield, and uses naturally occurring fuel that doubles up as a template for the growth of nanowires/nanorods. The schematic representation of the nanowire cluster formation is given in Fig. 4. The V₂O₅ NWC exhibited an initial discharge and charge capacity of 209 and 206 mA h g⁻¹, respectively with a corresponding coulombic efficiency of 96% and the obtained discharge capacity at the 30th cycle was 198 mA h g⁻¹. The discharge capacity fading was found be negligible (with a small fading rate of 0.1% per cycle) from the beginning to the 50th cycle. The capacity retention in the 30th cycle was 95% with respect to the first cycle capacity. Compared to previous literature,^70,71 the as-prepared structure behaved as a superior material with respect to capacity and cycling stability.

Fig. 4 Illustration of V₂O₅ NWC formation.⁶⁹

2.1.3 V₂O₅ nanotubes. Compared with nanowires, the nanotubes have different contact areas, such as the inner and outer walls of the tube. The tubular structure can serve as a channel for filling the electrolyte, which is conducive to the promotion of ion transport. For the preparation of vanadium oxide nanotubes, the hydrothermal method is the most commonly used method. However, the fabrication of vanadium oxide nanotubes by hydrothermal synthesis is a very complicated process. In principle, nanotube arrays have larger surface areas than nanorod arrays. In addition, the tubes can operate as electrolyte-filled channels for faster transport of ions to the intercalation sites. As for the previous literature, template-based electrodeposition is a simple and efficient route for producing oxidic nanorods and nanotubes. Wang and co-workers⁷² used this method to prepare well-defined V₂O₅ nanotubes. Investigation of its electrochemical properties showed that the nanotube array possessed a high Li⁺ intercalation capacity of 300 mA h g⁻¹. Although the capacities of the nanotube array decreased during cycling, the array achieved stabilized capacities within 10 cycles. The stabilized capacity of the nanotube was approximately 160 mA h g⁻¹, which was 30% higher than the stabilized capacity of the film. The initial high capacity of the nanotube was obviously ascribed to the large surface area and short diffusion distances provided by the nanostructure. Cui et al.⁷³ presented a convenient and controllable approach to synthesize lithium vanadium oxide nanotubes by a combined sol–gel reaction and hydrothermal treatment procedure.

Nadimicherla et al.⁷⁵ synthesized V₂O₅ nanotubes and PEG surfactant V₂O₅ nanotubes by a simple hydrothermal method. Batteries using V₂O₅ nanotube electrodes showed an initial specific capacity 192 mA h g⁻¹, whereas the PEG surfactant V₂O₅ nanotubes exhibited a value of 204 mA h g⁻¹. It was found that the PEG surfactant V₂O₅ nanotubes showed higher specific capacity at the initial stages and better stability as cycling progressed than the V₂O₅ nanotubes. This was due to the fact that polyethylene glycol occupied space between the vanadium oxide layers and contributed to the increase in Li⁺ intercalation. In addition, polyethylene glycol had a relatively strong interaction with the vanadium oxide layer and had a complex interaction with Li⁺, thus effectively alleviating the electrostatic effect of the vanadium oxide layer and Li⁺. Li et al.⁷⁶ chose commercial V₂O₅ powder and oxalic acid as the raw materials to obtain a low-cost inorganic vanadium oxalate solution as the electrospinning vanadium precursor, and adopted a facile electrospinning approach followed by annealing to fabricate porous V₂O₅ nanotubes. The schematic of the synthesis of the porous V₂O₅ nanotubes is shown in Fig. 5. The sample had highly porous and hollow nanostructures of the as-prepared V₂O₅, and the as-prepared V₂O₅ had diameters in the range of 300–500 nm with many pores on the surface. In the voltage window of 2.0–4.0 V, the cathode obtained an initial discharge capacity of 275.2 mA h g⁻¹ and 204 mA h g⁻¹ during the first and 50th cycles at a current density of 58.8 mA g⁻¹, respectively. Moreover, the coulombic efficiency maintained a value of >96% throughout the cycling test.

Fig. 5 Electrospinning preparation of porous V₂O₅ nanotubes: (a) electrospinning process; (b) annealing process.⁷⁶

2.1.4 V₂O₅ nanofibers. Compared with some other methods, electrospinning is a simple, cost-effective method that enables the synthesis of nanofibers with diameters ranging from tens of nanometers to several micrometers and a large surface area-to-volume ratio. Yu et al.⁷⁷ prepared mesoporous polycrystalline vanadium oxide nanofibers by electrospinning and high-temperature annealing with V₂O₅, H₂O₂ and polyvinylpyrrolidone (PVP) as the starting materials. The resulting nanofibers were 350 nm in diameter and consisted of a porous polycrystalline vanadium oxide with a specific surface area of ∼97 m² g⁻¹. The mesoporous V₂O₅ nanofibers demonstrated a significantly enhanced Li-ion storage capacity of above 370 mA h g⁻¹ and a high charge/discharge rate of up to 800 mA g⁻¹ with little cyclic degradation. Such mesoporous V₂O₅ nanofibers allowed easy mass and charge transfer with sufficient freedom for volume change accompanying the lithium ion intercalation and de-intercalation. Cheah et al.⁷⁸ fabricated single-phase polycrystalline high aspect ratio porous V₂O₅ nanofibers by a simple one-step electrospinning process using V₂O₅ precursor consisting of vanadium vanadyl acetylacetonate and poly(vinylpyrrolidone), followed by sintering at 400 °C for 15 min. Porosity was caused by the slight shrinkage of the fibers due to the thermal decomposition of the PVP polymer. In the voltage window of 1.75–4.0 V, the cathode achieved an initial discharge capacity of 316 mA h g⁻¹ and 158 mA h g⁻¹ during the first and 50th cycles at 0.1C rate, respectively. In the voltage window of 2.0–4.0 V, the cathode obtained an initial discharge capacity of 310 mA h g⁻¹ and 229 mA h g⁻¹ during the first and 50th cycles at 0.1C rate, respectively. Compared with in the 1.75–4 V voltage range, the specific capacity was greatly improved, which was due to the formation of nanoparticles connected to the nanofiber morphology.

