Comprehensive understanding of the mechanism and development trend of lithium-ion battery electrolyte

by:CTECHi     2021-07-07

As the blood of lithium-ion batteries, electrolyte is responsible for the transportation of lithium ions. Its quality will directly affect the performance of lithium-ion batteries and also affect the safety of lithium-ion batteries to a certain extent.

(1) Good electrochemical stability, does not react with cathode materials, anode materials, separators, current collectors, binders, etc.;

(2) The ion conductivity is new, the dielectric constant is high, the viscosity is low, and the resistance to ion migration is small;

(3) Keep liquid in the temperature range, the general temperature range is -40℃~70℃, which is suitable for improving the high and low temperature characteristics of the battery;

(4) It can best promote the electrode reversibility The progress of the reaction means that it has a higher circulation efficiency;

(5) Environmentally friendly, preferably non-toxic or low-toxic.

The physical properties of common solvents are as shown in the above table. According to the selection principle of electrolyte and the system in which the appropriate solvent is selected, the basic solvents include cyclic, chain and Carboxylic acid ester series. At present, the commonly used lithium salt is LiPF6, which is very sensitive to moisture. Once in contact with moisture, it will react, causing problems such as gas production, battery swelling, and serious cycle attenuation. In the temperature range of 20-60 ℃, LiPF6 and The reaction rate constant k of water is: EC+DMC<EC+DEC<EC+DEC+DMC (as shown in Table 1); the reaction rate of LiPF6 and water increases greatly with the increase of temperature. The reaction rate constant at 40℃ is 3 to 4 times at 20°C, and 8 to 12 times at 60°C. Therefore, the temperature and humidity of the environment must be controlled when the electrolyte is configured. The electrolyte used in mass production generally controls moisture The content is less than 20ppm.

Some commonly used additives are shown in the above table, and use a smaller amount to improve certain aspects of performance.

(1) Film-forming additives: VC is widely used, and its important mechanism is the radical polymerization reaction on the surface of the carbon negative electrode to generate polyalkyl lithium carbonate compound, which is effective Inhibit the co-intercalation reaction of solvent molecules; PS, ES, DES, DMS and other substances whose important mechanism is reduction and decomposition to form the important components of the SEI film are the inorganic salt Li2S, Li2SO3 or Li2SO4 and the organic salt ROSO2Li, which greatly enhances the stability of the SEI film ;

(2) Safety additives: flame-retardant additives, reduce the electrolyte exothermic value and self-heating rate, the most important is organic compounds containing P or F, such as organic phosphorus Chemicals, organic fluorides, and fluoroalkyl phosphates, etc. The important mechanism of overcharging additives is redox shuttle reaction (ferrocene) and electropolymerization (biphenyl, cyclohexylbenzene);

(3) Multifunctional Additives: for comprehensive purposes such as water removal, conductivity, and film formation, amide additives, which form hydrogen bonds with water, and contain lone pairs of electrons to stabilize the SEI film.

Comparing the performance of graphite anode with film-forming additives after cycling, it can be clearly seen that after adding film-forming additives, the surface of the anode material is much smoother after cycling, but there is no The negative electrode with film-forming additives is much rougher, and the cycle decays faster. Potential

After adding flame retardant additives, it is obvious that the electrolyte is no longer combustible after a certain amount of addition, which brings certain safety guarantee to high energy density batteries . An article to understand the mechanism and development trend of lithium-ion battery electrolyte

As the nickel content and the upper limit voltage of charging increase, the requirements for the electrolyte of the cathode material are also getting higher and higher. NiO will appear in the high nickel material during the cycle, which will cause the battery to fail due to water absorption and gas production.

Some polyphosphate esters can significantly improve the performance of high nickel materials.

LiPO2F2 can form a film on the surface of the positive and negative electrodes, significantly improving the performance of high nickel and high voltage materials. Now it has been widely used as a common additive.

With the increase in energy density, the wide application of high voltage, high nickel cathode materials, and silicon carbon anodes, more and more functional additives will be used.

According to the technical route given by the expert group, it can be seen that, at present, the development of high-purity and high-stability electrolytes will be gradually carried out according to the material development. The development of high-voltage, composite lithium salts and all-solid electrolytes. As far as China’s current electrolyte market is concerned, the barriers to entry are not high, but there are barriers to invisible technologies. With the localization of key raw materials, the current The cost of the electrolyte has also been further reduced, and Japanese and Korean companies have also begun to transfer their manufacturing plants to the country. It is believed that in the near future, China's electrolyte will go abroad and into the world.

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