Na₃V₂(PO₄)₂F₃ is obtained by regulating anions to expand the NASICON structure of Na₃V₂(PO₄)₃. Studies have found that introducing ions with stronger electronegativity (F^-, N^3-, P₂O₇^4-) to replace some (PO₄) groups in Na₃V₂(PO₄)₃ results in Na₃V₂(PO₄)₂F₃, which not only enhances the induction effect and improves the material's crystal structure but also increases the material's redox potential^[37]. The charge-discharge voltage platform of Na₃V₂(PO₄)₃ material is about 3.4 V, with a theoretical specific capacity of 117 mAh·g^-1, while F-substituted Na₃V₂(PO₄)₂F₃ cathode material still has a three-dimensional structure. The average operating voltage of this material is 3.95 V, with a theoretical specific capacity of 128 mAh·g^-1, which makes it more significant for research compared to Na₃V₂(PO₄)₃^[38].

Na₃V₂(PO₄)₂F₃ belongs to NASICON-type materials, and its crystal structure can be described as having the P4₂/mnm space group (tetragonal system), as shown in Fig. 1(a~c). At room temperature, it is in the β-Na₃V₂(PO₄)₂F₃ phase^[39], with lattice parameters a=9.047(2) Å and c=10.705(2) Å. The [V₂O₈F₃] octahedral units are bridged by [PO₄] tetrahedral units, and both units contribute to expanding the 3D framework. Along the (110) and (001) directions, sodium ions can freely migrate through large tunnels^[40-41].

Na₃V₂(PO₄)₂F₃ was first synthesized in 1999. Meins et al.^[40] prepared a series of fluorophosphate materials (Na₃M₂(PO₄)₂F₃; M = Al³⁺, V³⁺, Cr³⁺, Fe³⁺) using hydrothermal and solid-state methods, and systematically studied their crystal structures. The results showed that NVPF with the P4₂/mnm space group belongs to the tetragonal system, consisting of [V₂O₈F₃] double octahedra and [PO₄] tetrahedra. The double octahedra [V₂O₈F₃] are connected by F atoms, while the tetrahedra [PO₄] are connected by O atoms to form a three-dimensional network structure. Charge exchange is usually accompanied by the redox of transition metal ions in the crystal structure, so the electron diffusion path depends on the interconnection between [V₂O₈F₃] double octahedra. The presence of [PO₄] enables rapid migration of sodium ions in the framework structure and maintains structural stability during the redox process. However, the insulating [PO₄] tetrahedral structure also blocks electron transport. These insulators are uniformly connected to the [V₂O₈F₃] double octahedra, causing obstruction of electron transport paths during charging and discharging processes, significantly affecting the electrochemical reaction performance. To further study the sodium ion deintercalation mechanism of NVPF, Zhao et al.^[42] used a combination of first-principles calculations and experimental investigations to study the deintercalation and intercalation behavior of sodium ions in Na₃V₂(PO₄)₂F₃. The research indicated that there are two positions in each unit's crystal structure for accommodating sodium ions, namely Na(1) and Na(2) sites. Through the intercalation/deintercalation of sodium ions during charge and discharge, two voltage plateaus can be formed. Due to the high chemical potential of Na(2) site ions, they are inserted first and extracted last. The charge/discharge plateau in the lower potential range (3.3~3.6 V) is caused by the intercalation/deintercalation of sodium ions at the Na(2) site in the NVPF crystal, while the second charge/discharge plateau (4.0 V) is due to the intercalation/deintercalation of sodium ions at the Na(1) site^[43]. Despite the many ion diffusion channels in NVPF, the slow diffusion rate of sodium ions and low electronic conductivity severely limit its electrochemical performance.

As the research on NVPF deepens, its preparation methods are becoming more diverse, with the most representative ones being the high-temperature solid-phase method, hydrothermal method, and sol-gel method. Additionally, spray drying^[44], electrospinning^[45], soft templating^[46], among others, are also commonly used methods.

The high-temperature solid-state method is the main preparation method for cathode materials of sodium-ion batteries, synthesizing the target product through means such as ball milling and sintering. Gover et al^［47］ first synthesized vanadium phosphate using carbon as a reducing agent, then mixed the vanadium phosphate with a certain amount of sodium fluoride, used argon as a protective gas, and sintered it at 600~800 ℃, ultimately obtaining NVPF. The high-temperature solid-state method has a simple process and is easy to operate, but the reaction temperature is high, the raw material solid particles do not react sufficiently, resulting in irregular morphology and uneven distribution of the obtained material particles, as shown in Figure 2a, it can be found that the particle size consistency is poor, uneven, and agglomerated; these irregular morphologies limit the electrochemical performance of the material^［48-49］.

The hydrothermal method, also known as the solvothermal method, utilizes the characteristic that the vast majority of reactants can partially dissolve under high pressure, using a PTFE-lined autoclave to employ the solvent as a pressure-transmitting medium, promoting the reaction to occur in the liquid or gas phase^[50]. Zhu et al.^[36] prepared NVPF/graphene composite nanomaterials via a one-step hydrothermal method. Without subsequent heat treatment, they obtained highly porous materials where nanoparticles are connected to graphene, enabling a charge-discharge cycle completed in 6 minutes with an energy density as high as 348 Wh·kg^-1, showing high specific capacity, excellent cycling stability, and rate performance. Ye Fan et al.^[51] used ascorbic acid, citric acid, oxalic acid, and tartaric acid as complexing agents respectively, to prepare a series of NVPF through a one-step hydrothermal method, discovering that the type of complexing agent affects the material's phase, morphology, and electrochemical properties. Among them, the sample with ascorbic acid as the complexing agent is cubic-shaped with the highest crystallinity and optimal electrochemical performance. The products obtained by the hydrothermal method are not only of high purity and have fewer defects, but also small particle size with uniform distribution. The particle morphology is shown in Fig. 2b, however, its reaction time is relatively long and it requires high equipment sealing^[49].

