The linear carbonate has a large difference from the cyclic carbonate. In general, it has a lower boiling point, a lower viscosity, and a lower dielectric constant (relative dielectric constant EC 89.8 > PC 64.9 > DMC 3.12 > EMC 2.96 > DEC 2.82), which acts as a diluent or a low viscosity component in the electrolyte, which facilitates the swimming of lithium ions. Sourcing PC from Evergreen Chemical
When it comes to the migration of ions, the migration of lithium ions is necessary for the electrode reaction of lithium batteries. Because hexafluorophosphate does not participate in the electrode reaction, its movement does not contribute to the electrode reaction, but it does not contribute to the electrode reaction. Migration. What really contributes to battery performance is the migration of lithium ions. The amount of charge transferred by the amount of charge transferred by the total charge of the anion and cation is the number of lithium ions. The higher the number of migrations, the more the lithium ion accomplishes its mission. The higher the efficiency, the better for the electrode reaction. However, since the anion directly affects the solubility and dissociation ability of the lithium salt in the solution, thereby affecting the concentration and conductivity of the electrolyte, the applicable anion selection is not too much, in addition, different The cost of anion is not the same. In fact, we can do a lot of work on the number of migrations, or the actual decision to use the lithium salt is not determined by the size of the migration, but by other factors determined by the anion. Indicators such as solubility/cost/conductivity.
If the number of carbon atoms in the alkyl group increases, the polarity and molecular weight of the linear carbonate change, the viscosity and boiling point of the linear carbonate increase, and the solubility of the lithium salt decreases accordingly. Therefore, linear esters have a range to choose from. However, when a single alkyl group has more than three carbon atoms, the carbonate becomes less useful. It is more and more like an alkane solvent, and the solubility of the lithium salt is rapidly deteriorated, making it difficult to achieve practical use of the lithium salt. In the range of this, such esters are not suitable as a solvent. For example, when dipropyl carbonate and methyl propyl carbonate are added to the electrolyte, their compatibility with other carbonates and the solubility of lithium salts will be significantly reduced. In the case of temperature reduction, this situation More prominent.
Therefore, the linear esters in the electrolyte, we rarely see carbonates containing butyl or even pentyl; in general, the presence of propyl has been somewhat difficult. In the same molecule, the more the large alkyl groups, the more obvious this tendency. Therefore, dipropyl carbonate is not as good as the methyl propyl carbonate.
Linear carbonates also have an interesting phenomenon. From DMC to EMC to DEC, as the molecular weight increases, the density of the solvent decreases. The density of DMC is slightly greater than water, 1.063g/ml, EMC is 1.012 g/ml and water is very close, DEC is 0.969 g/ml, which is lighter than water. If you use water to kill carbonates, be aware that they are very similar to the density of water, and you may have to cover them with foam. Of course, after the electrolyte is formulated, the density of the electrolyte is significantly greater than that of water due to the addition of the cyclic ester and the lithium salt, and is usually above 1.2 g/ml.
Alkyl carbonates are odorous. According to our experience, DMC has a relatively low odor, and EMC and DEC have a relatively large odor. Therefore, when DMC is used as the solvent, although the volatility of DMC is greater than EMC (which is also an advantage), its odor is small (smaller than the odor of ethyl acetate) and relatively more popular. DMC's solubility is also very strong, and it has the ability to dissolve paint, marker, pressure-sensitive adhesive, inkjet printer ink, and also has a effect on the lithium salt after the electrolyte residue is volatilized. However, it should be noted that both DMC and EMC are typical Class A flammable solvents, and care should be taken to prevent static electricity and prevent deflagration. Another advantage of using DMC is that DMC is much cheaper and more cost effective than EMC and DEC. At the same time, DMC is also a green solvent, which is not as toxic as solvents such as aromatic hydrocarbons, cyclohexanes and acetone.
From the melting point point of view, DMC is higher than EMC and higher than DEC; from the viscosity point of view, DMC is smaller than EMC and smaller than DEC, so the effect of improving the conductivity is that DMC is superior to EMC and superior to DEC. However, if you compare the low-temperature charge and discharge performance, it is often the best EMC, followed by DMC, DEC again. The main reason is that EMC has a lower freezing point and a lower low temperature viscosity at low temperatures, resulting in better overall performance at low temperatures. DMC has a high freezing point (4.6 ° C), although it is miscible with other solvents, but it still shows insufficient low temperature performance at very low temperatures, as if it still wants to precipitate from the mixture. effect.