Modified polypropylene polymer material, binder, negative plate and battery

Abstract

The invention relates to a modified polypropylene polymer material, a binder, a negative plate and a battery. The structural formula of the modified polypropylene high polymer material is shown as a formula 1, in the formula 1, R is selected from one of carboxyl, hydroxyl, an ether bond-containing group and an aldehyde group, S is selected from one of C1-C20 alkylamino, cyano and C1-C20 alkyimino, T is selected from one of C1-C20 ester groups and C1-C20 alkenyl, m, n and k are independently selected from integers in the range of 0-100000, and m, n and k are not 0 at the same time. According to the scheme provided by the invention, the structural stability of the negative plate can be ensured, and the battery shows excellent normal-temperature cycle performance and low-temperature discharge performance.

Description

Technical Field

[0001] This application relates to the technical field of batteries, and particularly to modified polypropylene-based polymer materials, binders, negative electrode sheets, and batteries. Background Art

[0002] Lithium-ion batteries are widely used in fields such as 3C digital products, power tools, aerospace, energy storage, and electric vehicles due to their advantages of high specific energy, no memory effect, and long cycle life. The rapid development of electronic information technology and consumer products has put forward higher requirements for the high voltage and high energy density of lithium-ion batteries.

[0003] In related technologies, commercially available binders mainly use polyvinylidene fluoride (PVDF). However, the production cost of PVDF remains high, mainly due to its complex synthesis process. In terms of the impact on battery performance, as a binder, PVDF will form a certain microstructure during the electrode forming process. Its molecular structure characteristics make it easy to generate more pores inside the electrode. Although these pores contribute to the infiltration of the electrolyte to a certain extent, they also increase the tortuosity of the electron transport path, resulting in more obstacles to the electron transport inside the electrode, thereby increasing the internal resistance of the battery. In a low-temperature environment, the chemical reaction kinetics inside the battery is inhibited, and the migration rate of ions decreases significantly. The molecular chains of PVDF become more rigid at low temperatures, and its binding effect on active material particles may weaken, leading to a decrease in the stability of the electrode structure. In addition, the interaction between PVDF and the electrolyte also changes at low temperatures, making the wettability of the electrolyte on the electrode surface poor, further hindering the transport of lithium ions between the electrode and the electrolyte interface, and seriously affecting the charge and discharge performance of the battery in a low-temperature environment.

[0004] Generally speaking, it is urgent to develop a binder material with better performance, which has become a key problem to be solved in the battery field. Summary of the Invention

[0005] To solve or partially solve the problems existing in related technologies, this application provides a modified polypropylene-based polymer material, a binder, a negative electrode sheet, and a battery, which can ensure the structural stability of the negative electrode sheet and enable the battery to exhibit excellent room-temperature cycle performance and low-temperature discharge performance.

[0006] In the first aspect of this application, a modified polypropylene-based polymer material is provided, wherein the structural formula of the modified polypropylene-based polymer material is as follows: Formula 1 In Formula 1, R is selected from one of a carboxyl group, a hydroxyl group, an ether bond-containing group, and an aldehyde group; S is selected from one of an alkylamino group having 1 to 20 carbon atoms, a cyano group, and an alkylimino group having 1 to 20 carbon atoms; T is selected from one of an ester group having 1 to 20 carbon atoms and an alkenyl group having 2 to 20 carbon atoms; m, n, and k are each independently selected from integers in the range of 0 to 100000, and m, n, and k are not simultaneously 0.

[0007] The modified polypropylene-based polymer material as described above, wherein 5000 ≤ m ≤ 50000, 10000 ≤ n ≤ 30000, and 5000 ≤ k ≤ 20000.

[0008] The modified polypropylene-based polymer material as described above, wherein the number-average molecular weight of the modified polypropylene-based polymer material is 500000 to 2000000.

[0009] The modified polypropylene-based polymer material as described above, wherein the viscosity of a 6 wt% aqueous solution of the modified polypropylene-based polymer material is 3000 Pa·s to 25000 Pa·s; and / or, The tensile strength of the modified polypropylene-based polymer material is 10 MPa to 150 MPa; and / or, The elongation at break of the modified polypropylene-based polymer material is 0.5% to 30%.

[0010] The second aspect of the present application provides a method for preparing a modified polypropylene-based polymer material, which includes the following steps: Polymerize an acrylic acid monomer and at least one graft monomer under the action of an initiator to obtain a mixture; Terminate the reaction of the mixture under the action of a terminator to obtain the modified polypropylene-based polymer material; Wherein, each of the graft monomers contains an R group, an S group, or a T group.

[0011] The method for preparing a modified polypropylene-based polymer material as described above, wherein the reaction conditions of the polymerization reaction include: In an inert atmosphere, stir the reactants and heat at a preset temperature for a preset duration until a viscous mixture is obtained; preferably, the preset temperature is 55°C to 90°C, the preset duration is 3 h to 7 h, and the stirring rate is 200 rpm to 3000 rpm.

[0012] The method for preparing a modified polypropylene-based polymer material as described above, wherein the mass ratio of the acrylic acid monomer, the monomer including R, the monomer including S, and the monomer including T is (30 to 70):(1 to 30):(1 to 30):(1 to 30); and / or, The initiator includes at least one of benzoyl peroxide, lauroyl peroxide, azobisisobutyronitrile, azobisisoheptonitrile, cumene hydroperoxide, and tert-butyl hydroperoxide; preferably, the mass ratio of the initiator to the total mass of the acrylic monomer and the graft monomer is 0.005 to 0.03; and / or, The terminator includes at least one of hydroquinone, p-tert-butylcatechol, wood tar, sodium dimethyldithiocarbamate, sodium polysulfide, and sodium nitrite; preferably, the mass ratio of the terminator to the total mass of the acrylic monomer and the graft monomer is 0.005 to 0.03.

