Advances in Vanadium-Redoxed Polyanions for High-Voltage Sodium-Ion Batteries

Wu, Honglun; Chen, Yiqing; Wen, Tianzhuo; Chen, Long; Pu, Xiangjun; Chen, Zhongxue

doi:10.3390/batteries9010056

Open AccessReview

Advances in Vanadium-Redoxed Polyanions for High-Voltage Sodium-Ion Batteries

by

Honglun Wu

¹,

Yiqing Chen

¹,

Tianzhuo Wen

¹,

Long Chen

¹,

Xiangjun Pu

^2,* and

Zhongxue Chen

^1,*

¹

School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China

²

Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong 999077, China

^*

Authors to whom correspondence should be addressed.

Batteries 2023, 9(1), 56; https://doi.org/10.3390/batteries9010056

Submission received: 9 November 2022 / Revised: 8 January 2023 / Accepted: 10 January 2023 / Published: 12 January 2023

(This article belongs to the Special Issue Advanced Electrode Materials for High-Performance Sodium-Ion Batteries)

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Abstract

:

Large-scale energy storage using sodium ion batteries (SIBs) as a hub for the conversion of renewable energy has become a topic of great importance. However, the application of SIBs is hindered by low energy density arising from inferior capacity and operation voltage. In this regard, vanadium-based phosphate polyanions with multiple valence changes (III–V), high redox potential, abundant resources, spacious frame structure, and remarkable thermal stability are promising avenues to address this dilemma. In this review, following the principle of electronic structure and function relationship, we summarize the recent progress in phosphates, pyrophosphates, fluorophosphates, and mixed polyanions of vanadium-centered polyanionic materials for SIBs. This review may provide comprehensive understanding and guidelines to further construct high performance, low-cost sodium-ion batteries.

Keywords:

high-voltage; V-based polyanions; sodium-ion batteries

Graphical Abstract

1. Introduction

In recent years, as the price of lithium resources has rapidly grown, sodium ion batteries (SIBs) have become more competitive in the field of large-scale electrochemical energy storage (EES) [1,2]. Sodium is abundant in the Earth’s crust and in sea water, and is thus more available than lithium (element concentrations in the upper continental crust: Li = 22 ppm, Na = 25,670 ppm) [3]. As shown in Figure 1a, the working principle of SIBs is similar to that of lithium-ion batteries (LIBs) [4]. However, SIBs have been plagued by low theoretical capacity and sluggish insertion/extraction kinetics, owing to the larger ionic radius and greater atomic mass of Na⁺ [5]. Considering that large-scale EES demands higher power density to conduct frequent peak regulation, SIBs have encountered challenges with respect to their practical application [6]. Cathode materials are also an essential aspect of the electrochemical performance and reliability of SIBs. Their inherent crystal structure determines essential attributes such as electron transfer number, redox potential, and the charge-transfer kinetics.

There are four primary kinds of cathode materials for SIBs: Prussian blue, transition metal oxides, organic compounds, and polyanionic compounds. The comparison of these four types of compounds is displayed in Figure 1b. Generally, Prussian blue (A_xM[Fe(CN)₆]_y·zH₂O) has an open framework structure and good structural stability [7,8]. However, the crystal water in the structure results in large amounts of Fe(CN)₆ vacancies, largely restricting the material’s electrochemical properties. Transition metal oxides possess a high specific capacity and a moderate working voltage, but are also limited by the structural changes and phase transitions that occur during electrochemical processes and by inferior air stability [9]. Organic compounds have also been considered as potential cathodes for SIBs owing to their structural diversity, micro-regulatory properties, and resource sustainability, but their widespread application has been limited by their low intrinsic electronic conductivity and low operating voltage [10]. Compared with these other candidates, polyanionic compounds possess a comprehensive manifestation of relatively high operating voltage, specific energy (vs. Na⁺/Na), operation reliability, and safety [11,12,13].

Among polyanionic compounds, vanadium (V)-based polyanionic compounds have tended to receive more attention in recent years due to their many merits [14]. On one hand, V has a higher abundance in the upper continental crust as compared to Co, Ni, Cu, and Cr, making it more available (Figure 1c) [3]. On the other hand, vanadium possesses a valence electron configuration of 3d³4s², which allows it to experience a multivalent electrochemical reaction from V³⁺ to V⁵⁺. Additionally, many V-based polyanionic compounds exhibit a high operating voltage of over 3.4 V (vs. Na⁺/Na) owing to the inductive effect of polyanionic groups and V redox (see Table 1). A schematic diagram of inductive effects is presented in Figure 1d [15]. When the M–O bond is more covalent, the antibonding orbitals will be pushed up, resulting in larger energy splitting between antibonding and bonding orbitals, as well as lower redox potential. As shown in Figure 1e, with a similar theoretical capacity of ~97 mAh g⁻¹, a higher operating voltage remarkably increases the theoretical specific energy (vs. Na⁺/Na). When another atom X with stronger electronegativity is introduced to form M–O–X bonds, the inductive effect begins to take effect since the covalency between M–O will be lowered, leading to higher voltage [16]. Hence, the redox voltage can be effectively improved by introducing different polyanionic groups in cathode materials. In addition, the unique open framework often endows V-based polyanionic compounds with outstanding cycle stability and rate performance [4].

To date, several reviews of polyanionic compounds have been conducted with respect to their classification, crystal structures, and common electrochemical performance [4,14,17,18,19]. Nevertheless, to the best of our knowledge, there has not been a unique summary and interpretation of the high voltage properties of V-based polyanionic compounds, although this type of material is an essential candidate for SIBs. For a simple sample, and to have a comprehensive comparison of typical cathode materials for SIBs, we included their voltages and specific energies (vs. Na⁺/Na) in Figure 1f [4,7,8]. As can be seen, V-based polyanionic compounds have a moderate specific energy (vs. Na⁺/Na) but a relatively high voltage; thus, a systematic interpretation is necessary to build better a understanding of the causes, effects, and strategies for further enhancing V-based electrodes.

In this review, we will concentrate on high-voltage V-based polyanionic compounds (≥3.4 V) and present a comprehensive interpretation in terms of their structure, charge transfer kinetics, Na⁺ storage mechanisms, and electrochemical performance. In addition, modification approaches, major defects, challenges, and perspectives on the application of V-based electrode materials for SIBs will be also presented.

Figure 1. (a) Working principle of room-temperature SIBs [20]. (b) Comparison of various cathode materials for SIBs [20]. (c) Element concentrations of Co, Cu, Ni, Cr, V, Mn, and Fe (in ppm) in the upper continental crust [3]. (d) Schematic diagram of the influence of the M-O bond on the orbital energy level and operating potential (Voc). (e) Schematic illustration showing the relationship of Voc and theoretical specific energy (vs. Na⁺/Na). (f) Potential and specific energies (vs. Na⁺/Na) of typical cathode materials for SIBs.

Table 1. A comparison of V-based polyanionic compounds as cathode materials for SIBs.

