Synthesis and characterization of Na 0.44 MnO 2 nanorods/graphene composite as cathode materials for sodium-ion batteries

Na 0.44 MnO 2 nanorods have been prepared by a hydrothermal method. The experimental parameters have been systematically investigated and optimized. The results show that Na 0.44 MnO 2 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 0.44 MnO 2 /graphene composite. The initial discharge capacity of Na 0.44 MnO 2 /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 0.44 MnO 2 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 0.44 MnO 2 /graphene composite are about 89 and 78 mA·h/g, respectively, indicating outstanding rate capability.