Using a combination of first-principles total energies, a cluster expansion
technique, and Monte Carlo simulations, we have studied the Li/Co ordering in
LiCoO_2 and Li-vacancy/Co ordering in CoO_2. We find: (i) A ground state search
of the space of substitutional cation configurations yields the (layered) CuPt
structure as the lowest-energy state in the octahedral system LiCoO_2 (and
CoO_2), in agreement with the experimentally observed phase. (ii) Finite
temperature calculations predict that the solid-state order- disorder
transitions for LiCoO_2 and CoO_2 occur at temperatures (~5100 K and ~4400 K,
respectively) much higher than melting, thus making these transitions
experimentally inaccessible. (iii) The energy of the reaction E(LiCoO_2) -
E(CoO_2) - E(Li) gives the average battery voltage V of a Li_xCoO_2/Li cell.
Searching the space of configurations for large average voltages, we find that
CuPt (a monolayer superlattice) has a high voltage (V=3.78 V), but that
this could be increased by cation randomization (V=3.99 V), partial disordering
(V=3.86 V), or by forming a 2-layer Li_2Co_2O_4 superlattice along
(V=4.90 V).Comment: 12 Pages, RevTeX galley format, 5 figures embedded using epsf Phys.
Rev. B (in press, 1998