Aqueous lithium-ion batteries (LIBs) have been highlighted as being applied for low-cost and safe energy storages. However, a conventional positive electrode used in LIBs, representatively layered LiCoO2 (LCO), has low...

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.