Cost‐Effective Na₄Fe₃(PO₄)₂P₂O₇ Cathode Materials for Sodium‐Ion Batteries in Large‐Scale Energy Storage Applications

Abstract

The rising interest in sodium‐ion batteries (SIBs) has led to significant research on cathode materials, given their essential function in SIBs. Among various cathode materials, Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) stands out with its NASICON structure, offering high structural stability, rapid ion transport, and high operating potential. With its ultralong cycle life, cost‐effectiveness, high safety, and superb temperature adaptability, NFPP is an attractive choice for large‐scale energy storage. Despite these advantages, the widespread application of NFPP is limited by low electronic conductivity and impurity issues. In this review, we discussed the crystal structure, properties, and diverse synthetic approaches of NFPP. Additionally, various modification strategies are also looked into including impurity adjustment, doping, and surface modification. The full cells of NFPP were further analyzed, underscoring its potential for commercialization. In conclusion, we outline our opinions on the obstacles and future research priorities for NFPP. This review aims to assess the potential of NFPP for large‐scale energy storage applications, sparking greater interest and progress in NFPP cathode materials.

Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) cathodes for sodium‐ion batteries are systematically reviewed, covering crystal structure, synthesis strategies, and electrochemical properties. Key challenges, including low electronic conductivity and impurity control, are critically analyzed alongside recent modification approaches. Insights into structure–property relationships provide guidance for rational design and support the development of scalable, safe, and durable energy storage systems.

1. Introduction

As part of the global energy transformation plan, wind and solar power generation are advancing rapidly, accelerating to replace coal‐fired power generation [1, 2]. Nonetheless, grids cannot fully take advantage of wind and solar power due to their inconsistency and instability, resulting in significant electricity waste [3, 4]. To address this challenge, lithium‐ion batteries (LIBs) are currently considered the leading energy storage technology because of their high energy density and cycle stability [5, 6]. Although widely used, LIBs face challenges in large‐scale energy storage due to limited lithium resources and cost concerns [7]. To overcome these barriers, sodium‐ion batteries (SIBs), a more affordable and resource‐abundant alternative, are gaining increasing attention [4, 8]. The advancement of SIB technology can be expedited by directly applying strategies designed for LIBs, as they both operate on the ’rocking chair’ mechanism [9, 10]. Nonetheless, enhancing the energy density and cycling stability of SIBs remains crucial for meeting the demands of practical applications.

SIBs require Na⁺ ions to be reversibly embedded and de‐embedded between cathode and anode electrodes to achieve energy conversion during the charging and discharging [11]. Consequently, the electrochemical performances of SIBs are significantly affected by the properties of the electrode materials, particularly cathode materials [12]. Different cathode materials have been designed, including layered oxides, Prussian blue analogs, and polyanion‐type compounds [13, 14, 15]. Although layered oxides exhibit exceptional energy density, their high sensitivity to humidity and low redox potential limit their suitability for extensive energy storage applications [16, 17]. Besides, Prussian blue analogs garnered attention for being cost‐effective, while their stability can be compromised by ligand water and lattice defects [18, 19]. In contrast, polyanion‐type compounds are increasingly studied and utilized due to their high safety and superior electrochemical properties [20, 21, 22]. The outstanding voltage platform, cycling performance, and thermal stability of polyanion compounds also make them promising candidates for energy storage despite their low conductivity issues [23, 24].

Among various polyanion types, iron‐based cathode materials, particularly Na₄Fe₃(PO₄)₂P₂O₇ (NFPP), distinguish them from other polyanionic options because of their affordability and abundant resources [25]. In recent years, NFPP cathode material has gained prominence as a leading candidate for commercial energy storage technology in SIBs. With its unique structure, NFPP offers a voltage of around 3.2 V and a theoretical capacity of 129 mAh g⁻¹, demonstrating exceptional stability beyond 10 000 cycles [26]. Although NFPP is still a developing material, significant advancements toward its commercialization have been achieved [27, 28]. Both academic and industrial communities continue to monitor its long‐term performance closely, despite the rapid pace of commercialization [29]. Despite its promising properties, NFPP still faces several critical challenges that limit its practical application. First, its intrinsically low electronic conductivity leads to poor rate capability, requiring additional modification strategies. Second, impurity phase formation during synthesis is difficult to control and can significantly affect electrochemical performance. Third, achieving scalable and cost‐effective production while maintaining phase purity and structural uniformity remains a major engineering challenge. Addressing these issues is essential for the practical deployment of NFPP‐based cathodes and constitutes the main focus of current research. The lack of comprehensive information has impeded a deeper understanding of NFPP. To bridge this gap and guide future material design, it is crucial to provide a thorough summary and analysis of the research progress on NFPP.

This review reveals the fundamental details of NFPP by analyzing its crystal structure, material properties, and synthesis methods. Strategies for enhancing its electrochemical performance, including impurity adjustment, doping, and surface modification, are also discussed. The full cell and commercialization of NFPP were also examined, underscoring its potential for energy storage applications. Finally, the challenges and future research emphasis for NFPP are proposed. This review aims to provide readers with a thorough understanding of NFPP and inspire further research efforts.

2. Crystal Structure and Properties of Na₄Fe₃(PO₄)₂(P₂O₇)

In 2012, the crystal structure (Figure 1a) and properties of Na₄Fe₃(PO₄)₂(P₂O₇) were studied by Kim et al. [30]. It was discovered that NFPP possesses an orthorhombic crystal system with a Pn2₁a space group and structural features similar to NASICON‐type crystals. Rietveld refinement provided further information about the cell parameters and volume characteristics of NFPP (a = 18.07517 Å, b = 6.53238 Å, c = 10.64760 Å, and V = 1257.204 ų). In this structure, an infinite layer structure [Fe₃P₂O₁₃]∞ is formed within the bc plane, composed of FeO₆ octahedra and (PO₄)³⁻ groups, which is interconnected by P₂O₇ groups along the a‐axis, resulting in an open and free three‐dimensional (3D) framework [32]. The [PO₄] and [P₂O₇] units within the open frame make NFPP unique by allowing Na⁺ ions to diffuse through a 3D pathway, in contrast to many polyanion compounds that restrict diffusion to one or two‐dimensional [33, 34, 35]. Nonetheless, these insulating units simultaneously hinder electron transport, negatively impacting the electrochemical performance of NFPP.

FIGURE 1.

(a) Schematic illustration of Na₄Fe₃(PO₄)₂(P₂O₇) [30]. Reproduced with permission [31]. Copyright 2012, American Chemical Society. (b) Schematic depiction of coordination number and local structure of Na⁺ in NFPP [31]. (c) 23Na MAS NMR spectra of NFPP electrodes at various charge states [31]. Reproduced with permission [32]. Copyright 2019, Elsevier. In Na _x Fe₃(PO₄)₂(P₂O₇), (d) x = 4 and (e) x = 1 display integrated spin as a function of integration radius around Fe [32]. Reproduced with permission [33]. Copyright 2013, American Chemical Society. (f) Schematic of the local structure around Fe1 and the P(3)−O−P(4) bond angle comparison diagram as a function of Na content x in Na _x Fe₃(PO₄)₂(P₂O₇) from ND analysis [32]. Reproduced with permission [33]. Copyright 2013, American Chemical Society. (g) The lattice parameters of NFPP as a function of sodium concentration x, estimated from the electrochemical charging current during the initial charging process [31]. Reproduced with permission [32]. Copyright 2019, Elsevier.

Four Na sites (Na1, Na2, Na3, and Na4) are located around Fe sites in NFPP (Figure 1b), indicating the electrochemical capacity of NFPP due to their distinct coordination environments and exchangeability [31]. As illustrated by the ex situ solid‐state nuclear magnetic resonance (NMR) spectra (Figure 1c), Na⁺ ions in the NFPP structure experience different coordination settings: Na1 and Na4 are coordinated with six atoms, Na2 with seven, and Na3 with five. Notably, Na⁺ ions that have lower coordination numbers are more easily extracted than those with higher ones. Specifically, the extraction sequence is based on the increase in coordination number: The five‐coordinated Na3 is extracted first due to its higher chemical potential, followed by the six‐coordinated Na1 and Na4, whereas the seven‐coordinated Na2 primarily stabilizes the crystal structure and seldom engages in the reaction. It is worth noting that, unlike the extraction order of Na⁺ ions by the sol–gel method, Kim et al. demonstrated a distinct extraction order using the solid‐state method combined with theoretical calculations [32]. Their study indicates that the Na⁺ ions from Na3 (five‐coordination) and Na1 (six‐coordination) were initially extracted. Subsequently, some Na⁺ ions from Na4 (six‐coordination) and Na2 (seven‐coordination) sites are removed, ultimately leaving half of the Na4 and Na2 sites in the NaFe₃(PO₄)₂(P₂O₇) structure. To ensure the consistency and accuracy of these findings, additional experimental validation and theoretical analysis may be necessary to explain the differences.

The oxidation state of the Fe sites in the NFPP structure changes simultaneously with Na extraction. As illustrated in Figure 1d,e, the oxidation state of the transition metal is indicated by its magnetic moment around it. Due to electrostatic interactions between Fe and their relative position to Na sites, the Fe1 site oxidizes first to Fe³⁺, followed by the Fe2 site, with the Fe3 site oxidizing at the last step [32]. First‐principles calculations revealed that P₂O₇ undergoes significant distortion as the x‐value changes from 2 to 1 (Figure 1f). Simultaneously, the FeO6 polyhedra connecting the Fe1 and Fe3 sites change from edge‐sharing to corner‐sharing, primarily because of electrostatic repulsion among the Fe3, causing an unusual decrease in the c‐lattice parameter depicted in Figure 1g. The diffusion tunneling of Na narrows due to local structural deformations, particularly the movement of Fe in Fe1 and P₂O₇ dimers, leading to decreased Na de/embedding kinetics in certain regions and increased cell polarization. These findings shed light on the structure basis and kinetic mechanism of NFPP, which exhibit unique electrochemical performance in SIBs and provide valuable theoretical insights for optimizing cathode material design.

To further understand the discrepancies in Na⁺ extraction sequences reported for NFPP synthesized via different routes (e.g., solid‐state vs. sol–gel methods), it is important to consider both kinetic and thermodynamic factors. From a kinetic perspective, synthesis‐dependent variations in particle size, morphology, and crystallinity can significantly influence Na⁺ diffusion pathways and activation barriers. For instance, sol–gel‐derived NFPP typically exhibits smaller particle sizes and shorter diffusion lengths, which may favor kinetically accessible Na⁺ extraction pathways that differ from those in bulkier solid‐state materials. In contrast, thermodynamic contributions arise from subtle differences in crystal chemistry, including lattice distortions, defect concentrations (e.g., Na vacancies or Fe nonstoichiometry), and trace impurities introduced during synthesis. These factors can modify the relative stability of intermediate phases during desodiation, thereby altering the preferred Na⁺ extraction sequence. In particular, defect‐induced local structural heterogeneity and cation disorder may redistribute the Na⁺ site energies, leading to deviations from the ideal extraction order predicted for a perfectly ordered lattice. Therefore, the observed differences in Na⁺ extraction behavior are likely governed by a coupling of kinetic pathway dependence and synthesis‐induced thermodynamic perturbations, rather than a single dominant mechanism.

The localized migration energy barriers within the crystal structure of NFPP have been investigated using bond valence sum (BVS) and density functional theory (DFT) calculations, revealing the fundamental mechanism behind its exceptional rate performance [36]. It has been demonstrated that the crystal structure of NFPP offers a multidimensional pathway for the diffusion of Na⁺ ions diffusion (Figure 2a). Surprisingly, the a‐axis direction presents an insignificant diffusion energy barrier for Na⁺ ions, measuring only 0.02 eV. The calculated 3D diffusion paths for various types of Na⁺ ions in NFPP indicate that all Na⁺ ions exhibit diffusion energy barriers below 0.9 eV (Figure 2b,c). This evidence highlights mechanisms for the remarkable high‐rate performance of the modified materials. Furthermore, the study discovered that the overall conductivity of NFPP material is comparable to the well‐known NASICON‐type Na₃V₂(PO₄)₃ cathode material, as demonstrated through ionic conductivity tests, providing strong support for the fast ion transport properties of NFPP [38, 39].

FIGURE 2.

(a) Three‐dimensional diffusion path and migration barrier of Na⁺ ions in Na₄Fe₃(PO₄)₂(P₂O₇) structure [37]. Reproduced with permission [38]. Copyright 2024, Wiley. (b) Illustration of NFPP's crystal structure featuring three different types of Na⁺ ions [36]. Reproduced with permission [37]. Copyright 2019, Springer Nature. (c) Migration energy barrier between different Na⁺ ion groups corresponding to a three‐dimensional diffusion pathway [36]. Reproduced with permission [37]. Copyright 2019, Springer Nature. (d) Ex situ XRD patterns of NFPP during the charge/discharge process and amplified XRD patterns in the 2θ range 8°−19° [32]. Reproduced with permission [33]. Copyright 2013, American Chemical Society. (e) C‐rate capability of NFPP‐E electrodes in various states: fresh, exposed to air for 3 months, and fresh at both −20°C and 50°C [36]. Reproduced with permission [37]. Copyright 2019, Springer Nature. (f) XRD comparison of NFPP‐E powder in its fresh state and after 3 months of air exposure [36]. Reproduced with permission [37]. Copyright 2019, Springer Nature.

