Recent Progress on Sodium Vanadium Fluorophosphates for High Voltage Sodium-Ion Battery Application

2.1 Crystal structure, synthesis strategy and its electrochemical performance of Na₃V₂(PO₄)₂F₃

The NASICON-type Na₃V₂(PO₄)₂F₃ crystal structure is simply derived from the crystal structure of Na₃V₂(PO₄)₃ by introducing three F⁻ instead of one (PO₄)³⁻ to form Na₃V₂(PO₄)₂F₃ [26]. NVPF belongs to tetragonal system with the space group of P4₂/mnm. It is composed of VO₄F₂ octahedral units that forming [V₂O₈F₃] bi-octahedral units and is alternatively bridged by [PO₄] tetrahedral units that form a three dimensional network with large voids in the [110] and [110] directions which provide easy pathways for Na⁺ migration in the crystal structure (Fig. 1a). In NVPF, two Na(1) sites are fully occupied while the other two Na(2) sites are half occupied by Na ions [26–28]. Compared to NVP, the potential of NVPF is higher owing to the enhanced inductive effect of the (PO₄)³⁻ polyanion and the larger ionicity of the F-V bond [29]. The NVPF cathode exhibits an average working potential of 3.9 V (two potential plateaus at 3.7 and 4.2 V vs. Na/Na⁺) and its corresponding theoretical capacity is 128 mAh g⁻¹, which is a result of the extraction of two Na⁺ from the crystal structure. In addition, the energy density of NVPF (~507 Wh/kg) is similar to those of the commercially used Li-ion cathode materials such as LiFePO₄ (580 Wh/kg) and LiMn₂O₄ (480 Wh/kg) [26–31].

Fig. 1

(a) Crystal structure of Na₃V₂(PO₄)₂F₃ and (b) electrochemical voltage-composition curve. Reprinted with permission from Ref. [29]. Copyright © 2014 American Chemical Society.

In 2014, Song et al. investigated the electrochemical behavior of Na₃V₂(PO₄)₂F₃ [29]. The electrochemical voltage-composition curve of Na₃V₂(PO₄)₂F₃ is shown in Fig. 1b. Two voltage plateaus at 3.7 and 4.1 V can be inferred for the charging process, during which 1.9 Na⁺ could be extracted from the crystal structure to forming the Na_1.1V₂(PO₄)₂F₃. During the discharging process, only 1.8 Na⁺ are inserted back into the structure, resulting in the formation of Na_2.9V₂(PO₄)₂F₃. This difference in Na⁺ insertion and extraction is associated with an irreversible electrochemical reaction and some side reaction occurs at the electrode/electrolyte interfaces that result in capacity losses [29].

The electrochemical insertion/extraction process is mainly attributed to the V³⁺/V⁴⁺ reaction (eq. 1), which is not related to V⁴⁺/V⁵⁺ because the plateau region at different voltage corresponds to the insertion/extraction of Na ions at different crystallographic sites [28,29]. In the voltage-composition curve, plateau at 3.4 V is corresponded to the extraction of Na⁺ ions from the Na₃V₂(PO₄)₃ and the latter plateaus are attributed to the extraction Na⁺ ions from the Na₃V₂(PO₄)₂F₃ [32,33]. It is assumed that the NVPF prepared by Shakoor et al. was the mixed phase of Na₃V₂(PO₄)₂F₃ and Na₃V₂(PO₄)₃. Furthermore, the reported XRD profile contained small diffraction peaks at 20, 23 and 32° are well matched with the Na₃V₂(PO₄)₃ XRD peaks [34]. Here, the first plateau region originated from the extraction of Na⁺ from the Na(2) site, because the Na(2) site is less stable than the Na(1) site, and therefore, the Na ions are not very the stable position due to the electrostatic repulsion of Na-Na that facilitate the extraction of the Na ion from the Na(2) site during the initial stage of charging. The calculated structural energy values for Na⁺ extraction from the Na(1) and Na(2) sites are - 1006.00 and -1009.05 kJ mol⁻¹ respectively [28]. After complete extraction of the Na⁺ ion from the Na(2) site, the unstable of Na₂V₂(PO₄)F₃ structure is formed as a result of shorter Na-Na distance (2.92 Å). After reorganization the second Na-ion is extracted from the Na(1) sites at a higher potential (~4.1 V), which results in the formation of NaV₂(PO₄)₂F₃ at the end of the charging process [26,28,29]. After charging/discharging process, there is no change in the crystal structure of NVPF, which be explained by Shakoor et al. through their ex-situ XRD results [28]. The NVPF cathode material delivered an initial capacity of 92 mAh g⁻¹ at 0.91 C current rate [29]. In addition, they also estimate the diffusion coefficient of Na ions by using CV analysis based on the relationship between peak current (i_p) and the square root of scan rate (v^1/2). The calculated diffusion coefficient was 2.04 × 10⁻¹¹ cm² s⁻¹ [29]. Generally, the NVPF electrode shows poor electronic conductivity owing to the insulating nature of PO₄ ³⁻, therefore, it is very difficult to attain high rate capability [35].

