CuS-based composite for high-performance magnesium storage with a clarified charge-discharge mechanism

Rechargeable magnesium batteries (rMBs) are regarded as an alternative to lithium-ion batteries but are hindered by low cycling stability and sluggish kinetics at present. Herein, we report a sandwich-structured CuS/hNCNC@rGO cathode for rMBs by immobilizing CuS nanoparticles on the hierarchical nitrogen-doped carbon nanocages (hNCNCs) followed by wrapping the reduced graphene oxide (rGO). The charge/discharge mechanism is established based on the unique mixed-valence CuS double-salt structure, i.e., Cu2+S22−⋅Cu+2S2−, which was neglected in the previous studies of CuS-based rMBs. The cathode achieves a high reversible specific capacity of 285 mAh g−1 at 100 mA g−1, an outstanding rate capability of 158 mAh g−1 at 1,000 mA g−1, and ultralong cycling stability at room temperature, superior to most reported cathode materials to date. This study provides a fundamental understanding on the Mg storage mechanism of CuS and suggests a promising strategy for constructing high-performance conversion-type cathodes for rMBs.