Lithium ion battery conductive agent and preparation method thereof

Abstract

The invention provides a lithium ion battery conductive agent and a preparation method thereof in order to solve the technical problems that in the prior art, transition metal ions of a high-nickel positive electrode material are dissolved out seriously, and existing inhibition measures affect conductivity or cause side reactions due to dissolution and migration in an electrolyte. The conductive agent comprises a carbon nanotube base material subjected to dilute nitric acid oxidation treatment; a polyethyleneimine network grafted on the surface of the carbon nanotube; a polyacrylic acid network is formed by interpenetrating polymerization in the polyethyleneimine network layer, and polyethyleneimine and polyacrylic acid form a space interpenetrating network; and a graphene microchip coupled to the interpenetrating network. According to the technical scheme, a three-dimensional composite structure with a carbon nano tube as a framework, a polyethyleneimine-polyacrylic acid interpenetrating network as a functional layer and a graphene microchip as a conductive reinforcement is constructed, so that the unification of conductivity and a chelating function is realized.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and in particular to a lithium-ion battery conductive agent and its preparation method. Background Technology

[0002] As the energy density requirements of lithium-ion battery technology continue to increase, the application of high-nickel ternary cathode materials (NCM811, NCM90 and above) is becoming increasingly widespread. However, high-nickel materials suffer from severe transition metal ion (Ni²⁺, Mn²⁺, Co²⁺) dissolution during cycling. These dissolved metal ions migrate to the negative electrode surface and deposit, damaging the SEI film structure and leading to increased battery impedance and accelerated capacity decay. Studies have shown that after 500 cycles at 60℃, the Ni deposition on the negative electrode of an NCM811 battery can reach 80-150 ppm, and the capacity retention rate decreases to below 70%.

[0003] To suppress metal ion dissolution, existing technologies mainly employ two strategies: 1. Surface coating of the cathode material: Coating the surface of cathode particles with oxides such as Al2O3 and TiO2. However, this method hinders lithium ion insertion / extraction, leading to a 15-20% decrease in rate performance, and the coating layer is prone to breakage and failure during cycling. 2. Adding transition metal complexes: Adding small molecule complexes such as EDTA and crown ethers to the cathode slurry. However, these complexes have a high solubility of 5-15 g / L in the electrolyte and will migrate to the anode to catalyze side reactions, thus accelerating performance degradation. Existing conductive agents (such as carbon black and carbon nanotubes) only possess conductive properties and do not inhibit metal ion dissolution. Therefore, there is an urgent need for a novel multifunctional conductive agent that can conduct electricity efficiently, stably capture metal ions, and not affect lithium ion transport. Summary of the Invention

[0004] In view of the technical problems of severe dissolution of transition metal ions in high-nickel cathode materials in the prior art, and the fact that existing suppression measures affect conductivity or cause side reactions due to dissolution and migration in the electrolyte, the present invention provides a lithium-ion battery conductive agent, its preparation method and application.

[0005] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: The first aspect of this application provides a conductive agent for lithium-ion batteries, the conductive agent comprising: a carbon nanotube substrate treated with dilute nitric acid oxidation; a polyethyleneimine network grafted onto the surface of the carbon nanotubes; a polyacrylic acid network formed by interpenetrating polymerization in the polyethyleneimine network layer, wherein the polyethyleneimine and polyacrylic acid constitute an interpenetrating network; and graphene microsheets bonded to the interpenetrating network.

[0006] Furthermore, the conductive agent comprises: oxygen-containing functional groups formed on the surface of the carbon nanotube substrate by oxidation treatment with dilute nitric acid; the polyethyleneimine covalently grafted onto the surface of the carbon nanotube via a free radical chain transfer reaction; the polyacrylic acid interpenetrating with the polyethyleneimine via free radical polymerization, and there is no covalent bond between the polyacrylic acid and the carbon nanotube; and the graphene microsheets are used to construct conductive pathways across the interpenetrating network.

[0007] Furthermore, in some embodiments of this application, the carbon nanotube substrate is a multi-walled carbon nanotube with a diameter of 20-30 nm, a length of 10-30 μm, and a specific surface area ≥200 m² / g.

[0008] Furthermore, in some embodiments of this application, the thickness of the interpenetrating network is 5-10 nm, and the mass ratio of polyethyleneimine to polyacrylic acid in the interpenetrating network is 3:1-5:1. The interpenetrating network has a mesoporous structure of 5-10 nm and an amino group density of 6-12 mmol / g, and is used to capture transition metal ions.

[0009] Furthermore, the graphene microsheets are in-situ reduced graphene oxide with a sheet diameter of 3-5 μm and an electrical conductivity of ≥800 S / cm.

[0010] Furthermore, the polyethyleneimine has a weight-average molecular weight of 8,000-12,000, wherein the molar ratio of primary amine to secondary amine is 1:1.5-1:2.0.

