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

## TL;DR

This paper shows how measuring pressure changes during battery charging helps evaluate the effectiveness of redox mediators in lithium-oxygen batteries.

## Contribution

A novel operando pressure measurement method is introduced to assess redox mediator performance and track activity loss during battery cycling.

## Key findings

- Pressure measurements revealed distinct behaviors of glyme- and sulfone-based electrolytes with TEMPO as a redox mediator.
- Parasitic reactions were more pronounced in the mediated diglyme system during early cycles.
- The method identified exact points where redox mediator efficacy diminishes during battery charging.

## Abstract

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.

Redox mediator (RM) efficacy during Li–O2 cell charging is maximised when gas evolution is centred around the RM oxidation potential.