Recently, Yan's group⁷⁹ has proposed a modified, facile and effective electrospinning method to synthesize V₂O₅ nanofibers using commercial vanadium pentoxide as the precursor material, which is illustrated in Fig. 6. Compared with commercial V₂O₅ at various current densities, it is evident that V₂O₅ nanofibers exhibited much better rate capability and cyclic stability. For instance, at a current density of 100 mA g⁻¹, the commercial V₂O₅ electrode exhibited continuous capacity fading in the initial four cycles. Interestingly, the specific capacity for V₂O₅ nanofibers was recovered after the cathode electrode worked at various current densities for 45 cycles; this performance was significantly better than that of the commercial V₂O₅ electrode. Dewangan et al.⁸⁰ proposed a simple mild one-step hydrothermal technique to prepare uniform V₂O₅ nanofiber bundles (NBs). The method used a vanadium(V) hydroxylamido complex as the vanadium source in the presence of HNO₃. A bundle is made of an indefinite number of homogeneous V₂O₅ nanofibers with lengths up to several micrometres and widths ranging from 20 to 50 nm. The as-prepared V₂O₅ NBs displayed a high electrochemical performance in a non-aqueous electrolyte as a cathode material for lithium-ion batteries.

Fig. 6 Schematic showing the strategy for fabricating hierarchical V₂O₅ nanofibers.⁷⁹

2.1.5 V₂O₅ nanobelts. Using one-dimensional vanadium pentoxide material, nanobelt structures have also been synthesized. Qin's group⁸¹ proposed a template-free hydrothermal method to synthesize ultra-thin VO₂(B) nanobelt arrays with a thickness of about 10 nm, which could be converted to V₂O₅ porous nanobelt arrays by calcining in air at 400 °C. In the voltage window of 2.5–4.0 V, a specific discharge capacity of 146 mA h g⁻¹ at the 50th cycle as achieved, which is very close to the theoretical capacity of V₂O₅ crystallite electrodes (147 mA h g⁻¹). The coulombic efficiency was nearly 100%, suggesting the good reversibility of the Li-ion insertion/deinsertion process. Even at a much higher current density of 1000 mA g⁻¹, a specific discharge capacity of 130 mA h g⁻¹ and capacity retention of 128 mA h g⁻¹ after 100 cycles could be achieved. The capacity fading rate was calculated to be 0.015% per cycle, suggesting superior capacity retention capability. The excellent rate performance and good capacity retention at various discharge rates are attributed to the interesting porous V₂O₅ nanobelt array structures. The nano-sized belts reduce the distance of Li-ion diffusion and electron transportation. Moreover, the porous structures of the nanobelt arrays facilitate electrolyte penetration and increase the contact area between the active material and the electrolyte. Furthermore, the porous structure might also be of advantage to accommodate the volume variations during Li-ion intercalation and de-intercalation. Niu et al.⁸² successfully fabricated hierarchical V₂O₅ nanobelts by an economical and facile hydrothermal strategy followed by annealing treatment, as illustrated in Fig. 7. The self-template synthesized hierarchical nanobelt assembled by plate-like nanocrystallines had an open structure with tiny haircuts and a large contract area. When the battery was cycled between 2 and 4 V at a current density of 50 mA g⁻¹, the initial discharge capacity was 288 mA h g⁻¹. The discharge capacity was 246 mA h g⁻¹ at the end of the 50th cycle, with a capacity retention of 85.4%. The good performance of the hierarchical nanobelts can be attributed to their advantages of a hierarchical structure, which is open with tiny haircuts, providing more pathways for the immersion of liquid electrolyte. Also, these nanobelts have a large and effective surface area, facilitating the electrochemical reaction between the active material and the electrolyte and enable easy strain relaxation to accommodate volume variations during lithiation/delithiation, thus enhancing the structural stability and cyclability.

Fig. 7 (a) Schematic of possible formation process of V₂O₅-HNbs, and (b) advantages of hierarchical nanobelt structures when applied in lithium-ion batteries.⁸²

Wang and co-workers⁸³ fabricated a self-standing V₂O₅ nanobelt electrode free of conductive agents, binders and current collectors by a simple one-step hydrothermal reaction. The length of the V₂O₅ nanobelts was up to several hundreds of micrometers and the thickness was around 40 nm. The as-prepared sample demonstrated better cycling performance than commercial conventional V₂O₅ electrodes with a high reversible capacity of 114.4 mA h g⁻¹ after 200 charge–discharge cycles. The superior cycling performance of the nanobelt network electrode can be attributed to the excellent structural stability provided by the crosslinked nanobelt network. Rui et al.⁸⁴ demonstrated a cost-effective and green process for the large-scale synthesis of single-crystalline V₂O₅ nanobelts (in kilogram scale) under ambient conditions by simply vigorous stirring the commercially available V₂O₅ powder in an aqueous NaCl solution. The as-prepared sample exhibited a higher reversible specific capacity of 242 mA h g⁻¹ up to the 50th cycle with a capacity retention at 86%.

For one-dimensional nanostructured materials, the preparation methods are mostly the sol–gel method and hydrothermal method, because these processes are simple and feasible and the requirements of the experimental conditions are not too high. Common nanofibers, nanowires, nanotubes, etc. to some extent have improved the performance of vanadium oxide as a cathode material, such as shortening the diffusion distance of lithium ions and electrons, improving the charge and discharge properties of the electrode material, or increasing the effective contact area to improve the storage of lithium ions and to improve the specific capacity of the material. The superior electrochemical performance of one-dimensional nanostructures can be attributed to the following aspects.

First, one-dimensional nanostructures of V₂O₅ provide an effective electron transport pathway along the one-dimensional direction. Second, the one-dimensional nanostructures of V₂O₅ nanotubes with large electrode/electrolyte contact area and hierarchical porous channels provide short Li⁺ diffusion distances. Finally, the existence of pyrolyzed carbon improves the conductivity of the one-dimensional nanostructures. These nanomaterials have a high specific surface area when they are prepared alone. However, when the electrode is prepared, the one-dimensional nanostructures are greatly reduced due to the large surface energy and low mechanical stability, likely resulting in agglomeration and a significant decrease in the specific surface area. Their initial discharge specific capacity will be higher in the charge and discharge tests, but they will produce significant capacity attenuation; also, the number of cycles will not be high in the long cycle performance without further improvement.

