Alkaline Excess Strategy to NASICON-Type Compounds towards Higher-Capacity Battery Electrodes

The sodium excess strategy was applied to NASICON-type compound to form Na 3+x M 3+ 2− x M ' 2+ x (PO 4 ) 3 , towards a new family of sodium-ion battery cathode materials with increased reversible capacity. Here we chose M 3+ = V 3+ and M ' 2+ = Fe 2+ and successfully synthesized sodium enriched mixed valence compounds up to x = 0.5, i.e., Na 3.5 V 3+ 1.5 Fe 2+ 0.5 (PO 4 ) 3 . It shows reversible Na + (de)intercalation centred at 3.4 V vs. Na/Na + , involving the Fe 3+ /Fe 2+ , V 4+ /V 3+ and V 5+ /V 4+ redox couples, demonstrating that the excess sodium contributes to the electrode reaction.