Binder, positive electrode sheet, and secondary battery

Abstract

The present application provides a binder, a positive electrode sheet, and a secondary battery. The binder of the present application comprises a fluorine-free polymer, and the fluorine-free polymer comprises a first structural unit and a second structural unit. In addition, the pH of the binder is 6-10. The binder of the present application not only has good bonding performance, but also has the effects of inhibiting collapse of a main material structure under high temperatures and high voltages and reducing heat production inside a secondary battery, such that a positive electrode sheet can have good safety performance and cohesion, thereby improving the thermal safety performance and 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] Due to their advantages such as high voltage, good cycle performance, high specific energy, and low environmental pollution, secondary batteries, represented by lithium-ion batteries, have become the power source for most electronic devices and the energy storage power source for peak shaving and frequency regulation. With the development of battery material technology, the energy density and electrochemical performance of secondary batteries have been greatly improved, but this often leads to a decline in safety performance and an increased risk of battery fires and explosions.

[0003] Currently, PVDF (polyvinylidene fluoride) binders are used in the electrodes of secondary batteries to improve their stability and reliability. However, excessive PVDF can also affect the energy density of secondary batteries and cause environmental pollution. Summary of the Invention

[0004] This application provides an adhesive, a positive electrode sheet, and a secondary battery. The adhesive of this application has good bonding properties and can suppress the collapse of the main material structure under high temperature and high voltage, reduce the internal heat generation of the secondary battery, and enable the positive electrode sheet to have good safety performance and cohesion, thereby improving the thermal safety performance and energy density of the secondary battery.

[0005] In a first aspect, this application provides an adhesive comprising a fluorine-free polymer, the fluorine-free polymer comprising a first structural unit and a second structural unit, the first structural unit having the following structural formula:

[0006] ;

[0007] The structural formula of the second structural unit is:

[0008] ;

[0009] The pH of the adhesive is 6-10.

[0010] In the above technical solution, the inventors discovered that in the fluorine-free polymer, the trihydroxybenzene ring in the first structural unit has excellent adhesive properties, enabling the binder to have good dispersibility and adhesion; the cyano group in the second structural unit is a strongly polar group with a short bond length, exhibiting good high voltage stability and can play a role in conducting Li +The binder not only protects the positive electrode material, but also possesses flexible long carbon chains in the second structural unit, which endow the fluorinated polymer with good flexibility, further enhancing the adhesive properties of the binder. Good adhesive properties can suppress the collapse of the main material structure under high temperature and high voltage, reducing heat generation inside the secondary battery. Furthermore, the cyano groups can interact with the positive electrode material in the secondary battery, forming a strong adsorption layer on its surface, inhibiting the release of oxygen from the positive electrode material under high voltage and high temperature, and reducing factors that could lead to fire. Therefore, when the binder of this application is used in the positive electrode sheet and secondary battery, it can not only improve energy density but also enhance thermal safety performance. Moreover, the binder's pH of 6-10 is beneficial for improving its solubility in organic solvents, thereby facilitating the preparation of a uniform positive electrode slurry, and ultimately improving the energy density and thermal safety performance of the secondary battery.

[0011] In one possible implementation, 100≤m+n≤10000; the amount of the first structural unit is 55%~65% of the amount of the binder, and the amount of the second structural unit is 15%~30% of the amount of the binder.

[0012] In one possible implementation, the adhesive satisfies at least one of the following conditions: (1) 500≤m+n≤8000; (2) x is any integer from 4 to 11; (3) the amount of the first structural unit is 58% to 62% of the amount of the adhesive; (4) the amount of the second structural unit is 20% to 25% of the amount of the adhesive.

[0013] In the above technical solutions, the adhesive has better bonding performance, and its use in the positive electrode sheet can further enhance the cohesion of the positive electrode sheet.

[0014] In one possible implementation, the pH of the adhesive is 6 to 9.

