Electronic-Structure-Driven Regulation of Water Hydrogen-Bond Dynamics Revealed by Ultrafast IR Spectroscopy for Stable Aqueous Sodium-Ion Batteries

Cathode dissolution in aqueous sodium-ion batteries (ASIBs) is governed not only by the thermodynamic activity of water but also, more critically, by the dynamics of the hydrogen-bond network at the electrolyte–electrode interface. However, how subtle electronic-structure variations in electrolyte additives regulate these dynamics and thereby control cathode stability remains poorly understood. Here, we elucidate an oxidation-state-dependent modulation of water hydrogen-bond dynamics by comparing two structurally similar cyclic sulfoxide additives, sulfolane (SFL) and tetramethylene sulfoxide (TMSO). Despite their comparable molecular frameworks, combined vibrational spectroscopic measurements and theoretical calculations reveal that the lower sulfur oxidation state in TMSO redistributes the electron density of the S═O moiety, strengthening its hydrogen-bond accepting capability while inducing a moderated slowing of water reorientation dynamics relative to SFL. Ultrafast IR spectroscopy further shows that, in contrast to the more rigid hydrogen-bond network imposed by SFL, TMSO stabilizes the water network without excessive dynamical constraint. This balanced dynamic regulation effectively suppresses water-driven dissolution of manganese-based Prussian white cathodes. Electrochemical measurements in Mn–Prussian white/NaTi2(PO4)3 full cells employing TMSO-regulated NaClO4 aqueous electrolytes validate the proposed mechanism, exhibiting stable cycling behavior in both coin and pouch cell configurations. These findings establish electronic-structure-driven regulation of hydrogen-bond dynamics as a promising design principle for stabilizing aqueous battery chemistries.