Materials	Structure	Redox (V)	Redox Couple	Theoretical Capacity (mAh g⁻¹)	Theoretical Specific Energy (vs. Na⁺/Na) (Wh kg⁻¹)	Electrochemical Activity
Na₃V₂(PO₄)₃ [21,22]	Rhombohedral	3.4	V⁴⁺/V³⁺	117.6	400	117 mAh g⁻¹ at 1 C 82 mAh g⁻¹ at 100 C
Na₃V₃(PO₄)₄ [23]	Monoclinic (layered structure)	3.9	V⁴⁺/V³⁺	44.5	174	~40 mAh g⁻¹ at 0.6 C
Na₃V(PO₄)₂ [24]	Monoclinic (layered structure)	3.5	V⁴⁺/V³⁺	90	315	~90 mAh g⁻¹ at 0.2 C
VOPO₄ [25]	Tetragonal (layered structure)	3.4	V⁵⁺/V⁴⁺	165.5	563	150 mAh g⁻¹ at 0.05 C
NaVOPO₄ [26]	Monoclinic	3.6	V⁵⁺/V⁴⁺	145	522	101 mAh g⁻¹ at 5 mA g⁻¹
NaVOPO₄ [27]	Orthorhombic	3.3	V⁵⁺/V⁴⁺	145	479	115 mAh g⁻¹ at 0.1 C
NaVOPO₄ [28]	Triclinic	3.5	V⁵⁺/V⁴⁺	145	508	144 mAh g⁻¹ at 0.05 C
Na₄VO(PO₄)₂ [29,30]	Orthorhombic	3.5	V⁵⁺/V⁴⁺	78	273	41.3 mAh g⁻¹ at 10 C
NaVPO₄F [31]	Tetragonal (NASICON)	3.7	V⁴⁺/V³⁺	142.5	527	120.9 mAh g⁻¹ at 0.05 C 70.1 mAh g⁻¹ at 0.5 C
NaVPO₄F [32,33,34]	Monoclinic (NASICON)	3.4	V⁴⁺/V³⁺	142.5	484.5	135 mAh g⁻¹ at 0.2 C 86.5 mAh g⁻¹ at 100 C
Na₅V(PO₄)₂F₂ [35]	Trigonal/Orthorhombic	3.4/3.5	V⁴⁺/V³⁺	68	231.2/238	61 mAh g⁻¹ at 0.1C
Na₃V₂(PO₄)₂F₃ [36,37]	Tetragonal (NASICON)	3.9	V⁴⁺/V³⁺	128.3	500	125 mAh g⁻¹ at 0.2 C
Na₃V₂(PO₄)₂O₂F [38,39]	Tetragonal	3.8	V⁵⁺/V⁴⁺	130	494	127.4 mAh g⁻¹ at 0.2C
Na₃V₂(PO₄)₂O_1.6F_1.4 [40,41]	Tetragonal	3.8	V⁵⁺/V⁴⁺	129.7	492.9	134 mAh g⁻¹ at 0.1 C
NaVP₂O₇ [42]	Monoclinic	3.9	V⁴⁺/V³⁺	108.1	421	104 mAh g⁻¹ at 0.1 C
Na₇V₃(P₂O₇)₄ [43,44]	Monoclinic	4.13	V⁴⁺/V³⁺	79.6	329	67.2 mAh g⁻¹ at 8 C
Na₂VOP₂O₇ [45]	Tetragonal	3.8	V⁵⁺/V⁴⁺	93.4	355	80 mAh g⁻¹ at 0.05 C
Na₇V₄(P₂O₇)₄(PO₄) [46,47]	Tetragonal	3.85	V⁴⁺/V³⁺	92.7	357	92 mAh g⁻¹ at 0.05 C 70.2 mAh g⁻¹ at 10 C
Na₂(VO)₂(HPO₄)₂(C₂O₄) [48]	Monoclinic	4.0	V⁵⁺/V⁴⁺	116.6	466.4	105 mA h g⁻¹ at 0.1 C

2. Phosphate

V-based phosphates have been widely investigated owing to their high operating potential, and outstanding thermal and structural stability. The inductive effect of the (PO₄)³⁻ polyanion endows them with high potential, while the strong P–O bond ensures a stable structure during the charge–discharge process. To date, numerous compounds including Na₃V₂(PO₄)₃, Na₃V₃(PO₄)₄, VOPO4, NaVOPO₄, and Na₄VO(PO₄)₂ have been confirmed to have high operation potential.

2.1. Na₃V₂(PO₄)₃ (3.4 V vs. Na⁺/Na, the Same Hereinafter)

The well-known Na₃V₂(PO₄)₃ (NVP) has the rhombohedral NASICON structure with the space group R

\bar{3}

c [49,50]. Figure 2a shows its 3D structure; the isolated VO₆ octahedra and PO₄ tetrahedra are connected to form the framework [V₂(PO₄)₃]³⁻ along the c direction. The unit cell of Na₃V₂(PO₄)₃ is constructed of six formula units and two kinds of Na⁺ sites with different coordination environments, which are referred to as the Na1 site and Na2 site. The structure can provide spacious diffusion paths to ensure sodium ions’ intercalation/deintercalation in the structure [50]. Meanwhile, two different types of Na⁺ occupy two oxidation state channels in the lattice, namely the Na1 site and the Na2 site. The skeleton structure is prone to be stable during the extraction of two Na⁺ ions because of the tightly bonded covalent influence of (PO₄)₃. Figure 2b displays the galvanostatic cycling profile within the voltage range of 2.3–3.9 V for a Na₃V₂(PO₄)₃/C electrode [51]. With a flat plateau at 3.4 V vs. Na⁺/Na, two Na⁺ ions can be released from the host material to form NaV₂(PO₄)₃, resulting in a high theoretical capacity of 117.6 mAh g⁻¹ [20]. During the charge/discharge process, a highly reversible bi-phase reaction based on V³⁺/V⁴⁺ has been observed [52]. When charge from Na₃V₂(PO₄)₃ to form NaV₂(PO₄)₃, calculations using density functional theory (DFT) have indicated that Na⁺ prefers to diffuse through the inside Na layers, such that Na⁺ ions located at the Na1 site tend to remain while the rest of the Na⁺ ions at the Na2 site can be extracted [53,54].