[0013] The third aspect of the present application provides an adhesive, which includes the modified polypropylene-based polymer material as described above or the modified polypropylene-based polymer material prepared by the preparation method as described above.

[0014] The fourth aspect of the present application provides a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector, and the negative electrode active material layer includes the adhesive as described above.

[0015] The fifth aspect of the present application provides a battery, which includes the negative electrode sheet as described above.

[0016] The technical solution provided by the present application may include the following beneficial effects: In the modified polypropylene-based polymer material, the R group is an oxygen-containing group, the S group includes a carbon-nitrogen bond, and the T group includes an ester group and an alkenyl group. The R group, S group, and T group can act synergistically, enabling hydrogen bonds to be formed between the modified polypropylene-based polymer material and the negative electrode active material, greatly enhancing the interaction between the modified polypropylene-based polymer material and the negative electrode active material. This enhanced adhesive force significantly improves the peel strength of the negative electrode active material layer on the surface of the negative electrode current collector, effectively ensuring the structural integrity of the negative electrode sheet during the charge and discharge cycles of the battery, and thus improving the cycle performance of the battery; and it can make the modified polypropylene-based polymer material have good affinity with lithium ions, effectively promoting the migration of lithium ions in the electrode material, increasing the migration rate of lithium ions in the battery, and thereby improving the normal temperature cycle performance and low-temperature discharge performance of the battery; at the same time, it also makes the modified polypropylene-based polymer material have good affinity with the electrolyte, significantly enhancing the wettability of the electrolyte on the electrode surface, improving the transport of lithium ions at the interface between the electrode and the electrolyte, and thus improving the cycle performance and low-temperature discharge performance of the battery.

[0017] It should be understood that the above general description and the following detailed description are only exemplary and explanatory, and cannot limit the present application. Detailed Embodiments

[0018] To make this application easy to understand, the following will describe this application in detail. However, before describing this application in detail, it should be understood that this application is not limited to the specific embodiments described. It should also be understood that the terms used herein are only for describing specific embodiments and do not represent limitations.

[0019] Where a numerical range is provided, it should be understood that each intermediate value between the upper and lower limits of the range and any other specified or intermediate value in the specified range is encompassed within this application. The upper and lower limits of these smaller ranges may be independently included in the smaller ranges and are also encompassed within this application, subject to any explicit exclusions in the specified range. Where the specified range includes one or both of the limits, ranges excluding either or both of the included limits are also included in this application.

[0020] Unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of this application, the preferred methods and materials are now described.

[0021] In the related art, commercial binders mainly use polyvinylidene fluoride (PVDF). As a binder, the molecular structure characteristics of PVDF make it easy to generate more pores inside the electrode. Although these pores contribute to the infiltration of the electrolyte to a certain extent, they also increase the tortuosity of the electron transport path and the internal resistance of the battery. In a low-temperature environment, the molecular chains of PVDF become more rigid at low temperatures, and its binding effect on the active material particles may weaken, resulting in a decrease in the stability of the electrode structure. In addition, the interaction between PVDF and the electrolyte also changes at low temperatures, making the wettability of the electrolyte on the electrode surface poor, seriously affecting the charge and discharge performance of the battery in a low-temperature environment.

[0022] In view of the above problems, an embodiment of this application provides a modified polypropylene-based polymer material, and the structural formula of the modified polypropylene-based polymer material is as follows: Formula 1 In Formula 1, R is selected from one of a carboxyl group, a hydroxyl group, an ether bond-containing group, and an aldehyde group, S is selected from one of an alkylamino group having 1 to 20 carbon atoms, a cyano group, and an alkyleneimine group having 1 to 20 carbon atoms, T is selected from one of an ester group having 1 to 20 carbon atoms and an alkenyl group having 2 to 20 carbon atoms, and m, n, and k are each independently selected from integers in the range of 0 to 100,000, and m, n, and k are not simultaneously 0.

[0023] The ether bond-containing group in the present application refers to a methoxy group (-OCH3) or an ether bond connected to other functional groups in Formula 1. The C1-C20 alkylamino group in the present application refers to an alkane group with 1-20 carbon atoms including an amino group, such as -CH2-NH2, -C2H4-NH2, -CH(NH2)-CH3, etc. The C1-C20 alkylimino group in the present application refers to an alkane group with 1-20 carbon atoms including an imino group, such as -CH=NH, -CH2-CH=NH, -CH2-C(=NH)-CH3, etc. The C1-C20 ester group in the present application refers to an ester group with 1-20 carbon atoms, such as -COO-CH3, -COO-C2H5, -CH2-COO-CH3, etc. The C2-C20 alkenyl group in the present application refers to an alkene group with 2-20 carbon atoms, such as -CH=CH2, -CH2CH=CH2, -CH=CH-CH3, etc. In the present application, m, n, and k are each independently selected from integers in the range of 0-100000, and m, n, and k are not simultaneously 0. For example, m, n, and k can each independently be selected as 0, 1, 2, 5, 10, 50, 100, 500, 1000, 5000, 10000, 50000, or 100000, etc., but m, n, and k are not simultaneously 0.

[0024] In the present application, there is no limitation on the arrangement order of the propylene groups connecting the R, S, and T groups in the modified polypropylene-based polymer material, as long as the structural formula 1 is satisfied.

