Surface engineering of core-shell MoS2@N-doped carbon spheres as stable and ultra-long lifetime anode for sodium-ion batteries

MoS2 is regarded as a hopeful anode candidate for sodium-ion batteries (SIBs) due to their various merits such as high specific capacity, abundant raw material reserves and low cost. However, their practical application is impeded by unsatisfied cycling ability due to the intense mechanical stress and unstable solid electrolyte interphase (SEI) during Na+ insertion/extraction process. Herein, spherical MoS2@polydopamine derived highly conductive N-doped carbon (NC) shell composites (MoS2@NC) are designed and synthesized to promote the cycling stability. The internal MoS2 core is optimized and restructured from the original micron-sized block to the ultra-fine nanosheets during initial 100-200 cycles, which not only improves the utilization of electrode materials but also shortens the ion transport distance. The outer flexible NC shell effectively maintains the original spherical structure of the overall electrode material and prevents the occurrence of large-scale agglomeration, which is conducive to form a stable SEI layer. Therefore, the core-shell MoS2@NC electrode presents a remarkable cyclic stability and a capable rate performance. Under a high rate of 20 A g-1, the high capacity of 428 mAh g-1 can be acquired after over ultra-long 10,000 cycles without obvious capacity loss. Moreover, the MoS2@NC‖Na3V2(PO4)3 full-cell assembled by employing commercial Na3V2(PO4)3 cathode can achieve a high capacity retention of 91.4% after 250 cycles at 0.4 A g-1. This work reveals the promising prospect of MoS2-based materials as anode of SIBs, and also has some inspirations on the structural design for conversion-type electrode materials.