[0015] In the above technical solutions, the adhesive has better solubility.

[0016] In one possible implementation, the adhesive has a D90 of D1μm, and 1.1≤D1≤1.6.

[0017] In the above technical solutions, the adhesive has better solubility.

[0018] In one possible implementation, based on X-ray diffraction characterization, the binder has four peaks in the diffraction angle range of 50° to 70°.

[0019] In one possible implementation, based on differential scanning calorimetry characterization, the thermal weight loss temperature of the binder is 131℃~170℃.

[0020] In one possible implementation, the adhesive of this application satisfies: 70≤m≤8000, and / or, 30≤n≤3000.

[0021] 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.

[0022] In one possible implementation, the positive electrode active layer further includes a positive electrode active material, the surface of which is coated with a positive electrode conductive agent, and the positive electrode active material includes 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, which can further improve 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 initial cohesive force and initial resistance of the positive electrode are F1N / m and R1Ω, respectively. After the positive electrode is baked at 131℃~170℃ for t hours, the second cohesive force and second resistance of the positive electrode are F2N / m and R2Ω, respectively, 0.25≤t≤0.5. The positive electrode satisfies the following conditions: (1) 45≤F1≤65; (2) 0.001≤R1≤0.5; (3) 1.5F1≤F2≤100; (4) R1(1+t)≤R2≤0.8.

[0026] Among the above technical solutions, the positive electrode sheet has better safety performance.

[0027] In one possible implementation, after the positive electrode sheet is baked at 131℃~170℃ for 0.25h~0.5h, a portion of the surface area of ​​the positive electrode active layer is recessed inward to form a channel. The channel has a length of 0.5mm~1mm, an outer diameter of 0.1mm~1mm, an inner diameter of 0.01mm~0.1mm, and the total area of ​​the recessed region is 40%~60% of the surface area of ​​the positive electrode active layer.

[0028] Among the above technical solutions, the positive electrode sheet has better thermal safety performance.

[0029] 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.

[0030] The beneficial effects of this application are:

[0031] This application provides a binder, a positive electrode sheet, and a secondary battery. The binder comprises a fluorinated polymer, which includes a first structural unit and a second structural unit. The pH of the binder is 6-11. In the fluorinated polymer, specific amounts of the first and second structural units work together to give the binder good bonding properties, which can improve the cohesive force of the positive electrode sheet, thereby improving the thermal safety performance and energy density of the secondary battery. Attached Figure Description

[0032] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0033] Figure 1 is a CCD image of the positive electrode sheet after baking in Embodiment 1-1 of this application.

[0034] Figure 2 is an XRD pattern of the adhesive of Example 1-1 of this application. Embodiments of the present invention

[0035] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0036] In a first aspect, this application provides an adhesive comprising a fluorine-free polymer, the fluorine-free polymer comprising a first structural unit and a second structural unit, the first structural unit having the following structural formula:

[0037] ;

[0038] The structural formula of the second structural unit is:

[0039] ;

[0040] Where x can be any integer from 4 to 11, preferably 4, specifically 4, 5, 6, 7, 8, 9, 10, or 11; the pH of the adhesive is 6 to 10, preferably 6 to 9.

[0041] The binder in this application is a fluorine-free polymer that contains virtually no fluorine, which is environmentally friendly. Furthermore, the trihydroxybenzene ring in the first structural unit of the fluorine-free polymer exhibits excellent adhesive properties, resulting in good dispersibility and adhesion of the binder; the cyano group in the second structural unit is a strongly polar group with a short bond length, providing good high-voltage stability and serving as a conductor for Li. + The binder serves to protect the positive electrode material. Furthermore, the flexible long carbon chains in the second structural unit give the fluorinated polymer good flexibility, further enhancing the adhesive properties of the binder. When the binder of this application is used in the positive electrode sheet and secondary battery, its excellent adhesive properties can suppress the collapse of the main material structure under high temperature and high voltage, reducing heat generation inside the secondary battery. Moreover, the cyano groups can react with the positive electrode material in the secondary battery to form a strong adsorption layer on its surface, inhibiting the release of oxygen from the positive electrode material under high voltage and high temperature, reducing factors that could lead to fire. Thus, the binder not only improves the energy density of the secondary battery but also enhances its thermal safety performance. In addition, the binder's pH range of 6-10 ensures good solubility in organic solvents such as N-methylpyrrolidone (NMP), which is beneficial for preparing a uniform positive electrode slurry, thereby improving the energy density and thermal safety performance of the secondary battery.

