Materials science: Look inside an overheating Li-ion battery

For the first time, the failure of a lithium ion (Li-ion) battery due to overheating has been recorded, reports a paper published in Nature Communications. Li-ion batteries are vulnerable to thermal runaway, which occurs when the rate of heat generation is greater than the rate of heat lost. Although battery failure is rare, prevention of thermal runaway presents one of the greatest challenges for the safe operation of lithium-ion batteries.

Paul Shearing and colleagues subjected two commercial Li-ion batteries (Cell 1 and Cell 2) to external heat and used thermal imaging and non-invasive high speed imaging techniques to observe the internal structure. Cell 1 remained intact during battery failure which allowed the heat-generating reactions to run to completion, resulting in hot gas and then molten material jetting out through the battery’s vent. Copper material inside Cell 1 melted, indicating internal temperatures of at least 1,085°C. The rapid pressure rise in Cell 2 caused the entire cap of the cell to detach, which in real-world situations, could increase thermal runaway by allowing oxygen into the cell.

The authors found that thermal and electrochemical reactions inside both cells produced gas pockets that deformed the spiral wound layers of the cells. Cell 2 showed severe internal distortion of its architecture, which the authors suggest compromised the safety of the cell. Unlike Cell 2, Cell 1 was designed with a central cylindrical support, which seemed to help maintain structural integrity. The authors hope that their observations of the response of cells to thermal runaway will lead to improvements in the design of Li-ion batteries and improve their safety features.