We examine the lithiation behavior of silicon-graphene (Si-Gr) composites using density functional theory calculations. Our calculations demonstrate charge transfer from Li to both Si and C (in graphene); the excess electrons on graphene create an electric field, which attracts Li cations while repelling Si anions and thus results in a distinct alternative Li-Si layering structure near graphene. The interfacial Li ions exhibit substantially higher mobility along the Si/Gr interface in comparison to bulk diffusion in Si; such facile interfacial diffusion could contribute toward high performance anodes with fast charge/discharge rates. However, the presence of graphene tends to have no significant impact on the structural evolution of Si during lithiation, as Li atoms are mostly incorporated in the Si matrix rather than at the Si/Gr interface. Consequently, the theoretical capacity and voltage profile of the Si-Gr composite are predicted to be close to those of pure Si.