Research progress in electrolytes for lithium-ion batteries-Industry dynamics-Yantai Haibo Electrical Equipment Co., Ltd-Haibo Electric

Research progress in electrolytes for lithium-ion batteries

The four key materials of lithium-ion batteries include positive electrode, negative electrode, separator, and electrolyte. The electrolyte transports ions and ion compounds between the positive and negative electrodes of the battery, and its performance directly determines the conductivity, capacity, and output voltage of the lithium-ion battery. The main component of electrolyte is electrolyte. According to the state, lithium ion battery electrolytes can be divided into liquid, gel polymer, dry ionic conductive polymer and inorganic ceramics.Electrolytes should meet the following requirements as much as possible

1. Good electrochemical stability, without serious side reactions with positive electrode, negative electrode, separator, current collector, binder, battery shell, etc;

2. High ion conductivity, good electronic insulation, high dielectric constant, conducive to reversible electrochemical reactions;

3. Wide temperature range and voltage window for use;

4. Low production cost and environmentally friendly.

一、 Electrolytes for traditional electrolytesUsually, the electrolyte needs to be dissolved in an appropriate solvent to make an electrolyte for use. Electrolyte is the ionic conductive medium between the positive and negative electrodes inside a battery, mainly including coordinated phosphates, coordinated boric acids, sulfonamide salts, and other lithium salts, with a concentration typically ranging from 0.5 to 2.0 mol/L.

Coordination phosphates are lithium salts of LiPF6 and its derivatives, mainly including Li (C3F7) 2PF4, Li (C3F6) PF5, Li (C2O4) PF4, Li (C2F5) PF3, etc. This type of electrolyte has good oxidation stability, high ion conductivity, but is extremely sensitive to water, and the choice of solvent has a significant impact on the temperature of use. Among them, LiPF6 is currently the most widely used lithium-ion battery electrolyte in laboratory and industrial applications.Coordination boric acids are LiBF4 and its derivatives, mainly including LiBF2C2O4LiB (C2H5) 3 (C4H4N), LiB (C6F5) 3 (CF3), LiB (C2O4) 2, etc. This type of electrolyte has good low-temperature performance, wide electrochemical window, high conductivity, and better thermal stability than coordinated phosphates, but its production cost is high and has not been widely applied.Sulfonimide salts LiN (SO2CF3) 2 and its derivatives mainly include Li [N (SO2F2)] 2, LiN (SO2F) (SO2C4F9)], Li [N (SO2F) (SO2C2F5)], etc. This type of electrolyte has good antioxidant and thermal stability, but is prone to corroding the positive electrode current collector aluminum foil, and has not been widely applied. Other types include LiClO4, LiAsF6, etc.Electrolytes containing As are highly toxic and have few practical applications. Electrolytes with the anion ClO4- have strong oxidizing properties and are prone to side reactions at higher voltages, causing battery bloating and safety issues. Currently, they are only used in small quantities in laboratories.During the initial discharge process of lithium-ion batteries, most negative electrode materials are prone to forming a solid Li+conductive film (solid electrolyte film SEI film). The SEI film is very thin and has poor mechanical strength. For materials with significant volume changes before and after charging and discharging, the SEI film will continuously break and form during the charging and discharging process, seriously consuming electrolyte and electrode active materials, leading to battery capacity degradation. Adding a small amount of functional additives to the electrolyte can delay this phenomenon.

Many compounds containing sulfur, nitrogen, boron, and phosphorus elements can be used as additives. For example, when the concentration of lithium difluorophosphate (LiDFP) reaches 0.15mol/L, the main components of the SEI membrane are LiF and phosphate, which can effectively inhibit the growth of lithium dendrites. The use of dimethyl sulfate as an additive can make the SEI film contain Li2S/Li2O, which can significantly increase the utilization rate of the metal lithium negative electrode. By using a mixture of fluorinated ethylene carbonate (FEC) and LiNO3 as additives, the main components of the SEI film can be LiF and LiNxOy, allowing the metal lithium negative electrode to maintain a smooth surface after multiple charges and discharges.The above additives are all controlled by using anionic compounds to form lithium salts with Li+to control the composition of the SEI membrane. Recent reports have shown that the use of additives containing cations such as In3+, Sn2+, Bi3+, Mg2+, Zn2+can deposit cations on the negative electrode surface, form alloys with lithium ions, and thus inhibit the growth of lithium dendrites.In addition to changing the composition of the SEI film, adding flame retardants can also improve the safety of the battery. At present, there are four main types of flame retardants: ① phosphorus containing compounds; ② Halogenated compounds; ③ Phosphorus nitrogen compounds; ③ Other compounds that do not contain phosphorus, nitrogen, or halogens.Phosphorus containing flame retardants mainly include fluorinated phosphate esters, phosphoronitrile, and alkyl phosphate ester compounds, which have good flame retardant effects. Trimethyl phosphate, triethyl phosphate, dimethyl methylphosphonate, tributyl phosphate, diethyl ethylphosphate, and tris (2,2,2-trifluoroethyl) phosphite are widely studied flame retardants containing phosphorus compounds. Halogenated flame retardants mainly include organic compounds such as fluorocarbonates and fluorochloroalkanes. This type of flame retardant has the advantages of low viscosity and high solubility. In addition, fluorinated flame retardants can increase the flash point of electrolyte solvents. The phosphorus nitrogen compound series flame retardants combine the advantages of phosphorus containing flame retardants and halogenated flame retardants, and have good flame retardant effects. Phosphorus nitrogen compound flame retardants have the advantages of low smoke and low toxicity due to their halogen-free nature. Their flame retardant mechanism is to decompose upon heating and attach a dense layer of carbon black to the surface of the object, isolating oxygen. Non phosphorus, nitrogen, and halogenated flame retardants are generally additives such as organosilanes and soluble metal salts (such as CuCl2). They mainly increase the flash point of the electrolyte solvent to raise the ignition temperature.

