Secondary battery and manufacturing method therefor, and electric device

Abstract

The present application provides a secondary battery and a manufacturing method therefor, and an electric device. The secondary battery comprises a positive electrode sheet. The positive electrode sheet comprises a current collector and a positive electrode film layer located on the surface of at least one side of the current collector. The positive electrode film layer comprises a conductive aid, a dispersant, and a binder; the conductive aid comprises carbon nanotubes; the dispersant comprises a first polymer, wherein the first polymer comprises a first structural unit and a second structural unit, the first structural unit is as shown in formula I, the second structural unit is derived from butadiene or vinylidene fluoride, or the second structural unit is absent; and the binder comprises a second polymer, wherein the second polymer comprises a structural unit derived from vinylidene fluoride and a structural unit as shown in formula II. The positive electrode film layer has excellent binding performance and conductivity, thereby improving the cycle performance of the secondary battery.

Description

Secondary batteries, their preparation methods and electrical devices

[0001] Cross-references

[0002] This application references Chinese Patent Application No. 2024114416726, filed on October 15, 2024, entitled "Secondary Battery and Method for Preparation Thereof and Electrical Device Thereof", which is incorporated herein by reference in its entirety. Technical Field

[0003] This application relates to the field of secondary battery technology, and in particular to a secondary battery, its preparation method, and an electrical device thereof. Background Technology

[0004] In recent years, lithium secondary batteries have been increasingly widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power plants, as well as in many fields such as power tools, electric bicycles, electric motorcycles, electric cars, military equipment, and aerospace. The market has also put forward higher requirements for the cycle life of secondary batteries, making the development of secondary batteries with high energy density an urgent matter.

[0005] Among the positive electrode active materials for lithium-ion batteries, lithium-rich manganese-based materials have a theoretical specific capacity greater than 250 mAh / g and a charging voltage above 4.5V, making them candidate materials for high-energy-density batteries. However, lithium-rich manganese-based materials have poor electronic and ion conduction properties, and under high-voltage conditions, the side reactions between lithium-rich manganese-based materials and the electrolyte are intensified, resulting in poor cycle performance of secondary batteries based on lithium-rich manganese-based materials.

[0006] Therefore, improving the cycle performance of secondary batteries based on lithium-rich manganese materials is an urgent problem to be solved. Summary of the Invention

[0007] Based on the above situation, this application optimizes the conductive network of the positive electrode film by rationally compounding general binders, dispersants and conductive additives in the positive electrode sheet of lithium-rich manganese-based material system, improves the bonding strength of the positive electrode film, and improves the cycle performance of the secondary battery.

[0008] A first aspect of this application provides a secondary battery, including a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive electrode film layer located on at least one side surface of the positive current collector, the positive electrode film layer including a conductive additive, a dispersant and a binder, the conductive additive including carbon nanotubes.

[0009] The dispersant comprises a first polymer, which includes a first structural unit and a second structural unit, the first structural unit being shown in Formula I.

[0010] In Formula I, R1, R2, and R3 each independently include one of hydrogen or a C1-C3 alkyl group, and R4 includes one of an ester group, a carboxyl group, or a nitrile group.

[0011] The second structural unit is derived from butadiene or vinylidene fluoride, or the second structural unit is not present;

[0012] The adhesive comprises a second polymer, the second polymer comprising structural units derived from vinylidene fluoride and structural units shown in Formula II.

[0013] In Formula II, M1, M2, and M3 each independently include one of hydrogen and C1-C3 alkyl groups, and M4 includes at least one of ester group, carbonyl group, C1-C3 alkyl group, and single bond.

[0014] The dispersant uniformly disperses the conductive additives within the positive electrode film, constructing a robust conductive network that significantly enhances the conductivity of the positive electrode film and reduces the internal resistance of the cell. The binder improves the bonding strength of the positive electrode film, inhibits volume expansion, reduces defects such as cracking, detachment, or internal microcracks, and decreases the contact area and side reaction degree between the positive electrode active material and the electrolyte. The synergistic effect of the conductive additives, dispersant, and binder improves the cycle performance of the secondary battery.

[0015] In any embodiment, the first polymer has a weight-average molecular weight of 2W-150W, exhibiting good dispersibility and facilitating the dispersion of conductive additives.

[0016] In any embodiment, the weight-average molecular weight of the second polymer is 100W-500W; or, based on the total number of moles of structural units in the second polymer, the molar content of the structural unit shown in Formula II is 0.1%-2%.

[0017] The structural unit shown in Formula II contains carboxyl groups. The hydrogen bonds formed between molecules enable the binder molecules to adhere more tightly to other materials in the electrode, effectively improving the suspension performance and viscosity of the positive electrode slurry, and enhancing the electrode coating quality and the adhesion strength of the positive electrode film. Simultaneously, the structural unit shown in Formula II provides additional steric hindrance, reducing the approach of alkaline substances in the positive electrode slurry to the structural unit shown in Formula II, reducing binder degradation, and giving the second polymer good adhesion properties, thus contributing to improved adhesion strength of the positive electrode film.

[0018] In any embodiment, in the first polymer, R1, R2, and R3 in the first structural unit are each independently selected from hydrogen, and R4 is selected from a nitrile group; the second structural unit is derived from butadiene; or,

[0019] The first polymer comprises a first structural unit and a hydrogenated second structural unit; or,

[0020] The weight-average molecular weight of the first polymer is 10W-30W; or,

[0021] Based on the total mass of the first and second structural units in the first polymer, the mass content of the first structural unit is 30%-45%.

[0022] In any embodiment, in the first polymer, R1, R2, and R3 in the first structural unit are each independently selected from hydrogen, and R4 is selected from a carboxyl group; the second structural unit is derived from vinylidene fluoride; or,

[0023] The weight-average molecular weight of the first polymer is 50W-150W; or,

[0024] Based on the total mass of the first and second structural units in the first polymer, the mass content of the first structural unit is 0.2%-3%.

[0025] In any embodiment, in the first polymer, R1 and R3 in the first structural unit are each independently selected from hydrogen, R2 is selected from methyl, and R4 is selected from -C(O)O-(C1-C3 alkyl); the second structural unit is absent; or,

[0026] The weight-average molecular weight of the first polymer is 2W-6W.

[0027] In any embodiment, the first polymer includes at least one of hydrogenated acrylonitrile-butadiene copolymer, vinylidene fluoride-acrylic acid copolymer, and polymethyl methacrylate.

[0028] The first polymer can encapsulate and wet the conductive additive, allowing the conductive additive to be fully dispersed, thereby improving the conductive network of the positive electrode film and improving the cycle performance of the lithium secondary battery.

[0029] In any embodiment, in the second polymer, M1, M2, and M3 are each independently selected from hydrogen, and M4 is selected from -C(O)O-(C1-C3 alkyl)-. The ester group helps to improve the polarity of the second polymer, and the alkyl chain helps to improve the flexibility of the binder molecule. While improving the bonding strength of the positive electrode film, it also has good elongation at break, so that the positive electrode sheet has good flexibility.

[0030] In any embodiment, the monomer represented by Formula II includes at least one of acrylic acid, methacrylic acid, β-acryloyloxypropionic acid, and maleic acid. The second polymer containing the monomer represented by Formula II exhibits excellent adhesion properties and stability, which helps to improve the adhesion strength of the positive electrode film.

[0031] In any embodiment, the carbon nanotubes have a Buerger specific surface area of ​​200 m². 2 / g-1500m 2 / g can significantly increase the charge transport interface, giving the carbon nanotubes higher conductivity.

[0032] In any embodiment, the Raman spectrum of the carbon nanotubes shows a peak at 1560 cm⁻¹. -1 -1600cm -1 The maximum peak intensity within the range is set to G, 1310 cm⁻¹. -1 -1350cm -1 The maximum peak intensity within the range is set as D, and the ratio of G / D is 2-100, which is beneficial to balance the conductivity and fabrication feasibility of carbon nanotubes.

[0033] In any embodiment, the carbon nanotubes have a diameter of 1nm-7nm, which helps to disperse the carbon nanotubes in the positive electrode slurry, reduces agglomeration, and allows them to be uniformly distributed or attached to the surface of the positive electrode active material, increasing the charge transport interface and forming a uniform and good conductive network, thereby further improving the conductivity of the positive electrode film.

[0034] In any embodiment, the carbon nanotubes include at least one of single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes.

[0035] Carbon nanotubes have a two-dimensional layered structure similar to graphite. Carbon nanotubes with different numbers of wall layers can provide different numbers of electron conduction channels and have high electron mobility, giving carbon nanotubes excellent conductivity and thus improving the conductivity of the positive electrode film.

[0036] In any embodiment, based on the total mass of the positive electrode film, the mass content of the conductive additive is 0.1%-1%, which can improve the conductive network in the positive electrode film, promote the transport of electrons between active material particles, and reduce the internal resistance of the cell.

[0037] In any embodiment, based on the total mass of the positive electrode film, the mass content of the conductive additive is 0.2%-0.6%, which can further improve the conductive network in the positive electrode film and enhance the conductivity of the positive electrode film.

