Lithium ion battery electrolyte additive and electrolyte

Abstract

The application relates to a lithium ion battery electrolyte additive 2-(dimethyl phosphate ester)-18-crown-6-ether and a corresponding high-energy-density lithium ion battery electrolyte. The components of the electrolyte include a lithium salt, an anhydrous organic solvent and an electrolyte additive; wherein the electrolyte additive includes 2-(dimethyl phosphate ester)-18-crown-6-ether; the amount of 2-(dimethyl phosphate ester)-18-crown-6-ether in the electrolyte is 0.1%-10% of the total mass of the electrolyte. The 2-(dimethyl phosphate ester)-18-crown-6-ether prepared by the application introduces a phosphate ester functional group on the basis of a crown ether; not only can the conductivity be improved to meet the charging and discharging requirements, the viscosity can be reduced to reduce ion migration resistance (especially at low temperatures), the chemical stability can be enhanced, the thermal stability can be improved, the positive and negative electrode compatibility can be improved, the interface reaction and SEI film rupture problems can be reduced and the like, the battery performance and safety can be ensured, and thus the lithium ion battery technology development is promoted.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a high energy density lithium ion battery electrolyte, and more particularly to the application of 2-(dimethylphosphonate)-18-crown-6-ether in electrolyte. BACKGROUND

[0002] With the global emphasis on environmental protection and sustainable development, the electric vehicle (EV) market is growing rapidly. In order to improve the cruising range of electric vehicles and make them competitive with traditional fuel vehicles in terms of convenience of use, it is necessary to improve the energy density of lithium ion batteries. As a key component of lithium ion batteries, the performance of electrolyte directly affects the energy density of the battery. For example, long-range electric vehicles require batteries to store more energy in a limited volume and weight, which puts higher demands on electrolyte.

[0003] Smartphones, tablets and laptops and other portable electronic devices are constantly pursuing thinner designs, longer battery life. Consumers want devices to work longer without increasing the size and weight of the device. High energy density lithium ion batteries are the key to meeting this demand, and electrolyte innovation is crucial to achieving battery energy density improvement. For example, in smartphones, higher energy density batteries can reduce charging frequency and improve user experience.

[0004] Electrolyte plays an important role in transporting lithium ions in lithium ion batteries. The solvation structure of lithium ions in electrolyte, migration mechanism and other theories are the basis for designing high energy density electrolyte. For example, understanding how lithium ions interact with solvent molecules and lithium salt anions in electrolyte helps to select the right combination of solvents and lithium salts to improve the migration rate of lithium ions. Studies have shown that by adjusting the dielectric constant and electron-donating ability of the solvent, the solvation sheath of lithium ions can be optimized, thereby improving its transport efficiency in electrolyte.

[0005] Electrolyte must have sufficient electrochemical stability to prevent oxidative or reductive decomposition within the operating voltage range of the battery. In theory, the redox potential window of the electrolyte determines the highest and lowest operating voltage that the battery can achieve. High energy density batteries often require higher operating voltage, which requires electrolyte to have a wider electrochemical stability window. For example, in high-voltage lithium ion batteries, electrolyte needs to be stable at voltages above 4.5V, which involves the selection and optimization of solvents, lithium salts and additives in the electrolyte to improve its oxidative stability.

[0006] Current lithium-ion battery electrolytes are mainly based on a combination of carbonate solvents (such as ethylene carbonate, diethyl carbonate, etc.) and lithium salts (such as lithium hexafluorophosphate). This traditional electrolyte meets the needs of existing lithium-ion batteries to some extent, but has limitations in terms of energy density. For example, lithium hexafluorophosphate is prone to decomposition at high voltage, limiting the further improvement of battery operating voltage and energy density.

[0007] In the development of high-energy-density power lithium-ion battery electrolytes, major chemical companies, battery manufacturers, and research institutions around the world are actively competing. For example, Japanese companies are leading in the early development of lithium-ion battery electrolytes, with companies such as Mitsubishi Chemical having a deep accumulation in the basic formulation development of electrolytes. In recent years, Chinese and Korean companies and research institutions have made considerable progress in the development and industrialization of new electrolyte materials. Chinese companies such as Tianci Materials and Suzhou New Area have expanded their share of the electrolyte market and are actively investing in the development of high-energy-density electrolytes.

[0008] As mentioned earlier, increasing the operating voltage of the battery is an important way to improve energy density, but existing electrolytes are prone to decomposition at high voltage, etc. This not only leads to a decline in battery performance, but also may cause safety hazards.

[0009] High-energy-density batteries often perform poorly in low-temperature environments, and the viscosity of the electrolyte will increase significantly as the temperature decreases, making it difficult for lithium ions to transport. Improving the low-temperature performance of the electrolyte so that it can still maintain good ionic conductivity at low temperatures is a technical challenge that needs to be addressed.

[0010] As energy density increases, the safety risk of the battery also increases. The electrolyte may burn or explode under abnormal conditions such as overcharging, overdischarging, short circuiting, or high temperature. Therefore, while improving energy density, ensuring the safety of the electrolyte, such as improving its flame retardancy, inhibiting lithium dendrite growth, etc., is also an important challenge.

[0011] In view of the above technical problems to be solved, the existing technologies are currently available. For example, CN116344943A discloses a lithium manganese iron phosphate battery electrolyte, a secondary battery and an electric device. The composition of the lithium manganese iron phosphate battery electrolyte includes: lithium salt, selected from lithium hexafluorophosphate, lithium tetrafluoroborate and the like; organic solvent, including ethylene carbonate, propylene carbonate and the like; additives, including crown ether compounds and boron trifluoride-pyridine, and can also include vinylene carbonate. Each additive has a certain mass percentage content range in the electrolyte. The secondary battery includes a positive electrode sheet, a negative electrode sheet (made of graphite or the like), a separator and the above electrolyte. The electrolyte includes lithium salt, solvent and additives, which can well solve the problems of poor high-temperature cycle and storage performance of LMFP battery. The additives include crown ether compounds, which can significantly reduce the residual amount of Mn ions in the electrolyte. Due to the strong coordination environment of its structure, it can form a complex with Mn ions, well inhibit the migration of dissolved Mn ions in the solution, effectively remove Mn ions in the electrolyte, and improve the thermal stability of the electrolyte. In addition, by selecting additives containing Si-N characteristic structure, HF can be removed, and the synergistic effect of low impedance additives can be combined to comprehensively improve the cycle performance of the battery, while reducing the interface impedance and improving the conductivity. However, in the existing technology, crown ethers or phosphates are added alone to the electrolyte, which plays a corresponding role, but also brings corresponding performance risks, as described below.

