When talking about electric vehicles, people often worry about their cruising range and charging time. With the continuous advancement of technology, the current range of electric vehicles has reached more than 300km, even more than 400km, close to the level of fuel vehicles. The only shortcoming of a car compared to a fuel car is that it takes too long to charge. The key to reducing the charging time of an electric car is to increase the charging speed of the power battery. However, we all know that Li+ is pulled out from the positive electrode when the power battery is charged, and migrates after solvation. On the surface of the negative electrode, after desolvation on the surface of the negative electrode, it is embedded in the inside of the graphite negative electrode. At an excessively fast charging speed, the polarization of the negative electrode increases remarkably, resulting in a decrease in the potential of the negative electrode, causing precipitation of metal Li on the surface of the negative electrode. This leads to a decrease in the coulombic efficiency of the lithium ion battery, which affects the cycle performance of the battery, and in severe cases, causes a short circuit in the lithium ion battery.
In the past, we improved the fast charge performance of lithium-ion batteries mainly from the choice of anode materials, using a smaller particle size anode material, increasing the active area to reduce the current density, and reducing the diffusion distance of Li+ in the graphite anode to achieve improved charging. The purpose of the magnification. However, if the negative electrode particles are too small, there will be some problems, such as a decrease in tap density and a decrease in efficiency for the first time. Hye Bin Son (first author) and Nam-Soon Choi of UNIST, Ulsan National Science and Technology Research Institute, Korea (Corresponding author) But I turned my attention to the choice of electrolyte solvents and additives that we pay less attention to. If we carefully analyze the structure of the negative electrode, we will find that there is an inert layer between the graphite negative electrode and the electrolyte. It is the SEI film that we often say that the Li+ in the electrolyte can only be embedded in the layered structure of the graphite negative electrode after passing through this inert layer, so the characteristics of the SEI film will also produce the fast charge performance of the lithium ion battery. Significant influence.
The functional additive of the electrolyte has a significant effect on the structure and composition of the SEI of the negative electrode. Therefore, the authors studied and analyzed the effects of three additives of EC, FEC and VC on the fast charge performance of NCM622 lithium ion battery, in order to analyze the solvation pair. The effect of lithium-ion battery on fast charge new energy, the author also analyzed the effect of different electrolyte solvent formulations on fast charge performance.
The positive electrode material used in the test was NCM622 from L&F, the negative electrode material was artificial graphite, and the coating amount of positive and negative electrodes was 18 mg/cm. 2And 8.3mg/cm 2Molecular orbital theory is an important tool for electrolyte research. Generally, the lower the LUMO energy of a molecule, the easier it is to get electrons, and the easier it is to reduce the surface of the negative electrode. The higher the molecular HOMO energy, the higher. It is easy to lose electrons, and the easier it is to oxidize on the surface of the positive electrode. Therefore, LUMO and HOMO energy become important parameters for additive selection. The following figure shows the LUMO and HOMO energy spectra of different molecules calculated by the density function theory of Hye Bin Son. From the figure, it can be seen that VC is easier to reduce on the surface of the negative electrode, and it is easier to oxidize on the surface of the positive electrode.
Figure a below shows the charge and discharge curves of electrolytes with different electrolyte additives. From the figure, it can be seen that the electrolyte with EC added (EC: DMC=5:95) has a higher voltage platform during charging and constant voltage charging. The process is also longer, which indicates that the ohmic impedance inside the lithium-ion battery is larger than that of the other two additives. Considering that the EC solvent has a higher dielectric constant (89.8) and a higher donor constant than the DMC solvent ( 16.4), therefore, the solvation process of Li+ is mainly that EC plays a role, so a layer of EC-based solvation sheath is formed around Li+. As Li+ migrates to the surface of the negative electrode, the LUMO energy of EC is lower. Therefore, it is easier to obtain an electron decomposition reaction on the surface of the negative electrode to form an SEI film, which results in the formation of a thicker SEI film on the surface of the negative electrode using the EC additive electrolyte, which also leads to an increase in ohmic impedance and first efficiency. Compared with the first efficiency of the three additives, we found that the first efficiency of the electrolyte of VC and FEC additives is about 90.6%, the discharge specific capacity of NCM material is 172mAh/g, and the first efficiency of the electrolyte using EC additive is only 85.2%, the capacity of the positive electrode material is only 163mAh / g, which is also the fact that the above speculation is basically in line with the fact. Hye Bin Son believes that this is mainly because the LUMO energy of FEC and VC is relatively low, so it can be higher. Reductive decomposition occurs at the potential to form a more stable SEI film with a composition and structure. Generally, a better SEI film means better cycle performance. From the following figure c, the cycle of the electrolyte using the EC additive can be seen. The performance is poor. After 200 cycles (0.5C charge and discharge), the capacity retention rate is only 88.3%, and the coulombic efficiency is only 96.9%. However, the battery retention rate of the electrolyte using FEC additive is up to 98% after 200 cycles. It shows that the SEI film formed in the electrolyte with FEC has more stable characteristics.