[0038] In any embodiment, the dispersant has a mass content of 0.1%-1% in the positive electrode film layer, which can effectively disperse the conductive additive, allowing the conductive additive to be fully and uniformly dispersed in the positive electrode film layer, forming a well-distributed and uniformly conductive network, and improving the conductivity of the positive electrode film layer.

[0039] In any embodiment, the mass content of the dispersant in the positive electrode film layer is 0.2% - 0.6%, which can further improve the dispersion of the conductive additive in the positive electrode film layer.

[0040] In any embodiment, the mass content of the binder in the positive electrode film layer is 0.1% - 2%, which can improve the bonding strength and cohesion of the positive electrode film layer, help to inhibit the volume expansion of the electrode plate during charge and discharge, reduce the generation of defects such as film layer cracking, peeling or internal microcracks, and improve the cycle performance while enhancing the battery safety.

[0041] In any embodiment, the mass content of the binder in the positive electrode film layer is 0.5% - 2%, which can further improve the bonding performance of the positive electrode film layer and the cycle performance of the secondary battery.

[0042] In any embodiment, the positive electrode film layer includes a positive electrode active material, and the positive electrode active material includes a lithium manganese-based composite oxide. Alternatively, the lithium manganese-based composite oxide includes a general formula of xLi2MnO3·(1 - x)LiNi y Co z Mn a M 1-y-z-a O2, where 0 < x < 1, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1, 0 ≤ a ≤ 1, and 0 < y + z + a ≤ 1; M is at least one of Mg, B, Al, V, Ti, Zr, Sn, and Mo.

[0043] The positive electrode active material has a relatively high specific capacity, which helps to improve the capacitance of the secondary battery and the cycle performance.

[0044] In any embodiment, the bonding strength of the positive electrode film layer is 10 N / m - 35 N / m, and the positive electrode film layer has excellent bonding performance, which can effectively inhibit the expansion and contraction of the film layer during the cycle.

[0045] In any embodiment, the elongation at break of the positive electrode film layer is 25% - 500%, which can effectively inhibit the expansion and contraction of the film layer during the cycle, reduce film layer fracture, and improve the service life and safety of the secondary battery.

[0046] In any embodiment, the film layer resistance of the positive electrode film layer is 0.05 Ω - 1 Ω, which can reduce the DC internal resistance and internal energy loss of the secondary battery and improve the cycle capacity retention rate of the secondary battery.

[0047] In any embodiment, the lithium-rich manganese-based material comprises single-crystal particles with a particle size distribution Dv90 of 4 μm-15 μm, a particle size distribution Dv50 of 2 μm-10 μm, and a particle size distribution Dv10 of 1 μm-8 μm. These single-crystal particles help improve the energy density of the secondary battery and the mechanical properties of the positive electrode active material, reduce particle breakage and interfacial side reactions, thereby improving the cycle performance of the secondary battery.

[0048] A second aspect of this application provides a method for preparing a secondary battery, the secondary battery comprising a positive electrode sheet, the positive electrode sheet being prepared by comprising the following steps:

[0049] The conductive additive and dispersant are dispersed in a solvent to obtain a conductive additive dispersion;

[0050] The conductive additive dispersion is mixed evenly with the positive electrode active material, binder, conductive agent and solvent to obtain a positive electrode slurry, which is then coated, dried, cold pressed and slit to obtain a positive electrode sheet;

[0051] The conductive additive includes carbon nanotubes.

[0052] The dispersant comprises a first polymer, which includes a first structural unit and a second structural unit, the first structural unit being shown in Formula I.

[0053] In Formula I, R1, R2, and R3 each independently include one of hydrogen or a C1-C3 alkyl group, and R4 includes one of an ester group, a carboxyl group, or a nitrile group.

[0054] The second structural unit is derived from butadiene or vinylidene fluoride, or the second structural unit does not exist;

[0055] The adhesive comprises a second polymer, the second polymer comprising structural units derived from vinylidene fluoride and structural units shown in Formula II.

[0056] In Formula II, M1, M2, and M3 each independently include one of hydrogen and C1-C3 alkyl groups, and M4 includes at least one of ester group, carbonyl group, C1-C3 alkyl group, and single bond.

[0057] The preparation method is simple and mature, has a wide range of applications, and has relatively mild requirements for existing production equipment.

[0058] The third aspect of this application provides an electrical device, including a secondary battery provided in the first aspect or a secondary battery prepared by the method provided in the second method. Attached Figure Description

[0059] Figure 1 is a schematic diagram of a secondary battery according to an embodiment of this application.

[0060] Figure 2 is an exploded view of a secondary battery according to an embodiment of this application shown in Figure 1.

[0061] Figure 3 is a schematic diagram of a battery module according to one embodiment of this application.

[0062] Figure 4 is a schematic diagram of a battery pack according to one embodiment of this application.

[0063] Figure 5 is an exploded view of the battery pack of one embodiment of this application shown in Figure 4.

[0064] Figure 6 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of this application.

[0065] Explanation of reference numerals in the attached diagram: 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Housing; 52 Electrode assembly; 53 Cover plate. Detailed Implementation

[0066] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the positive electrode active material, its manufacturing method, positive electrode sheet, secondary battery, battery module, battery pack, and electrical device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0067] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60–120 and 80–110 are listed for a specific parameter, it is understood that ranges of 60–110 and 80–120 are also expected. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1–3, 1–4, 1–5, 2–3, 2–4, and 2–5. In this application, unless otherwise stated, the numerical range "a–b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0~5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0068] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0069] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0070] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0071] Lithium-rich manganese-based materials possess the properties of multiple redox reactions involving transition metals and oxygen ions, resulting in high specific capacity and energy output, making them candidate materials for high-energy-density batteries. However, during the sintering process, residual lithium compounds from the lithium source react with moisture or other gases in the air to form alkaline substances such as lithium hydroxide or lithium carbonate, which remain on the material surface. In the cathode slurry homogenization process, these residual alkaline substances react with the commonly used binder, polyvinylidene fluoride (PVDF), leading to PVDF dehydrogenation, defluorination, and cross-linking denaturation. This results in poor suspension and weak adhesion of the prepared cathode slurry, causing uneven distribution of active materials and binders in the cathode film. During battery operation, this can easily lead to film cracking, detachment, or internal microcracks, failing to contain the expansion stress of the electrode during charge and discharge, especially the cyclic expansion stress under high voltage conditions above 4.5V. This increases the interface between the active material and the electrolyte in the cathode film, intensifies the side reactions, negatively impacts the cycle performance of the secondary battery, and severely affects its lifespan and safety.

[0072] [Lithium-ion rechargeable battery]

[0073] The first aspect of this application discloses a lithium secondary battery, specifically comprising a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive and negative electrodes. During the charging and discharging process, active ions (e.g., lithium ions) repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, disposed between the positive and negative electrodes, primarily serves to prevent short circuits between the positive and negative electrodes while allowing ions to pass through.

[0074] In some embodiments, the positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0075] In some embodiments, the positive electrode film layer includes a positive electrode active material, a conductive additive, a dispersant, and a binder.

[0076] In some embodiments, the conductive agent comprises carbon nanotubes, and the dispersant comprises a first polymer, the first polymer comprising a first structural unit and a second structural unit, the first structural unit being as shown in Formula I.

[0077] In Formula I, R1, R2, and R3 each independently include one of hydrogen or a C1-C3 alkyl group, and R4 includes one of an ester group, a carboxyl group, or a nitrile group.

[0078] The second structural unit is derived from butadiene or vinylidene fluoride, or the second structural unit is not present;

[0079] The adhesive comprises a second polymer, the second polymer comprising structural units derived from vinylidene fluoride and structural units shown in Formula II.

[0080] In Formula II, M1, M2, and M3 each independently include one of hydrogen and C1-C3 alkyl groups, and M4 includes at least one of ester group, carbonyl group, C1-C3 alkyl group, and single bond.

[0081] In this article, the "conductive additive" refers to a material with conductive electron capability that is added to the positive electrode slurry of a secondary battery in addition to the conductive agent.

[0082] In this article, "dispersant" refers to a material that can fully disperse conductive additives in a solvent.

[0083] In this article, "binder" refers to a polymer used to bond positive electrode solid materials together.

[0084] In this article, "polymer" refers to a high molecular weight compound or its derivative obtained through polymerization reaction and formed by repeated chemical bonds. The derivative is usually chemically modified by the functional groups of the high molecular weight compound.

[0085] In this document, "alkyl" refers to a straight-chain or branched saturated hydrocarbon group. C1-C3 alkyl refers to a straight-chain or branched saturated hydrocarbon group having 1 to 3 carbon atoms, such as methyl, ethyl, and n-propyl.

[0086] In this document, "ester group" refers to a group containing -C(O)O-, such as -C(O)O-CH3 or -C(O)O-CH2CH3.

[0087] In this article, "carboxyl group" refers to a group containing -COOH, such as -CH2-COOH or -COOH.

[0088] In this document, "nitrile group" refers to a group containing -CN, which, for example, can be -CN or -CH2-CN.

[0089] In this document, "carbonyl" refers to a group containing -C(O)-, such as -C(O)-CH3 or -C(O)-CH2CH3.

