Types of electrolyte additives
Film forming additive
Excellent SEI film (solid electrolyte film) has the property of organic solvent impermeability, which allows lithium ions to enter and leave the electrode freely, but the solvent molecules can not pass through, thus preventing the destruction of the electrode caused by the co insertion of solvent molecules, improving the cycle efficiency and reversible capacity of the battery.
They are mainly divided into inorganic film-forming additives
(SO2, CO2, CO and other small molecules, as well as lithium halide, etc.) and organic film-forming additives
(fluorinated, chlorinated and brominated carbonates, etc.), which can improve the ability of central atoms to obtain electric power by virtue of the electron absorption effect of halogen atoms, reduce and effectively passivate the electrode surface at higher electric potential, and form a stable SEI film In addition, Sony reported that adding trace anisole or its halogenated derivatives to the non-aqueous electrolyte of lithium-ion battery can improve the cycle performance of the battery and reduce the loss of irreversible capacity.
The research of additives
to improve the conductivity of electrolyte mainly focuses on improving the dissolution and ionization of conductive lithium salt and preventing the damage of solvent co insertion to the electrode.
According to the type of interaction, it can be divided into cation interaction type (mainly including some amines and aromatic heterocyclic compounds with more than two nitrogen atoms in the molecule, crown ethers and hole like compounds), anion interaction type (anion ligands are mainly some anion receptor compounds, such as boron based compounds) and electrolyte ion interaction type (neutral ligands are mainly some rich compounds) Compounds formed by bonding electron deficient atoms n or B with electronic groups, such as azaethers and alkyl boron.
Flame retardant additive
As a commercial application, the safety of lithium-ion battery is still an important factor restricting its application and development. Lithium ion battery itself has many security risks, such as high charging voltage, and most of the electrolytes are organic flammable materials. If used improperly, the battery will be dangerous or even explode. Therefore, improving the stability of electrolyte is an important method to improve the safety of lithium-ion battery. Adding some solvent with high boiling point, high flash point and nonflammability can improve the safety of the battery.
It is mainly divided into: (1) organic phosphates (2) organic fluorinated compounds (3) halogenated alkyl phosphates
Overcharge protection additive
The principle of this method is that the redox pair does not participate in any chemical or electrochemical reaction during normal charging. When the cell is fully charged or slightly higher than this value, the additive begins to oxidize on the positive electrode and then diffuses to the negative electrode The reduction reaction takes place as follows.
Positive electrode: R → O + ne-
Negative electrode: O + ne - → R
The best overcharge protection additive should have a cut-off voltage of 4.2 ~ 4.3v, so as to meet the requirements of more than 4V voltage for lithium-ion batteries. In general, this part of the research work needs to be further studied.
for controlling water and HF content in electrolyte
Trace water and HF in the organic electrolyte have a certain effect on the formation of excellent SEI film, which can be seen from the reaction of EC, PC and other solvents at the electrode interface. However, if the content of water and acid (HF) is too high, it will not only lead to the decomposition of LiPF6, but also damage the SEI membrane. When Al2O3, MgO, Bao and carbonate of lithium or calcium are added into the electrolyte as additives
, they will react with trace HF in the electrolyte, reduce the content of HF, prevent the destruction of the electrode and the catalytic effect on the decomposition of LiPF6, improve the stability of the electrolyte and improve the performance of the battery. However, the removal rate of HF by these substances is slow, so it is difficult to prevent the damage of HF to battery performance.
Although some anhydride compounds can remove HF quickly, they can also produce other acidic substances which can destroy the performance of the battery. Alkane diimine compounds can form weak hydrogen bonds with water molecules through the hydrogen atoms in the molecules, thus preventing the reaction between water and LiPF6 to produce HF.
for improving low temperature performance
Low temperature performance is one of the important factors to broaden the application range of lithium-ion batteries, and it is also necessary in current aerospace technology. N. The results show that n-dimethyltrifluoroacetamide has low viscosity (1.09mpa · s, 25 ° C), high boiling point (135 ° C) and flash point (72 ° C), good film-forming ability on graphite surface and good oxidation stability on positive electrode. The assembled battery has excellent cycle performance at low temperature. Organic borides and fluorinated carbonates are also beneficial to improve the low temperature performance of the battery.
are ideal additives
for lithium-ion batteries. They can improve the performance of electrolyte in many ways and play an important role in improving the overall electrochemical performance of lithium-ion batteries. It is becoming the main direction of additive research and development in the future.
In fact, some of the existing additives
are multifunctional additives
. For example, 12-crown-4 is added to PC solvent. While improving the self conductivity of Li +, the possibility of Li + reacting with solvent molecules at the electrode interface is greatly reduced by using the electrophilic effect of crown ligands on the electrode surface. The preferential and desolvation effect of crown ethers on Li + inhibits the co insertion of PC molecules, and the SEI film at the electrode interface is optimized, which reduces the first irreversible capacity loss of the electrode.
In addition, the addition of fluorinated organic solvents, Halogenated Phosphate such as BTE and TTFP into the electrolyte not only helps to form excellent SEI film, but also has certain or even obvious flame retardant effect on the electrolyte, which improves the performance of the battery in many aspects.