Abnormal Overcharging during Lithium-Ether Co-Intercalation in a Graphite System: Formation of Shuttling Species by the Reduction of the TFSI Anion

Abstract

Materializing an ultrafast charging system is one of the crucial technologies for next-generation Li-ion batteries (LIBs). Among many studies aimed at achieving fast charging systems, Li-ether solvent cointercalation into the graphite electrodes in LIB has been identified as a novel concept for achieving high power performance because this system does not consist of the sluggish desolvation step and a resistive solid-electrolyte interface (SEI) layer. Interestingly, while studying the Li-ether solvent cointercalation into graphite electrodes, employing lithium bis-trifluoromethane sulfonimide (LiTFSI) as the Li salt, we observed an abnormal overcharging phenomenon. Here, we screened the specific conditions, under which the abnormal overcharging occurred, and revealed that this abnormal overcharging was attributable to the shuttling mechanism. The formation of shuttling species could have been derived by the reduction of TFSI- anion. With this understanding of the underlying mechanism, we efficiently suppressed the abnormal overcharging by adding LiNO3 to the electrolyte. The shuttling and resulting overcharging could be prevented by the synergistic contributions of LiNO3 and SxOy, dissolved in the electrolyte, to the formation of a dense solid LiSxOy SEI layer on Li-metal. We expect that this work could be a great reference in analyzing many unsolved phenomena in systems utilizing TFSI-.