二、 Ionic liquid electrolyte

Ionic liquid refers to a liquid composed entirely of anions and cations, which is usually non combustible. Room temperature ionic liquids are those that appear in a liquid state at or near room temperature, while those that require high temperature to appear in a liquid state are called high-temperature ionic liquids. The ionic bonds of ionic liquids decrease with the increase of cation radius, and the melting point also decreases accordingly.

At present, the most studied ionic liquids in lithium-ion electrolytes are [Glyme Li]+TFSI - and [Glyme Li]+FSI -. Seki et al.'s research indicates that [Li (triglyme) x] TFSA ionic liquids can effectively inhibit the dendritic growth of metallic lithium anodes, and the inhibitory effect becomes more pronounced with the increase of lithium salt concentration. High polarity solvent molecules such as ethylene carbonate can disrupt the coordination between Li+and ethylene glycol dimethyl ether, thereby disrupting the structure of the ionic liquid.Kitazawa et al. obtained [Li (tetraglyme) 2] FSI-DOL ionic liquid electrolyte by adding non-polar dioxane (DOL) to Tetraglyme/LIFSI, resulting in a first-time efficiency of up to 99% for metallic lithium. Due to the decomposition of DOL, a uniform thin layer of SEI film was formed on the lithium surface.Ionic liquid electrolytes have many advantages compared to traditional electrolytes, but ionic liquids themselves are easy to absorb water, and the thermal stability and electrochemical window of the absorbed ionic liquid electrolyte are significantly reduced; And most ionic liquids have high viscosity, which limits the movement of Li+. In addition, the high price of ionic liquids and the high cost of purification processes during production limit the application of ionic liquid electrolytes.

三、 Polymer electrolyte

Liquid electrolytes are prone to leakage during use and cannot be made into thin films, which limits the energy density and power density of batteries. The use of gel electrolyte can improve the energy density and power density of the battery.The traditional gel electrolyte is mainly a complex formed by a polymer with electron donating groups and lithium salt. This electrolyte has elasticity similar to rubber and good processing properties. At present, the most studied gel electrolytes are gel electrolytes in the polyethylene oxide (PEO) system, gel electrolytes in the polyacrylonitrile system (PAN), gel electrolytes in the polymethyl methacrylate system (PMMA), Polyvinylidene fluoride (PVDF) and its copolymer system gel electrolyte.Usually, PEO systems have high viscosity, and in the process, PEO tends to be dissolved in the electrolyte, poured into a film, and then partially evaporated to obtain an ion conductive thin film; The viscosity of PAN, PMMA, and PVDF systems is much lower than that of PEO systems, so PAN, PMMA, and PVDF can be filmed through processes such as electrospinning and casting, and then absorbed into the electrolyte to obtain an ion conductive film. The part of gel electrolyte that plays the role of ionic conduction is the absorbed lithium salt/electrolyte, and the polymer acts as a membrane forming scaffold.

