Electrochemical insertion of potassium ions in Na4Fe3(PO4)2P2O7 mixed phosphate

Potassium-ion batteries (KIBs) can offer high-voltage performance and energy density similar to lithium-ion batteries with the added advantages of elemental abundance, materials economy, and efficient K+ intercalation due to smaller radius of solvated ions. This nascent field offers ample room to exploit open framework polyanionic compounds as efficient cathode materials. Due to similar ionic size, Na-based compounds can be employed as suitable cathodes for KIBs. In this work, iron-based mixed phosphate Na4Fe3(PO4)2P2O7 is demonstrated as a robust 3.0 V cathode for potassium-ion batteries. The in-situ carbon coated nanoscale Na4Fe3(PO4)2P2O7 cathode delivers a discharge capacity of ~120 mAh g−1 (i.e., 94% of its theoretical capacity) with excellent capacity retention and rate kinetics. With its three-dimensional open framework having multiple alkali sites, Na4-xFe3(PO4)2P2O7 undergoes a solid-solution Fe3+/Fe2+ redox mechanism acting as an efficient host for K+ (de)insertion. It forms the best Fe-based phospho-polyanionic cathode for potassium-ion batteries. A full cell comprising Na4-xFe3(PO4)2P2O7 cathode and graphite anode demonstrates the potential future application in KIBs. It marks the first demonstration of Na-based mixed polyanionic phosphates as insertion hosts for KIBs, which can be extended to various polyanionic insertion materials.