Positive pole piece, preparation method thereof and lithium ion battery

Abstract

The invention relates to the technical field of lithium ion batteries, and discloses a positive pole piece, which comprises a positive current collector and a positive coating coated on at least one side surface of the positive current collector, the thickness of the positive current collector is 5-15 [mu] m, and the thickness of the positive pole piece is 65-90 [mu] m; the positive electrode coating comprises a positive electrode active material which comprises nickel cobalt lithium manganate, and the mass percentage content of the positive electrode active material in the positive electrode coating is 93%-97%; the positive conductive agent comprises carbon nanotubes and carbon black in a mass ratio of (4: 1)-(1: 2); wherein the carbon nano tube is doped with a sulfur element, and the mass percentage of the positive conductive agent in the positive coating is 1%-2.5%; the positive electrode binder comprises polyvinylidene fluoride and polyimide in a mass ratio of (9: 1)-(1: 1); wherein the mass percentage content of the positive electrode binder in the positive electrode coating is 1.5%-4%. Therefore, the structural stability and conductivity of the lithium ion battery can be effectively balanced, and the electrochemical performance of the lithium ion battery is remarkably improved. The invention also discloses a preparation method of the positive pole piece and a lithium ion battery.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, specifically to a positive electrode sheet and its preparation method, and a lithium-ion battery. Background Technology

[0002] Currently, with the widespread application of lithium-ion batteries in new energy vehicles, energy storage systems, and portable electronic devices, the requirements for their energy density and rate performance are continuously increasing. Among them, lithium-ion batteries made of high-nickel ternary materials have advantages such as high energy density and fast charging efficiency.

[0003] However, these generally suffer from insufficient electronic conductivity and poor structural stability. For example, a related technology discloses a positive electrode sheet comprising a positive electrode material layer, which includes a positive electrode active material, single-walled carbon nanotubes, conductive carbon black, and a positive electrode binder. This allows for a significant reduction in the amount of conductive agent used while ensuring the positive electrode sheet has low resistance, thereby improving electronic conductivity.

[0004] In the process of implementing the embodiments of this disclosure, at least the following problems were found in the related art:

[0005] Although single-walled carbon nanotubes in related technologies have good electronic conductivity, their hydrophobicity limits their dispersibility and interfacial compatibility during the wetting process of aqueous slurries and electrolytes. As a result, it is difficult to adapt to the electrochemical stability of high-nickel systems. Especially in thick electrode, high rate and long cycle applications, problems such as electrode polarization, particle cracking and capacity decay are prone to occur, thus affecting the rate performance and cycle performance of lithium-ion batteries.

[0006] It should be noted that the information disclosed in the background section above is only used to enhance the understanding of the background of this application, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0007] To provide a basic understanding of some aspects of the disclosed embodiments, a brief summary is given below. This summary is not intended as a general commentary, nor is it intended to identify key / important components or describe the scope of protection of these embodiments, but rather as a prelude to the detailed description that follows.

[0008] This disclosure provides a positive electrode sheet and its preparation method, as well as a lithium-ion battery, which can form a composite network in which electrons and ions conduct synchronously, thereby effectively balancing the structural stability and conductivity of the lithium-ion battery, and thus improving the rate performance and cycle performance of the lithium-ion battery.

[0009] In some embodiments, the positive electrode sheet includes a positive current collector and a positive electrode coating coated on at least one surface of the positive current collector, wherein the thickness of the positive current collector is 5 μm to 15 μm, and the thickness of the positive electrode sheet is 65 μm to 90 μm; wherein the positive electrode coating includes:

[0010] Positive electrode active materials include lithium nickel cobalt manganese oxide, with the chemical formula LiNi. x Co y Mn z M b O2; wherein, 0.7≤x≤0.95, 0.15≤y<0.45, 0.05≤z<0.45, 0≤b≤0.25, and x+y+z+b=1, the element M includes one or more of Zr, W, Ti, Al, Sr, B and Nd, and the mass percentage of the positive electrode active material in the positive electrode coating is 93%~97%;

[0011] The positive electrode conductive agent comprises carbon nanotubes and carbon black in a mass ratio of 4:1 to 1:2; wherein the carbon nanotubes are doped with sulfur and have a specific surface area of ​​220 m². 2 / g~320m 2 / g, and the positive electrode conductive agent in the positive electrode coating has a mass percentage content of 1% to 2.5%;

[0012] The positive electrode binder includes polyvinylidene fluoride and polyimide; wherein the polyimide has a mass percentage content of 10% to 50% in the positive electrode binder, and the positive electrode binder has a mass percentage content of 1.5% to 4% in the positive electrode coating.

[0013] In some embodiments, the positive conductive agent satisfies one or more of the following conditions:

[0014] (1) The pore volume of the carbon nanotubes is 0.6 cm³. 3 / g~1.6cm 3 / g;

[0015] (2) The atomic percentage of sulfur in the carbon nanotubes is 0.3% to 0.7%;

[0016] (3) The water contact angle of the carbon nanotubes is ≤50°;

[0017] (4) The intensity ratio of the D peak to the G peak in the Raman spectrum of the carbon nanotube is ID / IG, which is 0.2 to 0.6.

[0018] In some embodiments, when the carbon nanotubes are fabricated into a predetermined circular positive electrode, their electronic conductivity σ ≥ 15 S·cm -1The area of ​​the pre-set circular positive electrode is 132.665 mm². 2 The thickness is 0.8mm.

[0019] In some embodiments, the carbon nanotubes exhibit characteristic absorption peaks in the X-ray photoelectron spectroscopy at 163.5 eV–164.5 eV, 283.5 eV–285.8 eV, and 528.5 eV–535.5 eV.

[0020] In some embodiments, the number-average molecular weight of polyvinylidene fluoride is 63.6 g / mol to 190.8 g / mol; and the number-average molecular weight of polyimide is 53 g / mol to 212 g / mol.

[0021] In some embodiments, a positive electrode slurry is coated on at least one side surface of the positive electrode current collector to form the positive electrode coating; wherein the viscosity of the positive electrode slurry is 5000 mPa·s to 8000 mPa·s, and the viscosity change rate V of the positive electrode slurry is <18% after standing for a preset time.

[0022] In some embodiments, the positive electrode sheet satisfies one or more of the following conditions:

[0023] (1) The interface resistance Rα is 1.0 × 10⁻⁶. -3 Ω·cm 2 ~1.0×10 -2 Ω·cm 2 ;

[0024] (2) Peel strength N is 8.5 N / m to 18.0 N / m;

[0025] (3) Compacted density D > 3.5 g / cm³ 3 .

[0026] In some embodiments, the method for preparing the positive electrode sheet includes the following steps:

[0027] MgSO4 is converted into sulfur element through thermal decomposition and / or chemical reaction. The sulfur element reacts with the surface of carbon nanotubes to obtain a matrix powder with sulfur atoms embedded in the tube wall structure of carbon nanotubes.

