Water-in-salt electrolytes are an appealing option for future electrochemical energy storage devices due to their safety and low toxicity. However, the physicochemical interactions occurring at the interface between the electrode and the water-in-salt electrolyte are not yet fully understood. Here, via in situ Raman spectroscopy and molecular dynamics simulations, we investigate the electrical double-layer structure occurring at the interface between a water-in-salt electrolyte and an Au(111) electrode. We demonstrate that most interfacial water molecules are bound with lithium ions and have zero, one, or two hydrogen bonds to feature three hydroxyl stretching bands. Moreover, the accumulation of lithium ions on the electrode surface at large negative polarizations reduces the interfacial field to induce an unusual “hydrogen-up” structure of interfacial water and blue shift of the hydroxyl stretching frequencies. These physicochemical behaviours are quantitatively different from aqueous electrolyte solutions with lower concentrations. This atomistic understanding of the double-layer structure provides key insights for designing future aqueous electrolytes for electrochemical energy storage devices.
The physicochemical properties of the solid−electrolyte interphase (SEI) at anodes of lithium-based batteries are crucial for achieving the highest performance, and therefore, accurate characterizations are necessary to reveal the molecular structure and chemical properties of SEI. Nanostructure-based plasmonenhanced Raman spectroscopy (PERS) techniques rely on localized surface plasmonic enhancement mechanism and have offered significant opportunities, albeit with challenges, for nondestructive and real-time studies of SEI over the past two decades. In this Perspective, we highlight the recent progress in PERS, including surface-enhanced Raman spectroscopy (SERS), tip-enhanced Raman spectroscopy (TERS), and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) as a class of highly sensitive chemical fingerprint techniques for investigating the structure and chemistry of SEI. We also discuss the advantages and limitations of PERS for characterizing SEI and related interfacial processes. Lastly, we provide possible directions on how PERS can be further effectively leveraged to advance the characterization of interfaces and interphases of Li-based batteries, which could also be challenges for physical chemistry and energy chemistry.
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