First-Principles Calculations of Ionic Conduction in Olivine-Type LixFePO4

Super ionic conductors are required for all solid Li ion batteries. Conduction mechanism of LixFePO4 has been revealed by first-principles calculations using plane-wave basis. Crystal structures were constructed using a unit cell and a 1 x 2 x 1 super cell. Transition of Li site occupation with the smallest energy fluctuation was searched through graph theory. Trajectories of Li ions and migration energies were obtained by Nudged Elastic Band Method. Results show the migrating atoms pass through faces of O polyhedra. At dilute limit of Li, a single ion migrated and had very low migration energy of 0.14 eV. Increasing the carrier concentration resulted in pairing of Li atoms and raised the migration energy. At higher concentration region, cooperative ionic conduction mechanisms of two Li ions occurred. Low migration energies were obtained to be 0.20 and 0.22 eV for x = 0.25 and 0.5 of LixFePO4, respectively. Considering the migration energy and carrier concentration, LixFePO4 with the concentration region 0.25 <= x <= 0.5 should have high ionic conductivity. These mechanisms would be applied to develop new superionic conductors.