A Microscopic Safety Mechanism Study for Li Ion Battery

As the large number of LiB has powering in wide ranges of devices in a variety of usage and environmental conditions, there were several LiB field incidents. Many LiB safety mechanisms studies were devoted to understand the safety hazard and to prevent field safety incidents. Many researches have focused on the inherent safety of the battery materials and on the simulation of the cell abuse conditions. The thermal behavior and stability of the LiB cell and its component were studies by the DSC (Differential Scanning Calorimetry), ARC (Accelerating Rate Calorimeter) and other tools. Many LiB degradation and safety mechanisms were proposed from those results. Meanwhile, some LiB safety standard and approval test are in place to minimize hazard or to prevent field safety incidents, which simulate batteries in the thermal, electrical and mechanical abuse conditions. However frequent LiB field incidents on the certified batteries have still causing big concerns on its safety.

Many field incident analysis reports identified that there are important difference between reported LiB safety mechanisms in papers, abuse testing in standards and the real field incidents. And those concluded the battery field incidents almost originated due to the cell internal short circuit that was not predictable at the point of cell manufacturing process and certifciation or approval testing process. There are many factors which can induce cell internal short circuit; -metal impurity in the electrode matrials, -metal particle during cell manufacturing process, -flaw in separator, - flaw in cell design and manufacturing, - defected by electric, mechanical and thermal impact and so on.   

This paper studied a mechanism of the cell internal short circuit by using microscopic techniques. This technique checks the status of a cell design and manufacturing status of a cell model, which may cause a cell internal short circuit during in use if the cell has flaw. We performed two types of the "Destructive Physical Analysis" testing to identify a cell's latent possibility of inducing internal short circuit; 1) by measuring the gap between separator and anode and cathode in the jellyroll or cell stacking (sepertor shrinkage allowance at room/high teperatture and cell electrode geometry) by microscopic techniques; 2) by checking the position and status of insulation mechanism in the cell (Electrode tab insulation, Application of insulation, Supplementary Insulation, Internal short avoidance, Position of insulation material).

This paper shows the relationship between the status of a cell model and the simulation result of the cell abuse testing (Electric testing, and thermal testing and mechanical testing). This paper identifies the battery safety mechanism which is representing the real field failures and incidents.

Attached image file shows relationships between cell design/manufacturing status of a LiB cell and cell abuse simulation testing result.