[0025] According to the above solution provided by the present application, when the modified polypropylene-based polymer material is used as a binder in the negative electrode sheet, it can maintain the structural stability of the negative electrode sheet and at the same time improve the normal-temperature cycle performance and low-temperature discharge performance of the battery. This is because the R group in the modified polypropylene-based polymer material is an oxygen-containing group, the S group includes a carbon-nitrogen bond, and the T group includes an ester group and an alkenyl group. The R group, S group, and T group can act synergistically, enabling the modified polypropylene-based polymer material to form hydrogen bonds with the negative electrode active material, greatly enhancing the interaction between the modified polypropylene-based polymer material and the negative electrode active material. This enhanced bonding force significantly improves the peeling force of the negative electrode active material layer on the surface of the negative electrode current collector, effectively ensuring the structural integrity of the negative electrode sheet during the charge and discharge cycle of the battery, and thus improving the cycle performance of the battery; and it can make the modified polypropylene-based polymer material have good affinity with lithium ions, effectively promoting the migration of lithium ions in the electrode material, increasing the migration rate of lithium ions in the battery, thereby improving the normal-temperature cycle performance and low-temperature discharge performance of the battery; at the same time, it also makes the modified polypropylene-based polymer material have good affinity with the electrolyte, significantly enhancing the wettability of the electrolyte on the electrode surface, improving the transport of lithium ions at the interface between the electrode and the electrolyte, and thus improving the cycle performance and low-temperature discharge performance of the battery.

[0026] In a specific embodiment, 5000 ≤ m ≤ 50000, 10000 ≤ n ≤ 30000, 5000 ≤ k ≤ 20000. For example, m can be 5000, 10000, 15000, 20000, 25000, 30000, 35000, 40000, 45000, 50000, etc.; n can be 10000, 15000, 20000, 25000, 30000, etc.; k can be 5000, 10000, 15000, 20000, etc. When m, n, and k are within the above ranges, the molecular weight of the modified polypropylene-based polymer material is reasonable, which can improve the adhesiveness of the modified polypropylene-based polymer material, and at the same time avoid the viscosity of the modified polypropylene-based polymer material from being too high, which is not conducive to the application and processing of the modified polypropylene-based polymer material in the negative electrode active material layer, thereby improving the cycle performance and low-temperature discharge performance of the battery.

[0027] In a specific embodiment, the number-average molecular weight of the modified polypropylene-based polymer material is 500000 - 2000000. For example, the number-average molecular weight of the modified polypropylene-based polymer material can be 500000, 1000000, 1500000, 2000000, etc. When the number-average molecular weight of the modified polypropylene-based polymer material is within the above range, the viscosity of the modified polypropylene-based polymer material is reasonable, which can ensure the high processability and high adhesiveness of the modified polypropylene-based polymer material, thereby improving the cycle performance and low-temperature discharge performance of the battery.

[0028] Specifically, the molecular weight of the modified polypropylene-based polymer material of the present application is obtained by gel permeation chromatography (GPC) test.

[0029] In a specific embodiment, the viscosity of the aqueous solution of the modified polypropylene-based polymer material at 6 wt% is 3000 Pa·s to 25000 Pa·s. The aqueous solution of the modified polypropylene-based polymer material at 6 wt% in this application refers to a solution formed by dispersing the modified polypropylene-based polymer material in water, and the mass percentage content of this aqueous solution is 6 wt%. For example, the viscosity of the aqueous solution of the modified polypropylene-based polymer material at 6 wt% can be 3000 Pa·s, 4000 Pa·s, 5000 Pa·s, 6000 Pa·s, 7000 Pa·s, 8000 Pa·s, 9000 Pa·s, 10000 Pa·s, 11000 Pa·s, 12000 Pa·s, 13000 Pa·s, 14000 Pa·s, 15000 Pa·s, 16000 Pa·s, 17000 Pa·s, 18000 Pa·s, 19000 Pa·s, 20000 Pa·s, 21000 Pa·s, 22000 Pa·s, 23000 Pa·s, 24000 Pa·s or 25000 Pa·s, etc. When the viscosity of the aqueous solution of the modified polypropylene-based polymer material is within the above range, the modified polypropylene-based polymer material can interact sufficiently with the negative electrode active material, improving the structural stability of the negative electrode active material layer, avoiding problems such as pulverization or shedding of the negative electrode active material layer during the cycling process, thereby enhancing the cycling performance of the battery. At the same time, it can ensure the efficient transmission of lithium ions in the electrode active material layer and at the interface between the electrode and the electrolyte, further enhancing the cycling performance and low-temperature discharge performance of the battery.

[0030] Specifically, the viscosity of the aqueous solution of the modified polypropylene-based polymer material at 6 wt% in this application is measured by a viscometer to obtain the viscosity of the aqueous solution of the modified polypropylene-based polymer material at 6 wt%.

[0031] In a specific embodiment, the tensile strength of the modified polypropylene-based polymer material is 10 MPa to 150 MPa. For example, the tensile strength of the modified polypropylene-based polymer material can be 10 MPa, 20 MPa, 30 MPa, 40 MPa, 50 MPa, 60 MPa, 70 MPa, 80 MPa, 90 MPa, 100 MPa, 110 MPa, 120 MPa, 130 MPa, 140 MPa or 150 MPa, etc.

[0032] In a specific embodiment, the elongation at break of the modified polypropylene-based polymer material is 0.5% to 30%. For example, the elongation at break of the modified polypropylene-based polymer material can be 0.5%, 1%, 5%, 10%, 15%, 20%, 25% or 30%, etc.

[0033] When the tensile strength and elongation at break of the modified polypropylene-based polymer material are within the above ranges, the mechanical properties of the modified polypropylene-based polymer material are high. When the modified polypropylene-based polymer material is applied to the negative electrode active material layer, the modified polypropylene-based polymer material can provide a stronger bonding force for the negative electrode active material layer, making the combination between the negative electrode active materials and between the active material and the current collector closer, ensuring the integrity of the negative electrode active material layer, and at the same time ensuring the wettability between the electrode and the electrolyte, improving the migration rate of lithium ions, thereby taking into account the cycle performance and low-temperature discharge performance of the battery.