To optimize the performance of high-voltage NVP, several types of modification strategies have been tested to address the poor intrinsic conductivity of Na₃V₂(PO₄)₃, including carbon coating, nano crystallization, and ionic doping. Carbon coating has been widely used to promote the electronic conductivity of electrode materials. Generally, construction of a carbon layer and conductive agents can enhance electronic conductivity, and thus the electrochemical performance of electrode materials [52,55,56]. Additionally, heteroatom doping (N, S, P, B, etc.) can also cause carbon layers by defects [57,58,59,60]. Nano crystallization of the cathode materials can increase the specific surface area and decrease ions’ diffusion paths [61,62,63]. Although carbon coating and nano crystallization are efficient approaches to improve electrochemical performance, they cannot change the inherent characteristics of a material. Hence, tests of anionic doping (Ti⁴⁺, Fe³⁺, Li⁺, Mn²⁺, Cr²⁺) and anionic ions (F⁻, Cl⁻) have been conducted to specifically modify the deep electrons’ structure. To enhance the voltage, Zakharkin et al. selected Mn²⁺ to partially replace V³⁺ and successfully synthesized Na_3+xMn_xV_2−x (PO₄)₃ (0 ≤ x ≤ 1) samples [57]. An additional high-voltage plateau (~3.9 V) was found in all Mn-substituted samples and Na_3+xMn_xV_2−x(PO₄)₃ (x = 0.4) exhibited 8–10% additional specific energy (vs. Na⁺/Na) in comparison to bare Na₃V₂(PO₄)₃ (Figure 2c). Further phase transformations in the voltage window of 2.5–3.8 V were also investigated, and it was found that several single- and two-phase actions exist during Na⁺ (de)intercalation (Figure 2d), thus leaving more opportunities for novel composition. For anionic ions, Chen et al. synthesized F-doped Na₃V₂(PO₄)₃/C composites [58]. The modified Na₃V₂(PO₄)_2.93F_0.07/C presented a higher electronic conductivity and better cycle performance (86% retention after 1000 cycles at 200 mA g⁻¹).

Figure 2. (a) Crystal structure of Na₃V₂(PO₄)₃ [50]. (b) Charge/discharge curves of Na₃V₂(PO₄)₃/C at various current rates in the voltage range 2.3–3.9 V [51]. (c) Improvement of the specific energy (vs. Na⁺/Na) for Mn-doped Na₃V₂(PO₄)₃. (d) Schematic diagram of the Na⁺ deintercalation mechanism for Na_3+xMn_xV_2−x(PO₄)₃ (0 ≤ x ≤ 1) during Na⁺ extraction [57]. (e) Charge/discharge curves of Na₃V₃(PO₄)₄ at different current densities between 3.0–4.3 V. Inset is the relative CV profile at 0.05 mV s⁻¹ [23]. (f) The quasi-open-circuit potential profile of Na₃V(PO₄)₂ tested at C/20 and calculated average voltage. Inset is the dQ/dV profiles of Na₃V(PO₄)₂ [24].

2.2. Na₃V₃(PO₄)₄ (3.9 V)

Recently, a novel 3.9 V layered Na₃V₃(PO₄)₄ was explored as a potential cathode material for SIBs. It is isostructural with Na₃Fe₃(PO₄)₄ (ICSD No. 95532) with space group C2/c [23]. As shown in Figure 2e, due to the inductive effect, nearly one Na⁺ ion can be extracted from the structure of Na₃V₃(PO₄)₄ with a high voltage plateau at 3.9 V, which is the highest V³⁺/V⁴⁺ couple among V-based phosphates. From the perspective of thermodynamics, DFT calculations have also confirmed the high theoretical potential of 4.03 V when the first Na atom is removed from the original unit cell. However, it exhibited a low discharge capacity of 34 mAh g⁻¹ and further exploration is still necessary.

2.3. Na₃V(PO₄)₂ (3.5 V)

Another ratio in this derivative is the peculiar Na₃V(PO₄)₂, which is identical to the structure of Na₃Fe(PO₄)₂ in the monoclinic (C2/c) space group [24,64]. It has a layered structure, consisting of VO₆ octahedra and PO₄ tetrahedra. As shown in Figure 2f, a flat voltage plateau at ∼3.5 V (vs. Na⁺/Na) can be observed in quasi-open-circuit potential and dQ/dV profiles. During Na (de)intercalation, it experiences a biphasic reaction between Na₃V(PO₄)₂ and Na₂V(PO₄)₂ with only one Na able to be exacted form the structure, leading to a theoretical capacity of 90 mAh g⁻¹, even when charged to 4.2 V [65,66]. To address this drawback, Liu et al. attempted to extend the upper cut-off voltage to 4.3 V [67]. ⁵¹V solid-state NMR was employed to confirm that the high-voltage plateau can be attributed to V⁴⁺/V⁵⁺ reactions. Therefore, Na₃V(PO₄)₂ has the potential to deliver two electrons through V³⁺/V⁴⁺/V⁵⁺ reactions with a higher theoretical specific energy (vs. Na⁺/Na) of 657 Wh kg⁻¹. However, the poor reversibility of V⁴⁺/V⁵⁺ severely influences the cycle performance, calling for more work to tackle this this dilemma.

2.4. VOPO₄ (3.4 V)

Besides Na–V orthophosphates with different element ratios, vanadyl orthophosphates are enriching the high-voltage family. Prior to its application to SIBs in the last decade, VOPO₄ had already been recognized as a potential cathode material in LIBs owing to its high operating voltage and specific capacity. There are several types of crystal structures for VOPO₄ in terms of layered α₁, α₂, δ, ω, and γ, as well as three-dimensional β and ε phases, and all of them are composed of independent PO₄ tetrahedra sharing vertices with VO₆ octahedra, but they differ with respect to the orientation of the vanadyl bond [68,69]. He et al. first prepared layered VOPO₄ via chemical delithiation of the tetragonal α₁-LiVOPO₄, delivering a reversible capacity of 110 mAh g⁻¹ at 0.05 C with an obvious potential plateau at 3.4~3.5 V (Figure 3a) [25]. Additionally, introducing highly conductive reduced graphene oxide (rGO) can significantly increase the specific capacity of α₁-VOPO₄ to 150 mAh g⁻¹, amounting to 0.9 Na⁺ per formula. Yu et al. synthesized tetragonal VOPO₄ to achieve a high-rate capability of 70 mAh g⁻¹ at 5 C, as well as 136 mAh g⁻¹ at 0.1 C [70]. Further, a Na₂Ti₃O₇/VOPO₄ full cell was tested by Yu’s group [71]. The full cell presented a high operating voltage (~2.9 V) and reversible capacity (114 mAh g⁻¹ at 0.1C and 74 mAh g⁻¹ at 2 C) with a high specific energy (vs. Na⁺/Na) of 220 Wh kg⁻¹. Very recently, Zhang et al. found that VOPO₄•2H₂O can fulfill consecutive V⁵⁺/V⁴⁺/V³⁺ redox reactions [72]. Abundant phases bring more possibilities but also troubles for synthesis, thus preparing the targeted phase without side products is a major challenge for VOPO₄.

