Molecular Engineering of Monofluorinated Siloxane Electrolytes for High‐Voltage Lithium Metal Batteries

Abstract Electrolytes engineering plays a crucial role in determining electrode/electrolyte interfacial chemistry for developing high‐voltage lithium metal batteries (HV‐LMBs). Although great progress has been made on electrolytes for lithium metal anodes, the realization of HV‐LMBs has been severely hindered due to the lack of advanced electrolytes that can simultaneously support a stable Li metal anode and high‐voltage cathode (> 4.6 V vs Li + /Li). Herein, through molecular engineering via strategic monofluorination design, two terminal monofluorinated siloxanes including (2‐fluoroethoxy)trimethylsilane (MFS) and bis(2‐fluoroethoxy)dimethylsilane (F2DEO) are designed and synthesized. Compared with the nonfluorinated counterparts, the monofluorinated siloxane‐based electrolytes not only exhibit higher dielectric constant and higher oxidative stability but also allow weaker solvation ability and better interfacial compatibility. With the “4S” (single salt and single solvent) electrolytes at standard concentration, 1.0 M LiFSI/F2DEO electrolyte endows the stable operation of 590 h in Li||Li symmetric cells and high coulombic efficiency of 99.3% in Li||Cu half cells. Moreover, 4.6 V Li||LiCoO 2 full cells achieve a high‐capacity retention of 85.9% after 200 cycles, which may be attributed to the higher oxidative stability of F2DEO and the synergistic effect of FSI − and F2DEO for regulating electrode/electrolyte interphase. This design strategy provides a promising approach for future exploration of advanced electrolytes for HV‐LMBs.