Redox shuttles and Positive Electrode Protection for Li-O₂ systems

The present PhD work focuses on solving two major issues of the Li-O₂ positive electrodes, both being linked with the nature of the discharge product formed during the Oxygen Reduction Reaction, in Lithium cation electrolyte: Lithium peroxide (Li₂O₂). The first issue is related to the Discharge mechanism (consecutive Electrochemical nucleation and chemical disproportionation of an intermediate, lithium superoxide), which lead to the formation of large particles of lithium peroxide on the electrode surface. Owing to their size and resistivity (bandgap of lithium peroxide: 5 eV), it is nearly impossible to re-charge efficiently the electrode. This issue can be solved, thanks to the dissolution of an additive in solution, that promote the transport of electrons, and allow the oxidation of large discharge particles (in theory, even the ones disconnected from the electrode). A very good compound was found to efficiently work as a redox shuttle (enhanced Oxygen Evolution reaction), with also a highly beneficial effect for the ORR, with a catalysis effect that allowed to increase the onset of the ORR of 230 mV. However, this solution require a engineering of the practical system as this additive could cross from the positive electrode to the negative side (lithium) and trigger capacity loss and infinite charging loop. The second issue is linked to its reactivity. As a matter of fact, it is an hard base (according to HSAB theory), which reacts readily with a large panel of electrodes component (reactivity toward the PVDF binder, solvent, salts, but also with the carbon material, used as the positive electrode). As such, it is necessary to find a way to protect the latter, and a solution proposed in this work was to use Atomic Layer deposition of Niobium pentoxide (Nb₂O₅), in order to form a very thin deposit, which was supposed to prevent any contact between the discharge product, and the carbon support (consumption of Carbon, with formation of a large bandgap compound: Lithium carbonate). The deposition was conducted onto a graphitized carbon cloth (Zoltek Panex 30), which surprisingly proved to be highly resistant toward lithium peroxide. Sadly, the presence of the deposit did not protect the electrode but rather made it weaker, with tracers of the formation lithium carbonate. This compound was thus not considered anymore, and others deposits are yet needed to be tested in future studies. (author)