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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Doping Modification of Sodium Vanadium Fluorophosphate as Cathode Material for Sodium Ion Batteries

  • Fangcheng Hu 1, 2, 3 ,
  • Junxian Hu , 1, 2, 3, 4 ,
  • Yang Tian , 1, 2, 3, 4 ,
  • Dong Wang 1, 2, 3 ,
  • Tingzhuang Ma 1, 2, 3 ,
  • Lipeng Wang 1, 2, 3
Expand
  • 1. Key Laboratory for Nonferrous Vacuum Metallurgy of Yunnan Province,Kunming University of Science and Technology,Kunming 650093,China
  • 2. National Engineering Research Center of Vacuum Metallurgy,Kunming University of Science and Technology,Kunming 650093,China
  • 3. Faculty of Metallurgical and Energy Engineering,Kunming University of Science and Technology,Kunming 650093,China
  • 4. State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization,Kunming University of Science and Technology,Kunming 650093,China
* (Yang Tian);
(Junxian Hu)

Received date: 2024-05-08

  Revised date: 2024-10-17

  Online published: 2025-03-10

Supported by

Yunnan Province Nonferrous Metals Vacuum Metallurgy Top Team(202305AS350012)

Basic Research Program of Yunnan Provincial(202301BE070001-014)

Basic Research Program of Yunnan Provincial(202301AT070150)

Abstract

With excellent multiplication performance, stable high and low-temperature performance, abundant sodium resources and low cost, sodium-ion batteries have good application prospects in the field of large-scale energy storage and low-speed electric vehicles. The cathode material determines the working voltage and cycling performance of sodium-ion batteries, and is the core component that directly affects the overall performance of sodium-ion batteries. Among them, Na3V2(PO42F3 (NVPF) has excellent structural stability and high working potential, but slow ion diffusion and low electronic conductivity, which need to be further improved by elemental doping and other modification means. This paper has introduced the background, crystal structure and preparation method of NVPF. Has summarized in detail the modification progress of doping at different doping sites, such as sodium, vanadium, and anionic sites in NVPF materials. The mechanisms of doping in NVPF materials were analyzed, which can optimize the particle size, enhance the lattice stability, change the lattice spacing to enhance the diffusion rate of sodium ions, and increase the electronic conductivity. Based on the above, this paper summarized the preparation, doping sites and effects of NVPF materials from the perspective of subsequent research, and have also looked ahead to the future prospects of doping modification.

Contents

1 Research background

2 Structural mechanism and preparation of vanadium sodium fluorophosphate

2.1 Structural Characteristics

2.2 Preparation methods

3 Doping modification of sodium vanadium fluorophosphate at different sites

3.1 Sodium site doping

3.2 Vanadium site doping

3.3 Anion site doping

3.4 Carbon layer heteroatom doping

4 Study on the doping mechanism of sodium vanadium fluorophosphate

4.1 Suppresses particle agglomeration and optimizes particle size

4.2 Enhance structural stability

4.3 Changing the lattice spacing to enhance ion diffusion rate

4.4 Improve the electronic conductivity

5 Summary and outlook

Cite this article

Fangcheng Hu , Junxian Hu , Yang Tian , Dong Wang , Tingzhuang Ma , Lipeng Wang . Doping Modification of Sodium Vanadium Fluorophosphate as Cathode Material for Sodium Ion Batteries[J]. Progress in Chemistry, 2025 , 37(3) : 439 -454 . DOI: 10.7536/PC240508