2.5. NaVOPO₄ (3.3 V/3.5 V/3.6 V Due to Phase Difference)

Na-deficient VOPO₄ needs to be pre-sodiated to become a Na-rich cathode for Na-ion battery applications, causing it to be quite costly, time consuming, and complex; however, NaVOPO₄ containing reasonable amounts of Na has been studied as well. Similar to VOPO₄, NaVOPO₄ also has abundant phase states, such as monoclinic lattice α (space group: P21/c), tetragonal lattice α₁ (space group: P4/nmm), orthorhombic lattice β (space group: Pnma), and layered structured NaVOPO₄. α-NaVOPO₄ was first explored as a cathode material for SIBs by Goodenough et al. [73]. As shown in Figure 3b, with an average potential of 3.6 V (vs. Na⁺/Na), it exhibits a reversible capacity of 90 mAh g⁻¹ at 1/15 C. As shown in Figure 3c, β-NaVOPO₄ possesses a moderate potential plateau at 3.3 V and a high theoretical capacity of 145 mAh g⁻¹ based on a V⁵⁺/V⁴⁺ redox couple [27,74]. However, its actual capacity was greatly affected by the invasion of protons into the structure, leading to large irreversible capacity. α₁-NaVOPO₄ was built up of VOPO₄ sheets stacked along the c axis to show a typical layered structure. Na⁺ ions are adequately accommodated in the space between two VOPO₄ planes because of the large interlayer spacing. On the basis of molecular dynamics simulations, it has been found to have the highest Na⁺ mobility among the α, β, and α₁ polymorphs [75]. DFT+U was also employed to investigate the influence of cell voltage on the NaVOPO₄ polymorphs when doping on the vanadium site. As shown in Figure 3d, the promotion of cell voltage is tend to occur with cation doping (Al³⁺, Fe³⁺, Mn⁴⁺, Co²⁺, Ni²⁺, and Ti⁴⁺) at the vanadium site. Fang et al. prepared another layered NaVOPO₄ with a triclinic lattice (Figure 3e) [28]. It attains a high voltage of ~3.5 V (vs. Na⁺/Na) and a high discharge capacity of 144 mAh g⁻¹ at 0.05 C. Besides these crystals, an amorphous NaVOPO₄ was reported to exhibit high reversible capacity (110 mAh g⁻¹ at 0.05 C) at ∼3.5 V (vs. Na⁺/Na), as well as a capacity retention of 96% after 2000 cycles, showing an outstanding cyclability [76]. Additionally, some derivatives of NaVOPO₄ have also been shown to be feasible hosts for Na ions. For instance, based on a multi-electron transfer with V³⁺/V⁵⁺ redox reactions, KVOPO₄ could exhibit an extremely high specific capacity of 235 mAh g⁻¹ at 2.56 V, which can be attributed to its unique polyhedral framework [77].

2.6. Na₄VO(PO₄)₂ (3.5 V)

Na₄VO(PO₄)₂, indexed into a space group of orthorhombic Pbca, is the Na sufficient member of V-based oxygenous phosphates [30]. As can be seen from the charge/discharge profile and the dQ/dV presented in Figure 3f, one Na ion in Na₄VO(PO₄)₂ can be reversibly (de)intercalated from the structure with an average voltage of ∼3.5 V (vs. Na⁺/Na) and a theoretical capacity of 78 mAh g⁻¹ [29]. Based on the V⁴⁺/V⁵⁺ redox reaction, the reaction mechanism turns out to be a one-phase reaction. Additionally, Deriouche et al. also examined the electrochemical performance of the Na₄VO(PO₄)₂ compound, and found that is has high ionic conductivity and stability up to 700 °C [78]. Recently, DFT calculations and molecular dynamics simulations were employed to further study the electronic structure and influence of cation doping for Na₄VO(PO₄)₂ [30]. The results indicated a diffusion coefficient of D_Na = 5.1*10⁻¹¹ cm² s⁻¹ at 300 K and a Na ion activation energy of 0.49 eV. Additionally, the cell voltage is also predicted to increase by cation doping at the vanadium site. Nevertheless, similar to the prediction of NaVOPO₄, the practical performance of cation doping should be confirmed by experiment.

In short, V-based phosphates have excellent durability and fast kinetics, which can be attributed to their open channel structure and compositional stability. However, the operation voltage is still limited due to the inherent nature of PO₄, limiting the feasibility of its introduction into other groups with strong inductive effect.

3. Fluorophosphate and Vanadyl Fluorophosphate

Fluorine substitution for phosphate groups is possible to promote the operating voltage because of the stronger inductive effect of F⁻. To date, several F-containing V-based polyanionic compounds, including NaVPO₄F, Na₅V(PO₄)₂F₂, Na₃V₂(PO₄)₂F₃, and Na₃V₂O_2x(PO₄)₂F_3−2x have been investigated as high voltage cathodes for SIBs.

3.1. NaVPO₄F (3.7 V)

Compared with the other members of V-based polyanionic compounds, NaVPO₄F has a higher theoretical specific capacity of 143 mAh g⁻¹ (compare 117 mAh g⁻¹ for Na₃V₂(PO₄)₃ and 128 mAh g⁻¹ for Na₃V₂(PO₄)₂F₃), making it a promising choice for SIBs. Currently, research shows that NaVPO₄F can exist in two phases: a tetragonal symmetric structure with space group I4/mmm, and a monoclinic structure with space group C2/c [79]. The tetragonal phase is isostructural with Na₃Al₂(PO₄)₂F₃ while the monoclinic phase presents an analogous structure to NaAlPO₄F (ICSD no. 040522) (Figure 4a) [80,81]. Even with exactly the same chemical composition, the sodium storage properties of the two compounds vary greatly. The tetragonal NaVPO₄F possesses a theoretical capacity of 143 mAh g⁻¹ based on a V⁴⁺/V³⁺ redox reaction at 3.7 V. The monoclinic NaVPO₄F shows a relatively low working plateau of 3.4 V with a single-phase reaction (Figure 4b) [32,34]. Further reports found that the tetragonal NaVPO₄F tends to experience an irreversible transition to the monoclinic phase when the sintering temperature reaches up to ~750 °C, due to lower formation energy (i.e., more stable nature) of monoclinic NaVPO₄F as compared to the tetragonal phase (Figure 4a) [79,82]. Additionally, after surface coating and structure modification, the monoclinic phase seems to have capacity and rate merits compared to the other phase [83,84].

In short, the tetragonal phase is more suitable as a high-energy-density cathode, mainly due to strong ionic bonding, while the monoclinic crystal is feasible for high power applications due to the intrinsically faster charge transfer kinetics.

3.2. Na₅V(PO₄)₂F₂ (3.4 V/3.5 V)

Layered Na₅V(PO₄)₂F₂ has recently been investigated as a potential cathode material for SIBs, owing to its high voltage and good cycling stability. Interestingly, as shown in Figure 4c, it has two types of structures depending on the calcination temperatures. Under high temperatures, reaching 800 °C, the obtained phase turns out to be identical with the orthorhombic Na₅Fe(PO₄)₂F₂ (PDF 87-1031) with a space group P3, which is referred as o-Na₅V(PO₄)₂F₂. The other phase, t-Na₅V(PO₄)₂F₂, is isostructural to the trigonal Na₅Cr(PO₄)₂F₂ (PDF 77-0506), indexed into space group Pbca, and can be formed at a relatively low temperature of 600 °C [35]. Due to the different sodium coordinative environments in the two phases, Na⁺ ions in t-Na₅V(PO₄)₂F₂ are more likely to migrate along the c axis, while they prefer to migrate along the ac plane in o-Na₅V(PO₄)₂F₂. The material presents an operating voltage of 3.4 V for trigonal-type and 3.5 V for orthorhombic-type, based on the V³⁺/V⁴⁺ redox couple (Figure 4d). During the charging process, a biphasic process followed by a monophasic process was confirmed by ex situ XRD and DFT calculations for both phases. However, only one Na ion can be reversibly extracted/inserted, resulting in a poor specific capacity of ~62 mAh g⁻¹ at 0.1C (1C = 136 mA g⁻¹). Further reports have indicated that higher sodium ion activation barriers and possible structural decomposition of the intermediate phase make it difficult to remove two sodium ions from Na₅V(PO₄)₂F₂ [85].