High rate capability is one of the important factors for practical application and is mainly associated with two factors: ionic diffusion and electronic conduction. Recent studies have suggested ideas on how to overcome this issue: (i) reducing the particle size to the nano level could effectively enhance ionic diffusion [36,37], and (ii) NVPF nanoparticles composite with carbonaceous material such as amorphous carbon and graphene can enhance the electronic conductivity. In addition, metal doping onto the vanadium site can considerably increase the intrinsic electronic conductivity of the material [26,35,38–40]. Recently, Chen’s group implemented a new synthesis strategy for boosting up the high power capability of the Na₃V₂(PO₄)₂F₃ cathode [39]. They prepared the core/double shell structured Na₃V₂(PO₄)₂F₃@C_D nanocomposite through in situ coating of the carbon and the prepared particles were uniformly distributed in the mesoporous carbon framework. The schematic representation describeing the detailed synthesis strategy is shown in Fig. 2(a) [39]. Fig. 2b shows the HRTEM image, in which NVPF@C nanoparticles are incorporated into a mesoporous carbon structure and the thickness of the carbon coating is observed to be around 2 nm. The Na₃V₂(PO₄)₂F₃@C_D nanocomposite shows smaller redox polarization than NVPF/C composite based on CV analysis (not shown here), which suggests that fast electronic and ionic transport is achieved through the well dispersed mesoporous carbon conducting network [39]. Furthermore, charge discharge analysis demonst rated that the Na₃V₂(PO₄)₂F₃@C_D composite delivered 125, 123, 121, 116, 100, 92, 84 and 63 mAh g⁻¹ at the current rate of 0.5, 1, 5, 10, 20, 30, 50 and 100 C respectively (Fig. 2c). The cycling stability was extended to 5000 cycles at 50 C rate, with a reversible capacity of 62 mAh g⁻¹ and a capacity retention of 65% (Fig. 2d). These result revealed that the high electrochemical performances arise from the uniform distribution of the NVPF@C nanoparticles on the mesoporous carbon that offers sufficient electrochemical interfaces and abundant electronic/ionic pathways [39]. Following this, many studies have attempted to enhance the rate capability of Na₃V₂(PO₄)₂F₃ by changing the conducting matrix to graphene, carbon nanofibers and carbon nanotubes [30,41–43]. All the composition remarkably enhanced the rate capability, Zhao et al. prepared an in situ carbon nanofibers coating on Na₃V₂ (PO₄)₂F₃@C particles through chemical vapour deposition(CVD) (Fig. 3a) by using Fe as the catalyst at different ratios of NVPF@C to Fe (1:100, 5:100 and 10:100) [42]. CVD is a convenient method for producing highly conducting carbon with different structures [44,45]. Here, the NVPF@C@CNF-5 (5:100) sample was considered as the optimum one; upon increasing the Fe content, more or thicker carbon layers are formed on the surface of the active material. The thick carbon layer act as a barrier for Na-ion diffusion and thus affects the electrochemical performance even at low current density. The TEM image (Fig. 3b) of the NVPF@C@CNF-5 obviously shows that the NVPF@C particles are completely wrapped by the CNFs to form a three-dimensional electronic transfer network and the measured thickness of the carbon coating is around 2–3 nm (Fig. 3c). The electrochemical performance of NVPF@C@CNF-5 was analyzed at the high current rate of 20 C over 5000 cycles, a reversible capacity 93.3 mAh g⁻¹ with a capacity retention of 86.3% at more than 99.5% coulombic efficiency were observed (Fig. 3d). The high rate capability and cyclability originated from the uniform wrapping of the CNFs on the NVPF@C particles. Here, the CNFs serve the dual role of a three-dimensional conducting framework and a buffering matrix. The first, role helps achieving a high rate charge/discharge ability, while the second role restricts the volume changes that occur during the insertion/extraction process [42].

Fig. 2

(a) Schematic illustration of NVPF@C_D nanocomposite, (b-c) TEM image, rate capability and cycling stability curve of NVPF@C_D nanocomposite. Reprinted with permission from Ref. [39]. Copyright © 2016 American Chemical Society.

Fig. 3

(a) Synthesis process of NVPF@C@CNFs, (b, c) TEM and HRTEM images of NVPF@C@CNF-5, (d) Cycling stability curve of NVPF@C@CNFs. Reprinted with permission from Ref. [42]. Copyright © 2018 Wiley-VCH verlag GmbH&Co. KGaA.

