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–1CH3COOK with different concentrations of CH3COONa. The most concentrated solution, 20 mol kg–1CH3COOK + 7 mol kg–1CH3COONa, gives the best compromise between transport properties and
electrochemical stability, displaying a conductivity of 21.2 mS cm–1at 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.