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

Lithium diffusion mechanisms in β-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 β-LiAlO₂ and β-LiGaO₂ polycrystalline films in the temperature range between 473 and 773 K. A thin layer of ion-beam sputtered isotope-enriched ⁶LiAlO₂ or ⁶LiGaO₂ was used as a tracer source. The diffusivities of β-LiGaO₂ polycrystalline films are in good agreement with those measured on single crystals of the same type. The diffusivities of β-LiAlO₂ are higher than in β-LiGaO₂ by almost 2 orders of magnitude. This can be traced back to a lower activation energy for diffusion in β-LiAlO₂. Our computational study shows that the formation energy of a Li vacancy (V_Li) is much higher than that of the Li Frenkel 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 β-LiAlO₂ than in β-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.