[0034] Specifically, the tensile strength and elongation at break of the modified polypropylene-based polymer material of the present application are tested based on a film of the modified polypropylene-based polymer material. The film of the modified polypropylene-based polymer material is prepared by the following steps: introducing a solution containing the modified polypropylene-based polymer material into a mold, drying it at 60 °C, and then cutting it into a rectangular film with a width of 2 cm. The film of the modified polypropylene-based polymer material is tested according to GB / T 6344-2008 to obtain the tensile strength and elongation at break of the modified polypropylene-based polymer material.

[0035] The second aspect of the present application provides a preparation method of a modified polypropylene-based polymer material, including the following steps: S1, polymerizing an acrylic acid monomer and at least one graft monomer under the action of an initiator to obtain a mixture; Among them, a single graft monomer contains an R group, an S group or a T group.

[0036] S2, terminating the reaction of the mixture under the action of a terminator to obtain a modified polypropylene-based polymer material.

[0037] Specifically, in S1, the acrylic acid monomer and the graft monomer are added to an aqueous solvent and stirred to form a uniform and stable monomer solution. Subsequently, an initiator is added to the monomer solution for polymerization reaction, and the solution gradually becomes viscous to obtain a viscous mixture.

[0038] The present application does not limit the mass ratio of the total amount of the acrylic acid monomer and the graft monomer to water, which can be selected according to actual needs. For example, the mass ratio of the total amount of the acrylic acid monomer and the graft monomer to water can be 1:3.

[0039] The graft monomers of the present application are selected according to the target structure of the modified polypropylene-based polymer materials. For example, when R is a carboxyl group, S is an amino group, and T is an ester group in the modified polypropylene-based polymer materials, vinylamine and vinyl acetate can be selected as the graft monomers; for example, when R is a hydroxyl group, S is an imino group, and T is an alkenyl group in the modified polypropylene-based polymer materials, vinyl alcohol, imide, butadiene or styrene can be selected as the graft monomers; for example, when R is an aldehyde group, S is a cyano group, and T is an ester group in the modified polypropylene-based polymer materials, acrolein, acrylonitrile and vinyl acetate can be selected as the graft monomers; for example, when R is an ether bond-containing group, S is a cyano group, and T is an ester group in the modified polypropylene-based polymer materials, dimethyl ether, acrylonitrile and vinyl acetate can be selected as the graft monomers.

[0040] The present application does not limit the parameters of the stirring treatment of the acrylic monomers and the graft monomers in water, as long as a uniform and stable monomer solution can be formed.

[0041] The parameters of the polymerization reaction of the present application can be selected according to actual needs. For example, the polymerization reaction can be carried out at 75 °C for 5 h.

[0042] In S2, a terminator is added to the mixture to terminate the polymerization reaction, and a modified polypropylene-based polymer material is obtained.

[0043] The present application does not limit the reaction parameters of the termination reaction, which can be selected according to actual needs. For example, after adding the terminator, heating can be stopped and stirring can be continued until the reaction solution cools down to room temperature, and then stirring can be stopped.

[0044] After the termination reaction, the pH of the reaction solution can be measured. If the pH of the reaction solution is acidic, 1 M sodium hydroxide solution can be added dropwise until the reaction solution is neutral.

[0045] After the termination reaction, the reaction solution can be slowly poured into anhydrous ethanol for purification, and through precipitation, suction filtration, washing, and vacuum drying, a white solid of the modified polypropylene-based polymer material can be obtained.

[0046] The present application enables the acrylic monomers and the graft monomers to carry out a polymerization reaction under the action of an initiator and a terminator. The R group, S group, and T group in the graft monomers are grafted onto the polypropylene main chain as branched chain groups, and a modified polypropylene-based polymer material with the structural formula shown in Formula 1 is prepared. When the modified polypropylene-based polymer material is applied to the negative electrode active material layer, the modified polypropylene-based polymer material can generate hydrogen bonds with the negative electrode active material, and at the same time form good affinity with lithium ions and the electrolyte, thereby being beneficial to improving the structural stability of the negative electrode sheet and enhancing the normal temperature cycle performance and low temperature discharge performance of the battery.

[0047] In a specific embodiment, the polymerization reaction is carried out in an inert atmosphere. Conducting the polymerization reaction in an inert atmosphere can effectively avoid the interference of oxygen in the air and prevent it from having a negative impact on the polymerization reaction, providing a stable and reliable reaction environment for the preparation of modified polypropylene-based polymer materials and ensuring the smooth progress of the preparation process.

[0048] In a specific embodiment, in an inert atmosphere, the reactants are stirred and heated at a preset temperature for a preset duration until a viscous mixture is obtained; preferably, the preset temperature is 55°C to 90°C, for example, the preset temperature can be 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C or 90°C, etc., the preset duration is 3h to 7h, for example, the preset duration can be 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h, 6.5h or 7h, etc., and the stirring rate is 200 rpm to 3000 rpm, for example, the stirring rate can be 200 rpm, 500 rpm, 1000 rpm, 1500 rpm, 2000 rpm, 2500 rpm or 3000 rpm, etc. When the reaction temperature, reaction time and stirring rate of the polymerization reaction are within the above ranges, the acrylic acid monomer and the graft monomer can undergo a sufficient polymerization reaction under the action of an initiator, thereby forming a modified polypropylene-based polymer material with the structural formula shown in Formula 1.

