Novel structurally-stable Na-rich Na 4 V 2 O 7 cathode material with high reversible capacity by utilization of anion redox activity

A sodium-rich vanadium compound, Na 4 V 2 O 7 , is investigated as a cathode material for sodium-ion batteries, which delivers a high reversible capacity of 194 mA h g −1 after activating to 4.7 V. By limiting the cut-off voltage to 4.4 V, a good capacity retention of 93% after 50 cycles is achieved. The material exhibits a negligible volume variation of 1.04% during Na + (de-)intercalation, demonstrating that Na 4 V 2 O 7 is structurally-stable.