Multinuclear NMR Study of the Solid Electrolyte Interface Formed in Lithium Metal Batteries

The composition of the solid electrolyte interphase (SEI) layers formed in CulLi cells, using lithium bis(fluorosulfonyi)-imide (LiFSI) and lithium big(triflnoromethanesulfonyl)imide (LiTFSI) in 1,2-dimethoxyethane (DME) electrolytes is determined by a multinuclear solid-state MAS NMR study at-high magnetic field. It is found that the "dead" metallic Li is largely reduced in the SEI layers fotmed in a 4 M LiFSIDME electrolyte system compared with those, formed in. a 1 M LiFSI DME electrolyte system. This finding relates directly to the safety of Li metal batteries, as one of the main safety concerns for these batteries is associated with the "dead" metallic Li formed after long-term: cycling: It is also found that a large amount of LiF, which exhibits superior mechanical strength and good Li+ ionic conductivity, is observed in the SEI layer formed in the concentrated 4 M LiFSI-DME and 3 M LiTFSI DME systems, but not in the diluted 1 M LiFSI DME system. Quantitative Li-6 MAS NMR results indicate that the SEI associated with the 4 M LiFSI DME electrolyte is denser than those formed in the 1 M LiFSI DME and 3 M LiTFSI DME systems. These studies reveal the fundamental mechanisins behind the excellent electrochemical performance associated with higher concentration LiFSI DME electrolyte systems.