High-Pressure-Induced Reorganization of Al-O Bonding and Its Influence on Li-Ion Transport in LiAlO2 Polymorphs

Li+ transport properties of LiAlO2 polymorphs (α, β, γ, and δ) were systematically investigated using high-pressure synthesis, electrochemical measurements, and density functional theory (DFT) calculations. Dense single-phase α-, β-, and γ-LiAlO2 and δ-rich LiAlO2 samples were obtained under optimized high-pressure conditions with SiO2 addition, enabling direct comparison of their ionic conductivities under identical conditions. The polymorphs exhibited conductivities in the 10-8 to 10-7 S cm-1 range at 150 °C and followed the Meyer-Neldel rule. DFT calculations revealed Li+ migration-barrier differences that could not be explained solely by geometric factors, including the bottleneck size and volume per oxygen atom. Bader charge and electron localization function analyses revealed enhanced electron redistribution toward oxygen in both α- and δ-LiAlO2, leading to electronically rigid Al-O bonding, whereas β- and γ-LiAlO2 retained more ionic and compliant bonding environments. Moreover, the activation volumes derived from the pressure dependence of the nudged elastic band barriers demonstrate that rigid Al-O bonding is associated with larger lattice deformations during Li+ migration. These findings indicate that Li+ mobility in LiAlO2 polymorphs is jointly governed by geometric openness, electronic bonding characteristics, and lattice relaxation ability, providing design considerations for oxide electrolytes.