The excellent structural stability of NFPP plays a significant role in its long‐term operational lifespan, resulting in an ultralong cycle life. The consistent patterns observed in ex situ XRD patterns (Figure 2d) indicate a topotactic Na removal, demonstrating that the structure of NFPP can be fully reversed. Despite the significant deformation of pyrophosphate (P₂O₇) polyhedra, the volume change of NFPP is constrained to under 4%, marking one of the lowest volume changes for the cathode materials to date [32]. This minimal volume change can be attributed to the openness and rigidity of the hybrid polyanion crystal structure. Specifically, the P₂O₇ dimer within the crystal structure can rotate and twist, effectively accommodating structural changes. Phase changes are associated with volume expansion and contraction, which can induce mechanical stresses leading to material structure degradation. Notably, the sodiation/desodiation redox process of NFPP is an imperfect single‐phase reaction, which is rare among polyanionic cathode materials. In contrast, other polyanion compounds based on iron redox reactions, such as NaFePO₄ and Na₂FeP₂O₇, are more likely to undergo biphasic reactions [40, 41]. These distinctive features provide a robust foundation for the long‐term stability of NFPP and highlight its great potential in energy storage systems.

The excellent temperature adaptability and thermal stability of NFPP distinguish it in the energy storage field, especially when compared with cobalt and nickel‐based materials. Nano‐Na₄Fe₃(PO₄)₂(P₂O₇) plates (NFPP‐E) were successfully synthesized via the sol–gel method, and their cycling stability was tested at extreme high or low temperatures of 50°C and −20°C (Figure 2e) [36]. Under these conditions, their capacity retention of NFPP‐E was still 91.4% and 92.1%, respectively, demonstrating that the crystal structures almost remain unchanged in both cases. In the low‐temperature test at −20°C, NFPP‐E maintained a specific capacity of 95.0 and 84.7 mAh g⁻¹ at 0.1C and 0.2C. Additionally, the thermal stability of NFPP was thoroughly investigated, revealing the decomposition temperature of NFPP is approximately 530°C, higher compared to the conventional operating temperatures [32]. This substantially reduces the risk of thermal runaway, providing NFPP with distinct safety advantages. This inherent safety feature, combined with its applicability across all climate conditions, establishes a robust foundation for the commercialization of NFPP.

The air stability of NFPP cathode material is another significant advantage for its commercial application. As demonstrated in Figure 2f, the crystal structure and electrochemical properties of NFPP remained stable even after 3 months of air exposure, confirming its excellent air stability [36]. This advantage can further reduce overall costs by simplifying the battery manufacturing process, minimizing storage requirements, and enhancing the material's practicality. Additionally, recent studies have further explored the air stability of NFPP, revealing that it maintains stability when exposed to room‐temperature air for an hour. However, its electrochemical performance significantly deteriorates when subjected to temperatures of 80°C, 160°C, and 240°C for the same duration. Notably, moisture can cause ion leaching, leading to impurities that degrade battery efficiency [42]. To preserve the integrity and performance of NFPP, researchers advise limiting air exposure before use and storing it in low humidity or inert gas environments.

3. Methods for the Synthesis of Na₄Fe₃(PO₄)₂(P₂O₇)

The synthesis process is crucial in determining the structure and morphology of NFPP material, which further affects its electrochemical performance. Several reliable synthesis methods have been established, including solid‐state, spray‐drying, sol–gel, electrospinning, solution combustion, and template methods. Each of these methods offers distinct features and provides a variety of approaches for preparing NFPP materials.

3.1. Solid‐State Method

The solid‐state method involves mixing and grinding solid raw materials, followed by high‐temperature sintering to synthesize the desired compound (Figure 3a). This method is commonly used for large‐scale production and is practically applied in the industry because of its simplicity, affordability, and environmental friendliness. Nonetheless, the solid‐state method may cause poor sample consistency, which hinders its commercialization potential. The process typically involves grinding raw materials to increase their specific surface area for activation before sintering. The represented particles are generally agglomerated and heterogeneous (Figure 3b), adversely affecting the electrochemical performance of NFPP.

FIGURE 3.

Preparation schematic of (a) the solid‐state method [43]. Reproduced with permission [45]. Copyright 2023, American Chemical Society. (c) The spray‐drying method [44]. Reproduced with permission [46]. Copyright 2024, MDPI. (e) The sol–gel method. (g) The electrospinning method [45]. Reproduced with permission [47]. Copyright 2023, Elsevier. (i) Template method [46]. Reproduced with permission [48]. Copyright 2019, Elsevier. The corresponding morphology of (b) NFMPP [47]. Reproduced with permission [49]. Copyright 2023, Elsevier. (d) NFPP@rGO [48]. Reproduced with permission [49]. Copyright 2019, Elsevier. (f) NFPP‐4.5 [49]. Reproduced with permission [51]. Copyright 2024, American Chemical Society. (h) NFPP@C [45]. Reproduced with permission [47]. Copyright 2023, Elsevier. (j) NFPP@HC [46]. Reproduced with permission [48]. Copyright 2019, Elsevier.

A straightforward two‐step solid‐state method was employed to synthesize NFPP [32]. Initially, Na₄P₂O₇, Fe₂C₂O₄·2H₂O, and NH₄H₂PO₄ were ball‐milled with acetone for 24 h, dried, and sintered at 300°C for 6 h. Subsequently, the calcined powders were regrounded, pelletized, and sintered at 500°C for 12 h to produce the final NFPP sample. Additionally, thermal stability analysis indicates that NFPP tends to decompose into two components (NaFePO₄ and Na₂FeP₂O₇) above 500°C. In another study, soot was added to raw materials to prepare NFPP [50]. The mechanical activation process was performed for 5 min using high‐energy planetary milling at 900 rpm, with a sintering process at 400–600°C for 4 h. This final product exhibited an initial discharge of 104 mAh g⁻¹ at 0.2C and maintained 79% of its capacity after 50 cycles. These findings highlight the potential of NFPP and suggest additional work to enhance its performance further.

NFPP with porous structures can be fabricated through the solid‐state method, enhancing the electrochemical kinetics of electrons and ions. A porous structure NFPP‐P was synthesized using NH₄H₂PO₄, FeC₂O₄·2H₂O, NaHCO₃, and lauric acid (C₁₂H₂₄O₂) as raw material, in which lauric acid was used as carbon source and surfactant [51]. After mixing raw materials with absolute ethyl alcohol in ball milling at 600 rpm for 6 h, the precursor was dried and heated at 500°C for 12 h, resulting in the NFPP‐P. Notably, NFPP‐P exhibits an initial discharge capacity of 124.5 mAh g⁻¹ at 0.1C with a capacity retention of 93.98% at 10C after 5000 cycles. In contrast, NFPP maintained only 92.1% at 10C after 1800 cycles, highlighting the significant impact of porous morphology. The porous NFPP‐P achieves high capacity and excellent cycle stability, proving the effectiveness of its structure.

The solid‐state method is cost‐effective and scalable, making it ideal for NFPP production. Despite sample consistency and particle uniformity issues, recent advancements in porous structure design have significantly enhanced electrochemical performance and capacity retention, suggesting a promising avenue for further refinement.

3.2. Spray‐Drying Method

The spray‐drying method involves atomizing a liquid feed into hot drying mediums to produce fine particles (Figure 3c). This technique can achieve high production capacity and excellent product quality, making it widely used in industrial production. Compared to the solid‐state method, spray drying can regulate the morphology and structure of NFPP by controlling process parameters, resulting in a final product with good consistency (Figure 3d). Despite these advantages, the spray‐drying method is more costly and energy‐intensive, which may limit its widespread use in certain cases.

A spray‐drying method was employed to synthesize NFPP@rGO using Fe(NO₃)₃·₉H₂O, NaH₂PO₄·2H₂O, and C₆H₈O₇·H₂O as raw materials [48]. The NFPP@rGO exhibited a capacity of 128 mAh g⁻¹ at 0.1C, a rate capability of 35 mAh g⁻¹ at 200C, and maintained 62.3% of its capacity after 6000 cycles at 10C. Another study designed a green synthesis approach for Na₃Fe₂(PO₄)(P₂O₇)@rGO, and all the atoms were utilized [52]. Sand milling and sonication ensure uniform mixing, while spray drying provides consistent particle size, enhancing conductivity and ion transport. Calcination at 500°C integrates rGO, boosting stability and conductivity. This results in a capacity of 105.5 mAh g⁻¹ at 0.2C, 72.4% retention after 8000 cycles at 20C, and 42.4 mAh g⁻¹ at 100C, demonstrating the method's efficiency and commercial potential. This environmentally friendly and low‐cost synthesis method is a reference for future commercialization.

Porous and hollow spherical morphology can be built through the spray‐drying method. By providing a shorter pathway for Na⁺ ion migration and enhancing electron transport, the porous and hollow spherical design effectively manages the volume expansion of NFPP. HS‐NFPP/C was obtained using rust as the iron resource [53]. Despite 10 000 cycles, the HS‐NFPP/C can still retain an impressive 87.5% capacity at 10C and its reversible capacities reached 86.5, 78.6, 68.9, and 57.1 mAh g⁻¹ at 1, 10, 50, and 100C. Another study prepared hollow sphere structured NFPP powders using Fe(NO₃)₃·9H₂O, CH₃COONa, NH₄H₂PO₄, and C₂H₂O₄·2H₂O, and glucose as raw material [54]. The solutions with raw material were stirred, spray‐dried at 260°C, and sintered at 550°C for 10 h. The optimal sample has a discharge capacity of 107.7 mAh g⁻¹ at 0.2C and retains a capacity of 92% after 1500 cycles. These works underscore the significance of morphology, providing insights for further optimization.

The spray‐drying method can produce high‐quality, consistent materials and allows precise control over morphology, making it suitable for industrial use. Nonetheless, it is more expensive and energy‐demanding than the solid‐state method. Its ability to create advanced structures like porous and hollow forms enhances ion transport and stability, highlighting its commercial potential.

3.3. Sol–Gel Method

The sol–gel method (Figure 3e) facilitates homogeneous mixing of precursors at the molecular or nanoscale level, typically yielding products with consistent quality. The distinct porous structure of the final product provides a high specific surface area, enhancing the electrochemical performance of NFPP (Figure 3f). Nonetheless, achieving precise control over material morphology and microstructure remains challenging. Generally, the sol–gel process for NFPP employs phosphates that partially decompose into pyrophosphate [36, 37, 46, 55] [25, 30, 31, 56]. The presence of impurities and the complexity of the system can make the sol–gel method more sensitive to conditions, requiring more precise control compared to other methods.

In the synthesis of NFPP/C nanocomposites, the sol–gel method has been employed to optimize the material's performance [36]. Iron metal powder and citric acid were mixed and stirred in 30 mL deionized water at 80°C for 15 h. NaH₂PO₄·2H₂O and ethylene glycol were then added to this solution and oil bath at 120°C for 1 h. After drying at 80°C in air and ground, the precursor was calcined for 10 h to obtain the final product. Although the crystal structure of NFPP can be damaged above 600°C, forming the unwanted maricite NaFePO₄ and Na₂FeP₂O₇, excessive phosphate can promote the complete formation of pyrophosphate. This prepared NFPP/C‐500 can deliver a discharge capacity of 99 mAh g⁻¹ at 0.2C. Another study synthesized NFPP using FeC₂O₄·2H₂O, Na₄P₂O₇, NH₄H₂PO₄, and sucrose, examining the unique pyrophosphate environment [57]. Their research found that Fe‐oxalate coordination dominates at low pH, while Fe(II)‐citric acid coordination occurs at neutral pH, indicating that PH adjustments influence reactant complexation and reaction pathways. The cathode material produced at pH = 7 exhibited superior electrical performance compared to that made at pH = 3 with Na₄P₂O₇. These studies highlight the importance of synthesis conditions and pH control in optimizing NFPP/C performance.

3D interconnected porous and nanoplate‐like morphology can be created through the sol–gel method. Porous materials possess extensive surface areas and numerous active sites, enhancing their mass transfer capabilities. 3DIP‐NFPP/C was synthesized using a combined sol–gel method and solid‐state calcination, resulting in NFPP‐550 with a discharge capacity of 104.6 mAh g⁻¹ at 0.1C [58]. In another study, nanoplate‐like NFPP with an average particle size of 150 nm was synthesized using CH₃COONa, (NH₄)₃PO₄, glucose, iron(II) acetate, ethylenediaminetetraacetic acid, cetyltrimethylammonium bromide, and stearic acid as raw materials [36]. This composite exhibited 108.3 mAh g⁻¹ discharge capacity at 0.1C and maintained 80.3 mAh g⁻¹ at 20C. Building on recent findings, the sol–gel method can enhance NFPP materials by creating 3D porous and nanoplate‐like structures, which significantly boost their discharge capacities.

While the sol–gel method excels in achieving uniform mixing and enhancing surface area for better electrochemical performance, it faces challenges in controlling morphology precisely and is sensitive to impurities. Nonetheless, its capacity to generate distinct structures makes it a key player in advanced material synthesis.

3.4. Electrospinning Method

The electrospinning method (Figure 3g) has become an important way to synthesize 1D nanomaterials (Figure 3h) with specific structures and morphologies due to its simplicity, versatility, and affordability. The final product is formed by activating the polymer solution in a high‐voltage electrostatic field, creating a jet stream that rapidly solidifies in the flight. While effective at the laboratory scale, scaling up the electrostatic spinning method for industrial production remains challenging in terms of quality and reliability.