[0090] The dispersant possesses excellent dispersing properties, enabling uniform dispersion of carbon nanotubes in the cathode slurry and reducing agglomeration. Furthermore, the dispersant exhibits high stability, tolerating residual alkaline substances on the surface of the cathode active material and high voltage plateaus, and is not easily decomposed or degraded. This helps maintain good uniformity of carbon nanotube distribution during cathode slurry preparation and under secondary battery operating conditions. Carbon nanotubes also possess excellent conductivity; uniformly distributed carbon nanotubes can form a good conductive network in the cathode film, thereby improving the electron transport performance of the cathode film, reducing electron transport resistance, and improving the cycle performance of the secondary battery.

[0091] Unbound by any theory, the sp_atoms of carbon atoms in carbon nanotubes 2 Hybrid orbitals form a planar hexagonal grid, similar to the two-dimensional layered structure of graphite. Each carbon atom contributes an unhybridized p electron, forming a delocalized π-electron system that provides a channel for electron conduction. In addition, carbon nanotubes have a low defect density, fewer scattering centers encountered by electrons during transmission, and high electron mobility, which gives carbon nanotubes excellent electrical conductivity.

[0092] The binder can withstand the damage to the molecular structure caused by residual alkaline substances on the surface of the positive electrode active material. This alkali resistance of the binder helps maintain its bonding performance, ensuring that the positive electrode slurry remains in a good suspension state during homogenization. This not only improves the uniformity of solid material distribution in the positive electrode slurry but also facilitates uniform coating of the positive electrode slurry and the quality of electrode coating. Consequently, it improves the bonding strength and cohesion of the positive electrode film, inhibits the volume expansion of the electrode during charging and discharging, reduces defects such as film cracking, peeling, or internal microcracks, and reduces side reactions between the electrolyte and the positive electrode active material. This enhances battery safety while improving cycle performance.

[0093] By using conductive additives, dispersants, and binders in combination, the volume expansion of the positive electrode film during cycling can be suppressed, reducing the rupture of active materials and the occurrence of side reactions. At the same time, the conductive network in the positive electrode film is improved, enhancing electron transport performance and comprehensively improving the cycle performance and service life of the secondary battery.

[0094] The mixing method and order of addition of conductive additives and dispersants with other solid materials of the positive electrode can be reasonably adjusted according to production conditions. For example, conductive additives and dispersants can be mixed together with the positive electrode active material and conductive agent; or the conductive additives and dispersants can be mixed evenly and then added to the positive electrode active material and conductive agent for mixing.

[0095] In some embodiments, carbon nanotubes can be dispersed by including the following steps: adding a predetermined amount of dispersant, all the carbon nanotubes, and a predetermined amount of solvent to a mixing tank and kneading them; then adding the remaining amount of solvent and dispersant, stirring until homogeneous, and dispersing to obtain a carbon nanotube dispersion. The dispersion treatment can be any of the methods in the prior art, such as stirring, ultrasonication, or homogenization. In some embodiments, the dispersion treatment can be homogenization under a set pressure.

[0096] In any embodiment, the weight-average molecular weight of the first polymer is 20,000 to 1,500,000 (2W-1,500,000, hereinafter, "ten thousand" is abbreviated as "W"). The dispersant has a suitable molecular chain length and appropriate steric hindrance, which can effectively reduce the aggregation and sedimentation between conductive additive particles, improve the stability of the positive electrode slurry, and ensure that the conductive additive is fully dispersed in the positive electrode slurry.

[0097] In this paper, the term "weight-average molecular weight" refers to the sum of the products of the weight fraction of molecules of different molecular weights in a polymer and their corresponding molecular weights.

[0098] In this application, the weight-average molecular weight of the polymer can be determined using methods known in the art, such as a Waters 2695 Isocratic HPLC gel chromatograph (differential refractive index detector 2141).

[0099] In some embodiments, the weight-average molecular weight of the first polymer is 2W, 5W, 10W, 20W, 40W, 50W, 60W, 80W, 100W, 120W, 130W, 150W, or any combination of the above values ​​or any value within any range.

[0100] In some embodiments, in the first structural unit of the first polymer, R1, R2, and R3 are each independently selected from hydrogen, and R4 is selected from a nitrile group; and the second structural unit is derived from butadiene. In some embodiments, in the first structural unit of the first polymer, R1, R2, and R3 are each independently selected from hydrogen, and R4 is selected from -CN; and the second structural unit is derived from butadiene.

[0101] The nitrile groups contained in the first polymer can increase the polarity of the first polymer, increase the surface activity and intermolecular interaction forces of the polymer, which helps the dispersant to be adsorbed on the surface of the conductive additive particles and dispersed in the solvent, thereby improving the degree of dispersion of the conductive additive.

[0102] In some embodiments, the first polymer includes a first structural unit and a hydrogenated second structural unit. In some embodiments, the first polymer includes a first structural unit and a hydrogenated butadiene structural unit.

[0103] After hydrogenation, the number of easily oxidized groups in the first polymer is reduced, and the polymer's antioxidant and alkali resistance are enhanced, making it more stable under high voltage conditions and less prone to chemical decomposition or degradation.

[0104] In some embodiments, the mass content of the first structural unit is 30%-45% based on the total mass of the first and second structural units in the first polymer. In some embodiments, the mass content of the first structural unit is 35%-45% based on the total mass of the first and second structural units in the first polymer. In some embodiments, the mass content of the first structural unit is 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, or any range of the above values, or any value within any range, based on the total mass of the first and second structural units in the first polymer.

[0105] The first structural unit can increase the polarity of the polymer. When the mass content of the first structural unit is within a suitable range, it can take into account both the solubility and molecular polarity of the copolymer, so that the first polymer has suitable solubility and surface activity, which can well encapsulate and wet the conductive additives, and make them fully dispersed in the positive electrode slurry.

[0106] In some embodiments, the first polymer is a hydrogenated acrylonitrile-butadiene copolymer, and the weight-average molecular weight of the first polymer is 10W-30W. In some embodiments, the weight-average molecular weight of the first polymer is 20W-30W, 20W-28W, or 22W-26W. In some embodiments, the weight-average molecular weight of the first polymer is 10W, 15W, 20W, 22W, 24W, 26W, 28W, 30W, or any combination of the above values, or any value within any range.

[0107] A suitable weight-average molecular weight is beneficial for reducing the viscosity of the first polymer, hydrogenated acrylonitrile-butadiene copolymer, and improving the dispersion performance of the hydrogenated acrylonitrile-butadiene copolymer.

[0108] In some embodiments, in the first polymer, R1, R2, and R3 in the first structural unit are each independently selected from hydrogen, and R4 is selected from carboxyl groups; and the second structural unit is derived from vinylidene fluoride. In some embodiments, in the first polymer, R1, R2, and R3 in the first structural unit are each independently selected from hydrogen, and R4 is selected from -COOH; and the second structural unit is derived from vinylidene fluoride.

[0109] In the first polymer, the first structural unit contains a polar carboxylic acid group, which increases the surface activity of the polymer, facilitates the adsorption of the dispersant on the surface of conductive additive particles, and improves dispersion performance. Simultaneously, the carboxylic acid group provides electrostatic repulsion, helping to reduce the aggregation of conductive additive particles and improve the stability of the positive electrode slurry. Furthermore, the first structure can increase the interaction between polymer molecular chains, helping to maintain the structural integrity of the dispersant under high voltage conditions. The strong electronegativity of the fluorine atom in the second structural unit causes the carbon atoms on the polymer chain to carry a partial positive charge. This structure helps to suppress the insertion of lithium ions and solvent molecules in the electrolyte, reduces side reactions with the electrolyte under high voltage conditions, and improves the electrochemical stability of the first polymer in an alkaline environment.

[0110] In some embodiments, the first polymer is a vinylidene fluoride-acrylic acid copolymer, and the weight-average molecular weight of the first polymer is 50W-150W, for example, it can be 80W-120W or 100W-120W. In some embodiments, the weight-average molecular weight of the vinylidene fluoride-acrylic acid copolymer is 50W, 60W, 70W, 80W, 85W, 90W, 95W, 100W, 105W, 110W, 120W, 130W, 140W, 150W, or any range of the above values ​​or any value within any range.

[0111] A suitable weight-average molecular weight is beneficial for reducing the viscosity of the first polymer, vinylidene fluoride-acrylic acid copolymer, and improving its dispersion performance.

[0112] In some embodiments, the first polymer is a vinylidene fluoride-acrylic acid copolymer, and the mass content of the first structural unit is 0.2%-3% based on the total mass of the first and second structural units in the first polymer. In some embodiments, the mass content of the first structural unit is 0.5%-3% based on the total mass of the first and second structural units in the first polymer. In some embodiments, the mass content of the first structural unit is 0.2%, 0.4%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, or any value within any range of the above values, based on the total mass of the first and second structural units in the first polymer.

[0113] In the first polymer, the mass content of the first structural unit in the vinylidene fluoride-acrylic acid copolymer is within a suitable range, which can take into account the polymer's solubility, molecular polarity, and stability, maintain good surface activity and alkali resistance, and improve dispersion performance.

