First-Principles Study of Effects of Combined Ti Supervalent Cations and Lithium Ion Vacancies Doping on Crystal and Electronic Structures and Conductivity in LiFePO<sub>4</sub>

Abstract

Olivine-type LiFePO4 is widely considered as a cathode for lithium-ion batteries owing to its environmental friendliness and low-cost, yet its applicability in the pristine state is limited due to poor electronic and ionic conductivity. To investigate the conductivity enhancement of LiFePO4, first-principles method under the GGA+U framework is implemented to study effects of doping with Ti4+ at Fe2+ sites under the lithium-deficient environment. LiFePO4 crystal and electronic structures as well as conductivity are investigated. Ti doping creates the impurity states at the acceptor level, which are normally degenerate states, but split into multiple states by the crystal field splitting. Doping under the lithium-deficient environment induces small hole polarons localizing at the Fe atoms and creates defect states located in the intermediate band. Both phenomena combine to facilitate charge carrier hopping. The climbing-image nudge elastic band (cNEB) calculation shows that Li hopping can be promoted by doping with high Ti concentration. This co-doping mechanism therefore can enhance both the electronic and ionic conductivities, which can be beneficial benchmark for cathode-material synthesis in the future.