Flexible copolymer binder as well as preparation method and application thereof in lithium ion battery

Abstract

The invention provides a flexible copolymer binder as well as a preparation method and application thereof in a lithium ion battery, and particularly relates to the technical field of lithium ion batteries. The flexible copolymer binder provided by the invention has a chemical structure as shown in a formula I: # imgabs0 #, in the formula I, R1 is hydrogen, alkyl, phenyl or carboxyl; r2 and R3 are independently hydrogen or alkyl; r4 is alkyl; m = 0-4, n = 4-40, x + y + z = 1, xgt; 0, y > = 0, z > = 0, y + zgt; m and n are the number of repetitive units, and x, y and z are the mole ratios of the corresponding units. Carboxyl in an olefine acid chain segment in the prepared copolymer binder can promote conduction of lithium ions; the polyethylene glycol acrylate chain segment can enhance the flexibility; the acrylate chain segment can enhance hydrophobicity, and the finally obtained flexible copolymer binder is good in flexibility, strong in binding performance and strong in lithium ion conductivity.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium - ion batteries, and particularly relates to a flexible copolymer binder, a preparation method thereof, and an application thereof in lithium - ion batteries. Background Art

[0002] Lithium - ion batteries have the advantages of high energy density, wide operating temperature range, long cycle life, and light weight, and are widely used in many fields such as portable electronic devices, new energy vehicles, and energy storage systems. A lithium - ion battery mainly consists of a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode is composed of a positive electrode active material, a conductive agent, a binder, and a current collector. The role of the binder is to bond the positive electrode active material and the conductive agent to the current collector and maintain the integrity of the electrode structure during charge and discharge. Although the amount of the binder is small, it is an indispensable important component in the preparation of the positive electrode sheet of the lithium - ion battery and plays a crucial role in maintaining the integrity of the electrode structure and improving the electrochemical performance of the lithium - ion battery.

[0003] At present, the commonly used binder in the positive electrode of lithium - ion batteries is polyvinylidene fluoride (PVDF). However, due to the relatively high Young's modulus of PVDF, the flexibility of the prepared electrode sheet is not good enough. Moreover, the binding effect of PVDF generally comes from the van der Waals force between molecules, and this van der Waals force is relatively weak, which will cause the phenomenon of the active material and the conductive agent falling off from the current collector during the cycling process and cannot meet the requirements of high - performance batteries. At the same time, the lithium - ion transport ability of PVDF is poor, which limits the transport of lithium ions in the electrode and affects the charge - discharge performance of lithium - ion batteries at high rates. In addition, the production process of PVDF is complex and the price is expensive.

[0004] Therefore, developing a positive electrode binder with good flexibility, strong binding performance, beneficial to lithium - ion conduction, and simple preparation method, so that the lithium - ion battery assembled with the electrode sheet prepared by this binder has a high discharge specific capacity and cycle stability, has become the current focus of work. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a flexible copolymer binder, a preparation method thereof, and an application thereof in lithium - ion batteries. The flexible copolymer binder provided by the present invention has good flexibility, strong binding performance, strong lithium - ion conduction ability, and a simple preparation method. The lithium - ion battery assembled with the positive electrode sheet prepared by the flexible copolymer binder of the present invention has a high discharge specific capacity and cycle stability.

[0006] In order to solve the above - mentioned technical problems, the present invention provides the following technical solutions:

[0007] The present invention specifically provides a flexible copolymer binder having a chemical structure shown in Formula I:

[0008]

[0009] In formula I, R 1 is hydrogen, alkyl, phenyl or carboxyl; R 2 and R 3 are independently hydrogen or alkyl; R 4 is alkyl; m = 0 to 4, n = 4 to 40, x + y + z = 1, x > 0, y ≥ 0, z ≥ 0, y + z > 0, where m and n are the number of repeating units, and x, y and z are the molar ratios of the corresponding units.

[0010] Preferably, when R 1 , R 2 , R 3 and R 4 are independently alkyl, R 1 , R 2 and R 3 are independently methyl, and R 4 is ethyl or butyl.

[0011] Preferably, the flexible copolymer binder has a chemical structure shown in any one of formulas II to VI:

[0012]

[0013] The present invention also provides a preparation method of the flexible copolymer binder described in the above technical solution, including the following steps:

[0014] Under an inert atmosphere, a compound shown in formula VII, formula VIII and formula IX is mixed with an initiator and a first organic solvent, and a radical polymerization reaction is carried out to obtain a flexible copolymer solution;

[0015] The chemical structures of the compounds shown in formula VII, formula VIII and formula IX are as follows:

[0016]

[0017] The flexible copolymer solution is mixed with a precipitant to precipitate the flexible copolymer, and the flexible copolymer binder is obtained.

[0018] Preferably, the mass of the initiator accounts for 0.1 to 2% of the total mass of the compounds shown in formula VII, formula VIII and formula IX;

[0019] The initiator is an azo initiator or a peroxide initiator.

[0020] Preferably, the total mass of the compounds shown in formula VII, formula VIII and formula IX accounts for 10 to 50% of the total mass of the compounds shown in formula VII, formula VIII, formula IX and the first organic solvent;

[0021] The first organic solvent includes at least one of 1,4-dioxane, dimethyl carbonate, diethyl carbonate, acetonitrile, toluene, acetone, N-methylpyrrolidone, tetrahydrofuran, dichloromethane, and chloroform.

[0022] Preferably, the precipitant is at least one of diethyl ether, n-hexane, cyclohexane, petroleum ether, toluene, and n-heptane;

[0023] The mass ratio of the precipitant to the flexible copolymer solution is 4-11:1.

[0024] The present invention also provides an application of the flexible copolymer binder prepared by the preparation method described in the above technical solution in the preparation of a positive electrode sheet of a lithium-ion battery.

[0025] Preferably, the preparation of the positive electrode sheet of the lithium-ion battery includes the following steps:

[0026] Mix the flexible copolymer binder and a second organic solvent to obtain a binder solution;

[0027] Perform first ball milling on the positive electrode active material and the conductive agent to obtain a mixed powder;

[0028] Mix the binder solution, the mixed powder, and the second organic solvent to obtain a first positive electrode slurry;

[0029] Mix the aziridine crosslinking agent and the second organic solvent to obtain a crosslinking agent solution;

[0030] Perform second ball milling on the crosslinking agent solution and the first positive electrode slurry to obtain a second positive electrode slurry;

[0031] Coat the second positive electrode slurry on a current collector to obtain the positive electrode sheet of the lithium-ion battery.

[0032] Preferably, the molar ratio of the compound shown in formula VII used for synthesizing the flexible copolymer binder to the aziridine crosslinking agent is 1:(0.1-0.3);

[0033] The mass ratio of the flexible copolymer binder to the positive electrode active material is 1-20:60-98.

