Recent research progress in the application of Na₃V₂(PO₄)₂F₃ cathode materials in sodium-ion batteries: synthesis, modification, and battery optimization

Solid-state method

The solid-state procedure is easy to operate with a low associated cost, typically involving mechanical grinding and sintering^[80]. An increase in calcination temperature and the use of nanoscale raw materials can improve product purity and reduce overall reaction time.

Minart et al.^[81] reported dense and crystalline carbon-coated Na₃V₂(PO₄)₂F₃ (NVPF@C) nanoparticles resulting from solid-state synthesis, with a tap density of 1.4 g cm^-3 (increased by 40%) following mechanical grinding, which represents the highest published value. The mechanical processing reduced the size of the particles and crystals while retaining the NVPF structure and surface chemical properties.

In the case of carbon thermal reduction (CTR), the carbon source is a critical consideration, serving as a reducing agent and coating layer to improve NVPF electronic conductivity. A number of organic carbon (glucose^[66] and bitumen^[64]) and inorganic carbon (acetylene black^[82] and super-phosphorus^[83]) sources have been considered. Different carbon sources were mixed with V₂O₅ and NH₄H₂PO₄ by ball-milling, and sintered at 800 °C to prepare carbon-coated NVPF materials (C-NVPF)^[84]. Samples prepared using glucose as the sole carbon source delivered the best electrochemical performance in SIBs. Tannic acid has been employed as a chelating agent and carbon source to prepare a carbon-coated composite [NVPF composite coated by tannic acid-derived carbon (NVPF@C-T)], using a combination of solid-state and sol-gel methods^[85]. By introducing tannic acid in two stages, a comprehensive and consistent carbon coating layer can be successfully created which significantly enhances the electronic conductivity of NVPF, facilitates the diffusion of Na⁺, and safeguards NVPF from corrosion caused by the electrolyte.

Citric acid was used as both a reducing agent and a carbon source in the synthesis of C-NVPF using surface ball-milling [Figure 2A]^[86]. The carbon coating improved conductivity and inhibited the aggregation of nanoparticles. The low voltage platform and decreased energy density due to the formation of NVP can be addressed by employing ball-milling to substitute Al for V in preparing NVAPF^[87]. The partial substitution of Al for V alters the local electronic states, enhancing the stability of F, where kinetic studies have confirmed an increase in Na⁺ migration rate.

Figure 2. (A) Schematic representation of carbon coated NVPF synthesis by facial ball-milling. Reproduced with permission^[86]. Copyright 2024, Royal Society of Chemistry; (B) Schematic representation of rectangular NVPF synthesis using a hydrothermal method. Reproduced with permission^[90]. Copyright 2023, Wiley -VCH; (C) SEM image of the materials synthesized by the sol-gel technique. Reproduced with permission^[97]. Copyright 2024, Royal Society of Chemistry; (D) Schematic representation of NVPF composite synthesis by spray drying. Reproduced with permission^[102]. Copyright 2024, Wiley; (E) SEM image of the materials synthesized by electrostatic spinning. Reproduced with permission^[105]. Copyright 2023, American Chemical Society; (F) SEM image of the materials synthesized using one-step chemical vapor replacement. Reproduced with permission^[107]. Copyright 2024, Wiley -VCH. NVPF: Na₃V₂(PO₄)₂F₃; SEM: scanning electron microscope.

Although the NVPF product generated by solid-state synthesis has a high phase purity, the electrochemical performance is affected by poor compositional homogeneity and a wide particle size distribution. It is necessary to further optimize the solid-state synthesis procedure in order to obtain NVPF with a uniform composition.

Hydrothermal/solvothermal method

Applying a hydrothermal/solvothermal synthesis enables materials synthesis to generate a product that effectively alleviates structural stress during electrochemical operation. For instance, a microwave-assisted hydrothermal procedure has been used to uniformly distribute NVPF on reduced graphene oxide (rGO), where NVPF nanocuboids coated with rGO (NVPF@rGO) were obtained after calcination^[88]. Al-Marri^[89] reported the synthesis of NVPF/rGO with a 3D conductive structure using a simple hydrothermal/solvent thermal reduction process. With the help of rGO, NVPF/rGO exhibited improved surface kinetics and diminished polarization, ultimately resulting in enhanced rate performance and extended cycle stability. The application of phosphomolybdic acid (PMA) as a crystal plane inducer served to increase adsorption and surface energy, accelerating the growth of NVPF crystals, and resulting in rectangular NVPF with continuous ionic diffusion paths. A nitrogen-doped carbon-coated rectangular NVPF cathode material (c-NVPF@NC) was synthesized by a PMA-assisted hydrothermal method [Figure 2B]^[90].

The grain size and vacancy defects can be effectively adjusted by varying the annealing temperature. For example, a NVPF electrode material obtained using hydrothermal method and subsequent thermal annealing at 350 °C exhibited a specific capacitance of 167.73 F g^-1 and a capacitance retention of 85%^[91]. Lin et al.^[92] studied the crystallization of NaH₂PO₄-VOSO₄-NaF-H₂O under hydrothermal conditions, where Na₃V₂O_2x(PO₄)₂F_3-2x was produced at 170 °C after 2 h with vanadium present in mixed valence states (V³⁺ and V⁴⁺). Varying the synthesis time, vanadium source, or the addition of reducing agents resulted in the formation of different NVPF pure phases. The unique pore structure of the NVPF cathode offers more active sites and promotes the diffusion of Na⁺. For example, tNVPF@C with a dense microsphere morphology was prepared by sodium dodecyl benzene sulfonate (SDBS)-assisted solvothermal treatment, exhibiting a stable structure, high conductivity and large specific surface area^[93]. By carbonizing polyacrylamide (PAM) to create a coating, the electronic conductivity of tNVPF was significantly enhanced, resulting in tNVPF@C with exceptional Na⁺ storage capabilities. In addition, a 3D porous carbon was employed as a nucleation substrate to synthesize two-dimensional (2D) NVPF nanosheets (NS) [Na₃V₂(PO₄)₂F₃ nanosheets on 3D porous carbon (NVPF@3Dc)] in a one-step hydrothermal method^[58]. A rapid ion transfer within the a-b plane of the NVPF@3Dc crystal, combined with a minimal volume change during Na insertion and extraction, contributed to enhanced electrode rate performance.

