Computational Studies of Lithium Intercalation in Model Graphite in the Presence of Tetrahydrofuran
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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 Li+C32 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 C46 layers with an effective force field bearing the ab initio distribution of charges is studied using molecular-dynamics simulations. The results show a relative slabilization 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 (THF) or Li+-(THF)2 interacts with a single-layer graphite cluster, the distance Li-O from the complex increases due to competing interactions with the carbon lattice; however, the presence of solvent molecules contributes to stabilize the system.
