Lithium iron phosphate battery cycle is aging different SOC status thermal characteristics research

Article Source: Key Laboratory of Chemistry and Physical Power, Eighteenth Research Institute of China Electronics Technology Group Corporation

The key issue of safety is the heat safety of the battery. During the long -term or high power charging and discharge of the battery, electrical energy and chemical energy can be converted to each other. Under the synergy of battery side reactions, electrode polarization, and battery internal resistance, the battery produces heat, especially when the power battery is when high -power discharge when high -power discharge The heat production behavior is more significant.

The accumulation of calories will inevitably lead to increased internal temperature of the battery. When the temperature reaches a certain limit, it will cause severe chemical reactions such as lithium salt, SEI membrane and electrolyte decomposition, thereby generating more calories. If there is no effective heat dissipation measures to cool down, as the heat continues to accumulate, the battery temperature continues to rise, and the battery burns and explosion. Therefore, in order to improve the safety of lithium -ion batteries, optimize its safety design, and prevent the heat loss of batteries, in -depth research on the thermal discharge mechanism and process of the battery needs to be studied.

At present, a large number of scholars have studied the safety mechanism and structural performance of lithium -ion batteries. Among them, the experiment of the thermal discharge of lithium -ion batteries using thermal heating acceleration heat meter (ARC) is an effective means.

Lithium iron phosphate (LIFEPO4) battery theory is 170mAh/g, and the working voltage is 3.4V. It has an olive structure and excellent safety performance. Heating to 200 ° C is still stable under normal voltage, and the electrode structure of the electrode structure in the charging and discharge process is Little change. Lithium phosphate 4 (LIFEPO4) positive material is widely used in electric vehicles due to its long cycling life, high structure safety, rich resources and low prices, and low prices.

This paper performs ARC experiments based on the lithium iron phosphate 18650 battery after the cycle is 100 times, and anatomy analysis of the battery after ARC experiments to study the thermal characteristics of the battery in different SOC states.

1 experiment

1.1 Experimental battery sample

Battery sample: The basic parameter information of the lithium ion battery sample used in the experiment is shown in Table 1.

Cycling system: Use 0.2c constant current discharge until the discharge termination voltage is 2.0V, 10 minutes; the constant voltage voltage of the constant voltage is charged at the charging termination voltage at 0.2C current, and the charging current is stopped when the charging termination current drops to 0.02C; After standing for 10 minutes; discharge from 0.2C to the discharge termination of the voltage 2.0V, and the circulation 2 to activate; then charge the constant voltage voltage to charge at the charging terminal with a 1C current to the charging termination voltage. After charging, after standing for 10 minutes, the voltage of 1.0V is discharged to the discharge end of the discharge, and the cycle is 100 times. Finally, the 1C current constant currently charges to the corresponding SOC capacity as an ARC experimental battery sample. Table 2 is the status parameter during the battery ARC experiment.

Accelerating thermal (ARCSYS-999, British THT) battery sample’s insulation reaction detection uses “Heating Heat-Waiting Wait-Search Seak” mode. The ARC starts to pre -heated the sample from room temperature. After reaching the setting temperature of the setting, enter the work mode and start heating the sample.

Rainers by gradient. When the temperature rises a step ladder, the instrument is turned into the waiting mode, and the wait sample and the system can reach the thermal balance. Finally enter the search stage, the search temperature change rate is also the heating rate (DT). If you search for the battery heating rate parameter at the preset temperature increase), it is determined that a self -heating reaction occurs inside the battery, the instrument stops active heating, and instead enters the heating mode. ; If the temperature increase rate of the battery DT <0.02 ° C/min, the instrument actively heats into the new round of “Heating Heat-Waiting Wait-Search Seak” mode until it is spontaneous or reached the preset temperature.

See Table 3 in the ARC experiment setting parameter information.

In order to avoid the impact of the weight of the sleeve at high temperature and the top pads on the battery, the coat tube and top pads that remove the battery before the experiment.

1.2 diaphragm test

Sample treatment: Wash the dissected battery diaphragm 6 times with DMC and dry 1h in a 100 ° C vacuum oven.

Breakthrough measurement: Full automatic breathable smoothness instrument (4340, American Gurley) the time required for 100ml volume gas to pass the 6.45cm2 area of ​​the area of ​​6.45cm2 under 1.215kPa pressure conditions. The breathability is also called the Gurley value, which characterizes the ability of the diaphragm.

Optimum measurement: Scan the electronic microscope (S-4800, Japan Hitachi) amplifying 10K times under 1KV voltage conditions to observe the surface of the diaphragm.

2 Results and discussion

2.1 Thermal test

Under different SOC states, the battery is used for heating heating with an accelerated heat meter. The thermal output of the battery is tested. The results are shown in Figure 1.

It can be seen from Figure 1 (a) that the shape of the temperature time curve of the battery under 10%, 50%, and 100%SOC when the temperature is lower than 159 ° C is basically the same. There is no mutation below 159 ° C. Undaled heating out of control. The 10%and 50%SOC status batteries have a cooling mutation at 159 ° C (a) curve, and then the battery temperature has not risen again. It is speculated that the battery pressure leakage valve occurs at 159 ° C. The rupture of the battery olty pressure valve will spray the internal substance of the battery (gas, liquid), and the high temperature spray out of some heat, resulting in the cooling of the battery. At the same time, 10%and 50%of SOCs are relatively slow in each side of the low -loading state, and the reaction rate is low.

