A binder, a negative electrode sheet, and a lithium ion battery

Abstract

This invention provides an adhesive, a negative electrode sheet, and a lithium-ion battery. The adhesive comprises a polymer, which includes 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 its 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. This adhesive can effectively improve the wear and collision problems of hard carbon particles caused by volume expansion, and can also improve the diffusion rate of lithium ions under high-rate charge and discharge, compensating for the low conductivity of hard carbon. Simultaneously, it can enhance the adhesion between the electrode material and the current collector, thereby effectively improving the rate performance and cycle performance of the battery.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and relates to a binder, and more particularly to a binder, a negative electrode sheet and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries, with their advantages of high energy density, environmental friendliness, and lack of memory effect, have been widely used in various fields, such as aerospace, new energy, electric vehicles, and portable electronic devices. As a key material in battery manufacturing, binders directly affect battery performance and stability. Therefore, selecting a suitable binder is crucial in lithium-ion battery manufacturing.

[0003] Currently, commonly used binders include styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC), and polyvinyl alcohol (PVA), but all of them suffer from problems such as poor shear resistance, easy demulsification, floating, and low mechanical strength. Furthermore, during the cyclic charging and discharging of lithium batteries, the electrode materials may undergo drastic changes. If the mechanical strength of the binder is insufficient, it may lead to the peeling or separation of the electrode materials, affecting the cycle stability of the battery. Currently, hard carbon is a commonly used negative electrode active material, but it also has some drawbacks in its application. On the one hand, during charging and discharging, the hard carbon material undergoes volume expansion due to the insertion and extraction of lithium ions. This volume expansion leads to collisions and wear between hard carbon particles, causing deactivation of the active sites in the hard carbon particles and reducing the battery's cycle life. On the other hand, hard carbon has low electrical conductivity, so the lithium ion diffusion rate may be limited during high-rate charging and discharging, thus affecting the battery's rate performance. Summary of the Invention

[0004] To address the aforementioned deficiencies, this invention provides an adhesive that not only effectively improves the wear and collision problems of hard carbon particles caused by volume expansion, but also increases the diffusion rate of lithium ions under high-rate charge and discharge, compensating for the low conductivity of hard carbon; simultaneously, it also enhances the peel 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, applying the negative electrode sheet to a lithium-ion battery can effectively improve the rate performance and cycle performance of the battery.

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

[0007] The first aspect of the present invention provides an adhesive comprising a polymer, the polymer comprising a first structural unit, a second structural unit, and a third structural unit; the first structural unit comprises an acrylate structural unit having a main chain of not less than 10 carbon atoms, 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 described above, wherein the weight-average molecular weight of the adhesive is 2 × 10⁻⁶. 5 g / mol~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%–36%):(12%–28%):(36%–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 includes at least one of hydroxyl, epoxy, amino, and isocyanate groups; 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, ether, and sulfonic acid groups.

[0011] The adhesive as described above, wherein the acrylate structural unit with a main chain of not less than 10 carbon atoms includes at least one of nonyl acrylate, decaacrylate, undecyl acrylate, dodecyl acrylate, and tridecyl acrylate.

[0012] And / or, the bifunctional acrylate structural unit containing at least one polar functional group includes at least one of the following: tricyclodecanedimethyl diacrylate structural unit, bisphenol A dimethacrylate structural unit, ethoxylated bisphenol A dimethacrylate structural unit, bisphenol F dimethacrylate structural unit, tetramethylolpropane tetraacrylate structural unit, and isophorone dimethacrylate structural unit.

[0013] And / or, the acrylate structural unit containing at least two polar functional groups includes at least one of N-hydroxymethylacrylamide structural unit, 2-hydroxyethylacrylamide structural unit, 2,3-dihydroxypropyl acrylate structural unit, 2-acryloyloxyethyl-2'-hydroxyethylmethylacrylamide structural unit, 3-hydroxypropyl methacrylate structural unit, and 2-hydroxypropyl acrylate structural unit.

[0014] A second aspect of the present invention provides a negative electrode sheet, the negative electrode sheet comprising the binder of the first aspect.

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

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

[0017] The negative electrode sheet as described above 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] A third aspect of the present invention provides a lithium-ion battery, the lithium-ion battery comprising a binder of the first aspect or a negative electrode sheet of the second aspect.

[0019] This invention comprises a polymer backbone (i.e., a first structural unit, including acrylate structural units with at least 10 carbon atoms in the backbone) and branches grafted onto the polymer backbone. The branches include second and third structural units, wherein the second structural unit includes a bifunctional 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 controlled to be between 55°C and 70°C. This not only reduces the collision and wear between hard carbon particles, thus lowering electrode wear, but also improves the conductivity of the electrode, compensating for the limited lithium-ion diffusion rate caused by the low conductivity of hard carbon. Simultaneously, it enhances the peel strength between the binder and the current collector, thereby comprehensively improving the rate performance and cycle performance of the battery. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0021] The first aspect of the present invention provides an adhesive comprising a polymer, the polymer comprising a first structural unit, a second structural unit, and a third structural unit; the first structural unit comprises an acrylate structural unit having a main chain of not less than 10 carbon atoms, 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] For example, 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 of any two of these values.

