Binder, negative electrode sheet, and secondary battery

Abstract

The present application provides a kind of binder, negative electrode sheet and secondary battery, the binder includes alkali metal ion, C5~C10 acrylic ester structural unit, contain polar group's propylene base structural unit and allyl ether structural unit, the glass transition temperature of the binder is 43~86 ℃.The binder can effectively improve the cycle stability of secondary battery.

Description

Technical Field

[0001] This invention belongs to the field of secondary battery technology, and relates to an adhesive, and more particularly to an adhesive, a negative electrode sheet and a secondary battery. Background Technology

[0002] Due to their recyclable nature, rechargeable batteries are widely used in consumer electronics, electric vehicles, and energy storage systems, such as sodium-ion batteries, lithium-ion batteries, and potassium-ion batteries. However, during the cyclic charging and discharging of rechargeable batteries, the negative electrode active material undergoes significant volume changes due to the insertion and extraction of active ions. If the mechanical strength of the binder is insufficient, the negative electrode active layer may peel off, affecting the cycle stability of the rechargeable battery. Furthermore, in the application of negative electrode active materials, natural graphite, as a commonly used material, may undergo irreversible reactions with active ions (such as sodium or lithium ions) during charging and discharging, leading to the loss of active ions, a decrease in battery capacity, and consequently affecting the cycle life of the rechargeable battery. Summary of the Invention

[0003] To address the aforementioned shortcomings, this invention provides an adhesive that can effectively improve the cycle stability of secondary batteries.

[0004] This invention provides a negative electrode sheet. Since the negative electrode sheet includes the above-mentioned binder, its application in a secondary battery can effectively improve the cycle stability of the secondary battery.

[0005] The present invention provides a secondary battery comprising the above-mentioned binder or the above-mentioned negative electrode sheet, and the secondary battery has high cycle stability.

[0006] The first aspect of the present invention provides an adhesive comprising alkali metal ions, C5-C10 acrylate structural units, propylene structural units containing polar groups, and allyl ether structural units, wherein the glass transition temperature of the adhesive is 43°C to 86°C.

[0007] 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.

[0008] In the adhesive described above, the mass ratio of the alkali metal ions, the C5-C10 acrylate structural units, the propylene structural units containing polar groups, and the allyl ether structural units is (1-10):(40-60):(10-30):(5-20).

[0009] In the adhesive described above, the C5-C10 acrylate structural units are derived from C5-C10 acrylate monomers, and the glass transition temperature of the C5-C10 acrylate monomers is -65℃ to -35℃.

[0010] In the adhesive described above, the propylene-based structural unit containing polar groups is derived from propylene-based monomers containing polar groups, and the glass transition temperature of the propylene-based monomers containing polar groups is 80°C to 180°C.

[0011] In the adhesive described above, the alkali metal ion is a lithium ion or a sodium ion;

[0012] And / or, the C5-C10 acrylate structural units include at least one of the following: n-amyl acrylate structural units, isoamyl acrylate structural units, hexyl acrylate structural units, and 2-ethylhexyl acrylate structural units;

[0013] And / or, the propylene-based structural unit containing polar groups includes at least one of acrylamide structural units, acrylonitrile structural units, and acrylic acid structural units;

[0014] And / or, the allyl ether structural unit includes at least one of the following: methyl allyl polyoxyethylene ether structural unit, allyl ethoxy ether structural unit, methoxy polyethylene glycol allyl ether-100 structural unit, methoxy polyethylene glycol allyl ether-200 structural unit, methoxy polyethylene glycol allyl ether-300 structural unit, and methoxy polyethylene glycol allyl ether-400 structural unit.

[0015] The adhesive as described above, wherein the acrylamide structural unit includes at least one of N,N-dimethylacrylamide structural unit, N,N-diethylacrylamide structural unit, N-isopropylacrylamide structural unit, N-(2-hydroxyethyl)acrylamide structural unit, and acrylamide structural unit;

[0016] And / or, the acrylonitrile structural unit includes at least one of acrylonitrile structural unit, methacrylonitrile structural unit, and 2-chloroacrylonitrile structural unit;

[0017] And / or, the acrylic structural unit includes at least one of acrylic structural units, methacrylic acid structural units, and itaconic acid structural units.

[0018] A second aspect of the present invention provides a negative electrode sheet, the negative electrode sheet comprising the binder described in the first aspect.

[0019] The negative electrode sheet as described above further includes a negative electrode active material, which includes natural graphite.

[0020] A third aspect of the present invention provides a secondary battery, comprising the binder described in the first aspect or the negative electrode sheet described in the second aspect.

[0021] The binder in this invention comprises alkali metal ions, C5-C10 acrylate structural units, propylene-based structural units containing polar groups, and allyl ether structural units. Simultaneously, the glass transition temperature of the binder is controlled to be 43°C-86°C, which effectively improves the adhesion and compatibility between the binder and other components in the negative electrode active layer (such as negative electrode active materials, conductive agents, etc.), enhances the mechanical strength of the negative electrode sheet, and compensates for the irreversible loss of active ions in natural graphite during charging and discharging, reducing capacity decay and thus effectively improving the cycle stability of the secondary battery. Detailed Implementation

[0022] 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.

[0023] The first aspect of the present invention provides an adhesive comprising alkali metal ions, C5-C10 acrylate structural units, propylene structural units containing polar groups, and allyl ether structural units, wherein the glass transition temperature of the adhesive is 43°C to 86°C.

[0024] For example, the glass transition temperature of the adhesive can be 43°C, 47°C, 51°C, 55°C, 59°C, 63°C, 67°C, 71°C, 75°C, 79°C, 83°C, 86°C, or a range of any two of these values.

[0025] 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).

[0026] Specifically, the aforementioned adhesive is a polymer comprising alkali metal ions, C5-C10 acrylate structural units, propylene structural units containing polar groups, and allyl ether structural units. The C5-C10 acrylate structural units serve as the polymer backbone, while the propylene structural units and allyl ether structural units containing polar groups serve as polymer branches. The alkali metal ions and the polar groups (such as ester groups) in the polymer backbone are embedded into the polymer structure through complexation or electrostatic adsorption, forming an alkali metal ion-doped polymer backbone.