1 Research Background

The development and utilization of new renewable clean energy sources can effectively alleviate the two major issues of "energy crisis" and "environmental pollution." With the increasing demand for renewable energy utilization, secondary battery energy storage technology plays an indispensable role in compensating for its intermittency and is also a key technology that urgently needs breakthroughs. Lithium-ion batteries, due to their high specific energy and long cycle life, dominate the secondary battery market. However, lithium resources are severely scarce (accounting for only 0.0017 wt% in the Earth's crust) and unevenly distributed, and recycling technology needs further breakthroughs, which cannot meet the growing market demand. The large-scale application of lithium-ion batteries is seriously restricted, and the next generation of secondary batteries that can replace or partially replace lithium-ion batteries is in urgent need of development. Sodium metal belongs to the same Group IA as lithium and has very similar physical and chemical properties. Moreover, sodium resources are abundant (with crustal reserves as high as 2.74%, about 350 times that of lithium), widely distributed, and inexpensive. Therefore, sodium-ion batteries exhibit great application potential in large-scale power storage, medium- and low-speed electric vehicles, 5G communication base stations, and other fields. Since the sodium ion radius is larger than the lithium ion radius, this leads to sluggish electrochemical reaction kinetics, and the reaction mechanism and electrode reaction activity differ from those of lithium-ion batteries. Therefore, research on high-performance electrode materials for sodium-ion batteries faces severe challenges1-3.
The cathode material, as a core component of sodium-ion batteries, has a significant impact on the operating voltage, energy density, cycle performance, and rate performance of the sodium-ion battery system4-5. Currently, the main types of cathode materials for sodium-ion batteries are layered metal oxides, Prussian blue derivatives, and polyanionic compounds. Layered metal oxides consist of alternating layers of transition metals and alkali metals, forming a layered structure. The transition metal layers are formed by repeated MO6 octahedra connected by shared edges, offering a high theoretical capacity (approximately 200 mAh·g-1) and ease of synthesis, but with a relatively low working potential (about 2.8 V), poor crystal stability, and high sensitivity to water6-10. Prussian blue derivatives feature a three-dimensional nano-crosslinked framework structure with ion diffusion channels, showing minimal shape changes during ion deintercalation processes. Their specific energy can reach 500~600 mWh·g-1, but most exhibit low Coulombic efficiency and poor cycle performance11-16. Polyanionic compounds mainly consist of a three-dimensional structure formed by covalent bonds between transition metal octahedra and polyanionic tetrahedra, featuring a high operating potential (3.8~3.9 V) and large reversible capacity (110~120 mAh·g-1), along with good rate performance and cycling stability17-27.
Common polyanionic compounds mainly include iron sodium sulfate [Na2Fe(SO4)2], iron sodium phosphate [NaFePO4], iron sodium fluorophosphate [Na2FePO4F], sodium vanadium phosphate [Na3V2(PO4)3], sodium vanadium fluorophosphate [Na3V2(PO4)2F3] etc.[[xref ref-type="bibr" rid="R28">28-32]. Among the polyanionic compounds, sodium vanadium fluorophosphate (Na3V2(PO4)2F3, NVPF) with Na Super Ionic Conductor (NASICON) structure has a high operating voltage (~3.95 V) and large theoretical energy density (507 Wh/kg). Its operating voltage is higher than that of metal oxides, and its coulombic efficiency and cycle performance are superior to Prussian blue derivatives. However, the low electronic conductivity and slow ion diffusion rate of NVPF severely limit its further application. Current research has shown that element doping can improve the electronic conductivity of cathode materials for sodium-ion batteries and enhance the diffusion rate of sodium ions[[xref ref-type="bibr" rid="R33">33-36]. In order to achieve better performance of sodium vanadium fluorophosphate in sodium battery cathode applications, researchers have conducted extensive exploration on the doping modification of sodium vanadium fluorophosphate.
This article first introduces the formation background, crystal structure, and preparation methods of NVPF, with a focus on discussing research into doping modifications at different sites. It reviews the mechanisms of various dopings on NVPF and proposes prospects for the development of doping modifications of NVPF materials.

2 Structural Mechanism and Preparation Method of Sodium Vanadium Fluorophosphate

2.1 Structural Properties of Sodium Vanadium Fluorophosphate

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-, N3-, P2O74-) 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].
图1 (a) Crystal Structure of NVPF with Tetragonal Space Group P42/mnm 41; (b) Refined XRD Pattern of Tetragonal Space Group P42/mnm 41; (c) Refined Neutron Diffraction Pattern of Tetragonal Space Group P42/mnm 41

Fig. 1 (a) Crystal structure of the tetragonal space group P42/mnm of NVPF41; (b) rietveld refined XRD patterns of the tetragonal space group P42/mnm 41; (c) rietveld refined neutron diffraction patterns of the tetragonal space group P42/mnm 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.

