Ab initio study of the migration of small polarons in olivine LixFePO4 and their association with lithium ions and vacancies

Using first-principles pseudopotential calculations, we investigate the formation and transport of small polarons in olivine LixFePO4. It is demonstrated that excess charge carriers form small polarons in LiFePO4 and FePO4. Lower limits to the activation barrier for small polaron migration are calculated within the GGA+U framework. Additionally, the interaction between lithium ions and polarons is investigated and estimates of binding energies between lithium ions and polarons are provided. Our results show that the binding energy between electron polarons and Li+ ions in FePO4 is lower than that between hole polarons and lithium vacancies in LiFePO4. The electron transfer rate is predicted to be higher in FePO4 than in LiFePO4.