Develop an Activation Pressure Measurement Method of the Safety Vent and Investigate the Relationship between Cell Safety Devices' Activation Pressure and LIB Cell Safety
Authors: Chung Jaesik , Kwak Gunho , Jung Kwang , Flores Giovanni 
Published: July 2021

The current interrupt device (CID) and vent mechanism in the Cylindrical cells are a fuse-type safety device that permanently disconnects the cell's electrical circuit and releases internal pressure when triggered by excessive cell pressure by abnormal conditions to keeps the cell safe from the thermal runaway risks. Compared to the importance of the safety devices' activation pressure, the test method and apply the techniques to the cell characterization was not defined adequately.

This research developed the test methods, procedures, and equipment to measure the activation pressure accurately of both the CID and vent mechanism simultaneously. We select four different cell markers' commercial 18650 cell models with about 3.5 Ah nominal capacity to evaluate the cell model's safety level and to identify the relationship between the activation pressure level and the safety test item: electrical, mechanical, environmental (Thermal) abnormal impacts.

To investigate the relationship between cell safety devices' activation pressure and the cell model's safety, we performed some cell safety tests and cell model design checks. The cell alignment check with the electrode gap measurement, including some electrical tests, checks the test samples' quality status. Cell safety tests include thermal safety, external short circuit, nail penetration, impact, and crush test. Each safety test item's test condition and procedure are referenced from widely accepted cell safety standards and battery industrial best practices for the lithium-ion battery.

The activation pressure of the CID and vent mechanism was measured simultaneously by detecting the cell voltage changes and applied pressure to vent, respectively. All four cell models have shown that the CID was always activated earlier at a lower pressure than the vent mechanism. The activation pressure of the CID and the vent mechanism of the four cell models were different by manufacturers. The tolerance and the activation pressure gap between CID and the vent mechanism of each cell model were different among the four cell models.

The cell safety test items can be classified by 1) thermal and electric abuse test and 2) mechanical abuse test. We used the EUCAR Hazard Level to evaluate the safety level of each test item items. The table explains each test item's safety level of the four cell models according to the EUCAR Hazard Level criteria.

The cell hazard levels of the thermal and electric abuse test items (thermal safety and external short circuit test) are strongly affected by the activation pressure level of the CID and vent mechanisms. The detailed investigation concluded that the CID's activation pressure level affects more than that of the vent mechanism to the thermal and electric abuse test items' hazard level. These results are derived from the fact that the safety device activates in advance at a lower pressure than the cell enclosure's opening.

Meanwhile, the cell hazard levels of the mechanical abuse test items (nail penetration, impact, and crush test) were not affected by the safety devices' activation pressure level. Because the mechanical abuse test items always opened the cell enclosure in advance compared to the safety device activation.

This research proposes to add the activation pressure level of the cell safety devices to the EUCAR Hazard Level criterion to enhance the efficiency of evaluating the cell models' hazard level. That can provide more worthy criteria for the cell selection and cell safety evaluation process for the multi-cell battery system design.

Figure 1