1. Introduction: Thermal Management in Electric Vehicle Batteries
Recently, there has been a strong and increasing demand for innovative manufacturing concepts for electric and hybrid vehicle batteries. The design of a battery system from lithium-ion cells presents special challenges to thermal management [1
]. As the performance and durability of the cells depend strongly on the temperature of their environment, the thermal management system has to care for the efficient dissipation of heat loss, as well as for the heat supply in case the batteries are cold. In operation, heat is generated when the system is being discharged due to accelerating, but also when charged at the charging station or during recuperation of braking energy. To avoid hot spots and to slow the thermal response within the battery pack, a large thermal mass and good inter-cell thermal conductivity are advantageous.
Heat delivery and dissipation can be provided in various ways [3
]. Liquid-cooled systems have heat exchangers joined to the cells where the cooling medium absorbs the heat and conveys it to an external chiller. The heat transfer can be accomplished, either directly from the cells into a cooled baseplate, or via active or passive cooling sheets (located in between the cells) that are in turn thermally connected to the baseplate. Both mechanical and thermal connection is usually done by mechanical joining (bolts, clamps) or various welding procedures. Alternatively, thermally conductive adhesives and thermal interface materials (TIMs) provide a novel but already proven bonding solution [4
]. There are numerous options for applying thermal adhesives and pastes in battery assembly [5
]. Prismatic (hard case) cells can be mechanically fixed onto the cooling baseplate using thermally conductive structural adhesives, as shown in Figure 1
a. When thermally connecting cooling sheets in between prismatic or pouch cells, as shown in Figure 1
b, dispensable adhesives and fillers will be advantageous over foils and pads, which will have to be manually processed. Assembling the resulting modules on the battery pack base-frame is also possible using thermal interface materials, as shown in Figure 1
2. Thermal Interface Materials
2.1. Composition of TIMs
Thermal interface materials (TIMs) are composites made up of two or more components. Because the organic matrix, which is predominantly a polymer or liquid, generally has a low thermal conductivity of about 0.1 to 0.5 W/mK, it is complemented with thermally conductive fillers, such as aluminum oxide, aluminum nitride, graphite, metal particles, or similar materials. This type of composite thus combines the advantages of the polymers, such as low weight, good processing features, and corrosion resistance, with the thermal conductivity provided by the inorganic fillers. The resulting thermal conductivity of a composite material can reach between ca. 1 and 5 W/mK and is a function of the different thermal conductivities and the volume fractions of both matrix and filler. The basic functional and processing properties of these materials include thermal resistance, thermal conductivity, rheological behavior, density, and mechanical strength.
2.2. Types of TIMs
Commercially available types of TIMs are either non-curing thermal pastes and phase change materials (PCMs), ready-to-use thermal pads, or various polymer systems, such as adhesives, gels, and silicones that cure in place by a chemical reaction. Each of them has special advantages and drawbacks. Thermally conductive pastes are well-known from computer technology. They are easy to apply and to remove, and they feature a permanent thermal contact to the substrate surface due to their good intrinsic wetting properties. The same goes for PCMs, which can additionally absorb excess heat by melting. However, voids in battery modules can reach several millimeters of gap width. This requires thixotropic materials that are mechanically stable and non-sag, especially when considering dynamic loads like operational vibrations, shock impacts due to road holes while driving, and the varying inclination of vehicles when parked. Thermal pads, in contrast, are reasonably stable against mechanical loads, and they won’t show any migration or separation during vehicle operation. However, they cannot be processed in an automated production line and they will not fit into gaps which exhibit high allowances. Thermally conductive adhesives and gels are also generally more resilient to aging than non-curing pastes but need a higher effort for metering and mixing before application. When cured, the parts are harder to disconnect in case for reworking or repairing.