[0023] In this invention, "a bifunctional acrylate structural unit containing at least one polar functional group" refers to a structural unit containing at least one acrylate functional group and at least one polar functional group.

[0024] In this invention, "acrylate structural unit containing at least two polar functional groups" refers to a structural unit containing one acrylate functional group and at least two polar functional groups.

[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 this invention may be a single structural unit.

[0026] Specifically, the first structural unit (including acrylate structural units with no less than 10 carbon atoms in the main chain) in the above polymer is the polymer main chain, and the second structural unit (including bifunctional acrylate structural units containing at least one polar functional group) and the third structural unit (including acrylate structural units containing at least two polar functional groups) are both polymer branches.

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

[0028] In this invention, "main chain" refers to the chain containing the most carbon atoms, including functional groups. Preferably, the main chain carbon number of the acrylate structural unit with at least 10 main chain carbon atoms is 10 to 20; similarly, the main chain carbon number of the acrylate monomer with at least 10 main chain carbon atoms is 10 to 20. Exemplarily, the main chain carbon number can be 10, 12, 14, 16, 18, or 20, etc.

[0029] This invention does not specifically limit the source of acrylate monomers with no less than 10 carbon atoms in the main chain, bifunctional acrylate monomers containing at least one polar functional group, or acrylate monomers containing at least two polar functional groups. Products prepared by commercially available products or conventional preparation methods well known to those skilled in the art are acceptable.

[0030] The present invention does not specifically limit the initiator in the above-mentioned free 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] This invention does not specifically limit the source of the initiator; any commercially available product or product prepared by conventional methods well known to those skilled in the art can be used.

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

[0033] In this invention, the "glass transition temperature" refers to the temperature at which the glassy state transitions to the elastic state, which can be obtained by differential scanning calorimetry (DSC).

[0034] This invention provides an adhesive comprising a first structural unit (an acrylate structural unit with at least 10 carbon atoms in its main chain), a second structural unit (an acrylate structural unit containing at least one polar functional group), and a third structural unit (an acrylate structural unit containing at least two polar functional groups). The first structural unit serves as the polymer's main chain, while the second and third structural units act as branches grafted onto the polymer main chain. By controlling the glass transition temperature of the adhesive to be between 55°C and 70°C, a flexible polymer structure can be obtained. Firstly, the second structural unit can form a network cross-linked structure. On the one hand, this network cross-linked structure contains numerous pores, which can adjust the gaps between hard carbon particles, allowing them more space to expand and contract during charging and discharging. This disperses the stress experienced by the hard carbon particles during charging and discharging, ensuring that the stress is evenly distributed throughout the electrode structure. This reduces the collisions and wear between hard carbon particles, lowers electrode wear, and effectively improves the battery's cycle life. On the other hand, this network cross-linked structure also provides support, providing more support points between hard carbon particles. This buffers the volume expansion of hard carbon particles during charging and discharging, reducing the impact of volume changes on the electrode, improving the structural stability of the electrode, and thus improving the battery's cycle performance. Furthermore, this network cross-linked structure can construct a good conductive network, making electron conduction within the electrode more uniform and efficient, helping to improve the electrode's conductivity, lower resistance, and effectively improve the battery's rate performance. Secondly, the third structural unit has at least two polar functional groups, exhibiting strong interaction with the hard carbon surface. It can form a coating layer on the surface of the hard carbon particles, further preventing collisions and wear between hard carbon particles and reducing electrode wear. Finally, the polar functional groups contained in the second structural unit can form hydrogen bonds or other chemical bonds with the current collector surface, thereby enhancing the adhesion between the binder and the current collector and improving the mechanical strength of the electrode. Therefore, the binder in this invention can comprehensively improve the rate performance and cycle performance of the battery.

[0035] Furthermore, the high crystallinity and strong intermolecular forces of hard carbon particles make them difficult to disperse in solvents, easily leading to agglomeration. Simultaneously, the poor hydrophilicity of hard carbon particles weakens their interaction with some solvents, hindering effective dispersion. However, the hydrophilic groups in the coating layer formed on the surface of hard carbon particles enhance the affinity between them and the solvent, thereby strengthening their interaction. This also helps to weaken the intermolecular interactions between hard carbon particles, improving their dispersibility in solvents and effectively reducing agglomeration and accumulation during slurry preparation, ultimately enhancing their overall dispersibility in the slurry.

[0036] In one specific embodiment, the weight-average molecular weight of the adhesive is 2 × 10⁻⁶. 5 g / mol~5×10 5 g / mol. Within this range, on the one hand, the binder has strong mechanical strength and toughness, which can maintain stable adhesion between the negative electrode active material and the current collector during charge-discharge cycles, reducing material shedding; on the other hand, it helps to form a uniform electrode coating, improves the structural stability of the electrode, and thus extends the cycle life of the battery; furthermore, it can provide good slurry dispersibility and coatability, ensure good rheological properties during production, and help to form a uniform electrode coating; at the same time, it can also improve the problem of increased resistance caused by excessively 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 adjusting the reaction temperature and time during the polymerization reaction, the mass ratio of the initiator to acrylate monomers with at least 10 carbon atoms in the main chain, the mass ratio of the initiator to bifunctional acrylate monomers containing at least one polar functional group, the mass ratio of the initiator to acrylate monomers containing at least two polar functional groups, and the mass ratio of the aforementioned three types of reactants. This ensures that the weight-average molecular weight of the binder is between 2 × 10⁻⁶. 5 g / mol~5×10 5 Between g / mol.

