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

The power battery system is generally mainly composed of a battery module, a battery management system BMS, a thermal management system, and some electrical and mechanical systems. Factors affecting the large-scale promotion and application of new energy vehicles include battery system cost, cruising range and battery system safety. With the development of new energy vehicle technology, safety has been paid more and more attention. Power lithium-ion batteries are prone to cause thermal runaway due to chain exothermic reaction under overcharge, acupuncture and collision, resulting in smoke, fire and even explosion. At the same time, the performance of the power battery, including energy density and service life, is affected by temperature changes, so the importance of thermal management is further reflected.

First, the importance of thermal management

Under different driving conditions, the single cell has a certain internal resistance due to its own internal resistance, which will generate a certain amount of heat while outputting electric energy, so that its own temperature becomes higher, when its temperature exceeds its normal working temperature range, Affects battery performance and life. The power battery system on the electric vehicle is composed of a plurality of power battery cells. The power battery system generates a large amount of heat accumulated in a small battery box during the working process, and if the heat cannot be quickly dissipated in time, the high temperature Will affect the life of the power battery or even thermal runaway, resulting in fire and explosion.

At present, domestic thermal management research pays more attention to heat dissipation, more specifically to the battery system cabinet and module level, such as the application of liquid cooling system. However, there is not much attention paid to the prevention and control of heat insulation at the cell level. It can be seen from the design of the power battery system that the structure of the battery cell and the battery module must be considered in the design of the thermal management system. Therefore, in the overall design of the battery system, it is necessary to consider the influence of the temperature environment of the cell unit and the location of the battery module. Therefore, when designing the battery module arrangement, if the cells are arranged in a compact manner and there is no heat dissipation and heat insulation measures, the temperature of the battery pack will rise sharply during charging and discharging, and there is a serious safety hazard.

Therefore, it is necessary to study the battery thermal management technology to enhance the heating and heat dissipation capability of the battery, ensure that the battery works within a suitable temperature range and maintain a reasonable temperature distribution within the battery box. The research needs to gradually expand from the monomer-level thermal runaway generation mechanism and characteristics to the thermal runaway level triggered by the thermal runaway of the monomer and then propagated to the entire battery system.

Second, the difference between the presence or absence of insulation measures

Previous studies have shown that a thermal insulation layer is placed between the battery cells to block the heat transfer of the uncontrolled monomer to the adjacent cells. At the same time, the heat insulation layer is not completely closed, and a convection channel is left between the cells, which is beneficial to the runaway single. The heat generated by the body dissipates heat throughout the battery pack to avoid local overheating. In the "Integration Study on Thermal Protection and Heat Dissipation of Vehicle Power Battery", four schemes are set for thermal performance analysis during thermal runaway. Scheme 1 represents no heat dissipation and thermal insulation between battery cells, and Scheme 2 represents battery cells. Place the heat insulation board, the third method represents the heat pipe group placed between the battery cells, and the plan 4 represents the insulation board and the heat pipe group.

The heat dissipation and thermal insulation performance of the battery pack under normal conditions and thermal runaway were analyzed. The thermal management performance of the integrated system was verified and the effect of the thickness of the thermal insulation board on thermal runaway propagation was investigated as follows:

(1) The comparison of the four schemes shows that the thermal resistance of the scheme 2 is outstanding, which can effectively delay the thermal runaway propagation, but the heat dissipation performance is poor. The thermal insulation board and natural heat dissipation alone cannot meet the thermal management requirements of the battery pack. Scheme 3 has good heat dissipation performance, but as the discharge rate increases, the maximum temperature difference rises sharply. At the same time, the thermal resistance after the thermal runaway trigger is much lower than that of the second and fourth schemes. The scheme 4 not only greatly enhances the heat dissipation capability of the battery pack and the temperature uniformity of each unit in the battery pack, but also has high heat insulation performance to effectively block the thermal runaway propagation.

(2) By changing the thickness of the heat shield and enhancing the heat dissipation capability of the battery pack, the thermal runaway propagation can be effectively blocked. When the thickness of the heat shield is increased from 1mm to 2mm, the thermal runaway can be blocked before the heat shield under the premise of ensuring the normal operation of the heat pipe.

(3) The combination of reasonable insulation measures and cooling methods can not only effectively improve the stability of the operating temperature range of the battery pack, but also effectively block the thermal runaway.

More classic is the General Motors Volt's battery thermal management system uses liquid cooling. A metal heat sink (thickness: 1 mm) is disposed between the cells, and a capillary structure is left on the heat sink so that the coolant can flow in the capillary to remove heat and achieve heat dissipation. The insulation scheme uses a way of placing foam between the cell and the cell.

Third, the application of foam

The battery system and the module are generally designed according to the structural shape of the battery core, and the battery cells are mainly divided into three types: a cylindrical battery core, a square battery core, and a soft package. Soft-package cells have a higher energy density than the other two, so the application of soft-packages will be relatively increased under the influence of the energy subsidy policy. The advantage of the soft package is that the external structure has little influence on the battery core, the battery core has excellent performance, and the quality of the material used for the package is small. However, the shortcomings are also obvious, and the difficulty of sealing the large-capacity battery is increased 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 bulging of the soft bag during charging and discharging. If the soft bag and the soft bag are directly rubbed for a long time, the aluminum plastic film may be damaged, resulting in battery failure or even out of control. Therefore, the application of foam in the middle of soft-packed batteries is very important, which is manifested in the following four aspects:

1. The foam has low hardness and high resilience, which can absorb the bulging stress of the battery and play a buffering role;

2. When the cell sends heat out of control, the foam can act as a heat insulator to inhibit heat diffusion and delay the accident;

3. When the battery core is on fire, the flame retardant effect of the foam can delay the spread of fire and increase the escape time;

4. Foam has excellent resilience and wide compression ratio, which can be used as positioning.

There are many types of foam, including PU, CR, EVA and PE. In the actual application process of the battery system, it was found that only foams capable of maintaining sufficient elastic recovery ability under long-time compression environment, such as PU, are suitable for use between soft-pack batteries, and others such as CR recover after long-term compression. Poor capacity leads to a situation in which the module structure is in a falling position. Therefore, in the application of foam in the module design, it is necessary to consider the elastic modulus of the foam and the rebound rate.

In addition, through the disassembly analysis of the VOLT module structure, the applied foam does not appear to be self-extinguishing from the fire, which means that it is not the V0 required by the national standard, which is also an interesting place. Perhaps it has a very good insulation capacity to match the effect of the liquid cooling system, which can avoid the occurrence of thermal runaway. Or if there is thermal runaway, anyway, it has already burned, and the flame retardant will not play much role. It is better to do better than its flame retardant.