[0034] The present invention provides a flexible copolymer binder having a chemical structure shown in formula I:

[0035]

[0036] In formula I, R 1 is hydrogen, an alkyl group, a phenyl group, or a carboxyl group; R 2 and R 3 are independently hydrogen or an alkyl group; R 4is an alkyl group; m = 0 to 4, n = 4 to 40, x + y + z = 1, x > 0, y ≥ 0, z ≥ 0, y + z > 0, where m and n are the number of repeating units, and x, y, and z are the molar ratios of the corresponding units. The carboxyl groups in the acrylic acid chain segment of the copolymer prepared in the present invention can promote the conduction of lithium ions; the polyethylene glycol acrylate chain segment can enhance the flexibility of the flexible copolymer, and at the same time, due to its good lithium ion transport ability, it can improve the ion transport performance between the positive electrode sheet and the electrolyte interface and reduce the interface impedance; the acrylate chain segment can improve the flexibility of the flexible copolymer, soften the electrode sheet, and also enhance the hydrophobicity of the flexible copolymer to avoid the problems of battery capacity and cycle performance decline caused by water absorption of the flexible copolymer. The flexible copolymer obtained in the present invention has good flexibility, strong adhesion performance, and strong lithium ion conduction ability. According to the test results of the examples, the glass transition temperature of the flexible copolymer binder provided by the present invention is -53.3 to -42.8 °C, and the peel strength is 1.29 to 2.67 N·cm -1 , and the prepared flexible copolymer binder has good flexibility at a use temperature above -53.3 °C.

[0037] The present invention also provides a preparation method of the flexible copolymer binder. By controlling the dosage ratio of each monomer, the free radical polymerization reaction can proceed sufficiently, and the flexible copolymer binder can have better adhesion performance.

[0038] In addition, the positive electrode sheet prepared by using the flexible copolymer binder prepared in the present invention has the advantages of low interface impedance, high discharge specific capacity, and good cycle stability when assembled into a lithium ion battery. The carboxyl groups in the acrylic acid chain segment of the flexible copolymer binder prepared in the present invention can crosslink with the aziridine crosslinking agent, thereby reducing the solubility of the positive electrode sheet in the electrolyte; and the interaction (including hydrogen bonds and covalent bonds) between the carboxyl groups in the acrylic acid chain segment and the positive electrode active material can enhance the adhesion between the materials and maintain the integrity of the electrode structure during charge and discharge. According to the test results of the application examples, the interface impedance of the lithium iron phosphate|metal lithium battery assembled with the positive electrode sheet prepared by the flexible copolymer binder provided by the present invention is 177.1 to 201.1 Ω after 400 cycles at a rate of 0.5C, and the initial discharge specific capacity is 149.1 to 154.4 mAh·g at a cut-off voltage of 2.5 to 4.2V and a rate of 0.5C -1 , and the discharge specific capacity after 500 cycles is 132.3 to 145.6 mAh·g -1 , and the capacity retention rate is 88.73 to 94.30%. Description of the Drawings

[0039] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0040] Figure 1 1H NMR spectrum of the flexible copolymer binder A4 prepared in Example 4 of the present invention;

[0041] Figure 2 Differential scanning calorimetry curves of the flexible copolymer binder A4 prepared in Example 4 of the present invention and the binder B1 prepared in Comparative Example 1;

[0042] Figure 3 Interface impedance curves of the lithium iron phosphate|metal lithium batteries D4 and D9 after 400 cycles at 25°C and a rate of 0.5C;

[0043] Figure 4 Long cycle charge-discharge curves of the lithium iron phosphate|metal lithium batteries D4 and D9 at 25°C, a cut-off voltage of 2.5 - 4.2V, and a rate of 0.5C;

[0044] Figure 5 Charge-discharge voltage curves of the lithium iron phosphate|metal lithium battery D4 at 25°C, a cut-off voltage of 2.5 - 4.2V, and a rate of 0.5C during the 100th and 400th cycles;

[0045] Figure 6 Charge-discharge curves of the lithium iron phosphate|metal lithium batteries D4 and D9 at 25°C, a cut-off voltage of 2.5 - 4.2V, and rates of 0.2C, 0.5C, 1C, 2C, and 3C; Detailed implementation manners

[0046] The present invention provides a flexible copolymer binder having a chemical structure shown in Formula I:

[0047]

[0048] R 1 is hydrogen, alkyl, phenyl, or carboxyl; R 2 and R 3 are independently hydrogen or alkyl; R 4 is alkyl; m = 0 - 4, n = 4 - 40, x + y + z = 1, x > 0, y ≥ 0, z ≥ 0, y + z > 0, where m and n are the number of repeating units, and x, y, and z are the molar ratios of the corresponding units.

[0049] In the present invention, the R 1is hydrogen, alkyl, phenyl or carboxyl, the carbon number of the alkyl is preferably 1-4, and in a specific embodiment, the alkyl is methyl. In the present invention, x is the molar proportion of the corresponding unit (acrylic acid chain segment), and in a specific embodiment, x can be 0.01, 0.02, 0.05, 0.07, 0.1, 0.12, 0.15 or 0.2.

[0050] In the present invention, the R 2 is hydrogen or alkyl, the carbon number of the alkyl is preferably 1-4, and in a specific embodiment, the alkyl is methyl. In the present invention, y is the molar proportion of the corresponding unit (polyethylene glycol acrylate chain segment), and in a specific embodiment, y can be 0, 0.1, 0.15, 0.2, 0.25, 0.27 or 0.3.

[0051] In the present invention, the R 3 is hydrogen or alkyl, the carbon number of the alkyl is preferably 1-4, and in a specific embodiment, the alkyl is methyl.

[0052] In the present invention, the R 4 is alkyl, the carbon number of the alkyl is preferably 1-4, and in a specific embodiment, the alkyl is ethyl or butyl. In the present invention, z is the molar proportion of the corresponding unit (acrylate chain segment), and in a specific embodiment, z can be 0, 0.5, 0.55, 0.6, 0.66, 0.7, 0.73, 0.77, 0.8, 0.85, 0.9, 0.95 or 0.98.

[0053] In the present invention, x + y + z = 1, x > 0, y ≥ 0, z ≥ 0, y + z > 0.

[0054] In the present invention, m and n are the number of repeating units, m is preferably 0-4, and in a specific embodiment, m can be 0, 1, 2, 3 or 4; n is preferably 4-40, and n can be 4, 6, 9, 12, 15, 20, 25, 30, 35 or 40.

[0055] In the present invention, the flexible copolymer binder preferably has a chemical structure shown in any one of Formulas II-VI:

[0056]

[0057] In a specific embodiment of the present invention, the flexible copolymer binder has a chemical structure shown in any one of Formulas 1-8:

[0058]

[0059] In the present invention, the number-average molecular weight of the flexible copolymer binder is preferably 5.90×104 ~7.90×10 4 kg / mol。

[0060] In the copolymer prepared by the present invention, the carboxyl groups in the acrylic acid chain segment can promote the conduction of lithium ions; the polyethylene glycol acrylate chain segment can enhance the flexibility of the flexible copolymer binder. At the same time, due to its good ability to transport lithium ions, it can improve the ion transport performance between the positive electrode sheet and the electrolyte interface and reduce the interface impedance; the acrylate chain segment can improve the flexibility of the flexible copolymer binder, soften the electrode sheet, and can also enhance the hydrophobicity of the flexible copolymer binder to avoid the problems of decreased battery capacity and cycling performance caused by the absorption of water by the flexible copolymer binder. The flexible copolymer binder obtained by the present invention has good flexibility, strong bonding performance, and strong lithium ion conduction ability.