[0038] For example, the weight-average molecular weight of the adhesive can be 2 × 10⁻⁶. 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 a range consisting of any two of these values.

[0039] In one 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, the mass ratio of acrylate monomers with at least 10 carbon atoms in the main chain, bifunctional acrylate monomers containing at least one polar functional group, and acrylate monomers containing at least two polar functional groups in the polymerization reaction can be controlled so that the mass percentage of the first structural unit, the second structural unit, and the third structural unit in the polymer is within the aforementioned range.

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

[0042] Within this range, the polymer backbone and its branches can work together more effectively, allowing the resulting cross-linked network structure to better integrate with the hard carbon particles. This further reduces wear and collision between the hard carbon particles, resulting in higher cycle stability. Moreover, a better conductive network can be formed within this range, further improving electron transport efficiency and achieving higher rate performance. Furthermore, the adhesion between electrode materials and between the electrode materials and the current collector can be further enhanced, improving the mechanical strength of the electrodes. Finally, the affinity between hard carbon particles and solvent water molecules can be further enhanced, improving the problem of hard carbon particle agglomeration or accumulation in the slurry. This allows the hard carbon particles to be more uniformly dispersed in the solvent, further improving the battery's cycle performance.

[0043] In one specific embodiment, the polar functional group in the bifunctional acrylate structural unit containing at least one polar functional group includes at least one of hydroxyl, epoxy, amino, and isocyanate groups. This facilitates the formation of cross-linked structures during polymerization, improves the mechanical strength, solvent resistance, and heat resistance of the binder, and provides higher strength and toughness to the electrode including the binder, reducing cracking and expansion of the electrode coating during cycling, thereby improving the cycle performance of the battery.

[0044] It is understandable that since a difunctional acrylate structural unit containing at least one polar functional group is obtained by polymerization of a difunctional acrylate monomer containing at least one polar functional group, the polar functional group in the difunctional acrylate monomer containing at least one polar functional group also includes at least one of hydroxyl, epoxy, amino, and isocyanate groups.

[0045] In one specific embodiment, the polar functional groups in the acrylate structural unit containing at least two polar functional groups include at least one of hydroxyl, carboxyl, ether, and sulfonic acid groups. These groups can form a stable interface with the hard carbon surface through hydrogen bonding, electrostatic interactions, or dipole-dipole interactions, thereby forming a continuous coating layer on the particle surface. This enhances interparticle adhesion and structural integrity, buffers stress changes during charge and discharge, significantly reduces the risk of particle detachment and interface damage, and ultimately improves the electrode's cycle stability and capacity retention.

[0046] It is understandable that since acrylate structural units containing at least two polar functional groups are obtained by polymerization of acrylate monomers containing at least two polar functional groups, the polar functional groups in acrylate monomers containing at least two polar functional groups also include at least one of hydroxyl, carboxyl, ether, and sulfonic acid groups.

[0047] In one specific embodiment, the glass transition temperature of acrylate monomers with a main chain of not less than 10 carbon atoms is -90°C to -40°C, and the glass transition temperature of bifunctional acrylate monomers containing at least one polar functional group is 100°C to 300°C.

[0048] For example, the glass transition temperature of acrylate monomers with a main chain of not less than 10 carbon atoms can be a range of -90℃, -85℃, -80℃, -75℃, -70℃, -65℃, -60℃, -55℃, -50℃, -45℃, -40℃, or any two of these values; the glass transition temperature of bifunctional acrylate monomers containing at least one polar functional group can be a range of 100℃, 120℃, 140℃, 160℃, 180℃, 200℃, 220℃, 240℃, 260℃, 280℃, 300℃, or any two of these values.

[0049] When the glass transition temperatures of the aforementioned two types of monomers are within the range described above, it helps to keep the glass transition temperature of the binder between 55°C and 70°C. This not only further reduces the mutual collisions and wear between hard carbon particles but also forms a better conductive network, improving electronic conductivity and contributing to higher rate performance and cycle performance of the battery. Simultaneously, it ensures the electrode has high toughness and mechanical strength and improves the uniformity of hard carbon particle dispersion in the solvent water, further enhancing the battery's cycle stability. Finally, it also improves the electrode's flexibility, preventing cracking due to the expansion of the negative electrode active material during cycling and improving the structural stability of the electrode.

[0050] In one specific embodiment, the acrylate structural unit with a main chain of not less than 10 carbon atoms includes at least one of the following: nonyl acrylate structural unit, decaacrylate structural unit, undecyl acrylate structural unit, dodecyl acrylate structural unit, and tridecyl acrylate structural unit.

[0051] Specifically, the acrylate structural unit with a main chain of no less than 10 carbon atoms is derived from an acrylate monomer with a main chain of no less than 10 carbon atoms; the acrylate monomer with a main chain of no less than 10 carbon atoms includes at least one of nonyl acrylate, decaacrylate, undecyl acrylate, dodecyl acrylate, and tridecyl acrylate.