[0028] The residual oxides in the matrix powder are removed by hydrochloric acid and washed with deionized water until the pH value is neutral. After filtration and drying, the positive electrode conductive agent is obtained.

[0029] The positive electrode active material, positive electrode conductive agent and positive electrode binder are stirred in an N-methylpyrrolidone solvent system to form a positive electrode slurry;

[0030] The positive electrode slurry is coated on at least one side of the positive electrode current collector, and then dried and cold-pressed to obtain the positive electrode sheet.

[0031] In some embodiments, the lithium-ion battery includes a negative electrode and a positive electrode as described in the foregoing embodiments.

[0032] In some embodiments, the lithium-ion battery satisfies one or more of the following conditions:

[0033] (1) The charging characteristics of the lithium-ion battery are as follows: after a lithium-ion battery with a 100% charge rate is left to stand at 25°C for 6 hours, it is discharged to 2.5V at a discharge rate of 0.1C, and the corresponding discharge capacity is Q1; after discharging to 2.5V at a discharge rate of 10C, the corresponding discharge capacity is Q. 10 Discharge capacity retention rate Q R ≥60%, Q R =Q 10 / Q1;

[0034] (2) At 25°C, after 600 cycles at a 1C rate, the capacity decay rate Q of the lithium-ion battery is... d <18%.

[0035] The positive electrode sheet and its preparation method, as well as the lithium-ion battery provided in this disclosure, can achieve the following technical effects:

[0036] By optimizing the material ratio of the positive electrode conductive agent and the positive electrode binder, and by using sulfur-doped carbon nanotubes in synergy with polyimide, the high conductivity, high specific surface area, and good hydrophilicity of carbon nanotubes enable the formation of a dominant electron transport network. This not only optimizes the connectivity of electron and lithium-ion transport paths but also enhances the mechanical integrity and electrochemical stability of the electrode under thick electrode and high-compact designs, achieving a synergistic improvement in kinetic performance and long lifespan.

[0037] Meanwhile, polyimide also possesses an imide ring and an aromatic backbone. Its N–H groups can form strong hydrogen bonds with the surface-active oxygen-containing functional groups of carbon nanotubes, and its own aromatic backbone undergoes π–π interactions with carbon nanotubes. Furthermore, the polyimide molecular chains can form a flexible ion-conducting moisture-wicking layer in situ within the electrode pores, significantly improving electrolyte wettability. Thus, the two are interpenetrating at the micro-nano scale, achieving a high degree of coupling between electron and ion transport, thereby simultaneously obtaining excellent rate performance and long-cycle stability in high-nickel ternary thick electrodes.

[0038] The above general description and the description below are exemplary and illustrative only and are not intended to limit this application. Attached Figure Description

[0039] One or more embodiments are illustrated by way of example with reference to the accompanying drawings. These illustrations and drawings do not constitute a limitation on the embodiments. Elements having the same reference numerals in the drawings are shown as similar elements. The drawings are not to be scaled. And wherein:

[0040] Figure 1 This is a schematic flowchart of a method for preparing a positive electrode sheet according to an embodiment of this disclosure;

[0041] Figure 2 This is a schematic diagram of the structure of a lithium-ion battery provided in an embodiment of this disclosure;

[0042] Figure 3 This is a schematic diagram of the structure of a lithium-ion battery provided in an embodiment of this disclosure;

[0043] Figure 4 This is a schematic diagram of a lithium-ion battery provided in an embodiment of this disclosure.

[0044] Figure label:

[0045] 1-Positive terminal; 10-Cell; 11-Positive terminal post; 12-Negative terminal; 2-Shell; 3-Negative electrode plate; 4-Separator; 5-Positive electrode plate. Detailed Implementation

[0046] To provide a more detailed understanding of the features and technical content of the embodiments of this disclosure, the implementation of the embodiments of this disclosure will be described in detail below with reference to the accompanying drawings. The accompanying drawings are for illustrative purposes only and are not intended to limit the embodiments of this disclosure. In the following technical description, for ease of explanation, several details are used to provide a full understanding of the disclosed embodiments. However, one or more embodiments may still be implemented without these details. In other cases, well-known structures and devices may be simplified in their depiction to simplify the drawings.

[0047] The terms "first," "second," etc., used in the specification, claims, and accompanying drawings of this disclosure are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate for the embodiments of this disclosure described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion.

[0048] In this disclosure, the terms "upper," "lower," "inner," "middle," "outer," "front," and "rear," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are primarily for better description of the embodiments of this disclosure and their implementations, and are not intended to limit the indicated devices, elements, or components to having a specific orientation, or to require them to be constructed and operated in a specific orientation. Furthermore, some of the aforementioned terms may be used to indicate other meanings besides orientation or positional relationship; for example, the term "upper" may in some cases indicate a dependency or connection relationship. Those skilled in the art can understand the specific meaning of these terms in the embodiments of this disclosure according to the specific circumstances.

[0049] Furthermore, the terms "set up," "connect," and "fix" should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral structure; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, or it can be an internal connection between two devices, components, or parts. Those skilled in the art can understand the specific meaning of the above terms in the embodiments of this disclosure according to the specific circumstances.

[0050] Unless otherwise stated, the term "multiple" means two or more.

[0051] In this embodiment of the disclosure, the character " / " indicates that the objects before and after it are in an "or" relationship. For example, A / B means: A or B.

[0052] The term "and / or" describes an association between objects, indicating that three relationships can exist. For example, A and / or B means: A or B, or A and B.

[0053] It should be noted that, unless otherwise specified, the embodiments and features described in the present disclosure can be combined with each other.

[0054] This disclosure provides a positive electrode sheet, including a positive current collector and a positive electrode coating coated on at least one surface of the positive current collector. The thickness of the positive current collector is 5 μm to 15 μm, and the thickness of the positive electrode sheet is 65 μm to 90 μm. The positive electrode coating comprises:

[0055] Positive electrode active materials include lithium nickel cobalt manganese oxide, with the chemical formula LiNi. x Co y Mn z M bO2; wherein, 0.70≤x≤0.95, 0.15≤y<0.45, 0.05≤z<0.45, 0≤b≤0.25, and x+y+z+b=1, the element M includes one or more of Zr, W, Ti, Al, Sr, B and Nd, and the mass percentage of the positive electrode active material in the positive electrode coating is 93%~97%;

[0056] The positive electrode conductive agent comprises carbon nanotubes and carbon black in a mass ratio of 4:1 to 1:2; wherein the carbon nanotubes are doped with sulfur and have a specific surface area of ​​220 m². 2 / g~320m 2 / g, and the positive electrode conductive agent in the positive electrode coating has a mass percentage content of 1% to 2.5%;

[0057] The positive electrode binder includes polyvinylidene fluoride and polyimide; wherein the polyimide has a mass percentage content of 10% to 50% in the positive electrode binder, and the positive electrode binder has a mass percentage content of 1.5% to 4.0% in the positive electrode coating.

