Thermal runaway in lithium-ion batteries occurs due to several causes, which can also result in very different worst case outcomes. In this method, we are trying to simulate a catastrophic hazard in a single cell, and the protocol has been shown to provide consistency in results for the simulation of the type of hazard we want to create. The main advantage of this test procedure is that it measures various parameters in situ in one single test.
The time result data comprehensively characterizes the transient event of lithium-ion battery thermal runaway and fires. This experiment requires a synchronization of data acquisition from many sensors, FDIR, and video recording. The operator has to follow the standard operating procedure to operate multiple devices step-by-step correctly.
This ensures the success of experiments with consistent results and without potential hazards to personnel and devices. Demonstrating the procedure will be Pushkal Kannan, a PD student, and Dr.Ankit Sharma, a post-doctoral researcher from my lab. To begin, install a new or clean filter in the filter valve unit.
Open the valve of the nitrogen cylinder connected to the gas analyzer, and adjust the nitrogen flow rate to 150 to 250 cubic centimeters per minute. With a cell preparation, measure the initial voltage and mass of the cell with a precision of 0.01 grams, and record them on the experiment log sheet. Attach a heating tape to the center of the cell.
Ensure that the heating tape wires point toward the negative side of the cell. Take a picture of the cell with the tape. Attach three thermocouples to the cell surface, one near the positive terminal, one in the middle, and one at the bottom near the negative terminal of the cell using high temperature resistant tape.
Use the thermocouple near the positive terminal to control the heating rate through the proportional integral derivative, or PID. All three thermocouples should be located five millimeters away from the edge of the heating tape. Take a picture of the cell with a ruler to confirm the distance from the heating tape.
Spot weld nickel tabs to the positive and negative terminals of the cell for the cell voltage measurement, then load the cell on the cell holder. Place the cell and the cell holder on the mass balance in the chamber. Connect the thermocouple connectors, heating tape, and nickel tabs to the chamber feed-through plugs and wires.
Turn on the PID controller for the heating tape, and set up the heating profile. Connect the cables for the PID controller, data acquisition, and the mass balance to a laptop, and start the data acquisition program on the laptop. Make sure all the sensor readings shown in the data acquisition program are reasonable.
After checking the measurements, turn off the data acquisition program. Adjust the front and side view camcorder settings, manual white balance, manual focus, auto exposure, auto iris, and auto shutter speed. Make sure that the camcorder battery is full.
Position the front view camcorder on a tripod outside the chamber, start recording on the side view camcorder, and place it inside a protection box in the chamber. After checking the side view camcorder angle and view, lock the protection box. Close the chamber, and ensure all the screws on the cover plates are tightly fastened.
Use the vacuum or diaphragm pump to conduct a leak check, and change the FTIR intake from ambient air to the chamber. Then connect the FTIR return line to the chamber. Set the PID controller to ramp soak mode, and turn off the light in the room and the LED light in the chamber.
Start the front view camcorder recording, and then record the following startup process to synchronize the data acquisition and video recording. Start the data recording and the data acquisition program on the laptop. Start the PID ramp soak mode at 10 seconds on the data acquisition program timer, turn on the chamber LED light, and start the FTIR recording.
Position the front view camcorder on the tripod, and continue recording the experiment. Move to a different room, and continue monitoring the data acquisition panel on the laptop through a remote controlled desktop program. When thermal runaway occurs, or after the PID controller has maintained the cell temperature at 200 degrees Celsius for 60 minutes, turn off the power to the heating tape, and set the PID controller to standby mode.
End the experiment and data recording when all three thermocouple readings become lower than 40 degrees Celsius. Purge the FTIR gas analyzer with nitrogen to clean the tube in the analyzer for around 15 minutes. After purging, stop the FTIR measurement.
Before the chamber cleanup vacuuming procedure, check if the FTIR sampling intake line is closed or open to the ambient air. Select ambient air on the Protea Analyser Software, or PAS-Pro Software, or shut down the FTIR entirely. Open valve one to prepare for partially vacuuming the chamber using the chemical resistant diaphragm pump, and run the diaphragm pump until the chamber pressure drops to 9.7 pounds per square inch absolute.
Turn off the diaphragm pump, and close valve one, then open valve three to fill the chamber with ambient air. Close valve three when the chamber pressure recovers to the ambient pressure. After lowering the concentration of toxic gases by partially vacuuming the chamber with the diaphragm pump, run a rotary vein pump until the chamber pressure drops to 4.7 pounds per square inch absolute to remove the rest of the toxic gases.
Open the chamber, and retrieve the camcorder and the cell. Take photos before, during, and after taking the cell off the cell holder. Weigh the cell, and record the post-test mass of the cell.
Finally, post-process the collected data, and generate plots to visualize the time evolution of all measurements. The cell temperature and mass loss data obtained for an 18650 cylindrical cell at 75%state of charge is shown in this figure. The mass loss indicates two distinct gas release periods, one during cell venting, and the other during thermal runaway.
The concentrations of major hydrocarbon and toxic gas species are shown here. The recorded current and voltage supplied to the heating tape can be used to calculate the power input to the cell. The representative data for voltage and current supplied to the heating tape and calculated energy and power supplied to the heating tape are presented here.
The most important factor here is to ensure safety during and post each experiment. The experiment must prevent an external short circuit of the test cell. And the other critical factors are confirming that the cell SoC and the heating rates are verified to be correct before the test.
It is also critical to seal the chamber completely to confine the exhaust of toxic gases, and follow the cleanup procedure exactly to remove the gases in a safe manner. The test procedure can be extended to study fire application in different cell formats and modules, advancing our understanding of thermal runaway and battery fire scaling in multi-cell batteries. The comprehensive time result data collected in this test procedure enables the development of future models and theories of lithium-ion batteries.
It will also help to understand how battery fire scales up.
