Triggering and Characterisation of Realistic Internal Short Circuits in Lithium-Ion Pouch Cells—A New Approach Using Precise Needle Penetration

The internal short circuit (ISC) in lithium-ion batteries is a serious problem since it is probably the most common cause of a thermal runaway (TR) that still presents many open questions, even though it has been intensively investigated. Therefore, this article focusses on the generation and characterisation of the local single-layer ISC, which is typically caused by cell-internal impurity particles that cannot be completely eliminated in the cell production. A new, very promising method of precise and slow (1 $\mathsf{\mu }$m s${}^{-1}$) needle penetration made it possible to generate the most safety-critical reliable short-circuit type—the contact between the Al-Collector and the graphite active material of the anode—as demonstrated on a 10 Ah Graphite/NMC pouch cell. The special efforts in achieving high reproducibility as well as the detailed analysis of the initiated internal short-circuit conditions led to more reliable and meaningful results. A comprehensive approach to characterisation has been made by detailed measurement of the dynamic short-circuit evolution and a subsequent post-characterisation, which included the application of different electrochemical measurement techniques as well as a post-abuse analysis. It was shown that the cells demonstrated a very individual and difficult-to-predict behaviour, which is a major challenge for early failure detection and risk assessment of cells with an existing or former ISC. On the one hand, it is found that despite high local temperatures of over 1260 ${}^{\circ }$C and significant damage to the cell-internal structure, the cell did not develop a TR even with further cycling. On the other hand, it was observed that the TR occurs spontaneously without any previous abnormalities. Based on the overall test results, it was shown that at the high state of charge (SOC = 100%), even small, dynamically developing voltage drops (<10 mV) must be classified as safety-critical for the cell. For reliable and early failure detection, the first voltage drops of the ISC must already be detected.