2.2 Preparation Method of Sodium Vanadium Fluorophosphate

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 al47 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 material48-49.
图2 (a) HRTEM Image of NVPF@C Material Prepared by High-Temperature Solid-State Method48; (b) SEM Image of NVPF@C Material Prepared by Hydrothermal Method1; (c) SEM Image of NVPF@C Material Prepared by Sol-Gel Method54; (d) Schematic Diagram of NVPF@C Preparation by Sol-Gel Method54

Fig. 2 (a) HRTEM image of NVPF@C material prepared by high-temperature solid-phase method48; (b) SEM image of NVPF@C material prepared by hydrothermal method1; (c) SEM image of NVPF@C material prepared by sol-gel method54; (d) schematic diagram of NVPF@C prepared by sol-gel method54

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 temperature49. 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 challenging50.
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.

3 Doping Modification Research on Different Sites of Sodium Vanadium Fluorophosphate

Doping is a method to change the intrinsic properties of materials, which can effectively adjust the intrinsic electronic conductivity of materials, regulate lattice spacing, create defects to generate more sodium storage sites, and control redox platforms, etc., significantly improving the performance of materials33-35. Doping ions at a single site is beneficial to improve the charge-discharge capacity and cycle performance of NVPF, and different ions produce different effects at different doping sites. In addition to doping the main structure of NVPF, researchers have also conducted a series of doping studies on the composite carbon layer of NVPF, finding that heteroatom doping of the carbon layer can eliminate particle agglomeration. Research on sodium site doping, vanadium site doping, anion site doping, and heteroatom doping of the carbon layer is shown in Table 1. Studies show that doping has made certain progress in the modification preparation of NVPF, and according to the different elements doped and different doping sites, NVPF with different performances can be obtained.
表1 NVPF的掺杂改性策略研究

Table 1 Research of NVPF doping modification strategies

Cathode material Doping sites Element Capacity (mAh·g-1 Cycle performance Ref
Na4V2(PO42F3 Na Na 0.2 C 130 0.2 C 100% (after 20 cycle) 55
Na2.9K0.1V2(PO43F3 Na K 0.1 C 120.8 1 C 97.5% (after 500 cycle) 56
Na0.92K0.08V2(PO42F3@CNT Na K 1 C 120 50 C 90% (after 6000 cycle) 53
Na3- x Li x V2(PO42F3 Na Li 0.2 C 106 20 C 80% (after 25 cycle) 57
Na3V1.93Al0.07(PO42F3 V Al 0.1 C 121.3 5 C 75% (after 400 cycle) 58
Na3V1.95Mn0.05(PO42F3/C V Mn 0.2 C 122.9 0.2 C 99.1% (after 500 cycle) 59
Na3V1.98Mn0.02(PO42F3@C V Mn 0.5 C 79.9 1 C 81.9% (after 1000 cycle) 60
Na3V1.97Fe0.03(PO42F3/C V Fe 0.1 C 126.7 0.2 C 97.1% (after 100 cycle) 61
Na3V1.95Cr0.05(PO42F3/C V Cr 10C 101.9 10 C 68.7% (after 1000 cycle) 62
Na3V1.98Zr0.02(PO42F3/NC V Zr 0.5 C 119.2 20 C 90.2 % (after 1000 cycle) 63
Na3V1.95Ca0.05(PO42F3/C V Ca 1 C 124 10 C 70 % (after 1000 cycle) 64
Na3V1.97Cr0.03(PO42F3@C V Cr 0.5 C 111 2 C 93% (after 125 cycle) 65
Na3V1.9Fe0.1(PO42F3@N-CNTs V Fe 0.1 C 105 2 C 74.53% (after 1000 cycle) 66
Na3V1.95Mg0.05(PO42F3/C V Mg 10 C 80 10 C 88% (after 500 cycle) 67
Na3V1.9Y0.1(PO42F3/C V Y 0.5 C 121.3 1 C 93.46% (after 200 cycle) 68
NVPF- T i 0.1 + 2 V Ti 1 C 117 40 C 91.3% (after 500 cycle) 69
Na3V1.96W0.04(PO42F3@C V W 0.1 C 153 2 C 97.4% (after 250 cycle) 70
Na3V1.9Co0.1(PO42F3 V Co 0.1 C 111.3 5 C 70% (after 80 cycle) 71
Na3V2(PO42FBr2/C Anion Br 1 C 116.1 10 C 98.3% (after 1000 cycle) 72
Na3V2O2(PO42F/rGO Anion O 1 C 100.4 0.1 C 91.4% (after 200 cycle) 73
Na3V2O2(PO42F/MWCNT Anion O 0.1 C 102 1 C 78% (after 400 cycle) 74
Na3V2O2 x (PO42F3-2 x Anion O 1 C 100 0.5 C 93% (after 50 cycle) 75
Na3(VO1- x PO42F1+2 x Anion O 0.2 C 112 2 C 73% (after 1200 cycle) 76
Na3V2(PO42F3/C-PDPA Carbon N 0.5 C 113.8 10 C 95.8% (after 800 cycle) 77
Na3V2(PO42F3-PCNB-20 Carbon N&B 0.5 C 109 0.5 C 93.2% (after 100 cycle) 78
Na3V2(PO42F3-NSC Carbon N&S 10 C 83 5 C 92.1% (after 500 cycle) 79

