Binder, negative plate and lithium ion battery

Abstract

The invention provides a binder, a negative plate and a lithium ion battery, the binder comprises a polymer, the polymer comprises a first structural unit, a second structural unit and a third structural unit; the first structural unit comprises an acrylate structural unit of which the main chain carbon atom number is not less than 10, the second structural unit comprises a bifunctional acrylate structural unit containing at least one polar functional group, and the third structural unit comprises an acrylate structural unit containing at least two polar functional groups; and the glass transition temperature of the binder is 55-70 DEG C. The binder not only can effectively improve the problems of hard carbon particle abrasion and collision caused by volume expansion, but also can improve the diffusion rate of lithium ions under high-rate charge and discharge, and makes up for the defect of low conductivity of hard carbon; and meanwhile, the binding power of the electrode material and the current collector can be enhanced, so that the rate capability and the cycle performance of the battery are effectively improved.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion batteries and relates to a binder, in particular to a binder, a negative electrode sheet and a lithium ion battery. Background Art

[0002] Lithium-ion batteries have the advantages of high energy density, environmental protection, and no memory effect, and have been widely used in various fields, such as aerospace, new energy, electric vehicles, portable electronic devices, etc. Binders, as key materials in battery manufacturing, directly affect the performance and stability of batteries. Therefore, in the manufacture of lithium-ion batteries, it is very important to choose a suitable binder.

[0003] At present, commonly used binders include styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), etc., but they all have problems such as shear resistance, easy demulsification, floating, and low mechanical strength. In addition, during the cyclic charge and discharge process of lithium batteries, the electrode materials may undergo drastic changes. If the mechanical strength of the binder is insufficient, it may cause the electrode materials to peel off or separate, affecting the cycle stability of the battery. At present, hard carbon is a commonly used negative electrode active material, but it also has some disadvantages in its application. On the one hand, as lithium ions are embedded and removed during the charge and discharge process, the hard carbon material will expand in volume, and this volume expansion will cause collision and wear between hard carbon particles, causing the active sites of the hard carbon particles to be inactivated, making the cycle life of the battery worse; on the other hand, the conductivity of hard carbon is low, so the problem of limited lithium ion diffusion rate may occur during high-rate charge and discharge, thereby affecting the rate performance of the battery. Summary of the invention

[0004] In view of the above-mentioned defects, the present invention provides a binder, which can not only effectively improve the wear and collision problems of hard carbon particles caused by volume expansion, but also increase the diffusion rate of lithium ions under high-rate charge and discharge, and make up for the defect of low electrical conductivity of hard carbon; at the same time, it can also enhance the peeling strength between the electrode material and the current collector, thereby effectively improving the rate performance and cycle performance of the battery.

[0005] The present invention also provides a negative electrode sheet. Since the negative electrode sheet includes the above-mentioned binder, the negative electrode sheet is applied to a lithium ion battery to effectively improve the rate performance and cycle performance of the battery.

[0006] The present invention also provides a lithium ion battery. Since the lithium ion battery includes the above-mentioned binder or the above-mentioned negative electrode sheet, the battery has higher rate performance and cycle stability.

[0007] In the first aspect of the present invention, an adhesive is provided. The adhesive comprises a polymer, and the polymer comprises a first structural unit, a second structural unit, and a third structural unit; the first structural unit comprises an acrylate structural unit with at least 10 carbon atoms in the main chain, the second structural unit comprises a bifunctional acrylate structural unit containing at least one polar functional group, and the third structural unit comprises an acrylate structural unit containing at least two polar functional groups; the glass transition temperature of the adhesive is 55°C to 70°C.

[0008] The adhesive as described above, wherein the weight-average molecular weight of the adhesive is 2×10 5 g / mol to 5×10 5 g / mol.

[0009] The adhesive as described above, wherein, based on the total mass of the polymer, the mass percentages of the first structural unit, the second structural unit, and the third structural unit are (22% to 36%):(12% to 28%):(36% to 60%).

[0010] The adhesive as described above, wherein the polar functional group in the bifunctional acrylate structural unit containing at least one polar functional group comprises at least one of a hydroxyl group, an epoxy group, an amino group, and an isocyanate group; and / or, the polar functional group in the acrylate structural unit containing at least two polar functional groups comprises at least one of a hydroxyl group, a carboxyl group, an ether group, and a sulfonic acid group.

[0011] The adhesive as described above, wherein the acrylate structural unit with at least 10 carbon atoms in the main chain comprises at least one of a nonyl acrylate structural unit, a decyl acrylate structural unit, an undecyl acrylate structural unit, a dodecyl acrylate structural unit, and a tridecyl acrylate structural unit;

[0012] and / or, the bifunctional acrylate structural unit containing at least one polar functional group comprises at least one of a tricyclodecane dimethanol diacrylate structural unit, a bisphenol A dimethacrylate structural unit, an ethoxylated bisphenol A dimethacrylate structural unit, a bisphenol F dimethacrylate structural unit, a pentaerythritol tetraacrylate structural unit, and an isophorone dimethacrylate structural unit;

[0013] and / or, the acrylate structural unit containing at least two polar functional groups comprises at least one of an N-methylolacrylamide structural unit, a 2-hydroxyethyl acrylamide structural unit, a 2,3-dihydroxypropyl acrylate structural unit, a 2-acryloyloxyethyl-2'-hydroxyethyl methacrylamide structural unit, a 3-hydroxypropyl methacrylate structural unit, and a 2-hydroxypropyl acrylate structural unit.

[0014] In a second aspect of the present invention, a negative electrode sheet is provided, and the negative electrode sheet includes the binder of the first aspect.

[0015] The negative electrode sheet as described above, wherein the negative electrode sheet further includes a negative electrode active material, and the negative electrode active material includes hard carbon.

[0016] The negative electrode sheet as described above, wherein the median particle size of the hard carbon is 5 μm to 30 μm.

[0017] The negative electrode sheet as described above, wherein the negative electrode sheet further includes a conductive agent; the mass ratio of the binder, the hard carbon, and the conductive agent is (0.8 - 3):(50 - 100):(1 - 3).

[0018] In a third aspect of the present invention, a lithium ion battery is provided, and the lithium ion battery includes the binder of the first aspect or the negative electrode sheet of the second aspect.

[0019] In the present invention, by making the binder include a polymer main chain (i.e., the first structural unit, including an acrylate structural unit with at least 10 carbon atoms in the main chain) and side chains grafted to the polymer main chain, the side chains include a second structural unit and a third structural unit, wherein the second structural unit includes a difunctional acrylate structural unit containing at least one polar functional group, and the third structural unit includes an acrylate structural unit containing at least two polar functional groups, and controlling the glass transition temperature of the binder to be 55 °C to 70 °C, not only can the mutual collision and wear between hard carbon particles be reduced, the loss of the electrode be decreased, but also the electrical conductivity of the electrode can be improved, compensating for the problem of limited lithium ion diffusion rate caused by the low electrical conductivity of hard carbon; at the same time, the peeling strength between the binder and the current collector can be enhanced, thereby comprehensively improving the rate performance and cycle performance of the battery. Detailed Embodiments

[0020] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0021] In the first aspect of the present invention, an adhesive is provided. The adhesive comprises a polymer, and the polymer comprises a first structural unit, a second structural unit, and a third structural unit; the first structural unit comprises an acrylate structural unit with at least 10 carbon atoms in the main chain, the second structural unit comprises a bifunctional acrylate structural unit containing at least one polar functional group, and the third structural unit comprises an acrylate structural unit containing at least two polar functional groups; the glass transition temperature of the adhesive is 55°C to 70°C.

