The effect of addition of the redox mediator dimethylphenazine on the oxygen reaction in porous carbon electrodes for Li/O2 batteries

Abstract

Secondary Li–O₂ batteries are promising due to their potentially high theoretical energy density. However, both the discharge (oxygen reduction reaction, ORR) and the recharge reaction (oxygen evolution reaction, OER) are associated with high irreversible losses, and multiple side reactions, depending on the electrolyte of choice. Addition of redox mediators is currently considered a promising route to combat the challenges of the highly irreversible ORR/OER. In this work, the effect of addition of the redox mediator 5,10-dimethylphenazine (DMPZ) on the capacity and reversibility of the oxygen reaction is investigated in porous carbon electrodes. The electrolytes are based on tetraethylene glycol dimethyl ether (TEGDME) as solvent, and either Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as salt, or a combination of LiTFSI and LiNO₃ salt, alternatively dimethyl sulfoxide (DMSO) as solvent, with LiTFSI salt. The addition of DMPZ results in a significant improvement of the reversibility of the ORR/OER reactions for electrolytes based on LiTFSI in DMSO, and LITFSI + LiNO₃ in TEGDME. This is attributed to a depression of the side reactions limiting the recharge reaction in these electrolytes. Post mortem analyses by XRD, SEM, as well as FIB-SEM investigations of cross sections, are used to characterize the products from the side reactions.