Review:New market opportunities and future development trends of carbon fiber composites (2020 year)

Summary

With the in-depth research on the electrical properties of carbon fiber materials and the continuous development of solid polymer electrolytes, integrated structural/energy storage composite materials have emerged and become a new type of material that has attracted much attention in the past two decades. Structural/energy storage integrated composite materials can realize electric energy storage in structural parts. In the current global environment of electricization of passenger cars and the booming of electric aircraft, this new material is gradually becoming a research hotspot for functional composite materials. The article focuses on the research progress of major scientific research institutions in the field of structural/energy storage integrated composite materials at home and abroad, analyzes the main research directions in this field, and looks forward to the future of structural/energy storage integrated composite materials.

Compared with metal materials, carbon fiber composite materials have the advantages of light weight, high specific strength, high specific stiffness, strong designability and corrosion resistance. It is an ideal structural weight loss material. With the increasing application of carbon fiber composite materials in aircraft, ships and automobiles year by year, its application site is transitioning from secondary bearing structure to main bearing structure, and from single structure bearing to structural/functional integration. Structural/energy storage integrated carbon fiber composite material is a new type of functional composite material that has attracted much attention in recent years. At present, the United States and the European Union have carried out a number of exploratory studies in this field. However, in China, there is less research on structural/energy storage integrated composite materials, and the research level is low, which is still different from the world’s advanced level.

Research progress on structural/energy storage integrated composite materials abroad

The research and development of structural/energy storage integrated composite materials began in the 1990s. In 1995, Takashi Iijim and others, a Japanese scientist of Nippon Steel, cooperated with Yamaguchi University to study the electrical characteristics of different carbon materials, proving that two commercial carbon fibers (asphalt-based carbon fiber and polyacrylonitrile-based carbon fiber) have the ability to adsorb lithium ions under specific conditions and can be used as lithium. The anode material of ionized batteries. Experiments have proved that carbon fiber electrodes have a good electrical capacity (350 m Ah/g) and battery cycle performance as good as graphite electrodes (375 m Ah/g) after heat treatment at high temperature (1000℃).

The good mechanical and electrochemical properties of carbon fiber materials make it possible to integrate structural/energy storage carbon fiber composites. Since 2000, the U.S. Army Research Laboratory, the Royal Swedish Institute of Technology, Luleå University of Technology, British Imperial University of Technology and other institutions have successively published research reports on the structure and related properties of various structural/energy storage integrated carbon fiber composites.

U.S. Army Research Laboratory (U.S.ARL)

U.S.ARL is the earliest research institution to successfully test sheet structure/energy storage composite materials. In order to meet the needs of the follow-up development of U.S. Army weapons and equipment, U.S.ARL designed and manufactured structural/energy-storage integrated composite batteries for the first time. A total of three prototypes of composite materials with carrying function have been designed. ( Figure 1) In the design of these integrated carbon fiber composites, the electrodes, electrolytes, diaphragms, catalysts and other components of the battery all have certain carrying functions.

In 2011-2015, U.S. ARL applied for patents for a number of structural capacitors. In 2011, U.S.ARL first applied for a patent for a class of structural capacitors (US7,864,505B1), which includes the design of a variety of structural capacitors. The stiffness of these structural capacitors can reach 10MPa ~ 1000GPa, and the fracture strength is 1MPa to 10GPa. One of the structural capacitors reinforced with polycarbonate can reach up to 575p F. In 2013, U.S.ARL invented a new structural electrochemical capacitor (US8,576,542B2), which consists of a pair of electrodes and solid electrolytes with an energy density of not less than 1n J/g. In 2015, U.S. ARL applied for a design method for structural electrochemical capacitors (US9,190,217B2), and the system summarized the method of improving structural electrochemical capacitors. From the patent publication in recent years, it can be seen that U.S.ARL has conducted in-depth research on structural/energy storage integrated capacitors and has accumulated rich experimental data and design experience.

Royal Swedish Polytechnic University (KTH) and Lülleo University (LTU)

Since 2008, SICOMP, a Swedish research institute, has organized a group of Swedish researchers to explore and research in the field of structural/energy storage integrated composite materials technology. The research is an important part of the Swedish KOMBATT project (lightweight structural energy storage materials) and is funded by the Swedish Strategic Research Fund (SSF).