[0042] As an example, in some embodiments of this application, m and n in the adhesive can satisfy the following relationship: 100≤m+n≤10000, preferably 500≤m+n≤8000, specifically 100, 300, 500, 1000, 6000, 8000, 9000, 10000, etc., or within the range of any two of the above values; the amount of the first structural unit is 55%~65% of the amount of the adhesive, preferably 58%~62%, specifically 55%, 56%, 58%, 60%, 62%, 64%, 65%, etc., or within the range of any two of the above values; the amount of the second structural unit is 15%~30% of the amount of the adhesive, preferably 20%~25%, specifically 15%, 18%, 20%, 23%, 25%, 28%, 30%, etc., or within the range of any two of the above values.

[0043] As an example, in some embodiments of this application, m and n in the adhesive can satisfy the following relationship: 70≤m≤8000, and / or, 30≤n≤3000. Specifically, m can be 70, 200, 400, 2000, 3000, 6000, 7000, 8000, etc., or within a range consisting of any two of the above values; n can specifically be 30, 100, 800, 1460, 2000, 3000, etc., or within a range consisting of any two of the above values.

[0044] It should be noted that in the adhesive of this application, the preparation process of the fluorine-free polymer typically involves homopolymerizing two different monomers, a first monomer and a second monomer, to obtain a first homopolymer and a second homopolymer, which are then linked together by carbon chain functional groups. The first monomer is acrylonitrile, the monomer corresponding to the first structural unit, and the second monomer is the second structural unit. The corresponding monomers, and the value of x during the homopolymerization of the second monomer, do not necessarily have to be exactly the same, as long as they satisfy the range of the general formula. This application does not limit the homopolymers or the specific reaction steps for linking the two homopolymers, as long as the purpose of this application can be achieved.

[0045] In addition, in some embodiments of this application, the D90 of the binder is D1μm, and 1.1≤D1≤1.6. This way, when using N-methylpyrrolidone (NMP) to prepare the positive electrode slurry, the particle size of the binder is small, making it easy to dissolve and disperse, and preventing the formation of large particles that affect the uniformity of coating.

[0046] In addition, the fluorine-free polymer in the binder can be determined based on X-ray diffraction characterization or glass transition temperature. Specifically, based on XRD (X-ray Diffractometer) characterization, the binder has a characteristic peak in the diffraction angle range of 2θ = (50, 70), and this characteristic peak is a quadruple peak.

[0047] Based on DSC (Differential Scanning Calorimeter) characterization, the thermal weight loss temperature of the binder is 131℃~170℃.

[0048] 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.

[0049] 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. Moreover, in this application, "the surface of the positive electrode current collector" can be the entire area of ​​the positive electrode current collector or a part of the positive electrode current collector. This application has no particular limitation, as long as the purpose of this application can be achieved.