[0012] Advantages of crown ethers: Adding crown ethers to lithium-ion battery electrolytes can improve electrolyte conductivity for the following two reasons: (1) Promote lithium salt dissociation: Lithium salts in lithium-ion battery electrolytes need to dissociate into lithium ions and anions to conduct electricity. In ordinary electrolytes, lithium salts may have some degree of association, which will limit the number of lithium ions and reduce the conductivity of the electrolyte. Crown ethers have a special ring structure, and the size of the cavity inside matches the size of lithium ions, allowing selective complexation with lithium ions. This complexation can break the association structure of lithium salts, allowing more lithium salts to dissociate into free lithium ions and anions, increasing the number of mobile charges in the electrolyte and improving conductivity. (2) Reduce electrolyte viscosity: The viscosity of the electrolyte has an important influence on the migration speed of ions. The higher the viscosity, the greater the resistance to ion movement in the electrolyte, and the lower the conductivity. The addition of crown ethers can interact with solvent molecules in the electrolyte, changing the structure and arrangement of solvent molecules and reducing the overall viscosity of the electrolyte. In this way, lithium ions can move more freely in low-viscosity electrolytes, speeding up the transmission of electrical charges and improving the conductivity of the electrolyte. But crown ethers also have performance risks: (1) Crown ethers have poor thermal stability when used in lithium-ion battery electrolytes. ① Instability of the macrocyclic structure: Crown ethers are macrocyclic compounds containing multiple oxygen atoms. This macrocyclic structure is prone to conformational changes or structural distortions when heated. The flexibility of the macrocyclic structure makes the molecule more susceptible to deformation under heat, causing stress on the chemical bonds within the molecule, which in turn reduces the thermal stability of the crown ether molecule. ② Influence of oxygen atoms: Crown ether molecules contain multiple oxygen atoms, which have high electronegativity and easily attract surrounding electrons. When heated, the electron cloud distribution around the oxygen atoms changes, causing the chemical bonds within the molecule to become unstable and prone to breaking, reducing the thermal stability of the crown ether. ③ Thermal sensitivity of complex structures: Crown ethers can complex with lithium ions to form complexes. This complex structure is sensitive to heat, as the complex bond between lithium ions and crown ethers is not very strong and can break at higher temperatures. When the complex bond breaks, lithium ions are released, and the structure of the crown ether molecule is also disrupted, resulting in a decrease in thermal stability. ④ Influence of energy changes: The complexation of crown ethers with lithium ions is an energy change process that absorbs or releases energy during the complexation process. When heated, this energy change is intensified, causing the interaction between crown ethers and lithium ions to change, affecting the thermal stability of crown ethers. (2) Crown ethers used in lithium-ion battery electrolytes result in poor SEI or CEI film formation. ① Interference with lithium ion transport: Crown ethers have strong complexing ability for lithium ions, forming complexes with lithium ions in the electrolyte. This complexation may interfere with the transport of lithium ions on the electrode surface, resulting in uneven distribution of lithium ion concentration on the electrode surface.During the formation of SEI or CEI films, lithium ions are required to participate in the reaction to form stable film structures. The complexation of crown ethers with lithium ions reduces the number of lithium ions participating in the film formation reaction or slows down the transmission speed, thereby affecting the normal growth and formation of the film, making the film formation effect worse. 2. Non-uniform film composition: The complex of crown ether and lithium ion may be embedded in the SEI or CEI film being formed, resulting in a non-uniform composition of the film. This non-uniform film structure may not effectively protect the electrode material during the charging and discharging process of the battery, and is prone to cracking or decomposition, thereby affecting the performance and life of the battery. 3. Solvent competition reaction: Crown ethers will interact with solvent molecules in the electrolyte, which may compete for reaction sites involved in the formation of SEI or CEI films. For example, in the initial reaction stage on the electrode surface, the reduction reaction of solvent molecules is an important step in the formation of SEI films. The presence of crown ethers may interfere with the adsorption and reaction of solvent molecules on the electrode surface, making it difficult for solvent molecules to participate in the formation of the film normally, resulting in an undesirable film formation effect.

