Boosting Low-Temperature Ionic Conductivity in MOF-Based Solid-State Electrolytes via Tailored Fluoro-Ligand Incorporation Into Crystal Frameworks.

The stable operation of solid-state electrolytes (SSEs) at low temperature is critical for expanding the application of solid-state lithium batteries (SSLBs) in cold climates. However, there are few relevant literature reports. Herein, a breakthrough is realized to directly address this challenge through tailored fluoro-ligand incorporation into the crystal framework of MOF-808. A series of fluorinated MOF-808 (MOF-808-хF, x = 3, 5, 7) are rationally constructed with fluorinated carboxylic acid of different chain lengths through fluorination engineering at the molecular level to precisely modulate the channels surface environment. Specifically, the chemical anchored F groups effectively promote the capture of anions and transport of Li⁺, while endowing the MOF-808 with excellent mechanical strength and flexibility, thereby improving the performances of SSEs in operation at low temperature. As a result, the optimized MOF-808-5F achieves remarkable ionic conductivity (1.25 × 10^-4 S cm^-1), high Li⁺ transference number (0.58) and wide electrochemical window (5.4 V) at -40 °C. Furthermore, fluorinated MOF-808 contributes to LiF solid electrolyte interphase (SEI) formation and effectively inhibits Li dendrites growth, enabling Li|MOF-808-5F|Li cell to realize a stable plating/stripping cycling over 1000 h at 0.2 mA cm^-2. Such results provide an insight on the design of electrolytes for SSLBs operating under low temperatures.