Atomic Level Investigations of Lithium Ion Battery Cathode Materials

The defect energetics, ion migration processes and surface structures of lithium ion battery cathode materials LiCoO2 and LiMPO4 (M = Mn, Fe, Co or Ni), probed using a Born model description of atomic interactions, are reported. The lowest energy intrinsic disorder types comprise lithium deficiency in the case of LiCoO2 and cation antisite defects in the case of the orthophosphates. Lithium diffusion in LiCoO2 is confirmed to be two dimensional, with a calculated activation barrier of 0.45 eV, whereas in LiMPO4 compounds diffusion is one dimensional only, with a barrier decreasing from 0.62 to 0.44 eV across the transition metal series. Unlike the linear path calculated for LiCoO2, in orthophosphates the Li ion follows a curved path between vacancies. Examination of low index surfaces in LiCoO2 and LiFePO4 further illustrates the utility of these methods for probing materials systems on the atomic level.