A Comprehensive Review of Na4Fe3(PO4)2P2O7 Cathode Materials for Sodium-Ion Batteries: From Crystal Structure and Phase Purification to Modification Strategies Progress

Na4Fe3(PO4)2P2O7 (NFPP), an iron-based mixed phosphate–pyrophosphate cathode material, has emerged as one of the most commercially promising candidates for large-scale sodium-ion battery (SIB) energy storage applications. Its exceptional characteristics—an ultralow volume change of less than 4% during Na+ de/intercalation, a three-dimensional open framework enabling rapid ionic diffusion, and the use of earth-abundant, low-cost iron as the redox center—collectively deliver a unique combination of structural stability, rate capability, and economic viability. However, the fundamental challenge of phase-purity control, arising from the three-phase thermodynamic competition among NFPP, electrochemically inert maricite-NaFePO4, and Na2FeP2O7 during synthesis, critically limits its electrochemical performance. This review provides a systematic overview of NFPP research progress from 2012 to 2026, covering crystal structure and sodium storage mechanisms, synthesis methodologies, and—most critically—Phase Adjustment and modification strategies including non-stoichiometric regulation, defect engineering, elemental doping, anionic substitution, and heterostructure design. Mechanistic insights into how each strategy addresses the phase-purity challenge and enhances electrochemical kinetics are critically examined. Industrialization progress, full-cell performance evaluation, cost analysis, and future research directions toward practical deployment are also discussed.