3.1 Sodium Site Doping

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 diffusion80-81. 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 Na2.90K0.10V2(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%.
图3 (a) TEM Image of NVPFCa-0.05/C64; (b) TEM Image of Na3V1.95Mg0.05(PO4)2F3/C67; (c) HRTEM Image of Na3V2- x Zr x (PO4)2F3 63; (d) First Cycle Charge-Discharge of Na4V2(PO4)2F3 55; (e) Cycling Performance of NKVPF@CNT at 50 C Rate53; (f) Schematic Diagram of Na3V2- x Ti x (PO4)2F3Preparation69

Fig. 3 (a) TEM image of NVPFCa-0.05/C64; (b) TEM image of Na3V1.95Mg0.05(PO42F3/C67; (c) HRTEM image of Na3V2- x Zr x (PO42F3 63; (d) First turn charging and discharging of Na4V2(PO42F3 55; (e) cycling performance of NKVPF@CNT at 50 C rate53; (f) schematic diagram of Na3V2- x Ti x (PO42F3 preparation69

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 researchers82 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 Na3-xLixV2(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.

3.2 Vanadium Site Doping

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 team64, 67 prepared Na3V2-xMgx(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 Na3V2-xCax(PO4)2F3/C, comparing the properties of the Ca2+ and Mg2+ doped NVPF/C structures. The study found that the optimally Mg2+ doped Na3V1.95Mg0.05(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 capability84. 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 conductivity84-85. 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 Na3V1.9Y0.1(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 Na3V2-xZrx(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 Tix+ with different valence states synthesized Na3V2-xTix(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-Ti0.12+ 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 Na3V1.96W0.04(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 al61 doped Fe into the vanadium sites in NVPF. When the optimal doping concentration of Fe ions was 3%, the corresponding material NV0.97Fe0.03PF/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 al66 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 Na3V2-xFex(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 Na3V2-xFex(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 Fe0.1-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 Fe0.1-NVPF@N-CNTs electrode reached 110~35 mAh·g-1, as shown in Figure 4a. After 50 cycles at 0.1 C, the Fe0.1-NVPF@N-CNT electrode maintained a discharge specific capacity of 108 mAh·g-1 with a capacity retention rate as high as 98%.
图4 (a) Rate Performance of Fe0.1-NVPF@N-CNT66; (b) Rate Performance of Na3V1.95Cr0.05(PO4)2F3/C62; (c) Rate Performance of Na3V1.98Mn0.02(PO4)2F360; (d) Rate Performance of NVAlPF-0.0758; (e) Capacity of Na3V1.98Mn0.02(PO4)2F3 at 1 C after 1000 Cycles60

