Ab initio study on interaction and stability of lithium-doped amorphous carbons

For the understanding of the Li storage mechanism of amorphous carbon materials utilized in Li-ion batteries, the interaction and relative stability of Li-doped aromatic carbon complexes were studied using the at? initio molecular orbital method. From the optimized geometry and the electronic structure of Li-doped pyrene (C16H10) and ovalene (C32H14) complexes, we have obtained the following results: (i) There exists not only ionic but also covalent character between Li dopants and an aromatic carbon molecule. The latter character originates in the overlap between Li 2p and carbon pi atomic orbitals and is prominent at the edge sites. (ii) The energy barrier for a Li dopant to travel from one site to others is so small that Li dopants in amorphous carbons exchange at room temperature with certain probabilities. We think many possible sites of amorphous carbons can be used efficiently through this exchange, realizing the high capacity in amorphous carbon materials. (iii) The substituted group affects the stability of a Li at the edges of a carbon plane. A hydroxyl group stabilizes the Li dopant through the electrostatic attraction between the ti and oxygen atoms, while a methyl group gives no significant effect. This stabilization by a hydroxyl group could increase the capacity through an increase in the number of Li storage sites. (C) 1999 The Electrochemical Society. S0013-4651(98)07-048-X. All rights reserved.