Investigation of the Influence of Compression of the Cell on Capacity Fade and Homogeneity of Lithium Distribution
Authors: Lewerenz Meinert , Rahe Christiane , Endisch Christian , Sauer Dirk Uwe 
Published: February 2020

In this contribution, the impact of external pressure on lithium-ion cells with respect to cycle life is demonstrated. Currently, cells for automotive applications are produced in three designs: pouch bag, cylindrical and prismatic housings.
During cycling especially the anode but also the cathode expands and contracts. Thus, the entire jelly roll consisting of the stack of electrodes including the separator is performing volume cycles. Pouch cells are hardly limited by the housing with respect to expansion. Especially, gas evolution occurring during aging reduces the limitation of the aluminum foil housing. For cylindrical cells with high utilization of space, the housing limits the expansion. Same is true for prismatic cells. In automotive applications, 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 shows the maximum difference of the impact of pressure. For these two extremes, the influence of shallow cyclic aging is investigated in this contribution.
Beside the standard characterization of aging like capacity loss and increase of internal resistance, the differential voltage analysis (DVA) is evaluated, too (10.1016/j.jpowsour.2017.09.059). In the DVA for the given cells LiNi1/3Mn1/3Co1/3O2 vs. graphite the strongest signatures in DVA can be attributed to the graphite anode via half-cell measurement (10.1016/j.est.2018.04.029, calendaric aging tests). The anode peaks 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 metallic lithium deposition on the anode (10.1016/j.jpowsour.2017.09.059, 10.1016/j.jpowsour.2017.10.003). However, this method is only applied with the systematic focus on HLD for cyclic aging tests for cylindrical LFP/graphite cells.
A comparison of DVA curve series of compressed prismatic cells and uncompressed pouch cells is presented for the first time (see figure pressure1). The cyclic aging tests are performed at five different SOCs at 6% and 12% DOD. It is shown that the DVA of the compressed cells smears out depending on the average SOC while the DVA remains nearly unchanged for the uncompressed pouch cells. Moreover, the compressed cells show a high capacity loss towards lower and higher SOCs. In a publication, that is currently under review, it is shown that, these inhomogeneities are reversible and can be reversed by giving the cell a rest for roughly 100 days at 20 °C.
These results are in line with publications of other authors for different cells, e.g.: An influence on DVA and a capacity loss to lower and higher SOCs is reported by Ecker et al. for cylindrical cells (10.1016/j.jpowsour.2013.09.143). Käbitz et al. tested an uncompressed pouch cell and the capacity loss increases with higher average test SOC. However, all reported findings have not been linked to the pressure due to the housing, so far.

Figure 1