Introduction: Compared with ternary lithium-ion batteries with higher energy density, lithium iron phosphate batteries are considered to have higher safety, so what is the experience of thermal runaway of safer lithium iron phosphate batteries?
In 2018, under the background of the first decline in the entire automobile market in more than 20 years, new energy vehicles still bucked the trend and increased by more than 60%, becoming a group of dark horses in the automobile market.
With the large-scale popularization of new energy vehicles, the safety of power batteries has also attracted more and more attention. Compared with ternary lithium-ion batteries with higher energy density, lithium iron phosphate batteries are considered to have higher energy density. Safety, so what is the experience of thermal runaway in a safer lithium iron phosphate battery?
Recently, PeterJ. Bugryniec (first author) and Solomon F. Brown (corresponding author) of the University of Sheffield in the United Kingdom used accelerated calorimetry (ARC) and hot box experiments for LFP batteries in different SoC states leading to thermal runaway The main reasons for the occurrence are analyzed, and the study shows that under high SoC, the decomposition reaction of cathode and anode is the main cause of thermal runaway of LFP battery, but the decomposition reaction of anode is the main cause of thermal runaway of LFP battery at lower SoC state. reason.
The LFP material has an olivine structure. We believe that due to the presence of more stable PO bonds, the LFP material has high stability at high temperatures. We take the 18650 structure battery as an example. If the LFP material is used, it can be at most in thermal runaway. 0.5g of O2 is released, but if we use LCO as the cathode material, as much as 3.25g of O2 can be released in thermal runaway, less O2 release means that the combustion reaction of the electrolyte is inhibited, releasing less heat, thus Suppresses the severity of thermal runaway in LFP cells.
The battery used in the experiment is a commercial LFP 18650 battery with a capacity of 1500mAh, and the thermal runaway behavior of the LFP battery is studied by ARC and hot box experiments respectively (as shown in the figure below), and the SoC of the LFP battery is controlled to be 0%, 28%, 63%, 100% and 110% were tested for ARC (accelerated calorimetry), and the control SoC was 100% for hot box test.
The ARC test is a common method to study the thermal stability of lithium-ion batteries. The basic operation method can be divided into three steps. The first step is to heat to a predetermined temperature, the second step is to wait, and the third step is to search, that is, the battery is at a certain temperature. When the heating rate of the battery temperature reaches a certain rate, it means that the battery starts to release heat. If the heating rate of the battery reaches a certain rate, it is considered that the battery starts to thermally runaway. Here, the author sets the starting temperature of ARC to 50°C and the end temperature to 315°C. The temperature increases by 5°C in each step, and waits for 60 minutes. If the heating rate of the battery reaches 0.02°C/min at this temperature, the temperature is the battery If the heating rate of the battery reaches 1°C/min, the temperature is the thermal runaway trigger temperature of the battery.
Figure a below is the ARC test curve of a 100% SoC battery. From the figure, it can be seen that the self-heating temperature of a 100% SoC LFP battery starts at 95 °C, and then the heating rate of the battery increases continuously, and reaches 3.7 °C at 230 °C /min, but then the heating rate of the battery began to decline, and a new high point appeared around 280 °C – 1.6 °C/min. Figure a below can be divided into four regions, in which region 1, 95-150 ℃, the battery starts to heat itself, which mainly corresponds to the decomposition of the SEI film on the surface of the negative electrode, accompanied by the negative electrode-electrolyte reaction, in region 3 At 150-255℃, the heat generated in this stage mainly comes from the side reactions of negative electrode-electrolyte and positive electrode-electrolyte, among which the heat released by negative electrode-electrolyte occupies most of it. In region 4 (>255 °C), the internal heat generation of the battery at this stage mainly comes from the oxidation reaction between the electrolyte and the O2 generated by the decomposition of LFP.
As can be seen from Figures b and c below, the shape of the ARC curve of the battery under 110% SoC and 63% SoC is basically the same as the shape of the ARC curve of the 100% SoC battery, but when the SoC of the battery drops further to 28%, Then the shape of the ARC curve of the battery will change significantly (as shown in Figure d below). From the start of self-heating of the battery until 190 ° C, the heating rate of the battery has been increasing, and reached a peak at about 190 ° C, and then began to heat up. decreased, and then the heating rate of the battery began to slowly increase again. In the low SoC state, the LFP positive electrode is relatively stable, so the heat production of the first half of the battery mainly comes from the decomposition reaction of the negative electrode and the electrolyte. decomposition reaction, but due to the relatively high stability of the positive electrode under this SoC, the heating rate of the battery is relatively slow.
