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Thermal Runaway Induced Casing Rupture: Formation Mechanism andEffect on Propagation in Cylindrical Lithium Ion Battery ModuleTo cite this article: Li Lao et al 2020 J. Electrochem. Soc. 167 090519

 

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Thermal Runaway Induced Casing Rupture: FormationMechanism and Effect on Propagation in Cylindrical Lithium IonBattery ModuleLi Lao,1,2 Yong Su,1 Qingchuan Zhang,1,z and Shangquan Wu1,z

1CAS Key Laboratory for Mechanical Behavior and Design of Materials, Department of Modern Mechanics, CAS Center forExcellence in Complex System Mechanics, University of Science and Technology of China, Hefei 230027, People’s Republicof China2SINOEV Technologies Inc., Hefei 230061, People’s Republic of China

With the wide-ranging and ever-increasing applications of lithium-ion batteries in electric vehicles (EV), thermal runaway (TR)-induced safety issues, such as fires and explosions, are raising more and more concerns. In this work, cylindrical 21700 batterieswere externally heated to conduct the TR experiment, and the casing rupture in the form of melting holes and tearing cracks wasfound to be one of the key factors that caused cell-to-cell TR propagation. The appearance and the cross-section microstructure ofthe ruptured casing showed that the melting hole is formed because of a large current short circuit and the tearing crack is due to adecrease in the mechanical strength at high temperatures. Experimental simulations were conducted to further demonstrate thecasing rupture mechanism. In addition, new designs with increased casing thickness were implemented to inhibit the occurrence ofcasing rupture, and their effectiveness was analyzed by performing numerous TR experiments. The improved casing was used in acommercial battery pack, and no TR propagation occurred during operation. Thus, in this study, the casing rupture mechanism wasfirst elucidated, and guidance for the design of lithium-ion batteries with improved safety was provided.© 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open accessarticle distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/1945-7111/ab8807]

Manuscript submitted February 14, 2020; revised manuscript received March 31, 2020. Published April 17, 2020. This paper ispart of the JES Focus Issue on Battery Safety, Reliability and Mitigation.

Since lithium-ion batteries were first used commercially in 1991,1

they have attracted significant attention for applications in electricvehicles (EV), power tools, portable devices, stationary storage, andso on, owing to the advantages of high specific energy, a long cyclelife, a wide operating-temperature range, low cost, and a low self-discharge rate.2,3 As lithium-ion batteries drive a revolution inelectrical energy storage, safety issues, especially the fire andexplosion accidents caused by thermal runaway (TR), have becomea critical obstacle impeding their applications. The TR is defined asthe self-accelerated degradation of lithium-ion batteries,4 which canbe induced by abuse5–9 or failure caused by internal defects ofbatteries.10 During the TR, a significant amount of burnable andharmful gas and a large amount of heat are generated, whichsubstantially increase the temperature of batteries.11 Furthermore,the TR of one cell could cause cell-to-cell TR propagation, resultingin the disastrous explosion of the battery pack.12 In EV, thepossibility of thermal runaway and the extent of destruction can bereduced by modifying the pack design (e.g., the battery managementsystem (BMS) suppresses the overcharge and overcurrent),10 butcomplete elimination of the self-induced failure caused by defectsgenerated in the manufacturing process is almost impossible. From aprobabilistic point of view, although the rate of self-induced failureof a lithium-ion battery is extremely low, the failure rate of a batterypack consisting of thousands of cells in series and parallel is muchhigher.10

To ensure the safe application of lithium-ion batteries, the TRprocess of lithium-ion batteries and the prevention of TR propaga-tion need to be investigated. The type of battery cells, such as pouch,prismatic, and cylindrical cells, is also an important factor affectingthe amount of energy released and the energy-release path.13–15

