Measuring the thickness of lithium-ion batteries is a reliable method to detect the intercalation stages of the electrodes, the state of charge and irreversible effects during aging [1]. When using tactile high-precision displacement sensors, the measurement is limited to only a few positions of the cell at the same time [2].
Experimental
In this work, we developed a test stand, which enables simultaneous precise thickness measurement at 100 positions of the cell. This allows for investigations of the homogeneity of mechanical expansion during charging and discharging, as well as non-destructive state-of-health (SOH) mapping. In a first step, inhomogeneity of mechanical expansion during cycling is studied. In a second step, the effects of expansion inhomogeneity during cycling on the aging behavior of the cells are investigated by determining the irreversible and reversible thickness change at each of the 100 positions every 100th cycle.
Results
We found that the cell contracts homogeneously during discharge, whereas a significant overshoot in the thickness increase during high-rate charging at the current collector tabs is observed at 25 °C. When the ambient temperature is increased to 40 °C, this inhomogeneity is no longer observable. This effect has been ascribed in literature to non-homogeneous intercalation of Li-ions due to the interplay of fast and slow Li ion diffusion processes [3]. The SOH distributions reveal a homogeneous aging of the cells cycled at low charging rate and high charging rate at 40 °C. Compared to that, the cells cycled at high charging rate at 25 °C show inhomogeneous degradation in areas close to the current collector tabs which show the largest degradation.
Literature
[1] J. Cannarella, C.B. Arnold, Journal of Power Sources 269 (2014) 7–14.
[2] K.-Y. Oh, J.B. Siegel, L. Secondo, S.U. Kim, N.A. Samad, J. Qin, D. Anderson, K. Garikipati, A. Knobloch, B.I. Epureanu, C.W. Monroe, A. Stefanopoulou, Journal of Power Sources (2014).
[3] L.W. Sommer, P. Kiesel, A. Ganguli, A. Lochbaum, B. Saha, J. Schwartz, C.-J. Bae, M. Alamgir, A. Raghavan, Journal of Power Sources 296 (2015) 46–52.