[0114] In some embodiments, in the first polymer, R1 and R3 in the first structural unit are each independently selected from hydrogen, R2 is selected from methyl, and R4 is selected from -C(O)O-(C1-C3 alkyl); and the second structural unit is absent. In some embodiments, in the first polymer, R1 and R3 in the first structural unit are each independently selected from hydrogen, R2 is selected from methyl, and R4 is selected from -C(O)OCH3; and the second structural unit is absent.

[0115] The ester groups contained in the first polymer can increase the polarity of the first polymer, increase the surface activity and intermolecular interaction forces of the polymer, which helps the dispersant to be adsorbed on the surface of the conductive additive particles and dispersed in the solvent, thereby improving the degree of dispersion of the conductive additive.

[0116] In some embodiments, the first polymer is polymethyl methacrylate, and the weight-average molecular weight of the first polymer is 2W-6W. In some embodiments, the weight-average molecular weight of the first polymer is 3W-5W. In some embodiments, the weight-average molecular weight of the first polymer is 2W, 2.5W, 3W, 3.5W, 4W, 4.5W, 5W, 5.5W, 6W, or any combination of the above values, or any value within any range, which can give the polyacrylate compound good dispersibility.

[0117] In some embodiments, the first polymer comprises at least one selected from hydrogenated acrylonitrile-butadiene copolymer, vinylidene fluoride-acrylic acid copolymer, and polymethyl methacrylate. In some embodiments, the first polymer comprises hydrogenated acrylonitrile-butadiene copolymer. In some embodiments, the first polymer comprises vinylidene fluoride-acrylic acid copolymer. In some embodiments, the first polymer comprises polymethyl methacrylate.

[0118] The first polymer can encapsulate and wet the conductive additive, allowing the conductive additive to be fully dispersed, thereby improving the conductive network of the positive electrode film and improving the cycle performance of the lithium secondary battery.

[0119] In some embodiments, the dispersant has a mass content of 0.1%-1% in the positive electrode film layer, which can fully disperse the conductive additive, allowing the conductive additive to be better distributed or attached to the surface of the positive electrode active material, so that it is uniformly dispersed in the positive electrode film layer and forms a good conductive network, thereby improving the conductivity of the positive electrode film layer, reducing the resistance to electron transport, reducing polarization and DC resistance of the cell, and improving the cycle capacity retention rate of the battery.

[0120] In some embodiments, based on the total mass of the positive electrode film, the mass content of the dispersant in the positive electrode film is 0.2%-0.6%, which can further disperse the conductive additives and improve the conductivity of the positive electrode film.

[0121] In some embodiments, based on the total mass of the positive electrode film, the mass content of the dispersant in the positive electrode film is 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%, 0.95%, 1%, or any combination of the above values ​​or any value within any range.

[0122] When the amount of dispersant is within a suitable range, it can effectively reduce the agglomeration of conductive additives, so that the conductive additives are evenly distributed in the positive electrode film layer, increase the contact sites between the conductive additives and the positive electrode active material, build a more effective conductive bridge between the current collector and the positive electrode active material, reduce the resistance of electron transmission, reduce polarization and reduce the DC resistance of the cell.

[0123] In some embodiments, in the second polymer, M1, M2, and M3 are each independently selected from hydrogen, and M4 is selected from -C(O)O-(C1-C3 alkyl)-. In some embodiments, in the second polymer, M1, M2, and M3 are each independently selected from hydrogen, and M4 is selected from -C(O)OCH2-. In some embodiments, in the second polymer, M1, M2, and M3 are each independently selected from hydrogen, and M4 is selected from -C(O)OCH2CH2CH2-. In some embodiments, in the second polymer, M1, M2, and M3 are each independently selected from hydrogen, and M4 is selected from -C(O)OCH2CH2CH2-. The ester group helps to improve the polarity of the second polymer, and the alkyl chain helps to improve the flexibility of the binder molecules, thus improving the bonding strength of the positive electrode film while also having good elongation at break, giving the positive electrode sheet good flexibility.

[0124] In some embodiments, the monomer represented by Formula II includes at least one of acrylic acid, methacrylic acid, β-acryloyloxypropionic acid, and maleic acid. The second polymer containing the monomer represented by Formula II exhibits excellent adhesion properties and stability, contributing to improved adhesion strength of the positive electrode film.

[0125] In some embodiments, the monomer represented by Formula II includes β-acryloyloxypropionic acid, which is beneficial for balancing the bonding properties and molecular flexibility of the second polymer, and helps to improve the bonding strength and elongation at break of the positive electrode film, thereby improving the bending resistance of the electrode sheet.

[0126] Without being bound by any theoretical constraints, the carboxyl groups in the β-acryloyloxypropionic acid structural unit are beneficial for increasing the polarity of polymer molecules and forming hydrogen bonds between polymer molecules. This results in a tighter bond between the binder and the positive electrode active material in the electrode sheet, effectively improving the adhesion strength and cohesion of the positive electrode film. This suppresses volume expansion of the electrode sheet during charge and discharge, reduces film cracking and active material particle breakage, lowers the degree of side reactions between the active material and the electrolyte, and improves the battery's cycle performance. Furthermore, the steric hindrance of the β-acryloyloxypropionic acid structural unit may help reduce the approach of alkaline substances in the positive electrode slurry to the ester bonds, reducing binder degradation and thus giving the second polymer higher alkali resistance.

[0127] In some embodiments, the weight-average molecular weight of the second polymer is 100W-500W. In some embodiments, the weight-average molecular weight of the second polymer is 110W-500W, 110W-400W, 110W-300W, or 110W-200W. In some embodiments, the weight-average molecular weight of the second polymer is 100W, 110W, 150W, 200W, 250W, 300W, 350W, 400W, 450W, 500W, or any range of the above values, or any value within any range.

[0128] When the weight-average molecular weight of the binder is within a suitable range, it can effectively increase the viscosity of the cathode slurry, improve the coating quality of the electrode, especially improve the processing quality of the thin coating process, enhance the adhesion performance of the cathode film, suppress the volume expansion of the cathode film, reduce the breakage of active particles and side reactions, and improve the cycle performance and cycle stability of the battery; it can also reduce the amount of binder used, which helps to increase the energy density of the electrode.

[0129] In some embodiments, the molar content of the structural unit represented by Formula II is 0.1%-2% based on the total molar number of structural units in the second polymer. In some embodiments, the molar content of the structural unit represented by Formula II in the second polymer is 0.5%-1.5%, 0.5%-1%, 1%-1.5%, 1%-2.0%, or 1.5%-2.0% based on the total molar number of structural units in the second polymer. In some embodiments, the molar content of the structural unit represented by Formula II is 0.1%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, or any range of the above values, or any value within any range, based on the total molar number of structural units in the second polymer.

[0130] The structural unit shown in Formula II contains carboxyl groups. The hydrogen bonds formed between molecules enable the binder molecules to adhere more tightly to other materials in the electrode, effectively improving the suspension performance and viscosity of the positive electrode slurry, and enhancing the electrode coating quality and the adhesion strength of the positive electrode film. Simultaneously, the structural unit shown in Formula II provides additional steric hindrance, reducing the approach of alkaline substances in the positive electrode slurry to the structural unit shown in Formula II, reducing binder degradation, and giving the second polymer good adhesion properties, thus contributing to improved adhesion strength of the positive electrode film.

[0131] In some embodiments, the binder has a mass content of 0.1%-2% in the positive electrode film layer, which can improve the bonding strength and cohesion of the positive electrode film layer, which is beneficial to suppress the volume expansion of the electrode during the charging and discharging process, reduce the generation of defects such as film layer cracking, peeling or internal microcracks, and improve cycle performance while improving battery safety.

[0132] In some embodiments, the binder has a mass content of 0.5%-2% in the positive electrode film layer, which can further improve the adhesion performance of the positive electrode film layer and the cycle performance of the secondary battery.

[0133] In some embodiments, the mass content of the binder in the positive electrode film layer is 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, or any combination of the above values ​​or any value within any range.

[0134] When the mass content of binder in the positive electrode film is within a suitable range, it can improve the suspension performance of the positive electrode slurry and the bonding strength and cohesion of the positive electrode film, making the positive electrode film uniform and suppressing the expansion of the electrode during charging and discharging, reducing the DC resistance and cycle polarization of the cell, and improving the cycle capacity retention rate of the battery.

[0135] In some embodiments, the carbon nanotubes have a Boehrette (BET) specific surface area of ​​200 m². 2 / g-1500m 2 / g, for example, 400m 2 / g-150m 2 / g、450m 2 / g-1200m 2 / g、500m 2 / g-1000m 2 / g、600m 2 / g-1500m 2 / g. In some embodiments, the carbon nanotubes have a Buerter specific surface area of ​​200 m². 2 / g、300m 2 / g、400m 2 / g、450m2 / g、500m 2 / g、550m 2 / g、600m 2 / g、650m 2 / g、700m 2 / g、750m 2 / g、800m 2 / g、900m 2 / g, 1000m 2 / g、1100m 2 / g、1200m 2 / g、1300m 2 / g, 1400m 2 / g, 1500m 2 / g or any range of the above values, or any value within any range.