[0013] In contrast, phosphates have the following advantages: (1) Phosphates applied in electrolyte can play a role in flame retardation. ① Phosphates will produce phosphorus-containing free radicals such as PO· during thermal decomposition. These free radicals can capture hydrogen radicals (H·) and hydroxyl radicals (HO·) generated during the combustion reaction of the electrolyte, and convert them into relatively stable substances such as HPO, thereby preventing or slowing down the combustion chain reaction, interrupting the free radical reaction chain that the flame relies on to continue burning, and achieving the effect of flame retardation. ② Phosphates will absorb a large amount of heat during decomposition and related chemical reactions, which can lower the temperature of the electrolyte system, making it difficult for battery materials to reach the temperature conditions required for combustion, thereby inhibiting the occurrence of combustion reactions. At the same time, the presence of phosphoric acid will prevent carbon monoxide (CO) from being oxidized to carbon dioxide (CO2), which will also consume heat and further lower the temperature of the system. (2) Improve the electrochemical window. The electrochemical window refers to the highest and lowest potential range in which redox reactions can occur in the electrolyte. The addition of phosphates can improve the electrochemical window of the electrolyte, allowing it to perform stable redox reactions in a wider potential range. A wider electrochemical window means that the battery can work at a higher voltage, thereby increasing the energy density of the battery; at the same time, it can also maintain good stability at a lower voltage, reducing the risk of damage to the battery in the case of overcharging or overdischarging. (3) Increase solvent stability. Phosphates have high chemical and thermal stability in electrolytes. It can react with water molecules in the electrolyte to form phosphates and esters, thereby reducing the content of water molecules. This helps to reduce the evaporation and decomposition rate of the electrolyte, improve the stability of the battery in harsh environments such as high temperature, and prolong the cycle life of the battery. Moreover, this improvement in stability is also conducive to maintaining good performance of the battery during long-term storage. (4) Assist in forming a stable interface film. On the surface of the positive and negative electrodes of the battery, a layer of solid electrolyte interface (SEI) film and positive electrolyte interface (CEI) film will be formed. Phosphates can participate in the formation process of these interface films, or adjust their structure and composition, making them more stable and having good ion conductivity and electronic insulation. Stable interface film can prevent continuous decomposition of electrolyte, inhibit the dissolution of positive metal ions, improve the cycle performance and safety of the battery. Phosphates also have some performance risks: (1) Reduced ionic conductivity: The molecular structure and properties of phosphates may affect the migration rate of lithium ions in the electrolyte. Compared with conventional electrolyte solvents, the viscosity of phosphates is usually higher, which will increase the resistance of ions in the electrolyte, resulting in a decrease in ionic conductivity. The decrease in ionic conductivity will increase the internal resistance of the battery, generate more heat during charging and discharging, and affect the power performance and energy efficiency of the battery. (2) Effect on lithium salt: Phosphates may interact with lithium salts in the electrolyte, affecting the solubility and ionization degree of lithium salts, thereby affecting the capacity of the battery.For example, phosphates may form complexes or precipitates with lithium salts, reducing the effective concentration of lithium salts, reducing the number of lithium ions participating in the charge and discharge reaction, and causing the battery capacity to decrease.(3) Freezing point rise: The molecular structure of phosphates makes them less mobile at low temperatures, prone to freezing or crystallization, causing the freezing point of the electrolyte to rise. In low temperature environments, the freezing of the electrolyte can severely affect the transport of lithium ions, significantly reducing the low temperature performance of the battery, including reduced discharge capacity, decreased discharge voltage platform, and increased internal resistance.

[0014] At the same time, in the field of electrolyte additive research, researchers are accustomed to improving within the same or similar functional compounds. The performance and reaction conditions of crown ethers and phosphates are very different, for example, crown ether synthesis requires specific cyclization reaction conditions, and phosphates have their own esterification requirements. Combining the two not only requires precise design of molecular structure and functional group connection method, but also requires appropriate reaction conditions. A slight deviation will not only fail to achieve functional integration, but also have a negative effect. That is, to ensure that the above functional groups are accurately formed and stably connected, it is a great challenge to overcome technical obstacles; there is no similar technical route in the prior art.

[0015] The present application aims to solve the technical problems of high energy density lithium ion battery electrolyte, mainly involving the following aspects:

[0016] (1) Improve conductivity: In order to meet the rapid charge and discharge requirements of high energy density batteries, the electrolyte needs to have high ionic conductivity. However, the ionic conductivity of traditional electrolyte systems will decrease under conditions such as high concentration electrolyte or low temperature.

[0017] (2) Reduce viscosity: High viscosity of electrolyte will increase the resistance of ion migration, reduce the charge and discharge performance of battery. Especially in low temperature environment, the viscosity of electrolyte will increase significantly, causing the performance of battery to decrease greatly.

[0018] (3) Chemical stability: In high energy density batteries, the working voltage of electrode materials is high and active, which is easy to react with electrolyte. For example, high nickel positive electrode material will accelerate the oxidative decomposition of electrolyte at high voltage, affecting the cycle life and safety of battery.

[0019] (4) Thermal stability: High energy density batteries generate a lot of heat during charging and discharging, causing the internal temperature of the battery to rise. If the thermal stability of the electrolyte is poor, it may decompose, burn, etc. at high temperature.

[0020] (5) Positive electrode compatibility: The positive electrode material of high-energy-density batteries usually has a high specific surface area and activity, which is prone to interface reaction with the electrolyte, leading to the destruction of the positive electrode surface structure and performance degradation. For example, high-nickel ternary positive electrode materials will generate side reactions with the electrolyte during the cycling process, generating gas and causing the battery to swell.

[0021] (6) Negative electrode compatibility: For high-capacity negative electrode materials such as silicon-based negative electrodes, a huge volume change occurs during the charging and discharging process, which easily leads to the rupture and reconstruction of the SEI film (solid electrolyte interface film) on the negative electrode surface, consumes a large amount of electrolyte, and reduces the cycle life of the battery. SUMMARY

[0022] The present application aims to solve the problems of high-energy-density lithium-ion battery electrolyte, including improving the conductivity to meet the charging and discharging requirements, reducing the viscosity to reduce the ion migration resistance (especially at low temperatures), enhancing the chemical stability, improving the thermal stability, improving the positive and negative electrode compatibility, reducing the interface reaction and SEI film rupture, etc., to ensure the battery performance and safety, and promote the development of lithium-ion battery technology.

[0023] Specifically, the present application provides an electrolyte additive; the additive is 2-(dimethyl phosphate)-18-crown-6-ether shown in formula I:

[0024]

[0025] The synthesis method of the above-mentioned lithium-ion battery electrolyte additive comprises the following steps: adding 18-crown-6-ether, N-bromosuccinimide and carbon tetrachloride in a dry reaction flask; then adding an initiator; under nitrogen protection, the reaction mixture is heated to reflux; during the reaction, N-bromosuccinimide generates bromine radicals under the action of the initiator, which selectively reacts with one carbon on 18-crown-6-ether to generate 18-crown-6-ether-Br and succinimide; in a dry reaction flask, add 18-crown-6-ether-Br, dimethyl phosphate potassium salt, phase transfer catalyst and organic solvent; under stirring, the reaction mixture is heated to 60-80℃ for reaction; the phosphate anion in dimethyl phosphate potassium salt reacts with the bromine atom in 18-crown-6-ether-Br under the action of the phase transfer catalyst to generate 2-(dimethyl phosphate)-18-crown-6-ether and potassium bromide; after the reaction is completed, the reaction mixture is cooled to room temperature; by extracting and combining the organic phase, drying and purifying, pure 2-(dimethyl phosphate)-18-crown-6-ether is obtained.

