What are the classifications of lithium battery electrolytes?

The role of electrolyte in lithium-ion batteries is like the importance of blood to the human body. It is the medium for lithium ions to move back and forth between the positive and negative electrodes. Without it, there will be no electron flow. There is no such battery, so its importance is self-evident, and electrolyte property analysis is explained in the original article.

The electrolyte acts as a charge transfer between the positive and negative electrodes and should be electrically conductive to ions and electrically insulated. It has a very important impact on battery cycling performance, operating temperature range, and battery durability. For lithium ion batteries, the composition of the electrolyte involves at least two aspects: solvent and lithium salt.

A. Liquid electrolyte

The choice of solvent is based primarily on the properties of the three aspects, namely the dielectric constant, viscosity and the electron donor nature of the solvent. In general, the high dielectric constant facilitates the dissociation of the lithium salt, while the strong electron donor capacity will facilitate the dissolution of the electrolyte salt. The electron donor property of the solvent is the electron deprivation ability inherent in the solvent molecule, and its ability determines the solvation ability of the electrolyte cation. Low viscosity increases the mobility of the ions and helps to increase the conductivity.

At present, a binary or multi-component mixed solvent in which two or more solvents are mixed is usually used. Common organic solvents are ethers, alkylcarbonates, lactones, ketals, and the like.

Lithium salts are mainly used to provide effective carriers. The choice of lithium salt generally follows the following principles:

Good stability (compatibility) with positive and negative materials, that is, during storage, the electrochemical reaction rate between the electrolyte and the active material is small, so that the self-discharge capacity loss of the battery is minimized; the specific conductivity is higher. The ohmic pressure drop of the solution is small; the safety performance is high, non-toxic and non-polluting.

Commonly used lithium salts are as follows: Lithium hexafluoroarsenate (LiPF6), LIAsF6 will release toxic arsenide during charge and discharge, and the price is relatively expensive. Lithium hexafluorophosphate (LiPF6), which has been widely used in commercial batteries, has a high electrical conductivity and has good compatibility with carbon materials. The disadvantage is that the price is relatively expensive, the stability in the solid state is poor, and it is very sensitive to water. Lithium trifluoromethanesulfonate LiCF3SO2 has good stability, but its conductivity is only half of that of LiPF6 based liquid electrolyte. Lithium tetrafluoroborate (LiBF4) and lithium perchlorate (LiCl04) are widely used salts. However, lithium lithium perchlorate-based lithium imide, typically lithium bisfluorosulfonimide (LiN(CF3SO2)2, has a conductivity comparable to that of a very dry LiPF6 electrolyte and has a stability exceeding that of FLiCF3SO2.

B. Solid electrolyte

Solid electrolyte, also known as "superionic conductor" or "fast ion conductor." it refers to a class of solid ionically conductive materials whose ionic conductivity approaches (or in some cases exceeds) the meltdown and electrolyte solution. It is a kind of strange solid material between solid and liquid. It is an abnormal state of matter. Some atoms (ions) have mobility close to liquid, while other atoms maintain their spatial structure (arrangement). This liquid-solid two-phase property, as well as its broad application prospects in various fields such as energy (including production, storage and energy saving), metallurgy, environmental protection, and electrochemical devices, has caused physicists and chemists and the materialist's extensive attention.

The polymer solid electrolyte is a solid electrolyte material formed by complexing a polymer containing a solvatable polar group with a salt. In addition to the properties of common conductivity systems such as semiconductors and ionic solutions, it also has plasticity that is not possible with inorganic solid electrolytes. This property makes the polymer solid electrolytes have three advantages in application:

Film of any shape and thickness, therefore, although the room temperature conductivity of the polymer electrolyte is not high, it is 2-3 orders of magnitude lower than the inorganic one, and the internal resistance of the battery is greatly reduced due to processing into a very thin film, so that the conductivity can be compensated by increasing the area/thickness ratio low; tightness - complete contact with the electrode, so that the charge and discharge current increases; should be - in the charge and discharge process can withstand the pressure changes well, to adapt to changes in electrode volume. The polymer solid electrolyte has a broader prospect for its application in terms of light weight, pressure resistance, shock resistance, fatigue resistance, non-toxicity, non-corrosion, and electrochemical stability when combined with electrodes. At present, scientists at home and abroad are working hard to make it applicable to energy storage, electrochemical components, sensors and other aspects of research, and have become the most powerful competitor in the development of high-energy lithium batteries.

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