Fluorinated Imidazolidinium Cations as a Fluorine-Lean Interface Repairing Agent for Li-Metal Batteries

Li-metal batteries promise ultrahigh energy density, but their application is limited by Li-dendrite growth. Theoretically, fluorine-containing anions such as bis(fluorosulfonyl)imide (FSI^-) in electrolytes can be reduced to form LiF-rich solid-electrolyte interphases (SEIs) with high Young's modulus and ionic conductivity that can suppress dendrites. However, the anions migrate toward the cathode during the charging process, accompanied by a decrease in the concentration of interfacial anions near the anode surface. This will create a fluorine-lean (F-lean) interface and an uneven SEI. Herein, we propose an F-lean interface repairing agent utilizing the fluorinated ionic liquid 2-fluoro-1,3-dimethylimidazolidinium hexafluorophosphate ([f-Im⁺][PF₆^-]). The [f-Im⁺] cations could migrate to the Li-metal anode along with Li⁺ during the Li-deposition process and be decomposed prior to Li deposition, thus repairing the F-lean interface and forming a dense and F-enriched SEI layer on the Li-metal anode. Further theoretical calculations suggest that after fluorine substitution, the fluorinated imidazolidinium cations are more thermally vulnerable to be decomposed to form a LiF-rich SEI. Consequently, an ultrahigh Coulombic efficiency (CE) of Li deposition over 99.5% and stable cycling of Li||NCM811 full cells over 100 times have been achieved even with an N/P ratio of 1.