Nanoribbons and nanofibers are constructed via the electrospinning method, enhancing the exceptional electrochemical performance of NFPP. The NFPP nanoparticles incorporated into carbon nanoribbons were prepared through polyvinylpyrrolidone‐assisted electrospinning [59]. This morphology facilitates rapid electron transfer along a 3D highway and prevents nanoparticle aggregation. This composite delivers a capacity of 128.6 mAh g⁻¹ at 0.1C and maintains 72% of its capacity after 5000 cycles at 50C. In another study, using the electrospinning method followed by pyrolysis, hierarchically carbon‐decorated nanofibers were designed [45]. These nanofibers form a 3D network, enhancing electron conductivity and Na storage kinetics. The prepared material achieved a reversible capacity of 118 mAh g⁻¹ at 0.2C and maintained 79.6% of its capacity after 10 000 cycles at 10C. Future research may consider optimizing these structures and exploring advanced modifications, further improving the performance of NFPP.

The electrostatic spinning method is simple and versatile, ideal for creating 1D nanomaterials with precise structures. It enhances electrochemical performance but faces challenges in scaling up to industrial production while maintaining quality.

3.5. Solution Combustion Method

The solution combustion method is favored due to its rapid reaction rate, simplicity, and high product purity. Completing a reaction within minutes is a significant advantage of this approach. Nonetheless, the high‐temperature reactions require stringent equipment and may generate harmful gases, posing potential risks to the environment and operator safety.

The solution combustion method, used for bulk material synthesis and thin film preparation, reveals the excellent performance of NFPP [39]. Initially, NaH₂PO₄, Fe(NO₃)₃·9H₂O, and C₆H₈O₆ were dissolved into 50 mL distilled water and stirred at 120°C for 2 h. The reactants were subsequently ignited on the hot plate at 300°C, and the final product was obtained by grinding and annealing at 600°C for 12 h. Thin films were then deposited onto stainless steel substrates using pulsed laser deposition (PLD). Notably, this material delivers a capacity of 125 mAh g⁻¹ at a low current rate of 2 µA cm⁻² and 110 mAh g⁻¹ at a high current rate of 10 µA cm⁻². After 500 cycles, no notable capacity fading was observed, with the Coulombic efficiency remaining at 98%. Interestingly, when used in potassium‐ion batteries, NFPP synthesized by solution combustion approach can achieve a discharge capacity of 120 mAh g⁻¹ at a robust 3.0 V [55]. Future research may explore combining solution combustion with other modification strategies for simple preparation and further optimization of NFPP's performance.

The solution combustion method stands out for its rapid reaction and high purity, making it ideal for efficient synthesis. Nonetheless, the high temperatures required can pose environmental and safety challenges. Despite these concerns, its ability to produce high‐performance materials quickly and effectively makes it a valuable technique in material synthesis.

3.6. Template Method

The template method utilizes template materials to control the microstructure of the final product, offering significant potential for producing materials with specific sizes and shapes. This method enables precise regulation over the pore structure and morphology, significantly enhancing the electrochemical performance. Despite all of the advantages, this method requires special chemical processing steps to use and remove templating agents, which increases production costs and complicates the overall synthesis process.

Nanospheres NFPP/C with adjustable carbon coating thickness and particle size were prepared via a template method (Figure 3i,j) [46]. F127, NH₄H₂PO₄, Na₄P₂O₇, and Fe(NO₃)₃·9H₂O were dissolved in deionized water, followed by the addition of HCl and a phenolic resol ethanol solution, which was stirred for 2 h. This emulsion was then dried for 24 h and subjected to heat treatment to obtain the final product. The nanosizing enhances electrochemical kinetics by shortening transport pathways for ions and electrons. These NFPP/C nanospheres exhibit a high discharge capacity of 128.5 mA g⁻¹ at 0.2C, with 63.5% capacity retention at 10C after 4000 cycles and an excellent reversible capacity of 79 mAh g⁻¹ at 100C. Future research may consider prioritizing enhancing specific capacity for NFPP/C nanospheres, followed by additional modifications to exceed theoretical capacity.

The template method precisely shapes cathode materials, improving their electrochemical performance. Although managing templates adds complexity and cost, its ability to create tailored structures makes it valuable for material design.

Different synthesis methods offer distinct advantages and uniquely impact the properties and performance of NFPP. Nonetheless, these methods also present numerous challenges that need careful consideration, especially regarding cost and impurity issues (Table 1).

TABLE 1.

Comparison of NFPP materials synthesized via different methods.

Synthesis method	Particle size	Carbon content, wt.%	Initial Coulombic efficiency (ICE), %	Specific capacity, mash g−1	Rate performance	Capacity retention, %	Key features/remarks
Solid state	1–10 μm	0–5	80–90	90–110	Moderate (≤10C)	70–85 (100–500 cycles)	High crystallinity, low cost, but large particle size limits kinetics
Sol–gel	100–500 nm	5–15	85–95	100–120	Good (20–50C)	80–95 (200–1000 cycles)	Fine particles, uniform carbon coating, improved kinetics
Spray drying	1–5 μm (secondary), nanosized primary	5–10	85–92	95–115	Good (≤20C)	80–90 (200–500 cycles)	Spherical morphology, scalable process, improved packing density
Hydrothermal/Solvothermal	50–300 nm	5–12	85–95	100–120	Excellent (50–100C)	85–95 (500–2000 cycles)	Controlled morphology, high surface area, enhanced ion transport
Electrospinning	Nanofibers (diameter ~100–300 nm)	10–20	85–95	100–115	Excellent (≤50C)	85–95 (500–1000 cycles)	1D conductive network, continuous electron pathways
Spray pyrolysis	Submicron to micron	5–10	80–90	90–110	Moderate–good (≤20C)	75–90 (200–500 cycles)	One‐step synthesis, scalable, but less control over defects

From a critical perspective, the electrochemical performance of NFPP is strongly governed by several key factors associated with synthesis methods, including particle size, crystallinity, phase purity, and carbon distribution. Methods that enable uniform particle size and intimate carbon coating generally result in improved rate capability and cycling stability. In contrast, approaches with limited control over morphology or phase composition often led to inferior performance. Therefore, precise control of microstructure and impurity phases is more critical than the choice of synthesis method itself.

From a practical perspective, the scalability of synthesis methods is a critical factor for the commercialization of NFPP cathode materials. Among the reported approaches, the solid‐state method and spray‐drying method are considered the most promising for large‐scale production. The solid‐state method offers advantages in terms of simplicity, low cost, and compatibility with existing industrial infrastructure; however, it often suffers from poor particle uniformity and limited control over morphology. In contrast, spray drying enables better control over particle size, morphology, and compositional uniformity, producing spherical and homogeneous materials suitable for industrial electrode fabrication. Despite its higher energy consumption and equipment cost, spray drying is widely regarded as a scalable and industrially viable technique. Other methods, such as sol–gel and electrospinning, provide superior control over microstructure and performance but are generally limited by higher complexity and lower throughput, making them less suitable for mass production at the current stage.

In summary, different synthesis methods offer distinct advantages for NFPP materials. The solid‐state method is cost‐effective and suitable for large‐scale production but provides limited control over particle morphology. Wet‐chemical approaches, such as sol–gel, enable better compositional uniformity and microstructure control, while spray drying offers a balance between scalability and morphology regulation. Advanced techniques, including electrospinning and template‐assisted methods, can further optimize nanostructures but are less practical for industrial‐scale production. Therefore, selecting an appropriate synthesis route requires balancing performance optimization and scalability.

4. Strategies for the Modification of Na₄Fe₃(PO₄)₂(P₂O₇)

The application of NFPP cathode materials is constrained by their large molecular mass and low electronic conductivity, which adversely affect their electrochemical performance, especially capacity and rate capability. Impurities are also a common issue during NFPP synthesis, negatively impacting the properties of NFPP. Therefore, handling these impurities and enhancing the electronic conductivity is a critical challenge for improving material performance. To date, researchers have explored various strategies to optimize the electrochemical performance of NFPPs, including doping, surface modification, and impurity modulation methods (Figure 4).

FIGURE 4.

Strategies for the modification of Na₄Fe₃(PO₄)₂(P₂O₇).

4.1. Impurity Adjustment

Impurities are a significant challenge in the NFPP system, hindering its overall electrochemical performance. Various modification strategies have been explored to address this issue, primarily focusing on two routes. One aims to suppress impurity generation by creating Fe defects or nonstoichiometric structures, while the other seeks to transform and utilize these impurities by constructing a heterostructure. Both strategies effectively address the impurity issue, offering novel solutions and providing innovative insights into NFPP research.

4.1.1. Impurity Suppression Route

The primary impurities in NFPP products are maricite NaFePO₄ (NFP) and Na₂FeP₂O₇ (NFPO), which possess suboptimal electrochemical properties, hindering the performance of the integrated NFPP system. Figure 5a illustrates the transitions among NFPO, NFPP, and NFP systems. Most reported NFPP cathode materials are not pure phases but mixtures, complicating the system. The formation energy (Figure 5b) of NFPP was calculated (−0.38 ev/f.u.), demonstrating synthesis tends to result in NFPP formation [60]. Nonetheless, trace amounts of the maricite NFP impurities often persist in the final product [30]. NFPP remains stable below 600°C, but a complete transformation to maricite NFP occurs above 800°C due to the better thermodynamic stability of NFP [39]. Additionally, the rough selection of raw material can increase impurities due to the sensitivity of reactions.

FIGURE 5.

(a) Structural schematic and transition relation among NFPO, NFPP, and NFP [60]. Reproduced with permission [61]. Copyright 2022, American Chemical Society. (b) Formation energy of NFPP [60]. Reproduced with permission [61]. Copyright 2022, American Chemical Society. (c) Complete and Fe2 defective crystal structures of the NFPP phase [60]. Reproduced with permission [61]. Copyright 2022, American Chemical Society. (d) X‐ray diffraction patterns and enlarged patterns from 31° to 34.5° of NFPP [25]. Reproduced with permission [26]. Copyright 2022, Elsevier. (e) XRD patterns and local enlargements of NFPPAl0, NFPPAl2, NFPPAl4, and NFPPAl6 [62]. Reproduced with permission [64]. Copyright 2024, Elsevier. (f,g) Rate capability and long‐term stability of Na_3.4Fe_2.4(PO4)_1.4P₂O₇ [61]. Reproduced with permission [63]. Copyright 2024, Wiley. (h) GITT curves of 1.0‐NFPP/C and 1.4‐NFPP/C [64]. Reproduced with permission [65]. Copyright 2024, American Chemical Society.

Different proportions of components have been tried in the NFPP system to obtain the high pure phase and understand the mechanism of impurities. Studies have investigated the Na _x + 2Fe _x + 1(PO₄) _x P₂O₇ (x = 1,2,3,4) system by varying the molar ratio of (PO₄)³⁻ to (P₂O₇)⁴⁻. It was discovered that a mixed phase appears when x is between 2 and 3, containing both NFPP and maricite NFP. Interestingly, some maricite NFP can be activated to become amorphous NFP, enhancing the capacity of mixed phases. To be more accurate, 17 Na₄Fe _x P₄O_12+x samples with varied x were studied [60]. It was verified only three phases are present in the Na₄Fe _x P₄O_12+x system and the possibility of producing a high‐purity Fe‐defective NFPP structure with x = 2.91 (Figure 5c). Although the theoretical capacity of NFPO electrodes can reach 98 mAh g⁻¹, NFPO has been shown to hinder system integral electrochemical performance [63]. Specifically, increasing x values (2.00, 2.11, 2.25, 2.40, 2.55, 2.70, 2.86, and 2.9) in the Na₄Fe _x P₄O_12+x system indicates a decrease in NFPO, resulting in an increase in discharge capacity from 85.7, 86.8, 88.4, 90.9, 93.2, 98.1, 105.0, to 110 mAh g⁻¹. Another study also explored NFPP‐x (x = 3.00, 2.90, 2.85, 2.70) and examined the relationship between Fe concentration and the content of NFPP, NFP, and NFPO. Among these, NFPP‐2.85 demonstrated the best performance, achieving 111.8 mAh g⁻¹ at 0.1C [65]. Future research may continue optimizing component ratios to achieve high purity and performance.

The optimal Fe content for achieving a high‐purity phase can be attained by incorporating a few Fe defects into the NFPP lattice [25]. Interestingly, X‐ray diffraction patterns in Figure 5d reveal that the NaFePO₄ impurity decreases as Fe defects increase, with 3% Fe defect indicating the high‐purity phase Na₄Fe_2.91(PO₄)₂(P₂O₇). These defects in NFPP lower the bandgap and migration energy barriers, enhancing Na⁺ ion and electron conductivity. This pure‐phase Na₄Fe_2.91(PO₄)₂(P₂O₇) material exhibits outstanding long‐cycle stability over 10 000 cycles, a high discharge capacity of 110.9 mAh g⁻¹ at 0.2C, and a rate performance (52 mAh g⁻¹ at 100C). Additionally, appropriate doping can suppress impurity production and enhance electrochemical performance [62, 66, 67]. For instance, the characteristic peak of impurity NaFePO₄ (Figure 5e) gradually decreased with the increased Al doping in NAFPP as reduced Fe content inhibits NaFePO₄ generation. Likewise, pure NFPP was observed with 6% nickel substitution in another study [62]. The cooperative interaction of doping and Fe content adjustment may offer valuable insight into NFPP's modification. The possible charge compensation mechanisms in nonstoichiometric NFPP include Fe valence adjustment (Fe²⁺/Fe³⁺ redox), sodium vacancy formation, and local structural rearrangements.

