Modified polypropylene binder as well as preparation method and application thereof

Abstract

The invention relates to the technical field of materials, and discloses a modified polypropylene binder and a preparation method and application thereof, the modified polypropylene binder comprises the following synthetic raw materials: a monomer A, a monomer B, a monomer C and N, N '-methylene bisacrylamide; the molar ratio of the monomer A to the monomer B to the monomer C is 1: (0.5-3): (0.2-1); the monomer A is an acrylic monomer; the monomer B is an acrylamide monomer; the monomer C has a structure shown as a formula (I), R1 is H or CH3, R2 is CnHxF2n + 1-x, n is less than or equal to 12, and x is less than 2n + 1. According to the modified polypropylene binder, the elongation at break of the polypropylene binder and the cycle capacity retention rate of a lithium ion battery using a silicon-based negative electrode material are improved;

Description

Technical Field

[0001] The present invention relates to the field of material technology, and more particularly to a modified polypropylene adhesive, a preparation method thereof, and an application thereof. Background Art

[0002] Silicon-based negative electrode materials have a high theoretical specific capacity. When used in the charge and discharge process of lithium-ion batteries, the silicon and lithium in the silicon-based negative electrode material will undergo alloying reaction to form a variety of lithium-silicon alloy phases, including Li 15 Si4 phase, and Li 15 Si4 exhibits a theoretical specific capacity of up to 3579 mAh / g. Furthermore, silicon-based anode materials offer advantages such as a low voltage platform and abundant sources, making them a strong contender for next-generation high-performance lithium-ion battery anode materials.

[0003] Silicon-based anode materials are prone to volume expansion during the lithium insertion cycle of lithium-ion batteries, leading to the continuous formation of the SEI film, which damages the electrode structure and causes severe capacity decay in lithium-ion batteries. Polyacrylic acid (PAA) is a linear polymer widely used as a binder for silicon-based anode materials due to its high elastic modulus and high molecular weight. It can restrain silicon-based anode materials to a certain extent and inhibit their volume expansion. However, PAA has poor toughness and cannot meet the requirements of long-term cycling of lithium-ion batteries, resulting in low cycle capacity retention.

[0004] Therefore, it is of great significance to develop a modified polypropylene binder to improve the elongation at break of the polypropylene binder and the cycle capacity retention rate of lithium-ion batteries using silicon-based negative electrode materials. Summary of the Invention

[0005] The purpose of the present invention is to solve the deficiencies in the prior art and to provide a modified polypropylene adhesive and a preparation method and application thereof.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] In a first aspect, the present invention provides a modified polypropylene adhesive comprising the following synthetic raw materials:

[0008] Monomer A, monomer B, monomer C and N,N'-methylenebisacrylamide;

[0009] The molar ratio of monomer A, monomer B and monomer C is 1:(0.5-3):(0.2-1);

[0010] The monomer A is an acrylic monomer; the monomer B is an acrylamide monomer;

[0011] The monomer C has a structure shown in formula (I):

[0012]

[0013] wherein R1 is H or CH3, R2 is C n H x F 2n+1-x , n≤12, x<2n+1.

[0014] The present application can form a stable cross-linking network structure when synthesizing the modified polypropylene adhesive by using N,N'-methylene bisacrylamide and monomer A, monomer B and monomer C in a specific molar ratio, which can not only improve the strength, adhesion and toughness of the modified polypropylene adhesive, realize the improvement of the elongation at break of the polypropylene adhesive, but also inhibit the volume expansion of the silicon-based negative electrode material during the lithium ion battery cycle lithium intercalation process, and can also enhance the ion conductivity of the modified polypropylene adhesive, which is beneficial to maintaining the stability and kinetic performance of the silicon-based negative electrode material during long-term charge and discharge cycles, and can improve the cycle capacity retention rate of the lithium ion battery using the silicon-based negative electrode material.

[0015] Preferably, the molar ratio of the monomer A, the monomer B and the monomer C is one of 1:0.5:0.6, 1:1:0.2, 1:1:0.6, 1:1:1, 1:1.25:0.2, 1:1.25:0.6, 1:1.25:1, 1:3:0.6 or a range value between any two of them.

