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The application of aerogel in Thermal Protection of Power batteries

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1. Requirements and characteristics of thermal insulation materials for power batteries
In the first ten months of 2024, production and sales reached 9.779 million and 9.75 million respectively, achieving year-on-year growth of 33% and 33.9%. In addition, it is expected that this year's annual production will exceed the 12 million vehicle mark. In 2024, there were over 1630 fires in new energy vehicles across the country, with fires caused by thermal runaway of power batteries being one of the main factors. During the use of power batteries, due to the influence of heat conduction and flames, adjacent cells may experience thermal runaway, leading to thermal runaway of the entire battery module or pack, and ultimately causing safety accidents such as car fires and explosions. After thermal runaway, the surface temperature of the battery cell can reach over 800 ℃, and the jet flame temperature can even reach over 1200 ℃. In order to block the spread of thermal runaway and reduce or prevent its harm, it is necessary to add insulation pads or insulation layers inside the battery cells, modules, and PACKs. Thermal insulation materials should have the following characteristics:
1) Long term use temperature of at least 800 ℃, short-term tolerance to high temperature of 1200 ℃
2) Low thermal conductivity to accommodate high energy density and limited insulation space of power batteries.
3) Under the expansion contraction phenomenon that occurs during the charging and discharging process of the battery cell, it has good compression performance and deformation rate (greater than 30%).
At present, researchers are committed to developing new high-performance fiber reinforced aerogel composites to further improve its thermal protection performance and mechanical strength. For example, by introducing nanomaterials such as carbon nanotubes and graphene, the thermal conductivity and mechanical properties of composite materials can be significantly enhanced. High performance fiber reinforced aerogel composites have attracted extensive attention in the field of thermal protection of power batteries due to their excellent thermal insulation performance and mechanical strength. With the rapid development of the electric vehicle market, the safety requirements for power batteries are increasingly increasing. Aerogel composites have become the ideal materials for thermal management of power batteries due to their low density, high porosity, excellent thermal stability and excellent thermal insulation performance.


2. Thermal runaway phenomenon and protective measures of power batteries
In the application of thermal protection of power batteries, aerogel composites can effectively reduce the temperature of batteries during charging and discharging, prevent overheating, thus extending the service life of batteries and improving their safety. In addition, the aerogel composite material also has good mechanical properties, which can withstand the vibration and impact that the battery may suffer during use, and ensure the structural stability of the battery pack.
The active protection strategy includes using the cooling water circuit in the heat pump system to cool or heat the three electric systems, and adjusting the temperature through active system control; The passive protection strategy mainly involves embedding insulation materials in the battery cells or module PACKs to block and delay the spread of thermal runaway, thereby improving the operational safety of the battery pack. Although proactive protective measures can effectively curb battery thermal runaway, the timing and quantity of coolant injection control are complex, technically challenging, and there is a risk of coolant leakage. Additionally, the product has high additional power consumption, resulting in expensive project implementation costs. In contrast, the method of adding insulation materials to battery cells or module PACKs is more convenient and effective. These types of insulation materials typically have low thermal conductivity, excellent fire and flame retardant properties, low density, good electrical insulation performance, and dimensional stability. Traditional insulation materials for power batteries include foam, high silica foam, ultra-fine glass wool, and vacuum insulation panels. Fiber reinforced aerogel composite is a new and efficient thermal insulation material. Its substrate is SiO2 aerogel with very low density. Compared with traditional thermal insulation materials for power batteries, fiber reinforced aerogel composites have obvious advantages in low thermal conductivity, V0 grade flame retardancy, wide range of application temperature, adjustable tensile and compressive strength, etc., and have become the preferred thermal insulation material to inhibit thermal runaway of ternary lithium-ion power batteries.
The thermal runaway phenomenon of power batteries is usually triggered by internal or external short circuits, causing the battery to release a large amount of heat in a very short period of time, resulting in a sharp rise in temperature. Once a certain battery cell experiences thermal runaway, the heat may quickly spread to adjacent cells through thermal conduction and flame propagation, causing thermal runaway of the entire battery module or battery pack, which may lead to serious consequences such as car fire or even explosion. In order to effectively address the problem of thermal runaway in power batteries, insulation pads or layers can be added inside the battery cells, modules, and PACKs to effectively block the propagation path of thermal runaway, thereby reducing or avoiding the harm caused by thermal runaway, as shown in Figure 3.
After in-depth analysis of the characteristics of the power battery, it was found that the surface temperature of the battery cell can soar to over 800 ℃ under thermal runaway, and the temperature of the jet flame may even exceed 1200 ℃. Therefore, insulation materials must be able to withstand high temperatures of 800 ℃ for a long time and cope with extreme temperatures of 1200 ℃ in the short term. Given the high energy density of power batteries, the space for insulation between battery cells is limited, which requires insulation materials with low thermal conductivity. In addition, during the charging and discharging process, the battery cells will expand and contract. In order not to affect the service life of the battery, the insulation pad usually needs to have a deformation rate of over 30% under a pressure of 1 MPa. Fiber reinforced SiO2 aerogel composite, whose base material is composed of inorganic fiber and SiO2 aerogel, has excellent high temperature resistance and low thermal conductivity. The toughening effect of fibers makes the compression performance of composite materials designable. By adjusting the arrangement and proportion of fibers, precise control of compression performance can be achieved, meeting the strict requirements of thermal insulation materials for power batteries. For example, the long-term service temperature of basalt fiber reinforced aerogel material can reach 850 ℃, and the short-term service temperature can exceed 1200 ℃. Its thermal conductivity at room temperature is only 0.0161 W/(m · K). Under the same insulation effect, its thickness is only one-fifth of traditional insulation materials. Under a pressure of 1 MPa, the deformation rate exceeds 30% and it has good resilience, which will not affect its effective insulation thickness after compression.


