Atomic Structure Evolution during First Charge of Layered-Layered Composite Lithium Ion Battery Cathode Materials

Layered-layered composites with composition xLi2MnO₃•(1-x)LiMO₂ enable high capacity and energy density Li-ion batteries, but suffer from degradation with cycling. Evidence of atomic instabilities during the first charge are addressed in this work with X-ray absorption spectroscopy, first principles simulation at the GGA+U level, and existing literature. The pristine material of composition xLi₂MnO₃•(1-x)LiMn_0.5Ni_0.5O₂  is assumed in the simulations to have the form of LiMn₂ stripes, alternating with NiMn stripes, in the metal layers. The charged state is simulated by removing Li from the Li layer, relaxing the resultant system by steepest descents, then allowing the structure to evolve by molecular dynamics at 1000 K, and finally relaxing the evolved system by steepest descents. The simulations show that about ¼ of the oxygen ions in the Li₂MnO₃ domains are displaced from their original lattice sites, and form oxygen-oxygen bonds, which significantly lowers the energy, relative to that of the starting structure in which the oxygen sublattice is intact. The oxygen displacement facilitates the displacement of about (1/3) of the (Li₂MnO₃ domain) Mn ions to migrate to the delithiated Li layers.