Thermal Runaway of Li-Ion Cells: How Internal Dynamics, Mass Ejection, and Heat Vary with Cell Geometry and Abuse Type

The Li-ion cells chosen for this study were the KULR 18650-K330, the KULR 21700-K500, the LG 21700-M50, and the Saft D-Cell-VES16. All cells were tested within one year of manufacture and were supplied directly from the manufacturers. These cylindrical cells represent a range of geometries for examining how thermal runway varies within cells of different designs and dimensions. It should be mentioned that all these cells are high energy-density cells. With that said, repeat tests were conducted for each cell type, and with the continued expansion of the Battery Failure Databank, further cells will be added for comparison in the future. At the time that this study was conducted, the aforementioned cells have the most data available and can provide the most complete picture for cells of different geometries. The properties of each cell examined as well as the total number of tests for each cell type under each trigger methods are provided in Table I.

The FTRC was designed to facilitate various abuse test conditions for comparing thermal runaway response to different trigger methods including nail penetration, internal short circuit (ISC) device combined with external heat, ¹⁴ and thermal abuse. The nail penetration trigger method was performed using an adapter that was attached to the side of the cell chamber of the FTRC. This adapter enabled the nail to be inserted and retracted from the cell body on demand. To perform thermal abuse tests on the cells, heating elements were incorporated into the walls of the cell chamber of the calorimeter, a design which is described in more detail in previous work. ⁸ The heaters provided around 960 W until the cells underwent thermal runaway at which point the heaters were switched off. The thermal runaway reaction was then able to continue unimpeded to completion. The ISC trigger method consisted of an ISC device described in previous work. ¹⁶ Test cells were specially manufactured to incorporate the ISC device six layers into the wound electrode assembly. To activate the ISC device, the heating elements were switched on until the wax layer separating the current collectors melted (at approximately 57 °C), creating an internal short circuit that consistently initiated thermal runaway.

The FTRC was used to decouple the heat ejected from the cells and the heat emitted from the casing of the cells. The design of the FTRC to facilitate these distinct measurements is described in previous work. ^7,8 The FTRC is also constructed of aluminum, allowing simultaneous X-ray imaging during tests (Fig. 1a). This provided distinct measurements for heat ejected from the positive end, heat emitted from the cell casing, and heat ejected from the negative end. For nail penetration tests, a modified nail with an embedded thermocouple near its tip was used for simultaneous internal temperature measurement during the tests. The design of the temperature-measuring nail is described in more detail in previous work. ^17,20 By slowing the flow of ejected contents via a system of baffles and copper mesh in the ejecta bore chamber of the FTRC (see previous work ^17,20 ), the FTRC also captured ejected collectible mass such as aerosol matter, particulate, carbonaceous material, and shreds of current collector (Fig. 1b). Composition analysis and fractional contributions of particulate, aerosols, and carbonaceous matter in the collected mass are beyond the scope of this work, but a detailed breakdown of ejected solids from Li-ion cells undergoing thermal runaway can be found in work by Barone et al. ²² and Premnath et al. ²³ The collected mass may also include some absorbed volatiles but most volatiles and aerosol particulate of <100 nm diameter were likely lost through the fume extraction. Following the completion of thermal runaway, the FTRC was disassembled and the cell mass distribution throughout the calorimeter was assessed. The mass of the collected material was measured gravimetrically on a microbalance. A photograph of a recovered cell, as well as an example of mass collection from the bore chamber are shown in Figs. 1c–1d. Any mass caught in the baffles and copper mesh is recovered and measured. This is defined as positive or negative ejecta, with positive or negative being determined by which terminal the mass was ejected from. Any mass remaining in the cell casing is defined as cell body mass. By finding the difference between the pre-test mass and the sum of cell body mass, positive ejecta, and negative ejecta, the mass of gases released is determined and defined as unrecovered mass; note that FTRC calculations are designed to estimate the energy contained within these unrecovered components. This mass accounts for the gases and solids (such as aerosols) which left the system during the tests. Data from the specific tests can be extracted from the Battery Failure Databank where specific test identifiers are listed in Supplementary Information (available online at stacks.iop.org/JES/169/020526/mmedia).

