Deciphering the role of LiBr as redox mediator in Li-O2 Aprotic Batteries

Abstract

Lithium-oxygen batteries (Li-O2) represent a highly promising category of energy storage systems, primarily owing to their elevated theoretical energy density. Nevertheless, their effective deployment is significantly impeded by challenges such as inadequate reversibility and the presence of undesirable parasitic reactions. Recent investigations have turned to redox mediators, specifically lithium bromide (LiBr), as a potential solution to improve reaction kinetics and minimize overpotentials in these systems. This research presents a comprehensive analysis of the effects of three distinct solvents—dimethoxyethane (DME), tetraethylene glycol dimethyl ether (TEGDME), and dimethyl sulfoxide (DMSO)—on both the electrochemical performance and reaction mechanisms of LiBr-mediated lithium-oxygen cells. The findings indicate that a channel for the evolution of singlet oxygen (1O2), which subsequently leads to cell degradation due to singlet oxygen-induced reactions, is exclusively activated in the TEGDME electrolyte environment. In contrast, both DME and DMSO facilitate an oxygen evolution reaction devoid of singlet oxygen: however it is notably only DME that demonstrates chemical stability under the operational conditions of the LiBr-mediated Li-O2 cells. Furthermore, a comparative analysis of the redox mediation effects arising from lithium iodide (LiI) and LiBr across various solvent environments reveals that the activation of the singlet oxygen release pathway is consistently observed when the Lewis acidity and basicity of the oxidized forms of the redox mediators (RM) and the solvent are congruous (for instance, exhibiting weak acid/weak base or strong acid/strong base characteristics). This study elucidates the nuanced interactions between solvents and redox mediators, thereby contributing to the advancement of more efficient lithium-oxygen battery systems.

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