The sol-gel method is a commonly used approach to prepare inorganic materials through a series of chemical reactions such as hydrolysis, coordination, and cross-linking in solution at low temperatures using metal organic compounds or inorganic salts. This process initially forms a sol, then transforms into a gel, and finally sinters at a lower temperature^［49］. The preparation process is shown in Figure 2d. Jiang et al.^［52］ used citric acid as a chelating agent and carbon source to first synthesize the NVPF precursor via the sol-gel method. Then, under the protection of nitrogen, the NVPF/C composite material was obtained through two-step calcination. In the material, the carbon formed by the high-temperature pyrolysis of citric acid can not only inhibit the excessive growth of NVPF particles but also ensure good contact between NVPF particles, thereby significantly improving the electrical conductivity of the material. Therefore, the initial discharge specific capacity of the material is 127 mAh·g^-1, close to the theoretical capacity of NVPF (128 mAh·g^-1). Li et al.^［53］ prepared potassium-doped NVPF composite materials coated with carbon nanotubes (CNT) (NKVPF@CNT) using the sol-gel method. This material exhibits excellent specific capacity, with a discharge specific capacity of 120 mAh·g^-1 at 1 C, and the discharge specific capacity after 1600 cycles remains above 90 mAh·g^-1. The sol-gel method is usually carried out under low temperature or mild conditions, with low energy consumption, and it easily achieves uniform mixing of reactants at the molecular level, so the product has good uniformity, as shown in Figure 2c. However, its synthesis cycle is long, steps are cumbersome, and industrialization is more challenging^［50］.

In summary, the high-temperature solid-state method, hydrothermal method, and sol-gel method are common approaches for preparing NVPF. The high-temperature solid-state method is simple to operate, but the material particles have irregular morphology, are prone to contamination by impurities, tend to agglomerate, and exhibit a wide particle size distribution. The hydrothermal method can produce materials with regular morphology, and its composition and structure are easily adjustable, but the process is relatively complex and time-consuming. The sol-gel method can yield nano-materials with uniform size, but it requires subsequent heat treatment, which may lead to the formation of impurity phases, making it unsuitable for large-scale production in practical applications. During the actual preparation process of Na₃V₂(PO₄)₂F₃ materials, fluctuations in factors such as raw materials, methods, and sintering temperatures can easily result in the formation of impurity phases. These impurity phases generally lack electrochemical activity or have poor activity, which reduces the overall electrochemical performance of the material. In addition to selecting different preparation methods, element doping at various sites to modify the material is also an effective approach for producing NVPF with ideal morphology and excellent electrochemical performance.

Generally speaking, incorporating elements with larger ionic radii into the sodium sites of NVPF can expand the lattice spacing and broaden the pathways for ion diffusion^［80-81］. Li et al.^［53］ conducted relevant research on K⁺ doping at the sodium sites of NVPF and synthesized NKVPF. The study found that K⁺ with a larger ionic radius partially replaced Na⁺, broadening the sodium-ion diffusion channels. The material NKVPF@CNT, after being composited with carbon nanotubes, exhibited excellent rate capability and cycling performance. The cycling performance of the doped NKVPF is shown in Figure 3e. After 1600 cycles at a 10 C rate, the material delivered a capacity of over 90 mAh·g^-1, and after 6000 cycles at 50 C, the capacity retention was still as high as 90%. Zhang et al.^［56］ prepared Na_2.90K_0.10V₂(PO₄)₃F₃ with K⁺ doping at the sodium sites through a solid-state method. To determine the possible occupation sites of K⁺ in the lattice, density functional theory (DFT) calculations were performed to demonstrate K⁺ doping at the Na(1) site. To verify whether K⁺ could remain in the structure during the charge-discharge process, the potassium concentration in the electrolyte of cycled half-cells was measured using inductively coupled plasma optical emission spectrometry (ICP). It was proven that K⁺ consistently acted as a pillar within the structure throughout the cycling process, supporting the cycling stability of K⁺-doped NVPF. First-principles calculations were used to compute the sodium-ion migration barriers and band gaps before and after doping. After K⁺ doping, the migration barrier decreased from 187 meV to 128 meV, and the band gap reduced from 2.75 eV to 2.68 eV, indicating that K⁺ doping facilitated Na⁺ migration and enhanced the intrinsic conductivity of NVPF. The doped material exhibited a high discharge specific capacity of 120.8 mAh·g^-1 at 0.1 C, demonstrating high capacity characteristics; it maintained a capacity of 66 mAh·g^-1 under high-rate charge-discharge at 30 C, and the capacity retention after 500 cycles at 1 C reached 97.5%.

Doping elements with different ionic radii at the sodium site will affect the lattice spacing, thereby leading to different electrochemical properties. In addition to the expansion of lattice spacing caused by doping elements with larger ionic radii at the sodium site of NVPF as mentioned above, some researchers^［82］ have also studied the doping of elements with smaller ionic radii at the sodium site. Kosova et al.^［57］ prepared lithium-ion doped sodium sites Na_3-xLi_xV₂(PO₄)₂F₃ through a chemical ion exchange method, and experiments and analysis proved that when the value of x is close to 0.5, significant Na/Li doping occurs in the chemical ion exchange, while the degree of Na/Li doping in samples prepared by solid-state synthesis is very small. The research shows that the conductivity of the doped cathode material has increased by about four orders of magnitude, significantly reducing the activation energy of conductivity, and demonstrating good reversible cycle performance.