[0022] Exemplarily, the glass transition temperature of the adhesive can be 55°C, 56°C, 57°C, 58°C, 59°C, 60°C, 61°C, 62°C, 63°C, 64°C, 65°C, 66°C, 67°C, 68°C, 69°C, 70°C or a range composed of any two of these values.

[0023] The "bifunctional acrylate structural unit containing at least one polar functional group" in the present invention refers to a structural unit containing at least one acrylate functional group and at least one polar functional group at the same time.

[0024] The "acrylate structural unit containing at least two polar functional groups" in the present invention refers to a structural unit containing one acrylate functional group and at least two polar functional groups at the same time.

[0025] It should be noted that the "bifunctional acrylate structural unit containing at least one polar functional group" and the "acrylate structural unit containing at least two polar functional groups" in the present invention may be the same structural unit.

[0026] Specifically, the first structural unit (including the acrylate structural unit with at least 10 carbon atoms in the main chain) in the above polymer serves as the main chain of the polymer, and the second structural unit (including the bifunctional acrylate structural unit containing at least one polar functional group) and the third structural unit (including the acrylate structural unit containing at least two polar functional groups) both serve as the side chains of the polymer.

[0027] Among them, the acrylate structural unit with at least 10 carbon atoms in the main chain is obtained by free radical polymerization of acrylate monomers with at least 10 carbon atoms in the main chain under the action of an initiator. Similarly, the bifunctional acrylate structural unit containing at least one polar functional group is obtained by free radical polymerization of bifunctional acrylate monomers containing at least one polar functional group under the action of an initiator, and the acrylate structural unit containing at least two polar functional groups is obtained by free radical polymerization of acrylate monomers containing at least two polar functional groups under the action of an initiator.

[0028] The "main chain" in the present invention refers to the chain with the largest number of carbon atoms including functional groups. Preferably, the main chain of the acrylate structural unit with no less than 10 carbon atoms in the main chain has 10 to 20 carbon atoms; similarly, the main chain of the acrylate monomer with no less than 10 carbon atoms in the main chain has 10 to 20 carbon atoms. Exemplarily, the number of carbon atoms in the main chain can be 10, 12, 14, 16, 18, 20, etc.

[0029] The present invention does not specifically limit the sources of acrylate monomers with no less than 10 carbon atoms in the main chain, difunctional acrylate monomers containing at least one polar functional group, and acrylate monomers containing at least two polar functional groups. Products commercially available to those skilled in the art or products prepared by conventional preparation methods can be used.

[0030] The present invention does not specifically limit the initiator in the above radical polymerization reaction. It can be a commonly used initiator in the art. For example, the initiator includes at least one of ammonium persulfate, potassium persulfate, sodium persulfate, ammonium sulfate, trimethylamine, benzoyl peroxide, and tert-butyl peroxide.

[0031] The present invention does not specifically limit the source of the initiator. Products commercially available to those skilled in the art or products prepared by conventional preparation methods can be used.

[0032] Specifically, during the preparation process, the glass transition temperature of the binder can be further controlled by controlling the glass transition temperature of the acrylate monomer with no less than 10 carbon atoms in the main chain and the difunctional acrylate monomer containing at least one polar functional group, or the mass ratio of the acrylate monomer with no less than 10 carbon atoms in the main chain, the difunctional acrylate monomer containing at least one polar functional group, and the acrylate monomer containing at least two polar functional groups, so that the glass transition temperature of the binder is within 55°C to 70°C.

[0033] The "glass transition temperature" in the present invention refers to the temperature corresponding to the transition from the glassy state to the high elastic state, which can be obtained by differential scanning calorimetry (DSC).

[0034] In the present invention, the binder comprises a first structural unit (acrylate structural unit with at least 10 carbon atoms in the main chain), a second structural unit (bifunctional acrylate structural unit containing at least one polar functional group), and a third structural unit (acrylate structural unit containing at least two polar functional groups). The first structural unit serves as the main chain of the polymer, and the second and third structural units serve as the branches grafted onto the polymer main chain. By controlling the glass transition temperature of the binder to be 55 °C to 70 °C, a flexible polymer structure can be obtained. First of all, the second structural unit can form a network crosslinked structure. On the one hand, there are a large number of pores in this network crosslinked structure, and these pores can adjust the gaps between the hard carbon particles, enabling them to have more space to expand and contract during charge and discharge, dispersing the stress received by the hard carbon particles during charge and discharge, and making the stress evenly distributed throughout the electrode structure, thereby reducing the mutual collision and wear between the hard carbon particles, reducing the loss of the electrode, and effectively improving the cycle life of the battery. On the other hand, this network crosslinked structure can also provide a supporting effect, enabling there to be more support points between the hard carbon particles, having a buffering effect on the volume expansion of the hard carbon particles during charge and discharge, thereby reducing the impact of volume change on the electrode sheet, improving the structural stability of the electrode, and further improving the cycle performance of the battery. On the further hand, this network crosslinked structure can construct a good conductive network, making the electron conduction inside the electrode more uniform and efficient, contributing to improving the conductive performance of the electrode, reducing the resistance, and effectively improving the rate performance of the battery. Secondly, the third structural unit has at least two polar functional groups, with a strong interaction force with the hard carbon surface, and can form a coating layer on the surface of the hard carbon particles, which can further prevent the collision and wear between the hard carbon particles and reduce the electrode loss. Finally, the polar functional groups contained in the second structural unit can form hydrogen bonds or other chemical bonds with the surface of the current collector, thereby enhancing the adhesion between the binder and the current collector and improving the mechanical strength of the electrode sheet. Therefore, the binder in the present invention can comprehensively improve the rate performance and cycle performance of the battery.

[0035] In addition, the high crystallinity and strong intermolecular interaction force of the hard carbon particles result in the difficulty of the hard carbon particles to disperse in the solvent and easy aggregation into clusters. At the same time, the surface of the hard carbon particles has poor hydrophilicity, which makes the interaction between them and some solvents weak and difficult to be effectively dispersed in the solvent. The hydrophilic groups contained in the coating layer formed on the surface of the hard carbon particles can enhance the affinity between the hard carbon particles and the solvent, and then enhance the interaction between them. It also helps to weaken the interaction force between the hard carbon particles and improve the dispersibility of the hard carbon particles in the solvent, effectively improving the aggregation and accumulation of the hard carbon particles during the preparation of the slurry and enhancing the dispersibility of the hard carbon particles in the slurry.

[0036] In a specific embodiment, the weight-average molecular weight of the binder is 2×10 5 g / mol to 5×10 5 g / mol. Within this range, on the one hand, the binder has strong mechanical strength and toughness, can maintain stable adhesion between the negative electrode active material and the current collector during the charge and discharge cycle, and reduce material shedding; on the other hand, it helps to form a uniform electrode coating, improve the structural stability of the electrode, and thus extend the cycle life of the battery; on the third hand, it can provide good slurry dispersibility and coatability, ensure good rheological properties during the production process, and help to form a uniform electrode coating; at the same time, it can also improve the problem of increased resistance caused by too high weight-average molecular weight, which is beneficial to improving the rate performance and electrochemical activity of the battery.