[0061] The present invention also provides a preparation method of the flexible copolymer binder described in the above technical solution, including the following steps:

[0062] Under an inert atmosphere, the compounds represented by Formula VII, Formula VIII, and Formula IX are mixed with an initiator and a first organic solvent, and a radical polymerization reaction is carried out to obtain a flexible copolymer solution;

[0063] The chemical structures of the compounds represented by Formula VII, Formula VIII, and Formula IX are as follows:

[0064]

[0065] The flexible copolymer solution is mixed with a precipitant to precipitate the flexible copolymer, and the flexible copolymer binder is obtained.

[0066] In the present invention, unless otherwise specified, the raw materials and equipment used are all well-known commercially available products in the art.

[0067] In the present invention, the compounds represented by Formula VII, Formula VIII, and Formula IX are mixed with an initiator and a first organic solvent, and a radical polymerization reaction is carried out to obtain a flexible copolymer solution.

[0068] In the present invention, the chemical structure of the compound represented by Formula VII is as follows:

[0069]

[0070] In a specific embodiment, the compound represented by Formula VII is preferably at least one of acrylic acid, 3-butenoic acid, 4-pentenoic acid, 5-hexenoic acid, 6-heptenoic acid, 3-phenylacrylic acid, crotonic acid, and itaconic acid.

[0071] In the present invention, the chemical structure of the compound represented by Formula VIII is as follows:

[0072]

[0073] In a specific embodiment, the compound represented by Formula VIII is preferably at least one of methoxypolyethylene glycol acrylate and methoxypolyethylene glycol methacrylate.

[0074] In the present invention, the chemical structure of the compound represented by Formula IX is as follows:

[0075]

[0076] In a specific embodiment, the compound represented by Formula IX is preferably at least one of ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, 2-ethylbutyl acrylate, and amyl acrylate.

[0077] In the present invention, the initiator is preferably an azo initiator or a peroxide initiator. The azo initiator is preferably at least one of azobisisobutyronitrile, azocyclohexanecarbonitrile, and dimethyl azobisisobutyrate. The peroxide initiator is preferably benzoyl peroxide. In the present invention, the mass of the initiator preferably accounts for 0.1-2% of the total mass of the compounds represented by Formula VII, Formula VIII, and Formula IX. In a specific embodiment, the mass of the initiator preferably accounts for 0.1%, 0.2%, 0.5%, 1%, 1.2%, 1.5%, 1.8%, or 2% of the total mass of the compounds represented by Formula VII, Formula VIII, and Formula IX. By using the above initiator in the present invention, the monomers can be initiated to undergo free radical polymerization, and the free radical polymerization reaction can be promoted to proceed fully by controlling the dosage.

[0078] In the present invention, the first organic solvent preferably includes at least one of 1,4-dioxane, dimethyl carbonate, diethyl carbonate, acetonitrile, toluene, acetone, N-methylpyrrolidone, tetrahydrofuran, dichloromethane, and chloroform. The above first organic solvent in the present invention has good compatibility with each raw material.

[0079] In the present invention, the total mass of the compounds represented by Formula VII, Formula VIII, and Formula IX preferably accounts for 10-50% of the total mass of the compounds represented by Formula VII, Formula VIII, and Formula IX and the first organic solvent. In a specific embodiment, the total mass of the compounds represented by Formula VII, Formula VIII, and Formula IX can account for 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, or 50% of the total mass of the compounds represented by Formula VII, Formula VIII, and Formula IX and the first organic solvent. By controlling the dosage of the first organic solvent in the present invention, the free radical polymerization reaction can be ensured to proceed fully.

[0080] The present invention has no special requirements for the manner of mixing, and well-known technical means in the art can be used.

[0081] In the present invention, the radical polymerization reaction is preferably carried out under an inert atmosphere. The inert atmosphere is preferably at least one of nitrogen and argon. The inert atmosphere can avoid the interference of air on the radical polymerization reaction.

[0082] In the present invention, the temperature of the radical polymerization reaction is preferably 45 to 80 °C. In a specific embodiment, the temperature of the radical polymerization reaction can be 45 °C, 50 °C, 55 °C, 60 °C, 65 °C, 70 °C, 75 °C or 80 °C; the time of the radical polymerization reaction is preferably 6 to 24 h. In a specific embodiment, the time of the radical polymerization reaction can be 6 h, 10 h, 15 h, 18 h, 20 h, 22 h or 24 h.

[0083] After obtaining the flexible copolymer solution, in the present invention, the flexible copolymer solution is mixed with a precipitant to precipitate the flexible copolymer, and the flexible copolymer binder is obtained.

[0084] In the present invention, the mixing of the flexible copolymer solution and the precipitant is preferably dropping the flexible copolymer solution into the precipitant. The present invention has no special requirements for the dropping method, and common technical means in the art can be used.

[0085] In the present invention, the precipitant is preferably at least one of ether, n-hexane, cyclohexane, petroleum ether, toluene and n-heptane. When the precipitant is two or more of the above, the present invention has no special limitation on the proportional relationship between the two or more, and any ratio can be mixed. In the present invention, the precipitant is used to separate the flexible copolymer from the reaction solution.

[0086] In the present invention, the mass ratio of the precipitant to the flexible copolymer solution is preferably 4 to 11:1. In a specific embodiment, the mass ratio of the precipitant to the flexible copolymer solution can be 4:1, 6:1, 8:1, 10:1 or 11:1. The present invention controls the amount of the precipitant to fully precipitate the product of the radical polymerization reaction.

[0087] In the present invention, the operations after mixing the flexible copolymer solution and the precipitant preferably further include filtration and drying.

[0088] In the present invention, there is no special limitation on the filtration operation, and the operations well-known to those skilled in the art can be used.

[0089] In the present invention, the drying method is preferably vacuum drying, and the temperature of the vacuum drying is preferably 30 to 80 °C. In a specific embodiment, the temperature of the vacuum drying can be 30 °C, 40 °C, 50 °C, 60 °C, 70 °C or 80 °C; the time of the vacuum drying is preferably 1 to 24 h. In a specific embodiment, the time of the vacuum drying can be 1 h, 5 h, 10 h, 12 h, 15 h, 20 h or 24 h.

[0090] By controlling the dosage ratios of the corresponding compound raw materials of the acrylic acid chain segment, the polyethylene glycol acrylate chain segment and the acrylate chain segment, the free radical polymerization reaction in the present invention proceeds sufficiently, and the flexible copolymer binder can have better binding performance.

[0091] The present invention also provides an application of the flexible copolymer binder prepared by the preparation method described in the above technical solution in the preparation of a positive electrode sheet of a lithium ion battery.

[0092] In the present invention, the preparation of the positive electrode sheet of the lithium ion battery includes the following steps:

[0093] Mix the flexible copolymer binder and a second organic solvent to obtain a binder solution;

[0094] Perform first ball milling on the positive electrode active material and the conductive agent to obtain a mixed powder;

[0095] Mix the binder solution, the mixed powder and the second organic solvent to obtain a first positive electrode paste;

[0096] Mix the aziridine crosslinking agent and the second organic solvent to obtain a crosslinking agent solution;

[0097] Perform second ball milling on the crosslinking agent solution and the first positive electrode paste to obtain a second positive electrode paste;

[0098] Coat the second positive electrode paste on a current collector to obtain the positive electrode sheet of the lithium ion battery.

