An Analysis of Calendaric Aging over 5 Years of Ni-rich 18650-Cells with Si/C Anodes

Calendaric lifetime testing of lithium-ion cells is time-consuming and resource-intensive. As Dubarry et al.[1] state, testing is often limited to a few aging conditions. This paper presents the results of a long-term aging study on lithium-ion cells with a nickel-rich NCA cathode and a graphite-silicon anode. 69 cells were stored at 5 different voltages and under 4 different temperatures for 5 years. Regular reference performance tests (RPT) provide insights for State of Health (SoH) calculation and further analysis through differential voltage analysis. The results are verified against post-mortem analyses. The long aging period enables accurate determination of aging rates. Our results demonstrate that the storage voltage level strongly influences the degradation rate, with temperature playing a minor role. The identified aging effects include loss of active material on the cathode side and loss of lithium inventory. Initial degradation follows a t ${\sqrt t }$ -trajectory but is caused by overhang effects. The long-term aging is rather linear. This paper presents the calendaric aging of NCA and Si/C containing cells aged for 5 years. The differential voltage analyses shows loss of lithium and cathode degradation. Additionally, post-mortem results, including coin-cell measurements and SEM images are discussed. Si plays its role in a continuous SEI growth, while the NCA material already at 80 % SoC significantly influences aging speed. image