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The role of LiO2 solubility in O2 reduction in aprotic solvents and its consequences for Li–O2 batteries

Nature Chemistry volume 6, pages 1091–1099 (2014)Cite this article

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An Erratum to this article was published on 17 December 2014

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Abstract

When lithium–oxygen batteries discharge, O2 is reduced at the cathode to form solid Li2O2. Understanding the fundamental mechanism of O2 reduction in aprotic solvents is therefore essential to realizing their technological potential. Two different models have been proposed for Li2O2 formation, involving either solution or electrode surface routes. Here, we describe a single unified mechanism, which, unlike previous models, can explain O2 reduction across the whole range of solvents and for which the two previous models are limiting cases. We observe that the solvent influences O2 reduction through its effect on the solubility of LiO2, or, more precisely, the free energy of the reaction LiO2* ⇌ Li(sol)+ + O2−(sol) + ion pairs + higher aggregates (clusters). The unified mechanism shows that low-donor-number solvents are likely to lead to premature cell death, and that the future direction of research for lithium–oxygen batteries should focus on the search for new, stable, high-donor-number electrolytes, because they can support higher capacities and can better sustain discharge.

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Figure 1: CVs demonstrating the significant effect that solvent DN and cation type have on O2 reduction.
Figure 2: CVs showing the first step of O2 reduction and plots of shifts of E°1 (O2/O2−) with In[Li+] concentration for Me-Im and DMSO.
Figure 3: SER spectra demonstrating that at high voltages (low overpotentials) O2− and LiO2 species are observed on the electrode surface at short times in high- and low-DN solvents, respectively, to be replaced by Li2O2 over time.
Figure 4: Evidence from RRDE experiments showing the presence of O2− in solution in high-DN solvents (Me-Im and DMSO), some in intermediate-DN solvent (DME) and essentially none in low-DN CH3CN.
Figure 5: Schematic of the O2 reduction mechanism and plot showing how it is affected by DN and potential.
Figure 6: Potential versus time at a planar Au electrode in various O2-saturated aprotic solvents and 100 mM LiClO4.
Figure 7: SEM images showing the Li2O2 morphologies obtained in different solvents and at different potentials.

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  • 20 November 2014

    In the version of this Article originally published, the author list was incorrectly ordered. Jean-Marie Tarascon should have appeared as the penultimate name. This has now been corrected in all online versions of the Article.

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Acknowledgements

P.G.B. acknowledges financial support from the Engineering and Physical Sciences Research Council (including the SUPERGEN programme). S.A.F. acknowledges financial support from the Austrian Federal Ministry of Economy, Family and Youth and the Austrian National Foundation for Research, Technology and Development as well as the Austrian Science Fund (FWF): P26870-N19. K.D. thanks the UK EPSRC for funding and the European Union project FAMOS (FP7 ICT, contract no. 317744). The authors thank D. Larcher for discussions.

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Author notes

  1. Lee Johnson and Chunmei Li: These authors contributed equally to this work

Authors and Affiliations

  1. EastChem, School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9ST, UK

    Lee Johnson, Chunmei Li, Zheng Liu & Yuhui Chen

  2. Departments of Materials, University of Oxford, Parks Road, OX1 3PH, Oxford, UK

    Lee Johnson, Zheng Liu, Yuhui Chen & Peter G. Bruce

  3. Laboratoire de Réactivité et Chimie des Solides — UMR CNRS, 6007, 33 rue Saint-Leu, Amiens Cedex, 80039, France

    Chunmei Li

  4. Christian Doppler Laboratory for Lithium Batteries, and Institute for Chemistry and Technology of Materials, Graz University of Technology, Stremayrgasse 9, Graz, 8010, Austria

    Stefan A. Freunberger

  5. SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9SS, UK

    Praveen C. Ashok, Bavishna B. Praveen & Kishan Dholakia

  6. Collège de France, 11 place Marcelin Berthelot, Paris Cedex, 75231, France

    Jean-Marie Tarascon

  7. Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR

    Peter G. Bruce

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Contributions

L.J. and C.L. designed and performed electrochemical and Raman spectroscopy experiments and analysed the data. Z.L. discharged and performed microscopy of Li2O2 on high-surface-area cathodes. P.C.A. and B.B.P. built and maintained the Raman microscope and contributed to the Raman measurements and analysis. Y.C. performed the UV–vis spectroscopy experiments and analysed the data. P.G.B., L.J., Y.C. and S.F. interpreted the data. P.G.B. wrote the paper with contributions from L.J. The project was supervised by P.G.B., J-M.T. and K.D.

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Correspondence to Peter G. Bruce.

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Johnson, L., Li, C., Liu, Z. et al. The role of LiO2 solubility in O2 reduction in aprotic solvents and its consequences for Li–O2 batteries. Nature Chem 6, 1091–1099 (2014). https://doi.org/10.1038/nchem.2101

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