2.2 2D nanostructures

Two-dimensional nanostructures increase the effective contact area between the electrolyte and the active material, increase the storage capacity of the lithium ions, and shorten the diffusion distance of the lithium ions to meet the demand for high charge and discharge and thus form an ideal structure for the rapid storage of lithium ions. However, the mechanical stress produced by the nanosheets in the process of circulation is easy to make the nanosheets overlap and thicken, weakening the effective contact area of the nanosheets. Therefore, the dispersibility of two-dimensional nanosheets is restricted as an excellent key factor for electrode material performance. It is helpful to effectively improve the electrochemical performance of two-dimensional vanadium pentoxide nanosheets electrode materials by introducing a conductive agent between the nanosheet and increasing the spacing of the nanosheets. Huang et al.⁸⁵ developed an additive-free ultrasonic method with subsequent thermal decomposition to synthesize V₂O₅ self-assembled nanosheets. The loose V₂O₅ nanosheets were stacked as 4–6 layers and each layer (∼50 nm in thickness) was assembled using abundant nanoparticles. The as-prepared sample delivered a reversible capacity of 185.6 mA h g⁻¹ and 179.5 mA h g⁻¹ at the 1st and 50th cycles, respectively, corresponding to a capacity retention of 96.7%. However, NPs V₂O₅ only had a reversible capacity of 121.4 mA h g⁻¹. After 50 cycles, 87.4% of the capacity (∼106.1 mA h g⁻¹) was retained. All the results demonstrated that the unique structure of SANs V₂O₅ resulted in improved rate performance and high cycling stability at higher current densities, which was expected to satisfy the requirement of a long cycle life for LIBs.

Rui's group⁸⁶ proposed a liquid phase separation method to prepare ultrathin V₂O₅ nanosheets. In the range of 2.05–4 V, the first reversible specific discharge capacity of was 290 mA h g⁻¹ at 0.2C. When the rate increased to 50C, the capacity was 117 mA h g⁻¹. This is mainly due to the deposition of lithium ions on the surface of these ultra-thin nanosheets; ultra-thin nanosheets shorten the embedded path of lithium ions to meet the requirements of rapid charge and discharge. An et al.⁸⁷ successfully prepared ultrathin V₂O₅ nanosheets through a supercritical solvothermal reaction followed by annealing, as illustrated in Fig. 8. As a cathode material for lithium batteries, the ultrathin V₂O₅ nanosheets exhibited a capacity of 108 mA h g⁻¹ at a high rate of up to 10C at 2.4–4 V and excellent cyclability with little capacity loss after 200 cycles. The enhanced rate performance is attributed to the shortened diffusion distance and the increased electrode–electrolyte contact area of the ultrathin nanosheet structure. Subsequently, Liang's group⁸⁸ fabricated nanosheet-structured vanadium pentoxide by a sol–gel method. As a cathode material, the V₂O₅ nanosheets exhibited enhanced cycling stability and rate capability. The as-prepared NSs showed better lithium storage properties than commercial V₂O₅, with a reversible capacity of 237 mA h g⁻¹ after 50 cycles.

Fig. 8 Schematic illustration of the procedure for fabricating the ultrathin vanadium oxide nanosheets.⁸⁷

Song et al.⁸⁹ prepared mesoporous V₂O₅ nanosheets by a hydrothermal method followed by instantaneous heating and calcination in air, as illustrated in Fig. 9. The as-prepared V₂O₅ nanosheets were composed of several well-defined porous nanosheets that assembled themselves together and formed a highly mesoporous nanosheet structure. In the voltage window of 2.5–4.0 V, the cathode obtained a reversible capacity of 147 mA h g⁻¹ in the 2nd cycle and 144 mA h g⁻¹ in the first and 100th cycles at a current density of 100 mA h g⁻¹. The good performance of mesoporous V₂O₅ nanosheets can be attributed to the advantages of this novel structure. Xu's group⁹⁰ developed a one-step polymer-assisted chemical solution method to synthesize two-dimensional V₂O₅ sheet networks. This unique network structure provided an interconnected transportation pathway for lithium ions. The as-prepared sample exhibited a high capacity of about 300 mA h g⁻¹ at a current density of 100 mA g⁻¹. Recently, Liang et al.⁹¹ proposed a bottom-up solvothermal method to synthesize ultra-large V₂O₅ nanosheets. The thickness of the large nanosheet was around 4 nm and the thickness of the parallel stacked nanosheets was about 3–5 nm. Moreover, the cathode obtained a capacity of is 135 mA h g⁻¹ in the 2nd cycle and a retention of 93.8% after 200 cycles at a current density of 100 mA h g⁻¹. The coulombic efficiency was close to 100% during the whole discharge/charge process. The excellent rate performance and remarkable cyclic stability are due to the structural advantages of this uniform 2D ultrathin nanosheet with an ultra-large lateral size: (1) the void space between the nanosheets can facilitate the penetration of electrolyte; (2) the ultra-thin feature can reduce Li⁺ ion diffusion distance and electron transportation distance; (3) the flexible nanosheets can accommodate the volume changes caused by repeated lithium ion insertion and removal; and (4) the ultra-large nanosheets and layer-by-layer stacking structures can better maintain the integrity of the nanosheet electrodes. Li and co-workers⁹² prepared 2D leaf-like V₂O₅ nanosheets by a novel and facile green method. The unique nanoscale characteristics, including 2D morphology, hierarchical porous structure, and large specific surface, of these 2D V₂O₅ nanosheets led to superior electrochemical performance in terms of their specific capacity, rate capability, and cyclability.

Fig. 9 Schematic of the synthesis route of V₂O₅ mesoporous nanosheets in this work.⁸⁹

As the cathode material of the two-dimensional nanosheet, lamellar nanostructures have a large specific surface area and good structural flexibility, which provide a larger space for the diffusion of ions and can adapt to the volume changes in the cathode material during the lithiation and de-lithiation of lithium, thus improving the cycling stability of the cathode material. If a conductive active material such as carbon nanotubes and carbon black is compounded in the structural material of the nanosheets, it is possible to prevent not only the agglomeration of the nanosheets but also the high conductivity of the active material. At the same time, this reduces the impedance of ions on nanosheets and makes the lithium ions diffuse faster in the electrode material, thus obtaining excellent charge and discharge performance, which is helpful to improve the electrochemical performance of the two-dimensional nanosheet cathode materials. Compared with one-dimensional nanostructured materials, the initial discharge specific capacity of the nanosheets is significantly lower. The reason may be that the vanadium oxide nanosheets are agglomerated during the charge and discharge process, resulting in a significant reduction in the effective contact area, which cannot effectively maintain the high specific surface area of the nanosheets. As with the one-dimensional vanadium pentoxide nanostructures, the long-term cycling performance of nanosheet cathode materials reported in the relevant literature has not improved.

