Research progress on vanadium-based cathode materials for sodium ion batteries

Abstract

Sodium ion batteries (SIBs) have attracted increasing attention as one of the most promising candidates for cost-effective, high-energy rechargeable batteries. Owing to their high theoretical capacity and energy density, and rich electrochemical interaction with Na⁺ (V²⁺–V⁵⁺), a large number of vanadium(V)-based cathode materials, including vanadium oxides (e.g., V₂O₅ and VO₂), vanadium bronzes (e.g., Na_xVO₂, NaV₃O₈, NaV₆O₁₅ and δ-NH₄V₄O₁₀), V-based phosphates (e.g., Na₃V₂(PO₄)₃, VOPO₄, NaVOPO₄, Na₇V₃(P₂O₇)₄ and Na₂(VO)P₂O₇) and F-containing V-based polyanions (e.g., NaVPO₄F, Na₃V₂(PO₄)₂F₃ and Na₃(VO_x)₂(PO₄)₂F_3−2x), have been explored for SIBs. In this review, we mainly summarize the basic structures, modified/optimized structures, synthetic methods and morphology control of V-based cathode materials for SIBs. Additionally, major drawbacks, emerging challenges and some perspectives on the development of V-based cathode materials for SIBs are also discussed.

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1. Introduction

In recent years, efficient energy storage systems (ESSs) have attracted significant attention with the increasing exploration of clean and renewable energy sources (e.g., solar, wind, tide, hydropower, biomass and geothermal energy).¹ Lithium ion batteries (LIBs) have dominated the market of portable and smart electronic devices for nearly three decades. This is mainly due to their high energy and power densities, long cycle life and environmental friendliness.² However, owing to the resource shortage, high cost and geographic distribution of Li in the Earth's crust, a candidate for LIBs is needed.³ Alternatively, sodium ion batteries (SIBs) have recaptured attention mainly due to the abundance of Na (2.83 wt% vs. 0.01 wt% for Li), as well as similar chemical properties of Na to Li (the alkali metal family) and electrochemical mechanisms of SIBs to LIBs.⁴ Therefore, SIBs have been considered as an alternative to LIBs for ESSs in the future.

The initial research on room-temperature SIBs dates back to the 1980s,⁵ not far behind that of LIBs initially developed in the 1970s. However, the studies on SIBs were almost interrupted in the following three decades. This is mainly attributed to the lower energy density of SIBs than LIBs, and the safety issues caused by the use of Na in SIBs. The former results from the higher potential of Na (∼0.3 V, vs. Li⁺/Li) and higher mass of the Na counterpart (∼3 times heavier than Li). The latter is due to the low melting point (∼97.7 °C) of Na, Na dendrite formation and interface aging problems. On the other hand, the exploration of available electrodes for SIBs that could accommodate Na⁺ with the larger radius of 1.02 Å (vs. 0.76 Å for Li⁺) during the discharge–charge process, especially the anode materials, was also a big challenge.⁶ For example, only a few studies focused on lead⁷ and hard carbons⁸ were reported. The reported electrochemical performance of these electrode materials was also far from expectation. Recently, the smart design and nano-engineering of electrode materials for high-performance LIBs are being applied for SIBs.⁹ However, similar to LIBs, SIBs are often limited by the energy and power densities of cathode materials.¹⁰ The exploration and optimization of cathode materials with a suitable voltage window, high reversible capacity and stable structure are of significant importance toward the applications of SIBs.¹¹

To date, a large number of cathode materials for SIBs, including layered transition-metal oxides, transition-metal fluorides, phosphates, fluorophosphates, pyrophosphates, sulphides, sulphates, Prussian blue and organic polymers, have been explored and well summarized in some reviews.¹² Among these reported cathodes, vanadium(V)-based cathode materials have attracted more attention due to their high operating voltages, theoretical capacities and energy densities (Fig. 1), as well as their rich electrochemical reaction with Na⁺. V (atomic no. 23 in the periodic table) from VB group has a valence electron layer of 3d³4s². All the five valence electrons of V atom can take part in bonding, thereby giving rise to a multivalent V from V²⁺ to V⁵⁺. Apart from the rich electrochemical properties of V (V²⁺ to V⁵⁺), the low price and abundant source also make V-based materials promising cathode candidates for SIBs. To date, the reported V-based materials can be mainly divided into four categories: vanadium oxides (e.g., V₂O₅ and VO₂), vanadium bronzes (e.g., Na_xVO₂, NaV₃O₈, NaV₆O₁₅ and δ-NH₄V₄O₁₀), V-based phosphates (e.g., Na₃V₂(PO₄)₃, VOPO₄, NaVOPO₄, Na₇V₃(P₂O₇)₄, Na₂(VO)P₂O₇) and F-containing V-based polyanions (e.g., NaVPO₄F, Na₃V₂(PO₄)₂F₃ and Na₃(VO_x)₂(PO₄)₂F_3−2x). In this review, we will summarize the basic structures, modified/optimized structures, synthetic methods and morphology control of V-based cathode materials for SIBs. In addition, major drawbacks, emerging challenges and some perspectives on the development of V-based electrode materials for SIBs will also be discussed.

Fig. 1 Average discharge potential (V, vs. Na⁺/Na) as a function of theoretical capacity (mA h g⁻¹) and energy density (W h kg⁻¹) of representative V-based materials as cathodes for SIBs.

2. Vanadium oxides

For the construction of vanadium oxides, the multiple oxidation states of V, adjusting various coordination polyhedra and abundant two-dimensional structural frameworks are key factors.¹³ Generally, two-dimensional (2D) layered structures of vanadium oxides are formed by two units of [VO₆] octahedra sharing corners, faces and/or edges. The [VO₆] octahedra tend to be distorted into a square pyramid with a long V–O bond (2.1–2.79 Å, e.g., 2.79 Å for V₂O₅) and a short V=O bond (1.55–1.75 Å). As a result of this distortion, many layered structures appeared to form vanadium oxides. These layered structures together with large interlayer spacing are beneficial to accommodate Na⁺ and facilitate Na⁺ diffusion.¹⁴

2.1. V₂O₅

V₂O₅ possesses a typical layered structure. The single-layered V₂O₅ is formed by two units of “double chain of edge-sharing VO₆ octahedra” connected to each other.^14a Within an α-V₂O₅ (orthorhombic, space group Pmmn) crystal structure, one of the terminal bonds (weak V–O bond) is shorter than other V–O bonds, which causes the vanadium coordination polyhedron to form a square pyramid (Fig. 2a). Due to the corner sharing of the double chains, adjacent [VO₅] pyramids alternate in an up-up-down-down arrangement with every third row being vacant, as shown in Fig. 2b. On the other hand, because each single-layer of V₂O₅ is interconnected by the weak V–O bond, the interlayer spacing can be adjusted by intercalate species (e.g., organic or inorganic molecules) to form double layer δ-M_xV₄O₁₀ (Fig. 2c).^14a,15 When the intercalate species are removed, the δ-M_xV₄O₁₀ structure collapses to form metastable vanadium dioxide VO₂(B) (Fig. 2d).^14a

Fig. 2 Crystal structures of vanadium oxides: (a) single layer and (b) layered-structure of orthorhombic V₂O₅, (c) δ-M_xV₄O₁₀, (d) VO₂(B).

The layered V₂O₅ as a host material for Na⁺ storage was initially proposed early in 1988,¹⁶ however, very few reports focused on the study of the sodium storage properties of V₂O₅ in the following decades.¹⁷ Recently, V₂O₅ has been recaptured as a cathode material for SIBs due to its high theoretical capacity, as well as tremendous progress achieved towards its application in LIBs. Additionally, Mai's group investigated the Na⁺ storage mechanism of α-V₂O₅ on the basis of ex situ X-ray diffraction (XRD) measurements and Fourier transform infrared (FTIR) spectroscopy.¹⁸ They found that the initial phase transformation from α-V₂O₅ to NaV₂O₅, corresponding to a discharge capacity of 170 mA h g⁻¹, was not completely reversible after subsequently charging back to 4.0 V. The irreversible phase change was suggested to be responsible for the quick capacity fading of α-V₂O₅ in the following cycles. Ali et al. also employed ex situ XRD and near edge X-ray absorption fine structure (NEXAFS) spectroscopy to study the Na⁺ insertion/extraction behavior of V₂O₅.¹⁹ They revealed that the full-discharged products of V₂O₅ were both NaV₂O₅ and Na₂V₂O₅, indicating an oxidation state mixture of V (V⁴⁺/V⁵⁺) within the discharge product (Fig. 3a and b). Based on the redox reaction of V (V⁴⁺/V⁵⁺), V₂O₅ almost reached its theoretical capacity of ∼235 mA h g⁻¹ (Fig. 3c). During the Na⁺ insertion process, V₂O₅ retained the high integrity in its structure, as evidenced by the increase of its lattice parameter c (also the unite cell) by only 9.09% after the full-sodiation process. The clear Na⁺ storage mechanism of α-V₂O₅ inspires more researchers to explore related studies.

Fig. 3 (a) XRD patterns and (b) NEXAFS of V₂O₅/C at the fully charged and discharged states. (c) Discharge–charge profiles and crystal structures (inset of (c)) at different voltage steps of V₂O₅/C. Reproduced with permission.¹⁹ Copyright 2016, American Chemical Society.

Although the layer spacing of orthorhombic V₂O₅ (4.37 Å) is conducive for Li⁺ insertion/extraction, it is difficult for Na⁺ because of the larger radius of Na⁺ than Li⁺.^17a,20 Moreover, the intrinsic low conductivity and diffusion coefficient of Na⁺ and large volume expansion also make V₂O₅ exhibit poor electrochemical performance.²¹ To address these issues, many strategies have been developed, including the morphological control of V₂O₅, modification of the crystal structure, surface and crystallinity of V₂O₅, and the hybridization of V₂O₅ with other conductive agents.