Because the cylindrical lithium-ion battery is the most ubiquitous inthe market (e.g., the commercial Panasonic NCR18650B),16 manystudies have focused on these batteries. During the TR, the batterycould reach a high temperature and release a significant amount ofburnable gas.5,14,17 In order to prevent the build-up of the high gaspressure, venting elements exist in the battery. When the inner

pressure is larger than the threshold of the vent, the vent opens.Typically, the 18650 battery generates about 40–75 kJ of energyaccording to its capacity, and most of this energy is released througha top and bottom vent.14 The venting mechanisms of five different18650 cell designs from four leading manufacturers were analyzedby using high-speed X-ray radiography and CT;16 in some cases,ruptures are observed near the vent when the vent itself does notallow sufficient flow of gas to escape.14 However, when a largenumber of TR tests were performed, in addition to the opening of thevent and the rupture near the vent for most batteries, we observed thenon-standard failure mode: the side-wall of some battery casingsbroke during TR. This failure mode, referred to as casing rupture,provides an additional energy-release path. Although this rupture ismentioned in certain scenarios, it has not been studied system-atically.

While the TR of a single cell may not necessarily lead to a safetydisaster (although it would affect the consistency of the batterypack), the cell-to-cell propagation could lead to fire and explosion ofthe battery pack and vehicle, and it thus poses a great threat topeople’s health and life. Thus, in addition to studies on the TRperformance of a cylindrical battery at a cell level, it is essential tostudy the process and mechanism of TR propagation and preventivemeasures as well. Many studies have been carried out to investigatethe propagation process in the battery module or system.12,18–20

These studies have focused on the thermal properties during thepropagation, mainly the heat transfer between cells, between cellsand modules, and between batteries and the environment. The heattransfer is affected by the configuration of the battery modules, e.g.,the materials in the battery gaps,21 water-cooling system,22 andtopology of the electrical circuit.12 However, in most of thesestudies, the venting design of the battery was considered, while fewstudies considered casing rupture, which could cause TR propaga-tion due to the generation of an unexpected energy release path. Ourexperiments show that although the possibility of casing rupture islow, it would inevitably lead to immediate TR propagation, and thus,it is critical to inhibit casing rupture.

In this work, a significant number of TR experiments usingcommercial cylindrical 21700 lithium-ion cells were conducted byexternally heating, to investigate the casing rupture mechanism ofzE-mail: zhangqc@ustc.edu.cn; wushq@ustc.edu.cn

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batteries. The casing rupture in the form of a melting hole and atearing crack were found in the TR tests at the cell level, and theeffects on the TR propagation at the module level were also studied.The formation mechanism of casing rupture was analyzed based onthe post-mortem casing appearance, electrode winding state, andrupture-edge cross-section microstructure. Simulative experimentswere carried out to support the proposed explanation of the TRpropagation mechanism. Finally, a feasible design was proposed toinhibit the casing rupture and was validated by performing extensiveTR experiments.

Experimental

Based on our experiments, almost all the commercial cylindricallithium-ion batteries have a certain possibility of casing rupture in alarge number of external heating TR tests, and this possibility isaffected by the design of the battery, such as the anode/cathodematerial composition, the vent threshold, and the electrode thick-ness. In this study, commercial cylindrical 21700 (21 mm diameterand 70 mm height) lithium-ion batteries with different casingthicknesses (0.22 and 0.30 mm) were selected for the experiments.These batteries were used to ensure consistency, because they aremass produced with the production of their components using a moldand the assembly process being automated. The specifications of thetested cell are presented in Table I. The capacity of all cells wasmeasured, with an error within 1.4%. Moreover, the operatingpressure of the current interrupt device (CID) and vent are measuredto ensure consistency. As the cell with 100% state of charge (SOC;constant-current constant-voltage charging to 4.2 V) has the highestenergy that can cause the most serious damage to the casing, all thecell TR experiments were carried out at 100% SOC.

