Binder, positive electrode sheet and secondary battery

Abstract

Provided in the present application are a binder, a positive electrode sheet and a secondary battery. The binder of the present application comprises a first polymer as represented by formula I, wherein R1 is a C5-C12 alkyl, and R2 is any one of a C1-C3 alkyl, a hydrogen atom or an aryl; and the number-average molecular weight of the first polymer is 6×104 to 2×106. The binder of the present application substantially does not contain fluorine, and has a low swelling rate and high adhesion; and when used in a positive electrode sheet and a secondary battery, the binder can inhibit the expansion of the positive electrode sheet and improve the energy density of the secondary battery.

Description

An adhesive, a positive electrode sheet, and a secondary battery Technical Field

[0001] This application relates to the field of secondary batteries, and more specifically, to an adhesive, a positive electrode sheet, and a secondary battery. Background Technology

[0002] The electrode sheets of secondary batteries, such as lithium-ion batteries, include current collectors and an active layer on the surface of the current collectors. After charging and discharging, the thickness of the electrode sheets in secondary batteries expands and rebounds significantly; the thickness rebound of the positive electrode sheet is approximately 5% of its original thickness, while that of the anode electrode sheet is approximately 20%. Given a limited battery thickness, if the thickness rebound of the electrode sheets can be reduced, one or more additional electrode sheets can be wound around the bare battery cell, thus significantly improving the battery's volumetric energy density.

[0003] Currently, PVDF (polyvinylidene fluoride) is used as a binder to improve the adhesion between layers in the electrode, thereby suppressing electrode expansion and increasing the energy density of the secondary battery. However, the fluorine in PVDF will pollute the environment and is not conducive to environmental protection. Therefore, there is an urgent need for a solution that can replace PVDF to suppress electrode expansion. Summary of the Invention

[0004] To address the aforementioned issues, this application provides an adhesive, a positive electrode sheet, and a secondary battery. The adhesive in this application is essentially free of fluorine and has a low swelling rate and high adhesion, which can improve the adhesion of the positive electrode sheet, suppress its expansion, and increase the energy density of the secondary battery.

[0005] In a first aspect, this application provides an adhesive comprising a first polymer, the first polymer comprising a structure as shown in Formula I:

[0006]

[0007] Formula I

[0008] Wherein R1 is a C5-C12 alkyl group, and R2 is any one of a C1-C3 alkyl group, a hydrogen atom, or an aryl group; the number average molecular weight of the first polymer is 6 x 10. 4 ~2x10 6 .

[0009] In the above technical solution, the inventors discovered that when the number-average molecular weight of the first polymer is limited to 6 x 10... 4 ~2x10 6In this process, the amide groups in the first polymer can exert excellent dispersibility and adhesion of the binder through the dual effects of dipole-dipole bonding and intermolecular hydrogen bonding. The cyano groups in the first polymer are strongly polar groups with short bond lengths, which not only give the binder good high-voltage stability, but also, when the binder is used in a secondary battery, the cyano groups in the first polymer can act as conductors of Li. + The first polymer has a protective function for the positive electrode material; the main chain of the first polymer contains flexible long carbon chains such as R1 groups, which gives the first polymer good flexibility, which not only further enhances the adhesion of the binder, but also gives the binder good elasticity. Therefore, the binder in this application not only has good adhesion, but also has a low swelling rate; in addition, the binder in this application contains virtually no fluorine, which is also beneficial to environmental protection.

[0010] In one possible implementation, the binder satisfies at least one of the following conditions: (1) R1 is a C5-C8 alkyl group; (2) 500 ≤ m ≤ 20000; (3) 300 ≤ n ≤ 10000; (4) 1.25n ≤ m ≤ 3n; (5) the number average molecular weight of the first polymer is 3 × 10. 5 ~2x10 6 .

[0011] In the above technical solutions, the adhesive has higher bonding strength and lower swelling rate.

[0012] In one possible implementation, the adhesive satisfies at least one of the following conditions: (1) 800 ≤ m ≤ 13960; (2) 400 ≤ n ≤ 10000.

[0013] In one possible implementation, the polydispersity of the first polymer is 1.3 to 1.5.

[0014] In the above technical solution, the polydispersity is in the range of 1.3 to 1.5, the properties of the first polymer are relatively uniform, which can better improve the adhesion of the adhesive and reduce the swelling rate of the adhesive.

[0015] In one possible implementation, based on infrared spectroscopy characterization, the first polymer is detected at 1100 cm⁻¹. -1 ~1300cm -1 It contains characteristic peaks.

[0016] In one possible implementation, the glass transition temperature of the first polymer is -45°C to -25°C.

[0017] Secondly, this application provides a positive electrode sheet, which includes a positive current collector, and at least one surface of the positive current collector is provided with a positive active layer, the positive active layer including the aforementioned binder. Therefore, the positive electrode sheet of this application also has good performance in use.

