Aging mechanism of MoS₂ nanosheets confined in N-doped mesoporous carbon spheres for sodium-ion batteries

N-doped hollow carbon nanospheres (NHCS) with mesoporous carbon shells is selected as nanoreactors and encapsulated with few-layered MoS₂ nanosheets to monitor its electrochemical properties and structural conversion in long-term cycling. Mesoporous walls of NHCS can facilitate electrolyte penetration and provide conductive shells for superior mass diffusion and charge transfer. When serving as SIB anodes, MoS₂@NHCS has a capacity value of 371 mAh g^-1 at 1 A g^-1 with a capacity retention of 94.9% after 100 cycles which is higher than powder form MoS₂ and is among the highest values of earlier reported MoS₂ electrodes. The origin of gradual capacity decay of MoS₂@NHCS anodes in-between 50 to 150 cycles has been investigated. Transmission electron microscopy and ex-situ X-ray diffraction have shown significant morphology change of space-confined MoS₂ and the production of MoO₃ together with oxygen-deficient MoO_x after continuous charge/discharge test. After long-term cycle test of MoS₂@NHCS, the ratios of Mo⁶⁺ and Mo⁵⁺ increased significantly with the decreasing of Mo⁴⁺ ratio, while the sulfur from MoS₂ transformed from S²⁻ to SO₄ ²⁻. This work highlight the phase transformation and composition change of MoS₂ for MoS₂-based anode in long-duration charge/discharge tests.