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Measuring inhomogeneous pressure distribution in lithium-ion pouch cells during long-term ageing
Pack design & thermal management

In terms of batteries, lithium-ion technology is currently the most widely used and researched technology. By extending the lifetime of these batteries, this technology becomes more environmentally friendly and cost-efficient. This can be achieved either by the development of new battery chemistries and designs or by operating them lithium-ion batteries closer to ideal conditions. One parameter of to approach these conditions is a life-extending pressure distribution on the battery cells. For the pouch cell format, this pressure is usually applied homogeneously on the battery surface. However, large-format battery cells show inhomogeneous current and heat distribution across the electrode surfaces which results in inhomogeneous ageing.
During electrical cycling, the active materials of lithium ion batteries will change their volume regarding the lithiation of the material. In addition to this reversible volume change, irreversible expansion due to ageing can be observed. Applying the correct locally resolved pressure on the battery surfaces could contribute to optimize the current distribution and lead to a homogenized ageing behaviour.
For the optimization of this locally resolved pressure a test setup has been developed. In this test setup, pouch cell batteries are braced between two stiff aluminium plates with a milled height profile and ten integrated resistance-based thin film force sensors. The correct pressure is maintained within a predefined linear range despite volume changes by using silicone based cushioning foams.
During the ageing of the cell, the pressure distribution is tracked using custom-made electronics and the integrated thin-film force sensors. After the completion of the long-term tests, the cells are opened and analysed using various chemical, mechanical and optical analysis methods. The resulting observations are then checked for correlations in terms of the temporally and locally resolved pressure distribution.
This work discusses the long-term stability and accuracy of the pressure measurement regarding the sensors and the electronics. For this purpose, stress test measurements of different sensors are compared. Furthermore, the associated post mortem analysis methods are shown. Results of the first batch of long-term tests are presented and correlations with data from the literature are debated.

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Hendrik Laufen, Julian Engeser, Fabian Frie, Egbert Figgemeier

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