Demand for lightweight energy storage technology is driving the development of carbon fiber composites for automotive batteries and other electronics.

　　Driven by the explosion of mobile and portable electronics and the proliferation of drones and electric vehicles (EVs), the research race is on to develop new lightweight materials for energy storage technologies, especially ones with longer lifespan and higher High weight and volume efficient materials. Carbon and composite materials have been an integral part of energy storage systems for decades, with a notable example being graphitic carbon, which contains the anode of lithium-ion batteries. The anode is usually made of a carbon fiber composite made of metal or metal oxide, plus a polymer coating, a barrier layer and some type of cathode to create an electrical potential that allows electrons to flow through the circuit. Carbon fiber/polymer matrix composites filled with conductive materials are also used as electromagnetic interference (EMI) shielding materials and are widely used in special aerospace, automotive, consumer electronics and other fields.

　　An emerging research approach to composite energy storage is to minimize the mass of batteries, fuel cells, and capacitors through state-of-the-art materials, with the ultimate goal of increasing overall power density. “One of the problems today is that around 50% of a battery’s weight can go into components that don’t produce energy,” said Dr. Rid Collins, senior technology analyst at IDTechEx (London, UK), which conducts research on emerging technologies, including advanced materials. Independent market research.

　　For example, in the battery packs that power electric and hybrid vehicles, a necessary component of the total power system mass is the battery casing. Simon Hiiemae, technical director at automotive thermal management specialist Zircotec Group (Abingdon, UK), said: “High voltage batteries, performance EVs can reach over 1,000 volts, generate a lot of radio frequency noise.” Hiiemae points out that protecting sensitive controls integrated into EVs One approach for electronic devices is to place the battery in a metal box, usually aluminum, that acts as a Faraday cage to absorb electromagnetic fields or radio frequency interference from the battery itself or surrounding sources; however, this adds considerable weight. Replacing metal with lightweight carbon fiber composites can save weight, but most composites are transparent to radio frequency noise and do not provide the required level of interference protection.

　　Zircotec Group has introduced a solution to this technical challenge: a conductive two-part coating system that can be easily applied to battery housings made of carbon fiber or glass fiber filled composites. The first layer of the coating is a thin aluminum alloy adhesive layer that is applied directly to the composite material without any solvents or adhesives and provides the conductivity needed to protect the battery from interference. Second, a ceramic layer is applied over the first metal layer to protect it from wear and corrosion; additionally, this ceramic layer can be modified to incorporate thermal barrier protection to protect the battery from external heat sources. Hiiemae reports that many car manufacturers are testing the coating; he says composite battery casings can save up to 4kg/m2 in weight compared to aluminum.

　　multifunctional materials

　　Also gaining traction in the energy storage industry are structural laminates capable of generating electrical current or storing charge, an approach that attempts to integrate structural and electrical systems into power-consuming products. This type of multifunctional material was first proposed and explored in research more than 20 years ago. It is mainly aimed at special and advanced special aerospace applications. It still faces huge technical challenges on the road to full commercialization. However, in the past five to six years, the development of multifunctional materials has made substantial progress on several fronts.

　　Composite structural supercapacitors (SSCs) have special development potential, in part because of their relatively simple structure. SSCs store energy through electrostatic charge accumulation at the electrode/electrolyte interface and are typically designed as a sandwich structure of two carbon porous electrodes separated by a membrane and embedded in a liquid electrolyte with high ionic conductivity. Traditional commercial supercapacitors offer high energy and power density, long cycle life, and reliable operation over a wide range of temperatures and conditions. While SSC could one day be used to power trains or cars, the earliest commercial SSC applications are likely to be in special-purpose unmanned aerial vehicles (UAVs), idtechex's Collins said. "SSCs integrated into UAVs can significantly extend range and mission time, which is a critical performance requirement," Collins said. "Additionally, the first commercial SSCs are expensive to build, so they are likely to be the most suitable Special type of project.”

　　The accelerated pace of research into multifunctional materials is helping to move this technology toward commercialization. For example, an EU-funded project called Wizard is supporting research into lightweight structural energy storage materials for electric and hybrid electric aircraft. The project aims to develop advanced materials and technologies that can be used in structural batteries and structural supercapacitors.

　　Another project involves a group of researchers from the IMDEA Institute of Materials (Getafe, Spain) in collaboration with some special aerospace projects funded by the European Union, such as Project Wizard, including the Airbus project. The team demonstrated the structure of a new type of electric double-layer capacitor (EDLC) made from thin sandwich structures. The structures are staggered and include an ionic, liquid-based polymer electrolyte between a veil of carbon nanotube (CNT) fibers and a carbon fiber layer infused with epoxy resin. Dr. Juan Jose Vilatela, a member of the multifunctional nanocomposites group, said the composites produced in this project are noteworthy for high specific capacitance and power density of 88 mf/g and 30 wh/kg respectively. This is one to three orders of magnitude higher than the best performing structural materials. The material also has an energy density of 37.5Wh/kg, one of the highest measured values for structural supercapacitors studied so far.

　　Carbon nanotubes have the inherent advantage of having a surface area 1,000 times greater than carbon fiber fabrics, Vilatela said. Carbon nanotubes also have high electrochemical stability. To fabricate EDLC composites, a thin EDLC sandwich was set between eight layers (4+4) of Hexcel (Stamford, CT, US) Hexforce high-strength carbon fiber fabric (G0926), vacuum infused by Ashland Chemical (Columbus, OH, US) ) is supplied with epoxy vinyl ester resin Derakane 8084 and cures completely within 48 hours. room temperature. The middle layer consists of a sandwich structure that includes a polymer electrolyte membrane about 100-120 microns thick from the middle outward, set between two CNT fiber plates, both of which are fixed to thin aluminum current collector plates. superior. Carbon nanotube fibers were synthesized by direct spinning method using iron, sulfur catalyst and butane as carbon sources.

