A low-temperature electrolyte additive and preparation method thereof

Abstract

The invention discloses a low-temperature electrolyte additive and a preparation method thereof, belonging to the technical field of lithium batteries. The low-temperature electrolyte additive comprises the following raw materials in parts by weight: 10 parts of potassium bromide, 1-3 parts of an organosilicon phosphorylcholine auxiliary agent, 5-10 parts of a phenylpropyl crown ether-based unsaturated hydrocarbon compound, and 0.5-1.5 parts of fluoroethylene carbonate; the organosilicon phosphorylcholine auxiliary agent is obtained by a thiol-ene click reaction of a vinyl-terminated silicone oil and 11-mercaptoacyl phosphorylcholine; the preparation method of the low-temperature electrolyte additive comprises the following steps: uniformly mixing the components according to a formula ratio; and the invention provides a method for significantly improving the capacity, cycle life and flame retardancy of a lithium battery at low temperatures by mixing and adding multiple component raw materials.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium batteries, and in particular relates to a low-temperature electrolyte additive and a preparation method thereof. Background Art

[0002] The operating temperature affects the performance and life of lithium-ion batteries, and is one of the key factors limiting the use of lithium-ion batteries in high latitudes and high altitudes. Conventional lithium-ion batteries perform well at 0-40°C, but their discharge capacity decays severely below 0°C, and their cycle performance decreases significantly.

[0003] Studies have shown that the capacity of a 18650 lithium-ion battery with a nominal voltage of 2.4V at -20°C and discharged to 1.2V at 0.5C is only 69.7% of that at 25°C. The capacity of a 18650 lithium-ion battery with a nominal voltage of 3.6V at 0°C and 0.5C at 2.5-4.2V for 90 cycles has a capacity decay of about 14%. The capacity decay exceeds 25% after 10 cycles at -20°C. The main factors causing this phenomenon are: (1) As the temperature decreases, the viscosity of the electrolyte gradually increases. , the migration rate of lithium ions in the electrolyte becomes lower and lower; (2) During the reciprocating process of lithium ions migrating from the negative electrode to the electrolyte, they need to pass through the SEI membrane. The composition and thickness of the SEI membrane greatly affect the migration rate of lithium ions; (3) During low-temperature charging, especially low-temperature high-rate charging, active lithium is likely to precipitate on the surface of the negative electrode. The active lithium reacts irreversibly with the electrolyte, further increasing the thickness of the SEI membrane, making lithium ion migration more difficult, and the low-temperature performance of the lithium battery worse, even leading to combustion and explosion.

[0004] In order to improve the poor low-temperature performance of lithium-ion batteries, additives such as vinylene carbonate (VC), methylene carbonate or vinyl carbonate are currently added to the electrolyte of lithium-ion batteries. However, these additives have obvious disadvantages. The SEI film they form has a large impedance, which leads to an increase in the DC internal resistance of the battery and cannot significantly improve the low-temperature performance of lithium-ion batteries. Summary of the invention

[0005] The object of the present invention is to provide a low-temperature electrolyte additive and a preparation method thereof, so as to solve the problem that the existing low-temperature electrolyte additives do not significantly improve the low-temperature performance of lithium-ion batteries.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] A low-temperature electrolyte additive comprises the following raw materials in parts by weight:

[0008] Potassium bromide 10 parts;

[0009] 1-3 parts of organic silicon phosphorylcholine auxiliary agent;

[0010] 5-10 parts of benzocrown ether unsaturated hydrocarbon compound;

[0011] Fluoroethylene carbonate 0.5-1.5 parts.

