The key safety issue is the thermal safety of the battery. In the long term or high rate charging and discharging process of the battery, electric energy and chemical energy are converted to each other, and under the synergistic action of side reactions, electrode polarization and internal resistance of the battery, the heat generation behavior of the battery is more significant, especially when the power battery is discharged at high power.
The build-up of heat will inevitably lead to a rise in the temperature inside the battery. When the temperature reaches a certain limit, violent chemical reactions such as decomposition of lithium salts, SEI films and electrolytes result in more heat. If there is no effective cooling measures, with the accumulation of heat, the battery temperature continues to rise, eventually causing battery combustion and explosion. Therefore, in order to improve the use safety of lithium-ion batteries, optimize the safety design and prevent the occurrence of thermal runaway, it is necessary to conduct in-depth research on the occurrence mechanism and process of thermal runaway of batteries.
At present, a large number of scholars have studied the safety mechanism and structural performance of lithium ion batteries, among which ARC is an effective method to conduct thermal runaway experiments of lithium ion batteries and then study the relevant reaction mechanism.
Lithium iron phosphate (LiFePO4) battery theoretical specific capacity of 170mAh/g, working voltage is 3.4V, with olivine structure, excellent safety performance, atmospheric pressure heating to 200℃ is still stable, and in the process of charging and discharging electrode structure changes very little. Lithium iron phosphate (LiFePO4) cathode material is widely used in electric vehicles due to its unique advantages of long cycle life, stable olivine structure, high safety, rich resources and low price.
Based on the ARC experiment of lithium iron phosphate 18650 battery after 100 cycles, and the anatomical analysis of the battery after ARC experiment, the thermal characteristics of the battery at different SOC states were studied.
1 Experiment
1.1 Experimental battery sample
Battery sample: The basic parameters of the lithium-ion battery sample used in the experiment are shown in Table 1.
Cyclic system: at 0.2C constant discharge to discharge termination voltage 2.0V, 10min later; At 0.2C constant current charging to 3.65V charging termination voltage, turn to constant voltage charging, stop charging when the charging termination current drops to 0.02C; After standing for 10min; Discharge at 0.2C to discharge termination voltage 2.0V, and activate twice; Then, the battery was charged at 1C constant current until the termination voltage of charge was 3.65V, and the battery was charged when the termination current of charge dropped to 0.02C. After standing for 10min, the battery was discharged at 1C until the termination voltage of discharge was 2.0V, and the battery was cyclic 100 times. Finally, the battery was charged at 1C constant current to the corresponding SOC capacity as the ARC experimental battery sample. Table 2 shows the state parameters of battery ARC experiment.
The exothermic reaction detection of accelerated calorimeter (ARCSYS-999, UK THT) battery samples was carried out in the "heat- wait- search seak" mode. ARC preheats the sample from room temperature and enters the operating mode to heat the sample after reaching the set initial temperature.
The temperature is heated according to the gradient. When the temperature rises by one step, the instrument is transferred to the waiting mode, waiting for the thermal balance between the sample and the system. Finally, we enter the search phase, and search the temperature change rate, which is the temperature rise rate (dT). If the temperature rise rate preset by the battery's temperature rise rate parameter is found), it is determined that an autoexothermic reaction has occurred inside the battery. The instrument stops active heating and turns into an adiabatic mode. The system temperature rises synchronously with the temperature rise of the battery until thermal runaway occurs. If the heating rate dT of the battery is found to be <0.02℃/min, the instrument will actively heat into a new round of "heat- wait- search seak" mode until the self-heat release or the preset end temperature is reached.
See Table 3 for information about ARC experiment Settings.
In order to avoid the effect of melting of the casing and top gasket on the weight loss of the battery at high temperature, the outer casing and top gasket of the battery were removed before the experiment.
1.2 Diaphragm Test
Sample treatment: The dissected battery diaphragm was washed with DMC for 6 times and dried in a vacuum oven at 100℃ for 1 hour.
Breathability measurement: The time required for a fully automatic breathability smoothness meter (4340, Gurley USA) to pass a 100mL volume of gas through a 6.45cm2 area diaphragm at 1.215kPa pressure. Breathability, also known as Gurley value, indicates the permeability of the diaphragm.
Morphology determination: Scanning electron microscope (S-4800, Hitachi, Japan) was used to observe the surface morphology of the diaphragm under the condition of 1kV and increased by 10K times.
2 Results and discussion
2.1 Thermal properties test
Under different SOC states, the battery was heated adiabatically by an accelerated calorimeter to test the thermal runaway phenomenon of the battery. The results are shown in Figure 1.
As can be seen from Figure 1(a), the shape of temperature and time curve of batteries under 10%, 50% and 100%SOC conditions is basically the same when the temperature is lower than 159℃. Lithium iron phosphate battery has no sudden change and does not trigger thermal runaway when the temperature is lower than 159℃. FIG. 1(a) shows a sudden change in temperature of 10% and 50%SOC batteries at 159℃, and then the battery temperature does not rise again. It is speculated that the battery pressure relief valve breaks at 159℃. The rupture of the battery pressure relief valve will cause the battery internal materials (gas, liquid) to eject. The high-temperature ejecta will take away some heat, resulting in the battery cooling. At the same time, when 10% SOC and 50%SOC are in low charge state, all side reactions are gentle, reaction rate is low, heat generation is slow, and the battery does not heat up again after cooling.
