Nanosheets-assembled hierarchical NiCo(CO3)(OH)2 microspheres with superhydrophilicity and enhanced battery-supercapacitor properties

Battery-type NiCo-based compounds have attracted extensive interest owing to their high theoretical capacity, which are increasingly being used for high energy density supercapacitor. However, the sluggish ions-diffusion/transferability between NiCo-based electrodes and electrolyte limit the achieved energy density. Hydrophilicity is one of the key factors affecting the diffusion resistance for aqueous electrolyte, which is important for electrochemical reactions taking place in aqueous media. Herein, for the first time, hierarchical NiCo(CO3)(OH)2 microspheres assembled from ultrathin nanosheets with superhydrophilic surface/interface and fast ions-diffusion/transferability was synthesized by decorating with electrochemically active oxygen groups in the forms of conjugated carbonyl (-C=O) and hydroxyl (-OH). Benefiting from the superhydrophilicity merit, NiCo(CO3)(OH)2 exhibits excellent electrochemical reactions kinetics and superior energy storage performance. Impressively, the NiCo(CO3)(OH)2 exhibits unprecedented specific areal capacitance (7.5 F cm−2 at 2 mA cm−2), which outperforms state-of-art NiCo-based electrode. An asymmetric supercapacitor assembled with the optimized NiCo(CO3)(OH)2 electrode delivers a high energy density of 85.8 W hkg−1 and excellent cycling lifespan with 91.5 % capacitance retention after 7000 cycles. The superior energy storage performance is mainly attributed to the superhydrophilicity of NiCo(CO3)(OH)2 that significantly improve the faradaic redox reaction.