Synthesis and characterization of Na_0.44MnO₂ nanorods/graphene composite as cathode materials for sodium-ion batteries

Na_0.44MnO₂ nanorods have been prepared by a hydrothermal method. The experimental parameters have been systematically investigated and optimized. The results show that Na_0.44MnO₂ nanorods obtained via the hydrothermal treatment at 200 °C for 16 h show the best electrochemical properties, which deliver the high initial discharge capacity of 110.7 mA·h/g at 50 mA/g in potential window 2.0–4.0 V To further improve their electrochemical properties, a ball milling process with graphene has been carried out to obtain Na_0.44MnO₂/graphene composite. The initial discharge capacity of Na_0.44MnO₂/graphene composite is 106.9 mA·h/g at a current density of 50 mA/g. After 100 cycles, the residual discharge capacity is 91.8 mA·h/g and the capacity retention rate is 85.9%, which is much higher than that of pristine Na_0.44MnO₂ nanorods (74.7%) at the same condition. What is more, when the current density reaches 500 and 1000 mA/g, the corresponding discharge capacities of Na_0.44MnO₂/graphene composite are about 89 and 78 mA·h/g, respectively, indicating outstanding rate capability.