[0058] The positive electrode sheet provided in this disclosure, through optimized material ratios of the positive electrode conductive agent and the positive electrode binder, and the synergistic use of carbon nanotubes doped with sulfur and polyimide, leverages the high conductivity, high specific surface area, and good hydrophilicity of carbon nanotubes. This forms a dominant electron transport network, optimizing the connectivity of electron and lithium-ion transport paths and enhancing the mechanical integrity and electrochemical stability of the electrode sheet under thick electrode and high-compaction design, achieving a synergistic improvement in kinetic performance and long lifespan.

[0059] Meanwhile, polyimide also possesses an imide ring and an aromatic backbone. Its N–H groups can form strong hydrogen bonds with the surface-active oxygen-containing functional groups of carbon nanotubes, and its own aromatic backbone undergoes π–π interactions with carbon nanotubes. Furthermore, the polyimide molecular chains can form a flexible ion-conducting moisture-wicking layer in situ within the electrode pores, significantly improving electrolyte wettability. Thus, the two are interpenetrating at the micro-nano scale, achieving a high degree of coupling between electron and ion transport, thereby simultaneously obtaining excellent rate performance and long-cycle stability in high-nickel ternary thick electrodes.

[0060] In some embodiments, the positive current collector is aluminum foil or perforated aluminum foil.

[0061] In some embodiments, the pore volume of the carbon nanotubes is 0.6 cm³. 3 / g~1.6cm 3 / g.

[0062] In some embodiments, the atomic percentage of sulfur in the carbon nanotubes is 0.3% to 0.7%.

[0063] In some embodiments, the water contact angle of the carbon nanotubes is ≤50°.

[0064] In some embodiments, the intensity ratio ID / IG of the D peak to the G peak in the Raman spectrum of carbon nanotubes is 0.2 to 0.6.

[0065] In some embodiments, when the carbon nanotubes are fabricated into a predetermined circular positive electrode, their electronic conductivity σ ≥ 15 S·cm -1 The area of ​​the pre-set circular positive electrode is 132.665 mm². 2 The thickness is 0.8mm.

[0066] In some embodiments, the carbon nanotubes exhibit characteristic absorption peaks in the X-ray photoelectron spectroscopy at 163.5 eV–164.5 eV, 283.5 eV–285.8 eV, and 528.5 eV–535.5 eV.

[0067] In the embodiments disclosed herein, a characteristic absorption peak is observed in the energy spectrum from 163.5 eV to 164.5 eV, which corresponds to a –C–S–C– covalent bond, indicating that the carbon nanotube is doped with sulfur.

[0068] In this embodiment, a characteristic absorption peak is observed in the energy spectrum from 283.5 eV to 285.8 eV, indicating the intrinsic carbon-carbon hybridization of the carbon nanotube; that is, sp 2 Hybridized C=C (~284.0 eV), sp 3 Hybridized C–C (~285.2 eV).

[0069] In this embodiment of the disclosure, a characteristic absorption peak is present in the energy spectrum from 528.5 eV to 535.5 eV, corresponding to –SO x Functional groups.

[0070] In some embodiments, the number-average molecular weight of polyvinylidene fluoride is 63.6 g / mol to 190.8 g / mol; and the number-average molecular weight of polyimide is 53 g / mol to 212 g / mol.

[0071] In some embodiments, a positive electrode slurry is coated on at least one side surface of the positive electrode current collector to form the positive electrode coating; wherein the viscosity of the positive electrode slurry is 5000 mPa·s to 8000 mPa·s, and the viscosity change rate V of the positive electrode slurry is <18% after standing for a preset time.

[0072] In this embodiment of the disclosure, the positive electrode slurry is formed by mixing the positive electrode active material, the positive electrode conductive agent and the positive electrode binder in deionized water.

[0073] In some embodiments, the interface resistance Rα of the positive electrode is 1.0 × 10⁻⁶. -3 Ω·cm 2 ~1.0×10 -2 Ω·cm 2 .

[0074] In some embodiments, the peel strength N of the positive electrode sheet is 8.5 N / m to 18.0 N / m.

[0075] In some embodiments, the compaction density D of the positive electrode sheet is greater than 3.5 g / cm³. 3 .

[0076] Combination Figure 1 As shown in the embodiments of this disclosure, a method for preparing a positive electrode sheet is also provided, including the following steps:

[0077] S101. MgSO4 is converted into sulfur element through thermal decomposition and / or chemical reaction. The sulfur element reacts with the surface of carbon nanotubes to obtain a matrix powder with sulfur atoms embedded in the tube wall structure of carbon nanotubes.

[0078] S102. Remove residual oxides from the matrix powder with hydrochloric acid and wash with deionized water until the pH value is neutral. After filtration and drying, a positive electrode conductive agent is obtained.

[0079] S103. The positive electrode active material, positive electrode conductive agent and positive electrode binder are stirred in an N-methylpyrrolidone solvent system to form a positive electrode slurry;

[0080] S104. The positive electrode slurry is coated on at least one side of the positive electrode current collector, and after drying and cold pressing, a positive electrode sheet is obtained.

[0081] In some embodiments, MgSO4 is converted into sulfur element through thermal decomposition and / or chemical reaction. The sulfur element reacts with the surface of carbon nanotubes to obtain a matrix powder with a sulfur atom embedded in the tube wall structure of carbon nanotubes, comprising:

[0082] Carbon nanotubes were dispersed in an aqueous solution of MgSO4 according to a preset mass ratio. After ultrasonic dispersion, a uniform suspension was formed. Then, the suspension was dried and ground to obtain the initial matrix powder.

[0083] In an inert gas environment, the initial matrix powder is placed in the reaction chamber, heated to a preset temperature, and then kept at that temperature. This allows SO2 and SO3 generated by the thermal decomposition of MgSO4 to be converted into sulfur (elemental S) through a reduction reaction; or, MgSO4 reacts with C to generate sulfur (elemental S), and finally, the sulfur reacts with the surface of carbon nanotubes to obtain a matrix powder with a sulfur atom embedded in the wall structure of carbon nanotubes.

[0084] In this embodiment, carbon nanotubes are treated with acid or plasma to introduce oxygen-containing functional groups and structural defects onto their surface. In this way, the functional groups can serve as active sites for chemical reactions, forming chemical bonds with sulfur, while the lattice gaps or vacancies at the defect sites provide space for the insertion of sulfur atoms.

[0085] This disclosure also provides a lithium-ion battery, including a negative electrode and a positive electrode as described in the foregoing embodiments.

[0086] In some embodiments, the charging characteristics of the lithium-ion battery are as follows: after a lithium-ion battery with a 100% charge rate is left to stand at 25°C for 6 hours, it is discharged to 2.5V at a discharge rate of 0.1C, with a corresponding discharge capacity of Q1; and discharged to 2.5V at a discharge rate of 10C, with a corresponding discharge capacity of Q. 10 Discharge capacity retention rate Q R ≥60%, Q R =Q 10 / Q1.