2.3 3D nanostructures

In recent years, three-dimensional structures have attracted considerable attention as an important role of electrode materials. Compared with low-dimensional nanomaterials, three-dimensional nanostructured materials with desired morphologies have been prepared for their superior electrochemical properties as they not only inherit the advantages of low-dimensional nanomaterials, but also maintain the structural stability of the electrode during the Li⁺ intercalation and de-intercalation process. In addition, the three-dimensional nanostructures are self-supporting, and the nanocomposites are not prone to agglomeration during circulation. Also, the pore structure of the prepared electrode material is maintained for a long time. Thus, under the same electrochemical cycling test, vanadium pentoxide electrode materials have a three-dimensional or graded structure exhibiting better electrochemical performance than the low-dimensional electrode materials and better cycling stability. Compared with one-dimensional nanowires, nanotubes and two-dimensional nanosheets, the three-dimensional structure of vanadium oxide shows a variety of peculiar morphologies, such as loose nanoscale structures, and antennaed nanospheres, similar to the structure of sea urchins; these strange nanostructures can generally be prepared by adding a templating agent, such as dodecylamine and dodecyl mercaptan. For the three-dimensional nanomaterials of vanadium pentoxide, the most commonly used preparation method is the solvothermal method.

2.3.1 Hollow porous nanostructures. A few years ago, Mai et al.⁹⁵ used carbon microspheres as the template to synthesize 3D hollow porous V₂O₅ quasi-microspheres by a facile solvothermal and annealing method. By selecting an appropriate voltage window, the as-prepared sample exhibited good cycling performance of 283 mA h g⁻¹ at a current density of 100 mA g⁻¹. The excellent electrochemical performance is attributed to the nanosized building blocks of the 3D V₂O₅ hollow porous structure, which provides a short Li-ion diffusion distance, effective strain relaxation, and large active contact area. By using P123 surfactant as the soft template, Zhang's group⁹⁶ successfully obtained three-dimensional V₂O₅ hollow spheres by a simple synthesis strategy combining the solvothermal treatment and subsequent thermal annealing. As shown in Fig. 10a, the V₂O₅ precursor was composed of three-dimensional microspheres with diameters in the range of two to three microns. The reversible capacity was 189 mA h g⁻¹ at the end of 50 charge–discharge cycles. It also exhibited satisfactory lithium-ion storage performance. In another similar system, Uchaker and co-workers⁹⁷ synthesized nanostructured V₂O₅ hollow microspheres via a solvothermal route followed by thermal treatment. Previously, Cao et al.⁹⁸ prepared self-assembled V₂O₅ nanorods into microspheres by a mediated polyol process. The method represents a substantial simplification over more conventional methods such as electrostatic spray deposition or thermal evaporation. The as-prepared sample also exhibited good electrochemical performance when they were used as the cathode material in lithium-ion batteries.

Fig. 10 (a) SEM images of V₂O₅ precursor.⁹⁶ (b) Multilayer V₂O₅ hollow sphere arrays (cross-sectional SEM images in inset).⁹⁹ (c) SEM images of the V₂O₅ product synthesized by annealing the corresponding VEG precursors in air.¹⁰¹ (d) FESEM images of ammonium vanadium oxide octahedrons.¹⁰²

Chen and co-workers⁹⁹ developed a facile polystyrene sphere template-assisted electrodeposition method for the fabrication of porous multilayer V₂O₅ hollow sphere arrays on graphite paper substrates. We could clearly see that a 3D porous hollow sphere structure was well formed and the individual hollow sphere exhibited a size of ∼500 nm (Fig. 10b). We also can reach the conclusion that it is reasonable for the obtained porous multilayer V₂O₅ hollow sphere arrays to possess impressive porous systems, as these systems will be beneficial for fast ion/electron transfer, leading to fast reaction kinetics. This is especially important for high-rate LIB applications. Wu et al.¹¹² reported the controllable synthesis of novel rattle-type V₂O₅ hollow microspheres with the assistance of carbon colloidal spheres as hard templates. Carbon spheres@vanadium-precursor (CS@V) core–shell composite microspheres were first prepared through a one-step solvothermal method. V₂O₅ hollow microspheres with various shell architectures could be obtained after removing the carbon microspheres by calcination in air. When evaluated as a cathode material for LIBs, the rattle-type V₂O₅ hollow microspheres exhibited superior cycling stability and rate capability. Pan and co-workers¹¹⁴ proposed a one-pot template-free solvothermal method for the controllable synthesis of uniform VO₂ microspheres with different complex interiors, including yolk–shell and multi-shell structures. The VO₂ hollow spheres could be readily transformed into V₂O₅ hollow spheres without any structural deformation by calcination in air. The resulting V₂O₅ hollow microspheres exhibited a high initial reversible capacity of 256 mA h g⁻¹ at a current density of 300 mA g⁻¹ and good cycling performance over 50 cycles. Pan et al.¹¹⁶ also reported the synthesis of hierarchical nanosheet-constructed hollow V-glycolate microspheres by a template-free method using a cost-effective VOC₂O₄ solution as the precursor. After calcination in air, V₂O₅ hollow hierarchical microspheres with well-preserved structures could be obtained and they were found to exhibit superior rate capability and cycling stability as cathode materials for LIBs.

2.3.2 Hierarchical and porous nanostructures. Wang and co-workers¹⁰¹ developed a reflux approach followed by annealing to synthesise hierarchical and porous V₂O₅ microspheres. As is shown in Fig. 10c, the V₂O₅ microsphere had rough and textured surfaces with porous structures. The superior Li⁺ storage performance of the hierarchical and porous V₂O₅ microspheres could be mainly ascribed to the improved electrode/electrolyte interface, reduced Li⁺ diffusion paths, and relieved volume variation during lithiation and delithiation processes. An et al.¹⁰² reported the synthesis of porous V₂O₅ hierarchical octahedrons via a solid-state conversion process. The surface of the octahedrons was smooth, with a diameter range of 3–6 μm (Fig. 10d). Zhang et al.¹⁰³ synthesized novel 3D porous V₂O₅ hierarchical microspheres by a solvothermal method and subsequent calcination (Fig. 11). These 3D microspheres were constructed by nanoporous fibers, which endowed the electrode with shorter diffusion paths and higher conductivities. When evaluated as a cathode material for lithium-ion batteries, the V₂O₅ microspheres displayed relatively stable capacity retention at different current rates. They also showed excellent rate capability, with a capacity of 105 mA h g⁻¹ at a 30C rate. The excellent electrochemical performance suggests that these unique hierarchical V₂O₅ microspheres can be a promising cathode material for lithium-ion batteries.

