Lithium Diffusion Mechanisms in β-LiMO2 (M = Al, Ga): A Combined Experimental and Theoretical Study

Lithium diffusion mechanisms in beta-LiMO2 (M = Al, Ga) were studied in a combined experimental and theoretical approach based on Li tracer diffusion experiments and climbing-image nudged-elastic-band (cNEB) calculations at density functional theory (DFT) level, respectively. Secondary ion mass spectrometry (SIMS) investigations were carried out for beta-LiAlO2 and beta-LiGaO2 polycrystalline films in the temperature range between 473 and 773 K. A thin layer of ion-beam sputtered isotope-enriched (LiAlO2)-Li-6 or (LiGaO2)-Li-6 was used as a tracer source. The diffusivities of beta-LiGaO2 polycrystalline films are in good agreement with those measured on single crystals of the same type. The diffusivities of beta-LiAlO2 are higher than in beta LiGaO2 by almost 2 orders of magnitude. This can be traced back to a lower activation energy for diffusion in beta-LiAlO2. Our computational study shows that the formation energy of a Li vacancy (Vu) is much higher than that of the Li Frenke pair (V-Li + Li-i) showing that Li vacancies are not abundant in both systems. Irrespective of the defect types, the defect formation energy values are smaller in beta-LiAlO2 than in beta-LiGaO2, indicating that Li ion migration could be facile for the former case. In both systems, the most likely Li migration pathways involve Li diffusion from a regular LiO4 tetrahedral location to the first and/or second nearest tetrahedral sites by octahedral interstitial sites. On the basis of calculated activation energies it is concluded that Li diffusion is faster in LiAlO2 than in LiGaO2. Our calculated data are in good accord with the experiments.