Thermal Runaway Energy Release as a Function of the State of Charge

Editor’s Note: The paper on which this article is based was originally presented at the 2023 IEEE Product International Symposium on Product Compliance Engineering (ISPCE), held in Dallas, TX in May 2023, where it received the 2023 Best Paper Award. It is reprinted here with the gracious permission of the IEEE. Copyright 2024, IEEE.

Introduction

Advancement in lithium-ion (Li-ion) batteries has resulted in them being the power source of choice in consumer electronics. These types of batteries have the advantage of high power and energy densities. As the chemistry of Li-ion batteries advances, so does their capacity and energy density. The higher the energy content and the smaller and tighter the internal components, the higher the potential risk, magnitude, and consequences associated with battery failure events.

A major failure mechanism that can lead to fires and explosions is a thermal runaway event. In large battery packs, many cells can be packed in close proximity to each other. If one of the cells goes into thermal runaway, the energy released can heat up neighboring cells, which may lead to a thermal cascade throughout the battery pack. A pack design that mitigates this hazard may incorporate cell-to-cell gaps filled with insulating materials, potting compounds, or specialty materials designed to transfer the energy generated during the battery failure to less critical areas of the pack to be dissipated safely. From a risk assessment standpoint, it is generally expected that a single-cell failure within a large multi-cell battery pack might be rare but inevitable. This potential for failure propagation introduces an increased risk of property damage and safety issues. Thermal runaway events can result in the venting of flammable gases, which can generate fire or even an overpressure event if ignited in a confined space. The propagation of failures increases the total energy released during the event, as well as the volume of flammable gases and ejecta released.

As such, an accurate evaluation of the energy yielded during a thermal runaway event is beneficial for the design of battery-powered consumer electronics for both performance and safety. Accurate energy yield estimates are a valuable parameter for a wide range of design tasks, such as but not limited to:

• Comparing failure characteristics of cells of different formats, batches, or from different vendors
• Evaluating the energy release mechanism (i.e., Is the energy released within the cell casing or in the gas and ejecta?)
• Designing safer battery packs that minimize propagation of failure events to neighboring cells, and
• Generating reliable inputs for mechanical/thermal models of devices and/or battery packs.

The energy released during a battery thermal runaway event can be assessed by evaluating the sensible and chemical energy during the failure. Traditionally, two techniques are used to quantify the energy released such as (1) oxygen consumption calorimetry (OCC) [1], and (2) accelerating rate calorimetry (ARC).

OCC has been used for many years to estimate the energy released during combustion by collecting and analyzing the oxygen, carbon dioxide, and carbon monoxide concentrations of the exhaust gases. This technique can be used to obtain an estimate of the chemical energy associated with the combustion of the flammable vent gases released during a thermal runaway event. It should be noted that additional complexity is associated with OCC testing of Li-ion cells given cell composition, non-standard reaction paths, and generation of oxygen during the thermal runaway failure.

An ARC is an instrument designed to characterize the self-heating behavior of materials and reaction kinetics. This method has become highly utilized to understand the thermal runaway processes of batteries. ARC can be used to study a variety of variables that affect thermal decomposition and thermal runaway characteristics. If ARC testing is performed with the battery sample in a sealed vessel, the overall energy release from the thermal runaway event can be estimated using the heat capacity of the sample in conjunction with the temperature rise of the sample, the temperature rise of the ARC vessel, and the known heat input into the system. It should be noted that depending on the amount of vent gases released and the nature of the ARC test conducted (e.g., in an inert environment or in air), the combustion energy associated with the flammable gaseous species may not be fully captured.

This work provides an overview of a relatively novel experimental apparatus, fractional thermal runaway calorimetry (FTRC), designed to measure the energy output and mass ejections associated with a thermal runaway event. Compared to ARC, which relies on relatively coarse temperature measurements in a sealed vessel, the FTRC provides better estimates of the thermal runaway energy given the high-fidelity temperature mapping of each section of the apparatus. For this reason, the FTRC can provide additional information on the energy fractions of failures associated with (1) vent gases and ejecta compared to the cell body and (2) the positive and negative terminals of the cell.

Battery Thermal Runaway

Thermal runaway occurs when the internal temperature of a cell increases in an uncontrolled manner, leading to the cell’s failure and unintended side effects, such as the release of dangerous gases, fire, or an explosion. In the initial stages of thermal runaway, the solid electrolyte interface (SEI) layer decomposes in an exothermic reaction, followed by an exothermic reaction between the intercalated Li-ion cell and electrolyte. As positive electrode material reacts with the electrolyte, oxygen is generated inside the cell and the electrolyte decomposes and the cell disintegrates. During the process of thermal runaway initiation, the rising temperature generates gases, which are released through pressure relief vents in the cell when the internal pressure reaches the design relief pressure or if the cell enclosure fails. For Li-ion cells, these gases are hot and combustible, which is not only a hazard in itself but one that can propagate the failure to other cells in the pack.

