Towards Improved Li-O2 Batteries: Understanding the Role of Dissolved Redox Mediators

Abstract

Aprotic lithium-oxygen (Li-O2) batteries emerged as promising energy stores as they exhibit an exceptionally high theoretical energy density. However, their practical application is still hindered by high charging overvoltages arising from the electronically insulating nature of the insoluble reaction product lithium peroxide (Li2O2). Recently, dissolved redox mediators (RMs) like tertrathiafulvalene (TTF), lithium iodide (LiI) and 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) have been introduced as an efficient tool to reduce the charging overvoltages and increase the lifetime [1,2,3]. Herein, we present a systematic study on the underlying principles of redox mediators in Li-O2 batteries and introduce an advanced cell system providing improved capacity and cycling stability. At first, the impact of the different cell parameters on the charging profile of Li-O2 batteries with redox mediators is elucidated and an appropriate model is derived. The model is validated by potentiostatic measurements using a modified cell setup which enables the precise control of the different cell parameters. According to the model, the electrochemical properties of the redox mediator and the morphology of the reaction products are identified as the major impact factors. Subsequently, the model is applied to different cyclic nitroxides, which show a direct relation between structure and performance as redox mediator in Li-O2 batteries. Herein, the nitroxide 1-Methyl-2-azaadamantane N-oxyl emerged as the most efficient redox mediator, as it enables charging of Li-O2 batteries completely below 3.6 V vs. Li+/Li [4]. The concept of redox mediators in aprotic Li-O2 batteries has been further advanced by incorporating a solid Li+ conducting membrane between cathode and anode. This hybrid setup improves the cycling stability and increases the discharge capacity. The Li+ conducting membrane prevents the deactivation of RM+ at the anode and the unwanted shuttling of the redox mediator between both electrodes. It is demonstrated by means of UV-Vis spectroscopy, XRD and pressure monitoring that the redox mediator oxidation and reduction reactions (RMOR and RMRR) are included into the charge cycling of hybrid Li-O2 batteries. Depending on the concentration of the mediator, the capacity is raised by up to 500 mAh/gC in this study. Unexpectedly, the implementation of the Li+ selective membrane additionally improves the cycling stability, as the unwanted evolution of CO2 is distinctly reduced. These results indicate that the lithium anode contributes to the cathodic decomposition reaction in regular Li-O2 batteries without a membrane. In summary, this work provides a comprehensive understanding of the reactions in Li-O2 batteries with dissolved redox mediators and introduces a sustainable concept for their further development. [1] Chen et al., Nat. Chem. 2013, 5, 489. [2] Lim et al., Angew. Chem. Int. Ed. 2014, 53, 4007. [3] Bergner et al., J. Am. Chem. Soc. 2014, 136, 15054. [4] Bergner et al., Phys. Chem. Chem. Phys. 2015, 17, 31769.