In general, the electrochemical performance not only is related to the surface electronic conductivity but also depends on the intrinsic or bulk electronic conductivity of the electrode material [35]. The intrinsic electronic conductivity could be enhanced by ion doping onto the Na or vanadium sites in the NVP and NVPF structure [26,35,46–48]. Zhang et al. prepared Ti-doped Na₃V_2-xTi_x(PO₄)₂F₃ and investigated it by experimental and DFT analysis [35]. The pristine NVPF shows a bandgap of 1.167 eV and it decreased to 1.049 eV with the doping of titanium ions onto the vanadium sites suggests that electronic conduction is easier in Ti-doped NVPF compared to pristine NVPF. The electrochemical results also confirmed that enhancements in cycling stability (91 mAh g⁻¹ at 40 C over 500 cycles) and rate capability (81 mAh g⁻¹ at 80 C) relative to the pristine material [35].

Barker et al. initially investigated NaVPO₄F for application in Na-ion batteries. They refined the crystal structure the tetrahedral symmetry of Na₃Al₂(PO₄)₃F₂ (space group I4/mmm) and explained the electrochemical reaction that occurred in a similar fashion to that involving Al compounds. Here, the extended three-dimensional network of [AlO₄F₂] octahedra and [PO₄] tetrahedra that formation creates cavities, in which the Na ions are distributed. The full cell was fabricated using hard carbon and delivered an initial capacity of 79 mAh g⁻¹ with an output voltage of 3.7 V [49]. Many groups investigated the electrochemical performance of NVPO₄F by using different synthesis processes which are listed in Table 1 [50–53]. However, the electrochemical performance was not appropriately enhanced compared to the Na₃V₂(PO₄)₂F₃ cathode material.

Table 1

Electrochemical properties of sodium vanadium fluorophosphates.

2.2 Crystal structure, synthesis and electrochemical performance of Na₃V₂O_2x(PO₄)₂F_3-2x

During the synthesis of Na₃V₂(PO₄)₂F₃, a small amount of oxygen is incorporated into its crystal structure, which is typically denoted as Na₃V₂O_2x(PO₄)₂F_3-2x (x= 0 to 2); this amount mainly depends on the atmospheric condition and the carbon content during the synthesis process [54–64]. Fig. 4(a, b) indicates the structural correlation between Na₃V₂(PO₄)₂F₃ (x=0) and Na₃V₂O₂(PO₄)₂F (x=2). The structural and electrochemical performances of Na₃V₂O_2x(PO₄)₂F_3-2x have been analyzed by different groups [56–59]. These two structure shows a similar framework, because only the F atom is replaced by the O atom, which results in the vanadium existing in +3 and +4 oxidation states in Na₃V₂O₂(PO₄)₂F, and in a mixed valence state on intermediate compounds (0<x<2). This structure mainly arises from the [VO₅F] octahedrons and [PO₄] tetrahedrons that form a layer by sharing one oxygen atom. Here, Na(1) and Na(2) are occupied by disordered Na⁺ in a circular geometry in the ab plane and the adjacent layer is connected through the F ions that are coordinated to two [VO₅F] octahedrons; this structural arrangement presents a broad channel for the migration of Na+. In addition, Na⁺ exist in two types of coordination environment; the Na ion in the Na(1) site is coordinated by six O one F ions, whereas that in the Na(2) site is coordinated to six O ions [58,59].

Fig. 4

Crystal structure correlation between (a) Na₃V₂(PO₄)₂F₃ and (b) Na₃(VO)₂(PO₄)₂F. Reprinted with permission from Ref. [59]. Copyright © 2012 The Royal Society of Chemistry.

A detailed investigation of the insertion/extraction reaction of Na₃V₂O_2x(PO₄)₂F_3-2x was carried out by Serras [56], through electrochemical measurements, ex-situ XRD and XAS analysis. In this compound, the vanadium exists as V^3.77+, which was deduced by bond valence sum analysis (BVS) and substantiated by Vegard’s law. After the analysis, they proposed Na₃V₂O_1.6(PO₄)₂F_1.4 as the final composition. The electrochemical reaction involving the proposed compound was examined under different charge/discharging states and is shown in Fig. 5a. The electrochemical reaction is either single or multiphase, and was analyzed by ex-situ XRD analysis. It was confirmed that a single phase reaction occurred because the diffraction peaks and lattice parameter were unchanged during the charging/discharging process owing to the expansion and contraction of lattice (Fig. 5b). After the extraction of Na⁺, the Na-ion tunnels became smaller in the x and y-direction and wider in the z-direction (Fig. 5c) which agreed well with the lattice parameter values. In addition, the valence state was analyzed through XANES analysis, in which the pre-edge zone was assigned to V³⁺ (5468 eV), V⁴⁺ (5469.5 eV) and V⁵⁺ (5471 eV), which are indicated by arrows (Fig. 5d). The intensity of the lowest energy V³⁺ peak remained constant throughout the charging process, which corroborated that V³⁺ was not involved in the oxidation process. On the other hand, the intensity of the V⁴⁺ peak decreased, while that of V⁵⁺ increased subsequently during the charging process to suggest that V⁴⁺ oxidized to V⁵⁺. Further, they calculated the percentage of vanadium state by linear fitting analysis to obtain values of ~82% for V⁴⁺ and 18% for V³⁺. Based on the oxidation of vanadium, they proposed the following reaction mechanism:

Fig. 5

(a) Electrochemical voltage-composition curve and post-mortem points, (b) Lattice parameter evolution of Na₃V₂O_2x(PO₄)₂F_3-2x, (c) evolution of F(1)-F(1) and F(2)-F(2) distances during charging, and (d) Vanadium K-edge preedge feature of the samples and the standards. Reprinted with permission from Ref. [56]. Copyright © 2013 American Chemical Society.

Dai’s group recently synthesized Na₃V₂O₂(PO₄)₂F nanoparticles at low temperature and their synthesis process is shown in Fig. 6a [57]. The charge-discharge profile of the Na₃V₂O₂(PO₄)₂F nanoparticles, shown in Fig. 6b, reveals two redox potentials at 3.65/3.60 V and 4.07/4.00 V that corresponds to the extraction/insertion of Na⁺ into the Na(1) and Na(2) sites of the crystal structure. The Na₃V₂O₂(PO₄)₂F nanoparticles delivered a high reversible capacity of 112 mAh g⁻¹ at 0.5 C rate and retained 90% of the capacity over 1200 cycles at 2 C rate (Fig. 6c). The excellent cyclability and high rate performance are attributed to the nanosized particles that effectively shorten the electronic/ionic diffusion pathways and buffer the volume change occurring during the cycling process. Finally, we have summarized the electrochemical performance of previously reported NVPF in Table 1.

Fig. 6

(a) Synthesis process of Na₃V₂O₂(PO₄)₂F nanoparticles, (b) charge-discharge profile in the potential window 2.5-4.5 V at C/5 rate, and (c) cycling stability curve of Na₃V₂O₂(PO₄)₂F nanoparticles at 2 C rate. Reprinted with permission from Ref. [57]. Copyright © 2015 Wiley-VCH Verlag GmbH&Co. KGaA.

2.3 Crystal structure and electrochemical performance of Na₃V₂O_2x(PO₄)₂F_3-2x

In 2013, Kang’s group introduced the cathode material Na_1.5VPO₄F_0.7 for high-energy Na-ion batteries [62]. Na_1.5VPO₄F_0.7 belongs to the P4₂/mnm space group and is isostructural with Na_1.5VPO₅ and Na₃V₂(PO₄)₂F₃ [63]. The structure of Na_1.5VPO₄F_0.7 consists of corner sharing between the PO₄ tetrahedral and VO₅F/VO₄F₂ octahedral units that form a three-dimensional open framework, with the Na ions forming the Na layers on the ab plane of the structure (Fig. 7a). Noticeably, vanadium has two types of local environments, namely VO₅F and VO₄F₂ octahedra, in the presence of F⁻. The researchers examined the vanadium state and structural stability during the chargingdischarging process through XANES and ex-situ XRD, respectively. In XANES analysis (Fig. 7c), the edge of the spectrum shifted towards higher energy and completely reverted to the original state, which implied that the oxidation of vanadium is reversible during the charging/discharging process. The ex-situ XRD pattern shows that during the charging process (corresponding to the extraction of 0.5 Na⁺), the peak is continuously shifted and peak splitting is observed in a 30% state of charging (SOC), which indicates the coexistence of two phases. For 50% SOC, the final phase Na_0.5VPO_4.8F_0.7 is formed, which could be reversed during the discharging process (Fig. 7d). Moreover, a high average voltage of 3.8 V with 83% capacity retention over 500 cycles at 1 C current rate is observed. According to its theoretical capacity (155.6 mAh g⁻¹ for 1.2 Na⁺ extraction), the energy density is nearly 600 Wh kg⁻¹. These findings concluded that the Na_1.5VPO_4.8F_0.7 cathode is one of the highest energy density cathode materials reported so far (Fig. 7e) [62].

Fig. 7

(a) Crystal structure of Na_1.5VPO_4.8F_0.7 and the local environment of the vanadium ions, (b, c) XANES and ex-situ XRD patterns of the charging-discharging process, and (d) comparison of the energy density with those of different cathode materials. Reprinted with permission from Ref. [62]. Copyright © 2013 American Chemical Society.