[0087] In some embodiments, after a lithium-ion battery is cycled 600 times at 1C rate at 25°C, the capacity decay rate Q is... d <18%.

[0088] The specific structure of the positive electrode sheet is as described in the above embodiments. Since this lithium-ion battery adopts all the technical solutions of all the above embodiments, it has at least all the technical effects brought about by the technical solutions of the above embodiments, and will not be described in detail here.

[0089] In this embodiment of the disclosure, the lithium-ion battery further includes a casing, in which a positive electrode and a negative electrode are encapsulated. Figures 2 to 4 As shown, the lithium-ion battery specifically includes a cylindrical casing 2, which houses the battery cells. The top is the positive terminal 1, and the bottom is the negative terminal 12. A positive electrode post 11 is disposed on the positive terminal 1. Specifically, Figure 3 A schematic diagram of the lithium-ion battery structure in this application is shown. Figure 4 A schematic diagram of the lithium-ion battery described in this application is shown. The positive electrode 5, negative electrode 3, and separator 4 are shown as follows: Figure 4 The layers are stacked and then wound to form... Figure 3 The cylindrical battery cell 10 shown is initially wound to the electrode end at the cylindrical axis, and ends to the electrode end on the outer surface of the cylinder after winding. The positive electrode 5 includes a strip-shaped positive electrode foil, a positive electrode coating on the surface of the strip-shaped positive electrode foil, and a first empty foil area. The negative electrode 3 includes a strip-shaped negative electrode foil, a negative electrode coating on the surface of the strip-shaped negative electrode foil, and a second empty foil area. The first and second empty foil areas are perpendicular to the winding direction and are formed into the top or bottom end face of the lithium-ion battery by methods such as flattening or folding.

[0090] Based on this, the present disclosure provides a method for preparing a lithium-ion battery, comprising:

[0091] Preparation of positive electrode sheet: The positive electrode sheet prepared by the method described in the foregoing embodiments;

[0092] Preparation of negative electrode sheet: Mix negative electrode slurry, coat it on both sides of the negative electrode current collector, and obtain negative electrode sheet after drying and cold pressing;

[0093] Battery cell preparation: The positive and negative electrode sheets are rolled and slit respectively, and then wound together with the separator to obtain the battery cell;

[0094] Assembling lithium-ion batteries: The tabs of the lithium-ion battery are welded to the electrical connectors, installed into the battery casing, and electrolyte is injected, sealed, and formed to obtain the lithium-ion battery.

[0095] Furthermore, embodiments of this disclosure provide an electrical device including a lithium-ion battery for providing power as described in this application.

[0096] The present invention will be further explained and illustrated below with reference to embodiments.

[0097] Example 1

[0098] This embodiment 1 provides a method for preparing a lithium-ion battery as follows:

[0099] Preparation of the positive electrode sheet: This includes preparing a positive conductive agent by dispersing carbon nanotubes (CNTs) in an aqueous solution of MgSO4, wherein the carbon nanotubes are 0.8 g / mL and the mass ratio of carbon nanotubes to MgSO4 is 8:1. The dispersion is ultrasonically carried out for 60 min to form a uniform suspension. The mixed liquid is transferred to an evaporating dish and dried at 100 °C for 24 h to form a gray solid mixture. The gray solid mixture is ground into a uniform fine powder to obtain the initial matrix powder.

[0100] High-temperature sulfur doping reaction: The initial matrix powder is loaded into a quartz boat and placed in the center of a horizontal tube furnace. High-purity argon (Ar) gas is introduced at a flow rate of 100 sccm, and the temperature is raised to 800℃ and held for 1 h. The SO2 and SO3 generated by the thermal decomposition of MgSO4 are then reduced to generate elemental S. Elemental S reacts with the surface of carbon nanotubes to obtain a matrix powder with a tube wall structure in which sulfur atoms are embedded.

[0101] Acid washing and purification: After cooling the above matrix powder to room temperature, it was transferred to a three-necked flask, concentrated hydrochloric acid (HCl) was added to dilute the powder, and the mixture was refluxed at 90°C for 3 hours to remove residual MgO and other reactants. Subsequently, it was repeatedly rinsed with deionized water until pH≈7, filtered, and dried in a vacuum drying oven at 60°C for 12 hours to obtain a sulfur-doped carbon nanotube positive conductive agent (CNT@S). Here, the sulfur atom content in the carbon nanotubes is 0.53%.

[0102] Select positive electrode active material (LiNi) 0.8 Co 0.1 Mn 0.1 O2, NMC811, D 50 A positive electrode slurry was obtained by thoroughly mixing an 8.5 μm aluminum foil, a positive electrode conductive agent (CNT@S), carbon black (SuperP), polyimide (PI), and polyvinylidene fluoride (PVDF) in an N-methylpyrrolidone solvent system at a mass ratio of 96:1:1:0.6:1.4. The slurry was then coated onto a 12.0 μm aluminum foil, and after drying, cold pressing, slitting, and cutting, a compacted density of 3.62 g / cm³ was obtained. 3 The positive electrode sheet. Here, the mass percentage of polyimide in the positive electrode binder is 30%.

[0103] Preparation of the negative electrode sheet: Graphite, carbon nanotubes, sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were selected and stirred in deionized water at a mass ratio of 96:1.5:1:1.5 to form a negative electrode coating material with a solid content of 40%. This negative electrode coating material was then coated onto both sides of a copper foil, dried, and cold-pressed to achieve a compaction density of 1.5 g / cm³. 3 The negative electrode.

[0104] Electrolyte preparation: Lithium hexafluorophosphate (LiPF6), ethylene carbonate (EC), dimethyl carbonate (DMC), fluoroethylene carbonate (FEC), ethylene sulfate (DTD), and vinylene carbonate (VC) are mixed in a mass percentage ratio of 10:20:55:2:8:5 to obtain the electrolyte.

[0105] Preparation of the diaphragm: A high-porosity diaphragm is selected, in which the thickness of the polyethylene (PE) base membrane is 9 μm, the thickness of the ceramic coating on both sides of the base membrane is 1 μm, and the thickness of the polyvinylidene fluoride (PVDF) coating is 1 μm.

[0106] Assembling a lithium-ion battery: After the positive and negative electrode sheets are rolled, slit, and then wound together with the separator, a cylindrical battery core is obtained. The battery core is then welded to the electrical connectors and installed into the battery casing. After completing the electrolyte injection, sealing, and formation processes, the lithium-ion battery of Example 1 is obtained. The casing of this lithium-ion battery is cylindrical, with dimensions of 21.0 mm in diameter and 70.0 mm in length.

[0107] Example 2

[0108] Example 2 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass ratio of carbon nanotubes to carbon black is 1:2, the mass percentage of carbon nanotubes in the positive electrode coating is 0.67%, and the mass percentage of carbon black in the positive electrode coating is 1.33%. Everything else is the same as in Example 1.

[0109] Example 3

[0110] Example 3 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass ratio of carbon nanotubes to carbon black is 2:1, the mass percentage of carbon nanotubes in the positive electrode coating is 1.33%, and the mass percentage of carbon black in the positive electrode coating is 0.67%. Everything else is the same as in Example 1.

