Research and Analysis on Thermal Diffusion Protection Methods for Power Batteries—Insulation and Flame-Retardant Materials

Research and Analysis on Thermal Diffusion Protection Methods for Power Batteries—Insulation and Flame-Retardant Materials

I. Introduction
In recent years, safety accidents caused by battery thermal runaway in new energy vehicles have been occurring frequently. The safety of power batteries has become a key concern for automobile manufacturers, the market, and relevant departments. To promote the improvement of this issue, the Ministry of Industry and Information Technology (MIIT) issued the "Normative Conditions for the Automobile Power Battery Industry" in 2015. Since then, the MIIT, the Ministry of Environmental Protection, the Ministry of Commerce, and the General Administration of Quality Supervision, Inspection and Quarantine have all raised standards for new energy vehicles, reflecting the government’s high emphasis on power battery safety, especially the safety protection of battery thermal runaway.

II. Factors Affecting Thermal Runaway
The safety issues of power batteries can be summarized as “thermal runaway,” which refers to the uncontrollable rise in temperature within the battery cell once a certain temperature is reached. This can lead to rapid temperature increase, combustion, and even explosion. Key factors triggering thermal runaway include overheating, overcharging, internal short circuits, and external impacts. Unreasonable battery selection and thermal design can cause internal battery temperatures to rise excessively. Alternatively, severe external impacts (such as needle piercing, crushing, or collision) can cause battery short circuits. Many researchers are studying and analyzing the causes of battery thermal runaway. Once one battery cell undergoes thermal runaway, adjacent cells may also be affected and subsequently experience thermal runaway, leading to the spread of thermal runaway and ultimately causing safety accidents.

III. Thermal Runaway Protection Methods
Based on the causes of thermal runaway, battery selection is a critical internal factor. However, to ensure overall battery performance and cost-effectiveness, the internal material ratio has largely been determined. As a result, optimizing the structure and enhancing thermal protection have become the preferred methods for battery manufacturers and automakers to improve thermal diffusion performance.

1. Weakening Top Sealing and Enhancing Ventilation and Exhaust
The high-voltage end of the battery must be protected. Exhaust holes are typically positioned at the top. Conventionally, mica boards and insulating membranes are placed at the top to ensure thermal insulation. The positioning of the top exhaust holes should cover all internal battery cells to ensure that any cell triggering thermal runaway can release energy promptly. The number and area of exhaust holes need to be evaluated to maintain overall structural integrity. The primary optimization involves cutting the mica board at the exhaust hole location without changing the overall module structure, while keeping the mica coverage consistent. Figure 1(a) and Figure 1(b) compare mica boards without and with pre-cutting (the cutting shape selected for this module is a double Y-shape on both sides). Local weakening facilitates high-temperature exhaust release while maintaining the module’s overall insulation and heat-resistant protection.

2. Increasing Internal Thermal Insulation to Slow Down the Spread Rate
As the operating conditions of power batteries are complex, thermal runaway cannot be completely avoided. Suppressing the heat spread between cells—or even blocking it entirely within cells, modules, and battery packs—is a critical means of ensuring passenger safety. Domestic and international scholars have studied many insulating materials, including asbestos and graphite composite boards. Based on the characteristics of aerogels, certain insulating effects can also be achieved. In this study, ceramic fiber aerogel material from a domestic manufacturer was added between soft-pack batteries to slow down the rate of heat spread.

IV. Test Setup and Scheme Description

1. Test Sample Description
The test selected a 78Ah ternary soft-pack battery as the research object, consisting of a soft-pack cell, a heater, test aerogel, an insulating pad, a mica board, foam, temperature sensors, and other components. The heater power was set to 500W to trigger thermal runaway in the battery through heating. The monitoring temperature threshold for the thermocouples was 1400°C, with each thermocouple placed between single battery cells or separated by a layer of test aerogel. The test aerogel, made of ceramic fiber, had dimensions matching the battery cells with a thickness of approximately 1mm. Mica boards were placed on both sides of the battery for thermal insulation and to protect the test fixture. Foam was placed inside the fixture shell to adjust dimensions. The two battery cells were connected in parallel. The specific test setup, including the heater, thermocouples, and other test equipment and fixture arrangements, is shown in Figure 2, which compares the configurations with and without aerogel between battery cells.