Hydrothermal/solvothermal methods can enhance reaction rates and generate specific structures, offering significant application potential. However, the synthesis results in variable product morphology and performance. Further research is required to tune the procedure in order to control material uniformity and stability, particularly in large-scale synthesis of NVPF.

Sol-gel method

The sol-gel method represents a simple process, requiring a relatively low reaction temperature, and is widely applied in materials synthesis^[94,95]. Sun et al.^[96] have reported a dual-functional N-doped carbon and β’’-Al₂O₃ coated NVPF (NC/β’’@NVPF) using the sol-gel procedure with subsequent calcination. The NC/β’’@NVPF cathode exhibited a high capacity (126.7 mAh g^-1) at 0.2 C, maintaining 95.5% of cpacity after 500 cycles (at 10 C). This response was attributed to compensation for the loss of Na⁺ by Na⁺ in β’’-Al₂O₃ during the formation of the cathode/electrolyte interface film.

Citric acid and oxalic acid are typically used as cross-linking agents and carbon sources in sol-gel synthesis. Yang et al.^[97] prepared a carbon-coated Na₃V₂(PO₄)₂F₃·3Na₃V₂(PO₄)₃ composite (NVPF·3NVP/C) composite [Figure 2C] where citric acid was employed to fabricate a 3D conductive carbon layer. The synergistic effect of the composites facilitated rapid ionic migration and enhanced the kinetic reactions. Notably, NVP, serving as the primary phase, reinforced structural stability and substantially boosted the capacitive contribution. In a separate study, citric acid was utilized as a chelating agent to synthesize NVPF by sol-gel^[98] where an excess or insufficient chelating agent resulted in F atom deficiency with a loss of crystallinity. It was found that the specific capacity, rate capability, and cycling performance were significantly influenced by the crystallinity of NVPF.

Recent work has considered synergistic strategies involving sol-gel treatment directed at surface modification. For instance, Mahato et al.^[99] have provided the first report of rota-tumbler assisted sol-gel to synthesize a series of carbon-coated Fe and Cr co-doped NVPF (NVFCPF). An enlargement in the size of the saddle point, along with the planes of monocapped trigonal prisms and the hexagonal cavities, aided in reducing the ion diffusion activation energy, thereby enhancing the diffusivity of the doped material. In addition, a carboxymethyl cellulose (CMC)-assisted sol-gel strategy was utilized to prepare cross-linked carbon-coated porous NVPF composites (NVPF@C@CMC)^[100]. The CMC provided additional Na⁺ to compensate for sodium loss during the formation of the solid electrolyte interface. Structural stability was enhanced by the cross-linked carbon, and the porous structure of the coating facilitated Na⁺ transfer.

Although the sol-gel method can be used to prepare porous NVPF, additional reaction conditions should be investigated to further improve material pore structure and morphology, such as pH, precursor composition, and temperature.

Spray drying

Spray drying method is commonly used to prepare powder particles or films, offering notable commercial benefits, such as low cost, short processing time, and ease of production. Different morphologies and structures can be achieved by adjusting the preparation conditions and employing auxiliary reagents. For example, a Br-doped spherical NVPF/C was prepared using polytetrafluoroethylene (PTFE) and cetyltrimethylammonium bromide (CTAB) as F supplements and regulators, respectively, in a one-step spray drying procedure^[101]. During the synthesis process, the hard PTFE template supplied additional F, and the resultant porous structure facilitated electrolyte permeation. The soft CTAB template provided Br, and the self-assembly process resulted in a strong spherical structure while refining the particle size. In another study, Zhou et al.^[102] synthesized copper substitution NVPF microspheres with an eggshell structure using polyvinyl pyrrolidone (PVP)-assisted spray drying [Figure 2D]. The high electronic conductivity of the cathode material and rapid Na⁺ diffusion were ascribed to the unique eggshell structure and crystal lattice.

Carbon-coated NVPF/carbon nanotube (CNT) nanospheres were synthesized by spray drying followed by calcination^[103]. The 3D CNT network and carbon coated layer enhanced electronic conductivity, increasing the contact area between the cathode and electrolyte, and improving overall the structural stability. Applying sugarcane bagasse as raw material, carbon-coated NVPF nanospheres (NVPF/C) with high capacity were prepared by spray drying combined with high-temperature calcination^[104].

Although the NVPF particles prepared by spray drying exhibited a high degree of uniformity, the structures reported to date are relatively simple, and the ability to control structure by adjusting preparation conditions warrants further investigation.

Electrostatic spinning

Electrostatic spinning involves a relatively simple operation that produces materials with a uniform morphology, and is particularly suitable for synthesizing materials with a one-dimensional linear/tubular structure. These structures can facilitate an oriented transmission of electrons, enhancing reaction kinetics via a conductive network. Electrodes prepared by electrostatic spinning possess a self-supporting structure that serves to increase the specific energy by reducing the use of binders and conductive carbon.

Sodium storage may be improved by adjusting spinning parameters, combined with high-temperature calcination, hydrothermal synthesis, chemical deposition, and other techniques. The porous structure and tubular characteristics can provide channels for Na⁺ transfer, enhancing conductivity. Liang et al.^[105] utilized electrostatic spinning to prepare self-standing NVPF cathodes [Figure 2E]. The NVPF nanoparticles were evenly distributed on the surface and inside the carbon nanofibers (CNFs), which effectively limited particle volume expansion during charging and discharging. Moreover, the CNF network facilitated electron and ion transport. The NVPF self-standing cathode exhibited a high-voltage platform of ~ 4.07 V, and the assembled full cell demonstrated an energy density as high as 185.6 Wh kg^-1.

Electrostatic spinning requires stringent experimental conditions as the process is sensitive to temperature and humidity, and involves extended preparation times which have limited large-scale application. In addition, the inorganic fibers produced are susceptible to damage where the fiber length may be significantly decreased during grinding.