First of all, in order to mostly expose the active (110) plane of V₂O₅ to the electrolyte, Su et al.²² realized orthorhombic V₂O₅ hollow nanospheres with more exposure of (110) planes achieving good rate capability (e.g., 93.1 mA h g⁻¹ at 320 mA g⁻¹ after 100 cycles). Moreover, V₂O₅ with various other nanostructures also presented improved performance, like V₂O₅ nanobelts. V₂O₅ nanobelts with reduced particle size and carbon coating were prepared by an ambient dissolution–recrystallization process. Compared to the commercial V₂O₅ powder, orthorhombic V₂O₅ nanobelts exhibited a higher discharge capacity of 216 mA h g⁻¹ at 0.5C.²³ Furthermore, orthorhombic V₂O₅ encapsulated in nanoporous carbon, prepared by ambient hydrolysis deposition with a subsequent hydrolysis reaction, also delivered a high discharge capacity of 276 mA h g⁻¹ at 40 mA g⁻¹.²⁴ The morphology control realizes the performance improvement of orthorhombic V₂O₅via the exposure of the active plane. Solution-phase synthetic methods are efficient to control the morphology of V₂O₅ nanostructures, however, the yield is usually low. Therefore, it is important to explore efficient methods for the morphological control of V₂O₅ for practical applications.

On the other hand, several studies have demonstrated that bilayered-V₂O₅ (with a larger interlayer spacing, and also belonging to an orthorhombic system) exhibited better Na⁺ storage performance than normal orthorhombic V₂O₅ due to the larger interlayer spacing of bilayered-V₂O₅.²⁵ The large interlayer spacing of (001) provides bilayered-V₂O₅ with open channels for facile Na⁺ insertion and extraction. A bilayered-V₂O₅ electrode was prepared by using chemical deposition from VOSO₄ solution on Ni foil, followed by a vacuum annealing process.²⁶Fig. 4 shows the short/long range order of the bilayered-V₂O₅, which was well characterized by high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), X-ray absorption near-edge spectra (XANES), and small and wide angle X-ray scattering (SAXS and WAXS). As shown in Fig. 4a, the bilayered-V₂O₅ showed void space between randomly spaced bilayers with much larger layer spacing (13.5 Å) than orthorhombic V₂O₅ (4.4 Å), as well as short-range order and shorter V=O bond. As a result, sodium insertion induces the formation of long- and short-range order in the bilayered-V₂O₅ structure, whereas sodium deintercalation causes the loss of long-range order and preserves the short-range order (Fig. 4b). When measured at 1.5–3.8 V (vs. Na⁺/Na), the bilayered-V₂O₅ delivered a high initial discharge capacity of 250 mA h g⁻¹ at 20 mA g⁻¹ and showed an initial capacity retention of 85% after 320 cycles. Another study also indicated that the interlayer spacing of bilayered-V₂O₅ nanobelts could be adjusted up to 11.53 Å (Fig. 4c). After the uptake of Na⁺, the interlayer spacing can be further expanded to 15.35 Å.²⁷ Benefiting from the enlarged interlayer, the bilayered-V₂O₅ nanobelts displayed promising rate capability (e.g., reversible capacities of 231.4 mA h g⁻¹ at 80 mA g⁻¹ and 134 mA h g⁻¹ at 640 mA g⁻¹), as shown in Fig. 4d. The enlarged interlayer spacing endows bilayered-V₂O₅ with excellent rate capability. Therefore, it is necessary to explore the synthetic methods for preparing bilayered-V₂O₅ for cost-effective high-power SIBs. Apart from the crystalline V₂O₅ as electrode materials for SIBs, disordered or amorphous V₂O₅ also showed promising electrochemical performances.²⁸ This is attributed to the isotropic characteristics of amorphous materials with a short-range order, which provide V₂O₅ with abundant-opening ion diffusion channels. In addition, amorphous materials can also be easily obtained at low temperature and in a short-time process.²⁹ For example, the amorphous V₂O₅ electrode on Ni foam exhibited high reversible capacity of 241 mA h g⁻¹ at the 2^nd cycle and retained 160 mA h g⁻¹ after 100 cycles.³⁰ Despite the high capacity obtained, the cycling stability of amorphous V₂O₅ electrode should be further improved.

Fig. 4 (a) HRTEM images and SAED pattern of V₂O₅. (b) SAXS (right) and WAXS (left) spectra of V₂O₅: electrochemically deposited vacuum annealed sample (blue) discharged at different currents of 630 μA (black), 120 μA (gray), 20 μA (light gray); and then charged at 120 μA (red). Inset of (b): model structures and interlayer spacing depicting transformations occurring upon Na⁺ intercalation and deintercalation. (c) TEM image (top-left), HRTEM (top-right and bottom-left) images and geometrical model (bottom-right) and (d) rate capability at various current densities from 80 to 640 mA g⁻¹ of V₂O₅. (a, b) Reproduced with permission.²⁶ Copyright 2013, American Chemical Society. (c, d) Reproduced with permission.²⁷ Copyright 2012, American Chemical Society.

The layered structure endows V₂O₅ with an ability to intercalate various organic or inorganic composites to form intercalated V₂O₅. The electrochemical performance of intercalated V₂O₅ is highly dependent on the types and quantities of inserted species.^14b,31 The intercalated V₂O₅ xerogel with an interlayer spacing ranging from 8.8 to 13.8 Å, which usually originates from δ-H_xV₄O₁₀, favors large amounts of Na⁺ storage within the V₂O₅ xerogel.³² A V₂O₅·nH₂O (n = ∼0.55) xerogel with interconnected nanowire networks was prepared by a freeze-drying process.¹⁸ The amount of intercalated H₂O (n) largely determines the interlayer spacing of V₂O₅·nH₂O. Because large amounts of H₂O may be detrimental to the electrode performance, an appropriate amount of water is highly required. As a result, benefiting from the large xerogel interlayer spacing of 12.3 Å (n = ∼0.55) and the nanowire networks, the V₂O₅·0.5H₂O xerogel exhibited a higher initial discharge capacity of 338 mA h g⁻¹ at 0.05 A g⁻¹ and a high rate capacity of 96 mA h g⁻¹ at 1.0 A g⁻¹, which are comparable to those of orthorhombic V₂O₅. Both cyclic voltammetry (CV) measurements and theoretical calculation indicate that the open layer structure effectively ensures rapid Na⁺ diffusion and also provides V₂O₅ extra pseudocapacity. Nevertheless, the severe “lattice breathing” of V₂O₅ xerogel could cause the shrinkage and collapse of the layered V₂O₅ structure, leading to the deactivation of V₂O₅ and fast capacity decay.^15,33 Besides, the conductivity of V₂O₅ xerogel (10⁻²–10⁻³ S cm⁻¹) is also relatively low.³⁴ Metal ions (e.g., Li⁺, Na⁺, Mg²⁺, Cu²⁺ and Zn²⁺) are also often intercalated within the V₂O₅ xerogel to improve the conductivity and stability of the V₂O₅ xerogel, leading to the high-performance of intercalated V₂O₅.^14a,35 In particular, δ-Na_xV₂O₅ nanowires were prepared by a chemical pre-intercalation synthesis approach with the use of orthorhombic α-V₂O₅ as the precursor, displaying a high discharge capacity of 365 mA h g⁻¹ at 20 mA g⁻¹.³⁶ Therefore, it is important to explore optimized methods to realize the electrochemically stable V₂O₅ xerogel with high conductivity for high-energy/power cathode materials.

V₂O₅ is a promising cathode material with high capacity, as well as easy synthesis, however, several issues should still be addressed. First of all, how to delicately pre-sodiate V₂O₅ in practical full-cells is a great challenge when V₂O₅ and its composites do not contain sodium. Moreover, it is easy to synthesize V₂O₅ without using V⁵⁺ based salts as the precursors, however, it is not easy to achieve highly desirable performance through morphological and crystalline control as well as the intercalation of inserted species. Furthermore, the cycling stability of amorphous or intercalated V₂O₅ should also be considered in practical applications.

2.2. VO₂

Although VO₂ has a similar layered structure to V₂O₅, there are no vacancies in VO₂, which is composed of layers of V₂O₄ pyramids with the up-down-up-down chess-board-like arrangement (Fig. 2d). There are four phases for VO₂: VO₂(R), VO₂(M), VO₂(A) and VO₂(B). Among these phases, metastable VO₂(B), which can be converted irreversibly to the thermodynamically stable VO₂(M) or VO₂(R) above 300 °C,³⁷ has been widely investigated in the electrochemistry field. Within the VO₂(B) crystal structure, the double layers of V₄O₁₀ share corners to form a shear-type structure containing a one-dimensional (1D) tunnel.³⁸ Owing to the favorable characteristics of this 1D tunnel parallel to the [010] crystal orientation for insertion/extraction of Na⁺, VO₂(B) has been investigated as the cathode for SIBs.³⁹ During the Na⁺ intercalation process, the edge sharing along the direction of [010] increases the resistance to V–O bond cleavage, thereby effectively preventing the structural collapse.

