Binder, negative electrode sheet and lithium ion battery

Abstract

The present invention provides a binder, a negative electrode sheet, and a lithium-ion battery. The binder comprises C5-C15 alkyl acrylate structural units, polar structural units, and polyurethane acrylate structural units; the binder has a glass transition temperature of -10°C to 40°C. The binder significantly improves the peel strength between the electrode material and the current collector, as well as the dispersibility of artificial graphite in the slurry, thereby effectively improving the battery's cycling stability.

Description

Technical Field

[0001] The present 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 batteries, one of the current mainstream battery technologies, play a vital role in mobile devices, electric vehicles, and energy storage systems. In lithium-ion batteries, binders, a key material in battery manufacturing, primarily serve to secure active materials, enhance electrode conductivity, strengthen the bond between the electrode and the current collector, and improve battery cycling stability. They effectively and firmly bond electrode materials to the electrode surface, forming a uniform and dense electrode structure, thereby enhancing the battery's charge transfer efficiency and cycle life. Therefore, selecting the right binder is crucial in lithium battery manufacturing.

[0003] Commonly used binders include styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC), and polyvinyl alcohol (PVA). However, SBR and PVA are not shear-resistant during processing, prone to demulsification and floating. CMC also has low mechanical strength and is not shear-resistant. During the charge and discharge cycle of lithium batteries, the volume of the electrode material changes significantly, which can cause delamination or separation of the electrode material, or even cracking of the electrode, resulting in capacity loss and reduced cycle performance. Furthermore, artificial graphite, a commonly used negative electrode active material, is difficult to disperse and takes a long time to disperse during the slurry preparation process. This leads to easy sedimentation of the slurry and wastes energy, which can also significantly affect the battery's cycle performance. Summary of the Invention

[0004] In response to the above-mentioned defects, the present invention provides a binder that can significantly improve the peel strength between the electrode material and the current collector, as well as the dispersibility of artificial graphite in the slurry, thereby effectively improving the cycle stability of the battery.

[0005] The present invention provides a negative electrode sheet comprising the above-mentioned binder. Since the binder can improve the peel strength between the electrode material and the current collector, as well as the dispersibility of the artificial graphite in the slurry, the negative electrode sheet can be applied to a lithium-ion battery to effectively improve the cycle performance of the battery.

[0006] The present invention provides a lithium ion battery. Since the lithium ion battery includes the binder or the negative electrode sheet, the battery has high cycle stability.

[0007] A first aspect of the present invention provides an adhesive comprising a C5-C15 alkyl acrylate structural unit, a polar structural unit and a polyurethane acrylate structural unit; the adhesive has a glass transition temperature of -10°C to 40°C.

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

[0009] In the adhesive as described above, the mass percentages of the C5-C15 alkyl acrylate structural unit, the polar structural unit, and the polyurethane acrylate structural unit are (30%-70%): (10%-50%): (5%-30%).

[0010] The adhesive as described above, wherein the C5~C15 alkyl acrylate structural unit includes at least one of a pentyl acrylate structural unit, a hexyl acrylate structural unit, a heptyl acrylate structural unit, an octyl acrylate structural unit, an isononyl acrylate structural unit, a cyclohexyl acrylate structural unit, and an isooctyl acrylate structural unit.

[0011] In the binder as described above, the polar structural unit includes at least one of an acrylamide structural unit, an acrylonitrile structural unit, and an acrylic acid structural unit.

[0012] The adhesive as described above, wherein the acrylamide structural unit includes at least one of an acrylamide structural unit, a methacrylamide structural unit, an N-isopropylmethacrylamide structural unit, and an N,N-dimethylacrylamide structural unit;

[0013] And / or, the acrylonitrile structural unit includes at least one of an acrylonitrile structural unit, a methacrylonitrile structural unit, and a phenylacrylonitrile structural unit;

[0014] And / or, the acrylic structural unit includes an acrylic structural unit and / or a methacrylic structural unit.

[0015] The adhesive as described above, wherein the polyurethane acrylate structural unit includes at least one of an HDI-polyurethane acrylate structural unit, an IPDI-polyurethane acrylate structural unit, a PPG-polyurethane acrylate structural unit, a PEG-polyurethane acrylate structural unit, a BDO-polyurethane acrylate structural unit, an HDO-polyurethane acrylate structural unit, a polyadipate polyurethane acrylate structural unit, a trifunctional polyurethane acrylate structural unit, and a tetrafunctional polyurethane acrylate structural unit.

[0016] A second aspect of the present invention provides a negative electrode sheet, comprising the binder described in the first aspect.

[0017] The negative electrode sheet as described above, wherein the negative electrode sheet further comprises a negative electrode active material; the negative electrode active material comprises artificial graphite.

[0018] A third aspect of the present invention provides a lithium-ion battery, comprising the binder described in the first aspect, or the negative electrode sheet described in the second aspect.

[0019] The present invention provides a binder comprising C5-C15 alkyl acrylate structural units, polar structural units and polyurethane acrylate structural units, while controlling the glass transition temperature of the binder to be between -10°C and 40°C. This ensures good adhesion and compatibility between the binder and the current collector, and simultaneously improves the dispersion uniformity of the artificial graphite in the solvent and the stability of the dispersion system, thereby effectively enhancing the cycle performance of the lithium-ion battery. DETAILED DESCRIPTION

[0020] To make the objectives, technical solutions, and advantages of the present invention more clear, 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 part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0021] A first aspect of the present invention provides an adhesive comprising a C5-C15 alkyl acrylate structural unit, a polar structural unit and a polyurethane acrylate structural unit; the adhesive has a glass transition temperature of -10°C to 40°C.

[0022] Illustratively, the glass transition temperature (Tg) of the binder may be -10°C, -5°C, 0°C, 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, 40°C, or a range consisting of any two of these values.

[0023] Specifically, the C5-C15 alkyl acrylate structural units are obtained by free radical polymerization of C5-C15 alkyl acrylate monomers in the presence of an initiator. Similarly, the polar structural units are obtained by free radical polymerization of acrylic monomers containing at least one polar group in the presence of an initiator, and the polyurethane acrylate structural units are obtained by free radical polymerization of polyurethane acrylate monomers in the presence of an initiator.

[0024] In the present invention, "C5-C15 alkyl acrylate monomers" refer to alkyl acrylate monomers having 5 to 15 carbon atoms in the main chain. For example, the main chain has 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 carbon atoms. "Main chain" refers to the chain with the largest number of carbon atoms, including the functional groups.