[0050] In the positive electrode sheet of this application, the positive active layer contains a binder, which can enhance the cohesive force of the positive active layer in the positive electrode sheet. Moreover, under high temperature conditions, the cohesive force and resistance of the positive electrode sheet of this application will increase, which is beneficial to improving the thermal safety performance of the secondary battery. Typically, in this application, the initial cohesive force F1 of the positive electrode sheet is between 45 N / m and 65 N / m, and the initial resistance R1 is between 0.001 Ω and 0.5 Ω. Furthermore, the cohesive force and resistance of the positive electrode sheet of this application will increase under high temperature conditions, which is beneficial to enhancing the cohesive force of the positive electrode sheet, thereby improving the energy density of the secondary battery. Specifically, after the positive electrode sheet of this application is baked at 131℃~170℃ for t hours, the relationship between the second cohesive force F2 and F1 of the positive electrode sheet generally satisfies the following condition: 1.5F1≤F2≤100, and the relationship between the second resistance R2 and the initial resistance R1 will also satisfy the following condition: R1(1+t)≤R2≤0.8. The principle is as follows: Since the thermal weight loss temperature of the binder is within the range of 131℃ to 170℃, when the positive electrode sheet of this application is baked at 131℃ to 170℃, the binder will change from a powder solid state to a hollow foam state. In this way, some areas on the surface of the positive electrode active layer of the positive electrode sheet will form dense channels that are recessed, causing the bonding points in the positive electrode active material to be squeezed and merged, which increases the cohesion and resistance of the positive electrode sheet. With the increase in resistance, the current density passing through is smaller under the same voltage conditions, the heat generation inside the secondary battery is lower, and it is less likely to cause the separator to shrink, thus reducing the risk of short circuit and fire. The channel length after baking is 0.5mm to 1mm, the outer diameter is 0.1mm to 1mm, and the inner diameter is 0.01mm to 0.1mm. Moreover, the total area of ​​the recessed region of the positive electrode active layer is 40% to 60% of the surface area of ​​the positive electrode active layer. The measurement methods of cohesion, resistance, and channels are detailed in the following content of this application.

[0051] This application does not impose any particular limitation on the content of the binder, as long as it meets the purpose of this application. As an example, in some embodiments of this application, the mass of the binder in the positive electrode active layer of the positive electrode sheet is generally 0.5% to 2% of the mass of the positive electrode active layer, specifically 0.5%, 1%, 1.5%, 2%, or any combination of the above 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 electrode active material and reducing the likelihood of energy density loss in the secondary battery.

[0052] In addition, in the positive electrode sheet of this application, besides the binder, the positive electrode active layer also includes a positive electrode active material, a conductive agent, etc., wherein the positive electrode active material can be any material capable of reversibly intercalating and deintercalating Li. + Na +Substances containing alkali metal ions are used to ensure the normal charging and discharging of the positive electrode and the secondary battery. Positive electrode active materials include, but are not limited to, lithium cobalt oxide (LiCoO2), and may also include, but are not limited to, at least one of lithium iron phosphate (LiFePO4), lithium manganese oxide, lithium nickel oxide, and ternary materials. Ternary materials include, but are not limited to, at least one of LiNixCoyMnzO2 and LiNixCoyAlzO2, and the contents of Ni, Co, Mn, Al, etc., can be adjusted to ensure x+y+z=1. For example, the ternary material could be LiNi... 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.88 Co 0.08 Mn 0.04 O2, LiNi 0.8 Co 0.15 Mn 0.05 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.88 Co 0.1 Mn 0.02 O2, LiNi 0.8 Co 0.15 Al 0.05 O2, LiNi 0.88 Co 0.1 Al 0.02 O2, etc. 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 and the secondary battery. The principle is that the fluorine-free 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 less likely to escape and generate oxygen, thereby reducing the probability of thermal runaway at high temperatures. Moreover, the inventors have also found that the higher the amount of binder added in the positive electrode, the higher the thermal shock pass temperature. Under high voltage system (not less than 4.5V), the highest thermal shock pass temperature for lithium cobalt oxide and ternary systems is 150°C.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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, NMP, ethylene carbonate (EC), etc. Alternatively, in preparing the positive electrode sheet, the various components of the positive electrode active layer can be dry-mixed to form a sheet, and then the resulting sheet can be pressed onto the positive electrode current collector.

