Role of the solvent in the activation of Li₂S as cathode material: a DFT study

Lithium-sulfur batteries are considered one of the possible next-generation energy-storage solutions, but to be commercially available many drawbacks have yet to be solved. One solution with great potentiality is the use of lithium sulfide as cathode material since it can be coupled to Li-free anodes, such as graphite, Si or Sn. Nevertheless, Li₂S, like sulfur, is electronically and ionically insulating, with a high activation potential for its initial oxidation step. To overcome this issue, different strategies have been explored, one of them being the use of catalytic surfaces. In the present article, we study using first principles calculations the effect of the dielectric constant of the solvent on the activation energy of the cleavage reaction of Li₂S on different catalytic surfaces. To the best of our knowledge, this is the first time that such a study is undertaken. We find that the effect of the solvent should be twofold: on one side, it should decrease the interaction between the Li₂S molecule and the surface. On the other side, since the species arising in the dissociation reaction are charged, the solvent should decrease the activation barrier for the dissociation of the Li₂S molecule, when compared with the reaction in vacuum. These theoretical findings are discussed in connection with experimental results from the literature, where the behaviour of the Li-S cathode is studied in different solvents.