Publish Time: 2025-01-15 Origin: Site
Since the beginning of the 21st century, the maturation of lithium-ion battery technology has fueled the rapid development of electric vehicles (EVs). In recent years, EV penetration has accelerated, creating a disruptive trend against traditional internal combustion engine vehicles. However, challenges such as range anxiety, reduced performance in winter, and battery safety still hinder broader market acceptance of EVs. Addressing these issues requires further innovation in power battery technology, which is closely tied to the development and application of new materials. These materials include not only electrode materials within battery cells but also structural materials at the system integration level, such as battery housing materials.
Power battery housings, including system enclosures and covers, are commonly made of metal materials such as steel and aluminum. These materials offer high strength and established manufacturing processes, meeting the mechanical performance requirements of battery housings. However, as demands for energy density, thermal insulation, and other attributes increase, lightweight composite materials have started replacing or partially replacing metals. This has become a significant technological trend in battery housing development, gaining increasing attention and exploratory applications. In particular, composite covers have achieved mass production in market-ready vehicle models, with their usage and application scope continuously expanding and set to play an even more critical role in the future.
In the automotive industry, fiber-reinforced polymer/plastic (FRP) composites have been widely used. Their most common applications involve replacing traditional metal materials to achieve weight reduction in components such as vehicle bodies, interior and exterior trim, and underbody panels. Depending on the resin matrix's processing characteristics, FRPs are classified into thermosetting and thermoplastic composites, both of which have been extensively adopted in the automotive field.
Thermosetting Composites
Common thermosetting resins include epoxy resin, characterized by one-time heat curing, high strength, excellent heat resistance, superior electrical properties, corrosion resistance, aging resistance, and dimensional stability.
Thermoplastic Composites
Common thermoplastic resins include polypropylene (PP), nylon/polyamide (PA), polycarbonate (PC), and polyethylene (PE). These materials soften when heated and harden upon cooling, allowing for repeated processing. They offer impact resistance, ease of processing, and recyclability.
Common reinforcing fibers used in automotive FRPs include carbon fiber and glass fiber. While carbon fiber has superior strength, its complex manufacturing processes and high costs limit its large-scale application in EVs. Glass fiber is less strong but more cost-effective. However, recycling and reusing both carbon and glass fiber composites remain challenging, potentially posing environmental concerns.
Reinforcing fibers are categorized based on retained fiber dimensions in the composite product: short fibers, long fibers, and continuous fibers. Continuous fiber-reinforced composites exhibit the best strength, stiffness, and impact resistance, presenting significant potential for lightweight automotive applications.
Resin-based composite materials can be shaped through processes such as compression molding, resin transfer molding (RTM), filament winding, and pultrusion. For large panel structures like battery covers, the primary methods are compression molding and RTM.
Compression Molding: A defined amount of molding material is placed in a metal mold, then heated and pressed to cure into shape. Subcategories include:
Discontinuous Fiber Thermosetting Composites: SMC (Sheet Molding Compound), BMC (Bulk Molding Compound), TMC (Thick Molding Compound).
Discontinuous Fiber Thermoplastic Composites: GMT (Glass Mat Thermoplastics), LFT-D (Direct Long Fiber Thermoplastic), LFT-G (Long Fiber Thermoplastic Granule Injection).
Continuous Fiber Composites: PCM (Prepreg Compression Molding), WCM (Wet Compression Molding).
Resin Transfer Molding (RTM): This process involves injecting resin into a closed mold to impregnate reinforcement materials and cure the product. Traditional RTM has limitations, such as low resin impregnation rates causing porosity, resin flow disrupting fiber alignment, and uneven resin distribution in large products. These issues have led to improved processes such as High-Pressure RTM (HP-RTM) and Vacuum-Assisted Resin Transfer Molding (VARTM). HP-RTM, for instance, enhances resin injection pressure, creating products with low porosity and high fiber volume fractions.
Common materials for power battery covers include steel, aluminum alloys, and composites:
Steel: Steel covers offer high strength and low cost. High-strength steels (e.g., HC340, DP590) enable thicknesses of 0.8mm or 0.7mm for lightweighting. Surface treatments such as electrophoresis improve corrosion resistance, while fireproof coatings enhance thermal protection.
Aluminum Alloys: Aluminum offers higher specific strength than steel, enabling further weight reduction. Typically, 5-series aluminum alloys are used, with thicknesses as low as 1.2mm or 1.5mm. While aluminum forms a natural oxide layer for corrosion resistance, treatments like electrophoresis, spray coating, or applying protective layers improve insulation and thermal protection.
Composites: Early applications of composites in battery covers involved SMC processes using discontinuous glass fibers, such as in the battery covers of BAIC EU5 vehicles. However, SMC materials' low strength (tensile strength < 100MPa) required thicknesses of 2mm or more, limiting lightweighting benefits. Recent advances in continuous fiber molding processes (e.g., PCM and HP-RTM) have extended carbon fiber composite techniques to more cost-effective glass fiber composites.
Continuous glass fiber-reinforced composites now achieve higher strength (tensile strength > 400MPa) than aluminum alloys, with lower density (~1.9g/cm³). Thicknesses can be reduced to 1.2mm or thinner, enabling significant lightweighting. Furthermore, the material's inherent fire resistance and insulation properties enhance safety compared to aluminum. However, costs remain higher than steel or aluminum.
Mass production of continuous glass fiber-reinforced composite covers primarily uses PCM and HP-RTM processes.
PCM: Lower upfront investment, manual prepreg layering, slower production, ideal for small batches or prototypes.
HP-RTM: Higher equipment and mold costs, dry fiber fabric materials, vacuum high-pressure resin injection, faster production rates, and superior surface quality.