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

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.