Molecular Design of Asymmetric Difluorinated Ether Electrolytes for Stable Operation of High-Voltage Lithium Metal Batteries.

Fluorination of electrolyte solvents is key to improving the cycling stability of high-voltage lithium metal batteries (LMBs), yet the critical role of fluorinated alkyl chain length in governing solvation and interphase chemistries remains unclear. Herein, we systematically engineered a series of methoxy ethoxy methane derivatives with tailored fluorinated alkyl groups to screen the optimal asymmetric difluorinated electrolytes for high-voltage LMBs. Spectroscopic and computational studies show that the ─CF2H group in difluoroethoxy methoxy methane (DFME) facilitates balanced Li─F interactions, which are essential for ensuring efficient ion transport and maintaining oxidation stability. In contrast, the elongated ─CF2CF2H group in tetrafluoropropyl methoxy methane (TFME) fails to coordinate with Li+ ions, which hampers ion transport and leads to interfacial instability. The DFME electrolyte facilitates the spontaneous formation of a protective dual-layer interface featuring a LiF-rich inner phase and a Li2CO3-dominated outer phase. Consequently, Li||LiCoO2 cells (2.5 mAh cm-2) using DFME exhibit remarkable cycling stability, retaining 92% capacity after 180 cycles. This is further corroborated by a 140-mAh pouch cell, which retains 91% capacity after 140 cycles. Our study offers fundamental insights into the design of advanced fluorinated electrolytes for the stable operation of high-voltage LMBs.