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Bosch, TUM neutron imaging study shows electrodes wetted twice as fast under vacuum

Developers from Bosch and scientists at the Technical University of Munich (TUM) are using neutron imaging to analyze the filling of lithium-ion batteries for hybrid cars with electrolytes. Their experiments, reported in the Journal of Power Sources, show that electrodes are wetted twice as fast in a vacuum as under normal pressure.

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Filling of a lithium-ion cell under vacuum: The wetting of the electrode (dark area) proceeds evenly from all sides. Left: after 33 min, right after 41 min. After approx. 50 min., the electrode is completely wetted. (Image: Wolfgang Weydanz / Bosch / TUM) Click to enlarge.

One of the most critical and time-consuming processes in battery production is the filling of lithium cells with electrolyte fluid following the placement the of electrodes in a battery cell.

While the actual filling process takes only a few seconds, battery manufacturers often wait several hours to ensure the liquid is fully absorbed into the pores of the electrode stack.

The fact that neutrons are hardly absorbed by the metal battery housing makes them ideal for analyzing batteries. That is why Bosch employees, in collaboration with scientists from the TU Munich and the University of Erlangen-Nuremberg, investigated the wetting process at the cold neutron radiography and tomography facility ANTARES of the research neutron source FRM II.

(FRM II is the most powerful neutron source in Germany. Relative to its thermal power (20 MW), it is capable of producing the highest neutron flux (8 × 1014 neutrons cm-2s-1) in the world.)

Manufacturers of lithium cells often fill the empty cells in a vacuum. The process is monitored indirectly using resistance measurements.

To make sure that all the pores of the electrodes are filled with the electrolyte, manufacturers build in large safety margins. That costs time and money.

—Dr. Wolfgang Weydanz, Bosch, lead author

Using neutron imaging, the scientists recognized that in a vacuum the electrodes were wetted completely in just over 50 minutes. Under normal pressure, this takes around 100 minutes. The liquid spreads evenly in the battery cell from all four sides, from the outside in.

In addition, the electrodes absorb ten percent less electrolyte under normal pressure. The culprit is gases that hinder the wetting process, as the scientists were able to demonstrate for the first time using the neutrons.

Resources

  • W.J. Weydanz, H. Reisenweber, A. Gottschalk, M. Schulz, T. Knoche, G. Reinhart, M. Masuch, J. Franke, R. Gilles (2018) “Visualization of electrolyte filling process and influence of vacuum during filling for hard case prismatic lithium ion cells by neutron imaging to optimize the production process” Journal of Power Sources, Volume 380, Pages 126-134 doi: 10.1016/j.jpowsour.2018.01.081

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