Fig. 11 Schematic of the formation of 3D porous V₂O₅ hierarchical microspheres.¹⁰³

Recently, Dong et al.¹⁰⁴ developed a facile two-step strategy to synthesise V₂O₅ microspheres with a hierarchical structure. First, they synthesized a vanadium glycolate precursor by a facile template-free and polyol-mediated solvothermal method and then, they obtained V₂O₅ microspheres by thermal annealing. Bai and co-workers¹⁰⁵ facilely fabricated hierarchical 3D microspheres consisting of 2D V₂O₅ (vanadium pentoxide) nanosheets by a low-temperature hydrothermal method. The novel hierarchical micro-/nano-V₂O₅ electrode exhibited excellent electrochemical performance in terms of high-energy and high-power applications owing to its unique structural properties. These hierarchical 3D micro-/nano-V₂O₅ possess many unique features advantageous for LIBs: (1) 2D V₂O₅ nanosheets facilitate Li⁺ diffusion and electron transport; (2) hierarchical 3D micro-/nano-cathode structures built up by V₂O₅ nanosheet spheres lead to close and sufficient contact between the electrolyte and activate materials and at the same time create a buffer to accommodate volume changes during discharge/charge process; and (3) micro-scale V₂O₅ spheres easily induce high cell packing density beneficial for high-power batteries. Pan et al.¹⁰⁶ developed a facile solvothermal method to synthesize hierarchical vanadium oxide with various nano/microstructures by simply varying the concentration of the precursor (VOC₂O₄) solution. The V₂O₅ prepared by solvothermal method for 2 h showed good lithium storage properties with a high reversible capacity of 219 mA h g⁻¹ after 50 cycles. In a follow-up study based on the above experimental results, Pan¹⁰⁷ synthesized hierarchically hollow microflowers composed of nanosheets via a one-pot solvothermal method. After annealing, the sample delivered a high initial discharge capacity of 277 mA h g⁻¹, and the value slightly increased to 284 mA h g⁻¹ in the second cycle probably due to improved electrolyte penetration. With a voltage window of 2.0–4.0 V, the V₂O₅ hollow microflowers retained a remarkable reversible capacity of 211 mA h g⁻¹ at the end of the 100th cycle.

Chen et al.¹⁰⁹ successfully synthesized hierarchical V₂O₅ microflowers by a solvothermal reaction followed by a calcination process at 350 °C. The product consisted of uniform flower-like micro-spheres with an average size of around 2 μm. After annealing, the as-prepared hierarchical V₂O₅ microflowers showed good lithium storage properties with a high reversible capacity of 128 mA h g⁻¹ after 150 cycles at a current density of 200 mA g⁻¹. It was obvious that these capacities were higher than those of bulk V₂O₅. Ma et al.¹¹¹ developed a facile template-less approach for the synthesis of various V₂O₅ hierarchical structures by calcining the solvothermally prepared VO₂ with different morphologies and structures, which could be simply tailored by adjusting the solvothermal reaction duration. As the cathode materials for lithium-ion batteries, the electrode delivered reversible capacities of 119.2 and 87.3 mA h g⁻¹ at high current densities of 2400 and 3600 mA g⁻¹, respectively, as well as a capacity retention of 78.31% after 80 cycles at 1200 mA g⁻¹. The excellent electrochemical performance could be attributed to the purity of the phase and the synergistic effect between the yolk–shell structure and hierarchical structure of the sub-microspheres. Thus, the three-dimensional nanostructures are more stable than the low-dimensional structures. Nanocomponents are less prone to agglomeration, and the active specific surface area can be maintained for a long time. As can be seen from Table 4, the three-dimensional structures show better cycling performance over prolonged cycling than the low-dimensional structures.

Table 4 Electrochemical performance of carbonaceous materials used with V₂O₅ electrodes for Li-ion battery cathode materials

Precursor material	Nanocomposites	Initial capacity/mA h g^−1	Current density/mA g^−1	Cycles (times)/capacity (mA h g^−1)	References
NH4VO3	Mesoporous carbon foam	268	29.4	30/220	117
NH4VO3	Carbon	292	29.4	30/261	118
Vanadium pentoxide nanofibers	Amorphous carbon	300	29.4	50/201	119
V2O5 powder	Mesoporous carbon	247	500	100/163	120
V2O5 powder	Porous carbon	297	294	50/288	122
V2O5 powder	Multiwalled carbon nanotube	About 152	25	50/143	126
Vanadium oxytriisopropoxide	Multiwalled carbon nanotubes	285	294	100/190	127
Sodium metavanadate (NaVO3)	Single-walled carbon nanotubes	250	14.7	30/225	128
V2O5 powder	Multiwalled carbon nanotube	110	200	500/about 110	129
V2O5 powder	Multiwalled carbon nanotubes	238	50	50/151	130
V2O5 nanoparticles	Multi-graphitic nanotubes	224	150	200/211	131
VO(acac)2	CNTs	250	200	40/190	132
Vanadium isopropoxide	Multiwalled carbon nanotubes	292	29.4	50/275	133
V2O5 powder	Multiwalled carbon nanotubes	243	50	50/209	135
NH4VO3	Carbon tube-in-tube	280	58.8	20/265	136
V2O5 powder	Multiwalled carbon nanotubes	402	100	50/222	137
Vanadium oxytriisopropoxide	Single-walled carbon nanotubes	About 598	300	40/548	138
Ammonium metavanadate	Reduced graphene oxide (RGO)	287	100	50/207	140
Vanadium oxytriisopropoxide	Reduced graphene oxide (RGO)	196	600	160/102	141
VCl4	Graphene nanoribbons	278	29.4	100/217	142
NH4VO3	Graphene oxide	160	70	200/133	143
Vanadyl acetylacetonate	Reduced graphene oxide (RGO)	225	58.8	60/125	144
Vanadyl(IV) sulfate (VOSO4)	Graphene oxide	240	100	20/220	145
V2O5 powder	Reduced graphene oxide (RGO)	235	20	100/171	146
V2O5 powder	Graphene oxide	190.9	100	150/182.4	147
Vanadium(IV) acetylacetone	Graphene oxide	255	100	100/153	148

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3. V₂O₅-Based nanocomposites

In addition to phase-pure V₂O₅, V₂O₅-based nanocomposites have also been intensively studied as positive electrodes for LIBs. In this section, composites containing V₂O₅ and other materials will be discussed. In this part, V₂O₅ with different carbonaceous supports, including amorphous carbon, carbon nanotubes (CNTs), and graphene (Gr) will be surveyed (Table 4).

