Lithium-ion batteries are a key technology for the global transition to green energy. However, ensuring the safety of these batteries is essential for widespread adoption. Safe operation throughout the entire lifecycle is a primary concern of product developers. Of particular interest is the safety of aged lithium-ion cells, as their behavior in degraded state can be complex and unpredictable due to various aging mechanisms. Therefore, a comprehensive understanding of the factors which can lead to safety-critical issues is of utmost importance.

In this study, commercial cylindrical cells with a capacity of 4.9 Ah and an NCA / C-Si chemistry were subjected to both calendar and cyclic aging protocols. During the experiments, certain cells exhibited a charging anomaly characterized by sudden, excessive heat generation and a significant overcharge. This abnormal behavior intensified over the course of the experiments. Ultimately, it led to charging factors of up to 1.6 and cell surface temperatures of up to 95 °C, exceeding the safe operation range. The event did not directly lead to a thermal runaway, but the cell’s state stabilized at high temperatures. Moreover, it did not occur during every charging cycle but only under specific operational histories, which were given in this study during the checkups. A total of 21 cells from the test matrix showed this phenomenon, enabling a detailed investigation of the underlying causes.

Firstly, the exact conditions under which this charging anomaly occurs are presented. The short-term history, which is required to trigger this phenomenon, involves a period of around 1 day in low state-of-charge. From the test matrix it can be concluded that the charging anomaly occurs for cells operated at high state-of-charges (calendar or cyclic). The degradation characteristics of affected cells are examined by using differential voltage analysis, electrochemical impedance spectroscopy (EIS), and post-mortem analysis, with comparisons to non-affected cells. The occurrence of this anomaly is associated with a passivation of the active material: As soon as the cells increase their resistance by approximately 20 %, the charging anomaly evolves and intensifies with proceeding degradation. Insights from the post-mortem analysis provide the basis to propose a mechanism explaining the observed phenomenon. The work concludes by discussing the implications for the application and safe operation of aged NCA / C-Si lithium-ion cells.