[0025] The present invention does not impose any specific limitation on the molecular weight of the polyurethane acrylate monomer. For example, a polyurethane acrylate monomer with a weight average molecular weight of 500 g / mol to 3000 g / mol may be selected.

[0026] The present invention does not specifically limit the polar group in the "acrylic monomer containing at least one polar group", and it can be, for example, a hydroxyl group, an amino group, a cyano group, a carboxyl group, and the like.

[0027] The present invention does not specifically limit the sources of C5-C15 alkyl acrylate monomers, acrylic monomers containing at least one polar group, and polyurethane acrylate monomers. Commercially available products or products prepared by conventional preparation methods familiar to those skilled in the art may be used.

[0028] The present invention does not specifically limit the initiator in the above-mentioned free radical polymerization reaction, and can be an initiator commonly used 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.

[0029] The present invention does not specifically limit the source of the initiator, and any commercially available product or a product prepared by a conventional preparation method well known to those skilled in the art may be used.

[0030] Specifically, during the preparation process, the glass transition temperature of the adhesive can be further controlled by controlling the glass transition temperature of the C5~C15 alkyl acrylate monomer and the acrylic monomer containing at least one polar group, or the mass ratio of the C5~C15 alkyl acrylate monomer, the acrylic monomer containing at least one polar group and the polyurethane acrylate monomer, so that the glass transition temperature of the adhesive is between -10°C and 40°C.

[0031] The “glass transition temperature” in the present invention refers to the temperature corresponding to the transition from a glassy state to a highly elastic state, which can be obtained by differential scanning calorimetry (DSC) testing.

[0032] In the present invention, the polyurethane acrylate structural unit is preferably an aliphatic polyurethane acrylate structural unit. It is understood that the aliphatic polyurethane acrylate structural unit is derived from an aliphatic polyurethane acrylate monomer.

[0033] The present invention achieves excellent flexibility and plasticity by combining a binder composed of C5-C15 alkyl acrylate structural units (derived from C5-C15 alkyl acrylate monomers), polar structural units (derived from acrylic monomers containing at least one polar group), and polyurethane acrylate structural units (derived from polyurethane acrylate monomers), with a glass transition temperature controlled between -10°C and 40°C. Furthermore, the multiple polar groups (such as hydroxyl, amino, cyano, and carboxyl groups) contained in the branched polar structural units can form hydrogen bonds or other chemical bonds with the current collector surface, improving adhesion and compatibility between the binder and the current collector and enhancing the mechanical strength of the electrode. Furthermore, the polyurethane acrylate structural units possess both lipophilic (such as cyano, long-chain alkyl, and aromatic groups) and hydrophilic (such as hydroxyl and carboxyl groups) groups, enabling simultaneous interaction with the artificial graphite surface and solvent water molecules. This enhances interactions between the binder, the solvent, and the artificial graphite particles, helping to evenly disperse the artificial graphite in the solvent and form a stable dispersion. On the other hand, polyurethane acrylate structural units are derived from polyurethane acrylate monomers through free radical polymerization. Polyurethane acrylate monomers are derived from aliphatic polyols (such as poly(1,6-hexanediol adipate)) and long-chain alkyl acrylate monomers. The aliphatic structure is typically saturated, soft, and contains no aromatic rings. Therefore, this polyurethane acrylate structural unit can undergo physical adsorption or chemical reactions with the surface of artificial graphite, such as adsorption between lipophilic groups (such as long-chain alkyl or aliphatic segments) and hydrophobic regions on the surface of artificial graphite. The soft aliphatic chains can approach the surface of artificial graphite, generating weak intermolecular forces and forming a coating on the surface of artificial graphite. It can be understood that this coating is covered on the surface of artificial graphite particles through physical adsorption and chemical bonding, forming a molecular film on the surface of artificial graphite. The main functions of this coating layer include: 1) reducing the polarity or hydrophobicity mismatch problem on the surface of artificial graphite, introducing soft aliphatic segments, and adjusting the flexibility and interfacial compatibility of the artificial graphite surface; 2) the aliphatic segments of the coating layer can reduce the direct contact between artificial graphite particles, making the artificial graphite particles more dispersed and reducing the attraction between artificial graphite particles; 3) improving surface hydrophilicity: the side chains of the polyurethane acrylate structural units contain polar groups that can adsorb solvent molecules, enhance the interaction between the artificial graphite surface and the solvent, and form a stable dispersion system; 4) through cross-linking or bonding, improve the stability of the chemical bonds on the surface of artificial graphite and reduce agglomeration. The above functions all help to achieve uniform dispersion of artificial graphite, improve the stability of the slurry, and ultimately improve electrode performance.

[0034] Therefore, the binder in the present invention can improve the peel strength between the electrode material and the current collector, as well as the dispersibility of the artificial graphite in the slurry, thereby effectively improving the cycle performance of the battery.

[0035] In one embodiment, the weight average molecular weight of the binder is 2×10 5 g / mol~5×10 5 g / mol. Within this range, the polymer chain length is sufficiently long to provide good mechanical strength and toughness, while avoiding excessive rigidity or brittleness. It also maintains a certain degree of flexibility, helping to absorb electrode expansion and stress changes, and improving the cycling stability of the electrode. At the same time, the binder has good solubility or dispersibility in the solution, which facilitates its uniform distribution in the electrode. It also balances the mechanical strength and interfacial compatibility of the electrode, preventing peeling or shedding.

[0036] When the weight average molecular weight of the binder is less than 2×10 5 When the molecular weight is too low, the molecular chain will be too short, which will not only reduce the mechanical strength of the polymer and make the electrode prone to cracking or peeling under stress, affecting the cycle performance, but also cause insufficient flexibility and fail to effectively buffer the volume expansion and contraction in the electrode. In addition, it may also cause the binder to fail to form a sufficiently strong interaction with the current collector and the active material surface, resulting in insufficient interfacial bonding. On the other hand, a binder with too low a molecular weight may exhibit low viscosity in the solution, resulting in an unstable dispersion system and difficulty in coating the artificial graphite particles. It also cannot effectively reduce the attraction between the artificial graphite particles, causing the slurry to agglomerate and affecting the electrode coating quality.