[0018] In one possible implementation, the mass content of the binder is a%, 0.5≤a≤2, based on the mass of the positive electrode active layer.

[0019] In the above technical solution, the mass content of the binder is within the above range, which can ensure that the positive electrode sheet has a low expansion rate, and at the same time will not have a significant adverse effect on the energy density of the positive electrode sheet.

[0020] In one possible implementation, the positive electrode active layer further includes a positive electrode active material and a positive electrode conductive agent. The positive electrode active material is coated with a positive electrode conductive agent, and the coverage of the positive electrode conductive agent on the surface of the positive electrode active material is S%, 73.6+2a≤S<83.7.

[0021] In the above technical solution, the binder can make the conductive agent adhere to the surface of the positive electrode active material, which is beneficial to improving the conductivity of the positive electrode sheet. The coverage of the conductive agent on the surface of the positive electrode active material meets the above conditions, which is beneficial to the deintercalation of lithium ions from the positive electrode active material.

[0022] In one possible implementation, the positive electrode active material contains transition metal elements.

[0023] In the above technical solution, the nitrile functional groups in the binder can form a complex adsorption structure with the ions of transition metal elements, making the structure of the positive electrode active material less likely to be destroyed, thus improving the thermal safety performance of the positive electrode sheet.

[0024] In one possible implementation, the transition metal element includes at least one of nickel, cobalt, or manganese.

[0025] In one possible implementation, the adhesive coating density on at least one surface of the positive electrode is 0.129 mg / mm². 2 ~0.228mg / mm 2 .

[0026] In the above technical solutions, the positive electrode sheet has a high bonding retention rate, a relatively stable structure, and a long service life.

[0027] Thirdly, this application provides a secondary battery comprising the aforementioned positive electrode. Therefore, the secondary battery provided by this application has a high energy density.

[0028] This application provides an adhesive, a positive electrode sheet, and a secondary battery. The adhesive of this application includes a first polymer, which has the structure shown in Formula I. In the first polymer, cyano groups, amide groups, and R1 groups of the main chain work together to give the adhesive good adhesion and low swelling rate. When the adhesive is used in the positive electrode sheet and the secondary battery, it can suppress the expansion of the positive electrode sheet and improve the energy density of the secondary battery. Attached Figure Description

[0029] Figure 1 is the infrared spectrum of the first polymer in Example 1-1 of this application. Embodiments of the present invention

[0030] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0031] In existing technologies, the electrodes of secondary batteries tend to expand after charging and discharging, which is detrimental to the energy density of the battery. Although the expansion of the electrodes can be suppressed by using PVDF in the binder, PVDF contains fluorine, which causes significant environmental pollution. To replace PVDF, current methods include thinning the aluminum foil and increasing the compaction density of the electrodes to suppress expansion. However, thinning the aluminum foil can easily lead to breakage and wrinkling; excessive compaction density can make the electrodes brittle, prone to breakage, and also cause poor electrolyte wetting, resulting in low capacity and lithium plating.

[0032] To address the aforementioned technical problems, this application provides an adhesive with low swelling ratio, high adhesion, and virtually no fluorine content. When used in positive electrode sheets and secondary batteries, it can suppress electrode expansion and improve the energy density of the secondary battery. The technical solution of this application is described in detail below:

[0033] In a first aspect, this application provides an adhesive comprising a first polymer, the first polymer comprising a structure as shown in Formula I:

[0034]

[0035] Formula I

[0036] Where R1 is a C5-C12 alkyl group, R2 is any one of a C1-C3 alkyl group, hydrogen atom, or aryl group, and m and n are the number of corresponding repeating units, respectively; the number average molecular weight of the first polymer is 6 x 10. 4 ~2x10 6 3x10 is preferred 5 ~2x10 6 .

[0037] The binder of this application contains virtually no fluorine in its first polymer, which is environmentally friendly. Compared to other groups, the amide groups in the first polymer exhibit excellent adhesion and dispersibility through both dipole-dipole interactions and intermolecular hydrogen bonding. The cyano groups in the first polymer are strongly polar groups with short bond lengths, which not only contribute to the binder's good high-voltage stability but also, when used in secondary batteries, allow the cyano groups to conduct Li₂. + This also serves to protect the positive electrode material. Furthermore, since the main chain of the first polymer also contains flexible long carbon chains such as R1 groups, the first polymer possesses good flexibility, which not only further enhances the adhesive force of the binder but also gives the binder good elasticity. Preferably, the R1 group can be a straight-chain alkyl group or a C5-C8 alkyl group, such as a straight-chain C5 alkyl group. In addition, the branched R2 group connected to the amide group can be any of a C1-C3 alkyl group, a hydrogen atom, or an aryl group. This results in a shorter chain length, which can control steric hindrance effects and reduce the risk of binder agglomeration and sedimentation. Therefore, the R2 group is preferably any of a C1-C3 alkyl group or a hydrogen atom.