[0136] Excessive Buerter surface area of ​​carbon nanotubes can lead to aggregation, reducing their dispersibility in the cathode slurry. Conversely, insufficient Buerter surface area hinders the improvement of contact points and charge transport interfaces between carbon nanotubes, resulting in minimal improvement in the conductive network's transport path. A suitable Buerter surface area allows carbon nanotubes to maintain high conductivity while achieving good dispersibility, thus balancing the conductivity and fabrication feasibility of the cathode film.

[0137] The Buerter specific surface area of ​​carbon nanotubes can be determined using any known method in the prior art. For example, the Buerter specific surface area of ​​carbon nanotubes can be determined using a fully automated specific surface area measuring device (manufactured by MOUNTECH, HM model 1208).

[0138] In the Raman spectrum of carbon nanotubes, at 1560 cm⁻¹ -1 -1600cm -1 The maximum peak intensity within the range is set as G, which can characterize the integrity of carbon nanotubes; 1310 cm⁻¹ -1 -1350cm -1 The maximum peak intensity within the range is set as D, which can characterize the degree of defect in carbon nanotubes. G / D can be used to evaluate the structural quality and defect density of carbon nanotubes.

[0139] In some embodiments, the G / D ratio is 2-100. In some embodiments, the G / D ratio is 5-100, 10-100, 20-100, 30-100, 40-100, 50-100, 60-100, 70-100, 80-100, or 90-100. In some embodiments, the G / D ratio Ig / Id is 2, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, or any value within any range of the above values. When the G / D ratio is within a suitable range, carbon nanotubes possess good electron transport channels and electron mobility, resulting in excellent electrical conductivity.

[0140] In some embodiments, the diameter of the carbon nanotubes is 1 nm-7 nm. In some embodiments, the diameter of the carbon nanotubes is 1 nm-5 nm, 1 nm-4 nm, or 1 nm-3 nm. In some embodiments, the diameter of the carbon nanotubes is 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm, 5 nm, 5.5 nm, 6 nm, 6.5 nm, 7 nm, or any combination of the above values, or any value within any range.

[0141] A suitable diameter range helps reduce the aggregation of carbon nanotubes, which in turn helps them to be evenly distributed or attached to the surface of the positive electrode active material, improving the uniformity of carbon nanotube distribution and forming a uniform and good conductive network, thereby further improving the conductivity of the positive electrode film.

[0142] The diameter of carbon nanotubes can be determined using any known method in the prior art. For example, the microstructure of carbon nanotubes can be directly observed and its diameter measured by observing and photographing them using a transmission electron microscope (e.g., a JEM 1400 transmission electron microscope from NEC Corporation).

[0143] In some embodiments, the carbon nanotubes include at least one of single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes. In some embodiments, the carbon nanotubes include single-walled carbon nanotubes. In some embodiments, the carbon nanotubes include double-walled carbon nanotubes. In some embodiments, the carbon nanotubes include multi-walled carbon nanotubes.

[0144] In this article, the term "single-walled carbon nanotube" refers to a carbon nanotube with a single wall layer.

[0145] In this article, the term "double-walled carbon nanotube" refers to a carbon nanotube with two wall layers.

[0146] In this paper, the term "multi-walled carbon nanotube" refers to carbon nanotubes with three or more wall layers.

[0147] Carbon nanotubes with different wall layers can provide varying numbers of electron conduction channels and possess high electron mobility, giving them excellent conductivity. Adding a small amount of carbon nanotubes can form a good three-dimensional conductive network between the positive electrode active materials, improving the electronic conductivity and electrical conductivity of the positive electrode film, and thus enhancing the battery's cycle performance. This, in turn, improves the conductive electronic properties of the positive electrode film.

[0148] In some embodiments, based on the total mass of the positive electrode film, the mass content of the conductive additive is 0.1%-1%, which can improve the conductive network in the positive electrode film, promote the transport of electrons between active material particles, and reduce the internal resistance of the cell.

[0149] In some embodiments, the mass content of the conductive additive is 0.2%-0.6% based on the total mass of the positive electrode film. In some embodiments, the mass content of the conductive additive in the positive electrode film is 0.3%-0.6%, 0.2%-0.5%, or 0.2%-0.4%. In some embodiments, the mass content of the conductive additive in the positive electrode film is 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, or any combination of the above values, or any value within any range.

[0150] Too low a carbon nanotube content prevents the formation of a continuous conductive network, affecting conductivity; too high a content exacerbates nanotube aggregation, hindering dispersion. Appropriate amounts of carbon nanotubes can improve the continuity of the conductive network, reduce electron transport resistance and DC resistance of the cell, and improve cycle performance. Appropriate amounts of conductive additives can increase the transport interface between positive electrode active material particles, promoting electron transport between these particles, improving electron transport performance, and thus reducing cell internal resistance and improving cycle performance.

[0151] In some embodiments, based on the total mass of the positive electrode film, the positive electrode film includes 0.1%-1% by mass of a conductive additive, 0.1%-1% by mass of a dispersant, and 0.1%-2% by mass of a binder.

[0152] In some embodiments, based on the total mass of the positive electrode film, the positive electrode film includes 0.1%-0.6% by mass of a conductive additive, 0.1%-0.6% by mass of a dispersant, and 0.5%-1.5% by mass of a binder.

[0153] In some embodiments, the positive electrode active material includes a lithium-rich manganese-based material. In some embodiments, the positive electrode active material includes a lithium manganese composite oxide.

[0154] In some embodiments, the lithium manganese composite oxide includes a general formula of xLi2MnO3·(1-x)LiNi y Co z Mn a M 1-y-z-a O2, where 0 < x < 1, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1, 0 ≤ a ≤ 1, 0 < y + z + a ≤ 1; M is at least one of Mg, B, Al, V, Ti, Zr, Sn, and Mo.

[0155] In some embodiments, 0 < x < 1, 0 ≤ y ≤ 1, 0 ≤ z ≤ 0.8, 0 ≤ a ≤ 1, 0 < y + z + a ≤ 1.

[0156] In some embodiments, the positive electrode active material includes, but is not limited to: 0.35Li2MnO3·0.65LiNi 0.46 Co 0.08 Mn 0.46 O2, 0.35Li2MnO3·0.65LiNi 0.45 Co 0.08 Mn 0.46 Al 0.01 O2, 0.35Li2MnO3·0.65LiNi 0.45 Co 0.08 Mn 0.46 Al 0.01 O2, 0.35Li2MnO3·0.65LiNi 0.45 Co 0.08 Mn 0.46 Ti 0.01 O2, 0.35Li2MnO3·0.65LiNi 0.45 Co 0.08 Mn 0.46 V 0.01 O2.

[0157] The positive electrode active material has a high specific capacity, which helps to improve the capacitance of the secondary battery and improve the cycling performance. However, the electron-conducting and ion-conducting properties of the positive electrode active material are low, resulting in a high DC internal resistance of the secondary battery, which is not conducive to the cycling performance of the battery. The conductive additive added to the positive electrode film layer in this application can improve the electron-conducting ability of the film layer. The binder can inhibit the shrinkage and expansion of the electrode during cycling, inhibit the cracking and side reaction degree of the positive electrode active material particles, and can significantly improve the cycling performance of the lithium-rich manganese-based positive electrode active material.

[0158] In some embodiments, the positive electrode active material includes single-crystal particles, also known as primary particles, which typically refer to individual, fine particles with good mechanical properties. During cycling, these particles help reduce grain boundary breakage and cracking in the active material, thereby reducing interfacial side reactions and the consumption of active ions. Single-crystal particles also help improve the energy density of the secondary battery, thus jointly improving the cycle performance of the secondary battery.

[0159] Whether a positive electrode active material contains or is a single crystal particle can be analyzed, tested, or determined using methods known in the art. For example, transmission electron microscopy diffraction can be used for testing.

[0160] In some embodiments, the particle size distribution Dv90 of the single crystal particles is 4μm-15μm. In some embodiments, the particle size distribution Dv90 of the single crystal particles is 4μm-13.5μm. In some embodiments, the particle size distribution Dv90 of the single crystal particles is 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, or any range of the above values, or any value within any range.

[0161] In some embodiments, the particle size distribution Dv50 of the single crystal particles is 2μm-10μm. In some embodiments, the particle size distribution Dv50 of the single crystal particles is 2μm-8μm. In some embodiments, the particle size distribution Dv50 of the single crystal particles is 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, or any combination of the above values, or any value within any range.

[0162] In some embodiments, the particle size distribution Dv10 of the single crystal particles is 1μm-8μm. In some embodiments, the particle size distribution Dv10 of the single crystal particles is 1μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm, 8μm, or any combination of the above values, or any value within any range.

[0163] Positive electrode active materials with a suitable particle size distribution help increase the number of active sites and the migration path of active ions, improve the migration rate of active ions and reduce migration resistance, reduce cell internal resistance and improve cycle performance.

[0164] The particle size distribution of the positive electrode active material can be tested using methods known in the art; laser scattering may be used as a suitable method.

[0165] In some embodiments, the bonding strength of the positive electrode film is 10 N / m to 35 N / m. In some embodiments, the bonding strength of the positive electrode film is 10 N / m, 12 N / m, 14 N / m, 16 N / m, 18 N / m, 20 N / m, 22 N / m, 24 N / m, 26 N / m, 28 N / m, 30 N / m, 32 N / m, 35 N / m, or any value within any range of the above values. This can reduce desorption, breakage, or slight cracking of the positive electrode film during battery operation, maintain the structural integrity of the electrode during charge and discharge, reduce electrode structural damage caused by the expansion or contraction of active materials, and improve the cycle capacity retention rate of the battery.