KTH tested the basic electrochemical properties of different grades of commercial PAN-based carbon fiber as the anode of lithium-ion batteries. Experiments have proved that some commercial carbon fibers have good electrochemical properties. Among the commercial carbon fibers, the medium-mode carbon fiber IMS65 (tensile modulus 290 GPa, tensile strength 6000 MPa) produced by Dongbang Tennax Company has a reversible capacity of 350 at a charging rate of 0.1C. m Ah/g, close to the theoretical capacity of the graphite electrode (375 m Ah/g). In 2012, KTH researchers explored the impact of lithium reaction and electrochemical cycle on the tensile properties of carbon fiber. Research shows that the ultimate tensile strength of carbon fiber is lost in the lithium-embedded reaction and expands in the direction of the fiber. During the delithium reaction, the ultimate strength of the material partially recovers and the fiber shrinks. After 1000 electrochemical cycles, the tensile properties and microscopic shape of the carbon fiber electrode have not changed significantly. These research results lay the foundation for the subsequent design and manufacture of structural/energy storage integrated composite materials.

KTH then carried out research on solid polymer electrolytes (SPE) with carrying function. KTH disperses lithium salt and photoinitiator in a monomer mixture through a rapid solvent-free process, synthesizing a variety of photocurable epoxy acrylic solid electrolytes. The lithium salt mass fraction of these solid electrolytes can reach 4%, and the Young’s modulus ranges from 0.8MPa to 1.5 GPa at 20℃. The conductivity of the electrolyte is correlated with the stiffness of the material, and the conductivity of the synthesized solid electrolyte can reach up to 1.5×10-6 S/cm. In 2013, KTH published a synthesis method based on the photocuring reaction of thiolene. By adding a small amount of thiol, it improves the conductivity of SPE without losing stiffness. The conductivity of a variety of solid electrolytes synthesized can reach 8×10-7S/cm, 20℃. The Young’s modulus is from 2 MPa to 2 G Pa doesn’t wait.

In 2013, LTU’s L.E.Asp and SICOMP researchers produced two carbon fiber reinforced structure/energy storage integrated laminated batteries to verify the battery design of ARL. The battery structure consists of three parts, namely, the anode of the carbon fiber woven cloth, the fiberglass woven cloth diaphragm and the positive electrode made of aluminum woven cloth coated with lithium iron phosphate (Li Fe PO4). The battery structure is composed of solid polymer electrolytes and polymer gel electrolyte respectively. Figure 3) The tensile modulus of this structure/energy storage integrated laminated battery is better than that of glass fiber/epoxy composite material (23 GPa), reaching 35 GPa, the open circuit voltage (OCP) of the battery is 3.3 V, and the energy density is 116Wh/kg, which is basically close to the performance of lithium cobalt batteries (OCP=3 .3 V, energy density 130 Wh/kg).

In the same year, LTH and SICOMP also published a carbon fiber reinforced structure/energy storage integrated laminated capacitor preparation method developed for the automobile manufacturing industry. This method uses a vacuum molding process to make a composite laminate with capacitor characteristics by placing three dielectric polymer (PA; PET; PC) diaphragms between two layers of carbon fiber woven epoxy prepreg. The researchers compared the electrical properties of the capacitors in the third middle and found that the thinner the thickness of the diaphragm, the higher the capacitance and the lower the dielectric strength. When using the PET diaphragm, the capacitance can reach up to 1860 n F/m2. At the same time, the mechanical properties test also proves that the structural/energy storage capacitor has better mechanical properties than the fiberglass composite material.

Since 2014, researchers from KTH, LTH and SICOMP have continued to promote the research of structural/energy storage integrated composite batteries and capacitors. In 2014, Eric Jacques of KTH further studied the impact of lithium-embedding process on the mechanical properties of carbon fiber, and analyzed the changes in tensile stiffness and ultimate tensile strength of carbon fiber electrodes with different degrees of lithium-inlay. It is believed that the reasons for the decline in mechanical properties of carbon fiber electrodes after multiple charge and discharge cycles. It is caused by some lithium ions stranded in the defective area of the carbon fiber bundle during the delithium removal process. In 2015, the Leif Asp team studied the impact of the fatigue performance of carbon fiber surface coated with polymer coating. It was found that the polymer coating can effectively improve the fatigue performance of carbon fiber, and the coating itself is not affected by long-term mechanical fatigue. This discovery can be used to improve the fatigue and electrical properties of structural/energy storage integrated composite batteries in the future.

In 2018, with the support of the Swedish Energy Agency, KTH, together with Chalmers University of Technology and the University of Paderborn, designed a Ultra-thin one-way carbon fiber reinforced composite electrode. The mechanical properties of the ultra-thin electrode have not decreased after 10 charge and discharge cycles, and the capacity is stable at 200m Ah/g. In the same year, KTH published a comprehensive design method for integrated structural/energy storage composite batteries, and designed three different new structural batteries through calculation. The test results of three different structural batteries prove that when designing a new structural battery, the classical laminated theory can be used to estimate the elastic performance of the integrated composite battery, or the electrical parameters of carbon fiber electrodes and structural electrolytes can also be used to estimate the electrical performance of the overall battery structure.