2.Test Environment Description
Battery testing must be conducted in a safety laboratory. In addition to the test fixture, temperature and pressure sensors are placed around the test environment to monitor the surrounding conditions of the battery. For the aforementioned tests, temperature and pressure sensors are also arranged to detect the state of the battery surroundings, as shown in Figure 3.

Thermocouples are positioned at the exhaust port, the front positive and negative terminals, and the upper and lower surfaces. The open environment primarily detects the state of thermal diffusion in the module, including the direction of high-temperature gas ejection, the propagation direction of thermal diffusion, and its speed. The compare tests on battery cells are divided into two groups to verify the impact of weakened insulating pads at the exhaust port and the addition of aerogel between cells on thermal diffusion. Three test scenarios are conducted, as detailed in Table 1. Comparisons ① and ② show the differences in ejection behavior before and after weakening the insulating pad at the exhaust port. Comparisons ① and ③ illustrate the differences in heat spread speed before and after adding aerogel.

3.Test System Setup
The data acquisition system includes a video recorder, a data acquisition device, thermocouples, voltage lines, pressure sensors, and other components. Before testing, the battery is charged to 100% SOC (with a cell voltage of approximately 4.25V) and left to rest for 4–8 hours. After completing the test setup, heating begins at 220V until thermal runaway is detected, at which point heating is stopped. All data acquisition devices, thermocouples, voltage lines, and sensors are wrapped in fire-resistant tape to ensure that data information is not destroyed by high temperatures.

V. Test Results and Analysis
The tests primarily compare the performance of battery structures before and after optimization under identical thermal triggering conditions to detect, contrast, and analyze the test states and information.

1. Directed Heat Dissipation and Stable Ejection
Using the same testing method but with optimized battery protection structures, the ejection characteristics during thermal runaway show significant improvement. Although the tested soft-pack battery is designed with an ejection port, unlike prismatic batteries, it is difficult to add specific exhaust channels internally. The optimized structure involves weakening the protective mica at the ejection port, effectively guiding the release of high-temperature gases. Before optimization, when heated to a certain temperature, high-temperature gases begin to release. In addition to the preset ejection port, many gases escape from the sides. If this occurs within a module, it greatly increases the likelihood of short-circuiting at the tab connections to the high-voltage end. If it occurs within a battery pack, the sudden ejection of large amounts of high-temperature gas from the sides can easily ignite surrounding modules and cause short-circuiting. After optimization, during thermal runaway, high-temperature gases are directed to exit through the designed path. Even in space-constrained modules or full battery packs, gases can be smoothly discharged with minimal impact on surrounding modules.

2. Extended Time for Battery Voltage to Drop to Zero
Battery voltage is one of the criteria for evaluating whether thermal triggering has occurred. A voltage drop to zero indicates that the battery has been triggered, and the duration reflects the speed of heat spread. Using the same testing method, the time for single-cell voltage to drop to zero was compared between the optimized and unoptimized thermal protection structures. The average time for single-cell voltage to drop to zero was extended by 3–4 times. Figures 4(a) and 4(b) show the comparison results. The data indicates that the time for single-cell voltage to drop to zero increased from 15–18 seconds to 58–60 seconds. The optimized test time was significantly longer, and the voltage drop curve was more stable, indicating a sequential triggering of cells within the module rather than simultaneous thermal runaway across all cells within a short period.

