Cation and Vacancy Ordering in LixCoO2

Using a combination of first-principles total energies, a cluster expansion technique, and Monte Carlo simulations, we have studied the Li/Co ordering in LiCoO2 and Li-vacancy/Co ordering in the squareCoO2. We find: (i) A Ground-state search of the space of substitutional cation configurations yields the CuPt structure as the lowest-energy state in the octahedral system LiCoO2 (and squareCoO2),in agreement with the experimentally observed phase, (ii) Finite-temperature calculation predict that the solid-state order-disorder transitions for LiCoO2 and squareCoO2 occur at temperatures (more or less 5100 K and more or less 4400 K, respectively) much higher than melting, thus making these transitions experimentally inaccessible, (iii) The energy of the reaction Etot (oxsquareCoO2)-Etot(Li, bee) gives the average battery voltage V of a LixCoO2/Li cell for the cathode in the structure o. Searching the space of configurations o for large average voltages, we find that o-CuPt [a monolayer (111) superlattice} has a high voltage (V=3.78 V), but that this could be increased by cation randomization (V=3.99 V), by partial disordering (V=3.86 V), or by forming atwo-layer Li2Co2O4 superlattice along (111) (V-4.90 V).