Critical appraisal on the role of catalysts for the oxygen reduction reaction in lithium-oxygen batteries

This work reports a detailed characterization of the reduction of oxygen in pyrrolidinium-based ionic liquids for application to lithium-oxygen batteries. It is found that, in the absence of Li⁺, all electron transfer kinetics are fast, and therefore, the reactions are limited by the mass transport rate. Reversible reduction of O₂ to O₂^{• −} and O₂^{• −} to O₂²⁻ take place at E⁰ = 2.1 V and 0.8 V vs. Li⁺/Li, respectively. In the presence of Li⁺, O₂ is reduced to LiO₂ first and then to Li₂O₂. The solubility product constant of Li₂O₂ is found to be around 10⁻⁵¹, corroborating the hypothesis that electrode passivation by Li₂O₂ deposition is an important issue that limits the capacity delivered by lithium-oxygen batteries. Enhancing the rate of Li₂O₂ formation by using different electrode materials would probably lead to faster electrode passivation and hence smaller charge due to oxygen reduction (smaller capacity of the battery). On the contrary, soluble redox catalysts can not only increase the reaction rate of Li₂O₂ formation but also avoid electrode passivation since the fast diffusion of the soluble redox catalyst would displace the formation of Li₂O₂ at a sufficient distance from the electrode surface