Magical battery cell foam! Talking about the application of thermal management materials in battery systems

The power battery system is generally composed of battery modules, battery management system BMS, thermal management system, and some electrical and mechanical systems. At present, factors affecting the large-scale promotion and application of new energy vehicles include battery system cost, driving range, and battery system safety. With the development of new energy vehicle technology, safety has received increasing attention. Power lithium-ion batteries are prone to chain exothermic reactions under overcharging, puncture, and collision, causing thermal runaway, smoke, fire, and even explosion. At the same time, the performance of power batteries, including energy density and service life, is affected by temperature changes, so the importance of thermal management is further reflected.

1. The Importance of Thermal Management

Under different driving conditions, the single cell will generate a certain amount of heat while outputting electricity due to its own internal resistance, which will increase its own temperature. When its own temperature exceeds its normal operating temperature range, it will affect the performance and life of the battery. The power battery system on electric vehicles is composed of multiple power battery single cells. The power battery system generates a lot of heat during operation and accumulates in the narrow battery box. If the heat cannot be dissipated quickly and in time, the high temperature will affect the life of the power battery and even cause thermal runaway, leading to fire and explosion.

At present, domestic thermal management research focuses more on heat dissipation, or more accurately, on the level of battery system boxes and modules, such as the application of liquid cooling systems. However, not much attention has been paid to thermal insulation prevention and control at the cell level. From the design of the power battery system, it can be seen that the structures of the two levels of battery cells and battery modules need to be considered when designing the thermal management system. Therefore, the influence of the temperature environment of the battery cells and battery modules must be taken into account in the overall design of the battery system. Therefore, when designing the arrangement of battery modules, if the single cells are arranged compactly and there are no heat dissipation and insulation measures, the temperature of the battery pack will rise sharply during charging and discharging, posing serious safety hazards.

Therefore, it is necessary to study battery thermal management technology to enhance the heating and heat dissipation capabilities of the battery, ensure that the battery operates within a suitable temperature range, and maintain a reasonable temperature distribution in the battery box. The research needs to be gradually expanded from the mechanism and characteristics of thermal runaway at the single-cell level to the thermal runaway level triggered by thermal runaway of the single cell and then propagated to the entire battery system.

2. The difference between having or not having insulation measures

Studies have shown that setting a heat insulation layer between battery cells can block the heat transfer from the runaway cell to the adjacent cell. At the same time, the heat insulation layer is not completely closed, and there are convection channels between the cells, which is conducive to the heat generated by the runaway cell to be dissipated in the entire battery pack to avoid local overheating. In the "Research on Thermal Protection and Heat Dissipation Integration of Automotive Power Batteries", four schemes are set up to analyze the thermal performance during thermal runaway. Scheme 1 means that no heat dissipation and heat insulation measures are added between battery cells, Scheme 2 means that heat insulation plates are placed between battery cells, Scheme 3 means that heat pipe groups are placed between battery cells, and Scheme 4 means that heat insulation plates and heat pipe groups are placed in a staggered manner between cells.

The heat dissipation and thermal insulation performance of the battery pack under normal operating conditions and thermal runaway under four schemes were analyzed, the thermal management performance of the integrated system was verified by comparison, and the barrier effect of the thickness of the heat shield on the propagation of thermal runaway was explored. The conclusions are as follows:

(1) The comparison of the four schemes shows that Scheme 2 has outstanding heat resistance and can effectively delay the propagation of thermal runaway, but its heat dissipation performance is poor. Relying solely on heat insulation panels and natural heat dissipation cannot meet the thermal management requirements of the battery pack. Scheme 3 has good heat dissipation performance, but the maximum temperature difference rises sharply as the discharge rate increases. At the same time, the heat resistance performance after thermal runaway is triggered is much lower than that of Scheme 2 and Scheme 4. Scheme 4 not only greatly enhances the heat dissipation capacity of the battery pack and the temperature uniformity of each cell in the battery pack, but its high thermal insulation performance can also effectively block the propagation of thermal runaway.

(2) By changing the thickness of the heat shield, the heat dissipation capacity of the battery pack can be enhanced, which can effectively block the propagation of thermal runaway. When the thickness of the heat shield increases from 1 mm to 2 mm, thermal runaway can be blocked before the heat shield while ensuring the normal operation of the heat pipe.

(3) Reasonable thermal insulation measures combined with cooling methods can not only effectively improve the stability of the battery pack's operating temperature range, but also effectively prevent thermal runaway.

A more classic example is the battery thermal management system of the Volt of General Motors, which uses liquid cooling. A metal heat sink (1mm thick) is placed between the cells, and a capillary structure is left on the heat sink so that the coolant can flow in the capillary to take away the heat and achieve the purpose of heat dissipation. The heat insulation solution uses foam placed between the cells.

3. Application of Foam

Battery systems and modules are generally designed according to the structural shape of the battery cell. The battery cell monomers are mainly divided into three types: cylindrical battery cells, square battery cells and soft packs. Since the energy density of soft pack batteries is higher than the other two types, the application of soft packs will increase relatively under the influence of energy subsidy policies. The advantages of soft packs are that the external structure has little impact on the battery cell, the battery cell performance is excellent, and the quality of the material used for packaging is relatively small. However, the disadvantages are also obvious. The sealing process of large-capacity batteries is more difficult and the reliability is relatively poor. In addition, the aluminum-plastic composite packaging film used has low mechanical strength, and the life of the aluminum-plastic composite film restricts the service life of the battery.

Therefore, it is necessary to consider the swelling of the soft pack during charging and discharging. If the soft packs rub against each other for a long time, the aluminum-plastic film may be damaged, causing battery failure or even loss of control. Therefore, the application of foam in the soft pack battery core is very important, which is reflected in the following four aspects:

1. Foam has low hardness and high resilience, which can absorb the battery expansion stress and play a buffering role;

2. When the battery cell is in thermal runaway, the foam can act as a heat insulator, inhibit heat diffusion, and delay the occurrence of accidents;

3. When a fire occurs in the battery cell, the flame retardant effect of the foam can delay the spread of the fire and increase the escape time;

4. Foam has excellent resilience and a wide compression ratio, and can be used for positioning.

There are many types of foam, including PU, CR, EVA and PE. In the actual application of battery systems, it is found that only foams that can maintain sufficient elastic recovery ability under long-term compression, such as PU, are suitable for use between soft-pack batteries. Others, such as CR, have poor recovery ability after long-term compression, causing the module structure to fall apart. Therefore, when designing and applying foam in modules, it is necessary to consider the elastic modulus and rebound rate of the foam.

In addition, through the disassembly and analysis of the VOLT module structure, the foam used did not extinguish itself when leaving the fire, which also means that it is not V0 as required by the national standard. This is also an interesting point. Perhaps it is because its heat insulation ability is quite good and the effect of the liquid cooling system can avoid the occurrence of thermal runaway. Or if thermal runaway occurs, it has already burned anyway, and flame retardancy will not play a big role. Instead of flame retardancy, it is better to do a good job of heat insulation.