Classification of lithium battery electrolytes

The role of electrolytes in lithium-ion batteries is like the importance of blood to the human body. It is the medium through which lithium ions in lithium-ion batteries can move between positive and negative poles. Without it, there will be no electron flow, and there will be no such batteries. The existence of, Therefore, its importance is self-evident. The analysis of electrolytic fluid properties was explained in the previous article.

Electrolytic fluid plays a charge role between positive and negative poles and should conduct electricity to ions and insulate electrons. It has a very important influence on the battery cycle performance, operating temperature range, and durability of the battery. For lithium-ion batteries, the composition of the electrolyte involves at least two aspects: solvents and lithium salts.

A. Liquid electrolytes

The choice of solvent is mainly based on the property requirements of three aspects, namely dielectric constant, viscosity and electronic donor properties of solvent. In general, the high dielectric constant is conducive to the dissociation of lithium salts, while the strong electron donor ability will be conducive to the dissolution of electrolytic liquid salts. The electron donor property of the so-called solvent is the inherent electron-loss ability of the solvent molecule, and its capacity determines the degree of solvent capacity of electrolytic liquid ions. Low viscosity can increase ion fluidity and help increase conductivity.

At present, two or more solvents are usually mixed into binary and multivariate mixed solvents. Common organic solvents include ether, alkylcarbonate, lactone, ketal, and the like.

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

Good stability(compatibility) with positive and negative polar materials, that is, during the storage period, the electrochemical reaction between the electrolyte and the active substance is small, so that the loss of self-discharge capacity of the battery is minimized; Higher than conductivity, the ohmic pressure drop of the solution is small; High safety, non-toxic, 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 electrolytes

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.

Polymer solid electrolyte is a solid electrolyte material formed by the combination of polymer and salt containing solvable polar groups. In addition to showing the properties of common conductive systems such as semiconductors and Ionic solutions, it also has plasticity that is not available in inorganic solid electrolytes. This characteristic makes polymer solid electrolytes show three major 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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