[0099] In the present invention, the molar ratio of the compound shown in Formula VII used for synthesizing the flexible copolymer binder to the aziridine crosslinking agent is preferably 1:0.1 to 0.3. In a specific embodiment, the molar ratio of the compound shown in Formula VII used for synthesizing the flexible copolymer binder to the aziridine crosslinking agent can be 1:0.1, 1:0.15, 1:0.2, 1:0.25 or 1:0.3.

[0100] In the present invention, the second organic solvent is preferably at least one of N-methylpyrrolidone, tetrahydrofuran, N,N-dimethylformamide, dimethyl sulfoxide and acetonitrile. The second organic solvent has excellent solubility for the flexible copolymer binder.

[0101] In the present invention, the positive electrode active material preferably includes lithium iron phosphate (LiFePO 4 ), lithium cobalt oxide (LiCoO 2 ), lithium manganese oxide (LiMn 2 O 4 ), ternary nickel cobalt manganese 811 (LiNi 0.8 Co 0.1 Mn 0.1 O 2 ), ternary nickel cobalt manganese 523 (LiNi 0.5 Co 0.2 Mn 0.3 O 2 ), ternary nickel cobalt aluminum 811 (LiNi 0.8 Co 0.1 Al 0.1 O 2 ), ternary nickel cobalt manganese 622 (LiNi 0.6 Co 0.2 Mn 0.2 O 2 ), ternary nickel cobalt manganese 613 (LiNi 0.6 Co 0.1 Mn 0.3 O 2 ), and lithium titanate (Li 4 Ti 5 O 12 ). The positive electrode active material can endow the lithium ion battery with good electrochemical performance.

[0102] In the present invention, the mass ratio of the flexible copolymer binder to the positive electrode active material is preferably 1-20:60-98. In specific embodiments, the mass ratio of the flexible copolymer binder to the positive electrode active material can be 20:60, 15:70, 10:80, 5:90, or 1:98.

[0103] In the present invention, when the flexible copolymer binder is mixed with the second organic solvent, there is no special limitation on the amount of the second organic solvent used, as long as the viscosity of the binder solution can be within the range of 0.1-10 Pa·s. Controlling the viscosity of the binder solution can enable the flexible copolymer binder to be fully dissolved and is beneficial to controlling the slurry at an appropriate viscosity.

[0104] In the present invention, the temperature of the mixing is preferably 25°C; the mixing time is preferably 8-12 h. In specific embodiments, the mixing time can be 8 h, 10 h, or 12 h.

[0105] The present invention performs first ball milling on the positive electrode active material and the conductive agent to obtain a mixed powder.

[0106] In the present invention, the conductive agent preferably includes at least one of superconducting carbon black, carbon nanotubes, acetylene black, and Ketjen black.

[0107] In the present invention, the mass ratio of the positive electrode active material to the conductive agent is preferably 60-98:1-20. In a specific embodiment, the mass ratio of the positive electrode active material to the conductive agent can be 60:20, 70:15, 80:10, 90:5, or 98:1.

[0108] In the present invention, the rotation speed of the first ball milling is preferably 600-1200 rpm. In a specific embodiment, the rotation speed of the first ball milling can be 600 rpm, 700 rpm, 800 rpm, 900 rpm, 1032 rpm, or 1200 rpm; the time of the first ball milling is preferably 0.5-4 h. In a specific embodiment, the time of the first ball milling can be 0.5 h, 1 h, 2 h, 3 h, or 4 h.

[0109] After obtaining the mixed powder, the present invention mixes the binder solution, the mixed powder, and a second organic solvent to obtain a first positive electrode slurry.

[0110] In the present invention, the total mass concentration of the mixed powder and the flexible copolymer binder in the first positive electrode slurry is preferably 30-95%. In a specific embodiment, the total mass concentration of the mixed powder and the flexible copolymer binder in the first positive electrode slurry can be 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 95%.

[0111] The present invention mixes an aziridine crosslinking agent and a second organic solvent to obtain a crosslinking agent solution.

[0112] In the present invention, the mass of the aziridine crosslinking agent preferably accounts for 30-95% of the mass of the crosslinking agent solution. In a specific embodiment, the mass of the aziridine crosslinking agent accounts for 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 95% of the mass of the crosslinking agent solution.

[0113] After obtaining the crosslinking agent solution, the present invention performs a second ball milling on the crosslinking agent solution and the first positive electrode slurry to obtain a second positive electrode slurry.

[0114] In the present invention, the rotation speed of the second ball milling is preferably 600-1200 rpm. In a specific embodiment, the rotation speed of the second ball milling can be 600 rpm, 700 rpm, 800 rpm, 900 rpm, 1032 rpm, or 1200 rpm; the time of the second ball milling is preferably 0.2-0.5 h. In a specific embodiment, the time of the second ball milling can be 0.2 h, 0.3 h, 0.4 h, or 0.5 h.

[0115] After obtaining the second positive electrode slurry, the present invention coats the second positive electrode slurry on a current collector to obtain the positive electrode sheet of the lithium-ion battery.

[0116] In the present invention, the current collector is preferably aluminum foil. The present invention has no special limitation on the size of the aluminum foil, and it can be adjusted according to actual needs.

[0117] In the present invention, the coating device is preferably a doctor blade. The present invention has no special limitation on the model of the doctor blade, and any instrument and equipment well-known to those skilled in the art can be used.

[0118] In the present invention, the coating thickness is preferably 60 - 300 μm.

[0119] In the present invention, after coating, a coated product is obtained. Preferably, the coated product is successively dried, roll-pressed, and cut. By drying, roll-pressing, and cutting, a flat and appropriately sized positive electrode sheet is obtained.

[0120] In the present invention, the drying preferably includes atmospheric drying and vacuum drying. The temperature of the atmospheric drying is preferably 60 - 120 °C. In specific embodiments, the temperature of the atmospheric drying can be 60 °C, 80 °C, 100 °C, or 120 °C; the time of the atmospheric drying is preferably 6 - 24 h. In specific embodiments, the time of the atmospheric drying can be 6 h, 12 h, 18 h, 20 h, or 24 h; the temperature of the vacuum drying is preferably 60 - 80 °C. In specific embodiments, the temperature of the vacuum drying can be 60 °C, 70 °C, or 80 °C; the time of the vacuum drying is preferably 6 - 24 h. In specific embodiments, the time of the vacuum drying can be 6 h, 12 h, 18 h, 20 h, or 24 h. By controlling the drying temperature and time, it is beneficial for the organic solvent to volatilize fully and can avoid the problems of cracking and curling of the positive electrode sheet of the lithium-ion battery caused by the volatilization of the organic solvent.

