Mechanism and Performance of a CO2-Driven Li-O2 Battery Containing LiBr

Abstract

The environmental friendly aprotic lithium–oxygen (Li–O2) battery with high theoretical energy density has received great attention in the past years. A necessary shift to a direct exploitation of atmospheric air to assure the practicality of these devices has, however, demonstrated that the presence of CO2 and moisture fundamentally alters the O2 electrochemistry of the cell. The uncovered effect of CO2 in Li–O2 cells in a tetraglyme-electrolyte under a wide O2:CO2 mixture range revealed that with up to 30% CO2, the latter acts as an O2 − scavenger during discharge steps, leading to the formation of representative CO4 −/C2O6 2−-based species correlated with enhanced full capacity and cell cyclability. The CO2-driven Li–O2 device suffers, however, from unpractical recharge potentials at ~4.5 V required for the decomposition of Li2CO3. In this work, we investigate the performance of these devices in cells containing LiBr acting as a redox mediator. The electrochemically-generated Br3 –/Br2 redox couple offers a promising pathway to facilitate the decomposition of Li2CO3. We show that hindered overcharges (~0.5 V) are not only a function of the introduced LiBr concentration, but also depend on the crystallinity and morphology of the insulating Li2CO3 discharge product. Coupled electrochemical and spectroscopic analyses shed light on the redox shuttling steps at progressive states-of-recharge, revealing the presence of Br2···Br3 − loosely-bound anionic complexes in the electrolyte. As opposed to discrete polar Br2 irreversibly precipitated on the carbon-based cathode surface, the facile diffusion of the soluble Br2···Br3 − complex pinpoints these species as operating redox mediators. These findings offer insight toward the rational design of efficient mobile mediators beyond an overcharge decrease in the CO2-driven Li–O2 device.