The schematic diagram of TR triggering at the cell and modulelevels and the photograph of the heated cell are shown in Figs. 1aand 1b. The TR of the batteries was triggered by using a Cr20Ni80heating wire with a diameter of 0.25 mm. The heating wire has alength of exactly 567.4 mm, to ensure a resistance of 15 Ω. Further,

it is powered with a specific voltage of 39 V through an auto rangeDC power supply (IT6720, ITECH), which is equivalent to a heatingpower of 100 W. A high-temperature insulating tape (3 M 7413D)was stuck on the surface of the cell and over the heating wire toavoid the heating-wire short circuit with the casing as well as toensure the fixture of the heating wire and the thermocouple. Toensure consistency in the experiments, the method of winding andlocation of the heating wire were fixed, as shown in Fig. 1a. Thethermocouple was installed 2 mm from the heating wire, in order toexclude the influence of the heating wire. The TR of the heated cellwas triggered to analyze the probability of casing rupture in the formof a melting hole and a tearing crack. In order to guaranteeconsistency in the experiments, the setups and operations were

Table I. Specifications of cylindrical 21700 lithium-ion cell used forthermal runaway test.

Characteristic 21700 cell

Manufacturer Lishen BatteryType LR2170SFRated capacity [mAh] 4500 ± 60Casing thickness [mm] 0.22/0.30Positive electrode NCANegative electrode Graphite+SiOSeparator 12um PE + 4um Al2O3

Charging cut-off voltage [V] 4.2Discharging cut-off voltage [V] 2.5Mass [g] 70Mandrel YesNegative vent YesCID bursting pressure [MPa] 2.1 ± 0.15Negative vent bursting pressure [MPa] 3.1 ± 0.15Casing material Low-carbon steel

Figure 1. (a) Schematic diagram of thermal runaway (TR) test set-up (cell and battery module levels). (b) Photograph of triggering cell. (c) Schematic diagramof battery module structure. The inset shows the TR experiment location, and the #1 cell is the triggering cell. (d) Explosive view of battery module.

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strictly controlled to exclude bias. The effect of the casing rupture onthe battery module were also verified. The structure of the batterymodule (27P6S), the location of the triggering cell, and theconfiguration of the thermocouple are shown in Figs. 1c and 1d.The module is composed of an Al polar sheet (connected to the cellsand current collector), Al current collector, Al water pipe (thecoolant flows through the pipe to cool down cells), and ABS plate(fixing the cells) and cells, as shown in Fig. 1d, and the shortestdistance between cells is 2.5 mm. The #1 cell was heated till TR wastriggered, and the temperatures of heated cell and the adjacent sixcells were monitored and recorded during heating and TR. Thelocation of thermocouples on #2 to #7 cells is the same as that on #1cell. The #2 and #7 cells are in the same series, while #1, #3, #4, #5and #6 cells are in the other series. The temperatures of #1 to #7 cellsare denoted as T1 to T7, respectively, as shown in Fig. 1c. Anexternal liquid cooling system is connected to the module, with theflow rate of 0.5 L min−1 and inlet temperature of 25 °C.

Flat dog-boned tensile specimens of the casing sheet with 80 mmgauge length and 20 mm × 0.30 mm cross sections were cut andthen polished with sand paper to remove surface asperities. Tensile

tests from room temperature to 1073 K with a strain state of 10−4

before the yield stage and a strain state of 10−3 during the yield stagewere carried out on an INSTRON-3367 multifunctional materialtesting system with a high-temperature heating chamber. The yieldstrength and tensile strength were calculated over three repeatedtests. Simulative experiments of the generation of the casing meltinghole and tearing crack were carried out as follows. The melting holeexperiment was conducted by using a pulse current generator(current: 2000 A, duration: 5 ms, IT6532D, ITECH), as shown inFig. 2a, and Cu probes with a contact area of 0.2 mm2. The casingtearing crack experiment was carried out using the stretchingequipment shown in Fig. 2b at room temperature. The casingspecimens with 50 mm gauge length and 5 mm × 0.22 mm crosssection were cut, and tensile test was carried out at room tempera-ture. The melting hole and tearing crack samples were immersed intransparent resin materials, cut, and polished. Subsequently, thecross-section microstructural observations of samples were per-formed using optical microscopy (XZJ-2030B, Phenix).