To reveal the Na⁺ insertion/extraction mechanism of single crystalline VO₂ nanosheets, the theoretical calculation was performed based on the first-principles plane-wave pseudo-potential method implemented in the Cambridge Sequential Total Energy Package (CASTEP).⁴⁰ As shown in Fig. 5a, the phase transformation of Na_xVO₂ during the Na⁺ insertion process becomes stable until x = 1, as evidenced by the lowest formation energy of Na_xVO₂ with x = 1. From x = 0.25 to x = 1.0, the volume change of Na_xVO₂ is only 7%, indicating the excellent structural stability of VO₂ (Fig. 5b). The stable structure of VO₂ nanosheets makes it possible for Na⁺ transportation (x < 1.0) even at high current rates, with a theoretical capacity of 323 mA h g⁻¹ (i.e., VO₂ + Na⁺ + e⁻ = NaVO₂). It should be noted that the Na⁺ storage mechanism in the following cycles may be expressed as NaVO₂ ↔ Na_xVO₂ (0 < x < 0.5) + Na⁺ + e⁻ (Fig. 5c), as indicated by the X-ray photoelectron spectroscopy (XPS) measurements on the discharged product of Na_xVO₂ (0 < x < 0.5) after charging to 4 V. Similar to the cases of V₂O₅/VO_x,^41–43 the hybridization of nanosized VO₂ with carbon/graphene is an effective way to improve the electrochemical properties of VO₂.⁴⁴ For example, a “binder-free and flexible” electrode based on graphene quantum dots (GQDs) coated VO₂(B) arrays/graphene displayed a high initial discharge capacity of 306 mA h g⁻¹ at C/3 (i.e., ∼100 mA g⁻¹) in the voltage range of 1.5–3.5 V (vs. Na⁺/Na). At a high rate of 5C, the electrode still delivered a high capacity of 247 mA h g⁻¹ and retained a remarkable capacity retention of 91% after 500 cycles.⁴⁵ Similar to V₂O₅, delicate pre-sodiation of VO₂ for its application in full-cells is a great challenge. In addition, it also requires more strategies to effectively control the phases and valance of V in VO₂.

Fig. 5 (a) The formation energy and (b) cell volume of Na_xVO₂ as a function of x in Na_xVO₂. The insets of (b): atomic structures of Na_xVO₂ with x = 0.25, 1 and 1.25. (c) Schematic presentation of the Na⁺ insertion/extraction mechanism of VO₂. Reproduced and updated with permission.⁴⁰ Copyright 2014, Elsevier.

3. Vanadium bronzes

3.1. Na_xVO₂

Sodium layered oxides Na_xMO₂, (M = transition metal) are considered to be promising cathode candidates for SIBs mainly due to their simple layered structures, high theoretical capacities and easy synthesis routes.⁴⁶ Na_xMO₂ is built up of edge-sharing MO₆ octahedra. According to the stacking orientations of MO₆ octahedral layers along the c-axis direction, Na_xMO₂ can be classified into O3-, P2-, and P3-type phases.⁴⁷ The letters “O” and “P” represent the Na⁺ location site while the numbers “2” and “3” stand for distinguishable sodium layers. Electrochemical kinetics and performance are highly reliable on the phase of Na_xMO₂. This is because different contents of Na⁺ occupying various sites will result in different host structural stabilities. For example, Na⁺ within the O3-phase Na_xMO₂ (0.7 < x < 1) occupies octahedral sites and MO₂ layers stack in an ABCABC pattern (Fig. 6a). Different from that for O3-phase Na_xMO₂, MO₂ layer stacking in an ABBA pattern within P2-phase Na_xMO₂ occurs with x = ∼0.7 (Fig. 6b) and MO₂ layer stacking of P3-phase Na_xMO₂ with an ABBCCA pattern occurs with x = ∼0.5 (Fig. 6c).^12e Usually, O3-type Na_xMO₂ has the highest initial discharge capacity but poor cycling stability. This is due to the serious slab-gliding during the charge–discharge process. P3-phase Na_xMO₂ has the lowest capacity due to the smallest x. Compared to O3 type and P3-phase Na_xMO₂, P2-type Na_xMO₂ might be a better candidate.⁴⁸

Fig. 6 Typical structures of (a) O3-, (b) P2-, and (c) P3-phase layered oxides.

It is difficult to obtain single Na_xVO₂ phase without impurities due to the possible phase conversion between oxygen and V³⁺ with high reducing activity.⁴⁹ As a result, there were few studies on the structures of Na_xVO₂ and its related electrochemical properties.⁵⁰ Recently, O3-phase and P2-phase Na_xVO₂ cathode materials have been investigated. Hamani et al.⁵¹ reported that both P2-Na_0.7VO₂ and O3-NaVO₂ could reversibly accommodate 0.5 Na⁺ per unit formula at a voltage range of 1.6–1.7 V (vs. Na⁺/Na), giving rise to a reversible capacity of ∼120 mA h g⁻¹. Delmas' group⁵² prepared pure NaVO₂ by chemical reduction of NaVO₃ in a H₂ atmosphere and studied its phase transition behavior during the Na-deintercalation process. The Na-deintercalation process was reversible up to x = 0.5, corresponding to the capacity of 126.4 mA h g⁻¹. Single-phase Na_2/3VO₂ and Na_1/2VO₂ can be electrochemically obtained at ∼1.8 V and 2.3 V, respectively. Furthermore, they also studied the structural change of P2–Na_0.71VO₂ by in situ XRD measurements.⁵³ As shown in Fig. 7a, there are four single-phase-transformation steps observed during the sodium insertion/de-intercalation process (0.5 ≤ x ≤ 0.92). In situ XRD patterns (Fig. 7b) clearly show the presence of additional weak Bragg peaks in the 2θ-range of 23.4–23.8° (0.61 < x < 0.70), and 28.9–29.8° and 33.8–34.7° (0.53 < x < 0.57), indicating the existence of superstructures or modulated structures during the discharge–charge process. From the electrochemical point of view, however, the wide voltage variation and poor electrochemical performances of Na_xVO₂ still cannot satisfy the demand for practical application. Additionally, the control of phase purity is a requirement to realize the consistent performance of electrode materials, so the complexity in its phases also limits its applications.

Fig. 7 (a) Charge–discharge profile of Na_xVO₂ with the assigned region of biphasic domains and monophasic domains and (b) in situ XRD patterns of Na_xVO₂ recorded during the galvanostatic intermittent titration technique (GITT) experiments. (a, b) Reproduced and modified with permission.⁵³ Copyright 2013, Nature Publishing Group.

3.2. NaV₃O₈

Within the NaV₃O₈ crystal structure, Na⁺ occupies the octahedral sites between V₃O₈ polyhedral layers.⁵⁴ Na⁺ can act as pillar cations to maintain the superior stability of the layered structure and thus allow NaV₃O₈ with Na⁺ insertion to form Na_1+xV₃O₈. He et al.⁵⁵ for the first time reported the Na⁺ storage in NaV₃O₈ based on a single-phase charge–discharge mechanism: NaV₃O₈ + xNa⁺ + xe⁻ ↔ Na_1+xV₃O₈. In order to further improve the electrochemical performance of NaV₃O₈, the composites and morphology of NaV₃O₈ were studied. A core–shell NaV₃O₈@ppy composite showed a discharge capacity of 99 mA h g⁻¹ at 80 mA g⁻¹ after 60 cycles.⁵⁶ Na_1.25V₃O₈ nanowires with a hierarchical zigzag structure were synthesized by a simple topotactic intercalation method.⁵⁷ The hierarchical zigzag structure endows Na_1.25V₃O₈ with a short Na⁺ diffusion pathway and prevents Na_1.25V₃O₈ from structural collapse and self-aggregation during the Na⁺ insertion/extraction process. As a result, Na_1.25V₃O₈ displayed a high initial capacity of 158.7 mA h g⁻¹ at 200 mA g⁻¹ in the voltage range of 1.5–4.0 V, with a capacity retention of 95% after 200 cycles. The good reversibility of Na_1.25V₃O₈ was further revealed by in situ XRD measurements. At present, it remains challenging to precisely identify the occupancy site of Na⁺ in the insertion/extraction process and further determine the exact phase corresponding to each charge–discharge plateau.

Similar to the case of V₂O₅·nH₂O, Na₂V₆O₁₆·nH₂O with a typical layered structure is expected to provide large interstices between V₃O₈ layers to accommodate sodium ions. Deng et al. prepared a 1D nanostructured Na₂V₆O₁₆·nH₂O through a hydrothermal method.⁵⁸ Na₂V₆O₁₆·nH₂O as an anode for aqueous LIBs displays a low reversible capacity of only ∼45 mA h g⁻¹ and limited cycling performance in an aqueous full LIB using Na_0.44MnO₂ as the cathode. The poor electrochemical performance of Na₂V₆O₁₆·nH₂O is mainly due to its low electronic conductivity of 2.46 × 10⁻¹⁴ cm² s⁻¹ and the irreversible phase transition as evidenced by ex situ XRD. In order to achieve high performance Na₂V₆O₁₆·nH₂O for LIBs, how to stabilize the crystal structures of Na₂V₆O₁₆·nH₂O during the Na⁺ intercalation process in the initial cycles is important.

3.3. NaV₆O₁₅

To construct the NaV₆O₁₅ crystal structure, both VO₅ pyramid and VO₆ octahedra build up the ordered (V₂O₅)_x frameworks with a tunnel along the b axis for Na⁺ location.⁵⁹ Na⁺ can reversibly insert in/extract from the NaV₆O₁₅ nanorods along the b orientation. Several reports predicted that a maximum amount of 1.6 Na⁺ could be inserted into bulk NaV₆O₁₅, corresponding to a high theoretical capacity of 225 mA h g⁻¹.⁶⁰ So far, the obtained capacity of NaV₆O₁₅ is lower than the theoretical capacity, including NaV₆O₁₅ nanorods⁶¹ and NaV₆O₁₅ nanoflakes.⁶² For example, NaV₆O₁₅ nanorods showed a capacity of 142 mA h g⁻¹ at 20 mA g⁻¹ with a charge/discharge plateau at ∼2.5 V (vs. Na⁺/Na). Nevertheless, they still displayed reasonable rate capability due to the small size of the nanorods.