As the electrostatic repulsion between sodium ions in Na₃V₂(PO₄)₂F₃ is weakened and the interlayer spacing increases, an additional sodium ion can be inserted, forming Na₄V₂(PO₄)₂F₃. Peng et al.^[55] prepared nano Na₃V₂(PO₄)₂F₃ material using an improved sol-gel method. The cathode material synthesized by this method, which enhances sodium-ion insertion performance, can increase the theoretical capacity of sodium-ion batteries. As shown in Figure 3d, after inserting an additional sodium ion into nano Na₃V₂(PO₄)₂F₃, the material's initial discharge capacity reached a higher value of 250 mAh·g^-1, and it still maintained 72% capacity retention after 20 cycles at 0.2 C.

In NVPF, due to the redox of V³⁺/V⁵⁺ at the vanadium site, some metal ions with similar valence states can be doped to improve the overall performance of NVPF. These metal ions themselves are electrochemically active and can also participate in redox reactions^[83]. Compared with sodium site doping, vanadium site doping offers a wider selection of elements, including Mg²⁺, Ca²⁺, Y³⁺, Zr⁴⁺, Ti²⁺, W⁶⁺, Fe³⁺, Cr³⁺, Mn²⁺, Al³⁺, etc.

The Puspitasari team^{［64, 67］} prepared Na₃V_2-xMg_x(PO₄)₂F₃/C using the sol-gel method, and then doped Ca²⁺ into the NVPF/C structure using the sol-gel and carbothermal reduction methods to prepare Na₃V_2-xCa_x(PO₄)₂F₃/C, comparing the properties of the Ca²⁺ and Mg²⁺ doped NVPF/C structures. The study found that the optimally Mg²⁺ doped Na₃V_1.95Mg_0.05(PO₄)₂F₃/C exhibited excellent rate performance of 80 mAh·g^-1 at 10 C, with a capacity retention rate still reaching 88% after 500 charge-discharge cycles, and an average coulombic efficiency of 99.9%. Due to the larger ionic radius of Ca²⁺, Ca²⁺ doping would cause lattice expansion, ultimately widening the sodium ion diffusion channels in the lattice and accelerating the insertion/extraction of Na⁺; Ca²⁺ doping also facilitated the formation of a stable passivation layer, promoting rapid charge transfer. The TEM images of NVPF doped with Ca²⁺ and Mg²⁺ are shown in Fig. 3a, b. The synthesized NVPFCa-0.05/C provided excellent high-rate performance, with discharge specific capacities of 124 and 86 mAh·g^-1 at 0.1 and 10 C respectively, and a capacity retention rate of 70% after 1000 cycles at 10 C, showing excellent cycling stability. The doping of Ca²⁺ and Mg²⁺ optimized the particle size of NVPF, enhanced structural stability, accelerated the diffusion speed of sodium ions, and improved electronic conductivity.

Rare earth elements possess many excellent properties, such as high charge, large radius, and high self-polarization capability^［84］. Among all rare earth ions, yttrium (Y) has a relatively large radius and high affinity for oxygen and has been reported as an effective dopant for the LiCoO₂ cathode material in LIBs. Liu et al.^［68］ selected rare earth element doping to improve the reversible deintercalation ability of sodium ions. Y³⁺ was introduced into the NVPF/C composite via the sol-gel method as a partial substitute for V. When Y atoms with a larger atomic radius (the radius of Y³⁺ is 90 μm, and the radius of V³⁺ is 64 μm) are doped at vanadium sites in the NVPF crystal structure, the larger interstitial space facilitates rapid diffusion of Na⁺. Compared to the V—O bonds formed by Y doping, the weaker Y—O bonds enhance the intrinsic electronic conductivity^［84-85］. At a low rate of 0.5 C, the discharge capacity of the sample doped with 5 mol% is 121.3 mAh·g^-1, which is very close to the theoretical specific capacity. Even at a high rate of 50 C, its discharge capacity remains above 80 mAh·g^-1. After 200 cycles, the capacity retention rate of Na₃V_1.9Y_0.1(PO₄)₃/C at 1 C remains as high as 93.46%.

