Power CR2032 button cell battery pack process series - Introduction to the basics of thermal conductive adhesive

The simple mechanical assembly method has exposed more and more shortcomings and cannot meet the requirements of the ever-increasing safety of power lithium batteries. Adhesive assembly or cooperative assembly makes up for the shortcomings of mechanical assembly. The types of adhesives used in power lithium battery assembly include structural adhesives, thermal conductive adhesives, solder joint protection adhesives and sealants, etc. Adhesives are used in many aspects to improve the performance and safety of power lithium batteries. The purpose of using adhesives can be roughly divided into four categories: fixation, heat transfer, flame retardancy, and shockproof, and the specific forms of adhesive use include gaskets, potting, and filling.

Let's start with the basic properties of thermal conductive adhesives today.

In thermal design, it is often necessary to consider the balance between battery charging and discharging power and heat generation and heat dissipation capabilities. The performance of lithium-ion batteries is extremely sensitive to temperature. Obtaining an appropriate operating temperature is of great significance to fully exerting battery performance and maintaining a reasonable battery life. Reasonable selection of heat transfer media should not only consider its heat transfer capacity, but also take into account the process, maintenance operability, and excellent cost performance in production. Let's start with the principle.

Why does thermal conductive adhesive conduct heat?

Thermal conductive adhesive is mainly composed of resin matrix [EP (epoxy resin), silicone and PU (polyurethane), etc.] and thermal conductive filler. The type, dosage, geometry, particle size, mixed filling and modification of thermal conductive filler all have an impact on the thermal conductivity of thermal conductive adhesive. Thermal conductive principle of thermal conductive adhesive: The thermal conductive carrier inside the solid is mainly electrons and phonons (in dielectrics, heat conduction is achieved through the vibration of the lattice, and the energy of the lattice vibration is quantized. This lattice vibration quantum is called phonon). There are a large number of free electrons inside the metal, and heat can be transferred through the mutual collision between electrons; inorganic non-metallic crystals conduct heat through the thermal vibration of neatly arranged grains, which is usually described by the concept of phonons; since amorphous solids can be regarded as crystals with extremely fine grains, the thermal conductivity of amorphous solids can also be analyzed by the concept of phonons, but its thermal conductivity is much lower than that of crystals; most polymers are saturated systems without free electrons, so adding high thermal conductive fillers to adhesives is an important method to improve their thermal conductivity. Thermally conductive fillers are dispersed in the resin matrix and contact each other to form a thermal network, so that heat can be quickly transferred along the thermal network, thereby achieving the purpose of improving the thermal conductivity of the adhesive.

What are the general forms of thermal conductive adhesives?

In order to adapt to various environments and requirements, there are proper countermeasures for possible thermal conductivity problems. There are many subdivided types of thermal conductive products, which are not limited to application scenarios in power lithium battery systems.

1) Phase change thermal insulation materials use the characteristics of the substrate to undergo phase change at the working temperature, so that the material fits the contact surface more closely, and also obtains ultra-low thermal resistance, which allows for smoother heat transfer. It can be used to fill the gap between modules and transfer heat to the outside of the module.

2) Thermally conductive pads

Thermally conductive materials with high thermal conductivity and low resistance are generally used inside electronic appliances. Their thermal conductivity and the flexibility of the material itself are well suited to the heat dissipation and installation requirements of power devices.

3) Thermal conductive tape is used for bonding between heating devices and radiators. It can achieve the functions of heat conduction, insulation and fixation at the same time, reduce the size of the equipment, and is an option to reduce equipment costs.

4) Thermally conductive insulating elastic rubber has good thermal conductivity and high-level pressure resistance, which meets the current demand of the electronics industry for thermal conductive materials. It is the best product to replace the binary heat dissipation system of silicone grease thermal paste plus mica sheet. This type of product is easy to install, conducive to automated production and product maintenance, and is a new material with great processability and practicality.

5) Flexible thermal conductive pad is a thicker thermal conductive pad specially produced for the design method of transferring heat through gaps. It can fill the gaps and complete the heat transfer between the heating part and the heat dissipation part. At the same time, it can also play a role in shock absorption, insulation, sealing and other purposes. This is very suitable for the application inside the battery module. 6) Thermal conductive filler can also be used as thermal conductive glue, which not only has the effect of heat conduction, but also is a bonding and sealing potting material. The heat of the heating component is transferred by filling the contact surface or can-shaped body. Cylindrical battery module is a typical application.

