Research has found that Li-containing organic matter formed on the surface of the SEI film is one of the important factors causing thermal runaway of 3.7v 2200mah 18650 lithium battery.

　　Graphite anode is the current mainstream anode material for lithium-ion batteries. Its lithium insertion potential is low (<0.2VvsLi+/Li), which makes lithium-ion batteries have high-voltage characteristics that other types of batteries do not have. However, the low potential of graphite anode makes Commercial carbonate solvents will become very unstable on the negative electrode surface. For example, common EC solvents begin to decompose at around 1.2V. Part of the decomposition products of the electrolyte will be converted into gas, and the other part will be converted into solid phase substances and deposited on the surface. The surface of the negative electrode becomes what we usually call the SEI film.

　　The SEI film is a passive film that can effectively inhibit the decomposition of the electrolyte and improve the cycle life of lithium-ion batteries. However, the SEI film is not completely stable. Generally, we believe that the SEI film begins to decompose when the battery temperature reaches above 60°C. Above 90°C, spontaneous exothermic reactions begin to occur, causing the life of the lithium-ion battery to decline and even trigger thermal runaway. Therefore, the thermal and chemical properties of the SEI film are crucial to the safety and cycle stability of the lithium-ion battery. Impact.

　　Recently, KihyunSon (first author), EuiHwanSong (corresponding author), Young-JunKim (corresponding author) and others from Sungkyunkwan University in South Korea used the separator peeling method to analyze the thermal and electrochemical properties of independent SEI films.

　　Although there are currently many studies on SEI films, most of them are conducted in battery systems. Due to the presence of interference factors such as active materials, binders, and current collectors, it is difficult to accurately measure the thermal characteristics of SEI films. Therefore, KihyunSon adopted the separator-assisted peeling method to obtain separator + SEI film samples and tested them to obtain more accurate thermal characteristics of the SEI film.

　　In the experiment, the author used to obtain the SEI film sample. The positive electrode of the 33mAh soft pack battery was Mg-doped LCO material, the negative electrode was artificial graphite material, the electrolyte was provided by Samsung SDI, and the separator was 10um thick and had an alumina-polymer coating. The above-mentioned batteries are first precharged to 2.5V using a 0.5C rate, and then formed (2.75-4.4V) after aging for one day. In order to obtain a sufficient number of SEI film samples, the author cycled the above-mentioned battery 400 times in the range between 2.75-4.4V (as shown in Figure a below). After the battery was cycled 100 times, 200 times and 400 times The capacity retention rates are 90.2%, 78.7% and 64.7% respectively. Figure c below shows the EIS test results of the battery before cycling, after 200 cycles and 400 cycles. It can be seen that the battery EIS curve mainly consists of a compressed semicircle in the high frequency range and a diffusion curve in the low frequency range. The EIS test After fitting the results, we can find that as the number of battery cycles increases, the ohmic impedance, SEI film impedance and charge exchange impedance of the battery all increase significantly (as shown in Figure d below).

　　After the above-mentioned battery was cycled, the author dissected the battery and found that the separator and the negative electrode had been completely stuck together. After separating the separator from the negative electrode, a layer of SEI film would stick to the surface of the separator. Usually we think this is mainly because The outer structure of the SEI membrane mainly contains porous organic components. The author made the above-mentioned separator with SEI film into a sample with a diameter of 4mm for DSC testing (the test results are shown in Figure f below). From Figure f below, you can see that all diaphragm samples have a small endothermic peak near 140°C, which corresponds to the melting of the diaphragm. At the same time, we noticed that the cycled separator samples had a relatively obvious exothermic peak, while the uncycled separators did not show an obvious exothermic peak. Therefore, the author judged that the exothermic peaks of these separator samples mainly came from the adhesion of the separator surface. The decomposition reaction of SEI components. The heat release test of different separator samples shows that the heat release of separator 1# and 2# samples after 200 cycles are 322.4mJ and 81.2mJ respectively, while the heat release of the separator sample after 400 cycles is The heat reached 751.1mJ, indicating that samples with longer cycle times produced more SEI.

　　In order to analyze the composition of the SEI film, the author used XPS to analyze the SEI film sample stuck on the separator (the results are shown in the figure below). From the figure below, we can see that the main components on the sample without circulation are ROCO2Li-, and Some O-containing polymer components, such as polyethylene oxide (PEO), also contain small amounts of Li2CO3 and polyVC. In the samples after cycling, we observed considerable amounts of Li2CO3 and ROLi, as well as ROCO2Li-, PEO and Li2O, etc., which indicates that during the cycle, part of the ROCO2Li- reacts and decomposes with traces of H2O or CO2 gas in the electrolyte. It becomes Li2CO3, and the small amount of Li2O in the SEI film mainly comes from the decomposition of trace amounts of H2O or Li2CO3.

　　The figure below shows the infrared absorption spectra of different SEI film samples. From the figure below, we can see that several relatively weak peaks can be observed in the uncycled samples. The peaks at 1776 and 1805/cm are respectively the peaks in polycarbonate. The stretched C=O bond and the Li-containing product after EC/DMC decomposition, the peak at 1404/cm is the -CH3 or -CH2 bond in ROCO2Li, the peak at 1271/cm is the O-C-O bond energy in Li2CO3, 1182/ The peak at cm mainly comes from the decomposition products of LiPF6 and EC/DMC, and the peak at 1078/cm is mainly from the C-O bonds in ROCO2Li and Li2CO3. After 200 cycles, the intensity of these peaks increased significantly. After 400 cycles, the intensity of these peaks further increased. In particular, the peaks of ROCO2Li and Li2CO3 were significantly enhanced. At the same time, we also observed some newly formed characteristic peaks in the electrode after 400 cycles, such as the C-H bond in ROLi near 1456/cm and 1483/cm, and the Li-COO bond in lithium carboxylate, and in ROCO2Li Stretched C=O bonds (1650/cm), and C-H bonds in -CH3/ROCO2Li/ROLi/polycarbonate. This finding shows that the SEI film on the negative electrode surface thickens significantly after long-term cycling, and its surface layer contains a large number of organic components, such as polycarbonate, ROCO2Li, and Li-EC and Li2CO3.

　　Therefore, the exothermic reaction in the SEI film sample below 140°C that we observed in the previous DSC experiment mainly comes from the decomposition of organic components in the SEI film, such as ethylene carbonate Li (from the decomposition of the EC solvent), The specific reaction is shown in the following formula.

　　KihyunSon's research shows that after long-term cycling, the surface layer of the SEI membrane will form a large amount of Li-containing organic components, and these components will begin to undergo exothermic decomposition reactions at a lower temperature (140°C lower than the melting temperature of the separator). , which is also the main source of heat released by the SEI film decomposition reaction at low temperatures, and is one of the important factors that may cause thermal runaway of lithium-ion batteries.

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