[0037] Specifically, the weight-average molecular weight of the binder can be further controlled by controlling the reaction temperature and reaction time in the polymerization reaction, the mass ratio of the initiator to the acrylate monomer with at least 10 main-chain carbon atoms, the mass ratio of the initiator to the bifunctional acrylate monomer containing at least one polar functional group, the mass ratio of the initiator to the acrylate monomer containing at least two polar functional groups, and the mass ratio of the above three reaction monomers, so that the weight-average molecular weight of the binder is between 2×10 5 g / mol and 5×10 5 g / mol.

[0038] Exemplarily, the weight-average molecular weight of the binder can be 2×10 5 g / mol, 2.5×10 5 g / mol, 3×10 5 g / mol, 3.5×10 5 g / mol, 4×10 5 g / mol, 4.5×10 5 g / mol, 5×10 5 g / mol or the range composed of any two of these values.

[0039] In a specific embodiment, based on the total mass of the polymer, the mass percentages of the first structural unit, the second structural unit, and the third structural unit are (22% - 36%):(12% - 28%):(36% - 60%).

[0040] Specifically, by controlling the mass ratio of the acrylate monomer with at least 10 main-chain carbon atoms, the bifunctional acrylate monomer containing at least one polar functional group, and the acrylate monomer containing at least two polar functional groups in the polymerization reaction, the mass percentage contents of the first structural unit, the second structural unit, and the third structural unit in the polymer can be within the aforementioned ranges.

[0041] Exemplarily, the mass percentage content of the first structural unit in the polymer can be 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, or a range composed of any two of these values; the mass percentage content of the second structural unit in the polymer can be 12%, 14%, 16%, 18%, 20%, 22%, 24%, 28%, or a range composed of any two of these values; the mass percentage content of the third structural unit in the polymer can be 36%, 38%, 40%, 42%, 44%, 46%, 48%, 50%, 52%, 54%, 56%, 58%, 60%, or a range composed of any two of these values.

[0042] Within this range, the main chain and each side chain of the polymer can cooperate better, enabling the formed network cross-linked structure to better cooperate with the hard carbon particles, further reducing the wear and collision between the hard carbon particles, and endowing the battery with higher cycle stability performance. Moreover, within this range, a better conductive network can be formed, further improving the electron transfer efficiency and achieving higher rate performance. In addition, the adhesion between electrode materials and between electrode materials and current collectors can be further enhanced, further improving the mechanical strength of the electrode sheet. Finally, the affinity between hard carbon particles and solvent water molecules can be further enhanced, improving the problem of agglomeration or accumulation of hard carbon particles in the slurry, so that the hard carbon particles are more evenly dispersed in the solvent, further improving the cycle performance of the battery.

[0043] In a specific embodiment, the polar functional groups in the bifunctional acrylate structural unit containing at least one polar functional group include at least one of hydroxyl, epoxy, amino, and isocyanate groups. At this time, it is beneficial to the formation of the cross-linked structure during the polymerization process, improving the mechanical strength, solvent resistance, and heat resistance of the binder, and enabling the electrode including this binder to have higher strength and toughness, reducing the cracking and swelling of the electrode coating during the cycling process, thereby improving the cycle performance of the battery.

[0044] It can be understood that since the bifunctional acrylate structural unit containing at least one polar functional group is obtained by polymerization of a bifunctional acrylate monomer containing at least one polar functional group, the polar functional groups in the bifunctional acrylate monomer containing at least one polar functional group also correspondingly include at least one of hydroxyl, epoxy, amino, and isocyanate groups.

[0045] In a specific embodiment, the polar functional groups in the acrylate structural unit containing at least two polar functional groups include at least one of hydroxyl group, carboxyl group, ether group, and sulfonic acid group. The aforementioned groups can form a stable interface with the hard carbon surface through hydrogen bonding, electrostatic interaction, or dipole-dipole interaction, thereby forming a continuous coating layer on the particle surface, enhancing the adhesion and structural integrity between particles, buffering the stress change during the charge and discharge process of the electrode, significantly reducing the risk of particle shedding and interface damage, and further improving the cycle stability and capacity retention rate of the electrode.

[0046] It can be understood that since the acrylate structural unit containing at least two polar functional groups is obtained by the polymerization reaction of an acrylate monomer containing at least two polar functional groups, the polar functional groups in the acrylate monomer containing at least two polar functional groups also correspondingly include at least one of hydroxyl group, carboxyl group, ether group, and sulfonic acid group.

[0047] In a specific embodiment, the glass transition temperature of the acrylate monomer with no less than 10 main chain carbon atoms is -90°C to -40°C, and the glass transition temperature of the bifunctional acrylate monomer containing at least one polar functional group is 100°C to 300°C.

[0048] Exemplarily, the glass transition temperature of the acrylate monomer with no less than 10 main chain carbon atoms can be -90°C, -85°C, -80°C, -75°C, -70°C, -65°C, -60°C, -55°C, -50°C, -45°C, -40°C, or a range composed of any two of these values; the glass transition temperature of the bifunctional acrylate monomer containing at least one polar functional group can be 100°C, 120°C, 140°C, 160°C, 180°C, 200°C, 220°C, 240°C, 260°C, 280°C, 300°C, or a range composed of any two of these values.

[0049] When the glass transition temperatures of the aforementioned two types of monomers are within the above ranges, it helps to make the glass transition temperature of the binder between 55°C and 70°C, which can not only further reduce the mutual collision and wear between hard carbon particles, but also form a good conductive network, enhance the electron conductivity, and help the battery achieve higher rate performance and cycle performance. At the same time, it can also ensure that the electrode has high toughness and mechanical strength, improve the dispersion uniformity of hard carbon particles in solvent water, and further improve the cycle stability of the battery. Finally, it can also improve the flexibility of the electrode, avoid cracking due to the expansion of the negative active material during the cycle, and improve the structural stability of the electrode.

[0050] In a specific embodiment, the acrylate structural unit having at least 10 main-chain carbon atoms includes at least one of nonyl acrylate structural unit, decyl acrylate structural unit, undecyl acrylate structural unit, dodecyl acrylate structural unit, and tridecyl acrylate structural unit.

[0051] Specifically, the acrylate structural unit having at least 10 main-chain carbon atoms is derived from an acrylate monomer having at least 10 main-chain carbon atoms; the acrylate monomer having at least 10 main-chain carbon atoms includes at least one of nonyl acrylate, decyl acrylate, undecyl acrylate, dodecyl acrylate, and tridecyl acrylate.

[0052] When the acrylate structural unit having at least 10 main-chain carbon atoms simultaneously includes the foregoing various specific structural units, the present invention does not specifically limit the proportion of each specific structural unit.

[0053] In a specific embodiment, the difunctional acrylate structural unit containing at least one polar functional group includes at least one of tricyclodecane dimethanol diacrylate structural unit, bisphenol A dimethacrylate structural unit, ethoxylated bisphenol A dimethacrylate structural unit, bisphenol F dimethacrylate structural unit, pentaerythritol tetraacrylate structural unit, and isophorone dimethacrylate structural unit.

