Separator wettability is vital to electrochemical performance of lithium metal batteries. This study demonstrates that the wettability of the polyolefin separator is enhanced by adding lithium bis(trifuoromethanesulfonyl)imide (LiTFSI) in the electrolyte and the good wettability has the positive impacts on the rate performance and lithium dendrite suppression of lithium metal batteries. Moreover, the underlying wetting mechanism of the LiTFSI has been investigated by the comparison with its cousin lithium bis(fluorosulfonyl)imide (LiFSI). It is revealed that perfluorinated alkyl (-CF3) groups in TFSI- anions can greatly reduce the surface tension of the electrolyte, and thus favor the good wettability of the separator. The better separator wettability of 1 M LiTFSI/propylene carbonate (PC) than those of 1 M LiFSI/PC and 1 M LiPF6/PC contributes to the higher rate capability in Li parallel to LiFePO4 batteries. Introduction of the LiTFSI into the LiPF6-based electrolyte can obtain comparable separator wettability with the LiTFSI-based electrolyte. Owing to the greatly enhanced wettability of the separator, the electrolyte of 0.4 M LiPF6-0.6 M LiTFSI/PC as well as 1 M LiTFSI/PC has the positive effect on the lithium dendrite suppression in the lithium metal batteries.