Polarizability-Enhanced Ionic Transport in Rare-Earth-Free Halide–Sulfide Electrolytes: Li2ZrSCl4-xBrx

Abstract

Commercially viable all-solid-state batteries (ASSBs) rely on solid electrolytes (SEs) that combine high ionic conductivity, electrochemical stability, low cost, and scalable production. Here, we report a series of rare-earth-free solid electrolytes, Li2ZrSCl4-xBrx (0 ≤ x ≤ 4), synthesized via a rapid, energy-efficient mechanochemical route. The optimized composition, amorphous Li2ZrSCl1.3Br2.7, exhibits an ionic conductivity of ~1.03 mS cm-1, one order of magnitude higher than crystalline Li2ZrCl6. Structural analyses (XRD, SEM/EDS, and 6Li MAS NMR) reveal that progressive Br⁻ substitution drives amorphization, enhances anion polarizability, and yields dynamically disordered Li⁺ environments conducive to rapid ion migration. Compared to the semi-crystalline Li2ZrSCl4 and fully brominated Li2ZrSBr4 end members, Li2ZrSCl1.3Br2.7 achieves an optimal balance between structural disorder and diffusion-pathway connectivity, increasing the effective population of mobile Li⁺ ions and promoting percolative transport, resulting in enhanced ionic conductivity. When used as a catholyte in an ASSB with TiS2 as the cathode active material, Li2ZrSCl1.3Br2.7 exhibits good rate capability and stable long-term cycling performance. This work highlights the viability of Li2ZrSCl4-xBrx as a high-performance and inexpensive solid electrolyte, combining fast Li-ion transport, electrochemical stability, and scalable synthesis, making it a promising candidate for commercial ASSBs.