3.4. δ-NH₄V₄O₁₀

δ-NH₄V₄O₁₀ is formed by V₄O₁₀ layers accommodating NH₄⁺ between them (Fig. 8a).⁶³ Each V₄O₁₀ layer is composed of two single-layer V₂O₅ sheets with short V=O bonds (1.6–1.7 Å). The adjacent V₂O₅–V₂O₅ sheets are linked by long trans bonds V⋯O.⁶⁴ The interlayer spacing of δ-NH₄V₄O₁₀ is 6.8 Å, indicating the ability for the accommodation of not only Li⁺ but also Na⁺ (Fig. 8b).⁶³ Recently, δ-NH₄V₄O₁₀ with various structures (e.g., nanorods, nanoflowers and nanobelts) have been prepared by hydrothermal methods and used as cathodes for SIBs.⁶⁵ Fei et al.^65b reported δ-NH₄V₄O₁₀ nanorods with a high capacity of 186 mA h g⁻¹ at 40 mA g⁻¹ and good cycling stability in the voltage range of 1.5–3.5 V. Mitra's group⁶⁶ prepared δ-NH₄V₄O₁₀ nanobelts on a carbon coated Al-current collector (Fig. 8c). When it was used as a cathode for SIBs, the δ-NH₄V₄O₁₀ electrode showed a high reversible capacity of 190 mA h g⁻¹ at 200 mA g⁻¹ (Fig. 8d) and 120 mA h g⁻¹ at 1000 mA g⁻¹ after 50 cycles. Sarkar et al.⁶³ investigated the sodiation/desodiation mechanism of Na_xNH₄V₄O₁₀ by density functional theory (DFT) calculation. Theoretical cyclic voltammetry (CV) analysis indicated that Na_xNH₄V₄O₁₀ experienced multiple phase transformation in the voltage range of 1.5–3.5 V, especially with x > 1.0 at <1.88 V (Fig. 8e), which agrees well with previous experimental results. Moreover, it was found that the lattice volume of Na_xNH₄V₄O₁₀ decreased only by 3% at x = 1. When Na⁺ concentration increased from x > 1 to x = 3, the lattice volume of Na_xNH₄V₄O₁₀ increased by 17%, indicating the large distortion of NH₄V₄O₁₀ frameworks after large uptake of Na⁺.

Fig. 8 (a) Crystal structure of NH₄V₄O₁₀ with short apical V–O and long trans V–O bonds, and (b) scheme of NH₄V₄O₁₀ interaction with inserted ions (yellow solid spheres). (c) Field emission-scanning electron microscopy (FESEM) image and (d) initial charge–discharge profile of NH₄V₄O₁₀ at 200 mA g⁻¹. (e) Calculated potential of Na_xNH₄V₄O₁₀ with multiple phases in the voltage range of 1.5–3.5 V. (a, b and e) Reproduced with permission.⁶³ Copyright 2016, Royal Society of Chemistry. (c, d) Reproduced with permission.⁶⁶ Copyright 2015, American Chemical Society.

4. V-based phosphates

V-based phosphates have been widely considered to be one of the most promising types of cathodes for SIBs because of their high operating potential, excellent thermal and structural stability, and high theoretical capacity.⁶⁷ The distinguished features are derived from the inductive effect of the PO₄³⁻ polyanion and strong P–O bond, ensuring the high working potential vs. Na⁺/Na and the stable structure during the charge–discharge process.⁶⁸ To date, a large number of V-based phosphates have been investigated, including V-based single-phosphates (Na₃V₂(PO₄)₃, NaVOPO₄, VOPO₄), pyrophosphates (NaVP₂O₇, Na₇V₃(P₂O₇)₄, t-Na₂(VO)P₂O₇), fluorophosphates (NaVPO₄F, Na₃V₂(PO₄)₂F₃) and mixed-polyanions Na₇V₄(P₂O₇)₄(PO₄).

4.1. V-based single-phosphates

4.1.1. NASICON Na₃V₂(PO₄)₃. Due to the fast Na⁺ conduction and stable 3D framework structures with expedite channels for Na⁺ diffusion, NASICON (Na Super Ionic Conductor)-type Na₃M₂(PO₄)₃ (M = V, Fe, Ti) materials have been intensively studied as cathodes for SIBs.⁶⁹ In particular, Na₃V₂(PO₄)₃ displays a high working potential of 3.4 V (vs. Na⁺/Na) and a high theoretical capacity of 117.6 mA h g⁻¹.⁷⁰Fig. 9a and b show the crystal structure of Na₃V₂(PO₄)₃.⁷¹ As can be seen, three PO₄ tetrahedra and two VO₆ octahedra share corners along the c-axis direction to assemble a 3D covalent [V₂(PO₄)₃] skeleton. There are two types of interstitial spaces with different coordination environments for Na⁺ location: one position per formula unit defined as Na(1) and three positions per formula unit defined as Na(2).⁷² Due to the strong and stable P–O bond, the Na₃V₂(PO₄)₃ material showed remarkable thermal stability up to 450 °C even in the desodiated state, demonstrating its high safety feature during the cell operation.⁷³ In contrast, in the temperature range of −30–225 °C, Na₃V₂(PO₄)₃ experienced four reversible phase transitions, namely, α-, β-, β′- and γ-Na₃V₂(PO₄)₃.⁷⁴

Fig. 9 Schematic representation of the NASICON-Na₃V₂(PO₄)₃ structure in different direction-views: (a) Na1 site and (b) Na2 site. (c) CV curves of Na₃V₂(PO₄)₃/C in the voltage range of 1.0–3.8 V. (d) In situ XRD patterns of Na₃V₂(PO₄)₃/C in the voltage range of 2.7–3.7 V (♣ NaV₂(PO₄)₃, ♦ Na₃V₂(PO₄)₃). (a, b) Reproduced with permission.⁷¹ Copyright 2015, Royal Society of Chemistry. (c) Reproduced with permission.⁷⁵ Copyright 2011, Elsevier. (d) Reproduced with permission.⁷⁷ Copyright 2015, Wiley-VCH.

As shown in Fig. 9c, CV curves of the Na₃V₂(PO₄)₃ cell present two pairs of redox peaks at ∼3.40 and ∼1.63 V (vs. Na⁺/Na), corresponding to the redox reactions of V³⁺/V⁴⁺ and V²⁺/V³⁺, respectively.⁷⁵ It demonstrates the potential use of Na₃V₂(PO₄)₃ as both cathode and anode for SIBs. To date, the electrochemical reaction mechanism of the Na₃V₂(PO₄)₃ cathode has been deeply investigated.⁷⁶In situ XRD measurements revealed that a typical two-phase reaction between Na₃V₂(PO₄)₃ and NaV₂(PO₄)₃ occurred in the voltage range of 2.7–3.7 V (vs. Na⁺/Na) during the charge/discharge process (Fig. 9d).⁷⁷ Further analysis revealed that only Na occupying Na2 sites can be extracted from Na₃V₂(PO₄)₃ at room temperature. After two Na⁺ were extracted from Na2 sites, the skeleton structure of the host material is still preserved well with only a slight volume change.^72d Based on the Na⁺ extraction from Na₃V₂(PO₄)₃ to NaV₂(PO₄)₃, Na₃V₂(PO₄)₃ has a theoretical capacity of 117.6 mA h g⁻¹.⁷⁸ Furthermore, DFT calculation also indicated that Na⁺ might diffuse through two intra-layer pathways and one inter-layer pathway.⁷⁹ The activated barriers for both intra-layer pathway and inter-layer pathway are 353 and 513 meV, respectively, which indicates that the diffusion of Na⁺ through the inside Na layers is more preferable.

Yamaki's group for the first time reported the electrochemical performance of Na₃V₂(PO₄)₃ with a high specific capacity of 140 mA h g⁻¹ at a low current density of 0.1 mA cm⁻² in the voltage range of 1.2–3.8 V (vs. Na⁺/Na).⁸⁰ Since then, numerous effective strategies have been applied to modify Na₃V₂(PO₄)₃ to improve the intrinsic poor conductivity of Na₃V₂(PO₄)₃, including carbon decoration, downsizing the particles, designing tailored nanostructures and metal ion doping. First of all, carbon coating is one of the most preferred options for improving the electronic conductivity of electrode materials. Various carbon coated Na₃V₂(PO₄)₃ composites have been synthesized through solid state reactions,^75,77 self-combustion method,⁸¹ pre-reduction-calcination process,⁸² glucose-assisted carbon-thermal reduction,⁸³ C₂H₂-pyrolysis reduction,⁸⁴ ball-milling–annealing process,⁸⁵ ultrasonic spray pyrolysis process,⁸⁶ sol–gel assisted method⁸⁷ and so on. The as-reported Na₃V₂(PO₄)₃/C composites exhibited promising electrochemical performances as summarized in Table 1. The conductive carbon coating layer not only greatly enhances the electric conductivity of Na₃V₂(PO₄)₃/C but also effectively prevents Na₃V₂(PO₄)₃/C from severe aggregation. Jian et al.⁷⁷ enhanced the electrochemical performance of Na₃V₂(PO₄)₃/C through the control of carbon coating with different contents, as well as the use of different electrolyte systems. Together with the use of sodium bis(fluorosulfonyl)imides/propylene carbonate (NaFSI/PC) as the electrolyte, a 3.8 wt% carbon coating makes Na₃V₂(PO₄)₃ deliver a high discharge capacity of 107.1 mA h g⁻¹ at 0.1C with a high initial coulombic efficiency of 98.7%. In addition, doping carbon with non-metallic elements (e.g., N, B and S) has also been widely employed to enhance the conductivity of pristine carbon materials.^71,88 It was found that doping carbon with BC₂O and BCO₂ could create numerous extrinsic defects and active sites, which allowed more surface Na⁺ absorption and fast diffusion of Na⁺ through the carbon coated layer, thus improving the rate performance of the Na₃V₂(PO₄)₃–C–B composites.⁷¹ Furthermore, reducing the particle size can further improve the utilization of active materials and shorten the diffusion path of Na⁺. Combined with an amorphous carbon shell with the thickness of few nanometers, a particle size of ∼40 nm allowed the nano-sized Na₃V₂(PO₄)₃ to deliver a high specific capacity of 94.9 mA h g⁻¹ at 5C and superior capacity retention of 96.1% after 700 cycles.^87b Recently, due to the abundant channels for Na⁺/electron transportation, Na₃V₂(PO₄)₃ with 3D hierarchitectures has attracted great attention.⁸⁹ For example, combined with carbon coating, a 3D “foam-like” structure provides Na₃V₂(PO₄)₃ with a high specific capacity of 73 mA h g⁻¹ at 100C and excellent cycle performance (∼100% capacity retention after 1000 cycles at 100C).^89a In addition to the 3D “form-like” structure, a 3D honeycomb-type hierarchical microball also made Na₃V₂(PO₄)₃/C exhibit satisfactory electrochemical performance due to the synergistic effect of the hierarchical porous nanostructure and 3D continuous conductive framework.^89b

