Investigation of a charging anomaly in aged commercial NCA / C-Si lithium-ion cells

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. Although the technology is safe and becoming safer, the exponential growth of the market has led to increased incident numbers. 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.

In the presented 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 charging surplus. 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 operation at low state-of-charge. From the test matrix, it can be concluded that the charging anomaly occurs for cells aged at high state-of-charges (calendar or cyclic). The degradation characteristics of affected cells are examined by presenting the development of the available capacity and internal resistance. The occurrence of the anomaly is associated with a passivation of the active material: When cells increase their resistance by approximately 25 %, the charging anomaly evolves and intensifies with proceeding degradation. Insights from the post-mortem analysis provide the basis to propose a theory explaining the observed phenomenon. The presentation concludes with a summary and implications for the safe operation of aged NCA / C-Si lithium-ion cells.