[0016] More preferably, the molar ratio of the monomer A, the monomer B and the monomer C is 1:(1-3):(0.6-1).

[0017] More preferably, the molar ratio of the monomer A, the monomer B and the monomer C is 1:(1.25-3):(0.6-1).

[0018] Preferably, the ratio of the total amount of substance of the monomer A, the monomer B and the monomer C to the amount of substance of N,N'-methylene bisacrylamide is one of (monomer A+monomer B+monomer C):N,N'-methylene bisacrylamide=100:0.1, 100:0.2, 100:0.3, 100:0.4, 100:0.5, 100:0.6, 100:0.7, 100:0.8, 100:0.9, 100:1, 100:1.1, 100:1.2, 100:1.3, 100:1.4, 100:1.5, 100:1.6, 100:1.7, 100:1.8, 100:1.9, 100:2 or a range value between any two of them.

[0019] More preferably, the ratio of the total amount of monomer A, monomer B, and monomer C to the amount of N,N'-methylenebisacrylamide is (monomer A+monomer B+monomer C):N,N'-methylenebisacrylamide=100:(0.1-2).

[0020] More preferably, the ratio of the total amount of monomer A, monomer B, and monomer C to the amount of N,N'-methylenebisacrylamide is (monomer A+monomer B+monomer C):N,N'-methylenebisacrylamide=100:(1-2).

[0021] Preferably, the acrylic monomer includes at least one of acrylic acid, methacrylic acid, 2-acetamidoacrylic acid, maleic acid, 2-ethylacrylic acid, 2-propylacrylic acid, methylenesuccinic acid, and 3-phenyl-2-propenoic acid.

[0022] Preferably, the acrylamide monomer includes at least one of acrylamide, methacrylamide, N-isopropylacrylamide, and N-tert-butylacrylamide.

[0023] Preferably, the monomer C is at least one of dodecafluoroheptyl methacrylate, heptadecafluorodecyl methacrylate, hexafluorobutyl acrylate, hexafluorobutyl methacrylate, octafluoropentyl acrylate, octafluoropentyl methacrylate, dodecafluoroheptyl acrylate, tridecafluorooctyl acrylate, tridecafluorooctyl methacrylate, hexafluoroisopropyl acrylate, 2-(perfluorobutyl)ethyl acrylate, and trifluoroethyl methacrylate.

[0024] In a second aspect, the present invention provides a method for preparing a modified polypropylene binder, comprising:

[0025] Monomer A, monomer B, monomer C and N,N'-methylenebisacrylamide are polymerized to obtain a modified polypropylene adhesive.

[0026] Preferably, the polymerization reaction is carried out by solution polymerization or emulsion polymerization.

[0027] More preferably, when the polymerization reaction is carried out as solution polymerization, the preparation method of the modified polypropylene binder is: mixing monomer A, monomer B, monomer C, N,N'-methylenebisacrylamide, solvent and initiator, and reacting to obtain the modified polypropylene binder.

[0028] More preferably, the solvent is at least one of water, acetone, DMSO, DMF, and ethyl acetate.

[0029] In the present invention, the initiator used in the solution polymerization is a commonly used initiator in the art, including but not limited to at least one of ammonium persulfate (APS), potassium persulfate (KPS), azobisisobutyronitrile (AIBN), azobisisoheptanenitrile (ABIN), and dimethyl azobisisobutyrate (AIBA).

[0030] More preferably, the reaction temperature is 50-120°C.

[0031] More preferably, the reaction time is 4-12 hours.

[0032] More preferably, the ratio of the total amount of monomer A, monomer B, monomer C and N,N'-methylenebisacrylamide to the amount of the initiator is 1:(0.001-0.01).

[0033] More preferably, the ratio of the total mass of the monomer A, monomer B, monomer C and N,N'-methylenebisacrylamide to the volume of the solvent is 1 g:(1-9) mL.