[0011] The second aspect of this application provides a method for preparing the conductive agent for lithium-ion batteries, comprising the following steps: (1) dispersing carbon nanotubes in dilute nitric acid for oxidation treatment, and obtaining surface-activated carbon nanotubes after neutralization and washing; (2) dispersing surface-activated carbon nanotubes, polyethyleneimine, and acrylic acid monomer in deionized water, adding persulfate initiator under nitrogen protection, and carrying out free radical polymerization reaction; (3) adding glutaraldehyde crosslinking agent and ethylene glycol diglycidyl ether crosslinking agent, adjusting pH to carry out double crosslinking reaction; (4) adding graphene oxide to the system, dispersing it ultrasonically, and then carrying out in-situ reduction; (5) obtaining the conductive agent by spray drying.

[0012] Further, the steps include: (1) dispersing multi-walled carbon nanotubes in 0.3-0.8M dilute nitric acid, oxidizing them at 50-70℃ for 1-2 hours, and obtaining surface-activated carbon nanotubes after neutralization and washing; (2) dispersing the surface-activated carbon nanotubes, polyethyleneimine, and acrylic monomer in deionized water, adding persulfate initiator under a nitrogen atmosphere, and carrying out free radical polymerization at 65-75℃; wherein the mass ratio of polyethyleneimine to carbon nanotubes is 0.3-0.5:1, and the mass ratio of acrylic monomer to polyethyleneimine is 0.2-0.4:1; the initiator is potassium persulfate or ammonium persulfate, and the amount used is 3-8% of the mass of acrylic monomer; (3) adding glutaraldehyde crosslinking agent and ethylene glycol diglycidyl ether crosslinking agent, adjusting the pH to 8.5-10.0, and carrying out a double crosslinking reaction at 70-85℃ for 1.5-2.5 hours; (4) Add graphene oxide to the system, with a mass ratio of graphene oxide to carbon nanotubes of 0.1-0.3:1. After ultrasonic dispersion, reduce the system in situ with hydrazine hydrate at 85-95℃ for 1-2 hours; (5) Obtain the conductive agent by spray drying.

[0013] Furthermore, the free radical polymerization reaction in step (2) is a stepwise grafting: First step: Polyethyleneimine and carbon nanotubes are pre-reacted at 65-75°C for 1.5-2 hours to form an anchoring layer; Second step: Acrylic acid monomers continue to polymerize in the pores of the anchoring layer for 3-4 hours to form an interpenetrating network.

[0014] Furthermore, in the pre-reaction, the amount of acrylic monomer added is 5-10 wt% of the total amount of acrylic monomer to form an anchoring layer; the remaining 90-95 wt% of acrylic monomer is added in the second step to form an interpenetrating network.

[0015] Furthermore, the double crosslinking reaction in step (3) is as follows: glutaraldehyde is used to crosslink the amino group of polyethyleneimine, with a mass ratio of glutaraldehyde to polyethyleneimine of 0.15-0.25:1; ethylene glycol diglycidyl ether is used to crosslink the carboxyl group of polyacrylic acid, with a mass ratio of ethylene glycol diglycidyl ether to polyacrylic acid of 0.10-0.15:1.

[0016] Furthermore, in step (4), the amount of reducing agent hydrazine hydrate is 8-10 times the mass of graphene oxide, and the electrical conductivity of the reduced graphene microsheets is ≥800 S / cm. The graphene microsheets are uniformly distributed on the carbon nanotube network nodes through electrostatic self-assembly.

[0017] A third aspect of this application provides a positive electrode sheet for a lithium-ion battery, comprising a positive current collector and a positive active material layer coated on the surface of the positive current collector, wherein the positive active material layer comprises a positive active material, a binder and the conductive agent, wherein the amount of the conductive agent added is 1.0-2.0% of the mass of the positive active material.

[0018] The fourth aspect of this application provides a lithium-ion battery comprising the aforementioned positive electrode, negative electrode, separator, and electrolyte, wherein the battery retains ≥85% of its capacity after 500 cycles at 60°C, and the electrode resistivity is 6-8 Ω·cm.

[0019] The beneficial effects of this application are as follows: Addressing the severe transition metal dissolution in existing high-nickel ternary cathode materials and numerous shortcomings of current solutions, this application provides a lithium-ion battery conductive agent that integrates high-efficiency conductivity and ion trapping functionality. First, by constructing a polyethyleneimine / polyacrylic acid (PEI / PAA) interpenetrating network on surface-activated carbon nanotubes and precisely controlling its mesoporous structure (5-10 nm) and amino density (6-12 mmol / g), a dual trapping mechanism of physical confinement and chemical chelation for transition metal ions is achieved. This network is stably fixed on the conductive framework, completely avoiding the dissolution and migration problems of traditional small-molecule complexing agents. Second, the pore size of this interpenetrating network is much larger than that of solvated lithium ions, ensuring efficient lithium ion migration and overcoming the drawback of traditional surface coatings hindering ion transport. Third, by introducing in-situ reduced highly conductive graphene microsheets, a three-dimensional conductive pathway across the polymer network is synergistically constructed with the carbon nanotube substrate. This not only compensates for the potential increase in resistance caused by the polymer layer but also improves the overall conductivity. Therefore, the technical solution of this application effectively suppresses the dissolution of transition metals and stabilizes the electrode / electrolyte interface while ensuring the excellent cycle stability of the battery. Detailed Implementation

[0020] To make the technical problems solved, the technical solutions, and the beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the present invention.

