Severe external conditions such as electricity, machinery, and heat are important factors that trigger safety issues in lithium-ion batteries, while safety issues such as ignition and explosion in lithium-ion batteries are generally attributed to thermal runaway.
For example, overcharging, internal short circuits, etc. can lead to thermal runaway and cause safety issues. When the safety limit is exceeded in the battery, thermal runaway of the battery can easily occur due to factors such as poor thermal stability of the separator and the possibility of intense oxidation reaction between the electrolyte and the positive electrode.
So the safety of lithium-ion batteries in thermal environments is an important aspect of lithium-ion battery safety research. This article will explore a new testing method for evaluating the safety of lithium-ion batteries when used under high temperature conditions.
At present, there are three main types of standards for assessing the safety of lithium-ion batteries: international standards (such as IEC standards), national or regional standards (such as JIS, GB, EN standards, etc.), and industry standards (such as UL,IEEE, SJ standards, etc.). Among these existing standards, the specified testing methods at high temperatures mainly include heat abuse test, temperature cycling test, high temperature placement test, etc
The heat abuse test method specified in IEC62133:2002 is to stabilize a fully charged battery at room temperature and place it in a natural or circulating air convection incubator. The incubator heats up to 130 ℃± 2 ℃ at a rate of 5 ℃/min ± 2 ℃/min. After maintaining this temperature for 10 minutes, stop the test and check if the battery is on fire or exploding. The heat abuse test of IEC62133 has been adopted by domestic and foreign standards such as JISC8712,YD1268 and UL1642.
The heat abuse test is generally used to assess the safety of battery cells when placed under high temperature conditions. During the test, the battery sample is in a fully charged state and the sample is not charged or discharged
The temperature cycling test method specified in IEC62281:2004 is to place the battery cell or battery pack in an environment of 75 ° C for at least 6 hours, and then place it in an environment of 40 ° C for at least 6 hours. The transition time between different temperatures does not exceed 30 minutes, and a total of 10 temperature cycles are carried out. After the test, the sample shall not ignite or explode, and there shall be no loss of quality or voltage. The temperature cycling test of IEC62281 is derived from UN38.3, and the test method is also adopted by domestic and foreign standards such as EN62281.
The temperature cycling test is generally used to simulate the safety of battery cells or battery packs under multiple alternating temperature changes during transportation. During the test, the battery sample was in a fully charged state, and the sample was not charged or discharged during the test process
The high-temperature stress relief test method specified in lEC62133 for high-temperature stress relief (the ability of molded shells to withstand high temperatures) is to place the battery pack in an environment at a temperature of 70 ℃± 2 ℃ for 7 hours. After the test, the battery pack casing should not undergo physical deformation that exposes the internal components. The stress relief test of IEC62133 has been adopted by standards such as EN62133 and JISC8712.
The high temperature stress relief test is generally used to assess the ability of the battery pack shell to maintain integrity in the high temperature environment. During the test, the battery pack sample is in the fully charged state. During the test, the sample is not charged or discharged. In combination with the high temperature related tests in domestic and foreign lithium ion battery standards, the heat abuse test is aimed at the ability of the battery cell to withstand high temperatures.
The temperature cycle is to simulate the temperature change during transportation, The high-temperature stress relief test is only used to assess the ability of the molded shell to withstand high temperatures. During these tests, the samples were in a fully charged state and were not charged or discharged during the testing process.
2.High temperature test
From the above analysis, it can be seen that the existing high-temperature tests in the standard do not consider the high-temperature safety issues faced by lithium-ion batteries used in automobiles, and lithium-ion batteries do not consider the temperature rise caused by charging and discharging during the testing process.
Considering the actual usage habits of users (in car charging and placing in the car), combined with the high temperature and holding time in the car, and taking into account the temperature rise caused by lithium-ion batteries during charging and discharging, a new lithium-ion battery safety testing method is proposed to evaluate the safety of lithium-ion batteries used in high-temperature environments.
The specific method is as follows: Place the fully charged battery pack sample in a high-temperature test box, with the temperature set to T. After the surface temperature of the sample stabilizes for 7 hours, the sample should meet one of the following requirements
- Cut off the circuit without fire, explosion, or leakage
- Without cutting off the circuit, continue a discharge charging cycle in accordance with the specified charge and discharge method during the high temperature test, and the sample should not be fired, not exploded, and no leakage.
Among them, the temperature value T is the maximum value specified by the manufacturer for the charging and discharging upper limit temperatures of the battery pack, the charging and discharging upper limit temperatures of the battery, and 80 ℃.
During the high-temperature use test mentioned above, attention should be paid to the following points:
① Due to the need for a certain amount of time for thermal balance, the starting point for the 7-hour test time is not when the test chamber reaches temperature T, but when the sample surface temperature reaches T;
② If the charging and discharging cycle of the lithium-ion battery is not completed within 7 hours, the test time should be extended until the end of this cycle;
③ Due to the accelerated heat transfer caused by air flow in the test chamber, which leads to the loss of surface heat from the battery during charging and discharging, measures such as reducing wind speed may be necessary to minimize the impact on the sample as much as possible;
④ If the protective circuit of the sample is cut off during the charging and discharging process (instead of immediately cutting off after temperature equilibrium), it is only necessary to continue to maintain high temperature until the end of the test (7 hours in total) and the battery is not in danger.