[0034] More preferably, when the polymerization reaction is carried out by emulsion polymerization, the preparation method of the modified polypropylene binder is:

[0035] S1. Mixing an emulsifier, part of the water, monomer A, monomer C and N,N'-methylenebisacrylamide to obtain a pre-emulsion;

[0036] S2. Part of the pre-emulsion and part of the initiator are prepolymerized to obtain a prepolymer solution;

[0037] S3. Add the remaining initiator, monomer B, the remaining pre-emulsion and the remaining water to the prepolymer solution and continue the reaction to obtain a modified polypropylene binder.

[0038] In the present invention, the emulsifier used in the emulsion polymerization is a common emulsifier in the art, including but not limited to at least one of sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS).

[0039] In the present invention, the initiator used in the emulsion polymerization is a commonly used initiator in the art, including but not limited to at least one of ammonium persulfate (APS), potassium persulfate (KPS), azobisisobutyronitrile (AIBN), azobisisoheptanenitrile (ABIN), and dimethyl azobisisobutyrate (AIBA).

[0040] More preferably, in step S2, the temperature of the prepolymerization reaction is 50-120°C.

[0041] More preferably, in step S2, the prepolymerization reaction time is 0.2-2 hours.

[0042] More preferably, in step S3, the reaction temperature for continuing the reaction is 50-120°C.

[0043] More preferably, in step S3, the reaction time for continuing the reaction is 2-6 hours.

[0044] More preferably, the ratio of the total amount of monomer A, monomer B, monomer C and N,N'-methylenebisacrylamide to the amount of emulsifier is (monomer A + monomer B + monomer C + N,N'-methylenebisacrylamide):emulsifier=1:(0.02-0.06).

[0045] More preferably, the ratio of the total mass of the monomer A, monomer B, monomer C and N,N'-methylenebisacrylamide to the volume of water is 1 g:(1-4) mL.

[0046] More preferably, the mass ratio of the partial water to the remaining water is (0.5-9):1.

[0047] More preferably, the ratio of the total amount of monomer A, monomer B, monomer C and N,N'-methylenebisacrylamide to the amount of the initiator is 1:(0.001-0.01).

[0048] More preferably, the mass ratio of the part of the initiator to the remaining initiator is 1:(0.5-9).

[0049] More preferably, the mass ratio of the partial pre-emulsion to the remaining pre-emulsion is (1-5):(1-5).

[0050] In a third aspect, the present invention provides an application of a modified polypropylene binder in a negative electrode.

[0051] Preferably, the application in a negative electrode is an application in preparing a negative electrode.

[0052] Preferably, the negative electrode contains a silicon-based negative electrode material.

[0053] Preferably, the negative electrode is a negative electrode of a lithium ion battery.

[0054] Compared with the prior art, the present invention has the following beneficial effects:

[0055] The present invention uses N,N'-methylenebisacrylamide and monomers A, B, and C in a specific molar ratio when synthesizing a modified polypropylene binder to form a stable cross-linked network structure. This not only improves the strength, adhesion, and toughness of the modified polypropylene binder, but also increases the elongation at break of the polypropylene binder, which is beneficial for suppressing volume expansion of silicon-based negative electrode materials during lithium insertion cycles in lithium-ion batteries. It also enhances the ionic conductivity of the modified polypropylene binder, which is beneficial for maintaining the stability and kinetic performance of the silicon-based negative electrode materials during long-term charge and discharge cycles, and can improve the cycle capacity retention rate of lithium-ion batteries using the silicon-based negative electrode materials. DETAILED DESCRIPTION

[0056] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0057] The experimental methods in the following examples and comparative examples, where specific conditions are not specified, are generally based on conventional conditions in the art or conditions recommended by the manufacturers; the raw materials and reagents used, unless otherwise specified, are all raw materials and reagents that can be obtained from commercial channels such as conventional markets.

[0058] Example 1

[0059] This embodiment provides a modified polypropylene binder. When emulsion polymerization is used, the preparation method thereof comprises the following steps:

[0060] S1. Mix an emulsifier (sodium dodecyl sulfate, SDS) and some deionized water at 50°C and 300 rpm, then add monomer A (2-acetamidoacrylic acid), monomer C (dodecafluoroheptyl methacrylate), and N,N'-methylenebisacrylamide. Disperse at 10,000 rpm for 20 minutes to obtain a milky white pre-emulsion.