[0026] Further, the initiator is azobisisobutyronitrile; the phase transfer catalyst is tetrabutylammonium bromide; the molar ratio of 18-crown-6-ether-Br to potassium dimethyl phosphate is 1:1.1-1.2, and the amount of the phase transfer catalyst is 0.1-0.2 times the amount of 18-crown-6-ether-Br.

[0027] Further, the purification is performed by column chromatography: silica gel is selected as the stationary phase, and ethyl acetate-petroleum ether mixed solvent is selected as the eluent, the ratio of the two is adjusted to 1:5-1:3, and TLC is monitored, and the fraction containing the target product is collected to obtain pure 2-(dimethyl phosphate)-18-crown-6-ether.

[0028] For example, the synthesis method of 2-(dimethyl phosphate)-18-crown-6-ether can be as follows:

[0029] (1) 18-crown-6-ether (high purity, dry treatment), N-bromosuccinimide (NBS), azobisisobutyronitrile (AIBN) as initiator, carbon tetrachloride (CCl4) as solvent.

[0030] (2) In a dry reaction flask, 18-crown-6-ether, N-bromosuccinimide and carbon tetrachloride are added. The molar ratio of 18-crown-6-ether to N-bromosuccinimide is 1:1.1. A small amount of azobisisobutyronitrile (mole fraction of 18-crown-6-ether is about 0.05-0.1) is added.

[0031] (3) Under the protection of nitrogen, the reaction mixture is heated to reflux. During the reaction, N-bromosuccinimide generates bromine radicals under the initiation of AIBN, and the bromine radicals selectively react with one carbon on 18-crown-6-ether to generate brominated product of 18-crown-6-ether and succinimide. The reaction equation is: 18-crown-6-ether + NBS→ 18-crown-6-ether-Br + succinimide. Reflux for 3-6 hours.

[0032] (4) Potassium dimethyl phosphate (which can be prepared by reacting dimethyl phosphate with potassium hydroxide), phase transfer catalyst (tetrabutylammonium bromide, TBAB), organic solvent (dimethylformamide, DMF).

[0033] (5) In a dry reaction flask, 18-crown-6-ether-Br, potassium dimethyl phosphate, phase transfer catalyst and organic solvent are added. The molar ratio of 18-crown-6-ether-Br to potassium dimethyl phosphate is 1:1.1-1.2, and the amount of the phase transfer catalyst is 0.1-0.2 times the amount of 18-crown-6-ether-Br.

[0034] (6) Under stirring, the reaction mixture is heated to 60-80℃ for reaction. The phosphate anion in the potassium salt of dimethyl phosphate reacts with the bromine atom in 18-crown-6-ether-Br to form 2-(dimethyl phosphate)-18-crown-6-ether and potassium bromide under the action of the phase transfer catalyst. The reaction equation is: 18-crown-6-ether-Br + (CH3O)2PO2K→ 2-(dimethyl phosphate)-18-crown-6-ether + KBr. The reaction time is 12-24 hours.

[0035] (7) After the reaction is completed, the reaction mixture is cooled to room temperature.

[0036] (8) Then pour into a large amount of water, and extract the product with an organic solvent (dichloromethane). Extract 3-5 times, and the amount of organic solvent used each time is about 1 / 3-1 / 2 of the volume of the aqueous phase.

[0037] (9) Combine the organic phases, and wash with saturated brine 2-3 times to remove residual water-soluble impurities.

[0038] (10) Dry the organic phase with anhydrous sodium sulfate overnight, and filter to remove the drying agent.

[0039] (11) Remove the organic solvent by a rotary evaporator to obtain a crude product.

[0040] (12) Purify the crude product by column chromatography. Select silica gel as the stationary phase, and select ethyl acetate-petroleum ether mixed solvent as the eluent. Adjust the volume ratio of ethyl acetate and petroleum ether to 1:5-1:3, and monitor by TLC. Collect the fraction containing the target product to obtain pure 2-(dimethyl phosphate)-18-crown-6-ether.

[0041] The present application relates to a high-energy-density lithium-ion battery electrolyte containing 2-(dimethyl phosphate)-18-crown-6-ether, and the components of the electrolyte include a lithium salt, an anhydrous organic solvent, and an electrolyte additive; wherein the electrolyte additive includes 2-(dimethyl phosphate)-18-crown-6-ether; the amount of 2-(dimethyl phosphate)-18-crown-6-ether in the electrolyte is 0.1%-10% of the total mass of the electrolyte, preferably 0.1%-5%, and further preferably 0.1%-2%.

[0042] Distinguished from the prior art, the 2-(dimethylphosphonate)-18-crown-6-ether introduced phosphate functional groups on the basis of crown ether. In practical applications, the crown ether group in 2-(dimethylphosphonate)-18-crown-6-ether can promote the dissociation of lithium salt, reduce the viscosity of electrolyte, and thus improve the conductivity of electrolyte. The phosphate group in 2-(dimethylphosphonate)-18-crown-6-ether can play a flame-retardant effect, while improving the electrochemical window, increasing the solvent stability, and assisting in the formation of a stable interface film.

[0043] Further, the present application relates to a high-energy-density lithium ion battery electrolyte containing 2-(dimethylphosphonate)-18-crown-6-ether, wherein the additive accounts for 0.1% to 20% of the total mass of the lithium ion battery electrolyte.

[0044] As a preferred, in addition to 2-(dimethylphosphonate)-18-crown-6-ether, the other additive is selected from at least one of fluoroethylene carbonate, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, vinylene carbonate, 1,3-propane sultone, vinyl sulfate, methane disulfonate methylene, tris(trimethylsilyl)phosphate, 1,3-propylene sultone, ethoxy pentafluorocyclophosphazene, tetraethenylsilane, fluorobenzene, tripropargyl phosphate, dicyclohexyl carbodiimide, lithium difluorophosphate, lithium difluoro oxalate borate, lithium difluoro dioxalate phosphate.

[0045] Further, the present application relates to a high-energy-density lithium ion battery electrolyte containing 2-(dimethylphosphonate)-18-crown-6-ether, wherein the concentration of lithium salt in the electrolyte is 0.8M-1.4M.

