Understanding lithium-ion battery degradation in vehicle applications: Insights from realistic and accelerated aging tests using Volkswagen ID.3 pouch cells

In this paper, we provide an in-depth analysis of long-term aging tests of over three years of continuous cycling using lithium-ion battery cells of the 2020 Volkswagen ID.3 Pro Performance. The study examines the degradation of state-of-the-art NMC/graphite pouch cells and compares it with the degradation behavior in the field application under real-world conditions. The cell level tests include two realistic edge-case scenarios, namely a long-distance highway driving pattern as well as a short trip urban-interurban commuter pattern, alongside a synthetic, accelerated 1C/1C test case. Identifying the related dominating degradation modes reveals four key findings: first, the battery cells demonstrate exceptional cycle life performance, achieving nearly 400 000 km and 1100 fast-charge cycles between 20 % and 80 % state of charge in the highly demanding edge-case scenario. Second, accelerated synthetic load conditions significantly reduce testing time, with 1C/1C constant current cycles covering the OEM's cycle aging warranty of 160 000 km within 14 weeks, compared to 24 weeks for the highway and 1.5 years for the commuter driving scenarios. Third, a notable mismatch in the degradation behavior of the three examined test cases is observed, with hardly any degradation of the negative electrode and only a slight loss of lithium inventory in the commuter scenario. In contrast, the elevated stresses acting in the highway scenario and 1C/1C cycling cause a considerably higher loss of lithium inventory in combination with severe degradation of the negative electrode, ultimately resulting in a knee point. Fourth, inhomogeneous lithium distribution in the negative electrode is found due to omitted rest periods, depending on the stress level. This inhomogeneous lithium distribution further limits the transferability of synthetic accelerated laboratory test results to real-world vehicle applications, as these tests do not adequately replicate the degradation behavior observed in field applications. Nonetheless, a high level of agreement between the cell test applying the commuter driving pattern and the field application could be proven.