This article investigates the current applications and research progress of carbon fiber composites in hydrogen storage and battery pack enclosures for new energy vehicles. It discusses the classification and development trends of high-pressure gas cylinders and battery pack enclosures, analyzes the advantages and disadvantages of carbon fiber composites, and anticipates the future applications and prospects of high-performance fiber composites in the field of new energy vehicles.

Overview of Carbon Fiber Composites

Using lightweight materials to reduce vehicle weight has become a crucial method to achieve the lightweighting of new energy vehicles. With the continuous development in materials science, various lightweight fiber composites, such as glass fiber-reinforced composites and carbon fiber-reinforced composites, have begun to be used in the field of new energy vehicles.

Carbon fiber composites, known for their low density, high strength, corrosion resistance, and fatigue resistance, are the most widely used high-performance fiber composites in the automotive sector. They are extensively used in various automotive systems, such as doors and roofs in the car body, pushrods and rockers in the engine system, drive shafts and clutch blades in the transmission system, and chassis components like underbody frames and suspension parts.

With the rapid development of new energy vehicles, the safe storage of their power energy has become a key research focus. High-pressure gas cylinders for hydrogen energy vehicles and battery pack enclosures for electric vehicles are the main energy storage methods currently. Carbon fiber composites, with their numerous advantages, are starting to gain prominence in this field.

Introduction to Carbon Fiber

Carbon fibers are generally used as reinforcement materials, combined with resin, metal, or ceramic matrices to form carbon fiber composites. Figure 1 shows examples of carbon fiber fabrics and carbon fiber composite profiles.

Carbon fibers possess the following advantages:

1. Low density and high strength: With a density of only 1.5~2.0 g/cm³, they are about half the density of lightweight aluminum alloys, but 4-5 times stronger than steel and 6-7 times stronger than aluminum.

2. High temperature and low-temperature resistance: Carbon fibers do not melt or soften in non-oxidizing atmospheres at 3000°C and do not become brittle at liquid nitrogen temperatures.

3. Good electrical conductivity: At 25°C, high modulus carbon fibers have a specific resistance of 775Ω·cm, while high strength carbon fibers have a specific resistance of 1500Ω·cm.

4. Acid corrosion resistance: Carbon fibers resist corrosion from concentrated hydrochloric acid, phosphoric acid, and sulfuric acid.

Based on precursor types, mechanical properties, and filament bundle sizes, carbon fibers can be categorized into several types, as shown in Table 1.

Carbon fibers are typically classified by their mechanical properties, mainly tensile strength and modulus. High-strength types have a strength of 2000 MPa and a modulus of 250 GPa, while high-modulus types exceed 300 GPa. Ultra-high strength types have a strength greater than 4000 MPa, and ultra-high modulus types have a modulus greater than 450 GPa.

Current Applications of Carbon Fiber Composites in the Automotive Field

With increasing demands for green energy and efficiency, the level of automotive lightweighting continues to rise. According to data from the European Aluminium Association, reducing a vehicle's weight by 10% can improve energy efficiency by 6% to 8% and reduce pollutant emissions by 10% per 100 kilometers. For new energy vehicles, reducing weight by 100 kg can increase their range by about 6% to 11%.

Lightweight and high-strength carbon fiber composites have a wide range of applications in automobiles. Table 2 lists some vehicle models that use carbon fiber composites, and Figure 2 shows the market size and forecast of the global automotive carbon fiber market, which is expected to reach 20,100 tons by 2025.

Applications of Carbon Fiber Composites in Hydrogen Storage

Due to their high strength, corrosion resistance, fatigue resistance, good flame retardancy, and dimensional stability, carbon fiber composites are ideal materials for hydrogen storage in new energy vehicles and lightweight battery pack enclosures.

High-Pressure Hydrogen Storage Tanks

High-pressure gas cylinders are the widely adopted method for hydrogen storage by domestic and international manufacturers. Depending on the materials, high-pressure hydrogen storage tanks are classified into Type I, II, III, and IV, made from pure steel, steel liners with fiber wrapping, metal liners with fiber wrapping, and plastic liners with fiber wrapping, respectively, as shown in Figure 3.

Table 3 compares the performance of different types of hydrogen storage tanks. High-pressure hydrogen storage can be divided into fixed high-pressure storage, lightweight vehicle-mounted high-pressure storage, and transport high-pressure storage. Fixed high-pressure storage tanks, typically steel hydrogen tanks and steel pressure vessels, are mainly used at hydrogen refueling stations, offering low cost and mature development.

Vehicle-mounted lightweight high-pressure storage tanks primarily use aluminum alloy or plastic liners with carbon fiber wrapping to enhance structural strength and reduce overall weight. Internationally, 70 MPa carbon fiber-wrapped Type IV tanks are widely used in hydrogen fuel cell vehicles, while domestically, 35 MPa carbon fiber-wrapped Type III tanks are more common, with fewer applications for 70 MPa carbon fiber-wrapped Type III tanks.

Carbon Fiber Composites in Vehicle-Mounted High-Pressure Hydrogen Storage Tanks

Types III and IV tanks are the mainstream for vehicle-mounted high-pressure hydrogen storage, consisting mainly of liners and fiber-wrapped layers. Figure 4 shows a cross-section of a carbon fiber composite Type IV high-pressure hydrogen storage tank. The fiber composites, wound helically and hoop-wise around the liner, primarily increase the liner's structural strength.

Currently, the common fibers used in vehicle-mounted high-pressure hydrogen storage tanks include carbon fibers, glass fibers, silicon carbide fibers, aluminum oxide fibers, aramid fibers, and poly(p-phenylene benzobisoxazole) fibers. Among these, carbon fibers are gradually becoming the mainstream fiber material due to their excellent properties.

