Morphology control of CoCO3 crystals and their conversion to mesoporous Co3O4 for alkaline rechargeable batteries application

Abstract

Shape-controlled synthesis of CoCO3 crystals is achieved by a mild template- and surfactant-free solvothermal process. On switching the volume ratio of ethylene glycol (EG) in the mixed solvent, the structures of the CoCO3 crystals change from cantaloupe-like patterns to microcubes. The formation pathway of CoCO3 was discussed from the viewpoint of Ksp and the equilibrium of water ionization. After calcination in air at 500 °C, the as-prepared CoCO3 crystals convert to porous Co3O4 microstructures. When used as negative electrodes of alkaline rechargeable batteries, these Co3O4 samples display high discharge capacities and good cycle stability. The electrochemical reactions occurring on the Co3O4 electrode are investigated by XRD, cyclic voltammetry (CV) and charge–discharge curves. In the activation process for about 20 cycles, Co3O4 transforms to Co(OH)2 in the charged process, then a large discharge capacity is obtained through Faradaic reaction between Co and Co(OH)2. Experimental results indicate that the discharge capacities of the obtained Co3O4 samples are significantly influenced by their surface area and microstructures. Through lowering the calcination temperature to 300 °C, a mesoporous sample with a high BET surface area of 105 m2 g−1 and narrow particle size distribution is obtained. At a current density of 100 mA g−1, the discharge capacity of this Co3O4 sample reaches 490.2 mA h g−1. After 50 cycles it can still achieve 436.9 mA h g−1. Meanwhile, the Co3O4 sample shows enhanced rate performance, indicating great potential application in alkaline rechargeable batteries.