[0012] Furthermore, the preparation method of the benzocrown ether-based unsaturated hydrocarbon compound comprises the following steps:

[0013] 4-Formyl-15-crown-5 and terephthalonitrile are added to anhydrous ethanol, and then piperidine is added. The mixture is heated under reflux for reaction for 12-14 hours. After the reaction is completed, the mixture is cooled to room temperature, filtered, and purified by silica gel column chromatography using dichloromethane and methanol in a volume ratio of 98:2 to obtain a solid product, which is a benzocrown ether-based unsaturated hydrocarbon compound. The amount ratio of anhydrous ethanol, 4-formyl-15-crown-5, terephthalonitrile, and piperidine is 10 mL: 0.4 g: 0.1 g: 0.15 mL. 4-Formyl-15-crown-5 and terephthalonitrile are used as raw materials, and the benzocrown ether-based unsaturated hydrocarbon compound is obtained through a Knoevenagel condensation reaction.

[0014] Furthermore, the preparation method of the organosilicon phosphorylcholine adjuvant comprises the following steps:

[0015] Add terminal vinyl silicone oil and tetrahydrofuran into a flask, stir and dissolve for 12 hours, then add tetrahydrofuran solution of 11-mercaptoacylphosphorylcholine, stir for 4-6 hours, add benzoin dimethyl ether, stir for 5-10 minutes, and then place under ultraviolet light to react for 30 minutes. The reaction product is precipitated with methanol 3-5 times, the solvent is evaporated at room temperature, and then placed in a vacuum oven at 65°C for 12 hours to obtain an organosilicon phosphorylcholine auxiliary agent.

[0016] Furthermore, the feeding amount of 11-mercaptoacylphosphorylcholine is 1-1.2 times the molar amount of vinyl groups in the vinyl-terminated silicone oil.

[0017] Furthermore, the feeding amount of benzoin dimethyl ether is 0.3 times the molar amount of vinyl groups in the vinyl-terminated silicone oil.

[0018] Furthermore, the viscosity of the vinyl-terminated silicone oil is 100-700 mPa·s.

[0019] The present invention also provides a method for preparing the low-temperature electrolyte additive, comprising the following steps:

[0020] Potassium bromide, organosilicon phosphorylcholine auxiliary agent, benzocrown ether-based unsaturated hydrocarbon compound, and fluoroethylene carbonate are weighed according to the formula amount, stirred and mixed evenly to obtain the low-temperature electrolyte additive.

[0021] Beneficial effects of the present invention:

[0022] The present invention provides a low-temperature electrolyte additive, including potassium bromide, organosilicon phosphorylcholine auxiliary agent, benzocrown ether unsaturated hydrocarbon compound, fluoroethylene carbonate; by mixing and adding multiple component raw materials, the capacity, cycle life and flame retardancy of lithium batteries at low temperatures are significantly improved; the specific analysis is as follows:

[0023] Potassium bromide reduces the freezing point of the electrolyte by acting on the organic solvent in the electrolyte, thereby avoiding the problem of poor wettability of the electrolyte under low temperature conditions, so that the electrolyte still has good conductivity under low temperature conditions;

[0024] The molecular structure of benzocrown ether-based unsaturated hydrocarbon compounds contains a crown ether structure, a cyano group, and an unsaturated double bond. The cavity in the crown ether structure can + Included, it is beneficial to increase Li + The dissociation degree and conductivity of the electrolyte can be improved, and the precipitation of lithium salts at low temperatures can be reduced. The cyano group can form hydrogen bonds with the organic solvent in the electrolyte, thereby replacing the hydrogen bonding between the organic solvent molecules and improving the low-temperature fluidity of the organic solvent. The presence of unsaturated double bonds allows the benzocrown ether-based unsaturated hydrocarbon compound to participate in the formation of the SEI film together with fluoroethylene carbonate, improving the composition of the SEI film, reducing its internal resistance, and improving the migration and deintercalation ability of lithium ions under low temperature conditions. It also plays a coordinated role with the organosilicon phosphorylcholine auxiliary agent to improve the stability of the electrolyte under low temperature conditions.

