Research On Thermal Runaway Resistance Of Composite Material Box Cover Of Power Battery

 

 

From the perspective of environmental protection and energy development, new energy pure electric vehicles have extremely broad application prospects and are the key development goal of my country’s transportation. For new energy electric vehicles, the power battery pack is the core component, which directly affects the vehicle’s cruising range, speed and acceleration performance.

 

Therefore, the higher the battery energy demand means the more battery usage, which leads to a higher battery box mass, which accounts for about 25% to 30% of the vehicle mass. The overweight battery box assembly also causes it to consume a lot of battery efficiency. Therefore, the lightweighting of the power battery box assembly itself is the core issue that needs to be faced in the development of new energy electric vehicles.

 

The lightweighting of the power battery box is an inevitable development direction, but it is also restricted by materials. Improving the energy density of the battery and using a high specific energy electrode material system are important ways to reduce weight.

 

The selection of battery positive electrode materials is crucial to energy density. Low cobalt and high nickel ternary materials are the current main direction, with characteristics such as high energy density and low temperature stability.

 

However, the higher the energy density of the material, the greater the energy when the thermal runaway is triggered, so the safety risk also increases accordingly. At present, the technology of using steel materials for the box cover is relatively mature. Through structural improvement and optimization, the wall thickness can be reduced to reduce weight, but the weight reduction effect of only 20% has not yet achieved the expected effect.

 

In addition, although there are many cases of using aluminum alloy box covers, the melting point of aluminum alloy is only about 660℃, which cannot meet the requirement of the maximum temperature exceeding 700℃ when the ternary lithium battery is in thermal runaway.

 

In short, the development of power battery box covers needs to take into account both the lightweight of the material and the safety during thermal runaway. As a lightweight material with high specific strength, high specific modulus, corrosion resistance and integrated design, continuous fiber composite materials have certain advantages in the application of battery box covers. In order to meet the requirements of thermal runaway of power battery packs, it is of great significance to study the corresponding thermal runaway resistance performance.

 

The resin selected in this study is a thermosetting resin, which has reached the UL94V-0 test method 1.5mm thickness flame retardancy. This study selected high-performance continuous fibers, compared different types of fiber fabrics, and selected fireproof felt composed of disordered fiber felt, expanded graphite and a small amount of binder. Its excellent fireproof performance has been maturely applied in construction.

 

As a glass fiber felt, it can be injected with resin and has good bonding with composite materials. Therefore, it can be used in composite box cover structures to improve fire resistance and thermal runaway performance. This study uses a relatively thin glass fiber fireproof felt with good paving properties and a thickness of about 0.3mm.

 

1 Thermal Runaway Requirements for Power Battery Packs

 

The State Administration for Market Regulation and the National Standardization Administration issued the mandatory national standard “Safety Requirements for Power Batteries for Electric Vehicles” (GB38031-2020) on May 12, 2020 to replace the original recommended national standard GB/T31467.3-2015. The new national standard adds the “thermal diffusion” item in the “thermal stability” section, requiring the battery pack or system to perform thermal diffusion occupant protection analysis and verification in accordance with Appendix C.

 

It is stipulated that a thermal event alarm signal should be provided by the battery pack or system 5 minutes before the thermal runaway of a single battery causes heat diffusion and thus causes danger to the passenger compartment [9].

 

Appendix C stipulates that manufacturers can choose one of the two methods of triggering thermal runaway: needle puncture or heating, or they can choose other methods to trigger thermal runaway.

 

Thermal Runaway Trigger Judgment Conditions:

 

1) The trigger object produces a voltage drop, and the drop value exceeds 25% of the initial voltage;

 

2) The temperature of the monitoring point reaches the maximum operating temperature specified by the manufacturer;

 

3) The temperature rise rate dT/dt of the monitoring point ≥ 1℃/s, and lasts for more than 3s. When 1) and 3) or 2) and 3) occur, thermal runaway is determined to have occurred.

 

If the recommended method is used as the thermal runaway trigger method and thermal runaway does not occur, in order to ensure that thermal diffusion will not cause danger to vehicle occupants, it is necessary to prove that thermal runaway will not occur when the above two recommended methods are used.

 

2 Fire Test Setup

 

For the study of thermal runaway resistance of composite materials used in battery box covers, this paper simplifies the test to simulate the thermal runaway test requirements of power battery packs in order to save costs and facilitate research. According to relevant experience, after the thermal runaway of the battery cell is triggered, combustible materials with both high temperature and pressure will be ejected [9]. Static fire (simulated temperature) and impact fire test (simulated temperature and pressure) are set up to provide important reference significance for the selection of box cover materials to meet the thermal runaway test of the whole pack.