[0038] It should be noted that in the adhesive of this application, the first polymer is typically prepared by homopolymerizing two different monomers separately to obtain two homopolymers, which are then linked together. The two monomers are the monomers corresponding to the structural units of the first polymer; that is, one monomer is acrylonitrile, and the other monomer is... The monomers corresponding to the structural units, and the types of R1 and R2 in the homopolymerization process do not necessarily have to be exactly the same, as long as they meet the range of the general formula. This application does not limit the homopolymer or the specific reaction steps for linking two homopolymers, as long as the purpose of this application is achieved. Furthermore, in order to make the number average molecular weight of the first polymer 6 x 10... 4 ~2x10 6 Within the range, as an example, in some embodiments of this application, 500≤m≤20000, preferably 800≤m≤13960, specifically can be 500, 600, 800, 1000, 5000, 10000, 13960, 20000, 36926, etc., or within the range of any two of the above values; 300≤n≤10000, preferably 400≤n≤10000, specifically can be 300, 350, 400, 800, 1000, 5000, 10000, 15274, etc., or within the range of any two of the above values.

[0039] In addition, in some embodiments of this application, in order to further improve the adhesive strength and reduce its swelling rate, m and n can satisfy the following relationship: 1.25n≤m≤3n.

[0040] To improve the uniformity of the adhesive, thereby further enhancing its adhesion and reducing its swelling rate, the polydispersity of the first polymer is typically 1.3 to 1.5 in some embodiments of this application.

[0041] Alternatively, the first polymer in the binder can be determined based on infrared spectroscopy or glass transition temperature. Specifically, as shown in Figure 1, based on infrared spectroscopy characterization, with the amide group as a specific control functional group, the first polymer at 1100 cm⁻¹... -1 ~1300cm -1 It contains characteristic peaks; based on DSC (Differential Scanning Calorimeter) characterization, the glass transition temperature of the first polymer is -45℃ to -25℃.

[0042] Secondly, this application provides a positive electrode sheet, which includes a positive current collector, and at least one surface of the positive current collector is provided with a positive active layer, the positive active layer including the aforementioned binder.

[0043] It should be noted that, in this application, "a positive electrode active layer disposed on at least one surface of the positive electrode current collector" means that the positive electrode active layer can be disposed on one surface or on two surfaces in the thickness direction of the positive electrode current collector. Furthermore, in this application, "the surface of the positive electrode current collector" can be the entire area of ​​the positive electrode current collector or only a portion thereof; this application has no particular limitation, as long as the purpose of this application is achieved. As an example, in some embodiments of this application, the adhesive coating density on the surface coated with the positive electrode active layer is 0.129 mg / mm². 2 ~0.228mg / mm 2 At this point, the adhesive retention rate f can reach 0.95~1. The specific method for measuring the adhesive retention rate can be found in the subsequent content of this application.

[0044] In the positive electrode sheet of this application, the positive active layer contains a binder, which can improve the adhesion between the positive active layer and the positive current collector, and reduce the swelling rate of the positive active layer. Specifically, the swelling rate of the positive electrode sheet can be measured in the following ways:

[0045] Select a positive electrode with an initial resistance R1 of 0.005Ω~1Ω and an initial weight W1 of 5g~90g. After immersing the positive electrode in electrolyte in an 85℃ oven for 48h, measure the second resistance R2 and the second weight W2 of the positive electrode. R2 will be in the range of 0.001Ω~0.9Ω, and W2 will be in the range of 8g~100g. Moreover, the four parameters will satisfy the following relationship: This indicates that the positive electrode has high adhesion and low swelling rate. Furthermore, an empirical formula exists: for every 1 wt% increase in electrolyte retention of the positive electrode, its resistance decreases by 0.02 Ω. The maximum electrolyte absorption percentage of the positive electrode in this application does not exceed 30%, hence the above relationship. Specific methods for measuring resistance and weight can be found in the subsequent content of this application.

[0046] In the positive electrode sheet, the positive active layer includes positive active material, conductive agent, etc., in addition to binder. The mass of binder is generally 0.5% to 2% of the mass of the positive active layer, specifically 0.5%, 1%, 1.5%, 2%, or any combination of these values. Compared to other binders, this application only requires 0.5% to 2% binder to effectively suppress the expansion of the positive electrode sheet, thus not reducing the content of the positive active material and reducing the risk of energy density loss in the secondary battery. Furthermore, the inventors have discovered that the higher the amount of binder added to the positive electrode sheet, the higher the thermal shock withstand temperature of the secondary battery during subsequent fabrication. Under high voltage systems (not less than 4.5V), lithium cobalt oxide and ternary systems can withstand thermal shock tests up to 140℃.

[0047] In the positive electrode of this application, the positive active material can be any material capable of reversibly inserting and de-intercalating Li. + Na + The positive electrode and the secondary battery contain alkali metal ions to ensure that they can be charged and discharged normally. In addition, in the positive electrode of this application, the amide groups in the first polymer of the binder will form hydrogen bonds with the positive electrode material, thereby further enhancing the adhesion between the positive electrode active layer and the positive electrode current collector.

