Thermal management mode of pure electric vehicle: battery immersion cooling

In the new application of automobile electrification, it is very important to cool and heat the electrical components to keep them at the optimum working temperature, because this can guarantee the service life and efficiency of the electrical components. Therefore, an appropriate thermal management system is essential. In other words, it is necessary to design a suitable thermal management system for the electrical components used.
If the operating temperature of the battery is too high, it may lead to the loss of battery capacity, and in extreme cases, it may lead to thermal runaway. If the operating temperature of the battery is too low, it may lead to lower battery efficiency, higher resistance, lower battery capacity and formation of lithium dendrite (lithium plating layer). Lithium plating will lead to accelerated aging and failure of the battery core.
The goal of thermal management is to ensure that the system is at the optimal operating and safe temperature. More complex is that the optimal temperature of the battery system may change with the change of the operation mode. The optimum temperature for fast charging may be different from that for driving or parking (parking).
At present, the battery thermal management systems used mainly include gas cooling, indirect liquid cooling, direct liquid cooling (also known as immersion cooling) and phase change materials. [1]

Description of various cooling systems
Air cooling systems are most widely used because of their simple design, low cost and no leakage. Air cooling is divided into active type with forced convection and passive type with natural convection. Compared with liquid and other media, air has small thermal capacity (Cp=1.006 kJ/kgK at standard temperature) and low thermal conductivity, so air cooling is unlikely to become the preferred technology for the next generation of electric vehicles with larger battery packs and faster charging speed. [1]
Liquid cooling can be divided into indirect and direct modes. Compared with air, coolant has greater thermal capacity and higher thermal conductivity. Due to the balanced temperature control, indirect liquid cooling is currently one of the most commonly used solutions in battery thermal management. The most commonly used coolant is a mixture of water and glycol. The principle of indirect cooling is to let the coolant flow through the channel at the bottom or side of the cell/battery module to transfer heat from the system.
The cooling effect can be improved by using specific thermal interface materials (TIM).
Compared with air cooling, the disadvantage of indirect liquid cooling is the complexity of the system. More components and channels/pipelines may cause more failures, extra weight and leakage problems.
Another emerging cooling technology is direct liquid cooling, also known as immersion cooling, which completely immerses the battery in the dielectric liquid. This is a non-conductive liquid with high resistance to electrical breakdown. The introduction of this technology means that the complexity of battery process and component design can be greatly reduced, and it also helps to reduce the weight and volume of the system, significantly improving the stability and balance of battery temperature control. Immersion cooling can heat or cool the battery as required without using heat exchanger, which brings significant efficiency improvement. Immersion cooling of electric vehicle batteries is still in its infancy, but there have been some use cases, such as the patented full-immersion battery core system of Faraday Future [2], the immersion cooling technology of the Dakar Rally car Audi RS Q e-tron [3], or the IMMERSIO created by Panasonic and Tesla's former employees ™ System [4].

 Immersion cooling system for Dakar Rally in Audi RS Q e-tron
The dielectric liquid commonly used in immersion cooling is flame retardant, which can inhibit the thermal runaway event. At present, there are several groups of cooling media available on the market - hydrofluoroether, hydrocarbon oil, silicone oil and fluorinated hydrocarbon. People pay more and more attention to biodegradable dielectric liquids.
The characteristics of coolant play an important role in thermal management and should meet the following requirements:
-Good electrical insulation
-High specific heat capacity and high thermal conductivity
-Nonflammable and/or high flash point
-Easy to produce and can be supplied in large quantities
-There is a suitable operating temperature range
-Long shelf life of liquid
In addition to the above requirements, material compatibility, low density, low viscosity and environmental protection must also be considered when selecting the appropriate immersion coolant.

Description of various coolants
Hydrofluoroethers - Hydrofluoroethers from power electronics applications have received great attention in the field of immersion cooling of pure electric vehicles. The literature shows that its cooling efficiency is significantly improved compared with the air cooling system. Due to the non-flammability of hydrofluoroether and almost no flash point, the system safety has also been improved. The performance of this hydrofluoroether immersion cooling system in the whole life cycle remains to be studied. In addition, the density of HFE is about 40% higher than that of water-glycol system, which has a negative impact on the weight and range of pure electric vehicles. In addition, material cost and environmental protection are also important factors that hinder the use of hydrofluoroether in the thermal management system of electric vehicles.
Hydrocarbons - including mineral oil, polymer α- Olefin (PAO) and synthetic hydrocarbon oil. Hydrocarbons are distillation products of petroleum, which makes them low cost and low toxicity, and suitable for immersion cooling in the appropriate operating temperature range. The disadvantage of these liquids is that they may be flammable and have a flash point.

