PC is a solvent with obvious advantages, high chemical stability, and is not easily decomposed during rectification. It has a boiling point of 242 ° C, a melting point of -48.8 ° C, and a liquid range of up to 290 ° C, which is a very large advantage. It can be used regardless of whether it is hot or cold. It is very rare for solvents. In addition, the solubility of PC is very good, and there are many substances that can be dissolved, and it has excellent versatility. At the same time, the PC is also non-toxic, low-priced, and so on. PC also has a very interesting feature. It has a strong ability to dissolve CO2. It can dissolve 8.5 liters of CO2 per liter of PC at 2.8 atmospheres at 25 ° C, and the solubility increases rapidly when the pressure rises. Therefore, it is used industrially to absorb carbon dioxide in the mixed gas. I have seen a technical solution in the past. In the process of making a soft-pack battery, carbon dioxide is used as a shielding gas. When the electrolyte contains PC, the carbon dioxide is dissolved and absorbed by the PC after the injection, and the internal pressure of the soft-packed battery is lowered. It is tightly attached to the battery core. The idea is to take full advantage of the PC's ability to dissolve carbon dioxide.
PC has many disadvantages, one of which is that the viscosity is relatively high compared to other solvents, which leads to an electrolyte containing a high PC, although it is less likely to condense at low temperatures, but it is sticky (the actual feeling is not So exaggerated, but this is the case for lithium ion migration), which is not conducive to the migration of lithium ions. The migration of ions cannot keep up with the need, resulting in a large concentration polarization, and the rate performance will be poor. This is also confirmed by experiments. Therefore, generally in the low temperature electrolyte, if it is a small current application, PC can be used, but once the magnification is high, the PC has to carefully control the amount, otherwise the performance is difficult to see.
In the research, people gradually found that in lithium ion batteries, ethylene carbonate (EC) performed better than PC. In addition to the relative dielectric constant (EC 89) is greater than PC (PC 65), it is made into the cycle performance of the battery. It is also better than PC, so EC has slowly developed. For reasons of this reason, the film formation potential of EC on the negative electrode is relatively high. When charging, the potential of the negative electrode is continuously decreased, and the EC with higher reduction potential preferentially precipitates, which participates in the formation of the SEI film, which is advantageous for stabilizing the negative electrode and impedance. Moderate, battery performance is better.
Another advantage of EC is that after dissolution, its viscosity is smaller than PC. For lithium battery electrolytes, a low viscosity means lower ion mobility resistance, which means better battery performance. In addition, there is a hidden reason, that is, the co-embedding side effect of PC on graphite anode materials makes PC not suitable for high concentration use, and EC does not have this limitation. Slow EC replaces PC. It is an essential solvent in the electrolyte and an indispensable component.
But EC is also a problem, and the problem is not small - the melting point is as high as 37 ° C. This is a "class" with the PC. From the structural point of view, PC only has one more methyl group on the ring, but this methyl group brings about molecular asymmetry, which makes the polarity of PC smaller, and it is more difficult to crystallize, so the melting point is greatly reduced.
Among the cyclic carbonates, in addition to EC and PC, there is also a butylene carbonate BC. Since there are two more carbon atoms, BC has two structures, one is one more ethyl than EC, namely 1,2-butanediol cyclic carbonate, or abbreviated as 1,2-BC (CAS 4437-85- 8); The other is two more methyl groups than EC, namely 2,3-butanediol cyclic carbonate, or simply 2,3-BC (CAS 4437-70-1). At present, there are few studies in BC, and samples are not easy to obtain.
In addition, the structure of PC also exists an isomer, namely 1,3-propanediol cyclic carbonate (CAS 2453-03-4), whose molecular structure is a six-membered ring. Generally this compound is not called propylene carbonate (theoretically you can't get it by propylene and carbonic acid addition), but it is called trimethylene carbonate, or TMC for short. However, the TMC compound seems to be less stable and it is easy to form a polymer. By transesterification of 1,3-propanediol with dmc, TMC can theoretically be obtained, but in fact, TMC polymers are produced. Therefore, the reported method of synthesizing TMC is basically to first manufacture TMC polymer. Depolymerized under certain conditions. However, the TMC compound is relatively friendly to the human body. Many people currently study its application in the medical industry, and rarely see electrolyte research.
Of course, cyclic carbonates are far more than these. There is also a reported carbonate called catechol carbonate, also known as catechol carbonate (abbreviated CC), which is equivalent to eccentricity outside the structure of EC. This compound was obtained on the previous benzene ring and was reported as a lithium electrolyte additive many years ago.