Nonstoichiometric NFPP material can achieve high‐purity phases and exhibit remarkable electrochemical performance. Stoichiometric polyanions generally introduce impurities, restricting their commercial viability. A high‐purity Na_3.4Fe_2.4(PO₄)_1.4P₂O₇ cathode material was prepared through precise molecular structure design [61]. This nonstoichiometric material demonstrates excellent electrochemical performance, with a capacity of 50.5 mAh g⁻¹ at 100C and a cycle life of over 14 000 cycles at 20C (Figure 5f,g). In another study, a pure‐phase Na_3.4Fe_2.4(PO₄)_1.4P₂O₇/C(1.4‐NFPP/C) material was developed using a spray‐drying method [64]. As shown in the galvanostatic intermittent titration testing (GITT) curves in Figure 5h, 1.4‐NFPP/C displayed a lower overpotential than 1.0‐NFPP/C during discharge, leading to faster redox kinetics. The 1.4‐NFPP/C exhibits a discharge capacity of 112.2 mAh g⁻¹ at 0.1C, a rate performance reaching 70 mAh g⁻¹ at 30C, and outstanding long‐term cycle performance, remaining 99.7% after 1000 cycles at 10C. It should be noted that beyond the stoichiometric system, there remains a significant research gap in nonstoichiometric modulation, which could address existing challenges and unlock new opportunities.

While the introduction of Fe defects and nonstoichiometry can effectively suppress impurity phase formation and, in some cases, enhance electrochemical activity, excessive defect concentrations may adversely affect structural stability. From a crystallographic perspective, a high density of defects (e.g., Fe vacancies, Na deficiencies, or antisite disorder) can induce significant lattice distortion and local strain accumulation, which may weaken the robustness of the polyanion framework. During repeated Na⁺ insertion/extraction, such structurally perturbed regions are more susceptible to irreversible rearrangement, potentially triggering phase transitions or partial amorphization. In addition, excessive defects may disrupt long‐range Na⁺ diffusion pathways, leading to increased polarization and deteriorated rate capability over extended cycling. There is therefore a critical balance between beneficial defect engineering and structural integrity: moderate defect concentrations can facilitate charge compensation and suppress parasitic phase formation, whereas overaccumulation may accelerate capacity fading and compromise long‐term cyclability. Accordingly, careful control of defect chemistry is essential to optimize both electrochemical performance and structural durability in NFPP systems.

Other emerging approaches are also effective for suppressing the impurities. Small amounts of LaBr₃ were added as an additive to the synthesis of Na_4−2x Fe_3–1.5x La _y (PO_4−x Br _x )₂P₂O₇ with dual‐site defects. In this composite, the introduction of Fe/Na dual‐site defects narrows the bandgap and creates efficient pathways for Na⁺ ions, enhancing the rate capability and stability of NFPP. The cathode achieves impressive results, delivering 55.2 mAh g⁻¹ at 50C and retaining 93% of its capacity after 2000 cycles at 10C [68]. Another study developed Na_4−x Fe₃(PO₄)₂P₂O₇ to achieve a pure‐phase NFPP. This approach created Na ion vacancies, which were filled by iron ions, resulting in an optimal Na_4−x Fe₃(PO₄)₂P₂O₇ with a specific capacity of 127.2 mAh g⁻¹ [69]. Adding LaBr₃ and engineering Na ion vacancies are promising methods to improve NFPP purity and performance, leading to better electrochemical applications.

To sum up, these innovative strategies for impurity suppression and phase optimization in NFPP materials not only enhance electrochemical performance but also lay the groundwork for future breakthroughs. By refining these methods, we can unlock new potentials in energy storage technologies, driving advancements in battery efficiency and sustainability.

4.1.2. Impurity Transformation Route

The crystalline structure of NaFePO₄ includes a maricite phase and an olivine phase [70]. As an independent electrode, the olivine NFP cathode can achieve a large theoretical capacity of 154 mAh g⁻¹ and an operational potential of 2.9 V [8]. Nonetheless, maricite NaFePO₄ is the thermodynamically stable phase [71]. Olivine NaFePO₄ can only be synthesized through specialized methods, initially involving the electrochemical delithiation of LiFePO₄ and electrochemical sodiation of FePO₄ to form NaFePO₄. While maricite NaFePO₄ is generally an electrochemically inactive electrode for SIBs, nanosized maricite NaFePO₄ can demonstrate remarkable electrochemical activity because of structure transformation into the amorphous phase (Figure 6a). The potential to activate maricite NaFePO₄ into amorphous NaFePO₄ reveals a potential route for impurity transformation.

FIGURE 6.

(a) Schematic diagram illustrating the structural evolution of maricite‐type NaFePO₄ during electrochemically induced amorphization [72]. (b) Schematic illustration of the NFPP‐4.5 cathode with a two‐phase intergrown heterostructure of NFP and NFPP‐4 [49]. Reproduced with permission [51]. Copyright 2024, American Chemical Society. (c) Operando XRD patterns of NFPP‐4.5 electrode, featuring 2D contour plots within a selected 2θ range and the corresponding charge/discharge curves during the first cycle [49]. Reproduced with permission [51]. Copyright 2024, American Chemical Society. (d,e) Corresponding FFT patterns in crystalline and amorphous regions in NFPP‐4.5 [49]. Reproduced with permission [51]. Copyright 2024, American Chemical Society. (f) HR‐TEM illustrations of Na₃V₂ (PO₄)₃‐Na₃Fe₂(PO₄)P₂O₇ [73]. Reproduced with permission [74]. Copyright 2022, Wiley. (g) EDS mapping and SEM image. (h) Rate performance of Na₄Fe₃(PO₄)₂(P₂O₇)/Na₂VTi(PO₄)₃ [75]. Reproduced with permission [76]. Copyright 2024, Wiley.

Nanosized maricite NaFePO₄ can be stably contained as the shell of Na₄Fe₃(PO₄)₂(P₂O₇), promoting its electrochemical activation. A core‐double shell Na₄Fe₃(PO₄)₂(P₂O₇)@NaFePO₄@C was prepared using a versatile sol–gel fabrication method followed by heat treatment [74]. Notably, the maricite NaFePO₄ shell maintains a thickness of 15 nm, enabling its electrochemical activation. The joint effort of the activated NaFePO₄ and the rapid 3D carbon shell allows Na₄Fe₃(PO₄)₂(P₂O₇)@NaFePO₄@C to deliver an exceptional discharge capacity of 136 mAh g⁻¹ at 0.1C, beyond the theoretical capacity of NFPP. Additionally, this material demonstrates outstanding cycling stability with no degradation for over 3000 cycles at 10C and an excellent rate capacity of 68 mAh g⁻¹ at 100C. This innovative strategy highlights the potential to utilize NaFePO₄, indicating the possibility of using impurities.

While nanosized NaFePO₄ can be activated as a shell of NFPP, maricite‐type NaFePO₄ can also intergrow with Na₄Fe₃(PO₄)₂(P₂O₇) phase to form a heterostructure. Na_4.5Fe_3.5(PO₄)_2.5P₂O₇ composites with a heterostructure were prepared using sol–gel method (Figure 6b) [49]. As shown in the operando XRD patterns in Figure 6c, the typical shoulder peaks of maricite NFP's (220) and (121) faces disappear in the fully discharged state at 2.0 V, indicating a spontaneous amorphization transition of the NFP phase. The FFT electron diffraction patterns in Figure 6d,e further confirm the high‐crystallinity NFPP‐4 phase and the amorphous NFP structure. Despite 11 000 cycles at 20C, the capacity of Na_4.5Fe_3.5(PO₄)₂P₂O₇ remains at 88%. Notably, this composite exhibits a reversible capacity of over 130 mAh g⁻¹ and an energy density of 400 W h kg⁻¹, the highest among iron‐based polyanionic cathodes for SIBs.

Besides heterogeneous structures with maricite NaFePO₄, other heterogeneous structures can also affect impurity production while enhancing the performance of NFPP. Notably, the intergrowth structure helps improve the purity and crystallization of NFPP. A heterogeneous Na₃V₂(PO₄)₃‐Na₃Fe₂(PO₄)P₂O₇ composite with a 1:1 molar ratio was prepared using a site preparation strategy [73]. In Figure 6f, areas with different lattice spacings of 0.374 and 0.559 nm, correspond to the (113) plane of the Na₃V₂(PO₄)₃ phase and (101) plane of the Na₃Fe₂(PO₄)(P₂O₇) phase, indicating successful preparation of the heterogeneous products. This composite delivers a reversible capacity of 113.1 mAh g⁻¹ at 0.1C and an energy density of 357 W h kg⁻¹. The preparation of heterogeneous composites effectively enhances NFPP performance, and future research can focus on exploring more effective cathode material combinations.

Another heterogeneous material, Na₄Fe₃(PO₄)₂(P₂O₇)/Na₂VTi(PO₄)₃ (NFPP/NVTP), was prepared through the spray‐drying method [75]. Two distinct crystal orientations with a clear region interface and tight contact were discovered. Additionally, Figure 6g provides direct evidence of the cross‐distribution of NFPP and NVTP in the heterostructure, as the abundance of Fe coincides with the scarcity of V and Ti. It should be mentioned that the NFPP/NVTP cathode delivers a high capacity of 155.3 mAh g⁻¹ at 20 mA g⁻¹ (Figure 6h) and an outstanding energy density of up to 417 W h kg⁻¹. This material demonstrates a capacity retention of 82.9% after 2500 cycles at 1 A g⁻¹. While these heterogeneous structures effectively improve the energy density of NFPP, a significant knowledge gap remains to be explored.

The impurity issue is a major distinction between Na₄Fe₃(PO₄)₂(P₂O₇) and other polyanionic materials, introducing new variables to the synthesis process and increasing the complexity of the NFPP system. Both impurity suppression and transformation strategies can enhance the electrochemical performance of NFPP. Nonetheless, impurity control can be integrated with other modification strategies to enhance the electrochemical performance of NFPP while controlling the impurity content.

4.2. Doping

As an effective modification strategy, doping can help enhance the electrochemical performance of NFPP materials by adjusting the crystal structure. To date, researchers have investigated a wide range of elements as dopants including titanium (Ti), manganese (Mn), nickel (Ni), aluminum (Al), magnesium (Mg), cobalt (Co), molybdenum (Mo), copper (Cu), zinc (Zn), fluorine (F), vanadium (V), silicon (Si), and so on (Figure 7a).

FIGURE 7.

(a) Statistical diagram of doped elements. (b) Design concept illustrating entropy increase with Ti doping in NFPP [76]. Reproduced with permission [77]. Copyright 2024, American Chemical Society. (c) Calculation of configurational entropy upon Ti substitution at Fe sites in the NFPP lattice [76]. Reproduced with permission [77]. Copyright 2024, American Chemical Society. (d) Operating voltages of NFPP and NFM _x PP at 0.1C in Na half‐cells [78]. Reproduced with permission [79]. Copyright 2024, Wiley. (e) Nyquist plots from EIS and fitting results for NFPP/rGO and Mn0.3‐NFPP/rGO [77]. Reproduced with permission [80]. Copyright 2022, Elsevier. (f) Comparison between THoCS and THoCS‐0.6Ni samples [79]. Reproduced with permission [81]. Copyright 2024, Wiley. (g) Cycle performance of Na_3.94Fe_2.94V_0.06(PO₄)₂(P₂O₇) material at 20C over 10 000 cycles [66]. Reproduced with permission [68]. Copyright 2024, American Chemical Society. (h) EIS spectra for NFPP and NFAPP cells [80]. Reproduced with permission [82]. Copyright 2024, Elsevier. (i) Na⁺ ion diffusion dynamics of NFPP and NFAPP at various voltages based on GITT measurement [80]. Reproduced with permission [82]. Copyright 2024, Elsevier.

4.2.1. Single‐Ion Doping

Single‐cation doping, due to its simplicity and ease of control, has emerged as a crucial strategy for enhancing the electrochemical performance of NFPP materials. This approach can significantly optimize the electrochemical properties of NFPP cathode materials in SIBs, including stabilizing the operating voltage plateau, boosting the reversible capacity, and improving cycling stability.

The nontoxic and safe properties of titanium (Ti) have made it one of the most popular active elements. Titanium doping enhances the electrochemical activity of NFPPs by improving their electronic conductivity. The synthesized Na₄Fe_2.98Ti_0.01(PO₄)₂(P₂O₇) material exhibits a discharge capacity of 112.5 mAh g⁻¹ at 0.1C and a capacity of 97.2 mAh g⁻¹ at 10C over 5000 cycles [81]. During the de/sodiation process, a volumetric change of merely 2.98% was recorded, signifying an improved reversibility of the crystal lattice. As shown in Figure 7b,c, the 5% Ti‐substituted lattice (NTFPP‐0.05) causes entropic augmentation, enhancing electronic conductivity and inhibiting the impure phases of NaFeP₂O₇ and NaFePO₄. The optimal NTFPP‐0.05 composite demonstrates a high initial discharge capacity (118.5 mAh g⁻¹ at 0.1C) [76]. Another study indicates that the synthesized Ti‐NFPP/C cathode demonstrates outstanding rate performance, achieving 66.7 mAh g⁻¹ at 30C, and exhibits remarkable cycling stability with a capacity retention of 93.8% after 2000 cycles at 10C [82]. These findings highlight the potential of Ti doping to enhance NFPP performance.