Domestically, the development of high-pressure hydrogen storage tanks lags behind international advancements. The United States, Canada, and Japan have achieved mass production of 70 MPa hydrogen storage tanks and have begun using Type IV tanks. U.S. companies like General Motors enhance the structure of carbon fiber-wrapped layers, while Canada's Dynetek improves the winding and transition layers, enhancing the composite strength of carbon fibers with resin matrices. However, due to issues like plastic and metal sealing, Chinese regulations currently do not permit their widespread use.

Domestic institutions like Zhejiang University and Tongji University have successfully developed 70 MPa hydrogen storage tanks, and companies like Blue Sky Energy under Bohong Energy have broken through the 70 MPa vehicle hydrogen storage system. Additionally, companies like Shenyang Starling, Beijing Ketaike, and Beijing Tianhai have also developed and tested 70 MPa hydrogen storage tanks.

Due to the immature technology and difficulty in mass production of 70 MPa carbon fiber-wrapped Type IV tanks domestically, the high preparation costs greatly inhibit the demand and development of Type IV tanks. According to research by the U.S. Automotive Research Council, the larger the production scale of high-pressure hydrogen storage tanks, the lower the costs. When the production scale increases from 10,000 to 500,000 sets, costs can drop by one-fifth. Therefore, with the advancement of preparation technology and the expansion of production scale, high-level carbon fiber-wrapped vehicle-mounted high-pressure hydrogen storage tanks are bound to shine in the future.

Applications of Carbon Fiber Composites in Battery Pack Enclosures

Development of Battery Pack Enclosures

The stability and safety of new energy power batteries have always been focal points of concern. Battery pack enclosures are key components of the new energy vehicle battery system, closely related to the electrical system and vehicle safety. The power battery pack, covered by the enclosure, forms the main body of the battery pack.

The battery pack enclosure plays a crucial role in the safe operation and protection of battery modules, requiring materials with corrosion resistance, insulation, resistance to normal and low-temperature impacts (-25°C), and flame retardancy. Figure 5 shows a new energy vehicle power battery pack and its decomposition.

As the carrier of battery modules, the battery pack enclosure ensures the stable operation and safety protection of the battery modules, generally installed at the bottom of the vehicle to protect lithium batteries from damage due to external collisions and compressions. Traditional vehicle battery enclosures are cast from materials like steel plates and aluminum alloys, with surface coatings for protection. With the development of energy-saving and lightweight vehicles, battery enclosure materials have seen lightweight alternatives like glass fiber-reinforced composites, sheet molding compounds, and carbon fiber-reinforced composites.

Steel battery pack enclosures are the original materials used for power battery packs, typically made from welded steel plates, offering high strength and rigidity but also high density and mass, requiring additional corrosion protection processes. Aluminum alloy enclosures are the mainstream material for power battery packs, offering lightweight (only 35% of steel density), easy processing and forming, and corrosion resistance.

With the development of lightweight vehicles and the advancement of thermosetting plastic molding technologies, new plastics and composites are gradually being used as battery pack enclosure materials. Thermosetting plastic battery pack enclosures weigh 35 kg, about 35% lighter than metal enclosures, and can carry 340 kg of batteries.

Prospects of Carbon Fiber Composites in Battery Pack Enclosures

Carbon fiber composites, with their numerous advantages, have become ideal substitutes for traditional metal battery enclosures and have already seen preliminary applications in some vehicle models. For instance, NIO, in collaboration with Germany's SGL Carbon, developed an 84 kWh carbon fiber battery pack, reducing the shell weight by 40% compared to aluminum structures, with an energy density exceeding 180 (W·h)/kg. The Tianjin Institute of Advanced Technology and Lishen jointly developed a carbon fiber composite battery pack enclosure weighing approximately 24 kg, reducing the weight by 50% compared to aluminum alloy structures, with an energy density of up to 210 (W·h)/kg.

Researchers like Duan Duanxiang et al. have conducted lightweight designs and ply process optimizations for carbon fiber composite battery pack enclosures, reducing the enclosure weight by 66% compared to steel structures while meeting relevant working conditions. Zhao Xiaoyu et al. used carbon fiber composites and the stiffness equivalent design method for lightweight battery pack enclosures, achieving a weight reduction of 64% to 67.6% compared to steel structures.

LIU et al. addressed the lightweight design problem of carbon fiber composite battery pack upper covers using the RBDO method, achieving a 22.14% weight reduction while meeting performance requirements. Tan Lizhong et al. compared three solutions: a 1.5 mm thick aluminum upper cover (Scheme 1), a 1.5 mm thick carbon fiber upper cover (Scheme 2), and a 0.5 mm carbon fiber + 3 mm thick honeycomb panel + 0.5 mm thick carbon fiber composite upper cover (Scheme 3). They found that Scheme 3 was optimal, reducing the weight by 31% compared to Scheme 1.

Metal-liner fiber-wrapped tanks (Type III) and plastic-liner fiber-wrapped tanks (Type IV) are the mainstream fiber composite-wrapped gas cylinders. Fibers such as glass fiber, silicon carbide fiber, aluminum oxide fiber, boron fiber, carbon fiber, aramid fiber, and poly(p-phenylene benzobisoxazole) fiber have been used to manufacture fiber composite-wrapped gas cylinders. Lightweight, impact-resistant, and flame-retardant fiber composites are also expected to become important materials for future lightweight battery pack enclosures.

However, due to cost constraints, high-performance fiber composites dominated by carbon fiber composites have not been widely applied in battery pack enclosures. It is believed that with the development of new energy and the expansion of fiber composite applications, the cost of using fiber composites will gradually decrease. Fiber composites are set to shine in the future new energy market.