Thermal Runaway is one of the major safety risks in lithium ion batteries. One of the prime reasons for thermal runaway are Internal Short Circuits (ISCs). The ISC phenomenon is very fast and the battery discharges in a matter of seconds, resulting in an explosion which can propagate to other cells, resulting in additional thermal runaway. Hence it is important to understand which sensors can give earliest warning (integrated with voltage measurement) to give a higher confidence level of detecting the imminent evolution of the thermal runaway due to the ISC. Leveraging prior work in [1] where battery swelling was used to improve SOC estimation, we consider the benefits of force measurement along with temperature measurement for detecting the onset of thermal runaway. As it is shown in Figure 1, the measured force increases at the same instance when there is a voltage drop due to the short. The measured force changed earlier than the installed RTD (resistance thermometer) or thermocouple. The mechanical measurement, therefore, can be used in conjunction with electric measurements to generate a robust detection of internal shorts.

A 4.5 Ah NCM - Graphite pouch cell with a built-in ISC device was fabricated for the experiment at the University of Michigan Battery Lab. The ISC device used is similar to the one developed by NREL [2]. During the battery fabrication, a hole is cut in the separator, which is then covered by a Phase Change Material (PCM) trigger. The PCM melts when the temperature reaches its melting point at 55^oC, thus creating a gap in the separator, allowing the electrodes to contact and triggering the ISC.

This pouch cell was inserted in a cell holder shown in Figure 2. A RTD strip with six sensing elements [3] was placed on the cell. A thermocouple was placed between the current collecting tabs (at the position where the venting will occur). There are four load cells (which measure force) attached to the four corners of the cell holder. The instrumented cell fixture was heated in an Accelerating Rate Calorimeter until the ISC is triggered. The temperatures, terminal voltage and load cells were all sampled at 80 Hz during the experiment.

The chronology of events shown in Fig.2 suggests that the ISC leads to local heating and gas generation, which results in battery swelling before the elevated temperature can reach to the surface of the cell. The measured voltage exhibits an initial drop (indicated by t=0 in the graph) but subsequently rises again and continued to fluctuate until reaching zero 6.5s later. The force started rising immediately after the initial voltage drop. Even the thin film RTD located on top of the ISC trigger responded slower than the force, at 0.2s after the initial voltage drop. The accumulated pressure inside the cell then caused the pouch to rupture between the electrical contacts (at the weakest part of the pouch cell). The ultimate burst pressure exceeded the measurement range of the sensor, but the timing of the event was observed by the rapid depressurization at 0.55 s. Finally, the thermocouple temperature increased at 0.60s, most likely due to hot gases venting out over the thermocouple placed between the tabs.

An ISC event and the eventual rupture can be detected using measured force earlier than using temperature sensors on the surface of the battery. The actual implementation of the mechanical measurement can be achieved with load sensors in the end plates of a battery module [1], or with strain sensors embedded in the module [3].

References

[1] S Mohan et al. “A phenomenological model of bulk force in a li-ion battery pack and its application to state of charge estimation” Journal of The Electrochemical Society, 161(14): A2222-A2231, 2014

[2] M. Keyser et al., “Development of a Novel Test Method for On-Demand Internal Short Circuit in a Li-Ion Cell”, Advanced Automotive Battery Conference, Pasadena, CA, 2011.

[3] A. Knobloch et al., “Fabrication of Multi-Measurand Sensor for Monitoring of a Li-ion Battery”, ASME Journal of Electronic Packaging, 2017 (Submitted)