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 K_sp 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.