Solid Electrolyte Interphase Revealing Interfacial Electrochemistry on Highly Oriented Pyrolytic Graphite in a Water-in-Salt Electrolyte

We present a fundamental study of solid-electrolyte interphase (SEI) layers toward a better understanding of interfacial electrochemistry. In particular, water-in-salt electrolytes yield SEIs with a simple composition that describes the electrolyte-electrode interface explicitly. The 21 m lithium bis(trifluoromethanesulfonyl)imide formed a porous SEI film on a highly oriented pyrolytic graphite (HOPG) electrode at -2 V (vs Ag/AgCl). The significant hydrogen evolution reaction (HER) made holes in a thin SEI film and defect sites in the HOPG. In addition, the SEI comprised fragmented TFSI without including any Li compounds. We suggested that fragments of TFSI- were precipitated out by the addition of the hydrogen atoms, which were yielded through the Volmer step and detached from the HOPG surface before HER. Subsequently, a nonporous and LiOH-rich film was formed by -4 V. The OH- and Li+ ions were enriched during the continuous HER, and their chemical reaction produced a thick film and nanoneedles. However, there was no evidence of Li+ intercalation into graphitic layers of the HOPG, presumably caused by sluggish Li+-ion transport in the Li-deficient SEI layer. This study shows variable interfacial reactions over a wide range of applied potential and the HER impact on SEI films associated with the performance of aqueous Li-ion batteries.