Divalent anion-driven framework regulation in Zr-based halide solid electrolytes for all-solid-state batteries

Research into solid electrolytes for all-solid-state batteries has intensified due to demand for safer and higher-energy-density batteries. Halide solid electrolytes are valued for their high ionic conductivity, oxidative stability, and ductility. Among them, Li₂ZrCl₆ is cost-effective but has a relatively lower Li⁺ ionic conductivity (0.4 mS cm^-1 at 25 °C) compared to other halides, such as Li₃InCl₆ (> 1 mS cm^-1 at 25 °C). Here, we elucidate a fundamental mechanism of divalent-anion-driven framework modification that enables enhanced ionic conduction in Zr-based halides. Specifically, we demonstrate enhanced Li⁺ conductivities for oxygen- (0.8Li₂O-ZrCl₄: 1.78 mS cm^-1 at 25 °C) and sulfur- (0.8Li₂S-ZrCl₄: 1.01 mS cm^-1 at 25 °C) substituted lattices. Synchrotron-based X-ray analyses identify distinct anionic sublattices and first-principles calculations reveal that divalent anions locally cluster within the lattice, inducing structural distortion and Li-site destabilization. These changes widen lithium conduction channels and alter the bonding environment, weakening and diversifying Li-Cl interactions. As a result, the energy landscape for lithium migration is flattened, leading to improved ionic conduction. These findings highlight design strategies for divalent-anion-driven framework regulation in halide solid electrolytes.