[0027] Among them, C5-C10 acrylate structural units are obtained by polymerization of C5-C10 acrylate monomers under the action of an initiator; similarly, propylene structural units containing polar groups are obtained by polymerization of propylene monomers containing polar groups under the action of an initiator, and allyl ether structural units are obtained by polymerization of allyl ether monomers under the action of an initiator.

[0028] This invention does not specifically limit the source of C5-C10 acrylate monomers, propylene monomers containing polar groups, and allyl ether monomers; products prepared using commercially available products or conventional preparation methods well known to those skilled in the art are acceptable.

[0029] The present invention does not specifically limit the initiator in the above polymerization reaction, and 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, and tert-butyl peroxide.

[0030] 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.

[0031] In this invention, the C5-C10 acrylate structural unit refers to an acrylate structural unit with 5 to 10 carbon atoms in its main chain, wherein the main chain refers to the chain containing the most carbon atoms, including functional groups. For example, the number of carbon atoms in the main chain of the C5-C10 acrylate structural unit can be 5, 6, 7, 8, 9, or 10.

[0032] This invention does not impose any particular limitation on the method of controlling the glass transition temperature of the adhesive, as long as the objective of this invention can be achieved. For example, the glass transition temperature of the adhesive can be controlled by adjusting the glass transition temperature of C5-C10 acrylate structural units and propylene structural units containing polar groups, or by adjusting the mass ratio of C5-C10 acrylate structural units, propylene structural units containing polar groups, and allyl ether structural units, thereby controlling the glass transition temperature of the adhesive to be between 43°C and 86°C.

[0033] The binder in this invention comprises alkali metal ions, C5-C10 acrylate structural units, propylene-based structural units containing polar groups, and allyl ether structural units. The binder has a glass transition temperature of 43°C to 86°C and exhibits good flexibility and plasticity. Specifically, the polar groups (such as amino, carboxyl, and cyano groups) in the polymer branches (i.e., propylene-based structural units containing polar groups) can form hydrogen bonds or other chemical bonds with the negative electrode current collector and natural graphite, effectively enhancing the adhesion between natural graphite particles and between natural graphite and the negative electrode current collector, thereby improving the mechanical strength of the negative electrode sheet. Furthermore, the polymer branches (i.e., allyl ether structural units) can also form large-area van der Waals adhesion with the surface of natural graphite through branched alkyl segments, and the ether oxygen bonds provide weakly polar adsorption sites, allowing the polymer molecules to be firmly adsorbed at the interlayer or edge positions of natural graphite. During the preparation of the negative electrode sheet, after the negative electrode slurry including the binder is coated on the negative electrode current collector, this strong adsorption force enables alkali metal ions to be uniformly dispersed around the surface of natural graphite. After the prepared negative electrode sheet is made into a secondary battery, the interfacial potential drives the alkali metal ions to embed into the natural graphite during the first charge cycle, directly compensating for the loss of active ions in the natural graphite during the first charge cycle. Compared with traditional external compensators (such as lithium or sodium supplements), it has the advantages of low ion diffusion resistance, short diffusion path, and more efficient compensation. At the same time, this strong adsorption force also enables the binder to form a protective layer on the surface of natural graphite, effectively weakening the attraction between natural graphite particles, reducing the risk of agglomeration or accumulation between natural graphite particles, improving the dispersibility of natural graphite in the negative electrode slurry, and further improving the cycle stability of the secondary battery.

[0034] Therefore, the binder in this invention can effectively improve the adhesion between the negative electrode active layer and the negative electrode current collector, prevent the negative electrode active layer from peeling or falling off, and at the same time compensate for the irreversible loss of active ions in natural graphite during charging and discharging, thereby improving the cycle stability of the secondary battery.

[0035] 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, it is advantageous for the binder to maintain sufficient chain entanglement, peel strength, and processability, and it also facilitates good coating at high solids content. When the weight-average molecular weight of the binder is greater than 5 × 10⁻⁶ g / mol... 5At a molecular weight of g / mol, the binder's molecular chains are too long, resulting in severe chain entanglement. This leads to a higher load on the disperser during the negative electrode slurry preparation process, increased energy consumption, and localized heating in the disperser, inducing water separation or thickening of the negative electrode slurry. Furthermore, under high shear, air bubbles are difficult to remove, increasing vacuum degassing time and resulting in poor uniformity of the negative electrode slurry coating thickness. During drying, interlaced stripes resembling fish scales appear on the surface of the negative electrode sheet, also known as "fish scale pattern." Simultaneously, the excessively long molecular chains bridge and block the pores, reducing the porosity of the negative electrode sheet, decreasing electrolyte wettability, increasing polarization, and increasing the battery's internal resistance. When the binder's weight-average molecular weight is less than 2 × 10⁻⁶, the negative electrode slurry becomes more difficult to remove. 5 When the viscosity is g / mol, the molecular chains of the binder are too short and the chain entanglement is insufficient, resulting in a significant decrease in the zero-shear viscosity of the negative electrode slurry. This can easily lead to problems such as slurry sedimentation, low peeling force, and low cycle performance.

[0036] 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.

[0037] This invention does not impose any particular limitations on the method of controlling the weight-average molecular weight of the binder, as long as the objective of this invention can be achieved. For example, the weight-average molecular weight of the binder can be further controlled by adjusting the reaction temperature, the mass ratio of the initiator to C5-C10 acrylate monomers, the mass ratio of the initiator to propylene monomers containing polar groups, the mass ratio of the initiator to allyl ether monomers, or the mass ratio of the aforementioned three types of monomers, so that the weight-average molecular weight of the binder is 2 × 10⁻⁶. 5 g / mol~5×10 5 g / mol.

[0038] In one specific embodiment, the mass ratio of alkali metal ions, C5-C10 acrylate structural units, propylene structural units containing polar groups, and allyl ether structural units is (1-10):(40-60):(10-30):(5-20). More specifically, based on the total mass of alkali metal ions, C5-C10 acrylate structural units, propylene structural units containing polar groups, and allyl ether structural units, the mass fraction of alkali metal ions is 1-10 parts, the mass fraction of C5-C10 acrylate structural units is 40-60 parts, the mass fraction of propylene structural units containing polar groups is 10-30 parts, and the mass fraction of allyl ether structural units is 5-20 parts. Within this range, the alkali metal ions in the binder and the various structures of the polymer can better synergize and cooperate, interacting with natural graphite. This not only helps to further improve the adhesion between natural graphite particles and between natural graphite and the negative electrode current collector, effectively improving the problem of peeling or detachment of the negative electrode active layer, but also helps to make natural graphite have better dispersibility in the negative electrode slurry system. Furthermore, the alkali metal ions in the binder can also better compensate for the loss of active ions in natural graphite during cycling, thereby comprehensively improving the cycle performance of the secondary battery.