Other methods

A number of researchers have considered alternative routes to NVPF materials synthesis. In order to improve the low tap density and poor cycling performance, Song et al.^[106] synthesized NVPF materials (HTS-NVPF) with a uniform conductive network and high tap density using a high-temperature shock (HTS) strategy. The ultra-fast high-temperature synthesis (heating rate = 1,100 °C s^-1 and heating time = 10 s) circumvented F loss and the formation of impurities, while enhancing cycling stability. The conductive network associated with the large synthesized particles was uniform, contributing to increased conductivity. The HTS-NVPF system exhibited a wide range of operating temperatures (-45 °C to 55 °C).

In order to address Na⁺ diffusion rate, Wang et al.^[107] prepared Na_3-yVPO_2-xBr_xF by doping Br^- and pre-loading Na⁺ vacancies using a one-step chemical vapor replacement (CVR) method [Figure 2F]. The electrostatically shielded channels in Na_3-yVPO_2-xBr_xF reduced coulombic hindrance by Na⁺ during the insertion/extraction process, increasing Na⁺ diffusion rate by a factor of 5.

A flexible cathode with a high-rate capability and stability can satisfy the requirements of energy storage devices with respect to portability, high power and energy density. A flexible cathode material without binder (NVPF@C/CC) was prepared using a colloidal crystal template method, which anchored nanocarbon onto carbon cloth that was used to coat the NVPF, providing a 3D ordered microporous structure^[108]. The latter facilitated ion diffusion, while the combination of the carbon layer and carbon cloth provided a continuous electronic transmission path, which contributed to a high-rate performance and conductivity.

The electrochemical properties of the NVPF materials synthesized using a range of methods are presented in Table 1.

Carbon coated NVPF

One of the key approaches to optimizing electrode structure is to construct an electron transfer system with high electrical conductivity. It is possible to effectively enhance ion and electron conductivity, and increase electrode stability using surface coating techniques^[125]. Carbon materials are commonly used in coating, offering high conductivity and porous characteristics that provide effective transport channels, increasing electrochemical reaction rates while effectively avoiding direct contact with the electrolyte, which mitigates volume changes during Na⁺ extraction/insertion^[126]. Sun et al.^[127] prepared carbon-coated NVPF composites using the sol-gel method, employing PTFE as the carbon source and a fluorine supplement. The presence of the NVP impurity in these carbon-coated NVPF composites was markedly diminished or entirely eradicated.

Carbon-based materials can control NVPF morphology and structure during synthesis and sintering, providing a scaffold that reduces the stress impact caused by electrical effects. Hu et al.^[128] have prepared microsphere composite materials (HM-NVPF@CN) assembled from carbon-coated NVPF NS [Figure 3A]. Subsequently, a cathode-electrolyte interface (CEI) layer rich in B and F was constructed on the surface. The unique 2D NS structure of HM-NVPF@CN can shorten the diffusion paths of Na⁺ and e^-, increasing the reaction contact area while inhibiting parasitic reactions. The thin carbon coating is beneficial for improving electronic conductivity and mitigating structural stress. The optimized electrode exhibited superior cycling stability over a wide temperature range (-25 °C to 60 °C). In addition, NVPF with a porous microsphere structure was synthesized using the deep eutectic solvent method^[129]. The composite carbon coated Na₃V₂(PO₄)₂F₃ microspheres with fluorocarbon layer NVPF/C-F was prepared by forming an amorphous fluoroarbon layer via PTFE surface modification. The study demonstrated that the amorphous fluoroarbon layer promoted Na⁺ diffusion and charge transfer.

Figure 3. (A) Finite element simulation models and design strategies for NVPF microspheres assembled using carbon coated NVPF nanosheets. Reproduced with permission^[128]. Copyright 2024, Elsevier B.V.; (B) Schematic representation of materials preparation; (C) TEM images of a dual-function carbon network composed of nitrogen-doped carbon layers and carbon bridges. Reproduced with permission^[133]. Copyright 2024, American Chemical Society. NVPF: Na₃V₂(PO₄)₂F₃; TEM: transmission electron microscope.

Layered carbon materials provide a bridge between electrons and active substances, and effectively promote the migration of Na⁺ between the electrolyte and electrodes. Carbon-coated NVPF with a microcube structure (NVPF/C) was prepared using an open system synthesis method, involving in situ growth on graphene oxide (GO) microcubes^[130]. The NVPF/C has an average particle size of ~ 2 μm and a carbon layer thickness of ~ 5 nm. Due to the structure-induced electrochemical reversibility and improved reaction kinetics, it demonstrated the superior electrochemical properties. The carbon coatings prepared in situ from inorganic or organic carbon sources exhibit different structural characteristics. For example, carbon-coated NVPF composites were synthesized using branched starch as carbon source and structural template^[131]. Two-dimensional nanostructures were formed where the branched starch induced NVPF nucleation along its main chain. This structural characteristic can shorten the Na⁺ diffusion path and enhance electron transfer.

Carbon coating doped with heteroatoms can significantly improve electronic conductivity and structural stability where the dopants introduce numerous active sites and structural defects, which enhance ion transport efficiency between the electrode and electrolyte. N-doping provides abundant active sites for electrochemical reactions and enhances surface hydrophilicity. An N-doped carbon-coated NVPF composite was prepared using PVP as nitrogen source using the sol-gel method^[132]. The N doping introduced exogenous defects in the carbon layer and additional active sites, increasing conductivity and promoting Na⁺ diffusion.

It is known that dopamine-based coatings can spontaneously accumulate on the surface of the materials. Sun et al.^[133] fabricated a bifunctional carbon network by pyrolysis of dopamine (PDA) and PTFE, encapsulating NVPF particles and carbon bridges with an N-doped carbon layer [Figure 3B and C]. The PTFE served to supplement F loss during high-temperature calcination, inhibiting the formation of NVP impurities. The dopamine provided N for the carbon coating layer, creating defects and additional Na storage sites. The bifunctional N-doped carbon network can optimize electron/ion transfer at the interface of the electrode material, preventing NVPF corrosion by the electrolyte.

