Ultrahigh Ionic Conductivity in Halide Electrolytes Enabled by Anion Framework Flexibility Engineering

Chloride-based solid electrolytes are promising for all-solid-state batteries owing to their favorable oxidative stability and mechanical deformability. However, most chlorides exhibit only moderate ionic conductivity, primarily due to the restricted ion transport imposed by their close-packed anion frameworks. In this work, we address this limitation by enhancing anion framework flexibility through lowering the negative charge on chloride anions, achieved by incorporating high-valent, highly electronegative cations, accompanied by a reduction in the lithium content. Computations reveal that this strategy substantially decreases the energy barriers for anion reorientation, leading to a more flexible anion framework characterized by intensified libration and even activated rotation. These anion dynamics transiently widen Li⁺-ion transport bottlenecks and distort local coordination environments, thereby flattening the energy landscape and enabling fast ion diffusion. The effectiveness of this strategy was experimentally validated, with the tailored chloride electrolytes achieving ionic conductivities as high as 10.3 mS cm^-1 at room temperature. Solid-state batteries utilizing Li_1.25Zr_0.25Ta_0.75Cl₆ as the catholyte deliver outstanding rate capacity and cycling performance, retaining 82.5% capacity after 20,000 cycles at 4C. These findings offer new insights into the ion transport mechanism in close-packed chlorides and provide guidelines for designing superionic conductors.