Speaking of the negative of Si reminds me of the gorilla George, a mutant gorilla in the Hollywood giant “The Behemoth Beast” that was recently shown in the movie theater. It contains amazing power, but it is extremely violent until the person who can control it appears. The negative electrode of Si is just like the variant George. It has tremendous strength. The theoretical capacity of Si material can reach 4200mAh/g (Li4.4Si), which is more than ten times that of graphite material, and even more than the theoretical specific capacity of metal Li (3800mAh). /g) is even higher, and the lithium potential of Si material is close to the graphite material, which is a perfect negative electrode material, but the negative electrode of Si is like a fierce wild beast, and the volume of Si negative electrode is fully inserted into lithium. Expansion of more than 300% will not only result in the pulverization of the active material particles, but also damage the structure of the negative electrode and cause the loss of active materials, resulting in rapid decline in the capacity of lithium ion batteries using a silicon negative electrode. We need to wear a film like this one. Weiss’s role helped us to tamed such a beast!
To solve the problem of large volume expansion of silicon anodes, the following methods are mainly used: 1) Mixing with graphite materials, use of graphite materials with less volume expansion to absorb the volume expansion of silicon materials; 2) New type of binders, through adoption of new types Binder, reduce the destruction of electrode structure due to volume expansion of Si anode and reduce the loss of active material; 3) New conductive agent, refers to the large expansion characteristic of silicon anode, formed by using new conductive agent, such as carbon nanotubes, etc. A stable conductive network reduces the damage caused by Si volume expansion to the conductive network, and improves the cycling performance of batteries using Si negative electrodes.
Recently, Dawei Li and others from Shanghai University of China and the University of Kentucky in the United States studied the effects of sodium alginate SA, perfluorosulfonic acid and PVDF adhesives on the mechanical and electrochemical properties of Si anodes. The study found that the use of sodium alginate SA and Compared with Si negative electrode using PVDF adhesive, Si anode of perfluorosulfonic acid can better maintain the stability of the electrode structure, reduce the loss of active material, and improve the cycle performance of the electrode.
Through analysis, it was found that the SEI films of the electrodes with the three adhesives have the same composition, so Dawei Li believes that the main reason for the different electrochemical properties of the three adhesives is that the electrodes with the three adhesives have different mechanical properties. Characteristics. In order to study the effect of three adhesives (sodium alginate SA, perfluorosulfonic acid, and PVDF) on the mechanical properties of the electrode, Dawei Li used a single-layer coated electrode for lithium insertion experiments because of the lithium-inserted active material layer. The volume expansion will occur, and the Cu foil will not swell, so it will cause the electrode to bend (as shown in the figure below). According to the degree of bending of the electrode, the volume expansion of the electrode can be judged. The picture below shows the photo of lithium-encapsulated 30% silicon anode. From the figure, we can see that the electrode with sodium alginate SA has the largest bend, which means that the electrode with sodium alginate has the largest volume expansion. This is also better understood. The three adhesives, sodium alginate, are the most 'hard', PVDF. Second, perfluorosulfonic acid is the softest, so electrodes with sodium alginate will produce greater volumetric expansion. However, the second 'hard' PVDF adhesive has the smallest volume expansion caused by lithium insertion, the most 'soft' fluorine Volume expansion acid binder but higher than the PVDF binder, does not meet our conventional understanding, so there may be other factors that affect the volume expansion of the electrode.
The following figure shows the charge-discharge voltage curve and the curve of curvature of the electrode in the first three cycles of the electrode with three adhesives (large curvature means greater volume expansion). Since the cycle system uses the lithium-intercalation 6h, then take off. Lithium to 2V system, so all electrodes have the same capacity, so the curvature of the electrode using three adhesives is comparable, from the following figure b we can notice that the electrode using sodium alginate SA is embedded The volume expansion in the lithium process is the largest, the volume expansion of the electrode using perfluorosulfonic acid is secondarily followed, and the volume expansion of the electrode using the PVDF adhesive is the smallest. At the same time, the curvature of the electrode from the second and third lithium insertion processes is also changed. It was found that the embedded Li did not completely escape during charging, but that part of it remained. Therefore, the volume expansion of the lithium insertion of the electrode for the third time was actually higher than that of the second insertion of lithium. However, PVDF adhesive was used. When the electrode is embedded for the third time, the expansion of the electrode is less than the second time. Dawei Li thinks that this shows that a crack has appeared in the electrode using the PVDF adhesive, and the electrode releases some of the stress through these cracks.
