CoSe hollow microspheres, nano-polyhedra and nanorods as pseudocapacitive Mg-storage materials with fast solid-state Mg diffusion kinetics.

CoSe materials with different nanostructures are used as pseudocapacitive Mg-storage cathodes, which exhibit fast solid-state Mg ions diffusion kinetics. In this work, CoSe with different nanostructures including hollow microspheres (H-CoSe), nano-polyhedra (P-CoSe) and nanorods (R-CoSe) are fabricated by using facile one-step hydrothermal methods, and used as pseudocapacitive electrodes for rechargeable Mg batteries. It is observed that R-CoSe exhibits the highest reversible capacity of 233 mA h g at 50 mA g and an excellent rate capability of 116 mA h g at 500 mA g, ascribing to the 1D nanorod structure which facilitates the solid-state Mg diffusion. Benefitting from the stable hierarchical structure, H-CoSe exhibits a superior long-term cycling stability of 350 cycles. A mechanism study indicates that the redox reaction reversibly occurs between CoSe and metallic Co. Further investigation demonstrates that the fast solid-state Mg diffusion kinetics and surface-controlled pseudocapacitive behavior enhance the electrochemical performance. This work highlights a novel and efficient Mg-storage strategy of using pseudocapacitive materials, and the performance and solid-state Mg diffusion kinetics of CoSe could be optimized by rational structural tailoring.