Electrolyte engineering is crucial for developing high-performance lithium metal batteries (LMB). Here, we synthesized two cosolvents methyl bis(fluorosulfonyl)imide (MFSI) and 3,3,4,4-tetrafluorotetrahydrofuran (TFF) with significantly different reduction potentials and add them into LiFSI-DME electrolytes. The LiFSI/TFF-DME electrolyte gave an average Li Coulombic efficiency (CE) of 99.41 % over 200 cycles, while the average Li CEs for MFSI-based electrolyte is only 98.62 %. Additionally, the TFF-based electrolytes exhibited a more reversible performance than the state-of-the-art fluorinated 1,4-dimethoxylbutane electrolyte in both Li||Cu half-cell and anode-free Cu||LiNi0.8Mn0.1Co0.1O2 full cell. More importantly, the decomposition product from bis(fluorosulfonyl)imide anion could react with ether solvent, which destroyed the SEI, thus decreasing cell performance. These key discoveries provide new insights into the rational design of electrolyte solvents and cosolvents for LMB.