[0111] Example 4

[0112] Example 4 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass ratio of carbon nanotubes to carbon black is 3:1, the mass percentage of carbon nanotubes in the positive electrode coating is 1.5%, and the mass percentage of carbon black in the positive electrode coating is 0.5%. Everything else is the same as in Example 1.

[0113] Example 5

[0114] Example 5 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass ratio of carbon nanotubes to carbon black is 4:1, the mass percentage of carbon nanotubes in the positive electrode coating is 1.6%, and the mass percentage of carbon black in the positive electrode coating is 0.4%. Everything else is the same as in Example 1.

[0115] Example 6

[0116] Example 6 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass ratio of carbon nanotubes to MgSO4 is 10:1, and the sulfur atom content in the carbon nanotubes is 0.32%. Everything else is the same as in Example 1.

[0117] Example 7

[0118] Example 7 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass ratio of carbon nanotubes to MgSO4 is 9:1, and the sulfur atom content in the carbon nanotubes is 0.41%. Everything else is the same as in Example 1.

[0119] Example 8

[0120] Example 8 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass ratio of carbon nanotubes to MgSO4 is 7:1, and the sulfur atom content in the carbon nanotubes is 0.65%. Everything else is the same as in Example 1.

[0121] Example 9

[0122] Example 9 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass ratio of carbon nanotubes to MgSO4 is 5:1, and the sulfur atom content in the carbon nanotubes is 0.74%. Everything else is the same as in Example 1.

[0123] Example 10

[0124] Example 10 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass percentage of polyvinylidene fluoride in the positive electrode coating is 1.8%, the mass percentage of polyimide in the positive electrode coating is 0.2%, and the mass percentage of polyimide in the positive electrode binder is 10%. Everything else is the same as in Example 1.

[0125] Example 11

[0126] Example 11 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass percentage of polyvinylidene fluoride in the positive electrode coating is 1.6%, the mass percentage of polyimide in the positive electrode coating is 0.4%, and the mass percentage of polyimide in the positive electrode binder is 20%. Everything else is the same as in Example 1.

[0127] Example 12

[0128] Example 12 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass percentage of polyvinylidene fluoride in the positive electrode coating is 1.2%, the mass percentage of polyimide in the positive electrode coating is 0.8%, and the mass percentage of polyimide in the positive electrode binder is 40%. Everything else is the same as in Example 1.

[0129] Example 13

[0130] Example 13 provides a lithium-ion battery. The difference between this example and Example 1 is that the mass percentage of polyvinylidene fluoride in the positive electrode coating is 1%, the mass percentage of polyimide in the positive electrode coating is 1%, and the mass percentage of polyimide in the positive electrode binder is 50%. Everything else is the same as in Example 1.

[0131] Example 14

[0132] Example 14 provides a lithium-ion battery. The difference between this example and Example 1 is that, during the acid washing and purification process of the matrix powder, concentrated hydrochloric acid diluent is added, and the mixture is refluxed at 90°C for 1 hour. Everything else is the same as in Example 1.

[0133] Example 15

[0134] Example 15 provides a lithium-ion battery. The difference between this example and Example 1 is that, during the acid washing and purification process of the matrix powder, concentrated hydrochloric acid diluent is added, and the mixture is refluxed at 90°C for 2 hours. Everything else is the same as in Example 1.

[0135] Example 16

[0136] Example 16 provides a lithium-ion battery. The difference between this example and Example 1 is that, during the acid washing and purification process of the matrix powder, concentrated hydrochloric acid diluent is added, and the mixture is refluxed at 90°C for 4 hours. Everything else is the same as in Example 1.

[0137] Example 17

[0138] Example 17 provides a lithium-ion battery. The difference between this example and Example 1 is that, during the acid washing and purification process of the matrix powder, concentrated hydrochloric acid diluent is added, and the mixture is refluxed at 90°C for 5 hours. Everything else is the same as in Example 1.

[0139] Comparative Example 1

[0140] Comparative Example 1 provides a lithium-ion battery. The difference between this comparative example and Example 1 is that the carbon nanotubes are not doped with sulfur. Everything else is the same as Example 1.

[0141] Comparative Example 2

[0142] Comparative Example 2 provides a lithium-ion battery. The difference between this comparative example and Example 1 is that polyimide was not added to the positive electrode binder. Everything else is the same as Example 1.

[0143] The lithium-ion batteries of Examples 1 to 17, as well as Comparative Examples 1 and 2, were tested accordingly.

[0144] The lithium-ion battery is discharged at a constant current to 2.5V; this ensures the lithium-ion battery is in a safe state to reduce the risk of short circuits or thermal runaway during disassembly. The battery is disassembled inside a glove box, and the positive electrode of the cylindrical cell is removed; the glove box is in a pure argon atmosphere or other inert gas atmosphere.

[0145] Cut the removed positive electrode sheet to an appropriate size and immerse it in anhydrous dimethyl carbonate (DMC) solution for 30 minutes to dissolve and remove residual electrolyte and possible byproducts. Remove the positive electrode sheet and wipe its surface with a lint-free wiping paper. Then replace the anhydrous dimethyl carbonate (DMC) solution and repeat the immersion and wiping process three times to ensure that there are no residual contaminants on the electrode sheet surface.

[0146] Rinse the positive electrode with anhydrous ethanol and wipe it again to ensure that there are no residual contaminants on the surface of the positive electrode. Place the cleaned positive electrode in a glove box and let it stand for 48 hours to ensure that the electrode is completely dry, so as to prevent subsequent tests from being interfered with by solvent residue.

[0147] This embodiment provides a method for testing the peel force of a positive electrode sheet, including the following steps:

[0148] Cut the positive electrode sheet into a standard-sized strip sample, with a length of 2cm and a width of 10cm;

[0149] Use double-sided tape to fix the test sample to a flat, thin steel plate, ensuring that the tape is stuck in the center of the steel plate;

[0150] Then peel off the protective layer of the double-sided tape, stick the test sample on the double-sided tape, and use a pressure roller to evenly press the test sample to ensure that the sample is in full contact with the steel plate surface and is firmly adhered;

[0151] At the unattached end of the positive electrode sheet, it is naturally folded 180° and fixed to the fixture on the electronic tensile testing machine for a 180° peel test. The tensile speed is 100 mm / min. The peel force-displacement curve is recorded during the tensile process, and data is selected in the stable stage where the force value fluctuation does not exceed 10%.

[0152] Calculate the average tensile force of this segment and the ratio of the average tensile force to the width of the positive electrode sheet to obtain the peel strength of the positive electrode sheet;

[0153] Peel force tests were conducted on the front, middle, and rear sections of the aforementioned positive electrode sheet, and the resulting peel force values ​​were denoted as N1, N2, and N3, respectively. The average peel force N of the actual tested positive electrode sheet was obtained by calculating the average of the above peel force values, where N = (N1 + N2 + N3) / 3.

