Redox Shuttles for Sustained Oxygen Reduction in Non-Aqueous Lithium Electrolyte

Direct electrochemical oxygen reduction in non-aqueous electrolytes results in the deposition of an insoluble product on the electrode surface, thereby blocking access of oxygen to the surface for continued reaction (Fig. 1). Once the surface is completely covered, the reaction stops and the discharge is ended. Alternatively, in the presence of a soluble redox shuttle, oxygen reduction can occur homogeneously in solution near the air interface (Fig. 2). This has the effect of displacing oxygen reduction away from the electrode surface, which remains active to regenerate the shuttle to the reduced form. A further possibility is mediation of the oxygen reduction reaction in order to favour the formation of lithium peroxide rather than radical products that may degrade the electrolyte (Fig. 3). Given a suitable shuttle reagent, this scheme may have several potential advantages, for example,

• Avoidance of electrode passivation by insoluble lithium oxides
• An increased, chemical driving force for oxygen adsorption into the electrolyte
• Increased mass transfer to the electrode surface
• Enhanced kinetics of lithium peroxide formation
• Minimisation of side reactions that may degrade the electrolyte

We will present results on  Ethyl Viologen acting as a shuttle, including its reduction at the electrode, its reactions with oxygen, and its stability to the products of oxygen reduction under various conditions.

Acknowledgement

This work was supported by the European Union’s Seventh Framework Programme under EC-GA No. 265971 ‘LABOHR’ and the University of Southampton.