[0037] When the weight average molecular weight of the binder is higher than 5×10 5 When the molecular weight is too high at 0.05477 g / mol, on the one hand, the viscosity of the binder will increase significantly, making it difficult to disperse evenly in the solvent, affecting the slurry preparation and coating process; it may also cause the rheological properties of the slurry to deteriorate, making it difficult to achieve uniform coating and good thickness control. On the other hand, due to the high viscosity, the binder may be difficult to distribute evenly between the artificial graphite particles, affecting the dispersion effect and causing particle agglomeration or stratification. On the other hand, excessively long molecular chains may lead to increased stress in the pole piece, and in extreme cases may cause cracks or stratification of the pole piece, affecting battery life and performance; and high molecular weight may cause the pole piece to adhere too strongly, increasing resistance during pole piece processing and battery assembly, affecting production efficiency.

[0038] Specifically, the weight average molecular weight of the binder can be further controlled by controlling the reaction temperature in the polymerization reaction, the mass ratio of the initiator to the C5-C15 alkyl acrylate monomer, the mass ratio of the initiator to the acrylic monomer containing at least one polar group, the mass ratio of the initiator to the polyurethane acrylate monomer, and the mass ratio of the C5-C15 alkyl acrylate monomer, the acrylic monomer containing at least one polar group, and the polyurethane acrylate monomer, so that the weight average molecular weight of the binder is within the range of 2×10 5 g / mol~5×10 5 Between g / mol.

[0039] For example, the weight average molecular weight of the binder may be 2×10 5 g / mol, 2.5×10 5 g / mol, 3.0×10 5 g / mol, 3.5×10 5 g / mol, 4.0×10 5 g / mol, 4.5×10 5 g / mol, 5.0×10 5 g / mol or a range consisting of any two of these values.

[0040] In a specific embodiment, the mass percentages of the C5-C15 alkyl acrylate structural units, the polar structural units, and the polyurethane acrylate structural units are (30%-70%): (10%-50%): (5%-30%).

[0041] Specifically, the mass percentage of the C5-C15 alkyl acrylate structural units in the binder is 30%-70%, the mass percentage of the polar structural units is 10%-50%, and the mass percentage of the polyurethane acrylate structural units is 5%-30%.

[0042] For example, the mass percentage of the C5-C15 alkyl acrylate structural unit in the binder may be 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, or a range consisting of any two values ​​therein; the mass percentage of the polar structural unit in the binder may be 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range consisting of any two values ​​therein; the mass percentage of the polyurethane acrylate structural unit in the binder may be 5%, 10%, 15%, 20%, 25%, 30%, or a range consisting of any two values ​​therein.

[0043] Within this range, the C5~C15 alkyl acrylate structural units, polar structural units and polyurethane acrylate structural units can better cooperate and coordinate with each other, further enhancing the peel strength and compatibility between the binder and the current collector, and improving the problem of electrode material falling off from the current collector surface; at the same time, the polyurethane acrylate structural units can be better coated on the surface of the artificial graphite particles, further reducing the agglomeration and aggregation of the artificial graphite particles, and improving the stability and uniformity of the dispersion system, so that the lithium-ion battery has higher cycle performance.

[0044] When the mass percentage of C5~C15 alkyl acrylate structural units in the binder is lower than 30%, the binder is not flexible enough and the electrode is prone to cracking or peeling; when it is higher than 70%, the electrode is not mechanically strong enough to support electrode expansion and stress changes, interface damage increases, and active material shedding intensifies, resulting in a significant decrease in battery cycle performance; at the same time, it will also lead to a decrease in the hydrophilicity of the binder, insufficient interaction between the active material and the binder, and easy sedimentation of particles.

[0045] When the mass percentage of polar structural units in the binder is lower than 10%, the peel strength between the binder and the current collector and active material will decrease, increasing the risk of electrode peeling; when it is higher than 50%, it will lead to excessive polar groups, which may cause the hygroscopicity of the binder to increase, the solubility of the binder to decrease, the stability of the slurry to decrease, and the performance of the battery to deteriorate.

[0046] When the mass percentage of polyurethane acrylate structural units in the binder is less than 5%, the flexibility of the binder and its dispersibility in the solvent are insufficient, which can easily lead to agglomeration of the slurry; when it is higher than 30%, it may introduce too much lipophilicity, resulting in poor compatibility with aqueous solvents, thereby affecting the uniformity and processing performance of the slurry, and may even cause phase separation of the slurry, reduce the stability of the slurry, reduce the overall bonding performance of the binder, and also increase costs.

[0047] Specifically, the mass ratio of each structural unit can be controlled within the range of (30%~70%): (10%~50%): (5%~30%) by controlling the mass ratio of C5~C15 alkyl acrylate monomers, acrylic monomers containing at least one polar group, and polyurethane acrylate monomers in the polymerization reaction.

[0048] In one embodiment, the binder is prepared by a method comprising the following steps:

[0049] After the C5-C15 alkyl acrylate monomer undergoes a first polymerization reaction, an acrylic monomer containing at least one polar group is added to the reaction system to initiate a second polymerization reaction. Subsequently, a polyurethane acrylate monomer is added to the reaction system to initiate a third polymerization reaction to obtain a binder.

[0050] Specifically, a C5-C15 alkyl acrylate monomer and a first initiator are mixed in deionized water to obtain a mixed system. The mixed system is deoxygenated and then subjected to a first polymerization reaction, during which a C5-C15 alkyl acrylate structural unit is formed. After the reaction is completed, an acrylic monomer containing at least one polar group and a second initiator are added to the reaction system to carry out a second polymerization reaction, during which the acrylic monomer containing at least one polar group is grafted onto the aforementioned C5-C15 alkyl acrylate structural unit to form a polar structural unit. After the reaction is completed, a polyurethane acrylate monomer and a third initiator are added to the reaction system to carry out a third polymerization reaction, during which the polyurethane acrylate structural unit is grafted onto the aforementioned C5-C15 alkyl acrylate structural unit to obtain a binder.

[0051] Furthermore, the temperature of the first polymerization reaction is 50°C to 70°C, the temperature of the second polymerization reaction is 70°C to 90°C, and the temperature of the third polymerization reaction is 80°C to 100°C.

[0052] Illustratively, the temperature of the first polymerization reaction can be 50°C, 55°C, 60°C, 65°C, 70°C or a range consisting of any two of them; the temperature of the second polymerization reaction can be 70°C, 75°C, 80°C, 85°C, 90°C or a range consisting of any two of them; the temperature of the third polymerization reaction can be 80°C, 85°C, 90°C, 95°C, 100°C or a range consisting of any two of them.

