Interaction between functionalized graphene and sulfur compounds in a lithium-sulfur battery - a density functional theory investigation.

Lithium-sulfur (Li-S) batteries are emerging as one of the promising candidates for next generation rechargeable batteries. However, dissolution of lithium polysulfides in the liquid electrolyte, low electrical conductivity of sulfur and large volume change during electrochemical cycling are the main technical challenges for practical applications. In this study, a systematic first-principles density functional theory calculation is adopted to understand the interactions between graphene and graphene with oxygen containing functional groups (hydroxyl, epoxy and carboxyl groups) and sulphur (S) and long chain lithium polysulfides (LiS and LiS). We find the adsorption is dominated by different mechanisms in sulphur and lithium polysulfides, van der Waals attraction and formation of coordinate covalent Li-O bonds. The adsorption strength is dependent on the inter-layer distance and electron rich functional groups. Through these mechanisms, sulphur and lithium polysulfides can be successfully retained in porous graphene, leading to improved conductivity and charge transfer in the cathode of Li-S batteries.