When compared with single atom doping, double atom doping can introduce more active sites, effectively promoting electron and ion transport^[134]. This is particularly the case for doping with N, S, and B, which serves to improve Na⁺ storage capacity and electronic conductivity in diverse modification processes^[135]. In a recent study, a N, B-doped carbon-coated NVPF composite (NVPF@NBC) was prepared by a mechanical ball-milling assisted sol-gel procedure, employing polyacrylonitrile (PAN) and boric acid as nitrogen and boron sources, respectively^[136]. The co-doping of N and B atoms increased the number of defects in the carbon, enhancing Na⁺ storage and transport. The N, B co-doped carbon layer can reduce particle agglomeration and maintain structural stability. Yu et al.^[137] synthesized a N/B co-doped carbon coated NVPF (NVPF-CNB) using the sol-gel method with 1-vinyl-3-methyl imidazole tetrafluoroborate as the nitrogen and boron source, and a carbon-coated NVPF composite using ionic liquid-acrylic copolymer as a modification reagent^[138]. A N/S co-doped carbon-coated NVPF (NVPF@NSC) was also prepared by sol-gel combined with mechanical ball-milling which employed thiourea as the nitrogen and sulfur source^[139]. The study revealed that the doping of N and S had no impact on the crystal structure of NVPF_, but demonstrated excellent reversibility and minimal polarization.

Despite the widespread use of carbon-coated NVPF composite materials, graphitization in carbon layers remains a challenge that impairs electronic conductivity. Moreover, it is difficult to form uniform carbon layers using current preparation procedures, which has a significant impact on Na⁺ migration between particles and electrolytes. It should also be noted that carbon layers formed from different carbon sources have varying electrochemical properties. Preparation conditions and the use of regulatory factors warrant further investigation.

Carbon composite NVPF

When NVPF particles experience fracture or loss of potential contact, the resistance between them intensifies, resulting in decreased capacity. It is essential to construct a stable NVPF structure and maintain a consistent and reliable electrochemical output. Three-dimensional conductive carbon materials, notably CNTs, are recognized as the most suitable candidates for this purpose. Research has conclusively demonstrated the efficacy of CNTs in boosting ionic transport^[140]. By embedding CNTs within NVPF particles and constructing an interconnected network, the material can better accommodate Na⁺ volumetric fluctuations during repeated charging and discharging, maintaining the electrical conductivity between self-assembled particles. For example, Ma et al.^[141] synthesized a CNT-hybrid dual-phase anionic phosphate material (denoted as NFC) via a one-pot sintering method. The material consisted of NVPF with interwoven CNTs and a minor amount of NVP. This cathode material demonstrated a wide operating temperature range (-20 °C to 50 °C). A NVPF@CNT composite was prepared by anchoring NVPF nanoparticles on CNTs with a low number of walls [Figure 4A-C]^[142]. Ion transmission via the CNTs promoted the growth of primary NVPF crystal particles. The high crystallinity and anchoring effect enhanced structural stability, while the 3D network composed of CNTs and amorphous carbon layers increased conductivity. This strategy can improve material crystallinity and increase grain size. CNTs were utilized as an effective medium to adjust the electronic conductivity of NVPF in a recent study^[143].

Figure 4. (A) Schematic representation of ion transport by CNTs to promote NVPF grain growth; (B) Synthesis process of NVPF@CNT composite materials; (C) Cycle performance of different materials. Reproduced with permission^[142]. Copyright 2023, Elsevier; (D) SEM and (E) HRTEM images of NVF-GEL/rGO. Reproduced with permission^[149]. Copyright 2022, Elsevier. (F) Schematic representation of the synthesis process. The illustration is TEM image of NVPF@rGO/CNTs. Reproduced with permission^[150]. Copyright 2023, Wiley. CNT: Carbon nanotube; NVPF: Na₃V₂(PO₄)₂F₃; NVPF@CNT: Na₃V₂(PO₄)₂F₃ coated with carbon nanotubes; SEM: scanning electron microscope; HRTEM: high resolution transmission electron microscope; NVF-GEL/rGO: NVPF modified reduced graphene oxide using GEL method.

Research has shown that the formation of a 3D cross-linked carbon network can enhance charge transfer kinetics and shorten the ionic transfer pathway. For instance, a composite with a cross-linked dual-carbon framework structure [Na₃V₂(PO₄)₂F₃ caged in 3D cross-linked carbon networks composed of carbon layer and intertwined CNTs (NVPF@C@CNTs)] was constructed by cross-linking NVPF with amorphous carbon and CNTs^[144], and NVPF@C/CNTs composites with a carbon-coated 3D conductive network were prepared by solvothermal method ^[145].

Introducing graphene is an effective strategy to improve electrode performance. GO can serve as an efficient carrier for immobilizing nanoparticles due to its unique 2D structure^[146,147]. A high-temperature heat treatment is used to prevent particle agglomeration and lower the GO content with a stable anchoring of NVPF, and an extended contact area between the electrode and electrolyte^[148]. For instance, NVPF-gel/rGO composites were synthesized using a hydrothermal procedure with addition of gelatin and rGO^[149]. The morphology of the material exhibited a high degree of uniformity where the addition of gelatin reduced impurities, and the presence of graphene enhanced Na⁺ diffusion [Figure 4D and E]. Shi et al.^[150] prepared a composite material (NVPF@rGO/CNT) by self-assembly, generating a 3D cross-linked conductive network containing 2D rGO and 1D CNT [Figure 4F], which makes it feasible to operate in a wide temperature range (-40 °C to 50 °C).

Combining conductive carbon with NVPF is an effective strategy to enhance electronic conductivity. This is particularly the case when the carbon materials form 3D or dual-carbon layer wrapping structures.

Element doping of NVPF

NVPF materials typically exhibit lower intrinsic electronic conductivity and limited rate performance due to the insulating properties of the [PO₄] tetrahedron. Element doping enables a flexible adjustment of material properties, such as electronic conductivity, structural defects, crystallinity, and redox potential. The doping elements can be classified into three categories based on differences in the substitutional sites: sodium ion site substitution; vanadium ion site substitution; anion site substitution. Recent research has mainly focused on the first two types of substitution.