Manganese (Mn), with the features of being environmentally friendly and cost‐effective, is widely used in doping strategies. The operating voltage of NFM _x PP was enhanced by increasing the Mn content (Figure 7d). Introducing Mn (0.6 ≤ x  ≤ 1.2) into NFM _x PP can also weaken the Fe–O bonding interaction, allowing full utilization of the Mn³⁺/Mn²⁺ redox couple. A series of Na₄Fe_3−x Mn _x (PO₄)₂(P₂O₇) cathode have been studied revealing that the NFM_1.2PP cathode exhibited a high operating voltage of approximately 3.3 V with a reversible capacity of 109.2 mAh g⁻¹ [78]. Mn doping at the Fe site decreases Na⁺ ion diffusion energy barrier and the electronic bandgap, thereby synchronizing rapid electron and Na⁺ ion mobility. In another investigation, a 5% Mn‐doped Na₄Fe_2.85Mn_0.15(PO₄)₂(P₂O₇) composite was prepared, exhibiting ultralong‐lasting cyclability with 88.1% capacity retention over 10 000 cycles at 50C and an impressive rate capability of 42.7 mAh g⁻¹ at 200C [47]. The optimal Na₄Fe_2.7Mn_0.3(PO₄)₂(P₂O₇)/rGO provides an initial discharge capacity of 131.5 mAh g⁻¹ at 0.1C and excellent rate performance of 70.2 mAh g⁻¹ at 50C [77]. The Mn0.3‐NFPP/rGO demonstrates superior ion conductivity, which is advantageous for rapid ion diffusion. It maintains a discharge capacity of 85.3 mAh g⁻¹ at 0.2C even at −20°C, along with good rate performance (Figure 7e). Additionally, Na₄Fe_2.9Mn_0.1(PO₄)₂(P₂O₇)@C composites was prepared by mechanochemically assisted approach, releasing 119.6 mAh g⁻¹ at 0.1C and 84.8% after 3000 cycles at 10C [43]. Future research on Mn doping can explore new composites and optimize doping levels for better performance.

Doping with nickel (Ni) heteroatoms can enhance intrinsic electron conductivity and refine the Na⁺ ion diffusion pathway and energy barrier, facilitating rapid charging and Na storage at low temperatures (Figure 7f). A hollow core‐shelled Na₄Fe_2.4Ni_0.6(PO₄)₂(P₂O₇) with minimal void space has been developed, exhibiting a superior rate capability of 86.4 mAh g⁻¹ at 25C and exceptional cycling stability at −20°C, maintaining a capacity of 43.6 mAh g⁻¹ after 2500 cycles at 5C [79]. Additionally, Ni can stabilize the crystal structure and expand the Na⁺ ion migration. The Na₃Fe_1.9Ni_0.1(PO₄)(P₂O₇) was synthesized through the sol–gel method delivering a discharge capacity of 100.7 mAh g⁻¹ at 0.1C and excellent cycling stability throughout 5000 cycles at 10C [83]. Interestingly, Ni substitution can also inhibit electrochemical inert maricite‐NaFePO₄ impurity generation [67]. The optimal Na₄Fe_2.82Ni_0.18(PO₄)₂(P₂O₇)@C‐N cathode delivers an exceptionally high discharge capacity of 128.4 mAh g⁻¹ at 0.2C, outstanding rate performance of 62.5 mAh g⁻¹ at 50C, and impressive long‐term cycling stability, maintaining approximately 83% capacity retention after 3000 cycles at 10C. These advancements underscore the efficacy of Ni doping in optimizing the performance of NFPP.

Tiny amounts of vanadium (V) doping can also significantly improve the electrochemical performance of NFPP without excessive costs. NFPP doped with vanadium demonstrates lower barriers for sodium‐ion mobility and increased conductivity, resulting in enhanced electrochemical performance. The Na_3.94Fe_2.94V_0.06(PO₄)₂(P₂O₇) exhibits a reversible capacity of 123.4 mAh g⁻¹ at 0.1C [66]. Even at 20C, this composite maintains a reversible capacity of 99.6 mAh g⁻¹ and maintains 81.65% capacity after 10 000 cycles (Figure 7g). Another study on vanadium doping confirmed that the appropriate introduction of V³⁺ can suppress the formation of NFP impurities, resulting in Na_3.95Fe_2.95V_0.05(PO₄)₂P₂O₇ composite with a capacity of 114 mAh g⁻¹ at 0.1C [84]. While cost reduction is a primary driver for the commercialization of NFPP, incorporating a small amount of V proves feasible if it substantially enhances electrochemical performance.

Aluminum (Al) doping has garnered extensive attention as a dopant for NFPP. Doping Al elements can effectively enhance NFPP's electronic conductivity, as illustrated in Figure 7h. NFAPP exhibits higher Na⁺ ion diffusion coefficients (D _Na) compared to NFPP at the same voltage, indicating that Al substitution promotes Na⁺ ion dynamics (Figure 7i). A microporous Na_3.9Fe_2.9Al_0.1(PO₄)₂(P₂O₇) (NFAPP) has been developed, demonstrating an overall volume change of around 3.5% and maintaining 85.1% capacity over 10 000 cycles at 50C [80]. Na₄Fe_2.94Al_0.04(PO₄)₂(P₂O₇)/C was synthesized via a straightforward ball‐milling technique [62]. The average length of the Al–O bond is a little shorter than that of the Fe–O bond, which contributes to structure stability. By minimally substituting Al³⁺ for Fe²⁺ in the NFPP structure, Fe defects are introduced, which impede the NaFePO₄ formation and improve Na⁺ ion diffusion kinetics and conductivity, resulting in Na₄Fe_2.94Al_0.04(PO₄)₂P₂O₇/C cathode a discharge capacity of 128.1 mAh g⁻¹ at 0.2C. Another Al doping study emphasizes the impact of low‐energy 3p orbitals and transition metal vacancies. These low‐energy p‐orbitals can lower the conduction band and enhance metal–oxygen covalent bonds, resulting in a reduced bandgap and a reinforced crystal structure [85]. This leads to Na₄Fe_2.85Al_0.1(PO₄)₂(P₂O₇) achieving exceptional cycling stability, enduring over 10 000 cycles without significant degradation.

Magnesium (Mg), copper (Cu), and molybdenum (Mo) have been used in NFPP doping due to their distinct roles [86, 87]. The incorporation of Cu into NFPP enhances the local electron density and reduces the bandgap. Specifically, the synthesized Na₄Fe_2.7Cu_0.3(PO₄)₂P₂O₇ achieves a discharge capacity of 119.01 mAh g⁻¹ at 1C and retains 82.76% of its capacity after 3000 cycles at 20C [88]. Additionally, pure‐phase Mo⁶⁺‐doped Na₄Fe_3−x Mo _x (PO₄)₂(P₂O₇)/C was synthesized via spray drying and annealing. DFT calculations demonstrate that Mo⁶⁺ doping enhances the intrinsic electron conductivity of Mox‐NFPP by facilitating the transition of electrons from the valence band to the conduction band. At 0.1C, the Mo0.10‐NFPP cathode delivers an initial discharge capacity of up to 123.9 mAh g⁻¹, nearly its theoretical capacity [67]. Even at a high rate of 10C, it achieves a substantial discharge capacity of 86.09 mAh g⁻¹ and retains 96.18% of its capacity after 500 cycles. The successful application of Mg, Cu, and Mo doping in NFPP highlights a clear path forward for optimizing its performance in advanced energy storage devices.

Anionic substitution is an effective strategy to expand the modification strategies for NFPP materials. The electrochemical properties can be fine‐tuned by replacing anions in the NFPP structure, providing a more favorable migration path for Na⁺ ions. As illustrated in Figure 8a, the substitution of the (PO₄)³⁻ group with the (SiO₄)⁴⁻ group enhances diffusion kinetics and electronic conductivity, while maintaining structural integrity [89]. The incorporation of (SiO₄)⁴⁻ into the NFPP matrix has been confirmed by XPS analysis as shown in Figure 8b. By substituting at a nonactive site, the electrochemical performance is improved without affecting the theoretical capacity. Consequently, this (SiO₄)⁴⁻‐doped Na₄Fe₃(PO₄)₂(P₂O₇) exhibits a capacity of 119.4 mAh g⁻¹ at 0.1C. Another study prepared the NASICON‐Na₄Fe₃(PO₄)_1.9(SiO₄)_0.1P₂O₇ was prepared. The substitution of SiO₄ ⁴⁻ slightly enlarges the crystal lattice, thereby widening the Na⁺ diffusion channel, resulting in NFPP‐Si0.1 exhibiting a high rate capability of 77.6 mAh g⁻¹ at 50C with a retention rate of 79.4% after 7000 cycles at 10C. These findings underscore the substantial potential of anionic substitution and offer valuable insights into the modification of NFPP materials.

FIGURE 8.

(a) Mechanism of (SiO₄)⁴⁻ doping into NFPP [89]. Reproduced with permission [92]. Copyright 2023, American Chemical Society. (b) XPS spectra of NFPP‐Si0.05 sample showing Fe2p and Si2p peaks [89]. (c) Na⁺ ion diffusion pathways in NCMFPP and NFPP analyzed using the BVEL method [91]. Reproduced with permission [92]. Copyright 2024, Elsevier. (d) Schematic illustration of Fe spin transition. (e) Four‐point probe conductivity measurements for NFPP, NFPP‐Mn, NFPP‐F, and NFPP‐Mn‐F [92]. Reproduced with permission [93]. Copyright 2024, Wiley. (f) Long‐term cycling stability of HE‐NFPP at 5 A g⁻¹ [90]. Reproduced with permission [94]. Copyright 2024, Wiley. (g) Volumetric variation comparison between HE‐NFPP and other sodium‐ion cathode materials [90]. Reproduced with permission [92]. Copyright 2024, Wiley.

4.2.2. Multiple‐Ion Doping

Innovative doping techniques, including dual and high‐entropy strategies, are at the forefront of enhancing the electrochemical performance of NFPP materials. These approaches improve capacity and stability and open new avenues for developing advanced energy storage systems.

Dual doping is a strategy to optimize the electrochemical properties of NFPP by simultaneously introducing two doping elements into their structures. This approach can induce a synergistic effect, significantly enhancing the overall performance of the material through interactions between different doping elements [93]. Dual‐cation doping of cobalt (Co) and manganese (Mn) effectively increases the voltage and electrochemical capacity of NFPP. Na₄Co_0.5Mn_0.5Fe₂(PO₄)₂(P₂O₇) is prepared to facilitate a redox reaction involving 3.5 electrons during the charge/discharge process [91]. NCMFPP exhibits a more consistent Na⁺ ion diffusion path than NFPP and a broader Na⁺ ion diffusion channel along the c‐axis direction, as shown by the bond‐valence energy landscape (BVEL) analysis in Figure 8c. It was found that the NCMFPP provides a high initial discharge capacity of 139 mA g⁻¹ at 0.1C with rate capacity of 75 mA g⁻¹ at 10C.

In another study, a novel Na₄Fe_2.96Mg_0.02Cu_0.02(PO₄)₂P₂O₇ cathode material was synthesized. The dual doping of Mg and Cu significantly enhanced the electronic conductivity by narrowing the bandgap, achieving a discharge capacity of 112.3 mAh g⁻¹ [92]. As shown in Figure 8d, dual doping with Mn and fluorine (F) can modulate the electronic structure, thus enhancing electronic conductivity. The material's bandgap was reduced from 1.01 to 0.80 eV, while its electronic conductivity increased from 8.5 to 24.4 µS cm⁻¹, resulting in competitive electrochemical performance (Figure 8e) [92]. This composite demonstrates excellent rate performance, achieving 121.0 mAh g⁻¹ at 0.1C and 104.9 mAh g⁻¹ at 5C, and retaining 88.5% of its capacity after 1000 cycles at 1C. Future research can explore more dual‐doping combinations to further enhance NFPP performance.

High‐entropy doping is a strategy that produces a high‐entropy effect by introducing multiple doping elements into a material, showing promise in enhancing the electrochemical properties of NFPP. This approach increases the conformational entropy of the material, thereby improving its thermodynamic stability and kinetic properties [94, 95, 96]. Na₄Fe_2.5Mn_0.1Mg_0.1Co_0.1Ni_0.1Cu_0.1(PO₄)₂(P₂O₇) cathode delivers a rate performance of 55 mAh g⁻¹ at 10 A g⁻¹ and an ultralong cycling lifespan exceeding 18 000 cycles at 5 A g⁻¹ (Figure 8f) [90]. The increased configurational entropy in HE‐NFPP results in an improved structure, with a minimal volume variation of only 1.83% (Figure 8g). Likewise, the high‐entropy doped Na₄Fe_2.85(Ni,Co,Mn,Cu,Mg)_0.03(PO₄)₂(P₂O₇) cathode achieves 122 mAh g⁻¹ at 0.1C and 85 mAh g⁻¹ at 50C [97].

