Positive electrode sheet and secondary battery

Abstract

The present application provides a positive electrode sheet and a secondary battery. The positive electrode sheet of the present application comprises a first carbon nanotube and a binder, the binder comprising a first polymer. The first polymer of the present application contains both a cyano group and an amide group, which not only form an excellent conductive network path within the positive electrode sheet, but also reduce an ion transport distance, thereby allowing for easy deintercalation and intercalation of lithium ions, and a low direct current resistance value. Therefore, the secondary battery of the present application exhibits good safety performance in addition to good capacity and room-temperature cycling performance.

Description

A positive electrode and a secondary battery Technical Field

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

[0002] To ensure good cohesion in the electrodes of secondary batteries, PFAS (per- and polyfluoroalkyl substances) such as polyvinylidene fluoride (PVDF) are usually added as binders. However, PFAS have a significant environmental impact, so fluorine-free binders, such as polyacrylonitrile (PAN), can be used instead.

[0003] While the presence of PAN-based substances in the binder of the electrode can enhance the cohesion of the electrode, it also increases the direct current resistance (DCR) of the electrode, thus adversely affecting the capacity and room temperature cycle performance of the secondary battery. Summary of the Invention

[0004] This application provides a positive electrode and a secondary battery. The positive electrode of this application contains materials with good cohesion and low DC resistance. Therefore, the positive electrode of this application can not only improve the safety performance of the secondary battery, but also improve the capacity and room temperature cycle performance of the secondary battery.

[0005] In a first aspect, this application provides a positive electrode sheet comprising a first carbon nanotube and a binder, the binder comprising a first polymer as shown in Formula I:

[0006] 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 .

[0007] The inventors discovered that the cyano groups in the binder readily form a strong adsorption layer with the current collector containing transition metals in the positive electrode, thereby hindering the extraction and insertion of lithium ions in the positive electrode. In the above technical solution, the first polymer contains amide groups, which can form hydrogen bonds with the polar hydrogen on the surface of the first carbon nanotube. Simultaneously, the cyano groups in the binder can also form a strong adsorption layer with the current collector. Thus, the binder connects the current collector and the first carbon nanotube, forming an excellent conductive network pathway; at the same time, it shortens the distance between the current collector and the first carbon nanotube, thereby reducing the ion transport distance in the positive electrode. Due to the excellent conductive network pathway and the reduced ion transport distance, lithium ions in the positive electrode of this application are easily extracted and inserted, resulting in a lower DCR. When the positive electrode of this application is used in a secondary battery, it can not only improve the safety performance of the secondary battery but also enhance its capacity and room-temperature cycle performance.

[0008] In one possible implementation, R1 is a C5-C8 straight-chain alkyl group; the number average molecular weight of the first polymer is 8 x 10. 4 ~9x10 5 .

[0009] In the above technical solutions, the positive electrode sheet has better adhesion and lower DCR.

[0010] In one possible implementation, the diameter of the first carbon nanotube is 0.1 nm to 0.9 nm.

[0011] In the above technical solution, the first carbon nanotube has a smaller diameter, better conductivity, higher electron transport efficiency, and is also easier to form a three-dimensional conductive network in the positive electrode. Therefore, the positive electrode material has a higher current carrying capacity and a greater current density.

[0012] In one possible implementation, the positive electrode comprises single-walled carbon nanotube bundles, each bundle containing 300 to 40,000 first carbon nanotubes.

[0013] In the above technical solution, the first carbon nanotubes aggregate to form single-walled carbon nanotube bundles, which are less prone to breakage during the preparation and processing of the positive electrode, thus better utilizing their ion conduction performance. Moreover, since the first carbon nanotubes are aggregated into bundles, they can also provide pores in the positive electrode, reducing the difficulty of lithium ion extraction and making it easier for them to be transported to the negative electrode.

[0014] In one possible implementation, each bundle of single-walled carbon nanotubes has a length of L μm and a diameter of D μm, with 0.01 ≤ D ≤ 400 and 1.3 ≤ L / D ≤ ​​61.

[0015] In the above technical solutions, single-walled carbon nanotube bundles have a longer length, a larger aspect ratio, and better conductivity, thus resulting in a smaller DCR.

[0016] In one possible implementation, based on the mass of the positive electrode sheet, the mass content of the binder is a%, the mass content of the first carbon nanotube is b%, 0.5≤a≤3, and 0.5≤b≤2.

[0017] In the above technical solution, since the binder forms hydrogen bonds with the first carbon nanotube, when the content of the binder and the first carbon nanotube is within the above range, the DCR of the positive electrode sheet will be further reduced.