Results and Discussion

To investigate the mechanism of casing rupture, cell-level TRtests were conducted (casing thickness D = 0.22 mm) 100 times.Besides the normal venting observed in 91 cells, a non-standardfailure mode was discovered in the test: nine cells exhibited casingrupture, with four cells showing a melting hole (Figs. 3b–3d) andfive cells showing a tearing crack (Figs. 3e and 3f). Notably, themelting whole could be in the upper, middle, and lower side of thecell, showing no relation to the location of heating wire. The presentresults indicate that the probability of casing rupture during cell-level TR is relatively low. The three different types of casing TR areshown in Figs. 3a–3d: TR with no casing rupture (Fig. 3a), TR witha melting hole (Figs. 3b–3d), and TR with a tearing crack (inFigs. 3e and 3f). The size of the tearing crack is much larger thanthat of the melting hole. Further, Fig. 3e presents the typicaltemperature curves corresponding to no-rupture, melting hole, andtearing crack samples during TR. The TR triggering temperature(∼150 °C) and time (∼120 s) for the three types of samples wereapproximately equal, indicating that there was no difference in thetest method. Furthermore, the maximum temperature of the tearingcrack sample (∼400 °C) is much less than those of the no-rupture(∼670 °C) and melting hole (∼630 °C) samples. This might bebecause a part of the electrode winding was ejected from the cellunder the influence of the tearing crack and internal pressure duringTR, leading a fraction of the energy to be dissipated into theenvironment. Further, the maximum temperature of the melting holesample is slightly lower than that of the no-rupture sample, and the

Figure 2. Schematics of (a) melting-hole and (b) tearing-crack simulativeexperiments.

Figure 3. Photographs of the (a) no-rupture sample, (b)–(d) melting hole in different locations, and (e) and (f) tearing crack sample after TR tests. (g)Temperature curves of no-rupture, melting hole, and tearing crack samples during TR.

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rate of decrease in the temperature for the melting hole sample isfaster than that of the no-rupture one. This is possibly because mostof the solid materials remain in the casing in both samples, and themelting hole provides an extra energy-release path for the hot gas.As the TR of the battery involves complex chemical and electro-chemical reactions, the electrode winding shows a nondirectivemovement under the effect of the inner pressure, leading touncertainty in terms of locations of the melting hole and tearingcrack.

To further verify the effect of casing rupture in the form of themelting hole and tearing crack on the TR propagation, 50 module-level TR experiments are conducted. The #1 cell in each module washeated until TR was triggered (Fig. 1c), and then, the liquid coolingsystem started operation. This is because a TR monitor exists in thebattery pack, which can work even if the vehicle engine is in the key-off state. Once the TR event is detected, the cooling system would

begin and continue operation. The modules that experienced TRpropagation had water poured on them at the initial propagationstage to stop further propagation and preserve the status. Based onthe present module design, no TR propagation was observed in themodules except for one module with a melting hole in the #1 cell andone module with a tearing crack in the #1 cell (casing thickness of0.22 mm (Table II)). The three types of module-level TR withcorresponding temperature data are shown in Fig. 4. As shown inFig. 4a, there is no casing rupture in the heated cell and no TRpropagation occurred. Moreover, the battery module is not damagedwith an almost intact ABS plate and Al current collector; however,the surface of the module is covered by the materials ejected fromthe heated cell. After TR was initiated in the #1 cell, the maximumtemperature of the adjacent six cells increased slowly at the rate ofabout 0.03 °C s−1 and were below 80 °C (Fig. 4b), which wasattributed to the fact that the liquid cooling system could take the

Figure 4. (a) Module without TR propagation and (b) corresponding temperature data, (c) module with TR propagation caused by the melting hole and (d)corresponding temperature data, and (e) module with TR propagation caused by the tearing crack and (f) corresponding temperature data. The insets in (a), (c),and (e) represents the corresponding heated cell. (T1–T7 represent the temperatures of the triggering cell and the adjacent six cells in the module, respectively).