3.1 V₂O₅ with amorphous carbon

Amorphous carbon is an inexpensive, frequently used carbonaceous material that is easy to produce in industrial quantities. In LIB applications, amorphous carbon is used to produce a conductive compact outer layer on the surface of V₂O₅ NPs, which not only serves as a buffer layer to accommodate the volume expansion during cycling but also contributes to the formation of a stable solid electrolyte interface (SEI) layer. In past research, the use of a carbon layer as a matrix buffer has exhibited superb properties. The carbon layer can also promote the electronic conductivity of the electrodes. As early as 2013, carbon-coated V₂O₅ nanoparticles,¹¹⁷ produced via a facile and scalable method via the route illustrated in Fig. 12, demonstrated a first charge capacity of 268 mA h g⁻¹ and a capacity of 220 mA h g⁻¹ in the 30th cycle at a current density of 29.4 mA g⁻¹. This carbon-coated V₂O₅ showed improved cycling stability and rate performance compared to nanosized V₂O₅. This approach can be extended to enhance the electrochemical performances of other alternative cathodes. Shin and co-workers¹¹⁸ reported a facile and scalable method for the synthesis of carbon-coated V₂O₅ nanoparticles through a carboxylic acid-assisted sol–gel method and controlled calcination. NH₄VO₃ was used as the vanadium source. Oxalic acid (C₂O₄H₂), tartaric acid (C₄H₆O₆), and citric acid (C₆H₈O₇) were used as both the carbon sources and chelating/reducing agents for preparing three different carbon-coated V₂O₅ composites. In the voltage window of 2.1–4.0 V, VCA11 had a discharge capacity of 292 mA h g⁻¹ for the first cycle and a specific capacity of 261 mA h g⁻¹ in the 30th cycle, with a fading rate of 0.36% per cycle. Cheah et al.¹¹⁹ developed a plasma-enhanced chemical vapour deposition (PECVD) method to synthesize carbon-coated V₂O₅ nanofibers. The carbon-coated VNFs exhibited better capacity retention characteristics than bare VNFs irrespective of the initial capacity values during prolonged cycling. Such an enhancement is mainly attributed to the homogeneous carbon coating by PECVD, which improves the electronic conducting profiles of VNF and prevents undesired side-reactions with electrolyte counterparts.

Fig. 12 Schematic of the preparation of carbon-fabricated V₂O₅ samples (CF300 and CF400).¹¹⁷

Ihsan et al.¹²⁰ have recently developed an ultrasound assisted method followed by sintering to fabricate a V₂O₅/mesoporous carbon composite. Mesoporous carbon, which has pore size between 2 and 50 nm, facilitates electrolyte diffusion into the bulk of the electrode material and hence provides fast transport channels for the conductive ions. Obviously, amorphous structures of carbon and lattice fringes of V₂O₅ are observed. After sintering, the as-prepared V₂O₅/mesoporous carbon showed good lithium storage properties with a high reversible capacity of 163 mA h g⁻¹ after 100 cycles. The capacity retention of V₂O₅/mc was considerably higher than that of V₂O₅ np. Yu et al.¹²¹ rationally designed and synthesized V₂O₅/ordered mesoporous carbon (CMK-3) composites via an ultrasonic method. The results showed that the ultrasonic synthesis method was flexible and efficient to highly disperse V₂O₅ nanoparticles in CMK-3. Zhang's group¹²² reported that carbon-coated V₂O₅ nanocrystals could be obtained via a unique capillary-induced filling strategy. The obtained nanocrystals exhibited markedly enhanced rate capability and excellent cyclability when used as the cathode material for Li-ion batteries. Guo et al.¹²³ demonstrated that the electrochemical performance and stability of V₂O₅ nanowires could be significantly improved by coating a thin carbon layer as the shell. The V₂O₅@C nanowires exhibited substantially enhanced capacitive performance compared to that of V₂O₅ nanowires.

3.2 V₂O₅ with carbon nanotubes

Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure. They find applications in LIB electrode materials owing to their extraordinary thermal, mechanical, and electrical properties. CNTs can be categorized as single-walled nanotubes (SWCNTs) and multi-walled nanotubes (MWCNTs). SWCNTs have high conductivity (≈104 S cm⁻¹) and provide electronic conduction without blocking electrolyte access to the active material. MWCNTs show high mechanical and chemical stability, and the mesoporous character favors the diffusion of the reacting species.^124,125 CNTs are generally employed as conducting agents to replace carbon black when used in the electrode of LIBs. Nanocomposites containing V₂O₅ and CNTs are reported to have better lithium storage properties than bare V₂O₅ materials. This can be attributed to the flexible nature of the CNTs, which alleviate the internal stress caused during the charge–discharge process. With increased conductivity and surface area, such nanocomposites show enhanced lithium storage capability.

By early 2011, Seng et al.¹²⁶ prepared free-standing V₂O₅/MWCNT films by a hydrothermal technique and filtration of ultra-long nanowires. The MWCNTs not only acted as conductivity modifiers, but also contributed to forming an integrated web-like structure. Yu's group¹²⁷ proposed a facile hydrothermal approach to fabricate CNTs covered by interconnected V₂O₅ nanosheets, as illustrated in Fig. 13. After calcination in air, flakelike nanosheets of V₂O₅ could be observed, which were highly interconnected and covered the CNT skeleton. The interconnected V₂O₅-NSs were in intimate contact with the CNTs, which was favorable for the enhancement of conductivity. The as-derived CNTs@V₂O₅ hybrid structure showed a larger reversible capacity over 100 cycles than the V₂O₅-mf sample. In the voltage window of 2.0–4.0 V, the CNTs@V₂O₅ electrode delivered a capacity of 285 and 190 mA h g⁻¹ during the 1st and 100th cycle at the current rate of 1C, respectively. Zhou and co-workers¹³⁰ used low-cost V₂O₅ powders and H₂O₂ as the raw materials to synthesize vanadium oxide nanosheet–MWCNT composite by a simple sol–gel method and hydrothermal process. It was found that V⁴⁺ was the predominant V element of the vanadium oxide nanosheet. The addition of carbon nanotubes improved the conductivity of the vanadium oxide nanosheets, exhibiting higher specific capacity and cycling stability than materials without carbon nanotubes. When the batteries were cycled between 1.5 V and 4 V at a constant current density of 50 mA g⁻¹, the maximum discharge capacity of the vanadium oxide nanosheet–MWCNT composite was 238 mA h g⁻¹ and 151 mA h g⁻¹, respectively, after the 50th cycle. The good performance of the vanadium oxide nanosheet–MWCNT composite can be attributed to the sheet-like nanostructure having a large specific surface area and good structural flexibility, which can provide more Li⁺ ion intercalation sites and accommodate large volume variations.