[0053] The present invention does not impose any specific restrictions on the reaction time of the first polymerization reaction, the second polymerization reaction, and the third polymerization reaction. Appropriate reaction time can be selected according to actual conditions, as long as the glass transition temperature of the finally prepared adhesive is -10°C to 40°C.

[0054] In one embodiment, the reaction time of the first polymerization reaction is 3 hours to 5 hours; for example, the reaction time is 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, or a range consisting of any two values ​​therein. The reaction time of the second polymerization reaction is 3.5 hours to 5.5 hours; for example, the reaction time is 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, or a range consisting of any two values ​​therein. The reaction time of the third polymerization reaction is 3 hours to 6 hours; for example, the reaction time is 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, or a range consisting of any two values ​​therein.

[0055] The present invention does not impose any specific limitation on the mixing method. It is only necessary to uniformly disperse the reaction materials in the mixing system. For example, the mixing can be performed by magnetic stirring or mechanical stirring.

[0056] The present invention does not specifically limit the deoxygenation method, and it is sufficient to completely remove the active oxygen in the mixed system. For example, nitrogen is introduced into the mixed system for deoxygenation, and the nitrogen introduction time is not less than 1 hour.

[0057] In the present invention, the types and sources of the first initiator, the second initiator and the third initiator are not specifically limited and can be consistent with the definition of the above-mentioned initiators, and are not described in detail here; and the first initiator, the second initiator and the third initiator can be the same or different.

[0058] Furthermore, the mass ratio of the C5-C15 alkyl acrylate monomer, the acrylic monomer containing at least one polar group, and the polyurethane acrylate monomer is (30-70):(10-50):(5-30). Within this range, the mass percentages of the C5-C15 alkyl acrylate structural units, polar structural units, and polyurethane acrylate structural units in the prepared binder are (30%-70%):(10%-50%):(5%-30%), resulting in higher cycle performance for the battery.

[0059] Furthermore, the mass ratio of the first initiator to the C5~C15 alkyl acrylate monomer is (0.1~2):100, the mass ratio of the second initiator to the acrylic monomer containing at least one polar group is (0.1~2):100, and the mass ratio of the third initiator to the polyurethane acrylate monomer is (0.1~2):100.

[0060] For example, the mass ratio of the first initiator to the C5~C15 alkyl acrylate monomer can be 0.1:100, 0.2:100, 0.5:100, 1:100, 1.5:100, 2:100, or a range consisting of any two ratios therein; the mass ratio of the second initiator to the acrylic monomer containing at least one polar group can be 0.1:100, 0.2:100, 0.5:100, 1:100, 1.5:100, 2:100, or a range consisting of any two ratios therein; the mass ratio of the third initiator to the polyurethane acrylate monomer can be 0.1:100, 0.2:100, 0.5:100, 1:100, 1.5:100, 2:100, or a range consisting of any two ratios therein.

[0061] Furthermore, the glass transition temperature of C5-C15 alkyl acrylate monomers is between -60°C and -10°C; and the glass transition temperature of propylene monomers containing at least one polar group is between 50°C and 150°C. Within this range, the binder's glass transition temperature is maintained between -10°C and 40°C, enhancing the peel strength between the binder and the current collector and increasing the uniformity of the dispersion of the artificial graphite in the aqueous solvent, thereby improving the battery's cycling stability. Furthermore, it optimizes thermal stability, electrolyte resistance, mechanical properties, and processing performance, ensuring a long battery life and high stability.

[0062] For example, the glass transition temperature of the C5~C15 alkyl acrylate monomer can be -60°C, -55°C, -50°C, -45°C, -40°C, -35°C, -30°C, -25°C, -20°C, -15°C, -10°C, or a range consisting of any two values ​​therein; the glass transition temperature of the acrylic monomer containing at least one polar group can be 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, 125°C, 130°C, 135°C, 140°C, 145°C, 150°C, or a range consisting of any two values ​​therein.

[0063] In a specific embodiment, the C5~C15 alkyl acrylate structural unit includes at least one of a pentyl acrylate structural unit, a hexyl acrylate structural unit, a heptyl acrylate structural unit, an octyl acrylate structural unit, an isononyl acrylate structural unit, a cyclohexyl acrylate structural unit, and an isooctyl acrylate structural unit.

[0064] Specifically, the C5~C15 alkyl acrylate structural unit is derived from a C5~C15 alkyl acrylate monomer, and the C5~C15 alkyl acrylate monomer includes at least one of pentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate, isononyl acrylate, cyclohexyl acrylate, and isooctyl acrylate.

[0065] When the C5-C15 alkyl acrylate structural unit includes multiple of the aforementioned specific structural units, the present invention does not impose any specific limitation on the ratio between the specific structural units.

[0066] In a specific embodiment, the polar structural unit includes at least one of an acrylamide structural unit, an acrylonitrile structural unit, and an acrylic acid structural unit.

[0067] Specifically, the polar structural unit is derived from an acrylic monomer containing at least one polar group, and the acrylic monomer containing at least one polar group includes at least one of an acrylamide monomer, an acrylonitrile monomer, and an acrylic acid monomer.

[0068] In a specific embodiment, the acrylamide structural unit includes at least one of an acrylamide structural unit, a methacrylamide structural unit, an N-isopropylmethacrylamide structural unit, and an N,N-dimethylacrylamide structural unit.

[0069] Specifically, the acrylamide structural unit is derived from an acrylamide monomer, and the acrylamide monomer includes at least one of acrylamide, methacrylamide, N-isopropylmethacrylamide, and N,N-dimethylacrylamide.

[0070] In a specific embodiment, the acrylonitrile structural unit includes at least one of an acrylonitrile structural unit, a methacrylonitrile structural unit, and a phenylacrylonitrile structural unit.

[0071] Specifically, the acrylonitrile structural unit is derived from an acrylonitrile monomer, and the acrylonitrile monomer includes at least one of acrylonitrile, methacrylonitrile, and phenylacrylonitrile.

[0072] In a specific embodiment, the acrylic structural unit includes at least one of an acrylic structural unit and a methacrylic structural unit.

[0073] Specifically, the acrylic structural unit is derived from an acrylic monomer, and the acrylic monomer includes at least one of acrylic acid and methacrylic acid.