Doping large cation radius in the NVPF cathode material has a positive impact on expanding lattice distance, enhancing intrinsic electronic conductivity, and stabilizing the NVPF structure. Guo et al.^[63] prepared Na₃V_2-xZr_x(PO₄)₂F₃ cathode material via the sol-gel method. As shown in the TEM of Figure 3c, a small amount of Zr doping can effectively suppress crystal growth, while excessive Zr may reduce crystallization points in some way, thereby promoting the growth of NVPF crystals. At 0.5 C during the first cycle, the charge-discharge capacities were 119.2 and 91 mAh·g^-1, respectively, with a Coulombic efficiency of 78% (initial capacity loss may stem from the formation of the cathode electrolyte interface (CEI) and partial electrolyte decomposition), simultaneously exhibiting excellent rate performance. After 1000 cycles, it demonstrated the highest capacity retention rate of 90.2%, showing an extremely low capacity decay rate of 0.0098% per cycle, visually indicating the effect of doping on structural stability. Yi et al.^[69] studied Ti^x+ with different valence states synthesized Na₃V_2-xTi_x(PO₄)₂F₃ materials, as illustrated in Figure 3f. Appropriate Ti doping can effectively suppress crystal growth and control morphology formation. They also compared the effects of VSC doping (valence state change during synthesis) and VSU doping (the valence state of dopant ions remains unchanged) on NVPF. The study proved that doping Ti into one-eighth of the vanadium sites in the NVPF crystal will lead to a reduced bandgap of the NVPF unit cell, facilitating electron movement from the valence band to the conduction band, thus enhancing material conductivity. Meanwhile, the VSC-doped samples (x=2, 3) have smaller particle sizes than the VSU-doped samples (x=4), improving electronic conductivity and sodium-ion diffusion rates. Finally, NVPF-\mathrm{T}{\mathrm{i}}_{\mathrm{0.1}}^{\mathrm{2}+} prepared by the VSC doping method exhibits the smallest particle size distribution (approximately 40 nm), achieving the highest reversible charge-discharge capacity of 125 mAh·g^-1 at 0.2 C and maintaining good performance under high currents: 104 mAh·g^-1 at 40 C, 81 mAh·g^-1 at 80 C, and 41 mAh·g^-1 at 200 C. After 500 cycles at 40 C, the capacity retention rate was 91.3%. Nongkynrih et al.^[70] prepared Na₃V_1.96W_0.04(PO₄)₂F₃@C via a sol-gel method followed by ball milling. The migration of Na⁺ within the W-doped NVPF material was enhanced. W has a higher equivalent state at the vanadium site, and the doping of high-valent W⁶⁺ at the low-valent V³⁺ position leads to the formation of vacancies to ensure charge balance, which is beneficial for ion diffusion, thereby improving the electrode's Coulombic efficiency. EIS studies and DFT calculations showed that W⁶⁺ doping reduces charge transfer resistance, narrows the bandgap, enhances intrinsic conductivity, and improves sodium-ion diffusion speed in the electrode/electrolyte interface, achieving a capacity retention rate of 97.4% after 250 cycles. The research on W-doped NVPF provides a direction for studying how high-valent metal ion doping improves the rate performance and stability of NVPF.

Early on, Li et al^［61］ doped Fe into the vanadium sites in NVPF. When the optimal doping concentration of Fe ions was 3%, the corresponding material NV_0.97Fe_0.03PF/C exhibited excellent capacity retention and cycling performance: the discharge specific capacity at 0.1 C was 126.7 mAh·g^-1, and it could still deliver specific capacities of 85.0 and 47.1 mAh·g^-1 at 10 and 80 C, even higher than the capacity of NVPF/C at 5 C (44.5 mAh·g^-1). The capacity retention after 100 cycles at 0.2 C was as high as 97.1%, and after 300 cycles at 1 C, it could still maintain 87.8% of its capacity. Recently, Yang et al^［66］ added NH₄VO₃, Na₂H₂PO₄, NaF, and Fe(CH₃COO)₃ into a hydrothermal reactor with C₈H₁₈O₅ as the solvent and C₃H₆O₃ as the reducing agent to prepare Fe-doped Na₃V_2-xFe_x(PO₄)₂F₃ cathode materials for sodium-ion batteries. Various characterization techniques were used to study the crystal phase, lattice fringes, micro-morphology, and changes in chemical bonds before and after doping of the Fe-doped Na₃V_2-xFe_x(PO₄)₂F₃ cathode material. The research showed that the introduction of Fe effectively controlled the particle size, ensuring a range of 100~200 nm and significantly reducing agglomeration. At an Fe content of 0.1, the particles showed the most uniform dispersion. When Fe was introduced at a content of 0.1, the Fe_0.1-NVPF@N-CNT electrode demonstrated excellent rate performance. After cycling at rates from 0.1 to 10 C, the average discharge specific capacities of the Fe_0.1-NVPF@N-CNTs electrode reached 110~35 mAh·g^-1, as shown in Figure 4a. After 50 cycles at 0.1 C, the Fe_0.1-NVPF@N-CNT electrode maintained a discharge specific capacity of 108 mAh·g^-1 with a capacity retention rate as high as 98%.

Criado et al.^［65］ performed Cr doping modification on NVPF and found that low-content Cr doping can reduce interface resistance and battery polarization, enhancing the kinetic performance of NVPF at high rates. The full cell assembled with this material, HC//NVPF-Cr_0.01, exhibited a discharge specific capacity of 66 mAh·g^-1 at 10 C, an average discharge voltage of 3.7 V, and an energy density as high as 277 Wh·kg^-1. Yi et al.^［62］ prepared Na₃V_2-xCr_x(PO₄)₂F₃/C nanomaterials using the sol-gel method and conducted systematic research on their structure, morphology, and electrochemical performance. The modified Na₃V_1.95Cr_0.05(PO₄)₂F₃/C material demonstrated optimal rate performance and cycling stability, with an initial discharge specific capacity of 101.9 mAh·g^-1 at 10 C and a capacity retention rate of 68.7% after 1000 cycles. The rate performance of Na₃V_1.95Cr_0.05(PO₄)₂F₃/C is shown in Figure 4b. Through DFT calculations and electronic conductivity measurements, it was revealed that Cr³⁺ doping significantly promotes the electronic conductivity of the Na₃V₂(PO₄)₂F₃/C material. Calculations of the density of states (DOS) and defect formation energy showed that Cr³⁺ doping reduces the bandgap of NVPF, allowing electrons to migrate more easily from the valence band to the conduction band, effectively improving the intrinsic electronic conductivity of NVPF.

