FRACTURE MODE ANALYSIS OF LITHIUM-ION BATTERY UNDER MECHANICAL LOADING

Due to its extensive application, the safety issue of lithium ion battery has received increasing attention. For crashworthiness design of battery in electric vehicles, it is of great importance to investigate the response of the battery under mechanical loading and understand the mechanism of internal short circuit. Quasi-static and intermediate strain rate uniaxial tension tests were conducted on the electrodes and the separators. The high speed camera and DIC (digital image correlation) method were adopted to measure the strain while a self-design load cell was used to measure force in dynamic test. Either loading velocity or loading direction was varied in different tests. The upper limit of the test strain rate achieved 66 /sec. All the component materials showed strain rate dependency and separators demonstrated noticeable anisotropy. Quasi-static penetration tests were conducted on two different types of pouch cell using steel and plastic punch heads. For both Type A pouch cell and Type B pouch cell, during penetration process using plastic punch head, no significant voltage drop or temperature rise was observed. During penetration process using steel punch head, only Type A pouch cell produced short circuit. When the punch head was removed, the voltage of the cells could recover to certain level. From post mortem examination, it was found that for a single pouch cell, all the electrodes presented the same fracture mode that the stacked anode and cathode formed several fragments in the penetration path, while the separators in between only formed a central crack when the punch head went through. Since the separators had a larger elongation ratio than the electrodes, the extended separators around the rupture location could block the direct and constant contact between anode and cathode, electrodes and steel punch, which explained why no massive internal short circuit was initiated. The drop tower was used to conduct dynamic penetration test. The 'results indicated that under dynamic loading, internal short circuit was more likely to be triggered which can be explained by the strain rate effect of the separators. This study highlighted the importance of the separator to the safety performance of pouch cells.