Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) cathode for sodium-ion batteries: from crystal structure to high-energy-density design and solid-state battery application prospects

Sodium-ion batteries have attracted significant attention as efficient energy storage devices to address contemporary energy challenges. The development of high-performance cathode materials is essential for the large-scale application of sodium-ion batteries. Among various cathode materials, Na₄Fe₃(PO₄)₂(P₂O₇), a typical iron-based polyanion compound, is regarded as one of the most promising sodium-ion cathode materials due to its low cost, excellent air stability, and superior electrochemical performance. However, Na₄Fe₃(PO₄)₂(P₂O₇) faces several limitations, including the presence of inert impurities, low intrinsic electrical conductivity, slow Na⁺ diffusion kinetics, and insufficient energy density, all of which significantly restrict its large-scale application. Although considerable progress has been made in Na₄Fe₃(PO₄)₂(P₂O₇) research, particularly over the past decade, a comprehensive and timely review summarizing the advancements in modification strategies, underlying mechanisms, and application prospects is still lacking. This study first investigates the structural framework and sodium storage mechanisms of Na₄Fe₃(PO₄)₂(P₂O₇)-based cathode materials. It then provides a detailed discussion of the current challenges and the corresponding modification strategies and mechanisms. Furthermore, regarding energy density enhancement, the review focuses on Na₄Fe_3-<i>x</i>Mn_<i>x</i>(PO₄)₂(P₂O₇), a promising candidate with improved application potential, and discusses the issues arising from the incorporation of Mn, along with proposed solutions. Furthermore, we include a detailed discussion on the prospective applications of NFPP-based cathode materials within the realm of solid-state batteries. Finally, the relationship between NFPP modification research and the practical applications of sodium-ion batteries is emphasized, and potential future research directions for pyrophosphate-based cathode materials in the large-scale deployment of sodium-ion batteries are proposed.