Zhang et al^［59］ utilized solvothermal and chemical vapor deposition to controllably prepare Mn²⁺-doped NVPF hollow microspheres (NVPF-Mn@C). After 500 cycles at 0.2 C, the material still exhibited a high discharge specific capacity of 109 mAh·g^-1, with a capacity retention rate of 99.1%. Meanwhile, NVPF-Mn@C also demonstrated excellent rate performance, maintaining a capacity of 60.7 mAh·g^-1 at 10 C, far exceeding that of the original NVPF (18.1 mAh·g^-1). Gu et al^［60］ enhanced the process based on solvothermal and chemical vapor deposition, simplified the synthesis route, and prepared Na₃V_1.98Mn_0.02(PO₄)₂F₃ cathode material using a more convenient hydrothermal method. The rate performance of NVPF-Mn_0.02 is shown in Figure 4c, with a capacity retention rate of 96.5% after 400 cycles at 25 C; the cycling performance of NVPF-Mn_0.02 is shown in Figure 4e, with a capacity retention rate exceeding 90% after 400 cycles at 0.5 C, and retaining 81.9% capacity after 1000 cycles at 1 C. The study shows that the introduction of Mn²⁺ strengthens the crystal structure of the material, enhancing the stability during charge-discharge cycling. Through SEM, XPS, GITT, CV, XRD, and other testing methods, it was confirmed that the lattice regulation of V³⁺ by Mn²⁺ reduces electrode polarization and increases electronic conductivity by generating crystal defects.

Among various metals, Al has attracted widespread attention due to its low cost, environmental friendliness, and stabilizing effect on materials during charge-discharge cycles. Zhuang et al^［58］ used NH₄VO₃, Al(NO₃)₃·9H₂O, NaF, NH₄H₂PO₄, and citric acid to prepare Al-doped Na₃V_2-xAl_x(PO₄)₂F₃ by doping an appropriate amount of Al into the vanadium site of the NVPF crystal structure using the sol-gel method. Compared with pure NVPF, Na₃V_1.93Al_.07(PO₄)₂F₃ with 0.07 Al doping exhibits the highest specific capacity and cycling stability. The rate performance of NVAlPF-0.07 is shown in Figure 4d. The discharge specific capacity of NVAlPF-0.07 is 86.4 mAh·g^-1, with a reversible capacity of 64.8 mAh·g^-1 after 400 cycles, and a Coulombic efficiency of 99.9%; even at 10 C, the discharge capacity remains above 60 mAh·g^-1. The corresponding capacity retention rates for pristine NVPF and NVAlPF-0.07 are 71.8% and 74.4%, respectively, and the Coulombic efficiency of NVAlPF-0.07 approaches 99% after 400 cycles, indicating that NVAlPF-0.07 demonstrates the best cycling reversibility and stability.

Gao et al^［71］ dissolved NH₄VO₃, NaF, NH₄H₂PO₄, Co(NO₃)₂·6H₂O in water and heated with stirring, then dried the water to obtain a precursor which was calcined at high temperature in an argon atmosphere. They first prepared Co-doped Na₃V_2-xCo_x(PO₄)₂F₃ as a cathode material for sodium-ion batteries using the sol-gel method. Studies have shown that the substitution of V³⁺ with larger ionic radius Co²⁺ can increase the unit cell volume of Na₃V₂(PO₄)₂F₃, broaden the sodium ion diffusion channels, thereby improving its sodium ion diffusion rate. In addition, Co doping effectively reduces the charge transfer impedance of the Na₃V₂(PO₄)₂F₃ material and enhances electronic conductivity. When x=0.1, the Na₃V_1.9Co_0.1(PO₄)₂F₃ material exhibits the most excellent electrochemical performance, with an initial discharge specific capacity of 111.3 mAh·g^-1 at 0.1 C, a reversible capacity of 91.9 mAh·g^-1 at 5 C, and a capacity retention rate of 70% after 80 cycles.

For phosphate NVPF, the anion is a key structural unit of the crystal. Na₃V₂(PO₄)₃ obtained by F substitution still has a three-dimensional structure, with enhanced induction effect and increased redox potential of the material^[86]. Researchers doped Br, which is in the same group as F, into the anion sites of Na₃V₂(PO₄)₂F₃. Hu et al.^[72] used a one-step spray drying method to prepare Br-doped porous spherical Na₃V₂(PO₄)₂F₃ particles. During the high-temperature sintering process, Br ions were released through the introduction of cetyltrimethylammonium bromide (CTAB), doping into the F atoms in the Na₃V₂(PO₄)₂F₃/C phase lattice. The preparation process is shown in Figure 5c. Studies have shown that the doped Br atoms are likely to replace F and reside at the dangling positions of F, forming double octahedral V₂O₈F₂Br. DFT calculations were used to analyze the impact of Br doping on the sodium ion state density and diffusion barrier, as shown in Figure 5a. With the introduction of CTAB, Br was incorporated into NVPF, significantly reducing its diffusion barrier and increasing sodium ion diffusion rate, enhancing the battery's rate performance. After Br doping in NVPF, its band gap narrowed, the V orbit near the Fermi level widened, charge localization weakened, and conductivity was enhanced. As shown in Figure 5b, porous spherical Br-doped Na₃V₂(PO₄)₂F₃/C exhibits excellent rate performance and long cycle performance when used as the cathode for sodium-ion batteries, showing discharge specific capacities of 116.1, 105.1, and 95.2 mAh·g^-1 at rates of 1, 10, and 30 C, respectively, and maintaining 98.3% capacity after 1000 cycles at 10 C.