[0121] When the present invention prepares the positive electrode sheet of the lithium-ion battery using a flexible copolymer binder, the carboxyl group in the acrylic acid segment of the flexible copolymer binder can crosslink with the aziridine crosslinking agent, thereby reducing the solubility of the positive electrode sheet in the electrolyte; and the interaction (including hydrogen bonds and covalent bonds) between the carboxyl group in the acrylic acid segment and the positive electrode active material can enhance the adhesion between the materials and maintain the integrity of the electrode structure during charge and discharge.

[0122] The present invention has no special limitation on the operation of the roll-pressing, and any operation well-known to those skilled in the art can be used.

[0123] The present invention has no special limitation on the operation of the cutting, and any operation well-known to those skilled in the art can be used.

[0124] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions of the present invention in conjunction with specific embodiments. The described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Any modifications, equivalent replacements, improvements, etc. made to the embodiments of the present invention based on the technical essence and general principles of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0125] Example 1

[0126] Under a nitrogen atmosphere, 250 g of acrylic acid, 8378 g of polyethylene glycol acrylate, 6225 g of butyl acrylate, 39 g of azobisisobutyronitrile, and 27580 g of dimethyl carbonate were mixed in a reaction kettle, stirred at 70 °C for 8 h, and subjected to a free radical polymerization reaction to obtain a flexible copolymer solution.

[0127] The flexible copolymer solution was dropped into 212000 g of ether for precipitation and filtration. The filtered product was vacuum dried at 50 °C for 12 h to obtain 13360 g of a flexible copolymer binder, named A1.

[0128] A1 has the chemical structure as shown in Formula 1:

[0129]

[0130] The structure of Formula 1 corresponds to R in Formula I 1 、R 2 and R 3 being hydrogen, R 4 being butyl, x = 0.05, y = 0.25, z = 0.7, m = 0, n = 9. The molecular weight of A1 is 6.81×10 4 kg / mol.

[0131] Example 2

[0132] Under a nitrogen atmosphere, 100 g of acrylic acid, 2584 g of polyethylene glycol acrylate, 1677 g of butyl acrylate, 21 g of azobisisobutyronitrile, and 8100 g of dimethyl carbonate were mixed in a reaction kettle, stirred at 70 °C for 8 h, and subjected to a free radical polymerization reaction to obtain a flexible copolymer solution.

[0133] The flexible copolymer solution was dropped into 62300 g of ether for precipitation and filtration. The filtered product was vacuum dried at 50 °C for 12 h to obtain 4055 g of a flexible copolymer binder, named A2.

[0134] A2 has the chemical structure as shown in Formula 2:

[0135]

[0136] The structure of Formula 2, corresponding to R in Formula I 1 , R 2 and R 3 is hydrogen, R 4 is butyl, x = 0.07, y = 0.27, z = 0.66, m = 0, n = 9. The molecular weight of A2 is 6.51×10 4 kg / mol.

[0137] Example 3

[0138] Under a nitrogen atmosphere, 200 g of acrylic acid, 1340 g of polyethylene glycol acrylate, 2846 g of butyl acrylate, 22 g of azobisisobutyronitrile, and 8145 g of dimethyl carbonate were mixed in a reaction kettle, stirred at 70 °C for 8 h, and subjected to a free radical polymerization reaction to obtain a flexible copolymer solution.

[0139] The flexible copolymer solution was dropped into 62660 g of ether for precipitation and filtration. The filtered product was vacuum dried at 50 °C for 12 h to obtain 4065 g of a flexible copolymer binder, named A3.

[0140] A3 has the chemical structure of Formula 3:

[0141]

[0142] The structure of Formula 3, corresponding to R in Formula I 1 , R 2 and R 3 is hydrogen, R 4 is butyl, x = 0.1, y = 0.1, z = 0.8, m = 0, n = 9. The molecular weight of A3 is 6.30×10 4 kg / mol.

[0143] Example 4

[0144] Under a nitrogen atmosphere, 200 g of acrylic acid, 6759 g of butyl acrylate, 14 g of azobisisobutyronitrile, and 12920 g of 1,4-dioxane were mixed in a reaction kettle, stirred at 70 °C for 8 h, and subjected to a free radical polymerization reaction to obtain a flexible copolymer solution.

[0145] The flexible copolymer solution was dropped into 99410 g of ether for precipitation and filtration. The filtered product was vacuum dried at 50 °C for 12 h to obtain 6880 g of a flexible copolymer binder, named A4.

[0146] A4 has the chemical structure of Formula 4:

[0147]

[0148] The structure of Formula 4, corresponding to R in Formula I 1 and R 3is hydrogen, R 4 is butyl, x = 0.05, y = 0, z = 0.95, m = 0. The molecular weight of A4 is 7.16×10 4 kg / mol.

[0149] Figure 1 is the proton nuclear magnetic resonance spectrum of the flexible copolymer binder A4 in Example 4 of the present invention. It can be seen from Figure 1 that the flexible copolymer binder is successfully prepared in the present invention.

[0150] Example 5

[0151] Under a nitrogen atmosphere, 200 g of acrylic acid, 2016 g of butyl acrylate, 12 g of azobis(cyclohexanecarbonitrile), and 4115 g of 1,4-dioxane were mixed in a reaction kettle, stirred at 70 °C for 8 h, and subjected to a free radical polymerization reaction to obtain a flexible copolymer solution.

[0152] The flexible copolymer solution was dropped into 31660 g of diethyl ether for precipitation, filtered, and the filtered product was dried in vacuo at 50 °C for 12 h to obtain 2010 g of a flexible copolymer binder, named A5.

[0153] A5 has the chemical structure as shown in Formula 5:

[0154]

[0155] The structure of Formula 5 corresponds to R in Formula I 1 and R 3 is hydrogen, R 4 is butyl, x = 0.15, y = 0, z = 0.85, m = 0. The molecular weight of A5 is 5.91×10 4 kg / mol.

[0156] Example 6

[0157] Under a nitrogen atmosphere, 100 g of acrylic acid, 12735 g of polyethylene glycol acrylate, 48 g of azobisisobutyronitrile, and 23830 g of dimethyl carbonate were mixed in a reaction kettle, stirred at 70 °C for 8 h, and subjected to a free radical polymerization reaction to obtain a flexible copolymer solution.

[0158] The flexible copolymer solution was dropped into 183350 g of diethyl ether for precipitation, filtered, and the filtered product was dried in vacuo at 50 °C for 12 h to obtain 12725 g of a flexible copolymer binder, named A6.

[0159] A6 has the chemical structure as shown in Formula 6:

[0160]

[0161] The structure of Formula 6 corresponds to R in Formula I 1and R 2 is hydrogen, x = 0.05, y = 0.95, z = 0, m = 0, n = 9. The molecular weight of A6 is 7.80×10 4 kg / mol.

[0162] Example 7

[0163] Under a nitrogen atmosphere, 200 g of itaconic acid, 9281 g of polyethylene glycol acrylate, 7172 g of butyl acrylate, 34 g of azobisisobutyronitrile, and 30960 g of dimethyl carbonate were mixed in a reaction kettle, stirred at 70 °C for 8 h, and subjected to a free radical polymerization reaction to obtain a flexible copolymer solution.

