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., Li₂S), indicating a fundamental way for improving both the cycling stability and sulfur utilization. Typically, a TiO₂–Ni₃S₂ heterostructure is prepared by in situ growing TiO₂ nanoparticles on Ni₃S₂ surface and the intimately bonded interfaces are the key for bidirectional catalysis. For reduction, TiO₂ traps while Ni₃S₂ catalytically converts polysulfides. For oxidation, TiO₂ and Ni₃S₂ both show catalytic activity for Li₂S dissolution, refreshing the catalyst surface. The produced sulfur cathode with TiO₂–Ni₃S₂ 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.