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High-speed radiography took place at ESRF—The European Synchrotron and Diamond Light Source Synchrotron. At the ESRF, a polychromatic beam of 76 keV was used for high-speed radiography at beamline ID19, with a high-speed PCO.Dimax (PCO AG, Germany) detector and LuAG:Ce (Lu₃Al₅O₁₂) scintillator. The FOV was 22.89 mm × 14.67 mm (horizontal × vertical) which consisted of 2016 × 1292 pixels with an isotropic pixel size of 11.35 μm. Images were captured at a rate of 2000 frames per second (fps) (with an exposure time of 461 μs). At Diamond Light Source, a monochromatic beam of 75 keV at beamline I12, along with a Vision Research Phantom Miro 310 high-speed detector (Vision Research, NJ, USA) and LuAG:Ce scintillator, were used. The cells were imaged with a FOV of 22.91 mm × 14.32 mm consisting of 1280 × 800 pixels (horizontal × vertical) with an isotropic pixel size of 17.9 μm, at 2000 fps using an exposure time of 490 μs. Radiography videos were flat-field corrected and timestamped using MATLAB, and are open access in the Battery Failure Databank. ²¹

The mass ejection distributions for each cell can be seen in Fig. 2. As seen in Fig. 2a, each cell exhibited different characteristics regarding the mass ejected between trials. Overall, the Saft D-Cell was the most consistent, generally ejecting about 40 g of mass. The other cells showed more variability, with the LG 21700 ejected mass values having the largest distribution, spanning from roughly 30 g to 54 g across trials. This variability could potentially be linked to the rate at which thermal runaway propagated through the cell. This relationship will be discussed further when discussing the internal velocities as determined from radiography video.

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To compare ejected mass values across cell type, the mass fraction ejected—that is the fraction of the cell's original mass that was ejected—is used in the remaining histograms. Figures 2b–2e display the mass fraction ejected from the positive end of the cell, the negative end, what remained within the cell casing, and the fraction of mass that went unrecovered (smoke, gas, and some particulate matter). While the data in Fig. 2 show the distribution of mass ejection behaviors, the donut plots in Fig. 3 provide better insight into the average behavior for each cell during thermal runaway.

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The post-test mass distribution for each cell and trigger method is shown in Fig. 3a. The Saft D-Cell generally had the largest percentage of mass remaining within the cell body following the test, regardless of trigger method. This could potentially be due to the lack of a bottom vent on the D-Cell, albeit the D-cell had 2 top vents for pressure relief. All other cells in this study had a bottom vent which could play a vital role in ejecting mass from the cell body. Another consideration is the rate at which thermal runaway progressed in the cell. The D-Cell had the slowest thermal runaway front velocities when compared to both the 18650 and 21700 cell formats, which will be discussed in more detail in a following section. This allowed for any gases that may have been produced during the reaction to escape the cell body at a rate that prevented the pressure within the casing from rising too high and causing a more violent cell rupture. This will be discussed further when addressing the velocities of thermal runaway propagation within each type of cell.

Referring to Fig. 3b, it is apparent the Saft D-Cell had the largest percentage of unrecovered mass of the four cells being considered. The Saft D-Cell was designed with more excess electrolyte than typical rechargeable 18650 cells to assist with achieving long cycle-life for space satellites. As the Saft D-Cell experiences high internal temperatures due to thermal runaway, the electrolyte evaporates by nature of its volatility, eventually venting from the cell and ejecting as unrecovered mass. The large quantity of unrecovered mass is unlikely to have been caused by relatively high internal temperatures during thermal runaway since radiography revealed that molten Cu was not observed to be widespread within the cell; Rather, the large quantity of unrecovered mass may be due to the cell having a higher ratio of electrolyte to active materials and hence a higher loss of volatile mass, the impact of which is discussed in greater detail by Ostanek et al. ¹²

As for the KULR 18650, the nail penetration trigger method yielded a different ejected mass distribution than either the ISC or external heating mechanisms. Specifically, a greater mass is ejected from the positive end of the cell. Of the three nail penetration tests that were performed on this cell, only one of them resulted in activation of the bottom vent of the cell. This would suggest that internal pressures within the cell did not typically exceed the burst pressure of the vent. Instead, gases that accumulate and cause this pressure rise in thermal and ISC abuse tests would be able to escape through the hole in the cell casing created by the nail. The hole produced by the nail may have acted as an additional vent, reducing the need for the bottom vent to activate.