[0021] Technical terminology definition: Carbon nanotube substrate: refers to one-dimensional carbon material that serves as a conductive framework and supporting base. In this application, it specifically refers to carbon nanotubes that have undergone surface oxidation treatment, preferably multi-walled carbon nanotubes.

[0022] Polyethyleneimine network: refers to a two-dimensional or three-dimensional network structure formed by chemical cross-linking or physical entanglement of polyethyleneimine molecular chains, characterized by being rich in amino functional groups.

[0023] Polyacrylic acid network: refers to a two-dimensional or three-dimensional network structure formed by chemical cross-linking or physical entanglement of polyacrylic acid molecular chains, characterized by being rich in carboxyl functional groups.

[0024] Interpenetrating spatial networks (IPNs) refer to composite polymer structures formed by the interpenetration and entanglement of two or more polymer networks, but without covalent bonds connecting the networks. In this application, it specifically refers to an IPN structure formed by a polyethyleneimine network and a polyacrylic acid network.

[0025] Graphene microsheets: refer to two-dimensional sheet-like nanomaterials composed of a single layer or few layers of carbon atoms. In this application, they specifically refer to graphene sheets obtained by in-situ reduction of graphene oxide.

[0026] Currently, with the widespread application of high-nickel ternary cathode materials in lithium-ion batteries, the problem of transition metal ion dissolution during cycling is becoming increasingly prominent. Dissolved Ni²⁺, Mn²⁺, and Co²⁺ ions can penetrate the separator, catalyze electrolyte decomposition on the negative electrode surface, and damage the SEI film, leading to a sharp increase in battery internal resistance and rapid capacity decay. Existing technologies, such as surface coating of the cathode material, sacrifice rate performance, while adding small molecule complexes carries the risk of self-dissolution and migration. Therefore, developing a multifunctional material that can both conduct electricity efficiently and stably capture dissolved metal ions in situ is a key challenge for improving the cycle performance of high-nickel batteries.

[0027] To address the aforementioned issues, this application provides a conductive agent for lithium-ion batteries, its preparation method, and its applications. The conductive agent comprises a carbon nanotube substrate as its core framework, with a polyethyleneimine network covalently grafted onto its surface. A polyacrylic acid network is formed by intercalation and polymerization within the pores of the polyethyleneimine network, together constituting a spatial interpenetrating network structure. This interpenetrating network structure possesses abundant mesopores and a high density of amino groups, enabling selective capture of transition metal ions. Furthermore, graphene microsheets are integrated with this interpenetrating network and traverse different carbon nanotube substrates, constructing highly efficient conductive pathways.

[0028] The first aspect of this application provides a conductive agent for lithium-ion batteries. This conductive agent comprises a carbon nanotube substrate treated with dilute nitric acid oxidation, a polyethyleneimine network grafted onto the surface of the carbon nanotube substrate, a polyacrylic acid network formed by interpenetrating polymerization within the polyethyleneimine network layer, and graphene microsheets bonded to the interpenetrating network. The polyethyleneimine and polyacrylic acid constitute a spatial interpenetrating network. This interpenetrating network has a mesoporous structure of 5-10 nm and an amino group density of 6-12 mmol / g, used to capture transition metal ions. This technical solution achieves a unity of conductivity and chelation function by constructing a three-dimensional composite structure with carbon nanotubes as the framework, a polyethyleneimine-polyacrylic acid (PEI-PAA) interpenetrating network as the functional layer, and graphene microsheets as the conductive reinforcement. Among them, carbon nanotubes provide a one-dimensional high-speed conductive pathway; the PEI-PAA interpenetrating network is rich in high-density amino groups, which can efficiently chelate dissolved transition metal ions; the 5-10nm mesoporous structure ensures the free transport of Li⁺ and avoids the loss of rate performance; and graphene microsheets construct a two-dimensional conductive plane across nodes, ensuring the overall impedance of the electrode.

