Influence of synthesis and substitution on the structure and ionic transport properties of lithium rare earth metal halides

Abstract

Lithium rare earth metal halides have emerged as attractive candidates for solid electrolytes in all-solid-state batteries due to their high ionic conductivities and stability against oxidation. Here, we study their electrochemical properties as a function of the synthesis procedure and post-synthetic treatment and report on the impact of iso- and aliovalent substitutions in the cation and anion sublattices of the lithium rare earth metal iodides Li₃MI₆. For selected compounds we have investigated the impact of the synthetic approach, i.e. of different solid-state synthesis protocols, and mechanochemical ball-milling without and with post-synthetic calcination on the resulting materials. Lithium rare earth metal iodides obtained from solid-state synthesis generally outperform the mechanochemical synthesized compounds in terms of ionic conductivity and activation energy for ion diffusion, but when mechanochemical ball-milling is combined with a post-synthetic calcination step, these iodides show similar ionic conductivites as their counterparts obtained from conventional solid-state synthesis. Furthermore, we report a series of new Li₃MI₆ compounds with M = Y, Sm, Gd–Lu, partially Cd²⁺-substituted Li_3+yGd_1−yCd_yI₆ and partially Cd²⁺-, Ca²⁺- and Zr⁴⁺-substituted Li_3±yY_1−yM^II/IV_yBr_6−xI_x phases. Using a combination of ssNMR, EIS and PFG-NMR we reveal the influence of structural parameters such as RE/Li radius ratio, intra-layer cation and stacking fault disorder on the ionic transport properties, obtained from in-depth PXRD analyses. We find that the ionic conductivity is strongly affected by the ratio of the RE/Li radius ratio as well as by the degree of intra-layer cation disorder. It ranges between 3.0 × 10⁻⁵ S cm⁻¹ and 4.6 × 10⁻⁴ S cm⁻¹ for M = Lu–Sm at 20 °C with activation energies between 0.20 eV to 0.33 eV. The combination of partial anion and cation substitution increases the ionic conductivity up to 3.0 × 10⁻³ S cm⁻¹ and leads to a lower activation energy of 0.17 eV. This study highlights the impact of microstructural effects on the electrochemical properties of solid electrolytes. The rational design and modification of solid electrolytes, along with their comprehensive (micro)structural analysis is thus crucial to optimize their ionic transport properties for applications in all-solid-state batteries.