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

The atomic order in layered–layered composites with composition x Li 2 MnO 3 ·(1 – x)LiCoO 2 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 Li 2 MnO 3 in a LiCoO 2 host is predicted. Calculations of Co vacancy formation and migration energies in LiCoO 2 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 = −3 e 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 Li 2 MnO 3 and LiCoO 2 phases. The relatively small (of the order of a few nm) Li 2 MnO 3 domain sizes observed with TEM in some x Li 2 MnO 3 ·(1 – x)Li M O 2 composites may result from other factors, such as coherency strain, which perhaps block further domain coarsening in these materials.