Molecular dynamics study of graphite/electrolyte interfaces

Molecular dynamics simulations of graphite-electrolyte interfaces are performed on 3D unit cells with periodic boundary conditions at lithium concentrations between 0 and 17% in the carbon phase. The liquid electrolyte consists of a mixture of cyclic carbonates and LiPF6. Staging phenomena, structural changes in the modeled graphite systems, charge distribution on the atoms, and lithium-ion diffusion coefficients are evaluated as a function of lithium concentration in the solid phase. Transitions between ordered carbon structures are detected in the model systems. Repulsive lithium-lithium interlayer interactions are predominant during the intercalation process. Calculated solid phase diffusion coefficients of lithium ions for a state of charge between 0 and 17% are in the range 10(-8) to 10(-9) cm(2)/s. The maximum increase of graphite interlayer spacing found when the lithium ions are intercalated varies from 6 to 10% depending on the degree of intercalation. An electrostatic double layer is formed between the solid and the electrolyte phase; the average charge at each side of the solid/liquid interface is strongly dependent on the composition and electronic properties of the electrolyte. (C) 2001 The Electrochemical Society.