The high voltage, high energy density and excellent cycling performance of lithium-ion batteries make it the most successful chemical energy storage battery at present. In particular, the rapid development of the electric vehicle industry in recent years has enabled lithium-ion batteries to usher in a period of rapid development. The huge market stimulated the rapid expansion of global lithium-ion battery production capacity. Compared to traditional chemical energy storage batteries, lithium-ion batteries have significant advantages in both energy density and cycle life, but lithium-ion batteries in the temperature adaptability There are still many gaps. Due to the deterioration of the kinetic conditions at low temperatures, the battery capacity will decrease, and even safety problems will arise. At high temperatures, the side reactions between the positive and negative electrodes of the lithium-ion battery and the electrolyte will increase, causing the battery to The increase in resistance can affect the cycling performance of lithium-ion batteries. This has led to the necessity of adopting complex temperature control systems in lithium-ion battery applications. This will increase the cost of lithium-ion battery packs and increase the consumption of electrical energy. , Reduce the cruising range of electric vehicles.
In response to the problem of gas production and cycling performance degradation of lithium-ion batteries at high temperatures, R. Genieser et al. of the University of Warwick, UK, developed electrolytes for NMC111/graphite cells at 80°C, increased internal resistance, and decreased capacity. Careful analysis was carried out and it was shown that the increase of battery internal resistance mainly originated from the increase of the charge exchange resistance of NCM111 positive electrode material. R. Genieser thought that the main factor that caused the increase of the charge exchange resistance of NCM111 material was the breakage of secondary particles, so the follow-up The optimization of the material's high temperature performance should be carried out for the structural stability of the secondary particles.
The electrolyte composition used by R. Genieser in the experiment is shown in the following table, wherein PES is 1,3-propenyl-sultone, DTD is methane methane disulfonate, and TTSPi is (trimethylsilyl). ) Phosphite.
The figure below shows the cycling curves of batteries with different electrolytes at high temperature of 80°C. It can be seen that the batteries with A electrolyte without any additive lost more than 90% of the initial capacity in 50 cycles. In addition, A electrolytics were used. The liquid battery also experienced significant bulging after cycling. This may be due to LiPF6 decomposing to LiF and PF5 at high temperatures. PF5 is a strong Lewis acid that reacts with water to produce HF and POF3 and causes further reactions. Produce PEO and CO2 (as shown in the formula below). In addition, the positive electrode can also cause oxidation of the electrolyte solvent, resulting in CO, CO2 and other gases.
Simply adding 1% of VC additive to the electrolyte can greatly improve the cycling performance of the electrolyte at high temperatures (curve B), while adding PES and DTD, electrolyte C of TTSPi additive is slightly better than electrolyte B, The electrolyte D in the early cycle is similar to the B and C electrolyte cycles, but after 200 cycles, the capacity of the battery suddenly drops. Adding more FEC (a common Si negative electrolyte Additives, which can produce SEI films with higher LiF content and thus improve the mechanical stability of the SEI film.) Electrolyte E also shows a rapid decrease in capacity at high temperatures, and there is a phenomenon of battery bulging, which may be FEC decomposition caused by high temperature (shown below).
The charge exchange resistance Rct obtained by EIS fitting is shown in Figure b below. It can be seen that the increase in the charge exchange impedance of the electrolyte C battery in the cycle is slow compared to other electrolytes. This may be due to the improved PES additive. The stability of the electrolyte. It can be seen from the series resistance Rs obtained from the fitting that only electrolytes B and C remain relatively stable in the cycle, and other electrolytes show a clear rise. Here we note the electrolyte. After D is about 200 times, there is a significant increase in Rs. At this time, the battery capacity just jumps. It indicates that the electrolyte D may have been depleted in the battery (the solvent PC is prone to co-embedding, DEC is in LiPF6. The electrolyte is not stable, these factors together cause the electrolyte D to consume faster.
Figure a shows the volume expansion of a battery with additive-free electrolyte A during cycling at 40°C and 80°C. The periodic volume change of the battery at low temperatures is mainly due to the positive and negative electrodes in the lithium insertion and delithiation process. Volume change (lithium expansion during graphite insertion of 8%, NCM volume expansion of about 2% in the delithiation process), so R. Genieser believes that the large volume expansion of NCM batteries at high temperatures is mainly caused by the positive electrode electrolyte oxidation production Gas (research shows that EC oxidation produces CO and CO2 is the most important source of battery gas production.) As can be seen in Figure b, the volume expansion of the battery with electrolyte B is significantly slower than that of the battery with electrolyte A, which may be Because VC helps to form a more stable passivation layer (research shows that VC not only forms a stable SEI film on the negative electrode, but also shows a thin layer of passivation layer on the positive electrode), while electrolyte C undergoes After the initial expansion, there was no significant expansion of the battery volume at the end of the cycle, indicating that the additives in electrolyte C can form a more stable passivation layer on the positive electrode surface, thus less oxidative decomposition of the electrolyte solvent.