Beyond high entropy, combining high entropy with an iron‐defect strategy can further enhance the electrochemical performance of NFPP. Na₄Fe_2.65(Ni,Cr,Mg,Co,Mn)_0.027(PO₄)₂P₂O₇ was engineered with Fe defects to reduce impurity phase formation. This design achieves a capacity of 57 mAh g⁻¹ at 100C and retains 80% of its capacity after 4000 cycles at 20C [98]. Another study developed a Na₄Fe_2.61(Ni,Co,Mn,Cu,Zn,Mg)_0.05(PO₄)₂P₂O₇ cathode that combines high entropy doping with Fe vacancy engineering. By providing additional active sites and enhancing the stability of the crystal structure, this approach achieves a discharge capacity of 101.9 mAh g⁻¹ and maintains 74.3% capacity retention after 15 000 cycles [99]. Combining high entropy with iron‐defect strategies can effectively enhance the electrochemical performance of NFPP, paving the way for more efficient and durable energy storage solutions.

While doping can significantly enhance various material properties, it also presents several potential challenges that are critical concerns in current research. The selection of doping elements offers substantial potential for expansion, especially for bi‐ionic doping, which remains relatively unexplored. For elements where doping has been implemented, the precision of doping levels often requires improvement, necessitating more accurate control of the ratios to achieve optimal performance. Additionally, doping can complicate material synthesis, especially in NFPP systems sensitive to impurities. There is a lack of in‐depth understanding of the specific mechanisms by which doping elements affect NFPP materials. Future research should focus on elucidating these mechanisms through theoretical calculations and experimental studies.

A comparative analysis of different modification strategies reveals their distinct roles in improving NFPP performance (Table 2). Carbon coating and conductive compositing are most effective in enhancing electronic conductivity, thereby significantly improving rate capability. Doping strategies primarily optimize the crystal structure and Na⁺ diffusion pathways, contributing to improved cycling stability and moderate rate performance. In contrast, impurity engineering and heterostructure design can introduce additional electrochemical activity or interfacial effects, in some cases leading to higher specific capacity. However, these strategies also present trade‐offs. Carbon‐based modifications may reduce tap density, while excessive doping can induce structural distortion. Impurity‐related approaches require precise control to avoid performance degradation. Both doping and impurity engineering are effective strategies for enhancing the electrochemical performance of NFPP; however, they involve distinct mechanisms and trade‐offs. Doping typically modifies the intrinsic electronic structure and ion diffusion pathways by introducing foreign elements into the lattice, leading to improved conductivity, structural stability, and redox activity. Nevertheless, excessive or inappropriate doping may introduce lattice distortion or increase synthesis complexity and cost. In contrast, impurity engineering focuses on controlling, suppressing, or even utilizing secondary phases (e.g., NaFePO₄ or Na₂FeP₂O₇) to enhance overall performance. While impurity suppression can improve phase purity and stability, impurity transformation strategies (e.g., heterostructure design) can sometimes deliver capacities beyond the theoretical limit of pure NFPP. However, controlling impurity phases precisely remains challenging, particularly in large‐scale synthesis. Therefore, the choice between doping and impurity engineering depends on the targeted performance metrics and application scenarios. In practice, combining these strategies may offer synergistic effects and represents a promising direction for future research. Overall, no single strategy is universally optimal; instead, combining multiple approaches often leads to synergistic improvements in conductivity, kinetics, and stability.

TABLE 2.

Comparison of the electrochemical performances of NFPP cathodes in SIBs modified by different strategies.

Composite	Preparation method	Specific capacity, mAh g−1	Cycle number	Capacity retention	Ref
Mesoporous sponge‐like Na4Fe3(PO4)2(P2O7)@C@rGO	Solid‐state method	111.7@0.05C	30 000@20C	86.70%	[100]
Na4Fe2.5Mn0.1Mg0.1Co0.1Ni0.1Cu0.1(PO4)2(P2O7)@C	Sol–gel method	108.9@0.02A g−1	18 000@5A g−1	70.50%	[90]
Na4Fe2.61(Ni,Co,Mn,Cu,Zn,Mg)0.05(PO4)2(P2O7)@C	Sol–gel method	129.5@0.01A g−1	15 000@5A g−1	74.2%	[99]
Na3.9Fe2.6V0.1Mn0.1Cu0.1Mg0.1(PO4)2(P2O7)@C	Sol–gel method	122.3@0.1C	14 000@50C	78.58%	[95]
Nonstoichiometric Na3.4Fe2.4(PO4)1.4(P2O7)@C	/	110.8@0.1C	14 000@20C	83.10%	[61]
Na4Fe3(PO4)2(P2O7)@C‐Mg5%	/	104@0.05A g−1	14 000@5A g−1	80.80%
Na4.5Fe3.5(PO4)2.5(P2O7)@C	Sol–gel method	130.5@0.1C	11 000@20C	88%	[49]
Na3.94Fe2.94V0.06(PO4)2(P2O7)@C	Spray‐drying method	126.4@0.1C	10 000@20C	81.65%	[66]
Micro porous Na3.9Fe2.9Al0.1(PO4)2(P2O7)@C	Ball milling and freeze‐drying method	115.2@0.1C	10 000@50C	85.10%	[80]
Na4Fe2.85Mn0.15(PO4)2(P2O7)@C	Solid‐state method	111.6@0.1C	10 000@50C	88.10%	[47]
Pure‐phase Na4Fe2.91(PO4)2(P2O7)@C	Spray‐drying method	110.9@0.2C	10 000@10C	Without decay	[25]
Hollow spherical Na4Fe3(PO4)2(P2O7)@C	Spray‐drying method	86.5@1C	10 000@10C	89.70%	[53]
Na4Fe3(PO4)2(P2O7)/Na2VTi(PO4)3@C	Spray‐drying method	155.3@0.02A g−1	2500@1A g−1	82.90%	[75]
Na4Co0.5Mn0.5Fe2(PO4)2(P2O7)@C	Solid‐state method	139@0.1C	2000@10C	65.20%	[91]
Na4Fe3(PO4)2(P2O7)@NaFePO4@C	Sol–gel fabrication method	136@0.1C	3000@10C	Without decay	[74]
Na4Fe3(PO4)2(P2O7)@C nanoparticles in carbon nanoribbons	Electrospinning method	128.6@0.1C	5000@50C	72%	[59]
Na4Fe3(PO4)2(P2O7)@C nanospheres	Template method	128.5@0.2C	4000@10C	63.50%	[46]
Na4Fe2.82Ni0.18(PO4)2(P2O7)@C‐N	Solid‐state method	128.4@0.2C	3000@10C	83%	[67]
Na4Fe2.94Al0.04(PO4)2(P2O7)@C	Solid‐state method	128.1@0.2C	3000@10C	83.70%	[62]
Na4Fe3(PO4)2(P2O7)@rGO microspheres	Spray‐drying method	128@0.1C	6000@10C	62.30%	[48]
Hollow Core‐Shelled Na4Fe2.4Ni0.6(PO4)2(P2O7)@C	Spray‐drying method	120.3@0.5C	5000@20C	72.96%	[79]
Na4Fe2.98Ti0.01(PO4)2(P2O7)@C	Spray‐drying method	112.5@0.1C	5000@10C	95.88%	[81]
Na3Fe2(PO4)(P2O7) (NFPP)@rGO	Spray‐drying method	105.5@0.2C	8000@20C	72.40%	[52]
Na4Fe2.95(NiCoMnMgZn)0.01(PO4)2(P2O7)@C	Spray‐drying method	105.3@0.1C	7000@10C	74.20%	[94]
Na4Fe3(PO4)1.9(SiO4)0.1(P2O7)@C	Solid‐state method	89.9@0.5C	7000@10C	79.40%	[92]

4.3. Surface Modification

Surface modification, particularly carbon modification, is a crucial method for enhancing the electrochemical performance of NFPP. Carbon modification is favored due to its affordability, high conductivity, and chemical stability. There are two primary approaches to achieving carbon coating: One involves introducing a carbon source during the synthesis process to form an in situ carbon coating on the NFPP surface via thermal decomposition. The other approach is to directly composite NFPP with conductive carbon materials to enhance its conductivity. These methods effectively optimize the performance of NFPP.

Various carbon sources have been explored to prepare in situ carbon coatings, including glucose, anthracite, folic acid, phytic acid (PA), citric acid, and organophosphonic acid in NFPP. These carbon sources can form a uniform carbon layer on the surface of NFPP during heat treatment, improving the conductivity and electrochemical properties of the material. By directly coating the surface of the material, the interfacial contact between the cathode material and the electrode can be improved, facilitating rapid electron transfer. This improvement in interfacial contact is crucial for enhancing the overall performance of the battery.

Interestingly, NFPP can achieve a capacity close to its theoretical maximum solely with an appropriate carbon coating, without additional modification methods. The synthesized NFPP/C nanospheres achieve a high discharge capacity of 128.5 mAh g⁻¹ at 0.2C, with a capacity retention of 63.5% after 4000 cycles at 10C [46]. Notably, a high reversible capacity of 79 mAh g⁻¹ is demonstrated at an ultrahigh current rate of 100C (Figure 9a). The nanospheres (~30 nm) and carbon coating layers (~3 nm) create efficient conductive channels for electrons, resulting in excellent performances.

FIGURE 9.

(a) Discharge capacities of NFPP/HC and NFPP/LC at various discharge current rates [46]. Reproduced with permission [48]. Copyright 2019, Elsevier. (b) SEM images of nanoflakes [101]. Reproduced with permission [102]. Copyright 2022, Elsevier. (c) SEM images of NFPP@MCNTs [103]. Reproduced with permission [104]. Copyright 2020, Elsevier. (d) Illustration of the synthesis process of NFPP@MCNTs [103]. Reproduced with permission [104]. Copyright 2020, Elsevier. (e,f) SEM images of mesoporous sponge‐like NFPP@C@rGO [100]. Reproduced with permission [105]. Copyright 2023, Elsevier. (g) Schematic illustration of the preparation process of NFPP@NFP@C‐CC flexible electrode [74]. Reproduced with permission [78]. Copyright 2019, Elsevier. (h) Schematic illustration of the synthesis of NFPP@MX [105]. Reproduced with permission [106]. Copyright 2024, MDPI.

Introducing heteroatoms into the in situ carbon coating can further optimize the electrochemical performance of NFPP. Generally, carbon materials contain numerous defects, seriously affecting their comprehensive performance. Nitrogen doping can effectively reduce these defects, improving their stability and conductivity. A hierarchical nitrogen–carbon‐coated NFPP nanoflake (Figure 9b) was prepared, in which oleic acid is the surfactant and carbon source [101]. These NFPP nanoflake composites operate over a wide temperature range from −20°C to 50°C, providing a capacity of 129 mAh g⁻¹ at 0.1C, retaining 81% capacity at 10C after 1500 cycles at 25°C. A low‐cost and simple nitrogen doping method is expected to fully exploit the potential of coating materials and enhance the performance of NFPP, which requires more attention.

Another simple but effective strategy involves a direct dispersion of carbon nanotubes in NFPP precursors and high‐temperature sintering. Carbon nanotubes can significantly enhance the electron conductivity and stability of NFPP, which enhances its electrochemical performance. NFPP nanospheres were developed and grown on multiwalled carbon nanotubes (MCNTs) (Figure 9c,d) [103]. The NFPP@MCNT composite exhibits a capacity of 115.7 mAh g⁻¹ at 0.1C and retains 95% of its capacity after 1200 cycles at 2C and the rate capability of 62.8 mAh g⁻¹ can be seen even at 20C. Another study also developed NFPP@CNT‐1%, which delivers a capacity of 103.9 mAh g⁻¹ at 0.1C and retains 99.9% of its capacity after 1000 cycles at 5C [102]. These advancements underscore the potential of carbon nanotubes in boosting the electrochemical performance of NFPP materials for better energy storage.

The synthesized graphene Na₃Fe₂(PO₄)(P₂O₇) cathode demonstrated an ultralong cycle life, retaining 72.4% of its capacity after 8000 cycles at 20C, and exhibited outstanding rate capability, achieving 42.4 mAh g⁻¹ at 100C, largely due to graphene's ability to provide a robust conductive network that facilitates rapid electron transfer [52]. In another study, 3D graphene‐decorated Na₄Fe₃(PO₄)₂(P₂O₇) microspheres were prepared using spray‐drying method. This composite achieved a capacity of 128 mAh g⁻¹ at 0.1C and maintained 62.3% capacity retention over 6000 cycles at 10C [48]. Numerous studies have highlighted the effectiveness of graphene in surface modification, primarily due to its ability to significantly enhance electronic conductivity and alleviate structural stress in NFPP, thereby boosting overall performance during charging and discharging cycles.