[0049] In a specific embodiment, the mass ratio of the acrylic acid monomer, the monomer including R, the monomer including S, and the monomer including T is (30 to 70):(1 to 30):(1 to 30):(1 to 30), for example, the mass ratio can be 30:1:1:1, 40:1:1:1, 50:1:1:1, 60:1:1:1, 70:1:1:1, 30:15:15:15, 50:15:15:15, 70:15:15:15, 30:30:30:30, 50:30:30:30, 70:30:30:30, etc. When the mass ratio of the acrylic acid monomer, the monomer including R, the monomer including S, and the monomer including T is within the above ranges, the acrylic acid monomer and the graft monomer can undergo a polymerization reaction, so that the contents of the R, S, and T groups in the structural formula of the prepared modified polypropylene-based polymer material satisfy the ranges of m, n, and k, thereby further improving the adhesion of the modified polypropylene-based polymer material and its affinity with lithium ions and electrolytes, and further improving the normal temperature cycling performance and low temperature discharge performance of the battery to a greater extent.

[0050] In a specific embodiment, the initiator includes at least one of benzoyl peroxide, lauroyl peroxide, azobisisobutyronitrile, azodiisooctanenitrile, cumene hydroperoxide, and tert-butyl hydroperoxide. The above-mentioned initiators have high activity and high stability, and can effectively initiate the polymerization reaction of acrylic monomers and graft monomers, thus ensuring the efficient preparation of modified polypropylene-based polymer materials.

[0051] In a preferred embodiment, the mass ratio of the initiator to the total mass of the acrylic monomer and the graft monomer is 0.005 to 0.03. For example, the mass ratio can be 0.005, 0.01, 0.015, 0.02, 0.025, or 0.03, etc. When the mass ratio of the initiator to the total amount of monomers is within the above range, the initiator can provide an appropriate amount of free radicals, enabling the polymerization reaction to proceed at a suitable rate, avoiding too slow or too violent reaction rates leading to explosive polymerization, and at the same time enabling the rate of free radical generation to reach equilibrium with monomer chain growth and chain termination reactions, controlling the modified polypropylene-based polymer material to reach the expected molecular weight, thereby obtaining a modified polypropylene-based polymer material with high adhesiveness and high affinity, and further enabling the battery to exhibit better cycle performance and low-temperature discharge performance.

[0052] In a specific embodiment, the terminator includes at least one of hydroquinone, p-tert-butylcatechol, wood tar, sodium dimethyldithiocarbamate, sodium polysulfide, and sodium nitrite. The above-mentioned terminator can quickly combine with free radicals in the polymerization reaction to terminate the growth of the polymerization chain. At the same time, the terminator has high stability and low cost, which is beneficial to the efficient preparation of modified polypropylene-based polymer materials.

[0053] In a preferred embodiment, the mass ratio of the terminator to the total mass of the acrylic monomer and the graft monomer is 0.005 to 0.03. For example, the mass ratio can be 0.005, 0.01, 0.015, 0.02, 0.025, or 0.03, etc. When the mass ratio of the terminator to the total amount of monomers is within the above range, the terminator can provide sufficient active ingredients to react fully with free radicals in the system, ensuring that the polymerization reaction stops quickly and completely, preventing the molecular weight of the modified polypropylene-based polymer material from becoming too large, the distribution from becoming wider, or other undesired side reactions from occurring due to the continued progress of the polymerization reaction, and also being able to avoid dangerous situations such as out-of-control temperature and pressure increase caused by excessive polymerization reaction, ensuring the safety of the preparation process.

[0054] In the third aspect of the present application, an adhesive is provided, which includes the above-mentioned modified polypropylene-based polymer material or the modified polypropylene-based polymer material prepared by the above-mentioned preparation method. When the modified polypropylene-based polymer material is used as an adhesive in the negative electrode active material layer, the adhesive has strong adhesiveness, which is beneficial to improving the structural stability of the negative electrode active material layer, avoiding problems such as pulverization or shedding of the negative electrode active material layer, and the adhesive has high affinity with lithium ions and electrolyte, thereby being able to improve the room temperature cycle performance and low temperature discharge performance of the battery.

[0055] In the fourth aspect of the present application, a negative electrode sheet is provided. The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector. The negative electrode active material layer includes the above-mentioned adhesive. The negative electrode sheet has high structural stability, making the room temperature cycle performance and low temperature discharge performance of the battery higher.

[0056] In a specific embodiment, the negative electrode active material includes at least one of natural graphite, artificial graphite, mesocarbon microbeads (abbreviated as MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, lithiated TiO2-Li4Ti5O with spinel structure 12 , Li-Al alloy, iron oxide, lithium titanate.

[0057] The negative electrode current collector in the embodiments of the present application is not particularly limited as long as the purpose of the present application can be achieved. For example, it can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam or composite current collector, etc.

[0058] The negative electrode active material layer in the embodiments of the present application further includes a dispersant and a conductive agent. The selection of the dispersant and the conductive agent in the negative electrode active material layer can be conventional materials in the art.

[0059] In the fifth aspect of the present application, a battery is provided, which includes the above-mentioned negative electrode sheet. The battery provided by the present application has excellent room temperature cycle performance and low temperature discharge performance.

[0060] In a specific embodiment, the battery further includes a positive electrode sheet. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on the surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes at least one of lithium cobaltate, lithium nickel cobalt manganate, lithium iron phosphate, and lithium-rich manganese-based material. When the positive electrode active material selects the above compounds, the positive electrode active material can fully exert its performance and improve the electrochemical performance of the lithium ion battery.

[0061] In the embodiments of the present application, there is no particular limitation on the type of the positive current collector, and it can be any material known to be suitable for use as a positive current collector. In one embodiment, the positive current collector includes metal materials such as aluminum, stainless steel, nickel plating, titanium, tantalum, etc., and carbon materials such as carbon cloth and carbon paper. Preferably, the positive current collector is a metal material.

