Zero-strain Na4Fe7(PO4)6 as a novel cathode material for sodium-ion batteries.

We report for the first time a zero-strain cathode, Na4Fe7(PO4)6, for sodium-ion batteries (SIBs). This new iron-based polyanionic cathode delivers a reversible capacity of 66.5 mA h g-1 at 5 mA g-1 with almost 100% capacity retention over 1000 cycles under 200 mA g-1, and the outstanding performance benefits from single-phase-transition processes with a tiny volume change of only ∼0.24%.