What is thermal out of control? How is highly safe lithium iron phosphate ion battery thermal out of control?

　　Author ：Iflowpower – ספק תחנת כוח ניידת

In 2018, the entire car market has declined in more than 20 years. New energy vehicles still rose more than 60%, become a group of black horses in the automotive market. With the large-scale popularization of new energy vehicles, the safety of dynamic lithium batteries has also caused more and more attention, compared to the higher energy density, the lithium-ion battery, the lithium iron phosphate ion battery is considered to have more High security, then a safer lithium iron phosphate ion battery happened to a how to experience? The LFP material has an olivine structure, which we believe that the LFP material has high stability due to the presence of more stable PO keys, which takes 18650 structural batteries as an example.

If the LFP material can be used in the heat loss in the heat loss. The 0.5 g of O2, but if we can release up to 3.

25 g of O2 in thermal out-of-control, less O2 release means that the combustion reaction of the electrolyte is suppressed, and less heat is released. Suppressing the intensity of thermal out of control of LFP batteries. The battery used in the experiment is commercial LFP18650 battery with capacity of 1500mAh, and the thermal displacement behavior of the LFP battery (as shown in the following figure) is used separately, respectively, and the SOC of the LFP battery is 0%, 28%, 63, respectively.

%, 100% and 110% of ARC (Acceleration Heat) test, control SOC 100% for hot box test. ARC test is a common method of thermal stability of lithium-ion batteries. The basic operation method can be divided into three steps.

First, heating to a predetermined temperature, the second step is to wait, the third step is to search, that is, the battery is at a certain temperature The temperature rise rate of the battery temperature means a rate means that the battery starts to be self-exclaimed, if the rate of warming rate of the battery reaches a certain rate, it is to start thermostat. Here the author sets the start temperature of Arc to 50 ¡ã C, the end temperature is set to 315 ¡ã C, and the temperature is 10 ¡ã C, waiting for 60 min, if the battery is at which the temperature rate is 0.02 ¡ã C / min, the temperature is a battery.

Self-heating start temperature, if the temperature of the battery has reached 1 ¡ã C / min, the temperature is the thermal out of control of the battery. The following figure A is a 100% SOC battery ARC test curve, which can see the self-heating start temperature of 100% SOC of 100% SOC is 95 ¡ã C, and then the temperature rise rate of the battery has increased to 3.7 ¡ã C at 230 ¡ã C.

/ Min, but then the temperature roll rate of the battery begins to fall, and a new high point 1.6 ¡ã C / min appears near 280 ¡ã C. The following figure A can be divided into four regions, wherein the region 1, 95-150 ¡ã C, the battery starts self-heating, which is an important corresponding to the SEI film of the negative electrode surface, and with the negative electrode-electrolyte reaction, in the region 3 In the middle, 150-255 ¡ã C, the heat in this stage is important from the negative electrode-electrolyte, the node-electrode-electrolytic solution, wherein the heat release of the electrode-electrolyte releases exerted.

In the region 4 (> 255 ¡ã C), the occurrence of heat in the internal heat of this stage is important from the oxidation reaction that occurs in the electrolyte and LFP decomposition. As can be seen from the following, B and C, the shape of the arc curve of the battery in 110% SOC and 63% SOC is basically the same, but the SOC of the battery is further reduced to 28%, Then there will be significant changes in the arc curve shape of the battery (as shown in Figure D below), from the start of the self-exothermic heat until 190 ¡ã C, the temperature rise rate of the battery is always increased, and the peak is reached at around 190 ¡ã C, then start Falling, then the rate of temperature rise of the battery will start slowly. In the lower SoC state, the LFP positive electrode is more stable, and since the heat of the battery is important from the forefront of the battery, the battery is referred to, and the heat of the battery is imported from the positive electrode-electrolyte after the temperature exceeds 200 ¡ã C.

