(Digital Presentation) In Situ Measurement of Internal Short Circuit Resistance during Nail Penetration of Lithium-Ion Cells
Authors: Liu Siyi , Huang Shan , Long Mary Kate , Zhou Qian , Snyder Kent , Zhang Guangsheng 
Published: July 2022

Internal short circuit (ISC) is one of the main causes of lithium-ion cell thermal runaway in field failures. Nail penetration has been widely used to trigger ISC for evaluation of thermal runaway risks. Previous modeling studies1
,
2 revealed that ISC resistance greatly influences heat generation and thermal runaway risk, but there are few reports on experimental measurement of ISC resistance during nail penetration3. Furthermore, ISC resistance was assumed to be constant in these modeling and experimental studies. However, our recent experiments4
,
5 suggested that ISC resistance changes dramatically during nail penetration. Therefore, it is important to measure the dynamic ISC resistance in situ during nail penetration of lithium-ion cells. Such measurement will help better understand if and how an ISC triggers thermal runaway. The experimental data will also be useful for validation and improvement of numerical models on lithium-ion cell nail penetration.
In this study we report a method for in situ measurement of dynamic ISC resistance during nail penetration. As shown schematically in Figure 1, a small test lithium-ion cell made of single unit of electrodes and four tabs are developed. Two tabs are connected to a large power supply lithium-ion cell through a low-resistance current sensor. The other two tabs are connected to a voltage sensor for measurement of test cell voltage. Then the test cell is penetrated using a small, slow and in situ sensing (3S) smart nail as reported in our earlier work4
,
5. A picture of such smart nail is shown in Figure 1. When an ISC is formed inside the test cell by the smart nail, external current will flow from the large power supply cell to the test cell. If the test cell is much smaller than the large power supply cell in dimension and capacity, it is assumed that the measured current is approximately equal to the ISC current and the measured test cell voltage (V2 in the schematic) approximately equal to the ISC voltage. Then the ISC resistance can be obtained through dividing the ISC voltage with the ISC current.
Figure 2 and Figure 3 show preliminary results of this study based on dry test cells without electrolyte. The cells were penetrated using a smart nail with outer diameter of 0.76 mm at a speed of ~0.03 mm/s. At the tip of the nail, a thermocouple with diameter of 0.5 mm was embedded for in situ measurement of ISC temperature. The area of cathode in the test cells is 10 mm by 10 mm with a nominal capacity of 1.8 mAh. The large power supply cell has a nominal capacity of 3,000 mAh. Figure 2 shows results of penetration from Al foil to Cu foil while Figure 3 shows results of penetration from Cu foil to Al foil. It can be seen that ISC resistance changed dramatically during nail penetration in both cases. The lowest ISC resistance was several orders of magnitude lower than the stable resistance after full penetration. The lowest ISC resistance also caused rapid increase of ISC temperature, indicating higher risk of thermal runaway. By comparing Figure 2 and Figure 3, a stable resistance after full penetration was similarly seen in the two cases, but the dynamic resistance responses were very different. The dynamic resistance can be attributed to the different resistivity of test cell components as well as the change of contact resistance between the nail and those components. Moreover, the lowest ISC resistance during penetration from Al to Cu is tens of times smaller than that during penetration from Cu to Al. These results provide new insights into nail penetration processes. They also imply that future modeling studies on nail penetration should consider the dynamic change of ISC resistance to more accurately reflect the physical behaviors. More details about these dry test cell results and new results of working test cells with electrolyte will be presented in the conference.

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Figure 1