The mass adoption of electric transportation technologies requires electric energy storage and traction drive systems that offer improved performance, extended range, and fast-charging options. The ever-increasing power density of electric vehicle (EV) battery packs necessitates advanced thermal interface materials (TIMs) to provide superior thermal management of the batteries and power electronics. State-of-the-art TIMs play a vital role in maintaining the safe operation and consistent performance of modern EVs.
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With the rapidly expanding EV market, the automotive industry is going through one of the most profound changes in its history. Improved dependability, distance range, and recharging speed, together with an attractive ownership cost, increasingly shift consumers' attention towards battery-powered and hybrid (combining electric and internal combustion engines) EVs.
A Fast-Growing Market
Along with consumer demand, the automotive industry must meet ever-stricter fuel economy and emission targets imposed by governments. The importance of these requirements becomes increasingly significant as the global number of light vehicles is forecasted to nearly double by 2045. As a result, by 2031, EV production is anticipated to reach 50% of all light vehicles manufactured globally.
At the core of the current e-mobility revolution lies the development of efficient energy storage technologies, particularly lithium-ion batteries (LIBs), that enable car manufacturers to respond to the societal demand for clean, efficient, and sustainable vehicles. Since the development of battery cells consisting of lithium cobalt oxide as a cathode and graphite as an anode by Sony in 1991, LIB technology has conquered the energy storage market due to its high-energy-density, prolonged cycle life, and rapid charge and discharge rates compared to other rechargeable battery systems (lead-acid, Ni-Cd, Ni-metal hydride).
Battery Developments Underpin E-Mobility Expansion
Modern commercially available LIBs can provide around 250-270 Whkg-1 (Watt-hours per kilogram). Just a decade ago, the most advanced LIBs had an energy density of approximately 110 Whkg-1 (comparable to lead-acid and Ni-metal hydride batteries). The battery manufacturers expect to increase LIBs' energy density up to 450 Whkg-1 by 2030, offering more compact and lightweight energy storage solutions.
At the same time, the battery cost decreased from 1000 USD per kWh to 150 USD per kWh at present and is expected to drop below 90 USD per kWh in the next decade.
Efficient Thermal Management for Safe and Reliable EVs
The increasing energy density in the modern EV propulsion systems (including battery packs, power converters, and electric motors) puts special demands on the thermal management of individual components. In particular, the performance, durability, and safety of the LIBs depend strongly on their operating temperature, which ideally needs to be in the range of 15-35 °C and should never exceed 80 °C.
To ensure safe operation and extend the battery life, EV manufacturers have developed various battery designs (including cylindrical, pouch, or prismatic individual cells) and thermal management systems (employing air or liquid external cooling). These systems provide efficient dissipation of the heat generated during the battery's normal operation (due to the discharge and charge currents), reduce uneven temperature distribution between the individual cells within the battery pack, as well as supply heat to the battery when the external temperature is too low.
Benefits of TIMs in EVs Production
Regardless of the electric powertrain design, TIMs are essential components of EVs' thermal management systems. These materials are placed between the heat-generating electrical and electronic components and the heatsinks that dissipate the heat into the environment. The main purpose of the TIMs is to improve the thermal contact between hot and cold surfaces and to maximize the heat transfer.
Quite often, however, TIMs also perform additional functions (electrical insulation or shielding, provide the structural integrity of the battery packs, etc.) and need to combine conflicting properties.
Currently, the most widely used TIMs are composite materials consisting of two or more components - an organic matrix (paste-like or liquid polymer) complemented with thermally conductive fillers, such as aluminum oxide, aluminum nitride, graphite, or metal particles.
The organic matrix has a relatively low thermal conductivity of around 0.1-0.5 Wm–1K–1 (Watts per meter-Kelvin), while the filler materials exhibit much higher thermal conductivity in the range 30-100 Wm–1K–1.
The thermal conductivity of the resulting composite TIMs is a function of the thermal conductivities and the volume fractions of matrix and filler materials, and is in the range of 1-5 Wm–1K–1 (compared to the air's thermal conductivity of approximately 0.02 Wm–1K–1). This type of TIM combines the advantages of polymers, such as low weight, effective processability, and corrosion resistance with the thermal conductivity provided by the inorganic fillers.