[0018] In one possible implementation, the resistance of the positive electrode after full discharge is R. x Ω, the cohesive force of the positive electrode after full discharge is FN / m, 20≤R x (1+a%)×F≤40,0<R x ≤0.5.

[0019] In the above technical solution, as the amount of the first polymer used increases, the cohesion and resistance of the positive electrode sheet after it is fully discharged will also increase accordingly.

[0020] In one possible implementation, after the positive electrode is fully charged, the porosity of the positive electrode in the 5–15 nm range is n%, with 20 ≤ n ≤ 27. When the porosity is controlled within the range of this application, the ion channels are suitable, the kinetics are better, and the positive electrode has a suitable pore structure, thereby improving the low-temperature fast-charging performance of the secondary battery.

[0021] In the above technical solution, the first carbon nanotube and single-walled carbon nanotube can increase the number of pores by 5-15 nm, which can increase the lithium-ion transport channels, thereby improving the internal polarization phenomenon of the positive electrode and the secondary battery, reducing internal resistance, and improving capacity performance and cycle performance.

[0022] In one possible implementation, the positive electrode sheet includes a positive active material, and when the positive electrode sheet is fully charged, the specific surface area of ​​the positive active material is sg / cm². 3 0.24≤s≤0.26. When the specific surface area of ​​the positive electrode active material is too large, it indicates that the positive electrode active material is more fragmented, with more exposed fresh surface. Under high voltage, more side reactions occur with the electrolyte, producing more dead lithium and affecting the fast charging capacity retention rate. Conversely, if the specific surface area is too small, it will affect the normal mass exchange reaction rate between the positive and negative electrodes.

[0023] In one possible implementation, the density of the adhesive is ρ g / cm³. 30.3≤ρ≤0.6. When the density of the binder meets the range of this application, it can have better low-temperature fast charging performance. When the density of the binder is too high, the strength of the binder film is greater, and a greater tonnage is required for cold pressing to press the electrode to the designed thickness, which has a great impact on the particle breakage of the main material. On the other hand, when the density of the binder is too low, the stability of the binder is not good, and side reactions will occur, producing dead lithium, which will affect the low-temperature kinetics and reduce the low-temperature fast charging performance.

[0024] In the above technical solution, since the binder can connect the positive current collector and the first carbon nanotube at the same time, the density of the binder within a suitable range can enable the formation of a wider conductive network path in the positive electrode sheet, which is beneficial to further reduce the DC resistance of the positive electrode sheet.

[0025] In one possible implementation, the thermal expansion coefficient of the binder is β, where 1 ≤ β ≤ 1.2. When the thermal expansion coefficient of the binder is properly controlled, a more suitable migration path can be provided for lithium-ion transport, thus more effectively improving low-temperature fast charging performance.

[0026] In one possible implementation, the adhesive has an electrical conductivity of 3.7 × 10⁻⁶. -10 S / cm~3.7×10 -3 S / cm. When the conductivity of the binder meets the range of this application, it can effectively improve the migration rate of lithium ions on the cathode material, thereby improving the fast charge cycle performance under low temperature conditions.

[0027] In the above technical solutions, the higher the conductivity of the binder, the more beneficial it is to reduce the DC resistance of the positive electrode.

[0028] Secondly, this application provides a secondary battery comprising the aforementioned positive electrode. Therefore, the secondary battery provided by this application also exhibits good capacity performance and cycle performance.

[0029] In one possible implementation, the DC resistance of the secondary battery at 50% SOC (State of Charge, remaining capacity) is X1 mΩ, and the DC resistance of the secondary battery at 100% SOC is X2 mΩ, where 0.7 ≤ X1 / X2 ≤ 0.9, and 12 ≤ X2 ≤ 35.

[0030] In the above technical solution, the difference between the DC resistance of the secondary battery at 100% SOC and at 50% SOC is small, indicating that the positive electrode of this application can also reduce the DC resistance of the secondary battery when fully charged (i.e., at 100% SOC).

[0031] The beneficial effects of this application are:

[0032] This application provides a positive electrode and a secondary battery. The positive electrode of this application includes a first carbon nanotube and a binder, the binder including a first polymer. The first polymer of this application contains both cyano groups and amide groups, which can form an excellent conductive network pathway in the positive electrode and reduce the ion transport distance, thereby making lithium ions easier to extract and insert, resulting in a low DCR. Therefore, the secondary battery of this application has good safety performance, as well as good capacity and room temperature cycle performance. Detailed Implementation

[0033] 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.

