Uncovering electrocatalytic conversion mechanisms from Li2S2 to Li2S: Generalization of computational hydrogen electrode

Lithium-sulfur (Li-S) batteries have been strongly considered as promising next-generation batteries due to their significant advantages on theoretical energy density. However, the practical application of Li-S batteries are seriously impeded by the sluggish conversion reaction from Li2S2 to Li2S. The solution of this problem relies on comprehensive understanding of Li2S2 conversion mechanisms. Herein, a theoretical model is established by generalizing the computational hydrogen electrode (CHE) approach. More specifically, Li2S2 conversion is redefined in elementary steps that includes the participation of each electron. Associative and dissociative reaction mechanisms with electrocatalysts are proposed and inspected by the free energy criterion. Single atom catalysts containing Fe, Co, Ni, and V are discussed separately. The effects of applied electrode potential are accurately described by the generalized CHE approach. It is predicted that associative reaction mechanism can provide the highest electrode potential with the existence of Fe single atom catalysts. Analysis on electronic structures further confirms the electrocatalysis activity originates from the chemical bonds between S and transition metal atoms. This work unveils the Li2S2 conversion mechanisms in elementary steps. More importantly, the generalized CHE approach opens a new frontier of exploration on redox reactions in Li-S battery and hopefully other energy storage systems.