As the energy density of lithium ion battery continues to improve, traditional graphite materials have not been able to meet the demand of high-proportional battery, and the theoretical capacity of Si material can reach 4200mAh / g (Li4.4Si), and the lithium lithium voltage and graphite materials are close, it is one Ideal negative material. However, the Si negative electrode is as high as more than 300%, which will cause the Si particles itself, but also destroy the structure of the electrode, resulting in peeling of the active material, and the repeated volume expansion can cause an electrode.
The damage of the conductive network causes partial active substances that cannot be refined and charged, while huge volume expansion can also cause damage and regeneration of the negative electrode SEI membrane, which will cause the cycle performance of the Si negative electrode to be far less than graphite materials.. Therefore, the application of the Si negative electrode should be launched from two aspects: 1) Do a good job in the stability of the electrode structure, reduce the loss of active substance caused by volume expansion; 2) Improve the structural stability of the SEI film, reduce the decomposition and activity of electrolyte Consumption.
TonyJaumann (IFW) of the German Leibniz Solid State and Materials Institute (IFW) is made of FEC additive, CMC / SBR and PAA adhesive to the formation and performance of SEI film of nanoscale Si material In-depth research and analysis, the study shows that the Si film formed by the Si negative surface after the addition of FEC, and the cycle performance of the Si negative electrode is also better.. The adhesive also has a certain effect on the SI membrane, and the Si negative of the Si negative electrode of the PAA adhesive is thicker, but the composition of the SEI film has no large changes.
. The stability of the SEI film is critical to the Si negative electrode, and the electrolyte additive is the key to improving SEI film stability. Many studies have shown that the addition of FEC in the electrolyte can significantly improve the cycle performance of the Si negative electrode, which is important Because FEC additives are capable of optimizing the structure and components of the SEI film.
Usually we believe that the electrolyte is the only factor affecting the SEI film, but in fact, in the adhesive in the Si negative electrode, some functional groups (e.g., RCOOH) can occur and the surface of the Si negative electrode, thereby change some of the electrochemical properties of the Si negative surface.
Therefore, there is also an effect on the composition and structure of the Si negative electrode SEI film.. In the experiment, Tonyjaumann studies analyzes FEC additives, and the effects of CMC / SBR and PAA binders about nano-stage Si negative electrode (5 nm) cycle performance, SEI film structure and components.
. The nanoscale Si particles are relatively large than the surface area, so slow oxidation in the air environment, so the surface characteristics of the nano-Si particles may change during the process of both slurry of the CMC / SBR system and PPA system..
From the XPS analysis results from the following figure B, we can see that we can observe two peaks at 99.9 EV and 104EV. These two peaks represent Si and SiO2, and some of them are Si.
Osy, this shows that in the experiment, whether using a CMC / SBR binder or a PAA binder in the electrode preparation process, the surface oxidation of nano Si. After the two electrodes described above were formed into a buckle battery, the control group electrolyte (1 mlIPF6, EC / DMC = 1: 1) and an electrolyte added FEC (1 mlIPF6, EC / DMC / FEC = 1: 2: 1), the figure below is the cycle performance of the four batteries. From the figure we can notice that the four batteries are almost the same, but there is a significant gap in subsequent cycles.
. From the figure we can notice that the battery cycle performance of the addition of the FEC electrolyte is obviously better than the battery of the control group electrolyte, and the battery using the control group electrolyte is reached at 400 times after the capacity decline is 75%, and added FEC rear battery cycle 400 capacity decline rate is only 36%, while the selection of the binder is not much affected by battery cycle performance..
The picture below shows the Cullen efficiency curve of the battery of the two electrolytes. It can see the battery library efficiency of the FEC electrolyte in the first charge is only 35%, while the Cohen efficiency of the control group electrolyte is 40%, The lower Kurlen efficiency indicates that the electrolyte of the FEC is added to the surface decomposition of the Si negative electrolyte during the first charging, but after the start of 10 cycles, the product of the battery of the FEC electrolyte is significantly higher than the adoption of the control. The battery of the electrolyte, which indicates that the stability of the SEI film formed in the addition of the FEC electrolyte is significantly better than the control group electrolyte.
. In order to analyze the mechanism of FEC improvement of Si negative electrode cycle performance, the author understands the analysis after the cycle, the negative electrode after the circulation tends to have some electrolyte, and the residual electrolyte will interfere with the analysis of the SEI membrane component, but routine The cleaning method will break the structure of the SEI film, so TonyJaumann cleans the surface of the Si negative electrode with ultrasonic treatment..
The following figure shows the XPS analysis result of the electrode surface after ultrasonic cleaning and the electrode surface of ordinary cleaning, which can see the LIPF6 content of the Si negative surface after ultrasound was 0.08 mol%, only for ordinary cleaning. One (0.
24mol%), indicating that the ultrasound cleaning can better remove the residue of the electrolyte on the surface of the electrode.. The following table shows the SEI film formed in the control group electrolytic solution and the composition analysis result of the SEI film formed in the FEC electrolyte, which can be seen that the C and O content in the SEI film after the addition of FEC, which also indicates SEI.
The organic ingredients in the membrane are significantly reduced, and the content of Si has increased, which indicates that the decomposition of the electrolytic solution after the addition of FEC is significantly reduced, and the SEI film is thinner.. At the same time, we noticed that the F element in the SEI membrane was significantly added.
It was found that the LIF diffraction peak in the surface of the Si negative electrode after the addition of FEC, and the transmission electron microscope analysis also further confirmed the Si negative surface LIF. The existence of the crystal, indicates that a large amount of LIF product can be formed on the surface of the Si negative electrode after adding FEC..
The following figure shows the XPS analysis results of the Si negative surface, and the SI2P diffraction results are shown below, and the Si negative electrode is important containing products such as LiXsioy, SiO2, Si, Sioxfy, Lixsiy, which is important to generate the reaction shown in the formula.. The contribution of the Si element in XPS can be found that the contribution of the Si element in the FEC electrolytic solution is 23%, and the contribution of the blank control group electrolyte is only 10%, indicating that the electrolytic solution of the addition of FEC can form a thinner SEI film.
. The XPS data further determines the presence of the LIF in the Si negative surface after the addition of the FEC, but the grain size of the LIF is about 4 nm, so that the presence of the LIF is not usually considered a layered structure distribution. In the innermost layer of the SEI film, it should be a particulate distribution in the SEI film.
As mentioned earlier, since the adhesive contained some functional groups to the Si negative electrode, the surface characteristics of the Si negative electrode were changed, thereby impact on the formation of the SEI film, and Tonyjaumann also compared CMC / SBR and PAA. Junction of the effect of the Si negative electrode SEI film. From the table below, we can notice that the Si element content is significantly lower than the CMC / SBR binder in the PAA binder, but the content of C element is significantly increased, which indicates that the Si negative surface is formed in the PAA binder.
Thicker SEI film, but the binder does not change to the components of the negative electrode SEI membrane, the species has a large change. TonyJaumann’s work indicates that the FEC can significantly improve the cycle performance of the Si negative electrode in the electrolyte, because FEC additives can help form thinner, more stable SEI films, prevent SEI membranes from continuous growth during cycle..
The selection of the binder has a certain effect on the growth of the SEI film, for example, a thicker SEI film in the PAA adhesive, but the choice of the binder does not affect the structure and ingredients of the SEI film.