[0034] In the prior art, to reduce the environmental impact of secondary batteries, fluorine-free binders are used instead of fluorine-containing binders such as PVDF in the electrode sheets, with PAN-based materials being particularly common. However, when the binder contains PAN-based materials, the electrode sheet has a high DCR (discharge rate), which affects the capacity and room temperature cycle performance of the secondary battery. Based on this, the applicant provides a positive electrode sheet and a secondary battery. The positive electrode sheet of this application contains PAN-based materials, which can effectively replace PVDF. Furthermore, the electrode sheet of this application also contains first carbon nanotubes, which can reduce the DCR of the electrode sheet. The positive electrode sheet and secondary battery of this application are described in detail below.

[0035] In a first aspect, this application provides a positive electrode sheet, which includes a positive current collector and a positive active layer disposed on at least one surface of the positive current collector.

[0036] The positive electrode active layer includes a first carbon nanotube and a binder, and the binder includes a first polymer as shown in Formula I:

[0037] 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 Preferably, R1 is a C5-C8 straight-chain alkyl group; the number average molecular weight of the first polymer is 8 x 10. 4 ~9x10 5 .

[0038] The inventors discovered that cyano groups readily form a strong adsorption layer with the positive current collector in the positive electrode sheet, making it difficult for ions to be extracted and inserted into the positive electrode sheet. Therefore, although the positive electrode sheet has good cohesion, its DCR (displacement coefficient of lithium) is high. Based on this, the inventors incorporated a first carbon nanotube and the aforementioned compound of formula I into the positive electrode sheet. The compound of formula I contains both cyano and amide groups. The cyano groups can form a strong adsorption layer with the positive current collector, and the amide groups can form hydrogen bonds with the polar hydrogen on the surface of the first carbon nanotube. In this way, the binder connects the current collector and the first carbon nanotube, forming an excellent conductive network pathway; simultaneously, it shortens the distance between the current collector and the first carbon nanotube, thereby reducing the ion transport distance in the positive electrode sheet. Due to the excellent conductive network pathway and the reduced ion transport distance, lithium ions in the positive electrode sheet of this application are easily extracted and inserted, resulting in a lower DCR.

[0039] In addition, in this application, the positive electrode sheet also includes a positive electrode active material, which is usually located in the positive electrode active layer. The positive electrode active material can be any material capable of reversibly inserting and de-inserting Li. + Na + Substances containing alkali metal ions are used to ensure the normal charging and discharging of the secondary battery. For example, positive electrode active materials include, but are not limited to, at least one of lithium iron phosphate (LiFePO4), lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium nickel oxide, and ternary materials. Ternary materials include, but are not limited to, LiNi x Co y Mn z O2, LiNi x Co y Al z At least one of O2, etc., and the contents of Ni, Co, Mn, Al, etc., can be adjusted to ensure that x+y+z=1. For example, the ternary material can 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 Co0.1 Al 0.02 O2, etc.

[0040] In some embodiments of this application, the structure of the first carbon nanotube can be formed by rolling up a single layer of carbon atoms, with a shape similar to a seamless channel structure. This structure gives them an extremely high aspect ratio and a perfect tubular structure, resulting in a smaller diameter compared to existing ordinary carbon nanotubes. For example, in some embodiments of this application, the diameter of the first carbon nanotube is generally 0.1 nm to 0.9 nm. Due to the single-wall structure and small diameter of the first carbon nanotube, electron transport is more direct and efficient, and conductivity is better. However, if the first carbon nanotube is too thin and long, it is easily broken during processing. Therefore, in some embodiments of this application, 300 to 40,000 first carbon nanotubes are bundled together to form a single-walled carbon nanotube bundle. This not only makes the first carbon nanotube less prone to breakage during processing, but the single-walled carbon nanotube bundle also provides pores, making it easier for lithium ions to escape and be transported to the negative electrode, thereby reducing the internal resistance of the negative electrode and the secondary battery. As an example, in some embodiments of this application, each bundle of single-walled carbon nanotubes has a length of L μm and a diameter of D μm, with 0.01≤D≤400 and 1.3≤L / D≤61.

[0041] Since the binder forms hydrogen bonds with the first carbon nanotube, the amount of binder and the first carbon nanotube, or even their relative amounts, can be further limited to further reduce the DCR of the positive electrode. Preferably, in some embodiments of this application, based on the mass of the positive electrode, the mass content of the binder is a%, and the content of the first carbon nanotube is b%, typically 0.5 ≤ a ≤ 3, 0.5 ≤ b ≤ 2. In this case, the positive electrode of this application typically satisfies the following relationship: 20 ≤ R x (1+a%)×F≤40,0<R x ≤0.5, where R x R is the resistance of the fully charged positive electrode, measured in Ω; F is the cohesive force of the fully charged positive electrode, measured in N / m. F and R x The value will increase as the amount of the first polymer used increases.

