Fluorinated Deep Eutectic Gel Electrolytes for Sustainable Lithium Metal Batteries

As an emerging class of eutectic mixtures, deep eutectic gel electrolytes (DEGEs) exhibit unique advantages in lithium metal batteries (LMBs), particularly due to their high ionic conductivity and tunable molecular interactions. However, it is challenging to realize high interfacial stability between DEGEs and lithium metal. We developed a series of DEGEs utilizing fluorinated amides, which leverage the electron-withdrawing effects of fluorine to precisely modulate the lowest unoccupied molecular orbital (LUMO) energy levels of amides and their binding affinities for Li⁺. A bidirectional screening criterion is proposed, integrating the LUMO energy levels of amides and the desolvation barriers of solvated Li⁺ ions. Among the tested amides, 2,2,2-trifluoro-N-methylacetamide (C═Oα3F) demonstrates the lowest LUMO energy level and desolvation energy barrier. The declined LUMO energy level facilitates the formation of a uniform and robust SEI layer enriched with LiF and Li₃N. Meanwhile, the reduced Li⁺ desolvation energy barrier renders more anions to participate in the solvation process, and this synergistic solvent-anion-derived SEI mechanism further reinforces SEI stability. Additionally, the C═Oα3F-DEGE exhibits superior nonflammability and a high ionic conductivity of 1.24 mS cm^-1. The enhanced properties enable the corresponding Li||Li symmetric battery to achieve stable cycling for over 9000 h and the Li||LiFePO₄ battery counterpart to deliver an impressive lifespan of 2500 cycles with 81.7% capacity retention. These results underscore the advantages of C═Oα3F-DEGE over other fluorinated amide-based systems, providing new insights into the development of high-performance deep eutectic gel electrolytes.