Internal short circuit (ISC) is a critical failure mechanism for lithium-ion cells that can lead to disastrous thermal runaway. Therefore, great effort has been made to understand its mechanisms by purposefully triggering ISC through various methods^{1, 2}, such as embedding a nickel particle³ or a wax covered copper pad⁴ inside lithium-ion cells. While these methods have been successful in triggering ISC, none of them allows measurement of ISC current, a critical parameter that influences heat generation and risk of thermal runaway. Alternatively, Feng et al.⁵ reported a method of placing a resistor between two parallel-connected cells and using the resistor to externally short circuit the cells. This method could allow measurement of short circuit current, but the resistor is placed between two cells rather than between electrode layers to create different types of ISC⁶. Inspired by this method⁵ and the established ISC methods^{3, 4}, here we report a new method that not only triggers lithium-ion cell ISC but also monitors ISC current.

As shown schematically in Figure 1, the method works by inserting a pair of small nickel pads inside a lithium-ion cell. One pad is between cathode and separator. The other is between anode and separator. Then the pads are connected electronically outside the cell through insulated metal wires, a switch and a current sensor. Once the switch is closed, the anode and cathode will be shorted at the pad region to form an Anode-Cathode type ISC. The ISC current is forced to flow through the external circuit to be measured by the current sensor. The resistance of the external circuit is much lower than the resistance between nickel pads and their corresponding electrode, so heat generation is focused inside the cell at the pad region as in an ISC scenario. By adjusting dimension of the nickel pads and/or the pressure applied to the pad region, the ISC resistance can be adjusted for investigation of its effects. Furthermore, by connecting one of the pads electronically to the opposite tab, the Anode-Aluminum type ISC and the Cathode-Copper type ISC, can be created. Alternatively, these types of ISC can be created by removing part of electrode coating and placing one of the Ni pads to be in direct contact with current collector. In addition, by embedding thermocouples inside the cell⁷, temperature distributions can be simultaneously measured for understanding of electrochemically-thermally coupled phenomena during ISC.

Figure 2 shows preliminary results obtained from a prototype lithium-ion cell using this new method. The cell has a single unit of electrodes and a nominal capacity of 0.02 Ah. Three types of ISC were created, including Anode-Cathode, Anode-Aluminum and Cathode-Copper. As expected, the Anode-Cathode type ISC had the smallest current while the Anode-Aluminum type ISC had the highest current⁶, which can be attributed to the high resistance of cathode and the contact resistance between nickel pad and electrode. More results and analysis will be presented.

References

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