[0057] Thirdly, this application provides a secondary battery comprising the aforementioned positive electrode. Because the positive electrode in the secondary battery of this application has high cohesion, the secondary battery of this application exhibits excellent energy density.

[0058] In the secondary battery of this application, the positive electrode is as shown above, and the other structures are as follows:

[0059] Negative electrode sheet

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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).

[0065] 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.

[0066] 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.

[0067] electrolyte

[0068] 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.

[0069] 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.

[0070] 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.

[0071] Additives include, but are not limited to, nitriles, sulfones, sulfoxides, fluoronitriles, and fluoroesters.

[0072] Separating membrane

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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).

[0080] 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.

[0081] 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.

[0082] Example

[0083] 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.

[0084] Test methods and equipment:

[0085] Cohesion test

[0086] (1) The cell was fully charged from 3V to 4.51V at room temperature (25℃) and a small current (0.2C). After the full charge was completed, the cell was discharged from 4.51V to 3V at 0.2C. Then, the positive electrode was removed from the fully discharged cell at 3V at 25℃. The residual electrolyte on the surface of the positive electrode was wiped off with lint-free paper, and the cell was soaked in dichloromethane (DMC) solvent for 2 hours and then air-dried.

[0087] (2) Bake the positive electrode in a muffle furnace at 131°C for 0.25 h.

[0088] (3) Under the universal tensile testing machine, the cohesive force of the baked positive electrode sheet and the fully loaded positive electrode sheet were tested respectively, as follows: First, a sample with a width of 30mm and a length of 100mm~160mm was cut with a blade. Then, double-sided tape was attached to the steel plate with a tape width of 20mm and a length of 90mm~150mm. The cut positive electrode sheet sample was attached to the double-sided tape with the test side facing down. Then, green adhesive (width of 20mm and length of 90mm~150mm) was attached tightly to the surface of the positive electrode sheet. Then, a paper tape with a width equal to that of the positive electrode sheet and a length greater than that of the sample by 80mm~200mm was inserted under the green adhesive and fixed with wrinkle glue. Finally, the tensile testing machine was turned on and the limit block was adjusted to the appropriate position.

[0089] Resistance test

[0090] The resistance values ​​of the fully loaded and baked positive electrode sheets were measured on a resistance meter. The results for the fully loaded and baked positive electrode sheets were obtained by referring to the "Cohesive Force Test." The specific resistance testing process is as follows:

[0091] First, cut a sample with a width of 60mm and a length of 80mm using a blade. Then, place the sample into the special test box of the resistance meter. Finally, close the opening of the resistance meter and perform the measurement. The measurement result is the average value of the resistance values ​​at 10 points.

[0092] The resistance of the positive electrode after it is fully charged is the initial resistance, denoted as R1Ω. The resistance of the positive electrode after baking is the second resistance, denoted as R2Ω.

[0093] Channel parameter test

[0094] The baked positive electrode sheet (obtained according to "cohesion test") was sent to the Cross-Section for testing. The inner diameter, outer diameter, and length of the channel were measured using a cross-section polisher and a scanning electron microscope. Subsequently, the positive electrode sheet was punched into a diameter of 11540.25 mm² on a punching machine. 2 For a circle, use a CCD (charge coupled device) camera to calculate the area and determine the coverage.

[0095] Hot box test

[0096] Ten samples of the same lithium-ion battery under test were taken and subjected to a hot chamber test at 145°C, as follows:

[0097] The lithium-ion battery was left to stand for 5 minutes; it was then charged at a constant current of 0.7C to 4.51V, followed by constant voltage charging to a current of 0.02C. The open-circuit voltage (OCV) and impedance (IMP) before heating were recorded, and the appearance was inspected and photographed. The temperature was then increased to 130℃±2℃ at a rate of 5℃ / min±2℃ / min and held for 60 minutes, with the surface temperature rise and voltage of the lithium-ion battery recorded. After the test, the OCV and IMP were recorded, and the appearance was inspected and photographed. The criteria for passing the hot chamber test were: no fire and no explosion. The hot chamber test pass rate = (number of lithium-ion batteries that passed the hot chamber test / 10) × 100%. The highest temperature at which the pass rate was 100% was recorded. The higher the highest temperature, the better the thermal safety performance.

