(Invited) Synthesis of LiAlO2 Polymorphs Under High Pressure and Their Lithium-Ion Conductivity
Authors: Nishimura Toshihiro , Ishii Akihiro , Oikawa Itaru , Takamura Hitoshi 
Published: December 2024

Bottleneck sizes for ion migration have been considered an important decisive factor for Li+ conductivity. Experimental verification was carried out by doping isovalent ions of different sizes into matrix crystals and measuring their conductivity. However, this approach affects not only the bottleneck size but also the softness of the ions comprising the bottleneck. Aiming for a detailed experimental verification of the impact of bottleneck size on Li+ conductivity, this work synthesized α-, β-, γ-, and δ-LiAlO2 polymorphs, which provide a variety of bottleneck sizes without changing the composition. Pseudo-isostatic high-pressure was applied using a Kawai-type multi-anvil apparatus to prepare dense bodies while controlling their defect levels. This enabled a comparison of the difference in Li+ conductivity based only on crystal structural factors.

First, the preparation conditions for α-, β-, γ-, and δ-LiAlO2 dense pellets were optimized. The γ-phase, which is stable under ambient pressure, was prepared by a solid-state reaction. The other phases of LiAlO2 were synthesized by subjecting the synthesized γ-LiAlO2 to high-pressure treatment; however, the resulting samples were fragile because of temperature quenching. Thus, by considering the low-temperature Li-Al-Si oxide melts may mitigate the mechanical stress, [2] 0.5 mol% SiO2 was added when preparing the g-LiAlO2. The SiO2 addition drastically increased the relative density of the γ-LiAlO2 pellets from 72% to 90%. This γ-LiAlO2 pellet was single-phase at the XRD level; however, a small amount of Al2O3 was detected by SEM and 27Al MAS NMR. The lattice volume of γ-LiAlO2 increased by 0.05 % with the addition of SiO2, and no Si segregation was observed by SEM, suggesting that Si dissolved into the γ-LiAlO2 lattice. Using SiO2-added γ-LiAlO2, dense pellets of LiAlO2 polymorphs were successfully synthesized under high pressure.

The α-phase is considered a high-temperature and high-pressure phase [1]. The α-LiAlO2 pellet was prepared by subjecting the synthesized γ-LiAlO2 to the heat treatment at 800 °C under 9.5 GPa. This α-LiAlO2 pellet was single-phase at the XRD level, but Al2O3 was detected using SEM and NMR. The δ-phase is a low-temperature and high-pressure phase [1]. In this study, δ-LiAlO2 pellets were prepared in the temperature range of 200–400 °C and 9.5 GPa. The δ-phase was dominant at 400 °C, but the β-phase appeared as a secondary phase. β-LiAlO2 is thermodynamically metastable and has been synthesized by heat treatment at intermediate temperatures and pressures [1]. In this study, the heat treatment at 500 °C and 3 GPa resulted in the formation of β-LiAlO2 as a single phase at the XRD level.

The Li+ conductivity of the synthesized α-, β-, γ-, and δ-LiAlO2 dense pellets was investigated by the AC impedance method using a Pt electrode. One semicircle was observed in the Nyquist plot for each sample, and its width was the total resistance of Li+ conduction, attributed to the sum of the bulk and grain boundary contributions. The Li+ conductivity at 150 °C and activation energy were similar: 6.96×10-8 S·cm-1 and 0.77 eV for α, 1.26×10-7 S·cm-1 and 0.87 eV for β, 1.91×10-7 S·cm-1 and 0.82 eV for γ, and 8.96×10-8 S·cm-1 and 0.82 eV for δ-phases. Meanwhile, their sizes of bottlenecks for Li+ conduction are different: 3.98 Å2 for α, 4.58 Å2 for β, 5,00 Å2 for γ, and 3.68 Å2 for δ-phases. The bottleneck sizes for the β and γ phases are comparable to those of the well-known Li+-conducting cubic garnet, NASICON, and LISICON-type oxides, although they typically show a smaller activation energy around 0.5 eV. This suggests that the activation energy is determined not by the bottleneck size, but by other factors. The similar Li+ conductivity among the LiAlO2 polymorphs implies that the softness of the cations has a larger impact on the activation energy compared to the bottleneck size. Further discussion with theoretical calculations is provided in the presentation.

[1] L. Lei et al., J. Solid State Chem.
181 (2008) 1810-1815.

[2] A. Ishii et al., Acta Mater.
224 (2022) 117529.

This work was supported by JST SPRING, Grant Number JPMJSP2114.