Imperial University of Technology (ICL)

ICL’s research on structural/energy storage integrated composite materials is more engineering, and has achieved certain engineering application research results. ICL designed a structural/energy storage integrated composite supercapacitor using modified carbon fiber material. In cooperation with Volvo, the integrated composite material of structure/energy storage was applied to the automobile structure for the first time, and the energy storage function was realized while reducing weight. Supercapacitors use the charge separation between the electrolyte and the electrode interface under applied voltage to achieve rapid energy storage. The contact area between the electrode and the electrolyte determines the capacity of the supercapacitor. Therefore, increasing the specific surface area of the electrode can greatly improve the energy storage effect of the super capacitor.

In order to develop high-performance structural/energy storage integrated composite supercapacitors, ICL researchers began to carry out research on the activation of carbon fiber electrodes. In the study, the effects of physical activation (activation in air and CO2 oxidation) and chemical activation (HNO3 pickling activation and KOH alkali washing activation) on common commercial carbon fibers were compared. Research shows that the use of KOH for chemical activation can increase the specific surface area of carbon fiber from 0.21 m2/g to 23.3 m2/g without damaging the tensile strength of carbon fiber, and its electrode performance can be increased by 50 times.

In 2013, ICL continued to propose a method of preparing capacitors using carbon aerogel (CAG) modified carbon fiber fabric as an electrode. The method of preparing CAG modified carbon fiber electrodes is to first fully mix the carbon aerogel precursor resorcinol-formaldehyde with the catalyst KOH, then fully soak the mixture into the carbon fiber fabric through impregnation/injection method, and finally the carbon fabrication in N2 environment for 800℃ carbonization for 30 minutes. Modified carbon fiber electrode. This modification method can greatly increase the electrode capacity of the electrode, up to 62 F/g. The energy density of capacitors prepared with modified electrodes can reach 1 Wh/Kg (3600 J/kg), which is greatly improved compared with the structural/energy storage integrated capacitors (energy density 10-6J/Kg) prepared by ARL. ( Figure 5)

ICL has also been studied in the modification of polymer electrolytes used in structural/energy storage integrated composites. A new structural electrolyte based on the dual continuous phase ion liquid-epoxy resin system has been developed. The ion conductivity of this structural electrolyte at room temperature is 0.8 m S/cm, and the Young’s modulus is 0.2GPa. The synthesis route is shown in Figure 5.

ICL cooperated with Volvo to use the CAG modified carbon fiber electrode and the above-mentioned new structural electrolyte to make large-size structure/energy storage integrated composite composite automotive components, realizing the engineering application of structural/energy storage integrated composite materials for the first time. The car tail box cover is 60% less than the traditional metal structure, and can provide continuous power for the car LED decorative lights. ( Figure 7) In 2014, this structural/energy storage integrated composite capacitor preparation technology has been applied for a U.S. patent. ( Patent No.: US8659874 B2)

To sum up, the research of structural/energy storage integrated composite materials by foreign research institutions is shifting from theoretical research in the laboratory to engineering research. Although the electrical properties of existing structural/energy storage integrated composite batteries and capacitors are still not ideal at this stage, the mechanical properties of the materials are compared with traditional composite materials. There is also a gap, but with the continuous development of relevant research, the development prospects of structural/energy storage integrated composite materials are broad. In particular, the fully electrified plan for passenger vehicles launched by the European Union in recent years and the promulgation of a number of new environmental protection laws will further promote the development of relevant research.

Domestic research progress of structural/energy storage integrated composite materials

The domestic (China)research on structural/energy storage integrated composite materials started late, and the relevant research reports began in 2014. In recent years, it has shown a trend of increasing year by year. The unit of the structural/energy storage integrated composite capacitor is Suzhou University in China. In 2017, Beijing University of Aeronautics and Astronautics published a preparation method for integrated structural/energy storage batteries.

In 2014, Dr. Li Sumin of Jiangsu University carried out research on activated carbon fiber electrodes and activated the T300 carbon fiber fabric (3K) produced by Dongli Company. The specific surface area of T300 woven cloth was increased by 45 times by chemical oxidation (HNO3) and then heat treatment, but the tensile strength of the treated carbon fiber was reduced by 20%. In 2016, Dr. Li Sumin published the use of epoxy colloidal polymer electrolyte and PEGDA (polyethylene glycol dishrinkage). After mixing, TBAPF6 (tetrabutylammonium hexafluorophosphate) ion salt is added to synthesize polymer electrolytes. This polymer electrolyte voltage window is 2.7V, and the ion conductivity at room temperature is 10-5S/cm. It is a new structure/energy storage integrated composite capacitor composed of activated T300 carbon fiber woven cloth with a capacity of 3F/g.

