Formation of dendrite on lithium metal anode (LMA) poses a threat to commercial prospects of rechargeable lithium metal batteries (LMBs). Electrolyte engineering, including but not limited to using new solvents, salts and additives, is an effective remedy to surmounting this vexing problem. In this work, a novel additive, trimethylsilyl(fluorosulfonyl)(n-nonafluorobutanesulfonyl)imide {(CH3)3Si-N[(FSO2)(n-C4F9SO2)], TMS-FNFSI}, is introduced to the ether-based electrolyte of LiN[(CF3SO2)2]-1,3-dioxolane/dimethoxyethane (LiTFSI-DOL/DME) for LMBs. It preferentially reduces on LMA prior to the main electrolyte components (ethers and LiTFSI), resulting in a LiF-rich, organic F- and Si-containing solid electrolyte interphase (SEI) film, being validated by X-ray photoelectron spectroscopy (XPS). Thus, dendrite growth is effectively suppressed. Li|Cu cells, with 1.0 M LiTFSI-DOL/DME (1:1, v/v) containing 5 wt% TMS-FNFSI, exhibit an average coulombic efficiency up to 96.5% for ca. 100 cycles at the current density and areal capacity of 1 mA cm−2 and 2 mAh cm−2, respectively. The Li|Li symmetric cells with TMS-FNFSI can run for more than 1200 h with a stable polarization voltage (ca. 25 mV) at 0.5 mA cm−2. The Li|LiFePO4 cells with TMS-FNFSI can operate steadily for 100 cycles with capacity retention of 92% at 0.2 C. The reduction decomposition mechanism of TMS-FNFSI is tentatively proposed.