[0046] As a preferred, the lithium salt is selected from at least one of lithium hexafluorophosphate and lithium bisfluorosulfonylimide.

[0047] Further, the present application relates to a high-energy-density lithium ion battery electrolyte containing 2-(dimethylphosphonate)-18-crown-6-ether, wherein the anhydrous organic solvent in the electrolyte accounts for 60% to 80% of the total mass of the lithium ion battery electrolyte.

[0048] As a preferred, the anhydrous organic solvent includes any one or a mixture of two or more of carbonates and fluorinated solvents.

[0049] As a preferred, the carbonates mainly include propylene carbonate, diethyl carbonate, methyl ethyl carbonate, dimethyl carbonate, and vinyl carbonate.

[0050] As preferred, the fluorinated solvent mainly includes 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, methyl trifluoroethyl carbonate, di(2,2,2-trifluoroethyl) carbonate, and difluoro(2,2-difluoroethyl) carbonate.

[0051] The additive structure of the present application improves the poor thermal stability and poor film forming effect of crown ether due to the introduction of phosphate groups. The reason mechanism is as follows: (1) structural rigidity is enhanced: the introduction of phosphate groups changes the original molecular structure of crown ether. The phosphate group has a certain rigid structure, which limits the conformational change and structural distortion ability of the crown ether macrocycle after being connected with the crown ether macrocycle. This structural restriction reduces the stress on the intramolecular chemical bonds of the crown ether when heated, thereby improving the thermal stability of the whole molecule. (2) electronic effect adjustment: the phosphate group has a unique electron cloud distribution. When connected with the crown ether, it can adjust the electron cloud around the oxygen atoms in the crown ether molecule. Due to the presence of the phosphate group, the energy level change of the oxygen atoms in the original crown ether molecule is changed. During the heating process, the phosphate group shares part of the influence of electron cloud redistribution, which improves the stability of the chemical bonds inside the crown ether molecule and reduces the possibility of chemical bond rupture caused by the change of oxygen atom electron cloud. (3) complex stability is improved: after introducing the phosphate group, the complex structure of crown ether and lithium ion changes. The phosphate group may have a certain interaction with lithium ion, which cooperates with the complexation of crown ether with lithium ion, forming a more stable complex structure of crown ether-lithium ion-phosphate. During heating, the complex bond in this complex structure is relatively not easy to break, reducing the destruction of the crown ether molecular structure caused by the rupture of the complex bond, thereby improving the thermal stability. At the same time, due to the presence of the phosphate group, the absorption and release of energy are more stable during the energy change process, reducing the influence of energy change on the interaction between crown ether and lithium ion. (4) lithium ion transmission is optimized: the introduction of the phosphate group changes the complexation mode of crown ether to lithium ion. On the one hand, the phosphate group may have a moderate interaction with lithium ion, reducing the excessive complexation of crown ether to lithium ion. This increases the number of freely movable lithium ions in the electrolyte, and the concentration distribution of lithium ions on the electrode surface is more uniform, which is beneficial to the participation of lithium ions in the reaction during the formation of SEI or CEI film, improves the uniformity of film growth and formation, and improves the film forming effect. (5) Due to the change of the complex state of crown ether and lithium ion caused by the phosphate group, the embedding of crown ether-lithium ion complex into the SEI or CEI film is reduced. At the same time, the phosphate group itself may participate in the film formation process, which can have better compatibility with other components in the film, making the film composition more uniform. This uniform film structure can more effectively protect the electrode material during the charging and discharging process of the battery, improving the performance and life of the battery.

[0052] Advantages:

[0053] The present application introduces phosphate functional groups on the basis of crown ether to synthesize a 2-(dimethyl phosphate)-18-crown-6-ether. In the structure of the additive, the problem of poor thermal stability and poor film forming effect of crown ether is improved due to the introduction of phosphate groups. In the application of lithium ion battery electrolyte, the crown ether group in 2-(dimethyl phosphate)-18-crown-6-ether can promote the dissociation of lithium salt, reduce the viscosity of electrolyte, and thus improve the conductivity of electrolyte; the phosphate group in 2-(dimethyl phosphate)-18-crown-6-ether can play a flame-retardant effect, improve the electrochemical window, increase the solvent stability, and assist in forming a stable interface film.

[0054] Experiments show that the high-energy density lithium ion battery electrolyte with 2-(dimethyl phosphate)-18-crown-6-ether added in the electrolyte for lithium ion battery can improve the first efficiency, rate performance, high and low temperature discharge performance of-30℃ to 55℃, storage performance, cycle performance and safety performance of the battery during operation, and has a good application prospect in high-energy density systems.

[0055] The present application has been described in detail in the foregoing, but the above-mentioned embodiments are only illustrative in nature and are not intended to limit the present application. In addition, the present application is not limited by any theory described in the foregoing prior art or summary of the invention or in the following examples. DETAILED DESCRIPTION

[0056] The present application will be further described in conjunction with the examples below, and it should be noted that the following examples are provided only for illustrative purposes and do not constitute a limitation on the scope of protection claimed by the present application.

[0057] Unless otherwise specified, the raw materials, reagents, methods, etc. used in the examples are conventional raw materials, reagents, methods in the art.

[0058] The structure of the additive 2-(dimethyl phosphate)-18-crown-6-ether is as follows:

[0059]

[0060] The synthesis method of 2-(dimethyl phosphate)-18-crown-6-ether is as follows:

[0061] (1) 18-crown-6-ether (high purity, dry treatment), N-bromosuccinimide (NBS), azobisisobutyronitrile (AIBN) as initiator, carbon tetrachloride (CCl4) as solvent.

[0062] (2) In a dry reaction flask, 18-crown-6-ether, N-bromosuccinimide and carbon tetrachloride were added. The molar ratio of 18-crown-6-ether to N-bromosuccinimide was 1:1.1. A small amount of azobisisobutyronitrile (0.08 mole fraction relative to 18-crown-6-ether) was also added.