[0166] The bonding strength of the positive electrode film can be determined using methods known in the art, for example, by referring to the national standard "Test method for 180° peel strength of adhesives".

[0167] In some embodiments, the elongation at break of the positive electrode film is 25%-500%. In some embodiments, the elongation at break of the positive electrode film is 25%, 50%, 70%, 100%, 130%, 150%, 200%, 250%, 300%, 350%, 400%, 450%, 480%, 500%, or any combination of the above values, or any value within any range. This can withstand the volume changes of the positive electrode film during cycling, reduce the risk of film cracking, and improve the service life and safety of the secondary battery.

[0168] In some embodiments, the film resistance of the positive electrode film is 0.05Ω-1Ω. In some embodiments, the film resistance of the positive electrode film is 0.05Ω, 0.08Ω, 0.1Ω, 0.2Ω, 0.3Ω, 0.4Ω, 0.5Ω, 0.6Ω, 0.7Ω, 0.8Ω, 0.9Ω, 1Ω, or any value within any range of the above values, thereby reducing the DC internal resistance and internal energy loss of the secondary battery and improving the battery cycle capacity retention rate.

[0169] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0170] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0171] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as lithium-rich manganese-based positive electrode material, carbon nanotube conductive additive, binder, dispersant and any other components, in a solvent (e.g. N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0172] In some embodiments, the positive electrode sheet can be prepared by dispersing a conductive additive and a dispersant in a solvent to obtain a conductive additive dispersion; mixing the conductive additive dispersion with a positive electrode active material, a binder, a conductive agent and a solvent to obtain a positive electrode slurry, which is then coated, dried, cold-pressed and slit to obtain the positive electrode sheet.

[0173] The conductive additive includes carbon nanotubes.

[0174] The dispersant comprises a first polymer, which includes a first structural unit and a second structural unit, the first structural unit being shown in Formula I.

[0175] In Formula I, R1, R2, and R3 each independently include one of hydrogen or a C1-C3 alkyl group, and R4 includes one of an ester group, a carboxyl group, or a nitrile group.

[0176] The second structural unit is derived from butadiene or vinylidene fluoride, or the second structural unit does not exist;

[0177] The adhesive comprises a second polymer, the second polymer comprising structural units derived from vinylidene fluoride and structural units shown in Formula II.

[0178] In Formula II, M1, M2, and M3 each independently include one of hydrogen and C1-C3 alkyl groups, and M4 includes at least one of ester group, carbonyl group, C1-C3 alkyl group, and single bond.

[0179] The preparation method is simple and mature, has a wide range of applications, and has relatively mild requirements for existing production equipment.

[0180] [Negative electrode plate]

[0181] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including the aforementioned carbon-based negative electrode active material.

[0182] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0183] In some embodiments, the negative electrode film layer may further include other negative electrode active materials besides the carbon-based negative electrode active materials described above. In some embodiments, the negative electrode active materials include, but are not limited to, one or more of conventional natural graphite, other artificial graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based materials may include one or more of elemental silicon, silicon oxide, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based materials may include one or more of elemental tin, tin oxide, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as battery negative electrode active materials may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0184] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0185] In some embodiments, the negative electrode film layer may optionally include an adhesive. The adhesive may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0186] In some embodiments, the negative electrode film may optionally include a conductive additive. The conductive additive may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0187] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0188] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, conductive additive, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto a negative electrode current collector, and after drying, cold pressing and other processes, a negative electrode sheet including a negative electrode film layer can be obtained.

[0189] [Isolation membrane]

[0190] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0191] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0192] [Electrolytes]

[0193] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific limitations on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or entirely solid.

[0194] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0195] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0196] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0197] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0198] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0199] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0200] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0201] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. In some embodiments, the secondary battery is square. For example, Figure 1 shows a square-structured secondary battery 5 as an example.

[0202] In some embodiments, referring to FIG2, the outer packaging may include a housing 51 and a cover plate 53. The housing 51 may include a bottom plate and side plates connected to the bottom plate, the bottom plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be placed over the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator can be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0203] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0204] Figure 3 shows a battery module 4 as an example. Referring to Figure 3, in the battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, the multiple secondary batteries 5 can be fixed in place using fasteners.

[0205] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0206] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0207] Figures 4 and 5 show a battery pack 1 as an example. Referring to Figures 4 and 5, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper box 2 and a lower box 3, with the upper box 2 covering the lower box 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0208] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0209] As the electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0210] Figure 6 shows an example of an electrical device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0211] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0212] Example

[0213] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0214] I. Preparation or source of auxiliary agents

[0215] 1) Preparation of conductive additive dispersion

[0216] One part by weight of dispersant solution with a mass content of 8% and one part by weight of conductive additive powder were added to a mixing tank. The rotation speed was set to 25 rpm and the revolution speed to 800 rpm, and the mixing time was 30 minutes for kneading. 86.5 parts by weight of N-methylpyrrolidone (NMP) were added, followed by 11.5 parts by weight of dispersant solution under stirring conditions. The stirring speed was set to 1000 rpm, and the stirring time was 60 minutes, with cooling water circulation until the mixture was homogeneous. The mixture was then dispersed using a high-pressure homogenizer (Antos Nanotechnology (Suzhou) Co., Ltd.; model: AH22-100) with a single nozzle chamber (nozzle diameter 0.25 mm, pressure 100 MPa) to obtain a conductive additive dispersion, wherein the mass content of the conductive additive and the dispersant was 1%.

[0217] The parameters of the conductive additive powder in the dispersion are shown in Table 1 below:

[0218] Table 1

[0219] The parameters of the dispersant in the dispersion are shown in Table 2 below:

[0220] Table 2

[0221] The degree of hydrogenation of dispersants 1-4 is above 98%. Among them, dispersant 1 was purchased from Zeon Japan, model 720H. Dispersants 1-4 can also be prepared by referring to [1] Guo Xue. Research and application of high-junction acrylonitrile butadiene rubber synthesis technology [D]. Qingdao University of Science and Technology, 2023.

[0222] Dispersant 5-8 can be prepared using the following method:

[0223] 3.2 kg of deionized water and 100 g of hydroxypropyl methylcellulose were added to a 10 L autoclave. The autoclave was evacuated and O2 was replaced three times with N2. Then, 50 g of diisopropyl peroxide dicarbonate was added, followed by the gradual addition of 1.5 kg of vinylidene fluoride monomer to reach a pressure of 5.0 MPa. The mixture was stirred for 30 min at a stirring speed of 550 rpm and heated to 40 °C. Simultaneously, an aqueous solution containing acrylic acid was added in multiple portions. The amount of acrylic acid was calculated based on the mass percentage of acrylic acid in Table 2 (mass percentage of acrylic acid structural units = mass of acrylic acid monomer / (mass of acrylic acid monomer + mass of vinylidene fluoride monomer)). The reaction was allowed to proceed for 5.5 h, maintaining the reaction pressure between 3.7 and 5.2 MPa. After the reaction was stopped, the reaction system was centrifuged, the solid phase was collected, washed, and dried to obtain a polyvinylidene fluoride copolymer with a weight-average molecular weight of 110 W.

[0224] 2) Preparation of adhesive

[0225] The parameters of the vinylidene fluoride-β-acryloyloxypropionic acid copolymer are shown in Table 3 below:

[0226] Table 3

[0227] The preparation method of adhesive 1 is as follows:

[0228] First stage polymerization reaction: 4 kg of deionized water and 2 g of methyl cellulose ether were added to 10 L autoclaves No. 1 and No. 2. The system was evacuated and O2 was replaced with N2 three times. 5 g of tert-butyl peroxypentanoate and 2 g of sodium bicarbonate were added again, and 1 kg of vinylidene fluoride monomer was added to bring the pressure to 5 MPa. The mixture was stirred for 30 min, heated to 38 °C, and an aqueous solution containing 22.5 g of β-acryloyloxypropionic acid was added in multiple portions. The reaction was carried out for 4.0 h.

[0229] Second stage polymerization reaction: Transfer the reaction liquid in reactors 1 and 2 to reactor 3, purge with nitrogen to a pressure of 7 MPa, heat to 70°C, and stir for 3 hours.

[0230] The third polymerization reaction: After adding 42g of cyclohexane, the reaction continued for 1 hour, then the reaction was stopped. The reaction system was centrifuged, the solid phase was collected, washed, and dried to obtain the polyvinylidene fluoride copolymer.

[0231] Preparation of adhesives 2-4:

[0232] The preparation methods of binders 2-4 are similar to those of binder 1, but with adjustments: in the first stage of polymerization, the temperature is raised to 48℃, 43℃, and 45℃ respectively, and the reaction time is 1.5h, 3.0h, and 2.0h respectively; in the second stage of polymerization, the reaction time is 2h, 2.5h, and 3.0h respectively; and in the third stage of polymerization, 55g, 46g, and 55g of cyclohexane are added respectively.