[0154] This embodiment also provides a method for determining the ID / IG ratio of a positive electrode binder using Raman spectroscopy. A Horiba LabRAM laser Raman spectrometer is used, with a 532nm laser as the excitation source, a spot diameter of less than 1μm, and a sampling range of 200cm. -1 ~3000cm -1 .

[0155] In the Raman spectrum, the D peak is located at approximately 1300 cm⁻¹. -1 ~1400cm -1 Corresponding defects or SP 3 Structure; G peak located at 1570 cm⁻¹ -1 ~1600cm -1 Corresponding to graphitized sp 3 Structure; the degree of defect in carbon nanotubes was assessed by peak intensity ratio ID / IG.

[0156] For each sample, three points were collected and the average value was taken to ensure data accuracy and statistical validity.

[0157] This embodiment also provides a method for testing the compaction density of a positive electrode sheet, including the following steps:

[0158] The positive electrode sheet, which had been washed with dimethyl carbonate and vacuum dried, was cut into 6 standard-sized square samples, each 2 cm long and 2 cm wide.

[0159] The active material on the front and back of three square samples was wiped off, rinsed with ethanol, dried, weighed and the average mass M1 was calculated. At the same time, the average thickness L1 of the sample was measured using a micrometer.

[0160] Weigh the other three square samples and calculate the average mass M2. At the same time, use a micrometer to measure the average thickness L2 of the samples.

[0161] Through formula Calculate the compaction density of the electrode.

[0162] This embodiment also provides a method for determining the sulfur doping in carbon nanotubes. The method involves analyzing the sulfur doping ratio in carbon nanotubes using X-ray photoelectron spectroscopy. Specifically:

[0163] Using an Al Kα monochromatic X-ray source, based on the calibration of the C1s (284.8 eV) peak position, the high-resolution spectrum of the S2p region was extracted. The atomic percentage of the characteristic peak area of ​​S in the range of 163.5 eV to 164.5 eV was calculated to finally determine the percentage of sulfur doped atoms in the carbon nanotubes.

[0164] This embodiment also provides a method for determining the conductivity of carbon nanotubes, specifically:

[0165] 100 mg of carbon nanotube powder was placed in a stainless steel mold with an inner diameter of 13 mm.

[0166] A circular electrode sample with a thickness of approximately 0.8 mm was formed by pressing under a pressure of 10 MPa.

[0167] The disc electrode samples were subjected to I–V tests on a CHI660D electrochemical workstation;

[0168] Using a symmetrical two-electrode configuration, the scanning voltage range is -1.0V to +1.0V with a step size of 0.01V, and the resulting current-voltage curve exhibits linear ohmic behavior.

[0169] Resistance of the disc electrode sample

[0170] Where V is the voltage of the disc electrode sample; I is the current of the disc electrode sample;

[0171] Electronic conductivity of disc electrode samples

[0172] Where L is the thickness of the circular electrode sample, and A is the cross-sectional area of ​​the circular electrode sample, that is, the area of ​​the circle corresponding to the diameter of the circular electrode sample.

[0173] Thus, the electronic conductivity of the disc electrode sample can be obtained by using the slope of the I–V curve.

[0174] This embodiment also provides a method for determining the specific surface area and pore volume of carbon nanotubes, specifically:

[0175] The specific surface area was calculated using the BET (Brunauer–Emmett–Teller) model.

[0176] The BJH (Barrett–Joyner–Halenda) method was used to evaluate pore volume and pore size distribution;

[0177] After degassing the disc electrode samples under vacuum at 120℃ for 12 h, they were immersed in liquid nitrogen at -196℃, and nitrogen gas was introduced. The adsorption amounts under different relative pressures were recorded. Subsequently, the pore size distribution and pore volume of carbon nanotubes were characterized based on the relationship between the volume of each pore size and the corresponding partial pressure. The relative pressure range for testing was P / P0, which was 0.01–0.99.

[0178] This embodiment also provides a method for testing the interface resistance of the positive electrode, using a resistance testing system of model RM2610 to measure the interface resistance of the positive electrode. The resistance testing system has 45 probes arranged in a square matrix, with one probe serving as a ground probe.

[0179] The testing method includes the following steps:

[0180] The positive electrode sheet, which had been washed with dimethyl carbonate and vacuum dried, was divided into 40 square grids to ensure that the sample surface was flat.

[0181] Place the positive electrode on the testing device and adjust the pressure applied by the probe using a pressure gauge to ensure good contact between the probe and the test sample, with a contact area of ​​0.01 cm². 2 ;

[0182] During the test, a constant current is applied to the 20 outer probes, allowing the current to flow through the surface, interface, and current collector of the positive electrode, while the 25 middle probes measure the voltage change in real time.

[0183] The interface resistance R1 is calculated based on Ohm's law and fitting analysis method.

[0184] Subsequently, using the same method, nine more square grids were randomly selected for interfacial resistance measurement, and the obtained values ​​were recorded as R2, R3, R4, R5, R6, R7, R8, R9, and R... 10 ;

[0185] By calculating the average value of the above resistance values, the average interface resistance Rα is obtained, so as to fully obtain the conductivity characteristics and uniformity of the positive electrode.

[0186] This embodiment also provides a method for testing the rate performance of lithium-ion batteries. The lithium-ion battery is placed in a constant temperature chamber at 25°C for 4 hours, and the test is carried out according to the following steps:

[0187] Under a charging rate of 0.1C, charge at constant current and constant voltage to 4.2V, cut off current of 0.05C, and let stand for 30 minutes;

[0188] Under a discharge rate of 0.1C, the capacitor is discharged at a constant current until it is cut off at 2.5V. The capacitance is recorded as Q1, and the capacitor is left to stand for 30 minutes.

[0189] Under a charging rate of 0.1C, charge at constant current and constant voltage to 4.2V, cut off current of 0.05C, and let stand for 30 minutes;

[0190] Under a discharge rate of 10C, constant current discharge is performed until a cutoff voltage of 2.5V. The capacitance is denoted as Q. 10 Let it stand for 30 minutes;

[0191] Here, through Q1 and Q 10 Calculate the discharge capacity retention rate Q of a lithium-ion battery. R , where Q R =Q 10 / Q1×100%.

[0192] This embodiment also provides a method for testing the cycle performance of a lithium-ion battery. The lithium-ion battery is placed in a constant temperature chamber at 25°C for 4 hours, and the test is carried out according to the following steps:

[0193] Under a charging rate of 0.1C, constant current charging is applied to 4.2V, followed by constant voltage charging until the current drops to 0.01C; after charging is complete, the device is left to stand for 30 minutes.

[0194] Discharge to 2.5V at a discharge rate of 0.1C.

[0195] Charging steps: Under a 1C charging rate, charge at a constant current to 4.2V and let stand for 30 minutes.

[0196] Discharge procedure: Discharge to 2.5V at a discharge rate of 1C.