The LG 21700 also displayed a disparity between the ejected mass distributions for the nail penetration and thermal trigger methods. The radiography footage shows that as the nail entered the cell, multiple instances of thermal runaway begin to propagate from various points of contact between the nail and the electrode assembly, a phenomenon that will be discussed in more detail in the following section on radiography. These multiple fronts of thermal runaway propagation would generate gas more quickly than a single front. Since the LG 21700 cell had an additional vent on the negative end of the casing, the pressure rose and activated both the positive and negative vents. Due to the larger area of the negative vent, it is expected that a higher rate of mass ejection occurred from the negative end of the cell. ¹⁸ For the thermal abuse test, the thermal runaway had a slower rate of propagation within the cell, producing gases at a lower rate. This resulted in activation of the negative vent at a later stage of the thermal runaway event, producing a more even distribution of mass between positive and negative ends of the cell.

Considering the mass ejection and the heat output from the cells in question, there are a few notable trends that appear. As seen in Fig. 4, the KULR 18650 displayed a wide range of mass ejections, with one group between 0.4 and 0.6 mass fraction ejected and another group situated from 0.6 to 0.8. The former group produced a lower heat output than the latter; that is, a higher heat output was observed for cells that ejected more mass. Referring to the associated radiography footage of the group producing larger heat output (radiographs can be found in the Battery Failure Databank ²¹ and specific test identifiers for the 18650 data plotted in Fig. 4 are provided in Supplementary Table SI), layers of the electrode assembly along the inner regions of the cell can be seen peeling away and flowing towards the positive or negative vent, whichever had opened. Eventually this process would essentially stop, suggesting flow through the vents had ceased, potentially due to clogging. ¹⁸ Clogging would have caused the pressure within the cell to build until rupturing, ejecting a higher mass fraction and heat output than cells whose vents remained clear throughout the entire reaction.

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Another notable trend displayed in Fig. 4 is the variability of ejected mass fraction with respect to the cell diameter. Referring to the distributions along the x-axis, the D-Cell had the narrowest distribution of mass fraction being ejected while the 21700 formats had more variability. The 18650 ultimately displayed the highest degree of variability. This variability could be related to the area of the vents for each cell type, where a larger area would imply less resistance for the material to be ejected from the casing. Another consideration would be the rate at which pressure builds up within different cell volumes. For instance, if the rate of thermal runaway were constant for all cell types, pressure would still increase more rapidly in a smaller volume as compared to larger volumes. The rapid increase in pressure would increase the risk of a more uncontrolled rupture. This expected wider variation in internal pressure for smaller cells is expected to influence the mass ejected and heat output accordingly.

The Saft D-Cell produced the most heat when thermal runaway was initiated with the nail penetration trigger method. This is likely due to the nail inducing multiple fronts of thermal runaway that begin to propagate at the same time, i.e. a front that is observed to propagate from the surface of the nail along each electrode layer that it penetrated. This would result in greater heat output as the thermal runaway reaction is able to go further to completion before being ejected than thermal runaway events initiated with either thermal or ISC trigger methods. This trend can be seen in the other cells as well. In fact, the larger the cell diameter, the larger the difference in heat output between nail penetration and thermal trigger methods.