 

2.1 Static Fire

Static fire only involves temperature, not pressure (or the pressure is very weak) or ejected materials, so it is not used as a method to simulate the thermal runaway performance of materials. However, the test tools are relatively easy to obtain and the test stability is relatively high, which can be used for the screening of early materials and verifying the fire resistance of the material itself.

 

1) Static Fire Setup

As shown in Figures 1 and 2, static burning uses a special horizontal iron burning stand to place the flame at the center of the test piece for burning; propane fuel is used, and the test setting temperature is 1450°C; the position of the flame blowtorch and the test piece stand are fixed, and the flame distance is maintained at 19mm; two types of thermocouples are used to test the initial flame temperature and monitor the back-fire surface temperature; if it does not burn through, the test is stopped after 5 minutes of timing.

 

 

2) Static Fire Test Objects.

Table 1 lists the test conditions, namely four combinations of different resins, fibers and fireproof felts, with the same flame distance.

 

 

2.2 Impact Burning

 

Since it is difficult to accurately and stably obtain the failure conditions of the box cover when the whole package is in real thermal runaway, including the high-pressure gas pressure, solid (crystal dendrite) and liquid ejection pressure, impact amount, frequency and temperature of the specific failure position.

 

Therefore, this study simplifies the above conditions and adopts the impact burning method that includes both temperature and gas impact to simulate the high temperature and high-pressure gas that the box cover material is subjected to during thermal runaway. The pressure is continuous pressure, but does not contain solid and liquid ejection.

 

Since the pressure and temperature are relatively high, the stability of the test is poorer than static burning due to the limitations of the test setting and site, so this study only conducts horizontal comparative analysis under a single variable.

 

1) Impact burning setting. Impact burning uses a special high-foot burning horizontal frame to fix the test piece at the top and burn it in the center; using acetylene fuel + oxygen mixed cutting torch, the maximum temperature can reach more than 3000℃; due to the extremely high impact force, the flame temperature cannot be stably measured, so the comparative test is carried out by adjusting and locking the flame distance.

 

2) Test objects with impact fire. Table 2 lists the test conditions, namely, 17 combinations of epoxy resin, different fibers and fireproof layer, fireproof felt, mica board, and different flame distances.

 

 

3 Test Results and Analysis

 

3.1 Typical Static Fire Test

During the static fire process of carbon fiber composites with fireproof felt, the fireproof felt on the fire-exposed surface was activated to form a typical worm-like graphite expansion; the resin on the back-fired surface was decomposed by heat and produced a large amount of white smoke in the early stage of the fire, and then a relatively stable carbonized layer was formed, and the white smoke gradually decreased and finally disappeared.

 

3.2 Static Fire Test Results

 

Table 3 shows the results of the static fire test. After testing, all samples did not burn through under 1450℃, 5min static fire, and the back-fired surface was far from being burned through except for carbonization and partial resin decomposition. The morphology of the A-1 sample after the test is shown in Figure 3.

 

 

 

3.3 Analysis Of Static Fire Test

The flame retardant resin systems selected in this study are epoxy or polyurethane, both of which have excellent fire resistance and are sufficient to withstand 5 minutes of static fire. Due to the short time of the static fire test, it is impossible to compare the difference between using carbon fiber and glass fiber. The results of A-4 without fireproof felt and other samples with fireproof felt did not burn through, indicating that the use of flame-retardant resin has provided certain fire resistance for the box cover.

 

3.4 Typical Impact Fire Test Situation

During the impact fire test, the continuous fiber composites showed obvious stratification at the beginning, which was consistent with static fire. Then a large amount of white smoke appeared on the back of the fire, mainly due to the decomposition of the resin, not a precursor to burn through. The position of the sample impacted by the fire turned red, which was a precursor to burn through. The red area expanded rapidly and finally burned through.

 

3.5 Results Of Impact Fire Test

1) Flame distance comparison. Since this test method cannot monitor the temperature and pressure of the sample, the temperature and pressure are adjusted by setting different flame distances. From the test results, it can be seen that for the same material, the farther the torch is, the later the back fire will start. Comparing samples B-2 and B-5, it is found that the smaller the flame distance, the shorter the burn-through time, the more concentrated the damaged area, and the smaller the area involved in the ablation, as shown in Figure 4, and both B-2 and B-5 show this rule.