[0042] Furthermore, this application does not have specific requirements for the physicochemical properties of the adhesive, as long as they meet the objectives of this application. As an example, in some embodiments of this application, the density of the adhesive is ρ g / cm³. 3The binder density is 0.3 ≤ ρ ≤ 0.6. Since the binder can simultaneously connect the positive electrode current collector and the first carbon nanotube, a suitable binder density can allow for the formation of a wider conductive network in the positive electrode sheet. This helps to further reduce the DC resistance of the positive electrode sheet and improve the low-temperature fast charging performance of the lithium-ion battery. In some embodiments of this application, the thermal expansion coefficient of the binder is β, 1 ≤ β ≤ 1.2. When the thermal expansion coefficient of the binder is appropriately controlled, it can provide a more suitable migration path for lithium-ion transport, thus more effectively improving the low-temperature fast charging performance. In some embodiments of this application, the conductivity of the binder is 3.7 × 10⁻⁶. -10 S / cm~3.7×10 -3 The higher the conductivity of the binder (S / cm), the better it is for reducing the DC resistance of the positive electrode.

[0043] Furthermore, in some embodiments of this application, in order to achieve a higher energy density in the positive electrode sheet, the compaction density of the positive electrode sheet is typically cold-pressed to 4.25–4.28 g / cc. At this point, the specific surface area of ​​the positive electrode active material in the positive electrode sheet is 5 g / cm³. 3 The value is 0.24 ≤ s ≤ 0.26, obtained after the positive electrode is fully discharged. Setting the specific surface area of ​​the positive electrode active material within this range can reduce the difficulty of preparing high-energy-density positive electrode sheets. However, when the specific surface area of ​​the positive electrode active material is 0.24–0.26 g / cm³, the specific surface area is less than 0.24–0.26 g / cm³. 3 At this time, the proportion of 5-15nm pores in the positive electrode sheet is relatively low, which affects ion transport performance. When the specific surface area of ​​the positive electrode active material is too large, it indicates that the positive electrode active material is more fragmented, with more exposed fresh surface. Under high voltage, more side reactions occur with the electrolyte, generating more dead lithium and affecting the fast charging capacity retention rate. Conversely, if the specific surface area is too small, it will affect the normal mass exchange reaction rate between the positive and negative electrodes. When the porosity is controlled within the range of this application, the ion channels are suitable, the kinetics are better, and the positive electrode sheet has a suitable pore structure, thereby improving the low-temperature fast charging performance of the secondary battery.

[0044] However, in this application, even though the specific surface area of ​​the positive electrode active material is 0.24–0.26 g / cm³, 3Because the positive electrode contains first carbon nanotubes and / or single-walled carbon nanotubes, a relatively large number of 5-15 nm pores are still generated in the positive electrode active layer, thereby increasing ion transport channels and facilitating ion extraction. This helps reduce the resistance of the positive electrode and improve the internal polarization of the secondary battery. In particular, single-walled carbon nanotubes are formed by the aggregation of multiple first carbon nanotubes into bundles, resulting in richer internal channels and pores, with a higher proportion of 5-15 nm pores. As an example, in some embodiments of this application, after the positive electrode is fully loaded, the porosity of 5-15 nm in the positive electrode is n%, 20≤n≤27. When the porosity is controlled within the range of this application, the ion channels are suitable, the kinetics are better, and the positive electrode has a suitable pore structure, thereby improving the low-temperature fast-charging performance of the secondary battery.

[0045] Furthermore, it should be noted that the "surface of the positive current collector" in this application can be the entire area of ​​the positive current collector or only a portion thereof; there are no particular limitations, as long as the purpose of this application is achieved. Moreover, the type of positive current collector in the positive electrode sheet of this application is not particularly limited; 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; and materials such as carbon cloth and carbon paper. In addition, to reduce the electronic contact resistance between the positive current collector and the positive active layer, a conductive additive or conductive coating can be provided on 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. 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 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.