[0039] Natural graphite has the characteristics of low surface energy, flat base surface and few functional groups. Ordinary linear binders with low glass transition temperature have poor bonding performance relying solely on van der Waals forces. Furthermore, after natural graphite is rolled, the graphite sheets are oriented in-plane, and "micro-slippage" is prone to occur between layers. If the binder is too soft at the process temperature, it will creep and migrate, leading to gradual relaxation of the interface.

[0040] In one specific embodiment, the C5-C10 acrylate structural units are derived from C5-C10 acrylate monomers, whose glass transition temperatures range from -65°C to -35°C. Within this range, the binder helps maintain a rubbery state at room temperature (25°C ± 5°C) or the operating temperature of the secondary battery, effectively buffering the "micro-slippage" of the natural graphite sheets and the volume changes of natural graphite during cycling. Simultaneously, it helps maintain the glass transition temperature of the binder between 43°C and 86°C, which not only improves the mechanical strength and toughness of the negative electrode sheet and the uniformity of natural graphite dispersion in the negative electrode slurry, but also enhances the active ion compensation effect of the binder on natural graphite, thereby giving the secondary battery higher cycle stability.

[0041] For example, the glass transition temperature of C5 to C10 acrylate monomers can be -65°C, -60°C, -55°C, -50°C, -45°C, -40°C, -35°C, or any combination thereof.

[0042] In one specific embodiment, the propylene-based structural unit containing polar groups is derived from propylene-based monomers containing polar groups, and the glass transition temperature of the propylene-based monomers containing polar groups is 80°C to 180°C; optionally, the glass transition temperature of the propylene-based structural unit containing polar groups is 100°C to 160°C. Within this range, it helps to maintain the binder in a glassy state within the temperature range of drying and / or rolling, achieving high-modulus anchoring of natural graphite and inhibiting binder migration and pore blockage; simultaneously, it also helps to keep the glass transition temperature of the binder between 43°C and 86°C, which not only improves the mechanical strength and toughness of the negative electrode sheet and the uniformity of natural graphite dispersion in the negative electrode slurry, but also improves the active ion compensation effect of the binder on natural graphite, thereby giving the secondary battery higher cycle stability.

[0043] For example, the glass transition temperature of a propylene monomer containing a polar group can be a range of 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, or any combination thereof.

[0044] In one specific embodiment, the alkali metal ion is either lithium ion or sodium ion; further, the alkali metal ion is lithium ion. During the preparation of the binder, the lithium ions originate from a lithium source, and the sodium ions originate from a sodium source; wherein the lithium source includes at least one of lithium hydroxide, lithium carbonate, and lithium chloride; and the sodium source includes at least one of sodium carbonate, sodium oxalate, and sodium citrate.

[0045] When the lithium source or sodium source is a mixture of the above-mentioned multiple (two or more) specific compounds, the present invention does not impose specific limitations on the proportions between the various specific compounds.

[0046] In one specific embodiment, the C5-C10 acrylate structural unit includes at least one of the following: n-amyl acrylate structural unit, isoamyl acrylate structural unit, hexyl acrylate structural unit, and 2-ethylhexyl acrylate structural unit.

[0047] Specifically, the C5-C10 acrylate structural units are derived from C5-C10 acrylate monomers; these C5-C10 acrylate monomers include at least one of n-amyl acrylate, isoamyl acrylate, hexyl acrylate, and 2-ethylhexyl acrylate. Specifically, n-amyl acrylate has a glass transition temperature of -65°C, isoamyl acrylate has a glass transition temperature of -60°C, hexyl acrylate has a glass transition temperature of -54°C, and 2-ethylhexyl acrylate has a glass transition temperature of -55°C.

[0048] When the C5 to C10 acrylate structural units are a mixture of the above-mentioned multiple (two or more) specific structural units, the present invention does not impose specific limitations on the proportions between the various specific structural units.

[0049] In one specific embodiment, the propylene-based structural unit containing polar groups includes at least one of acrylamide structural units, acrylonitrile structural units, and acrylic acid structural units.

[0050] Specifically, the propylene-based structural units containing polar groups are derived from propylene-based monomers containing polar groups; these propylene-based monomers containing polar groups include at least one of acrylamide monomers, acrylonitrile monomers, and acrylic monomers.

[0051] When the propylene-based structural unit containing polar groups is a mixture of the above-mentioned specific structural units (two or more), the present invention does not impose specific limitations on the proportion between the various specific structural units.

[0052] In one specific embodiment, the allyl ether structural unit includes at least one of the following: methyl allyl polyoxyethylene ether structural unit, allyl ethoxy ether structural unit, methoxy polyethylene glycol allyl ether-100 structural unit, methoxy polyethylene glycol allyl ether-200 structural unit, methoxy polyethylene glycol allyl ether-300 structural unit, and methoxy polyethylene glycol allyl ether-400 structural unit.

[0053] Specifically, the allyl ether structural unit is derived from an allyl ether monomer; the allyl ether monomer includes at least one of methyl allyl polyoxyethylene ether, allyl ethoxy ether (AEE), methoxy polyethylene glycol allyl ether-100 (mPEG-allyl-100), methoxy polyethylene glycol allyl ether-200 (mPEG-allyl-200), methoxy polyethylene glycol allyl ether-300 (mPEG-allyl-300), and methoxy polyethylene glycol allyl ether-400 (mPEG-allyl-400). The glass transition temperature of allyl ethoxy ether is -50℃, that of methoxy polyethylene glycol allyl ether-100 is -55℃, that of methoxy polyethylene glycol allyl ether-200 is -60℃, that of methoxy polyethylene glycol allyl ether-300 is -63℃, and that of methoxy polyethylene glycol allyl ether-400 is -65℃.

