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 Li₂ZrF₆ (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.