Meeting the Demands of the Future
To meet the demands of future electric propulsion systems, material scientists are focusing their efforts on the development of novel durable lightweight materials with an extremely low thermal resistance (or high thermal conductivity,) as well as reducing the manufacturing and processing cost of the existing (and newly developed) TIMs.
Metal foams, phase-change materials, and carbon nanotube (CNT)-based materials show a particular promise in thermal management applications.
Recently, a research collaboration between Tsinghua University in China and the Georgia Institute of Technology in the USA developed a low-cost and high-performance soft porous copper–indium foam-like structure, which exhibits a superior thermal conductivity of 50 Wm–1K–1 while possessing reduced weight, improved durability, and excellent vibration and thermal stress resistance compared to the traditional TIMs.
The utilization of wax-like phase-change materials (PCM) that undergo a solid-liquid phase transition as TIMs is another development that attracts attention from battery manufacturers. Phase-change TIMs with a phase transition temperature within the optimal temperature range for the battery pack enable the implementation of a semi-passive thermal management system (since the phase-change TIM can reversibly absorb/release large amounts of heat upon heating/cooling) that requires a less sophisticated external cooling system.
Space-Proven Thermal Management Solutions
Combining the unique properties of PCMs with carbon-based materials that are intrinsically highly thermally conductive, such as vertically aligned CNTs, carbon fibers, or graphene, shows enormous potential for the development of the next generation TIMs.
KULR Technology Group, based in San Diego USA, has developed and commercialized a range of flexible, ultra-lightweight high-performance fiber thermal interface materials (FTIMs) with a thermal conductivity that exceeds 10 Wm–1K–1. The PCM-infused carbon fiber material possesses a large heat capacity over a small temperature range and outstanding mechanical properties.
KULR's FTIMs are utilized in the thermal management system of NASA's Perseverance rover, currently operating on Mars (the EV operating in some of the harshest environments at present). In collaboration with EV and battery manufacturers, the company's engineers are already exploring ways to reduce the manufacturing cost and to make FTIM technology compatible with large-volume mass production, aiming to offer one of the most advanced TIMs on the market.
References and Further Reading
J. Van Mierlo, et al. (2021) Beyond the State of the Art of Electric Vehicles: A Fact-Based Paper of the Current and Prospective Electric Vehicle Technologies. World Electric Vehicle Journal 12(1), 20. Available at: https://doi.org/10.3390/wevj12010020
I. Husain, et al. (2021) Electric Drive Technology Trends, Challenges, and Opportunities for Future Electric Vehicles. Proceedings of the IEEE 109(6), 1039-1059. Available at https://doi.org/10.1109/JPROC.2020.3046112
P. Liu, et al. (2021) Laminar Metal Foam: A Soft and Highly Thermally Conductive Thermal Interface Material with a Reliable Joint for Semiconductor Packaging. ACS Applied Materials & Interfaces 13 (13), 15791-15801. Available at: https://doi.org/10.1021/acsami.0c22434
KULR Technology (2020) KULR’s Thermal Architecture Included In Upcoming Mars Perseverance Rover. [Online] www.globenewswire.com Available at: https://www.globenewswire.com/news-release/2020/06/04/2043744/0/en/KULR-s-Thermal-Architecture-Included-In-Upcoming-Mars-Perseverance-Rover.html (Accessed on 23 June 2021).
KULR Technology (2020) KULR Technology Group Announces Supplier Partnership of NASA Grade Carbon Fiber Cooling Technology For Drako Motors Electric Supercar. [Online] www.kulrtechnology.com Available at https://kulrtechnology.com/kulr-announces-supplier-partnership-of-nasa-grade-carbon-fiber-cooling-technology-for-drako-motors-electric-supercar (Accessed on 23 June 2021).
J. Khan, et al. (2020) A review on 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
M. -A. Raux, et al. (2020) An Innovative Way to Make Anisotropic Thermal Interface Materials. 19th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), pp. 970-974, Available at: https://doi.org/10.1109/ITherm45881.2020.9190254
G. Xia, et al. (2017) A review on battery thermal management in electric vehicle application. Journal of Power Sources, 367, 90-105. Available at: https://doi.org/10.1016/j.jpowsour.2017.09.046
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