[0025] The organosilicon phosphorylcholine auxiliary agent is the product of the thiol-ene click reaction of vinyl-terminated polydimethylsiloxane and 11-mercaptoacylphosphorylcholine, and has higher thermal stability and chemical stability. Introducing it into the electrolyte can improve the problem of high viscosity of the electrolyte components at low temperatures. The organosilicon phosphorylcholine auxiliary agent contains anions and cations on the molecular chain, which is beneficial to improving the mobility and conductivity of lithium ions under low temperature conditions. In addition, it also contains flame retardant elements such as nitrogen, phosphorus, silicon, and sulfur, which play a synergistic role with the benzocrown ether-based unsaturated hydrocarbon compound to effectively improve the flame retardant properties of the electrolyte. DETAILED DESCRIPTION

[0026] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0027] Example 1

[0028] This embodiment provides an organosilicon phosphorylcholine adjuvant, and the preparation steps are as follows:

[0029] 15 g of vinyl-terminated silicone oil and 250 mL of tetrahydrofuran were added to a flask, stirred and dissolved for 12 h, then a tetrahydrofuran solution of 11-mercaptoacylphosphorylcholine was added, the amount ratio of 11-mercaptoacylphosphorylcholine to tetrahydrofuran in the tetrahydrofuran solution of 11-mercaptoacylphosphorylcholine was 1 g:5 mL, stirred for 4 h, benzoin dimethyl ether was added, stirred for 5 min, and then placed under ultraviolet light for reaction for 30 min, the ultraviolet lamp was 1000 W, and the wavelength was 365 nm. The reaction product was precipitated with methanol 3 times, the solvent was evaporated at room temperature, and then placed in a vacuum oven at 65 ° C for 12 h to obtain an organosilicon phosphorylcholine auxiliary agent, the amount of 11-mercaptoacylphosphorylcholine was controlled to be 1 times the molar amount of vinyl in the vinyl-terminated silicone oil, the amount of benzoin dimethyl ether was controlled to be 0.3 times the molar amount of vinyl in the vinyl-terminated silicone oil, and the viscosity of the vinyl-terminated silicone oil was 100 mPa·s.

[0030] Example 2

[0031] This embodiment provides an organosilicon phosphorylcholine adjuvant, and the preparation steps are as follows:

[0032] 15 g of vinyl-terminated silicone oil and 250 mL of tetrahydrofuran were added to a flask, stirred and dissolved for 12 h, then a tetrahydrofuran solution of 11-mercaptoacylphosphorylcholine was added, the amount ratio of 11-mercaptoacylphosphorylcholine to tetrahydrofuran in the tetrahydrofuran solution of 11-mercaptoacylphosphorylcholine was 1 g:5 mL, stirred for 6 h, benzoin dimethyl ether was added, stirred for 10 min, and then placed under ultraviolet light for reaction for 30 min, the ultraviolet lamp was 1000 W, and the wavelength was 365 nm. The reaction product was precipitated with methanol 5 times, the solvent was evaporated at room temperature, and then placed in a vacuum oven at 65 ° C for 12 h to obtain an organosilicon phosphorylcholine auxiliary agent, the amount of 11-mercaptoacylphosphorylcholine was controlled to be 1.2 times the molar amount of vinyl in the vinyl-terminated silicone oil, the amount of benzoin dimethyl ether was controlled to be 0.3 times the molar amount of vinyl in the vinyl-terminated silicone oil, and the viscosity of the vinyl-terminated silicone oil was 700 mPa·s.

[0033] Comparative Example 1

[0034] The present embodiment is polydimethylsiloxane, and the viscosity is 100 mPa·s.

[0035] Example 3

[0036] A low-temperature electrolyte additive, comprising the following raw materials in parts by weight:

[0037] Potassium bromide 10g, organic silicon phosphorylcholine auxiliary agent of Example 1 1g, benzocrown ether unsaturated hydrocarbon compound 5g, fluoroethylene carbonate 0.5g.

[0038] The preparation method of a benzocrown ether-based unsaturated hydrocarbon compound comprises the following steps:

[0039] To 100 mL of anhydrous ethanol, add 4 g of 4-formyl-15-crown-5 and 1 g of terephthalonitrile, and then add 1.5 mL of piperidine, and heat to reflux for 12 hours. After the reaction is completed, cool to room temperature, filter and purify by silica gel column chromatography using dichloromethane and methanol in a volume ratio of 98:2 to obtain a benzocrown ether-based unsaturated hydrocarbon compound.

