Cation and vacancy ordering

Using a combination of first-principles total energies, a cluster expansion technique, and Monte Carlo simulations, we have studied the Li/Co ordering in (Formula presented) and Li-vacancy/Co ordering in the (Formula presented). 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 (Formula presented) (and (Formula presented), in agreement with the experimentally observed phase. (ii) Finite-temperature calculations predict that the solid-state order-disorder transitions for (Formula presented) and (Formula presented) occur at temperatures (Formula presented) and (Formula presented), respectively) much higher than melting, thus making these transitions experimentally inaccessible. (iii) The energy of the reaction (Formula presented) gives the average battery voltage (Formula presented) of a (Formula presented) cell for the cathode in the structure (Formula presented). Searching the space of configurations (Formula presented) for large average voltages, we find that (Formula presented) [a monolayer (Formula presented) superlattice] has a high voltage (Formula presented), but that this could be increased by cation randomization (Formula presented), by partial disordering (Formula presented), or by forming a two-layer (Formula presented) superlattice along (Formula presented).