Unveiling the Underlying Mechanism of CO₂-Assisted Li–O₂ Batteries in the Presence of a Br₃ ^–/Br₂ Redox Couple

A critical step towards the commercialization of Li-air batteries involves replacing O2 with air. The presence of CO2 in the air, however, has been established to substantially alter the O2 chemistry of the Li-air battery, emphasizing the need to pursue fundamental exploratory works prior to the development of practical devices [1]. In our preliminary studies, we have investigated the effect of an increasing CO2 incorporation in the O2 gas feed on the performance of the resulting devices with a tetraglyme-based electrolyte. When up to 30% CO2 was added to Li–O2 cells, CO2 acted as an O2 − scavenger, with emerging soluble discharge products (e.g., CO4 2− and C2O6 2−) being correlated with enhanced full capacity and cell cyclability of these devices. The CO2-assisted Li–air device was confirmed to suffer, however, from unpractical recharge potentials at ~4.5 V required for the decomposition of Li2CO3, equally formed during discharge [2]. In agreement, incorporating a Br3 –/Br2 redox mediator was proposed as a promising strategy to suppress the overcharge potential. Electrochemically-generated Br2 was shown in this work to remarkably hinder the overpotential during recharge (~0.5 V). In addition, the performance of the introduced redox mediator was disclosed to be a function of the introduced LiBr concentration and of the insulating Li2CO3 crystallinity and morphology. As a seminal methodology, coupled electrochemical and spectroscopic analyses were then pursued to discern involved redox shuttling steps at progressive states-of-charge. Remarkably, our work revealed here for the first time the presence of Br2···Br3 − loosely-bound anionic complexes in the electrolyte during the recharge step. As opposed to discrete polar Br2 irreversibly precipitated on the CNT surface, the facile diffusion of the soluble Br2···Br3 − complex pinpointed these species as operating redox mediators during cell cycling. These findings are believed to shed light over the rational design of efficient mobile mediators beyond an overcharge decrease in the unexplored CO2-assisted Li–O2 device. References 1. Takechi, T. Shiga and T. Asaoka, Chem. Commun. 47, 3463 (2011). 2. Qiao, J. Yi, S. Guo, Y. Sun, S. Wu, X. Liu, S. Yang, P. Hec and H. Zhou, Energy Environ. Sci. 5, 1211 (2018).