First-principles modelling techniques based upon density-functional theory and the nudged elastic band method were used to examine possible Li ion diffusion mechanisms in idealized crystals of this electrolyte material in both the γ and β forms. Both vacancy and interstitial processes were considered in order to find the migration energies, Em. It was found that interstitial diffusion via an interstitialcy mechanism involving the concerted motion of an interstitial Li ion and a neighboring lattice Li ion could provide the most efficient ion transport in Li3PO4. Ion transport in undoped crystals depended upon the formation of vacancy-interstitial pairs. This required an additional energy, Ef, which resulted in a thermal activation energy of EA = Em + Ef/2. The calculated values of EA were in excellent agreement with single crystal measurements on γ-Li3PO4. The results highlighted the similarities and differences between the diffusion processes in the γ and β crystal structures. In addition, the zone-center phonon modes in both crystals were analyzed in order to compare the calculations with experimental measurements and to determine the range of vibrational frequencies which were associated with Li ion motions that might contribute to the diffusion processes.

Mechanisms of Li+ Diffusion in Crystalline γ- and β-Li3PO4 Electrolytes from First Principles. Y.A.Du, N.A.W.Holzwarth: Physical Review B, 2007, 76[17], 174302 (14pp)