Molecularly Engineered Amphiphilic Anions Enable Flame-Retarding Fluorous Electrolytes for Lithium Metal Batteries

Developing high-energy-density lithium metal batteries (LMBs) is challenging due to critical safety concerns and cycling instability. A highly fluorinated diluent offers improved safety features but fails to form miscible electrolytes. Herein, we address these key issues through the design of miscible fluorous electrolytes enabled by molecular engineering of anions with fluoro-alkyl moieties, creating an effective molecular bridge between solvents and fluorous diluents. Detailed spectroscopy and molecular dynamics simulations reveal the critical amphiphilic anion chemistry inward and outward of the Li⁺ solvation sheath: fluorophilic interactions (F···F) with the diluent and atypical hydrogen-bonding (F···H) with the solvent. The designed miscible fluorous electrolyte, featuring diluents with ultrahigh F/H atomic ratios of 4.33 or higher, exhibits not only remarkable nonflammability safety properties, but also dendrite-free Li plating/stripping with a high Coulombic efficiency (CE) of 99.53% and long-term cycling stability in Li||NCM811 batteries. LiF-rich interphases formed at the electrode-electrolyte interface and the unique electrolyte formulation greatly enhance the battery performance and safety profile, as characterized by delayed onset and peak temperatures of thermal runaway reactions. This study demonstrates a general approach for engineering high-safety electrolytes, advancing next-generation LMBs that overcome the traditional trade-off between performance and safety.