[0029] In some embodiments of this application, the conductive agent comprises: oxygen-containing functional groups formed on the surface of a carbon nanotube substrate by oxidation treatment with dilute nitric acid; polyethyleneimine covalently grafted onto the surface of the carbon nanotubes via a free radical chain transfer reaction; polyacrylic acid interpenetrating with polyethyleneimine via free radical polymerization, and there is no covalent bond between the polyacrylic acid and the carbon nanotubes; and graphene microsheets used to construct conductive pathways across the interpenetrating network. This scheme ensures structural stability through a defined bonding method (PEI covalently linked to CNTs, PAA interpenetrating with PEI), preventing the functional components from dissolving in the electrolyte. Simultaneously, the graphene microsheets act as "conductive bridges," effectively connecting the carbon nanotubes encapsulated by the polymer network, ensuring overall conductivity.

[0030] In some embodiments of this application, the carbon nanotube substrate is preferably multi-walled carbon nanotubes. These nanotubes have a diameter of 20-30 nm, a length of 10-30 μm, and a specific surface area ≥200 m² / g. The technical basis for selecting this parameter range is that a diameter of 20-30 nm ensures good conductivity while possessing moderate flexibility, making it less prone to breakage during electrode rolling; a length of 10-30 μm provides a sufficiently large aspect ratio, which is beneficial for forming an effective conductive network at low content; and a specific surface area of ​​not less than 200 m² / g ensures sufficient surface sites for subsequent oxidation treatment and polymer grafting. Exemplarily, the diameter of the carbon nanotube substrate 10 can be 20 nm, 22 nm, 25 nm, 28 nm, or 30 nm; and the length can be 10 μm, 15 μm, 20 μm, 25 μm, or 30 μm.

[0031] In some embodiments of this application, the thickness of the interpenetrating network is 5-10 nm, and the mass ratio of polyethyleneimine to polyacrylic acid is 3:1-5:1. Controlling the thickness of the interpenetrating network to 5-10 nm is a key balance point. This thickness is sufficient to provide abundant amino functional groups (density 6-12 mmol / g) for efficient metal ion capture, while its mesopore size (5-10 nm) is much larger than the solvation sheath size of lithium ions, thus not significantly hindering lithium ion insertion / extraction. Exemplarily, the thickness of the interpenetrating network can be 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, or 10 nm; the amino group density can be 6 mmol / g, 8 mmol / g, 10 mmol / g, or 12 mmol / g. The mass ratio of polyethyleneimine to polyacrylic acid is a core parameter determining the network performance. Polyethyleneimine provides the main amino group capture sites, while polyacrylic acid mainly serves as the network framework and stabilizer. If the mass ratio is lower than 3:1 (e.g., 2:1), the amino density is insufficient, and the metal ion capturing ability decreases; if it is higher than 5:1 (e.g., 6:1), the polymer chains in the network structure are not sufficiently entangled, the mechanical stability deteriorates, and excessive PEI may block ion channels due to swelling. For example, the mass ratio of polyethyleneimine to polyacrylic acid can be 3:1, 3.5:1, 4:1, 4.5:1, or 5:1.

[0032] In some embodiments of this application, the weight-average molecular weight of the polyethyleneimine is 8,000-12,000, wherein the molar ratio of primary amine to secondary amine is 1:1.5-1:2.0. This molecular weight range is chosen because if the molecular weight is below 8,000, the PEI chains are too short to form an effective network structure; if it is above 12,000, the solution viscosity is too high, which is not conducive to uniform grafting and reaction on the carbon nanotube surface. Secondary amines generally have stronger coordination ability than primary amines and less steric hindrance. Controlling the molar ratio of primary amine to secondary amine at 1:1.5-1:2.0 can optimize the overall coordination microenvironment and improve the capture efficiency and selectivity of transition metal ions. Exemplarily, the weight-average molecular weight of polyethyleneimine can be 8,000, 9,000, 10,000, 11,000, or 12,000; the molar ratio of primary amine to secondary amine can be 1:1.5, 1:1.7, 1:1.9, or 1:2.0.

[0033] In some embodiments of this application, the graphene microsheets are in-situ reduced graphene oxide with a diameter of 3-5 μm and an electrical conductivity ≥800 S / cm. The in-situ reduction method allows for the retention of a small number of oxygen-containing functional groups (such as hydroxyl and epoxy groups) on the graphene surface. These functional groups can effectively bond with the PEI / PAA interpenetrating network through hydrogen bonding or electrostatic interactions, achieving uniform composite formation. The 3-5 μm diameter of the graphene microsheets is sufficient to effectively bridge adjacent carbon nanotube substrates, reducing contact resistance. An electrical conductivity of at least 800 S / cm is a prerequisite for its function as a highly efficient conductive "bridge." Exemplarily, the diameter of the graphene microsheets can be 3 μm, 3.5 μm, 4 μm, 4.5 μm, or 5 μm; the electrical conductivity can be 800 S / cm, 900 S / cm, 1000 S / cm, or 1100 S / cm.

