Unraveling concentration-dependent solvation structures and molecular interactions in water-in-salt electrolytes for enhanced performance of electric double-layer capacitors

Water-in-salt electrolytes (WiSEs) are increasingly recognized as advanced aqueous electrolytes for energy storage due to their inherent safety, wide electrochemical stability window (ESW) beyond 1.23 V, and relatively high ionic conductivity to viscosity ratio. Despite these advantages, a comprehensive understanding of the physicochemical and structural properties of WiSEs across a broad temperature spectrum remains elusive, thus limiting potential performance enhancements. In this study, the microscopic ion-ion and ion-water interactions in WiSE with potassium acetate (KOAc) as a model electrolyte were systematically analyzed in both bulk phase and electric double-layer (EDL). Concentration-dependent solvation structures, thermal and structural behaviors, and ionic transport properties were investigated through Raman spectroscopy, differential scanning calorimetry (DSC), X-ray diffraction (XRD), and analytical electrochemistry measurements. The phase diagram of the aqueous KOAc solution was constructed using low-temperature XRD to identify the solidified phases, and DSC was used to determine the phase transition temperatures in a wide temperature range from -120 °C to 80 °C. We also explored the temperature-dependent electrochemical performance of various concentrations of KOAc electrolytes, finding the 5 m KOAc shows optimal performance for EDLCs at room temperature and in sub-zero temperatures. Additionally, we proposed a novel concept of an optimal cutoff voltage for maximizing power, comparing it with other commonly used metrics for setting cutoff voltage. This study validates our approach and highlights its significance in balancing key performance parameters.