3.7V 18650 lifepo4 battery

　　Research and analysis of thermal runaway of 18650 lithium battery at 80℃

　　Inadequate thermal management of the battery during operation can cause it to heat up, entering a self-heating mode. When the temperature continues to rise uncontrollably, thermal runaway occurs, which ultimately endangers the safety of the system. When a battery is repeatedly charged/discharged or stored at high temperatures, the capacity of the battery decreases. High charge/discharge cycles for Li-ion batteries

　　Inadequate thermal management of the battery during operation can cause it to heat up, entering a self-heating mode. When the temperature continues to rise uncontrollably, thermal runaway occurs, which ultimately endangers the safety of the system. When a battery is repeatedly charged/discharged or stored at high temperatures, the capacity of the battery decreases. High charge/discharge cycle performance and high safety level of lithium-ion batteries are essential for their large-scale applications. Therefore, to meet these requirements, it is important to analyze the decay behavior of Li-ion batteries and its impact on safety.

　　Thermal runaway in lithium-ion secondary batteries can occur under various circumstances. The dependence of the state of charge (SOC) on the onset temperature of thermal runaway and areas of self-heating and thermal runaway in the battery can be identified using thermal mapping images. But only a few studies have focused 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 investigated.

　　Recently, Professor Minoru Umeda of Nagaoka Institute of Technology in Japan used 18650 LiCoO2 batteries as the research object, and stored the batteries at different SOC levels at 80°C for different lengths of time. Based on this, the author summarizes and analyzes the relationship between the thermal runaway onset 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. The battery charged to 100% SOC exhibited the lowest onset temperature of thermal runaway. That is, the higher the energy content, the easier it is for thermal runaway to occur.

　　Figure 1. Surface temperature of (a) 0%, (b) 25%, (c) 50%, (d) 75%, and (e) 100% SOC cells in heat-wait-search tests. C1 (143 days storage at 80°C), C2 (58 days), C3 (19 days), C4 (13 days), C5 (7 days)

　　Low temperature lithium iron phosphate battery 3.2V 20A -20℃ charge, -40℃ 3C discharge capacity ≥70%

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

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　　a shows that the first battery to reach 200C is C5, followed by C4, C3, C2 and C1. Under 0% and 25% SOC conditions, batteries with different storage times showed similar trends when reaching the 200C limit. But there is no clear trend for cells with 50%, 75% and 100% SOC.

　　. Heating rate dT/dt during thermal runaway tests of (a) 0%, (b) 25%, (c) 50%, (d) 75%, and (e) 100% SOC cells.

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

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

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

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

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

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　　For a 100% SOC battery, the thermal runaway onset temperature is about 150C.

　　Heat maps of (a) 0%, (b) 25%, (c) 50%, (d) 75%, and (e) 100% SOC batteries. The blue, yellow and red bars reflect no heating (dT/dt<0.05Cmin-1), self-heating (0.05Cmin-1dT/dt<1Cmin-1), and thermal runaway regions (1Cmin-1dT/dt).

　　There was 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 the battery SOC has a greater influence than the degree of storage decay.

　　Figure 4. Open circuit voltage and internal voltage during thermal runaway test of Li-ion battery at (a) 0%, (b) 25%, (c) 50%, (d) 75%, and (e) 100% SOC The relationship between resistance and battery surface temperature.

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

　　Self-heating rates of 0%, 25%, 50%, 75% and 100% SOC Li-ion batteries (left); 0%, 25%, 50%, 75% and 100% SOC Li-ion batteries at 180°C relative Heat production rate (right)

　　The calculation method of the self-heating rate is: Q=CpmΔT/Δt (ΔT is the temperature change of the battery; Δt is the time elapsed from the beginning of the 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 self-heating rate of the C2 cells in these groups is higher. On the other hand, with regard to the battery with SOC of 50%, the self-heating rate of the C4 and C5 batteries is higher. This change indicates that the self-heating rate is more strongly dependent on SOC than the storage condition at 80C. Regardless of the storage conditions at 80 °C, the relative heat generation rate of the SOC100% battery is the highest. In the calculated relative heat production versus SOC and storage conditions, the relative heat production rate at 100% SOC was twice that at 75% SOC. In addition, the fresh battery at 100% SOC exhibited a higher heating rate than the degraded battery. (Relative heat generation rate: during a thermal runaway test (100% SOC) by dividing the heating rate of the cell at 180C by the heating rate of a new reference cell at 180C)

　　(a) Self-heating rate and (b) relative heat generation rate versus onset temperature of thermal runaway. Symbols indicate storage conditions for C1 (●), C2 (■), C3 (◆), C4 (▲) and C5 (▼); cells are 0% (black), 25% (light blue), 50% (orange ), 75% (green) and 100% (red) SOC.

　　The onset temperature of 0~50% SOC battery is roughly in the same temperature range, while the onset temperature of 75% and 100% SOC is lowered. The relative heat production rate has a strong exponential correlation with the onset temperature of thermal runaway. Similar to the correlation of the self-heating rate, the thermal runaway onset temperature of 0~50% SOC is almost the same, where the relative heat generation rate is small. However, at 75% and 100% SOC, the relative heat generation rate increased exponentially with the transition from the thermal runaway onset temperature to lower temperatures. All data points show an exponential relationship between thermal runaway onset temperature and SOC, independent of storage conditions. By varying the storage conditions in this study, it was demonstrated that all batteries tested at 100% SOC were thermally unstable, exhibiting a lower thermal runaway onset temperature.