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Mechanism of mediated alkali peroxide oxidation and triplet versus singlet oxygen formation

Nature Chemistry volume 13, pages 465–471 (2021)Cite this article

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Abstract

Aprotic alkali metal–O2 batteries face two major obstacles to their chemistry occurring efficiently, the insulating nature of the formed alkali superoxides/peroxides and parasitic reactions that are caused by the highly reactive singlet oxygen (1O2). Redox mediators are recognized to be key for improving rechargeability. However, it is unclear how they affect 1O2 formation, which hinders strategies for their improvement. Here we clarify the mechanism of mediated peroxide and superoxide oxidation and thus explain how redox mediators either enhance or suppress 1O2 formation. We show that charging commences with peroxide oxidation to a superoxide intermediate and that redox potentials above ~3.5 V versus Li/Li+ drive 1O2 evolution from superoxide oxidation, while disproportionation always generates some 1O2. We find that 1O2 suppression requires oxidation to be faster than the generation of 1O2 from disproportionation. Oxidation rates decrease with growing driving force following Marcus inverted-region behaviour, establishing a region of maximum rate.

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Fig. 1: Thermodynamics of alkali (su)peroxides, 1O2 evolution thresholds and the used mediators.
Fig. 2: Singlet and triplet oxygen evolution upon mediated peroxide and superoxide oxidation.
Fig. 3: Kinetics of mediated (su)peroxide oxidation and superoxide disproportionation.
Fig. 4: Free energy dependence of superoxide oxidation kinetics.
Fig. 5: Mediated alkali (su)peroxide oxidation mechanism.

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Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author S.A.F. upon reasonable request. Source data are provided with this paper.

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Acknowledgements

S.A.F. is indebted to the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 636069) as well as IST Austria. O.F thanks the French National Research Agency (STORE-EX Labex Project ANR-10-LABX-76-01). We thank EL-Cell GmbH (Hamburg, Germany) for the pressure test cell. We thank R. Saf for help with the mass spectrometry, J. Schlegl for manufacturing instrumentation, M. Winkler of Acib GmbH, G. Strohmeier and R. Fürst for HPLC measurements and S. Mondal and S. Stadlbauer for kinetic measurements.

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Authors and Affiliations

  1. Institute for Chemistry and Technology of Materials, Graz University of Technology, Graz, Austria

    Yann K. Petit, Eléonore Mourad, Christian Prehal, Christian Leypold, Daniel Mijailovic, Christian Slugovc & Stefan A. Freunberger

  2. Institute of Solid State Physics, Graz University of Technology, Graz, Austria

    Andreas Windischbacher & Egbert Zojer

  3. University of Belgrade, Faculty of Technology and Metallurgy, Belgrade, Serbia

    Daniel Mijailovic

  4. Institute for Analytical Chemistry and Food Chemistry, Graz University of Technology, Graz, Austria

    Sergey M. Borisov

  5. Dipartimento di Chimica, Università di Roma La Sapienza, Roma, Italy

    Sergio Brutti

  6. Institut Charles Gerhardt Montpellier, Université Montpellier, Montpellier, France

    Olivier Fontaine

  7. School of Energy Science and Engineering, Vidyasirimedhi Institute of Science and Technology (VISTEC), Rayong, Thailand

    Olivier Fontaine

  8. IST Austria (Institute of Science and Technology Austria), Klosterneuburg, Austria

    Olivier Fontaine & Stefan A. Freunberger

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Contributions

S.A.F., Y.K.P., E.M., C.P., C.L., D.M. and S.M.B. performed the experiments and analysed the results. S.B., A.W. and E.Z. did the density functional theory calculations. C.S. helped with synthesis. O.F. helped with Marcus theory. S.A.F. conceived and directed the research, set up and performed experiments, analysed the results and wrote the manuscript. All authors contributed to the discussion and interpretation of the results.

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Correspondence to Olivier Fontaine or Stefan A. Freunberger.

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Extended data

Extended Data Fig. 1 3O2 loss upon superoxide disproportionation in presence of RMox.

KO2 powder was immersed in 0.1 M LiTFSI/TEGDME containing 0, 0.5, 1, or 2 equivalents of the indicated RMox. One equivalent is the amount to theoretically evolve all O2 (0.5 mol RMox/mol KO2) considering 0.5 mol O2/mol KO2 to evolve from disproportionation. Equal amounts of electrolytes were used and hence the RMox concentration adapted. a, The found amounts of 3O2 relative to the total amount expected from disproportionation and oxidation for the indicated RMox. The dashed lines are quadratic polynomial fits. To prove that RMox rather than RMred drives 3O2 loss, we also used the reduced form of DMPZ. b, The data in a plotted versus the redox potential of the RMs. The trendlines are to guide the eye. See Supplementary Note 7 for in-depth discussion.

Source data

Extended Data Fig. 2 Oxidation kinetics and RMox concentration.

a, Comparision of the mediated superoxidation kinetics k2 and apparent peroxide oxidation kinetics kapp including fits with the Marcus expression in equation (5). b, 1/k which is proportional to the required RMox concentration (\(c_{{{\rm{RM}}^{{\rm{ox}}}}}\) = ν/k) to drive a certain areal oxidation rate ν = k × \(c_{{{\rm{RM}}^{{\rm{ox}}}}}\).

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Supplementary information

Supplementary Information

Supplementary Methods, Figs. 1–11, Notes 1–8 and References.

Supplementary Table 4

xyz-Coordinates of the structures of the three molecules (TDPA, TDPA+ and TDPA2+) optimized in methanol, 76 atoms, coordinates in Å.

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Petit, Y.K., Mourad, E., Prehal, C. et al. Mechanism of mediated alkali peroxide oxidation and triplet versus singlet oxygen formation. Nat. Chem. 13, 465–471 (2021). https://doi.org/10.1038/s41557-021-00643-z

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