Decomposition reaction, but since the stability of the positive electrode under this SOC is relatively high, the temperature rate of the battery is also slower. The shape of the Arc curve of the LFP battery under 0% SOC can be further changed, and it is possible to note that the self-heating start temperature of the battery is shown in the drawing, and the temperature rise rate peak near 190 ¡ã C is also disappeared. It indicated that under low SOC, the battery is in a relatively stable state, the negative electrode has been completely dehydricate, so the speed of the negative electrode-electrolyte decomposition reaction is also greatly reduced, and the shape of the curve and 28% SOC after the temperature exceeds 200 ¡ã C.

The battery is basically the same, and the small amount of O2 released by the LFP positive decomposition promotes the decomposition of the electrolyte, so that the temperature rise rate of the battery is slowly increased. The following figure shows the temperature of the self-heating trigger temperature, the maximum temperature rise rate temperature and the maximum temperature rate of the battery according to the ARC test results, which can see the maximum temperature rise rate of the battery as the SOC of the battery is increased from the figure. The corresponding rise, this is important because more energy stored in the battery under higher SOC, and higher SOC also means that the stability of the positive and negative electrode of the battery is also lower, and it is important to store the LI stored in the negative electrode.

More, therefore the decomposition reaction of the negative electrode and the binder, the electrolyte, etc., thereby accelerating the rise of the temperature of the lithium ion battery. Since the maximum heating rate can reflect the stability of the internal positive and negative electrode inside the lithium ion battery, the maximum temperature rise rate can reflect the risk of thermal out of control of the lithium ion battery, and the following figure compares several common lithium ion battery positive electrode systems in different The maximum temperature rise rate in the SOC state, can see from the figure, in the SOC state, the maximum temperature rate of the LFP battery is higher than the other type of battery, which indicates that the LFP battery is compared to Other types of batteries have significant advantages in security.

The figure below shows the change curve (solid line) of the surface temperature of the LFP battery in the hot box test, and the internal temperature (dotted line) of the hot box, the temperature change curve of the battery can be divided into four regions, wherein the region A is a battery in the hot tank. The process of heating the temperature rise, the temperature of the battery is lower than 95 ¡ã C, the battery has not started. The area B continues to rise by about 180 ¡ã C for the surface temperature of the battery.

This stage SEI membrane begins to decompose, negative-electrolyte and positive electrode-electrolyte decomposition reactions begin, the battery starts with self-heating, the battery temperature rapidly rapidly exceeded Hot box temperature, the pressure relief valve of the final battery is oversized. After the region C is started to the battery relief valve to the battery thermal out of control, the area D is the end of the battery heat, the temperature of the battery is finally restored to the temperature of the hot box. Comparing the two different temperature hot tanks obtained by the battery surface temperature profile, the peak temperature in the thermal out of control in the 220 ¡ã C thermostat is significantly higher than the battery in the heat box of 180 ¡ã C, which indicates in the heat box at 220 ¡ã C.

Additional reactions occur in the thermal out-of control of the battery, the previous Arc analysis shows that the LFP positive decomposition reaction will occur after the surface of the battery reaches 210 ¡ã C, and the decomposition reaction of the electrolyte is only when the surface temperature of the battery exceeds 255 ¡ã C. At the end of the 180 ¡ã C hot tank test, the upper temperature of the battery is less than 230 ¡ã C, so at least the battery has not reached the decomposition temperature of the electrolyte, and the LFP positive is released at a lower temperature, which is significantly reduced. Reduce the heat production rate of the lithium ion battery, thereby suppressing elevation of the battery temperature.

Peterj.BugryNIEC's research shows that the SOC has a significant impact on the thermal displacement behavior of the LFP battery. As the SOC's new battery thermal loss has significantly added, the stability of the battery is significantly reduced.

Regarding the specific cause analysis of thermal out-of control indicates an important cause of the battery thermal out of control in a 100% and 110% SOC state as an anode-electrolyte and a positive electrode-electrolyte reaction, but in a lower SOC state, battery thermal out of control The important trigger factor is the decomposition reaction of the negative electrode-electrolyte, and the thermal stability of the LFP is significantly improved when the SOC is less than 28%, and heat loss will not occur. The hot box test indicates that the temperature of the hot tank can cause a more serious thermal out-of control of the lithium-ion battery, which is important because the better hot box temperature triggers the decomposition reaction of the electrolyte and the positive decomposition release O2 reaction, exacerbated the bat.