[0048]

[0049] In some embodiments of this application, a positive electrode active material containing a transition metal element is preferred. The transition metal element can be at least one of nickel, cobalt, or manganese; for example, it can be lithium cobalt oxide, ternary materials, etc. This can improve the thermal safety performance of the positive electrode sheet and the secondary battery. The principle is that the first polymer of the binder is rich in nitrile functional groups. The nitrile functional groups can form a complex adsorption structure with the ions of the transition metal element (such as cobalt ions), inhibiting the dissolution of metal ions and ensuring that the structure of the positive electrode active material is not easily destroyed. In this way, oxygen atoms in the positive electrode active material are not easily released to generate oxygen, thereby reducing the probability of thermal runaway at high temperature.

[0050] This application does not limit the type of positive electrode conductive agent; any known conductive material can be used. Specifically, the positive electrode conductive agent includes, but is not limited to, at least one of the following: acetylene black, Super-P carbon black, amorphous carbon such as needle coke, carbon nanotubes, or graphene. Due to the presence of the binder, in this application, the positive electrode conductive agent adheres to the surface of the lithium cobalt oxide particles, and the coverage rate of the positive electrode conductive agent on the lithium cobalt oxide surface is S%. The coverage rate S% and the binder content a% generally satisfy the following relationship: 73.6 + 2a ≤ S < 83.7. In this way, the carbon nanotubes can be uniformly distributed in the positive electrode active layer, which is beneficial for the intercalation and deintercalation of lithium ions from positive electrode active materials such as lithium cobalt oxide.

[0051] In addition, the positive electrode sheet of this application may also incorporate resin additives as needed to further enhance its safety performance. These resin additives include, but are not limited to, at least one of melamine-formaldehyde resin, phenolic resin, epoxy resin, and their derivatives. This application does not impose any particular restrictions on the type or amount of resin additives, as long as they meet the objectives of this application.

[0052] In the positive electrode sheet, there are no particular restrictions on the type of positive current collector; it can be any known material suitable for use as a positive current collector. Materials for the positive current collector include, but are not limited to, metals such as aluminum, stainless steel, nickel plating, titanium, and tantalum. Furthermore, to reduce the electronic contact resistance between the positive current collector and the positive active layer, conductive additives or conductive coatings can be applied to the surface of the positive current collector. Conductive additives include, but are not limited to, carbon and precious metals such as gold, platinum, and silver. The conductive coating can be a mixture of inorganic oxides, conductive agents, and positive electrode binders.

[0053] In preparing the positive electrode sheet, the components of the aforementioned positive electrode active layer can be dissolved or dispersed in a liquid solvent to form a positive electrode slurry. This slurry is then coated onto a positive electrode current collector and dried, thereby forming the positive electrode active layer on the current collector, thus obtaining the positive electrode sheet. When preparing the positive electrode sheet using this method, there are no particular limitations on the solvent in the positive electrode slurry, as long as it can dissolve or disperse the aforementioned components. Specifically, the solvent in the positive electrode slurry includes, but is not limited to, N-methylpyrrolidone (NMP) and ethylene carbonate (EC). Alternatively, in preparing the positive electrode sheet, the various components of the positive electrode active layer can be dry-mixed to form a sheet, which is then pressed onto the positive electrode current collector.

[0054] Thirdly, this application provides a secondary battery comprising the aforementioned positive electrode. Because the positive electrode in this secondary battery does not easily expand and rebound after charging and discharging, the secondary battery of this application exhibits excellent energy density.

[0055] In the secondary battery of this application, the positive electrode is as shown above, and the other structures are as follows:

[0056] Negative electrode sheet

[0057] The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector. The composition of the negative electrode active layer includes the negative electrode sheet active material. That is, in this application, the negative electrode active layer can be disposed on one surface or on two surfaces in the thickness direction of the negative electrode current collector. Moreover, in this application, the "surface of the negative electrode current collector" can be the entire area of ​​the negative electrode current collector or a part of the negative electrode current collector. This application has no particular limitation, as long as the purpose of this application can be achieved.

[0058] The negative electrode active layer generally contains a negative electrode active material, and this application does not impose any particular limitation on the negative electrode active material. Specifically, the negative electrode active material may include at least one of carbon materials or silicon-based materials. More specifically, carbon materials include, but are not limited to, at least one of natural graphite, artificial graphite, mesophase micro carbon spheres, hard carbon, or soft carbon; silicon-based materials include, but are not limited to, at least one of silicon, silicon-oxygen composite materials, or silicon-carbon composite materials.

[0059] In some embodiments, the negative electrode active layer typically also contains a negative electrode conductive agent. This application does not impose any particular limitation on the type of negative electrode conductive agent, as long as it achieves the purpose of this application. For example, negative electrode conductive agents include, but are not limited to, at least one of acetylene black, Ketjen black, carbon nanotubes, carbon fibers, carbon dots, or graphene.

