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Influence of compression on the accelerated aging of lithium ion battery cells

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In this contribution, the influence of external compression on lithium-ion cells with respect to cyclic aging is demonstrated. Currently, cells for EV applications are manufactured in three different designs: pouch bag, cylindrical and prismatic housings.
During charge and discharge especially the anode but as well the cathode expands and contracts. Thus, the entire jelly roll consisting of the stack of electrodes including the separator is undergoing volume cycles. Pouch cells are hardly limited by the cell’s casing with respect to expansion. Especially, gas evolution taking place during aging reduces the limitation of the aluminum foil case. For cylindrical cells with high utilization of space, the case limits the expansion. Same holds true for prismatic cells. In automotive use cases, the prismatic or pouch cells are further compressed by bracing plates. Thus, the difference between the uncompressed pouch cell and a compressed e.g. prismatic cell reveals the maximum difference of the impact of pressure. For these two extremes, the impact of shallow cyclic aging is investigated in this contribution.
Beside the typical characterization of degradation like capacity loss and increase of internal resistance, the differential voltage analysis (DVA, dV/dQ) is evaluated (10.1016/j.jpowsour.2017.09.059). In the DVA for the given cells LiNi1/3Mn1/3Co1/3O2 vs. graphite the strongest signatures in dV/dQ can be attributed to the graphite anode via half-cell measurement (10.1016/j.est.2018.04.029, calendric aging tests). The anode characteristics are indicator for the degree of homogeneity of lithium distribution (HLD) or in other words for the width of the SOC distribution function. A sharp peak is correlated to a high HLD. A flattening of the peak down to a complete smear out of the characteristics is indicator for a low to very low HLD. A very low HLD is crucial concerning safety as strong deviations in SOC increases the risk of local lithium plating on the anode (10.1016/j.jpowsour.2017.09.059, 10.1016/j.jpowsour.2017.10.003). However, this method is so far only applied with the systematic focus on HLD for cyclic aging tests for cylindrical LFP/graphite cells.
We present in the poster presentation a comparison of dV/dQ curve series of compressed prismatic cells and uncompressed pouch cells for the first time. The cyclic aging tests are performed at five different SOCs at 6% and 12% DOD. We show that the dV/dQ curves of the compressed cells smear out depending on the average SOC while the DVA remains nearly unchanged for the uncompressed pouch cells. Moreover, the compressed cells exhibit a high capacity loss towards lower and higher SOCs. In (10.1016/j.est.2019.01.004.) we reported that these inhomogeneities are reversible and can be reversed by giving the cell a rest for roughly 100 days at 20 °C.

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