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The large-scale adoption of electric transportation technology requires electric energy storage and traction drive systems to provide higher performance, greater range, and fast charging options. The increasing power density of electric vehicle (EV) battery packs requires advanced thermal interface materials (TIM) to provide superior thermal management for batteries and power electronics. The most advanced TIM plays a vital role in maintaining the safe operation and consistent performance of modern electric vehicles.
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With the rapid expansion of the electric vehicle market, the automotive industry is undergoing one of the most profound changes in its history. Increases in reliability, range and charging speed, coupled with attractive cost of ownership, are increasingly turning consumers’ attention to battery-powered and hybrid (combined electric and internal combustion) electric vehicles.
In addition to consumer demand, the automotive industry must also meet increasingly stringent fuel economy and emission targets set by the government. As the number of light vehicles worldwide is expected to nearly double by 2045, the importance of these requirements has become increasingly important. Therefore, by 2031, the output of electric vehicles is expected to reach 50% of all light vehicles manufactured globally.
The core of the current electric vehicle revolution lies in the development of high-efficiency energy storage technologies, especially lithium-ion batteries (LIB), so that automakers can respond to society's demand for clean, efficient and sustainable vehicles. Since Sony developed battery cells with lithium cobalt oxide as the positive electrode and graphite as the negative electrode in 1991, LIB technology has conquered the energy storage market with its high energy density, long cycle life, and fast charge and discharge characteristics. The rate compared to other rechargeable battery systems (lead acid, nickel cadmium, nickel metal hydride).
Hyundai's commercially available LIB can provide about 250-270 Whkg-1 (watt hour per kilogram). Just ten years ago, the energy density of the most advanced LIB was about 110 Whkg-1 (comparable to lead-acid and nickel-metal hydride batteries). Battery manufacturers expect to increase the energy density of LIB to 450 Whkg-1 by 2030, thereby providing a more compact and lighter energy storage solution.
At the same time, battery costs have fallen from the current US$1,000 per kWh to US$150 per kWh, and are expected to fall below US$90 per kWh in the next ten years.
The ever-increasing energy density in modern EV propulsion systems (including battery packs, power converters, and electric motors) places special requirements on the thermal management of individual components. In particular, the performance, durability and safety of LIBs depend to a large extent on their operating temperature, ideally within the range of 15-35 °C, and should not exceed 80 °C.
To ensure safe operation and extend battery life, electric vehicle manufacturers have developed various battery designs (including cylindrical, pouch or prismatic single cells) and thermal management systems (using air or liquid external cooling). These systems can effectively dissipate the heat generated by the battery during normal operation (due to the discharge and charging current), reduce the uneven temperature distribution between individual cells in the battery pack, and the external temperature is too low.
Regardless of the design of the electric powertrain, TIM is an important part of the thermal management system of electric vehicles. These materials are placed between the electrical and electronic components that generate heat and the radiator that dissipates the heat to the environment. The main purpose of TIM is to improve the thermal contact between hot and cold surfaces and maximize heat transfer.
However, TIMs usually also perform additional functions (electrical insulation or shielding, providing structural integrity of the battery pack, etc.) and need to incorporate conflicting characteristics.
Currently, the most widely used TIM is a composite material composed of two or more components-an organic matrix (paste or liquid polymer) supplemented by thermally conductive fillers such as alumina, aluminum nitride, graphite or metal particles .
The thermal conductivity of the organic matrix is relatively low, about 0.1-0.5 Wm–1K–1 (watts per meter Kelvin), while the filler material exhibits a higher thermal conductivity in the range of 30-100 Wm–1K–1.
The thermal conductivity of the resulting composite TIM is a function of the thermal conductivity and the volume fraction of the matrix and filling material, in the range of 1-5 Wm–1K–1 (by comparison, the thermal conductivity of air is approximately 0.02 Wm –1K– 1). This type of TIM combines the advantages of polymers, such as light weight, effective processability and corrosion resistance, and thermal conductivity provided by inorganic fillers.
