Multicycle operando pressure measurements enable assessment of redox mediator efficacy in lithium-oxygen batteries

Redox mediators (RMs) present a promising strategy for achieving low overpotential charging of lithium-oxygen (Li-O₂) batteries, thereby extending cycle life and improving overall energy efficiency. In this study, multi-cycle operando pressure measurement during galvanostatic Li-O₂ cell cycling was employed to assess the efficacy of 2,2,6,6-tetramethylpiperdinyloxyl (TEMPO) as a charge RM in sulfolane- and diglyme-based electrolytes. In both mediated electrolytes, electrochemical TEMPO oxidation coincided with gas evolution, validating TEMPO activity and revealing distinct behaviour in the reactions and stability of the glyme- and sulfone-based electrolytes. Pressure measurements showed a greater extent of parasitic reactions during charging in the mediated diglyme system during early cycles. In the sulfolane-based electrolyte, initial stable cycling was observed. However, a more rapid capacity fade was subsequently observed in the latter cycles, due to increasing parasitic chemistry on charge. Furthermore, highly sensitive pressure measurements enabled small changes in the pressure response to be correlated with transitions in the electrochemical cycling profile. Analyses of the dynamic rate of pressure changes within Li-O₂ cells and correlation with differential capacity was used to identify exact points within a charge step wherein RM efficacy is diminished, thereby tracking the evolution of RM activity loss during cycling. This approach provided a valuable indicator of RM efficacy, defined in terms of maximising the number of cycles for which gas evolution is centred around the RM oxidation potential. Importantly, this method directly assesses RM cyclability in the Li-O₂ cell environment and can be applied to any electrolyte-electrode combination, proving to be a versatile approach for identification of promising mediated electrolyte formulations for longer life Li-O₂ batteries.