[0062] In a specific embodiment, the positive active material layer further includes a conductive agent. The conductive agent may include, for example, carbon materials such as natural graphite, artificial graphite, acetylene black, needle coke, carbon nanotubes, graphene, vapor-grown carbon fiber (VGCF), etc.

[0063] In a specific embodiment, the battery of the present application further includes an electrolyte, which is an electrolyte well-known in the art that can be used in a battery and enables excellent electrochemical performance of the battery, including a lithium salt and an organic solvent, and can be specifically set according to needs.

[0064] In a specific embodiment, the battery further includes a separator. There is no particular limitation on the material and shape of the separator in the embodiments of the present application, as long as the effects of the present application are not significantly impaired. It may include substances in the form of porous sheets or non-woven fabrics with excellent liquid retention properties, etc. The materials of the resin or glass fiber separator include, but are not limited to, polyolefins, aromatic polyamides, polytetrafluoroethylene, polyethersulfone, etc., and can be specifically set according to needs.

[0065] In a specific embodiment, the battery may include an outer package, which can be used to encapsulate the above electrode assembly and electrolyte.

[0066] In a specific embodiment, the outer package of the battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery can also be a soft package, such as a pouch soft package. The material of the soft package can be plastic. As plastics, polypropylene, polybutylene terephthalate, and polybutylene succinate, etc. can be listed.

[0067] The present application has no particular limitation on the shape of the secondary battery, and it can be cylindrical, square, or any other arbitrary shape.

[0068] Hereinafter, the present application will be further described in detail through specific embodiments.

[0069] Example 1 Preparation of the Binder Step 1: Stir and mix acrylic acid monomer, vinylamine monomer, vinyl acetate monomer with deionized water to obtain a monomer solution. Add the initiator cumene hydroperoxide to the monomer solution, and then introduce nitrogen into the reaction solution. Perform a polymerization reaction under a nitrogen atmosphere to obtain a mixture.

[0070] Among them, the mass ratio of acrylic acid monomer, vinylamine monomer, and vinyl acetate monomer is 7:2:1, and the ratio of the total mass of acrylic acid monomer, vinylamine monomer, and vinyl acetate monomer to the mass of deionized water is 1:3. The mass ratio of cumene hydroperoxide to the total mass of acrylic acid monomer, vinylamine monomer, and vinyl acetate monomer is 0.01. The polymerization reaction is carried out under stirring conditions. The reaction temperature of the polymerization reaction is 75 °C, the reaction time is 5 h, and the stirring rate is 1000 rpm.

[0071] Step 2: Add sodium dimethyldithiocarbamate as a terminator to the mixture to terminate the reaction. Keep stirring until the reaction solution cools down to room temperature, and then stop stirring. Subsequently, pour the reaction solution into absolute ethanol, stir, precipitate, filter, wash, and dry under vacuum to obtain a solid modified polypropylene-based polymer material.

[0072] Among them, the mass ratio of sodium dimethyldithiocarbamate to the total mass of acrylic acid monomer, vinylamine monomer, and vinyl acetate monomer is 0.01.

[0073] Examples 2 to 31, Comparative Examples 1 to 4 are mainly different from Example 1 in the parameters of the preparation process. Refer to Table 1.

[0074] Table 1

[0075] Test Example 1 Carry out comprehensive tests of infrared spectroscopy, liquid chromatography, and nuclear magnetic resonance on the modified polypropylene-based polymer material to obtain the structural formula of the modified polypropylene-based polymer material. The test results are shown in Table 2.

[0076] Test the modified polypropylene-based polymer material by gel permeation chromatography (GPC) to obtain the molecular weight of the modified polypropylene-based polymer material. The test results are shown in Table 2.

[0077] Disperse the modified polypropylene-based polymer material in water to form an aqueous solution with a mass percentage content of 6 wt%. Test this aqueous solution with a viscometer to obtain the viscosity of the 6 wt% aqueous solution of the modified polypropylene-based polymer material. The test results are shown in Table 2.

[0078] The modified polypropylene-based polymer material is made into a modified polypropylene-based polymer material film through the following steps: The solution containing the modified polypropylene-based polymer material is introduced into a mold, dried at 60 °C, and then cut into rectangular films with a width of 2 cm. The modified polypropylene-based polymer material films are tested according to GB / T 6344-2008 to obtain the tensile strength and elongation at break of the modified polypropylene-based polymer material. The test results are shown in Table 2.

[0079] Table 2

[0080] Test Example 2 The modified polypropylene-based polymer material is applied to the negative electrode sheets to prepare the batteries of Examples 1 to 31 and Comparative Examples 1 to 5, including the following steps: 1. Preparation of the negative electrode sheet The negative electrode active material (90% graphite + 10% silicon-carbon material), the dispersant sodium carboxymethyl cellulose (CMC), and the above-mentioned modified polypropylene-based polymer material binder are stirred and mixed at a mass ratio of 98:1:1 for 15 min. Subsequently, deionized water is added to adjust the solid content to 55%, and then secondary dispersion is carried out for 60 min to obtain the negative electrode slurry. The negative electrode slurry is uniformly coated on one surface of a negative electrode current collector copper foil with a thickness of 6 μm and dried at 110 °C to obtain a negative electrode sheet with a negative electrode material layer coated on one side and a coating thickness of 70 μm. The above steps are repeated on the other surface of the negative electrode sheet to obtain a negative electrode sheet with negative electrode active material layers coated on both sides.