The following figure shows the SEM photographs of the positive and negative electrodes of the battery after the A and C electrolytes were circulated at 25 and 80°C. It can be clearly seen that the electrolyte of the NCM surface was decomposed after the high temperature cycle of the battery without the added electrolyte A. It is significantly thicker than 25°C cycle. No obvious change has occurred on the surface of NCM with electrolyte C. Of course, this does not mean that the electrolyte is not decomposed on the surface of the positive electrode because the decomposition product of the electrolyte at the positive electrode may migrate. SEI film is formed on the negative electrode surface.
From the SEI film on the surface of the negative electrode, it can be seen that electrolyte A without additives will form a thick electrolyte decomposition product (mainly LiF) at high temperature, and electrolyte solution decomposition products on the negative electrode surface of electrolytes B and E The smaller particles are smaller, which indicates that the two electrolytes can form a more stable SEI film or the two electrolytes decompose less on the positive electrode surface. The negative electrode of the electrolyte solution D shows an inflorescence-like state and cannot be dissolved. In DMC, it is indicated that this may be a decomposition product of LiPF6. However, the surface state of the negative electrode of the battery using the electrolytic solution C has no significant change compared with the battery circulating at room temperature.
From the results of EIS analysis, the impedance of various electrolyte NCM positive electrodes showed a significant increase after high temperature cycling, and the increase in the battery's positive electrode impedance using electrolyte A was mainly due to the decomposition of the electrolyte on its surface, using electrolysis. Although battery B of liquid B can suppress the decomposition of the electrolyte in the positive electrode, since the number of cycles is large (350 times), the impedance increases the most, and the resistance of NCM material in the C electrolyte with the same number of cycles is greater than that of the cycle. The smaller number of electrolytes A is still less, indicating that the electrolyte stability is better, but the NCM materials using electrolytes D and E have a shorter number of cycles (175 times), so the internal resistance increases little.
The following figure a shows the XRD diffraction pattern of the NCM positive electrode material in the electrolyte C after the formation and the high temperature cycling at 80°C. It can be seen that the crystal structure of the NCM material has not changed significantly after the high temperature cycle, but we note that The hkl values of the two diffraction peaks at 006/012 and 018/110 changed significantly, indicating a significant change in the a and c values of the material. Generally, we believe that a/c is linearly related to the active Li in the material. As shown in Figure b below, we can see from the figure that the stoichiometric x of the Li in the NCM material circulating in the electrolyte C is 0.952, indicating that most of the active Li (96.4%) is retained in the material. However, the NCM material with electrolyte B lost more active Li after high-temperature cycling. The remaining activity Li is equivalent to SoC = 74.4%. Here we can see that the battery with electrolyte C has almost no temperature cycling. The loss of active Li occurs, and then the loss of 35% capacity in the cycle comes from there? This is probably due to the increase in the surface charge exchange resistance of the positive electrode, resulting in increased battery polarization during charge and discharge.
In order to analyze the factors causing the increase of the charge exchange resistance of NCM using electrolyte C, R. Genieser observed and analyzed the cross-section of NCM material. From the figure below, we observed that NCM is formed by sintering a large number of primary particles together. The secondary particles, but after 80 °C high temperature cycle, NCM produced a lot of cracks between the primary particles, resulting in part of the primary particles separated from the main particles, causing increased contact resistance between particles.
R. Genieser's research results gave us insights into the deep mechanism of NCM/graphite cells' capacity decay and gas expansion at high temperatures. We found that electrolyte additives reduce the decomposition of electrolytes at high temperatures, reduce gas production, and increase the number of cells. The cycling performance has an important influence. R. Genieser's research also shows that although electrolyte additives such as PES, DTD and TTSPi can effectively improve the cycle performance of NCM/graphite cells, reduce battery gas production and reduce the loss of active Li, but Still can not avoid the increase of the positive charge exchange impedance, further research shows that the increase in NCM charge exchange impedance is mainly due to NCM secondary particle internal cracks, resulting in part of the separation between the primary particles and the main particles, resulting in increased contact resistance, resulting in The increase of the polarization leads to the decrease of the discharge capacity of the battery. Therefore, the optimization of the high temperature performance of the NCM material is mainly to improve the stability of the secondary particles of the NCM material.