[0054] Specifically, the difunctional acrylate structural unit containing at least one polar functional group is derived from a difunctional acrylate monomer containing at least one polar functional group; the difunctional acrylate monomer containing at least one polar functional group includes at least one of tricyclodecane dimethanol diacrylate, bisphenol A dimethacrylate, ethoxylated bisphenol A dimethacrylate, bisphenol F dimethacrylate, pentaerythritol tetraacrylate, and isophorone dimethacrylate. Among them, the glass transition temperature of tricyclodecane dimethanol diacrylate is 130 °C, the glass transition temperature of bisphenol A dimethacrylate is 115 °C, the glass transition temperature of ethoxylated bisphenol A dimethacrylate is 90 °C, the glass transition temperature of bisphenol F dimethacrylate is 130 °C, the glass transition temperature of pentaerythritol tetraacrylate is 110 °C, and the glass transition temperature of isophorone dimethacrylate is 120 °C.

[0055] When the difunctional acrylate structural unit containing at least one polar functional group simultaneously includes the foregoing various specific structural units, the present invention does not specifically limit the proportion of each specific structural unit.

[0056] In a specific embodiment, the acrylate structural unit containing at least two polar functional groups includes at least one of N-methylolacrylamide structural unit, 2-hydroxyethyl acrylamide structural unit, 2,3-dihydroxypropyl acrylate structural unit, 2-acryloyloxyethyl-2'-hydroxyethyl methacrylamide structural unit, 3-hydroxypropyl methacrylate structural unit, and 2-hydroxypropyl acrylate structural unit.

[0057] Specifically, the acrylate structural unit containing at least two polar functional groups is derived from an acrylate monomer containing at least two polar functional groups; the acrylate monomer containing at least two polar functional groups includes at least one of N-methylolacrylamide, 2-hydroxyethyl acrylamide, 2,3-dihydroxypropyl acrylate, 2-acryloyloxyethyl-2'-hydroxyethyl methacrylamide, 3-hydroxypropyl methacrylate, and 2-hydroxypropyl acrylate. Among them, the glass transition temperature of N-methylolacrylamide is 170 °C, the glass transition temperature of 2-hydroxyethyl acrylamide is 120 °C, the glass transition temperature of 2,3-dihydroxypropyl acrylate is 130 °C, the glass transition temperature of 2-acryloyloxyethyl-2'-hydroxyethyl methacrylamide is 180 °C, the glass transition temperature of 3-hydroxypropyl methacrylate is 105 °C, and the glass transition temperature of 2-hydroxypropyl acrylate is 100 °C.

[0058] When the acrylate structural unit containing at least two polar functional groups simultaneously includes the foregoing various specific structural units, the present invention does not specifically limit the ratio of each specific structural unit.

[0059] The present invention also provides a method for preparing the binder of the first aspect, comprising the following steps:

[0060] 1) At 50 °C to 70 °C, a first polymerization reaction occurs in a first raw material system including an acrylate monomer with at least 10 main chain carbon atoms and a first initiator to obtain a first reaction system;

[0061] 2) Add a second raw material system including a bifunctional acrylate monomer containing at least one polar functional group and a second initiator to the first reaction system, and initiate a second polymerization reaction at 65 °C to 75 °C to obtain a second reaction system;

[0062] 3) Add a third raw material system including an acrylate monomer containing at least two polar functional groups and a third initiator to the second reaction system, and initiate a third polymerization reaction at 78 °C to 85 °C to obtain the binder.

[0063] Specifically, in step 1), a raw material including an acrylate monomer with at least 10 main-chain carbon atoms and a first initiator is mixed in deionized water to obtain a first raw material system. After deoxygenating the first raw material system, the temperature of the system is raised to 50°C to 70°C for a first polymerization reaction to obtain a first reaction system. During this process, the acrylate monomer with at least 10 main-chain carbon atoms undergoes polymerization to obtain a first structural unit, that is, the polymer main chain.

[0064] Exemplarily, the reaction temperature of the first polymerization reaction can be 50°C, 55°C, 60°C, 65°C, 70°C or a range composed of any two of these values.

[0065] Furthermore, the mass ratio of the first initiator to the acrylate monomer with at least 10 main-chain carbon atoms is (0.1 - 2):100. Within this range, the reaction rate is relatively high, which can enable the monomers to polymerize fully and will not cause explosive polymerization.

[0066] Exemplarily, the mass ratio of the first initiator to the acrylate monomer with at least 10 main-chain carbon atoms can be 0.1:100, 0.2:100, 0.4:100, 0.6:100, 0.8:100, 1:100, 1.2:100, 1.4:100, 1.6:100, 1.8:100, 2:100 or a range composed of any two of these ratios.

[0067] The present invention does not specifically limit the type of the first initiator, which can be a commonly used initiator in the art. For example, it includes at least one of ammonium persulfate, potassium persulfate, sodium persulfate, ammonium sulfate, trimethylamine, benzoyl peroxide, and tert-butyl peroxide.

[0068] The present invention does not specifically limit the mixing method, as long as the raw materials are evenly dispersed in the first raw material system. For example, mixing can be carried out by magnetic stirring or mechanical stirring.

[0069] The present invention does not specifically limit the deoxygenation method, as long as the active oxygen in the first raw material system is completely removed. For example, nitrogen can be introduced into the first raw material system for deoxygenation, and the nitrogen introduction time is 30 min to 60 min.

[0070] The present invention does not specifically limit the heating rate, which can be adjusted according to the actual situation, as long as the temperature of the first raw material system is between 50°C and 70°C.

[0071] The present invention does not specifically limit the reaction time of the first polymerization reaction, and a suitable reaction time can be selected according to the actual situation, as long as the glass transition temperature of the finally prepared binder is between 55°C and 70°C.

[0072] In one embodiment, the reaction time of the first polymerization reaction is 3 h to 5 h; for example, the reaction time can be 3 h, 3.5 h, 4 h, 4.5 h, 5 h, or a range composed of any two of these values.

[0073] In step 2), a raw material including a difunctional acrylate monomer containing at least one polar functional group and a second initiator is mixed to obtain a second raw material system. The second raw material system is added to the above first reaction system, and a second polymerization reaction is carried out at 65°C to 75°C to obtain a second reaction system. During this process, the difunctional acrylate monomer containing at least one polar functional group reacts with the polymer main chain formed in step 1) and grafts onto the polymer main chain to form a branched chain.

[0074] For example, the temperature of the second polymerization reaction can be 65°C, 67°C, 69°C, 71°C, 73°C, 75°C, or a range composed of any two of these values.

[0075] Furthermore, the second raw material system is added to the first reaction system under stirring, so that the second raw material system is uniformly dispersed in the first reaction system, improving the uniformity of the reaction.

[0076] Furthermore, the addition time of the second raw material system to the first reaction system is 1.5 h to 3 h. Within this range, the occurrence of violent polymerization in the polymerization reaction can be avoided. It should be noted that the addition speed is a uniform addition, and it is only necessary to add the second raw material system to the first reaction system within the specified time. For example, the addition time can be 1.5 h, 2 h, 2.5 h, 3 h, or a range composed of any two of these values.

[0077] Furthermore, the mass ratio of the second initiator to the difunctional acrylate monomer containing at least one polar functional group is (0.5 to 1):100. Within this range, the monomer can react fully and violent polymerization will not occur. For example, the mass ratio of the second initiator to the difunctional acrylate monomer containing at least one polar functional group can be 0.5:100, 0.6:100, 0.7:100, 0.8:100, 0.9:100, 1:100, or a range composed of any two of these ratios.