[0063] (3) The reaction mixture was heated to reflux under nitrogen protection. During the reaction, N-bromosuccinimide produced bromine radicals under the initiation of AIBN, and the bromine radicals selectively reacted with one carbon on 18-crown-6-ether to produce brominated product of 18-crown-6-ether and succinimide. The reaction equation is: 18-crown-6-ether + NBS→ 18-crown-6-ether-Br + succinimide. The reflux time was 3-6 hours.

[0064] (4) Potassium dimethyl phosphate (which can be prepared by reacting dimethyl phosphate with potassium hydroxide), phase transfer catalyst (tetrabutylammonium bromide, TBAB), organic solvent (dimethylformamide, DMF).

[0065] (5) In a dry reaction flask, 18-crown-6-ether-Br, potassium dimethyl phosphate, phase transfer catalyst and organic solvent were added. The molar ratio of 18-crown-6-ether-Br to potassium dimethyl phosphate was 1:1.2, and the amount of phase transfer catalyst was 0.2 times the amount of substance of 18-crown-6-ether-Br.

[0066] (6) The reaction mixture was heated to 60-80°C under stirring for reaction. The phosphate anion in potassium dimethyl phosphate reacted with the bromine atom in 18-crown-6-ether-Br under the action of the phase transfer catalyst to produce 2-(dimethyl phosphate)-18-crown-6-ether and potassium bromide. The reaction equation is: 18-crown-6-ether-Br + (CH3O)2PO2K→ 2-(dimethyl phosphate)-18-crown-6-ether + KBr. The reaction time was 22 hours.

[0067] (7) After the reaction was completed, the reaction mixture was cooled to room temperature.

[0068] (8) Then pour into a large amount of water, and extract the product with an organic solvent (dichloromethane). Extract 3-5 times, and the amount of organic solvent used each time is about 1 / 2 of the volume of the aqueous phase.

[0069] (9) The organic phase was combined and washed with saturated brine 3 times to remove residual water-soluble impurities.

[0070] (10) The organic phase was dried with anhydrous sodium sulfate overnight, and the drying agent was filtered out.

[0071] (11) The organic solvent was removed by a rotary evaporator to obtain the crude product.

[0072] (12) The crude product was purified by column chromatography. Silica gel was selected as the stationary phase, and ethyl acetate-petroleum ether mixed solvent was selected as the eluent, the ratio of the two was adjusted to 1:5, and the target product was collected by TLC monitoring, and the pure 2-(dimethyl phosphate)-18-crown-6-ether was obtained.

[0073] Example

[0074] The positive active material of the lithium ion battery used in the embodiment of the application is selected as ternary material (LiNi0.8Co0.1Mn0.1O4), the negative active material is selected as artificial graphite, and the amount of electrolyte used is 2.3±0.5 g / Ah. The lithium ion battery assembled with high-energy-density lithium ion battery electrolyte containing 2-(dimethyl phosphate)-18-crown-6-ether is selected as the example, and the lithium ion battery assembled with conventional electrolyte not containing 2-(dimethyl phosphate)-18-crown-6-ether is selected as the comparative example.

[0075] The specific steps of preparing the electrolyte in the glove box (H2O < 5 ppm) are as follows:

[0076] (1) Under the protection of nitrogen or inert gas, anhydrous organic solvent molecular sieve is dehydrated to obtain anhydrous organic solvent;

[0077] (2) A suitable amount of lithium salt is added to the anhydrous organic solvent, stirred and cooled to obtain a mixed solution;

[0078] (3) A suitable amount of additive is added to the mixed solution to obtain 2000 g of electrolyte, which is packed in a fluorinated bottle.

[0079] The electrolyte is used to assemble a lithium ion battery. The positive active material of the lithium ion battery is ternary material (LiNi0.8Co0.1Mn0.1O4), the negative active material is artificial graphite, the design capacity is 60 Ah, and the amount of electrolyte injected is 2.3±0.5 g / Ah.

[0080] The electrolyte formula is shown in the comparative example or the example.

[0081] Comparative Example 1:

[0082] The anhydrous organic solvent used is: diethyl carbonate: ethyl methyl carbonate: ethylene carbonate = 30:40:30; lithium salt: lithium hexafluorophosphate (LiPF6) with a concentration of 1.2 M.

[0083] Comparative Example 2:

[0084] The anhydrous organic solvent used is: diethyl carbonate: ethyl methyl carbonate: ethylene carbonate = 30:40:30; lithium salt: lithium hexafluorophosphate (LiPF6) with a concentration of 0.8M, lithium bisfluorosulfonylimide (LiFSI) with a concentration of 0.4M.

[0085] Comparative Example 3:

[0086] The anhydrous organic solvent used is: diethyl carbonate: ethyl methyl carbonate: ethylene carbonate = 30:40:30; lithium salt: lithium hexafluorophosphate (LiPF6) with a concentration of 0.8M, lithium bisfluorosulfonylimide (LiFSI) with a concentration of 0.4M. Additives: 0.5% lithium difluoro(oxalato)borate, 0.5% lithium difluorophosphate, 0.3% lithium difluorodioxalate phosphate, 0.3% vinylene carbonate, 0.5% 1,3-propane sultone, 0.5% fluoroethylene carbonate, 1% vinyl sulfate, 0.5% tris(trimethylsilyl) phosphate, 0.5% methanedisulfonate methylene, (the above percentages are the percentage of the corresponding additive accounting for the total mass of lithium ion battery electrolyte, the same below).

[0087] Example 1:

[0088] The anhydrous organic solvent used is: diethyl carbonate: ethyl methyl carbonate: ethylene carbonate = 30:40:30; lithium salt: lithium hexafluorophosphate (LiPF6) with a concentration of 0.8M, lithium bisfluorosulfonylimide (LiFSI) with a concentration of 0.4M. Additives: 0.5% lithium difluoro(oxalato)borate, 0.5% lithium difluorophosphate, 0.3% lithium difluorodioxalate phosphate, 0.3% vinylene carbonate, 0.5% 1,3-propane sultone, 0.5% fluoroethylene carbonate, 1% vinyl sulfate, 0.5% tris(trimethylsilyl) phosphate, 0.5% methanedisulfonate methylene, 8% 2-(dimethylphosphato)-18-crown-6-ether.