[0164] The flexible copolymer solution was dropped into 238100 g of ether for precipitation and filtration. The filtered product was vacuum dried at 50 °C for 12 h to obtain 16550 g of a flexible copolymer binder, named A7.

[0165] A7 has the chemical structure as shown in Formula 7:

[0166]

[0167] The structure of Formula 7 corresponds to R in Formula I 1 is a carboxyl group, R 2 and R 3 are hydrogen, R 4 is butyl, x = 0.02, y = 0.25, z = 0.73, m = 1, n = 9. The molecular weight of A7 is 6.99×10 4 kg / mol.

[0168] Example 8

[0169] Under a nitrogen atmosphere, 100 g of methacrylic acid, 2805 g of polyethylene glycol acrylate, 2313 g of butyl methacrylate, 26 g of azobisisobutyronitrile, and 9688 g of dimethyl carbonate were mixed in a reaction kettle, stirred at 70 °C for 8 h, and subjected to a free radical polymerization reaction to obtain a flexible copolymer solution.

[0170] The flexible copolymer solution was dropped into 74500 g of ether for precipitation and filtration. The filtered product was vacuum dried at 50 °C for 12 h to obtain 5095 g of a flexible copolymer binder, named A8.

[0171] A8 has the chemical structure as shown in Formula 8:

[0172]

[0173] The structure of Formula 8 corresponds to R in Formula I 1 and R 3 are methyl, R 2 is hydrogen, R 4is butyl, x = 0.05, y = 0.25, z = 0.7, m = 0, n = 9. The molecular weight of A8 is 5.93×10 4 kg / mol.

[0174] Preparation of the positive electrode sheet of a lithium-ion battery

[0175] A1 - A8 prepared in Examples 1 - 8 were respectively used as the flexible copolymer binder in the following preparation method, and the obtained positive electrode sheets of lithium-ion batteries were denoted as C1 - C8 in sequence (the positive electrode sheet prepared using the flexible copolymer binder A1 in Example 1 was denoted as C1, and so on).

[0176] 10 g of the flexible copolymer binder and 90 g of N-methylpyrrolidone were mixed and stirred at 25°C for 10 h until all the polymer was dissolved to obtain a binder solution (viscosity: 100 mPa·s);

[0177] 80 g of lithium iron phosphate and 10 g of superconducting carbon black were ball-milled at a rotation speed of 1032 rpm for 1 h to obtain a mixed powder;

[0178] 100 g of the binder solution and 90 g of the mixed powder were mixed and ball-milled at a rotation speed of 1032 rpm for 6 h, and 80 g of N-methylpyrrolidone was added and ball-milled for another 2 h to obtain the first positive electrode slurry;

[0179] The aziridine cross-linking agent (the mass of the added aziridine cross-linking agent was calculated according to the molar ratio of the compound shown in Formula VII in the flexible copolymer binder to the aziridine cross-linking agent being 1:0.1 - 0.3) and 10 g of N-methylpyrrolidone were added to a container and stirred at 25°C until all the aziridine cross-linking agent was dissolved to obtain a cross-linking agent solution;

[0180] The cross-linking agent solution was mixed with the first positive electrode slurry and ball-milled at a rotation speed of 1032 rpm for 0.2 h to obtain the second positive electrode slurry;

[0181] The second positive electrode slurry was coated on aluminum foil with a doctor blade, dried at 60°C under normal pressure for 12 h and then dried at 80°C under vacuum for 12 h, and was rolled and cut in sequence to prepare the lithium iron phosphate positive electrode sheets C1 - C8.

[0182] Comparative Example 1

[0183] 1000 g of dry polyvinylidene fluoride PVDF (HSV900) and 9000 g of N-methylpyrrolidone were mixed in a reaction kettle and stirred at 25°C for 12 h to prepare a PVDF binder solution, denoted as B1.

[0184] Preparation of the positive electrode sheet of a lithium-ion battery

[0185] 80 g of lithium iron phosphate and 10 g of superconducting carbon black were ball-milled at a rotation speed of 1032 rpm for 1 h to obtain a mixed powder.

[0186] 100 g of B1 was mixed with 90 g of the mixed powder and ball-milled at a rotation speed of 1032 rpm for 6 h. Then, 100 g of N-methylpyrrolidone was added and ball-milled for another 2 h to obtain a positive electrode slurry.

[0187] The positive electrode slurry was coated on an aluminum foil using a doctor blade, dried at 60 °C under normal pressure for 12 h, and then dried at 80 °C under vacuum for 12 h. After rolling and cutting in sequence, a lithium iron phosphate positive electrode plate C9 was prepared.

[0188] Preparation of negative electrode binder solution

[0189] 400 g of sodium carboxymethyl cellulose powder (Shenzhen Kejing, MAC500LC), 1712 g of styrene-butadiene rubber latex (Shenzhen Kejing, S2919, mass fraction of polymer is 35%), and 9000 g of pure water (resistivity greater than 0.1 MΩ·cm) were mixed in a reaction kettle and stirred at 25 °C for 12 h to obtain a negative electrode binder solution, denoted as B2.

[0190] Preparation of negative electrode plate of lithium-ion battery

[0191] 188 g of artificial graphite and 6 g of superconducting carbon black were ball-milled at a rotation speed of 1032 rpm for 2 h to obtain a negative electrode mixed powder.

[0192] 60 g of B2 was ball-milled with the negative electrode mixed powder at a rotation speed of 1032 rpm for 6 h. Then, 136 g of pure water (resistivity greater than 0.1 MΩ·cm) was added and ball-milled for another 3 h to obtain a negative electrode slurry.

[0193] The negative electrode slurry was coated on a copper foil using a doctor blade, dried at 60 °C under normal pressure for 12 h, and then dried at 80 °C under vacuum for 12 h. After rolling and cutting in sequence, a lithium iron phosphate negative electrode plate denoted as C10 was prepared.

[0194] Test Example 1

[0195] The flexible copolymer binders prepared in Examples 1-8 and the binder prepared in Comparative Example 1 were subjected to the following performance tests: number-average molecular weight M n,GPCDetermined by gel permeation chromatography (GPC), with THF as the eluent and polystyrene samples with different molecular weights as the standard samples; the thermal decomposition temperature was measured by a thermogravimetric analyzer (Netzsch, Germany, TG209). Under a nitrogen atmosphere, the temperature was raised from 25 °C to 600 °C at a heating rate of 10 °C / min; the glass transition temperature was measured by a differential scanning calorimeter (TA DSC Q100). Under a nitrogen atmosphere, the temperature range was -80 °C to 180 °C, and the heating / cooling rate was ±10 °C / min. The test results of the binder performance are shown in Table 1.