3.Slowed Thermal Runaway Speed
Changes in cell temperature can clearly reflect the speed of heat transfer within the battery. In the thermal diffusion test of the optimized battery, after triggering the heated cell, high-temperature gases can be rapidly released externally, while the internal aerogel slows down the heat transfer between adjacent cells. Figure 5 shows the temperature change data of the cells. T1–T3 represent the three thermocouples placed within the triggered module (see Figure 2). Before optimization, the heat spread speed between single cells was 8–12 seconds. After optimization, the time for heat generated by a single cell undergoing thermal runaway to diffuse within the aerogel alone was extended to 68 seconds. This demonstrates that adding aerogel between cells significantly improves the prevention of heat diffusion between cells.

4. Comparison of Thermal Runaway Tests in Modules and Battery Packs
Based on the results of the cell-level tests, comparative tests were conducted within modules and battery packs, with optimizations referencing those made at the cell level. Under identical testing conditions, the total time for module voltage to drop to zero increased from 92 seconds to 447 seconds, with the voltage drop occurring in a stepwise manner, reflecting a sequential triggering of cells within the module. When the optimized and unoptimized modules were placed in a battery pack and subjected to thermal diffusion tests under the same conditions, the temperature rise speed detected by thermocouples positioned at the same locations within the triggered module showed significant differences. Thermocouples were placed on the same side of the first, middle, and last cells within the triggered module to collect temperature data. After optimization, the time interval between temperature rises at adjacent thermocouples increased from approximately 15 seconds to around 150 seconds, further indicating that the optimized design slowed down the speed of heat diffusion between cells.

VI. Test Sample Disassembly and Evaluation
Disassembling the tested samples allows for further evaluation of the test results. After testing battery cells, modules, and battery packs, a detailed disassembly analysis was conducted. By examining the protective design, structural components, gas flow paths within the cells, and the burn damage conditions of cells in different positions, the impact of thermal runaway within the module could be effectively assessed. Figures 6(a) and 6(b) show the internal states of modules after thermal diffusion tests before and after optimization, including the first cell (adjacent to the heater), the middle cell, and the last cell (farthest from the heater).

From the comparison of internal cell states, it is evident that before optimization, the ejection paths were scattered, with traces of ejection from the top and sides. The burn severity across all cells within the module was relatively uniform. In contrast, after optimization, the ejection paths within the module were more consistent, with gases primarily exiting through the top ejection port. The burn severity of cells gradually decreased from the position closest to the heater to the farthest position. The ejection traces in the last cell were much fainter compared to the first cell.

VII. Conclusion

As the new energy vehicle industry continues to develop rapidly, safety issues caused by power battery thermal runaway have become increasingly prominent. This paper systematically investigates thermal diffusion protection methods for power batteries. Starting with the factors affecting thermal runaway, it analyzes key measures such as optimizing battery structure and enhancing thermal protection. Experimental validation confirms the effectiveness of optimization schemes, such as weakening top sealing and increasing internal insulation. The results indicate that the optimized battery significantly improves thermal diffusion speed, extends the time for voltage to drop to zero, and enhances module thermal runaway propagation characteristics, providing important references for improving power battery safety.

In this field of research, Yinsu, as a leader in the flame-retardant agent industry, has made significant contributions to the development and application of battery flame-retardant materials through years of dedication and technical accumulation.Yinsu specializes in the development of high-performance flame-retardant agents, which are widely used in various insulation and flame-retardant materials, effectively enhancing their fire resistance and thermal stability. In battery thermal diffusion protection, Yinsu’s flame-retardant agents can significantly slow down the heat spread rate and improve the overall thermal safety of batteries, safeguarding the stable operation of new energy vehicles.

Looking ahead, as the new energy vehicle industry continues to raise requirements for battery safety performance, Yinsu will leverage its professional expertise in flame-retardant agents to collaborate closely with battery manufacturers, material suppliers, and automakers. Together, they will drive technological innovation and upgrades in battery flame-retardant materials. By providing more efficient and reliable flame-retardant solutions, Yinsu aims to support the new energy vehicle industry in achieving safer and more sustainable development, contributing significantly to global green transportation initiatives.