Atomic origins of electrochemical stability in acetate-based dual-cation water-in-salt electrolytes

A recent study reported a super-concentrated lithium-potassium acetate-based water-in-bisalt (WiBS) electrolyte. Notably, this system exhibits a high electrochemical stability window of ∼3 V despite the absence of a conventional solid electrolyte interphase. Intriguingly, vibrational spectroscopy revealed a systematic redshift of the O-D stretch of HOD in this electrolyte with increasing salt concentration, contradicting the idea that the enhanced electrochemical stability is positively correlated with the strength of the O-H bond. To explore the atomic factors determining such extended electrochemical stability of this WiBS, we investigate the molecular distributions and water reactivity at the electrified electrode-electrolyte interface using constant potential classical molecular dynamics simulations. By identifying truly interfacial molecules, we uncover a diminishing surface concentration of water molecules at the electrodes, which likely leads to a delayed onset of electrochemical processes. Furthermore, the reduced magnitude of the perturbing electric field along the reaction coordinates for electrochemical water splitting provides an atomic-level explanation for the enhanced electrochemical stability observed in this dual-cation system.