[0052] When the aforementioned acrylate structural unit with no less than 10 carbon atoms in the main chain includes multiple specific structural units, the present invention does not impose specific limitations on the proportion of each specific structural unit.

[0053] In one specific embodiment, the bifunctional acrylate structural unit containing at least one polar functional group includes at least one of the following: tricyclodecanedimethyl diacrylate structural unit, bisphenol A dimethacrylate structural unit, ethoxylated bisphenol A dimethacrylate structural unit, bisphenol F dimethacrylate structural unit, tetramethylolpropane tetraacrylate structural unit, and isophorone dimethacrylate structural unit.

[0054] Specifically, the bifunctional acrylate structural unit containing at least one polar functional group is derived from a bifunctional acrylate monomer containing at least one polar functional group; this bifunctional acrylate monomer includes at least one of tricyclodecanedimethyl diacrylate, bisphenol A dimethacrylate, ethoxylated bisphenol A dimethacrylate, bisphenol F dimethacrylate, tetramethylolpropane tetraacrylate, and isophorone dimethacrylate. The glass transition temperature of tricyclodecanedimethyl diacrylate is 130°C, that of bisphenol A dimethacrylate is 115°C, that of ethoxylated bisphenol A dimethacrylate is 90°C, that of bisphenol F dimethacrylate is 130°C, that of tetramethylolpropane tetraacrylate is 110°C, and that of isophorone dimethacrylate is 120°C.

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

[0056] In one specific embodiment, the acrylate structural unit containing at least two polar functional groups includes at least one of N-hydroxymethylacrylamide structural unit, 2-hydroxyethylacrylamide structural unit, 2,3-dihydroxypropyl acrylate structural unit, 2-acryloyloxyethyl-2'-hydroxyethylmethylacrylamide 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-hydroxymethylacrylamide, 2-hydroxyethylacrylamide, 2,3-dihydroxypropyl acrylate, 2-acryloyloxyethyl-2'-hydroxyethylmethylacrylamide, 3-hydroxypropyl methacrylate, and 2-hydroxypropyl acrylate. The glass transition temperature of N-hydroxymethylacrylamide is 170°C, that of 2-hydroxyethylacrylamide is 120°C, that of 2,3-dihydroxypropyl acrylate is 130°C, that of 2-acryloyloxyethyl-2'-hydroxyethylmethylacrylamide is 180°C, that of 3-hydroxypropyl methacrylate is 105°C, and that of 2-hydroxypropyl acrylate is 100°C.

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

[0059] The present invention also provides a method for preparing an adhesive according to a first aspect, comprising the following steps:

[0060] 1) At 50℃~70℃, a first raw material system including acrylate monomers with no less than 10 carbon atoms in the main chain and a first initiator undergoes a first polymerization reaction 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℃~75℃ to obtain the 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℃~85℃ to obtain the binder.

[0063] Specifically, in step 1), raw materials including acrylate monomers with a main chain of no less than 10 carbon atoms and a first initiator are mixed in deionized water to obtain a first raw material system. This first raw material system is then deoxygenated. After deoxygenation, the system temperature is raised to 50℃~70℃ to carry out a first polymerization reaction, obtaining a first reaction system. During this process, the acrylate monomers with a main chain of no less than 10 carbon atoms polymerize to obtain the first structural unit, i.e., the polymer backbone.

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

[0065] Furthermore, the mass ratio of the first initiator to an acrylate monomer with at least 10 carbon atoms in its main chain is (0.1–2):100. Within this range, the reaction rate is high, allowing the monomer to polymerize fully without triggering explosive polymerization.

[0066] For example, the mass ratio of the first initiator to an acrylate monomer with a main chain of not less than 10 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 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, such as 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 specify the mixing method, as long as the raw materials are uniformly dispersed in the first raw material system. For example, the mixing can be carried out by magnetic stirring or mechanical stirring.

[0069] This invention does not specify a particular method for deoxygenation; it is sufficient to completely remove the active oxygen from the first raw material system. For example, nitrogen gas can be introduced into the first raw material system for deoxygenation, and the nitrogen introduction time is 30 to 60 minutes.

[0070] The present invention does not impose a specific limit on the heating rate, which can be adjusted according to the actual situation. It is sufficient to keep the temperature of the first raw material system between 50°C and 70°C.

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

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

[0073] In step 2), a second raw material system is obtained by mixing a bifunctional acrylate monomer containing at least one polar functional group and a second initiator. This second raw material system is then added to the first reaction system, and a second polymerization reaction is carried out at 65°C–75°C to obtain the second reaction system. During this process, the bifunctional acrylate monomer containing at least one polar functional group reacts with the polymer backbone formed in step 1), grafting onto the polymer backbone to form side chains.

[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 of any two of these values.

[0075] Furthermore, a 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, thereby improving the uniformity of the reaction.

[0076] Furthermore, the second raw material system is added to the first reaction system over a period of 1.5 to 3 hours. Within this range, rapid polymerization can be avoided. It should be noted that the addition rate is uniform; the second raw material system simply needs to be added to the first reaction system within the specified time. For example, the addition time can be 1.5 hours, 2 hours, 2.5 hours, 3 hours, or any combination of 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 sufficiently without explosive polymerization. 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 any two of these ratios.