In FIG. 1(b) of 100%SOC battery, the heating rate is also negative when it is near 159℃, but it changes to a positive rate soon afterwards, and the temperature continues to rise. When the temperature rises to 174℃, the heating rate starts to increase continuously. When the temperature reaches 191℃, the heating rate is 1℃/min. When it reaches 220℃, the heating rate is 2.4℃/min(0.04K/s), and then the temperature rises sharply, eventually leading to the occurrence of thermal runaway. The highest temperature tested in ARC experiment reaches 340℃.
In Figure 1(b), the battery has a negative heating rate near 159℃. It can be inferred that the battery pressure relief valve breaks at 159℃. After the battery pressure relief valve breaks, materials inside the battery are ejected (gas and liquid). After pressure relief, although the battery is temporarily triggered to cool down, because at the same time, the diaphragm may be melted and damaged, the battery internal short circuit, the battery is in a state of high charge, the battery internal reaction intensifies, and the pressure relief battery continues to produce heat. When the temperature is 174℃, the heating rate begins to increase continuously, indicating that a large amount of heat is generated in a short time, and the heat begins to accumulate in large quantities, and the temperature continues to rise, and finally the thermal runaway is generated.
2.2 Quality loss
FIG. 2 shows the battery comparison after ARC experiment. As can be seen from the figure, there are obvious ejecta on the top of the three SOC batteries, and there are obvious marks on the outer wall of the battery after the evaporation of the electrolyte, indicating that the pressure relief valves of the batteries in the three SOC batteries were broken during ARC experiment, and substances (gas and liquid) in the batteries were ejected, and the quality of the batteries was reduced (see Table 4).
Comparison Table 4 With the increase of SOC, the battery mass loss increases, indicating that the intensity of internal reaction of batteries under different SOC conditions is different, and the internal reaction intensifies with the increase of SOC.
2.3 Comparative Analysis of Battery Disassembly
The battery after ARC experiment was disassembled, and a battery of the same type that was not tested was taken as a reference. The disassembly results are shown in Figure 3. When the battery is in 10%SOC state, the internal cell remains intact. When disassembled, the positive and negative electrode pieces and diaphragm are completely separated, and the diaphragm is grayish-white and transparent. For the battery under 50%SOC, the internal cell remains intact. When disassembled, the positive and negative electrodes and diaphragm are completely separated. The diaphragm is gray and transparent, and part of the negative electrodes fall off and adhere to the diaphragm.
In the battery under 100%SOC state, the internal cell remains intact, and the negative copper foil changes color. When disassembling, the positive and negative electrodes cannot be completely separated, the diaphragm disappears, the positive and negative electrodes stick together, and part of the positive and negative electrode materials peel off. Compared with the battery, the internal cell is complete. When disassembling, the positive and negative electrode pieces and the diaphragm are completely separated. The diaphragm is pure white.
It can be seen that with the increase of SOC, the internal cell of the battery changes greatly after ARC experiment, especially the diaphragm changes greatly, indicating that the intensity of the internal reaction of the battery under different SOC conditions is different, and the internal reaction is intensified with the increase of SOC.
2.4 Diaphragm Comparison
The disassembled diaphragm was washed with DMC and the breathability was measured. The results are shown in Table 5.
Compared with the breathability test, the breathability value of the reference battery diaphragm is 308s; The permeability value of 10%SOC state shows Dense999999, and the permeability increases sharply. It is speculated that during ARC test, the diaphragm is heated and melted, and the diaphragm is closed. The permeability value of 50%SOC state is 26s, which is far less than that of the reference battery diaphragm. It is speculated that the internal energy in higher SOC state will be higher during ARC test, and the internal reaction will be more intense at the same external temperature. The internal temperature will also be higher, and the diaphragm will be heated at a higher temperature, resulting in damage and larger holes, which will lead to internal short circuit.
The disassembled diaphragm was washed with DMC and observed by SEM. It can be seen from the SEM image that the diaphragm of the reference battery is a uniform small hole (Figure 4). After ARC experiment of 10%SOC battery, most of the pores of the diaphragm sample have disappeared, and the surface has been melted into a piece (Figure 5), indicating that the diaphragm is heated and melted during ARC, and the diaphragm is closed. After ARC test of 50%SOC battery, most holes of the diaphragm sample have been closed, and the diaphragm is locally damaged (Figure 6). The local damage of the diaphragm in the 10%SOC state and the 50%SOC state showed that the internal reaction was intensified with the increase of SOC.
3 Conclusion
The thermal runaway behavior of lithium iron phosphate batteries at different SOC after 100 cycles of 1C was studied by an accelerated calorimeter. By comparing the final temperature and heating rate of the battery, it is found that no thermal runaway is triggered when the battery is at 10%SOC and 50%SOC.
When the temperature reaches 174℃ at 100%SOC, the heating rate will continue to increase. When the temperature is 191℃, the heating rate is 1℃/min. When the temperature is 220℃, the heating rate is 2.4℃/min(0.04K/s). The temperature then rose sharply, eventually leading to thermal runaway, with the highest temperature reaching 340℃. The battery mass loss was 9.7%, 9.9% and 11.8% after ARC experiments with 10%SOC, 50%SOC and 100%SOC, respectively.
Through the analysis of diaphragm test, the diaphragm closed hole after ARC experiment at 10%SOC state; The diaphragm is damaged at 50%SOC state; At 100%SOC state, the diaphragm completely disappeared (melted) after ARC experiment, indicating that the risk of thermal runaway of the battery increases with the increase of the state of battery charge.