[0046] Secondly, this application provides a secondary battery, which includes the positive electrode sheet of the first aspect. Therefore, the secondary battery of this application has good performance, not only with good safety performance, but also with excellent capacity and room temperature cycle performance. In particular, the difference between the DC resistance after full charge (i.e., DC resistance at 100% SOC) and the DC resistance at 50% SOC is small. The specific principle is as follows:

[0047] Under normal conditions, in a fully charged state, the lithium-ion concentration gradient in the electrolyte increases significantly, leading to increased ion diffusion resistance and hindered lithium-ion transport, resulting in a rise in the DC resistance of the secondary battery. Typically, the DC resistance of a secondary battery at 100% SOC is more than 50% higher than that at 50% SOC. However, this application uses a first polymer and a first carbon nanotube. The amide groups of the first carbon nanotube readily adsorb onto the polar hydrogen on its surface to form hydrogen bonds. Simultaneously, the cyano groups of the first polymer readily form a strong adsorption layer with the positive electrode current collector. The binder containing the first polymer connects the positive electrode current collector and the first carbon nanotube (which is typically used as a conductive agent). Lithium ions released from the positive electrode active material can be transported through the first carbon nanotube and the voids in the single-walled carbon nanotube bundles. Thus, the ion diffusion resistance of the secondary battery in this application decreases under full charge, and the DC resistance under full charge also decreases. Specifically, in some embodiments of this application, the DC resistance of the secondary battery at 50% SOC is X1 mΩ, and the DC resistance at 100% SOC is R2 mΩ, with 0.7≤X1 / X2≤0.9 and 12≤X2≤35.

[0048] In addition to the positive electrode, the secondary battery of this application also includes a negative electrode, an electrolyte, and a separator membrane located between the positive and negative electrodes. These are described in detail below:

[0049] electrolyte

[0050] Electrolytes play a crucial role in transporting lithium ions and electrons, ensuring the formation of internal pathways within the secondary battery. Electrolytes typically contain lithium salts, solvents, and additives. It should be noted that this application does not impose specific limitations on the amounts of each component in the electrolyte, as long as the objective of this application is achieved.

[0051] 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.

[0052] 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 (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), 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.

[0053] Additives include, but are not limited to, nitriles, sulfones, sulfoxides, fluoronitriles, and fluoroesters.

[0054] Negative electrode sheet

[0055] The negative electrode includes a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector, wherein the composition of the negative electrode active layer includes a negative electrode 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. Furthermore, in this application, the "surface of the negative electrode current collector" can be the entire area of ​​the negative electrode current collector or only a portion thereof; there are no particular limitations, as long as the purpose of this application is achieved.

[0056] 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 microcarbon 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.

[0057] In some embodiments, the negative electrode active layer typically also contains a negative electrode conductive agent. This application does not particularly limit 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.

[0058] 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.

[0059] In the negative electrode sheet, the material of the negative electrode current collector includes, but is not limited to, copper foil, aluminum 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).

[0060] 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.

[0061] 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.

[0062] Separating membrane

[0063] 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.

[0064] 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, polytetrafluoroethylene, polyethersulfone, etc. 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] This application also provides an electronic device that includes a secondary battery according to this application.

[0070] 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, stereo 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.

[0071] Example

[0072] 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.

[0073] Test methods and equipment:

[0074] Capacity test

[0075] At room temperature, the battery cell using the positive electrode is fully charged to 4.51V at a constant current rate of 0.2C on a new machine, then fully charged to 0.5C at a constant voltage rate, and then discharged to 3V at a discharge rate of 0.2C. The capacity data collected during discharge is the discharge capacity, in mAh.

[0076] Cyclic performance test

[0077] At 25℃, the battery cell is charged at a constant current rate of 2C to 4.2V, then at a constant voltage rate of 0.8C, followed by constant current charging to 4.5V and constant voltage charging to 0.05C, and then allowed to rest for 10 minutes. The cell is then discharged at a rate of 1C to 3.75V, followed by discharge at a rate of 0.5C to 3V, and then allowed to rest for 10 minutes. This charge-discharge cycle is repeated 1000 times.

[0078] Number-average molecular weight test

[0079] The number-average molecular weight of the polymer was determined indirectly using a gel permeation chromatography (GPC) instrument.

[0080] Pipe diameter and length testing

[0081] Sample preparation: After fully discharging the secondary battery, disassemble the positive electrode plate, soak it in DMC for 2 hours, and then air dry it in a fume hood.

[0082] Then, the positive electrode sheet was cut into 2mm*2mm square pieces, placed on a glass slide, and observed using a transmission electron microscope (TEM). The diameter Lμm and length Dμm of the single-walled carbon nanotube bundle were then analyzed and measured using measurement software. The average value was calculated after multiple measurements, and the aspect ratio L / D was then calculated.

[0083] Similarly, the diameter of the first carbon nanotube can be measured, and the magnification of the TEM at this time is higher than that when measuring a single-walled carbon nanotube bundle.

