Theoretical study of lithium intercalated graphite

Abstract

We have performed a series of calculations on small models (number of atoms ranging from 10 to 34) of graphite and lithium intercalated graphite (LIG) at the UHF level with a minimal basis set for the valence electrons and an effective core potential for the core electrons (CEP-4G), where the basis and the CEP is optimal for free atoms. We have shown that small model hosts, such as C10 (Bernal i.e., AB) and C12 (primitive hexagonal, i.e., AA), enable us to make several predictions regarding LIG. Firstly, lithium looses its valence electron upon entering either type of host lattice and eventually falls into a body-centered position in an AA host lattice. Secondly, lithium strongly destabilizes the AB lattice, while it strongly stabilizes the AA lattice. Thirdly, the barrier for site hopping in the limit of infinite dilution (Ea) can be estimated along with a related quantity which we call the hilltop energy (see text). Further, we have shown that by building up to host models no larger than C32 (AA) we can make a better estimate of Ea (0.72 eV), determine that the dynamics of Li (within any two-dimensional solvated sheet) is largely determined by ionic interactions with screening from adjacent carbon layers effecting an approximately 20% reduction of naked two-dimensional Li Coulombic forces, and give a reasonable indication of how much energy is liberated as Li is moved from infinity to a vacant site in unsaturated LIG (1.1±0.7 eV).