Na4Fe3(PO4)2(P2O7) cathode for sodium ion batteries: From crystal structure to high-energy-density design and solid-state battery application prospect

Abstract

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. Therefore, addressing the challenges of pyrophosphate-based cathode materials through targeted modifications is key to enhancing the practical value of sodium-ion batteries. 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 paper 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 Na4Fe3-xMnx(PO4)2(P2O7), a promising candidate with improved application potential, and discusses the new issues arising from the incorporation of Mn, along with proposed solutions. Finally, the relationship between NFPP modification research and the practical application 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.