The critical role of phase-transfer catalysis in aprotic sodium oxygen batteries



In the search for improved energy storage, rechargeable metal–oxygen batteries are very attractive owing to their reliance on molecular oxygen, which forms oxides on discharge that decompose reversibly on charge. Much focus has been directed at aprotic Li–O2 cells, but the aprotic Na–O2 system is of equal interest because of its better reversibility. We report here on the critical role and mechanism of phase-transfer catalysis in Na–O2 batteries. We find that it is solely responsible for the growth and dissolution of micrometre-sized cubic NaO2 crystals and for the reversible cell capacity. In the absence of phase-transfer catalysis, quasi-amorphous NaO2 films are formed and cells exhibit negligible capacity. Electrochemical investigations provide a measure of the transportation of superoxide from the carbon electrode to the electrolyte phase by the phase transfer catalyst. This leads to a new understanding of the mechanism of Na–O2 batteries that, significantly, extends to Li–O2 cells and explains their different behaviour.

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Figure 1: Comparison of the electrochemistry, XRD patterns and morphology of NaO2 formed by discharge in pure and water-added NaOTf salt electrolyte.
Figure 2: Oxygen evolution monitored by online mass spectrometry.
Figure 3: Schematic illustration of the mechanism of proton phase-transfer catalysis.
Figure 4: RRDE analysis of superoxide transport in the electrolyte in the absence or presence of water.
Figure 5: Electrochemical results for Na–O2 cells.


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The authors acknowledge financial support from NRCan, through the EcoEII programme, and from NSERC, via the Canada Research Chair and Discovery programme (to L.F.N.) and scholarship programme (CGS-D, to R.B. and B.A.). The Waterloo Institute of Nanotechnology is acknowledged for a WIN fellowship to R.F. The authors thank G. Popov for assistance with the Rietveld refinement of pure NaOTf.

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C.X. and L.F.N. designed this study. C.X. and R.F. prepared materials and carried out the electrochemical experiments, together with R.B., who performed the online mass spectrometry studies. R.B. carried out the SEM measurements and B.A. performed the RRDE experiments. L.F.N., together with all co-authors, wrote the manuscript, and all authors contributed to the scientific discussion.

Correspondence to Linda F. Nazar.

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Xia, C., Black, R., Fernandes, R. et al. The critical role of phase-transfer catalysis in aprotic sodium oxygen batteries. Nature Chem 7, 496–501 (2015) doi:10.1038/nchem.2260

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