Mechanistic Insight on the Formation of a Solid Electrolyte Interphase (SEI) by an Acetonitrile-Based Superconcentrated [Li][TFSI] Electrolyte near Lithium Metal

Recently, superconcentrated electrolytes have been successfully employed in Li-ion batteries because of their enhanced reductive stability and formation of an excellent solid electrolyte interphase (SEI). Herein, we performed density functional theory-based molecular dynamics (DFT-MD) simulations to delineate the initial processes occurring at the interface of Li metal and superconcentrated electrolyte Li bis(trifluoromethanesulfonyl)imide ([Li][TFSI]) in acetonitrile (AN) and to thoroughly determine the structural evolution of the SEI that formed near the Li metal. For the first time, we show that in such superconcentrated salts the initial SEI formation primarily originates from the reductive decomposition of the TFSI anions and the AN molecules are stable against the redox processes occurring near the Li metal. We observe that the SEI layer is highly structured and inhomogeneous. While the atomized C, O, F, and S atoms prefer to stay in between the inner edge and middle portion of the SEI layer, the atoms of N-SO2-CF3 fragments and AN molecules are mostly found in the middle or outer edge of the layer.