Elsevier

Electrochimica Acta

Volume 247, 1 September 2017, Pages 610-616
Electrochimica Acta

The Role of Ionic Liquid in Oxygen Reduction Reaction for Lithium-air Batteries

https://doi.org/10.1016/j.electacta.2017.06.137Get rights and content

Abstract

We have investigated the oxygen reduction reaction (ORR) in the presence of non-aqueous electrolytes in an attempt to overcome the challenges related to lithium-air batteries, such as low reversibility, poor rate capability, and electrode/solvent stability. We have used glassy carbon as the working electrode in electrolytes composed of lithium bis(trifluoromethanesulfonyl)imide and 1,2-dimethoxyethane or N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI, ionic liquid). We have employed the kinetic model to treat the electrochemical impedance spectroscopy data. This approach provides the rate constants for each of the elementary steps and allows indirect investigation of the role played by the ionic liquid in the ORR. The ionic liquid shifts the onset potential of the ORR to more positive values. The presence of the large Pyr14+ cation increases the rate-determining step by approximately three orders of magnitude as compared to the ether-based electrolyte. This ionic liquid is chemically resistant to degradation reactions and increases the rate of the ORR, which makes it a promising candidate for use in 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].M+O2K1MO2(ads)M

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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