1.Safety of lithium-ion battery under overheating condition

The safety problems of lithium-ion batteries are mainly manifested by fire and explosion caused by uncontrolled heat. The causes of thermal runaway can be divided into external conditions and internal conditions. The internal temperature of the battery also keeps rising when the external temperature keeps rising. When the temperature rises to a certain temperature, the diaphragm will be thermally closed and the positive and negative poles will be isolated for safety protection. However, if the diaphragm fails to close effectively, or the diaphragm melts and breaks, or other exothermic reactions occur inside the battery at the same time, so that the battery temperature continues to be high, it may cause safety problems. It can be seen that the safety problems caused by overheating are the result of the comprehensive action of internal and external conditions.

For example, when the temperature of lithium cobaltate system battery exceeds the safety limit (such as 150 ℃), it is very easy to cause the battery fire or explosion due to the thermal stability of the diaphragm and the violent oxidation reaction of the electrolyte and positive electrode.

2.Test method for lithium ion battery overheating

At present, the standards for assessing the safety of lithium-ion batteries mainly include international standards (such as IEC standards), national or regional standards (such as JIS, GB, EN standards, etc.) and industrial standards (such as UL, IEEE, SJ, QB standards, etc.). In these standards, many have used the “thermal abuse test” to assess the safety of lithium-ion batteries under overheating conditions, For example, the heat abuse test method specified in IEC 62133:2002 is as follows: “After the fully charged battery is stabilized at room temperature, put it into a thermostat with natural or circulating air convection, and the thermostat is heated to 130 ℃ ± 2 ℃ at the rate of 5 ℃/min ± 2 ℃/min. Maintain this temperature, and stop the test after 10 minutes to check whether the battery is on fire or exploding.” These standards are basically consistent with the test method for heat abuse test, Only the test conditions are slightly different.

The lithium-ion battery standard, which was invented a few years after the invention of lithium primary battery, was also formulated after the lithium primary battery standard. Many of the lithium-ion battery safety tests also drew on the lithium primary battery. The heat abuse test of lithium-ion batteries originated from the heat abuse test of lithium primary batteries. For example, the heat abuse test of lithium-ion batteries in IEC 62133 is basically consistent with the heat abuse test of lithium primary batteries in IEC 60086-4 Primary Batteries – Part 4: Safety Requirements for Lithium Batteries, which was developed earlier.

3.Analysis of test conditions

3.1 Analysis of high temperature holding time

In the existing standards, the test method of lithium-ion battery is basically “the incubator heats up to a certain high temperature at a certain rate and maintains at this temperature for a period of time”. The holding time of this test method starts when the temperature of the incubator reaches a certain high temperature. However, this method lacks fairness for different batteries, especially for batteries of different sizes. The heating rate of different batteries is not necessarily the same. Due to the large difference between the heat transfer rate of lithium ion battery and air, the heating rate of battery is lower than that of thermostat. The temperature of the lithium-ion battery cannot be completely consistent with the temperature in the thermostat. Therefore, when the temperature in the incubator reaches a certain high temperature, it will take a period of time for the battery to maintain the same temperature with the incubator. Because the material, shape, size and quality of the battery are different, the required balance time is also different.

Four typical lithium-ion batteries are selected as samples: small capacity square battery, cylindrical battery, large capacity square battery and polymer battery. Put the sample into an incubator, which heats up to 130℃ at a rate of 5℃/min and keeps it for a period of time.

Through comparison, it can be concluded that the temperature rise rate of different batteries has a large difference, especially for large batteries, the thermal balance time is longer. After the temperature of the incubator rises to the maximum and remains for 10 min, the temperature of the surface of the other cells, except for the sample (polymer cell), has not yet risen to the maximum temperature.

To sum up, it is not the same for different batteries to start timing when the temperature of the incubator rises to a certain temperature. Especially for large batteries, the purpose of test and assessment has not been achieved because the battery has not reached the test temperature. However, it is more reasonable to monitor the surface temperature of the battery during the test and take the surface temperature of the battery reaches a certain temperature as the starting condition for timing, However, it is difficult to operate because the balance time of each sample may be slightly different when at least three samples are tested at the same time. The battery type used in current electronic products is comprehensively considered and strictly considered. It is more appropriate to take the temperature of the incubator as the test condition after rising to a certain temperature and maintaining it for 30 minutes.

3.2 Maximum test temperature analysis

The maximum temperature that different battery materials can withstand is different, and being able to withstand higher temperature means that the battery has higher safety under overheating conditions. The thermal characteristics of diaphragm materials are very important for the thermal safety of lithium-ion batteries, and the maximum temperature that different diaphragm materials can withstand is also different. The commonly used diaphragm materials include PE, PP and PE-PP-PE, which can withstand a maximum temperature of 130 ℃~150 ℃. The overheat safety of batteries manufactured by them is also inconsistent. The above four battery samples were subjected to heat abuse tests at 130 ℃, 140 ℃, 145 ℃ and 150 ℃ respectively.

It can be seen from the actual test that in the heat abuse test at 130 ℃, the battery almost did not have its own exothermic reaction after heat balance, which caused the temperature to rise spontaneously, while in the test at 135 ℃, 140 ℃, 145 ℃ and 150 ℃, its own exothermic reaction occurred to varying degrees, and the higher the test temperature, the more intense the exothermic reaction. With the increase of the test temperature, the self-heating reaction of the battery gradually intensifies, and the assessment of safety is more severe, and the risk of fire and explosion is increased.

It can be seen from the above that 130 ℃ can only be used as the most basic test temperature for heat abuse assessment. At the same time, the test temperature can also be increased according to the different use occasions of the battery. In addition, the test also shows that if the battery temperature rises to the set temperature, if the heating is not stopped immediately, it will be kept at the set temperature for a period of time to ensure that sufficient autothermic reaction can occur to better assess its overheating safety.

4.Conclusion

The safety problems of lithium-ion batteries are mostly caused by thermal runaway. For lithium-ion batteries used in special occasions, it can also be considered to further assess the safety of lithium-ion batteries by increasing the test temperature, extending the high temperature holding time and other methods to tighten the test conditions.