[0074] When the polar structural unit includes the aforementioned multiple specific structural units at the same time, the present invention does not impose any specific limitation on the ratio between the specific structural units.

[0075] In a specific embodiment, the polyurethane acrylate structural unit includes at least one of an HDI-polyurethane acrylate structural unit, an IPDI-polyurethane acrylate structural unit, a PPG-polyurethane acrylate structural unit, a PEG-polyurethane acrylate structural unit, a BDO-polyurethane acrylate structural unit, an HDO-polyurethane acrylate structural unit, a polyadipate polyurethane acrylate structural unit, a trifunctional polyurethane acrylate structural unit, and a tetrafunctional polyurethane acrylate structural unit.

[0076] Specifically, the polyurethane acrylate structural unit is derived from a polyurethane acrylate monomer, and the polyurethane acrylate monomer includes at least one of HDI-polyurethane acrylate (such as EBECRYL® 8402, CN983), IPDI-polyurethane acrylate (such as EBECRYL® 284, MIRAMER® PU210), PPG-polyurethane acrylate, PEG-polyurethane acrylate, BDO-polyurethane acrylate, HDO-polyurethane acrylate, polyadipate polyurethane acrylate, EBECRYL 284 (from Allnex, USA), EBECRYL 8411 (from Allnex, USA), trifunctional polyurethane acrylate (such as CN966J75), tetrafunctional polyurethane acrylate, MIRAMER PU210 (from Miwon Co., Ltd. of South Korea), and MIRAMERPU256 (from Miwon Co., Ltd. of South Korea).

[0077] When the polyurethane acrylate structural unit includes the aforementioned multiple specific structural units at the same time, the present invention does not impose any specific limitation on the ratio between the specific structural units.

[0078] The present invention also provides a method for preparing the adhesive of the first aspect, comprising the following steps:

[0079] 1) causing a first polymerization reaction of a first raw material system including a C5-C15 alkyl acrylate monomer and a first initiator at 50° C. to obtain a first reaction system;

[0080] 2) adding a second raw material system comprising an acrylic monomer containing at least one polar group and a second initiator to the first reaction system, initiating a second polymerization reaction at 70° C. to 90° C. to obtain a second reaction system;

[0081] 3) adding a third raw material system including a polyurethane acrylate monomer and a third initiator to the second reaction system, initiating a third polymerization reaction at 80° C. to 100° C. to obtain a binder.

[0082] Specifically, in step 1), raw materials including C5-C15 alkyl acrylate monomers 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°C to 70°C for a first polymerization reaction to obtain a first reaction system. During this process, the C5-C15 alkyl acrylate monomers polymerize to obtain C5-C15 alkyl acrylate structural units.

[0083] The present invention does not specifically limit the type of the first initiator, which can be consistent with the above-mentioned initiator and will not be described in detail here.

[0084] The present invention does not specifically limit the mixing method and the deoxygenation method. For example, the mixing method and the deoxygenation method defined above can be adopted, and will not be described in detail here.

[0085] The present invention does not impose any specific limitation on the heating rate, and it is sufficient that the temperature of the first raw material system is between 50° C. and 70° C.

[0086] The present invention does not impose any specific limitation on the reaction time of the first polymerization reaction, which may be consistent with the above-mentioned first polymerization reaction time, and will not be described in detail here.

[0087] In step 2), a raw material comprising an acryl-based monomer containing at least one polar group and a second initiator are mixed to obtain a second raw material system. This second raw material system is added to the first reaction system, and a second polymerization reaction is carried out at 70°C to 90°C to obtain a second reaction system. During this process, the acryl-based monomer containing at least one polar group reacts with the C5-C15 alkyl acrylate structural unit formed in step 1) and is grafted onto the C5-C15 alkyl acrylate structural unit.

[0088] Furthermore, the second raw material system is added to the first reaction system under stirring, so that the second raw material system is evenly dispersed in the first reaction system, thereby improving the uniformity of the reaction.

[0089] Furthermore, the addition time of the second raw material system to the first reaction system is 1.5h to 3.5h. Within this range, it is possible to prevent sudden polymerization during the polymerization reaction. 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 1.5h, 2h, 2.5h, 3h, 3.5h, or a range consisting of any two values ​​therein.

[0090] The present invention does not specifically limit the type of the second initiator, which can be consistent with the above-mentioned initiator and will not be described in detail here.

[0091] The present invention does not specifically limit the mixing method. For example, the mixing method defined above can be adopted, which will not be described in detail here.

[0092] The present invention does not impose any specific limitation on the reaction time of the second polymerization reaction, which may be consistent with the above-mentioned second polymerization reaction time, and will not be further described herein.

[0093] In step 3), raw materials including polyurethane acrylate monomers and a third initiator are mixed to obtain a third raw material system, and the third raw material system is added to the above-mentioned second reaction system, and a third polymerization reaction is carried out at 80°C to 100°C. During the reaction, the polyurethane acrylate monomers react with the C5~C15 alkyl acrylate structural units formed in step 1) and are grafted onto the C5~C15 alkyl acrylate structural units to prepare a binder.

[0094] Furthermore, the third raw material system is added to the second reaction system under stirring, so that the third raw material system is evenly dispersed in the second reaction system, thereby improving the uniformity of the reaction.

[0095] Furthermore, the addition time of the third raw material system to the second reaction system is 2h~4h. Within this range, it is possible to prevent sudden polymerization during the polymerization reaction. It should be noted that the addition speed is uniform, and it is only necessary to add the third raw material system to the second reaction system within the prescribed time. Exemplarily, the addition time can be 2h, 2.5h, 3h, 3.5h, 4h, or a range consisting of any two of these values.

[0096] The present invention does not specifically limit the type of the third initiator. For example, the third initiator defined above can be used, which will not be described in detail here.

[0097] The present invention does not specifically limit the mixing method. For example, the mixing method defined above can be adopted, which will not be described in detail here.

[0098] The present invention does not impose any specific limitation on the reaction time of the third polymerization reaction, which may be consistent with the above-mentioned third polymerization reaction time, and will not be further described herein.

[0099] The present invention does not impose any specific restrictions on the mass ratio of the C5-C15 alkyl acrylate monomer, the acrylic monomer containing at least one polar group, and the polyurethane acrylate monomer, which can be consistent with the above-mentioned restrictions and will not be repeated here.