[0061] S2. Part of the pre-emulsion and part of the initiator (ammonium persulfate, APS) were added to the reactor, nitrogen was introduced, and a prepolymerization reaction was carried out at 75 ° C and 200 rpm for 30 min to obtain a prepolymer solution;

[0062] S3. At 75°C and 200 rpm, monomer B (N-tert-butyl acrylamide), the remaining pre-emulsion and the remaining deionized water were mixed for 5 minutes to obtain a mixed solution. The remaining initiator (ammonium persulfate, APS) and the mixed solution were added to the prepolymer solution and the reaction was continued at 75°C, 200 rpm and nitrogen for 4 hours. The mixed solution must be added dropwise within 4 hours of reaction. After the reaction is completed, the temperature is kept for 1 hour, and then the system is cooled to 50°C. The system is adjusted to pH = 7.5 with 1 mol / L NaOH solution and 1 mol / L HCl solution, and filtered with a 250-mesh filter to obtain a modified polypropylene binder;

[0063] The molar ratio of monomer A (2-acetamidoacrylic acid), monomer B (N-tert-butylacrylamide) and monomer C (dodecafluoroheptyl methacrylate) is 1:1.25:0.6;

[0064] The ratio of the total amount of monomer A (2-acetamidoacrylic acid), monomer B (N-tert-butylacrylamide) and monomer C (dodecafluoroheptyl methacrylate) to the amount of N,N'-methylenebisacrylamide is (monomer A+monomer B+monomer C):N,N'-methylenebisacrylamide=100:1;

[0065] The ratio of the total amount of monomer A (2-acetamidoacrylic acid), monomer B (N-tert-butylacrylamide), monomer C (dodecafluoroheptyl methacrylate) and N,N'-methylenebisacrylamide to the amount of emulsifier SDS is (monomer A+monomer B+monomer C+N,N'-methylenebisacrylamide):emulsifier=1:0.03;

[0066] The ratio of the total mass of the monomer A (2-acetamidoacrylic acid), monomer B (N-tert-butylacrylamide), monomer C (dodecafluoroheptyl methacrylate) and N,N'-methylenebisacrylamide to the volume of deionized water is 1 g:1 mL;

[0067] The ratio of the total amount of monomer A (2-acetamidoacrylic acid), monomer B (N-tert-butylacrylamide), monomer C (dodecafluoroheptyl methacrylate) and N,N'-methylenebisacrylamide to the amount of initiator APS is (monomer A+monomer B+monomer C+N,N'-methylenebisacrylamide):initiator=1:0.005;

[0068] The mass ratio of the partial deionized water to the remaining deionized water is 4:1;

[0069] The mass ratio of the part of the initiator (ammonium persulfate, APS) to the remaining initiator (ammonium persulfate, APS) is 1:4;

[0070] The mass ratio of the partial pre-emulsion to the remaining pre-emulsion is 1:1.

[0071] Examples 2-5 and Comparative Examples 1-5

[0072] Examples 2-5 and Comparative Examples 1-5 provide different modified polypropylene binders. The difference between them and Example 1 is that the molar ratios of monomer A (2-acetamidoacrylic acid), monomer B (N-tert-butylacrylamide), and monomer C (dodecafluoroheptyl methacrylate) are different. The rest are consistent with Example 1, as shown in the following table:

[0073] Table 1 Molar ratios of monomer A (2-acetamidoacrylic acid), monomer B (N-tert-butylacrylamide) and monomer C (dodecafluoroheptyl methacrylate) in Examples 1-5 and Comparative Examples 1-5

[0074]

[0075]

[0076] Examples 6-7 and Comparative Example 6

[0077] Examples 6-7 and Comparative Example 6 provide different modified polypropylene binders, which differ from Example 1 in that the ratio of the total amount of monomer A (2-acetamidoacrylic acid), monomer B (N-tert-butylacrylamide), and monomer C (dodecafluoroheptyl methacrylate) to the amount of N,N'-methylenebisacrylamide is different, that is, the specific ratio of (monomer A + monomer B + monomer C): N,N'-methylenebisacrylamide is different. The rest is consistent with Example 1, as shown in the following table:

[0078] Table 2 Ratio of the total amount of monomer A, monomer B and monomer C to the amount of N,N'-methylenebisacrylamide in Examples 1, 6-7 and Comparative Example 6

[0079] (Monomer A+Monomer B+Monomer C): N,N'-methylenebisacrylamide Example 1 100:1 Example 6 100:2 Example 7 100:0.1 Comparative Example 6 1:0

[0080] Examples 8-9

[0081] Examples 8-9 provide different modified polypropylene adhesives. The difference between them and Example 1 is that the type of monomer A is different. The rest is consistent with Example 1, as shown in the following table:

[0082] Table 3 Type of monomer A in Examples 1, 8-9

[0083] Type of monomer A Example 1 2-Acetamidoacrylic acid Example 8 2-Ethylpropenoic acid Example 9 acrylic acid

[0084] Examples 10-11

[0085] Examples 10-11 provide different modified polypropylene adhesives. The difference between them and Example 1 is that the type of monomer B is different. The rest is consistent with Example 1, as shown in the following table:

[0086] Table 4 Type of monomer B in Examples 1, 10-11

[0087] Type of monomer B Example 1 N-tert-Butylacrylamide Example 10 Methacrylamide Example 11 Acrylamide

[0088] Examples 12-13

[0089] Examples 12-13 provide different modified polypropylene adhesives. The difference between them and Example 1 is that the type of monomer C is different. The rest is consistent with Example 1, as shown in the following table:

[0090] Table 5 Type of monomer C in Examples 1, 12-13

[0091] Types of monomer C Example 1 Dodecafluoroheptyl methacrylate Example 12 Hexafluorobutyl acrylate Example 13 Hexafluorobutyl methacrylate

[0092] Example 14

[0093] This embodiment provides a modified polypropylene binder. When solution polymerization is used, the preparation method thereof comprises the following steps:

[0094] Monomer A (2-acetamidoacrylic acid), monomer B (N-tert-butylacrylamide), monomer C (dodecafluoroheptyl methacrylate) and N,N'-methylenebisacrylamide were dissolved in ethyl acetate, transferred to a reactor, and introduced with nitrogen for 30 minutes. An initiator (ammonium persulfate, APS) was added and the mixture was heated to 75°C for 8 hours. After the reaction stopped, deionized water of the same volume as the system was added, stirred and washed, and then filtered. This stirring, washing and filtering step was repeated three times. The mixture was dried at 60°C and ground to obtain a modified polypropylene binder.

[0095] The molar ratio of monomer A (2-acetamidoacrylic acid), monomer B (N-tert-butylacrylamide) and monomer C (dodecafluoroheptyl methacrylate) is 1:1.25:0.6;

[0096] The ratio of the total amount of monomer A (2-acetamidoacrylic acid), monomer B (N-tert-butylacrylamide) and monomer C (dodecafluoroheptyl methacrylate) to the amount of N,N'-methylenebisacrylamide is (monomer A+monomer B+monomer C):N,N'-methylenebisacrylamide=100:1;

[0097] The ratio of the total amount of monomer A (2-acetamidoacrylic acid), monomer B (N-tert-butylacrylamide), monomer C (dodecafluoroheptyl methacrylate) and N,N'-methylenebisacrylamide to the amount of initiator APS is (monomer A+monomer B+monomer C+N,N'-methylenebisacrylamide):initiator=1:0.005;

[0098] The total mass ratio of the monomer A (2-acetamidoacrylic acid), monomer B (N-tert-butylacrylamide), monomer C (dodecafluoroheptyl methacrylate) and N,N'-methylenebisacrylamide to the volume of the solvent (ethyl acetate) is 1 g:1 mL.

