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

Highly concentrated aqueous binary solutions of acetatesalts arepromising systems for different electrochemical applications, forexample, 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 ofan aqueous electrolyte. At the same time, the presence of Li+ or Na+ makes these solutions compatible with intercalationmaterials for the development of rechargeable alkaline-ion batteries.Although there is a growing interest in these systems, a fundamentalunderstanding of their physicochemical properties is still lacking.Here, we report and discuss the physicochemical and electrochemicalproperties 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 andelectrochemical stability, displaying a conductivity of 21.2 mS cm(-1) at 25 degrees C and a stability window of up to 3 Vin ideal conditions, i.e., using a small surface areaand highly electrocatalytic electrode in a flooded cell. Careful Ramanspectroscopy analyses help to address the interaction network, thephase evolution with temperature, and the crystallization kinetics.

Automated summary

Highly concentrated aqueous binary solutions of acetate salts show potential for electrochemical applications, including energy storage. These solutions have high solubility and can reduce ion activity, leading to better stability of the electrolyte. The addition of Li+ or Na+ allows compatibility with intercalation materials, making them suitable for rechargeable alkaline-ion batteries. However, a deeper understanding of their physicochemical properties is still needed. In this study, we investigated the properties of solutions with different concentrations of CH3COONa and found that the solution with 20 mol kg(-1) CH3COOK + 7 mol kg(-1) CH3COONa achieved the best balance between transport properties and electrochemical stability.