Safety of Li batteries is a major concern for many applications. One of major safety concerns is thermal runaway due internal short circuits created because of dormant internal defects. New processes, designs, and materials are being investigated to improve safety of batteries, particularly to prevent thermal runaway due to internal short circuits (ISC). The challenge is how to evaluate these improvements as existing methods using external triggers, such as nail or blunt rod penetration, crush, and heating, are widely known to be deficient. 

In response to this challenge, we have developed a very thin and small “thermal switch” to be implanted in a Li-ion cell for triggering ISC on demand to see if the cell goes to thermal runaway. This “thermal switch” or “ISC emulator device” consists of very thin layers of copper, wax, aluminum, and separator that are all compatible with the Li-Ion chemistries. The ISC device is inactive when is placed anywhere in a cell during its fabrication; the cell could be cycles many times without any performance issues.  ISC device is activated by applying heat to raise the temperature of the thermal switch to tunable 45°C-60°C to melt the thin layer of wax and create a current path between positive and negative electrode and or current collectors.  The ISC device can create any of 4 types of internal short circuit: +ve current collector to –ve current collector, anode active to +ve current collector, cathode active to –ve current collector, and anode active to cathode active.  The ISC device can work in any li-ion cell format (cylindrical can and prismatic pouch/can).

With support from DOE’s Vehicle Technologies Office, we have demonstrated that more than ISC devices are triggered in a cell creating ISC. In laboratory testing, the activated device can handle currents in excess of 300 A to simulate hard shorts (< 2 mohms).  Phase change melting has been 100% successful during trigger tests to go from non-conducting to conducting. Experiments have shown that the creating ISC in cells can lead to thermal runaway depending on the type of internal short circuit is created, the safety measures in the cell, and the chemistry.  We have provided more than 300 ISC devices to several cylindrical and pouch cell manufacture to implant them into their cell for evaluation of their safety supplying them to their clients for evaluation.  The ISC device has been able to initiate thermal runaway in portion of the cells tested.  In this paper, we provide details of the ISC device and the results of triggering the device.  We believe that the ISC device a useful research tool for evaluating the safety of lithium ion batteries in response to inherent internal defects.