 

 

2) Comparison Of Specimen Thickness. The performance of the test is closely related to the thickness of the specimen itself. Comparison of specimens of the same material and different thicknesses shows that the thicker the specimen, the longer the burn-through time. Comparison of specimens B-11 and B-13 shows that under the same medium flame distance, although the ablation time is different, the appearance after ablation is similar, the resin is basically burned off, and the fiber fabric is exposed, as shown in Figure 5.

 

 

3) Comparison Of Different Fibers.

Comparing the composite material samples, there are certain differences in the burn-through time and damage form of different fibers, and their laws also show a certain relationship with the flame distance. As shown in Table 4, the comparison of samples B-2, B-7, and B-8, subjected to close-range impact fire, yielded the following conclusions: At a close distance of nearly 2 cm, the carbon fiber composite showed better ablation resistance; the appearance of the carbon fiber composite after ablation was quite different from that of the other two fiber composites, the resin at the damaged part had decomposed, exposing the broken carbon fiber tows, but no obvious perforation was observed; while the appearance of the glass fiber composite and basalt fiber composite after ablation was basically similar, with obvious perforations at the damaged part.

 

 

The ablation appearance of the carbon fiber composite sample B-9 at this flame distance is different from that of B-2. The fibers are obviously decomposed, as shown in Figure 6, and the closer the fiber layer is to the fire surface, the larger the decomposition area; although some fibers of the glass fiber composite sample B-13 are melted and broken at this flame distance, the basic structure of the fabric is still maintained, and it is not in the perforated state shown by B-7.

 

 

3.6 Analysis Of Impact Fire Test

 

1) Flame Distance Factor.

The performance difference caused by the flame distance factor is the largest. For the flame retardant resin system composites used in this study, under close-range impact fire, the sample was punched through by high pressure before forming a stable carbonized layer. Therefore, the flame retardant and fireproof properties of the material were not demonstrated; and when the flame distance became larger, the flame retardant and heat insulation properties of the material came into play. Whether it was the carbon layer produced by the aromatic ring and volatiles on the epoxy resin, or the dehydration brought by the phosphoric acid substances generated by the phosphorus-based flame retardant, which accelerated the dehydration and carbonization of the polymer surface, the composite had enough opportunities to form a favorable carbonized layer to protect the overall structure.

 

2) Fiber Type Factor.

As shown in Table 4, carbon fiber performs better at a small flame distance, while glass fiber performs better at a medium and long flame distance. The appearance of carbon fiber composites after ablation is different from that of other fiber composites because the destruction mechanism of their fibers is different.

 

At temperatures above 500°C, carbon fiber undergoes oxidation and decomposition, which manifests itself as gradual fiber breakage during ablation. Glass fiber and basalt fiber are mainly composed of SiO2, which manifests itself as melting at high temperatures.

 

When the flame distance is small, the impact has a greater impact; when the flame distance is large, the temperature is the main factor. Due to the long test time, the temperature of carbon fiber thermal decomposition is lower, while the temperature resistance of continuous high-performance glass fiber is above 900°C, and the molten glass solution further blocks the damage of the next layer of glass fiber fabric.

 

3) Material Thickness Factor.

Under the test conditions of this study, when the flame distance and material type are the same, the burn-through time of the sample increases with the increase of the sample thickness. Although in theory, the thicker the product, the better the thermal runaway resistance, the overall weight reduction effect and layout space of the product need to be considered at the same time.

 

4 Conclusion

Aiming at the thermal runaway requirements of power battery packs, this study uses simplified tests, namely static fire burning and fire burning with impact, to compare different composite material samples. There are significant differences in the burn-through time and ablation appearance presented by these two fire test methods. Since impact fire is closer to actual thermal runaway than static fire, the evaluation of thermal runaway resistance of composite materials should refer more to the test results of impact fire test.

 

This study only analyzes the influence of each single factor. The medium and long flame distance relatively better combines the comprehensive influence of temperature and impact pressure, and its results are more meaningful for reference; regardless of the material, the greater the thickness, the better the thermal runaway resistance, but it is not conducive to lightweight; in terms of fiber material, continuous high-performance glass fiber should be the best choice for box cover material selection in terms of cost and thermal runaway resistance performance; the use of fireproof felt can further improve the thermal runaway resistance.

 

In short, the selection of composite battery box cover materials should be based on a comprehensive trade-off between lightweight and thermal runaway resistance. In subsequent research, it can be considered to introduce surface materials that can be integrated with the box cover on the inner surface of the box cover. While taking into account impact resistance, the use of fireproof felt can further improve the thermal runaway resistance of the box cover.