Lithiation mechanism of sulfur surfaces during discharge of Li–S batteries from quantum chemical calculations†

We present a computational study based on quantum-chemical calculations to investigate the initial lithiation reactions on the (001) surface of α-sulfur. The study aims to explore the possible emerging structures during consecutive lithiation steps and to analyze their reaction enthalpies. Our results show that during the first lithiation reactions, S₈ rings in the lower layers of the (001) surface are preferentially lithiated. In subsequent lithiation steps, we find that S₈ rings on the upper layers, adjacent to previously lithiated molecules, may also undergo lithiation. Once Li₂S₈ dimers are formed, further reactions on the surface can proceed, leading to the formation of Li₂S₈ trimers in a lower/upper/lower layer arrangement or lower-order Li-polysulfides, such as Li₂S₆/Li₂S₂ and Li₂S₅/Li₂S₃. Notably, in contrast to sulfur reduction reactions in the electrolyte, the formation of Li₂S₄/Li₂S₄ does not occur on the (001) surface, likely due to the surface morphology, which prevents complete exposure of S₈ rings to lithium ions. This suggests that surface lithiation predominantly leads to the formation of high-order polysulfides in the early stages of discharge, while the dissolution of these higher-order polysulfides into the electrolyte may facilitate their reduction to Li₂S₄, a process observed experimentally. Our study provides an atomistic mechanism for the discharge process of Li–S batteries with a crystalline α-sulfur cathode, contributing to a deeper understanding of both solid- and liquid-phase reactions during the early discharge stages.