[0196] Table 1 Performance indicators of the binder

[0197] Binder <![CDATA[M n,GPC (kg / mol)]]> <![CDATA[T d (℃)]]> <![CDATA[T g (℃)]]> A1 <![CDATA[6.81×10 4 > 288 -48.8 A2 <![CDATA[6.51×10 4 > 272 -47.8 A3 <![CDATA[6.30×10 4 > 283 -43.6 A4 <![CDATA[7.16×10 4 > 295 -53.2 A5 <![CDATA[5.91×10 4 > 298 -49.4 A6 <![CDATA[7.80×10 4 > 302 -52.1 A7 <![CDATA[6.99×10 4 > 277 -42.8 A8 <![CDATA[5.93×10 4 > 282 -43.7 B1 <![CDATA[5.00×10 5 > 316 -47.0

[0198] As can be seen from Table 1, the molecular weight of the flexible copolymer binder prepared in the present invention is in the range of 5.91×10 4 ~7.80×10 4 kg / mol, which is much lower than the molecular weight of commercial PVDF. Therefore, it can be expected that under the same conditions, the slurry prepared with the flexible copolymer binder in the present invention has a lower viscosity and is easier to coat; the thermal decomposition temperature of the flexible copolymer binder prepared in the present invention is in the range of 272 - 302 °C. Compared with PVDF, although the flexible copolymer binder prepared in the present invention has a lower thermal decomposition temperature, it still meets the requirements for the battery to operate under high-temperature conditions. The glass transition temperatures of the flexible copolymer binder A4 prepared in Example 4 of the present invention and the binder B1 prepared in Comparative Example 1 were tested, and the Figure 2 curve shown was obtained. As can be seen from Table 1 and Figure 2 , the T g of the flexible copolymer binder prepared in the present invention is in the range of -53.3 to -42.8 °C, which is close to the T g of PVDF. However, the flexible copolymer binder in the prior art is an amorphous polymer, while PVDF is a crystalline polymer. Therefore, the flexible copolymer binder prepared in the present invention has good flexibility at use temperatures above -53.3 °C.

[0199] Test Example 2

[0200] The lithium iron phosphate electrodes C1 - C9 of the lithium-ion battery were characterized. Among them, the areal density of the active material of the lithium iron phosphate electrodes C1 - C9 of the lithium-ion battery is the mass of lithium iron phosphate per unit area, which is calculated; the peel strength between the dry slurry coating on the surface of the current collector and the current collector in the electrode was measured according to the test method of GB / T 2791 - 1995; the areal density and peel strength of the obtained lithium iron phosphate electrodes are shown in Table 2.

[0201] Using the prepared lithium iron phosphate electrodes C1 - C9 as the positive electrode of a lithium - ion battery and lithium metal as the negative electrode of the battery, a lithium iron phosphate|metal lithium battery was assembled. The assembly method is as follows:

[0202] Lithium hexafluorophosphate (LiPF 6 ) dissolved in a solution composed of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) with a volume ratio of 1:1:1 was used as the electrolyte (the concentration of LiPF 6 was 1 mol / L), and a polypropylene microporous membrane (Celgard 2325) was used as the separator; the prepared lithium iron phosphate electrodes C1 - C9 were used as the positive electrode of the battery, and lithium metal was used as the negative electrode of the battery. In the central position of the CR2032 - type negative electrode case, stainless - steel gaskets, lithium metal, electrolyte, separator, electrolyte, lithium iron phosphate positive electrode sheet, stainless - steel gaskets, and stainless - steel shrapnel were placed in sequence, and a CR2032 - type positive electrode case was placed on the top. Then, the battery was put into an MSK 110 - type battery encapsulation machine, and the pressure was increased to 50 psi for encapsulation in the locked state. After unlocking, the lithium iron phosphate|metal lithium battery was obtained. The amount of electrolyte used in each lithium iron phosphate|metal lithium battery was 40 μL, and it was required to be dropped equally on both sides of the separator to fully wet the separator. The batteries assembled with lithium iron phosphate electrodes C1 - C9 were lithium iron phosphate|metal lithium batteries, denoted as D1 - D9 respectively (the lithium iron phosphate|metal lithium battery assembled with the positive electrode sheet C1 was denoted as D1, and so on to D9).

[0203] The assembled batteries D1 - D9 were characterized: the specific capacity of the battery refers to the initial discharge specific capacity and the discharge specific capacity of the 500th cycle of the assembled lithium iron phosphate|metal lithium battery at a current density of 0.5C. The test instrument was a battery cycle tester (Wuhan Blue - Electric, CT3002A), the cut - off voltage was 2.5 - 4.2V, the test temperature was 25°C, and the theoretical specific capacity of the active material was 170 mAh / g. The initial discharge specific capacity, the discharge specific capacity of the 500th cycle, and the retention rate (the ratio of the discharge specific capacity of the 500th cycle to the initial discharge specific capacity) of the lithium iron phosphate|metal lithium battery were tested, and the test results are shown in Table 2.

[0204] Table 2 Performance indicators of lithium iron phosphate|metal lithium batteries

[0205]

[0206] Figure 3 It is the interfacial impedance curve of lithium iron phosphate|metal lithium batteries D4 and D9 after 400 cycles at 25°C and a 0.5C rate condition.

[0207] Figure 4Long cycle charge-discharge curves of lithium iron phosphate|metal lithium batteries D4 and D9 at 25 °C, with a cut-off voltage of 2.5 - 4.2 V and a rate of 0.5C.

[0208] Figure 5 Charge-discharge voltage curves of lithium iron phosphate|metal lithium battery D4 at the 100th and 400th cycles at 25 °C, with a cut-off voltage of 2.5 - 4.2 V and a rate of 0.5C.

[0209] Figure 6 Charge-discharge curves of lithium iron phosphate|metal lithium batteries D4 and D9 at 25 °C, with a cut-off voltage of 2.5 - 4.2 V, at rates of 0.2C, 0.5C, 1C, 2C, and 3C.

[0210] As can be seen from Table 2, under the condition of similar lithium iron phosphate surface density, the lithium iron phosphate positive electrode sheets (C1 - C8) prepared with the flexible copolymer binder of the present invention all have higher peel strength than the lithium iron phosphate positive electrode sheet (C9) prepared with the PVDF binder, indicating that this flexible copolymer binder has stronger adhesiveness.

[0211] From Table 2 and Figure 3 it can be seen that the interfacial impedance of the lithium iron phosphate|metal lithium batteries D1 - D8 assembled with the positive electrode sheets C1 - C8 prepared with this flexible copolymer binder after 400 cycles at a rate of 0.5C is lower than that of the lithium iron phosphate|metal lithium battery D9 assembled with the positive electrode sheet C9 prepared with the PVDF binder after 400 cycles at a rate of 0.5C, indicating that this flexible copolymer binder has better lithium ion transport ability.

[0212] From Table 2, Figure 4 and Figure 5 it can be seen that the lithium iron phosphate|metal lithium batteries D1 - D8 assembled with the positive electrode sheets C1 - C8 prepared with this flexible copolymer binder have higher initial discharge specific capacity and capacity retention rate than the lithium iron phosphate|metal lithium battery D9 assembled with the positive electrode sheet C9 prepared with the PVDF binder.

[0213] From Figure 6 it can be seen that the lithium iron phosphate|metal lithium battery D4 assembled with the positive electrode sheet C4 prepared with this flexible copolymer binder can exhibit a relatively high discharge specific capacity at various current densities. Even at a high current density of 3C, it still exhibits a discharge specific capacity of 112.8 mAh·g -1 ; when the current density returns to 0.2C again, the specific capacity also smoothly returns to the initial value.

