The Role of Ionic Liquid in Oxygen Reduction Reaction for Lithium-air Batteries
Section snippets
INTRODUCTION
Lithium-air batteries are promising energy storage systems to replace the state-of-the-art lithium ion batteries—lithium-air batteries rely on conversion cathode instead of conventional intercalation materials, in which molecules of atmospheric oxygen are the redox active species [1], [2], [3], [4]. Unfortunately, the large-scale use of this technology is limited by issues like poor rate capability, low life cycle, low energy efficiency, formation of lithium dendrite formation, and solvent
EXPERIMENTAL
The N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI) ionic liquid (IL) (Fig. 1) was synthesized according to the procedure described in detail elsewhere [31], [32], [33]. The electrolyte was prepared by dissolving the LiTFSI lithium salt (3 M, battery grade) in PYR14TFSI at 0.2 mol kg−1. The electrolyte was dried under vacuum at 90 °C until the water content was reduced to less than 2 ppm as determined by Karl Fischer titration.
1,2-Dimethoxyethane (DME) containing 4 ppm
RESULTS AND DISCUSSION
Literature works have proposed the following reaction mechanism for the ORR occurring in aprotic medium [29], [36], [37], [38]. In our case, the difference lies on the last chemical equilibrium, which considers partial dissolution of adsorbed lithium peroxide in the solvent, to renovate the active surface. We also considered reaction intermediates and products adsorbed onto the electrode sites, which depended on the electrolyte and on the investigated potential range [6], [39].
CONCLUSIONS
According to the model in the frequency domain, the rate constant of the RDS is three orders of magnitude higher for the process conducted in ionic liquid as compared to DME. Indeed, the rate of the ORR is higher at more positive potentials in the presence of Pyr14TFSI, as observed by the cyclic voltammograms. We suggested this increased catalytic activity to interaction between Pyr14+ and the superoxide anion, which also makes lithium superoxide energetically more stable. Consequently, lithium
ACKNOWLEDGMENTS
We are grateful to FAPESP (Projects numbers 2011/12668-0, 2014/15798-0, and 2011/21545-0). We acknowledge the financial support of Helmholtz Association to Karlsruhe Institute of Technology (KIT)/Helmholtz Institute Ulm (HIU) and Deutschen Bundesstiftung Umwelt (project 31452/01).
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