[0089] Example 2:

[0090] The anhydrous organic solvent used is: diethyl carbonate: ethyl methyl carbonate: ethylene carbonate = 30:40:30; lithium salt: lithium hexafluorophosphate (LiPF6) with a concentration of 0.8M, lithium bisfluorosulfonylimide (LiFSI) with a concentration of 0.4M. Additives: 0.5% lithium difluoro(oxalato)borate, 0.5% lithium difluorophosphate, 0.3% lithium difluorodioxalate phosphate, 0.3% vinylene carbonate, 0.5% 1,3-propane sultone, 0.5% fluoroethylene carbonate, 1% vinyl sulfate, 0.5% tris(trimethylsilyl) phosphate, 0.5% methanedisulfonate methylene, 5% 2-(dimethylphosphato)-18-crown-6-ether.

[0091] Example 3:

[0092] The anhydrous organic solvent used is: diethyl carbonate: ethyl methyl carbonate: ethylene carbonate = 30:40:30; lithium salt: lithium hexafluorophosphate (LiPF6) with a concentration of 0.8M, lithium bisfluorosulfonylimide (LiFSI) with a concentration of 0.4M. Additives: 0.5% lithium difluoro(oxalato)borate, 0.5% lithium difluorophosphate, 0.3% lithium difluorodioxalate phosphate, 0.3% vinylene carbonate, 0.5% 1,3-propane sultone, 0.5% fluoroethylene carbonate, 1% ethylene sulfate, 0.5% tris(trimethylsilyl)phosphate, 0.5% methanedisulfonate methylene, 1% 2-(dimethylphosphato)-18-crown-6-ether.

[0093] The lithium ion batteries assembled in the above examples and comparative examples were tested for constant volume capacity at room temperature (25°C) in the voltage range of 2.75-4.5V at 1 / 3C discharge, 2C charge capacity (%) = 2C charge capacity / 1C discharge capacity, 3C discharge capacity (%) = 3C discharge capacity / 1C discharge capacity, and the results are shown in Table 1:

[0094] Table 1 Short-term performance test

[0095] Liquid retention amount / g Volume to capacity / Ah Initial efficiency 2C charge 3C discharge Comparative Example 1 140.7 60.29 78.6％ 35.1％ 30.2％ Comparative Example 2 140.3 60.99 77.6％ 67.1％ 69.6％ Comparative Example 3 141.0 60.97 92.2％ 89.2％ 84.5％ Example 1 140.7 60.14 92.4％ 92.9％ 95.0％ Example 2 140.5 60.56 92.4％ 93.9％ 95.3％ Example 3 140.9 60.97 92.2％ 94.3％ 96.6％

[0096] The lithium ion batteries assembled in the above examples and comparative examples were tested for capacity at 1C or 1 / 3C discharge in the temperature range of -30°C to 55°C, and the voltage range was 2.75-4.5V. The battery gas production was observed, and the electrode sheet was observed after low-temperature discharge. The results are shown in Table 2:

[0097] Table 2 High and low temperature discharge performance test

[0098]

[0099]

[0100] The lithium ion batteries assembled in the above examples and comparative examples were tested for capacity retention rate and recovery rate at 100% SOC storage for 7 days at 55°C, and the voltage range was 2.75-4.5V. The battery gas production and lithium precipitation were observed, and the results are shown in Table 3:

[0101] Table 3 High temperature charge performance test

[0102] Capacity retention rate Capacity recovery rate Lithium precipitation Gas generation Comparative Example 1 26.4％ 24.2％ Lithium precipitation Gas generation Comparative Example 2 68.3％ 63.7％ Lithium precipitation Gas generation Comparative Example 3 86.7％ 81.0％ Lithium precipitation Gas generation Example 1 93.8％ 97.2％ No lithium precipitation No gas generation Example 2 95.0％ 98.9％ No lithium precipitation No gas generation Example 3 95.0％ 99.6％ No lithium precipitation No gas generation

[0103] The lithium ion batteries assembled in the above examples and comparative examples were tested for capacity retention rate and recovery rate at 100% SOC storage for 28 days at 25°C, and the voltage range was 2.75-4.5V. The battery gas production and lithium precipitation were observed, and the results are shown in Table 4:

[0104] Table 4 Normal temperature charge performance test

[0105] Capacity retention rate Capacity recovery rate Lithium precipitation Gas generation Comparative Example 1 22.9％ 24.4％ Lithium precipitation Gas generation Comparative Example 2 69.4％ 67.9％ Lithium precipitation Gas generation Comparative Example 3 87.7％ 88.9％ Lithium precipitation Gas generation Example 1 95.3％ 98.0％ No lithium precipitation No gas generation Example 2 96.9％ 99.9％ No lithium precipitation No gas generation Example 3 97.1％ 100.8％ No lithium precipitation No gas generation

[0106] The lithium ion batteries assembled in the above comparative examples and examples were tested for capacity retention rate at 45℃ environment, 1C charge 1C discharge cycle, voltage range 2.75-4.5V. The lithium precipitation of the batteries was observed, and the results are shown in Table 5:

[0107] Table 5 High temperature cycle performance test

[0108]

[0109]

[0110] The lithium ion batteries assembled in the above comparative examples and examples were tested for capacity retention rate at 25℃ environment, 1C charge 1C discharge cycle, voltage range 2.75-4.5V. The lithium precipitation of the batteries was observed, and the results are shown in Table 6:

[0111] Table 6 Normal temperature cycle performance test

[0112] 100 cycles 200 cycles 300 cycles 400 cycles 500 cycles Lithium precipitation Gas generation Comparative Example 1 28.2％ 15.8％ 12.5％ 0.0％ 0.0％ Lithium precipitation Gas generation Comparative Example 2 66.1％ 55.0％ 52.6％ 31.5％ 14.5％ Lithium precipitation Gas generation Comparative Example 3 81.9％ 80.6％ 67.6％ 57.6％ 34.0％ Lithium precipitation Gas generation Example 1 98.2％ 96.1％ 94.4％ 92.2％ 90.7％ No lithium precipitation No gas generation Example 2 99.2％ 97.4％ 96.0％ 93.1％ 92.0％ No lithium precipitation No gas generation Example 3 100.7％ 98.0％ 96.3％ 94.9％ 92.3％ No lithium precipitation No gas generation

