Achieving Fast and Reversible Sulfur Redox by Proper Interaction of Electrolyte in Potassium Batteries

Potassium–sulfur batteries have potential for low-cost and high-energy density energy storage. However, it is a challenge to find suitable electrolytes affording liquid environment for intermediate sulfur species to convert at high voltages. In this study, a series of ether/potassium salt systems were systematically studied to investigate the electrochemical stability and function of the electrolytes in sulfur electrochemistry by using in situ ultraviolet–visible and Fourier-transform infrared spectroscopies. Interactions of soluble polysulfides with the electrolyte were critical to the electrochemical performance. Under optimized conditions, the bis(trifluoromethanesulfonyl)imide anion demonstrated moderate interaction and reversible solvation/desolvation of polysulfides. Polar carboxyl groups in poly(acrylic acid) were effective for binding polysulfide in electrodes, enabling reversible sulfur conversions at high working voltages and improved initial Coulombic efficiency. This enhanced battery performance was achieved even using a conventional carbon host with a high sulfur loading of ∼69 wt %, i.e., ∼49 wt % in the cathode.