[0060] In some embodiments, the negative electrode active layer may also contain a negative electrode binder and a thickener. This application does not particularly limit the types of negative electrode binders and thickeners, as long as they can achieve the purpose of this application. For example, the negative electrode binder may include, but is not limited to, at least one of polyvinyl alcohol, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, styrene-butadiene rubber, or acrylated styrene-butadiene rubber; the thickener in the negative electrode slurry may include, but is not limited to, at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose.

[0061] In the negative electrode sheet, the material of the negative electrode current collector includes, but is not limited to, copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a polymer substrate coated with a conductive metal, etc., and this application does not have any particular limitations. Among them, the conductive metal includes, but is not limited to, copper, nickel, or titanium, and the material of the polymer substrate includes, but is not limited to, at least one of polyethylene, polypropylene, ethylene propylene copolymer, polyethylene terephthalate, polyethylene terephthalate, or poly(p-phenylene terephthalamide).

[0062] Furthermore, in this application, there are no particular limitations on the thickness of the negative electrode current collector and the negative electrode active layer, as long as the purpose of this application can be achieved. For example, the thickness of the negative electrode current collector is 4 μm to 12 μm, and the thickness of the single-sided negative electrode active layer is 30 μm to 160 μm.

[0063] Furthermore, similar to the preparation of the positive electrode sheet, the preparation of the negative electrode sheet can be achieved either by preparing a negative electrode slurry, coating the slurry onto a negative electrode current collector, and drying it to form a negative electrode active layer on the current collector, thus obtaining the negative electrode sheet; or by dry mixing the components of the negative electrode active layer to form a sheet, which is then pressed onto the negative electrode current collector to form the negative electrode active layer, thereby obtaining the negative electrode sheet. The solvent in the negative electrode slurry includes any one of aqueous solvents and organic solvents. Aqueous solvents include, but are not limited to, mixtures of alcohol and water or water itself. Organic solvents include, but are not limited to, aliphatic hydrocarbons such as hexane; aromatic hydrocarbons such as benzene, toluene, xylene, and methylnaphthalene; heterocyclic compounds such as quinoline and pyridine; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; esters such as methyl acetate and methyl acrylate; amines such as diethylenetriamine and N,N-dimethylaminopropylamine; ethers such as diethyl ether, propylene oxide, and tetrahydrofuran (THF); amides such as N-methylpyrrolidone (NMP), dimethylformamide, and dimethylacetamide; and aprotic polar solvents such as hexamethylphosphoramide and dimethyl sulfoxide. In some other embodiments, when using aqueous solvents, the negative electrode slurry composition may also include a thickener and styrene-butadiene rubber (SBR) emulsion to slurry the negative electrode slurry, thereby adjusting its viscosity. The types of thickeners in the positive electrode slurry include, but are not limited to, at least one of carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and their salts.

[0064] electrolyte

[0065] Electrolytes play a role in transporting lithium ions and electrons, ensuring the formation of internal pathways in secondary batteries. Electrolytes typically contain lithium salts, solvents, and additives. It should be noted that this application does not impose specific restrictions on the amount of each component in the electrolyte, as long as the purpose of this application can be achieved.

[0066] Specifically, lithium salts can dissolve in solvents to form ionic conductors and be used as conductive media and lithium-ion transport media; lithium salts include, but are not limited to, at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis(oxalate-borate), lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorooxalate-borate), and lithium bis(fluorosulfonyl)imide.

[0067] Solvents can dissolve lithium salts and additives. Solvents can be at least one of carbonates, carboxylic esters, ethers, and alcohols. Carbonates can be classified as cyclic carbonates and linear carbonates. Cyclic carbonates specifically include, but are not limited to, at least one of ethylene carbonate and propylene carbonate; linear carbonates specifically include, but are not limited to, at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, and methyl propyl carbonate; carboxylic esters include, but are not limited to, at least one of methyl formate, methyl acetate, methyl butyrate, ethyl propionate, propyl propionate, and propyl acetate; ethers include, but are not limited to, at least one of tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, and 4-methyl-1,3-dioxolane; and alcohols include, but are not limited to, at least one of ethanol, ethylene glycol, and glycerol.

[0068] Additives include, but are not limited to, nitriles, sulfones, sulfoxides, fluoronitriles, and fluoroesters.

[0069] Separating membrane

[0070] To prevent short circuits, a separator is typically placed between the positive and negative electrodes. In this case, the electrolyte of this application is typically used after penetrating into the separator.

