Lithium-ion battery safety is an important issue related to the safety of users' lives and property. Therefore, no matter what high performance indicators we pursue, safety is always a problem that we cannot avoid or avoid. Thermal runaway is the most serious problem for lithium-ion batteries. In the safety accident, the thermal runaway will cause the lithium ion battery to catch fire and explode, which will seriously threaten the safety of the user's life and property. Therefore, the lithium ion battery must fully consider the safety problem when designing.
Thermal runaway is mainly due to internal short circuit. External short circuit causes a large amount of heat inside the lithium ion battery within a short time, which leads to the decomposition of the positive and negative active materials and the electrolyte, causing the lithium ion battery to catch fire and explode. Different kinds of battery material heat Different stability, heat production in the thermal runaway is not the same, the following figure is the DSC test results of common materials inside the lithium-ion battery, first of all, we use the Li4Ti5O12 material in the lower left corner as an example to introduce the picture viewing method of this figure. First of all, we see In the figure, Q of LTO represents the heat release rate of LTO material, and H represents the total heat release of LTO. The three temperatures from left to right are Tonset trigger temperature, Tpeak peak temperature, and Tend final temperature, which means the closer the following figure is. The lower the material thermal stability in the lower right corner, the lower the heat production, the lower the height of its own 'color blocks', the lower the heat production power. This picture allows us to see the thermal stability of common lithium-ion battery materials more vividly. , which provides us with some references in the design of lithium-ion batteries.
Although there are many researches on the thermal stability of lithium-ion battery materials, there is not much research on the thermal stability of the whole battery. Recently, He Xiangming's research group of Tsinghua University used the accelerating calorimetry ARC and differential scanning calorimetry DSC to adopt. Lithium-ion batteries with different materials have been studied for the source of heat in thermal runaway. A total of four types of lithium-ion batteries were investigated in the experiment. The information of the four types of batteries is shown in the following table.
The temperature, voltage, and internal resistance of the four batteries in the accelerated calorimetry ARC test are shown in the figure below (all batteries were charged to 100% SoC before testing). First, let's take a look at the first battery, from In the following figure a, we can see that the battery starts to generate heat at 100°C. At 247°C, the battery runs out of control and the temperature suddenly rises to 866.3°C. The whole thermal runaway process is divided into four parts by the team. :
i. Stage 1 begins at 100°C and ends at 134.8°C. During this process, decomposition of the SEI film and self-discharge of the positive electrode material are the main source of heat.
Ii. Stage 2 starts at 134.8°C and ends at 173.4°C. During this process, the diaphragm starts to break down, the battery voltage begins to drop, the battery's temperature rise rate is significantly accelerated, and the internal short circuit eventually occurs at 173.4°C, and the voltage drops to At 0V, the internal short circuit is the main source of heat in the process.
Iii. Stage 3 starts at 173.4°C and ends at 247°C, eventually causing thermal runaway. The decomposition of the anode and cathode materials is the main source of heat.
Iv. Stage 4 starts at 247°C and ends at 886.3°C. The thermal runaway of the battery mainly occurs at this stage. At this stage, the reaction between the positive and negative electrode materials and the electrolyte is also triggered, causing the battery to generate more The heat.
For the second battery, the battery self-heated from 100°C, thermal runaway occurred at 208.8°C, and eventually reached 367.8°C. The thermal runaway of the battery was also divided into four phases, as shown below.
i. Stage 1, starting at 100°C and ending at 155.7°C, the decomposition of the SEI film and the self-discharge of the positive electrode are the main source of heat during this process.
Ii. Stage 2 starts at 155.7°C and ends at 170.3°C. The source of heat in this stage is mainly the reaction of the negative electrode with the electrolyte.
Iii. Stage 3 starts at 170.3°C and ends at 212°C. During this stage, the diaphragm begins to shrink and the voltage begins to fall. The main source of heat in this stage is the internal short circuit and the exothermic reaction of the negative electrode.
Iv. Stage 4 starts at 212.4°C and ends at 367.9°C. During this stage, the diaphragm is destroyed, causing a serious internal short circuit and the battery temperature rapidly rising. At the same time, according to the positive and negative DSC test data, it can be judged that the LFP positive electrode and the MCMB negative electrode At this stage also released a lot of heat.
The third battery began to self-heat at 85°C, and thermal runaway occurred at 190.6°C. The maximum temperature reached 634.6°C. The reaction of the third battery was divided into two phases, as shown below.
i. Stage 1 begins at 85°C and ends at 190.6°C. The negative electrode of the 3rd cell begins to exothermally react from 85°C, which is much lower than that of the 1st and 2nd cells, and the surface of the membrane Without coating, severe internal short-circuiting occurred quickly after the separator began to melt.
Ii. The second stage starts from 190.6°C, and the final battery reaches 634.6°C. During this stage, the battery heat mainly comes from the positive electrode and the reaction between the negative electrode and the electrolyte.
The fourth type of battery starts to generate heat at 116.5°C. The maximum temperature of the battery in thermal runaway reaches 215.5°C. The whole process can also be divided into two processes.
i. Stage 1 starts at 116.5°C and ends at 192.8°C. During this process, the heat mainly comes from the reaction between the anode and cathode materials and the electrolyte.
Ii. The second stage starts at 192.8°C and ends at 215.5°C. During this process, the temperature rise rate of the battery continues to decrease, indicating that the decomposition of positive and negative electrodes gradually stops at this stage.
Because the DSC test shows that the failure temperature of the coated separator reaches 290°C, the fourth battery will not have an internal short circuit in the ARC test. Therefore, the fourth battery in the test mainly comes from the positive and negative materials and the electrolyte. Reaction.
Some of the data for the four batteries in the test are shown in the table below.
From the above test results, we have seen that the thermal stability of lithium-ion batteries is closely related to the positive and negative electrode materials and the separators. Severe internal short-circuits in the first and third batteries cause severe reaction of the positive and negative electrode materials. As a result of thermal runaway, the battery emits a large amount of heat throughout the process, even more than the energy stored in the lithium-ion battery. The thermal runaway of the second type of battery is much milder, and the fourth kind of battery is in the heat out of control. No internal short circuit occurs, so the second and fourth types of batteries emit significantly less heat in the test than the energy stored in the batteries. Therefore, how to avoid serious internal short circuit is to improve the thermal stability of lithium-ion batteries The essential.