[0040] The preparation method of the low-temperature electrolyte additive comprises the following steps:

[0041] Potassium bromide, organosilicon phosphorylcholine auxiliary agent, benzocrown ether-based unsaturated hydrocarbon compound, and fluoroethylene carbonate are weighed according to the formula amount, stirred and mixed evenly to obtain the low-temperature electrolyte additive.

[0042] Example 4

[0043] A low-temperature electrolyte additive, comprising the following raw materials in parts by weight:

[0044] Potassium bromide 10g, organic silicon phosphorylcholine auxiliary agent of Example 2 2g, benzocrown ether unsaturated hydrocarbon compound 8g, fluoroethylene carbonate 1g.

[0045] The preparation method of a benzocrown ether-based unsaturated hydrocarbon compound comprises the following steps:

[0046] To 100 mL of anhydrous ethanol, add 4 g of 4-formyl-15-crown-5 and 1 g of terephthalonitrile, and then add 1.5 mL of piperidine, and heat to reflux for 13 hours. After the reaction is completed, cool to room temperature, filter and purify by silica gel column chromatography using dichloromethane and methanol in a volume ratio of 98:2 to obtain a benzocrown ether-based unsaturated hydrocarbon compound.

[0047] The preparation method of the low-temperature electrolyte additive comprises the following steps:

[0048] Potassium bromide, organosilicon phosphorylcholine auxiliary agent, benzocrown ether-based unsaturated hydrocarbon compound, and fluoroethylene carbonate are weighed according to the formula amount, stirred and mixed evenly to obtain the low-temperature electrolyte additive.

[0049] Example 5

[0050] A low-temperature electrolyte additive, comprising the following raw materials in parts by weight:

[0051] Potassium bromide 10g, organic silicon phosphorylcholine auxiliary agent 3g of Example 2, benzocrown ether unsaturated hydrocarbon compound 10g, fluoroethylene carbonate 1.5g.

[0052] The preparation method of a benzocrown ether-based unsaturated hydrocarbon compound comprises the following steps:

[0053] To 100 mL of anhydrous ethanol, add 4 g of 4-formyl-15-crown-5 and 1 g of terephthalonitrile, and then add 1.5 mL of piperidine, and heat to reflux for 14 hours. After the reaction is completed, cool to room temperature, filter and purify by silica gel column chromatography using dichloromethane and methanol in a volume ratio of 98:2 to obtain a benzocrown ether-based unsaturated hydrocarbon compound.

[0054] The preparation method of the low-temperature electrolyte additive comprises the following steps:

[0055] Potassium bromide, organosilicon phosphorylcholine auxiliary agent, benzocrown ether-based unsaturated hydrocarbon compound, and fluoroethylene carbonate are weighed according to the formula amount, stirred and mixed evenly to obtain the low-temperature electrolyte additive.

[0056] Comparative Example 2

[0057] A low-temperature electrolyte additive is disclosed. Compared with Example 3, the organosilicon phosphorylcholine auxiliary agent in Example 3 is replaced by the substance in Comparative Example 1, and the remaining raw materials and preparation process are the same as those in Example 3.

[0058] Comparative Example 3

[0059] A low-temperature electrolyte additive. Compared with Example 3, the benzocrown ether-based unsaturated hydrocarbon compound in Example 3 is replaced by a "mixture of 4-formyl-15-crown-5 and terephthalonitrile in a mass ratio of 4:1", and the remaining raw materials and preparation process are the same as Example 3.

[0060] Comparative Example 4

[0061] A low-temperature electrolyte additive, comprising the following raw materials in parts by weight:

[0062] Potassium bromide 10g, organic silicon phosphorylcholine auxiliary agent of Example 1 6g, benzocrown ether unsaturated hydrocarbon compound 0g, fluoroethylene carbonate 0.5g.

