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

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)(OTf)·1.5HO, 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 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.