Boosting ionic conductivity of fluoride electrolytes by polyanion coordination chemistry enabling 5 V-Class all-solid-state batteries

Fluoride solid-state electrolytes (SSEs) offer superior electrochemical stability, making them theoretically ideal for high-voltage all-solid-state batteries (ASSBs), but their low Li + conductivity hinders practical application. Here, we exploit the polyanion coordination strategy to develop a class of fluoride SSEs, Li x Ti(PO 4 ) x /3 F 4 , which exhibits high ionic conductivity and ultrahigh-voltage stability. Especially, Li 1.3 Ti(PO 4 ) 1.3/3 F 4 (1.16 × 10 −5 S cm −1 ) demonstrates a two-order-of-magnitude enhancement in Li + conductivity compared to analogous Li 2 TiF 6 . Combined experimental and theoretical analyses reveal that the high ionic conductivity of Li x Ti(PO 4 ) x /3 F 4 arises from weakened Li + -surrounding interactions and flexible network structures. Attractively, the exceptional ionic conductivity and ultrahigh-voltage stability of Li 1.3 Ti(PO 4 ) 1.3/3 F 4 enable a low-resistance and highly stable electrolyte-electrode interface, contributing to 91.6% capacity retention after 200 cycles for 5 V-class LiNi 0.5 Mn 1.5 O 4 -based ASSBs at 1 C and 30°C. The discoveries in this work open a new avenue for designing high-conductivity fluoride SSEs, paving the way for the application of high-voltage cathodes in high-energy-density ASSBs.