[0034] The second aspect of this application provides a method for preparing a conductive agent for lithium-ion batteries, comprising the following steps: Step S101: The carbon nanotube substrate is dispersed in dilute nitric acid for oxidation treatment, followed by neutralization and washing to obtain surface-activated carbon nanotubes. Preferably, multi-walled carbon nanotubes are dispersed in 0.3-0.8M dilute nitric acid and oxidized at 50-70℃ for 1-2 hours. The dilute nitric acid oxidation aims to introduce oxygen-containing functional groups such as carboxyl groups (-COOH) and hydroxyl groups (-OH) onto the carbon nanotube surface. These functional groups are the active sites for subsequent grafting of polyethyleneimine. The nitric acid concentration is a key parameter: below 0.3M, oxidation is insufficient, resulting in low functional group density; above 0.8M, oxidation is too strong, potentially severing the carbon nanotube structure and damaging its conductivity. The reaction temperature and time also need precise control; the combination of 50-70℃ and 1-2 hours achieves the optimal balance between introducing sufficient functional groups and maintaining structural integrity. For example, the concentration of dilute nitric acid can be 0.3M, 0.5M or 0.8M; the reaction temperature can be 50℃, 60℃ or 70℃; and the reaction time can be 1h, 1.5h or 2h.

[0035] Step S102: Surface-activated carbon nanotubes, polyethyleneimine, and acrylic monomers are dispersed in deionized water. Under nitrogen protection, a persulfate initiator is added to initiate a free radical polymerization reaction. Preferably, the reaction is carried out at 65-75°C under a nitrogen atmosphere. The mass ratio of polyethyleneimine to carbon nanotubes is 0.3-0.5:1, and the mass ratio of acrylic monomers to polyethyleneimine is 0.2-0.4:1. The initiator is potassium persulfate (KPS) or ammonium persulfate (APS), and its dosage is 3-8% of the mass of the acrylic monomers. This step is crucial for constructing the interpenetrating network. The mass ratio of PEI to CNTs determines the grafting density of PEI on the CNT surface; a range of 0.3-0.5:1 ensures effective coverage while avoiding over-coating. The mass ratio of AA to PEI directly affects the composition and pore structure of the interpenetrating network; an AA dosage of 0.2-0.4:1 can create effective interpenetration and support between PEI chains. 65-75℃ is the temperature window for the efficient decomposition of persulfate initiators to generate free radicals. For example, the mass ratio of PEI to CNT can be 0.3:1, 0.4:1, or 0.5:1; the mass ratio of AA to PEI can be 0.2:1, 0.3:1, or 0.4:1; and the amount of initiator can be 3%, 5%, or 8% of the mass of acrylic acid monomer.

[0036] In some preferred embodiments, the free radical polymerization reaction is a stepwise grafting process: First, polyethyleneimine (PEI) and carbon nanotubes (CNTs) are pre-reacted at 65-75°C for 1.5-2 hours to form an anchoring layer; second, acrylic monomers continue to polymerize in the pores of the anchoring layer for 3-4 hours to form an interpenetrating network. More preferably, 5-10 wt% of the total acrylic monomers are added in the pre-reaction, with the remaining 90-95 wt% added in the second step. In the first step, acrylic acid acts as a "molecular glue," promoting the uniform anchoring of PEI on the CNT surface via free radical chain transfer, forming a primary network with a thickness of approximately 5-8 nm. In the second step, most of the AA monomers are added, and bulk polymerization occurs under the action of an initiator. The grown PAA chains interpenetrate and entangle with the anchored PEI chains, forming a stable interpenetrating network. This stepwise method avoids the problem that homogeneous copolymerization of PEI and PAA cannot effectively graft onto the CNT surface.

[0037] Step S103: Add glutaraldehyde crosslinking agent and ethylene glycol diglycidyl ether crosslinking agent, and adjust the pH to carry out a double crosslinking reaction. Preferably, adjust the pH to 8.5-10.0 and react at 70-85℃ for 1.5-2.5 hours. Glutaraldehyde is used to crosslink the amino groups of polyethyleneimine, with a mass ratio of glutaraldehyde to polyethyleneimine of 0.15-0.25:1; ethylene glycol diglycidyl ether (EGDGE) is used to crosslink the carboxyl groups of polyacrylic acid, with a mass ratio of EGDGE to polyacrylic acid of 0.10-0.15:1. Double crosslinking is key to improving the stability of the interpenetrating polymer network. The two aldehyde groups of glutaraldehyde can react with the two amino groups on the PEI chain to form a Schiff base (-C=N-), achieving chemical crosslinking between PEI chains. The two epoxy groups of EGDGE can undergo ring-opening esterification with the carboxyl groups on the PAA chain under alkaline conditions, achieving chemical crosslinking between PAA chains. The pH environment of 8.5-10.0 is favorable for the nucleophilic attack of the amino groups on the aldehyde groups and the nucleophilic attack of the carboxyl groups on the epoxy groups. Precise crosslinking agent dosage can control crosslinking density, preventing the network from becoming too dense and affecting ion transport. For example, the pH can be 8.5, 9.0, 9.5, or 10.0; the reaction temperature can be 70°C, 75°C, 80°C, or 85°C; and the reaction time can be 1.5h, 2h, or 2.5h.

