Non-contacting chromatic confocal distance measurements of pouch cells during cycling

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Lithium-ion cells gain increasingly interest in e-mobility applications. During charging and discharging, these batteries experience reversible thickness changes and inhomogeneous temperature distribution causing a space-dependent aging. Additionally, irreversible expansion and temperature driven defects occur during operation, such as gassing, particle cracking and SEI growth. These aging effects occur inhomogeneously. As a result, the useful life of commercial lithium-ion batteries can be significantly shorter than the life estimation for a theoretical cell considering homogeneous aging. It is common knowledge that the battery cells swell over the course of their service life. So far, the industry is not further considering local swelling discrepancies. The optimization of the swelling distribution, and thus the aging distribution, has been counteracted by compressing battery cells and modules. Compression has shown an impact on the reduction of electrode delamination, gas evolution and SEI growth. However, the local distribution and detailed expansion has not been studied in detail until now. The current research is mostly focused on global cell expansion over cyclic and calendric lifespan.

Our goal was to fill the missing research gap by examining local volume changes in detail using several chromatic confocal distance sensors. The sensors have a lateral resolution of 1 µm. In this work, two commercial Ni-rich lithium-ion pouch cells were investigated during cycling. The aim was to measure the local expansion through the strategic positioning of the sensors above the pouch cells. In this way, the inhomogeneity could be confirmed. Therefore, a carbon fiber frame, a granite substrate and a climate chamber were used. This reduces thermal and mechanical interference factors. the pouch cells were cycled at low C-rate and the temperature was held at 25°C.
This research reveals detailed local expansion behavior of lithium-ion pouch cells and their correlations to geometric and manufacturing factors. In this poster we present the carbon fiber set-up, reference measurements and three sensor locations during charging and discharging. The reversible volume change varies between 140-165 µm. We can gain information for ideal compression strategies and local aging. In order to approach homogeneous aging and a longer cell service life, this information is of high value.

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