As the demand for increased driving range in electric vehicles (EVs) intensifies, automakers are relentlessly pursuing higher energy density battery cells and optimizing the space utilization within the EV battery pack to accommodate more modules. However, this push for greater capacity often leads to a significant increase in the overall weight of the EV battery pack, making lightweighting of structural components an urgent priority. The application of composite materials for the EV battery pack cover has emerged as a highly effective strategy in this endeavor.
Composite materials, prized for their high specific strength, high specific modulus, corrosion resistance, flame retardancy, thermal insulation, and design integration capabilities, are increasingly being adopted. Traditionally, thermoset composites such as Sheet Molding Compound (SMC), Prepreg Compression Molding (PCM), High-Pressure Resin Transfer Molding (HP-RTM), and Spray Transfer Molding (STM) have dominated this application. These materials, however, utilize thermosetting resins like unsaturated polyester, epoxy, or polyurethane, which present challenges related to non-recyclability and higher carbon emissions. In contrast, thermoplastic composites offer a compelling solution by addressing end-of-life recyclability and aligning with stringent carbon emission regulations. This article delves into the development and characterization of Continuous Glass Fiber Reinforced Polypropylene Thermoplastic Composites (CFRTP) for EV battery pack covers, comparing their properties against established thermoset systems and evaluating key performance criteria.
The core of CFRTP material begins with the manufacturing of unidirectional (UD) tapes. This is achieved through a melt impregnation process where continuous glass fiber rovings and polypropylene (PP) resin are combined. The fibers are fully impregnated with the molten polymer, resulting in a consolidated tape with aligned fibers. These UD tapes are then stacked in multiple layers with varying fiber orientations (e.g., 0°, 90°, ±45°) to create a laminate structure that provides balanced mechanical properties in multiple directions. Finally, this stacked layup is consolidated under heat and pressure, typically using a double-belt press process, to form a rigid, high-performance laminate sheet ready for shaping into an EV battery pack cover.
To establish a baseline, it is critical to understand the material composition and typical properties of thermoset composites currently used in EV battery pack covers across the industry. The following table summarizes key data for four prevalent systems.
| Composite Type | Resin Type | Fiber Type | Density (g/cm³) | Tensile Modulus (GPa) | Tensile Strength (MPa) | Flame Rating (UL-94) |
|---|---|---|---|---|---|---|
| SMC | Unsaturated Polyester | Chopped Glass (25 mm) | 1.8 | 9 | 90 | V0 @ 2.5mm |
| PCM | Epoxy | Glass Fiber Fabric | 1.95 | 25 | 450 | V0 @ 1.5mm |
| HP-RTM | Epoxy | Glass Fabric/NCF | 1.9 | 20 | 350 | V0 @ 1.5mm |
| STM | Polyurethane | Glass Fabric/Mat | 1.6 | 16 | 250 | V0 @ 1.5mm |
The data reveals distinct trade-offs. SMC offers lower mechanical properties, particularly a modulus of only 9 GPa, which would necessitate greater thickness for a given stiffness requirement in an EV battery pack cover. PCM provides the highest mechanical performance but at the highest density. HP-RTM strikes a middle ground, while STM offers the lowest density but also the lowest strength and modulus. Material selection for an EV battery pack cover thus involves a complex optimization between stiffness, strength, weight savings, and cost. The industry’s shift towards “carbon reduction” is steering attention towards lower-carbon footprint materials, and with advancing technology, thermoplastic composites like CFRTP are poised to become a key enabler for sustainable EV battery pack design.
Among various thermoplastic composites, CFRTP stands out due to its compelling cost-performance ratio, which is crucial in the cost-sensitive automotive sector. As battery cell technology evolves towards enhanced safety and diverse form factors, CFRTP presents a viable alternative for the EV battery pack cover. A fundamental requirement for any material in this application is flame retardancy. Pure polypropylene is highly flammable, necessitating the incorporation of flame retardant (FR) additives. We developed and characterized several CFRTP grades with different flame retardancy levels to understand this critical interplay. The rule of mixtures provides a foundational model for composite properties. For instance, the tensile modulus of a unidirectional composite can be estimated by:
$$ E_c = V_f E_f + (1-V_f) E_m $$
where $E_c$ is the composite modulus, $V_f$ is the fiber volume fraction, and $E_f$ and $E_m$ are the moduli of the fiber and matrix, respectively. However, the presence of additives like flame retardants can alter the effective properties of the matrix and the composite’s microstructure.
The performance of CFRTP with different FR systems is summarized below. The fiber content is a critical parameter, as it directly influences mechanical properties according to the rule of mixtures. The addition of FR additives often reduces the maximum achievable fiber loading due to processing viscosity and compatibility issues.
| UL94 Rating | Thickness (mm) | Density (g/cm³) | Glass Fiber Content (wt%) | Tensile Strength (MPa) | Tensile Modulus (GPa) | FR System Type |
|---|---|---|---|---|---|---|
| Non-FR | 1.5 | 1.60 | 65 | 320 | 20 | N/A |
| V2 | 1.5 | 1.62 | 50 | 270 | 17 | Halogenated |
| V1 | 1.5 | 1.65 | 50 | 260 | 15 | Halogenated |
| V0 | 1.5 | 1.76 | 50 | 260 | 16 | Halogenated |
| V0 | 1.5 | 1.62 | 50 | 250 | 15 | Halogen-Free |
Analysis of the data shows a clear trend: achieving higher flame retardancy levels (from non-FR to V0) necessitates a reduction in glass fiber content, from 65% to 50%. This reduction is the primary driver for the decrease in tensile strength (from 320 MPa to ~260 MPa, an 18.75% drop) and modulus (from 20 GPa to ~16 GPa, a 20% drop). The most significant property decline occurs between the non-FR and V2 grades. For grades with the same fiber content (V2 to V0), mechanical properties remain relatively stable, indicating that the FR additives themselves have a minor direct impact on tensile properties. The density changes are telling. While fiber content drops significantly from non-FR to V2, density remains nearly constant, implying the FR additives have a substantial mass contribution. Comparing the two V0 grades, the halogenated system has a much higher density (1.76 g/cm³) than the halogen-free system (1.62 g/cm³) at the same fiber content. This makes halogen-free FR systems more attractive for the EV battery pack cover from a lightweighting perspective, especially since they often come at a lower material cost. Therefore, for the EV battery pack cover application, the selection of an FR system significantly influences the weight and cost of the final component, with halogen-free systems offering clear advantages.
