A Review of Transition Metal Compounds as Functional Separators for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have great potential for the development of next-generation high-energy-density secondary batteries owing to their high theoretical energy density, active material (sulfur) environmental friendliness, and low cost. However, their application is still impeded by the inherent sluggish kinetics and solubility of intermediate products of the sulfur cathode. Interface design is an important direction to address challenges in the development of Li-S batteries. The modification of the separator has been shown to effectively suppress the shuttling effect of physical hindrance or chemical bonding without affecting the utilization of active materials. This review encompasses the application of nanostructured transition metal oxides (TMOs), transition metal sulfides (TMSs), transition metal nitrides (TMNs), transition metal phosphides (TMPs), such as incorporating functional separators beyond the approach for preparing novel cathodes, and discusses their composites in a new multifunctional barrier layer for Li-S batteries. The objective properties of various metal compounds and the effect of the shuttle effect in particular on the electrochemical performance in Li-S batteries are highlighted, and give an outlook on the promising approaches for the construction of reliable Li-S batteries.