[0078] The present invention does not specifically limit the type of the second initiator. It can be a commonly used initiator in the art, which can be the same as or different from the first initiator, and will not be elaborated here.

[0079] The present invention does not specifically limit the mixing method. For example, the mixing method in step 1) above can be adopted, and will not be elaborated here.

[0080] The present invention does not specifically limit the reaction time of the second polymerization reaction. The appropriate reaction time can be selected according to the actual situation, as long as the glass transition temperature of the finally prepared binder can be between 55 °C and 70 °C.

[0081] In one embodiment, the reaction time of the second polymerization reaction is 3.5 h to 6 h; for example, the reaction time can be 3.5 h, 4 h, 4.5 h, 5 h, 5.5 h, 6 h or the range composed of any two of these values.

[0082] In step 3), the raw materials including the acrylate monomer containing at least two polar functional groups and the third initiator are mixed to obtain a third raw material system. The third raw material system is added to the above-mentioned second reaction system, and a third polymerization reaction is carried out at 78 °C to 85 °C to obtain a binder. During this process, the acrylate monomer containing at least two polar functional groups reacts with the polymer main chain and grafts onto the polymer main chain to form branches.

[0083] For example, the temperature of the third polymerization reaction can be 78 °C, 79 °C, 80 °C, 81 °C, 82 °C, 83 °C, 84 °C, 85 °C or the range composed of any two of these values.

[0084] Furthermore, the third raw material system is added to the second reaction system under stirring, so that the third raw material system is uniformly dispersed in the second reaction system, improving the uniformity of the reaction.

[0085] Furthermore, the addition time of the third raw material system to the second reaction system is 2 h to 4 h. Within this range, the occurrence of violent polymerization in the polymerization reaction can be avoided. It should be noted that the addition speed is a uniform addition, and it is only necessary to add the third raw material system to the second reaction system within the specified time. For example, the addition time can be 2 h, 2.5 h, 3 h, 3.5 h, 4 h or the range composed of any two of these values.

[0086] Furthermore, the mass ratio of the third initiator to the acrylate monomer containing at least two polar functional groups is (0.5 - 1):100. Within this range, not only can the full reaction of the monomer be ensured, but also the occurrence of violent polymerization can be avoided. For example, the mass ratio of the third initiator to the acrylate monomer containing at least two polar functional groups can be 0.5:100, 0.6:100, 0.7:100, 0.8:100, 0.9:100, 1:100 or the range composed of any two of these ratios.

[0087] The present invention does not specifically limit the type of the third initiator. It can be a commonly used initiator in the art, which can be the same as or different from the first initiator or the second initiator, and will not be elaborated here.

[0088] The present invention does not specifically limit the mixing method. For example, the mixing methods in the above step 1) or step 2) can be adopted, which will not be elaborated here.

[0089] The present invention does not specifically limit the reaction time of the third polymerization reaction. A suitable reaction time can be selected according to the actual situation, as long as the glass transition temperature of the finally prepared binder can be between 55°C and 70°C.

[0090] In one embodiment, the reaction time of the third polymerization reaction is 3h to 4h. Exemplarily, the reaction time can be 3h, 3.2h, 3.4h, 3.6h, 3.8h, 4h, or a range composed of any two of these values.

[0091] In the preparation method of the binder in the present invention, first, an acrylate monomer with a main chain carbon atom number of not less than 10 is polymerized to form a polymer main chain (corresponding to the first structural unit). Then, a bifunctional acrylate monomer containing at least one polar functional group and an acrylate monomer containing at least two polar functional groups are successively added, and these two monomers are grafted onto the aforementioned polymer main chain to form branches respectively (wherein, the bifunctional acrylate monomer containing at least one polar functional group corresponds to the formation of the second structural unit, and the acrylate monomer containing at least two polar functional groups corresponds to the formation of the third structural unit), and the glass transition temperature of the binder is controlled to be between 55°C and 70°C, which can make the binder have good flexibility and plasticity. The second structural unit can form a kind of network cross-linked structure. The pores contained in this structure can adjust the gaps between hard carbon particles, relieve the stress suffered by the hard carbon particles during the charge and discharge process, improve the problem of collision and wear between hard carbon particles, reduce the electrode loss, and effectively improve the cycle life of the battery; and this network cross-linked structure can build a good conductive network inside the electrode, making the electron conduction more uniform and efficient, which helps to improve the conductive performance of the electrode, reduce the resistance, and improve the rate performance of the battery. At the same time, the polar functional groups in the second structural unit can interact with the surface of the current collector, and the formed hydrogen bonds or other chemical bonds can effectively enhance the adhesion between the binder and the current collector, and improve the mechanical strength of the electrode sheet. In addition, the third structural unit can promote the formation of a coating layer on the surface of the hard carbon particles. This coating layer can enhance the affinity between the hard carbon particles and the solvent water molecules, and weaken the interaction force between the hard carbon particles and the particles, improve the agglomeration or accumulation problem of the hard carbon particles during the slurry preparation process, and thus improve the dispersion of the hard carbon particles in the slurry.

[0092] Therefore, the binder prepared by the above preparation method can comprehensively improve the rate performance and cycle performance of the battery.

[0093] In a specific embodiment, the mass ratio of the acrylate monomer with no less than 10 main-chain carbon atoms, the bifunctional acrylate monomer containing at least one polar functional group, and the acrylate monomer containing at least two polar functional groups is (22-36):(12-28):(36-60). In this range, it helps to make the mass percentage ratio of the first structural unit, the second structural unit, and the third structural unit in the prepared polymer fall within (22%-36%):(12%-28%):(36%-60%).

[0094] The second aspect of the present invention provides a negative electrode sheet. Since the negative electrode sheet includes the binder of the first aspect, when the negative electrode sheet is used in a lithium-ion battery, the rate performance and cycle stability of the battery can be effectively improved.

[0095] The present invention does not specifically limit the structure of the negative electrode sheet. In one embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least a part of the surface of the negative electrode current collector; the negative electrode active layer includes a negative electrode active material.

[0096] The present invention does not specifically limit the material of the negative electrode current collector, and a commonly used negative electrode current collector in the art, such as copper foil, can be selected.

[0097] In a specific embodiment, the negative electrode sheet further includes a negative electrode active material, and the negative electrode active material includes hard carbon. When the negative electrode active material includes hard carbon, the binder can cooperate with the hard carbon. On the one hand, the network cross-linked structure can not only reduce the mutual collision and wear between hard carbon particles, reduce the loss of the electrode, but also form a good conductive network, reducing the negative impact on the rate performance of the battery due to the poor conductivity of hard carbon; on the other hand, the binder can also enhance the adhesion between electrode materials and between the electrode material and the current collector, and improve the dispersion uniformity of hard carbon particles in the slurry, thereby comprehensively improving the rate performance and cycle performance of the battery.