[0233] The preparation methods of binders 5-7 are the same as those of binder 2, but the molar content of β-acryloyloxypropionic acid is adjusted.

[0234] The parameters for other adhesives are shown in Table 4 below:

[0235] Table 4

[0236] II. Preparation of Secondary Batteries

[0237] Example 1

[0238] 1) Preparation of positive electrode sheet

[0239] The positive electrode active material is 0.35Li2MnO3·0.65LiNi 0.46 Co0.08 Mn 0.46 O2 (single crystal particles, Dv90 4.0μm, Dv50 2.7μm, Dv10 1.9μm), conductive carbon black, conductive additive carbon nanotubes, dispersant hydrogenated acrylonitrile-butadiene copolymer, and binder were mixed in a mass ratio of 97.8:1:0.1:0.1:1, wherein the conductive additive carbon nanotubes and the dispersant hydrogenated acrylonitrile-butadiene copolymer were added in the form of conductive additive dispersion. N-methylpyrrolidone solvent was then added, and the mixture was stirred until homogeneous to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated onto a positive electrode current collector aluminum foil, and after drying, cold pressing, and slitting, positive electrode sheets were obtained. The single-sided coating amount of the positive electrode film was 15 mg / cm². 2 .

[0240] 2) Preparation of negative electrode sheet

[0241] Artificial graphite (active material), carbon black (conductive additive), styrene-butadiene rubber (SBR) (binder), and sodium carboxymethyl cellulose (CMC-Na) (thickener) were dissolved in deionized water at a weight ratio of 96.2:0.8:0.8:1.2 and mixed thoroughly to prepare a negative electrode slurry. The negative electrode slurry was then uniformly coated onto a copper foil current collector (negative electrode) one or more times. After drying, cold pressing, and slitting, the negative electrode sheet was obtained. The single-sided coating weight of the negative electrode film was 10 mg / cm². 2 .

[0242] 3) Preparation of electrolyte

[0243] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), the organic solvents ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed evenly at a volume ratio of 3 / 7. 12.5% ​​LiPF6 lithium salt is added and dissolved in the organic solvent and stirred evenly to obtain the electrolyte.

[0244] 4) Separating membrane

[0245] A polyethylene film with a thickness of 13 μm was used as the separator.

[0246] 5) Preparation of secondary batteries

[0247] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. The electrode assembly is then wound up to obtain the electrode assembly. The electrode assembly is placed in an outer packaging, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium-ion battery is obtained.

[0248] Examples 2-4

[0249] The preparation methods of Examples 2-4 are similar to those of Example 1, except that the mass content of conductive additives and dispersants in the positive electrode slurry is adjusted, and the amount of positive electrode active material is adjusted accordingly. Specific parameters are shown in Table 5.

[0250] Examples 5-20 and Comparative Examples 1-4

[0251] Examples 5-20 and Comparative Examples 1-4 were prepared in a similar manner to Example 1, and the specific parameters are shown in Table 5.

[0252] Table 5: Parameters for the preparation of secondary batteries in Examples 1-20 and Comparative Examples 1-4

[0253] The positive electrode slurry prepared in Comparative Example 2 had almost no binding force and could not be used for the preparation of secondary batteries. Therefore, the performance of the secondary batteries prepared in Examples 1-20 and Comparative Examples 1 and 3-4 was tested using the following test methods:

[0254] 1) Electrode film adhesion strength test

[0255] Referring to the national standard "Test Method for 180° Peel Strength of Adhesives", cut a sample with a width of 30mm and a length of 100-160mm using a blade. Apply special double-sided adhesive tape, 20mm wide and 90-150mm long, to the steel plate. Place the previously cut electrode sample onto the double-sided tape, test side down, and then roll it three times in the same direction using a pressure roller. Insert a paper strip with a width equal to the electrode and a length 80mm-200mm longer than the sample length under the electrode and secure it with wrinkle adhesive. Turn on the tensile testing machine (think carefully, sensitivity is 1N). When the indicator light illuminates, adjust the limit block to the appropriate position and secure the end of the steel plate without the electrode attached using the lower clamp. Fold the paper strip upwards and secure it with the upper clamp. Use the "up" and "down" buttons on the manual controller provided with the tensile testing machine to adjust the position of the upper clamp. Then perform the test and read the values.

[0256] 2) Elongation at break test of electrode film layer

[0257] Cut a 20×100mm test specimen from the positive electrode sheet and set aside. Turn on the tensile testing machine (think carefully, sensitivity 1N). Once the indicator light illuminates, adjust the limit block to the appropriate position and secure both ends of the electrode sheet with clamps. Use the "up" and "down" buttons on the manual controller provided with the tensile testing machine to adjust the position of the upper clamp until the electrode sheet completely breaks. Then perform the test and read the values. The formula for calculating the elongation at break is: (Lm-L0) / L0, where L0 is the original length of the sample and Lm is the length of the sample after breakage.

[0258] 3) Electrode film resistance test

[0259] Cut 3mm diameter circular pieces from the left, center, and right sides of the electrode. Turn on the indicator light of the electrode resistance meter (Yuaneng Technology, model BER2500), place the disc at the appropriate position on the probe of the film resistance meter, and click the start button. Wait for the reading to stabilize before taking the reading. Test two positions for each small disc, and finally calculate the average of the six measurements, which is the film resistance of the electrode.

[0260] 4) Number of bends that can be achieved under extreme compression of the electrode sheet

[0261] Cut the positive electrode sheet into 20mm × 100mm test specimens for later use. Bend and fold the electrode sheet in half and fix it in place. Roll it once with a 2kg roller and check if light is transmitted through the folded part of the electrode sheet and metal is exposed. If no light is transmitted through the folded part and metal is exposed, turn the electrode sheet over and fold it in half again and fix it in place. Roll it once with a 2kg roller and check if light is transmitted through the folded part of the electrode sheet and metal is exposed. Repeat the above steps until light is transmitted through the folded part of the electrode sheet and metal is exposed.

[0262] 5) Electrode expansion force growth rate after 500 cycles

[0263] Before the cell cycle test, pressure sensors are installed in the center of both sides of the steel shell to monitor the change of the external pressure of the cell as the cycle progresses. The initial pressure is recorded as P0, and the cycle pressures are recorded as P1, P2, ..., Pn. The growth rate of the cycle expansion force corresponding to the nth cycle is (Pn-P0) / Pn×100%.

[0264] 6) The rate of increase of the DC resistance of the battery cell after 500 cycles

[0265] The battery DC impedance test procedure is as follows: At 25℃, the test battery is charged at a constant current of 1 / 3C to 3.65V, then charged at a constant voltage of 3.65V to a current of 0.05C. After resting for 5 minutes, the voltage V1 is recorded. Then, it is discharged at 1 / 3C for 30 seconds, and the voltage V2 is recorded. The internal resistance DCR1 of the battery after the first cycle is obtained by (V2-V1) / 1 / 3C. The above steps are repeated for the same battery, and the internal resistance DCRn of the battery after the nth cycle is recorded (n=1, 2, 3……500). In this test process, the first cycle corresponds to n=1, the second cycle corresponds to n=2, and so on, up to the 500th cycle, which corresponds to n=500. The DCR increase rate = (DCRn-DCR1) / DCR1×100%.

[0266] 7) Capacity retention rate of the battery after 500 cycles at 25℃

[0267] Under a constant temperature environment of 25℃, the secondary battery was charged at a constant current of 0.5C to 4.45V, and then charged at a constant voltage of 4.45V to a current of 0.05mA. After resting for 5 minutes, the secondary battery was discharged at a constant current of 0.5C to 2.5V, and the discharge capacity at this point was recorded, which is the discharge capacity of the first cycle. The secondary battery was subjected to a cyclic charge-discharge test using the above method, and the discharge capacity after each cycle was recorded. The capacity retention rate (%) of the secondary battery after 500 cycles at 25℃ = discharge capacity after 500 cycles / discharge capacity of the first cycle × 100%.

[0268] The test results of the secondary batteries prepared in each embodiment and comparative example are shown in Table 6 below:

[0269] Table 6: Performance Test Results

[0270] Comparing Examples 1-20 with Comparative Example 1, it is evident that the bonding strength of the positive electrode sheet is significantly improved, the elongation at break is greatly reduced, and the number of bends and capacity retention are improved. This indicates that the alkali resistance and bonding performance of the vinylidene fluoride-β-acryloyloxypropionic acid copolymer are superior to those of the vinylidene fluoride-acrylic acid copolymer. It can be understood that, compared to the vinylidene fluoride-acrylic acid copolymer, the β-acryloyloxypropionic acid structural unit of the vinylidene fluoride-β-acryloyloxypropionic acid copolymer contains two chemically reactive groups, allowing it to withstand more alkaline substances and improving the stability of the binder. Simultaneously, the enhanced bonding performance further inhibits the shrinkage and expansion of the electrode sheet during cycling, reducing electrode expansion and thus inhibiting and reducing the deformation and cracking of the positive electrode active material particles during cycling, reducing DC internal resistance, and improving the cycle performance of the secondary battery. Furthermore, the increased side chain length and flexible segments of the vinylidene fluoride-β-acryloyloxypropionic acid copolymer contribute to improved electrode toughness, elongation at break, and number of bends.