3.3. Na₃V₂(PO₄)₂F₃ (3.9 V)

Among V-based fluorophosphate, Na₃V₂(PO₄)₂F₃ displays the highest working voltage (~3.9 V vs. Na⁺/Na) due to the strong ionicity of the F–V bond, and high specific energy (vs. Na⁺/Na).

Na₃V₂(PO₄)₂F₃ has a tetragonal symmetry structure with a space group of P42/mnm, and is composed of [V₂O₈F₃] bi-octahedral units and [PO₄] tetrahedral units connected by shared O atoms (Figure 5a) [86]. This arrangement generates channels along the a and b directions with sodium located in the tunnel sites, providing apparent pathways for diffusion of Na ions [87,88]. Na₃V₂(PO₄)₂F₃ delivers a theoretical capacity of 128 mAh g⁻¹ with two potential plateaus at 3.7 V and 4.2 V, based on a two-step redox reaction of V³⁺/V⁴⁺ (Figure 5b) [86]. Two different environments for Na ions resulted in respective Na (de)insertion, which generated a difference of 0.4–0.5 V between the two voltage plateaus. Although the high working voltage (about 3.9 V), large theoretical capacity, and fast ionic mobility enables Na₃V₂(PO₄)₂F₃ to be a promising cathode material for SIBs, the poor intrinsic electronic conductivity often leads to high inner resistance, low coulombic efficiency, and limited rate performance, which seriously affects its applications.

Many approaches have been employed to enhance the electronic conductivity of Na₃V₂(PO₄)₂F₃, such as heteroatom-doping [89], carbon-coating [90], morphology design [91,92], etc. In order to explore the possibility of modifying the values of the redox couples, anionic substitution of oxygen for fluorine has also been conducted. Hence, several vanadyl fluorophosphates including Na₃V₂(PO₄)₂O₂F and Na₃V₂(PO₄)₂O_1.6F_1.4 have been further investigated.

Figure 5. (a) The crystal structure of Na₃V₂(PO₄)₂F₃ [86]. (b) CV curves of Na₃V₂(PO₄)₂F₃ for the first three cycles at 0.1 mV s⁻¹ [93]. (c) CV curves of Na₃V₂(PO₄)₂O₂F at a scan rate of 0.5 mV s⁻¹ [94]. (d) Tends in average voltages of the Na_y(VO_1−xPO₄)₂F_1+2x compound as x increases. [95]. (e) Charge/discharge profile at a C/10 rate for the Na₃V₂(PO₄)₂O_1.6F_1.4 cathode and the corresponding dQ/dV curve inset [40]. (f) The dQ/dV curves for Na₃V³⁺_2−yV⁴⁺_y(PO₄)₂F_3−yO_y electrode materials corresponding to the 5th cycle performed at C/20 [96].

3.4. Na₃V₂O_2x(PO₄)₂F_3−2x (0 < x ≤ 1)

Na₃V₂(PO₄)₂F₃ is actually the special member for x = 0 of a series of isostructural Na₃V₂O_2x(PO₄)₂F_3−2x (0 ≤ x ≤ 1) compounds, the latter of which has three-dimensional O₈F_3−2xO_2x bi-octahedral units linked together by phosphate groups via oxygen atoms, with the highly mobile Na ions present at the tunnel sites [97]. In spite of their similar crystal framework, the redox mechanism and phase reactions significantly deviate with the composition due to variations in covalent and ionic bonding, as we stated in the introduction section. Na₃V₂O_2x(PO₄)₂F_3−2x (0 < x < 1) exhibits a slightly lower operating voltage than Na₃V₂(PO₄)₂F₃, because of the weaker electronegativity of O than of F [98]. Fortunately, they present a high theoretical capacity due to the extra Na⁺ insertion/extraction below 4.5 V, since partial substitution of O for F can reduce the Na⁺–Na⁺ repulsion, thus allowing more Na to be accommodated. Here, two typical compounds in the series, Na₃V₂(PO₄)₂O₂F and Na₃V₂(PO₄)₂O_1.6F_1.4, are discussed in detail.

3.4.1. Na₃V₂(PO₄)₂O₂F (3.8 V)

When x = 1, the formula turns out to be Na₃V₂(PO₄)₂O₂F. It has a tetragonal symmetry within the I4/mnm space group [99]. Its crystal structure is constructed by layers of alternating [PO₄] tetrahedra and [VO₅F] octahedra. The layers in the structure are loosely interconnected over the F atoms along the c direction, forming a 3D open framework [100]. Two types of Na⁺ sites were found: Na1 sites coordinated by one F atom and six O atoms, and Na2 sites coordinated six O atoms. As shown in Figure 5c, the reversible extraction of two Na⁺ ions at voltages of ~3.6 and ~4.0 V (vs. Na⁺/Na) in Na₃V₂(PO₄)₂O₂F resulted in a theoretical capacity of 130 mA h g⁻¹ and a theoretical specific energy (vs. Na⁺/Na) of 486 Wh kg⁻¹ [94]. As for the Na⁺ storage mechanism, different hypotheses have been proposed based on characterizations. Based on an in situ XRD test, one typical statement indicates that the Na₃V₂(PO₄)₂O₂F experiences two completely reversible bi-phasic transitions from Na₃V₂(PO₄)₂O₂F to Na₂V₂(PO₄)₂O₂F, and then to NaV₂(PO₄)₂O₂F, as well as a low volumetric change during the charging and discharging processes [101]. Based on time-resolved in situ SXRD, another statement indicates a complex asymmetrical reaction during the Na⁺ insertion/extraction process [102].

Similar to Na₃V₂(PO₄)₂F₃, the sluggish Na⁺ diffusion kinetics and poor electronic conductivity of Na₃V₂(PO₄)₂O₂F cause a rapid fading of capacity during Na⁺ insertion/extraction and low specific energy (vs. Na⁺/Na). Therefore, multiple surfaces, morphology, and crystal structure modification strategies have been developed to address the problems [38,103,104,105]. Investigation on the family of Na₃(VO_1−xPO₄)₂F_1+2x (0 ≤ x ≤ 1) has been conducted to evaluate their performance. As shown in the Figure 5d, when x = 0 (or 1), the reaction between the charge/discharge process is only based on the V³⁺/V⁴⁺ (or V⁴⁺/V⁵⁺) redox couple [95]. Owing to the strong inductive effect of F⁻ in fluorine-rich samples, the V³⁺/V⁴⁺ redox potential of the latter (∼3.9 V) was higher by ∼0.13 V as compared to the V⁴⁺/V⁵⁺ redox potential of the former (∼3.77 V). The linear change in voltage may help us achieve a balance between voltage and capacity for higher specific energies (vs. Na⁺/Na) via changing the x.