Table 1 Representative electrochemical properties of Na₃V₂(PO₄)₃ based materials as electrodes for SIBs

Electrode materials	Plateau voltage/potential range (V)	Specific capacity (mA h g^−1) (1C = 117 mA g^−1)	Rate capacity (mA h g^−1)	Capacity retention	Reference
Na3V2(PO4)3 foam	∼3.4, 2.5–3.8	112 at 1C	51 at 200C	99.5% after 100 cycles at 1C	89a
Hierarchical Na3V2(PO4)3 particles	∼3.4, 2.5–4.0	114.1 at 0.1C	95.6 at 5C	74% after 600 cycles at 1C	89c
Na3V2(PO4)3 nanoparticles	∼3.4, 2.5–4.0	83.71 at 0.1C	48.87 at 10C	74.3% after 100 cycles at 0.1C	82b
Mesoporous Na3V2(PO4)3/C	∼3.4, 2.8–3.8	98 at 0.1 A g^−1	63 at 1 A g^−1	74% after 450 cycles at 0.4 A g^−1	82a
Na3V2(PO4)3/C	2.5–3.8	100.72 at 0.1C	61.76 at 30C	95% after 50 cycles at 0.1C	81
Na3V2(PO4)3/C	∼3.4, 2.7–3.7	107.1 at 0.1C	—	93% after 80 cycles at 0.1C	77
Porous Na3V2(PO4)3/C	∼3.4, 2.7–4.0	118.9 at 0.05C	97.6 at 5C	88.6% after 100 cycles at 5C	87a
Na3V2(PO4)3/C porous hollow spheres	∼3.4, 2.3–3.9	99.2 at 20 mA g^−1	9.6 at 1 A g^−1	90% after 300 cycles at 20 mA g^−1	86
Na3V2(PO4)3@C porous microspheres	∼3.4, 2.2–3.8	112.8 at 0.2C	70 at 10C	90% after 400 cycles at 1C	90a
Hierarchical porous Na3V2(PO4)3/C microballs	∼3.4, 2.5–3.7	97.2 at 5C	80.2 at 20C	93.6% after 200 cycles at 1C	89b
Na3V2(PO4)3/C nanofibers	∼3.4, 2.7–4.0	112.5 at 0.1C	∼88.9 at 50C	∼93% after 300 cycles at 1C	92a
CNTs modified Na3V2(PO4)3/C	∼3.4, 2.5–3.8	107.6 at 0.5C	81.7 at 10C	93.9% after 500 cycles at 10C	96b
Na3V2(PO4)3/C nanofibers	∼3.4, 2.7–3.8	101 at 0.1C	20 at 20C	∼97% after 100 cycles at 2C	92c
Na3V2(PO4)3@C@graphene	∼3.4, 2.5–3.8	115 at 20C	86 at 100C	64% after 10 000 cycles at 100C	94a
Na3V2(PO4)3/C	∼3.4, 2.5–3.8	117 at 0.33C	65 at 2.67C	—	76b
Na3V2(PO4)3/C	∼3.4, 2.5–4.0	91.6 at 1C	93.9 at 2C	65% after 700 cycles at 2C	84
Na3V2(PO4)3/acetylene carbon	∼3.4, 2.3–3.9	117.5 at 0.5	97 at 5C	96.4% after 200 cycles at 5C	91
Na3V2(PO4)3@C	∼3.4, 2.5–3.8	104.3 at 0.5C	94.9 at 5C	96.1% after 700 cycles at 5C	87b
Layer-by-layer Na3V2(PO4)3@rGO	∼3.4, 2.5–4.0	118 at 0.5C	73 at 100C	70% after 15 000 cycles at 50C	95
3D-skeleton-carbon coated Na3V2(PO4)3	∼3.4, 2.5–4.0	113 at 1C	60 at 50C	87.5% after 8000 cycles at 2C	90b
Na3V2(PO4)3/nitrogen-decorated carbon	∼3.4, 2.5–3.8	110.9 at 0.2C	76.6 at 5C	—	88a
Graphene-encapsulated Na3V2(PO4)3	∼3.4, 2.5–3.8	115.2 at 0.2C	70.1 at 30C	86.0% after 300 cycles at 5C	93b
Graphene-supported Na3V2(PO4)3	∼3.4, 2.5–3.8	90.6 at 0.2C	60.4 at 30C	93% after 100 cycles at 1C	93a
Na3V2(PO4)3/C nanofibers	∼3.4, 2.5–4.0	106.8 at 0.2C	103.0 at 2C	∼100% after 125 cycles at 0.2C	92b
Double-nanocarbon-modified Na3V2(PO4)3	∼3.4, 2.5–4.0	94.5 at 0.2C	70 at 70C	87% after 300 cycles at 30C	96c
Na3V2(PO4)3/C/graphene	∼3.4, 2.3–3.9	116.8 at 0.1C	80 at 30C	96.45% after 100 cycles at 0.2C	88b
Na3V2(PO4)3@C/3D Gr	∼3.4, 2.5–4.0	112 at 0.2C	76 at 60C	82% after 1500 cycles at 40C	94b
Na3V2(PO4)3@C@CMK-3	∼3.4, 2.3–3.9	115 at 1C	81 at 30C	68% after 2000 cycles at 5C	97
Mg^2+-Doped Na3V2(PO4)3/C	∼3.4, 2.5–4.0	112.5 at 1C	94.2 at 30C	81% after 50 cycles at 20C	99b
Na3V2−xFex(PO4)3/C (0 ≤ x ≤ 0.5)	∼3.4, 2.0–4.3	115 at 0.5C	∼110 at 2C	∼100% after 40 cycles at 0.5C	99a
Na3V1.8Al0.2(PO4)3/C	∼3.4, 2.0–4.3	107 at 0.2C	96.8 at 6C	96% after 60 cycles at 0.5C	99c
Na2.91K0.09V2(PO4)3/C	∼3.4, 2.5–3.8	110.4 at 0.2C	81.7 at 5C	>80% after 200 cycles at 1C	98
Mn-Doped Na3V2−xMnx(PO4)3/C	∼3.4, ∼3.85, 2.0–4.3	104 at 0.5C	104 at 2C	—	99e
Li-Doped Na3−xLixV2(PO4)3/C	∼3.4, 2.0–4.5	117.6 at 5C	102 at 20C	—	100
B-Doping Na3V2(PO4)3/C	∼3.4, 2.7–4.0	93.8 at 0.2C	87.1 at 10C	98% after 40 cycles at 0.2C	101
K3V2(PO4)3/C bundled nanowires	∼1.6, ∼3.6, 1.5–4.0	119 at 0.1 A g^−1	∼45 at 2 A g^−1	99.0% after 2000 cycles at 1 A g^−1	102
Carbon coated Na3V2(PO4)3	∼3.4, 2.7–3.8	93 at 0.05C	89 at 0.2C	99% after 10 cycles at 5C	75
Na3V2(PO4)3 : rGO–CNTs	∼3.4, 2.3–4.0	∼117 at 1C	∼82 at 100C	∼96% after 2000 cycles at 10C	89d
Na3V2(PO4)3/C composite	∼3.4, 2.5–3.8	98.17 at 0.1C	76.44 at 30C	95.02% after 50 cycles at 0.1C	83
Porous sponge-like Na3V2(PO4)3/C	∼3.4, 2.5–4.0	101.88 at 1C	75.54 at 80C	92.5% after 500 cycles at 70C	103
Na3V2(PO4)3 nanofibers/C	∼3.4, 2.6–3.8	114 at 0.2C	76 at 40C	99.9% after 10 000 cycles at 20C	104
Na3V2(PO4)3/rGO nanocomposites	∼3.4, 2.7–3.9	113 at 100 mA g^−1	98 at 2 A g^−1	96.6% after 120 cycles at 100 mA g^−1	105
Hierarchical carbon framework wrapped Na3V2(PO4)3	∼3.4, 2–3.9	115 mA h g^−1 at 0.2C	38 at 500C	54% after 20 000 cycles at 30C	106
Na3V2(PO4)3/C nanocomposite	∼3.4, 2.0–4.2	106 at 20 mA g^−1	77 at 2 A g^−1	90% after 50 cycles at 20 mA g^−1	107
N, S, F-doped carbon coated Na3V2(PO4)3	∼3.4, 2.5–3.8	117.5 at 1C	93.4 at 10C	∼95% after 1000 cycles at 10C	108
Na3V2(PO4)3/elastic carbon foam	∼3.4, 2.5–4.0	∼117 at 0.5C	>60 at 20C	—	109
Porous Na3V2(PO4)3/C	∼3.4, 2.3–3.9	114 at 1C	106 at 10C	50% after 30 000 cycles at 40C	110