[0084] Porosity testing of positive electrode sheet

[0085] Sample preparation: After fully discharging the secondary battery, disassemble the negative electrode plate, soak it in DMC for 2 hours, and then air dry it in a fume hood.

[0086] The prepared positive electrode sample is then placed into the sample chamber of the testing instrument. Mercury is introduced into the sample chamber, and the pressure is gradually increased. Under pressure, the mercury gradually enters the pores of the sample. The volume of mercury entering the sample and the corresponding pressure are recorded, and the porosity of the positive electrode is obtained using the formula:

[0087] P = 2γcosθ / r.

[0088] In the above formula, P is the pressure, γ is the surface tension of mercury, θ is the contact angle between mercury and the material, and r is the pore size. Plotting pore size and quantity yields the percentage of pores of different sizes, and then recording the porosity percentage from 5 to 15 nm.

[0089] Specific surface area testing of positive electrode active materials

[0090] Sample preparation: After fully discharging the secondary battery, disassemble the positive electrode plate, soak it in DMC for 2 hours, and then air dry it in a fume hood.

[0091] The muffle furnace was set to 600℃ for 2 hours for baking. The positive electrode was then placed in the muffle furnace, and the process was started. After cooling, the positive electrode was removed, and the coating powder was scraped off using a small knife on the experimental platform and collected in a sample bag. The collected powder was then placed in a TriStarⅡ3020 surface area analyzer for testing.

[0092] Measurement of DC resistance of secondary battery at 25℃

[0093] 1) The experiment uses a 4.52V voltage system. After the secondary battery has been formed to capacity, the gas bag is cut off and the excess electrolyte is extracted. The electrolyte retention coefficient is set to 1.0 (electrolyte retention coefficient = weight of retained electrolyte / theoretical discharge capacity of the cell * 1000).

[0094] 2) 25℃ DC resistance test method: In a 25℃ constant temperature chamber, fully charge the battery with a current of 0.7C (4.45V, 0.05C current cutoff), let it sleep for 10 minutes, and then discharge it to 3V with a current of 1C. Take a sample every 100ms to obtain the DC resistance X1 of the secondary battery at 50% SOC and the DC resistance X2 of the secondary battery at 100% SOC. The unit is mΩ.

[0095] Measurement of cohesive force of positive electrode sheet

[0096] The secondary battery was charged from 3V to 4.52V (fully charged) at room temperature and with a low current of 0.2C, then discharged back to 3V with a low current of 0.2C. Subsequently, the positive electrode was removed from the fully discharged 3V cell at 25℃. Residual electrolyte on the surface of the positive electrode was wiped away with lint-free paper. The cohesive force was then tested using a universal tensile testing machine, as detailed below:

[0097] First, use a blade to cut a sample 30mm wide and 100-160mm long. Next, attach double-sided tape to the steel plate, with the tape width being 20mm and the length 90-150mm. Place the cut positive electrode sample on the double-sided tape, with the test side facing down. Then, attach green adhesive (20mm wide and 90-150mm long) tightly to the surface of the electrode. Subsequently, insert a paper tape with a width equal to that of the positive electrode and a length 80-200mm longer than the sample length under the green adhesive and fix it with wrinkle glue. Finally, turn on the tensile testing machine and adjust the limit block to the appropriate position.

[0098] Resistance test of positive electrode

[0099] After fully discharging the secondary battery, disassemble it to obtain the positive electrode plate. Soak the positive electrode plate in DMC for 2 hours, then air dry it in a fume hood. The resistance of the fully discharged positive electrode plate was then measured using a resistance meter; the resistance was R. x Ω.

[0100] Adhesive density test

[0101] After fully discharging the secondary battery, disassemble it to obtain the positive electrode. Wipe away any residual electrolyte on the surface of the positive electrode with lint-free paper. Then, immerse the positive electrode in dimethyl carbonate for 1 hour, remove it, and air dry it in a fume hood. Next, immerse the positive electrode in NMP and stir it at 1500 rpm for 4 hours using a single-bar stirrer to fully disperse the positive active layer on the positive current collector into the NMP, forming a uniform slurry. Then, use a vacuum filter with a filter paper pore size of 100 nm to remove solids from the slurry, leaving a clear NMP solution. Then, place the obtained solution in a 120°C oven to bake and remove the NMP liquid, leaving a binder film. Finally, pulverize the obtained film using a pulverizer to obtain binder powder, and then use a powder metallurgy density tester to test its density.

[0102] Thermal expansion coefficient test of adhesive

[0103] After disassembling the secondary battery, an adhesive film was obtained. The initial volume V1 of the film was then measured at 25°C using the water displacement method. The film was then placed in a 15°C oven and baked for 30 minutes. The expansion volume V2 was then measured at 25°C, and the thermal expansion rate β of the film was obtained as β = V2 / V1-1.