3.4.2. Na₃V₂(PO₄)₂O_1.6F_1.4 (3.8 V)

When x = 0.8, the formula turns out to be Na₃V₂(PO₄)₂O_1.6F_1.4. It is isostructural to both Na₃V₂(PO₄)₂O₂F and Na₃V₂(PO₄)₂F₃ with a space group of P4₂/mnm [41]. Benefiting from the open framework, Na₃V₂(PO₄)₂O_1.6F_1.4 possesses a theoretical capacity of 129.7 mAh g⁻¹ based on one electron transfer with two potential plateaus at 3.61 and 4.02 V (vs. Na⁺/Na). However, it can actually exhibit a higher reversible capacity of 134 mAh g⁻¹ due to more electron transfer (Figure 5e) [40]. During the charge/discharge process, Na₃V₂(PO₄)₂O_1.6F_1.4 experiences a typical two-step solid-solution reaction while both V⁴⁺/V⁵⁺ and V³⁺/V⁴⁺ redox couples are active. Moreover, solid-state nuclear magnetic resonance and electron paramagnetic resonance have been employed to give a comprehensive understanding of the Na⁺ extraction/intercalation process [106]. The results indicated that V³⁺ mostly reacts in the high-potential charge region while V⁴⁺ continually translates to V⁵⁺ during the whole charge process. The influence of O content has also been investigated, and it was found that Na₃V₂(PO₄)₂O_1.6F_1.4 has broad and less defined peaks (Figure 5f) because of the solid solution reaction [96]. Meanwhile, the results indicated that the average redox potential of the second pseudo-plateau tends to rise with of the increase in fluorine content, which can be attributed to the more ionic V–F bond in comparison to the V=O one. Additionally, a small volume change (2.9%; 0.88 ≤ x ≤ 3) upon cycling was observed, which guarantees a remarkable cycling stability and rate performance. For instance, a nanostructured Na₃V₂(PO₄)₂O_1.6F_1.4 exhibited, via microwave-assisted solvothermal procedure, excellent rate capability (67.2 mAh g⁻¹ under 30 C) and long-term cycle performance (61 mAh g⁻¹ after 1000 cycles at 10 C) when tested as a cathode for Na-ion coin cells [107].

Compared with Na₃V₂(PO₄)₃, V-based fluorophosphates present an elevated specific capacity and an improved potential, which can be attributed to the lighter molecular mass and stronger inductive effect of F⁻. However, fluorine loss during heat treatment is a challenge, as fluorine is unstable in V-based fluorophosphates, and thus calls for proper preparation methods to precisely regulate material composition.

4. Pyrophosphates

Compared with phosphates and fluorophosphates, V-based pyrophosphates possess abundant crystal chemistry, robust structure, and superior Na⁺ mobility, manifesting potential in application. To date, NaVP₂O₇, Na₇V₃(P₂O₇)₄, and Na₂(VO)P₂O₇ are typical representatives.

4.1. NaVP₂O₇ (3.9 V)

NaVP₂O₇ presents a relatively high operating voltage of 3.9 V and a theoretical capacity of 108.4 mAh g⁻¹ [108,109]. As shown in Figure 6a, it has two types of crystal structures: α- and β-phase. α-NaVP₂O₇ is isostructural with the monoclinic NaMoP₂O₇, delivering a poor reversible capacity of 40 mAh g⁻¹ [42]. β-NaVP₂O₇ isotypic to the monoclinic KAlP₂O₇, and could be operated at 3.9 V vs. Na/Na⁺ (Figure 6b) with a reversible capacity of 104 mAh g⁻¹. The different structures of the two phases leads to lower migration barrier in the β-phase and thus disparate electrochemical performances. The diffusion of Na in α-NaVP₂O₇ was slower by several orders of magnitude than that of β-NaVP₂O₇, making β-NaVP₂O₇ a more suitable cathode material than the α-phase. β-NaVP₂O₇ also exhibited good high-rate performance (77 mAh g⁻¹ at 50 C) and low stable galvanostatic cycling with only 0.5% of volume change. Similar to Na₂(VO)P₂O₇, further research on NaVP₂O₇ is still limited.

4.2. Na₇V₃(P₂O₇)₄ (4.13 V)

Owing to the strong inductive effects of P₂O₇⁴⁻, Na₇V₃(P₂O₇)₄ possesses the highest redox potential to date among reported vanadium-based cathode materials for SIBs. It is isotypic to the disordered form of Na₇Fe₃(P₂O₇)₄ (ICSD:86437), which can be indexed into a monoclinic space group of C2/c [110]. Previous studies have shown that Na₇V₃(P₂O₇)₄ exhibits a reversibility of ~80 mAh g⁻¹ at an average voltage of 4.13 V (vs. Na⁺/Na) based on the V³⁺/V⁴⁺ redox couple (Figure 6c,d) [43]. Additionally, the electrode also presented a good rate capability with 75% initial capacity retention after 600 cycles at 1 C, which can be attributed to the open framework and low volume change (~1%) of the material during charge/discharge possess. Recently, detailed Na⁺ extraction/insertion processes of Na₇V₃(P₂O₇)₄ have been investigated. As shown in Figure 6e, the charge capacity can be increased up to 106 mA h g⁻¹ with extraction of about 4 Na⁺ ions via activation of the V⁴⁺/V⁵⁺ redox couple at higher voltages [44]. However, this Na⁺ insertion/extraction reaction at high voltage is not fully reversible. In a wider voltage window of 1.3–4.8 V vs. Na⁺/Na, operando X-ray diffraction was employed to determine the theoretical selective order of Na⁺ extraction/insertion, demonstrating that the capacity of polyanionic materials may be promoted by the activation of additional redox couples for metal ions.

4.3. Na₂(VO)P₂O₇ (3.8 V)

Introducing vanadyl into pyrophosphates may be another promising approach. Dating back to 1998, tetragonal Na₂(VO)P₂O₇ has been reported to have an excellent ionic conductivity of 3.05 × 10⁻⁵ S cm⁻¹, rendering it a potential candidate for cathode applications [111]. However, it has only been explored as a potential cathode material for SIBs in the last decade. Slabs of [VP₂O₈]_∞ are piled up along the c direction, intercepted by slabs of Na atoms, and thus a layered framework with Na atoms located along the tunnels is constructed. Moreover, the structure lets Na⁺ preferentially diffuse between the layers, which can be considered a 1D conductor for Na diffusion. As can be seen in the charge/discharge curves displayed in Figure 6f, the Na₂(VO)P₂O₇ can deliver a reversible capacity of ~80 mAh g⁻¹ at 3.8 V based on a V⁵⁺/V⁴⁺ redox reaction, while its theoretical capacity is 93.4 mAh g⁻¹ [45]. Different from Na₇V₃(P₂O₇)₄, the reaction of Na₂(VO)P₂O₇ is mainly based on the V⁵⁺/V⁴⁺ redox couple rather than activating it for higher voltage.

Generally, pyrophosphates possess a relatively high operating voltage but a low specific capacity because of the strong inductive effect and large molecular mass of P₂O₇⁴⁻. Additionally, low theoretical specific capacity and poor conductivity mainly limited their development. Therefore, partially (rather than completely) replacing PO₄ with P₂O₇⁴⁻ and other polyanions to generate mixed polyanions may be an effectively way to handle this dilemma.

