In recent years, continuous studies have highlighted the languished research state of Li–O₂, and the impelling development of energy storage devices with analogously high theoretical energy density [1]. In preliminary studies, we established that fed O₂ and CO₂ mixtures (Li–O₂/CO­₂) swiftly shift the reaction mechanism toward an exclusive formation of Li₂CO₃ in a TEGDME-based electrolyte. The proposed O₂-assisted pathway is initiated by an electron transfer from the cathode, with subsequent CO₄^– formation and complete Li₂O₂ suppression. Compared with O₂-driven counterparts, the CO₂-based Li–air cell suffers, however, from higher overcharges during the decomposition of Li₂CO_{3 at} ~4.5 V_, rendering impracticable a commercialization of these technologies [2]. Here we propose for the first time the incorporation of a redox mediator (RM) toward facile Li₂CO₃ oxidation in Li–O₂/CO­₂ devices. Lithium bromide (LiBr) possessing two redox couples (Br⁻/Br₃⁻ and Br₃⁻/Br₂) has gathered increasing attention in conventional Li–air cells and was considered a suitable candidate in this study. All galvanostatic measurements were carried out under 30% CO₂. At optimized concentrations, hindered recharge potentials (~0.5 V) at similarly sustained cyclability were demonstrated. Further dissection of the significance of involved Br⁻/Br₃⁻/Br₂ species inferred, however, an additional role of the RM in the stabilization of intermediate discharge species and in the undesirable formation of side-products. In this context, drawn conclusions reinforced the need to consider a deeper evaluation of the effects of a RM inclusion beyond an overcharge decrease and the urge to stabilize RM/RM⁺ species participating in deleterious side-reactions with neighboring species and cell components.

References

[1] Grande, E. Paillard, J. Hassoun, J.-B. Park, Y.-J. Lee, Y.-K. Sun, S. Passerini and B. Scrosati, Adv. Mater. 2015, 27, 784.

[2] Qiao, J. Yi, S. Guo, Y. Sun, S. Wu, X. Liu, S. Yang, P. Hec and H. Zhou, Energy Environ. Sci. 2018, 5, 1211.