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

For the understanding of the Li storage mechanism of amorphous carbon mater ials utilized in Li-ion batteries, the interaction and relative stability o f Li-doped aromatic carbon complexes were studied using the at? initio mole cular orbital method. From the optimized geometry and the electronic struct ure of Li-doped pyrene (C16H10) and ovalene (C32H14) complexes, we have obt ained the following results: (i) There exists not only ionic but also coval ent character between Li dopants and an aromatic carbon molecule. The latte r character originates in the overlap between Li 2p and carbon pi atomic or bitals 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 am orphous carbons exchange at room temperature with certain probabilities. We think many possible sites of amorphous carbons can be used efficiently thr ough this exchange, realizing the high capacity in amorphous carbon materia ls. (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 el ectrostatic attraction between the ti and oxygen atoms, while a methyl grou p gives no significant effect. This stabilization by a hydroxyl group could increase the capacity through an increase in the number of Li storage site s. (C) 1999 The Electrochemical Society. S0013-4651(98)07-048-X. All rights reserved.