5. Mixed Polyanions

As mentioned above, by combining various polyanionic units, the concept of “mixed polyanionic compounds” has been proposed. This kind of combination can not only result in multiple electron redox activity and abundant structural diversity, but also regulate the redox voltage via different degrees of inductive effect. Among V-based mixed-polyanionic compounds, Na₇V₄(P₂O₇)₄(PO₄) and Na₂(VO)₂(HPO₄)₂(C₂O₄) are two typical representatives.

5.1. Na₇V₄(P₂O₇)₄(PO₄) (3.85 V)

Na₇V₄(P₂O₇)₄(PO₄) is indexed to a tetragonal structure (P-421c) in which a 3D open framework and well-defined ionic channels can serve for Na (de)insertion. Lim et al. reported that Na₇V₄(P₂O₇)₄(PO₄) can exhibit an initial capacity of 91.0 mAh g⁻¹ with a retention of ~78% over 1000 cycles [112]. The CV curve and charge/discharge profiles of Na₇V₄(P₂O₇)₄(PO₄) are displayed in Figure 7a,b [46]. It exhibits a single-valued voltage plateau at 3.88 V vs. Na⁺/Na based on the V³⁺/V⁴⁺ redox reaction. Moreover, Deng et al. found the intermediate phase of Na₅V^3.5+₄(P₂O₇)₄(PO₄) during the sodium de/intercalation of a Na₇V₄(P₂O₇)₄(PO₄) nanorod [113]. As a result, the potential plateau can be subdivided into 3.87 V (V³⁺/V^3.5+) and 3.89 V (V^3.5+/V⁴⁺) (Figure 7c). Methods of carbon coating and structure have also been employed to improve the performance of Na₇V₄(P₂O₇)₄(PO₄) [47,114,115,116]. Recently, an interesting study indicated that Na₇V₄(P₂O₇)₄PO₄ can be used for both cathode and anode materials owing to the operating voltage of 3.85 V and 0.94 V, respectively [46]. They assemble a symmetric full battery which exhibits an initial capacity of 81.9 mAh g⁻¹ at 2.84 V. Although the compound possesses a high operating voltage and well-defined plateau, a relatively low capacity due to the heavy P₂O₇ group and the complexity still restricts its wide application.

5.2. Na₂(VO)₂(HPO₄)₂C₂O₄ (4.0 V)

Generally, the V⁵⁺/V⁴⁺ redox reaction suffers from poor reversibility in other polyanionic compounds, leading to rapid capacity degradation. A solution is to employ the coupling of (C₂O₄)²⁻ and (HPO₄)²⁻ to result in a smaller forbidden band gap and lower energy barrier 2D Na⁺ ion migration paths, as well as to stabilize reversible high-valent redox of V⁴⁺/V⁵⁺. Very recently, Li et al. reported a V-based mixed-polyanion sodium oxalate-phosphate compound, formulated Na₂(VO)₂(HPO₄)₂(C₂O₄) [48]. As shown in Figure 7d, it exhibited a high redox potential of 4.0/3.8 V and a considerable reversible capacity of 105.4 mAh g⁻¹ based on the V⁴⁺/V⁵⁺ redox couple. The high redox potential is confirmed by the dQ/dV plots (inset), which can be attributed to the accumulated inductive effect of mixed anions. As shown in Figure 7e, the corresponding dQ/dV curves of different cycles indicated the low voltage decay of only 0.08 V in the Na₂(VO)₂(HPO₄)₂(C₂O₄) electrode. Hence, this showed the decent cycling stability with an initial capacity retention of 74.7% after 100 cycles at 0.2 C and 61.2% after 1000 cycles at 5 C.

The concept of mixed polyanions brings us new ideas regarding cathode materials. However, the complexity of the components limits the preparation methods and influences the phase purity; thus, to determine their electrochemical performance, more work is necessary to decipher this puzzle.

6. Summary and Perspectives

In this review, the structure, Na⁺ storage mechanisms, charge transfer kinetics, and electrochemical performance of members among V-based polyanionic phosphates, fluorophosphates, vanadyl fluorophosphates, pyrophosphates, and mixed polyanions were systematically introduced and interpreted. A comprehensive comparison of their potential, specific capacity, and specific energy (vs. Na⁺/Na) is displayed in Figure 8.

V-based phosphates have been widely investigated because of their high operating potential, and outstanding structural and thermal stability. The inductive effect of the (PO₄)³⁻ polyanion endows them with high potential while the strong P–O bond ensures stable structure during charge–discharge process. However, they are short on efficient methods to further promote the working voltage, and the stronger inductive effect of F atoms made them a substitution alternative to further promote operating voltage. V-based fluorophosphates present a relatively higher operating voltage and specific capacity but also suffer from fluorine loss during heat treatment. Pyrophosphates possess rich crystal chemistry, stable structure, and superior Na⁺ mobility, manifesting potential in application. However, the high molecular mass of P₂O₇⁴⁻ leads to low theoretical specific capacity, although mixed polyanions seem to be a promising idea. Therefore, a balance of voltage and capacity is likely to be achieved using mixed polyanions, but the complexity of the components limits the synthetic methods and their electrochemical performance. In short, V-based phosphates are the most promising cathode material for massive applications, while fluorophosphates have the potential to achieve higher operating voltage and specific energy (vs. Na⁺/Na).

Although encouraging progress has been achieved in V-based polyanionic materials for SIBs, obstacles still remain that must be addressed. A comprehensive conclusion will help us to better understand the future of high-voltage V-based polyanionic materials:

