Abstract:

Transition metal sulfides have emerged as a desirable anode material for lithium-ion batteries in recent years. Among them, molybdenum disulfide(MoS₂) has received intensive research attention because of its unique 2D-layered structure, which can provide an effective diffusion path for the intercalation and exfoliation of lithium ions during the electrochemical reaction process and high theoretical specific capacities(670 mA·h/g). However, as an typical semiconductor material, MoS₂ suffers from the inherent low electrical conductivity and large volumetric expansion/shrinkage upon cycling, which will result in poor rate capability and rapid capacity decay that limit its large-scale applications. Much efforts have been devoted to passing these problems by optimizing MoS₂ materials to nanostructures and integrating MoS₂ with other conductive materials. Cobalt sulfide(Co₉S₈) is a metallic transition metal sulfide with relatively higher electrical conductivity but shows inferior electrochemical performance, which is possibly related to its sluggish ion transport kinetics. In this regard, the combination of MoS₂ and Co₉S₈ into rationally designed hybrid architectures may offer synergistic advantages, which manifest overall structural merits over the individual component. Herein, we report the synthesis of uniform MoS₂@Co₉S₈ yolk-shell spheres via solvothermal together with chemical vapor deposition method. The MoS₂ and Co₉S₈ are homogeneously distributed throughout the entire yolk-shell spheres, which leads to a faster electron and Li-ion transport and effectively improves the cycling stability and reversible capacity. The void space in the yolk-shell structure can efficiently cushion the volume change during the discharge/charge process. The uniform mixing of the Co₉S₈ and MoS₂ nanocrystals can also facilitate rapid ion/electron transportation and help to stabilize the cycling performance. Therefore, the as-prepared MoS₂@Co₉S₈ yolk-shell spheres deliver superior Li storage performance with good rate capability and stable cycling performance. Especially for the lithium ion battery application, the MoS₂@Co₉S₈ yolk-shell spheres show a remarkably reversible capacity of about 631.5 mA·h/g after 500th cycles at a current density of 0.2 A/g.

Key words: Co₉S₈, MoS₂, MoS₂@Co₉S₈, lithium ion batteries, anode material, yolk-shell structure