[0100] The present invention does not impose any specific restrictions on the mass ratio of the first initiator to the C5-C15 alkyl acrylate monomer, the mass ratio of the second initiator to the acrylic monomer containing at least one polar group, and the mass ratio of the third initiator to the polyurethane acrylate monomer. These ratios can be consistent with the above-mentioned restrictions and will not be further described here.

[0101] The preparation method of the adhesive of the present invention comprises the following steps: firstly, subjecting C5-C15 alkyl acrylate monomers to a first polymerization reaction to form C5-C15 alkyl acrylate structural units as a polymer main chain; then, adding an acrylic monomer containing at least one polar group, reacting the monomer with the C5-C15 alkyl acrylate structural units, and grafting the monomer onto the polymer main chain to form polar structural units as side chains; and finally, adding a polyurethane acrylate monomer, reacting the monomer with the C5-C15 alkyl acrylate structural units, and grafting the monomer onto the polymer main chain to form polyurethane acrylate structural units as side chains, thereby preparing an adhesive having a glass transition temperature of -10°C to 40°C. The binder has good flexibility and plasticity, and the polar groups contained in the side chains can form hydrogen bonds or other chemical bonds with the surface of the current collector, thereby improving the peel strength between the electrode material and the current collector; at the same time, the polyurethane acrylate structural units in the side chains have lipophilic and hydrophilic groups, which can act simultaneously with artificial graphite and water molecules to improve the dispersibility of artificial graphite in water; and the polyurethane acrylate structural units can form a uniform coating on the surface of artificial graphite, reducing the agglomeration of artificial graphite particles, improving the stability of the slurry, and thus comprehensively improving the cycle performance of lithium-ion batteries.

[0102] A second aspect of the present invention provides a negative electrode sheet, which includes the binder of the first aspect. Therefore, using the negative electrode sheet in a lithium-ion battery can effectively improve the cycle stability of the battery.

[0103] In one embodiment, the negative electrode sheet further includes a negative electrode active material; the negative electrode active material includes artificial graphite. When the negative electrode active material includes artificial graphite, the binder can significantly improve the dispersion of the artificial graphite in the slurry system and enhance the stability of the dispersion system, thereby further improving the cycle life of the lithium-ion battery.

[0104] 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.

[0105] The present invention does not impose any specific limitation on the material of the negative electrode current collector, and any negative electrode current collector commonly used in the art, such as copper foil, may be used.

[0106] In a specific embodiment, the negative electrode sheet further includes a conductive agent; the mass ratio of the binder, the artificial graphite, and the conductive agent is (0.8-3): (50-100): (1-3).

[0107] Specifically, the weight percentage of the binder in the negative electrode active layer is 0.8 to 3 parts, the weight percentage of the artificial graphite is 50 to 100 parts, and the weight percentage of the conductive agent is 1 to 3 parts.

[0108] For example, the mass fraction of the binder in the negative electrode active layer can be 0.8 parts, 1 parts, 1.5 parts, 2 parts, 2.5 parts, 3 parts, or a range consisting of any two of the values; the mass fraction of the artificial graphite can be 50 parts, 60 parts, 70 parts, 80 parts, 90 parts, 100 parts, or a range consisting of any two of the values; the mass fraction of the conductive agent can be 1 part, 1.5 parts, 2 parts, 2.5 parts, 3 parts, or a range consisting of any two of the values.

[0109] Within this range, the ratio of binder, artificial graphite and conductive agent is more appropriate, which can not only improve the uniformity of distribution of artificial graphite on the negative electrode sheet, but also make the electrode material and the current collector have stronger peeling strength, prevent the electrode material from falling off, and improve the cycle performance of the battery; at the same time, it can also make the negative electrode sheet have sufficient conductive network to ensure the rate performance of the battery; and the content of artificial graphite, the negative electrode active material, will not be too little, ensuring that the battery has a higher capacity.

[0110] In the present invention, the negative electrode sheet can be prepared by existing methods. In one embodiment, a negative electrode active slurry including a binder, a negative electrode active material, and a conductive agent can be applied to at least a portion of the surface of the negative electrode current collector, and then rolled, dried, and cut to obtain the negative electrode sheet.

[0111] A fourth aspect of the present invention provides a lithium-ion battery. Since the lithium-ion battery includes the binder of the first aspect, or the binder prepared by the preparation method of the second aspect, or the negative electrode sheet of the third aspect, the battery has high cycle performance.

[0112] Hereinafter, the binder of the present invention and the lithium ion battery including the binder will be described in detail through specific examples.

[0113] Example 1

[0114] 1) Binder Preparation: 65 parts by mass of octyl acrylate (glass transition temperature, -70°C), 0.13 parts by mass of ammonium persulfate (a first initiator), and deionized water were added to a reactor to obtain a first raw material system. Nitrogen was introduced to remove active oxygen in the reactor and pipelines. The temperature was raised to 65°C for a first polymerization reaction for 4 hours to obtain a first reaction system.

[0115] A second raw material system was obtained by mixing 20 parts by mass of acrylamide (glass transition temperature: 165°C) and 0.04 parts by mass of a second initiator, ammonium persulfate. The second raw material system was added to the first reaction system for 2 hours. A second polymerization reaction was carried out at 78°C for 4 hours to obtain a second reaction system.

[0116] A third raw material system was obtained by mixing 15 parts by mass of HDI-polyurethane acrylate (weight-average molecular weight of 1000 g / mol) and 0.03 parts by mass of a third initiator, ammonium persulfate. The third raw material system was added to the above-mentioned second reaction system for 3 hours. A third polymerization reaction was carried out at 90° C. for 4 hours to obtain a binder.

[0117] 2) Preparation of negative electrode sheet:

[0118] In parts by mass, 97 parts of artificial graphite, 2.5 parts of the binder prepared in step 1), and 1.5 parts of carbon black were mixed and stirred at a low speed of 15 rpm for 15 minutes to obtain a mixed solution, and water was added to make the solid content of the mixed solution 55 wt%; then, the mixture was pre-stirred at a stirring speed of 40 rpm for 15 minutes and kneaded, and then stirred at 40 rpm for 40 minutes, and then dispersed at a high speed of 15 m / s for 40 minutes, and then 0.5 parts of the binder was added. After dispersing at 5 m / s for 20 minutes, the mixture was vacuum defoamed and discharged to obtain a negative electrode active slurry; finally, the negative electrode active slurry was coated on both sides of the negative electrode current collector copper foil, and the single-sided coating surface density was 90 g / m 2 After drying, cold pressing, slitting and welding of the tabs, the compacted density is 1.6g / cm 3 The negative electrode.

