Understanding Origin of Voltage Hysteresis in Conversion Reaction for Na Rechargeable Batteries: The Case of Cobalt Oxides

Conversion reaction electrodes offer a high specific capacity in rechargeable batteries by utilizing wider valence states of transition metals than conventional intercalation-based electrodes and have thus been intensively studied in recent years as potential electrode materials for high-energy-density rechargeable batteries. However, several issues related to conversion reactions remain poorly understood, including the polarization or hysteresis during charge/discharge processes. Herein, Co3O4 in Na cells is taken as an example to understand the aforementioned properties. The large hysteresis in charge/discharge profiles is revealed to be due to different electrochemical reaction paths associated with respective charge and discharge processes, which is attributed to the mobility gap among inter-diffusing species in a metal oxide compound during de/sodiation. Furthermore, a Co3O4-graphene nanoplatelet hybrid material is demonstrated to be a promising anode for Na rechargeable batteries, delivering a capacity of 756 mAh g(-1) with a good reversibility and an energy density of 96 Wh kg(-1) (based on the total electrode weight) when combined with a recently reported Na4Fe3(PO4)(2)(P2O7) cathode.