[0099] Performance Testing

[0100] The performance tests of the modified polypropylene adhesives of the embodiments and comparative examples were carried out as follows:

[0101] 1. Test of elongation at break and elastic modulus of modified polypropylene adhesive

[0102] (1) Preparation of film

[0103] The modified polypropylene binders prepared by emulsion polymerization in Examples 1-13 and Comparative Examples 1-6 were diluted with water to form modified polypropylene binder emulsions with a solid content (solid content) of 15 wt %;

[0104] The modified polypropylene binder prepared by solution polymerization in Example 14 was dissolved in N-methylpyrrolidone (NMP) and diluted to a modified polypropylene binder solution having a solid content (solid content) of 15 wt %;

[0105] At 25°C, the modified polypropylene adhesive emulsion or modified polypropylene adhesive solution was poured into a PTFE dumbbell-shaped mold to form a film with a length of 50 mm, a width of 20 mm, and a thickness of 0.02 mm;

[0106] (2) Under an environmental condition of 25° C., the film is stretched at a stretching speed of 20 mm / min using an electronic universal testing machine to measure the elongation at break (%) of the film prepared in step (1);

[0107] (3) Under an environmental condition of 25° C., the film was stretched at a stretching speed of 5 mm / min using an electronic universal testing machine to measure the elastic modulus (GPa) of the film prepared in step (1);

[0108] 2. Cycle capacity retention, electrode adhesion, 5C rate discharge capacity retention and cycle expansion test (1) Preparation of lithium-ion batteries

[0109] S1. Preparation of negative electrode sheet

[0110] When the modified polypropylene binder is prepared by emulsion polymerization, the preparation method of the negative electrode plate is as follows:

[0111] A1. The modified polypropylene binder prepared by emulsion polymerization in Examples 1-13 and Comparative Examples 1-6 was diluted with water to a solid content (solid content) of 15 wt % of a modified polypropylene binder emulsion;

[0112] A2. The negative electrode active material artificial graphite, the negative electrode active material silicon-based negative electrode material silicon carbon (SiC), the conductive agent carbon black, and the modified polypropylene binder were mixed in a mass ratio of 80:15:1:4, wherein the amount of the modified polypropylene binder was calculated according to the solid content of the modified polypropylene binder emulsion to obtain a mixture, and then deionized water was added at a ratio of 1g of the mixture: 1mL of water. After thorough stirring and mixing, the mixture was coated on the negative electrode current collector copper foil, dried, cold pressed, and slit to obtain a negative electrode sheet;

[0113] When the modified polypropylene binder is prepared by solution polymerization, the preparation method of the negative electrode plate is as follows:

[0114] B1. The modified polypropylene binder prepared by solution polymerization in Example 14 was dissolved in N-methylpyrrolidone (NMP) and diluted to a solid content (solid content) of 15wt% of a modified polypropylene binder solution;

[0115] B2. The negative electrode active material artificial graphite, the negative electrode active material silicon-based negative electrode material silicon carbon (SiC), the conductive agent carbon black, and the modified polypropylene binder were mixed in a mass ratio of 80:15:1:4, wherein the amount of the modified polypropylene binder was calculated according to the solid content of the modified polypropylene binder solution to obtain a mixture, and then NMP was added at a ratio of 1g mixture: 1mL NMP. After thorough stirring and mixing, the mixture was coated on the negative electrode current collector copper foil, dried, cold pressed, and slit to obtain a negative electrode sheet;

[0116] S2. Preparation of positive electrode sheet

[0117] The positive electrode active material lithium cobalt oxide, the conductive agent single-walled carbon nanotubes, and the binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 97:1:2, and then N-methylpyrrolidone (NMP) was added at a ratio of 1g lithium cobalt oxide: 0.25mL N-methylpyrrolidone. After thorough stirring and mixing, the mixture was coated on the positive electrode current collector aluminum foil, dried, cold pressed, and slit to obtain a positive electrode sheet;

[0118] S3. Preparation of isolation membrane

[0119] A polyethylene (PE) porous polymer film is used as a separator;

[0120] S4. Preparation of electrolyte

[0121] Ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), propyl propionate (PP), and vinylene carbonate (VC) are mixed in a mass ratio of 30:20:20:28:2 to obtain a non-aqueous organic solvent; lithium salt LiPF6 and the non-aqueous organic solvent are mixed in a mass ratio of 8:92 to obtain an electrolyte;

[0122] S5. Preparation of lithium-ion batteries

[0123] The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in order with the separator located between the positive and negative electrodes, and then wound to obtain a bare battery cell; the bare battery cell is placed in the aluminum-plastic film outer packaging of the battery shell for packaging, and then placed in an 85°C vacuum oven for baking for 48 hours; the electrolyte is injected into the dried battery, and the battery is packaged, allowed to stand, formed, shaped, and capacity divided, and then sealed again to obtain a lithium-ion battery.

