No abstract

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 LiPF 6 . 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 Ϫ8 to 10 Ϫ9 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.

No abstract

Interactions of lithium ions with graphite clusters are studied by ab initio methods. Energies, electronic distributions, multipole moments, and molecular orbitals or ground-state clusters are calculated for systems containing up to 32 carbon atoms using density functional theory on geometries optimized with the Austin model 1 (AM1) semiempirical method.These systems are sufficiently large for the study of differential reactivity between edge and central sites. Li binds in out-of-plane locations, preferentially to armchair edge and basal plane sites; while at zigzag edge sites, the binding energy is about 21 kJ/mol lower. Calculations including electron correlation are necessary to detect binding to the basal plane. This binding is not revealed by the semiempirical method. The existence of preferred binding sites is in qualitative agreement with reported kinetic regions for the diffusion of Li in graphite structures. When a second graphite layer is added to the Lit3, system, the interlayer distance increases about 45% with respect to the experimental value in graphite, according to an AM1 optimization. A larger system composed of eight C66 layers with an effective force field bearing the ab initio distribution of charges is studied using molecular-dynamics simulations. The results show a relative stabilization of the interlayer distance with a maximum increase of 23% with respect to those in unlithiated graphite. These values overestimate the experimentally observed increase of about 10%. When the complex Li-tetrahydrofuran (THE') or Lit-(THF), interacts with a single-layer graphite cluster, the distance Li-O from the complex increases due to competing interactions with the carbon lattice; howevei the presence of solvent molecules contributes to stabilize the system. chemical methods, several levels of theoretical complexity are available: semiempirical parametrized models and ab

No abstract