Electrolyte leakage in lithium-ion cells can be compared to a situation where a tightly sealed container is repeatedly moved between a freezer & an oven. Just as the container's seal might weaken and eventually break under such extreme temperature fluctuations, leading to spills, li-ion batteries face similar challenges under severe thermal stress. This breakdown leads to reduced efficiency & the potential loss of electrolyte, posing safety risks.
One of the studies by Sahithi M et. al. addresses the significant issue of electrolyte leakage in cylindrical li-ion batteries subjected to extreme temperature cycling from 25 °C to 80 °C. Their research utilized capacity tests, electrochemical impedance spectroscopy (EIS), computed tomography (CT) scans, and thermal analysis to explore the integrity of the batteries under these conditions.
The investigation reveals that the primary factor behind these failures is the differential thermal expansion among the various materials within the battery cap. Different components, such as metals & polymers, expand & contract at different rates under thermal stress. As the temperature increases, the polymers within the seals expand much more than the metals. When cooled, these polymers contract faster than the metals can return to their original forms, potentially leaving gaps. These gaps can enlarge through repeated cycling, allowing the electrolyte to leak out. This not only degrades the battery's performance but also heightens the risk of hazardous outcomes like corrosion & fire.
This analysis also highlights inadequacies in the existing standards outlined in UN 38.3, which prescribes thermal testing. The study shows that the current max temperature limit of 72 °C & a subsequent 24 hr observation period are insufficient to fully capture the delayed effects of thermal stress that batteries might experience under actual conditions.
After extensive cycling tests, certain test batteries demonstrated significant structural failures: Weight measurements indicated that some batteries experienced a reduction of up to 11.7% post-test, suggesting electrolyte & possibly gas venting. EIS revealed increased impedance, especially in batteries that showed signs of corrosion at the positive terminals, further validating the leakage paths. CT scans particularly showed internal structural changes & confirmed leakage pathways & corrosion without invasive disassembly.
The study's insights emphasize the need for revising temperature limits & extending observation periods in regulatory standards to ensure that li-ion batteries are capable of withstanding real-world conditions without compromising safety.