context: Metal sulfides having various micro-/nanostructures and compositions (Fig. 2), such as spherical/non-spherical hollow materials, high surface area/porosity/selectivity structures, hierarchical carbon-based hybrids, and unique functional materials, have been successfully prepared in recent years through various synthetic approaches. These material systems offer the possibility to study the influence of these features on the electrochemical properties of Li–S batteries. Generally, there are two typical strategies to building metal sulfides and sulfur cathode composites: (i) one-step in situ fabrication and (ii) stepwise synthesis, which often involves injection of sulfur into a host via melt/solution-diffusion methods. Synthetic strategies involving the use of metal sulfides to protect Li anodes, modify separators, and engineer interlayers, are more flexible than those related to the use of metal sulfides and sulfur cathode composites, owing to the removal of the sulfur injection process. We consider the fundamental challenges facing Li–S batteries and discuss synthetic strategies to highlight their attractive features. Accordingly, the methods can be broadly categorized into five groups: (i) building buffer space to accommodate volume fluctuations; (ii) enhancing the binding ability for LiPSs; (iii) improving the electronic conductivity of the active material; (iv) increasing the sulfur loading; and (v) designing other specialized functions.
answer: Li