O-doped Na₃V₂(PO₄)₂F₃, also known as Na₃V₂(PO₄)F_3-yO_y (0≤y≤2), or Na₃V₂O_2y(PO₄)₂F_3-2y and Na₃(VO_1-yPO₄)₂-F_1+2y (where y varies from 0 to 1), can be obtained in a similar crystal framework, featuring large sodium ion diffusion channels during charge and discharge processes^[87]. Xu et al.^[73] prepared Na₃V₂O₂(PO₂)₂F/RGO with a theoretical specific capacity of 130 mAh·g^-1 and an energy density of 501 Wh·kg^-1 at voltage platforms of 3.7 and 4.0 V. To study the structural and electrochemical properties of NVPF doped with varying O contents, Park et al.^[88] synthesized a series of Na₃(VO_1-xPO₄)₂F_1+2x. It was found that Na₃(VO_1-xPO₄)₂F_1+2x exhibited similar average voltages (3.8~3.9 V) and discharge capacities (120~130 mAh·g^-1). Due to differences in redox pairs (V³⁺/V⁴⁺ or V⁴⁺/V⁵⁺) and F/O distribution, Na₃(VO_1-xPO₄)₂F_1+2x demonstrated different potential compositions and sodium ion diffusion mechanisms. Broux et al.^[89] prepared Na₃V₂(PO₄)F_3-yO_y (0≤y≤0.5), further confirming through crystal structure and electrochemical characteristics that as more oxygen atoms occupy fluorine sites, the hysteresis between charging and discharging significantly decreases, and the mixed valence state of V^3+/4+ enhances the material’s electronic conductivity and ion diffusion rate. This conclusion is consistent with the hypothesis that Na⁺ moves from the Na(1) site to the Na(2) site for deintercalation layers, with two different spatial structures of Na⁺ distribution shown in Figure 5d.

Kumar et al^［74］ prepared Na₃V₂O_2x（PO₄）₂F_3-2x/MWCNT cathode material. Na₃V₂O_2x（PO₄）₂F_3-2x is actually a mixed phase of NVPF and Na₃V₂O₂（PO₄）₂F, with an average oxidation state of V^3.46+（Na₃V₂O_0.92（PO4）₂F_2.08）. Na₃V₂O_2x（PO₄）₂F_3-2x/MWCNT exhibits excellent long-cycle performance and high-rate capacity in both aqueous and non-aqueous electrolytes. In the Na₃（VO_1-xPO₄）₂F_1+2x material prepared by Qi et al^［76］, Na₃（VOPO₄）₂F shows the best sodium storage performance, with a discharge specific capacity of 112 mAh·g^-1, and after 1200 cycles at 2 C, the capacity retention rate can still reach 90%, further confirming the effectiveness of oxygen doping in enhancing NVPF electrodes. Xu et al^［82］ attempted to replace F with Cl occupying the oxygen site to form Na₃V₂（PO₄）₂FCl₂. Compared with F, Cl has lower electronegativity, and this doping has little impact on the framework; the structure is also easier to synthesize. Due to the reversible deintercalation of three Na⁺, the battery operating voltage and energy density will not be significantly reduced.

In addition to doping the internal structure of NVPF, external doping can also be performed. After heteroatom doping in the NVPF composite carbon layer, it can not only eliminate particle agglomeration but also improve the electronic conductivity and rate performance of the material. Heteroatom doping introduces abundant active sites and non-intrinsic defects into the carbon skeleton, thus achieving better electronic conductivity and ion diffusion speed than the original coating. Nitrogen is the most widely studied heteroatom doped in the composite carbon layer. Nitrogen doping on the carbon layer can form abundant defects, adjust the electrical conductivity of carbon materials, create more active sites, enhance sodium-ion diffusion speed, and improve surface wettability. Zhang et al.^[77] used polydopamine as a carbon source and successfully prepared nitrogen-doped carbon-coated NVPF composites (NVPF/C-PDPA) through a simple self-polymerization reaction and high-temperature heat treatment method. Due to the excellent film-forming properties of dopamine, a complete and uniform carbon coating layer was formed on the surface of NVPF particles, effectively preventing the corrosion of NVPF in the electrolyte and improving the cycling stability of the material. As shown in Figure 6b, NVPF, NVPF/C, and NVPF/PDPA, NVPF/C-PDPA exhibit discharge specific capacities of 110 and 98 mAh·g^-1 at 0.5 and 10 C, respectively; as shown in Figure 6d, the capacity retention rate after 100 cycles at 0.5 C is 96.8%, and the capacity retention rate after 800 cycles at 10 C is still as high as 95.8%. Yi et al.^[90] utilized PECVD technology with nitrogen gas as the nitrogen source to achieve nitrogen incorporation. NVPF/N exhibits a capacity retention rate as high as 96.5% after 100 cycles at 1 C rate, and a discharge specific capacity as high as 102.6 mAh·g^-1 when charged and discharged at 10 C rate.

