Li _{1+ <i>x</i>} TaO _<i>x</i> F _{6– <i>x</i>} Oxyfluoride Solid Electrolytes with Amorphization-Driven Enhancement of Ion Conduction Channels for 5 V All-Solid-State Batteries

Fluoride solid electrolytes (SEs), despite their extremely low ionic conductivities, offer a promising pathway for enabling 5 V-class chemistries in all-solid-state batteries (ASSBs) owing to their exceptional oxidative stability. Herein, we report a new amorphous oxyfluoride SE, Li_1+<i>x</i>TaO_<i>x</i>F_6-<i>x</i> (<i>x</i> = 0.0-1.0), which exhibits over 3 orders of magnitude higher Li⁺ conductivity than crystalline LiTaF₆, reaching 1.08 × 10^-6 S cm^-1 at 30 °C (<i>x</i> = 1.0). Pair distribution function analysis, Raman spectroscopy, and X-ray absorption spectroscopy reveal an extended, corner-sharing chain of Ta(O/F)_6/7 polyhedra framework. Melt-quenching ab initio molecular dynamics simulations further demonstrate that this interconnected structure broadens Li⁺ diffusion pathways. Leveraging high oxidative stability (>5 V) and improved Li⁺ conductivity, Li₂TaOF₅ was implemented as a shielding layer for 5 V-class LiNi_0.5Mn_1.5O₄ cathodes, enabling exceptional cycling performance with 85.8% capacity retention after 1000 cycles at 1.0C and 30 °C. Even under high-mass-loading (49.3 mg cm^-2) or low-temperature (-20 °C) conditions, the modified LNMO electrodes with Li₂TaOF₅ exhibited promising performance, achieving >5.9 mAh cm^-2 with 94% retention. These findings underscore the efficacy of amorphization in advancing fluoride SEs and provide key design insights for advanced halide SEs in high-voltage ASSBs.