​A Comparative Showdown of Composite Materials for Battery Cover Lids: Who Will Set The New Standard for New Energy Vehicle Safety Performance?

A Comparative Showdown of Composite Materials for Battery Cover Lids: Who Will Set the New Standard for New Energy Vehicle Safety Performance?

New energy electric vehicles have an urgent need to improve driving range. In addition to continuously developing higher energy density battery cells, the lightweighting of battery pack structural components is particularly important. The application of composite materials for battery cover lids has become an effective method.

I. Material Composition and Performance Comparison of Different Composite Materials for Battery Covers
Research on composite materials used by domestic and international OEMs for battery cover lids identifies four commonly used types: SMC, PCM, HP-RTM, and STM. Their performance data is shown in Table 1.

From Table 1, it can be seen that the mechanical properties of SMC material are lower than the other three materials, with a Young's modulus of only 9 GPa. Therefore, considering product stiffness, the design thickness of the product will be higher than the other three materials. Among PCM, HP-RTM, and STM materials, PCM material has the highest mechanical properties but also the highest density. STM material has the lowest density and mechanical properties, while HP-RTM material falls between the two. Currently, thermoset composites dominate the market. However, with increasingly stringent "carbon emission reduction" requirements and the maturing technology of the thermoplastic composite industry, thermoplastic composites are becoming a key new material for the automotive industry.

II. Development of Thermoplastic Composites
There are various types of thermoplastic matrix composites on the market, but due to the cost sensitivity of the automotive industry, continuous glass fiber reinforced polypropylene composites (CFRTP) have garnered significant attention due to their cost competitiveness. Combined with the trend of battery cell technology moving towards higher safety performance and the diversification of cell types and layouts, CFRTP has the opportunity to become a key material choice for battery cover lids.
Continuous glass fiber (GF) reinforced polypropylene (PP) composites are first prepared into unidirectional tape (UD) by combining continuous glass fiber yarn and polypropylene resin through melt impregnation (Figure 1). Then, the tape layers are stacked at different angles (Figure 2) and formed into laminates via a double-belt press method (Figure 3).

Since polypropylene resin (PP) itself contains alkanes, making it highly flammable, and battery covers require a certain flame retardancy rating, flame retardants need to be added to the polypropylene composite to improve its flame resistance. Four CFRTP materials with different flame retardancy ratings were developed, and the results are shown in Table 2.

From Table 2, it can be seen that as the flame retardancy rating increases, the tensile strength and tensile modulus of the material decrease. The most significant drop in tensile strength and modulus occurs from non-flame retardant to UL94 V2 rating. subsequently, from UL94 V2 to V0, the tensile strength and modulus only slightly decrease. The impact of flame retardants on tensile performance is primarily due to a reduction in glass fiber filling content, leading to decreased tensile performance. Different types of flame retardants show little difference in tensile performance, but halogen-free flame retardants have a smaller impact on density.

III. Impact of Different Types of Flame Retardants on Material Performance After Damp Heat Aging
Battery cover lids need to withstand damp and hot environments throughout their product life cycle. Damp heat aging tests were conducted on materials with UL94 V0 rating using two different types of flame retardant additives. The test conditions were a temperature of 85°C, relative humidity of 85%, and an aging cycle of 1000 hours, referencing standard GB/T 2423.50-2012 "Environmental Testing - Part 2: Test Methods - Test Cy: Damp Heat, Steady State, Primarily for Equipment Accelerated Testing." Tensile performance tests were then conducted, referencing standard ISO 527-4 "Determination of Tensile Properties of Anisotropic Fiber-Reinforced Plastic Composites." The results are shown in Table 3.

From the above results, it can be seen that the strength retention rate of halogen-free flame retardant V0 material after damp heat aging is lower than that of halogenated flame retardant V0 material. It can be concluded that damp heat aging has a greater impact on the tensile properties of halogen-free CFRTP materials.

IV. Impact of Different Types of Flame Retardants on the Fire Resistance of CFRTP Materials
Fire tests were conducted on CFRTP sheets to evaluate their fire resistance. Two types of V0-rated CFRTP materials with different flame retardants were subjected to fire testing under the following conditions: 1200±100°C, nozzle distance from sheet 20 cm, fire pressure oxygen 0.5 MPa, propane 0.07 MPa. The specific fire process is shown in Figure 4, the results of the sheets after the fire test are shown in Figure 5, and the fire process is detailed in Table 4.

From the fire test results, although both materials achieved a V0 flame retardancy rating, there were differences in the actual simulated fire performance of the sheets. The halogen-free flame retardant V0 material performed significantly better than the halogenated flame retardant V0 material. Firstly, its burn-through time was 2 minutes longer. Secondly, as shown in Figure 5, the ablation condition of the halogen-free flame retardant material was significantly better than the halogenated material, and the burn-through hole was very small. The current test conditions are quite severe, with both the fire temperature and pressure being higher than those commonly used in market fire tests. However, based on the thermal runaway temperatures of different battery cells, the above test is relatively close to the performance of high-nickel NMC lithium-ion cells. For LFP cells, the jet temperature is estimated to be only 600~650°C, significantly lower than the conditions of this test. Therefore, based on the fire results, CFRTP materials are relatively more suitable for LFP cells. Of course, further fire testing of CFRTP materials at 600°C is needed to confirm whether they meet the performance requirements. For NMC cells, based on the current test results, the burn-through time is short, and further improvement in fire performance is needed. Overall, based on the fire results, the performance of halogen-free flame retardant CFRTP materials is superior to halogenated systems. Subsequent characterization of the fire performance of halogen-free CFRTP materials at 600°C is needed to confirm their suitability for LFP cell thermal runaway.

In summary, addressing the comprehensive requirements for flame retardancy, mechanical properties, and environmental durability of battery cover lid composite materials, Yinsu's PAADP-20 halogen-free flame retardant masterbatch and T3MB bromine-antimony masterbatch offer differentiated solutions. PAADP-20, based on an environmentally friendly halogen-free system, significantly enhances fire and ablation resistance while maintaining high mechanical properties, making it particularly suitable for battery systems with stringent thermal runaway requirements. The T3MB bromine-antimony masterbatch, with its highly efficient flame retardancy and excellent resistance to damp heat aging, ensures flame retardancy ratings while maintaining long-term material reliability, providing flexible and mature material choices for different battery types and OEM needs.