Structure-Property Correlations in Aqueous Binary Na/K-CHCOO 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 CHCOOK 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 CHCOOK with different concentrations of CHCOONa. The most concentrated solution, 20 mol kg CHCOOK + 7 mol kg CHCOONa, gives the best compromise between transport properties and electrochemical stability, displaying a conductivity of 21.2 mS cm 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.