The figure below shows the change of the thickness of the electrode in the thickness direction (below in figure a) and the change in the porosity of the electrode due to the volume change of the Si particles in the lithium intercalation process (below b). From figure a we can see that The largest relative volume change rate in the process was the electrode with PVDF adhesive, while the electrode with sodium alginate SA adhesive had the smallest relative volume change rate, indicating that the highest strength sodium alginate SA can be limited to some extent. The volume expansion of the Si particles maintains the stability of the electrode structure, thereby improving the cycle performance of the negative electrode of Si. The following figure b shows that although the volume change of the Si negative electrode during lithium insertion and delithiation is large, the porosity of the electrode is almost zero. Changes have taken place.
Figure a shows the elastic modulus of electrodes with three adhesives in different lithium intercalation states. From the figure we note that as the intercalation of lithium increases, the elastic modulus of the negative electrode containing Si gradually decreases. Interestingly, during the delithiation process, the elastic modulus of the Si negative electrode does not return to its original level. After delithiation, the elastic modulus of the electrode becomes even smaller. Dawei Li thinks that the main reason is that the volume of lithium in the electrode is inserted. After expansion, many open cracks are produced on the surface of the electrode, resulting in a decrease in the elastic modulus of the electrode. This can also be confirmed from the SEM image below. There is a large amount of cracks on the surface of the Si negative electrode after lithium insertion. The crack can release a part of the stress, so that the elastic modulus of the electrode decreases.
From the above experimental results, we can also find a strange phenomenon. From the viewpoint of the elastic modulus of the adhesive itself, SA is the most 'hard', perfluorosulfonic acid is the 'soft', and PVDF is between two. However, from the viewpoint of the elastic modulus of electrodes using three types of binders, we found that the electrode with PVDF binder has the lowest elastic modulus, that is, the electrode with PVDF binder is the softest. However, the elastic modulus of the electrode using the most soft binder perfluorosulfonic acid is even higher than that of the electrode using PVDF adhesive. This is a little unreasonable. To explain this phenomenon, Dawei Li uses scanning electron microscopy for three types of electrodes. The electrode was observed. It was found that the electrode using the PVDF adhesive before the cycle was the most flat and there was almost no crack, while the surface of the electrode using the SA and the perfluorosulfonic acid binder had many cracks, but after three cycles Later, Dawei Li discovered that a large number of new cracks appeared in the electrodes using PVDF adhesives. Instead, there were not many new cracks in the electrodes using SA and perfluorosulfonic acid binders. Dawei Li counted three electrode cracks in the electrode area. The proportion of the total area of the electrode (as shown below ) It was found that the area occupied by the cracks in the electrode with PVDF binder after the three cycles increased by more than 5 times, while the electrode change with SA and perfluorosulfonic acid binder was much smaller, and it was these increases. The cracks reduce the elastic modulus of the electrodes using PVDF binders.
Dawei Li's research shows that the choice of binder has a significant influence on the mechanical properties of Si negative electrodes. SA and perfluorosulfonic acid binders can better maintain the negative electrode of Si in the face of huge volume expansion of Si negative electrodes. The structural integrity reduces the generation of electrode cracks, thereby reducing the loss of active material and improving the cycling performance of the battery. On the contrary, the electrode using PVDF adhesive generates a large number of cracks on the surface of the electrode after cycling, thus destroying the structure of the electrode. Integrity, caused the loss of active material, the destruction of the conductive network, resulting in poor cycling performance of the battery.
