The Thermodynamic and Kinetic Influence of Solvent, Counter Anion and Salt Concentration on ORR in Li-O2 Batteries

Abstract

Aprotic Li-O2 batteries have been extensively studied due to their very high theoretical gravimetric energy density; potentially 2-3 times that of commercial Li-ion batteries1. Despite this significant research effort, practical rechargeable Li-O2 batteries remain elusive due to significant issues relating to high overpotential and parasitic side reactions that limit the cycling performance of the battery chemistry. In order to address these issues, a stronger fundamental understanding of the underlying oxygen redox reactions is needed. In this work, the complex influence of the solvent2, counter anion3 and salt concentration4 during the oxygen reduction reaction are unified within a single framework. Ionic coupling between Li+ ions and the superoxide (O2 -) anion formed following the first electron transfer is highlighted as the critical parameter for determining the selectivity between the two competitive discharges pathways. By weakening the Li+-O2 - coupling through interactions with solvent/counter anions with high donor number, or by lowering the Li+ activity, we observe a significant enhancement in the proportion of observed soluble discharge products. Additionally, electrolyte design strategies for suppressing the direct two electron transfer reaction to form an insulating film of Li2O2 will be discussed in their relation to enabling redox mediators during the reduction process, such as DBBQ5 and EV. References D. Aurbach, B. D. McCloskey, L. F. Nazar, and P. G. Bruce, Nature Energy, 1, 16128 (2016).D. G. Kwabi et al., Angewandte Chemie International Edition, 55, 3129–3134 (2016).C. M. Burke, V. Pande, A. Khetan, V. Viswanathan, and B. D. McCloskey, Proceedings of the National Academy of Sciences, 112, 9293–9298 (2015).R. Tatara et al., The Journal of Physical Chemistry C, 121, 9162–9172 (2017).X. Gao, Y. Chen, L. Johnson, and P. G. Bruce, Nature Materials, 15, 882–888 (2016).