[0197] Repeat the above charging and discharging steps 600 times.

[0198] The discharge capacities Q1 and Q2 of the battery after 1 cycle and 600 cycles respectively. 600 Calculate the capacity decay rate Q of a lithium-ion battery. d Q d =(Q1-Q 600 )×100%.

[0199] After performing the above tests on Examples 1 to 17, as well as Comparative Examples 1 and 2, the corresponding data were obtained.

[0200] The interface resistance, peel strength, capacity retention (10C capacity retention), and cycle retention (capacity retention after 600 cycles) measured according to Examples 1 to 5 are shown in the table below:

[0201] CNT@S:Superp Interface resistance Peel strength Capacity retention Cycle retention rate Example 1 1:1 <![CDATA[1.5mΩ·cm 2 ]]> 16.2 N·m 73.4％ 88.9％ Example 2 1:2 <![CDATA[4.2mΩ·cm 2 ]]> 14.1 N·m 64.3％ 82.6％ Example 3 2:1 <![CDATA[3.2mΩ·cm 2 ]]> 14.9 N·m 67.1％ 85.3％ Example 4 3:1 <![CDATA[2.0mΩ·cm 2 ]]> 15.1 N·m 69.5％ 87.2％ Example 5 4:1 <![CDATA[1.9mΩ·cm 2 ]]> 13.5 N·m 67.8％ 86.1％

[0202] Table 1

[0203] Table 1, comparing Examples 1 to 5, shows that appropriately increasing the proportion of carbon nanotubes significantly enhances the continuity and stability of the electron conduction path within the positive electrode. This is mainly because carbon nanotubes possess high conductivity and a long-range structure, facilitating the formation of a three-dimensional conductive network, while carbon black, primarily composed of particles, enhances the density and flexibility of the structure. The combination of these two components achieves synergistic optimization in conductivity, dispersibility, and adhesion.

[0204] If the proportion of carbon nanotubes is too high, the agglomeration effect may lead to uneven dispersion of the slurry and increased contact resistance, which may affect the overall performance. Therefore, when the mass ratio of carbon nanotubes to carbon black is 2:1, the positive electrode exhibits the lowest interface resistance and the highest peel strength, and the corresponding lithium-ion battery has the highest 10C capacity retention rate and the highest capacity retention rate after 600 cycles.

[0205] The interface resistance, peel strength, 10C capacity retention, and capacity retention after 600 cycles measured according to Examples 1, 6 to 9 are shown in the table below:

[0206]

[0207] Table 2

[0208] Table 2 shows that by comparing Example 1 and Examples 6 to 9, as the mass ratio of carbon nanotubes to sulfur source (MgSO4) increases, the proportion of sulfur increases from 0.32% to 0.53%, and the electrochemical performance of the positive electrode continuously improves, especially the capacity retention rate, which increases by more than 6%. When the sulfur doping exceeds 0.6%, both the capacity retention rate and the cycle retention rate decrease.

[0209] Among them, when the mass ratio of carbon nanotubes to sulfur source (MgSO4) is 10:1, the capacity retention rate decreases. The main reason is that doping with S may introduce C–S–C bonds, which enhances the surface polarity and electron density of carbon nanotubes, improves their wettability to electrolyte and compatibility with polyimide binders.

[0210] Furthermore, doped sulfur atoms can induce the formation of local defect sites, which helps to construct highly reactive channels. However, when the doping ratio of sulfur atoms is too high, it may cause excessive accumulation of defects in the carbon nanotube wall structure, resulting in a decrease in framework conductivity. In addition, residual sulfur clusters increase the risk of side reactions, thereby affecting stability and rate response.

[0211] The interface resistance, peel strength, capacity retention (10C capacity retention), and cycle retention (capacity retention after 600 cycles) measured according to Examples 1, 10 to 13 are shown in the table below:

[0212]

[0213] Table 3

[0214] Table 3, comparing Examples 1 and 10 to 13, shows that as the mass percentage of polyimide in the binder system is appropriately increased, the interfacial resistance of the positive electrode sheet decreases, the adhesion strength increases, and thus the rate performance (capacity retention) and cycle performance (cycle retention) of the lithium-ion battery are further improved. Therefore, this formulation indicates that it helps to construct a more stable electrode structure.

[0215] However, when the mass percentage of polyimide exceeds 40%, the relevant performance shows a downward trend. This is mainly because the polyimide molecular chain contains conjugated imide groups, which not only impart good mechanical strength to the binder but also enhance the bonding force with the positive electrode active material. At an appropriate ratio, the aromatic ring structure of polyimide can synergistically align with the carbon nanotube walls through π–π stacking interactions. Simultaneously, its polar imide groups can also undergo hydrogen bonding with oxygen-containing functional groups introduced into the carbon nanotubes, such as C–O–C and C=O, to synergistically construct an electron-ion composite conductive network, thereby improving conductivity continuity and the overall integrity of the electrode.

[0216] However, when the polyimide content is too high, the rigidity of the cathode slurry system increases while its flexibility decreases, and the rheological behavior of the cathode slurry deteriorates, resulting in poor dispersion uniformity and reduced film formation during the coating process, thereby causing enhanced interfacial polarization and reduced cycle stability.

[0217] The interface resistance, peel strength, capacity retention (10C capacity retention), and cycle retention (capacity retention after 600 cycles) measured according to Examples 1, 14 to 17 are shown in the table below:

[0218] Processing time Specific surface area Interface resistance Peel strength Capacity retention Cycle retention rate Example 1 3h <![CDATA[280m 2 / g]]> <![CDATA[1.5mΩ·cm 2 ]]> 16.2 N·m 73.4％ 88.9％ Example 14 1h <![CDATA[220m 2 / g]]> <![CDATA[3.3mΩ·cm 2 ]]> 15.1 N·m 68.2％ 86％ Example 15 2h <![CDATA[253m 2 / g]]> <![CDATA[2.7mΩ·cm 2 ]]> 15.8 N·m 70.1％ 87.5％ Example 16 4h <![CDATA[303m 2 / g]]> <![CDATA[2.1mΩ·cm 2 ]]> 15.5 N·m 69.6％ 88％ Example 17 5h <![CDATA[320m 2 / g]]> <![CDATA[2mΩ·cm 2 ]]> 14.6 N·m 68％ 86.2％

[0219] Table 4

[0220] Table 4 shows that, by comparing Examples 1 and 14 to 17, the specific surface area of ​​carbon nanotubes varies depending on the addition of concentrated hydrochloric acid diluent and the duration of reflux heating at 90°C during the acid washing and purification process. Here, the specific surface area increases to a maximum of 320 m² as the acid washing and purification treatment time increases. 2 / g, its cycling stability and rate performance actually decrease. The main reason is that a moderate specific surface area can provide a larger interfacial contact area, thereby enhancing the interfacial bonding strength between carbon nanotubes and binders / active materials, improving electronic coupling and ion wetting pathways, and helping to construct a multidimensional conduction network.

