Abstract: Sodium-ion batteries (SIBs) have been considered as a promising electrochemical system for large-scale energy storage due to their advantages of abundant sodium reserves, low cost, and reliable safety. As a typical polyanionic-type cathode material, Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) is regarded as an ideal cathode material for SIBs, featuring a stable three-dimensional NASICON-type framework that ensures robust structural integrity, distinct working voltage (~3.1 V), outstanding structural reversibility, and overall low-cost iron-based chemistry. Consequently, NFPP has attracted widespread attention from both academic and industrial communities. Nevertheless, NFPP still faces several intrinsic challenges that hinder its practical application, such as insufficient electronic conductivity, sluggish Na⁺ migration kinetics within the crystal lattice, sensitivity to moisture and atmospheric gases, and relatively low tap density. This review systematically summarizes the recent progress on NFPP cathode material for SIBs. Based on clarifying the fundamental relationship between its crystal/electronic structure and electrochemical performance, the regulatory mechanisms of various synthesis methods-including solid-phase reaction, sol-gel, electrospinning, and spray drying processes-on its microstructures, morphologies, and phase purity are discussed in detail. Furthermore, the synergistic mechanisms of advanced modification strategies are comprehensively analyzed, with a focus on elemental doping, carbon decoration, and non-stoichiometric design. Specifically, rational elemental doping can tailor the electronic structure and narrow the bandgap to enhance electronic conductivity, while the induced lattice distortion can simultaneously optimize Na⁺ diffusion pathways, thus enhancing electrochemical/ionic transport efficiency. Carbon-based compositing, through methods such as in situ carbon coating or hybridization with conductive carbons (e.g., carbon nanotube or graphene), constructs continuous conductive networks to facilitate electron transport and acts as a protective barrier against environmental degradation. Additionally, non-stoichiometric engineering can precisely regulate crystal defects and Na⁺ site occupancy, thereby enhancing transport kinetics and structural robustness during the repeated sodiation/desodiation process. Finally, future research directions including multi-scale architecture design, artificial intelligence-accelerated development, wide-temperature (especially low-temperature) operation, and industrial applications are prospected, aiming to provide theoretical guidance for the development of high-performance, cost-effective sodium-ion batteries based on NFPP cathodes.

Key words: Sodium-ion battery, Cathode materials, Na₄Fe₃(PO₄)₂P₂O₇, Electrochemical performance