Thermal runaway primarily occurs when internal or external short circuits cause a large amount of heat to accumulate within a lithium-ion battery in a short period, triggering the decomposition of the positive and negative electrode active materials and electrolyte, leading to fire and explosion. Different types of battery materials have varying thermal stability and generate different amounts of heat during thermal runaway. The following figure shows the DSC test results of common materials inside lithium-ion batteries. We will first use the Li4Ti5O12 material in the lower left corner as an example to illustrate how to interpret this figure. We can see that Q in the figure represents the heat release rate of the LTO material, and H represents the total heat release of the LTO. The three temperatures from left to right are the Tonset trigger temperature, Tpeak peak temperature, and Tend final temperature. In other words, materials closer to the lower right corner in the figure have better thermal stability and generate less heat. The lower the height of the colored block, the smaller the heat generation power. This figure vividly illustrates the thermal stability of common lithium-ion battery materials, providing some reference for lithium-ion battery design.
While there is extensive research on the thermal stability of lithium-ion battery materials, studies on the thermal stability of the entire battery are relatively few. Recently, Professor He Xiangming's research group at Tsinghua University used accelerated calorimetry (ARC) and differential scanning calorimetry (DSC) to investigate the heat sources in thermal runaway of lithium-ion batteries using different materials. Four types of lithium-ion batteries were studied in the experiment, and the information for the four batteries is shown in the table below.
The changes in temperature, voltage, and internal resistance of the four batteries during accelerated calorimetry (ARC) testing are shown in the following figure (all batteries were charged to 100% SoC before testing). First, let's look at the first battery. From Figure a below, we can see that this battery begins to self-heat at 100℃, and thermal runaway occurs at 247℃, with the temperature suddenly rising to 866.3℃. The Xiangming team divided the entire thermal runaway process into four parts:
i. The first stage starts at 100°C and ends at 134.8°C. During this process, the decomposition of the SEI film and the self-discharge of the cathode material are important sources of heat.
ii. The second stage begins at 134.8℃ and ends at 173.4℃. During this process, the separator begins to break down, the battery voltage begins to drop, the rate of temperature increase of the battery accelerates significantly, and finally an internal short circuit occurs at 173.4℃, causing the voltage to drop to 0V. The internal short circuit is an important source of heat during this process.
iii. Stage 3 begins at 173.4℃ and ends at 247℃, ultimately triggering thermal runaway. The decomposition of the positive and negative electrode materials is a significant source of heat in this process.
iv. Stage 4 begins at 247°C and ends at 886.3°C; thermal runaway of the battery primarily occurs in this stage. During this stage, reactions between the positive and negative electrode materials and the electrolyte are also triggered, leading to even more heat generation in the battery.
Regarding the second type of battery, it begins to self-heat at 100°C, experiences thermal runaway at 208.8°C, and eventually reaches 367.8°C. This battery's thermal runaway is also divided into four stages, as shown below.
i. Stage 1, starting at 100℃ and ending at 155.7℃, during which the decomposition of the SEI film and the self-discharge of the positive electrode are important sources of heat.
ii. The second stage begins at 155.7℃ and ends at 170.3℃. The main source of heat in this stage is the reaction between the negative electrode and the electrolyte.
iii. Stage 3 begins at 170.3℃ and ends at 212℃. During this stage, the diaphragm begins to shrink and the voltage begins to decrease. The main heat sources in this stage are the internal short circuit and the exothermic reaction at the negative electrode.
iv. Stage 4 begins at 212.4℃ and ends at 367.9℃. During this stage, the separator is damaged, leading to a severe internal short circuit and a rapid increase in battery temperature. Based on the DSC test data of the positive and negative electrodes, it can be determined that the LFP positive electrode and MCMB negative electrode also release a large amount of heat during this stage.
The third type of battery began to self-heat at 85°C and experienced thermal runaway at 190.6°C, with the highest temperature reaching 634.6°C. The reaction of the third type of battery was divided into two stages, as shown below.
i. Stage 1 starts at 85°C and ends at 190.6°C. The negative electrode of the third type of battery begins to undergo an exothermic reaction at 85°C, which is much lower than that of the first and second types of batteries. At the same time, since there is no coating on the surface of the separator, the separator melts quickly, leading to a serious internal short circuit.
ii. The second stage starts at 190.6°C and eventually reaches 634.6°C. In this stage, the heat of the battery mainly comes from the reaction between the positive electrode, the negative electrode and the electrolyte.
The fourth type of battery begins to self-heat at 116.5℃, and the highest temperature of the battery during thermal runaway reaches 215.5℃. The entire process can also be divided into two stages.
i. The first stage starts at 116.5℃ and ends at 192.8℃. During this process, the heat mainly comes from the reaction between the positive and negative electrode materials and the electrolyte.
ii. The second stage starts at 192.8℃ and ends at 215.5℃. During this process, the rate of temperature rise of the battery decreases significantly and continuously, indicating that the decomposition of the positive and negative electrodes gradually stops at this stage.
Since the DSC test showed that the failure temperature of the coated separator reached 290°C, the fourth type of battery did not experience an internal short circuit in the ARC test. Therefore, the heat generated by the fourth type of battery in the test mainly came from the reaction between the positive and negative electrode materials and the electrolyte.