[0054] When the allyl ether structural unit is a mixture of the above-mentioned multiple (two or more) specific structural units, the present invention does not impose specific limitations on the proportion between the various specific structural units.

[0055] In one specific embodiment, the acrylamide structural unit includes at least one of N,N-dimethylacrylamide, N,N-diethylacrylamide, N-isopropylacrylamide, N-(2-hydroxyethyl)acrylamide, and acrylamide structural units.

[0056] Specifically, the acrylamide structural unit is derived from acrylamide monomers; these acrylamide monomers include at least one of N,N-dimethylacrylamide, N,N-diethylacrylamide, N-isopropylacrylamide, N-(2-hydroxyethyl)acrylamide, and acrylamide. The glass transition temperature of N,N-dimethylacrylamide is 110°C, that of N,N-diethylacrylamide is 95°C, that of N-isopropylacrylamide is 132°C, that of N-(2-hydroxyethyl)acrylamide is 120°C, and that of acrylamide is 165°C.

[0057] In one specific embodiment, the acrylonitrile structural unit includes at least one of the following: acrylonitrile structural unit, methacrylonitrile structural unit, and 2-chloroacrylonitrile structural unit.

[0058] Specifically, the acrylonitrile structural unit is derived from acrylonitrile monomers; these acrylonitrile monomers include at least one of acrylonitrile, methacrylonitrile, and 2-chloroacrylonitrile. The glass transition temperature of acrylonitrile is 105°C, and the glass transition temperature of methacrylonitrile is 100°C.

[0059] In one specific embodiment, the acrylic structural unit includes at least one of acrylic structural units, methacrylic acid structural units, and itaconic acid structural units.

[0060] Specifically, the acrylic structural unit is derived from acrylic monomers; these acrylic monomers include at least one of acrylic acid, methacrylic acid, and itaconic acid. The glass transition temperature of acrylic acid is 105°C, that of methacrylic acid is 170°C, and that of itaconic acid is 180°C.

[0061] In one specific embodiment, the adhesive is prepared by a method comprising the following steps:

[0062] After the alkali metal ion source and C5-C10 acrylate monomers undergo a first polymerization reaction, a propylene monomer containing polar groups is added to the reaction system to initiate a second polymerization reaction. Subsequently, an allyl ether monomer is added to the reaction system to initiate a third polymerization reaction, resulting in an adhesive.

[0063] Specifically, raw materials including an alkali metal ion source, C5-C10 acrylate monomers, and a first initiator are mixed in deionized water to obtain a first raw material system. After deoxygenation, a first polymerization reaction is carried out on this first raw material system. During the first polymerization reaction, the C5-C10 acrylate monomers undergo polymerization to form C5-C10 acrylate structural units, which serve as the polymer backbone. Simultaneously, the alkali metal ion source dissociates in the aqueous phase to release alkali metal ions (such as Li). + Or Na + Alkali metal ions are incorporated into the polymer structure through complexation or electrostatic adsorption with polar groups (such as ester groups) in the polymer backbone, forming an alkali metal ion-doped polymer backbone. After the first polymerization reaction, a first reaction system is obtained. A second raw material system is obtained by mixing raw materials including propylene monomers containing polar groups and a second initiator. The second raw material system is then added to the first reaction system for a second polymerization reaction. During the second polymerization reaction, the propylene monomers containing polar groups react with the polymer backbone and graft onto it, forming branches. After the second polymerization reaction, a second reaction system is obtained. A third raw material system is obtained by mixing raw materials including allyl ether monomers and a third initiator. The third raw material system is then added to the second reaction system for a third polymerization reaction. During the third polymerization reaction, the allyl ether monomers react with the polymer backbone and graft onto it, forming branches, thus obtaining a binder.

[0064] In this invention, the alkali metal ion source refers to a raw material that provides alkali metal ions. As long as it contains the target element, it falls within the scope of this invention. For example, the alkali metal ion source can be a lithium source or a sodium source. The specific materials of the lithium source or sodium source can be the same as those listed above, and will not be elaborated here.

[0065] The present invention does not specifically limit the first initiator, the first initiator and the type of the first initiator, which may be the same or different. For example, it may include at least one of ammonium persulfate, potassium persulfate, sodium persulfate, ammonium sulfate, trimethylamine, benzoyl peroxide and tert-butyl peroxide.

[0066] The present invention does not specifically limit the sources of the alkali metal ion source, the first initiator, the second initiator and the third initiator, and products prepared by commercially available products or conventional preparation methods known to those skilled in the art can be used.

[0067] Further, the temperature of the first polymerization reaction is 50℃~70℃, the temperature of the second polymerization reaction is 55℃~70℃, and the temperature of the third polymerization reaction is 69℃~89℃. For example, the temperature of the first polymerization reaction can be 50℃, 52℃, 54℃, 56℃, 58℃, 60℃, 62℃, 64℃, 66℃, 68℃, 70℃, or any combination thereof; the time of the second polymerization reaction can be 55℃, 56℃, 58℃, 60℃, 62℃, 64℃, 66℃, 68℃, 70℃, or any combination thereof; and the temperature of the third polymerization reaction can be 69℃, 71℃, 73℃, 75℃, 77℃, 79℃, 81℃, 83℃, 85℃, 87℃, 89℃, or any combination thereof.

[0068] Further, the reaction time of the first polymerization reaction is 3h to 6h, the reaction time of the second polymerization reaction is 2h to 4h, and the reaction time of the third polymerization reaction is 2h to 4h; for example, the reaction time of the first polymerization reaction can be 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h or any two of these ranges, the reaction time of the second polymerization reaction can be 2h, 2.5h, 3h, 3.5h, 4h or any two of these ranges, and the reaction time of the third polymerization reaction can be 2h, 2.5h, 3h, 3.5h, 4h or any two of these ranges.

[0069] Furthermore, the mass ratio of alkali metal ions, C5-C10 acrylate monomers, propylene monomers containing polar groups, and allyl ether monomers in the alkali metal ion source is (1-10):(40-60):(10-30):(5-20). Within this range, it helps to ensure that the mass ratio of alkali metal ions, C5-C10 acrylate structural units, propylene structural units containing polar groups, and allyl ether structural units in the prepared binder is within the aforementioned range.

