Structure-Property Correlations in Aqueous Binary Na⁺/K⁺-CH₃COO^- Highly Concentrated Electrolytes

Highly concentrated aqueous binary solutions of acetate salts are promising systems for different electrochemical applications, for example, energy storage devices. The very high solubility of CH₃COOK allows us to obtain water-in-salt electrolyte concentrations, thus reducing ion activity and extending the cathodic stability of an aqueous electrolyte. At the same time, the presence of Li⁺ or Na⁺ makes these solutions compatible with intercalation materials for the development of rechargeable alkaline-ion batteries. Although there is a growing interest in these systems, a fundamental understanding of their physicochemical properties is still lacking. Here, we report and discuss the physicochemical and electrochemical properties of a series of solutions based on 20 mol kg^-1 CH₃COOK with different concentrations of CH₃COONa. The most concentrated solution, 20 mol kg^-1 CH₃COOK + 7 mol kg^-1 CH₃COONa, gives the best compromise between transport properties and electrochemical stability, displaying a conductivity of 21.2 mS cm^-1 at 25 °C and a stability window of up to 3 V in "ideal" conditions, i.e., using a small surface area and highly electrocatalytic electrode in a flooded cell. Careful Raman spectroscopy analyses help to address the interaction network, the phase evolution with temperature, and the crystallization kinetics.