[0098] In a specific embodiment, the median particle size of the hard carbon is 5 μm - 30 μm. In this range, the particle size of the hard carbon particles is more appropriate, which can not only ensure a better lithium ion diffusion rate, enhance the rate performance of the battery, but also not increase side reactions, contribute to the formation of the SEI film and the improvement of the first Coulomb efficiency. At the same time, it can effectively fill the electrode space, provide a higher compaction density, improve the volume energy density of the battery, and will not cause too much negative impact on conductivity and electrolyte wetting. In addition, it helps to maintain the structural stability of the negative electrode active material, reduce the pulverization phenomenon, enhance the cycle life of the battery. Finally, it can make the specific surface area of the hard carbon particles more moderate, reducing the side reactions between the hard carbon and the electrolyte.

[0099] Exemplarily, the median particle size of the hard carbon can be 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, or a range composed of any two of these values.

[0100] The "median particle size" in the present invention refers to the particle size corresponding to when the cumulative volume percentage of the hard carbon particles reaches 50%, and can be measured by a laser particle size analyzer.

[0101] In a specific embodiment, the negative electrode sheet further includes a conductive agent; the mass ratio of the binder, hard carbon, and conductive agent is (0.8 - 3):(50 - 100):(1 - 3). Within this range, the ratio of the binder, hard carbon, and conductive agent is more suitable, which can improve the rate performance and cycle life of the battery while ensuring the high capacity of the battery.

[0102] In a specific embodiment, the negative electrode sheet is prepared by a method including the following steps:

[0103] The negative electrode active material, the binder of the first aspect, the conductive agent, and a solvent (such as deionized water) are uniformly mixed to prepare a negative electrode active paste; the negative electrode active paste is uniformly coated on at least one surface of the negative electrode current collector, and then dried, rolled, and slit to obtain the negative electrode sheet.

[0104] The negative electrode sheet obtained by the above preparation method can not only achieve high rate performance and cycling performance; moreover, during the mixing process, it is not necessary to additionally reduce the rotation speed of the dispersion equipment, which can effectively shorten the process and time in the production process, simplify the process flow, and reduce energy consumption.

[0105] The third aspect of the present invention provides a lithium-ion battery. Since this lithium-ion battery includes the binder of the first aspect or the negative electrode sheet of the second aspect, this battery has high rate performance and cycling performance.

[0106] Hereinafter, the binder of the present invention and the lithium-ion battery including this binder will be introduced in detail through specific examples.

[0107] Example 1

[0108] 1) Preparation of the binder

[0109] 25 parts by mass of nonyl acrylate (glass transition temperature: -60°C to -50°C), 0.5 part by mass of ammonium persulfate, and 300 parts by mass of deionized water are added to a reaction kettle to obtain a first raw material system. Nitrogen is introduced for 30 minutes to drive away the active oxygen in the reaction kettle and the pipeline, and the temperature is raised to 60°C for the first polymerization reaction. The reaction time is 4 hours to obtain a first reaction system.

[0110] Mix 20 parts by mass of tricyclodecane dimethanol diacrylate (glass transition temperature: 135 °C) and 0.2 part by mass of ammonium persulfate to obtain a second raw material system. Add the second raw material system to the above-mentioned first reaction system over 2 h, and carry out a second polymerization reaction at 70 °C for 5 h to obtain a second reaction system.

[0111] Mix 55 parts by mass of N-methylolacrylamide (glass transition temperature: 170 °C) and 0.55 part by mass of ammonium persulfate to obtain a third raw material system. Add the third raw material system to the above-mentioned second reaction system over 3 h, and carry out a third polymerization reaction at 80 °C for 3.5 h to obtain a binder.

[0112] 2) Preparation of negative electrode sheet

[0113] Mix the binder prepared in step 1), negative electrode active material hard carbon (median particle size: 10 μm), and conductive agent carbon black in a mass ratio of 2:95:3, add deionized water, and obtain a negative electrode active paste under the action of a vacuum mixer; uniformly coat the negative electrode active paste on both side surfaces of a copper foil, with a single-sided areal density of 90 g / m 2 , and obtain a negative electrode sheet with a tap density of 1.6 g / cm 3 after drying, cold pressing, and slitting.

[0114] 3) Preparation of positive electrode sheet

[0115] Mix 96.80 wt% of positive electrode active material NCM622 (chemical composition: Li(Ni 0.6 Co 0.2 Mn 0.2 )O2), 1.5 wt% of conductive agent carbon black (Super. P Li), 0.5 wt% of conductive graphite (KS-6), 1.2 wt% of binder polyvinylidene fluoride (PVDF), and N-methylpyrrolidone to prepare a positive electrode active paste; then coat the positive electrode active paste on both side surfaces of an aluminum foil, with a single-sided areal density of 170 g / m 2 , and obtain a positive electrode sheet with a tap density of 3.4 g / cm 3 after drying, rolling, and slitting.

[0116] 6) Preparation of lithium-ion battery

[0117] Stack the above positive electrode sheet, separator film (PE film), and the above negative electrode sheet in sequence, with the separator film placed in the middle of the positive electrode sheet and the negative electrode sheet to play an isolation role, to form an electrode assembly. Place the electrode assembly in an outer package, inject electrolyte and seal it. After that, through processes such as formation and degassing, a lithium-ion battery is obtained. The electrolyte includes a lithium salt, an organic solvent, and an additive. The lithium salt is lithium hexafluorophosphate (LiPF6), the organic solvent is a mixed solvent of ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), and ethyl propionate (EP), the additive is vinylene carbonate (VC), and the volume ratio of EC, DEC, PC, and EP is 30:30:10:30; the concentration of the lithium salt in the electrolyte is 1 mol / L, the mass percentage content of the additive VC in the electrolyte is 5%, and the balance is the organic solvent.

[0118] Example 2

[0119] The preparation method of the lithium-ion battery in this example is basically the same as that in Example 1, except that in step 1), in the first raw material system, nonyl acrylate is replaced by decyl acrylate (glass transition temperature is -70°C to -60°C), and ammonium persulfate is replaced by potassium persulfate.

[0120] Example 3

[0121] The preparation method of the lithium-ion battery in this example is basically the same as that in Example 1, except that in step 1), in the first raw material system, nonyl acrylate is replaced by undecyl acrylate (glass transition temperature is -70°C), and ammonium persulfate is replaced by sodium persulfate;

[0122] In the second raw material system, tricyclodecane dimethanol diacrylate is replaced by bisphenol A dimethacrylate (glass transition temperature is 115°C), and ammonium persulfate is replaced by sodium persulfate.

[0123] Example 4

[0124] The preparation method of the lithium-ion battery in this example is basically the same as that in Example 1, except that in step 1), in the first raw material system, nonyl acrylate is replaced by dodecyl acrylate (glass transition temperature is -80°C), and ammonium persulfate is replaced by ammonium sulfate;

[0125] In the second raw material system, tricyclodecane dimethanol diacrylate is replaced by bisphenol A dimethacrylate (glass transition temperature is 115°C), and ammonium persulfate is replaced by ammonium sulfate;

[0126] In the third raw material system, N-methylolacrylamide is replaced by 2-hydroxyethyl acrylamide (glass transition temperature is 120°C), and ammonium persulfate is replaced by ammonium sulfate.

[0127] Example 5

[0128] The preparation method of the lithium-ion battery in this example is basically the same as that in Example 1, except that in step 1), in the first raw material system, nonyl acrylate is replaced by tridecyl acrylate (glass transition temperature is -80°C to -90°C), and ammonium persulfate is replaced by trimethylamine;

[0129] In the second raw material system, tricyclodecane dimethanol diacrylate is replaced by ethoxylated bisphenol A dimethacrylate (glass transition temperature is 90°C), and ammonium persulfate is replaced by trimethylamine;

[0130] In the third raw material system, N-methylolacrylamide is replaced by 2,3-dihydroxypropyl acrylate (glass transition temperature is 130°C), and ammonium persulfate is replaced by trimethylamine.

