Water-in-bisalt electrolytes with mixed hydrophilic and hydrophobic anions for enhanced transport and stability for potassium-ion batteries

Abstract

Water-in-salt electrolytes provide an expanded electrochemical potential window, thus enabling a wide range of battery chemistries based on readily available salts and water. This study introduces a binary salt approach for achieving high K⁺ concentration with a tunable solvation sphere composed of acetate (Ac⁻) and trifluoromethane sulfonate (OTf⁻) anions, and water. Combining the hydrophilic low-cost potassium acetate with hydrophobic potassium trifluoromethane sulfonate salts, 36 molal liquid electrolyte, K(Ac)_0.9(OTf)_0.1·1.5H₂O, is achieved with an electrochemical stability window spanning from −1.74 V on the Al electrode to over 3 V on the Ti electrode, exceeding a total of 4.74 V and an ionic conductivity of 18 mS cm⁻¹ at 25 °C. Both the Raman and NMR analyses show strong water-Ac⁻ interactions within the primary solvation shell of K⁺. In parallel, OTf⁻ is found to be outside of this shell disrupting the water O–H network, thus pushing water into the K⁺ solvation shell. Elimination of the free water molecules and the solvation disproportion resulting from the bi-salt approach underlie the enhanced transport properties and the electrochemical stability window. Therefore, the mixed bi-salt approach in water-in-salt electrolytes can be a promising and cost-effective solution for advancing potassium batteries.