3. Degradation of Thermal Interface Materials
A key risk factor in the development of thermal interface systems is the need to provide the material with sufficient thermal and mechanical stability to maintain its function, when used in a battery, during the vehicle’s lifetime of 10–15 years. Thermal interface materials are exposed to various operating conditions and environmental impacts during their service life. Both physical and chemical aging processes may occur, as shown in Figure 2
Polymer systems and composites generally have a finite lifetime under operating conditions. Chemical aging of a TIM will alter its properties, like its layer integrity, surface adhesion, elasticity, electrical insulation, thermal conductivity, and others. The alterations are due to changes in the molecular structure, the formation of new functional groups, chemical reactions between molecular components, or the breakdown of molecular bonds to form degradation products. All of these changes can be induced, or accelerated, by the absorption of energy in the form of heat. In addition, reactions with ambient media may occur. For example, organic compounds can react with oxygen from the air to form water and carbon dioxide; inorganic substances mostly form oxides. Physical, thermal, and thermomechanical loads may lead to the separation of the disperse and continuous phases [7
], agglomeration or aggregation of particles, evaporation of the continuous phase, and macroscopic changes like migration or crack formation [8
]. In particular, weakly or non-cross-linked materials tend to migrate, or separate, from fillers under vibrational stress and temperature changes. As a result, the thermal contact is interrupted under operating conditions. This can lead to the reduction, or in extreme cases, to the failure of heat transfer and therefore to a thermal overload within the battery. The following general failure mechanisms in TIMs are known or described in the literature [9
]: migration [10
], thermomechanical pump-out [11
], delamination, cracking [13
], decreasing coverage [14
], pore formation, bleeding [15
], dry-out [16
], swelling or leaching by media, and oxidation [17
In real-life operation, the influencing factors and aging mechanisms are complex. During vehicle use, the material is simultaneously subject to increased temperatures, temperature changes, shock/vibration, mechanical stress, and load changes as well as exposure to environmental media, such as atmospheric oxygen or air humidity. In the case of thermal pastes, which are classically used only for a layer thicknesses below 1 mm, the intended use in battery cooling and the resulting requirement for filling larger gaps up to several millimeters results in the additional risk of mechanical instabilities concerning inclination or vibration. Moreover, the TIM must be compatible with the common construction materials used in the automotive industry, like steel, aluminum, and various polymers and coatings, where no interactions or alterations of both the components and the TIM are allowed. Also, consideration must be given to aspects of electrochemical corrosion due to moisture loads and abrasion of the contact surfaces in reaction to vibration loads.
Unfortunately, the operational and environmental conditions within electric vehicle (EV) batteries have barely been investigated and described in the literature, and are mostly proprietary information of the manufacturers, although generally aging of batteries is of high research interest [18
] and life cycle testing of electric vehicle battery modules has been standardized [19
]. Despite ever-present operational vibrations and air moisture, batteries are subject to temperature changes resulting from both the environment and the heating/cooling during operation and charging. Figure 3
shows some temperature abundance distributions found in the literature regarding the typical operation of EV batteries within their lifetime, with most abundant temperatures typically between 10 °C and 50 °C.
The findings show that the overall lifetime prediction of TIMs used in EV batteries is complex. An iterative approach seems to be appropriate including the following steps. At first, key material properties of the TIMs under test are to be identified which would possibly deteriorate upon aging and therefore serve as failure criteria. Secondly, test procedures and respective boundary conditions have to be selected which give a reasonable representation of the environment found in the battery, which can be suitably amplified to accomplish accelerated aging. Applying these procedures, the key properties identified before must be monitored to determine and quantify possible alterations. Finally, an appropriate lifetime calculation model must be selected that responds to the respective loads (temperature, humidity, pressure, etc.) that were applied during the test. When extrapolating and drawing general conclusions from the results, additional care must be taken regarding the sample dimensions (sample area and bond line thickness) as well as the testing level applied (from bulk material test to lab or application-scale to field testing level). In summary, to obtain significant knowledge and experience regarding long-term TIM durability, there is still a strong need for research.