# Insight into the prospects and limitations of mechanochemically-synthesised lithium tetrahalogallates, LiGaX4 (X = Cl, Br, I), as Li-ion conductors

## TL;DR

This study explores the properties of mechanochemically synthesized lithium tetrahalogallates as potential solid-state electrolytes for Li-ion batteries.

## Contribution

The work presents the first detailed characterization of LiGaX4 materials and their ionic transport properties.

## Key findings

- LiGaBr4 shows the highest room-temperature ionic conductivity (4.87 × 10−6 S cm−1) among LiGaX4 compounds.
- LiGaX4 has lower ionic conductivity than LiAlX4 due to less dimensional diffusion pathways and higher activation energies.
- DFT calculations correlate band gaps with electrochemical stability in LiMX4 materials.

## Abstract

Halide solid-state electrolytes have attracted significant interest due to their appreciable Li+ conductivity at room temperature, good electrochemical stability against oxidation, and favourable compatibility with oxide cathodes. Nevertheless, the family of lithium tetrahalogallates, LiGaX4 (X = Cl, Br, I), has scarcely been studied and, consequently, their physicochemical properties remained largely unknown. In this work, we report the mechanochemical synthesis of high-purity LiGaX4 and investigate their crystal structures, thermal, electronic, vibrational, and ionic transport properties through a combination of advanced characterisation techniques and computational methods. Powder X-ray and neutron diffraction confirm that all three phases crystallise in a monoclinic unit cell (P21/c), isostructural to LiAlX4 analogues. Preliminary results indicate that LiGaBr4 exhibits the highest ionic conductivity at room temperature (4.87 × 10−6 S cm−1) among the series. Compared to LiAlX4, the diffusion pathways in LiGaX4 showed a lower dimensionality and higher activation energies for Li+ diffusion, which results in reduced ionic conductivities. Periodic density functional theory (DFT) based calculations indicate a general correlation between computed band gaps and electrochemical windows in LiMX4 materials (M = Al, Ga; X = Cl, Br, I). Additionally, μ+SR data demonstrate that softer lattices provide lower activation energies for Li+ migration and suggest that additional factors influence the results obtained through electrochemical impedance spectroscopy.

Advanced characterisation of mechanochemically synthesised LiGaX4 halides uncovers their structures, transport properties and key differences between short- and long-range conduction.