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

Abstract

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 CH 3 COOK 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 CH 3 COOK with different concentrations of CH 3 COONa. The most concentrated solution, 20 mol kg −1 CH 3 COOK + 7 mol kg −1 CH 3 COONa, 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.

Are NaTFSI and NaFSI Salt-Based Water-in-Salt Electrolytes Structurally Similar or Different?

“…Sodium ion batteries benefit from high natural abundance, low cost, and wide distribution in the Earth’s crust. − In the past, various studies have been focused on the utilization of water as a solvent medium in battery electrolytes. − These aqueous-based electrolytes are used more widely in supercapacitors, low-cost electronic devices, and grid storage. , Aqueous sodium ion rechargeable batteries are safer and provide nonflammability and high power density with an affordable cost. ,− The ionic conductivity of lithium ion based aqueous electrolytes is found to be higher than those in organic solvents conventionally used in battery electrolytes. , Dilute aqueous electrolytes suffer from severe drawbacks, including their narrow electrochemical stability window (<1.2 V). ,− Therein, low voltage causes deposition of water on the electrodes which leads to poor cycling life . In addition, there is a lack of stable solid electrolyte interphase (SEI) formation .…”

“…Further, they compared the solubility and electrochemical performance of different sodium salts dissolved in water and concluded that, apart from having higher salt solubility in water, the anions must involve in the SEI formation to achieve better battery performance. Later on, several research studies on WiSEs emphasized the role of higher salt solubility and the ability of anions to interact with cations in order to enhance the SEI formation as well as suppress the water activity. ,− In addition to this, in recent studies, various approaches have been adapted to increase the solubility limit of sodium salts in water. ,, The sodium salt having a bis(trifluoromethylsulfonyl)imide (TFSI) anion has a limited solubility of around 7 m in water, but it shows a dense SEI formation at a lower salt concentration as compared to LiTFSI salt . Also, sodium bis(fluorosulfonyl)imide (NaFSI) based aqueous electrolyte renders its electrochemical potential window up to 2.6 V at a high salt concentration of 35 m .…”

Unveiling the Structural and Dynamic Characteristics of Concentrated LiNO3 Aqueous Solutions through Ultrafast Infrared Spectroscopy and Molecular Dynamics Simulations

“…The hydration of ions significantly influences the electrochemical stability window of aqueous electrolytes to a large extent. 41 Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy were employed in this work to elucidate the hydrogen-bonding networks in LiNO 3 aqueous solutions. It should be noted that these solubility values represent the solubility limits of stable supersaturated solutions prepared by dissolving LiNO 3 at 35 °C and then cooling the solution to room temperature, as described in the literature.…”

“…The hydration of ions significantly influences the electrochemical stability window of aqueous electrolytes to a large extent . Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy were employed in this work to elucidate the hydrogen-bonding networks in LiNO 3 aqueous solutions.…”