Investigation of Capacity and Homogeneity Recovery of Commercial Cells after Cycle Life Tests
Authors: Lewerenz Meinert , Dechent Philipp , Endisch Christian , Sauer Dirk Uwe 
Published: February 2020

In this presentation, the irreversible capacity loss is determined by two strategies during shallow cycling of commercial Li-Ion cells at different average SOCs. According to the findings, superimposed reversible capacity effects lead to the need for a more complex evaluation of capacity (10.1016/j.est.2018.06.003). Reversible effects are split into contributions of the anode overhang (geometrical oversized anode) and into the homogeneity of lithium distribution (HLD) in the electrodes. The anode overhang leads to migration of lithium ions from or to the active anode leading to a superimposed decrease or even an increase in extractable capacity. The SOC before testing and the average SOC during testing define the direction of lithium migration. The HLD is a method to measure the uniformity of SOC distribution of the particles within the electrodes e.g. during a discharge process. One way to qualify HLD is by evaluating peak heights in differential voltage analysis (10.1016/j.jpowsour.2017.09.059). Hence, for a higher SOC spread within the electrodes, a lower HLD is obtained. One example is given in figure Relaxation1.

The cycle life test is followed by an additional rest period to recover extractable capacity, which is referred to as ‘relaxation method’ in the following. During this rest period, a certain superimposed calendar aging has to be considered. Here, two corner cases are studied by storing the cells at a fixed low and the same SOC as during cycling test. While the contribution of solely HLD can be obtained by storing the lithium ion cells at the same average SOC as during cycling, the contribution of the anode overhang is evaluated for a defined low SOC. In this presentation, both SOC strategies are examined. During the storage phase in all test cases, the capacity rises supporting the theory considering reversible capacity (see example in figure Relaxation2). Additionally, the HLD, measured via capacity difference analysis and differential capacity analysis, rises as well; this is found for all test conditions highlighting the reversible nature of HLD and its impact on extractable capacity.

Furthermore, the irreversible capacity losses are compared to an alternative method, called ‘slope method’. For this method, the aging is assumed to be nearly linear and the linear-like part, after the impact of the anode overhang is negligible, is mainly associated with irreversible aging. While the results of both methods are in the same order of magnitude, the relaxation method can be utilized, not only for static but also for any dynamic aging profile.

The findings are very important regarding application oriented testing like dynamic driving profiles for electric vehicles (EV). To accelerate the measurement time, non-operation periods are deleted and applied currents and temperatures are increased. This leads to more inhomogeneities within the cell and therefore to less extractable capacities. The relaxation method helps to distinguish the acceleration-induced inhomogeneities (HLD) and the accompanying effects from the anode overhang. This enhances the accuracy of lifetime prediction as only the irreversible losses are included in aging models and reversible effects are included in a separate step.

Figure 1