[0271] As can be seen from the comparison between Examples 1-20 and Comparative Example 3, single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes can better construct a conductive network in the positive electrode film layer, significantly reduce the film layer resistance and DC resistance growth rate, thereby improving the uniformity of current distribution in the electrode film layer, reducing electrode expansion, and improving the cycle capacity retention rate of the battery.

[0272] A comparison of Examples 1-20 with Comparative Example 4 shows that hydrogenated acrylonitrile-butadiene copolymer and vinylidene fluoride-acrylic acid copolymer have better alkali resistance than polyvinylpyrrolidone, and can fully disperse carbon nanotubes during the preparation of the positive electrode slurry. This improves the uniformity of current distribution in the electrode film, makes the deformation stress of the positive electrode active material particles more uniform during cycling, reduces the cracking of the positive electrode active material particles, reduces electrode expansion, and improves the cycle capacity retention rate of the battery.

[0273] Examples 1-4 show that a conductive additive content of 0.1%-0.6% in the positive electrode slurry can improve the film resistance, volume expansion, and cycle performance of the electrode. A conductive additive content of 0.3%-0.6% can further improve the film resistance, volume expansion, and cycle performance of the electrode.

[0274] Examples 3 and 5-7 show that a binder with a weight-average molecular weight of 110W-500W can improve the adhesion performance of the electrode film, thereby improving the electrode volume expansion rate. A binder with a weight-average molecular weight of 250W-500W can further improve the adhesion performance and elongation at break of the electrode film, and suppress electrode volume expansion.

[0275] Examples 3 and 10-12 show that a 30%-45% mass content of acrylonitrile structural units in the copolymer can improve the dispersion effect of hydrogenated acrylonitrile-butadiene copolymer on carbon nanotubes and reduce film resistance. A 35%-45% mass content of acrylonitrile structural units in the copolymer can further reduce film resistance.

[0276] Examples 13-17 show that vinylidene fluoride-acrylic acid copolymer and polymethyl methacrylate can withstand alkaline substances in the positive electrode slurry and play a good dispersing role for carbon nanotubes.

[0277] Examples 3 and 18-20 show that a molar content of β-acryloyloxypropionic acid structural units in the copolymer of 0.3%-2.0% can give the electrode good bonding strength, and a molar content of β-acryloyloxypropionic acid structural units in the copolymer of 1%-2.0% can significantly improve the bonding strength of the film and suppress the volume expansion of the electrode.

[0278] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A secondary battery characterized by comprising: The device includes a positive electrode sheet, which comprises a positive current collector and a positive electrode film layer located on at least one side surface of the positive current collector. The positive electrode film layer comprises a conductive additive, a dispersant, and a binder. The conductive additive includes carbon nanotubes. The dispersant includes a first polymer, the first polymer including a first structural unit and a second structural unit, the first structural unit being represented by Formula I, In Formula I, R1, R2, and R3 each independently include one of hydrogen or a C1-C3 alkyl group, and R4 includes one of an ester group, a carboxyl group, or a nitrile group. The second structural unit is derived from butadiene or vinylidene fluoride, or the second structural unit is not present; The binder comprises a second polymer comprising structural units derived from vinylidene fluoride and structural units of the formula II, In Formula II, M1, M2, and M3 each independently include one of hydrogen and C1-C3 alkyl groups, and M4 includes at least one of ester group, carbonyl group, C1-C3 alkyl group, and single bond.

2. The secondary battery according to claim 1, characterized in that, The weight-average molecular weight of the first polymer is 2W-150W; or, The weight-average molecular weight of the second polymer is 100W-500W; or, Based on the total number of moles of structural units in the second polymer, the molar content of the structural units shown in Formula II is 0.1%-2%.

3. The secondary battery according to claim 1 or 2, characterized in that, In the first polymer, R1, R2, and R3 in the first structural unit are each independently selected from hydrogen, and R4 is selected from a nitrile group; the second structural unit is derived from butadiene; or, The first polymer comprises a first structural unit and a hydrogenated second structural unit; or, The weight-average molecular weight of the first polymer is 10W-30W; or, Based on the total mass of the first and second structural units in the first polymer, the mass content of the first structural unit is 30%-45%.

4. The secondary battery according to claim 1 or 2, characterized in that, In the first polymer, R1, R2, and R3 in the first structural unit are each independently selected from hydrogen, and R4 is selected from a carboxyl group; the second structural unit is derived from vinylidene fluoride; or, The weight-average molecular weight of the first polymer is 50W-150W; or, Based on the total mass of the first and second structural units in the first polymer, the mass content of the first structural unit is 0.2%-3%.

5. The secondary battery according to claim 1 or 2, characterized in that, In the first polymer, R1 and R3 in the first structural unit are each independently selected from hydrogen, R2 is selected from methyl, and R4 is selected from -C(O)O-(C1-C3 alkyl); the second structural unit is absent; or, The weight-average molecular weight of the first polymer is 2W-6W.

6. The secondary battery according to any one of claims 1 to 5, characterized by The first polymer includes at least one of hydrogenated acrylonitrile-butadiene copolymer, vinylidene fluoride-acrylic acid copolymer, and polymethyl methacrylate.

7. The secondary battery according to any one of claims 1 to 6, characterized by In the second polymer, M1, M2, and M3 are each independently selected from hydrogen, and M4 is selected from -C(O)O-(C1-C3 alkyl)-.

8. The secondary battery according to any one of claims 1 to 7, characterized by, The monomer represented by Formula II includes at least one of acrylic acid, methacrylic acid, β-acryloyloxypropionic acid, and maleic acid.

9. The secondary battery according to any one of claims 1 to 8, characterized in that, The carbon nanotubes have a Brunauer-Emmett-Teller specific surface area of 200 m 2 / g-1500 m 2 / g; or, In the Raman spectrum of the carbon nanotubes, at 1560 cm⁻¹ -1 -1600cm -1 The maximum peak intensity within the range is set to G, 1310 cm⁻¹. -1 -1350cm -1 The maximum peak intensity within the range is set as D, and the ratio of G / D is 2-100; or, The diameter of the carbon nanotubes is 1nm-7nm.

10. The secondary battery according to any one of claims 1 to 9, characterized by The carbon nanotubes include at least one of single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes.

11. The secondary battery according to any one of claims 1 to 10, characterized by Based on the total mass of the positive electrode film, the mass content of the conductive additive is 0.1%-1%; and / or, The dispersant has a mass content of 0.1%-1%; and / or, The adhesive has a mass content of 0.1%-2%.

12. The secondary battery according to any one of claims 1 to 11, characterized in that, The conductive additive has a mass content of 0.2%-0.6% in the positive electrode film; and / or, The dispersant has a mass content of 0.2%-0.6% in the positive electrode film; and / or, The binder has a mass content of 0.5%-2% in the positive electrode film layer.

13. The secondary battery according to any one of claims 1 to 12, characterized by The positive electrode film layer includes a positive electrode active material, which includes a lithium manganese composite oxide, or... The lithium manganese composite oxide includes a general formula of xLi2MnO3·(1-x)LiNi y Co z Mn a M 1-y-z-a O2, where 0 < x < 1, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1, 0 ≤ a ≤ 1, 0 < y + z + a ≤ 1; M is at least one of Mg, B, Al, V, Ti, Zr, Sn, and Mo.

14. The secondary battery according to any one of claims 1 to 13, characterized in that, The bonding strength of the positive electrode film is 10 N / m-35 N / m; or... The elongation at break of the positive electrode film is 25%-500%; or, The film resistance of the positive electrode film is 0.05Ω-1Ω.

15. The secondary battery according to any one of claims 1 to 14, characterized by, The positive electrode active material comprises single-crystal particles, wherein the particle size distribution Dv90 of the single-crystal particles is 4μm-15μm; or, The particle size distribution Dv50 of the single crystal particles is 2μm-10μm; or, The particle size distribution Dv10 of the single crystal particles is 1μm-8μm.

16. A method for producing a secondary battery, characterized by, The secondary battery includes a positive electrode sheet, which is prepared by the following steps: The conductive additive and dispersant are dispersed in a solvent to obtain a conductive additive dispersion; The conductive additive dispersion is mixed evenly with the positive electrode active material, binder, conductive agent and solvent to obtain a positive electrode slurry, which is then coated, dried, cold pressed and slit to obtain a positive electrode sheet; The conductive additive includes carbon nanotubes. The dispersant includes a first polymer, the first polymer including a first structural unit and a second structural unit, the first structural unit being represented by Formula I, In Formula I, R1, R2, and R3 each independently include one of hydrogen or a C1-C3 alkyl group, and R4 includes one of an ester group, a carboxyl group, or a nitrile group. The second structural unit is derived from butadiene or vinylidene fluoride, or the second structural unit does not exist; The binder comprises a second polymer comprising structural units derived from vinylidene fluoride and structural units of the formula II, In Formula II, M1, M2, and M3 each independently include one of hydrogen and C1-C3 alkyl groups, and M4 includes at least one of ester group, carbonyl group, C1-C3 alkyl group, and single bond.

17. An electrical device, comprising: The secondary battery includes any one of claims 1 to 15 or the secondary battery prepared by the method described in claim 16.

Images