The high-voltage cathode data of lithium-ion batteries has been a research hotspot in recent years. The development of a new type of lithium-ion battery with better performance, higher energy and higher voltage is the current research hotspot in the field of power supply. In recent years, the data of high-voltage positive electrodes represented by LiNi0.5Mn1.5O4 and LiCoPO4 have developed rapidly, while the supporting electrolyte is relatively backward. Therefore, the research and development of 5V electrolyte system is a key issue that needs to be solved urgently.
1. Carbonate solvent
Traditional carbonate rock solvents have been considered the best choice for general electrolytes due to their high conductivity, good solubility of lithium salts, and the ability to form a stable solid electrolyte interface film (SEI) on the surface of the negative electrode. However, the applicability of traditional carbonates in high-voltage battery systems is not good. This is because traditional carbonate solvents have a low oxidation potential and are prone to attack in advance under high-potential oxidation and decomposition. In addition, the content of water in the electrolyte of lithium-ion batteries is regarded as a key criterion to determine the quality of the battery. High-voltage electrolytes have higher requirements for water. If the water content in the electrolyte is slightly higher, it will greatly reduce the resistance of the electrolyte. Oxidation performance.
2. Ionic liquid
Ionic liquid is a salt composed entirely of cations and anions. It is liquid at room temperature and can conduct electricity. Ionic liquids have the advantages of low volatility, low flammability, high ionic conductivity, and wide electrochemical window. Due to these characteristics of ionic liquids, ionic liquids have been extensively studied in recent years and used as a new electrolyte to improve the electrochemical and thermal stability of lithium-ion batteries at high capacity and high voltage. The results show that the pyrrole-piperidinyl difluoromethanesulfonimide salt ionic liquid is more suitable for 5V high voltage electrolyte data than the traditional lipf6 based electrolyte.
But ionic liquids have obvious disadvantages:
(1) The production cost is high and cannot be used in large-scale industrialization;
(2) Although the ionic conductivity of the ionic liquid is very high, the conductivity of the liquid electrolyte is still very low;
(3) The viscosity of the entire ionic liquid is relatively large, which is not conducive to the high rate charge and discharge of lithium ion batteries.
The development of new electrolyte systems is the most concerned direction in the research of lithium ion high-voltage electrolytes, including: fluorinated solvents, nitrile compounds, sulfone compounds and other new solvent compounds.
1. Fluorine solvent
Because fluorine atoms have strong electronegativity and weak polarity, fluorine solvents have high electrochemical stability. The researchers studied a series of fluorinated organic carbonate solvents and found that the fluorine element in the fluorocarbonate solvent significantly improved the antioxidant function of the fluorocarbonate. The oxidation potential of fluorinated ethylene, methyl 2,2,2-trifluoroethyl carbonate and ethyl 2,2,2-trifluoroethyl carbonate is significantly higher than that of ethyl carbonate (EC) and methyl ethyl carbonate. Carbonate (EMC) and Ethyl Carbonate (DEC). However, as the number of hydrogen atoms replaced by fluorine increases, the solubility of LiPF6 in solvents is greatly reduced.
2. Nitrile solvent
In the process of studying double-layer capacitors, researchers found for the first time that the antioxidant potential of glutaronitrile and adiponitrile is as high as 8.3v, and their electrochemical window is wider than that of all aprotic solvents. But the compatibility of nitrile solvent and graphite anode is poor. With the increase of cycles, the internal resistance of the battery also increases, which greatly reduces the cycle function of the battery. Using EC and LiBOB as electrolyte additives can improve the poor compatibility of nitrile solvent and graphite anode.
3. Sulfoxide solvent
Sulfoxide solvents are currently a research hotspot at home and abroad, and it is a research hotspot to replace traditional carbonate rock solvents. Sulfoxide solvents are widely used as electrolytes in lithium ion batteries, lithium sulfur batteries, lithium air batteries and other fields. The antioxidant potentials of methyl ethyl sulfone (EMS) and methyl ethyl sulfone both exceed 5.8v, and they have good compatibility with Mn series positive electrodes, but they have poor compatibility with graphite negative electrodes and cannot be used in batteries with graphite as the negative electrode. .