A new sodium iron phosphate as a stable high-rate cathode material for sodium ion batteries

Low-cost room-temperature sodium-ion batteries (SIBs) are expected to promote the development of stationary energy storage applications. However, due to the large size of Na⁺, most Na⁺ host structures resembling their Li+ counterparts show sluggish ion mobility and destructive volume changes during Na ion (de)intercalation, resulting in unsatisfactory rate and cycling performances. Herein, we report a new type of sodium iron phosphate (Na_0.71Fe_1.07PO₄), which exhibits an extremely small volume change (~ 1%) during desodiation. When applied as a cathode material for SIBs, this new phosphate delivers a capacity of 78 mA·h·g^-1 even at a high rate of 50 C and maintains its capacity over 5, 000 cycles at 20 C. In situ synchrotron characterization disclosed a highly reversible solid-solution mechanism during charging/discharging. The findings are believed to contribute to the development of high-performance batteries based on Earth-abundant elements.