[0119] 3) Preparation of positive electrode sheet:

[0120] The positive electrode active material NCM622 (chemical composition is Li(Ni 0.6 Co 0.2 Mn 0.2 )O2), conductive agent carbon black (SuperP), and binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 97.3:1.5:1.2, and N-methylpyrrolidone (NMP) is added and stirred evenly to prepare a positive electrode active slurry. The positive electrode active slurry is coated on both sides of the aluminum foil, with a single-side coating density of 170g / m 2 After drying, rolling, slitting and welding of the tabs, the compacted density is 3.4g / cm 3 The positive electrode.

[0121] 4) Lithium-ion battery preparation:

[0122] The negative electrode sheet, separator (PE film), and positive electrode sheet are wound in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. This forms an electrode assembly, which is then placed in an outer package. The electrolyte is then injected and packaged. After forming and exhausting, a secondary battery is obtained. The electrolyte comprises 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), and the additive is vinylene carbonate (VC). 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 of the VC additive in the electrolyte is 5 wt%, and the balance is the organic solvent.

[0123] Example 2

[0124] The binder and battery were prepared according to the method of Example 1, except that:

[0125] Octyl acrylate was replaced by heptyl acrylate (glass transition temperature of -50°C), acrylamide was replaced by methacrylamide (glass transition temperature of 118°C), and HDI-urethane acrylate was replaced by IPDI-urethane acrylate (weight average molecular weight of 800 g / mol). Other parameters remained unchanged.

[0126] Example 3

[0127] The binder and battery were prepared according to the method of Example 1, except that:

[0128] Isononyl acrylate (glass transition temperature of -70°C) was used to replace octyl acrylate, acrylonitrile (glass transition temperature of 105°C) was used to replace acrylamide, and PPG-polyurethane acrylate (weight average molecular weight of 1500 g / mol) was used to replace HDI-polyurethane acrylate. Other parameters remained unchanged.

[0129] Example 4

[0130] The binder and battery were prepared according to the method of Example 1, except that:

[0131] Cyclohexyl acrylate (glass transition temperature of -25°C) was used to replace octyl acrylate, acrylic acid (glass transition temperature of 105°C) was used to replace acrylamide, and PEG-urethane acrylate (weight average molecular weight of 1800 g / mol) was used to replace HDI-urethane acrylate. Other parameters remained unchanged.

[0132] Example 5

[0133] The binder and battery were prepared according to the method of Example 1, except that:

[0134] Octyl acrylate was replaced by amyl acrylate (glass transition temperature of -70°C), acrylamide was replaced by acrylic acid (glass transition temperature of 105°C), and HDI-urethane acrylate was replaced by BDO-urethane acrylate (weight average molecular weight of 800 g / mol). Other parameters remained unchanged.

[0135] Example 6

[0136] The binder and battery were prepared according to the method of Example 1, except that:

[0137] In step 1), in the first polymerization reaction, the amount of octyl acrylate added was adjusted to 30 parts, and the amount of ammonium persulfate added was adjusted to 0.06 parts;

[0138] In the second polymerization reaction, the amount of acrylamide added was adjusted to 50 parts, and the amount of ammonium persulfate added was adjusted to 0.1 parts;

[0139] In the third polymerization reaction, the addition amount of HDI-polyurethane acrylate was adjusted to 20 parts, the addition amount of ammonium persulfate was adjusted to 0.04 parts, and the other conditions remained unchanged.

[0140] Example 7

[0141] The binder and battery were prepared according to the method of Example 1, except that:

[0142] In step 1), in the first polymerization reaction, the amount of octyl acrylate added was adjusted to 70 parts, and the amount of ammonium persulfate added was adjusted to 0.14 parts;

[0143] In the second polymerization reaction, the amount of acrylamide added was adjusted to 10 parts, and the amount of ammonium persulfate added was adjusted to 0.02 parts;

[0144] In the third polymerization reaction, the addition amount of HDI-polyurethane acrylate was adjusted to 20 parts, the addition amount of ammonium persulfate was adjusted to 0.04 parts, and the other conditions remained unchanged.

[0145] Example 8

[0146] The binder and battery were prepared according to the method of Example 1, except that:

[0147] In step 1), in the second polymerization reaction, the amount of acrylamide added is adjusted to 30 parts, and the amount of ammonium persulfate added is adjusted to 0.06 parts;

[0148] In the third polymerization reaction, the addition amount of HDI-polyurethane acrylate was adjusted to 5 parts, the addition amount of ammonium persulfate was adjusted to 0.01 parts, and the other conditions remained unchanged.

[0149] Example 9

[0150] The binder and battery were prepared according to the method of Example 1, except that:

[0151] In step 1), in the first polymerization reaction, the amount of octyl acrylate added was adjusted to 50 parts, and the amount of ammonium persulfate added was adjusted to 0.1 parts;

[0152] In the third polymerization reaction, the addition amount of HDI-polyurethane acrylate was adjusted to 30 parts, the addition amount of ammonium persulfate was adjusted to 0.06 parts, and the other conditions remained unchanged.

[0153] Example 10

[0154] The binder and battery were prepared according to the method of Example 1, except that:

[0155] In step 1), in the first polymerization reaction, the amount of octyl acrylate added was adjusted to 20 parts, and the amount of ammonium persulfate added was adjusted to 0.04 parts;

[0156] In the second polymerization reaction, the amount of acrylamide added was adjusted to 60 parts, and the amount of ammonium persulfate added was adjusted to 0.12 parts;

[0157] In the third polymerization reaction, the addition amount of HDI-polyurethane acrylate was adjusted to 20 parts, the addition amount of ammonium persulfate was adjusted to 0.04 parts, and the other conditions remained unchanged.

[0158] Example 11

[0159] The binder and battery were prepared according to the method of Example 1, except that:

[0160] Replace artificial graphite with hard carbon, and keep everything else the same.

[0161] Comparative Example 1

[0162] The binder and battery were prepared according to the method of Example 1, except that:

[0163] Methyl acrylate (glass transition temperature is 10°C) was used to replace octyl acrylate, and the other conditions remained unchanged.

[0164] Comparative Example 2

[0165] The binder and battery were prepared according to the method of Example 1, except that:

[0166] Styrene (glass transition temperature of 100°C) was used to replace acrylamide, while other parameters remained unchanged.

