Single crystalline nanorods of Na0.44MnO2 enhanced by reduced graphene oxides as a high rate and high capacity cathode material for sodium-ion batteries

Abstract Sodium-ion battery (SIB) is one of the most promising alternatives to partly substitute for lithium-ion batteries (LIBs), yet the large radius of Na + leads to substantial volume change during sodiation/desodiation processes, resulting in inferior electrochemical performance. Na 0.44 MnO 2 (NMO) possesses S-shaped Na + diffusion pathway that can accommodate insertion/deinsertion of large Na + , though the poor electronic conductivity of NMO limits its ionic transport kinetics. Herein, single crystalline NMO nanorods (NRs) were synthesized, and their electronic conductivity was enhanced by one order of magnitude using reduced graphene oxide (rGO), as quantified by local conductive atomic force microcopy (c-AFM) measurement. When used as a cathode material for SIBs, NMO/rGO nanocomposite exhibits reversible specific capacities of 124mAh/g at 0.2C after 200 cycles and 70.8mAh/g at 15C, which are significantly enhanced over control cathode made of pure NMO NRs. Through detailed electrochemical impedance spectroscopy (EIS) analysis, it was found that NMO/rGO nanocomposite exhibits much reduced Warburg factor, resulting in enhanced Na + diffusion at each discharging voltage platform. This suggests that the enhanced electronic conductivity helps Na + insertion/deinsertion, resulting in improved electrochemical performance, and these insights help us understand how electronic conductivity improves ionic transport kinetics in SIBs.