[0131] Example 6

[0132] The preparation method of the lithium-ion battery in this example is basically the same as that in Example 1, except that in step 1), in the first raw material system, ammonium persulfate is replaced by benzoyl peroxide;

[0133] In the second raw material system, tricyclodecane dimethanol diacrylate is replaced by bisphenol F dimethacrylate (glass transition temperature is 130°C), and ammonium persulfate is replaced by benzoyl peroxide;

[0134] In the third raw material system, N-methylolacrylamide is replaced by 2-acryloyloxyethyl-2'-hydroxyethyl methacrylate (glass transition temperature is 180°C), and ammonium persulfate is replaced by benzoyl peroxide.

[0135] Example 7

[0136] The preparation method of the lithium-ion battery in this example is basically the same as that in Example 1, except that in step 1), in the first raw material system, ammonium persulfate is replaced by tert-butyl peroxide;

[0137] In the second raw material system, tricyclodecane dimethanol diacrylate is replaced by pentaerythritol tetraacrylate (glass transition temperature is 110°C), and ammonium persulfate is replaced by tert-butyl peroxide;

[0138] In the third raw material system, N-methylolacrylamide is replaced by 3-hydroxypropyl methacrylate (glass transition temperature is 105°C), and ammonium persulfate is replaced by tert-butyl peroxide.

[0139] Example 8

[0140] The preparation method of the lithium-ion battery in this example is basically the same as that in Example 1, except that in step 1), in the first raw material system, the mass fraction of nonyl acrylate is 22 parts, and the mass fraction of ammonium persulfate is 0.44 part;

[0141] In the second raw material system, the mass fraction of tricyclodecane dimethanol diacrylate is 28 parts, and the mass fraction of ammonium persulfate is 0.28 part;

[0142] In the third raw material system, the mass fraction of N-hydroxymethyl acrylamide is 50 parts, and the mass fraction of ammonium persulfate is 0.5 part.

[0143] Example 9

[0144] The preparation method of the lithium-ion battery in this example is basically the same as that in Example 1, except that in step 1), in the first raw material system, the mass fraction of nonyl acrylate is 28 parts, and the mass fraction of ammonium persulfate is 0.56 part;

[0145] In the second raw material system, the mass fraction of tricyclodecane dimethanol diacrylate is 12 parts, and the mass fraction of ammonium persulfate is 0.12 part;

[0146] In the third raw material system, the mass fraction of N-hydroxymethyl acrylamide is 60 parts, and the mass fraction of ammonium persulfate is 0.6 part.

[0147] Example 10

[0148] The preparation method of the lithium-ion battery in this example is basically the same as that in Example 1, except that in step 1), in the first raw material system, the mass fraction of nonyl acrylate is 36 parts, and the mass fraction of ammonium persulfate is 0.72 part;

[0149] In the second raw material system, the mass fraction of tricyclodecane dimethanol diacrylate is 28 parts, and the mass fraction of ammonium persulfate is 0.28 part;

[0150] In the third raw material system, the mass fraction of N-hydroxymethyl acrylamide is 36 parts, and the mass fraction of ammonium persulfate is 0.36 part.

[0151] Example 11

[0152] The preparation method of the lithium-ion battery in this example is basically the same as that in Example 1, except that in step 2), the median particle size of the hard carbon is 5.4 μm.

[0153] Example 12

[0154] The preparation method of the lithium-ion battery in this example is basically the same as that in Example 1, except that in step 2), the median particle size of the hard carbon is 29.5 μm.

[0155] Example 13

[0156] The preparation method of the lithium-ion battery in this example is basically the same as that in Example 1, except that in step 1), the reaction temperature of the second polymerization reaction is 65 °C, and the reaction temperature of the third polymerization reaction is 78 °C.

[0157] Example 14

[0158] The preparation method of the lithium-ion battery in this example is basically the same as that in Example 1, except that in step 1), in the first raw material system, the mass fraction of nonyl acrylate is adjusted to 40 parts, and the mass fraction of ammonium persulfate is adjusted to 0.8 part;

[0159] In the second raw material system, the mass fraction of tricyclodecane dimethanol diacrylate is adjusted to 30 parts, and the mass fraction of ammonium persulfate is adjusted to 0.3 part;

[0160] In the third raw material system, the mass fraction of N-methylol acrylamide is 30 parts, and the mass fraction of ammonium persulfate is 0.3 part.

[0161] Example 15

[0162] The preparation method of the lithium-ion battery in this example is basically the same as that in Example 1, except that in step 2), the hard carbon is replaced with artificial graphite, and the median particle size is 18 μm.

[0163] Example 16

[0164] The preparation method of the lithium-ion battery in this example is basically the same as that in Example 1, except that in step 2), the hard carbon is replaced with natural graphite, and the median particle size is 20 μm.

[0165] Example 17

[0166] The preparation method of the lithium-ion battery in this example is basically the same as that in Example 1, except that in step 2), the median particle size of the hard carbon is 35 μm.

[0167] Comparative Example 1

[0168] The preparation method of the lithium-ion battery in this comparative example is basically the same as that in Example 1, except that in step 1), the second polymerization reaction is not carried out, that is, the third polymerization reaction is directly carried out after the first polymerization reaction; at this time, in the first raw material system, the mass fraction of nonyl acrylate is 45 parts, and the mass fraction of ammonium persulfate is 0.9 part, and the rest remains unchanged.

[0169] Comparative Example 2

[0170] The preparation method of the lithium-ion battery in this comparative example is basically the same as that in Example 1, except that in step 1), the third polymerization reaction is not carried out, that is, the binder of this comparative example is obtained after the second polymerization reaction; in the first raw material system, the mass fraction of nonyl acrylate is 80 parts, and the mass fraction of ammonium persulfate is 1.6 parts, and the others remain unchanged.

[0171] Comparative Example 3

[0172] The preparation method of the lithium-ion battery in this comparative example is basically the same as that in Example 1, except that in step 1), the second polymerization reaction and the third polymerization reaction are not carried out, that is, the binder of this comparative example is directly obtained after the first polymerization reaction; at this time, the mass fraction of tricyclodecane dimethanol diacrylate is 100 parts, and the mass fraction of ammonium persulfate is 2 parts, and the others remain unchanged.

[0173] Comparative Example 4

[0174] The preparation method of the lithium-ion battery in this comparative example is basically the same as that in Example 1, except that in step 1), the first polymerization reaction is not carried out, that is, the second polymerization reaction and the third polymerization reaction are directly carried out to obtain the binder of this comparative example; at this time, the mass fraction of tricyclodecane dimethanol diacrylate is 45 parts, and the mass fraction of ammonium persulfate is 0.45 parts, and the others remain unchanged.

[0175] Test Example

[0176] 1. Test the weight-average molecular weight and glass transition temperature of the binders prepared in the above examples and comparative examples:

[0177] 1) Weight-average molecular weight

[0178] Use a gel permeation chromatograph to test the weight-average molecular weight of the binder.