[0038] Step S104: Add graphene oxide to the system, disperse it ultrasonically, and then perform in-situ reduction. Preferably, the mass ratio of graphene oxide to carbon nanotubes is 0.1-0.3:1, and the reduction is carried out in-situ at 85-95℃ with hydrazine hydrate for 1-2 hours. The amount of hydrazine hydrate used as the reducing agent is 8-10 times the mass of graphene oxide. The amount of graphene added can effectively improve conductivity without significantly affecting the cost. Hydrazine hydrate is a strong reducing agent, and an amount of 8-10 times ensures that graphene oxide is fully reduced to conductive graphene. The surface of the reduced graphene microflakes is negatively charged, while the PEI / PAA interpenetrating network is positively charged at pH 7-8. Through electrostatic self-assembly, the graphene microflakes are uniformly adsorbed and anchored on the surface of the interpenetrating network. For example, the reduction temperature can be 85℃, 90℃, or 95℃; the reduction time can be 1h, 1.5h, or 2h.

[0039] Step S105: The conductive agent is obtained by spray drying. Spray drying is a rapid and continuous powder preparation technology that can obtain conductive agent powder with uniform particle size and good dispersibility, which is convenient for subsequent dispersion in positive electrode slurry.

[0040] Examples of the applicable scope and types of raw materials, main materials, and auxiliary materials involved in the preparation process: Carbon nanotube substrate: may include, but is not limited to, multi-walled carbon nanotubes, such as products with a diameter of 20-30 nm and a length of 10-30 μm prepared by CVD.

[0041] Polyethyleneimine: This may include, but is not limited to, branched polyethyleneimine with a weight-average molecular weight in the range of 8,000-12,000, such as PEI Mw 10,000 produced by Sigma-Aldrich.

[0042] Acrylic acid monomers: may include, but are not limited to, analytical grade acrylic acid, which must be subjected to vacuum distillation to remove polymerization inhibitors before use.

[0043] Initiators may include, but are not limited to, potassium persulfate (KPS) and ammonium persulfate (APS).

[0044] Crosslinking agent: may include, but is not limited to, 25% glutaraldehyde aqueous solution.

[0045] Graphene oxide: This may include, but is not limited to, aqueous solutions of graphene oxide prepared by the Hummers method.

[0046] Reducing agent: may include, but is not limited to, hydrazine hydrate (80% aqueous solution).

[0047] pH adjusters may include, but are not limited to, ammonia water and sodium hydroxide solution.

[0048] A third aspect of this application provides a positive electrode sheet for a lithium-ion battery, comprising a positive current collector and a positive active material layer coated on the surface of the positive current collector. The positive active material layer comprises a positive active material, a binder, and a conductive agent as described in this application. The amount of conductive agent added is 1.0-2.0% of the mass of the positive active material. This range is the result of extensive experimental optimization. 1.0% is the threshold for constructing an effective conductive network and achieving ion trapping, while 2.0% is the upper limit for ensuring high energy density. Exemplarily, the amount of conductive agent added can be 1.0%, 1.2%, 1.5%, 1.8%, or 2.0%. The positive active material may include, but is not limited to, LiNi. 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.9 Co 0.05 Mn 0.05 High-nickel ternary materials such as O2 (NCM90). Adhesives may include, but are not limited to, polyvinylidene fluoride (PVDF) and styrene-butadiene rubber (SBR).

[0049] A fourth aspect of this application provides a lithium-ion battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte. The negative electrode may include, but is not limited to, a graphite negative electrode or a silicon-carbon negative electrode. The separator may include, but is not limited to, a polyethylene (PE) membrane, a polypropylene (PP) membrane, or a ceramic-coated separator. The electrolyte comprises a lithium salt, a non-aqueous organic solvent, and additives. The lithium salt may be, for example, LiPF6, and the solvent may be, for example, a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC). Due to the dual function of the conductive agent in this application, the battery exhibits a capacity retention of ≥85% after 1000 cycles at 60°C, and an electrode resistivity of 6-8 Ω·cm, demonstrating excellent high-temperature cycle stability and low impedance characteristics.

[0050] Example The technical solutions and effects of this application will be described in detail below through specific embodiments. It should be noted that the following embodiments are only used to illustrate this application and are not intended to limit its scope of protection.