[0070] For example, the mass fraction of alkali metal ions can be 1 part, 2 parts, 3 parts, 4 parts, 5 parts, 6 parts, 7 parts, 8 parts, 9 parts, 10 parts, or any two of these values; the mass fraction of C5-C10 acrylate monomers can be 40 parts, 42 parts, 44 parts, 46 parts, 48 ​​parts, 50 parts, 52 parts, 54 parts, 56 parts, 58 parts, 60 parts, or any two of these values; the mass fraction of propylene monomers containing polar groups can be 10 parts, 12 parts, 14 parts, 16 parts, 18 parts, 20 parts, 22 parts, 24 parts, 26 parts, 28 parts, 30 parts, or any two of these values; and the mass fraction of allyl ether monomers can be 5 parts, 6 parts, 8 parts, 10 parts, 12 parts, 14 parts, 16 parts, 18 parts, 20 parts, or any two of these values.

[0071] Furthermore, the mass ratio of the first initiator to C5-C10 acrylate monomers is (0.1-0.8):100, the mass ratio of the second initiator to propylene monomers containing polar groups is (0.1-0.8):100, and the mass ratio of the third initiator to allyl ether monomers is (0.1-0.8):100. Within this range, the reaction rate is high, allowing the monomers to polymerize fully without triggering explosive polymerization.

[0072] For example, the mass ratio of the first initiator to the C5-C10 acrylate monomer can be 0.1:100, 0.2:100, 0.3:100, 0.4:100, 0.5:100, 0.6:100, 0.7:100, 0.8:100, or any combination thereof; the mass ratio of the second initiator to the propylene monomer containing polar groups can be 0.1:100, 0.2:100, 0.3:100, or any combination thereof. The mass ratio of the third initiator to the allyl ether monomer can be 0.1:100, 0.2:100, 0.3:100, 0.4:100, 0.5:100, 0.6:100, 0.7:100, 0.8:100, or any two of these ranges.

[0073] 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.

[0074] Furthermore, the addition time of the second raw material system to the first reaction system is 1 hour to 4 hours. Within this range, the risk of explosive polymerization during the polymerization reaction can be reduced. For example, the addition time can be 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, or any combination thereof. It should be noted that the addition rate can be uniform; the second raw material system only needs to be added to the first reaction system within the specified time.

[0075] 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.

[0076] Furthermore, the third raw material system is added to the second reaction system at a time interval of 1 to 3 hours. Within this range, the risk of explosive polymerization during the polymerization reaction can be reduced. For example, the addition time can be 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, or any combination thereof. It should be noted that the addition rate can be uniform; the third raw material system only needs to be added to the second reaction system within the specified time.

[0077] This invention does not impose specific limitations on the mixing method, as long as the reactants are uniformly dispersed in the mixing system. For example, mixing can be carried out by magnetic stirring or mechanical stirring.

[0078] The present invention does not specify the method of deoxygenation, as long as the active oxygen in the mixed system is completely removed. For example, nitrogen gas is introduced into the mixed system for deoxygenation, and the nitrogen gas introduction time is not less than 0.5 hours.

[0079] The method for preparing the adhesive in this invention involves first polymerizing an alkali metal ion source and C5-C10 acrylate monomers under the action of a first initiator to obtain an alkali metal ion-doped polymer backbone. Then, an propylene monomer containing polar groups is introduced, and under the action of a second initiator, this monomer polymerizes and grafts onto the aforementioned polymer backbone. Finally, an allyl ether monomer is introduced, and under the action of a third initiator, this monomer polymerizes and grafts onto the aforementioned polymer backbone to obtain an adhesive with a glass transition temperature of 43°C to 86°C. This binder exhibits good flexibility and plasticity. Its polar groups can form hydrogen bonds or other chemical bonds with natural graphite and the surface of the negative electrode current collector, effectively improving the adhesion between electrode materials (such as the negative electrode active material and conductive agent) and between the negative electrode active layer and the negative electrode current collector, thus giving the negative electrode sheet high mechanical strength and toughness. Simultaneously, the polymer branches (i.e., allyl ether structural units) can form large-area van der Waals adhesion with the surface of natural graphite through branched alkyl segments, and the ether oxygen bonds provide weakly polar adsorption sites, allowing the polymer molecules to be firmly adsorbed at the interlayer or edge positions of natural graphite. This strong adsorption force enables alkali metal ions to be uniformly dispersed around the surface of natural graphite. During the first charge cycle of the secondary battery, the interfacial potential drives the alkali metal ions to embed into the natural graphite, directly compensating for the loss of active ions in the natural graphite during the first charge cycle. Compared with traditional external compensators, it has the advantages of low ion diffusion resistance, short diffusion path, and more efficient compensation. At the same time, this strong adsorption force also enables the binder to form a protective layer on the surface of natural graphite, reducing the risk of agglomeration or accumulation between natural graphite particles, improving the dispersibility of natural graphite in the negative electrode slurry, and further improving the cycle stability of the secondary battery.

[0080] Therefore, the binder prepared by the above preparation method can effectively improve the adhesion between the negative electrode active layer and the negative electrode current collector, prevent the negative electrode active layer from peeling or falling off, and at the same time compensate for the irreversible loss of active ions in natural graphite during the charging and discharging process, thereby improving the cycle stability of the secondary battery.

[0081] A second aspect of the present invention provides a negative electrode sheet comprising the binder of the first aspect. Therefore, using this negative electrode sheet in a secondary battery can effectively improve the cycle stability of the secondary battery.

[0082] In one specific embodiment, the negative electrode sheet further includes a negative electrode active material, which includes natural graphite. When the negative electrode active material includes natural graphite, the binder can synergistically and effectively improve the mechanical strength and toughness of the negative electrode sheet, and greatly enhance the dispersibility of natural graphite in the negative electrode slurry system. Moreover, the alkali metal ions in the binder can also play a role in active ion compensation, compensating for the irreversible loss of active ions in natural graphite during cycling, thereby effectively improving the cycle stability of the secondary battery.

