Adsorption and diffusion of Li with S on pristine and defected graphene

Li-S batteries are the promising high energy density alternative to current rechargeable battery technologies, particularly since it has been shown that the use of graphene, nanotubes and other nanostructured carbons in the cathode can improve the cyclability. We explore the microscopic interactions between Li_nS and graphene and diffusion of Li ions through pristine and defected graphene in the presence of S using first-principles methods. The introduction of Li weakens the interaction of atomic S with graphene, increasing the height of adsorbed S and leading to the formation of Li_nS clusters. These Li_nS clusters are adsorbed accompanied by charge transfer to the graphene. We find that double vacancies in the graphene are sufficient to allow Li ions to pass through the graphene plane. This is impeded in the presence of S due to the binding of Li to Li_nS clusters, but still can happen for larger clusters. The electronic properties confirm the excellent conductivity of pristine and defected graphene cathodes in contact with Li_nS clusters.