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

Abstract

Abstract 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 ─CF 2 H 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 ─CF 2 CF 2 H 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 Li 2 CO 3 ‐dominated outer phase. Consequently, Li||LiCoO 2 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.