In order to test the cycle performance of several electrolytes at large magnification, the authors will use the above-mentioned electrolytes for 2C charging 1C room discharge cycle 100 tests, it is worth noting that the performance is better in 0.5C charge and discharge. The VC electrolyte had the worst performance at 2C rate, and the capacity retention rate was only 61% after 100 cycles, while the electrolyte with FEC showed better performance. The capacity retention rate reached 90% after 100 cycles of 3C charge cycle. From the EIS test results, the SEI film formed in the electrolyte with VC additive has the largest charge exchange resistance, which leads to a significant increase in the polarization of the negative electrode during rapid charging, and it is easy to cause Li phenomenon in the negative electrode, which seriously affects Battery cycle performance.
The following figure shows the change of the anode potential when the battery with three electrolytes is charged at 10C rate. It can be seen from the figure that the potential of the anode drops rapidly to below 0V during the charging process of the battery using VC additive electrolyte. The precipitation of metal Li on the surface of the negative electrode.
Hye Bin Son will use different electrolytes and dissipate the battery after 2C charge 1C for 100 cycles. Observe the cross section of the negative electrode (as shown below). From the figure below, we can see that EC and VC additives are used for electrolysis. Li-dendrites of disordered growth appeared on the surface and inside of the negative electrode of the liquid battery. In particular, the surface of the negative electrode of the VC additive electrolyte grew a thick layer of metal Li dendrites, and the growth of Li dendrites was consumed. Part of the active Li leads to a decrease in the reversible capacity of the lithium ion battery. However, no metal Li dendrites are observed on the surface of the negative electrode using the FEC additive electrolyte, which is mainly due to the SEI impedance formed in the FEC additive electrolyte. Low, so the polarization of the negative electrode is small.
In order to further analyze the effect of SEI film composition formed by different additives on fast charge performance, the authors used XPS tool to analyze the negative electrode after 0.1C pre-circulation. It can be seen from the following figure e that the negative electrode is formed in VC additive electrolyte. The SEI film contains more poly-VC structure. This long-chain structure may lack mobility and may affect the diffusion of Li+, which may cause severe polarization of the negative electrode during high-rate charging, resulting in the occurrence of Li. The SEI film formed in FEC contains more Li2CO3, which can allow Li+ to diffuse through, thus improving the rate performance of the negative electrode.
It is not difficult to see from the above analysis that the FEC additive can form a more suitable SEI film, which can significantly improve the fast charge performance of the lithium ion battery. Therefore, the author has analyzed the addition ratio of FEC in the electrolyte, from the following figure a It can be seen that different proportions of FEC additives have little effect on the first efficiency of the battery, which is about 90.5%. From the following figure c, it can be seen that when charging at 5C large magnification, 30% of FEC added batteries can obtain the highest. The discharge capacity, and the electrolyte with FEC content of 70% will lead to a rapid decrease in the fast charge performance of the battery. This is mainly because the viscosity of the electrolyte increases as the amount of FEC increases, and the FEC content increases to 70%. In the future, the viscosity rises sharply, causing the ionic conductivity of the electrolyte to decrease. At the same time, we notice that when the FEC content is below 30%, although the viscosity of the electrolyte increases, the ionic conductivity of the electrolyte is still significantly increased. Mainly because FEC is a polar molecule, which can promote the solvation of Li+, thereby increasing the carrier concentration in the electrolyte and increasing the conductivity of the electrolyte.
The authors further analyzed the effect of solvent on battery rate performance. The test showed that the rate performance of FEC/DMC system was significantly better than that of FEC/EMC system. Further analysis found that the fast charge performance of graphite anode in these two systems is close. However, the fast charge performance of the NCM positive electrode is greatly affected by the electrolyte solvent system. Therefore, the effect of the two electrolyte systems on the fast charge performance of the battery is mainly reflected in the NCM positive electrode material. The author believes that this is mainly a process of rapid charging. In the process, as the large amount of Li+ enters the electrolyte, the FEC is gradually consumed in the process of solvation, so the electrolyte solvent and Li+ are required to be solvated, but since the molecular chain of EMC is long, it is not easy to form. The solvation outer sheath, while DMC can more easily arrange the space, so that the solvation ability is stronger, thus improving the fast charge capacity of the lithium ion battery.
The work of Hye Bin Son shows that FEC additive can form SEI film with smaller charge exchange resistance and higher ionic conductivity on the surface of the negative electrode, thereby reducing the polarization of the negative electrode during rapid charging, reducing the precipitation of metal Li, and improving the high charge rate. The cycle performance. The solvent research also found that the solvating ability of DMC solvent can significantly improve the cycle stability of lithium ion battery at high charge rate.