Fig. 13 Schematic of the synthesis process for CNTs@V₂O₅.¹²⁷

In contrast to the conventional strategy of coating the exterior and/or filling the interior of CNTs with V₂O₅, Chen et al.¹³⁴ proposed a novel strategy to fabricate MWCNT/V₂O₅ core/shell nanostructures. They first used chemical vapor deposition (CVD) to grow an MWCNT sponge in a quartz tube at 860 °C, which had very low density and high porosity. Then the sponge was cut into the desired size and placed in a commercial ALD reactor, where 1000 cycles of H₂O-based ALD V₂O₅ was deposited on it. The as-derived MWCNT/V₂O₅ core/shell nanostructures showed excellent electrochemical performance. Zhou's group¹³⁷ have developed a simple hydrothermal route and subsequent post-sintering to fabricate a novel MWCNTs–V₂O₅ composite with nanosized architecture. During the hydrothermal reaction, protonated hexadecylamine acted as an intermediator. The unique porous nanoarchitecture of MWCNTs–V₂O₅ provided a large specific surface area and a good conductive network, which facilitated fast lithium ion diffusion and electron transfer. Additionally, the uniformly dispersed MWCNTs conducting network also behaved as an effective buffer that could relax the strain generated during charge–discharge cycles. The as-prepared sample exhibited an initial specific capacity of 402 mA h g⁻¹, and it could remain 222 mA h g⁻¹ after 50 cycles.

3.3 V₂O₅ with graphene

The use of graphene can also enhance the cycling rate and electrochemical property of V₂O₅. As an important 2D carbonaceous material, graphene has quickly become an important focus of material science. It has good flexibility, high surface area, and excellent thermal and chemical stability and electrical conductivity. Compared with other carbon materials like amorphous carbon, graphite, and other carbon nanomaterials, graphene nanosheets (GNSs) can provide a higher buffer property for the deformation of active anode materials due to their flexibility reaction path and high volume expansion area; graphene also exhibits a better dispersion in nanotubes and nanoparticles.¹³⁹

Recently, Chen et al.¹⁴⁰ successfully prepared reduced graphene oxide (rGO)-encapsulated V₂O₅ nanocomposites by co-assembly between negatively charged GO and positively charged oxide nanorods (Fig. 14). The process was driven by the mutual electrostatic interactions of the two species and was followed by thermal reduction. The as-synthesized nanocomposites possessed flexible and ultrathin rGO shells that effectively enwrapped the oxide nanorods. After several electrochemical tests, the nanocomposites presented excellent results. In the voltage window of 2.0–4.0 V, the cathode achieved an initial charge and discharge capacity of 287 mA h g⁻¹ and 207 mA h g⁻¹ during the first and 50th cycles at a current density of 100 mA g⁻¹, respectively. Cheng's group¹⁴¹ reported a simple solvothermal method to directly self-assemble V₂O₅ nanosheets on reduced graphene oxide (rGO). The V₂O₅ nanosheets/rGO hierarchical nanocomposites exhibited high reversible capacity and good rate capability compared to the bulk material by taking advantage of the synergetic effect of the two components.

Fig. 14 Schematic of the fabrication of rGO enwrapped V₂O₅ nanorods.¹⁴⁰

Sun et al.¹⁴³ developed a hydrothermal method to fabricate hydrated vanadium pentoxide nanoribbons modified with reduced graphene oxide (rGO). The intertwining network structure provided efficient electron conduction pathways and short Li⁺ diffusion distances. Su and co-workers¹⁴⁷ proposed a hydrothermal method to synthesize flexible V₂O₅ nanowires and then V₂O₅/graphene composites were prepared via the dry-freezing step. It was found that the V₂O₅ nanowires were uniformly decorated on the surface of the graphene sheets. The unique nanocomposite expectedly exhibited an enhanced performance compared to pure V₂O₅ nanowires and V₂O₅/graphene composites. Chen et al.¹⁴⁸ developed a facile solvothermal approach to synthesize the composite of V₂O₅ nanoparticles anchored on graphene. The as-prepared sample showed higher capacity and better cyclic capacity retention compared to bare V₂O₅, delivering a high reversible capacity of 153 mA h g⁻¹ after 100 cycles.

4. V₂O₅-Metal composites

In order to overcome the shortcomings encountered in the application of V₂O₅ and improving its electrochemical performance, in addition to the above-mentioned nanocrystalline composites with different carbonaceous supports, cation doping has been an effective approach to improve lithium ion intercalation in V₂O₅, mainly because the introduction of alien ions into the V₂O₅ lattice may tune the ion occupation and the electronic structure of the host material.¹⁴⁹ Recently, utilization of metal nanoparticles to improve the electrical conductivity has attracted extensive attention. Crystal structure stability is important for the V₂O₅ cathode, which can be stabilized by doping of metallic elements such as Al,^150,151 Cu,^152,158,163 Fe,¹⁵³ Cr,¹⁵⁴ Mn,^155,156,159 Sn,^157,164 Ag,¹⁶¹ Mo,¹⁶⁰ and Ni.¹⁶² Table 5 summarizes the electrochemical properties of the cation doped V₂O₅ samples.