All thermal runaway events are a result of elevated temperature in the cell. This temperature rise can be caused in many ways, including, but not limited to:

• External heating from a high ambient temperature, thermal abuse, or external fire
• An internal cell defect resulting in an internal short circuit which can cause heating at the site of the defect
• A surge in charge/discharge current and the resulting heat generated
• Improper electrical connection at the tab of a battery that can cause increased electrical resistance, generating heat at the contact location or
• Mechanical damage to the cell which can lead to internal shorts, resulting in heat generation.

During thermal runaway, the cell produces gases that build up within the cell. Some cells, such as cylindrical cells, are designed with vents that open to release gases when a certain internal pressure is reached. Occasionally, these relief vents fail via obstruction or inadequate venting area, which may result in the rupture of the cell casing. For particularly energetic failures or for weak and/or flawed cell casing designs, the rupture of the cell casing may lead to the release of ejecta and vent gases from the negative terminal of the cell (i.e., in the case of negative side rupture) or from the side of the cell (i.e., side rupture). Such abnormal failure modes may result in a drastically different energy breakdown during failure and require careful consideration for the design of appropriate mitigation strategies to handle the thermal runaway energy. Cell formats, such as pouch cells, are not designed with vents but rather weak points in the external pouch that release gases when internal gas pressure rises. The peer-reviewed scientific literature provides insights into the typical vent gas volume released during a thermal runaway event involving cylindrical, pouch, and prismatic cells [2‑11]. The gas production ranges between approximately 1 L and 3 L per Ah of battery capacity. The major constituents include CO2, CO, and H2, together with non‑negligible amounts of hydrocarbons (e.g., CH4, C2H4, C2H6, plus longer chain hydrocarbons) [2]. The interested reader should refer to references [2-11] for a comprehensive overview of experimental data on vent gas volume and composition, vent gas release rates, as well as the effect of the state of charge (SOC) on the main thermal runaway parameters.

Methodology

An FTRC is a battery testing apparatus specifically designed by the National Aeronautics and Space Administration (NASA) to measure the energy output and mass ejections associated with a battery thermal runaway event [12][13]. The FTRC is equipped with interchangeable cell chambers that can accommodate cells with various form factors and capacity (e.g., 18650 cells, 21700 cells, D cells), as well as different cell triggering mechanisms ranging from external heating to nail penetration and internal short circuit devices. The cell chamber is centrally located and is interfaced on either side with (1) ejecta mating assemblies, (2) ejecta bore assemblies, and (3) rod‑and‑baffle assemblies. The cell chamber is equipped with four cartridge heaters that can be used to trigger a thermal runaway by raising the cell temperature. An FTRC apparatus equipped with a standard 18650 cell chamber is fundamentally a symmetric device that can evaluate energy releases associated with cell failures encompassing top venting, bottom venting, or both. Modifications to the cell chamber design can be implemented to provide a means to estimate energy releases associated with side casing ruptures. Figure 1 shows two close-up views of the 18650 cell chamber displaying the cartridge heaters and the four slots in the chamber for the heaters.

Figure 2 shows close-up views of the cell chamber containing an 18650 cell and displays both the positive and negative cell terminals.

The operation of the FTRC is based on simple physical principles. The various assemblies of the FTRC are all composed of known materials with known masses and thermal properties. The temperatures of these components are recorded throughout a test run. Since the material composition of the assemblies is well known, it is understood how much energy must be added to the assemblies to cause a given rise in temperature. Thus, by measuring the temperature of the components, it is simple to compute how much energy was transferred to each component (i.e., how much energy the cell released).

The FTRC cell chamber (see Figure 1 and Figure 2) is connected to the ejecta mating assemblies via ceramic bushings that provide a certain degree of thermal isolation between the sub-assemblies while guaranteeing the continuity of the flow path for the vent gases ejected during the battery failure event. The ejecta mating assemblies (see Figure 3) are designed to capture large debris and ejecta released during the cell failure. Figure 3 shows close-up views of an ejecta mating displaying (1) the ceramic bushing that interfaces with the cell chamber and (2) the opposite mating component that connects with the bore section of the calorimeter.

Figure 4 shows close-up views of a rod-and-baffle assembly and associated bore. The bore assemblies and rod-and-baffles assemblies are located downstream of the ejecta mating. They are designed to extract sensible energy from the vent gases by creating a tortuous flow path encompassing (a) a series of aluminum baffles and (b) copper mesh windings.

Figure 5 shows a photograph of a fully assembled FTRC equipped with an 18650 cell chamber. Note the two copper mesh windings are shown prior to their installation in the rod-and-baffle assembly of the FTRC. The fully assembled device is placed in a dedicated insulated casing to minimize heat loss during the test (see Figure 6).

