Polarizability-enhanced ionic transport in rare-earth-free halide-sulfide electrolytes: Li₂ZrSCl_4-x Br _x

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, Li₂ZrSCl_4-x Br _x (0 ≤ x ≤ 4), synthesized via a rapid, energy-efficient mechanochemical route. The optimized composition, Li₂ZrSCl_1.3Br_2.7, exhibits an ionic conductivity of ∼1.03 mS cm^-1, one order of magnitude higher than Li₂ZrCl₆. Structural and morphological analyses using XRD, SEM/EDS, and ⁶Li MAS NMR reveal that progressive Br^- substitution drives structural disorder, 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, Li₂ZrSCl_1.3Br_2.7 achieves an optimal structural disorder and Li⁺ ion mobility, resulting in enhanced ionic conductivity. When used as a catholyte in an ASSB with TiS₂ as the cathode active material, Li₂ZrSCl_1.3Br_2.7 exhibits good rate capability and stable long-term cycling performance. This work highlights the viability of Li₂ZrSCl_4-x Br _x 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.