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The importance of safety for lithium batteries is self-evident. Lithium batteries must undergo strict safety testing before formal production to ensure the safety in extreme use. However, the lithium battery is actually a metastable system. The existence of side reactions between electrolyte and the positive and negative electrode interfaces during the use process will lead to significant changes in the thermal and electrical characteristics of the lithium battery at the end of its life. Therefore, the thermal stability of the lithium battery will inevitably continue to change with the aging of the lithium battery. Although we have done a lot of research on the thermal runaway of lithium batteries, it is still important to focus on the fresh batteries at the beginning of their life, and the research on the thermal runaway of batteries at the end of their life is relatively less.

Shuichi Taniguchi (the first author) and Minoru Umeda (the corresponding author) of Nagaoka University of Technology and Science in Japan and the Japan Space Agency conducted in-depth research and analysis on the impact of high temperature aging on the thermal stability of lithium batteries.

The battery used in the experiment is CGR18650E battery from Panasonic, with LiCoO2 as the positive electrode and graphite as the negative electrode. The rated capacity of the battery is 2550mAh and the rated voltage is 3.7V. After the battery is adjusted to different SoC, the storage aging test is carried out at 80 ℃ according to the system shown in the following table. It can be seen from the table that the aging degree of C5 battery stored in 100% SoC is significantly greater than that of C1 battery stored in 0% SoC.

Accelerated calorimeter (ARC) is used to test the thermal stability of the battery after aging. The test is mainly divided into three steps. The first step is heating. The ARC equipment heats up at a temperature interval of 5 ℃. When it reaches a certain temperature, it goes to the next step: thermal balance. This process lasts about 30 minutes to make the battery reach thermal balance with the environment. Then it goes to the search step. If the temperature rise rate of the battery exceeds 0.05 ℃/min, It indicates that the battery starts the self heating mode and ARC starts the adiabatic mode. If the temperature rise rate of the battery exceeds 1 ℃/min, it indicates that the battery has entered the thermal runaway state.

The following figure shows the ARC test results of batteries aged at 80 ℃ at different SoC states as shown in Table 1. From Figure a, it can be seen that under 0% SoC state, the batteries that first reached 200 ℃ in the ARC test were C5, followed by C4 and C3, followed by C2, and finally C1, indicating that the more severely aged the batteries were at 0% SoC state, the worse their thermal stability was. We also observed this phenomenon under 25% SoC state, but with the SoC further increased to 50% After 75% and 100%, there is no clear relationship between thermal stability and battery aging.

In order to further analyze the influence of aging degree on the thermal stability of the battery, the author has drawn a temperature rise rate curve. From the following figure a, it can be seen that the starting temperature of thermal runaway of battery C4 and C5 (temperature rise rate>1 ℃/min) is 170 ℃, and the starting temperature of thermal runaway of battery C1, C2 and C3 is 180 ℃; The starting temperature of thermal runaway of C1-C5 battery is basically 180 ℃ under 25% SoC state, 160 ℃ under 50% SoC state for C5 battery, and 175 ℃ for other batteries; Under 75% SoC condition, the thermal runaway test temperature of C5 battery is 170 ℃, and that of other batteries is 160 ℃; The starting temperature of thermal runaway of all batteries under 100% SoC is about 150 ℃.

It is not difficult to see that the SoC state of the battery has the greatest impact on the thermal stability of the battery. With the improvement of the SoC state of the battery, the temperature of the thermal runaway of the battery decreases significantly. The influence of battery aging degree on the temperature of thermal runaway depends on the SoC state of the battery. The battery with large aging degree under 0% SoC state has poor thermal stability, while the battery with large aging degree under 75% SoC state has higher thermal stability.

In order to make the comparison of thermal runaway temperature and self heating temperature of batteries with different aging degrees more intuitive, the author drew a column chart (as shown in the figure below), in which the blue part is the non self heating area, the yellow part is the self heating area, and the red part is the thermal runaway area. The thermal stability of the battery can be directly compared by comparing the length of the yellow and red strips in the self heating and thermal runaway areas. The longer the red and yellow areas are, the worse the thermal stability of the battery is.

From the above analysis, it is easy to see that the battery's SoC state has the greatest impact on battery safety. The higher the SoC state is, the worse the thermal stability of the battery is, and the lower the temperature of thermal runaway of the battery is. The aging degree of the battery has little influence on the battery safety, and the relationship is not obvious. The thermal stability of the battery aging at 0% SoC is poor, while that of the battery aging at 75% SoC is poor.

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