[0214] Test Example 3

[0215] The preparation method of the lithium iron phosphate|artificial graphite battery is as follows:

[0216] Lithium hexafluorophosphate (LiPF 6 ) is dissolved in a solution composed of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) with a volume ratio of 1:1:1 to form an electrolyte solution (the concentration of LiPF 6 is 1 mol / L); a polypropylene microporous membrane (Celgard 2325) is used as the separator; C4 or C9 is used as the positive electrode plate; and an artificial graphite electrode plate C10 is used as the negative electrode plate. Place them in the center position of the CR2032-type negative electrode case in the order of stainless steel gasket, artificial graphite negative electrode plate, electrolyte, separator, electrolyte, lithium iron phosphate positive electrode plate, stainless steel gasket, and stainless steel spring piece. Place the CR2032-type positive electrode case on the top, and then put the whole into an MSK 110-type battery encapsulation machine. Apply pressure to 50 psi for encapsulation in the locked state, and after unlocking, obtain the lithium iron phosphate|artificial graphite batteries E1 (with C4 as the positive electrode) and E2 (with C9 as the positive electrode). The electrolyte dosage for each lithium iron phosphate|artificial graphite battery is 40 μL, and it is required to be dropped equally on both sides of the separator to fully wet the separator.

[0217] Perform battery cycle testing on the assembled lithium iron phosphate|artificial graphite full batteries E1 to E2 in a battery cycle tester (Wuhan Blue Electric, CT3002A). Among them, the cut-off voltage is 2.5 - 4.2 V, the test temperature is 25 °C, the rate is 0.5 C, and the standard specific capacity of the active material is 170 mAh / g. Test the initial specific capacity of the battery cycle, the discharge specific capacity at the 800th cycle, and the capacity retention rate (the ratio of the discharge specific capacity at the 800th cycle to the initial discharge specific capacity). The test results are shown in Table 3.

[0218] Table 3 Performance indicators of the lithium iron phosphate|artificial graphite battery

[0219]

[0220] From the full battery performance results in Table 3, it can be seen that the lithium iron phosphate|artificial graphite full battery assembled with the positive electrode plate C4 prepared by the flexible copolymer binder of the present invention has a higher initial discharge specific capacity and capacity retention rate at a current density of 0.5 C compared to the lithium iron phosphate|artificial graphite full battery assembled with the positive electrode plate C9 prepared by using PVDF binder.

[0221] As can be seen from the results of the above embodiments, the flexible copolymer binder provided by the present invention is a polymer formed by free radical polymerization of acrylic acid, polyethylene glycol acrylate and acrylate. The acrylic acid chain segment in the copolymer mainly has three functions: First, the carboxyl group in the acrylic acid chain segment can crosslink with the aziridine crosslinking agent, thereby reducing the solubility of the positive electrode sheet in the electrolyte; Second, the interaction (including hydrogen bonds and covalent bonds) between the carboxyl group in the acrylic acid chain segment and the positive electrode active material can enhance the adhesion between the materials and maintain the integrity of the electrode structure during charge and discharge; Third, the carboxyl group in the acrylic acid chain segment can promote the conduction of lithium ions. On the one hand, the polyethylene glycol acrylate chain segment in the copolymer can enhance the flexibility of the flexible copolymer; on the other hand, due to its good lithium ion transport ability, it can improve the ion transport performance between the positive electrode sheet and the electrolyte interface and reduce the interface impedance. The acrylate chain segment in the copolymer can not only improve the flexibility of the flexible copolymer binder, soften the electrode sheet, but also enhance the hydrophobicity of the flexible copolymer binder, avoiding the problem of decline in battery capacity and cycle performance caused by the absorption of water by the binder. The lithium ion battery assembled with the electrode sheet prepared by using the above flexible copolymer binder has the advantages of low interface impedance, high discharge specific capacity and cycle stability, etc.

[0222] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A flexible copolymer binder, characterized in that: It has the chemical structure shown in Formula I: In formula I, R1 is hydrogen, alkyl, phenyl or carboxyl; R2 and R3 are independently hydrogen or alkyl; R4 is alkyl; m=0-4, n=4-40, x+y+z=1, x>0, y≥0, z≥0, y+z>0, wherein m and n are the number of repeating units, and x, y and z are the molar proportions of the corresponding units.

2. The flexible copolymer binder according to claim 1, characterized in that: When R1, R2, R3 and R4 are independently alkyl groups, the number of carbon atoms of the alkyl groups is independently 1 to 4.

3. The flexible copolymer binder according to claim 1, characterized in that: The flexible copolymer binder has a chemical structure shown in any one of Formulas II to VI:

4. The method for preparing the flexible copolymer binder according to any one of claims 1 to 3, characterized in that: The following steps are involved: Under an inert atmosphere, the compounds represented by formula VII, formula VIII and formula IX are mixed with an initiator and a first organic solvent to perform a free radical polymerization reaction to obtain a flexible copolymer solution; The chemical structures of the compounds represented by formula VII, formula VIII and formula IX are as follows: The flexible copolymer solution is mixed with a precipitant to precipitate the flexible copolymer, thereby obtaining the flexible copolymer binder.

5. The preparation method according to claim 4, characterized in that: The mass of the initiator accounts for 0.1-2% of the total mass of the compounds represented by formula VII, formula VIII and formula IX; The initiator is an azo initiator or a peroxide initiator.

6. The preparation method according to claim 4, characterized in that: The total mass of the compounds represented by formula VII, formula VIII and formula IX accounts for 10 to 50% of the total mass of the compounds represented by formula VII, formula VIII, formula IX and the first organic solvent; The first organic solvent includes at least one of 1,4-dioxane, dimethyl carbonate, diethyl carbonate, acetonitrile, toluene, acetone, N-methylpyrrolidone, tetrahydrofuran, dichloromethane and chloroform.

7. The preparation method according to claim 4, characterized in that: The precipitant is at least one of ether, n-hexane, cyclohexane, petroleum ether, toluene and n-heptane; The mass ratio of the precipitant to the flexible copolymer solution is 4 to 11:

1.

8. Use of the flexible copolymer binder according to any one of claims 1 to 3 or the flexible copolymer binder prepared by the preparation method according to any one of claims 4 to 7 in the preparation of positive electrode sheets for lithium-ion batteries.

9. The use according to claim 8, characterized in that: The preparation of the positive electrode sheet of the lithium ion battery comprises the following steps: Mixing the flexible copolymer binder with a second organic solvent to obtain a binder solution; The positive electrode active material and the conductive agent are first ball-milled to obtain a mixed powder; Mixing the binder solution, the mixed powder and the second organic solvent to obtain a first positive electrode slurry; mixing an aziridine cross-linking agent and a second organic solvent to obtain a cross-linking agent solution; The cross-linking agent solution and the first positive electrode slurry are subjected to a second ball milling to obtain a second positive electrode slurry; The second positive electrode slurry is coated on the current collector to obtain the positive electrode plate of the lithium ion battery.

10. The use according to claim 9, characterized in that: The molar ratio of the compound represented by formula VII used to synthesize the flexible copolymer binder to the aziridine crosslinking agent is 1:(0.1-0.3); The mass ratio of the flexible copolymer binder to the positive electrode active material is 1-20:60-98.

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