Hydrogen Evolution in Lithium Water-in-Salt Electrolytes

The hydrogen evolution reaction (HER) is a persistent issue in water-in-salt electrolyte aqueous lithium-ion batteries. In this Perspective, we discuss that the HER can be reduced to a manageable kinetics-limited side process. We develop a bottom-up perspective: the bulk nanostructure sets the electric double layer, which, in turn, governs the HER kinetics and products at the interface. We map these interfacial constraints to the full-cell level, regarding self-discharge, HF-driven cathode degradation, and the fragile solid–electrolyte interphase, and suggest rationales on the design of materials. Dihydrate-melt electrolytes based on bulky imide anions (TFSI/BETI), paired with intrinsically HER-suppressing anodes (Wadsley–Roth niobates) and H-adsorption-poor surface layers (e.g., Ag or Zn oxides), are predicted to render the HER kinetically negligible under operational work conditions. This Perspective links nanostructure and Gibbs free energy descriptors to operational metrics.