Lithium-ion battery electrolyte additives, selection criteria for high-voltage electrolytes. With the continuous improvement of the energy density of lithium-ion batteries, people's expectations for the improvement of the energy density of lithium-ion batteries are getting higher and higher. Lithium battery electrolyte additives can effectively reduce the internal resistance of the battery, improve the battery's ability to accept large currents, and extend the service life of the battery. It is a good additive for quickly rechargeable batteries.

　　Introduction to lithium-ion battery electrolyte additives

　　1. Boron-containing additives

　　Boron-containing compounds are often used as additives in lithium-ion batteries with different cathode materials. During the battery cycle, many boron-containing compounds will form a protective film on the surface of the cathode to stabilize the interface between the electrode/electrolyte, thereby improving battery performance. . Considering this unique property of boron-containing compounds, many scholars have begun to try to apply them to high-voltage lithium-ion batteries to enhance the stability of the cathode interface.

　　2. Organophosphorus additives

　　According to the relationship between frontline orbital energy and electrochemical stability: the higher the HOMO of the molecule, the more unstable the electrons in the orbit are, and the better the oxidation property; the lower the LUMO of the molecule, the easier it is to obtain electrons, and the better the reduction property. Currently used organophosphorus additives also include phosphate ester compounds. XIA et al. applied triallyl phosphate (TAP) additive to Li[Ni0.42Mn0.42Co0.16]O2 (NMC442) graphite full battery and found that the Coulombic efficiency will be significantly improved when TAP is present. After long-term cycling, , still with high capacity retention. XPS results show that during the cycling process, the allyl group may undergo cross-linking electropolymerization reaction, and the resulting product covers the electrode surface to form a uniform SEI film.

　　3. Carbonate additives

　　Fluorine-containing (PFA) compounds have high electrochemical stability and possess both hydrophobic and oleophobic properties. When PFA is added to organic solvents, the solvent-phobic PFA will aggregate together to form micelles. Due to this characteristic of PFA, ZHU et al. tried to add ethylene carbonate substituted by perfluorohydrocarbon groups (TEM-EC, PFB-EC, PFH-EC, PFO-EC in the figure below) into the electrolyte of high-voltage lithium ion batteries. For Li1.2Ni0.15Mn0.55Co0.1O2 graphite batteries, when 0.5% (mass fraction) of PFO-EC is added, the performance of the battery is significantly improved during long-term cycling. This is mainly because the additive forms a double layer during the cycling process. layer of passivation film, while reducing the degradation of the electrode surface and the oxidative decomposition of the electrolyte.

　　4. Sulfur-containing additives

　　In recent years, there have been many reports on the application of organic sulfonate esters as additives in lithium-ion batteries. PIRES adds 1,3-propanesultone (PS) to the high-voltage lithium-ion battery electrolyte, which effectively inhibits the occurrence of side reactions on the electrode surface and the dissolution of metal ions. ZHENG et al. used dimethylsulfonylmethane (DMSM) as an electrolyte additive for high-voltage LiNil/3Col/3Mn1/3O2 graphite batteries. XPS, SEM and TEM analysis results showed that the presence of MMDS has a good modification effect on the positive electrode SEI film, even if It can also significantly reduce the electrode/electrolyte interface impedance under high pressure and improve the cycle stability of the cathode material. In addition, HUANG et al. studied the cycle performance of trifluoromethylphenylene sulfide (PTS) additive in high-voltage lithium-ion batteries at room temperature and high temperature. Analysis of theoretical calculation data and experimental results shows that PTS is oxidized preferentially than solvent molecules during the cycle, and the SEI film formed improves the cycle stability of the battery under high voltage. In addition, some thiophene and its derivatives are also considered for use as high-voltage lithium-ion battery additives. When these additives are added, a polymer film will be formed on the surface of the positive electrode to avoid oxidative decomposition of the electrolyte under high pressure.

　　5. Ionic liquid additives

　　Ionic liquid is a low-temperature molten salt that is widely used in lithium-ion batteries because of its advantages such as low vapor pressure, high conductivity, non-flammability, thermal stability and high electrochemical stability.

　　Selection criteria for high voltage lithium battery electrolyte

　　The performance of high-voltage lithium-ion batteries is mainly determined by the structure and properties of active materials and electrolytes. Among them, the matching of electrolyte is also very important. Because as the energy density increases, generally the compacted densities of the positive and negative electrodes are relatively large, the wettability of the electrolyte becomes worse, and the liquid retention capacity decreases. Low fluid retention capacity will result in poor cycle and storage performance of the battery.

　　1. Choose some solvents with higher oxidation potential and wider electrochemical window (such as sulfones, nitriles and fluorinated solvents).

　　2. Some positive electrode protection additives can be added to the electrolyte to improve the interface properties of the positive electrode material.

　　3. Add positive electrode film-forming additives to the electrolyte to inhibit the reaction between the electrolyte and the positive electrode material interface.

　　4. Add new high-voltage resistant lithium salt as an additive to the electrolyte. For example, adding bisoxaloboric acid (LiBOB) to the electrolyte can also form a film on the surface of the cathode material, preventing side reactions between the electrolyte and the electrode material.

　　High-voltage additives generally oxidize prior to solvent molecules during the circulation process, forming a passivation film on the surface of the positive electrode, stabilizing the electrode/electrolyte interface, and ultimately achieving the stable existence of the electrolyte under high pressure. I believe that with the advancement of technology, the use of high-voltage lithium batteries will be the future trend.

　　The electrolyte is very critical. We call the electrolyte the blood of the ion battery. On the one hand, the electrolyte is the bridge connecting the positive and negative electrodes. On the other hand, the electrolyte is also the medium for the migration and transmission of ions. We need to develop a high-voltage electrolyte system. , additives are very critical. Some additives can use competing ions to introduce ions and lithium ions for competitive solvation, thereby changing the solvation structure of the electrolyte.

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