[0167] Comparative Example 3

[0168] The binder and battery were prepared according to the method of Example 1, except that:

[0169] No HDI-polyurethane acrylate was added and the third polymerization reaction was not carried out; that is, the binder of this comparative example was obtained after the second polymerization reaction; then in the first polymerization reaction, the mass fraction of octyl acrylate was 80 parts and the mass fraction of ammonium persulfate was 0.16 parts; the others remained unchanged.

[0170] Comparative Example 4

[0171] The binder and battery were prepared according to the method of Example 1, except that:

[0172] No acrylamide was added and the second polymerization reaction was not carried out; that is, the third polymerization reaction was carried out directly after the first polymerization reaction to obtain the binder of this comparative example; then in the first polymerization reaction, the mass fraction of octyl acrylate was 85 parts and the mass fraction of ammonium persulfate was 0.17 parts; other factors remained unchanged.

[0173] Test example

[0174] 1. The weight average molecular weight and glass transition temperature of the binders prepared in the above examples and comparative examples were tested:

[0175] 1) Weight average molecular weight

[0176] The weight-average molecular weight of the binder was measured using gel permeation chromatography.

[0177] 2) Glass transition temperature

[0178] The glass transition temperature (Tg) of the adhesives prepared in the Examples and Comparative Examples was measured using a differential scanning calorimeter (DSC-100, Shanghai Qunhong Instruments Co., Ltd.). The following steps were used: High-purity nitrogen was supplied at a flow rate of 0.5 L / min to 0.6 L / min, the DSC was powered on, and the desktop instrument was operated. The temperature was set to -60°C, held constant for 10 minutes, within a temperature range of -60°C to 100°C, and at a heating rate of 10 K / min. After completing the settings, the prepared adhesive sample was placed in a heating furnace, the furnace cover was secured, and an appropriate amount of liquid nitrogen was added to the thermostat. The test was then run when the sample temperature reached -60°C.

[0179] The test results are shown in Table 1.

[0180] 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 examples and comparative examples was tested:

[0181] The coated single-sided negative electrode sheet was heated at 1.6 g / cm 3After compaction, the electrode is cut into 20 cm long and 3 cm wide, and 3M double-sided tape is attached to the steel plate. The electrode coating is fixed on the tape on the steel plate with the surface facing down. After rolling back and forth 6 times with a 2.5 kg roller, a tensile testing machine with a range of 20 N is used. The upper plate clamps the copper foil side, and the coating and copper foil are torn apart at a speed of 50 mm / min and stretched at 180 ° C. The data of the tensile plateau section is recorded as the peel strength (N / m).

[0182] The test results are shown in Table 1.

[0183] 3. The cycle performance of the lithium-ion batteries prepared in the above examples and comparative examples was tested:

[0184] At 25°C, charge at a constant current of 0.5C to 4.35V, and charge at a constant voltage to 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 above charge and discharge mechanism, and record the discharge capacity at 500 cycles as Q1. The capacity retention rate (%) of the battery after 500 cycles = (Q1 / Q0) × 100%.

[0185] The test results are shown in Table 1.

[0186] 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 C5-C15 alkyl acrylate structural unit, the polar structural unit, and the polyurethane acrylate structural unit.

[0187] Table 1

[0188]

[0189] From Table 1 we can see that:

[0190] The binder provided by the present invention can significantly increase the peel strength between the electrode material and the current collector, and at the same time improve the dispersibility of artificial graphite in the slurry, thereby effectively improving the rate performance and cycle stability of the battery.

[0191] 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 it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements 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. A binder, characterized in that: The adhesive comprises C5-C15 alkyl acrylate structural units, polar structural units and polyurethane acrylate structural units; the glass transition temperature of the adhesive is -10°C to 40°C; The polar structural unit includes an acrylamide structural unit; The preparation method of the adhesive comprises the following steps: firstly, subjecting C5-C15 alkyl acrylate monomers to a first polymerization reaction to form C5-C15 alkyl acrylate structural units as a polymer main chain; then, adding an acrylamide monomer containing at least one polar group, reacting the monomer with the C5-C15 alkyl acrylate structural units, and grafting the monomer onto the polymer main chain to form polar structural units as side chains; and finally, adding a polyurethane acrylate monomer, reacting the monomer with the C5-C15 alkyl acrylate structural units, and grafting the monomer onto the polymer main chain to form polyurethane acrylate structural units as side chains, thereby preparing an adhesive having a glass transition temperature of -10°C to 40°C.

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

3. The adhesive according to claim 1 or 2, characterized in that The mass percentages of the C5-C15 alkyl acrylate structural unit, the polar structural unit and the polyurethane acrylate structural unit are (30%-70%): (10%-50%): (5%-30%).

4. The adhesive according to claim 1, characterized in that The C5-C15 alkyl acrylate structural unit includes at least one of a pentyl acrylate structural unit, a hexyl acrylate structural unit, a heptyl acrylate structural unit, an octyl acrylate structural unit, an isononyl acrylate structural unit, a cyclohexyl acrylate structural unit, and an isooctyl acrylate structural unit.

5. The adhesive according to claim 1, characterized in that The acrylamide structural unit includes at least one of an acrylamide structural unit, a methacrylamide structural unit, an N-isopropylmethacrylamide structural unit, and an N,N-dimethylacrylamide structural unit.

6. The adhesive according to claim 1, characterized in that The polyurethane acrylate structural unit includes at least one of an HDI-polyurethane acrylate structural unit, an IPDI-polyurethane acrylate structural unit, a PPG-polyurethane acrylate structural unit, a PEG-polyurethane acrylate structural unit, a BDO-polyurethane acrylate structural unit, an HDO-polyurethane acrylate structural unit, a polyadipate polyurethane acrylate structural unit, a trifunctional polyurethane acrylate structural unit, and a tetrafunctional polyurethane acrylate structural unit.

7. A negative electrode sheet, characterized in that: The negative electrode sheet comprises the binder according to any one of claims 1 to 6.

8. The negative electrode sheet according to claim 7, characterized in that: The negative electrode sheet further includes a negative electrode active material; the negative electrode active material includes artificial graphite.

9. A lithium-ion battery, characterized in that: The invention comprises the binder according to any one of claims 1 to 6, or the negative electrode sheet according to any one of claims 7 or 8.

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