Computational Identification and Experimental Realization of Lithium Vacancy Introduction into the Olivine LiMgPO₄

Calculation of the energetics of aliovalent substitution into the olivine LiMgPO 4 suggests that replacement of Mg 2+ by In 3+ is the most effective way to introduce lithium vacancies and thus generate Li ion conductivity. Experimental synthesis accesses materials with up to 17% Li vacancy content. An order-of-magnitude increase in the high-temperature hopping rates probed by 7 Li NMR spin–lattice relaxation, and over 2 orders of magnitude increase in the room-temperature Li + ion conductivity measured by impedance spectroscopy is observed upon the introduction of In 3+ ions and Li vacancies. NMR spectroscopy and calculations reveal that the energy barrier to site-to-site hopping is 0.3–0.5 eV, comparable with best-in-class nonoxide systems such as argyrodite, but NMR-derived hopping rates, and impedance spectroscopy shows that longer range transport is less facile with activation energies in the range of 0.7–1 eV. Calculations suggest that this is because the Li vacancies are strongly bound to the In 3+ dopants, suggesting that high lithium mobilities in oxides are accessible but high conductivities require strategies to separate defect from dopant.