[0104] Low-temperature fast charging capacity retention test

[0105] At 0℃, the battery cell is charged at a constant current rate of 2C to 4.2V, then at a constant voltage rate of 0.8C, followed by constant current charging to 4.5V and constant voltage charging to 0.05C, and then placed in a sleep state for 10 minutes. The cell is then discharged at a rate of 1C to 3.75V, followed by discharge at a rate of 0.5C to 3V, and then placed in a sleep state for 10 minutes. This charge-discharge cycle is repeated 200 times.

[0106] Example 1-1

[0107] <Preparation of Electrolyte>

[0108] In a dry argon atmosphere glove box, EC, DEC, and EMC were mixed in a volume ratio of 3:5:2 to form a mixture. Then, lithium hexafluorophosphate (LiPF6), fluoroethylene carbonate (FEC), and 1,3-propanesulfonate lactone (PS) were added to the mixture to form an electrolyte. Based on the total volume of the electrolyte, the concentration of LiPF6 was 1 mol / L. Based on the total mass of the electrolyte, the mass contents of FEC and PS were 5% and 3%, respectively, with the remainder being the mixture.

[0109] <Preparation of the positive electrode>

[0110] Lithium cobalt oxide (CCO) as the positive electrode active material, single-walled carbon nanotube bundles (SHU) as the conductive agent, and a first polymer as the binder were mixed. N-methylpyrrolidone (NMP) was added, and the mixture was stirred evenly under vacuum to obtain a positive electrode slurry with a solid content of 70 wt%. The positive electrode slurry was uniformly coated onto the upper and lower surfaces of a 9 μm thick aluminum foil for the positive electrode current collector. After drying and pressure treatment, the foil was cut into the specified size to obtain the positive electrode sheet.

[0111] Based on the total mass of the positive electrode sheet, the mass of the conductive agent is a%, and the mass content of the binder is b%. The values ​​of a and b are detailed in Table 1. Additionally, in the positive electrode sheet binder, the number-average molecular weight (Mn) of the first polymer is... n The types of R1 and R2 are shown in Table 1. Furthermore, each bundle of single-walled carbon nanotubes consists of 300 to 40,000 first carbon nanotubes aggregated into a bundle.

[0112] <Preparation of Negative Electrode Sheets>

[0113] Artificial graphite, styrene-butadiene rubber (SBR), polyacrylic acid (PAA), carbon nanotubes (CNTs): carboxymethyl cellulose (CMC) were mixed in a mass ratio of 95.8:2.4:0.5:0.5:0.8. Deionized water was then added as a solvent and the mixture was 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 used as a negative electrode 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 ceramic-coated diaphragm with a thickness of 5μm is used.

[0116] <Preparation of Secondary Batteries>

[0117] The positive and negative electrode sheets prepared as described above are connected to the tabs. They are then laminated using a separator to obtain a laminate. The laminate, along with the electrolyte, is then housed in an aluminum laminated casing. The opening of the casing is heat-sealed, and the secondary battery is manufactured through formation and degassing processes.

[0118] Examples 1-2 to Examples 1-12

[0119] Compared to Examples 1-1, the main difference is that the R1 and R2 groups and the number-average molecular weight M of Compound I are adjusted according to Table 1. n .

[0120] Examples 1-13 to Examples 1-18

[0121] Compared to Examples 1-4, the main difference is that the parameters of the first carbon nanotube in the positive electrode sheet are adjusted according to Table 1.

[0122] Examples 1-19 to Examples 1-31

[0123] Compared to Examples 1-4, the main difference is that the parameters of the binder and the positive electrode active material in the positive electrode sheet are adjusted according to Table 1.

[0124] Comparative Example 1

[0125] Compared to Examples 1-1, the main difference is that in the <Preparation of Positive Electrode>, the same mass of PVDF is used instead of Compound I.

[0126] Comparative Example 2

[0127] Compared to Examples 1-4, the main difference is that in the <Preparation of Positive Electrode>, the same mass of conductive carbon black is used instead of the first carbon nanotube (i.e., instead of the single-walled carbon nanotube bundle).

[0128] Comparative Examples 3-4

[0129] Compared to Examples 1-4, the main difference is that the number-average molecular weight M of compound I was adjusted according to Table 1. n .

[0130] Comparative Examples 5 to 7

[0131] Compared to Examples 1-4, the main difference is that the types of R1 or R2 in Compound I are adjusted according to Table 1.

