Bidirectional Catalysts for Liquid–Solid Redox Conversion in Lithium–Sulfur Batteries

Abstract

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.