[0081] 2. Preparation of the positive electrode sheet The positive electrode active material lithium cobaltate, the conductive agent acetylene black, and the binder polyvinylidene fluoride PVDF are mixed evenly at a mass ratio of 97.3:1.5:1.2, and fully stirred and mixed in an N-methylpyrrolidone solvent to prepare a slurry with a solid content of 75%, and stirred evenly to obtain the positive electrode slurry. The positive electrode slurry is uniformly coated on one surface of an aluminum foil with a thickness of 12 μm and dried at 90 °C. After cold pressing, a positive electrode sheet with a positive electrode active material layer thickness of 100 μm is obtained. Then, the above steps are repeated on the other surface of the positive electrode sheet to obtain a positive electrode sheet with positive electrode active material layers coated on both sides.

[0082] 3. Preparation of the electrolyte In an environment with a water content of less than 10 ppm, propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) are mixed in a mass ratio of 1:3:6. Based on the total mass of the electrolyte, 8% of butyl butyrate is added. Then, lithium hexafluorophosphate (LiPF6) is added to the solvent, dissolved and mixed evenly, and then fluoroethylene carbonate (FEC) is added to obtain the electrolyte. Among them, the molar concentration of LiPF6 in the electrolyte is 1.15 mol / L, and the mass concentration of FEC in the electrolyte is 10.1%.

[0083] 4. Fabrication of Lithium-Ion Batteries A polyethylene porous polymer film is used as the separator. The positive electrode sheet, the separator, and the negative electrode sheet are stacked in sequence, with the separator in the middle of the positive and negative electrodes to play a separating role. Then, the stacked electrode sheets and the separator are wound to obtain an electrode assembly. The electrode assembly is placed in a formed aluminum-plastic film shell, dehydrated at 80 °C, injected with the prepared electrolyte, and then subjected to processes such as vacuum packaging, standing, formation, and shaping to obtain a lithium-ion battery.

[0084] Comparative Example 5 The preparation method of the battery in this comparative example is substantially the same as that in Example 1, except that polyvinylidene fluoride (PVDF) is used as the binder in the negative electrode sheet.

[0085] Test Example 3 The batteries prepared in the above-mentioned examples and comparative examples are tested for the following items.

[0086] Adhesion Test: 3M double-sided tape is attached to a steel plate with a thickness of 1 mm and a width of 3 cm. Subsequently, the negative electrode sheet is cut into a width of 2 cm and pasted onto the 3M double-sided tape, and then a 180° peel test is carried out using a high-speed tensile machine to obtain the adhesion value.

[0087] Battery Performance Test: 1. Room Temperature Cycle Performance Test In a constant temperature oven at (25 ± 2) °C, the lithium-ion battery is charged at a constant current and constant voltage of 0.2C to 4.3V, charged at a constant voltage to 0.05C, left standing for 5 minutes, and then discharged at 0.2C to 3V. The capacity obtained by this step is the initial capacity, and a cycle test is carried out with 1C charge / 1C discharge. When the cycle capacity drops to 80% of the initial capacity, the cycle life is recorded.

[0088] For each group of 5 batteries, the calculated cycle life is averaged and recorded in Table 3.

[0089] 2. Low-temperature discharge test at 0°C: The battery is fully charged at 1C at 25°C, with a cut-off current of 0.05C, then left standing for 10 min. It is discharged at 0.5C to 3V at 25°C, and the charge-discharge cycle is repeated for 3 weeks. Record the discharge capacity at 0.5C in the third week as the initial capacity. The battery is fully charged at 1C at 25°C, with a cut-off current of 0.05C. Leave it standing at 0°C for 4 h, and then discharge it at 0.5C to 3.0V. Record the discharge capacity. Calculate the ratio of the discharge capacity to the initial capacity to obtain the capacity retention rate.

[0090] For each group of 5 batteries, the average value of the calculated low-temperature discharge capacity retention rate is recorded in Table 3.

[0091] Table 3

[0092] As can be seen from Table 3, For Examples 1 to 5, when m is in the range of 0 to 100,000, the prepared modified polypropylene-based polymer material has high adhesiveness, which can enable the battery to exhibit long cycle life and high and low-temperature discharge performance. Further, when m is in the range of 5,000 to 50,000, the adhesiveness of the modified polypropylene-based polymer material is higher, and the cycle life and low-temperature discharge performance of the battery are higher; For Examples 1, 6 to 9, when n is in the range of 0 to 100,000, the modified polypropylene-based polymer material has high adhesiveness. When applied to the battery, it can enable the battery to exhibit long cycle life and high and low-temperature discharge performance. Further, when n is in the range of 10,000 to 30,000, the adhesiveness of the modified polypropylene-based polymer material is higher, and at the same time, the cycle life and low-temperature discharge performance of the battery are better; For Examples 1, 10 to 13, when k is in the range of 0 to 100,000, the modified polypropylene-based polymer material has high adhesiveness. When applied to the battery, it can enable the battery to exhibit long cycle life and high and low-temperature discharge performance. Further, when k is in the range of 5,000 to 20,000, the adhesiveness of the modified polypropylene-based polymer material is higher, and at the same time, the cycle life and low-temperature discharge performance of the battery are better; For Examples 14 to 20, when R is selected from carboxyl group, hydroxyl group, ether bond-containing group and aldehyde group, S is selected from amino group, imino group, cyano group, and T is selected from ester group and alkenyl group, the modified polypropylene-based polymer material can maintain high adhesiveness. When this modified polypropylene-based polymer material is applied to the battery, it can enable the battery to exhibit excellent cycle life and low-temperature discharge performance; From the comparison of Example 14, 21, 22 and Comparative Example 1, it can be seen that when the reaction temperature, reaction time and stirring rate of the polymerization reaction are within a suitable range, the viscosity of the prepared modified polypropylene-based polymer material is better, and at the same time, the cycle performance and low-temperature discharge performance of the battery can be ensured. From the comparison of Example 14, 23, 24 and Comparative Example 2, it can be seen that when the number-average molecular weight of the modified polypropylene-based polymer material is within a reasonable range, the viscosity of the aqueous solution of the modified polypropylene-based polymer material is better, the final adhesion is higher, and the cycle performance and low-temperature discharge performance of the battery are better.