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

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

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

[0082] In step 3), a third raw material system is obtained by mixing an acrylate monomer containing at least two polar functional groups and a third initiator. This third raw material system is then added to the second reaction system described above, and a third polymerization reaction is carried out at 78°C to 85°C to obtain the binder. During this process, the acrylate monomer containing at least two polar functional groups reacts with the polymer backbone and grafts onto the polymer backbone to form side chains.

[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 any two of these values.

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

[0085] Furthermore, the third raw material system is added to the second reaction system over a period of 2 to 4 hours. Within this range, rapid polymerization can be avoided. It should be noted that the addition rate is uniform; the third raw material system simply needs to be added to the second reaction system within the specified time. For example, the addition time can be 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, or any combination of 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 to 1):100. Within this range, not only is sufficient reaction of the monomer guaranteed, but explosive polymerization is also prevented. 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 any two of these ratios.

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

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

[0089] The present invention does not impose a specific limit on the reaction time of the third polymerization reaction. An appropriate reaction time can be selected according to the actual situation, as long as the glass transition temperature of the final adhesive is between 55°C and 70°C.

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

[0091] The method for preparing the adhesive in this invention involves first polymerizing acrylate monomers with at least 10 carbon atoms in the main chain 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 to the aforementioned polymer main chain to form branches (wherein, the bifunctional acrylate monomer containing at least one polar functional group corresponds to the second structural unit, and the acrylate monomer containing at least two polar functional groups corresponds to the third structural unit). The glass transition temperature of the adhesive is controlled to be 55°C to 70°C, which can give the adhesive good flexibility and plasticity. The second structural unit can form a network cross-linked structure. The pores in this structure can adjust the gaps between hard carbon particles, alleviating the stress on the hard carbon particles during charging and discharging, improving the problem of collision and wear between hard carbon particles, reducing electrode wear, and effectively improving the cycle life of the battery. Furthermore, this network cross-linked structure can build a good conductive network inside the electrode, making electron conduction more uniform and efficient. This helps improve the conductivity of the electrode, reduce resistance, and improve the rate performance of the battery. Simultaneously, the polar functional groups in the second structural unit can interact with the surface of the current collector, forming hydrogen bonds or other chemical bonds that effectively enhance the adhesion between the binder and the current collector, improving the mechanical strength of the electrode. 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 solvent water molecules, weaken the interaction forces between hard carbon particles, improve the agglomeration or accumulation of hard carbon particles during slurry preparation, and thus improve the dispersibility of hard carbon particles in the slurry.

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

[0093] In one specific embodiment, the mass ratio of an acrylate monomer with at least 10 carbon atoms in its main chain, a bifunctional acrylate monomer containing at least one polar functional group, and an acrylate monomer containing at least two polar functional groups is (22–36):(12–28):(36–60). Within this range, it helps to ensure that the mass percentage of the first structural unit, the second structural unit, and the third structural unit in the prepared polymer is within (22%–36%):(12%–28%):(36%–60%).

[0094] A second aspect of the present invention provides a negative electrode sheet. Since the negative electrode sheet includes the binder of the first aspect, using the negative electrode sheet in a lithium-ion battery can effectively improve the rate performance and cycle stability of the battery.

[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 portion 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 commonly used negative electrode current collectors in the art, such as copper foil, can be selected.

[0097] In one specific embodiment, the negative electrode sheet further includes a negative electrode active material, which includes hard carbon. When the negative electrode active material includes hard carbon, the binder can work synergistically 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 and reduce electrode loss, but also form a good conductive network, reducing the negative impact on the battery rate performance caused by the poor conductivity of hard carbon; on the other hand, the binder can also enhance the adhesion between electrode materials and between electrode materials and current collectors, 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 one specific embodiment, the median particle size of the hard carbon is 5 μm to 30 μm. Within this range, the particle size of the hard carbon particles is suitable, ensuring not only a good lithium-ion diffusion rate and enhanced battery rate performance, but also minimizing side reactions and contributing to SEI film formation and improved initial coulombic efficiency. Simultaneously, it effectively fills the electrode space, providing high compaction density and increasing the battery's volumetric energy density without significantly negatively impacting conductivity and electrolyte wetting. Furthermore, it helps maintain the structural stability of the negative electrode active material, reducing pulverization and enhancing battery cycle life. Finally, it allows for a moderate specific surface area of ​​the hard carbon particles, reducing side reactions between the hard carbon and the electrolyte.

[0099] For example, the median particle size of hard carbon can be 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm or any combination of two of these values.

[0100] In this invention, "median particle size" refers to the particle size corresponding to a cumulative volume percentage of 50% for hard carbon particles, which can be determined using a laser particle size analyzer.

[0101] In one specific embodiment, the negative electrode sheet further includes a conductive agent; the mass ratio of binder, hard carbon, and conductive agent is (0.8–3):(50–100):(1–3). Within this range, the ratio of 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 one specific embodiment, the negative electrode sheet is prepared by a method including the following steps:

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

[0104] The negative electrode sheet obtained by the above preparation method can not only achieve high rate performance and cycle performance, but also does not require additional reduction of the speed of the dispersion equipment during the mixing process, which can effectively shorten the steps and time in the production process, simplify the process flow, and reduce energy consumption.

[0105] A third aspect of the present invention provides a lithium-ion battery, which has high rate performance and cycle performance because it includes the binder of the first aspect or the negative electrode of the second aspect.

