Overlooked Decomposition of Organic Cosolvents Undermines Stability of Aqueous Hybrid Electrolytes

The rapid adoption of hybrid aqueous/organic electrolytes is driven by the quest for expanded electrochemical stability windows of aqueous electrolytes. However, the long-term chemical integrity of these systems under operating conditions remains notably absent. Here, we reveal that the very presence of cations, especially multivalent cations, triggers a Lewis acid-catalyzed hydrolysis of commonly used carbonate cosolvents. We demonstrate that this overlooked decomposition is not a marginal side reaction but a dominant failure mode, with propylene carbonate (PC) degrading by over 60% during 50 cycles at 60 °C. By establishing a direct link between solvent hydrolysis, parasitic gas evolution, and premature battery death, this work exposes a fundamental flaw in current electrolyte screening protocols. We propose a new design framework that integrates thermochemical stability descriptors and postmortem chemical analysis to ensure the realization of truly durable, next-generation aqueous batteries.