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 square CoO2. 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 square CoO2), in agreement with the experimentally observed phase. (ii) Finite-temperature calculations predict that the solid-state order-disorder transitions for LiCoO2 and square CoO2 occur at temperatures (similar to 5100 K and similar to 4400 K, respectively) much higher than melting, thus making these transitions experimentally inaccessible. (iii) The energy of the reaction E-tot(sigma,LiCoO2) - E-tot(sigma,square CoO2) - E-tot(Li,bcc) gives the average battery voltage (V) over bar of a LixCoO2/Li cell for the cathode in the structure a. Searching the space of configurations a for large average voltages,we find that sigma = CuPt [a monolayer [111] superlattice] has a high voltage ((V) over bar = 3.78 V), but that this could be increased by cation randomization ((V) over bar = 3.99 V), by partial disordering ((V) over bar = 3.86 V), or by forming a two-layer Li2Co2O4 superlattice along [111] ((V) over bar = 4.90 V).