Table 5 Electrochemical performance of cation-doped V₂O₅ samples for Li-ion battery cathode materials

Precursor material	Doped metal	Initial capacity/mA h g^−1	Current density/mA g^−1	Cycles (N)/capacity (mA h g^−1)	References
Vanadyl acetylacetonate	Al	250	35	50/157.5	150
V2O5 powders	Al	216	150	50/162	151
V2O5 powders	Cu	229	100	60/160	152
NH4VO3	Fe	255	58.8	48/195	153
V2O5 powders	Cr	About 272	29	50/200	154
V2O5 powders	Mn	138	680	50/135	155
Sodium metavanadate	Mn	120	50	30/about 120	156
NH4VO3	Sn	251.1	200	50/212	157
NH4VO3	Cu	196	300	70/186	158
VOSO4·3H2O	Mn	251	300	50/201	159
V2O5 powders	Mo	203	44.1	50/about 92	160
V2O5 powders	Ag	266.1	250	50/276.4	161
NH4VO3	Ni	262	300	50/238	162
V2O5 powders	Cu	266	58.8	50/226	163
V2O5 powders	Sn	350	500	40/355	164

---

Wei et al.¹⁵² proposed a precipitation method followed by heat treatment to synthesize crystalline Cu-doped V₂O₅. Cu_0.04V₂O₅ showed better electrochemical performance than V₂O₅ because of its high electronic conductivity and good structural stability. Li and co-workers¹⁵³ employed electrostatic spray deposition technique to synthesize 3D porous Fe_0.1V₂O_5.15 thin films. The thin films were composed of rather porous spheres with diameters around 8 and 10 μm for the Fe-doped samples. In the voltage window of 2.0–4.0 V, the cathode obtained an initial charge and discharge capacity of 255 mA h g⁻¹ and 195 mA h g⁻¹ during the first and 48th cycles at a current density of 58.8 mA g⁻¹, respectively. We could ascribe the better cycling performance of the Fe_0.1V₂O_5.15 electrode to the improved stability of its layered structure. Li's group¹⁵⁷ developed a microwave-assisted solvothermal synthesis to obtain a series of Sn-doped V₂O₅ microspheres. The doping of Sn⁴⁺ also induced the formation of oxygen vacancies that would allow more active sites for the intercalation/extraction reactions of Li⁺. Therefore, the as-prepared sample showed excellent electrochemical properties. It could be attributed primarily to the lattice expansion upon doping that led to higher lithium diffusion coefficient than that of V₂O₅ as well as the presence of oxygen-rich vacancies.

Zou and co-workers¹⁵⁸ developed a simple chemical reaction combined with ultrasonic mixing to synthesize nanostructured composites of V₂O₅ spheres decorated by electric Cu NPs. The decorated Cu NPs resulted in good contact with active materials and facilitated transportation of the electron into the inner region of the electrode. As expected, the V₂O₅/Cu cathodes could afford an obviously better electrochemical performance compared to pure V₂O₅, with a high reversible capacity of 186 mA h g⁻¹ after 70 cycles under a current density of 300 mA g⁻¹ and good rate performance. Zheng's group¹⁶² developed a facile template-free nickel-mediated polyol process to prepare a high-performance V₂O₅ hollow microsphere for LIBs, in which nickel acted not only as the mediator to tailor the interior hollow structures of V₂O₅ and build a favorable 3D hierarchical nano-micron combined architecture, but also as doping units to tune vanadium valence states and improve lithium storage properties. Similarly, V₂O₅ doped with Cu²⁺ demonstrated excellent cycling stability and rate capability owing to its modified electronic conductivity and improved structural stability.¹⁶³ Li et al.¹⁶⁴ developed a sol–gel method to synthesize the homogeneous Sn-doped V₂O₅ sol. Preliminary three-electrode tests of the Sn-doped V₂O₅ revealed good cycling performance. It was believed that Sn⁴⁺ would occupy the interstitial positions between VO₅ slabs and form SnO₆ octahedra with oxygen, leading to a slightly expanded lattice that facilitated Li⁺ intercalation/extraction.

5. Conclusion and outlook

We attempt to provide a comprehensive review about V₂O₅-based nanomaterials as cathode materials for lithium-ion batteries. Starting with phase-pure V₂O₅, we have covered a wide range of nanostructures, from 1D nanorods/nanowires/nanotubes/nanofibers/nanobelts to 2D nanosheets and 3D hollow and porous structures. For each category, we have discussed about the synthesis and lithium storage properties. For the 1D nanostructured materials, the most common synthesis methods are the hydrothermal method and sol–gel method. Hence, their preparation process is simple and feasible, and the requirements of the experimental conditions are not too high. For 2D V₂O₅ nanosheets, the most common synthesis method is the hydrothermal method. Template-free and template-based methods are the main approaches for the synthesis of V₂O₅ hollow and porous structures. However, all three categories of V₂O₅ nanomaterials demonstrate significantly better electrochemical properties than bare V₂O₅ nanoparticles. These can be attributed to the following aspects. First, the nanostructures promote sufficient contact between the electrolyte and active materials. Hence, the nanostructures facilitate rapid electron transfer and mostly avoid the generation of high contact resistance, like a nanoparticle aggregated structure, thus ensuring satisfactory capacity retention even at high current densities. Finally, the existence of pyrolysed carbon improves the conductivity of the one-dimensional nanostructures.

Nanocomposites consisting of V₂O₅ and different carbonaceous supports have also been surveyed. Amorphous carbon, graphene, and carbon nanotubes are widely used as supports for electroactive V₂O₅. In such systems, V₂O₅ is usually decorated on graphene and CNTs, while amorphous carbon is hydrothermally coated on pre-synthesized V₂O₅ nanostructures. As potential cathode materials for LIBs, the V₂O₅ and different carbonaceous composites exhibit highly reversible capacities and better cycling performance. This excellent electrochemical performance is ascribed to their unique structures, which improve electrolyte infiltration and facilitate Li ion diffusion in the electrode. In addition, cation doping of lithium-ion battery (LIBs) cathode materials is beneficial to both ion diffusion and charge transfer in the electrochemical intercalation processes and hence may improve battery rate capability. As the cathode material for lithium-ion batteries, the cation-doped V₂O₅ samples exhibit better electrochemical performance compared than undoped ones.

By synthesizing V₂O₅ nanomaterials of various structures, low electrical conduction, slow Li⁺ diffusion and irreversible phase transitions upon deep discharge can be alleviated to a certain extent. The cycling performance can be further improved effectively via creating unique nanocomposites. The significant progress achieved in the past decade re-assures the promising use of V₂O₅ as the cathode material in high-performance lithium-ion batteries. It is reasonable to confirm that V₂O₅-based nanocomposites with good electrochemical properties will find wide use in high-performance electrochemical energy storage devices in the near future. Nonetheless, it will still take a long time before metal oxide-based positive electrodes can be incorporated into commercial lithium-ion batteries. While the fundamental electrochemical processes are well understood for V₂O₅-based materials, the breakthrough will likely come from the delicate design and synthesis of nanocomposite electrode materials. Future research needs to deeply understand the structural evolution during cycling, possibly through in situ observation techniques and elaborate analysis of their atomic structure.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by a grant from the National Natural Science Foundation of China (No. 61504080 and No. 51676130).

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