[0051] Example 1 (Basic Scheme) A method for preparing a conductive agent for lithium-ion batteries includes: (1) dispersing 10g of multi-walled carbon nanotubes (Tiannai Technology, diameter 25nm, length 20μm, specific surface area 220 m² / g) in 500mL of 0.5M dilute nitric acid and magnetically stirring for oxidative treatment at 60℃ for 1.5 hours. After the reaction, centrifuging and washing with deionized water until neutral, then washing twice with ethanol, and vacuum drying at 80℃ for 12 hours to obtain surface-activated carbon nanotubes. (2) adding 5g of surface-activated carbon nanotubes, 2g of polyethyleneimine (Aladdin, weight average molecular weight 10000, primary amine: secondary amine = 1:1.8), and 0.6g of acrylic acid monomer into 200mL of deionized water at once, and ultrasonically dispersing for 30 minutes. Nitrogen gas is introduced for 30 minutes to remove oxygen. The temperature is raised to 70℃, and 0.03g of potassium persulfate (KPS) is added to initiate polymerization, and the reaction is carried out for 4 hours. (3) Maintain the temperature at 70℃, add 0.3g glutaraldehyde and 0.06g ethylene glycol diglycidyl ether, adjust the pH to 9.5 with 1M NaOH solution, and continue the reaction for 2 hours. (4) Add 1.5g graphene oxide (SixthElement, sheet diameter 3-5μm) to the system and ultrasonically disperse for 30 minutes. Raise the temperature to 90℃, add 0.4g hydrazine hydrate (8 times the mass of GO), and react for 1.5 hours. (5) Spray dry the reaction product at an inlet temperature of 180℃ and an outlet temperature of 90℃ to obtain the final conductive agent powder.

[0052] Example 2 (high PEI / AA ratio) is basically the same as Example 1, except that: in step (2), the amount of polyethyleneimine is 2.5g and the amount of acrylic monomer is 0.75g, that is, the mass ratio of PEI:CNT is 0.5:1 and the mass ratio of AA:PEI is 0.3:1.

[0053] Example 3 (high graphene content) is basically the same as Example 1, except that: in step (4), the amount of graphene oxide added is 2.8g and the amount of hydrazine hydrate is 25g.

[0054] Example 4 (Step-by-Step Grafting Optimization Process) is basically the same as Example 1, except that: Step (2) involves step-by-step grafting: Step 1: Disperse 5g of surface-activated carbon nanotubes and 2g of polyethyleneimine in 200mL of deionized water, add 0.06g of acrylic acid monomer (10wt% of total acrylic acid), and ultrasonically disperse for 30 minutes. Purge with nitrogen for 30 minutes to remove oxygen. Heat to 70℃, add 0.03g of potassium persulfate (KPS) to initiate polymerization, and react for 1.5 hours to form a PEI anchoring layer.

[0055] Step 2: Add the remaining 0.54g of acrylic acid monomer to the reaction system and continue the reaction for 3 hours to allow the acrylic acid to polymerize in the pores of the anchoring layer and form an interpenetrating network.

[0056] Comparative Example 1 Commercially available Super P conductive carbon black is used.

[0057] Comparative Example 2 (PEI grafted only, without graphene and interpenetrating network) It is basically the same as Example 1, except that: Without adding acrylic monomers and subsequent polymerization in step (2), only PEI-grafted carbon nanotubes are prepared, without forming an interpenetrating network.

[0058] Positive electrode preparation and battery assembly The positive electrode active material NCM811, conductive agent (Examples 1-3 and Comparative Examples 1 and 2), and binder PVDF were mixed uniformly in NMP solvent at a mass ratio of 96:2:2 (based on 2 parts of conductive agent). The mixture was coated onto aluminum foil, and then dried, rolled, and stamped to obtain the positive electrode sheet. A CR2032 type button cell was assembled in an argon-filled glove box using graphite as the negative electrode, Celgard 2400 as the separator, and 1M LiPF6 in EC / DEC / EMC (1:1:1 vol%) as the electrolyte.

[0059] Performance testing methods 1. Metal ion adsorption capacity test: Add 50 mg of conductive agent powder to 50 mL of solution containing 100 ppm Ni²⁺, and shake at 25 °C for 24 hours for adsorption. Measure the Ni²⁺ concentration in the solution before and after adsorption using ICP-OES, and calculate the adsorption capacity.

[0060] 2. Powder conductivity test: The conductive agent powder was pressed into a disc with a diameter of 13 mm under a pressure of 10 MPa, and its conductivity was tested using a four-probe tester.

[0061] 3. Battery cycle performance test: In a 60℃ constant temperature chamber, the assembled button batteries were charged and discharged at a 1C rate in the voltage range of 2.8-4.3V. The discharge capacity of the first and 500th cycles was recorded, and the capacity retention rate was calculated.

[0062] Test Results The table below summarizes the performance test data for each embodiment and comparative example.

[0063] Results Analysis The data in the table above shows that: The conductive agents in Examples 1-4 of this application all showed high adsorption capacity for Ni²⁺ (38-44 mg / g), which was much higher than that in Comparative Example 1 (Super P, 5 mg / g), demonstrating that the PEI / PAA interpenetrating network has superior capture ability for metal ions.