Doping elements with a larger ionic radius at the Na site in NVPF materials can lead to an increase in lattice spacing that, in turn, widens the channels for ionic diffusion^[151,152]. This ensures that the NASICON framework is not damaged or affected by internal stress and structural deformation during charge and discharge cycles. Moreover, the introduction of these elements results in a shortening of the vanadium-fluorine bond (V-F) and vanadium-oxygen bond (V-O) bond lengths in the lattice, enhancing NVPF structural stability. Wu et al.^[153] have prepared composite materials with a carbon coating by the sol-gel method, incorporating K⁺ to replace a Na⁺ component in NVPF crystals [carbon coated Na₃V₂(PO₄)₂F₃ doped with K+ (N_1-xK_xVPF/C)]. The doped K⁺, which was located at the Na1 site, did not participate in reaction during charge-discharge, ensuring high Na⁺ migration. The doping of K⁺ reduced the bandgap energy and increased electronic conductivity.

The introduction of high-entropy elements (Mg, Cr, Al, Nb) can improve the stability of the Na2 site in NVPF, and suppress the intermediate reaction of the NVPF phase at low potentials. High-entropy carbon-coated NVPF composites (NVPF-HE@C) [Figure 5A], synthesized by the sol-gel method, exhibited a significant improvement in the specific capacity, capacity retention, and Na⁺ diffusion coefficient^[154]. Doping with Fe serves to refine the particles, inhibiting particle agglomeration, and promoting Na⁺ diffusion. Yang et al.^[155] prepared NVPF composites [Fe-doped Na₃V₂(PO₄)₂F₃ coated by N-doped CNTs (Fe-NVPF@N-CNTs)] composed of Fe-doped NVPF and N-doped CNTs, which exhibited enhanced conductivity due to a structure of 3D carbon framework.

Figure 5. Schematic representation of ion-doped NVPF crystal structures: (A) High-entropy element. Reproduced with permission^[154]. Copyright 2024, Elsevier; (B) Cr³⁺. Reproduced with permission^[166]. Copyright 2024, Elsevier; (C) Mn²⁺. Reproduced with permission^[169]. Copyright 2023, MDPI; (D) Ti⁴⁺. Reproduced with permission^[170]. Copyright 2023, Chemistry Europe; Rate performance of ion-doped NVPF materials: (E) Ca²⁺. Reproduced with permission^[162]. Copyright 2024, American Chemical Society; (F) Al³⁺. Reproduced with permission^[15]. Copyright 2022, Elsevier; (G) Cr³⁺. Reproduced with permission^[166]. Copyright 2024, Elsevier. NVPF: Na₃V₂(PO₄)₂F₃.

When compared to doping at the Na site, the method of replacing V with inexpensive elements represents a more common approach that can reduce the toxicity due to V and lower overall costs. Inactive elements (such as Mg^2+[156,157], Al^3+[158,159], and Y^3+[117]) that do not participate in electrochemical reactions, can promote the formation of defects in the NVPF crystals, providing additional charge transfer pathways.

The holes that are generated by the substitution of V by Mg enhance conductivity. The Mg²⁺ dopant does not participate in redox reactions, but can suppress changes to the lattice structure. Wang et al.^[160] prepared a NVPF composite [Na_3.3K_0.2V_1.5Mg_0.5(PO₄)₂F₃] with N-doped carbon coating, employing a binary co-doping strategy. The surface of the N-doped carbon layer was rich in defects, providing additional active sites and enhancing conductivity. Such modifications accelerate the reaction kinetics and augment electrolyte interaction through an expanded specific surface area, thus streamlining the electrochemical process. Concurrently, strategic heteroatom substitution leads to a more efficient engagement of Na⁺ in the electrochemical activities, thereby bolstering the cathode’s structural integrity. The ionic radius of Bi³⁺ is larger than that of V, which is beneficial for Na⁺ intercalation by increasing the interplanar spacing. A Bi-doped carbon-coated NVPF [Na₃V_2-xBi_x(PO₄)₂F₃/C] was synthesized using the sol-gel method combined with high-temperature calcination^[161]. The results indicate that the incorporation of an optimal amount of Bi-doping and carbon coating significantly enhanced the conductivity, Na⁺ diffusion performance, and structural stability of the cathode material during the charge-discharge cycle.

Replacing V³⁺ with Ca²⁺ can reduce the bandgap, enhance electronic conductivity, and improve structural stability. Puspitasari et al.^[162] prepared a Ca-doped NVPF cathode material with a carbon coating [Ca-doped Na₃V₂(PO₄)₂F₃ with a carbon coating (NVPFCa-x/C)] using sol-gel and carbothermal reduction techniques. The results have shown that Ca²⁺ doping did not alter the crystal structure but expanded the lattice spacing. The incorporation of the Ca²⁺ ion leads to the emergence of two localized states within the band gap, thereby significantly improving the electrochemical performance in comparison to Mg-doped NVPF/C.

Doping trivalent rare-earth ions at the V³⁺ site is an effective strategy for improving NVPF performance. In a recent study, an atomic simulation method based on the minimization of lattice energy was used to explore the effects of rare-earth ion doping on the local structural deformation and intrinsic defect properties of the NVPF cathode material^[163]. The study found that defects formed by Na interstitials and vacancies (Na Frenkel defects) serve to promote Na⁺ diffusion. In addition, the concentration of Na vacancies and interstitials can be increased by doping rare-earth ions at Na and P sites, enhancing reaction kinetics.

The removal or insertion of excess Na⁺ in NVPF can be achieved within a relatively wide voltage range by adding Al. A study of Al-doped NVPF (NVAlPF-x) synthesis has demonstrated that Al doping reduced the band gap without causing a phase transition^[15]. Doping with Al increased the Na⁺ diffusion coefficient, and Al³⁺ successfully replaced the V³⁺ site within the NVPF lattice.

A high-entropy doping can suppress adverse phase transitions at low voltages, promoting Na⁺ storage at high voltages. Gu et al.^[60] have applied the concept of high entropy with respect to the PACs lattice, and prepared a carbon-free high-entropy Na₃V_1.9(Ca,Mg,Al,Cr,Ca)_0.1(PO₄)₂F₃ cathode material by partially substituting V atoms. Density functional theory (DFT) calculations have shown that high-entropy doping alters the electronic state, enhancing conductivity and Na⁺ migration rate, further elevating the average voltage platform to 3.81 V and increasing the energy density to 445.42 Wh kg^-1.