3. Application examples of fiber reinforced aerogel composites
Fiber reinforced aerogel composites have been widely used in the field of new energy vehicle power batteries. In December 2023, Xiaomi SU7 was officially released, and 165 pieces of fiber reinforced aerogel thermal insulation materials were filled on the side of its battery cell, which can resist up to 1000 ℃ high temperature and effectively prevent the battery from losing control of heat. Huawei Jujing battery adopts a five layer thermal safety protection design, including high-temperature resistant mica board, insulating mica paper, aviation grade fiber reinforced aerogel insulation material, nano grade ceramic insulation layer and liquid cooling cooling system. The fiber reinforced aerogel material is also used on the side of the Kirin battery cell in the Ningde era to prevent the longitudinal heat transfer between the cells. In addition, BYD has disclosed since 2022 that the application range of fiber reinforced aerogel will expand from high-end products to full range of products, mainly used in high nickel ternary lithium batteries.


4. Technological challenges and future development directions
Although fiber reinforced aerogel composites are widely used in the market, fiber reinforced aerogel gel composites in the market have a serious problem of powder loss. Due to vibration or compression during use, aerogel particles fall off and accumulate on one side of the insulation sheet, resulting in inconsistent and greatly weakened thermal insulation performance. How to enhance the cross-linking between fiber and aerogel by chemical cross-linking method is a technical difficulty to solve the problem of powder shedding of fiber reinforced aerogel in the future. Thermal insulation materials can delay the propagation of thermal runaway from the target cell to adjacent individual cells, but to some extent reduce the heat dissipation effect. Therefore, it is still necessary to pay attention to timely transferring heat to the external system to avoid exacerbating the risk of thermal runaway due to continuous heat accumulation. In addition, the use of insulation materials inevitably reduces the energy density of battery packs. Therefore, it is necessary to optimize material selection, thickness selection, and layout selection, taking into account their energy density and safety, and comprehensively consider and design insulation materials reasonably. At the same time, the use of insulation pads reduces the expandable space of the battery cells during long-term charging and discharging cycles, which needs to be considered comprehensively and reasonably. In addition, fiber reinforced aerogel composites are produced by supercritical drying. However, the supercritical high-pressure drying method and equipment are complex, the high-pressure process is dangerous, and the cost is high. Improving the supercritical drying equipment and process, or seeking a one-step atmospheric pressure drying method for production, will greatly reduce the production cycle and cost of composite materials, which is one of the key research directions in the future.

5 Reference Materials
[1] Yang Xu, Su Yuefeng, Ren Yong, etc Application progress of high-performance fiber reinforced aerogel composites in the field of thermal protection of power batteries [J]. Equipment Environmental Engineering, 2024,21 (07): 169-182
[2] Thermal insulation material for new energy battery cell - aero gel Zhihu (2003)


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