[0083] In one specific embodiment, the negative electrode further includes a conductive agent; the mass ratio of binder, natural graphite, and conductive agent is (0.8–3):(50–100):(0.5–3). Within this range, the ratio of binder, natural graphite, and conductive agent is suitable, which helps to uniformly distribute natural graphite in the negative electrode active layer and improve the toughness and mechanical strength of the negative electrode. At the same time, it can also achieve a better active ion compensation effect, thereby comprehensively improving the cycle stability of the secondary battery.

[0084] This invention does not impose any particular limitation on the type of conductive agent; it can be any conductive agent commonly used in secondary batteries. For example, the conductive agent may include at least one of carbon black, acetylene black, Ketjen black, carbon fiber, and graphene.

[0085] In one specific embodiment, the negative electrode sheet is prepared by a method including the following steps:

[0086] The negative electrode active material, conductive agent and binder of the present invention are dispersed in a solvent (such as deionized water) and thoroughly stirred to form a uniform negative electrode slurry; the negative electrode slurry is uniformly coated on the negative electrode current collector, and after drying, rolling and cutting, a negative electrode sheet is obtained.

[0087] The negative electrode sheet prepared by the above preparation method can not only effectively improve the cycle stability of secondary batteries, but also significantly improve the dispersibility of natural graphite in the negative electrode slurry due to the binder in this invention. Therefore, during the mixing process, it is not necessary to reduce the speed of the dispersion equipment, which can effectively reduce the number of steps and time in the production process, simplify the process flow, and reduce energy consumption.

[0088] A third aspect of the present invention provides a secondary battery comprising the binder of the first aspect or the negative electrode of the second aspect. Therefore, the secondary battery exhibits high cycle performance.

[0089] The following detailed description of a secondary battery including the binder of the present invention is provided through specific embodiments.

[0090] Example 1

[0091] 1) Binder preparation: Lithium hydroxide (7 parts by mass of lithium element), 50 parts by mass of n-amyl acrylate (glass transition temperature -65℃), 0.2 parts by mass of ammonium persulfate as the first initiator and 200 parts by mass of deionized water were added to the reactor to obtain the first raw material system. Nitrogen gas was introduced to remove active oxygen in the reactor and pipeline. The temperature was raised to 60℃ to carry out the first polymerization reaction for 5 hours to obtain the first reaction system.

[0092] 25 parts by mass of N,N-diethylacrylamide (glass transition temperature of 95°C) and 0.1 parts by mass of ammonium persulfate, the second initiator, were mixed to obtain a second raw material system. The second raw material system was added to the first reaction system for 2 hours. The second polymerization reaction was carried out at 65°C for 3 hours to obtain the second reaction system.

[0093] A third raw material system was obtained by mixing 18 parts by mass of allyl ethoxy ether (glass transition temperature of -50℃) and 0.072 parts by mass of the third initiator ammonium persulfate. The third raw material system was added to the above second reaction system over a period of 2 hours, and a third polymerization reaction was carried out at 80℃ for 3 hours to obtain the binder.

[0094] 2) Negative electrode preparation:

[0095] By weight, 97 parts by weight of natural graphite, 1.5 parts by weight of the above-mentioned binder, and 1 part by weight of carbon black were mixed and stirred at a low speed of 15 rpm for 15 minutes to obtain a mixture. Water was added to make the solid content of the mixture 55 wt%. After pre-stirring at 40 rpm for 15 minutes, the mixture was kneaded, stirred at 40 rpm for 40 minutes, and then dispersed at high speed of 15 m / s for 40 minutes. Then, 0.5 parts by weight of binder were added, and the mixture was dispersed at 5 m / s for 20 minutes. After vacuum defoaming, the mixture was discharged to obtain the negative electrode slurry. Finally, the negative electrode slurry was coated on both sides of the copper foil of the negative electrode current collector, with a single-sided coating surface density of 90 g / m². 2 After drying, cold pressing, slitting, and welding of electrode tabs, a compacted density of 1.6 g / cm³ is obtained. 3 The negative electrode.

[0096] 3) Preparation of positive electrode sheet:

[0097] The positive electrode active material NCM622 (chemical formula Li(Ni) 0.6 Co 0.2 Mn 0.2 O2), conductive carbon black (Super P), and binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 97.3:1.5:1.2, and N-methylpyrrolidone (NMP) is added. The mixture is stirred evenly to prepare a positive electrode slurry. The positive electrode slurry is coated on both sides of an aluminum foil, with a single-sided coating density of 170 g / m². 2 After drying, rolling, slitting, and welding of electrode tabs, a compacted density of 3.4 g / cm³ is obtained. 3 The positive electrode plate.

[0098] 4) Lithium-ion battery manufacturing:

[0099] The negative electrode, separator (PE film), and positive electrode are wound sequentially, with the separator positioned between the positive and negative electrodes to act as a separator, thus forming an electrode assembly. This assembly is then placed in an outer package, filled with electrolyte, and sealed. After formation and degassing processes, a secondary battery is obtained. The electrolyte contains 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 wt%, and the remainder is the mixed solvent.

[0100] Example 2

[0101] The binder and lithium-ion battery were prepared according to the method of Example 1, with the following differences:

[0102] In step 1), lithium chloride is used to replace lithium hydroxide, hexyl acrylate (glass transition temperature -54℃) is used to replace n-pentyl acrylate, acrylic acid (glass transition temperature 105℃) is used to replace N,N-diethylacrylamide, and methoxy polyethylene glycol allyl ether-400 (glass transition temperature -65℃) is used to replace allyl ethoxy ether. All other steps remain unchanged.

[0103] Example 3

[0104] The binder and lithium-ion battery were prepared according to the method of Example 1, with the following differences:

[0105] In step 1), lithium carbonate is used instead of lithium hydroxide, 2-ethylhexyl acrylate (glass transition temperature -55℃) is used instead of n-pentyl acrylate, itaconic acid (glass transition temperature 180℃) is used instead of N,N-diethylacrylamide, and methoxy polyethylene glycol allyl ether-100 (glass transition temperature -55℃) is used instead of allyl ethoxy ether. All other steps remain unchanged.

[0106] Example 4

[0107] The binder and lithium-ion battery were prepared according to the method of Example 1, with the following differences:

[0108] In step 1), in the first polymerization reaction, the amount of lithium added to lithium hydroxide is adjusted to 1 part by mass, the amount of n-pentyl acrylate added is adjusted to 60 parts by mass, and the amount of ammonium persulfate added is adjusted to 0.24 parts by mass.