During the electrode reaction, new active redox pairs can be formed by high-activity elements, such as Mn^2+[164], Cr^3+[118], and Ti^3+[63], which serve to appreciably enhance the specific capacity and output voltage^[165]. Chromium doping is particularly effective as Cr³⁺ has a similar ionic radius to V³⁺. A moderate level of Cr³⁺ doping can introduce crystal defects, reducing the Na⁺ transport distance, and enhancing diffusion with higher electron transfer and charge transfer rates. For instance, Cr-doped NVPF composites with a carbon coating were synthesized using Cr³⁺ as an iso-valent doping ion by the sol-gel method^[166] [Figure 5B]. The results established improved electrical conductivity as a result of lower charge transfer resistance (R_ct) due to Cr³⁺ doping. In addition, Yi et al.^[167] prepared Cr-doped NVPF composites [Na₃V_2-xCr_x(PO₄)₂F₃/C] by sol-gel. The enhanced electrochemical performance is attributed to the elevated crystallinity, optimized intrinsic electronic conductivity, and Na⁺ diffusion coefficient resulting from the doping of Cr³⁺. This doping accelerates the electron transport and charge transfer rate.

An appropriate level of Mn²⁺ incorporation in NVPF can effectively enhance the Na⁺ diffusion rate due to the larger radius of Mn²⁺ relative to V³⁺ that regulates the lattice and surface energy. Gu et al.^[168] synthesized NVPF-NS with a 2D NS structure, which enabled an adjustment of the surface energy and V site in NVPF by partially replacing V³⁺ with Mn²⁺. This led to combined presence of V³⁺ and V⁴⁺, which enhanced NVPF electronic conductivity and structural stability. NVPF materials doped with different valence Mn ions have been produced in a hydrothermal procedure^[169]. The results reported demonstrated that Mn doping altered the NVPF crystal structure and residual carbon content [Figure 5C]. The incorporation of Mn²⁺ resulted in a NVPF micro-rectangular morphology, restricting crystal size, reducing R_ct and promoting Na⁺ diffusion.

Doping with Ti can effectively inhibit the formation of an irreversible tetragonal phase, and achieve Na⁺ storage via a reversible transition from the Amam to Cmc21 phase. Tong et al.^[170] synthesized Ti-doped NVPF (NVTPF) cathode material by solid-state sintering [Figure 5D]. The incorporation of Ti stabilized the local structure via F-Ti bond formation by precipitation, with minimal changes during charge and discharge. In the electrochemical process, vanadium was reversibly transformed from V³⁺ to V⁵⁺, where Ti⁴⁺ was unchanged. The substitution of W⁶⁺ at the V site can improve Na⁺ diffusion and electronic conductivity, while inhibiting grain growth. W-doped carbon-coated NVPF synthesis using a sol-gel and ball-milling assisted by sonication has been shown to enhance structural integrity and improve Na⁺ mobility^[171].

The element-doped NVPF with excellent rate performance reported in recent studies is illustrated in Figure 5E-G. A comparison of the electrochemical performance of NVPF cathodes in SIBs prepared using different doping strategies is presented in Table 2. It is evident that ion doping can enhance the intrinsic conductivity of NVPF, maintaining the stability of the crystal structure. Doping with inexpensive metals can reduce production costs. However, the incorporation of ions with large radii and excessive doping may damage the crystal structure and lower material stability. In order to ensure effective doping, many factors such as element type, doping amount, distribution uniformity, dopant siting and cost require further comprehensive investigation.

Interface and defect engineering

In recent years, researchers have made numerous attempts to improve NVPF electrochemical performance by utilizing interface and defect engineering to enhance electronic transmission dynamics and ionic diffusion kinetics, mitigating the structural stress generated during electrochemical processes.

Defect engineering can optimize Na⁺ transport kinetics. By introducing ordered vacancies, Na⁺ can be uniformly adsorbed, alleviating mechanical stress during charging and discharging and significantly improving rate performance^[178]. Bulk nitrogen doping in NVPF can expand the interlayer spacing and enhance electronic conductivity, raising the voltage platform while achieving ultra-long cycle stability^[179]. Vacancies or doping defects can lower the migration energy barrier for sodium ions and increase the diffusion coefficient.

Interfacial engineering can enhance structural stability and interface compatibility. The construction of strong chemical bond interfaces can inhibit surface side-reactions and improve electron transport efficiency^[180]. Hard carbons are the most commonly used anode materials in SIBs, while they generally suffer from poor cycling and rate capability. Glyme-ether-based electrolytes enable long-term cycling of SIBs based on hard carbon anode and NVPF cathode, due to the formation of fluorine-rich SEI layer with superior stability^[181]. Applying carbon coating or compositing with conductive materials (such as graphene) reduces interface impedance and alleviates volume expansion, improving the cycling stability of NVPF^{[103,108,128,145,150]}.

Interface and defect engineering can significantly improve the rate performance, cycle life, and low-temperature adaptability of NVPF cathodes by regulating Na⁺ transport pathways, enhancing electronic conductivity, and optimizing interface stability. Future research should focus on a precise design of defects and interfaces, with an examination of synergistic optimization mechanisms.

Optimization of electrolyte composition

The electrolyte provides the necessary medium for ionic transfer between cathode and anode^[182]. There are currently two main approaches to optimizing SIB electrolytes. The first deals with ionic conductivity and viscosity, focusing on the type or composition of the electrolyte. The second considers the use of electrolyte additives to form a stable protective layer at the interface between the electrolyte and the electrode, enhancing battery cycling performance.

The operation of SIBs at temperatures above 50 °C presents challenges in terms of safety and stable cycling, requiring high-temperature-resistant liquid electrolytes. Liang et al.^[183] combined fluoroethylene carbonate (FEC) and trimethylsilyl phosphite (TMSPi) to prepare a non-flammable electrolyte. This dual-functional integrated strategy resulted in a non-combustible electrolyte with thermal stability that ensured safe and stable battery operation at high temperatures [Figure 6A and B]. In addition, the same research group considered a film-forming modulation strategy, which improved the ionic transport capability and structural/thermal stability of CEI/SEI (solid electrolyte interphase) by optimizing the interfacial formation of P/Si intermediates, enabling stable operation of the SIB over a wide temperature range (-25 °C to 75 °C), successfully achieving all-weather adaptability [Figure 6C]^[184]. The assembled HC//NVPF full cell maintained over 90% capacity retention following long-term testing at -25 °C and 50 °C. The addition of an organic co-solvent can improve the low temperature resistance of the battery. A fully-printed flexible aqueous rechargeable SIB (ARSIB) with superior mechanical properties was assembled using screen printing technology for the first time, with NVPF/C as the cathode material and Na₃MnTi(PO₄)₃/C as the anode material^[185]. A 17 m (mol kg^-1) NaClO₄-EG mixed electrolyte which can withstand low temperature (-20 °C) was prepared by incorporating ethylene glycol (EG) as co-solvent with the 17 m NaClO₄ electrolyte. This flexible battery had a capacity of 40.1 mAh g^-1, with outstanding mechanical and cycling performance [Figure 6D and E].