[0063] The preparation process of the low-temperature electrolyte additive is the same as that of Example 3.

[0064] Comparative Example 5

[0065] A low-temperature electrolyte additive, comprising the following raw materials in parts by weight:

[0066] Potassium bromide 10g, organic silicon phosphorylcholine auxiliary agent of Example 1 0g, benzocrown ether unsaturated hydrocarbon compound 6g, fluoroethylene carbonate 0.5g.

[0067] The preparation process of the benzocrown ether-based unsaturated hydrocarbon compound and the low-temperature electrolyte additive is the same as that of Example 3.

[0068] The low-temperature electrolyte additives prepared in Examples 3-5 and Comparative Examples 2-3 were combined with existing materials to prepare electrolytes, and the specific operations were as follows:

[0069] In an argon atmosphere glove box with water / oxygen index <0.1ppm, the lithium salt LiPF 6 Dissolved in a mixed solvent consisting of ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) in a mass ratio of 1:1:1, then add vinylene carbonate and low-temperature electrolyte additives, stir thoroughly to completely dissolve, and obtain an electrolyte, in which the lithium salt LiPF 6 The mass fractions of mixed solvent, vinylene carbonate and low-temperature electrolyte additive are 13.5%, 75%, 1.5% and 10% respectively.

[0070] The electrolytes prepared by the low-temperature electrolyte additives of Examples 3 to 5 and Comparative Examples 2 to 5 were respectively subjected to flame retardancy tests by a self-extinguishing method, and the test method is as follows:

[0071] Immerse a glass wool ball with a mass of m1 and a diameter of 0.3 mm in the electrolyte to be tested. After it is fully soaked, weigh its mass m2. Then place the glass wool ball in a wire ring and ignite it with an ignition device. Record the time T from ignition to flame extinguishment. The self-extinguishing time t per unit mass of electrolyte is used as a standard to measure the flame retardant performance of the electrolyte. The calculation formula is: t = T / (m2-m1). The average value of three measurements is taken for each sample measurement. The test results are shown in Table 1:

[0072]

[0073] It can be seen from Table 1 that the electrolyte embodiments 3-5 to which the low-temperature additive of the present invention is added are safer than comparative examples 2, 4, and 5, and the fire phenomenon during use is avoided. It can be seen from the experimental results of Example 3 and Comparative Example 2 that the flame retardant performance of the electrolyte obtained by using polydimethylsiloxane instead of the organosilicon phosphorylcholine auxiliary agent is reduced. It can be seen from the experimental results of Example 3 and Comparative Example 3 that the flame retardant performance of the electrolyte obtained by replacing the benzocrown ether-based unsaturated hydrocarbon compound with a mixture of 4-formyl-15-crown-5 and terephthalonitrile in a mass ratio of 4:1 does not change significantly. It can be seen from the experimental results of Example 3 and Comparative Examples 4 and 5 that in this case, there is a synergistic effect between the organosilicon phosphorylcholine auxiliary agent and the benzocrown ether-based unsaturated hydrocarbon compound, which effectively improves the flame retardant performance of the electrolyte.

[0074] A lithium iron phosphate pole piece was used as the positive electrode, an artificial graphite pole piece was used as the negative electrode, a 16 um PP / PE film was used as the separator, and the electrolyte prepared by the low-temperature electrolyte additives of Examples 3 to 5 and Comparative Examples 2 to 5 was respectively infused to prepare a lithium ion battery, and a discharge cycle capacity retention rate test at low temperature was performed. The specific operation was as follows:

[0075] The capacity retention rate (%) of the discharge capacity of the above lithium-ion battery at low temperature (-20°C) relative to the discharge capacity at normal temperature (25°C) was tested at a charge and discharge voltage of 2.5V to 3.65V and a charge and discharge rate of 1C / 1C. That is, the lithium-ion battery is first charged and discharged at normal temperature (25°C), and the measured discharge capacity is set to 1, and then charged and discharged at low temperature (-20°C) under the same conditions. The ratio of the measured discharge capacity to the discharge capacity at normal temperature is the capacity retention rate (%) at this temperature. The results are shown in Table 2 below:

