Impurity particle contamination in lithium-ion batteries – a risk for safety-critical internal short circuits

A possible contamination with impurities in the cell production of lithium-ion batteries increases the risk of spontaneous internal short circuits (ISC), so that these faults are especially feared. Since detection of ISC in time for warning and effective countermeasures is difficult the safety risk is also increased. Various trigger methods are currently being discussed to reproduce these failure cases. However, a considering a valid basis on the practice-relevant particle properties is not considered. In order to provide a realistic initial state for the evaluation of the trigger methods in this poster particles examined from pouch cells of an established cell manufacturer were analysed in detail by using EDX (energy dispersive X-ray spectroscopy) and optical microscopy as analysis method. The results indicate, that several metallic particles with a significant size up to 1.0 x 1.7 mm could be found between the cell layers. Furthermore, non-electrically conductive particles were also found in those cells. This fact shows that critical impurities occur in manufacture and thus contaminated lithium-ion batteries can be expected in practical use.

In field, safety-critical battery faults occur frequently during the charging process. One possible theory for this condition is that the force on a hypothetical foreign particle increases due to ageing processes in the cell. Consequently, the failure force of the separator decreases with increased battery age. Due to the overlapping increasing force during the charging process, the probability of a particle-induced ISC in the most critical high state of charge increases with the age of the cell. This theory, as well as the visual presentation of the found impurities, is presented in this poster.