[0071] There are no particular limitations on the material and shape of the separator, as long as it does not significantly impair the effectiveness of this application. The separator material can be resin, glass fiber, inorganic materials, etc., formed from materials stable to the electrolyte of this application. In some embodiments, the separator includes a porous sheet or non-woven fabric-like material with excellent liquid retention properties. Examples of resin or glass fiber separator materials include, but are not limited to, polyolefins, aromatic polyamides, polyimide (PI), polyamide (PA), polytetrafluoroethylene, polyethersulfone, spandex, or aramid. In some embodiments, the polyolefin is polyethylene or polypropylene. In some embodiments, the polyolefin is polypropylene. The above-mentioned separator materials can be used alone or in any combination.

[0072] The separator can also be a material formed by laminating the above-mentioned materials, examples of which include, but are not limited to, a three-layer separator formed by laminating polypropylene, polyethylene, and polypropylene in that order.

[0073] Inorganic materials include, but are not limited to, oxides such as alumina and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, and sulfates (e.g., barium sulfate, calcium sulfate, etc.). The forms of inorganic materials include, but are not limited to, particulate or fibrous forms.

[0074] The separator can be in the form of a thin film, including but not limited to non-woven fabric, woven fabric, and microporous membranes. In the thin film form, the pore size of the separator is 0.01 μm to 1 μm, and the thickness is 5 μm to 50 μm. In addition to the above-mentioned independent thin film separators, the following separators can also be used: separators formed by using a resin-based adhesive to form a composite porous layer containing the above-mentioned inorganic particles on the surface of the positive electrode and / or negative electrode, for example, a separator formed by using fluororesin as an adhesive to form a porous layer of alumina particles with a particle size of less than 1 μm on both sides of the positive electrode.

[0075] The thickness of the separator is arbitrary. In some embodiments, the thickness of the separator is greater than 1 μm, greater than 5 μm, or greater than 8 μm. In some embodiments, the thickness of the separator is less than 50 μm, less than 40 μm, or less than 30 μm. When the thickness of the separator is within the above ranges, insulation and mechanical strength can be ensured, and the rate characteristics and energy density of the secondary battery can be ensured.

[0076] In this application, the diaphragm may include a substrate and a surface treatment layer. The substrate may be a nonwoven fabric or composite membrane with a porous structure, and the material of the substrate may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be used. Optionally, a surface treatment layer is provided on at least one surface of the substrate. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic materials. For example, the inorganic layer includes inorganic particles and a binder. This application does not have any particular limitation on the aforementioned inorganic particles, and may include at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. This application does not have any particular limitation on the aforementioned binders, and may include at least one of the aforementioned binders. The polymer layer contains a polymer, the polymer material of which includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, or poly(vinylidene fluoride-hexafluoropropylene).

[0077] The secondary battery of this application also includes a packaging bag for containing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the art for secondary batteries. This application does not limit the aforementioned other components. This application does not have any particular limitation on the packaging bag; it can be any packaging bag known in the art, as long as it can achieve the purpose of this application.

[0078] The application of the secondary battery in this application is not particularly limited, and it can be used in any electronic device known in the prior art. In some embodiments, the secondary battery of this application can be used in, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and lithium-ion capacitors, etc.

[0079] Example

[0080] The following uses lithium-ion batteries as an example to illustrate this application in more detail with examples and comparative examples. Those skilled in the art will understand that the preparation methods described in this application are merely examples, and any other suitable preparation methods are within the scope of this application.

[0081] Test methods and equipment:

[0082] Adhesion test

[0083] (1) Discharge the cell of the secondary battery to 3V at 25°C and a small current (0.2C) to perform a full discharge process. After the full discharge process is completed, remove the positive electrode from the fully discharged cell at 3V in an environment of 25°C, and then wipe away the residual electrolyte on the surface of the electrode with a lint-free paper.

[0084] (2) Test the bonding force of the removed positive electrode sheet under a universal tensile testing machine as follows: First, cut a sample with a width of 30mm and a length of 100mm~160mm with a blade. Then, stick double-sided tape on the steel plate with a tape width of 20mm and a length of 90mm~150mm. Stick the cut electrode sheet sample on the double-sided tape with the test side facing down. Then, stick green glue (width of 20mm and length of 90mm~150mm) tightly on the surface of the electrode sheet. Then, insert a paper tape with a width equal to that of the electrode sheet and a length greater than the sample length by 80-200mm under the green glue and fix it with wrinkle glue. Finally, turn on the tensile testing machine, adjust the limit block to the appropriate position, and record the value of the tensile testing machine.

[0085] Adhesion retention test

[0086]

[0087] Resistance test

[0088] (1) Obtain the positive electrode using the method in step (1) of the "Adhesion Test".

[0089] (2) The resistance of the sample to be tested was tested using a diaphragm resistance meter (manufactured by Yuaneng Technology). Before testing, the test probe of the diaphragm resistance meter was wiped clean with alcohol, and then the pressure and resistance were zeroed. The cut positive electrode (60mm×80mm) was placed flat on the sample stage, and then the sample stage was placed in the test chamber. Twelve different positions of each positive electrode sample were tested, and the average value was calculated to obtain the resistance of the positive electrode in Ω.

