Theoretical Study of Lithium-Doped Polycyclic Aromatic Hydrocarbons

The interactions between carbon layers and lithium atoms are described with a semiempirical molecular orbital method in order to clarify the Li storage mechanism in amorphous carbon (a-C) materials used in Li ion rechargeable batteries. The general electronic and geometric structures of Li-doped a-C materials are investigated using ovalene (C32H14) as a model carbon structure. The following results are obtained. (i) The intercalation of Li atoms proceeds preferentially up to the C6Li configuration, followed by the absorption of Li atoms on the surface of the carbon layer. (ii) The adsorption of Li atoms can occur even on the nearest neighbor site and can form Li cation clusters commensurate with the carbon lattice. (iii) Li atoms located at the acene-edge sites are more stable than those at the phenanthrene-edge sites. This result suggests that the acene-edge structure is favorable for carbon anode materials. (iv) Substitution of the carbon skeleton by heteroatoms such as boron and nitrogen is not effective for Li storage.