COMPUTATIONAL STUDIES OF LITHIUM INTERCALATION IN MODEL GRAPHITE IN THE PRESENCE OF TETRAHYDROFURAN

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 sy stems containing up to 32 carbon atoms using density functional theory on geometries optimized with the Austin model 1 (AM1) semiempirical m ethod. These systems are sufficiently large for the study of different ial reactivity between edge and central sites. Li+ binds in out-of-pla ne locations, preferentially to armchair edge and basal plane sites; w hile 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 semiem pirical method. The existence of preferred binding sites is in qualita tive agreement with reported kinetic regions for the diffusion of Liin graphite structures. When a second graphite layer is added to the L i-C32 system, the interlayer distance increases about 45% with respect to the experimental value in graphite, according to an AM1 optimizati on. A larger system composed of eight C-66 layers with an effective fo rce field bearing the ab initio distribution of charges is studied usi ng molecular-dynamics simulations. The results show a relative stabili zation of the interlayer distance with a maximum increase of 23% with respect to those in unlithiated graphite. These values overestimate th e experimentally observed increase of about 10%. When the complex Li+- tetrahydrofuran (THF) or Li+-(THF)(2) interacts with a single-layer gr aphite cluster, the distance Li-O from the complex increases due to co mpeting interactions with the carbon lattice; however, the presence of solvent molecules contributes to stabilize the system.