The extraordinary characteristics of the lithium-ion battery have changed our daily lives like very few other inventions. However, over time, various effects occur that gradually reduce the large storage capacity of these batteries.
At the Heinz Maier-Leibnitz Research Neutron Source (FRM II) of the Technical University of Munich, Dr. Anatoliy Senyshyn, instrument scientist for the SPODI high-resolution powder diffractometer, used neutron scattering to investigate the cause of these effects in cylindrical lithium-ion batteries.
Together with other scientists, he seeks answers to fundamental questions about the structure and behavior of rechargeable lithium-ion batteries: Why does available capacity decrease over time? How is lithium distributed in the battery?
Why use neutrons to investigate a rechargeable battery?
Observing the inside of a lithium-ion cell during charging and discharging—for example, electrolyte decomposition or lithium distribution—is very difficult from outside the cell due to the high reactivity of its components with oxygen and humidity in the air.
Neutrons are sensitive to lighter elements, such as hydrogen and lithium. Therefore, they can make the lithium inside a cell visible, allowing for research under real operating conditions.
Another advantage of working with neutrons is that they can be used in non-destructive testing. For example, researchers can observe internal battery processes from the outside without interfering with the delicate system.
Why does battery capacity decrease?
Lithium-ion cells produce electricity when lithium atoms release electrons that flow through a connected device. At the same time, inside the battery, a lithium ion moves from one electrode to another; this means there are always only as many electrons available as there are lithium ions. When a rechargeable battery loses capacity, it can also be said that the lithium is "lost." But where does it go?
Neutron scattering experiments on the STRESS-SPEC and SPODI instruments indicated a linear relationship between the loss of mobile lithium ions and the decomposition of the electrolyte, which occurs, for example, as an unwanted side reaction during charging.
The products resulting from the breakdown of the electrolyte trap lithium atoms that are no longer available as mobile lithium to be exchanged between the two electrodes. As a result, the battery loses capacity: it ages.
How is lithium distributed?
Until now, theoretical models, calculations, and measurements have generally assumed a uniform distribution of lithium in these batteries. However, research indicates that lithium is distributed very unevenly from the start, and this lack of homogeneity increases over time.
Modeling lithium-ion batteries can be improved if developers take this uneven lithium distribution into account. Furthermore, claims can be made about the storage capacity of the lithium-ion cell based on this distribution. These findings provide an important foundation for manufacturing more efficient rechargeable batteries with longer lifespans and higher performance in the future.
Publications:
Dominik Petz, Martin J. Mühlbauer, Alexander Schökel, Klaus Achterhold, Franz Pfeiffer, Thilo Pirling, Michael Hofmann and Anatoliy Senyshyn.
Heterogeneity of graphite lithification in the most modern cylindrical lithium-ion cells.
Batteries and Supercaps, October 28, 2020 - DOI: 10.1002/batt.202000178
https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/batt.202000178
MJ Mühlbauer, D. Petz, V. Baran, O. Dolotko, M. Hofmann, R. Kostecki, A. Senyshyn
Non-homogeneous distribution of lithium and electrolyte in aged cylindrical Li-ion cells
Journal of Power Sources, 475 (2020) 228690 - DOI: 10.1016/j.jpowsour.2020.228690
https://doi.org/10.1016/j.jpowsour.2020.228690
More information:
The work has been funded by the Heinz Maier-Leibnitz Zentrum (MLZ), the German Research Foundation (DFG), the German Federal Ministry of Education and Research (BMBF) and the Bavarian-California Technology Center (BaCaTeC), as well as by the United States Department of Energy, as part of its research activities on advanced battery materials.
