Effects of Na content on structure and electrochemical performances of NaxMnO2+δ cathode material

Sodium manganese oxides, NaxMnO2+δ (x = 0.4, 0.5, 0.6, 0.7, 1.0; δ = 0-0.3), were synthesized by solid-state reaction routine combined with sol-gel process. The structure, morphology and electrochemical performances of as-prepared samples were characterized by XRD, SEM, CV, EIS and galvanostatic charge/discharge experiments. It is found that Na0.6MnO2+δ and Na0.7MnO2+δ have high discharge capacity and good cycle performance. At a current density of 25 mA/g at the cutoff voltage of 2.0-4.3 V, Na0.6MnO2+δ gives the second discharge capacity of 188 mA·h/g and remains 77.9% of second discharge capacity after 40 cycles. Na0.7MnO2+δ exhibits the second discharge capacity of 176 mA·h/g and shows better cyclic stability; the capacity retention after 40 cycles is close to 85.5%. Even when the current density increases to 250 mA/g, the discharge capacity of Na0.7MnO2+δ still approaches to 107 mA·h/g after 40 cycles.