[0106] The binder of the present invention and the lithium-ion battery including the binder are described in detail below through specific embodiments.

[0107] Example 1

[0108] 1) Adhesive preparation

[0109] 25 parts by mass of nonyl acrylate (glass transition temperature -60℃ to -50℃), 0.5 parts by mass of ammonium persulfate, and 300 parts by mass of deionized water were added to the reactor to obtain the first raw material system. Nitrogen gas was introduced for 30 minutes to remove active oxygen from the reactor and pipelines. The temperature was raised to 60℃ to carry out the first polymerization reaction for 4 hours to obtain the first reaction system.

[0110] 20 parts by mass of tricyclodecanedimethyl diacrylate (glass transition temperature 135℃) and 0.2 parts by mass of ammonium persulfate were mixed to obtain a second raw material system. The second raw material system was added to the first reaction system for 2 hours, and a second polymerization reaction was carried out at 70℃ for 5 hours to obtain the second reaction system.

[0111] A third raw material system was obtained by mixing 55 parts by mass of N-hydroxymethylacrylamide (glass transition temperature of 170°C) and 0.55 parts by mass of ammonium persulfate. The third raw material system was added to the second reaction system for 3 hours. The third polymerization reaction was carried out at 80°C for 3.5 hours to obtain the binder.

[0112] 2) Preparation of negative electrode sheet

[0113] The binder prepared in step 1), hard carbon (median particle size of 10 μm) and conductive agent carbon black were mixed at a mass ratio of 2:95:3. Deionized water was added, and the mixture was stirred under vacuum to obtain a negative electrode active slurry. The negative electrode active slurry was then uniformly coated on both sides of a copper foil, with a single-sided surface density of 90 g / m². 2 After drying, cold pressing, and slitting, a compacted density of 1.6 g / cm³ was obtained. 3 The negative electrode.

[0114] 3) Preparation of positive electrode sheet

[0115] The positive electrode active material NCM622 (chemical composition Li(Ni) 0.6 Co 0.2 Mn 0.2 A positive electrode active slurry was prepared by mixing 96.80 wt% O2, 1.5 wt% conductive carbon black (Super. P Li), 0.5 wt% conductive graphite (KS-6), 1.2 wt% binder polyvinylidene fluoride (PVDF), and N-methylpyrrolidone. The positive electrode active slurry was then coated onto both sides of an aluminum foil, with a single-sided areal density of 170 g / m³. 2 After drying, rolling, and slitting, a compacted density of 3.4 g / cm³ was obtained. 3 Positive electrode film.

[0116] 6) Lithium-ion battery manufacturing

[0117] The positive electrode, separator (PE film), and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. This forms an electrode assembly. The electrode assembly is then placed in an outer package, injected with electrolyte, and sealed. After formation and degassing processes, a lithium-ion battery is obtained. The electrolyte comprises lithium salt, organic solvent, and additives. The lithium salt is lithium hexafluorophosphate (LIPF6), the organic solvent is a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), and ethyl propionate (EP), and the additive is vinylene carbonate (VC). The volume ratio of EC, DEC, PC, and EP is 30:30:10:30. The concentration of lithium salt in the electrolyte is 1 mol / L, the mass percentage of additive VC in the electrolyte is 5%, and the remainder is organic solvent.

[0118] Example 2

[0119] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1. The difference is that in step 1), nonyl acrylate is replaced with decaacrylate (glass transition temperature is -70℃ to -60℃) in the first raw material system, and ammonium persulfate is replaced with potassium persulfate.

[0120] Example 3

[0121] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1. The difference is that in step 1), nonyl acrylate is replaced with undecyl acrylate (glass transition temperature is -70℃) in the first raw material system, and ammonium persulfate is replaced with sodium persulfate.

[0122] In the second raw material system, tricyclodecanedimethyl diacrylate is replaced with bisphenol A dimethacrylate (glass transition temperature is 115℃), and ammonium persulfate is replaced with sodium persulfate.

[0123] Example 4

[0124] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 1), ammonium persulfate is replaced with benzoyl peroxide in the first raw material system.

[0125] In the second raw material system, tricyclodecanedimethyl diacrylate is replaced with bisphenol F dimethacrylate (glass transition temperature is 130℃), and ammonium persulfate is replaced with benzoyl peroxide.

[0126] In the third raw material system, N-hydroxymethylacrylamide is replaced with 2-acryloyloxyethyl-2'-hydroxyethylmethylacrylamide (glass transition temperature is 180℃), and ammonium persulfate is replaced with benzoyl peroxide.

[0127] Example 5

[0128] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 1), ammonium persulfate is replaced with tert-butyl peroxide in the first raw material system.

[0129] In the second raw material system, tricyclodecanediethanol diacrylate is replaced with tetramethylolpropane tetraacrylate (glass transition temperature is 110℃), and ammonium persulfate is replaced with tert-butyl peroxide.

[0130] In the third raw material system, N-hydroxymethylacrylamide is replaced with 3-hydroxypropyl methacrylate (glass transition temperature is 105℃), and ammonium persulfate is replaced with tert-butyl peroxide.