Fig. 4 (a) Doubling performance of Fe0.1-NVPF@N-CNT66; (b) doubling performance of Na3V1.95Cr0.05(PO42F3/C62; (c) doubling performance of Na3V1.98Mn0.02(PO42F3 60; (d) doubling performance of NVAlPF-0.0758; (e) doubling performance of N Na3V1.98Mn0.02(PO42F3 capacity for 1000 cycles at 1 C60

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-Cr0.01, 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 Na3V2-xCrx(PO4)2F3/C nanomaterials using the sol-gel method and conducted systematic research on their structure, morphology, and electrochemical performance. The modified Na3V1.95Cr0.05(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 Na3V1.95Cr0.05(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 al59 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 al60 enhanced the process based on solvothermal and chemical vapor deposition, simplified the synthesis route, and prepared Na3V1.98Mn0.02(PO4)2F3 cathode material using a more convenient hydrothermal method. The rate performance of NVPF-Mn0.02 is shown in Figure 4c, with a capacity retention rate of 96.5% after 400 cycles at 25 C; the cycling performance of NVPF-Mn0.02 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 al58 used NH4VO3, Al(NO3)3·9H2O, NaF, NH4H2PO4, and citric acid to prepare Al-doped Na3V2-xAlx(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, Na3V1.93Al.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 al71 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 Na3V2-xCox(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 Na3V1.9Co0.1(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.

3.3 Anion Site Doping

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.
图5 (a) Diffusion Barrier After Incorporation of Br72; (b) Long Cycle Performance at 10 C after Br Doping72; (c) Schematic Diagram of Br-Doped Porous Carbon Sphere NVPF Preparation72; (d) Space Groups of Two Na+Distributions89

Fig. 5 (a) Diffusion barrier after Br doping72; (b) 1000 long cycles performance at 10C after Br doping72; (c) Br-doped porous carbon spheres NVPF preparation schematic72; (d) space group of two Na+ distributions89

O-doped Na3V2(PO4)2F3, also known as Na3V2(PO4)F3-yOy (0≤y≤2), or Na3V2O2y(PO4)2F3-2y and Na3(VO1-yPO4)2-F1+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(VO1-xPO4)2F1+2x. It was found that Na3(VO1-xPO4)2F1+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(VO1-xPO4)2F1+2x demonstrated different potential compositions and sodium ion diffusion mechanisms. Broux et al.[89] prepared Na3V2(PO4)F3-yOy (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 V3+/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 al74 prepared Na3V2O2x(PO42F3-2x/MWCNT cathode material. Na3V2O2x(PO42F3-2x is actually a mixed phase of NVPF and Na3V2O2(PO42F, with an average oxidation state of V3.46+(Na3V2O0.92(PO4)2F2.08). Na3V2O2x(PO42F3-2x/MWCNT exhibits excellent long-cycle performance and high-rate capacity in both aqueous and non-aqueous electrolytes. In the Na3(VO1-xPO42F1+2x material prepared by Qi et al76, Na3(VOPO42F 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 al82 attempted to replace F with Cl occupying the oxygen site to form Na3V2(PO42FCl2. 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.

3.4 Carbon Layer Heteroatom Doping

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.
图6 (a) Schematic Diagram of NVPF-PCNB-20 Microsphere Preparation78; (b) Rate Performance of NVPF/C-PDPA at Different Rates77; (c) Rate Performance of NVPF-PCNB at Different Rates78; (d) Performance of NVPF/C-PDPA Cycled 800 Times at 10 C Rate77

Fig. 6 (a) Schematic diagram of NVPF-PCNB-20 microsphere preparation78; (b) doubling performance of NVPF/C-PDPA at different rates77; (c) doubling performance of NVPF-PCNB at different rates78; (d) performance of NVPF/C-PDPA with 800 cycles at 10 C rate77

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 al78 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 al79 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.