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Na₃V₂(PO₄)₃ supported on or embedded in highly conductive carbon matrixes (e.g., 0D–2D) also exhibited enhanced electrochemical kinetics.⁹⁰ For example, Mai's group dispersed Na₃V₂(PO₄)₃ on 0D acetylene carbon (AC) nanospheres, 1D carbon nanotubes (CNTs) and 2D graphite nanosheets.⁹¹ These results indicated that the Na₃V₂(PO₄)₃ nanograins on the 0D AC displayed the best electrochemical performance with a high initial discharge capacity of 117.5 mA h g⁻¹ at 0.5C and an excellent capacity retention of 96.4% at 5C after 200 cycles. Moreover, Na₃V₂(PO₄)₃ nanoparticles partly embedded in carbon nanofibers (CNFs), which were prepared by an electrospinning method,^92a showed a high discharge capacity of ∼88.9 mA h g⁻¹ at 50C and excellent capacity retention of ∼93% after 300 cycles at 1C. Furthermore, the use of 2D graphene to support or encapsulate Na₃V₂(PO₄)₃ (ref. 93) or wrap carbon coated Na₃V₂(PO₄)₃ (ref. 94) also displayed enhanced rate performances and cycling stability. As shown in Fig. 10a, the Na₃V₂(PO₄)₃@rGO composite with the “layer-by-layer” structure was prepared through modifying the surface charge of the gel precursor of Na₃V₂(PO₄)₃. Because of the confinement of Na₃V₂(PO₄)₃ nanocrystals in highly conductive rGO layers, the Na₃V₂(PO₄)₃@rGO composite showed high capacity (e.g., 118 mA h g⁻¹ at 0.5C), excellent rate capability (73 mA h g⁻¹ at 100C) and ultra-long cycle life (e.g., 70.0% capacity retention after 15 000 cycles at 50C, Fig. 10b and c).⁹⁵ Carbon coated nanosized Na₃V₂(PO₄)₃ embedded in a low-dimensional or 3D carbon matrix also delivered good electrochemical properties.⁹⁶ Yu's group reported that carbon-coated Na₃V₂(PO₄)₃ nanoparticles impregnated into 3D mesoporous conductive and interconnected CMK-3 achieved a high reversible capacity of 81 mA h g⁻¹ at 30C (Fig. 10d and e).⁹⁷

Fig. 10 (a) Schematic illustration of the formation process, (b) rate performance and (c) ultra-long cycling stability of the layer-by-layer Na₃V₂(PO₄)₃@rGO composite. (d) Schematic illustration of the synthesis process and the reversibility of electrochemical reactions, (e) rate performance of Na₃V₂(PO₄)₃@C@CMK-3 composites. (a–c) Reproduced with permission.⁹⁵ Copyright 2016, Wiley-VCH. (d, e) Reproduced with permission.⁹⁷ Copyright 2015, Wiley-VCH.

On the other hand, the introduction of metal ion dopants to Na₃V₂(PO₄)₃ makes Na₃V₂(PO₄)₃ display improved electrochemical performance. For example, the doping of K⁺ to the Na site of Na₃V₂(PO₄)₃ can greatly enhance the rate performance of Na₃V₂(PO₄)₃. This is due to the larger atomic size of K⁺ than that of Na⁺, which can significantly enlarge the lattice volume of Na₃V₂(PO₄)₃ along the c-axis and thus increase the Na⁺ diffusion pathway.⁹⁸ As a result, K_0.09–Na₃V₂(PO₄)₃/C presented a high reversible capacity of 110.4 mA h g⁻¹ at 0.2C and ∼82 mA h g⁻¹ at 5C. The metal (e.g., Fe, Mg, Al and Mn) doping or substitution to V can affect the electrochemical performance of Na₃V₂(PO₄)₃.⁹⁹ For example, the Mg²⁺ doping in Na₃V_2−xMg_x(PO₄)₃/C composites did not alter the host structure, which can effectively improve ionic/electronic conductivity, thus enhancing the rate capability.^99b,d

4.1.2. VOPO₄ and NaVOPO₄. VOPO₄ is composed of VO₆ octahedra sharing vertices with independent PO₄ tetrahedra. There are seven types of VOPO₄ crystal structures (i.e., namely, layered α_I, α_II, δ, ω, γ, and three-dimensional β, ε) which are mainly formed by the different orientations of the vanadyl bond.¹¹¹ Generally, the β and ε polymorphs have a 3D network with an open tunnel structure, ensuring their use as electrodes for SIBs with higher operating voltage than other polymorphs.¹¹² VOPO₄ has a theoretical capacity of 165 mA h g⁻¹. He et al.¹¹³ for the first time prepared layered VOPO₄ through the chemical delithiation of α_I-LiVOPO₄ and investigated its cathode properties for SIBs. The layered α_I-VOPO₄ demonstrated a reversible capacity of 110 mA h g⁻¹ at 0.05C with a long potential plateau at around 3.4–3.5 V. By incorporating highly conductive reduced graphene oxide (rGO), the specific capacity of α_I-VOPO₄ was significantly increased to 150 mA h g⁻¹, equivalent to 0.9 Na⁺ per formula. Very recently, Yu's group¹¹⁴ reported that VOPO₄ with ultrathin nanosheets could display a high rate capability of 70 mA h g⁻¹ at 5C, together with 136 mA h g⁻¹ at 0.1C. This is mainly ascribed to the ultrathin VOPO₄ with rapid Na⁺ diffusion channels, as well as the large capacity contribution from intercalation pseudocapacitance. Considering the layered VOPO₄ with excellent electrochemical properties, a Na₂Ti₃O₇/VOPO₄ cell was further assembled by Yu's group (Fig. 11).¹¹⁵ As a result, the full cell presented remarkable electrochemical performance with a high operating voltage of ∼2.9 V, high reversible capacity of 114 mA h g⁻¹ at 0.1C, ∼74 mA h g⁻¹ at 2C, excellent cycling stability (92.4% after 100 cycles) and high energy density of 220 W h kg⁻¹.

Fig. 11 (a) CV curve of VOPO₄ (orange) and Na₂Ti₃O₇ (blue). (b) Schematic presentation of the potential of VOPO₄, Na₂Ti₃O₇ and the Na₂Ti₃O₇//VOPO₄ full cell. (c) Representative charge–discharge profiles of VOPO₄ (orange) and Na₂Ti₃O₇ (blue). (d) Charge–discharge profiles of the Na₂Ti₃O₇//VOPO₄-cell at 10 mA g⁻¹. (e) The capacity retention and coulombic efficiency of the Na₂Ti₃O₇//VOPO₄ full cell at 100 mA g⁻¹. Reproduced with permission.¹¹⁵ Copyright 2016, Royal Society of Chemistry.

NaVOPO₄ has several different crystal structures, such as triclinic α, tetragonal α-1 and orthorhombic β-NaVOPO₄. The sodiation of VOPO₄ (Na_xVOPO₄) with x = 1 yields NaVOPO₄. The NaVOPO₄ framework is composed of chains consisting of corner-sharing VO₆ octahedra along the c axis in which the channels around each chain are filled by Na and P atoms.¹¹⁶ The framework offers NaVOPO₄ 3D tunnels for Na⁺ migration. The theoretical capacity of NaVOPO₄ is 145 mA h g⁻¹ (1 Na⁺ per formula unit).¹¹⁷ A series of β-Na_xVOPO₄ (x = 0, 0.1, 0.3, 0.5, 0.8 and 1.0) were prepared by a microwave-assisted solvothermal process.¹¹⁸ From the experimental result, β-Na_xVOPO₄ (x = 0 and 1.0) as a cathode material for SIBs displayed flat charge–discharge plateaus at the voltage range of 3.3–3.4 V (vs. Na⁺/Na) and involved the V⁴⁺/V⁵⁺ redox reactions.¹¹⁸ However, β-NaVOPO₄ suffered from large irreversible capacity of each cycle for 50 cycles.

