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Revealing the reaction mechanisms of Li–O2 batteries using environmental transmission electron microscopy

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Abstract

The performances of a Li–O2 battery depend on a complex interplay between the reaction mechanism at the cathode, the chemical structure and the morphology of the reaction products, and their spatial and temporal evolution1,2,3,4; all parameters that, in turn, are dependent on the choice of the electrolyte5,6,7,8. In an aprotic cell, for example, the discharge product, Li2O2, forms through a combination of solution and surface chemistries9,10,11 that results in the formation of a baffling toroidal morphology12,13,14,15. In a solid electrolyte, neither the reaction mechanism at the cathode nor the nature of the reaction product is known. Here we report the full-cycle reaction pathway for Li–O2 batteries and show how this correlates with the morphology of the reaction products. Using aberration-corrected environmental transmission electron microscopy (TEM) under an oxygen environment, we image the product morphology evolution on a carbon nanotube (CNT) cathode of a working solid-state Li–O2 nanobattery16 and correlate these features with the electrochemical reaction at the electrode. We find that the oxygen-reduction reaction (ORR) on CNTs initially produces LiO2, which subsequently disproportionates into Li2O2 and O2. The release of O2 creates a hollow nanostructure with Li2O outer-shell and Li2O2 inner-shell surfaces. Our findings show that, in general, the way the released O2 is accommodated is linked to lithium-ion diffusion and electron-transport paths across both spatial and temporal scales; in turn, this interplay governs the morphology of the discharging/charging products in Li–O2 cells.

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Figure 1: In situ observation of the morphological evolution of the discharge/charge product.
Figure 2: In situ TEM observation of the morphological evolution of the discharge–charge products.
Figure 3: In situ SAED analysis of phase evolution and the corresponding coupled reaction mechanisms.
Figure 4: Conformal coatings of discharging product.

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Acknowledgements

This work is supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the US Department of Energy under Contract no. DE-AC02-05CH11231, Subcontract no. 18769 and DE-AC-36-08GO28308 under Advanced Batteries Materials Research. The work was conducted in the William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy (DOE) Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for the DOE under Contract DE-AC05-76RLO1830.

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L.L., W.X., J.-G.Z. and C.M.W. conceived the experiment, B.L. and S.D.S. prepared the sample, L.L. carried out the in situ experiments, C.M.W. led the project and L.L. and C.M.W. wrote the manuscript with input from all the authors.

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Correspondence to Chongmin Wang.

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The authors declare no competing financial interests.

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Luo, L., Liu, B., Song, S. et al. Revealing the reaction mechanisms of Li–O2 batteries using environmental transmission electron microscopy. Nature Nanotech 12, 535–539 (2017). https://doi.org/10.1038/nnano.2017.27

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