Exploring the Janus Structure to Improve Kinetics in Sulfur Conversion of Li-S Batteries

Lithium-sulfur batteries, which have a larger theoretical capacity compared to lithium-ion batteries, have been attracting attention as a next-generation energy storage system. A typical cathode of a lithium-sulfur battery uses sulfur loaded on a porous or high specific surface area carbon support. During charging and discharging, the electrolyte soluble lithium polysulfide (LiPS) is converted to an electrically insulating solid lithium sulfide (Li₂S), where random deposition of lithium sulfide on the cathode substrate sluggish complete conversion of sulfur. Many previous efforts to introduce metal compounds to strongly adsorb LiPS and/or to promote conversion do not entail morphological control over Li₂S deposition. In this study, we introduce a Janus structure of a metal compound/carbon to simultaneously control the growth of Li₂S as well as promote the adsorption/conversion of LiPS. The Janus particles are prepared by directional thermal deposition of MoO₃ onto CNT-agglomerated spherical particles. Under potentiostatic discharge, we observe that Li₂S is deposited predominantly on MoO₃-coated regions on CNT particles, accompanied by high discharge capacity. As the deposition of Li₂S is microscopically controlled, the diffusion path of ions/charges in the electrode is preserved. Moreover, it is confirmed that the adsorption and catalytic properties of MoO₃ are greatly promoted by its lithiation during the charge/discharge process. Janus particle cathode cells achieve excellent performance, especially under kinetically limited conditions, such as ultra-high C-rates or high cathode sulfur densities.