　　A small amount of pressure is applied to the sandwich to impregnate the soft electrolyte membrane into the porous CNT fiber sheet before being positioned between the outer layers of carbon fiber fabric. The EDLC samples prepared in this study were approximately 4 centimeters square (the size of a typical laminate structural beam), although Vilatela said free-standing EDLCS can be used without the need for additional structural support and can be as large as 100 square centimeters. Field electromechanical measurements of EDLC samples in four-point bending tests showed that the electrochemical performance was maintained up to the breaking point. The trial is a key validation of the material's performance, given its applications as a structural and energy storage material.

　　IMDEA Group's use of unidirectional CNT fabrics to build the SSC distinguishes the project from similar parallel work using various "active" carbon fiber fabrics as energy storage materials. One such project cited by Vilatela et al. A paper published in Scientific Reports (February 2018) uses infusion technology to grow high-surface-area carbon aerogels (CAG) around carbon fiber fabrics. Combined with an ethylene glycol matrix containing 10% lithium ions, this technique produces a material with a calculated energy density of only 0.84 m wh kg, which is low compared to that obtained with EDLC containing CNT fibers. This technology, particularly when using high-surface-area composites, resulted in a composite with a shear modulus of 895 MPa, which is comparable to conventional structural composites. In comparison, the CNT composite in the IMDEA study had a flexural modulus of 60 gpa and a flexural strength of 153 mpa, values that are comparable to typical unfilled polyimide and lower than those used in composites used in major structural applications. required strength and stiffness properties.

　　The results of each project illustrate, in broad strokes, one of the challenges in developing fully commercialized multifunctional materials suitable for a wide range of applications: namely, it is difficult to build materials with adequate electrical and structural properties. composite materials.

　　Collins believes that the current trade-off between structural and electrical performance will not be a major barrier to adoption and commercialization of the technology. "I don't think the degradation in the structural properties of these new composite energy storage materials will limit their effectiveness, because even the slightly inferior carbon fiber reinforcements are still strong and stiff enough to be useful in some applications," he said. Greater The problem, he believes, is how to design and mechanically integrate solid polymer electrodes and separators that are electrically insulating but ionically conductive. Other challenges include cost-effective manufacturing and safety.

　　Leading automotive energy technology

　　In automotive racing, however, the future of advanced materials energy storage is already here. The first all-electric Formula E car in the FIA racing series since 2014, it is powered by an advanced multifunctional composite 800V structural battery. The battery technology was developed and manufactured by Williams Advanced Engineering (Grove, UK), the sole supplier of vehicle batteries for the Formula E grid. A typical Formula E car has about 250 horsepower (190 kilowatts), can accelerate from 0 to 100 km/h (0 to 62 mph) in 3 seconds, and has a maximum speed of 225 km/h (140 mph). For more information about Williams Advanced Engineering’s structural battery systems for consumer vehicles, such as the battery-electric FW-EVX, see CW’s November 2018 Focus on Design Story Design Story "Pushing EVS Forward".

　　On the other hand, Hyundai Motor Group (Seoul, South Korea) is using hydrogen fuel cell technology provided by SGL Group (Wiesbaden, Germany) to manufacture its zero-emission car Nexo. The production vehicle covers years of development work and collaboration between the automaker and SGL, during which Hyundai optimized the hydrogen powertrain and other components on the Ix35 demonstration fuel cell vehicle. Since starting production in March 2018, Hyundai has reported average sales of about 45 units per month, and if it can maintain this pace, it will reach sales of more than 500 units in 2018, which is a new year for hydrogen fuel cell vehicle sales. Record.

　　NEXO's core technology is the Sigracet fuel cell, a polymer electrolyte membrane fuel cell (PEMFC) developed and marketed by SGL. Proton exchange membrane fuel cells (PEMFC) generate electrochemical power by converting hydrogen fuel into electricity, with the only by-products being water and heat. A single PEMFC cell consists of two flow fields, including two gas diffusion layers (GDL) and two carbon-supported precious metal catalyst layers, each separated by a proton exchange membrane. GDL, in turn, is a two-layer structure consisting of a macroporous backing material (carbon fiber paper) and a microporous carbon base layer. The membrane electrode assembly is constructed as a layered laminate with one GDL layer serving as the anode and another GDL layer serving as the cathode. PEMFCs are sandwiched between two bipolar plates (BPP) made of graphite, coated steel or titanium. BPP forms the structural components of the chimney and is designed with channels to accommodate coolant flow and water outlets. Automotive PEMFC systems typically consist of up to 400 battery packs with a power output of approximately 80 to 120 kilowatts.

　　Composites are making progress in energy storage, but making cleaner, lighter energy a large-scale reality will depend on the details of advanced technologies such as fuel cells, structural batteries and structural supercapacitors.

　　IDTechEx studies and evaluates the Technology Readiness Level (TRL) of a variety of multifunctional polymer composites. The rating system is based on a range of criteria, including whether the research remains primarily academic (resulting in a lower rating), whether the technology is being prototyped or tested (resulting in a higher rating), or whether the technology has reached commercialization (the highest rating). Data mining shows that multifunctional materials for energy storage and energy storage are still in a relatively early stage of development by IDTechEx standards - just ahead of self-healing materials and fully embedded circuits, but behind power transmission and embedded sensors . "When you look at electrodes for SSCs, for example, one of the challenges is how to improve the surface area appropriately," Collins noted. "CNTs show promise, but there are still ways to prove this commercially."

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