There appears to be a positive correlation between the ejected mass and heat output of the cells during thermal runaway for the 18650 and 21700 cells, with the D-cell being the exception to this correlation. As ejected mass increases, the heat output generally increases regardless of trigger method. This could potentially be due to a lack of oxygen within the cell. A lack of oxygen would stunt thermal runaway and prevent the reaction from running to completion, i.e. oxygen-limited reactions. If a cell containing such a stunted reaction were to rupture, its contents would then be exposed to atmospheric oxygen allowing the reaction to run closer to completion. This would result in a relationship as seen in Fig. 3, where higher ejected mass values correspond to higher heat output during thermal runaway.

Radiography provides further insight into thermal runaway and the events occurring inside the cell. By comparing radiography with the thermal response of cells for each thermal runaway event, relationships can be brought to light. To assess the correlation between the propagation of thermal runaway throughout the cell and mass ejected, reaction front velocity was estimated using the radiography, as shown in Fig. 5. The front in this instance is taken to be the leading edge of the area within the cell that has thermally decomposed. Knowing the diameter of the cells, the distance of the reaction front from the outer radius of the cell was obtained with a corresponding timestamp. After some time had passed, the distance of the reaction front from the cell wall was again assessed, and a new timestamp was obtained. Using these values, the velocity of the front was roughly calculated. There is a large degree of uncertainty in these values, but they provide a qualitative comparison of rates of thermal runaway propagation within cells. It should be mentioned that not all tests had an accompanying radiography video that allowed for the calculation of velocities. The tests used to obtain velocity information are listed in Supplemental Information with their associated Battery Failure Databank ¹⁸ test identifiers.

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After obtaining the speed of the fronts for as many tests as possible, the Saft D-Cell had the slowest thermal runaway propagation compared to any other cells while the LG 21700 had the fastest propagation. As previously discussed, the Saft D-Cell also has the largest percentage of post-test mass remaining within the cell casing while the LG 21700 has the smallest percentage. Taking both into consideration, the faster front propagation as seen in the LG 21700 produced gases at a more substantial rate than the cell was able to vent. As a consequence of internal pressure build-up, the cell would rupture carrying more of its mass out with it. Conversely, the slower thermal runaway propagation as seen in the Saft D-Cell created gases at a slower rate allowing the vents to properly expel the gases and prevent a more violent cell rupture.

Analysis of unrecovered mass across the different cells shows the Saft D-Cell created the largest percentage of unrecovered mass while the LG 21700 produced the least. Comparing this with the associated thermal runaway front velocities, it is apparent there exists an inverse relationship. While the LG 21700 may produce a large amount of gas within a small period of time, following ejection thermal runaway tapers off and gas production is reduced until it ceases altogether. In contrast, the Saft D-Cell produces gas for a longer period of time at a lower rate. This lower rate avoids rapid pressure build-up and bursting, preventing thermal runaway from prematurely ending and ultimately resulting in a larger unrecovered mass being generated.

While the general trend for thermal runaway velocity within the cell seems unrelated to cell geometry, there does seem to be a relationship with the cell energy density (Wh/kg). Plotting energy density against the average of the axial and radial velocities for each cell, a potential relationship can be seen (Fig. 6). As cell energy density increases, the average velocity of thermal runaway seems to increase, albeit with some exceptions. More information on electrode and electrolyte chemistry needs to become available before looking further into this possible relationship, which is challenging to determine in commercial cells.

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Having access to high-speed radiography provides a window into the internal happenings within the cell while thermal runaway is progressing. Looking at the radiography video for each cell being considered displays unique characteristics that would otherwise go unnoticed. For instance, whenever thermal runaway was initiated in the LG 21700 with the nail penetration trigger method, the layers of the jellyroll closest to the center of the cell always developed cracks and split before the nail was able to penetrate the layers itself. While this does not seem to have a noticeable impact on the mass ejected or remaining in the cell, this feature can have implications when attempting to model cells for simulations. Another notable feature that was seen in the radiography was the lack of molten copper globules in the Saft D-Cell tests (Fig. 7). Copper is evident in radiographs due to its relatively high attenuation of X-rays, while all other materials within Li-ion cells are much less attenuating. Molten copper, which is highly attenuating and shows up as white beads in radiography, was visible in only 5.5 percent of all the Saft D-Cell tests performed. This is opposed to molten copper being visible in 90 percent, 93.3 percent and 100 percent of the tests for the LG 21700, KULR 18650, and KULR 21700 cells, respectively. This is significant as it suggests the internal temperature of the cell as thermal runaway was progressing rarely exceeded 1080 °C (the melting point of copper) in the D-cells, regardless of trigger method. This disparity could potentially be due to the higher ratio of electrolyte to active material within the Saft D-Cell. This electrolyte would consume latent heat that would otherwise create higher internal temperatures in order to evaporate and exit the cell as unrecovered mass. The ability to obtain this result from the radiography becomes even more important when considering the lack of reliable internal temperature measurement techniques for these types of tests.

