Revealing reaction mechanisms of nanoconfined Li₂S: implications for lithium-sulfur batteries

Using Li₂S as an active material and designing nanostructured cathode hosts are considered as promising strategies to improve the performance of lithium-sulfur (Li-S) batteries. In this study, the reaction mechanisms during the delithiation of nanoconfined Li₂S as an active material, represented by a Li₂₀S₁₀ cluster, are examined by first-principles based calculations and analysis. Local reduction and disproportionation reactions can be observed although the overall delithiation process is an oxidation reaction. Long-chain polysulfides can form as intermediate products; however they may bind to insoluble S^2-via Li atoms as mediators. Activating the charging process only requires an overpotential of 0.37 V if using Li₂₀S₁₀ as the active material. Sulfur allotropes longer than cyclo-S₈ are observed at the end of the charge process. Although the discharge voltage of Li₂₀S₁₀ is only 1.27 V, it can still deliver an appreciable theoretical energy density of 1480 W h kg^-1. This study also suggests that hole polarons, in Li₂₀S₁₀ and intermediate products, can serve as carriers to facilitate charge transport. This work provides new insights toward revealing the detailed reaction mechanisms of nanoconfined Li₂S as an active material in the Li-S battery cathode.