[0132] Comparative Example 8

[0133] Compared to Examples 1-4, the main difference is that in the <Preparation of the Positive Electrode>, the same mass of Compound II is used instead of Compound I; the general structural formula of Compound II is shown below:

[0134] To control for a single variable, the number-average molecular weight, R1, and R2 of the compound of formula II are the same as those of the compounds of formula I in Examples 1-4; and since the molecular weight of -NH- is close to that of -O-, x is close to m and y is close to n.

[0135] Table 1

[0136] Examples 2-1 to 2-10

[0137] Compared to Example 1-1, the other parameters were adjusted according to Table 2.

[0138] Table 2

[0139] Referring to Table 2, as seen from Examples 1-1, 2-1 to 2-10, the positive electrode sheet in this application exhibits low DC resistance due to the simultaneous presence of the first polymer and the first carbon nanotubes, resulting in good capacity and cycle performance of the secondary battery. Furthermore, the difference in DC resistance between the secondary battery at full charge and 50% SOC is relatively small. According to Examples 1-1, 2-1 to 2-2, and 2-5 to 2-6, when the thermal expansion rate of the binder in the positive electrode sheet meets the range of this application, the low-temperature fast-charge cycle capacity retention rate of the secondary battery can be effectively improved. According to Examples 1-1, 2-3 to 2-4, when the conductivity of the binder meets the range of this application, the migration rate of lithium ions on the positive electrode material can be effectively improved, thereby improving fast-charge cycle performance under low-temperature conditions. According to Examples 1-1, 2-9 to 2-10, when the specific surface area of ​​the positive electrode active material meets the range of this application, the secondary battery exhibits better low-temperature fast-charge cycle performance.

[0140] 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. A positive electrode sheet, characterized by, It comprises first carbon nanotubes and a binder, the binder comprising a first polymer of the following formula I: wherein R1 is a C5 to C12 linear alkyl group, R2 is any one of a C1 to C3 alkyl group, a hydrogen atom, or an aryl group; the first polymer has a number average molecular weight of 6 x 10 4 to 2 x 10 6 .

2. The cathode electrode of claim 1, wherein, It satisfies at least one of the following conditions: (1) R1 is a C5-C8 linear alkyl group; (2) the first polymer has a number average molecular weight of 8 x 10 4 ~ 9 x 10 5 .

3. The cathode electrode of claim 1, wherein The first carbon nanotube has a tube diameter of 0.1-0.9 nm.

4. The positive electrode sheet according to any one of claims 1 to 3, characterized by The positive electrode tab comprises single-walled carbon nanotube bundles, each of the single-walled carbon nanotube bundles comprising 300-40,000 first carbon nanotubes.

5. The cathode electrode of claim 4, wherein, Each of the single-walled carbon nanotube bundles has a length of L μm and a tube diameter of D μm, 0.01≤D≤400, and 1.3≤L / D≤61.

6. The cathode sheet of claim 1, wherein, Based on the mass of the positive electrode tab, the mass content of the binder is a%, and the mass content of the first carbon nanotubes is b%, 0.5≤a≤3, and 0.5≤b≤2.

7. The cathode electrode of claim 6, wherein, The resistance of the positive electrode tab after full discharge is R x Ω, the cohesion of the positive electrode tab after full discharge is F N / m, 20≤R x (1+a%)×F≤40, 0 x ≤0.

5.

8. The cathode sheet of claim 1, wherein, After full discharge of the positive electrode tab, the porosity ratio of the positive electrode tab within 5-15 nm is n%, and 20≤n≤27.

9. The cathode sheet of claim 1 or 8, wherein, The positive electrode plate comprises a positive electrode active material, and after full discharge of the positive electrode plate, the specific surface area of the positive electrode active material is s g / cm 3 , 0.24≤s≤0.

26.

10. The cathode sheet of claim 1, wherein, The density of the binder is ρ g / cm 3 0.3 < ρ < 0.

6.

11. The cathode electrode of claim 1, wherein, The binder has a thermal expansion rate of β, 1≤β≤1.

2.

12. The cathode sheet of claim 1, wherein, The electrical conductivity of the binder is 3.7 x 10 -10 S / cm ~ 3.7 x 10 -3 S / cm.

13. A secondary battery characterized by comprising: The secondary battery comprises the positive electrode tab of any one of claims 1-12.

14. The secondary battery according to claim 13, characterized by The secondary battery has a direct current resistance of X1 mΩ at 50% remaining capacity and a direct current resistance of X2 mΩ at 100% remaining capacity, 0.7≤X1 / X2≤0.9, and 12≤X2≤35.

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