The 100%SOC status battery Figure 1 (b) also occurred at a temperature increase rate at nearly 159 ° C, but then quickly transformed into a positive temperature increase rate, and the temperature continued to rise. When the temperature rises to 174 ° C, the heating rate will continue to rise; when the temperature reaches 191 ° C is 1 ° C/min; when it reaches 220 ° C, the temperature increase rate is 2.4 ° C/min (0.04k/s), and the temperature then temperature Rising sharply, eventually leading to the occurrence of heat out of control. The maximum temperature tested in ARC experiments reached 340 ° C.

In Figure 1 (b), the battery has a negative temperature rate near 159 ° C. It is inferred that when the battery pressure leakage valve is ruptured at about 159 ° C, the internal material of the battery is sprayed (gas, liquid), high temperature spray after the battery release valve is ruptured. Part of the heat from the outlet causes the battery to cool down; after the pressure is discreated, although the battery is transmitted for a long time, because the septum is also molten to damage, the battery is short -circuited, the battery is in the high -load state battery, the internal response of the battery is intensified, and the battery reveals the battery. Continue heat production; when the temperature is 174 ° C, the heating rate begins to increase, indicating that a large amount of heat generates in a short period of time.

2.2 Quality loss

Figure 2 is a battery comparison after ARC experiment. It can be seen from the picture that the top of the three SOC status batteries has obvious spraying objects. The outer wall of the battery has obvious marks after the electrolyte evaporated, indicating that the three SOC status batteries have ruptured by the battery olfactory valve during the ARC experiment. , Battery’s internal substances are sprayed (gas, liquid), and the battery quality is reduced (see Table 4).

Comparison Table 4 With the increase of the quality loss of the battery in SOC, it shows that the internal reaction of different SOC state batteries is different, and the internal response is intensified as the SOC increases.

2.3 Battery disassembly comparison analysis

Disassemble the battery after the ARC experiment, and take a battery that is the same but not experiment as a reference at the same time. Positive and negative chips and diaphragm are completely separated, and the diaphragm is gray and white and transparent. The battery in the 50%SOC state remains complete. When disassembling, the positive and negative chips and the diaphragm are completely separated.

The battery in the 100%SOC state, the internal battery is kept complete, the negative polarized copper foil is discolored, and the positive and negative chips cannot be completely separated during disassembly. The diaphragm disappears. Reference batteries, the internal battery cells are complete, the positive and negative plates, the diaphragm are completely separated during disassembly, and the diaphragm is pure white.

It can be seen that as the SOC increases the ARC experiment, the internal battery cells in the battery have changed significantly, especially the changing diaphragm changes hugely, indicating that the internal reaction of different SOC state batteries is different, and the internal response is intensified as the SOC increases.

2.4 Comparison of diaphragm

The disassembled diaphragm was washed with DMC and the breathability was determined. As a result, it was shown in Table 5.

Compared with the breathability test, the breathable value of the battery diaphragm is 308S; the 10%SOC status breathable value shows the DENSE999999, which increases sharply. The breathability value is 26s, which is far less than the reference battery diaphragm. It is speculated that the internal energy of the higher SOC state in the ARC test will be higher. The internal reaction will be more intense at the same external temperature, and the internal temperature will be higher. The higher temperature is heated, damage is caused, and the holes become larger, and the large pores will cause short circuits inside.

Wash the disassembly septum with DMC and perform SEM observation. From the SEM diagram, it can be seen that the reference battery diaphragm is a uniform hole (Figure 4); the 10%SOC state battery ARC experiment most of the pores have disappeared, and the surface has been melted into a piece (Figure 5), indicating that ARC The diaphragm is hot -melt, and the diaphragm is closed; the 50%SOC state battery ARC experiment most of the pores of the diaphragm sample have been closed, and the diaphragm is damaged (Figure 6). Compared with a 10%SOC state closure and a 50%SOC state partial partial partial damage indication shows that the internal response intensified as the SOC increases.

3 conclusions

After the acceleration thermal meter was used to conduct a thermal output behavior of lithium iron phosphate batteries in different SOC states after 100 times of 1C cycle. Compared with the final temperature and temperature increase rate of the battery, it is found that when the battery is 10%SOC and 50%SOC, it is not triggered to lose control;

At the 100%SOC state, the temperature increase rate will continue to increase after the temperature reaches 174 ° C; the temperature increase rate at 191 ° C is 1 ° C/min; the temperature increase rate at 220 ° C is 2.4 ° C/min (0.04k/s s s (0.04K/s s s s s ); Then the temperature rose sharply, which eventually led to the occurrence of heat out of control, the highest temperature reached 340 ° C. The battery was dissected and analyzed, and the batteries after 10%SOC, 50%SOC, and 100%SOC for ARC experiments was 9.7%, 9.9%, and 11.8%, respectively.

Through the analysis of the diaphragm test, the ARC experiment was closed after the ARC experiment at the 10%SOC state; the 50%SOC state septum is damaged; the ARC experiment after the 100%SOC state completely disappeared (melting) The heat -out risk of batteries increases.

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