At 0% SoC, the shape of the ARC curve of the LFP battery is further changed. It can be noticed from the figure that on the one hand, there is a significant delay in the self-heating start temperature of the battery, and secondly, the heating rate peak of the battery near 190 °C also disappears. It shows that under low SoC, the battery is in a relatively stable state, and the negative electrode has been completely delithiated, so the speed of the negative electrode-electrolyte decomposition reaction is also greatly reduced. Basically the same, a small amount of O2 released from the decomposition of the LFP cathode promotes the decomposition of the electrolyte, which makes the heating rate of the battery increase slowly.
The figure below shows the battery’s self-heating trigger temperature, maximum heating rate temperature and temperature corresponding to the maximum heating rate based on the ARC test results. It can be seen from the figure that as the SoC of the battery increases, the maximum heating rate of the battery also increases. It rises accordingly, mainly because more energy is stored in the battery under the higher SoC, and the higher SoC also means that the stability of the positive and negative electrodes of the battery is also lower, mainly because the stored Li in the negative electrode is more. Therefore, the decomposition reaction of the negative electrode with the binder, the electrolyte, etc. releases more heat, thereby accelerating the temperature increase of the lithium-ion battery.
Since the maximum heating rate can reflect the stability of the positive and negative electrodes inside the lithium-ion battery, the maximum heating rate can well reflect the risk of thermal runaway in the lithium-ion battery. The following figure compares several common lithium-ion battery positive systems in different The maximum heating rate in the state of Other types of batteries offer significant safety advantages.
The figure below shows the change curve of the surface temperature of the LFP battery in the hot box test (solid line), and the temperature inside the hot box (dotted line). During the process of temperature increase under heating, the surface temperature of the battery is lower than 95°C, and the battery has not yet begun to release heat. In area B, the surface temperature of the battery continues to rise to about 180°C. At this stage, the SEI film begins to decompose, the negative electrode-electrolyte and positive electrode-electrolyte decomposition reactions begin to occur, the battery begins to heat itself, and the battery temperature rises rapidly and quickly exceeds Hot box temperature, and eventually the battery’s pressure relief valve burst due to overpressure. In area C, the battery thermal runaway reaches the peak temperature after the battery pressure relief valve is activated, and in area D, the battery thermal runaway ends, and the temperature of the battery finally returns to the temperature of the hot box.
Comparing the battery surface temperature curves obtained from two hot boxes with different temperatures, it can be found that the peak temperature of the battery in the 220 °C hot box during thermal runaway is significantly higher than that of the battery in the 180 °C hot box, which indicates that in the 220 °C hot box Additional reactions will occur in the thermal runaway of the battery. The previous ARC analysis shows that the LFP cathode decomposition reaction will only occur when the battery surface reaches 210 °C, while the electrolyte decomposition reaction will only occur when the battery surface temperature exceeds 255 °C. Occurs, and the maximum temperature of the battery surface in the 180°C hot box test is less than 230°C, so at least the battery has not yet reached the decomposition temperature of the electrolyte, and the O2 released by the LFP positive electrode will also be significantly reduced at lower temperatures, which are all significant It reduces the heat generation rate of the lithium-ion battery, thereby suppressing the increase of the battery temperature.
Peter J. Bugryniec’s research shows that SoC has a significant impact on the thermal runaway behavior of LFP batteries. With the increase of SoC, the severity of thermal runaway of the battery increases significantly, and the stability of the battery decreases significantly. The analysis of the specific causes of thermal runaway shows that the main cause of battery thermal runaway in 100% and 110% SoC states is the decomposition reaction of anode-electrolyte and cathode-electrolyte, but the battery thermal runaway in lower SoC states The main trigger factor is the decomposition reaction of the anode-electrolyte. When the SoC is lower than 28%, the thermal stability of LFP is significantly improved, and thermal runaway will not occur. The hot box test shows that higher hot box temperature will lead to more serious thermal runaway of Li-ion batteries, mainly because better hot box temperature triggers the decomposition reaction of electrolyte and the decomposition of cathode to release O2 reaction, which intensifies the battery Rise in temperature.
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