As the global push towards electric vehicles (EV) is gaining momentum, rapid innovations are continuously reshaping this domain. Among these advancements, thermal management of EV batteries emerges as a critical area. Effective thermal management is vital because extreme temperatures can adversely affect battery performance, longevity, and safety. Overheating can lead to reduced efficiency, faster degradation, and in severe cases, thermal runaway, where the battery could catch fire. Conversely, low temperatures can diminish the battery's ability to hold a charge and reduce its power output. Addressing these thermal management challenges is a critical area of research and development in the EV industry, as it directly impacts the vehicle's performance, range, and overall safety.
The current standard for cooling EV batteries primarily involves two methods: air cooling and liquid cooling. Air cooling can be either passive, using air from outside the cabin, or active, supported by a heating, ventilation, and air conditioning (HVAC) system. Liquid cooling involves circulating a coolant liquid through tubes and cooling plates to reduce the battery temperature, transferring residual heat away from the battery to components like a radiator or heat exchanger.
One such emerging liquid cooling technology is immersion-cooling. Immersion-cooling in EV batteries involves a process where battery cells and their associated components are submerged in a dielectric coolant fluid. This method enhances the cooling efficiency by ensuring direct and uniform contact of the coolant with the battery components. The dielectric nature of the fluid means it does not conduct electricity, ensuring safe interaction with the electrical components of the battery.
Immersion-cooling technology is advantageous for a number of reasons. It eliminates the need for complex cooling channels and plates and therefore more cells can be packed within the same battery volume resulting in an increased energy density. Further, experiments have shown that immersion-cooling technology significantly increases the speed of peak charging, allowing batteries to charge much faster. It also enhances peak power density, extends battery life, and reduces the cost of battery packs.
Notably, in early 2021, MAHLE Germany introduced their own compact and efficient immersion-cooling system for EV batteries to enable fast charging in EVs.
Later in 2022, XING Mobility, a Taiwanese supplier of EV technologies, introduced the IMMERSIO" XM25 immersion-cooling technology for heavy duty vehicles. The immersion-cooling battery system was touted to be the world's first production-ready immersion-cooling battery system. Taiwanese patent TW202318711A by Xing Mobility Inc. relates to an immersion-cooled battery module and battery system. The patented Cell-To-Pack (CTP) battery system for EVs uses nickel cathode cylindrical lithium-ion cells that are cooled by mineral oil. The CTP architecture achieves a gravimetric energy density of up to 200 Wh/kg and a volumetric energy density of up to 400 Wh/l. This would allow EVs to be equipped with larger capacity batteries in almost the same space. Further, the CTP battery system also boasts of a fast charge capability of less than 15 minutes from 20-80% State of Charge (SoC).
In 2023, Ricardo engineers worked alongside TotalEnergies in the UK to retrofit a Volvo XC90 plug-in hybrid EV with an immersion alternative. Performance improvements were quickly evident as testing showed a sevenfold increase in cooling efficiency. Additionally, it was possible to reduce the vehicle's weight by 4% and costs by 5.6%. In tests for thermal runaway, the modified Volvo pack with immersion-cooling and dielectric fluid successfully prevented gas from a triggered cell from spreading to adjacent cells, stopping thermal propagation throughout the pack.
However, immersion-cooling technology for EV batteries is still in the relatively early stages of development, and current solutions are not without their downsides. Dielectric fluids in immersion-cooling have lower thermal conductivity and specific heat compared to water-glycol, requiring a higher flow rate. These fluids are often more viscous and chemically distinct, demanding more from fluid pumps and coolant systems. Additionally, typical battery modules aren't built for fluid containment, necessitating redesigns to prevent leaks and corrosion. Further, there is still concern about the weight of the coolant itself as certain heavy hydrofluoroether products often have densities over 1.5g/cm3 which is nearly 40-50% heavier than water-glycol based coolant.
XING Mobility showcased their IMMERSIO" CTP immersion-cooling battery system at CES 2024. They showcased a real sample and a video of the three-nail penetration safety test at 100% SOC in Li battery safety testing. Thermal runaway due to the punctured batteries was contained by the immersion-cooling technology within 30 seconds, restoring the temperature around the battery to normal levels within 20 seconds.
Such specifications mean that passenger vehicles can now have larger battery capacities in almost the same space, addressing range and safety concerns for drivers. The design removes battery modules, lowering weight and maintaining precise thermal management. This boosts the battery's overall energy density and prolongs its lifespan. XING Mobility has also introduced a new Cell-to-Chassis (CTC) design, tailored to the needs of both passenger and commercial vehicles.
The future of immersion-cooling technology in EVs appears promising, with its potential to revolutionise battery efficiency and reliability. As this technology continues to evolve, it's likely to become a leading choice for future EVs, offering solutions to current limitations such as range anxiety and long charging times. Its ability to enhance energy density, reduce costs, and extend battery life while ensuring safety positions immersion- cooling as a key player in the sustainable advancement of EV technology.
The ongoing innovations in this field suggest a future where EVs are more efficient, accessible, and environmentally friendly, marking a significant step forward in the automotive industry.
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