Cobalt porphyrin and Salcomine as novel redox shuttle species to enhance the oxygen evolution reaction in Li-O2 batteries
Résumé
Li-O2 secondary batteries outclass other battery systems in terms of energy density, at least on a theoretical point of view. However, these systems suffer issues from both the metallic lithium electrode and the oxygen electrode. For example, the cathode discharge product (lithium peroxide) is a semiconductor of large bandgap (5 eV), which yields severe cyclability issues, as the re-charge is limited. Using electrolyte additives may be one option to mitigate (if not solve) this issue. Herein, a new redox mediator compound (N,N′-Bis(salicylidene)ethylenediaminocobalt (II), noted Co-salen) is studied with regards to its impact on the oxygen reduction and evolution reactions in non-aqueous medium. Its performances are compared to those of a Cobalt porphyrin (noted Co(II)-Po). Electrochemical experiments in three-electrode setup, Differential Electrochemical Mass Spectrometry (DEMS) and UV–Visible spectroscopy, demonstrate that Co-salen can also greatly improve the oxygen reduction kinetics (the ORR onset potential is positively-shifted by 230 mV in the presence of Co-salen), thanks to the formation of a complex with oxygen in solution. Secondly, Co-salen enables to efficiently transport electrons in the solution, and help to recover the electrode surface (the re-charge of large Li2O2 particles becomes possible). Therefore, enhancing both the oxygen reduction and the oxygen evolution reactions (ORR and OER) with only one additive in the electrolyte is feasible, and places Co-salen among the best compounds to be used as redox shuttles for non-aqueous Li-O2 batteries.