Understanding Li-Ion Cell Internal Short Circuit during Nail Penetration By Simultaneous in Situ Measurement of Local Current, Resistance and Temperature
Authors: Liu Siyi , Huang Shan , Zhou Qian , Snyder Kent , Long Mary Kate , Zhang Guangsheng 
Published: November 2022

Internal short circuit (ISC) can be a main cause of Li-ion cell thermal runaway in field failures, but its mechanisms still require better understanding1
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2. Due to the highly localized and electrochemical-thermal coupled nature of ISC, it is important to use in situ/operando measurement of critical parameters for insightful understanding of the mechanisms. We recently reported a method for in situ measurement of dynamic ISC resistance and ISC current during nail penetration as schematically shown in Figure 1(a)3. When an ISC is formed inside the small test cell by nail penetration, external current will flow from the large power supply cell to the test cell. The test cell capacity is much smaller than the large power supply cell, so the measured current (A) can be assumed to be equal to the ISC current. The ISC voltage (V) is measured directly. Then the ISC resistance can be obtained through dividing the ISC voltage by the ISC current. The local temperature at the ISC spot is directly measured by a thermocouple embedded at the tip of the smart nail4.
The previous method enabled insightful understanding of ISC3. It was observed that the ISC resistance changed by several orders of magnitude during nail penetration and dramatically influenced heat generation and local temperature rise. In some cases, the local ISC temperature increased more than 500 ℃, and even caused melting of Al foil in contact with the nail and rapid recovery of ISC resistance. But the previous method has a shortcoming. As noted in Figure 1(a), the small test cell with ISC is chemically and thermally disconnected from the large power supply cell. Such disconnection makes the thermal behaviors of the cells different from a real-world large Li-ion cell in which the ISC location is electrochemically and thermally connected to the entire cell. In particular, it does not allow investigation of thermal runaway propagation from the ISC location to the large cell.
Built on our previous work while addressing its shortcoming, here we report an improved method. As shown schematically in Figure 1(b), a small cell and a large cell are fabricated inside the same pouch, sharing the same electrolyte and the same separator. They are not only electrically connected, but also chemically and thermally connected. The measurement of ISC current, resistance and temperature is similar to our previous work. When the smart nail penetrates the small cell, the ISC current flows from the large cell to the small cell through the external wire and can be measured by a current sensor (A). The ISC resistance is obtained from directly measured ISC current and ISC voltage. The local temperature can be measured not only by the thermocouple embedded at the tip of the smart nail, but also by thermocouples embedded at different locations inside the large cell5. This new method will enable insightful understanding of the highly localized and electrochemical-thermal coupled ISC phenomena under conditions closer to field failures. The experimental data can also be used for validation and improvement of numerical models of ISC.

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Figure 1