A Li‐Rich Fluorinated Lithium Zirconium Chloride Solid Electrolyte for 4.8 V‐Class All‐Solid‐State Batteries

Chloride solid-state electrolytes (SEs) represent an important advance for applications in all-solid-state batteries (ASSBs). Among various chloride SEs, lithium zirconium chloride (Li₂ZrCl₆) is an attractive candidate considering the high natural abundance of Zr. However, Li₂ZrCl₆ meets the challenge in practical ASSBs because of its limited ionic conductivity and instability when paired with high-voltage cathodes. This is a major drawback, which can result in a high internal resistance, a low capacity utilization of cathode, and poor cycle stability, especially at high voltage. Existing methods cannot achieve simultaneous enhancement on both ionic conductivity and high-voltage stability due to a trade-off between lithium-ion migration and structural stability. Here a two-pronged strategy based on partial fluorination and incorporation of lithium ions in excess of stoichiometric ratios is introduced that enables high-voltage stability while increasing ionic conductivity concurrently. The Li-rich fluorinated halide SE (Li_2.3ZrCl_6.1F_0.2) exhibits a significant advancement in performance, with an ionic conductivity that is double that of the pristine Li₂ZrCl₆ and much better high-voltage stability. By leveraging Li_2.3ZrCl_6.1F_0.2 with the LiCoO₂ cathode and the Li-In anode, the all-solid-state cell exhibits a remarkable initial specific capacity (198.0 mAh g^-1 at 0.1 C) and a high capacity retention (78.5% after 150 cycles) within 3.0-4.8 V.