[0131] Example 6

[0132] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1. The difference is that in step 1), the mass fraction of nonyl acrylate in the first raw material system is 22 parts and the mass fraction of ammonium persulfate is 0.44 parts.

[0133] In the second raw material system, the mass fraction of tricyclodecanediethanol diacrylate is 28 parts, and the mass fraction of ammonium persulfate is 0.28 parts;

[0134] In the third raw material system, N-hydroxymethylacrylamide has a mass fraction of 50 parts, and ammonium persulfate has a mass fraction of 0.5 parts.

[0135] Example 7

[0136] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1. The difference is that in step 1), the mass fraction of nonyl acrylate in the first raw material system is 28 parts and the mass fraction of ammonium persulfate is 0.56 parts.

[0137] In the second raw material system, the mass fraction of tricyclodecanediethanol diacrylate is 12 parts, and the mass fraction of ammonium persulfate is 0.12 parts;

[0138] In the third raw material system, N-hydroxymethylacrylamide has a mass fraction of 60 parts, and ammonium persulfate has a mass fraction of 0.6 parts.

[0139] Example 8

[0140] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1. The difference is that in step 1), the mass fraction of nonyl acrylate in the first raw material system is 36 parts and the mass fraction of ammonium persulfate is 0.72 parts.

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

[0142] In the third raw material system, N-hydroxymethylacrylamide has a mass fraction of 36 parts, and ammonium persulfate has a mass fraction of 0.36 parts.

[0143] Example 9

[0144] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 2), the median particle size of hard carbon is 5.4 μm.

[0145] Example 10

[0146] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1, except that in step 2), the median particle size of hard carbon is 29.5 μm.

[0147] Example 11

[0148] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1. The difference is 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.

[0149] Example 12

[0150] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1. The difference is that in step 1), the mass fraction of nonyl acrylate in the first raw material system is adjusted to 40 parts, and the mass fraction of ammonium persulfate is adjusted to 0.8 parts.

[0151] In the second raw material system, the mass fraction of tricyclodecanediethanol diacrylate was adjusted to 30 parts, and the mass fraction of ammonium persulfate was adjusted to 0.3 parts;

[0152] In the third raw material system, N-hydroxymethylacrylamide has a mass fraction of 30 parts, and ammonium persulfate has a mass fraction of 0.3 parts.

[0153] Example 13

[0154] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1. The difference is that in step 2), hard carbon is replaced with artificial graphite with a median particle size of 18 μm.

[0155] Example 14

[0156] The preparation method of the lithium-ion battery in this embodiment is basically the same as that in Example 1. The difference is that in step 2), hard carbon is replaced with natural graphite with a median particle size of 20 μm.

[0157] Example 15

[0158] The preparation method of the lithium-ion battery in this embodiment 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.

[0159] Comparative Example 1

[0160] The preparation method of the lithium-ion battery in this comparative example is basically the same as that in Example 1. The difference is that in step 1), the second polymerization reaction is not carried out, that is, the third polymerization reaction is carried out directly after the first polymerization reaction. At this time, in the first raw material system, the mass fraction of nonyl acrylate is 45 parts, the mass fraction of ammonium persulfate is 0.9 parts, and the rest remain unchanged.

[0161] Comparative Example 2

[0162] The preparation method of the lithium-ion battery in this comparative example is basically the same as that in Example 1. The difference is 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; then in the first raw material system, the mass fraction of nonyl acrylate is 80 parts, the mass fraction of ammonium persulfate is 1.6 parts, and the others remain unchanged.

[0163] Comparative Example 3

[0164] The preparation method of the lithium-ion battery in this comparative example is basically the same as that in Example 1. The difference is 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 obtained directly after the first polymerization reaction. At this time, the mass fraction of nonyl acrylate is 100 parts, the mass fraction of ammonium persulfate is 2 parts, and the others remain unchanged.

[0165] Comparative Example 4

[0166] The preparation method of the lithium-ion battery in this comparative example is basically the same as that in Example 1. The difference is that in step 1), the first polymerization reaction is not carried out, that is, the second polymerization reaction and the third polymerization reaction are carried out directly to obtain the binder of this comparative example. At this time, the mass fraction of tricyclodecanediethanol diacrylate is 45 parts, the mass fraction of ammonium persulfate is 0.45 parts, and the others remain unchanged.

[0167] Test case

[0168] 1. The weight-average molecular weight and glass transition temperature of the binders prepared in the above examples and comparative examples were tested:

[0169] 1) Weight-average molecular weight

[0170] The weight-average molecular weight of the binder was determined using gel permeation chromatography.

[0171] 2) Glass transition temperature

[0172] The glass transition temperature (Tg) of the binders prepared in the examples and comparative examples was detected using a differential scanning calorimeter (Shanghai Qunhong Instrument Equipment Co., Ltd., model: DSC-100). The procedure included: turning on high-purity nitrogen, setting the nitrogen flow rate to 0.5 L / min to 0.6 L / min, turning on the DSC power, and running the desktop program. The temperature was set to -60℃, held for 10 min, ranging from -60℃ to 160℃, with a heating rate of 10 K / min. After setting, the prepared sample was placed on the heating furnace, the furnace cover was closed, and an appropriate amount of liquid nitrogen was added to the thermostatic container. The test was conducted when the sample temperature reached -60℃.

