Abstract
Lithium–oxygen cells, in which lithium peroxide forms in solution rather than on the electrode surface, can sustain relatively high cycling rates but require redox mediators to charge. The mediators are oxidised at the electrode surface and then oxidise lithium peroxide stored in the cathode. The kinetics of lithium peroxide oxidation has received almost no attention and yet is crucial for the operation of the lithium–oxygen cell. It is essential that the molecules oxidise lithium peroxide sufficiently rapidly to sustain fast charging. Here, we investigate the kinetics of lithium peroxide oxidation by several different classes of redox mediators. We show that the reaction is not a simple outer-sphere electron transfer and that the steric structure of the mediator molecule plays an important role. The fastest mediator studied could sustain a charging current of up to 1.9 A cm–2, based on a model for a porous electrode described here.
Highlights
Lithium–oxygen cells, in which lithium peroxide forms in solution rather than on the electrode surface, can sustain relatively high cycling rates but require redox mediators to charge
We investigate the kinetics of Li2O2 oxidation by several classes of redox mediators, which differs in the Eo and ko values, to ascertain the factors that control the rate of Li2O2 oxidation by the mediators
The tip approaches the Li2O2 disk and at small separation distances, the mediator oxidised at the tip diffuses to the Li2O2 disk where it oxidises Li2O2, regenerating itself and contributing to a feedback loop, while concurrently, diffusion of the mediator to the tip is blocked by the surface
Summary
Lithium–oxygen cells, in which lithium peroxide forms in solution rather than on the electrode surface, can sustain relatively high cycling rates but require redox mediators to charge. Use of redox mediators (RMs) on discharge, such as 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ), which are reduced at the electrode surface on discharge and go on to reduce O2 to Li2O2 in solution, can help to mitigate these problems, but result in the formation of Li2O2 disconnected from the electrode surface and electronically isolated during charging[19] This introduces the need for a redox mediator to be employed on charging that can oxidise Li2O220–33. Such mediators are molecules capable of oxidation at the surface of the pores in the porous positive electrode on charging and transfer of holes to the electronically isolated Li2O2 particles within the pores. We investigate the kinetics of Li2O2 oxidation by several classes of redox mediators, which differs in the Eo (standard redox potential) and ko (standard heterogeneous electron transfer rate constant) values, to ascertain the factors that control the rate of Li2O2 oxidation by the mediators
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