Unlocking Fast Lithium Ion Migration in Zirconium‐Based Fluoride Solid Electrolytes

Abstract Fluoride solid‐state electrolytes (SSEs) have attracted significant interest due to the extremely high oxidation limits, excellent air stability, and favorable compatibility with metallic lithium, while the structure‐property relationship remains unknown. Although crystalline Li 2 ZrF 6 (LZF) synthesized at high temperatures exhibits extremely low ionic conductivity, the ionic conductivity of its quasicrystalline counterpart synthesized via lithium‐rich strategies can be enhanced by at least an order of magnitude. The enhanced ionic conductivity is attributed to effective modulation of 0‐, 1‐, and 2D defects in the structure, which manifests as an optimized carrier‐vacancy concentration equilibrium, structural rearrangement of framework units, and adjustment of dislocation and grain boundary configurations. Driven by these mechanisms, the sample with x = 0.5 exhibits the highest ionic conductivity and lowest activation energy. To elucidate this trend, combining simple theoretical models and experiments demonstrates the guiding role of the carrier‐vacancy theory, unit cell distortion theory, and defect theory in advancing ion transport of fluoride SSEs. In addition, the zirconium‐based fluoride exhibits superior oxidation stability and excellent compatibility with lithium metal, enabling the all‐solid‐state lithium batteries (ASSLBs) fabricated with it as a cathode additive to achieve a capacity retention rate of 66.83% after 1000 cycles.