[0173] The test results are shown in Table 1.

[0174] 2. The peel strength between the negative electrode active layer and the negative electrode current collector in the negative electrode sheets prepared in the above embodiments and comparative examples was tested:

[0175] The coated single-sided negative electrode sheet was prepared at a ratio of 1.6 g / cm². 3 After compaction, the electrode is cut into 20cm long x 3cm wide pieces. 3M double-sided tape is attached to the steel plate, and the coated side of the electrode is fixed to the tape on the steel plate with the coated side facing down. After rolling back and forth 6 times with a 2.5kg roller, a tensile testing machine with a range of 20N is used. The upper plate clamps the copper foil side, and the coating and copper foil are torn apart at a speed of 50mm / min and a tensile test at 180°. The data of the stable tensile section is recorded as the peel strength (N / m).

[0176] The test results are shown in Table 1.

[0177] 3. The rate performance and cycle performance of the lithium-ion batteries prepared in the above embodiments and comparative examples were tested:

[0178] (1) Ratio performance test

[0179] Step 1: At 25℃, first charge to 4.2V with a constant current at a rate of 0.5C, then charge at 4.2V with a constant voltage and cut-off current of 0.05C; then discharge to 3.0V with a constant current at a rate of 0.2C, end the discharge, let stand for 10 minutes, and record the baseline discharge capacity.

[0180] Step 2: Charge to 4.2V with a constant current at a rate of 0.5C, then charge at a constant voltage of 4.2V with a cutoff current of 0.05C; discharge to 3.0V with a constant current at a rate of 0.5C, then stop discharging and let stand for 10 minutes.

[0181] Step 3: Charge to 4.2V with a constant current at a rate of 0.5C, then charge at a constant voltage of 4.2V with a cutoff current of 0.05C; discharge to 3.0V with a constant current at a rate of 1C, then stop discharging and let stand for 10 minutes.

[0182] Step 4: Charge to 4.2V with a constant current at a rate of 0.5C, then charge at a constant voltage of 4.2V with a cutoff current of 0.05C; discharge to 3.0V with a constant current at a rate of 2C, then stop discharging and let stand for 10 minutes.

[0183] Step 5: Charge to 4.2V with a constant current at a rate of 0.5C, then charge at a constant voltage of 4.2V with a cutoff current of 0.05C; discharge to 3.0V with a constant current at a rate of 3C, then stop discharging and let stand for 10 minutes.

[0184] Step 6: Charge to 4.2V with a constant current at a rate of 0.5C, then charge at a constant voltage of 4.2V with a cutoff current of 0.05C; discharge to 3.0V with a constant current at a rate of 5C, then stop discharging and let stand for 10 minutes; record the 5C discharge capacity.

[0185] The capacity retention rate at 5C is calculated as follows: (5C discharge capacity / reference discharge capacity) × 100%.

[0186] (2) Cyclic performance test

[0187] At 25℃, charge at a constant current of 1C to 4.5V, then charge at a constant voltage of 4.5V with a cutoff current of 0.05C; then discharge at a discharge rate of 1C to 3.0V, and 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%.

[0188] In Table 1, Tg represents the glass transition temperature of the adhesive, Mw represents the weight-average molecular weight of the adhesive, P represents the peel strength of the adhesive, and A represents the mass percentage of the first, second, and third structural units in the polymer.

[0189] The test results are shown in Table 1.

[0190] Table 1

[0191]

[0192] As shown in Table 1:

[0193] When the binder of the present invention is used in anode sheets 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.

[0194] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An adhesive, characterized in that, The adhesive comprises a polymer, the polymer comprising a first structural unit, a second structural unit, and a third structural unit; 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. The first structural unit is at least one of nonyl acrylate, decaacrylate, undecyl acrylate, dodecyl acrylate, and tridecyl acrylate. The bifunctional acrylate structural unit containing at least one polar functional group includes at least one of the following: tricyclodecanediethanol diacrylate structural unit, bisphenol A dimethacrylate structural unit, ethoxylated bisphenol A dimethacrylate structural unit, and bisphenol F dimethacrylate structural unit. The acrylate structural unit containing at least two polar functional groups includes at least one of N-hydroxymethylacrylamide, 2-hydroxyethylacrylamide, 2,3-dihydroxypropyl acrylate, 2-acryloyloxyethyl-2'-hydroxyethylmethylacrylamide, 3-hydroxypropyl methacrylate, and 2-hydroxypropyl acrylate. 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%).

2. The adhesive according to claim 1, characterized in that, The weight-average molecular weight of the adhesive is 2 × 10⁻⁶. 5 g / mol~5×10 5 g / mol.

3. A negative electrode sheet, characterized in that, The negative electrode sheet includes the binder as described in claim 1 or 2.

4. The negative electrode sheet according to claim 3, characterized in that, The negative electrode sheet also includes a negative electrode active material, which includes hard carbon.

5. The negative electrode sheet according to claim 4, characterized in that, The median particle size of the hard carbon is 5 μm to 30 μm.

6. The negative electrode sheet according to claim 5, characterized in that, The negative electrode also 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).

7. A lithium-ion battery, characterized in that, The lithium-ion battery includes the binder as described in claim 1 or 2, or the negative electrode sheet as described in any one of claims 3-6.

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