Elements such as nitrogen, sulfur, and boron not only increase the active sites within the carbon layer but also enhance the sodium-ion diffusion rate and electronic conductivity of NVPF materials during the modification process. Compared with single-atom doping, multi-atom doped carbon-based materials possess more active sites and greater electronegativity, which can further improve the electrochemical performance of the material. Yu et al^［78］ synthesized NVPF with nitrogen and boron co-doped carbon nanotube layers using the sol-gel method, and the preparation process is shown in Figure 6a. The non-metal heteroatoms nitrogen and boron co-doped carbon layer effectively reduced particle agglomeration and increased the number of active sites, improving the material's conductivity. As shown in Figure 6c, NVPF-PCNB-20 (with polymer added at 20% of NVPF mass) exhibited discharge specific capacities of 109 and 90 mAh·g^-1 at 0.5 and 10 C, respectively, a capacity retention rate of 93.2% after 100 cycles at 0.5 C, and a capacity retention rate of 92.8% after 750 cycles at 10 C. The modified composite materials all demonstrated high electrical conductivity, indicating that the nitrogen and boron co-doped carbon layer significantly enhanced the material's electronic conductivity. Lu et al^［79］ successfully prepared nitrogen and sulfur co-doped carbon-coated Na₃V₂(PO₄)₂F₃ (NVPF-NSC) using the sol-gel method combined with freeze-drying technology, forming an in-situ nitrogen and sulfur co-doped conductive carbon layer, providing a three-dimensional framework to restrict particle agglomeration for NVPF. Due to the tight integration of the NSC skeleton and NVPF, its charge transfer resistance and polarization were also reduced, enhancing the material's conductivity. NVPF-NSC-25 exhibited a high discharge specific capacity of 109 mAh·g^-1 and excellent cycling performance, with a capacity retention rate of 92.1% after 500 cycles at 5 C and a capacity retention rate of 87.6% after 500 cycles at 10 C, with a discharge specific capacity of 83 mAh·g^-1.

During the synthesis of NVPF, factors such as reaction time and sintering temperature can cause excessive growth of crystal grains and agglomeration of small particles, which greatly affects the electrochemical performance of the material. Researchers often suppress grain growth and avoid secondary particle agglomeration by controlling grain growth and reducing particle size, making the particles exhibit the most uniform dispersion to enhance sodium ion diffusion speed and electronic conductivity. The Puspitasari team^{［64, 67］} conducted Ca²⁺ and Mg²⁺ doping, optimizing the NVPF particle size, enhancing structural stability, accelerating sodium ion diffusion speed, and improving electronic conductivity. The larger ionic radius of Ca²⁺ promotes lattice space expansion, reduces particle size, thus shortening the sodium ion diffusion path. The SEM of NVPF-Ca-0.05/C is shown in Figure 7a. Guo et al.^［63］ prepared Na₃V_2-xZr_x(PO₄)₂F₃ research shows that a small amount of Zr doping can effectively inhibit crystal growth, while excessive Zr will reduce the crystallization point in some way, promoting the growth of NVPF crystals. Yi et al.^［69］ prepared Na₃V_2-xTi_x(PO₄)₂F₃ materials and found that the prepared NVPF-\mathrm{T}{\mathrm{i}}_{\mathrm{0.1}}^{\mathrm{2}+} has the smallest particle size distribution (about 40 nm), indicating that an appropriate amount of Ti doping can also effectively inhibit crystal growth and control morphology formation. Yang et al.^［66］ prepared Na₃V_2-xFe_x(PO₄)₂F₃ materials confirmed that the introduction of Fe can suppress grain growth and avoid secondary particle agglomeration, ensuring that the material particle size is within the range of 100~200 nm and significantly reducing agglomeration phenomena, among which Fe_0.1-NVPF particles show the most uniform dispersion.

Yu et al^［78］ prepared NVPF-PCNB-20 using the sol-gel method, in which the non-metal heteroatoms nitrogen and boron co-doped carbon layer not only effectively reduced particle agglomeration and increased the number of active sites but also improved the material's conductivity. Its microscopic particles are shown in Figure 7b. Lu et al^［79］ successfully prepared nitrogen and sulfur co-doped carbon-coated Na₃V₂(PO₄)₂F₃ (NVPF-NSC) using the sol-gel method combined with freeze-drying technology. The in-situ formed nitrogen and sulfur co-doped conductive carbon layer provided a three-dimensional framework for NVPF to limit particle agglomeration and improved the electrochemical performance of the material.

Na₃V₂(PO₄)₂F₃ undergoes certain structural changes during the charge-discharge process, leading to capacity and voltage decay as well as unstable charge-discharge cycling. Element doping can provide structural support to the material during charge-discharge processes, preventing structural changes caused by sodium ion deintercalation and ensuring the cycling stability of the material during charge-discharge. The Puspitasari team^{［64, 67］} compared the structural properties of Ca²⁺ and Mg²⁺ doped NVPF/C, and found that Mg²⁺ doping can prevent crystal structure collapse during charge-discharge cycling because the size (radius 0.71 μm) of Mg²⁺ is larger than that of V⁴⁺. Inactive Mg²⁺ does not participate in redox reactions, which reduces crystal volume changes and enhances the cycling stability of the material. In the case of Ca²⁺ doping, the breaking of P—O and V—O covalent bonds is difficult, thus stabilizing the oxygen atoms in the lattice and enhancing structural stability, as shown in Fig. 7c, d, which provides the XRD patterns of Na₃V_1.95Mg_0.05(PO₄)₂F₃/C and NVPF-Ca-0.05/C, proving that Ca²⁺ and Mg²⁺ occupy the same positions as V³⁺, with Ca²⁺ and Mg²⁺ successfully replacing V³⁺ at vanadium sites. Additionally, Ca²⁺ can act as a pillar to prevent deformation of the NVPF/C structure during charge-discharge processes, thereby ensuring excellent cycling performance of the material. When V³⁺ is oxidized to V⁴⁺ (0.67 μm), its reduced ionic radius (0.78 μm) causes structural degradation, while Ca²⁺ (1 μm), a non-active component with a larger ionic radius, prevents structural deformation during charge-discharge cycling, maintaining structural stability even after long cycles at high rates. The rare earth element Y can also replace V in the NVPF crystal structure^［68］, acting as a pillar and buffering the deformation of the main NVPF crystal during charge-discharge cycling, improving the structural stability of the material.