[0076]

[0077] It can be seen from Table 2 that the -20°C capacity retention rate of the lithium ion battery of the electrolyte embodiments 3-5 to which the low-temperature additive of the present invention is added is better than that of the comparative examples 2-5, and has better performance under low temperature conditions. It can be seen from the experimental results of Example 3 and Comparative Example 2 that the -20°C capacity retention rate of the prepared lithium ion battery is reduced by using polydimethylsiloxane instead of the organosilicon phosphorylcholine auxiliary agent. It can be seen from the experimental results of Example 3 and Comparative Example 3 that the benzocrown ether-based unsaturated hydrocarbon compound is replaced with a mixture of 4-formyl-15-crown-5 and terephthalonitrile in a mass ratio of 4:1, and the -20°C capacity retention rate of the prepared lithium ion battery is also reduced. It can be seen from the experimental results of Example 3 and Comparative Examples 4 and 5 that there is a synergistic effect between the organosilicon phosphorylcholine auxiliary agent and the benzocrown ether-based unsaturated hydrocarbon compound in this case, which effectively improves the -20°C capacity retention rate of the lithium ion battery.

[0078] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0079] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A low-temperature electrolyte additive, characterized in that: It includes the following raw materials in parts by weight: Potassium bromide 10 parts; 1-3 parts of organic silicon phosphorylcholine auxiliary agent; 5-10 parts of benzocrown ether unsaturated hydrocarbon compound; Fluoroethylene carbonate 0.5-1.5 parts; The organosilicon phosphorylcholine auxiliary agent is obtained by a thiol-ene click reaction of vinyl-terminated silicone oil and 11-mercaptoacyl phosphorylcholine; The preparation method of a benzocrown ether-based unsaturated hydrocarbon compound comprises the following steps: Add 4-formyl-15-crown-5 and terephthalonitrile to anhydrous ethanol, then add piperidine, heat and reflux for 12-14 hours. After the reaction is completed, cool to room temperature, filter and purify by silica gel column chromatography using dichloromethane and methanol in a volume ratio of 98:

2.

2. A low-temperature electrolyte additive according to claim 1, characterized in that: The usage ratio of anhydrous ethanol, 4-formyl-15-crown-5, terephthalonitrile, and piperidine is 10 mL: 0.4 g: 0.1 g: 0.15 mL.

3. A low-temperature electrolyte additive according to claim 1, characterized in that: The preparation method of the organosilicon phosphorylcholine adjuvant comprises the following steps: Mix the vinyl-terminated silicone oil and tetrahydrofuran, add the tetrahydrofuran solution of 11-mercaptoacylphosphorylcholine, stir for 4-6 hours, add benzoin dimethyl ether, stir for 5-10 minutes, and then react under ultraviolet light for 30 minutes to obtain the organosilicon phosphorylcholine auxiliary agent.

4. A low-temperature electrolyte additive according to claim 3, characterized in that: The feeding amount of 11-mercaptoacylphosphorylcholine is 1-1.2 times of the molar amount of vinyl group in the vinyl-terminated silicone oil.

5. The low-temperature electrolyte additive according to claim 3, characterized in that: The feeding amount of benzoin dimethyl ether is 0.3 times the molar amount of vinyl in the vinyl-terminated silicone oil.

6. The low-temperature electrolyte additive according to claim 3, characterized in that: The viscosity of the vinyl-terminated silicone oil is 100-700 mPa·s.

7. A method for preparing a low-temperature electrolyte additive according to any one of claims 1 to 6, characterized in that: The following steps are involved: Potassium bromide, organosilicon phosphorylcholine auxiliary agent, benzocrown ether-based unsaturated hydrocarbon compound, and fluoroethylene carbonate are weighed according to the formula amount, stirred and mixed evenly to obtain the low-temperature electrolyte additive.

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