Figure 6. (A) The TMSPi-induced film formation process; (B) Flame test carried out with separators soaked in different electrolytes. Reproduced with permission^[183]. Copyright 2024, Elsevier; (C) Galvanostatic charge-discharge (GCD) curves associated with continuous temperature testing simulating a wide range of all-weather conditions. Reproduced with permission^[184]. Copyright 2024, American Chemical Society; (D) Power to electronic devices at 25 °C and -20 °C; (E) GCD curves of a battery with different bending angles at 0.1 C. Reproduced with permission^[185]. Copyright 2024, Wiley; (F) V dissolution of a charged NVPF powder in EC-PC-DMC with different sodium salts at 55 °C for 21 days. Reproduced with permission^[186]. Copyright 2023, Elsevier; (G) Schematic representation of solvation structure; (H) TEM image of a circulating NVPF cathode in NPP6 electrolyte (PFPN as a co-solvent in electrolyte at molar ratio of PC:PFPN = 7.5:1). Reproduced with permission^[188]. Copyright 2024, Wiley -VCH. TMSPi: trimethylsilyl phosphite; NVPF: Na₃V₂(PO₄)₂F₃; EC: ethylene carbonate; PC: propylene carbonate; DMC: dimethyl carbonate; TEM: transmission electron microscope; NPP6: 6th sodium-ion electrolyte formulation with PC and PFPN; PFPN: ethoxy (pentafluoro) cyclotriphosphazene.

The dissolution of V and resultant parasitic reactions are widely recognized as key factors leading to declining battery performance. A combined use of surface coatings and changes in electrolyte formulation was proposed to reduce V dissolution [Figure 6F]^[186]. It was shown that electrolytes based on sodium trifluoromethane sulfonimide (NaTFSI) inhibited V dissolution, but degradation must be prevented at high potentials. In addition, a carbon coating of NVPF particles with the addition of electrolyte surface protective agents is effective in minimizing V dissolution.

The formation of an anion-derived CEI layer improves interfacial stability and promotes rapid transfer of Na⁺ at the interface. The NVPF cathode exhibited greater stability in the diglyme-based electrolyte. Zheng et al.^[187] have reported a thorough examination of the Na storage mechanism by NVPF in diglyme and propylene carbonate(PC)/ethylene carbonate (EC) electrolytes containing 0.5 M NaPF₆ at low temperatures. This electrolyte system significantly enhanced the rate of Na⁺ diffusion in the bulk electrolyte with a reduction in the charge transfer barrier to 158.6 meV, which is only one-third of that in a PC/EC-based electrolyte. Moreover, the battery can function normally and maintain stable performance at -60 °C.

Recently, a novel ternary low-concentration electrolyte and cation/anion solvation strategy was proposed, which utilized ethoxy (pentafluoro) phosphazene (PFPN) as a weakly polar solvent to improve the electrode/electrolyte interfacial stability and enhance ion conductivity and oxidation stability [Figure 6G and H]^[188]. This strategy enabled increased cycling performance and energy density, facilitating durable high-voltage SIBs. The ionic conductivity of the low-concentration electrolyte was increased to 6.12 mS cm^-1, and the electrochemical stability window was raised to 4.84 V.

The fundamental cause of the significant capacity decline of NVPF₃-based batteries is associated with the continuous electrolyte oxidation process rather than F loss or the collapse of the crystal structure. The capacity retention of NVPF₃ batteries can be improved by optimizing the electrolyte. Jiang et al.^[189] designed an electrolyte additive containing two -C≡N groups that enabled a stable SIB cycling at high voltage.

Sulfonate anion-based ionic liquids have been widely used as electrolytes for secondary batteries. An inhibitory effect of the Na[FSA]-[C2C1im][FSA] ionic liquid on Al corrosion in the presence of [FAP]^- has been reported^[190]. A uniform deposition passivation layer formed by the decomposition products of [FSA]^- and [FAP]^- mitigated the corrosion of Al after adding [FAP]^-. The study found that Na[FSA]-[C2C1im][FSA] moderately inhibited Al corrosion at 25 °C, but a severe pitting corrosion of the Al electrode was observed at 90 °C when the potential was higher than 4 V vs Na⁺/Na. In the charge-discharge tests of NVPF electrodes, the addition of [FAP]^- elevated the electrochemical performance of the ionic liquid electrolyte, particularly at high temperatures.

Exogenous additives

Strategies for increasing the exposure of active crystal facets that promote Na⁺ transport involving selective crystal growth are crucial to developing cathodes for high-performance SIBs^[191]. For example, NVPFs with different (002) face exposure rates were synthesized using carbon clusters (c-clusters) as inducers to regulate the surface energy [Figure 7A]^[192]. The study demonstrated that the (002) face possessed more stable Na⁺ storage sites and lower diffusion barriers, where the adsorption of c-clusters generated 2D particles that influenced the crystal growth direction. The optimized NVPFs exhibited a high reversible capacity (123 mAh g^-1) and energy density (431 Wh kg^-1).

Figure 7. (A) Schematic representation of NVPF growth. Reproduced with permission^[192]. Copyright 2024, Royal Society of Chemistry; (B) Electron cloud density of associated materials. Reproduced with permission^[193]. Copyright 2024, Wiley -VCH; (C) TEM image of NVPF particle; (D) Schematic representation of the effects of NaPAA and PVDF binders on the formation of CEI layers. Reproduced with permission^[194]. Copyright 2024, Royal Society of Chemistry; (E) Schematic representation of the Na₄C₆O₆ loss compensation mechanism for the cathode and anode. Reproduced with permission^[195]. Copyright 2022, Royal Society of Chemistry. NVPF: Na₃V₂(PO₄)₂F₃; TEM: transmission electron microscope; NaPAA: sodium polyacrylate; PVDF: polyvinylidene fluoride;‌ CEI: cathode-electrolyte interface.