The dual‐carbon modification enhances the performance of NFPP by improving electronic conductivity and facilitating Na⁺ ion diffusion. An NFPP composite, integrated with in situ carbon and expanded graphite, achieved a rate capability of approximately 80.5 mAh g⁻¹ at 50C and cycling stability with 89.85% capacity retention over 11 000 cycles [104]. Another mesoporous NFPP structure with a carbon layer and graphene nanosheets resulted in 86.7% capacity retention at 20C for over 30 000 cycles and excellent thermal stability at 60°C (Figure 9e,f) [100]. Furthermore, NFPP@C/HG, synthesized through a scalable ball milling strategy and modified with 3D holely graphene, delivers a capacity of 118 mAh g⁻¹ at 0.2C and maintains 53 mAh g⁻¹ even at 100C [106]. Additionally, NFPP@AC/rGO exhibits 78 mAh g⁻¹ at 20C and 30°C and 42 mAh g⁻¹ at 20C and 15°C [107]. These dual‐carbon modification strategies significantly enhance the electrochemical performance of NFPP, particularly in long‐term stability, highlighting their potential for practical applications.

Other strategies for surface modification were employed in the fabrication of the NFPP cathode material. Metal‐phosphate capping can improve electronic conductivity and provide more active sites [74]. The Na₄Fe₃(PO₄)₂(P₂O₇)@NaFePO₄@C was prepared using a sol–gel fabrication strategy (Figure 9g). This composite exhibits a discharge capacity of this composite is 136 mAh g⁻¹ at 0.1C, with no capacity decay over 3000 cycles at 10C and an excellent rate capability of 68 mAh g⁻¹ at 100C. Additionally, 3D network structures assisted by MXene represent another important modification method to enhance the electrochemical performance of NFPP. The Na₄Fe₃(PO₄)₂(P₂O₇)@C/Ti₃C₂T _x was synthesized through an electrostatic self‐assembly method (Figure 9h) [105]. By creating this cross‐linked structure, the interparticle contact area is increased, and electrical conductivity is enhanced, leading to a discharge capacity of 106.1 mAh g⁻¹ at 0.2C and a rate capability of 60.4 mAh g⁻¹ at 10C.

The advancement of NFPP cathode materials necessitates precise control over the thickness and uniformity of the carbon cladding layer on the active material's surface. These properties of the carbon layer directly influence the Na⁺ migration efficiency between the particles and the electrolyte, affecting the charging and discharging performance of the battery. An inhomogeneous or excessively thick carbon layer may hinder ion transport, whereas a uniform and moderately thick carbon layer optimizes the ion and electron transport paths and improves the electrochemical activity of the material.

5. From Na₄Fe₃(PO₄)₂(P₂O₇) Material to Practical Application

We have reviewed the properties, synthesis, and modification strategies of NFPP materials and evaluated their electrochemical performance in half‐cells. Nonetheless, data from half‐cell tests may not apply to sodium‐ion full battery (SIFC) systems. To fully assess NFPP cathodes, further discussion on electrolytes and anodes is needed. The commercialization of NFPP is advancing rapidly, showing strong market potential and innovation. Challenges such as compaction density, presodium strategies, and cost issues will be mentioned.

5.1. Full Cell

The commercial use of NFPP materials for SIB applications is progressing, with potential applications in multiple battery forms being investigated. Beyond half‐cells, NFPP is adapted for button, pouch, and cylindrical cells. Selecting appropriate anode materials and electrolytes with adequate initial Coulomb efficiency and operating potential is essential for the full cell.

Prototypes of SIFC batteries include button, pouch, and cylindrical cells, each differing in internal structure and encapsulation. As shown in Figure 10a, button cells are the most common form in research, aiding in evaluating the performance of the NFPP cathode. Compared to button cells, pouch cells are gaining attention because of their high safety, low internal resistance, and design flexibility. A pouch cell was prepared with 2 kg of Na₄Fe_2.91(PO₄)₂(P₂O₇) to assess its electrochemical performance, maintaining a capacity of 85 mAh g⁻¹ after 1000 cycles at 1C with no observable degradation [25]. Another study prepared 2 kg of Na_4.5Fe_3.5(PO₄)_2.5(P₂O₇) to examine practical applications. The capacity of the small pouch can be reversed at approximately 550 mAh at 0.1C between 1.5 and 4.2 V, retaining 88% capacity even after 2000 cycles (Figure 10b,c).

FIGURE 10.

(a) The button cell schematic [69]. Reproduced with permission [78]. Copyright 2025, Royal Society of Chemistry. (b,c) Initial charge/discharge profile and cycling performance at 3C for NFPP‐4.5 pouch cells [49]. Reproduced with permission [51]. Copyright 2024, American Chemical Society. (d) Cointercalation process of solvated Na⁺ ions [108]. Reproduced with permission [109]. Copyright 2022, Wiley. (e) Galvanostatic charge/discharge curves of Bi//NFPP@C battery at 60°C, 80°C, and 100°C [108]. Reproduced with permission [109]. Copyright 2022, Wiley. (f) Photo of the fully charged HC//NFPP pouch cells after package clipping and under a flame [110]. Reproduced with permission [111]. Copyright 2024, American Chemical Society.

Cylindrical cells can exhibit higher energy density, more consistent product quality, and reduced cost. Notably, flatulence issues remain a challenge for SIBs. Compared to square and pouch cells, cylindrical cells have a rigid structure that enhances resistance to swelling. Their flexibility allows cylindrical cells to form various modules, facilitating the expansion of SIBs across multiple applications and supporting commercialization. Although data on cylindrical cells with NFPP cathodes is lacking, these cells hold strong potential for future commercialization.

Compatibility between the anode and cathode is crucial for achieving excellent electrochemical performance in batteries. Hard carbon (HC) is frequently utilized as an anode material in full cells because of its high reversible capacity and excellent cycling stability. The NFPP/HC anode is highly effective in SIBs, offering a power density of 24.1 kW kg⁻¹ and an energy density of 146.6 Wh kg⁻¹, resulting in high power and energy density. Additionally, compounds like NaTi₂(PO₄)₃ (NTP) have been investigated for use with NFPP cathode materials. A recent study reported the HE‐NFPP//HC battery has a specific capacity of 82.8 mAh g⁻¹ at 0.2 A g⁻¹, while the HE‐NFPP//NTP cell exhibits a capacity of 85.7 mAh g⁻¹ at 50 mA g⁻¹ current [90]. After 200 cycles, the HE‐NFPP/NTP cell remains at 99.2% of its capacity, highlighting the cycling stability advantages of NTP as an anode material. Another study used a bismuth (Bi) anode in a Bi//NFPP@C battery [108]. At −70°C, the Bi//NFPP battery retains 70.19% of its capacity at room temperature and performs well at temperatures up to 100°C, benefiting from its superior electrolyte boiling point (Figure 10d,e).

Selecting the appropriate electrolyte is essential for SIB performance, as it requires high ionic conductivity to ensure efficient sodium ions transport between electrodes. Various electrolyte formulations have been studied for NFPP cathode materials, including NaClO₄ dissolved in EC/DEC carbonate electrolyte [53], NaPF₆ in EC/PC (with FEC additive) [66], PD, PTD [112], and NaPF₆/DEGDME. A cell with EC/PC/1M NaClO₄ and an NFPP cathode exhibits a highly reversible capacity of 122 mAh g⁻¹ [112]. After 100 cycles, the discharge capacity remains nearly constant with a high Coulombic efficiency of 99%. Replacing NaPF6/DEGDME with NaPF₆/PC 5% FEC for the NFPP‐Mg5% cathode results in a higher operating voltage and improved discharge plateau. Despite the initial performance, cells with NaPF₆/DEGDME electrolyte face decreasing discharge capacity after 100 cycles. Optimizing the electrolyte is crucial for enhancing the electrochemical performance of NFPP full cells, requiring further attention to address commercial use challenges. An electrolyte (NaClO₄:TMP:TFEP = 1:3:2, by mol, 1.22 M) that is nonflammable, cost‐effective, and low salt concentration was developed [110]. The Ah‐level HC/NFPP pouch cell achieves CE of over 99.9% and retains 84.5% capacity over 2000 cycles, showing excellent performance across temperatures from −20°C to 60°C. This pouch cell maintains a steady voltage output despite packaging clipping and flame exposure, indicating its safety (Figure 10f).

5.2. Commercialization

Against the background of global energy structure transformation and upgrading, the demand for energy storage technologies is increasing. These energy storage technologies prioritize cycle life, safety, cost‐effectiveness, and environmental friendliness (Figure 11a). These technologies are widely applied in fields such as large‐scale energy storage systems, low‐speed vehicles, two‐wheelers, household storage, outdoor base station energy storage, and photovoltaic energy storage (Figure 11b). Among various competing cathode materials for SIBs, NFPP stands out due to its ultralong cycle life, climate adaptability, and high safety in energy storage systems. Additionally, NFPP is cost‐effective and environmentally friendly as it lacks toxic or expensive elements like vanadium (V), cobalt (Co), nickel (Ni), chromium (Cr), and fluorine (F). With these remarkable properties, NFPP is an attractive candidate for energy storage applications and has become a prominent focus of research and commercialization in academia and industry.

FIGURE 11.

(a) The advantages of NFPP for commercialization. (b) NFPP's application for commercialization. (c) The challenges and complementary strategies of NFPP. (d) The cost percentage analysis for SIBs with polyanion cathodes and LiFePO₄ cells.

The industrial progress of NFPP cathode materials in SIBs has shown a positive trend and is currently in a critical stage of its industrialization process. This period has seen a steady increase in the technological maturity of NFPP, with the industry recognizing its potential in SIBs. The latest information demonstrates that many companies are actively promoting the development and industrialization of NFPP materials. Although NFPP is less mature than some traditional cathode materials and still facing challenges, its unique advantages and complementary strategies strongly support its potential for large‐scale commercialization (Figure 11c). The pace of industrialization of NFPP materials is expected to accelerate significantly as work progresses and technical difficulties are systematically overcome. This will boost the development of SIB technology and contribute to the development of the entire energy storage industry.

5.2.1. A Comparative Examination of Potential Competitive Cathodes

Among the promising cathode materials for SIBs, which compete as potential alternatives, there is no clear consensus on which material offers superior performance. To fully understand the advantages and limitations of NFPP cathode materials, it is necessary to compare other polyanion cathode materials with commercialization prospects. Recent studies indicate that the most promising polyanion cathode materials include phosphate, pyrophosphate, and sulfate.

Lead‐acid and lithium‐iron phosphate batteries are significant competitors to NFPP, influencing each other's commercialization. As demand for lithium batteries rises, SIBs are primarily used as complementarity and effective substitutes. Nonetheless, SIBs, particularly those with iron‐based NFPP cathodes, are more cost‐effective than lithium‐iron‐phosphate cathodes. Additionally, LIBs cannot match the all‐weather resilience of SIBs. For instance, the Bi/NFPP@C battery, whose capacity maintains 70.19% of its room temperature even at −70°C, still excels at temperatures up to 100°C. Compared to lead‐acid batteries, NFPP offers advantages in energy density, life cycle, and cost, potentially leading to the gradual phase‐out of lead‐acid batteries.

Since their introduction, sodium vanadium phosphate cathodes have garnered significant attention for their excellent electrochemical properties. Additionally, cathode materials with commercial potential, such as Na₃V₂(PO₄)₂F₃ (NVPF) and Na₃(VO)₂(PO₄)₂F (NVOPF), have been developed and applied in practical applications. The energy density of NFPP cathode materials is lower than that of vanadium‐based due to their low redox potential. The cost‐effectiveness and environment‐friendly properties of NFPP make it more competitive than vanadium‐based phosphates, especially in applications that do not require high energy density. NFPP materials are expected to gain popularity in low energy density applications, while V‐based materials will remain important for high energy density needs.

Sodium iron sulfate is increasingly becoming a focus of research and application among sulfate cathode materials due to its unique advantages. Both sodium iron sulfate and sodium iron phosphate pyrophosphate offer benefits such as lower cost, medium energy density, and environmental friendliness. Compared to sodium iron phosphate pyrophosphate, sodium iron sulfate faces a technical challenge in its water absorption. At high temperatures, this problem worsens, leading to low‐capacity and iron ion leaching, harming the stability of the electrode–electrolyte interface. Despite this, the relatively low cost of sodium iron sulfate makes it suitable for small power applications, such as two‐wheeled vehicles, while NFPP may be more appropriate for energy storage needs. Notably, there is no consensus on whether sodium iron sulfate or sodium iron phosphate pyrophosphate is superior in commercial applications. NFPP and Na₂Fe₂(SO₄)₃ are expected to follow the commercially successful LiFePO₄ preparation method. As technology advances, both materials are expected to achieve their commercialization potential in appropriate applications.

Compared with other cathode materials for SIBs, NFPP exhibits a unique balance of safety, cost, and cycling stability. Layered oxide cathodes generally offer higher energy density and better electronic conductivity; however, they suffer from structural instability, air sensitivity, and safety concerns, especially under high voltage and long‐term cycling conditions. Prussian blue analogs, on the other hand, are attractive due to their low cost and facile synthesis, but their performance is often limited by structural defects, lattice water, and relatively low volumetric energy density. In contrast, NFPP, as a polyanion‐type cathode material, benefits from a robust 3D framework, high thermal stability, and excellent cycling durability, making it particularly suitable for large‐scale energy storage applications where safety and lifespan are critical. However, its relatively low electronic conductivity and the presence of impurities remain key challenges. NFPP occupies a distinctive position among sodium‐ion cathodes, offering a compelling trade‐off between performance, safety, and cost, especially for grid‐scale energy storage systems.