[0109] In the second polymerization reaction, the amount of N,N-diethylacrylamide added was adjusted to 30 parts by mass, and the amount of ammonium persulfate added was adjusted to 0.12 parts by mass;

[0110] In the third polymerization reaction, the amount of allyl ethoxy ether added was adjusted to 9 parts by mass, the amount of ammonium persulfate added was adjusted to 0.036 parts by mass, and the others remained unchanged.

[0111] Example 5

[0112] The binder and lithium-ion battery were prepared according to the method of Example 1, with the following differences:

[0113] In step 1), in the first polymerization reaction, the amount of lithium added to lithium hydroxide is adjusted to 10 parts by mass, the amount of n-pentyl acrylate added is adjusted to 40 parts by mass, and the amount of ammonium persulfate added is adjusted to 0.16 parts by mass.

[0114] In the second polymerization reaction, the amount of N,N-diethylacrylamide added was adjusted to 30 parts by mass, and the amount of ammonium persulfate added was adjusted to 0.12 parts by mass;

[0115] In the third polymerization reaction, the amount of allyl ethoxy ether added was adjusted to 20 parts by mass, the amount of ammonium persulfate added was adjusted to 0.08 parts by mass, and the others remained unchanged.

[0116] Example 6

[0117] The binder and lithium-ion battery were prepared according to the method of Example 1, with the following differences:

[0118] In step 1), in the first polymerization reaction, the amount of lithium added to lithium hydroxide is adjusted to 5 parts by mass, the amount of n-pentyl acrylate added is adjusted to 60 parts by mass, and the amount of ammonium persulfate added is adjusted to 0.24 parts by mass.

[0119] In the second polymerization reaction, the amount of N,N-diethylacrylamide added was adjusted to 30 parts by mass, and the amount of ammonium persulfate added was adjusted to 0.12 parts by mass;

[0120] In the third polymerization reaction, the amount of allyl ethoxy ether added was adjusted to 5 parts by mass, the amount of ammonium persulfate added was adjusted to 0.02 parts by mass, and the others remained unchanged.

[0121] Example 7

[0122] The binder and lithium-ion battery were prepared according to the method of Example 1, with the following differences:

[0123] In step 1), in the first polymerization reaction, the amount of lithium added to lithium hydroxide is adjusted to 10 parts by mass, the amount of n-pentyl acrylate added is adjusted to 60 parts by mass, and the amount of ammonium persulfate added is adjusted to 0.24 parts by mass.

[0124] In the second polymerization reaction, the amount of N,N-diethylacrylamide added was adjusted to 10 parts by mass, and the amount of ammonium persulfate added was adjusted to 0.04 parts by mass;

[0125] In the third polymerization reaction, the amount of allyl ethoxy ether added was adjusted to 20 parts by mass, the amount of ammonium persulfate added was adjusted to 0.08 parts by mass, and the others remained unchanged.

[0126] Example 8

[0127] The binder and lithium-ion battery were prepared according to the method of Example 1, with the following differences:

[0128] In step 1), in the first polymerization reaction, the amount of lithium added to lithium hydroxide is adjusted to 15 parts by mass, the amount of n-pentyl acrylate added is adjusted to 35 parts by mass, and the amount of ammonium persulfate added is adjusted to 0.14 parts by mass.

[0129] In the second polymerization reaction, the amount of N,N-diethylacrylamide added was adjusted to 35 parts by mass, and the amount of ammonium persulfate added was adjusted to 0.14 parts by mass.

[0130] In the third polymerization reaction, the amount of allyl ethoxy ether added was adjusted to 15 parts by mass, the amount of ammonium persulfate added was adjusted to 0.06 parts by mass, and the others remained unchanged.

[0131] Example 9

[0132] The binder and lithium-ion battery were prepared according to the method of Example 3, with the following differences:

[0133] Replace 2-ethylhexyl acrylate with n-amyl acrylate, adjust the amount of n-amyl acrylate to 40 parts by mass, and adjust the amount of the first initiator to 0.16 parts by mass; adjust itaconic acid to 30 parts by mass, and adjust the amount of the second initiator to 0.12 parts by mass; adjust methoxy polyethylene glycol allyl ether-100 to 23 parts by mass, and adjust the amount of the third initiator to 0.092 parts by mass.

[0134] Example 10

[0135] The binder and lithium-ion battery were prepared according to the method of Example 1, with the following differences:

[0136] In step 2), natural graphite is replaced with artificial graphite.

[0137] Example 11

[0138] The binder and lithium-ion battery were prepared according to the method of Example 1, with the following differences:

[0139] In step 2), natural graphite is replaced with hard carbon.

[0140] Comparative Example 1

[0141] The binder and lithium-ion battery were prepared according to the method of Example 1, with the following differences:

[0142] In step 1), methyl acrylate (glass transition temperature is 10℃) is used to replace n-amyl acrylate, while the rest remains unchanged.

[0143] Comparative Example 2

[0144] The binder and lithium-ion battery were prepared according to the method of Example 1, with the following differences:

[0145] In step 1), undecyl acrylate (glass transition temperature -70℃) is used to replace n-amyl acrylate, while the rest remains unchanged.

[0146] Comparative Example 3

[0147] The binder and lithium-ion battery were prepared according to the method of Example 1, with the following differences:

[0148] In step 1), styrene (glass transition temperature 100℃) is used to replace N,N-diethylacrylamide, while the rest remains unchanged.

[0149] Comparative Example 4

[0150] The binder and lithium-ion battery were prepared according to the method of Example 1, with the following differences:

[0151] In step 1), no allyl ethoxy ether 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; in the first polymerization reaction, the mass fraction of n-pentyl acrylate was 68 parts, the mass fraction of ammonium persulfate was 0.272 parts, and the others remained unchanged.

[0152] Comparative Example 5

[0153] The binder and lithium-ion battery were prepared according to the method of Example 1, with the following differences:

[0154] In step 1), the second and third polymerization reactions are not carried out, that is, the comparative binder is obtained directly after the first polymerization reaction; at this time, in the first polymerization reaction, the mass fraction of n-amyl acrylate is 93 parts, the mass fraction of ammonium persulfate is 0.372 parts, and the others remain unchanged.