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While the radiography videos provide evidence that all cells, with the exception of the Saft D-Cells, experienced internal temperatures greater than 1080 °C, thermocouples embedded in nails used for nail penetration tests show conflicting results. By inserting thermocouples into the center of hollowed out nails used in nail penetration tests, internal temperatures were recorded throughout the test. This practice is used to gather data on internal temperatures during thermal runaway which is useful for cell modelling. As seen in Figs. 8a–8b, these thermocouple readings were unreliable and rarely represented the actual internal cell temperatures. It should be mentioned that these data have been selectively chosen so that any discontinuous data (e.g. generated when a thermocouple broke or showed abnormal readings) is not presented. The IDs of the tests used in these plots can be found in Supplementary Information. Referencing the radiography video, molten copper can be seen flowing around the cell within 2 s of thermal runaway initiation for all but one of these tests (Fig. 8c). This suggests internal temperatures exceeded the 1080 °C melting point of copper, a temperature that was only recorded once by a thermocouple embedded within a nail (green in Figs. 8d–8e). The data from this single test is considered to be the most representative of internal temperatures during thermal runaway. The red plot in Figs. 8d–8e shows an example of a cell that clearly displayed molten copper, but whose thermocouple reading did not exceed 300 °C.

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The disparity between the thermocouple data and the radiography videos demonstrates a glaring reality regarding the experimental reliability of using nails embedded with thermocouples for internal temperature measurement during thermal runaway. It is clear that this methodology, which is often used to gather data for modelling and simulation, does not provide an accurate measurement of the internal temperature of the cell during thermal runaway. This is likely due to the relatively low thermal conductivity of steel, which means that the temperature of the tip of the nail is unable to reach thermal equilibrium with the cell within the timescale of the experiment. This work therefore demonstrates that temperature measurements conducted in this manner may lead to non-representative data and should be treated with caution. Since the data from the thermocouple within the nail was distinct from the rest of the thermocouples attached around the calorimeter, this erroneous temperature measurement did not influence the error of other heat output measurements.

When comparing to previous literature that used similar nail designs, ^17,20,25 it is seen that the peak temperature recorded by the nail reaches its maximum temperature around 5–15 s following initiation of thermal runaway. Here, we see via radiography that Cu melts within around 1 s following initiation of thermal runaway and that thermal runaway lasts for around 2 s. This indicates that the response time of thermocouples embedded in nails in previous work was likely slower than that needed to record the actual peak temperature that most likely occurred around 1–2 s following initiation of thermal runaway. It is seen in Fig. 8a that in general, plots with greater times to peak temperature had lower peak temperature readings. Since all nails here and in previous work were custom made, it is likely that high consistency during manufacturing was not achieved which may affect the response time. For example, differences in the distance of the thermocouple from the tip of the nail of just 100's of micrometers may affect the response time. A thermocouple with a slower response time will record later temperatures where some cooling may have already occurred, thus underestimating the actual peak temperature achieved. Therefore, to assist with future designs of nails embedded with thermocouples, it is advisable for researchers to pay attention to the time to peak temperature following penetration of cells and aim to minimize this time for highest accuracy of peak temperature readings. The green plot in Fig. 8d has a peak time of around 1 s and is therefore considered a reliable result.