The separator is an important part of lithium-ion batteries. The separators currently used are porous polypropylene. It is estimated that for rechargeable lithium-ion batteries, its material cost accounts for 65% of the total battery cost, and polymer separators account for about 11% of the material cost; for sodium-ion batteries, the cost of polymer separators is estimated to be Accounted for 20% of material costs. In past research, people always look forward to reducing the price of lithium batteries from the aspects of electrodes and electrolytes, but the cost of separators has been ignored for a long time. In addition, another question has been overlooked: Is the traditional polymer separator the best separator material?
In fact, long ago, people have used nanoparticles as coatings for polymer separators or hybrids with polymer frames as separators. In these cases, nanoparticles are used as additives to improve the cycle stability of batteries, and polymerize Matter is the real diaphragm. Although it is a supporting role, nanoparticles have shown their unique "super power" in these examples: it can improve the wetting ability of the electrolyte, can withstand higher temperatures, and can absorb hydrogen fluoride produced by certain lithium salts. And, compared with polymer separators, silica nanoparticles have another biggest advantage-cheaper. Lithium-ion battery John B. Goodenough recently published an article in "Advanced Functional Materials", stating that silicon dioxide nanoparticles, which are cheaper and easier to mass-produce, can be used instead of polymers as battery separators. Compared with the traditional polypropylene separator, after using silica nanoparticles as the separator, the cycle stability of the battery has been greatly improved.
1. The process of self-assembly of silica and the formation of a separator on the electrode surface
The author first coated silicon dioxide on the surface of the lithium cobaltate positive electrode through multiple coatings, and then added the electrolyte and negative electrode to assemble a lithium ion battery (Figure 1).
It can be seen from the SEM that the thickness of the coated silica nanoparticle layer is 12.3 microns, which is equivalent to the thickness of a single layer of a polypropylene separator commonly used in lithium batteries (Figure 2).
2. Characterization of electrochemical performance
After that, they performed a series of performance characterizations of the assembled batteries. Through the charge and discharge cycle tests under different current densities (Figure 3a, 3b is 1mA cm-2; Figure 3c, 3d is 2mA cm-2), it can be seen that the battery capacity and coulomb efficiency have been very good in 100 cycles. Okay. Compared with the polypropylene separator, the cycle stability of the battery prepared by the author has been greatly improved, and the impedance of the entire battery has also been reduced (Figure 3e, 3f).
3. Stability of silica diaphragm
Through high-resolution infrared spectroscopy and XPS spectroscopy after argon sputtering, the authors proved that there are no water or hydroxyl groups on the surface of silica nanoparticles, so it will not affect the cycle stability of the entire battery. The author assembled the silicon dioxide before and after the sputtering into a battery. After 50 charge and discharge cycles, XPS showed that the peak positions of Si 2p and O 1s did not change significantly, indicating that the silicon dioxide separator has Good stability (Figure 4).
To sum up, Professor Goodenough gave full play to his imagination. When most people were studying the positive electrode, negative electrode, and electrolyte, he initiated a new study. Silica nanoparticle is a material with a mature preparation route, which is simple to prepare and low in cost. Using it on a battery separator can make the battery have better cycle stability. Perhaps in the near future, we will see lithium batteries using silicon dioxide separators.