5.2.2. Compaction Density

Compaction density is a critical parameter for the commercialization of NFPP, significantly affecting its electrochemical performance. A higher compaction density allows for more active material, thus increasing the energy density of SIBs. Nonetheless, excessively high compaction density can reduce specific capacity and cycling stability. Therefore, understanding and controlling compaction density is essential for optimal performance. Reports indicate that the compaction density of NFPP in commercial products has reached 2.2 g/cm³, with the particles remaining intact and processable. Another study reported a sample of NFPP, with a compacted density of 1.92 g/cm³ [25]. Surface coating and morphology control can enhance the electrochemical properties of phosphate‐based polyanionic compounds [8]. Nonetheless, these strategies ultimately impact the performance of actual SIBs by reducing the electrode capacity or compact density, compromising the energy and volume density of the final product. To enhance the performance of NFPP full cells without affecting compaction density, greater attention should be given to modification strategies such as electrolyte optimization and element doping.

In addition to electrochemical performance, practical deployment of NFPP cathodes requires careful consideration of tap density and slurry processability. Phosphate‐based polyanion materials, including NFPP, typically rely on carbon coating or carbon compositing to improve electronic conductivity; however, this often leads to reduced tap density due to the low intrinsic density and porous nature of carbon. A low tap density directly limits the achievable volumetric energy density, which is a critical parameter for commercial SIBs. Moreover, synthesis routes that produce highly porous or nanosized particles, while beneficial for rate capability, may further decrease packing efficiency and increase electrode tortuosity. From a processing standpoint, particle morphology, size distribution, and surface chemistry strongly influence slurry rheology and electrode fabrication. For instance, excessive surface area and irregular particle shapes can increase binder demand and slurry viscosity, leading to poor coatability and inhomogeneous electrode films. Conversely, spherical secondary particles (e.g., obtained via spray drying) with controlled size distribution can improve flowability, packing density, and electrode uniformity. Therefore, future development of NFPP materials should aim to balance nanoscale electrochemical advantages with microscale engineering strategies, such as secondary particle design and optimized carbon distribution, to simultaneously achieve high gravimetric and volumetric performance while maintaining good manufacturability.

5.2.3. Presodiation Strategy

In the practical application of NFPP full batteries, the presodiation strategy is crucial for enhancing the electrochemical performance of SIFCs, as it compensates for the initial loss of sodium ions and extends battery cycle life. Presodiation techniques include three main strategies: electrochemical presodiation process, chemical presodiation strategies, and the addition of additives to the cathode material. Electrochemical presodiation has advanced in laboratory settings for presodiation electrodes, but commercialization remains challenging due to complexity and performance limitations. Chemical presodiation is effective for sodium‐ion prestorage; however, their industrial viability is constrained by the limited reduction capacity of the solution and the time‐consuming treatment process.

To compare, cathode additives have been widely utilized in commercial LIBs, providing valuable insights into the presodiation strategies of NFPP. In a recent study, introducing commercial Na₂C₂O₄ into the NFPP cathode increased the initial capacity from 111 to 140 mAh g⁻¹ [109]. At a current density of 0.1C, the reversible specific capacity of the HC||NFPP increased from 83 to 103 mAh g⁻¹, resulting in an energy density of 305 W h kg⁻¹ at 0.2C. After 400 stable cycles, the capacity retention remains 81% at 1C. Cathode additives can directly enhance the energy density of SIB systems, offering significant potential for commercialization. Despite the importance of presodiation strategies for NFPP, relevant studies remain relatively limited. Therefore, developing cathode additives suitable for NFPP cathode materials with excellent stability in air and low cost is crucial for their commercialization process.

We believe that NFPP will become increasingly prominent, with more companies engaging in its commercialization. While one material may eventually dominate, the likely trend is not the dominance or elimination of a single cathode material, but rather complementary roles and differentiated competition among various cathode materials. Each cathode material has unique strengths and limitations, and the most suitable choice should be based on a comprehensive assessment of performance needs and the specific application environment.

5.2.4. Cost Analysis

The cost of batteries is heavily influenced by cathode materials, which is crucial in determining their commercialization scalability. The absence of costless cathode materials was a barrier to SIBs commercialization in the past. Nonetheless, NFPP material cathode has made SIBs commercialization feasible due to its affordability and adaptability to large‐scale production.

The cost of SIBs is influenced by various factors, including raw material costs, electrode electrochemical performance, and packaging expenses. For instance, the cathode accounts for 21.51% of the cost in SIBs with polyanion cathodes compared to 41.17% in LiFePO₄ cells (Figure 10d) [8]. The use of cheaper materials is a key reason for the cost advantage in commercializing SIBs, especially for NFPP cathodes. Despite the low cost of its raw materials, NFPP incurs significantly higher expenses in manufacturing, packaging, and ancillary components due to its inferior volumetric capacity density relative to LIBs. Indeed, these combined costs for SIBs are nearly double those of LIBs (38.6% vs. 19.7%). The cathode cost contribution is also highly dependent on technological readiness, though ongoing advancements in the NFPP supply chain and fabrication processes promise continued cost reductions alongside scaling. Although current low Li₂CO₃ prices challenge SIB competitiveness, the inherent cost volatility of lithium reinforces NFPP as the superior long‐term solution for grid‐scale storage. Any substantial increase in lithium carbonate prices would therefore catalyze a decisive market shift toward NFPP.

5.2.5. TRL Comparison

As illustrated in Figure 12a, LiFePO₄ has reached Technology Readiness Level (TRL) 9, indicating full technological maturity from material design and scalable synthesis to electrode fabrication, cell integration, and commercial deployment. Its success stems not only from the intrinsic structural robustness of the olivine framework and superior thermal stability but also from long‐term engineering optimization, including carbon coating and particle downsizing strategies, standardized slurry and electrode processing, highly compatible pairing with graphite anodes, and the establishment of a complete and resilient supply chain. Consequently, LiFePO₄ has achieved strong cross‐scale integration across materials, cells, battery systems, and end‐use scenarios, with validation at the GW–TWh scale in electric mobility and stationary storage applications, thereby substantially mitigating both technical and commercial risks.

FIGURE 12.

The comparison of TRL for LiFePO₄ and Na₄Fe₃(PO₄)₂P₂O₇.

In contrast, Na₄Fe₃(PO₄)₂P₂O₇ is currently positioned at TRL 4–6. This level reflects successful laboratory validation and prototype demonstration in relevant environments, yet it remains short of large‐scale commercial deployment. At the materials level, although its 3D open framework enables favorable Na⁺ diffusion kinetics and structural stability, further advances are required in enhancing electronic conductivity, achieving reproducible and scalable synthesis with controlled particle morphology, and ensuring stability under high‐voltage operation. At the electrode and full‐cell level, challenges persist in optimizing compatibility with HC anodes, stabilizing the electrolyte window, suppressing interfacial side reactions, and mitigating polarization under high mass loading. From an engineering perspective, industrial‐scale consistency, energy‐efficient production routes, comprehensive cost modeling, and long‐term cycling reliability under practical conditions remain to be systematically validated—key factors limiting its progression beyond TRL 6.

Importantly, the developmental trajectory of Na₄Fe₃(PO₄)₂P₂O₇ may draw instructive parallels from that of LiFePO₄. The commercialization of LiFePO₄ demonstrates that once a polyanionic cathode system achieves a balanced interplay among structural stability, safety, and cost, sustained advances in interface engineering and electrode architecture can enable rapid TRL acceleration. For Na₄Fe₃(PO₄)₂P₂O₇, the intrinsic advantages of earth‐abundant iron and sodium resources, combined with its favorable safety characteristics, align particularly well with the demands of large‐scale stationary energy storage, where cost efficiency, durability, and resource sustainability are prioritized over extreme energy density. In such grid‐oriented applications—including peak shaving and distributed storage—the material's technoeconomic profile is strategically compelling.

Therefore, the current TRL gap between Na₄Fe₃(PO₄)₂P₂O₇ and LiFePO₄ does not fundamentally arise from insurmountable materials‐science limitations, but rather from the absence of fully established engineering frameworks and industrial ecosystems. With continued progress in standardized large‐scale synthesis, holistic full‐cell design, long‐term reliability assessment, and pilot‐scale demonstration projects, Na₄Fe₃(PO₄)₂P₂O₇ is well positioned to transition from laboratory validation to system‐level demonstration and early‐stage commercialization. Such advancement will be pivotal in determining whether this cost‐effective sodium‐ion cathode can replicate—or in specific stationary storage scenarios, even surpass—the mature development pathway exemplified by LiFePO₄.

6. Summary and Outlooks

From a broader perspective, SIB cathodes can be broadly classified into layered oxides, polyanion‐type compounds, and Prussian blue analogs, each with distinct advantages and limitations. Among them, NFPP, as a representative polyanion‐type material, exhibits several intrinsic merits, including a robust 3D framework, high thermal stability, and excellent cycling durability. These features make NFPP particularly attractive for large‐scale energy storage applications, where safety, lifespan, and cost are prioritized over energy density.

NFPP with a NASICON structure has garnered increasing attention as a promising SIB cathode material due to its low cost and excellent electrochemical performance. This review summarizes the research progress on NFPP, focusing on its crystal structure, electrochemical properties, and synthesis methods. Additionally, it provides an in‐depth analysis of modification strategies aimed at enhancing NFPP's electronic conductivity for improved electrochemical performance. The review also examines the development of NFPP full cells and the current state of commercialization. However, NFPP also faces several challenges that hinder its practical deployment. Its intrinsically low electronic conductivity necessitates additional modification strategies, such as carbon coating or compositing, which may increase processing complexity. Moreover, impurity phase formation during synthesis remains difficult to control and can significantly influence electrochemical performance. Compared with layered oxides, NFPP generally exhibits lower energy density, while compared with Prussian blue analogs, its synthesis process is relatively more complex and less flexible. To address these gaps, the following three research priorities are proposed:

Green and cost‐effective synthetic approaches for impurity‐free, large‐scale production need further exploration to enable commercialization. Current synthesis techniques for NFPP material are inadequate to ensure the purity required for practical applications. Additionally, the challenge of fully utilizing or suppressing impurities to achieve high‐performance material remains unresolved and requires further research. By employing customized synthesis conditions, it is possible to investigate the mechanism behind the synergistic interaction of heterostructure.

Machine learning is expected to help predict nonstoichiometric pure phases of NFPP material and assist in doping processes. Traditionally, the electrochemical performance of new systems is verified through repeated experiments, which, although accurate, involve a lengthy trial‐and‐error process. In contrast, machine learning offers a more efficient approach to exploring multiple systems and can provide valuable insights for researchers. By combining DFT calculation and machine learning with various arithmetic, it is possible to predict the properties and performance of the new system rapidly.

A rational NFPP morphology and size design needs more investigation, which can favor electron and ion diffusion paths. Nanostructures can significantly enhance the initial Coulombic efficiency of SIBs by increasing surface area and reaction sites, facilitating ion diffusion, and optimizing the solid electrolyte interphase. Additionally, porous structures further aid by improving ion and electron transport and accommodating volume changes, enhancing overall stability and efficiency. Nonetheless, limited NFPP morphologies have been reported to date. The design of NFPP can be accelerated by taking advantage of the experience of LIBs and other polyanionic cathodes in controlling their morphology. To sum up, the industrialization of NFPP cathode materials is presently at TRL 4–6. Although laboratory validation and prototype testing are established, significant advances in technological maturity and market readiness are necessary. Despite existing hurdles, emerging practical applications will undoubtedly drive continued research momentum.

In summary, although significant progress has been made, the practical application of NFPP is still constrained by several key challenges, including low electronic conductivity, difficulty in impurity control, and limitations in scalable synthesis. Future efforts should focus on developing strategies that simultaneously address these issues, particularly through scalable processing techniques combined with effective structural and compositional optimization. A clear understanding of these challenges will be crucial for advancing NFPP toward real‐world energy storage applications.

Looking forward, future research should focus on developing scalable and cost‐effective synthesis routes, achieving precise control over phase purity and microstructure, and designing synergistic modification strategies to enhance electronic conductivity and sodium‐ion diffusion kinetics. With continued progress, NFPP is expected to play a significant role in next‐generation SIBs, particularly for grid‐scale energy storage and other applications requiring high safety and long service life. NFPP represents a promising candidate for safe, low‐cost, and durable SIBs, particularly in stationary energy storage systems where long‐term reliability is critical. In addition, there are several key directions for the development of NFPP cathodes. First, advanced characterization techniques, particularly in situ/operando methods, are needed to better understand phase evolution and sodium‐ion storage mechanisms during cycling. Second, interface engineering, including surface coating and electrode/electrolyte optimization, is critical for improving interfacial stability and long‐term performance. Third, the development of scalable and cost‐effective synthesis routes with precise control over phase purity and microstructure remains essential for industrial application. At the same time, several challenges must be addressed for practical commercialization, including intrinsically low electronic conductivity, difficulty in impurity control, and the trade‐off between performance and processing cost. Addressing these issues through integrated material design and engineering strategies will be crucial for advancing NFPP toward large‐scale energy storage applications.

Conflicts of Interest

The authors declare no conflicts of interest.

Shen Qinghua, Qin Yonghong, Pang Boyi, Mai Ruilin, Song Jin, Li Huanxin, Fu Chaopeng, Cost‐Effective Na4Fe3(PO4)2P2O7 Cathode Materials for Sodium‐Ion Batteries in Large‐Scale Energy Storage Applications, Small Science 2026, 6, e70316. 10.1002/smsc.70316

Data Availability Statement

Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.