[0098] Energy density test

[0099] 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.

[0100] Energy density = capacity * platform voltage / cell volume, in Wh / L.

[0101] Characterization of diffraction angle and thermogravimetric temperature

[0102] (1) Sample preparation: Under an environment of 25℃, the positive electrode sheet was removed from the battery cell. The full-load procedure is shown in (1). The residual electrolyte on the surface of the positive electrode sheet was wiped off with lint-free paper. Then, the positive electrode sheet was immersed in dimethyl carbonate for 1 hour and then air-dried in a fume hood. First, the positive electrode sheet was immersed in 500 mL of NMP and stirred for 4 hours at 1500 r / min using a single bar stirrer to fully disperse the coating layer on the aluminum foil into the NMP to form a uniform slurry. Next, a vacuum filtration device with a filter paper pore size of 100 nm was used to remove the solid substances lithium cobalt oxide, conductive carbon powder, and carbon nanotubes from the slurry, leaving a clear NMP solution. Then, the obtained liquid was placed in a 120℃ oven to bake and remove the NMP liquid, leaving the binder film. Finally, the obtained film was crushed using a pulverizer to obtain binder powder.

[0103] (2) The diffraction angle 2θ and thermogravimetric temperature of the binder powder were tested by XRD and DSC respectively.

[0104] pH test

[0105] (1) Obtain the binder powder by referring to step (1) of “Characterization of Diffraction Angle and Thermogravimetric Temperature”.

[0106] (2) Dissolve the adhesive powder in pure NMP and stir until clear. Use a pH meter at 25°C, with pure NMP as the standard solution, to calibrate the pH meter. Use the calibrated pH meter to test the pH of the NMP and adhesive mixture solution, and the obtained pH value is the pH of the adhesive.

[0107] Example 1-1

[0108] <Preparation of Electrolyte>

[0109] 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.

[0110] <Preparation of the positive electrode>

[0111] 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. Then, N-methylpyrrolidone was added as an organic solvent and stirred until homogeneous to prepare a positive electrode slurry with a solid content of 70%. The positive electrode slurry was uniformly coated on the upper and lower surfaces of a 9 μm thick aluminum foil for the positive electrode current collector. After drying and pressure treatment, it was cut into the specified size to obtain the positive electrode sheet.

[0112] <Preparation of Negative Electrode Sheets>

[0113] 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.

[0114] <Isolation membrane>

[0115] A porous polyethylene film with a thickness of 15μm was used as the separator.

[0116] <Preparation of Lithium-ion Batteries>

[0117] 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.

[0118] Examples 1-2 to Examples 1-18

[0119] Except for adjusting the relevant parameters of the adhesive according to Table 1, the rest is the same as in Example 1-1.

[0120] Comparative Examples 1 to 2

[0121] Except for adjusting the relevant parameters of the adhesive according to Table 1, the rest is the same as in Example 1-1.

[0122] Comparative Example 3

[0123] Compared to Examples 1-1, this comparative example uses PVDF as a binder in the <Preparation of Positive Electrode Sheet>.

[0124] Comparative Example 4

[0125] Compared to Examples 1-7, in this comparative example, the second structural unit in the binder during the <Preparation of the Positive Electrode> is:

[0126] .

[0127] Comparative Example 5

[0128] Compared to Example 1-1, the second structural unit has the structural formula of hydroxyl.

[0129] Comparative Example 6

[0130] Compared to Example 1-1, the first structural unit has the structural formula of hydroxyl.

