A presodiation strategy to extend the cycle life of iron phosphate sodium-ion full cell

Loss of active sodium (Na) during the initial charge process is inevitable in Na-ion batteries due to the formation of solid electrolyte interphase and irreversible capacities of anodes, resulting in a low energy density and limiting cycle life at the full-cell level. Herein, we report a versatile and straightforward physical presodiation strategy to solve these issues. Directly adding sodium oxalate (Na2C2O4, denoted NCO) in the cathode electrode can provide additional Na (399 mAh g−1) during initial charging. A full cell using Na4Fe3(PO4)2P2O7 with 5 wt% NCO and hard carbon anode provides 19% higher capacity than that of hard carbon||Na4Fe3(PO4)2P2O7. The extended cycle life of full cells has been demonstrated, which can stably cycle 400th with a capacity retention rate of 81%, much higher than hard carbon||Na4Fe3(PO4)2P2O7 full cell (27% capacity retention rate). Good rate capability and high energy density have been demonstrated.