Theoretically revealing the major liquid-to-solid phase conversion mechanism of the second plateau in lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries are considered promising new energy storage devices due to their high theoretical energy density, environmental friendliness, and low cost. The sluggish reduction kinetics during the second half of the discharge hampers the practical applications of Li-S batteries. Although the reaction kinetics has been improved by various advanced cathode materials, the complex mechanism of the sulfur reduction reaction (SRR) leaves many obstacles to guide people in improving Li-S battery performances. An in-depth investigation of its electrocatalytic mechanism is an vital link to guide the design and application of cathode materials. Herein, the chemical mechanism from Li₂S₃ to Li₂S₂/Li₂S will be revealed, which also means that SRR from liquid-phase polysulfides to solid-phase polysulfides will be unveiled. An electrocatalytic model based on systematic density-functional theory calculations was developed using single-atom catalysts involving Ti, V, Fe, Co, and Ni as the cathodic catalytic materials. Intermediate products *LiS and *LiS₂ are used as descriptors to predict reaction pathways, rate-determining steps and overpotentials. This work explains the conversion mechanism of soluble to insoluble polysulfides at the late discharge stage and guides for the design of advanced high-performance lithium-sulfur battery electrocatalysts.