7) Thermally conductive insulating potting glue Thermally conductive insulating potting glue is suitable for potting electronic components with high requirements for heat dissipation. After curing, the glue has good thermal conductivity, excellent insulation, excellent electrical properties, good adhesion and good surface gloss. However, if the amount of glue is too large, the energy density of the battery pack will be lowered.

What are the factors affecting the performance of thermally conductive glue?

The thermal conductivity of the filled adhesive depends mainly on the resin matrix, the thermally conductive filler and the interface formed by the two, and the type, amount, particle size, geometry, mixed filling and surface modification of the thermally conductive filler will affect the thermal conductivity of the adhesive.

1) Type and amount of thermally conductive filler

The type and amount of filler will affect the thermal conductivity of the adhesive. When there is less filler, the filler is completely wrapped by the matrix resin, and most of the filler particles are unable to directly contact each other; at this time, the adhesive matrix becomes a heat flow barrier between the filler particles, inhibiting the transmission of filler phonons, so no matter what kind of filler is added, the thermal conductivity of the adhesive cannot be significantly improved. As the amount of filler increases, the filler gradually forms a stable thermal conductivity network in the matrix. At this time, the thermal conductivity increases rapidly, and filling with high thermal conductivity fillers is more conducive to improving the thermal conductivity of the adhesive. However, too high thermal conductivity of the filler is not conducive to improving the thermal conductivity of the system. Studies have shown that when the ratio of the thermal conductivity of the filler to the matrix resin exceeds 100, the improvement of the thermal conductivity of the composite material is not significant.

The data shown in the previous research example are used to illustrate the relationship between the amount of filler and the heat transfer performance. After adding high thermal conductivity fillers to the adhesive, the thermal conductivity of the composite material increases significantly with the increase in the amount of filler. Studies have shown that when w (artificial diamond SD) = 20% (relative to the mass of epoxy resin EP), the thermal conductivity is 0.335W (/mK); when w (SD) = 50%, the thermal conductivity is 1.07W (/mK), which is 3.5 times higher than that of pure resin; when w (SD) < 20%, the thermal conductivity of the system increases slowly; when w (SD) > 20%, the thermal conductivity of the system rises rapidly. This is because when w (SD)> 20%, the particles begin to contact each other and gradually form a thermal conductive chain; when w (SD) = 50%, the particles contact each other in large quantities to form a thermal conductive network, so the thermal conductivity is significantly improved.

2) Particle size and geometric shape of thermal conductive fillers

When the amount of filler is the same, nanoparticles are more conducive to improving the thermal conductivity of the adhesive than micron particles. The quantum effect of nanoparticles increases the number of grain boundaries, thereby increasing the specific heat capacity and covalent bonds become metal bonds, and heat conduction changes from molecular (or lattice) vibration to free electron heat transfer, so the thermal conductivity of nanoparticles is relatively higher; at the same time, the particle size of nanoparticles is small and the number is large, resulting in a large specific surface area, which is easy to form an effective thermal conductive network in the matrix, so it is conducive to improving the thermal conductivity of the adhesive. For micron particles, when the amount of filler is the same, the specific surface area of the thermal conductive filler with a large particle size is small, and it is not easy to be wrapped by the adhesive, so the probability of connecting with each other is greater (it is easier to form an effective thermal conductive path),

which is conducive to improving the thermal conductivity of the adhesive. A specific case, the study shows that: when the amount of filler is the same, the thermal conductivity of the Al2O3 system containing 30nm is relatively the highest, the thermal conductivity of the Al2O3 system containing 20μm is second, and the thermal conductivity of the Al2O3 system containing 2μm is relatively the lowest. This is because when the amount of filler is the same, the specific surface area of nanoparticles is larger than that of micron particles, and the huge specific surface area makes it more likely to form a thermal network than micron particles; for the 20 and 2μm Al2O3 filling systems, smaller particle sizes have larger specific surface areas, more phase interfaces in contact with the matrix, and are more likely to be wrapped by the matrix, unable to form an effective thermal network, so the thermal conductivity of the 2μm Al2O3 filling system is relatively the lowest.