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energy released from the #1 cell. The battery module with TRpropagation was damaged (Fig. 4c); the post-mortem analysisshowed that the melting hole was formed on the casing of the #1cell, and this hole directs to the #5 cell in the module. The #5 celltemperature sharply rose to 600 °C in 3 s after TR was triggered inthe #1 cell, indicating the TR occurrence in the #5 cell (Fig. 4d). Thetemperature of #4 and #6 cells then increased quickly to about160 °C at the rate of 1 °C s−1, and then, TR was triggered in them.The results of the post-mortem analysis and the temperature dataindicated that the melting hole in the #1 cell resulted in a flow of hotmolten materials ejected towards the #5 cell, causing TR to beinstantaneously triggered in this cell with the release of more hot andmolten materials. The liquid cooling system was not adequate to takeso much energy, so the temperatures of adjacent cells, especially the#4 and #6 cells, kept rising and exceeded the TR threshold. Asshown in Fig. 4e, the battery module displays more serious damage.The post-mortem analysis shows a tearing crack in the #1 celldirecting to the #2 cell. Owing to the large size of this tearing crack,most of the hot molten materials are instantly ejected to the adjacentcells, and TR is triggered in these cells instantaneously, as indicatedby the temperature data shown in Fig. 4f. Compared with Figs. 4band 4d, the smaller number of results in this data set is due to thebreakdown of thermocouples because of the dangerous TR propaga-tion. Out of the 50 module-level TR experiments, in 48 experiments,the TR-triggered cells vented normally, and there was no propaga-tion (Fig. 4a). On the other hand, in the other two experiments, amelting hole and a tearing crack were observed, and along withimmediate propagation (Figs. 4c and 4e); in particular, in the case ofthe tearing crack, rapid propagation and serious damage were

observed. These results show that, in order to prevent TR propaga-tion in the battery module, there should be no rupture in the casing,so it is significant to investigate the mechanism of casing rupture andpropose inhibition measures.

The internal electrode winding states in the cell-level test with amelting hole and an approximate crack phenomenon on the casingsurface are shown in Fig. 5. As shown in Fig. 5a, a change in theglossiness of the casing is observed near the melting hole, and someblack materials stick to the edge of the melting hole, which couldbe related to the combustion of the electrode winding. A large holeappears on the electrode winding located below the melting hole inthe disassembled cell presented in Fig. 5b. As the electrode windingsof batteries with cracked casing are seriously damaged, a quasi-crackbattery was chosen for post-mortem analysis, with a locally convextrace on the casing but without the crack, as shown in Fig. 5c. At thecorresponding position below the convex trace, there is a large crackin the electrode winding, extending along the axial direction, asshown in Fig. 5d. The deformations of both the casing and electrodewinding indicate that the internal pressure inside the cell during theTR could be very high.

The cross-section microstructure of the melting hole and tearingcrack are shown in Figs. 6b and 6c, respectively. The edge of themelting hole is relatively smooth and covered by a thin layer ofmetal. Further, the diameter of the melting hole generally decreasesfrom the inner to outer surface of the casing (Fig. 6b). In contrast, thetearing surface is rather rough without a metal layer on the edge(Fig. 6c). Notably, the necking phenomenon is observed, indicatingthat the tensile stress should be the major reason for the tearingcrack. Further, Fig. 6d exhibits the tensile and yield strength of thecasing material from room temperature to 1073 K. Both the tensileand yield strength monotonously decrease with increasing tempera-ture, and when the temperature reaches 1073 K, the tensile strengthdrops to 5% of that at room temperature.

