Dimethyl sulfoxide-based electrolytes for high-current potassium-oxygen batteries

The thermodynamic stability and relatively low free energy of formation (ΔG⁰ = -239.4 kJ mol^-1) of KO₂ offer the possibility of K-O₂ cells as a catalyst-free, low overpotential energy storage system. Having identified dimethyl sulfoxide (DMSO) as a solvent that promotes KO₂ production due to its high donor number, the present study elucidates the oxygen reduction reaction mechanism of the K-O₂ cell with a DMSO electrolyte. The use of DMSO-based electrolytes led to distinct first and second electron-transfer peaks, suggesting the possibility of facile voltage-based control of the cathode reaction to selectively produce KO₂ as the product. However, the observed low overpotential i-E behavior on a rotating ring-disk electrode could only be accounted for by postulating further chemical reactions (disproportionation on the electrode surface and in the electrolyte) of KO₂ to form K₂O₂. The rate of the surface disproportionation reaction to produce K₂O₂ was found to be competitive with the KO₂ desorption step, whereas the solution disproportionation step was found to be an order of magnitude slower. Thus, DMSO is proposed as a solvent that will allow the selective production of KO₂ as the reduction product in a K-O₂ cell, thereby improving the reversibility of the cell. Further, the first electron-transfer rate constant in DMSO was found to be 4 orders of magnitude higher than literature values for the same in diglyme, allowing us to show that DMSO-based K-O₂ cells can achieve rate capability superior to diglyme-based K-O₂ cells, significantly improving on the current state-of-the-art.