4.1.3. Sodium vanadium pyrophosphates. Due to their rich crystal chemistry, stable structure and good Na⁺ mobility, sodium vanadium pyrophosphates are also considered to be potential cathode candidates for SIBs.¹¹⁹ V-based pyrophosphates are composed of VO₆ and P₂O₇ groups connected to form stable 3D frameworks and large tunnels, which serve as ion diffusion pathways (Fig. 12a). In particular, tetragonal Na₂(VO)P₂O₇ was found to show a high operating potential of ∼3.8 V (vs. Na⁺/Na) with a reversible capacity of ∼80 mA h g⁻¹ at 0.05C, not far away from its theoretical capacity of 93.4 mA h g⁻¹ based on the redox reaction of V⁴⁺/V⁵⁺.¹²⁰ Recently, Kim et al.¹²¹ successfully synthesized Na₇V₃(P₂O₇)₄ and applied it as a cathode for SIBs. The Na₇V₃(P₂O₇)₄ electrode displayed a high discharge capacity of ∼80 mA h g⁻¹ at 0.1C, close to the theoretical capacity of 79.6 mA h g⁻¹ on the basis of the redox couple of V³⁺/V⁴⁺, and excellent cyclability (75% retention after 600 cycles at 1C) (Fig. 12b and c). Moreover, the Na₇V₃(P₂O₇)₄ electrode achieved a higher potential of 4.13 V (vs. Na⁺/Na) than Na₂(VO)P₂O₇, corresponding to a redox reaction of V³⁺/V⁴⁺. Unlike the synthesis of micro-size Na₇V₃(P₂O₇)₄ particles by a ball-milling method,¹²¹ Deng et al. simultaneously employed a sol–gel method to obtain highly phase-pure Na₇V₃(P₂O₇)₄ ultrafine particles.¹²² The Na₇V₃(P₂O₇)₄ ultrafine particles also showed good high rate capability with ∼92% initial capacity retention after 100 cycles at a higher rate of 10C. In addition, they further confirmed the redox reaction of Na₇V₃(P₂O₇)₄ using in situ XRD measurements,¹²² which displayed continuous change during the charge/discharge process (Fig. 12d).

Fig. 12 (a) Schematic illustration of the Na₇V₃(P₂O₇)₄ structure together with Na⁺ diffusion path. (b) Charge–discharge profile of Na₇V₃(P₂O₇)₄ with the calculated voltage (red dash line). Inset of (b): dQ/dV of the charge–discharge curve. (c) Cycling performance of Na₇V₃(P₂O₇)₄ at 1C for 600 cycles. Inset of (c): charge–discharge profiles of Na₇V₃(P₂O₇)₄ over 600 cycles. (d) Ex situ XRD patterns of Na₇V₃(P₂O₇)₄ at different voltage steps of the corresponding initial charge–discharge curve. (a–c) Reproduced with permission.¹²¹ Copyright 2016, Wiley-VCH. (d) Reproduced with permission.¹²² Copyright 2016 Elsevier.

4.2. V-based fluorophosphates

F-substitution to V-based phosphate cathodes for SIBs can significantly elevate the operating voltage of V-based phosphates due to the strong inductive effect of F anion.¹²³ Recently, several F-containing V-based polyanions, such as NaVPO₄F, Na₃V₂(PO₄)₂F₃ and Na₃(VO_x)₂(PO₄)₂F_3−2x, have been reported as cathode materials for SIBs.

4.2.1. NaVPO₄F. NaVPO₄F has two types of phases that exhibit good sodium storage performances: tetragonal symmetry structure (space group I4/mmm) and monoclinic structure (space group C2/c). The tetragonal NaVPO₄F phase structure is in agreement with its related counterpart, Na₃Al₂(PO₄)₃F₂ structure in which [Al₂O₈F₃] bioctahedra and [PO₄] tetrahedra are connected to each other by a corner-sharing O vertex. Na⁺ distributes through the produced cavities. The structure of monoclinic NaVPO₄F is similar to that of Na₃Al₂(PO₄)₂F₂, which is composed of two [PO₄] tetrahedra sharing two corner-oxygen atoms with two different [VO₄F₂] octahedra. Na⁺ is statistically filled in the 3D spatial channels.¹²⁴ Based on one Na⁺ per NaVPO₄F unit, the theoretical capacity of NaVPO₄F is 143 mA h g⁻¹. Barker et al. for the first time reported tetragonal NaVPO₄F, prepared by a direct incorporation reaction using VPO₄ and NaF at 750 °C, as the cathode for SIBs.¹²⁵ The phase transformation of NaVPO₄F from a monoclinic crystal to a tetragonal structure occurs at 750 °C.¹²⁶ Coupled with hard carbon as the anode, the NaVPO₄F/hard carbon full cell delivered a reversible specific capacity of 82 mA h g⁻¹, with an average discharge voltage of 3.7 V (vs. Na⁺/Na), however, with inferior cycling stability. To improve the cycling stability, several effective ways, including carbon coating,¹²⁷ graphene modification¹²⁸ and ion substitution (e.g., Cr³⁺ and Al³⁺)¹²⁹ were adopted. Very recently, Jin et al.^127c used the electrospinning method to synthesize 1D NaVPO₄F/C nanoparticles (∼6 nm). The nanoparticles display long cyclability with 96.5% initial capacity retention after 1000 cycles at 2C, as well as high capacity of 126.3 mA h g⁻¹ at 1C and outstanding rate capability with 61.2 mA h g⁻¹ at 50C.

4.2.2. Na₃V₂(PO₄)₂F₃. Na₃V₂(PO₄)₂F₃ possesses a tetragonal crystal structure with a space group of P4₂/mnm.¹²⁴ As shown in Fig. 13a and b, Na₃V₂(PO₄)₂F₃ is composed of [V₂O₈F₃] bi-octahedral (pairs of corner shared VO₄F₂ octahedra) and [PO₄] tetrahedral units connected by sharing oxygen atoms. Na⁺ occupies the channel cavities along the a- and b-axes, which are surrounded by four P and three O. There are two types of Na sites (fully occupied site of Na1 and half occupied site of Na2) present in the Na₃V₂(PO₄)₂F₃ structure, in which Na⁺ at Na2 sites have higher chemical potential than that at the Na1 site.¹³⁰ As the cathode for SIBs, two potential plateaus at 3.7 V and 4.2 V were observed in the voltage range of 2.0–4.5 V, corresponding to the two-step redox reaction of V³⁺/V⁴⁺. In the voltage range of 1.6–4.6 V, two of three Na⁺ can be reversibly extracted from/inserted into Na_xV₂(PO₄)₂F₃ (1 ≤ x ≤ 3) with a theoretical capacity of 128 mA h g⁻¹, corresponding to an energy density of ∼507 W h kg⁻¹.¹³¹ In order to better understand the sodium extraction mechanism of Na₃V₂(PO₄)₂F₃, high resolution synchrotron XRD was carried out.¹³¹ It was revealed that four intermediate phase transformations occurred from Na₃V₂(PO₄)₂F₃ to NaV₂(PO₄)₂F₃, as shown in Fig. 13c.¹³² The detailed route of sodium rearrangement inside the unit cell is illustrated in Fig. 13d. Park et al.¹³³ reported that Na_1.5VPO_4.8F_0.7 showed a high voltage of ∼3.8 V (vs. Na⁺/Na) with a large theoretical energy density of ∼600 W h kg⁻¹. Na_1.5VPO_4.8F_0.7 displayed outstanding long-term cycling stability with high capacity retention of ∼95% after 100 cycles and ∼84% after 500 cycles. The excellent electrochemical performance may be ascribed to the small volume change (2.9%) upon cycling. Liu et al.¹³⁴ proposed that the rapid Na motion during the extraction process may favor Na₃V₂(PO₄)₂F₃ with good rate performance. This is mainly because more Na⁺ extraction resulted in more Na vacancies and thus led to the coalescence of the resonances from the two Na sites. Also, morphological control (e.g. nanoflower-like structure¹³⁵) and carbon coating¹⁰⁸ have been employed to improve the cycling stability and rate capability of Na₃V₂(PO₄)₂F₃.

Fig. 13 Schematic representation of the Na₃V₂(PO₄)₂F₃ crystal structure viewed along the (a) a axis and (b) c axis. Na1 and Na2 in (b) are fully occupied Na sites and half occupied Na sites, respectively. (c) Electrochemical curves of potential-composition and (d) sodium distribution within Na_xV₂(PO₄)₂F₃ (1 ≤ x ≤ 3). The red/black circles (extreme right of (d)) indicate that only two positions out of four are expected to be occupied. The orange dotted circle (bottom-left of (d)) shows that the sodium site in NaV₂(PO₄)₂F₃ is empty. (a, b) Reproduced with permission.¹³⁰ Copyright 2012, Royal Society of Chemistry. (c, d) Reproduced with permission.¹³² Copyright 2015, American Chemical Society.