[0124] (2) Cyclic capacity retention test

[0125] In an environment of 25°C, the lithium ion battery prepared in step (1) was charged to 4.4V at a constant current and constant voltage of 0.5C, with a cut-off current of 0.05C, and then discharged to 3.0V at a constant current of 1C. This cycle was repeated 800 times. The discharge capacity of the first cycle and the discharge capacity at the 800th cycle were recorded respectively. The experiment was repeated 5 times, and the average value was taken. The cycle capacity retention rate (%) was calculated according to the following formula:

[0126] Cycle capacity retention (%)=(average discharge capacity at the 800th cycle / average discharge capacity at the first cycle)×100%.

[0127] (3) Pole adhesion test

[0128] Cut the negative electrode into a 20mm wide and 180mm long electrode sample. Prepare a clean, dust-free, and uneven glass plate. Stick a 24mm wide and 100mm long 3M scotch double-sided tape on the glass plate. Align one end of the electrode sample with one end of the double-sided tape and stick it to ensure that the area where the electrode sample and the double-sided tape are bonded is 20mm wide and 100mm long. Use a roller to roll the bonding section of the electrode sample and the double-sided tape 4 times to make the electrode sample fit tightly to the double-sided tape and the glass plate. Use an electronic universal testing machine to perform a peel test on the electrode sample to measure the electrode adhesion of the electrode sample. Make sure that one end of the electrode sample is fixed on the glass plate facing up, and the other end of the glass plate is fixed with the lower clamp. The free end of the electrode sample (that is, the part of the electrode sample that is not attached to the double-sided tape) is folded upward 180° and fixed with the upper clamp. At 25°C, keep the lower clamp stationary and stretch the upper clamp upward at a tensile speed of 100 mm / min to separate the electrode sample from the double-sided tape, reduce the bonding section between the electrode sample and the double-sided tape, and take the average peel force in the process of shrinking the bonding section from 75 mm to 0 mm as the electrode bonding force of the electrode sample, in gf.

[0129] (4) 5C rate discharge capacity retention test

[0130] ① At 25°C, charge the lithium-ion battery to 4.5V at 0.5C, with a cut-off current of 0.05C. Leave it for 5 minutes and measure the discharge capacity of the lithium-ion battery at this time, which is recorded as D前 ;

[0131] ②Discharge the lithium-ion battery at 0.8C to 3V, leave it for 5 minutes, then charge it at 5C to 4.5V with a cut-off current of 0.05C and leave it for 5 minutes;

[0132] ③ After repeating step ② 300 times, measure the discharge capacity of the lithium-ion battery at this time, and record it as D 后 ; and calculate the 5C rate discharge capacity retention rate (%) according to the following formula:

[0133] 5C rate discharge capacity retention rate (%) = D 后 / D 前 ×100%.

[0134] The experimental results are shown in the following table:

[0135] Table 6 Performance test results of lithium-ion batteries of various embodiments and comparative examples

[0136]

[0137]

[0138] As can be seen from Table 6, the modified polypropylene binder of the present invention improves the elongation at break of the polypropylene binder and the cycle capacity retention rate of the lithium ion battery using the silicon-based negative electrode material; among them, the elongation at break of the embodiment of the present invention can reach more than 10%, the elastic modulus can reach more than 4 GPa, the cycle capacity retention rate can reach more than 80%, the electrode bonding force can reach more than 40 gf, and the 5C rate discharge capacity retention rate can reach more than 80%.

[0139] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A modified polypropylene adhesive, characterized in that: Including the following synthetic raw materials: Monomer A, monomer B, monomer C and N,N'-methylenebisacrylamide; The molar ratio of monomer A, monomer B and monomer C is 1:(0.5-3):(0.2-1); The monomer A is an acrylic monomer; the monomer B is an acrylamide monomer; The monomer C has a structure shown in formula (I): Wherein, R1 is H or CH3, R2 is C n H x F 2n+1-x , n≤12, x<2n+1.