[0221] When the specific surface area of ​​carbon nanotubes exceeds 280m² 2 When the density is / g, it will cause a decrease in pore size and a tendency to agglomerate, increase the area of ​​SEI film formation, enhance side reactions, and reduce conductivity and electrochemical stability.

[0222] The interface resistance, peel strength, capacity retention (10C capacity retention), and cycle retention (capacity retention after 600 cycles) measured according to Example 1, Comparative Example 1, and Comparative Example 2 are shown in the table below:

[0223]

[0224] Table 5

[0225] Table 5 shows that by comparing Example 1 with Comparative Examples 1 and 2, it can be seen that in Comparative Example 1, the carbon nanotubes were not doped with sulfur, lacked polar group regulation, had poor interfacial compatibility and many defects in the conductive network, resulting in an overall decline in performance; in Comparative Example 2, the positive electrode binder did not contain polyimide, its flexibility and interfacial bonding were insufficient, its film-forming properties and electrolyte wettability were poor, and its rate performance and long-cycle stability were significantly deteriorated.

[0226] In summary, by synergistically controlling the mass ratio of carbon nanotubes and carbon black, the proportion of sulfur doped in carbon nanotubes, the polyimide content in the binder, and the specific surface area of ​​carbon nanotubes, optimal synergy of conductivity, adhesion, and structural stability can be achieved, significantly reducing interfacial resistance and improving peel strength, rate performance, and cycle performance.

[0227] The foregoing description and accompanying drawings fully illustrate embodiments of the present disclosure to enable those skilled in the art to practice them. Other embodiments may include structural and other changes. The embodiments represent only possible variations. Individual components and functions are optional unless explicitly required, and the order of operation may vary. Parts and features of some embodiments may be included or substituted for parts and features of other embodiments. Embodiments of the present disclosure are not limited to the structures described above and shown in the accompanying drawings, and various modifications and changes may be made without departing from its scope. The scope of the present disclosure is limited only by the appended claims.

Claims

1. A positive electrode plate, characterized in that, The device includes a positive current collector and a positive electrode coating applied to at least one surface of the positive current collector. The thickness of the positive current collector is 5 μm to 15 μm, and the thickness of the positive electrode sheet is 65 μm to 90 μm. The positive electrode coating comprises: Positive electrode active materials include lithium nickel cobalt manganese oxide, with the chemical formula LiNi. x Co y Mn z M b O2; wherein, 0.7≤x≤0.95, 0.15≤y<0.45, 0.05≤z<0.45, 0≤b≤0.25, and x+y+z+b=1, the element M includes one or more of Zr, W, Ti, Al, Sr, B and Nd, and the mass percentage of the positive electrode active material in the positive electrode coating is 93%~97%; The positive electrode conductive agent comprises carbon nanotubes and carbon black in a mass ratio of 4:1 to 1:2; wherein the carbon nanotubes are doped with sulfur and have a specific surface area of ​​220 m². 2 / g~320m 2 / g, and the positive electrode conductive agent in the positive electrode coating has a mass percentage content of 1% to 2.5%; The positive electrode binder includes polyvinylidene fluoride and polyimide; wherein the polyimide has a mass percentage content of 10% to 50% in the positive electrode binder, and the positive electrode binder has a mass percentage content of 1.5% to 4% in the positive electrode coating.

2. The positive electrode sheet according to claim 1, characterized in that, The positive electrode conductive agent satisfies one or more of the following conditions: (1) The pore volume of the carbon nanotubes is 0.6 cm³. 3 / g~1.6cm 3 / g; (2) The atomic percentage of sulfur in the carbon nanotubes is 0.3% to 0.7%; (3) The water contact angle of the carbon nanotubes is ≤50°; (4) The intensity ratio of the D peak to the G peak in the Raman spectrum of the carbon nanotube is ID / IG, which is 0.2 to 0.

6.

3. The positive electrode sheet according to claim 1, characterized in that, When the carbon nanotubes are fabricated into a pre-defined circular positive electrode, their electronic conductivity σ ≥ 15 S·cm -1 The area of ​​the preset circular positive electrode is 132.665 mm². 2 The thickness is 0.8mm.

4. The positive electrode sheet according to claim 1, characterized in that, The carbon nanotubes exhibit characteristic absorption peaks in the X-ray photoelectron spectroscopy of 163.5 eV to 164.5 eV, 283.5 eV to 285.8 eV, and 528.5 eV to 535.5 eV.

5. The positive electrode sheet according to claim 1, characterized in that, The polyvinylidene fluoride has a number-average molecular weight of 63.6 g / mol to 190.8 g / mol; the polyimide has a number-average molecular weight of 53 g / mol to 212 g / mol.

6. The positive electrode sheet according to claim 1, characterized in that, A positive electrode slurry is coated on at least one side of the positive electrode current collector to form the positive electrode coating; wherein the viscosity of the positive electrode slurry is 5000 mPa·s to 8000 mPa·s, and the viscosity change rate V of the positive electrode slurry is less than 18% after standing for a preset time.

7. The positive electrode sheet according to claim 1, characterized in that, The positive electrode sheet satisfies one or more of the following conditions: (1) The interface resistance Rα is 1.0 × 10⁻⁶. -3 Ω·cm 2 ~1.0×10 -2 Ω·cm 2 ; (2) Peel strength N is 8.5 N / m to 18.0 N / m; (3) Compacted density D > 3.5 g / cm³ 3 .

8. A method for preparing a positive electrode sheet according to any one of claims 1 to 7, characterized in that, Includes the following steps: MgSO4 is converted into sulfur element through thermal decomposition and / or chemical reaction. The sulfur element reacts with the surface of carbon nanotubes to obtain a matrix powder with sulfur atoms embedded in the tube wall structure of carbon nanotubes. The residual oxides in the matrix powder are removed by hydrochloric acid and washed with deionized water until the pH value is neutral. After filtration and drying, the positive electrode conductive agent is obtained. The positive electrode active material, positive electrode conductive agent and positive electrode binder are stirred in an N-methylpyrrolidone solvent system to form a positive electrode slurry; The positive electrode slurry is coated on at least one side of the positive electrode current collector, and then dried and cold-pressed to obtain a positive electrode sheet.

9. A lithium-ion battery, characterized in that, It includes a negative electrode and a positive electrode as described in any one of claims 1 to 7.

10. The lithium-ion battery according to claim 9, characterized in that, The lithium-ion battery meets one or more of the following conditions: (1) The charging characteristics of the lithium-ion battery are as follows: after a lithium-ion battery with a 100% charge rate is left to stand at 25°C for 6 hours, it is discharged to 2.5V at a discharge rate of 0.1C, and the corresponding discharge capacity is Q1; after discharging to 2.5V at a discharge rate of 10C, the corresponding discharge capacity is Q. 10 Discharge capacity retention rate Q R ≥60%, Q R =Q 10 / Q1; (2) At 25°C, after 600 cycles at a 1C rate, the capacity decay rate Q of the lithium-ion battery is... d <18%.

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