In order to meet the needs of future electric propulsion systems, materials scientists are working to develop new durable lightweight materials with extremely low thermal resistance (or high thermal conductivity) and reduce existing manufacturing and processing costs. And the newly developed) TIM.
Metal foams, phase change materials, and carbon nanotube (CNT)-based materials show particular promise in thermal management applications.
Recently, a research collaboration between Tsinghua University in China and Georgia Institute of Technology in the United States has developed a low-cost, high-performance soft porous copper-indium foam-like structure with a thermal conductivity of up to 50 Wm-1K-1. Compared with TIM, it has lighter weight, higher durability, and excellent vibration resistance and thermal stress resistance.
The use of a waxy phase change material (PCM) that undergoes a solid-liquid phase transition as a TIM is another development that has attracted the attention of battery manufacturers. The phase change TIM with the phase change temperature within the optimal temperature range of the battery pack can realize a semi-passive thermal management system (because the phase change TIM can reversibly absorb/release a large amount of heat during heating/cooling), which requires a less complicated external cooling system.
Combining the unique characteristics of PCM with intrinsically high thermal conductivity carbon-based materials (such as vertically aligned CNTs, carbon fibers, or graphene) shows great potential for the development of next-generation TIMs.
Headquartered in San Diego, USA, KULR Technology Group has developed and commercialized a series of flexible, ultra-lightweight high-performance fiber thermal interface materials (FTIM) with a thermal conductivity of more than 10 Wm-1K-1. The carbon fiber material injected with PCM has a larger heat capacity and excellent mechanical properties in a smaller temperature range.
KULR's FTIM is used in the thermal management system of the NASA Perseverance rover, which is currently operating on Mars (the electric car is currently operating in some of the harshest environments). Through cooperation with electric vehicle and battery manufacturers, the company's engineers have been exploring ways to reduce manufacturing costs and make FTIM technology compatible with mass production, aiming to provide one of the most advanced TIMs on the market.
J. Van Mierlo et al. (2021) Beyond the most advanced electric vehicles: a fact-based paper on current and future electric vehicle technologies. World Electric Vehicle Magazine 12(1), 20. See: https://doi.org/10.3390/wevj12010020
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P. Liu, et al. (2021) Laminated metal foam: a soft and highly thermally conductive thermal interface material with reliable joints for semiconductor packaging. ACS Applied Materials and Interface 13 (13), 15791-15801. Available at: https://doi.org/10.1021/acsami.0c22434
KULR Technology (2020) KULR's thermal architecture is included in the upcoming Mars Perseverance Rover. [Online] www.globenewswire.com URL: https://www.globenewswire.com/news-release/2020/06/04/2043744/0/en/KULR-s-Thermal-Architecture-Included-In-Upcoming- Mars-Perseverance-Rover.html (accessed June 23, 2021).
KULR Technology (2020) KULR Technology Group announced a partnership with Drako Motors as a supplier of NASA-grade carbon fiber cooling technology for electric supercars. [Online] www.kulrtechnology.com is available at https://kulrtechnology.com/kulr-announces-supplier-partnership-of-nasa-grade-carbon-fiber-cooling-technology-for-drako-motors-electric -supercar (accessed June 23, 2021).
J. Khan et al. (2020) Overview of advanced carbon-based thermal interface materials for electronic devices. Carbon, 168, 65-112. Available at: https://doi.org/10.1016/j.carbon.2020.06.012
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Cvetelin Vasilev has a degree and PhD in physics and is currently working as a biophysicist at the University of Sheffield. As a research scientist, he has more than 20 years of experience and is an expert in applying advanced microscopy and spectroscopy techniques to better understand the organization of "soft" complex systems. Cvetelin has published more than 40 papers in peer-reviewed journals (h index 17) in the fields of polymer science, biophysics, nanomanufacturing, and nanobiophotonics.
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