Verification of O2-• Formation during Oxygen Reduction Reaction in Li-O2 Batteries

Li-O₂ battery has been regarded as one of the most promising energy storage systems for the next generation of electrical vehicles and large scale stationary applications.¹  It has very high theoretical energy density compared to those of other rechargeable batteries. However, the research for Li-O₂ batteries so far is still in its early stage. Significant problems on the stability of electrolyte, cathode and Li anode used in Li-O₂ battery still need to be solved before its practical applications.²

In this regards, selection of stable electrolytes, substrates, and efficient catalysts are at the heart of all concerns, without which the rechargeable Li- O₂ batteries will not be successful. In order to search stable electrolyte and develop highly efficient catalyst and consequently improve the cycle life of the Li-O₂ battery, it is necessary to have a deep understanding on the mechanism of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in non-aqueous electrolytes. The mechanism of ORR and OER greatly depends on the activity of catalysts and the types of electrolyte solution environments. Superoxide radical anion (O₂^-•) has been proposed as an intermediate during the discharge process, followed by a disproportionation reaction. However, no direct evidence has been found to prove this mechanism because the lifetime of superoxide radical is extremely short at room temperature and it is difficult to visualize the existence of O₂^-• by using spectroscopic techniques.  Recently, we have studied ORR and OER mechanisms in a non-aqueous Li-O₂ battery using ex-situ electron paramagnetic resonance (EPR) technique and have successfully detected the formation of O₂^-• during the ORR process (Figure 1). In the charging process, nearly no formation of O₂^-•has been found during the OER process. Details of the investigations will be reported and discussed in the presentation.

Acknowledgements

This work was supported by the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the Basic Energy Sciences, Office of Science of the U.S. DOE. The EPR measurements were performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at PNNL.

1.             P. G. Bruce, S. A. Freunberger, L. J. Hardwick and J.-M. Tarascon, Nat Mater, 2012, 11, 19-29.

2.             Y. Shao, F. Ding, J. Xiao, J. Zhang, W. Xu, S. Park, J.-G. Zhang, Y. Wang and J. Liu, Adv Funct Mater, 2013, 23, 987-1004.