The Influence of Discharge Mediators on Singlet Oxygen Generation in Lithium-Air Batteries

Abstract

Next-generation metal-air batteries are limited by factors such as high overpotential, and low energy efficiency. These factors lead to significant electrode and electrolyte decomposition, and consequently, limit the cell life. The primary cause of these parasitic reactions has been identified as the generation of singlet oxygen (1O2) 1 from the disproportionation of superoxide during both charge and discharge.2 Nonetheless, in addition to singlet oxygen generation, in a lithium air battery the insulating products such as lithium peroxide (Li2O2) tend to accumulate on the cathode overtime and passivate the active surface. This results in early cell death and significantly hinders the overall efficiency of the battery. 3, 4 Redox mediators (RMs) are one potential solution to these problems. For example, by mediating the charge and discharge pathway, they can shuttle electrons back-and-forth between the electrodes and the charge and discharge products. This prevents passivation of the porous cathode by facilitating both a greater distribution of the discharge products, as well as an easier means by which to re-oxidise these insulating materials. The literature shows several examples of redox couples, including quinones, that inhibit the formation of a passivating film to some extent and result in higher rates, higher capacities and longer cycle lives.5, 6 Previously, our group has shown that the choice of RM can either enhance or suppress 1O2 formation during charge.7 We have now expanded this investigation to discharge mediators. In this work, we explore the primary reason for singlet vs triplet oxygen formation and identify if it is possible to suppress 1O2 formation completely via RMs. We leverage mass spectrometry, pressure cell measurements, and spectro-electrochemistry in this work to gain insight into the reaction kinetics and 1O2 generation mechanism. These experimental results are further supported by density functional theory (DFT) to help elucidate the reaction pathway.