[0155] Comparative Example 6

[0156] The binder and lithium-ion battery were prepared according to the method of Example 1, with the following differences:

[0157] In step 1), the first polymerization reaction is not carried out; that is, the second and third polymerization reactions are carried out directly. Lithium hydroxide is added in the second polymerization reaction to prepare the comparative binder. At this time, in the second polymerization reaction, the mass fraction of N,N-diethylacrylamide is 75 parts, the mass fraction of ammonium persulfate is 0.3 parts, and the others remain unchanged.

[0158] Comparative Example 7

[0159] The binder and lithium-ion battery were prepared according to the method of Example 1, with the following differences:

[0160] In step 1), if lithium hydroxide is not added in the first polymerization reaction, the mass fraction of n-pentyl acrylate is 57 parts, the mass fraction of ammonium persulfate is 0.228 parts, and the others remain unchanged.

[0161] Test case

[0162] 1. The weight-average molecular weight and glass transition temperature of the binders prepared in the above examples and comparative examples were tested:

[0163] 1) Weight-average molecular weight

[0164] The weight-average molecular weight of the binder was tested using gel permeation chromatography, and the results are shown in Table 1.

[0165] 2) Glass transition temperature

[0166] The glass transition temperature (Tg) of the binders in the examples and comparative examples was determined using a differential scanning calorimeter (Shanghai Qunhong Instrument Equipment Co., Ltd., model: DSC-100). The steps 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; setting the temperature to -20℃, holding for 10 min, with a temperature range of -20℃ to 120℃, and a heating rate of 10 K / min; placing the prepared binder sample on the heating furnace, covering the furnace with the protective cover, adding an appropriate amount of liquid nitrogen to the constant temperature container, and running the test when the sample temperature reached -20℃. The test results are shown in Table 1.

[0167] 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:

[0168] The coated single-sided negative electrode sheet was prepared at a ratio of 1.6 g / cm². 3After compaction, the electrode sheet was cut into 20cm long and 3cm wide pieces. 3M double-sided tape was attached to the steel plate, and the coated side of the electrode sheet was 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 was used. The upper plate clamped the copper foil side, and the coating and copper foil were torn apart at a speed of 50mm / min and a 180° stretch. The data of the stable tensile section was recorded as the peel strength (N / m). The test results are shown in Table 1.

[0169] 3. The cycle performance of the lithium-ion batteries prepared in the above embodiments and comparative examples was tested:

[0170] At 25℃, the battery was charged at a constant current of 0.5C to 4.2V and then charged at a constant voltage to a cutoff current of 0.05C. Subsequently, it was discharged at a discharge rate of 1C to 3.0V, and the initial discharge capacity was recorded as Q0. The battery was cycled 500 times according to the aforementioned charge and discharge mechanism, and the discharge capacity after 500 cycles was recorded as Q1. The battery's capacity retention rate after 500 cycles (%) = (Q1 / Q0) × 100%. The test results are shown in Table 1.

[0171] In Table 1, Tg represents the glass transition temperature of the binder, Mw represents the weight-average molecular weight of the binder, A represents the mass ratio of alkali metal ions, C5-C10 acrylate structural units, propylene structural units containing polar groups, and allyl ether structural units in the binder, and P represents the peel strength between the negative electrode active layer and the negative electrode current collector in the negative electrode sheet.

[0172] Table 1

[0173]

[0174] As shown in Table 1, when the binder in this invention is used in a negative electrode sheet with natural graphite as the negative electrode active material, it can effectively improve the cycle stability of lithium-ion batteries.

[0175] 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 alkali metal ions, C5-C10 acrylate structural units, propylene-based structural units containing polar groups, and allyl ether structural units. The glass transition temperature of the adhesive is 43°C to 86°C. The C5-C10 acrylate structural units are derived from C5-C10 acrylate monomers, and the glass transition temperature of the C5-C10 acrylate monomers is -65°C to -35°C. The propylene-based structural units containing polar groups are derived from propylene-based monomers containing polar groups. The glass transition temperature of the propylene monomer is 80℃~180℃; the mass ratio of the alkali metal ion, the C5~C10 acrylate structural unit, the propylene structural unit containing polar groups, and the allyl ether structural unit is (1~10):(40~60):(10~30):(5~20); the propylene structural unit containing polar groups is at least one of acrylamide structural unit and acrylic structural unit; the alkali metal ion is lithium ion; the binder is prepared by a method including the following steps: After the alkali metal ion source and C5-C10 acrylate monomers undergo a first polymerization reaction, a propylene monomer containing polar groups is added to the reaction system to initiate a second polymerization reaction. Subsequently, an allyl ether monomer is added to the reaction system to initiate a third polymerization reaction, resulting in an adhesive.

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. The adhesive according to claim 1, characterized in that, The C5-C10 acrylate structural units include at least one of the following: n-amyl acrylate structural units, isoamyl acrylate structural units, hexyl acrylate structural units, and 2-ethylhexyl acrylate structural units. And / or, the allyl ether structural unit includes at least one of the following: methyl allyl polyoxyethylene ether structural unit, allyl ethoxy ether structural unit, methoxy polyethylene glycol allyl ether-100 structural unit, methoxy polyethylene glycol allyl ether-200 structural unit, methoxy polyethylene glycol allyl ether-300 structural unit, and methoxy polyethylene glycol allyl ether-400 structural unit.

4. The adhesive according to claim 3, characterized in that, The acrylamide structural units include at least one of N,N-dimethylacrylamide, N,N-diethylacrylamide, N-isopropylacrylamide, N-(2-hydroxyethyl)acrylamide, and acrylamide structural units. And / or, the acrylic structural unit includes at least one of acrylic structural units, methacrylic acid structural units, and itaconic acid structural units.

5. A negative electrode sheet, characterized in that, The negative electrode sheet includes the binder as described in any one of claims 1-4.

6. The negative electrode sheet according to claim 5, characterized in that, The negative electrode sheet also includes a negative electrode active material, which includes natural graphite.

7. A secondary battery, characterized in that, Includes the binder as described in any one of claims 1-4, or the negative electrode sheet as described in claim 5 or 6.

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