Chemical pre-precipitation (CP) is an effective method to enhance SIB energy density. Current sodium agents are mostly polycyclic aromatic hydrocarbons with low reduction potential, which can readily result in excessive sodium insertion in the cathode with structural deterioration. As novel CP agents, aromatic ketones can endow the agents with mild reducibility by introducing the carbonyl (C=O) function to balance conjugation and inductive effects [Figure 7B]^[193]. While maintaining structural stability, the charge capacity of the positive electrode material was significantly improved.

Introducing a sodium polyacrylate (NaPAA) binder to the NVPF cathode can overcome the instability of the interface^[194]. The NaPAA binder interacted with the NVPF cathode via ionic-dipole interactions, improving the distribution of active materials and forming a stable ionic conductive NaPO₂F₂-rich CEI layer that effectively inhibited electrolyte decomposition [Figure 7C and D]. Zhang et al.^[195] prepared a novel high-capacity self-sacrificial Na₄C₆O₆ additive_, which contained a large number of sulfonic carboxyl groups [Figure 7E]; the resultant capacity exceeded 400 mAh g^-1. This additive can provide sufficient Na to compensate for Na loss during the initial charge-discharge cycle. The energy density of the full cell with 9% additive (HC//NVPF/rGO) increased from 154.5 Wh kg^-1 to 210.8 Wh kg^-1.

Mechanism of SIB performance

As the battery interphase properties and chemical composition vary with temperature and the dissolution and recombination of the interphase are affected by temperature, the desired characteristics can be achieved by combining high and low temperatures. A recent study has illustrated the effect of temperature on the interphase properties of Na₃V₂(PO₄)₂F₃|hard carbon (NVPF|HC) batteries [Figure 8A]^[196]. The results demonstrated that cycle stability, impedance, and rate performance were sensitive to temperature, leading to different SEI formation pathways. The battery capacity retention and coulomb efficiency values were lowest at 25 °C. At higher temperatures, the battery was stable but exhibited a high impedance, whereas the NaF content was high with minimal transition metal dissolution at low temperatures. Superior reversible capacity, cycle stability, and high-power rate capability can be achieved by adjusting temperature.

Figure 8. (A) Effect of temperature on different NVPF|HC cells parameters. Reproduced with permission^[196]. Copyright 2023, IOP Publishing; (B) Performance comparison at different temperatures. Reproduced with permission^[197]. Copyright 2023, American Chemical Society; (C) Rate capacity of the batteries; (D) Nyquist plots for NVPF-HU and NVPF-UU; (E) Effect of heat treatment on the failed NVPF-H cathode. Reproduced with permission^[200]. Copyright 2022, Wiley -VCH. NVPF|HC: Na₃V₂(PO₄)₂F₃|hard carbon.

Komayko et al.^[197] investigated the effects of two different intercalation approaches on the performance of SIB cathode materials at low temperatures and high discharge current limits [Figure 8B]. The research established that NaVPO₄F, which followed a solid solution mechanism, was significantly superior to NVPF which exhibited phase separation during Na⁺ intercalation. The energy density retention of NaVPO₄F at 10 C and -30 °C was 387 and 308 Wh kg^-1, respectively, compared with 259 and 184 Wh kg^-1 for NVPF. Although the Na⁺ diffusion coefficient in the single-phase region was similar for both materials, the decline in performance from the quasi-equilibrium charge-discharge state was caused by concentration polarization effects. In the case of NVPF, the slow movement of phase boundaries during phase transformation was the principal cause of the observed rapid capacity decay.

Molecular dynamics studies of the local nanostructure associated with a series of carbonate-based sodium electrolytes at the NVP and NVPF interfaces were conducted with an analysis of de-solvation^[198]. The results have indicated that the solvent filler behavior at the electrode surface is a key factor affecting the energy barrier height related to de-solvation. On a graphite electrode without SEI, no dissolution was observed due to the absence of specific oriented interactions between the solvent and the carbon sheet. However, in polyanion nested compounds with uneven surfaces and donor and acceptor groups, the closest layer of solvent molecules exhibited a distinct arrangement, with a partial loss of the Na⁺ solvation shell on passage through the layer. This effect was particularly pronounced on the NVP interface, and the high energy barrier was attributed to the highly ordered and well-oriented first solvent layer. The arrangement of the solvent was less pronounced at the NVPF interface due to the presence of non-polar fluorine atoms which interfered with the electrostatic attraction between the solvent and electrode.

Semykina et al.^[199] studied the mechanism and kinetics of the solid-phase reaction between NaF and VPO₄, along with the effect of high-energy ball-milling (HEBM). The results showed that the reaction proceeded in a “dimensionality reduction” manner, where the rate-limiting step involved the diffusion of Na⁺ and F^- through the single VPO₄ side. The macroscopic HEBM mechanism followed a third-order reaction model, enabling a significant increase in the rate constant and diffusion coefficient. The apparent activation energy was approximately 290 kJ mol^-1. The similarity in the structure of reactant (VPO₄) and product (NVPF) facilitated interaction with a partial occupation of Na⁺ sites that favored Na⁺ migration. The electrochemical performance of the prepared NVPF matched conventional materials.

An analysis of the interface between a high-voltage NVPF cathode and electrolyte in SIBs has revealed that the interface phase of high-concentration electrolytes was primarily composed of anions, including a significant presence of anion derivatives (C_xClO_y) and inorganic substances (Na₂CO₃ and NaF), which increased resistance to Na⁺ transport and exacerbated gas evolution^[200]. In low-concentration electrolytes, the solvation structure of the contact ion pairs was conducive to forming a stable anion-regulated and solvent-derived composite interface phase, enhancing the cycling stability and the interfacial kinetic performance of the battery at 55 °C [Figure 8C and D]. In addition, the performance of the failed NVPF cathode could be restored by annealing [Figure 8E]. The C_xClO_y phase decomposed in argon at 300 °C, reconstructing the interface phase.