According to the presented results, the formation of the meltinghole and tearing crack have two different mechanisms. The radialsection of the battery with the electrode winding and casing beforeTR is triggered is shown in Fig. 6a. Further, as shown in Fig. 6b,during the TR, large amounts of heat and gas are produced viavarious chemical and electro-chemical reactions inside the cell,leading to the increase in the internal stress and the possible ruptureof the electrode winding. Thereafter, when both the cathode andanode are short circuited through the casing, as shown in Fig. 5b, alarge current will flow through the loop composed of the casing andelectrodes due to the relatively low resistance of the involvedmaterials. As a result, based on the Joule law, heat energy will begenerated at the short circuit point, resulting in the local melting ofthe casing and formation of the melting hole. Since the melting startsfrom the inner surface of the casing, the diameter of the melting holegradually decreases from the inner to the outer surface of the casing,with a thin metal layer coated at the edge of the melting hole. Asshown in Fig. 6c, a high internal expansion pressure is generatedinside the cell during TR. Moreover, due to the non-uniformity ofthe internal reaction of the cell during TR, more heat and gas isgenerated in the area where the reaction is more volatile, possiblyleading to the crack in the electrode winding, and the temperature ofthe casing at the corresponding position is much higher, which

Table II. Experimental results of thermal runaway of battery with casing thicknesses of 0.22 and 0.30 mm.

0.22 mm 0.30 mm

Casing thickness Cell level Module level Cell level Module level

Test number 100 50 100 50No casing rupture 91 48 NTRP 100 50 NTRPMelting hole number 4 1 TRP 0 0 —

Tearing crack number 5 1 TRP 0 0 —

NTRP = no thermal runway propagation; TRP = thermal runaway propagation.

Figure 5. (a) Photograph of the cell with the melting hole, (b) electrodewinding state at the position of the melting hole after a part of the cell casingwas removed, (c) photograph of the cell with near-to-crack casing surface,and (d) electrode winding state at the position of the crack after a part of thecell casing was removed.

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results in a significant decrease in the tensile strength of the localcasing. The combination of the internal high pressure and decreasedtensile strength causes the formation of the tearing crack in thecasing. Therefore, the edge of the tearing crack is rough and thenecking phenomenon can be observed in the cross-section micro-structure.

In Figs. 7a and 7b, the cross-section microstructures of themelting hole and tearing crack produced in the simulative experi-ments, respectively, are shown. Notably, the surface of the meltinghole presented in Fig. 7a is smooth and covered by a thin metallayer. Further, the diameter of the melting hole monotonouslydecreases from the probe side to the opposite side of the casing.

Figure 6. (a) Radial section of the battery with the electrode winding and casing. (b) Formation mechanism of the melting hole formed by the short circuit ofelectrodes on the casing and the cross-section microstructures of the melting hole. The temperature in the red area is higher than that in the yellow area.(c) Formation of the tearing crack caused by a high pressure and decreased tensile strength of the casing at high temperatures and cross-section microstructure ofthe tearing crack. (d) Temperature dependence of tensile and yield strength of the casing material.

Figure 7. Cross-section microstructures of the (a) the melting hole and (b) tearing crack samples in simulative experiments.

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These observations are highly consistent with those shown inFig. 6b. From Fig. 7b, it can be seen that the edge of the tearingcrack with the necking phenomenon is serrated, which is similar tothat in Fig. 6c. The simulative experiments thus support theformation mechanisms of the melting hole and tearing crack.

Based on the present analysis results, the formations of themelting hole and tearing crack are principally attributed to theinhomogeneity of the cell internal reactions and destruction ofthe electrode winding integrity; therefore, casing rupture can bemitigated by (I) increasing the strength of the electrode winding byincreasing the strength of the Al and Cu current collectors to preventthe rupture of the electrode winding and (II) enhancing themechanical strength of the casing by increasing its thickness orusing a material with a high tensile strength. From the perspective ofpractical applications, in order to minimize the impact on otheraspects of the battery design, the improvement measure of increasingthe casing thickness is more practical and convenient to implement.The corresponding tests are carried out to verify the effectiveness ofthe improvement measure.

