Direct Monitoring of Li₂ S₂ Evolution and Its Influence on the Reversible Capacities of Lithium-Sulfur Batteries

The polysulfide (PS) dissolution and low conductivity of lithium sulfides (Li₂ S) are generally considered the main reasons for limiting the reversible capacity of the lithium-sulfur (Li-S) system. However, as the inevitable intermediate between PSs and Li₂ S, lithium disulfide (Li₂ S₂ ) evolutions are always overlooked. Herein, Li₂ S₂ evolutions are monitored from the operando measurements on the pouch cell level. Results indicate that Li₂ S₂ undergoes slow electrochemical reduction and chemical disproportionation simultaneously during the discharging process, leading to further PS dissolution and Li₂ S generation without capacity contribution. Compared with the fully oxidized Li₂ S, Li₂ S₂ still residues at the end of the charging state. Therefore, instead of the considered Li₂ S and PSs, slow electrochemical conversions and side chemical reactions of Li₂ S₂ are the determining factors in limiting the sulfur utilization, corresponding to the poor reversible capacity of Li-S batteries.