Inadequate thermal management during battery operation can cause its temperature to rise and enter a self-heating mode. When the temperature continues to rise uncontrolled, thermal runaway occurs, ultimately endangering the safety of the system. When a battery is repeatedly charged/discharged or stored at high temperatures, the battery's capacity decreases. High charge/discharge cycles of lithium-ion batteries

　　Inadequate thermal management during battery operation can cause its temperature to rise and enter a self-heating mode. When the temperature continues to rise uncontrolled, thermal runaway occurs, ultimately endangering the safety of the system. When a battery is repeatedly charged/discharged or stored at high temperatures, the battery's capacity decreases. The high charge/discharge cycle performance and high safety level of lithium-ion batteries are essential for their large-scale application. Therefore, in order to meet these requirements, it is important to analyze the degradation behavior of lithium-ion batteries and its impact on safety.

　　Thermal runaway in lithium-ion secondary batteries can occur under various environments. Use thermal mapping images to identify the dependence of the state of charge (SOC) on the onset temperature of thermal runaway and the areas of self-heating and thermal runaway in the battery. But only a few studies focus on the thermal behavior of lithium battery decay. Furthermore, most discussions on their thermal properties have focused on heat yield or specific calorific value, and the correlation between these parameters has not been studied.

　　Recently, Professor Minoru Umeda of Nagaoka Institute of Technology in Japan used 18650 LiCoO2 batteries as research objects and stored the batteries at 80°C at different SOC levels for different lengths of time. In this way, the author summarizes and analyzes the relationship between the thermal runaway starting temperature of the battery, the self-heating rate, and the heating rate of each battery. It was found that the self-heating rate is linearly related to the thermal runaway onset temperature, while the relative heat generation rate is exponentially related to it. Batteries charged to 100% SOC exhibit the lowest onset temperature of thermal runaway. That is, the higher the energy content, the easier it is for thermal runaway to occur.

　　Charging temperature: -20~45℃ -Discharge temperature: -40~+55℃ -40℃ Support maximum discharge rate: 3C -40℃ 3C discharge capacity retention rate ≥70%

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　　Regarding 0% SOC batteries, the starting temperature of thermal runaway for C4 and C5 batteries is about 170C, while the starting temperature for C1, C2 and C3 batteries is about 180C.

　　Regarding the 25% SOC battery, the thermal runaway onset temperature of all batteries (C1-C5) is approximately 180C. In 50% SOC batteries, the starting temperature of C5 battery is ~160C, and the starting temperature of other batteries is about 175C.

　　Regarding the 75% SOC battery, the starting temperature of the C5 battery is 170C, and the starting temperature of other batteries is about 160C.

　　For 100% SOC batteries, the starting temperature of thermal runaway is about 150C.

　　Low temperature and high energy density 18650 3350mAh-40℃ 0.5C discharge capacity ≥60%

　　Charging temperature: 0~45℃ Discharge temperature: -40~+55℃ Specific energy: 240Wh/kg -40℃ Discharge capacity retention rate: 0.5C Discharge capacity ≥ 60%

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　　There is no correlation between the onset temperature of self-heating and SOC or storage degradation conditions. Higher SOC results in lower thermal runaway onset temperature). This trend indicates that battery SOC has a greater impact than storage degradation.

　　When 0%, 25%, 50%, and 75% SOC batteries are at about 120°C, the internal resistance suddenly increases and the OCV decreases. As the temperature further increases, the OCV decreases and the internal resistance increases. A sharp increase in internal resistance was observed for cells using 100% SOC at temperatures below 100°C, but the subsequent behavior was identical to that of other cells. During ARC measurements, all cells showed similar internal resistance and OCV trends, independent of SOC.

　　The calculation method of self-heating rate is: Q=CpmΔT/Δt (ΔT is the battery temperature change; Δt is the time from the beginning of battery thermal runaway to 200C; Cp is the specific heat capacity 0.85kJkg−1K−1; m is the battery mass). The self-heating rate increases at higher SOC (75% and 100%), and the C2 cells in these groups have higher self-heating rates. On the other hand, regarding batteries with an SOC of 50%, C4 and C5 batteries have higher self-heating rates. This change suggests that the self-heating rate is more strongly dependent on SOC than storage conditions at 80C. Regardless of the storage conditions at 80°C, the SOC100% battery has the highest relative heat production rate. Calculated relative heat production versus SOC and storage conditions, the relative heat production rate at 100% SOC is twice that at 75% SOC. Furthermore, fresh cells with 100% SOC exhibit higher heating rates than degraded cells. (Relative heat production rate: calculated by dividing the heating rate of the cell at 180C by the heating rate of the new reference cell at 180C during a thermal runaway test (100% SOC))

　　The starting temperatures of 0~50% SOC batteries are roughly in the same temperature range, while the starting temperatures of 75% and 100% SOC decrease. The relative heat production rate has a strong exponential correlation with the onset temperature of thermal runaway. Similar to the correlation with self-heating rate, the thermal runaway onset temperatures of 0~50% SOC are almost the same, in which the relative heat generation rate is very small. However, under SOC conditions of 75% and 100%, the relative heat production rate increases exponentially as the thermal runaway onset temperature shifts to lower temperatures. All data points show an exponential relationship between thermal runaway onset temperature and SOC, regardless of storage conditions. By varying the storage conditions in this study, it was demonstrated that all cells tested at 100% SOC were thermally unstable, exhibiting lower thermal runaway onset temperatures.

　　In this study, the author used 18650 batteries with different SOCs to be stored at 80°C for different times, and through thermal runaway characteristic tests, the relationship between self-heating rate, relative heat production rate, and thermal runaway onset temperature was studied. It was concluded that the self-heating rate and the relative heat generation rate increased with the decrease of the thermal runaway onset temperature, and their dependence on SOC was stronger than the storage conditions. This is of great significance to lithium-ion batteries and can prevent potential safety hazards of lithium-ion batteries to a certain extent.

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