The operational environment of an EV battery pack demands long-term reliability under various conditions, including exposure to heat and humidity. Therefore, evaluating the hygrothermal aging resistance of CFRTP materials is essential. We subjected both halogenated and halogen-free V0-grade CFRTP laminates to accelerated aging at 85°C and 85% relative humidity for 1000 hours, following the principles of accelerated testing where degradation mechanisms are accelerated. The degradation of strength over time in such an environment can sometimes be modeled empirically. After aging, the tensile strength retention was calculated. The strength retention ratio $R$ is given by:
$$ R = \frac{\sigma_{aged}}{\sigma_{initial}} \times 100\% $$
where $\sigma_{initial}$ and $\sigma_{aged}$ are the tensile strengths before and after aging, respectively.
| UL94 V0 Grade | Initial Tensile Strength (MPa) | Tensile Strength After 1000h Aging (MPa) | Strength Retention Ratio, R (%) |
|---|---|---|---|
| Halogenated | 260 | 195 | 75.0 |
| Halogen-Free | 250 | 137.5 | 55.0 |
The results indicate a notable difference between the two FR systems. The halogenated CFRTP retained 75% of its original strength, whereas the halogen-free version retained only 55%. This suggests that the chemical composition and interaction of the halogen-free FR additives with the polypropylene matrix and glass fibers may make the composite more susceptible to hydrolysis or other degradation mechanisms under hot and humid conditions. For an EV battery pack cover designed to last the vehicle’s lifetime, this property is critical and indicates a need for further formulation development for halogen-free systems to improve their long-term durability.
Material-level flammability ratings like UL94 do not fully capture the performance under the extreme thermal conditions of a battery thermal runaway event within an EV battery pack. Therefore, we conducted direct flame impingement tests on CFRTP plates to simulate such a scenario. The test conditions were severe: a flame temperature of 1200±100°C, a nozzle distance of 20 cm, with oxygen at 0.5 MPa and propane at 0.07 MPa pressure. The time to burn-through was recorded, and the post-test specimens were examined. The heat flux $q”$ in such a test can be conceptually related to the temperature rise and material ablation, though a full model is complex. The time to failure $t_f$ under a constant heat flux can be inversely related to the material’s thermal ablation resistance and effective heat of gasification.
| UL94 V0 Grade | Time to Burn-Through (minutes) | Observations |
|---|---|---|
| Halogenated | 3 | Large, irregular hole; significant charring and delamination. |
| Halogen-Free | 5 | Small, localized hole; more intact structure; better char cohesion. |
Despite both materials achieving a V0 rating, their performance under intense, direct flame differs markedly. The halogen-free CFRTP resisted burn-through for two minutes longer and exhibited a much smaller, neater hole with less structural degradation compared to the halogenated version. This superior fire barrier performance is crucial for an EV battery pack cover, as it could potentially delay the propagation of a thermal event. It is important to contextualize this test: 1200°C is representative of the jet fire from a high-nickel NMC cell thermal runaway. For lithium iron phosphate (LFP) cells, the estimated ejection temperature is lower, around 600–650°C. Consequently, CFRTP materials appear more suitable for EV battery pack covers housing LFP cells based on this high-temperature test. Further characterization at around 600°C is warranted to confirm suitability for LFP-based EV battery packs. For high-nickel ternary chemistries, the relatively short burn-through time at 1200°C indicates that further enhancement of the fire resistance, perhaps through intumescent coatings or different material architectures, would be necessary for the EV battery pack cover.
In developing materials for the EV battery pack cover, we must also consider manufacturing and design aspects. The melt processability of CFRTP enables faster cycle times compared to thermoset composites, which require cure cycles. This can improve production throughput for EV battery pack assemblies. Furthermore, the potential for welding and integrated design with other thermoplastic components in the EV battery pack system offers additional avenues for weight reduction and cost-effective assembly.
To summarize the key findings from our development work on CFRTP for the EV battery pack cover: The incorporation of flame retardants primarily affects CFRTP by limiting the maximum achievable glass fiber content, which in turn dictates the mechanical performance ceiling. The FR additives themselves have a marginal direct effect on tensile properties. Both halogenated and halogen-free V0-grade CFRTP materials can meet the basic mechanical and flammability requirements for an EV battery pack cover. From a density and cost standpoint, halogen-free FR systems are preferable, offering better lightweighting potential for the EV battery pack. However, the hygrothermal aging performance of current halogen-free CFRTP formulations needs improvement to ensure long-term reliability of the EV battery pack cover over the vehicle’s lifespan. In simulated fire tests relevant to thermal runaway, halogen-free CFRTP demonstrated superior resistance to burn-through compared to its halogenated counterpart, making it a more promising candidate for enhancing the safety of the EV battery pack, particularly with LFP cell chemistries. The evolution of the EV battery pack towards higher integration and sustainability will continue to drive material innovation, and thermoplastic composites like CFRTP are well-positioned to contribute significantly to the next generation of lightweight, recyclable, and high-performance EV battery pack covers.