High-voltage cathode materials for lithium-ion batteries have been a hot topic of research in recent years, and the corresponding high-voltage electrolyte has also become a focus of research. Designing and preparing new lithium-ion batteries with better performance, higher energy and voltage is a research hotspot in the power supply field. In recent years, high-voltage cathode materials represented by LiNi0.5Mn1.5O4 and LiCoPO4 have developed rapidly, while the matching electrolytes have lagged behind. Therefore, the development of a 5V electrolyte system is a key issue that urgently needs to be solved.

　　Traditional electrolyte

　　1. Carbonate solvents

　　Traditional carbonate solvents have always been regarded as the best choice for general-purpose electrolyte solvents due to their high conductivity, good solubility of lithium salts, and ability to form a stable solid electrolyte interface film (SEI film) on the surface of the negative electrode. However, the applicability of traditional carbonates in high-voltage battery systems is not good. This is because the oxidation potential of traditional carbonate solvents is low and it is easy to oxidize and decompose prematurely at high potentials. In addition, the moisture content in the electrolyte of lithium-ion batteries has always been considered a key criterion for determining battery quality. High-voltage electrolytes have higher requirements for moisture. If the water content in the electrolyte is slightly higher, the electrolyte's performance will be greatly reduced. Oxidation resistance function.

　　2. Ionic liquid

　　Ionic liquids are salts that are completely composed of cations and anions, are liquid near room temperature, and can conduct electricity. Ionic liquids share many common advantages such as low volatility, low flammability, high ionic conductivity and wide electrochemical window. Due to these characteristics, ionic liquids have been extensively studied in recent years and used as new electrolytes to improve the electrochemical and thermal stability properties of lithium-ion batteries under high capacity and high voltage. Studies have shown that ionic liquids of pyrrole and piperidine bis(trifluoromethylsulfonyl)imide salts are more suitable for use as 5V high-voltage electrolyte materials than conventional LiPF6-based electrolyte systems.

　　However, ionic liquids have very obvious shortcomings:

　　(1) The preparation cost is high and it cannot be used on a large scale in industry;

　　(2) Although ionic liquids have high ionic conductivity, their conductivity is still low compared to liquid electrolytes;

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　　(3) Ionic liquids generally have high viscosity, which is not conducive to high-rate charging and discharging of lithium batteries.

　　New liquid electrolyte

　　The development of new electrolyte systems is the most concerned direction in the study of lithium-ion high-voltage electrolytes, which mainly include: fluorinated solvents, nitrile compounds, sulfone compounds and other new solvent compounds.

　　1. Fluorinated solvent

　　Because fluorine atoms have strong electronegativity and weak polarity, fluorinated solvents have high electrochemical stability. The researchers studied a series of fluorinated organic carbonate solvents and found that after introducing fluorine element into the carbonate solvent, the antioxidant function of the fluorine-containing carbonate will be significantly improved. The oxidation potentials of fluorinated ethylene carbonate, methyl-2,2,2-trifluoroethyl carbonate and ethyl-2,2,2-trifluoroethyl carbonate are much higher than those of unfluorinated ethylene carbonate. (EC), ethyl methyl carbonate (EMC) and ethyl carbonate (DEC). However, as the number of hydrogen atoms replaced by fluorine atoms increases, the solubility of LiPF6 in the solvent will be greatly reduced.

　　2. Nitrile solvents

　　In the process of studying electric double layer capacitors, researchers first discovered that glutaronitrile and adiponitrile have an antioxidant potential as high as 8.3V, and their electrochemical windows are wider than all aprotic solvents. However, nitrile solvents have poor compatibility with graphite negative electrodes. As cycles increase, the internal resistance of the battery also increases, greatly reducing the cycle performance of the battery. The use of EC and lithium bisoxalate difluoroborate (LiBOB) can form a stable SEI film on the surface of the graphite negative electrode. Adding EC and LiBOB as additives to the electrolyte can effectively improve the poor compatibility between nitrile solvents and graphite negative electrodes. One question.

　　3. Sulfone solvents

　　Sulfone solvents are a solvent that researchers are currently focusing on to replace traditional carbonates. Sulfone solvents are widely used as electrolytes in different fields, such as lithium-ion batteries, lithium-sulfur batteries and lithium-air batteries. The oxidation resistance potential of methyl ethyl sulfone (EMS) and methoxyethyl methyl sulfone exceeds 5.8V, and they have good compatibility with Mn-based positive electrodes. However, their compatibility with graphite negative electrodes is very poor, so they cannot Used in batteries with graphite as the negative electrode.

　　4. Other high voltage systems

　　LiBOB and lithium oxalate difluoroborate (LiODFB) are electrolyte lithium salts that can form a stable SEI film on the surface near graphite in a pure propylene carbonate (PC) solvent system. They are common negative electrode surface components like VC. Membrane additives. The study found that compared with TMS and LiPF6, TMS has very good compatibility with LiBOB and LiODFB. The electrolyte composed of TMS, LiBOB, and LiODFB can not only form a stable SEI film, but also effectively reduce the internal resistance of the battery. In recent years, LiBOB's ability to protect high-voltage cathode materials on the surface of the cathode has attracted more and more attention. For example, adding LiBOB to the electrolyte that matches traditional high-voltage cathode materials can enhance the cycle performance of the battery. . By studying the phenomenon of SEI film formed by LiBOB on the surface of the cathode, it was found that adding LiBOB can effectively protect the oxidative decomposition of the electrolyte. At the same time, through theoretical calculations, the oxidation and decomposition mechanism of LiBOB on the electrode surface was studied: LiBOB ring opening causes pairwise polymerization to form a protective film, and the B atoms exposed due to ring opening on the outside can attract PF-6 and F- to combine with it. , and then achieve the purpose of preventing the electrolyte from decomposing.

　　In general, high-voltage electrolytes for lithium-ion batteries have attracted the attention of many researchers and have become the most important research direction for lithium-ion battery electrolytes. Different high-voltage electrolyte systems have their own advantages and disadvantages. Due to the high cost of development and use of new solvent systems, there is currently no new solvent that can completely replace carbonate-based solvents; if the production and use costs can be significantly reduced and the compatibility with negative electrode materials can be improved, new solvent system electrolytes will have It is hoped that it can completely replace carbonate-based solvents. Therefore, more detailed research is needed. As the research continues to deepen, the new solvent system electrolyte will definitely have wider application prospects.

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