An Amorphous Sn 2+ -Based Perfluoride Solid-State Electrolyte with High Bulk and Total Ionic Conductivity

Ideal solid-state electrolytes (SSEs) combine high ionic conductivity and a wide electrochemical window. Fluoride SSEs offer exceptional electrochemical stability (0–6 V vs Li/Li+), yet they suffer from low room-temperature ionic conductivity that severely hinders their applications. To address this challenge, we designed an amorphous Sn2+-based perfluoride SSE (Li1.7Sn1.7La0.3F6, SFLL-La15) via element confusion of Sn2+ and La3+. This confusion-principle-inspired strategy significantly increases the amorphous content, thus reducing the grain boundary resistance by 2 orders of magnitude. Hence, the cold-pressed pellet of SFLL-La15 achieves a total ionic conductivity of 2.84 × 10–6 S cm–1, the highest reported for perfluoride SSEs. The bulk ionic conductivity of SFLL-La15 reaches 2.8 × 10–3 S cm–1, comparable with that of sulfide SSEs. Practicality is validated with lithium symmetric cells cycling stably at 0.01 mA cm–2 for over 500 h. Our work establishes both a high-performance perfluoride SSE and a design principle for overcoming grain boundary limitations in ceramic electrolytes.