[0093] From the comparison of Example 14, 25, 26 and Comparative Example 3, it can be seen that when the usage amount of the initiator is within a reasonable range, a modified polypropylene-based polymer material can be prepared, and the prepared modified polypropylene-based polymer material has high adhesiveness, which can make the cycle performance and low-temperature discharge performance of the battery better.

[0094] From the comparison of Example 14, 27, 28 and Comparative Example 4, it can be seen that when the usage amount of the terminator is within a reasonable range, a modified polypropylene-based polymer material can be prepared, and the prepared modified polypropylene-based polymer material has high adhesiveness, which can make the cycle performance and low-temperature discharge performance of the battery better.

[0095] From the comparison of Example 14 and 29, it can be seen that when cumene hydroperoxide and azobisisobutyronitrile are selected as the initiator, the preparation of the modified polypropylene-based polymer material can be ensured, and the adhesiveness of the modified polypropylene-based polymer material and the cycle performance and low-temperature discharge performance of the battery can be improved.

[0096] From the comparison of Example 14 and 30, it can be seen that when sodium dimethyldithiocarbamate and hydroquinone are selected as the terminator, the modified polypropylene-based polymer material can be successfully prepared, thereby ensuring the adhesiveness of the modified polypropylene-based polymer material and the cycle performance and low-temperature discharge performance of the battery.

[0097] From the comparison of Example 14 and 31, it can be seen that when the R, S, and T groups exist simultaneously, the structure of the modified polypropylene-based polymer material is better, thereby improving the adhesiveness of the modified polypropylene-based polymer material and the cycle performance and low-temperature discharge performance of the battery.

[0098] From the comparison of Examples 1 to 31 and Comparative Example 5, it can be seen that the prepared modified polypropylene-based polymer material can be better applied to the battery compared with PVDF, improving the cycle performance and low-temperature discharge performance of the battery.

Claims

1. A modified polypropylene-based polymer material, characterized in that, The structural formula of the modified polypropylene-based polymer material is as follows: Formula 1 In Formula 1, R is selected from one of a carboxyl group, a hydroxyl group, an ether bond-containing group, and an aldehyde group; S is selected from one of an alkylamino group having 1 to 20 carbon atoms, a cyano group, and an alkylimino group having 1 to 20 carbon atoms; T is selected from one of an ester group having 1 to 20 carbon atoms and an alkenyl group having 2 to 20 carbon atoms; m, n, and k are each independently selected from integers in the range of 0 to 100,000, and m, n, and k are not simultaneously 0.

2. The modified polypropylene-based polymer material according to claim 1, wherein 5000 ≤ m ≤ 50000, 10000 ≤ n ≤ 30000, 5000 ≤ k ≤ 20000.

3. The modified polypropylene-based polymer material according to claim 1, characterized in that, The number-average molecular weight of the modified polypropylene-based polymer material is 500,000 to 2,000,000.

4. The modified polypropylene-based polymer material according to claim 1, wherein, The viscosity of a 6 wt% aqueous solution of the modified polypropylene-based polymer material is 3000 Pa·s to 25000 Pa·s; and / or, The tensile strength of the modified polypropylene-based polymer material is 10 MPa to 150 MPa; and / or, The elongation at break of the modified polypropylene-based polymer material is 0.5% to 30%.

5. A method for preparing the modified polypropylene-based polymer material according to any one of claims 1 to 4, characterized in that, Comprising the following steps: Polymerizing an acrylic acid monomer and at least one graft monomer under the action of an initiator to obtain a mixture; Terminating the reaction of the mixture under the action of a terminator to obtain the modified polypropylene-based polymer material; Wherein, each of the graft monomers contains an R group, an S group, or a T group.

6. The preparation method of the modified polypropylene-based polymer material according to claim 5, wherein The reaction conditions of the polymerization reaction include: In an inert atmosphere, stirring the reactants and heating at a preset temperature for a preset time until a viscous mixture is obtained; preferably, the preset temperature is 55 °C to 90 °C, the preset time is 3 h to 7 h, and the stirring rate is 200 rpm to 3000 rpm.

7. The preparation method of the modified polypropylene-based polymer material according to claim 5, characterized in that, The mass ratio of the acrylic acid monomer, the monomer including R, the monomer including S, and the monomer including T is (30 to 70):(1 to 30):(1 to 30):(1 to 30); and / or, The initiator includes at least one of benzoyl peroxide, lauroyl peroxide, azobisisobutyronitrile, azobisisoheptonitrile, cumene hydroperoxide, and tert-butyl hydroperoxide; preferably, the mass ratio of the initiator to the total mass of the acrylic acid monomer and the graft monomer is 0.005 to 0.03; and / or, The terminator includes at least one of hydroquinone, p-tert-butylcatechol, wood tar, sodium dimethyldithiocarbamate, sodium polysulfide, and sodium nitrite; preferably, the mass ratio of the terminator to the total mass of the acrylic acid monomer and the graft monomer is 0.005 to 0.

03.

8. An adhesive, characterized in that, Comprising the modified polypropylene-based polymer material according to any one of claims 1 to 4 or the modified polypropylene-based polymer material prepared by the preparation method according to any one of claims 5 to 7.

9. A negative electrode sheet, characterized in that, Comprising a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer includes the binder according to claim 8.

10. A battery, characterized in that, Comprising the negative electrode sheet according to claim 9.

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