Recent developments of Na4M3(PO4)2(P2O7) as the cathode material for alkaline-ion rechargeable batteries: challenges and outlook

Sodium ion batteries (SIB) present one of the most perspective post lithium technology and their progress strongly depends on the development of compounds having the structure which enables fast sodium insertion/deinsertion reactions. Polyanion compounds have been widely investigated as cathode materials for SIBs where they compete effectively to the usually used layered oxides. This survey is focused on the development of specific family of isostructural polyanion phases encompassed by the common chemical formula Na4M3(PO4)2(P2O7). The comprehensive retrospective of their synthesis procedures, the kinetics and mechanism of sodiation/desodiation reactions, based on both experimental and theoretical results, is provided. First, the review summarizes the structural properties of variety of Na4M3(PO4)2(P2O7) compounds in terms of its electrical, vibrational and surface properties. Then, the synthesis methods and sodium/lithium storage performance, of each type of Na4M3(PO4)2(P2O7) compounds, are chronologically presented and discussed. Finally, the strengths and weaknesses of these mixed polyanion cathodes are outlined, with the aim to explain some discrepancies and unclarified issues encountered in the literature. Besides, this survey will make room for future development. It can be very useful for the future design of high-performance mixed polyanionic compounds as cathodes for alkaline-ion rechargeable batteries.