4.2.3. Na₃(VO_x)₂(PO₄)₂F_3−2x. Na₃(VO)₂(PO₄)₂F is a tetragonal symmetry compound with the I4/mmm space group. It possesses a similar crystal framework to Na₃V₂(PO₄)₂F₃, in which two fluorine atoms are replaced by two oxygen atoms (Fig. 14a).¹³⁶ Sauvage et al. revealed the crystal structure of Na_1.5VOPO₄F_0.5 and its Na storage properties, and found that Na₃(VO)₂(PO₄)₂F presented two potential plateaus at ∼3.6 and 4.0 V (vs. Na⁺/Na) based on two bi-phasic transitions.¹³⁷ Rojo's group did several studies on the electrochemical reaction mechanism of Na₃(VO_x)₂(PO₄)₂F_3−2x.¹³⁸ The Na⁺ extraction/insertion mechanism of Na₃(VO_x)₂(PO₄)₂F_3−2x was a solid-solution process involving the V⁴⁺/V⁵⁺ redox couple with a theoretical capacity of 130 mA h g⁻¹. More studies also suggested that V³⁺ of the original cathode was inactive in the voltage range of 2.8–4.3 V.¹³⁹ Park et al.¹⁴⁰ prepared a series of Na₃(VO_1−xPO₄)₂F_1+2x (x = 0.0, 0.2, 0.5, 0.8, and 1.0) using a solid-state reaction method. Theoretical and experimental results indicated that the electrochemical behavior of Na₃(VO_1−xPO₄)₂F_1+2x (x = 0.0, 0.2, 0.5, 0.8, and 1.0) was highly associated with the V³⁺/V⁴⁺ or V⁴⁺/V⁵⁺, Na⁺–Na⁺ ordering, and F/O distribution. Due to the small volume (∼2%) change during the cycles, the Na_y(VO_1−xPO₄)₂F_1+2x electrode can achieve a high energy density of ∼520 W h kg⁻¹ and good stability for 150 cycles. Broux et al.¹³⁶ investigated the impact of the oxygen content on the structure and electrochemical properties of Na₃V₂(PO₄)₂F_3−yO_y (0 ≤ y ≤ 0.5). They found that the increase of oxygen substitution to V could decrease the lattice parameters and unit cell volume. This is because large amounts of oxygen could result in partial oxidation of V³⁺ to V⁴⁺ as evidenced by magic-angle spinning (MAS)-nuclear magnetic resonance (NMR) spectroscopy (Fig. 14b and c). Moreover, a possible “multi-electron transfer” mechanism within Na_y(VO_1−xPO₄)₂F₁₊₂x (0 ≤ x ≤ 1) was proposed; Na_y(VO_1−xPO₄)₂F₁₊₂x (0 ≤ x ≤ 1) displayed a similar average voltage of 3.8–3.9 V (vs. Na⁺/Na) to Na₃V₂(PO₄)₂F_3−yO_y (0 ≤ y ≤ 0.5). Apart from the study on the electrochemical mechanisms, the design and fabrication of nanostructured Na₃(VO_x)₂(PO₄)₂F_3−2x,¹⁴¹ Na₃(VO_x)₂(PO₄)₂F_3−2x/C,^138a,142 Na₃(VO_x)₂(PO₄)₂F_3−2x/MWCNTs,¹⁴³ Na₃(VO_x)₂(PO₄)₂F_3−2x/graphene¹⁴⁴ and RuO₂-coated Na₃V₂O₂(PO₄)₂F¹⁴⁵ with improved electrochemical performances have been intensively investigated. For example, as shown in Fig. 15a–c, Deng et al.^142a reported graphene quantum dot (GQD)-shielded Na₃(VO)₂(PO₄)₂F@C nanocuboids with a high discharge capacity of 127 mA h g⁻¹ at 0.2C and 70 mA h g⁻¹ at 45C. The hybrid also showed excellent cycling stability with an initial capacity retention of 80% after 2000 cycles at 45C (Fig. 15d).

Fig. 14 (a) Crystal structure of Na₃V₂(PO₄)₂F_3–yO and possible Na Sites in Na₃V₂(PO₄)₂F_3–yO. (b) ³¹P MAS NMR spectra of Na₃V₂(PO₄)₂F_3–yO_y (0 ≤ y ≤ 0.5). (c) Comparison of V⁴⁺/V³⁺ environments around P sites of Na₃V₂(PO₄)₂F_3–yO_y (0 ≤ y ≤ 0.5) obtained from the experimental and calculated results. Reproduced with permission.¹³⁶ Copyright 2016, American Chemical Society.

Fig. 15 (a) TEM and (b) HRTEM of Na₃(VO)₂(PO₄)₂F@C. (c) HRTEM of GQD-anchored Na₃(VO)₂(PO₄)₂F@C with selected fast Fourier transform patterns of GQDs (inset of (c)). (d) Relatively long-term cycling performance of the GQD-anchored Na₃(VO)₂(PO₄)₂F@C electrode. Reproduced with permission.^142a Copyright 2016 Elsevier.

4.3. Mixed-polyanion Na₇V₄(P₂O₇)₄(PO₄)

The 3D framework of Na₇V₄(P₂O₇)₄(PO₄) can be described by the repeating (VP₂O₇)₄PO₄ unit in fourfold symmetry. In a (VP₂O₇)₄PO₄ unit, a central tetrahedron [PO₄] shares corners with four [VO₆] octahedra, and each diphosphate group [P₂O₇] bridges the two adjacent [VO₆] octahedra by sharing the corners. Na⁺ occupies three different crystallographic positions (Na1, Na2 and Na3), adopting bipyramidal square planar and tetrahedral coordinations, respectively. Lim et al.¹⁴⁶ reported that Na₇V₄(P₂O₇)₄(PO₄) displayed an initial capacity of 91.0 mA h g⁻¹ and maintained it with an initial capacity retention of >78% over 1000 cycles. Na₇V₄(P₂O₇)₄(PO₄) exhibited a single potential plateau at ∼3.88 V (vs. Na⁺/Na), which can be attributed to the V³⁺/V⁴⁺ redox couple. However, Deng et al.¹⁴⁷ found that the sodium de/intercalation within Na₇V₄(P₂O₇)₄(PO₄) nanorods not only involved the redox reaction of V⁴⁺/V³⁺ but also generated Na₅V₄^3.5+(P₂O₇)₄(PO₄) as the intermediate phase. This can be confirmed by the two pairs of potential plateaus at 3.87 V (V³⁺/V^3.5+) and 3.89 V (V^3.5+/V⁴⁺). However, the randomly distributed Na₇V₄(P₂O₇)₄(PO₄) nanorods suffer from limited electronic conductivity. In order to decrease the inconsecutive pathways for electron transport, they further reported a hierarchical Na₇V₄(P₂O₇)₄(PO₄)/C hybrid in which the Na₇V₄(P₂O₇)₄(PO₄)/C nanorod was coated by in situ residual carbon and enclosed by highly conductive 2D graphene.¹⁴⁸ Benefiting from its hierarchical structure, the hybrid displayed outstanding rate capability (e.g., reversible capacities of 80.2 and 70.3 mA h g⁻¹ at 10C and 20C, respectively). In addition, they further found that a “free-standing” 3D Na₇V₄(P₂O₇)₄(PO₄)–carbon foam used as a cathode for SIBs displayed a high initial capacity retention of 94% after 800 cycles at alternative rates of 20 and 3C.¹⁴⁹ The complexity of the component limits the synthetic methods that could be used, and affects the phase purity, which in turn determines the electrochemical performance as a cathode material for SIBs.

5. Conclusion and perspectives

In summary, as one of the most promising alternatives to LIBs, SIBs have attracted increasing attention for next-generation high-energy batteries. This is mainly due to the rich natural resources of Na and similar electrochemistry of SIBs to LIBs. Similar to their LIB counterparts, V-based electrode materials for SIBs have been widely demonstrated to deliver promising electrochemical performance. This is mainly attributed to their high operating voltage, theoretical capacity and energy density, along with the rich electrochemical interaction between Na and V with multiple oxidation states (V²⁺–V⁵⁺). Over the past several years, a large number of V-based electrode materials have been developed. They can be simply categorized into: vanadium oxides (e.g., V₂O₅, VO_x and VO₂), vanadium bronzes (e.g., Na_xVO₂, NaV₃O₈, NaV₆O₁₅ and δ-NH₄V₄O₁₀), vanadium-based phosphates (e.g., Na₃V₂(PO₄)₃, VOPO₄, NaVOPO₄, Na₇V₃(P₂O₇)₄, Na₂(VO)P₂O₇) and F-containing V-based polyanions (e.g., NaVPO₄F, Na₃V₂(PO₄)₂F₃ and Na₃(VO_x)₂(PO₄)₂F_3−2x). In this review, we have mainly summarized research on the structural and electrochemical features of these vanadium based materials, as well as recent progress in their applications in SIBs. Despite the encouraging progress achieved in the development of V-based electrodes for SIBs, several scientific and practical issues remain, as follows:

(1) The electrochemical behavior and performance of V-based materials are not only dependent on their inherent Na⁺ intercalation–deintercalation chemistry but also the occupancy and distribution of the constituent elements (e.g., Na, V, transition metals, O, F). Theoretical calculation is an effective tool to predict novel or optimized structures of V-based materials as high-performance cathodes for SIBs. As expected, it is highly desirable to explore novel V-based materials using theoretical calculation together with experiments.

(2) Morphology control (e.g., reduced particle-size, nanotubes, nanobelts, core–shell) and hybridization with other highly conductive materials (e.g., carbon-based materials) using various synthesis routes also significantly influence the electrochemical performance of V-based electrodes for SIBs. Therefore, novel, simple, eco-friendly and low cost synthesis methods for the preparation of V-based materials should be well developed.

(3) In order to achieve high capacity and energy density, V-based materials containing PO₄⁻ and F⁻ anions are often employed at much higher voltages (vs. the potentials of conventional electrolytes). Thus, appropriate working electrolytes for those cell systems with the merits of larger electrochemical window and high anodic voltage, in addition to other merits (e.g., low melting point, low vaporization point, high ionic conductivity, safety and environmental friendliness), should be widely explored.

(4) The configuration of a SIB full-cell, which is based on V-based materials as both anode and cathode for SIBs, or as a high voltage cathode coupled with other promising negative anodes for SIBs, offers great advances in practical applications in the future. Although there have been a few reports on the full-cell configuration with the use of vanadium based materials (e.g., Na₂Ti₃O₇//VOPO₄ and Na₃V₂(PO₄)₃//Na₃V₂(PO₄)₃), the rate capability and cycling stability are still very limited, especially compared to their LIB counterparts. Therefore, rapid development of the high-performance SIB full-cell configuration with appropriate cathode materials (including V-based materials), as well as anodes and electrolytes, is highly necessary.

(5) Although there are a number of advantages for the development of V-based materials for SIBs, as well as LIBs, the disadvantages of the large-scale use of vanadium, in terms of vanadium being a “non-green” element (maybe harmful to the human body and the environment), and its relatively high cost should also be carefully considered in the future.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51302079, 51702138), the Natural Science Foundation of Hunan Province (2017JJ1008), the Natural Science Foundation of Jiangsu Province (BK20160213), the “Young Talent Fellowship” program from the South China University of Technology, the Australian Renewable Energy Agency (ARENA) S4 project and the 111 project (B12015).

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