Gu et al^［60］ investigated the lattice regulation effect of Mn²⁺ on V³⁺ and found that the crystal structure of Na₃V_1.98Mn_0.02(PO₄)₂F₃ is more stable than that of NVPF. The introduction of Mn²⁺ strengthens the crystal structure of the material, and the lattice distortion during the cathode charge-discharge cycling process is also suppressed. NVPF-Mn_0.02 exhibits stable cycling performance with a capacity retention rate exceeding 90% after 400 cycles at 0.5 C. Zhang et al^［77］ prepared nitrogen-doped carbon-coated NVPF composite material (NVPF/C-PDPA) using polydopamine as the carbon source. Due to the excellent film-forming properties of dopamine, a complete and uniform carbon coating layer was formed on the surface of NVPF particles, effectively preventing the main structure of NVPF from being corroded by the electrolyte, thus improving the cycling stability of the material.

Doping elements with different ionic radii can alter the lattice spacing of materials, influence the diffusion path of sodium ions, enhance the diffusion rate of sodium ions, and improve the rate performance and cycling stability of NVPF. Peng et al.^［55］ prepared nano Na₄V₂(PO₄)₂F₃ material by inducing a certain degree of lattice distortion and defects. As shown in Fig. 8a, the material's structure exhibits lattice distortion, lattice defects, and an increase in the lattice (002) spacing, resulting in a decrease in the activation energy for sodium ion intercalation and a shorter diffusion path, thereby accelerating ion diffusion. Doping with elements of larger ionic radius can cause an expansion of lattice spacing; in the Na₃V_2-xCa_x(PO₄)₂F₃/C prepared by the Puspitasari team^［64］, the larger ionic radius of Ca²⁺ leads to lattice expansion, ultimately widening the sodium ion diffusion channels in the lattice. As shown in Fig. 8b, the sodium ion diffusion coefficient of the pristine NVPF/C electrode is 6.38×10^-13 cm²·s^-1, while that of the NVPF-Ca-0.05/ electrode is 2.30×10^-12 cm²·s^-1, indicating a significant enhancement in sodium ion diffusion speed. In the Na₃V_2-xY_x(PO₄)₂F₃/C samples prepared by Liu et al.^［68］, due to the larger radius of Y³⁺ (~90 pm) compared to V³⁺ (~64 pm), when Y³⁺ is doped into the crystal structure at the V³⁺ site, the lattice spacing of the material expands, accelerating sodium ion diffusion. As shown in Fig. 8c, in the Na₃V_2-xZr_x(PO₄)₂F₃ material prepared by Guo et al.^［63］, compared to NVPF, all peaks of Zr-doped NVPF on the (220) crystal plane show a slight shift in angle. According to Bragg's law, as the diffraction angle decreases, the interplanar spacing of the NVPF-Zr-X/NC crystal increases. A theoretical model of ab initio molecular dynamics (AIMD) calculations was established for it, using the Forcite module to optimize the system’s geometry, selecting Dynamic for dynamic calculations, and obtaining the mean square displacement (MSD) and sodium ion diffusion coefficient (D_{\mathrm{N}\mathrm{a}}₊) as shown in Fig. 8d. Studies have shown that doping with large cationic radius Zr has a positive impact on expanding lattice distance, enhancing sodium ion diffusion speed, and stabilizing the NVPF structure.

Element doping can effectively improve the electronic conductivity of materials, thereby enhancing their rate performance. Zhang et al.^［56］ prepared Na_2.90K_0.10V₂(PO₄)₃F₃ and used the DFT method to calculate the density of states (DOS) of NVPF before and after doping, studying the effect of K doping on electronic conductivity. The band gap decreased from 2.75 eV for the undoped material to 2.68 eV, indicating that K doping enhances the electronic conductivity of NVPF. Yi et al.^［62］ prepared Na₃V_2-xCr_x(PO₄)₂F₃/C material and calculated the DOS before and after Cr doping. The band gap of doped NVPF decreased from 3.29 eV to 1.35 eV. The reduced band gap of Cr³⁺-doped NVPF allows electrons to migrate more easily from the valence band to the conduction band, effectively improving the electronic conductivity of NVPF. Zhuang et al.^［58］ demonstrated through AC impedance testing that the charge transfer resistance of NVAlPF-0.07 is smaller than that of pure NVPF, resulting in better electronic conductivity. The resistance value of the NVAlPF-0.07 electrode at room temperature is 0.11 Ω·cm^-2, significantly lower than that of the NVPF electrode (0.27 Ω·cm^-2). The electronic conductivity of the NVAP-0.07 electrode at room temperature is 3517 S·cm^-1, while that of the NVPF electrode is 1040 S cm^-1.

The Br-doped porous spherical Na₃V₂(PO₄)₂F₃ particles prepared by Hu et al^［72］ show that after being doped with Br, the band gap of NVPF narrows, the V orbitals near the Fermi level widen, charge localization decreases, and electronic conductivity improves. The Na₃V₂(PO₄)F_3-yO_y (0≤y≤0.5) prepared by Broux et al^［89］ further confirms that as the oxygen content increases, the hysteresis during the charge-discharge cycle is significantly reduced, and the mixed oxidation state V³⁺/⁴⁺ helps enhance electronic conductivity. Additionally, the research on Na₃V₂O₂(PO₄)₂F by Xu et al^［73］ also demonstrates that the mixed valence state of V(V³⁺/⁴⁺) induced by partial doping of oxygen in Na₃V₂(PO₄)F₃ can enhance the electronic conductivity of the electrode.