In situ small-angle X-ray scattering reveals solution phase discharge of Li–O2 batteries with weakly solvating electrolytes

C. Prehal, A. Samojlov, M. Nachtnebel, L. Lovicar, M. Kriechbaum, H. Amenitsch, S.A. Freunberger, PNAS 118 (2021).


Journal Article | Published | English
Author
Prehal, Christian; Samojlov, Aleksej; Nachtnebel, Manfred; Lovicar, LudekIST Austria; Kriechbaum, Manfred; Amenitsch, Heinz; Freunberger, Stefan AlexanderIST Austria
Abstract
Electrodepositing insulating lithium peroxide (Li2O2) is the key process during discharge of aprotic Li–O2 batteries and determines rate, capacity, and reversibility. Current understanding states that the partition between surface adsorbed and dissolved lithium superoxide governs whether Li2O2 grows as a conformal surface film or larger particles, leading to low or high capacities, respectively. However, better understanding governing factors for Li2O2 packing density and capacity requires structural sensitive in situ metrologies. Here, we establish in situ small- and wide-angle X-ray scattering (SAXS/WAXS) as a suitable method to record the Li2O2 phase evolution with atomic to submicrometer resolution during cycling a custom-built in situ Li–O2 cell. Combined with sophisticated data analysis, SAXS allows retrieving rich quantitative structural information from complex multiphase systems. Surprisingly, we find that features are absent that would point at a Li2O2 surface film formed via two consecutive electron transfers, even in poorly solvating electrolytes thought to be prototypical for surface growth. All scattering data can be modeled by stacks of thin Li2O2 platelets potentially forming large toroidal particles. Li2O2 solution growth is further justified by rotating ring-disk electrode measurements and electron microscopy. Higher discharge overpotentials lead to smaller Li2O2 particles, but there is no transition to an electronically passivating, conformal Li2O2 coating. Hence, mass transport of reactive species rather than electronic transport through a Li2O2 film limits the discharge capacity. Provided that species mobilities and carbon surface areas are high, this allows for high discharge capacities even in weakly solvating electrolytes. The currently accepted Li–O2 reaction mechanism ought to be reconsidered.
Publishing Year
Date Published
2021-04-06
Journal Title
PNAS
Acknowledgement
S.A.F. and C.P. are indebted to the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement no. 636069) and the Austrian Federal Ministry of Science, Research and Economy and the Austrian Research Promotion Agency (grant No. 845364). The authors acknowledge A. Zankel and H. Schroettner for support with SEM measurements. C.P. thanks N. Kostoglou, C. Koczwara, M. Hartmann, and M. Burian for discussions on gas sorption analysis, C++ programming, Monte Carlo modelling and in stu SAXS experiments, respectively. We thank S. Stadlbauer for help with Karl-Fischer titration, R. Riccò for gas sorption measurements and acknowledge TU Graz for support through the Lead Project LP03. Likewise, the use of SOMAPP Lab, a core facility supported by the Austrian Federal Ministry of Education, Science and Research, the Graz University of Technology, the University of Graz, and Anton Paar GmbH is acknowledged.
Volume
118
Issue
14
Article Number
e2021893118
IST-REx-ID

Cite this

Prehal C, Samojlov A, Nachtnebel M, et al. In situ small-angle X-ray scattering reveals solution phase discharge of Li–O2 batteries with weakly solvating electrolytes. PNAS. 2021;118(14). doi:10.1073/pnas.2021893118
Prehal, C., Samojlov, A., Nachtnebel, M., Lovicar, L., Kriechbaum, M., Amenitsch, H., & Freunberger, S. A. (2021). In situ small-angle X-ray scattering reveals solution phase discharge of Li–O2 batteries with weakly solvating electrolytes. PNAS. Proceedings of the National Academy of Sciences. https://doi.org/10.1073/pnas.2021893118
Prehal, Christian, Aleksej Samojlov, Manfred Nachtnebel, Ludek Lovicar, Manfred Kriechbaum, Heinz Amenitsch, and Stefan Alexander Freunberger. “In Situ Small-Angle X-Ray Scattering Reveals Solution Phase Discharge of Li–O2 Batteries with Weakly Solvating Electrolytes.” PNAS. Proceedings of the National Academy of Sciences, 2021. https://doi.org/10.1073/pnas.2021893118.
C. Prehal et al., “In situ small-angle X-ray scattering reveals solution phase discharge of Li–O2 batteries with weakly solvating electrolytes,” PNAS, vol. 118, no. 14. Proceedings of the National Academy of Sciences, 2021.
Prehal C, Samojlov A, Nachtnebel M, Lovicar L, Kriechbaum M, Amenitsch H, Freunberger SA. 2021. In situ small-angle X-ray scattering reveals solution phase discharge of Li–O2 batteries with weakly solvating electrolytes. PNAS. 118(14), e2021893118.
Prehal, Christian, et al. “In Situ Small-Angle X-Ray Scattering Reveals Solution Phase Discharge of Li–O2 Batteries with Weakly Solvating Electrolytes.” PNAS, vol. 118, no. 14, e2021893118, Proceedings of the National Academy of Sciences, 2021, doi:10.1073/pnas.2021893118.
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