Esters - Because of their low cost, high flash point, good dielectric properties and biodegradability, they have been widely used in various industries. Esters are divided into synthetic and natural. Synthetic ester is the product of chemical reaction between polyol and carboxylic acid, while natural ester is the product of vegetable oil. The two sources also correspond to different attributes. The synthetic ester has good oxidation stability, which has a positive impact on extending the vehicle maintenance period; However, compared with natural ester, its flash point is usually lower. The disadvantage of ester based system is that the viscosity increases with the aging of the material, and then its cooling capacity decreases.
The main advantage of silicone oil - silicone oil is that it has good temperature resistance and dielectric properties at high and low temperatures.
Water/glycol - mixture of water and glycol. Compared with other systems, water/glycol has relatively high conductivity and thermal conductivity, and has cost advantages. However, the conductivity of water-type glycol mixture limits its use only for indirect liquid cooling. In the process of use, the sealing measures of indirect liquid cooling system are very important, because this can prevent the mixture from leaking to the battery or wiring, and then can prevent short circuit and final thermal loss of control. [5]

In short, there are many solutions for thermal management; At Detweeler, we believe that the thermal management system of pure electric vehicles can be improved through the proper use of materials and composites. With the steady development of the mobile travel industry in the direction of electrification, the focus now is to support the coolant manufacturers, suppliers at all levels and OEMs to upgrade their thermal management systems; In this regard, the material expertise of Detweller has been applied at a high level. Chemical compatibility is the key to the direct immersion cooling method, because different coolant may lead to different sealing solutions.
When selecting the best sealing solution for each immersion cooling system, tests must be carried out. As far as the electric vehicle industry is concerned, perfluoropolyether (PFPE) seems to be at the forefront of coolant application because of their non-flammable and low viscosity characteristics. However, no matter what kind of coolant is used on electric vehicles, it is necessary to ensure compatibility with sealing solutions to avoid corrosion/degradation of seals over time in this harsh environment and prevent the resulting problems. [1]
Safety is critical

The most important safety problem of battery system is to prevent fire and thermal runaway. Because the battery system with immersion cooling is directly immersed in the coolant, the whole area must be sealed with special sealing elastomer components, and the selection of the elastomer must be correct. It is better to have a certain level of chemical resistance and weather resistance throughout the vehicle's service life cycle,.
At Detwelle, we are conducting experiments and accurate analysis on various types of coolant in contact with different materials; Through these tests, we can determine which types of polymers or elastomers are most suitable for sealing these liquids.
Even in the simulation analysis stage, an important parameter that needs to be controlled is thermal aging. The material is tested at high temperature (up to 100 ° C) for a long time (up to 1000 hours). According to the first group of tests for PFPE, silicone oil and seed oil, the formula based on IC-DAT10 and IC-DAT30 of DETWELLER performs well in most coolants and should generally be used as the choice of sealing materials. Each IC-DAT code represents different elastomer series, so this study compares the performance of different polymer series in the selected liquid.
According to this strict test process, the physical properties of the material before and after immersion in these liquids were compared. The volume change shown in the figure below is used as an indicator of chemical stability over time after immersion in coolant. In addition, we also checked and understood the thermal stability, tensile property changes, compression set and leakage.

 Chemical stability results of standard rubber formula immersed in different types of coolant (seed oil, silicone oil and various PFPE) for immersion cooling system
In the second group of tests shown in Figure 2, we focus on halogen-free, non-toxic biological base and biodegradable coolant. All measured coolants are divided into two categories with global warming potential (GWP) of 0 and<1. In addition, this study also includes so-called mixed liquids, which not only provide cooling effect, but also provide lubrication effect for the propulsion system.
The volume change of the standard rubber formula after immersion in these liquids shows that most of the tested rubber formulas have obvious changes. It can be seen that among all the tested materials, only the formula with the code of IC-DAT41 is the suitable candidate formula.

 Chemical stability results of standard rubber formula immersed in specific type of bio-based coolant for immersion cooling system
It is speculated that the polarity of the sealing material will affect the chemical stability of the sealing element. In addition to IC-DAT41, Detweller is studying other candidate materials suitable for use as bio-based immersion coolants. Detweller uses different proprietary formulations for testing, and finally selects which formulation is the best sealing material for the specific coolant tested.