The results of the 100 cell-level TR tests with casing thicknessof 0.30 mm are shown in Fig. 8a. The improved cell is the same asthe original cell, except for the 1.7 g increase in mass, which isequivalent to an increase of 2.5%. The strength of the casingcorrespondingly increases by 36% owing to the increased thick-ness, which would inhibit the tearing crack.23 Moreover, theincreased thermal capacity of the casing requires more energy forthe hot spot to reach the melting point, and the energy acquired tomelt a hole from inside to outside would be larger due to theincreased distance. The energy density changed from 668.2 Wh l−1

to 658.1 Wh l−1 for the cell. For the battery pack, the massincreases by about 1.6%. None of the cells have melting holes ortearing cracks on the casing, implying that the increase in thecasing thickness is significantly effective in inhibiting casingrupture. The TR triggering temperature and maximum temperatureof cells with 0.22-mm-thick and 0.30-mm-thick casings are shownin Fig. 8b. No obvious difference is observed, indicating that thechange in the casing thickness has no effect on the TR mechanismand performance. Furthermore, in the 50 battery-module-level TRexperiments with a casing thickness of 0.30 mm, no TR propaga-tion occurred, and analysis after disassembly indicated that nocasing rupture occurred in the heated cell. The experimental resultsof battery TR with different casing thicknesses are listed inTable II. The results further confirm the effectiveness of theincrease in the casing thickness in inhibiting the casing ruptureand reducing the probability of TR propagation. The cells with thethicker casing were used in the commercial battery pack with about60 million cells, for use in EVs for more than 150 millionkilometers with no TR propagation, which indicates the effective-ness of the thicker casing in improving safety.

Conclusions

In summary, the rupture of the casing of a cylindrical 21700battery during TR and its effect on the TR propagation wasinvestigated, and a feasible inhibition method has been proposed.The casing rupture occurred in two forms, namely, a melting holeand a tearing crack, which inevitably caused TR propagation in thebattery module and pack. The formation mechanism of the casingrupture was investigated by triggering TR in commercial cylindrical21700 lithium-ion batteries. The edge of melting hole was found tobe relatively smooth and covered by a thin layer of metal, while thetearing crack surface was rather rough with the necking phenom-enon. It was concluded that the formation of the melting hole wasinduced by the large current flowing through the casing, and theoccurrence of the tearing crack was primarily attributed to thedecreased mechanical strength of the casing because of the highlocal temperature. The formation mechanisms of the melting holeand tearing crack were demonstrated by performing a large currentshort-circuit experiment and a casing tensile test, respectively. Basedon the formation mechanism, an increase in the casing thickness toimprove its mechanical strength was proposed to inhibit the casingrupture and improve the battery safety. The new design wasvalidated by performing a large number of TR tests, and imple-mented in a commercial 21700 battery. Further, 60 million improvedcells were used in EVs for more than 150 million kilometerswith no TR propagation. Thus, this work provides guidance forthe design and application of batteries with increased safety, whichcan facilitate the prevention of TR-propagation-induced fire andexplosion.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (grant nos. 11872355, 11872354, and11627803) and the Strategic Priority Research Program of theChinese Academy of Sciences (grant no. XDB22040502).

ORCID

Li Lao https://orcid.org/0000-0003-4184-0877

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Figure 8. (a) No casing rupture observed in 100 cell-level TR tests with casing thickness of 0.30 mm. (b) TR triggering temperature and maximum temperatureof cells with 0.22-mm-thick and 0.30-mm-thick casing during TR at the cell level: 0.22-TT and 0.30-TT represent the TR triggering temperatures correspondingto a casing thickness of 0.22 mm and 0.30 mm, respectively; 0.22-MT and 0.30-MT represent the maximum temperature during the TR for a casing thickness of0.22 mm and 0.30 mm, respectively.

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