In 2017, Beijing University of Aeronautics and Astronautics used T700 carbon fiber (12K) as electrodes and reinforcing materials, and epoxy resin and liquid electrolyte mixture as the matrix to prepare structural/energy storage integrated composite batteries through vacuum-assisted injection molding process. In the test, the resin system is composed of a mixture of E51, AG80 and curing agent, liquid electrolyte is a mixture of 1-ethyl-3-methylimidazole distrifluoromethylsulfonimide salt, propylene carbonate and ditrifluoromethane sulfonimide lithium. The material preparation route is shown in Figure 8. Four structural/energy storage integrated composite batteries were prepared by adjusting the ratio of liquid electrolyte to epoxy resin. Their first discharge capacity ranged from 12 m Ah/g to 25 n Ah/g. However, the battery cycle performance was not good. After 20 cycles, the battery charge and discharge capacity decreased significantly. In 2018, Zhao Danni published the preparation and performance test results of lithium ion electrolyte/epoxy vinyl ester resin solid electrolyte, and verified the influence of different proportions of ion electrolyte on the electrical and mechanical properties of solid electrolytes. The test results proved that when 40% electrolyte is added, the solid electrolyte as a whole The best performance.

Development trend of structural/energy storage integrated composite materials

Research on electrodes for structurally carrying composite materials

The electrodes of composite materials should have excellent energy storage and mechanical properties. However, the electrochemical properties of traditional carbon fiber materials are low and cannot meet the requirements of high-performance carbon fiber composite electrodes. Therefore, according to the needs of electrode materials, the surface modification of carbon fiber, the specific surface area and electrochemical properties of carbon fiber are improved, and the mechanical properties of carbon fiber are the main focus of research.

Research on structural electrolyte

Electrolytes are key materials for providing ion transfer channels in energy storage structures. In the research of structural energy storage integrated composite materials, structural electrolytes must have both high ionic conductivity and reasonable mechanical properties. The main means of structural electrolyte preparation is to mix liquid, gel, solid electrolyte with structural resin (epoxy resin, etc.) to form a structural electrolyte resin matrix with high ionic conductivity. Foreign studies have found that the mechanical properties of structural electrolytes with high ion conductivity are poor, while the ion conductivity of structural electrolytes with excellent mechanical properties is low. The functional and mechanical properties of structural electrolytes are negatively correlated. Therefore, seeking a balance between the functional and mechanical properties of resins is the focus of the research of structural energy storage integrated composite materials.

Study on the interface performance of fiber/structural electrolyte

The research on the structural design and preparation technology of battery structure of integrated composite materials with integrated energy storage is a study that combines battery structure design and preparation technology. On the basis of traditional composite material manufacturing technology, the results of composite electrodes and structural electrolyte research are integrated to design structural energy storage integrated composite batteries. Composite batteries should have the ability to stabilize the output current under a certain load, and the battery capacity should meet the design requirements. The focus of the study is how to match composite electrodes, resin-based structural electrolytes and positive materials to obtain higher energy density and stronger mechanical properties.

Research on structural/energy storage integrated battery/capacitor

Structural energy storage integrated composite battery/supercapacitor is a research focus at this stage. To study the influence of different combinations of different structural electrodes, diaphragms and structural electrolytes on the energy storage effect of batteries/capacitors. Optimize the energy storage efficiency of structural/energy storage integrated composites. At the same time, actively explore the design method of new structure/energy storage integrated battery/capacitor.

Safety and engineering research

Structural/energy storage integrated composite materials need to have two functions: bearing and energy storage, so we should continue to study the energy storage effect of materials under load and the impact of energy storage on the mechanical properties of composite materials. At the same time, we need to pay attention to the study of material safety, such as whether the integrated structure/energy storage battery will spontaneous combustion under extreme working conditions.

Prospects

In 1971, after Toli and United Carbon Valley Company realized the industrial production of T300-class carbon fiber for the first time, carbon fiber composite materials have gradually grown into key industrial materials second only to metals because of their light weight and high strength. In the next decade, in addition to focusing on improving the mechanical properties of carbon fiber itself, another major research direction will be to develop and improve functional composites by using the multi-layer designable characteristics of composite materials. Structural/energy storage integrated composite materials expand the application of composite materials in the field of energy storage. Under the background that the energy density of lithium batteries is gradually approaching the theoretical upper limit, structural/energy storage integrated composite materials will be an ideal energy storage expansion scheme for new energy transportation vehicles such as electric aircraft and electric vehicles in the future. With the continuous advancement of relevant research, the development of structural/energy storage integrated composite materials with high strength and high energy storage density is just around the corner.

Original text: Gu Jianxiao. Overview of the research progress of structural/energy storage integrated composites [J]. Metallurgy and Materials, 2020,40(03):59-63.