2. The modified polypropylene adhesive according to claim 1, wherein The molar ratio of monomer A, monomer B and monomer C is 1:(1-3):(0.6-1).

3. The modified polypropylene adhesive according to claim 1, wherein The ratio of the total amount of monomer A, monomer B, and monomer C to the amount of N,N'-methylenebisacrylamide is (monomer A+monomer B+monomer C):N,N'-methylenebisacrylamide=100:(0.1-2).

4. The modified polypropylene adhesive according to claim 3, wherein: The ratio of the total amount of monomer A, monomer B, and monomer C to the amount of N,N'-methylenebisacrylamide is (monomer A+monomer B+monomer C):N,N'-methylenebisacrylamide=100:(1-2).

5. The modified polypropylene adhesive according to claim 1, wherein Include at least one of the following (1)-(3): (1) The acrylic monomer includes at least one of acrylic acid, methacrylic acid, 2-acetamidoacrylic acid, maleic acid, 2-ethylacrylic acid, 2-propylacrylic acid, methylenesuccinic acid, and 3-phenyl-2-propenoic acid; (2) The acrylamide monomer includes at least one of acrylamide, methacrylamide, N-isopropylacrylamide, and N-tert-butylacrylamide; (3) The monomer C is at least one of dodecafluoroheptyl methacrylate, heptadecafluorodecyl methacrylate, hexafluorobutyl acrylate, hexafluorobutyl methacrylate, octafluoropentyl acrylate, octafluoropentyl methacrylate, dodecafluoroheptyl acrylate, tridecafluorooctyl acrylate, tridecafluorooctyl methacrylate, hexafluoroisopropyl acrylate, 2-(perfluorobutyl)ethyl acrylate, and trifluoroethyl methacrylate.

6. A method for preparing the modified polypropylene adhesive according to any one of claims 1 to 5, characterized in that: include: Monomer A, monomer B, monomer C and N,N'-methylenebisacrylamide are polymerized to obtain a modified polypropylene adhesive.

7. The method for preparing the modified polypropylene adhesive according to claim 6, wherein: The polymerization reaction is carried out in the form of solution polymerization or emulsion polymerization.

8. The method for preparing the modified polypropylene adhesive according to claim 7, wherein: Including the following (1) or (2): (1) When the polymerization reaction is carried out in solution polymerization, the preparation method of the modified polypropylene binder is as follows: monomer A, monomer B, monomer C, N,N'-methylenebisacrylamide, a solvent and an initiator are mixed and reacted to obtain the modified polypropylene binder; (2) When the polymerization reaction is carried out by emulsion polymerization, the preparation method of the modified polypropylene binder is: S1. Mixing an emulsifier, part of the water, monomer A, monomer C and N,N'-methylenebisacrylamide to obtain a pre-emulsion; S2. Part of the pre-emulsion and part of the initiator are prepolymerized to obtain a prepolymer solution; S3. Add the remaining initiator, monomer B, the remaining pre-emulsion and the remaining water to the prepolymer solution and continue the reaction to obtain a modified polypropylene binder.

9. The method for preparing the modified polypropylene adhesive according to claim 8, wherein: Include at least one of the following (1)-(5): (1) The solvent is at least one of water, acetone, DMSO, DMF, and ethyl acetate; (2) The initiator includes at least one of ammonium persulfate, potassium persulfate, azobisisobutyronitrile, azobisisoheptanenitrile, and dimethyl azobisisobutyrate; (3) The emulsifier includes at least one of sodium lauryl sulfate and sodium dodecylbenzene sulfonate; (4) When the polymerization reaction is carried out as solution polymerization, the reaction temperature is 50-120°C; (5) When the polymerization reaction is carried out by emulsion polymerization, the temperature of the prepolymerization reaction is 50-120°C.

10. Use of the modified polypropylene binder according to any one of claims 1 to 5 in a negative electrode.

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