Dual-Polyhedral Distortion for Low-Cost Amorphous Halide Electrolytes with Improved Ionic Conductivity

Amorphous halide solid electrolytes (AHSEs) have emerged as promising candidates for high-performance all-solid-state Na batteries (ASSNBs) owing to their balanced comprehensive performance. However, rational design principles assisting the precursor selection are still limited for developing low-cost Na-based AHSEs with improved ionic conductivity. Herein, a series of low-cost AHSEs, xNaBF4-ZrCl4 (x = 0.3, 0.5, 1, and 2), were synthesized via a one-step mechanochemical process. Competitive coordination between the Zr4+ and B3+ cations under mechanochemical activation is proposed to promote the rearrangement of F- and Cl- anions within the halide framework. The resulting dual-distorted polyhedral coordination facilitates the formation of long-range disordered structures that are highly favorable for isotropic Na+ transport, exhibiting a high Na+ ionic conductivity up to 1.01 mS cm-1 at 30 °C with excellent oxidative stability of 4.33 V vs Na+/Na. When assembled into all-solid-state cells, the 0.5NaBF4-ZrCl4 electrolyte enables stable cycling between 2.5 and 4.2 V with a capacity retention of 71.5% after 400 cycles at 0.2 C rate. This work provides a novel design paradigm for AHSEs and highlights the promising potential of high-performance ASSNBs.