The energy generated during the battery failure can be evaluated in terms of total energy yield, fractional energy yields associated with the battery body and positive/negative vent gas, and ejecta. The cell energy yield is obtained by solving an energy balance equation for all the sub-components of the calorimeter based on the mass, specific heat, and temperature increase experienced by each sub-assembly. More specifically, the sub-assembly temperature increase is measured by type-K thermocouples attached to the hardware of the calorimeter in multiple locations. The data collection system relies on seven multipin connectors that deliver the signals to several National Instrument data acquisition cards. The post-processing algorithm utilizes the temperature data collected by 104 temperature channels logged at a frequency of 20 Hz and six voltage channels logged at a frequency of 1600 Hz.

Test Results

This section summarizes the FTRC tests performed on 17 cells at different SOCs. Testing conditions and cell properties are summarized in Table 1. All tests utilized 18650 format lithium-ion cells with nominal capacities of 2.6 and 3 Ah. Cells were charged to a target SOC by first putting each cell through a full discharge-charge cycle and measuring the full cell charging capacity. Each cell was then charged to the desired fraction of the measured capacity using coulomb counting of the applied current.

Table 1: Test conditions and properties of tested cells

Charging to 100% SOC was done using a constant current constant voltage (CCCV) charging protocol:

• Cells A were charged at a constant current of 1.3 A, a voltage limit of 4.2 V, and a current cutoff of 0.13 A.
• Cells B were charged at a constant current of 3 A, a voltage limit of 4.2 V and a current cutoff of 0.15 A.

Discharge to 0% SOC was done using a constant current (CC) discharge:

• Cells A were discharged at a CC of 1.3 A and a lower voltage limit of 2.75 V.
• Cells B were discharged at a CC of 3 A and a lower voltage limit of 2.5 V.

Cells were tested utilizing two different abuse methods:

• Nail penetration: Cells were penetrated using a conductive stainless-steel nail with a diameter of 3 mm. The nail was sharpened to a 30-degree angle. The nail was inserted into the middle of the cylindrical face of the cell at a rate of 80 mm/s. In some cases, nail penetration tests at low SOCs did not result in thermal runaway. Therefore, only 100% SOC nail penetration tests are reported here.
• External heating: FTRC cell chamber is equipped with four cartridge heaters with the total rated power of 1 kW. These heaters are used to increase the temperature of the chamber, which in turn heats the cells. Heaters were operated at their maximum power until cell failure occurred, or until a cell surface temperature reached 300 °C. The energy supplied by the heaters was measured during the test and was ultimately subtracted from the total energy of the system to calculate the energy released by the cell.

Energy Yield as a Function of SOC

Figure 7 shows the total energy release by two different cell models tested under different abuse conditions. Figure 7a shows the energy as a function of SOC, while Figure 7b shows the energy release as a function of charging capacity measured by coulomb counting during the charging process. We observed that the energy release increases with SOC. Additionally, tested cells can release a significant amount of energy even at 0% SOC. Furthermore, we observed that the nail penetration abuse method resulted in higher total energy release for the tests done at 100% SOC on cells A.

Figure 8 shows variation in the total and fractional energy release as a function of SOC for cells B. Higher SOC led to more material being ejected from the cell casing (positive ejecta), demonstrating that the majority of the energy release was associated with ejecta. In contrast, lower SOCs saw the majority of the energy release remaining within the cell body, showing a higher cell body energy release for 0% and 30% SOC compared to 100% SOC. This finding indicates that understanding the energy release as a function of SOC could be important for system designs where the energy release in the cell body is of high importance, since low SOC failures may represent a worst-case scenario for such applications.

The voltage and current during cell charging were logged as a function of time, making it possible to measure the total amount of energy delivered to the cell during charging. An important metric to consider with respect to the energy release is the comparison between the total energy delivered to the cell during charging and the energy released by the cell as a result of the abuse. Figure 9 shows the ratio between the energy released during the thermal runaway and the energy supplied to the cell during charging. We observed a decreasing trend of this ratio with the increase in SOC, with the ratio approaching 1 – 1.5 at 100% SOC.

Conclusions

This work presents an experimental framework to characterize the energy released during a thermal runaway event of a lithium-ion battery cell that is agnostic of the battery chemistry. This fractional characterization of energy during thermal runaway is an important parameter that can inform the design of battery-powered consumer electronics from both safety and performance standpoints. The application of FTRC to the study of the energy release at different states of charge provides a unique look at the cell failure mechanisms. Several findings are identified as a result of this study:

• While the energy release by the cell increases at higher SOC, a significant portion of energy is still released even at 0% SOC
• At high SOCs, most of the energy is associated with ejecta and gases, while at low SOCs, the majority of energy is contained within the cell body and
• The ratio between the energy released by the cell and the energy supplied to the cell during charging decreases with the increasing SOC

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