First-Principles Analysis of Phase Stability in Layered–Layered Composite Cathodes for Lithium-Ion Batteries

The atomic order in layered–layered composites with composition xLi2MnO3·(1 – x)LiCoO2 is investigated with first-principles calculations at the GGA+U level. This material, and others in its class, are often regarded as solid solutions; however, only a minute solubility of Li2MnO3 in a LiCoO2 host is predicted. Calculations of Co vacancy formation and migration energies in LiCoO2 are presented to elucidate the rate of vacancy-mediated ordering in the transition-metal-layer. The neutral Co vacancy formation energy is predicted to be in a range centered slightly above 1 eV but varies widely with the oxygen chemical potential. The calculated migration energy for the vacancy with charge q = −3e is approximately 1 eV. These values are small enough to be consistent with rapid ordering in the transition metal layer at typical synthesis temperatures and, therefore, separated Li2MnO3 and LiCoO2 phases. The relatively small (of the order of a few nm) Li2MnO3 domain sizes observed with TEM in some xLi2MnO3·(1 – x)LiMO2 composites may result from other factors, such as coherency strain, which perhaps block further domain coarsening in these materials.