Weakening Ion Transport Anisotropy Activates Superionic Conduction in Li ₂ ZrCl ₆

ABSTRACT Halide solid‐state electrolytes (SSEs) are promising for all‐solid‐state batteries (ASSBs) but are often limited by the challenge of simultaneously achieving sufficient ionic conductivity and cost‐effectiveness. Herein, we demonstrate that weakening ion transport anisotropy is an effective route to achieve superionic conduction in low‐cost Li ₂ ZrCl ₆ through an anion‐cation co‐doping strategy using InF ₃ . Mechanistic analysis reveals that moderate co‐doping reduces the intrinsic anisotropy of Li‐ion transport, thereby facilitating faster long‐range diffusion, whereas excessive doping introduces localized migration bottlenecks in the monoclinic lattice, leading to diminished ionic conductivity. The optimized Li _2.1 Zr _0.9 In _0.1 Cl _5.7 F _0.3 exhibits a high ionic conductivity of 1.03 × 10 ⁻³ S cm ⁻¹ with a low activation energy of 0.357 eV. As a result, the ASSBs using this optimized halide SSE deliver high reversible capacity, excellent rate capability, and long‐term stability, retaining 92.5% capacity after 500 cycles at 1 C and operating stably over 2500 cycles at 4 C. Moreover, the enhanced ionic conductivity enables stable operation at a high cathode loading of 31.2 mg cm ⁻² . This work highlights weakening ionic transport anisotropy as an effective strategy for designing high‐performance halide SSEs.