Long-Chain Polysulfide Retention at the Cathode of Li-S Batteries

Lithium-sulfur batteries present a complex interconnected chemistry where the three components anode, electrolyte, and cathode strongly interact with each other. One of the main issues associated with these interactions is the dissolution in the electrolyte solution of part of the, sulfur reduction products (mainly long-chain polysulfides) during the discharge reactions at the cathode. These dissolved species can migrate and react at the anode surface producing undesired insulating films. A potential solution to mitigate this problem is to resort to additional materials which can act as anchors of the soluble species thus avoiding their migration. Density functional theory and ab initio molecular dynamics simulations are employed to investigate the ability of certain substrates to retain long-chain polysulfides (Li2S6 and Li2S8) at their surfaces in the presence of a pure solvent or a lithiated solution. Nanopores of graphene are first tested because the cathode is usually a mix of sulfur and carbon. Then, MoS2 and Mo-doped graphene are evaluated because of the well-known Mo-S affinity. Finally, a material which has been reported successful in experimental studies, MnO2, is analyzed and compared with another oxide surface, Fe2O3. Adsorption energies of the polysulfides to the surfaces and the detailed interactions of the Li ions and S atoms with the substrate are characterized via charge and geometric analyses. Both the Mo-containing materials and the oxides adsorb the polysulfides much more strongly than graphene nanopores do. However, some of these surfaces are found to be excessively reactive. A balance between affinity for S and moderate surface reactivity is found as a promising guideline for designing these materials.