Nanosheets assembling hierarchical starfish-like Cu2–xSe as advanced cathode for rechargeable Mg batteries

Abstract Rechargeable Mg battery is a low-cost and highly safe energy storage technique suitable for large-scale application; however, searching for high-performance cathodes remains a large challenge blocking their development. Non-stoichiometric Cu2–xSe has extraordinarily high conductivity and its unique crystal structure could better accommodate Mg2+ cations. In this work, Cu2–xSe with nanosheets assembling starfish-like hierarchical structure (S-Cu2–xSe) and nanocrystal (C-Cu2–xSe) are synthesized by facile one-step hydrothermal methods and used as cathode for rechargeable Mg batteries. It is observed that S-Cu2–xSe exhibits a high specific capacity of 210 mAh g-1 at 100 mA g-1, an excellent rate capability of 100 mA h g-1 at 1000 mA g-1, and a long-term cycling stability over 300 cycles. Mechanism investigation demonstrates that the Cu2–xSe cathodes experience a two-step magnesiation/demagnesiation process during cycling, in which phase transformation is involved. The high-performance of S-Cu2–xSe is owing to the fast solid-state Mg2+ diffusion in the thin nanosheets and the material integrity of the hierarchical structure. This work not only delivers facile approaches for the construction of high-performance Cu2–xSe Mg cathodes, but also highlights scientific insights for the exploration of conversion-type Mg-storage materials.