The need of more powerful systems with higher energy density raises a lot of interest in lithium metal batteries (LMBs). As LMBs suffer from safety concerns due to the dendrite growth, several strategies have been studied to limit this growth. Using a highly concentrated electrolyte allows a homogeneous lithium plating that delays the formation of dendrites. Herein, different techniques are used in order to better understand the beneficial role of the salt concentration in the lithium plating/stripping. Operando Fourier transform infrared spectroscopy highlights the better reversibility of the Li+ solvation in the 5 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1,2-dioxolane/1,3-dimethoxyethane electrolyte in comparison with the 1 M electrolyte. This obviously leads to different electrolyte decompositions during the lithium plating/stripping and changes the nature of the electrode solid electrolyte interphase (SEI) depending on the salt concentration. Gas chromatography coupled with mass spectrometry as well as X-ray photoelectron spectroscopy confirms that with the 5 M LiTFSI electrolyte the salt is preferentially reduced during the plating/stripping, leading to a more inorganic SEI on the lithium metal electrode.