[0179] 2) Glass transition temperature

[0180] Use a differential scanning calorimeter (Shanghai Qunhong Instrument and Equipment Co., Ltd., model: DSC-100) to detect the glass transition temperature (Tg) of the binders prepared in the examples and comparative examples. The steps include: turn on high-purity nitrogen, set the nitrogen flow rate to 0.5 L / min to 0.6 L / min, turn on the DSC power supply, and run the desktop. Set the temperature: -60 °C, hold for 10 min, from -60 °C to 160 °C, the heating rate is 10 K / min. After setting, put the prepared sample on the heating furnace, cover the furnace body protection cover, and add an appropriate amount of liquid nitrogen to the constant temperature tank. When the sample temperature reaches -60 °C, run the test.

[0181] The test results are shown in Table 1.

[0182] 2. Test the peel strength between the negative electrode active layer and the negative electrode current collector in the negative electrode sheets prepared in the above examples and comparative examples:

[0183] After compressing the coated single-sided negative electrode sheet at 1.6 g / cm 3 The electrode sheet is cut into a length of 20 cm × width of 3 cm. Stick the 3M double-sided tape on the steel plate, fix the coated side of the electrode sheet downward on the tape of the steel plate. After rolling back and forth 6 times with a 2.5 kg roller, use a tensile machine with a range of 20 N. Clamp the copper foil side with the upper plate, tear the coating and the copper foil, at a speed of 50 mm / min and stretch at 180 °C. Record the data in the stable tensile force section as the peel strength (N / m).

[0184] The test results are shown in Table 1.

[0185] 3. Test the rate performance and cycle performance of the lithium-ion batteries prepared in the above examples and comparative examples:

[0186] (1) Rate performance test

[0187] First step: At 25 °C, first charge at a constant current of 0.5C to 4.2V, then charge at a constant voltage of 4.2V, with a cut-off current of 0.05C; then discharge at a constant current of 0.2C to 3.0V, end the discharge, and leave it standing for 10 min. Record the reference discharge capacity.

[0188] Second step: Charge at a constant current of 0.5C to 4.2V again, then charge at a constant voltage of 4.2V, with a cut-off current of 0.05C; discharge at a constant current of 0.5C to 3.0V, end the discharge, and leave it standing for 10 min.

[0189] Third step: Charge at a constant current of 0.5C to 4.2V again, then charge at a constant voltage of 4.2V, with a cut-off current of 0.05C; discharge at a constant current of 1C to 3.0V, end the discharge, and leave it standing for 10 min.

[0190] Fourth step: Charge at a constant current of 0.5C to 4.2V again, then charge at a constant voltage of 4.2V, with a cut-off current of 0.05C; discharge at a constant current of 2C to 3.0V, end the discharge, and leave it standing for 10 min.

[0191] Fifth step: Charge at a constant current of 0.5C to 4.2V again, then charge at a constant voltage of 4.2V, with a cut-off current of 0.05C; discharge at a constant current of 3C to 3.0V, end the discharge, and leave it standing for 10 min.

[0192] Sixth step: Charge at a constant current of 0.5C to 4.2V again, then charge at a constant voltage of 4.2V, with a cut-off current of 0.05C; discharge at a constant current of 5C to 3.0V, end the discharge, and leave it standing for 10 min; record the 5C discharge capacity.

[0193] The capacity retention rate (%) at 5C = (5C discharge capacity / reference discharge capacity) × 100%.

[0194] (2) Cycle performance test

[0195] At 25°C, charge at a constant current of 1C to 4.5V, then charge at a constant voltage of 4.5V with a cut-off current of 0.05C; then discharge at a discharge rate of 1C to 3.0V, record the initial discharge capacity as Q0, cycle 500 times according to the aforementioned charge-discharge mechanism, and record the discharge capacity after 500 cycles as Q1. Then the capacity retention rate after 500 cycles (%) = (Q1 / Q0) × 100%.

[0196] In Table 1, Tg represents the glass transition temperature of the binder, Mw represents the weight-average molecular weight of the binder, P represents the peel strength of the binder, and A represents the mass percentages of the first structural unit, the second structural unit, and the third structural unit in the polymer.

[0197] The test results are shown in Table 1.

[0198] Table 1

[0199]

[0200] As can be seen from Table 1:

[0201] When the binder of the present invention is used in a negative electrode sheet with hard carbon as the negative electrode active material, it has excellent peel strength and can improve the rate performance and cycle performance of the battery.

[0202] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An adhesive, characterized in that, The binder includes a polymer, and the polymer includes a first structural unit, a second structural unit, and a third structural unit; the first structural unit includes an acrylate structural unit with at least 10 carbon atoms in the main chain, the second structural unit includes a difunctional acrylate structural unit containing at least one polar functional group, and the third structural unit includes an acrylate structural unit containing at least two polar functional groups; the glass transition temperature of the binder is 55°C to 70°C.

2. The binder according to claim 1, wherein, The weight-average molecular weight of the binder is 2×10 5 g / mol to 5×10 5 g / mol.

3. The binder according to claim 1 or 2, characterized in that, Based on the total mass of the polymer, the mass percentages of the first structural unit, the second structural unit, and the third structural unit are (22% to 36%):(12% to 28%):(36% to 60%).

4. The binder according to claim 1, characterized in that, The polar functional group in the difunctional acrylate structural unit containing at least one polar functional group includes at least one of hydroxyl, epoxy group, amino group, and isocyanate group; and / or, the polar functional group in the acrylate structural unit containing at least two polar functional groups includes at least one of hydroxyl, carboxyl group, ether group, and sulfonic acid group.

5. The binder according to claim 1, characterized in that The acrylate structural unit with at least 10 carbon atoms in the main chain includes at least one of nonyl acrylate structural unit, decyl acrylate structural unit, undecyl acrylate structural unit, dodecyl acrylate structural unit, and tridecyl acrylate structural unit; and / or, the difunctional acrylate structural unit containing at least one polar functional group includes at least one of tricyclodecane dimethanol diacrylate structural unit, bisphenol A dimethacrylate structural unit, ethoxylated bisphenol A dimethacrylate structural unit, bisphenol F dimethacrylate structural unit, pentaerythritol tetraacrylate structural unit, and isophorone dimethacrylate structural unit; and / or, the acrylate structural unit containing at least two polar functional groups includes at least one of N-hydroxymethyl acrylamide structural unit, 2-hydroxyethyl acrylamide structural unit, 2,3-dihydroxypropyl acrylate structural unit, 2-acryloyloxyethyl-2'-hydroxyethyl methacrylamide structural unit, 3-hydroxypropyl methacrylate structural unit, and 2-hydroxypropyl acrylate structural unit.

6. A negative electrode sheet, characterized in that, The negative electrode sheet includes the binder according to any one of claims 1-5.

7. The negative electrode sheet according to claim 6, characterized in that The negative electrode sheet further includes a negative electrode active material, and the negative electrode active material includes hard carbon.

8. The negative electrode sheet according to claim 7, characterized in that, The median particle size of the hard carbon is 5μm to 30μm.

9. The negative electrode sheet according to claim 8, wherein, The negative electrode sheet further includes a conductive agent; the mass ratio of the binder, the hard carbon, and the conductive agent is (0.8 to 3):(50 to 100):(1 to 3).

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes the binder according to any one of claims 1-5, or the negative electrode sheet according to any one of claims 6-9.

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