Quadra layer core−shell−core−shell structured Co9S8@MnS-NC nanorod composites for enhanced sodium-ion batteries

The dramatic volume expansion of transition metal sulfides has inhibited their widely usage despite of high theoretical capacity. In this work, the Co9S8@MnS-NC composite with novel sandwich-like structure is fabricated by a multi-step reaction. The micro/nano-sized Co9S8@MnS-NC is characteristic with stable hierarchical structure, the inner Co9S8 particles are protected by carbon rod, which is also covered by MnS shell and an external conductive carbon shell. The unique double core-shell structure promotes double protective effect, making it better to adapt to the volume expansion and maintain the structural integrity of the active Co9S8 core. The synergistic effect induced by bimetallic sulfides and N-doped carbon helps to enhance conductivity, cyclic stability and transport efficiency of charge carrier. As a result, the corncob-like Co9S8@MnS-NC delivers a high reversible capacity (444.7 mAh·g−1 at 100 mA·g−1) and a well-designed cyclic stability of 96% after 2000 cycles at 3000 mA g−1. Notably, the matching cathode electrode Na3V2(PO4)3 is assembled with Co9S8@MnS-NC to achieve the full cell with a high-performance reverse capacity (248.7mAh·g−1). The huge improvement of electrochemical activity and structural stability could be attributed to the MnS nanoparticles anchored on Co9S8 and the protective effect of the double carbon layer. This study suggests an effective approach to improve the electrochemical performance of bimetallic sulfide.