[0131] Comparative Example 7

[0132] Except for adjusting the relevant parameters of the adhesive according to Table 1, the rest is the same as in Example 1-1.

[0133] Table 1

[0134]

[0135] Note: In Table 1, the value of x in the main chain of the second structural unit is 4.

[0136] Examples 2-1 to 2-8

[0137] Except for adjusting the relevant parameters of the cathode material according to Table 2, the rest is the same as in Examples 1-4.

[0138] Table 2

[0139]

[0140]

[0141] Examples 3-1 to 3-22

[0142] Except for adjusting the relevant parameters according to Table 3, the rest is the same as in Examples 1-9.

[0143] Table 3

[0144]

[0145] Note: The “area percentage” in Table 3 refers to the proportion of the total area of ​​the inwardly recessed region to the surface area of ​​the positive electrode active layer.

[0146] 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. In particular, when the value of D1 is 1.1~1.6, the energy density of the secondary battery is even higher.

[0147] The above are merely embodiments of this application and are not intended to limit the scope of protection of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. An adhesive, characterized in that, It includes a fluoropolymer, which comprises a first structural unit and a second structural unit, wherein the first structural unit has the following structural formula: ; The structural formula of the second structural unit is as follows: ； The pH of the adhesive is 6-10.

2. The adhesive according to claim 1, characterized in that, 100≤m+n≤10000, and the amount of the first structural unit is 55%~65% of the amount of the adhesive, and the amount of the second structural unit is 15%~30% of the amount of the adhesive.

3. The adhesive according to claim 2, characterized in that, It satisfies at least one of the following conditions: (1) 500≤m+n≤8000; (2) x is any integer from 4 to 11; (3) The amount of the first structural unit is 58% to 62% of the amount of the adhesive; (4) The amount of the second structural unit is 20% to 24% of the amount of the adhesive.

4. The adhesive according to claim 1, characterized in that, The pH of the adhesive is 6-9.

5. The adhesive according to claim 1, characterized in that, The adhesive has a D90 of D1μm and 1.1≤D1≤1.

6.

6. The adhesive according to claim 1, characterized in that, Based on X-ray diffraction characterization, the adhesive has four peaks in the diffraction angle range of 50° to 70°.

7. The adhesive according to claim 1, characterized in that, Based on differential scanning calorimetry characterization, the thermal weight loss temperature of the binder is 131℃~170℃.

8. The adhesive according to claim 2, characterized in that, It satisfies: 70≤m≤8000, and / or, 30≤n≤3000.

9. 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 8.

10. The positive electrode sheet according to claim 9, characterized in that, The positive electrode active layer also includes a positive electrode active material, which includes transition metal elements.

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

12. The positive electrode sheet according to claim 9, characterized in that, The initial cohesive force and initial resistance of the positive electrode sheet are F1 N / m and R1 Ω, respectively. After the positive electrode sheet is baked at 131℃~170℃ for t hours, the second cohesive force and second resistance of the positive electrode sheet are F2 N / m and R2 Ω, respectively, and 0.25 ≤ t ≤ 0.

5. The positive electrode sheet satisfies the following conditions: （1）45≤F1≤65； （2）0.001≤R1≤0.5； (3) 1.5F1≤F2≤100; (4) R1(1+t)≤R2≤0.

8.

13. The positive electrode sheet according to claim 9, characterized in that, After the positive electrode sheet is baked at 131℃~170℃ for 0.25h~0.5h, a portion of the surface area of ​​the positive electrode active layer is recessed inward to form a channel. The length of the channel is 0.5mm~1mm, the outer diameter of the channel is 0.1mm~1mm, and the inner diameter of the channel is 0.01mm~0.1mm. The total area of ​​the recessed region is 40%~60% of the surface area of ​​the positive electrode active layer.

14. A secondary battery, characterized in that, It includes the positive electrode sheet as described in any one of claims 9 to 13.

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