4 Study on the Doping Mechanism of Sodium Vanadium Fluorophosphate

At present, element doping has made great progress in the field of material modification. Studies have found that different valence states, types, sizes, doping amounts, and positions of doped ions can all have different modification effects on the crystal structure and electrochemical performance of electrode materials. The action mechanisms of different element-doping modification strategies vary, but the mechanism of action of element doping on materials follows certain rules. The main mechanisms of action can be divided into the following categories.

4.1 Inhibition of Particle Agglomeration and Optimization of Particle Size

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 team64, 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 Na3V2-xZrx(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 Na3V2-xTix(PO4)2F3 materials and found that the prepared NVPF-Ti0.12+ 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 Na3V2-xFex(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 Fe0.1-NVPF particles show the most uniform dispersion.
图7 (a) SEM Image of NVPF-Ca-0.05/C64; (b) EDS Mapping of NVPF-PCNB-2078; (c) Refined XRD Pattern of NVPF-Ca-0.05/C Material64; (d) Refined XRD Pattern of Na3V1.95Mg0.05(PO4)2F3/C Material67

Fig. 7 (a) SEM image of NVPF-Ca-0.05/C64; (b) EDS mapping of NVPF-PCNB-2078; (c) XRD plots of NVPF-Ca-0.05/C material Rietveld after refinement64; (d) XRD plots of Na3V1.95Mg0.05(PO42F3/C material Rietveld after refinement67

Yu et al78 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 al79 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.

4.2 Enhancing Structural Stability

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 team64, 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 Na3V1.95Mg0.05(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 structure68, 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 al60 investigated the lattice regulation effect of Mn2+ on V3+ and found that the crystal structure of Na3V1.98Mn0.02(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-Mn0.02 exhibits stable cycling performance with a capacity retention rate exceeding 90% after 400 cycles at 0.5 C. Zhang et al77 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.

4.3 Altering Lattice Spacing to Enhance Ion Diffusion Rate

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 Na3V2-xCax(PO4)2F3/C prepared by the Puspitasari team64, 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 Na3V2-xYx(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 Na3V2-xZrx(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 (DNa+) 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.
图8 (a) Lattice Spacing of Na4V2(PO4)2F3[55]; (b) Z, and ω1/2 Inverse Function Plot of Na3V2-xCax(PO4)2F3/C[64]; (c) XRD Patterns of NVPF and NVPF-Zr-x/NC (x=0, 0.01, 0.02, 0.05, and 0.1) Samples[63]; (d) Kinetic Parameters of NVPF-Zr-0.02/NC[63]

Fig. 8 (a) Lattice spacing of Na4V2(PO42F3 55; (b) Z , versus ω 1 / 2 inverse function image of Na3V2- x Ca x (PO42F3/C64; (c) XRD patterns of NVPF and NVPF-Zr-x/NC (x = 0, 0.01, 0.02, 0.05 and 0.1) samples63; (d) kinetic parameter map of NVPF-Zr-0.02/NC63

4.4 Enhancement of Electronic Conductivity

Element doping can effectively improve the electronic conductivity of materials, thereby enhancing their rate performance. Zhang et al.56 prepared Na2.90K0.10V2(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 Na3V2-xCrx(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 al72 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)F3-yOy (0≤y≤0.5) prepared by Broux et al89 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 al73 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.

5 Conclusion and Prospect

In the doping modification research of sodium vanadium fluorophosphate, studies have found that alkali metals (such as Li, K, etc.) are more likely to be doped at sodium sites, transition metals (such as Fe, Ti, Cr, Mg, etc.) are more likely to be doped at vanadium sites, and non-metal elements (such as Br, O, etc.) are more likely to be doped at anion sites, depending on the differences of various elements and sites. From the perspective of commercializing sodium-ion batteries, selecting the solid-phase method to synthesize sodium vanadium fluorophosphate is an economical and effective approach. Meanwhile, it is required that sodium-ion batteries should possess features such as high specific capacity and power density, good cycle life, safety, non-toxicity, and low cost. In the doping modification research of sodium vanadium fluorophosphate, the electronic conductivity and sodium ion diffusion rate of sodium vanadium fluorophosphate can be enhanced by doping continuously variable-valence metal elements, such as common elements Fe and Mn, at vanadium sites.
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