Analysis and discussion of lithium-ion battery electrolyte

Electrolyte is one of the core materials that constitute the capacity of lithium secondary batteries and lithium primary batteries that are the energy sources of mobile phones, notebooks, video recorders, etc. It also improves the fluidity between the mobile anode and cathode and acts as a medium. This paper discusses the classification, characteristics, pre-charge conditions, and safety performance of lithium-ion electrolytes.

1. Classification of lithium-ion battery electrolytes

The electrolyte plays a role in transferring charge between the positive and negative electrodes. It should conduct ions and insulate electrons. It has a very important influence on the battery cycle 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 selection of solvents is mainly based on three aspects of property requirements, namely dielectric constant, viscosity and the electron donor property of the solvent. Generally speaking, a high dielectric constant is conducive to the dissociation of lithium salts, while a strong electron donor ability will be conducive to the dissolution of electrolyte salts. The so-called electron donor property of the solvent is the inherent electron loss ability of the solvent molecule, and its ability determines the solvation ability of the electrolyte cation. Low viscosity can increase the mobility of ions and help improve conductivity.

Currently, binary or multi-component mixed solvents formed by mixing two or more solvents are generally used. Common organic solvents include ether, alkyl carbonate, lactone, ketal, etc.

Lithium salts are mainly used to provide effective carriers. The following principles are generally followed when selecting lithium salts:

It has good stability (compatibility) with positive and negative electrode materials, that is, during storage, the electrochemical reaction rate at the interface between the electrolyte and the active material is low, so that the self-discharge capacity loss of the battery is minimized; the specific conductivity is high and the ohmic voltage drop of the solution is small; it has high safety performance, is non-toxic and pollution-free.

Commonly used lithium salts are as follows: lithium hexafluoroarsenate (LiPF6), which releases toxic arsenic compounds during charge and discharge, and is relatively expensive. Lithium hexafluorophosphate (LiPF6), which has been widely used in commercial batteries, has high conductivity and good compatibility with carbon materials, but is relatively expensive, has poor stability in solid state, and is very sensitive to water. Lithium trifluoromethanesulfonate (LiCF3SO2) has good stability, but its conductivity is only half that of liquid electrolytes based on LiPF6. Lithium tetrafluoroborate (LiBF4) and lithium perchlorate (LiCl04) are widely used salts. However, lithium perchlorate-containing imide lithium salts, typically lithium bis(fluorosulfonyl)imide (LiN(CF3 SO2)2, have conductivity comparable to that of very dry LiPF6 electrolytes and are more stable than FLiCF3SO2.

B. Solid Electrolyte

Solid electrolytes, also known as "superionic conductors" or "fast ion conductors".

It refers to a class of solid ion conductive materials whose ion conductivity is close to (or in some cases exceeds) that of molten and electrolyte solutions. It is a class of peculiar solid materials between solid and liquid, an abnormal state of matter, in which some atoms (ions) have a mobility close to that of liquid, while other atoms maintain their spatial structure (arrangement). This liquid-solid duality, as well as its broad application prospects in various fields such as energy (including generation, storage and energy saving), metallurgy, environmental protection, electrochemical devices, etc., has attracted widespread attention from physicists, chemists and materials scientists.

Polymer solid electrolyte is a solid electrolyte material formed by the complexation of polymers containing solvatable polar groups and salts. In addition to showing the properties of common conductivity systems such as semiconductors and ionic solutions, it also has plasticity that inorganic solid electrolytes cannot match. This feature makes polymer solid electrolytes show three major advantages in application:

Films of any shape and thickness. Therefore, although the room temperature conductivity of polymer electrolytes is not high, 2 to 3 orders of magnitude lower than that of inorganic electrolytes, the internal resistance of the battery is greatly reduced due to the processing into a very thin film, so that the low conductivity can be compensated by increasing the area/thickness ratio; tightness - complete contact with the electrode, so that the charge and discharge current increases; adaptability - can withstand pressure changes well during the charge and discharge process and adapt to changes in electrode volume. The polymer solid electrolyte is light, pressure-resistant, shock-resistant, fatigue-resistant, non-toxic and non-corrosive, and the electrochemical stability shown when it forms a battery with the electrode has opened up a wider prospect for its application. At present, scientists at home and abroad are committed to making it applicable to research in energy storage, electrochemical components, sensors and other aspects, and it has become the strongest competitor in the development process of high-energy lithium batteries.

2. Composition and characteristics of electrolyte

The electrolyte is one of the four key materials of lithium-ion batteries (positive electrode, negative electrode, separator, and electrolyte). It is known as the "blood" of lithium-ion batteries. It plays the role of conducting electrons between the positive and negative electrodes in the battery and is the guarantee for lithium-ion batteries to obtain advantages such as high voltage and high specific energy. The electrolyte is generally made of high-purity organic solvents, electrolyte lithium salts (lithium hexafluorophosphate, LiFL6), necessary additives and other raw materials, prepared in a certain proportion under certain conditions.

The electrolyte has the following major characteristics: resistance to reduction and oxidation; high-efficiency ion conductivity; high permeability (the more free ions there are, the lower the ion conductivity); low viscosity (the ease of movement of free ions); and low freezing point.

3. Purpose and conditions of electrolyte pre-charging

In order to improve the safety performance of the battery, prevent overcharging and heat exposure, increase capacity and extend life, we need to inject electrolyte and pre-charge the electrolyte at the same time. The purpose is to increase the electrolyte impregnation, remove moisture and reduce the thickness of the battery. After charging, it helps the film to form at high temperature. After the electrolyte is injected, before the pre-charge is put into operation, set a waiting time for the electrolyte to be absorbed. The average waiting time is set to about 30 minutes. In order to allow the electrolyte to be fully absorbed, wait for a certain period of time and pre-charge the battery cell that has been injected with electrolyte.

It is charged by static current. When charging is almost completed, the voltage, current and charging capacity are used to determine whether it is a good product. The charging conditions of each model are very different.

4. Safety performance of electrolyte

During the first charge of a liquid lithium-ion battery, the negative electrode material graphite reacts with the electrolyte for the first time to form a SEI film. A good SEI film can improve the safety performance of the battery. The following are the results of quantum chemical calculations using Vinylene carbonate (VC) as an additive. Its main function is to inhibit the decomposition of the electrolyte, so that the graphite negative electrode forms a good SEI film, and improve the electrode reversible capacity and stability. In addition, overcharge protection additives and high boiling point and high flash point flame retardants can be added as additives to the electrolyte to ensure safety performance. This article was originally published in the Global Market Information Guide, sponsored by the Documentation and Information Center of the Chinese Academy of Social Sciences, Issue 445, Issue 08, 2012 (published on February 28)

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