Fluorinated Plastic Crystal Cation Enables Stable Inorganic-Rich Interphase in Lithium Metal Batteries

The construction of robust, fluorine-rich electrode/electrolyte interfaces is key to stabilizing lithium metal anodes and enabling high-voltage cathodes. While fluorine sources are typically derived from anions or neutral molecules, the potential of fluorinated cations for solid polymer electrolyte interfacial regulation remains unexplored. Herein, an ionic design that incorporates fluorine into the cationic structure of plastic crystals is proposed. Together with a fluorinated polymer and lithium salt, we obtained a new electrolyte, the fluorinated cation is the novel fluorine source at the interface, bis(fluorosulfonyl)imide (FSI^-) is the conventional anionic source, and the fluorinated polymer acts as a mechanical scaffold. The prepared electrolyte delivers high room-temperature ionic conductivity (0.9 mS cm^-1), Li⁺ transference number (0.51), and enables ultra-stable Li plating/stripping (>1400 h). As a consequence, Li/LiFePO₄ cells retain ∼100% capacity after 2500 cycles at 5C. Remarkably, high-voltage Li/LiNi_0.6Co_0.2Mn_0.2O₂ cells maintain >80% capacity after 450 cycles at 2C, and Li/LiCoO₂ cells can operate stably at a cut-off voltage of 4.45 V. This work establishes fluorinated cations as effective interfacial modifiers and advances cationic fluorination as an ionic-level paradigm for engineering next-generation solid-state cells.