Mg secondary batteries are promising scalable secondary batteries for next-generation energy storage.However,Mg-storage cathode materials are greatly demanded to construct high-performance Mg batteries.Electrochemical conversion reaction provides plenty of cathode options,and strategy for cathode selection and performance optimization is of special significance.In this work,Ni0.85Se with nanostructures of dispersive hexagonal nanosheets (D-Ni0.85Se) and flower-like assembled nanosheets (F-Ni0.85Se)is synthesized and investigated as Mg-storage cathodes.Compared with F-Ni0.85Se,D-Ni0.85Se delivers a higher specific capacity of 168 mAh g-1 at 50 mA g-1 as well as better rate performance,owing to its faster Mg2+ diffusion and lower resistance.D-Ni0.85Se also exhibits a superior cycling stability over 500 cycles.An investigation on mechanism indicates an evolution of Ni0.85Se towards NiSe with cycling,and the Mg-storage reaction occurs between NiSe and metallic Ni0.The present work demonstrates that advanced conversion-type Mg battery cathode materials could be constructed by soft selenide anions,and the electrochemical properties could be manipulated by rational material morphology optimization.