The gel electrolyte made of unmodified PEO has high lithium ion conductivity, but poor mechanical properties, mainly because some PEO is soluble in solvents. The lower the molecular weight of PEO, the easier it is to dissolve, and increasing its molecular weight can improve its coagulation properties. The PEO based gel electrolyte modified by crosslinking or adding filler has high mechanical strength and good processing performance, and the lithium ion conductivity is close to 10-3S/cm, but the problem of PEO molecular chain dissolution is still not fundamentally solved.PAN based gel electrolyte has good processability. After being heated, cyclization reaction takes place to form a cross-linked structure, which is flame retardant. After modification, the lithium ion conductivity can approach 10-3S/cm. Because the PAN molecular chain does not contain oxygen atoms, and the interaction between nitrogen atoms and lithium ions is weak, the lithium ion migration number can reach 0.5, which is larger than that of PEO based gel electrolyte, but the decomposition voltage is low, generally between 4.3 and 5.0 V.The decomposition voltage of PMMA based gel electrolyte is more than 4.6V, but it is difficult for unmodified PMMA to form a self-supporting membrane. PVDF based gel electrolyte has the advantages of weather resistance, drug resistance, heat resistance, solvent resistance and so on. It is currently the most widely used polymer electrolyte in industry.

According to the amount of liquid absorbed, gel electrolytes can be divided into solvent polymers and polymer solvents. In the solvent polymer, the solvent is the main component, and the role of the polymer is to make the solvent (dissolved lithium salt electrolyte) gelled to avoid leakage. In polymer solvents, polymers are the main component, and this type of electrolyte has good flexibility, mechanical strength, and processing performance. Industrial polymer lithium-ion batteries often use this type of electrolyte.

During the preparation of gel electrolyte, organic or inorganic particles are added to make it flame retardant. Inorganic additives include TiO2, SiO2, Al2O3 and other oxides. The oxide particles are filled in the gel electrolyte, reducing the content of the electrolyte and making the combustibility worse, but this composite effect is not obvious. The oxide coating on the surface of gel polymer fiber can prevent the electrolyte from burning. Inorganic fillers can reduce the crystallinity of polymer membranes and improve the ion conductivity of polymer bodies. However, the inorganic filler and the polymer body are in a phase separation state, and lithium ions can only conduct in the polymer body, resulting in a slight decrease in the ion conductivity of the polymer electrolyte.

四、Inorganic Solid Electrolyte

Inorganic solid electrolytes have the advantages of high thermal stability (solid-phase sintering synthesis temperature generally greater than 600 ℃), non combustion, wide electrochemical window, high mechanical strength, and basically no lithium dendrites penetrating the electrolyte layer causing internal short circuit problems in the battery. Therefore, lithium-ion batteries using inorganic solid electrolytes generally have excellent cycling stability and capacity retention. The working temperature range of lithium-ion batteries using solid electrolytes can be extended to -50~200 ℃, which is much wider than the temperature range of traditional lithium-ion batteries -20~60 ℃.In addition, solid-state battery technology can simplify packaging processes and use high-voltage positive electrode materials, thereby significantly improving the specific energy density of the battery. Inorganic solid electrolytes can be divided into crystalline and amorphous types. Crystalline electrolytes have fixed lithium ion transport channels, but the presence of grain boundaries significantly reduces the lithium ion conductivity; The amorphous class does not have a clear lithium ion transport channel, but the long-range disordered arrangement of atoms can also hinder lithium ion transport.The key to practical applications of crystalline and amorphous electrolytes is to determine the appropriate ratio of components. At present, the four most studied lithium-ion inorganic solid electrolytes are lithium phosphorus oxide nitrogen (LiPON), perovskite (Perovskite), garnet (Garnet), and anionic polymer (LISICON) electrolytes.

The ion conductivity of LiPON is low, but its chemical and electrochemical properties are stable, making it an electrolyte material for thin film batteries; The Perovskite type solid electrolyte has a high conductivity, reaching 10-3S/cm, but its electrochemical performance and chemical stability are poor; Garnet type inorganic electrolytes have advantages such as excellent ion conductivity, good chemical stability, good electrode material compatibility, and wide operating temperature range; LISICON type electrolytes are structurally similar to γ- Li3PO4 is similar and can achieve excellent ion conductivity through ion doping.

In addition to the four typical crystalline solid electrolytes mentioned above, hydride, halide, boride, and phosphate solid electrolytes are mostly amorphous electrolytes. Due to their relatively easy preparation, they have attracted the interest of scientific researchers in recent years.In general, solid electrolytes have their unique advantages, but compared with liquid and gelled electrolytes, ionic conductivity is low. The densification of solid electrolyte films can minimize the negative impact of grain boundaries on ion transport, and the coating method is currently a mature industrial method for preparing complete all solid state batteries. From the perspective of production costs, solid electrolytes are expected to become the electrolyte for the next generation of safe lithium-ion batteries if they achieve technological breakthroughs and production costs are reduced.

(Reference: Feng Dong, Hao Siyu, Xie Yuhui, Xie Delong, Zeng Tianbiao. Research progress in lithium-ion battery electrolytes [J/OL]. New chemical materials.)

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