When the amount of filler is the same, the probability of the same filler of different geometric shapes forming a thermal network in the matrix is different. Thermally conductive fillers with larger aspect ratios are more likely to form a thermal network, which is more conducive to improving the thermal conductivity of the matrix. The above figures show that when φ(nanoscale silver wire) = 26% (relative to the volume of epoxy resin EP adhesive), the percolation threshold is reached, and the thermal conductivity increases from 5.66W (/mK) to 10.76W (/mK); when φ(nanoscale silver rod) = 28% and φ(nanoscale silver block) = 38%, the percolation threshold is reached; the larger the aspect ratio, the smaller the percolation threshold. Compared with silver rods and silver blocks, silver wires with large aspect ratios increase the probability of forming a thermal conductive network chain in the resin system due to their orientation, and a higher thermal conductivity can be achieved when the filler is less.

3) Mixed filling of thermal conductive fillers

Compared with a filler filling system with a single particle size, mixed filling of fillers of the same type with different particle sizes is more conducive to improving the thermal conductivity of the adhesive. Mixed filling of the same filler in different forms is easier to obtain an adhesive with high thermal conductivity than filling with a single spherical filler. When different types of fillers are properly proportioned, mixed filling is also better than filling with a single type of filler. This is attributed to the fact that the above-mentioned mixed fillings are easy to form a densely packed structure, and when mixed filling, the high aspect ratio particles are easy to bridge between spherical particles, thereby reducing the contact thermal resistance, and thus making the system have a relatively higher thermal conductivity. Studies have shown that when w (AlN) = 80% (relative to the mass of silicone rubber) and the particle sizes are 15 and 5 μm, the thermal conductivity of the system is 1.83 and 1.54 W (/mK), respectively; when the total amount of AlN is kept constant and the mass ratio of the two particle sizes is 1:1, the thermal conductivity of the system is 1.85 W (/mK). The thermal conductivity of large and small particle size doping is higher than that of a single particle size. This is because when large and small particle size doping, small particle size particles are easier to fill into the gaps of large particle size particles (density increases), making the contact between particles closer, and the arrangement density of fillers inside the matrix is increased (reducing contact thermal resistance), thereby adding thermal conductivity to the system.

4) Surface modification of thermally conductive fillers

There is a polarity difference between the interface of inorganic particles and the resin matrix, resulting in poor compatibility between the two, so the fillers are easy to aggregate in the resin matrix (not easy to disperse). In addition, the large surface tension of inorganic particles makes it difficult for the surface to be wetted by the resin matrix, and there are gaps and defects between the phase interfaces, thereby increasing the interfacial thermal resistance. Therefore, modifying the surface of inorganic filler particles can improve their dispersibility, reduce interface defects, enhance interfacial bonding strength, inhibit the scattering of phonons at the interface and increase the propagation free path of phonons, which is beneficial to improve the thermal conductivity of the system.

A thermal conductive adhesive influence test

By using experiments and simulations to check each other, the discharge requirements of power lithium batteries under different working conditions are compared, and the maximum temperature and maximum temperature difference of the system are investigated.

The case comparison and analysis of three working conditions: uniform speed driving, continuous acceleration and NEDC. The temperature rise and temperature difference of the battery pack with thermal conductive adhesive filled in the gap between the battery cells are significantly smaller than those of the battery pack with air in the gap. It can be seen that thermal conductive adhesive has obvious uses in reducing the temperature rise of battery packs and balancing the temperature field of battery packs. When conducting thermal design of battery packs, if the battery pack structure cannot be changed, the temperature rise and temperature difference of the battery pack can be reduced by filling thermal conductive adhesive between battery cells. In the case where the battery pack structure can be changed, the battery pack can be placed in a suitable working environment by changing the battery pack structure and filling thermal conductive adhesive between battery cells. When electric vehicles accelerate, the temperature rise and temperature difference of the battery are small due to the short acceleration time, that is, when the battery pack is discharged with a large current for a short time. However, when driving at a high speed and at a constant speed, the temperature rise and temperature difference of the battery pack increase significantly due to the accumulation of heat and long-term constant current discharge. The factory currently has 500 employees and its annual output can meet the demand of 200,000 new energy vehicles. However, new energy vehicles include hybrid vehicles, plug-in hybrid vehicles and pure electric vehicles, so it is not clear what the proportion of each type of vehicle is among the 200,000 new energy vehicles. Panasonic said that this new plant, which was built with an investment of 50 billion yen (about 2.96 billion yuan), can not only meet the rising market demand in my country, but also supply battery products to the North American market. This square power lithium battery is safer and is expected to be sold all over the world in the future to promote the development of new energy vehicles.

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