[0090] Weight and resistance testing

[0091] Obtain a fully loaded positive electrode sheet (see "Adhesion Test" for acquisition method), air dry the electrolyte on the surface of the positive electrode sheet, and test the initial weight and initial resistance; then immerse the positive electrode sheet in the electrolyte, bake it in an 85℃ oven for 48 hours, and test the second weight and second resistance. The rate of change of weight can represent the swelling rate of the binder and the expansion rate of the positive electrode sheet.

[0092] Coverage test of conductive agent on the surface of positive electrode active material

[0093] (1) Obtain the positive electrode using the method in step (1) of the "Adhesion Test".

[0094] (2) Coverage test using a surface Brewster thin film analyzer: Divide the narrow edge of the electrode into three equal parts. Select the upper left corner area of ​​each part and cut one pc of 2mm*2mm square electrode. Place the sample on the measuring stage, ensuring that the incident light is perpendicular to the sample surface. Record the intensity of the reflected light and pay attention to the maximum value of the light intensity. By measuring the change in light intensity, obtain the light intensity reflectance. Calculate the coverage based on the measured reflectance change and the known Brewster angle conditions.

[0095] Energy density test

[0096] The prepared, formed, and aged battery cells were charged from 3V to 4.51V at 0.2C at room temperature (25℃), left to stand for 5 minutes, and then discharged from 4.51V back to 3V at a rate of 0.2C. The discharge capacity and discharge plateau voltage were recorded. The volume of the fully discharged cell was measured using the water displacement method, and the data were recorded.

[0097] Energy density = capacity * platform voltage / cell volume, in Wh / L.

[0098] Infrared spectroscopy characterization

[0099] (1) Obtain the binder powder using the method in step (1) of the “Polydispersity Test”.

[0100] (2) Take 2 mg of adhesive powder, compress it into a tablet using the potassium bromide tableting method, and then put the tablet into a Fourier transform infrared spectrometer for testing to obtain the infrared spectrum of the adhesive.

[0101] polydispersity test

[0102] (1) Preparation of binder powder: Full loading was carried out at 25°C (see "Adhesion Test" for the full loading procedure), then the positive electrode was removed, and the residual electrolyte on the surface of the electrode was wiped off with lint-free paper. The positive electrode was then immersed in dimethyl carbonate for 1 hour, and then removed and air-dried in a fume hood. After air-drying, the following operations were performed:

[0103] First, the electrode was immersed in 500 mL of N-methylpyrrolidone (NMP) using a single-bar stirrer at 1500 rpm for 4 hours to fully disperse the coating layer on the aluminum foil into the NMP, forming a uniform slurry. Next, a vacuum filtration device with a filter paper pore size of 100 nm was used to remove solids from the slurry, leaving a clear NMP solution. The resulting liquid was then placed in a 120°C oven to remove the NMP liquid, leaving a binder film. Finally, the resulting film was pulverized using a pulverizer to obtain binder powder.

[0104] (2) Use tetrahydrofuran (THF) as solvent, and set the column temperature (25℃ to 40℃) and flow rate (0.5 mL / min).

[0105] Detector configuration: Differential refractive index (RI) detector. Polymer sample is injected into the system, its chromatogram is recorded, retention time and detector response are obtained, and the retention time is converted to molecular weight based on the calibration curve. Number-average molecular weight and weight-average molecular weight are then calculated, along with the polydispersity index.

[0106] Example 1-1

[0107] <Preparation of Electrolyte>

[0108] In an argon-atmospheric glove box with a water content of less than 10 ppm, methyl ethyl carbonate and ethyl acetate were mixed at a mass ratio of 1:1 to prepare a base solvent, and then lithium hexafluorophosphate (LiPF6) was added. Based on the total mass of the electrolyte, the mass percentage of LiPF6 was 12.5%, with the remainder being the base solvent.

[0109] <Preparation of the positive electrode>

[0110] Lithium cobalt oxide, conductive carbon SP, binder, and phenolic resin were mixed in a mass ratio of 97.5:1.1:1.3:0.1. N-methylpyrrolidone was then added as an organic solvent and stirred until homogeneous, resulting in a positive electrode slurry with a solid content of 70%. The positive electrode slurry was uniformly coated onto the upper and lower surfaces of a 9 μm thick aluminum foil used as a positive electrode current collector. After drying and pressure treatment, the foil was cut to the specified size to obtain the positive electrode sheet. The single-sided coating density of the binder was 0.129 mg / mm². 2 .

[0111] <Preparation of Negative Electrode Sheets>

[0112] Artificial graphite, sodium carboxymethyl cellulose, negative electrode conductive agent, and styrene-butadiene rubber were mixed in a mass ratio of 95:2:1:2. Distilled water was then added as a solvent and stirred until homogeneous, resulting in a negative electrode slurry with a solid content of 45 wt%. The negative electrode slurry was uniformly coated onto the upper and lower surfaces of a 6 μm thick copper foil current collector. After drying and pressure treatment, the foil was cut into specified sizes to obtain the negative electrode sheet.

