Bidirectional Catalysts for Liquid-Solid Redox Conversion in Lithium-Sulfur Batteries.

Accelerated conversion by catalysis is a promising way to inhibit shuttling of soluble polysulfides in lithium-sulfur (Li-S) batteries, but most of the reported catalysts work only for one direction sulfur reaction (reduction or oxidation), which is still not a root solution since fast cycled use of sulfur species is not finally realized. A bidirectional catalyst design, oxide-sulfide heterostructure, is proposed to accelerate both reduction of soluble polysulfides and oxidation of insoluble discharge products (e.g., Li2 S), indicating a fundamental way for improving both the cycling stability and sulfur utilization. Typically, a TiO2 -Ni3 S2 heterostructure is prepared by in situ growing TiO2 nanoparticles on Ni3 S2 surface and the intimately bonded interfaces are the key for bidirectional catalysis. For reduction, TiO2 traps while Ni3 S2 catalytically converts polysulfides. For oxidation, TiO2 and Ni3 S2 both show catalytic activity for Li2 S dissolution, refreshing the catalyst surface. The produced sulfur cathode with TiO2 -Ni3 S2 delivers a low capacity decay of 0.038% per cycle for 900 cycles at 0.5C and specially, with a sulfur loading of 3.9 mg cm-2 , achieves a high capacity retention of 65% over 500 cycles at 0.3C. This work unlocks how a bidirectional catalyst works for boosting Li-S batteries approaching practical uses.