Improvement in the operating voltage. Although V-based polyanionic compounds present relatively high operating voltages, more work is still urgently needed to further enhance the voltage of Vⁿ⁺¹/Vⁿ⁺ (n = 2,3,4) redox couples as well as specific energies (vs. Na⁺/Na). There are three primary strategies to improve the operating voltage: utilization of inductive effect, activation of the V⁵⁺/V⁴⁺ redox couple, and substitution of functional elements. On one hand, owing to the inductive effect, the stronger electronegativity of X and a more ionic M–O bond will always result in a higher working voltage for the Mⁿ⁺¹/Mⁿ⁺ redox couple. For the V³⁺/V⁴⁺ redox couple, different groups exhibit diverse operating voltages, following the order of P₂O₇ > PO₄F ≈ (PO₄)_m(P₂O₇) > (PO₄). On the other hand, it is possible for V-based compounds are to exhibit a multi-electron reaction (involving V³⁺/V²⁺, V⁴⁺/V³⁺, and V⁵⁺/V⁴⁺ redox couples). V⁴⁺/V³⁺ is the most widespread redox couple with a moderate voltage, while an efficient way to achieve a much higher voltage is to activate the V⁵⁺/V⁴⁺ redox couple at a higher voltage. Lastly, partially substituting vanadium with other high-voltage redox elements is also regarded as an efficient approach to promote voltage. Mn²⁺ and Co²⁺ may be ideal options and further investigation is necessary.
Developing new V-based polyanionic compounds. To achieve higher specific energy (vs. Na⁺/Na), partially substituting heavy polyanionic groups (PO₄³⁻, P₂O₇⁴⁻) with other light groups (CO₃²⁻, BO₃³⁻, etc.) is a viable option. The oxalate–phosphate compound Na₂(VO)₂(HPO₄)₂(C₂O₄) mentioned above is a referential instance. Additionally, considering the higher electronegativity for SO₄²⁻, V-based sulfates may also an available way to achieve high-voltage material. Vanadium-based phosphate polyanionic compounds are competitive candidates for application in SIBs because of their high voltage, high power density, and cycle stability.
Although high-voltage V-based polyanionic compounds seem to be competitive candidate for SIBs, there is still a long way to go before their wide application (Figure 9). For instance, Li₃V₂(PO₄)₃ is a well-studied cathode material in Li-ion batteries but has not been commercialized for large-scale EES; however, the analysis of it may help us figure out the obstacles towards commercialization. Li₃V₂(PO₄)₃ provides a high capacity of 156.9 mAh g⁻¹ within the voltage range of 3.0–4.8 V. However, its instability under high pressure, which may be caused by lithium–vanadium antisite mixing, necessitates researchers to limit the upper cut-off voltage to 4.3 V, resulting in uncompetitive specific energy (380 Wh kg⁻¹ for Li₃V₂(PO₄)₃, ~496 Wh kg⁻¹ for LiFePO₄, ~430 Wh kg⁻¹ for LiMn₂O₄). In short, the performance gap with rival products makes it difficult to commercialize. Even if V-based compounds have comparative advantages over other transition metal compounds, challenges still remain before their mass production. First, while vanadium resources are abundant in the upper continental crust, the price of vanadium is higher when compared to Fe, etc. Additionally, the recycling of vanadium and vanadium dissolution are also urgent problems before commercialization. Lastly, the pollution and toxicity of vanadium have also been noteworthy issues. Research indicates that vanadium enrichment will become an ecological problem around vanadium-production areas. Meanwhile, vanadium is moderately toxic, and excess vanadium can cause certain damage to the body’s organs and tissues. In general, if we can improve the production process of vanadium resources to reduce pollutants, and recycle V-based batteries at the end of their life, then vanadium-based cathode materials will usher in a promising future and maintain exuberant vitality.

Author Contributions

Conceptualization, X.P. and Z.C.; investigation, H.W. and Y.C.; data curation, H.W., T.W. and L.C.; writing—original draft preparation, H.W. and X.P.; writing—review and editing, H.W., X.P. and Z.C.; supervision, X.P. and Z.C.; funding acquisition, Z.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (21875171 and U22A20438) and the Key R&D Plan of Hubei Province (2020BAA030).

Data Availability Statement

No data support.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 3. (a) Typical charge–discharge curves and CV (inset) between 1.5–4.3 V of the layered VOPO₄ electrodes [25]. (b) CV profiles (scan rate at 0.1 mV s⁻¹) and crystal structure of α-NaVOPO₄ [73]. (c) Charge–discharge curves of β-VOPO₄ at C/20 (8 mA g⁻¹) between 4.3 and 1.5 V. Insets show the crystal structure and cycling capacities [74]. (d) How the type and concentration of dopants on the V site influence the cell voltage (vs. Na⁺/Na) in the α-NaVOPO₄ polymorphs (M = Al³⁺, Co²⁺, Fe³⁺, Mn⁴⁺, Ni²⁺, or Ti⁴⁺) [75]. (e) Lattice structure and galvanostatic charge–discharge profiles of the layered NaVOPO₄ electrode at a current rate of 0.05 C (1 C = 145 mA g⁻¹) [28]. (f) Charge/discharge curve of Na₄VO(PO₄)₂. Inset is dQ/dV [29].

Figure 4. (a) The formation energy calculated by DFT methods and (b) galvanostatic charge/discharge curves at 0.1C of NaVPO₄F-M and NaVPO₄F-T [79]. (c) The crystal structure and (d) cyclability, as well as CV curves, of o-Na₅V(PO₄)₂F₂/C and t-Na₅V(PO₄)₂F₂ [35].

Figure 6. (a) The crystal structure, shortest Na⁺ migration pathways, and migration barriers of β-NaVP₂O₇ and α-NaVP₂O₇. (b) Galvanostatic charge–discharge curves for β-NaVP₂O₇ collected at 10 mA g⁻¹ current density between 2.0 and 4.4 V vs. Na⁺/Na. Inset is the CV measurement for β-NaVP₂O₇ performed within 2.0–4.5 V [42]. (c) Calculation results of average redox potential for Na_xV₃(P₂O₇). (d) Charge/discharge curves for Na₇V₃(P₂O₇)₄. Inset is the dQ/dV profile of Na₇V₃(P₂O₇)₄ [43]. (e) First two cycles of Na₇V₃(P₂O₇)₄ in the extended range of 1.3–4.8 V [44]. (f) Galvanostatic characterization of Na₂(VO)P₂O₇ cathode [45].

Figure 7. (a) CV curve at a scan rate of 0.1 mV s⁻¹ in 3.3–4.2 V and (b) the charge/discharge curves at various rates of the Na₇V₄(P₂O₇)₄(PO₄)/C-GA cathode [46]. (c) The GITT charge and discharge curves of a Na₇V₄(P₂O₇)₄(PO₄) nanorod [115]. (d) Galvanostatic charge/discharge curves and the corresponding dQ/dV patterns of Na₂(VO)₂(HPO₄)₂(C₂O₄). (e) The dQ/dV plots of different cycles of Na₂(VO)₂(HPO₄)₂(C₂O₄) and Na₃V₂(PO₄)₃ at 0.2 C [48].

Figure 8. Comparisons among V-based polyanionic compounds for SIBs.

Figure 9. Perspective on the application of high-voltage V-based compounds [117].

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Wu, H.; Chen, Y.; Wen, T.; Chen, L.; Pu, X.; Chen, Z. Advances in Vanadium-Redoxed Polyanions for High-Voltage Sodium-Ion Batteries. Batteries 2023, 9, 56. https://doi.org/10.3390/batteries9010056

AMA Style

Wu H, Chen Y, Wen T, Chen L, Pu X, Chen Z. Advances in Vanadium-Redoxed Polyanions for High-Voltage Sodium-Ion Batteries. Batteries. 2023; 9(1):56. https://doi.org/10.3390/batteries9010056

Chicago/Turabian Style

Wu, Honglun, Yiqing Chen, Tianzhuo Wen, Long Chen, Xiangjun Pu, and Zhongxue Chen. 2023. "Advances in Vanadium-Redoxed Polyanions for High-Voltage Sodium-Ion Batteries" Batteries 9, no. 1: 56. https://doi.org/10.3390/batteries9010056

APA Style

Wu, H., Chen, Y., Wen, T., Chen, L., Pu, X., & Chen, Z. (2023). Advances in Vanadium-Redoxed Polyanions for High-Voltage Sodium-Ion Batteries. Batteries, 9(1), 56. https://doi.org/10.3390/batteries9010056

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Advances in Vanadium-Redoxed Polyanions for High-Voltage Sodium-Ion Batteries

Abstract

1. Introduction