[0113] <Isolation membrane>

[0114] A porous polyethylene film with a thickness of 15μm was used as the separator.

[0115] <Preparation of Lithium-ion Batteries>

[0116] The prepared positive electrode, separator, negative electrode, and separator are stacked in sequence, with the separator positioned between the positive and negative electrodes to act as a separator. The electrodes are then wound to obtain the electrode assembly. After welding the tabs, the electrode assembly is placed in an aluminum-plastic film packaging bag and dried in an 85°C vacuum oven for 12 hours to remove moisture. The prepared electrolyte is then injected, and the lithium-ion battery is obtained through vacuum sealing, settling, formation, shaping, and capacity testing.

[0117] Examples 1-2 to 1-26

[0118] Except for adjusting the relevant parameters of the first polymer in the binder according to Table 1, the rest is the same as in Examples 1-1.

[0119] Comparative Examples 1 to 5

[0120] Except for adjusting the relevant parameters of the first polymer in the binder according to Table 1, the rest is the same as in Examples 1-1.

[0121] Table 1

[0122]

[0123]

[0124]

[0125]

[0126] Note: In Table 1 and subsequent Tables 2 and 3, "whether there is a characteristic peak" refers to the first polymer at 1100 cm⁻¹. -1 ~1300cm -1 Does it contain characteristic peaks?

[0127] Examples 2-1 to 2-6

[0128] Except for adjusting the adhesive content according to Table 2, the rest is the same as in Examples 1-26.

[0129] Table 2

[0130]

[0131] Examples 3-1 to 3-6

[0132] Except for adjusting the single-sided coating density of the adhesive according to Table 3, the rest is the same as in Examples 1-26.

[0133] Table 3

[0134]

[0135] As shown in the table, the binder in this application has good adhesion and a low swelling rate, which can reduce the expansion rate of the positive electrode and improve the energy density of the secondary battery. Specifically, when the mass content of the binder is 0.4%~2%, or the coating density of the binder is 0.129 mg / mm², the effect is even better. 2 ~0.228mg / mm 2 At that time, the energy density of secondary batteries is higher.

[0136] The above are merely preferred embodiments of this application and are not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the principles of this application should be included within the protection scope of this application.

Claims

1. An adhesive, characterized in that, It includes a first polymer, which has a structure as shown in Formula I: Formula I Wherein R1 is a C5-C12 straight-chain alkyl group, and R2 is any one of a C1-C3 alkyl group, a hydrogen atom, or an aryl group; the number average molecular weight of the first polymer is 6 x 10. 4 ~2x10 6 .

2. The adhesive according to claim 1, characterized in that, It satisfies at least one of the following conditions: (1) R1 is a C5~C8 alkyl group; (2) R2 is any one of C1~C3 alkyl or hydrogen atoms; （3）500≤m≤20000； （4）300≤n≤10000； (5) 1.25n≤m≤3n; (6) The number average molecular weight of the first polymer is 3 × 10 5 ~2x10 6 .

3. The adhesive according to claim 2, characterized in that, It satisfies at least one of the following conditions: （1）800≤m≤13960； （2）400≤n≤10000。 4. The adhesive according to any one of claims 1 to 3, characterized in that, The polydispersity of the first polymer is 1.3 to 1.

5.

5. The adhesive according to any one of claims 1 to 3, characterized in that, Based on infrared spectroscopy characterization, the first polymer was observed at 1100 cm⁻¹. -1 ~1300cm -1 It contains characteristic peaks.

6. The adhesive according to claim 1, characterized in that, The glass transition temperature of the first polymer is -45℃ to -25℃.

7. A positive electrode sheet, characterized in that, It includes a positive current collector, at least one surface of which is provided with a positive active layer, the positive active layer comprising the binder according to any one of claims 1 to 6.

8. The positive electrode sheet according to claim 7, characterized in that, Based on the mass of the positive electrode active layer, the mass content of the binder is a%, 0.5≤a≤2.

9. The positive electrode sheet according to claim 8, characterized in that, The positive electrode active layer further includes a positive electrode active material and a positive electrode conductive agent. The positive electrode conductive agent is attached to the surface of the positive electrode active material, and the coverage rate of the positive electrode conductive agent on the surface of the positive electrode active material is S%, 73.6+2a≤S<83.

7.

10. The positive electrode sheet according to claim 9, characterized in that, The positive electrode active material contains transition metal elements.

11. The positive electrode sheet according to claim 10, characterized in that, The transition metal includes at least one of nickel, cobalt, or manganese.

12. The positive electrode sheet according to claim 7, characterized in that, On at least one surface of the positive electrode, the adhesive coating density is 0.129 mg / mm². 2 ~0.228mg / mm 2 .

13. A secondary battery, characterized in that, It includes the positive electrode sheet as described in any one of claims 7 to 12.

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