[0113] The lithium ion batteries assembled in the above comparative examples and examples were tested for needle puncture test at 25℃ environment, 100% SOC, and the results are shown in Table 7:

[0114] Table 7 Needle puncture test

[0115] Needle punching Comparative Example 1 Fire Comparative Example 2 Fire Comparative Example 3 Fire Example 1 No fire Example 2 No fire Example 3 No fire

[0116] From the results, it can be seen that the high-energy-density lithium ion battery electrolyte with 2-(dimethylphosphonate)-18-crown-6-ether added to the electrolyte can improve the first efficiency, rate performance, -30℃-55℃ high and low temperature discharge performance, storage performance, cycle performance and safety performance of the lithium ion battery when working at 4.5V, and has a good application prospect in high-energy-density systems.

[0117] The above examples are only used to illustrate the technical solutions of the present application, and are not intended to limit the present application. Although the present application has been described in detail with reference to the foregoing examples, it should be understood by those skilled in the art that the technical solutions described in the foregoing examples can be modified or some or all of the technical features can be replaced by equivalents without departing from the spirit and essence of the present application defined in the claims of the present application; and these modifications or replacements are still within the scope defined by the claims of the present application.

Claims

1. A lithium-ion battery electrolyte additive, characterized in that, The additive includes 2-(dimethylphospho)-18-crown-6-ether as shown in Formula I: （Ⅰ）。 2. The method for synthesizing a lithium-ion battery electrolyte additive as described in claim 1, characterized in that, The reaction includes the following steps: In a dry reaction flask, 18-crown-6-ether, N-bromosuccinimide, and carbon tetrachloride are added; then an initiator is added; under nitrogen protection, the reaction mixture is heated to reflux; during the reaction, N-bromosuccinimide generates bromine radicals under the action of the initiator, and the bromine radicals selectively react with one carbon atom on the 18-crown-6-ether to generate 18-crown-6-ether-Br and succinimide; in a dry reaction flask, 18-crown-6-ether-Br, potassium dimethyl phosphate, and a phase transfer catalyst are added. The reaction mixture was heated to 60-80°C with stirring. The phosphate anion in potassium dimethyl phosphate reacted with the bromine atom in 18-crown-6-ether-Br under the action of a phase transfer catalyst, generating 2-(dimethyl phosphate)-18-crown-6-ether and potassium bromide. After the reaction was complete, the reaction mixture was cooled to room temperature. The organic phases were combined by extraction, dried, and purified to obtain pure 2-(dimethyl phosphate)-18-crown-6-ether. The initiator was azobisisobutyronitrile.

3. The method for synthesizing a lithium-ion battery electrolyte additive as described in claim 2, further characterized in that the phase transfer catalyst is tetrabutylammonium bromide; the molar ratio of 18-crown-6-ether-Br to potassium dimethyl phosphate is 1:1.1-1.2, and the amount of phase transfer catalyst used is 0.1-0.2 times the amount of 18-crown-6-ether-Br.

4. The method for synthesizing a lithium-ion battery electrolyte additive as described in claim 2 or 3, further characterized in that the purification is carried out by column chromatography: silica gel is selected as the stationary phase, ethyl acetate-petroleum ether mixed solvent is selected as the eluent, the volume ratio of the two is adjusted to 1:5-1:3, and the fraction containing the target product is collected by TLC monitoring to obtain pure 2-(dimethyl phosphate)-18-crown-6-ether.

5. A lithium-ion battery electrolyte containing 2-(dimethylphosphate)-18-crown-6-ether, characterized in that, The electrolyte comprises lithium salt, anhydrous organic solvent, and electrolyte additives; wherein the electrolyte additives include 2-(dimethylphosphate)-18-crown-6-ether; the amount of 2-(dimethylphosphate)-18-crown-6-ether in the electrolyte is 0.1%-10% of the total mass of the electrolyte.

6. The lithium-ion battery electrolyte containing 2-(dimethylphosphate)-18-crown-6-ether as described in claim 5, further characterized in that the amount of 2-(dimethylphosphate)-18-crown-6-ether in the electrolyte is 0.1%-5% of the total mass of the electrolyte.

7. The lithium-ion battery electrolyte containing 2-(dimethylphosphate)-18-crown-6-ether as described in claim 6, further characterized in that the amount of 2-(dimethylphosphate)-18-crown-6-ether in the electrolyte is 0.1%-2% of the total mass of the electrolyte.

8. A lithium-ion battery electrolyte containing 2-(dimethylphosphate)-18-crown-6-ether as described in claim 5, 6, or 7, characterized in that, The amount of electrolyte additive used is 0.1% to 20% of the total mass of the lithium-ion battery electrolyte.

9. A lithium-ion battery electrolyte containing 2-(dimethylphosphate)-18-crown-6-ether as described in claim 5, 6, or 7, characterized in that, Except for 2-(dimethylphosphoyl)-18-crown-6-ether, the other additives are selected from at least one of fluoroethylene carbonate, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, vinylene carbonate, 1,3-propanesulfonate lactone, vinyl sulfate, methanedisulfonate methylene, tri(trimethylsilyl) phosphate, 1,3-propenesulfonate lactone, ethoxypentafluorocyclotriphosphazene, tetravinylsilane, fluorobenzene, triargyl phosphate, dicyclohexylcarbodiimide, lithium difluorophosphate, lithium difluorooxalate borate, and lithium difluorodioxalate phosphate.

10. A lithium-ion battery electrolyte containing 2-(dimethylphosphate)-18-crown-6-ether as described in claim 5, 6, or 7, characterized in that, The concentration of lithium salt in the electrolyte is 0.8M-1.4M; the lithium salt is selected from at least one of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide.

11. A lithium-ion battery electrolyte containing 2-(dimethylphosphate)-18-crown-6-ether as described in claim 5, 6, or 7, characterized in that, The anhydrous organic solvent includes at least one of carbonates and fluorinated solvents.

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