[0064] The powder conductivity (165-210 S / cm) of Examples 1-4 of this application is significantly higher than that of Comparative Examples 1-2, indicating that the introduction of graphene microsheets effectively compensates for the negative impact of the polymer network on conductivity, achieving high conductivity. Example 3 has the highest conductivity due to its higher graphene content.

[0065] Regarding the critical battery cycle performance, the batteries using the conductive agent of this application (Examples 1-4) maintained a capacity retention rate of over 90% after 500 cycles at 60°C. This fully demonstrates the synergistic effect of the conductive agent of this application, effectively suppressing the capacity decay of high-nickel batteries.

[0066] Example 4 employs a stepwise grafting process, and its Ni²⁺ adsorption capacity (41 mg / g) and retention rate after 500 cycles (93.5%) are both superior to those of Examples 1-3, demonstrating that stepwise grafting enables more sufficient PEI anchoring and a more regular interpenetrating network structure, thereby improving capture efficiency and structural stability.

[0067] Comparative Example 1 (Super P) has no capture capability and the worst battery cycle performance.

[0068] Although Comparative Example 2 (PEI only) had a high adsorption capacity, it had poor conductivity due to the lack of PAA network stability and graphene conductive bridging, and the cycle capacity retention was also affected, resulting in poor battery performance.

[0069] The above description is merely a preferred embodiment of this application and is not intended to limit the scope of protection of this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A conductive agent for lithium-ion batteries, characterized in that, The conductive agent includes: Carbon nanotube substrate treated with dilute nitric acid oxidation; Polyethyleneimine network grafted onto the surface of carbon nanotubes; A polyacrylic acid network, polymerized and interspersed within the polyethyleneimine network layer, The polyethyleneimine and polyacrylic acid form a spatial interpenetrating network; And graphene microsheets combined with the interpenetrating network.

2. The conductive agent according to claim 1, characterized in that, The conductive agent includes: The carbon nanotube substrate surface is oxidized with dilute nitric acid to form oxygen-containing functional groups; the polyethyleneimine is covalently grafted onto the carbon nanotube surface via a free radical chain transfer reaction; the polyacrylic acid interpenetrates with the polyethyleneimine via free radical polymerization, and there is no covalent bond between the polyacrylic acid and the carbon nanotube; the graphene microsheets are used to construct conductive pathways across the interpenetrating network.

3. The conductive agent according to claim 1, characterized in that, The carbon nanotube substrate is a multi-walled carbon nanotube with a diameter of 20-30 nm, a length of 10-30 μm, and a specific surface area of ​​≥200 m² / g.

4. The conductive agent according to claim 1, characterized in that, The interpenetrating network has a thickness of 5-10 nm, and the mass ratio of polyethyleneimine to polyacrylic acid in the interpenetrating network is 3:1-5:1; the interpenetrating network has a mesoporous structure of 5-10 nm and an amino density of 6-12 mmol / g.

5. The conductive agent according to claim 1, characterized in that, The graphene microsheets are in-situ reduced graphene oxide with a sheet diameter of 3-5 μm and an electrical conductivity ≥800 S / cm; the graphene content is 5.0-15.0 wt% of the carbon nanotube content.

6. The conductive agent according to claim 1, characterized in that, The polyethyleneimine has a weight-average molecular weight of 8,000-12,000, wherein the molar ratio of primary amine to secondary amine is 1:1.5-1:2.

0.

7. A method for preparing the lithium-ion battery conductive agent according to any one of claims 1-6, characterized in that, The process includes the following steps: (1) Dispersing carbon nanotubes in dilute nitric acid for oxidation treatment, followed by neutralization and washing to obtain surface-activated carbon nanotubes; (2) Dispersing surface-activated carbon nanotubes, polyethyleneimine, and acrylic monomer in deionized water, adding persulfate initiator under nitrogen protection, and carrying out free radical polymerization; (3) Adding glutaraldehyde crosslinking agent and ethylene glycol diglycidyl ether crosslinking agent, adjusting pH to carry out double crosslinking reaction; (4) Adding graphene oxide to the system, ultrasonically dispersing, and then carrying out in-situ reduction; (5) Obtaining the conductive agent by spray drying.

8. The preparation method according to claim 7, characterized in that, The steps include: (1) Disperse multi-walled carbon nanotubes in 0.3-0.8M dilute nitric acid, oxidize them at 50-70℃ for 1-2 hours, and obtain surface-activated carbon nanotubes after neutralization and washing. (2) Surface-activated carbon nanotubes, polyethyleneimine, and acrylic monomers are dispersed in deionized water. A persulfate initiator is added under a nitrogen atmosphere, and a free radical polymerization reaction is carried out at 65-75℃. The mass ratio of polyethyleneimine to carbon nanotubes is 0.3-0.5:1, and the mass ratio of acrylic monomers to polyethyleneimine is 0.2-0.4:

1. The initiator is potassium persulfate or ammonium persulfate, and its dosage is 3-8% of the mass of the acrylic monomers. (3) Add glutaraldehyde crosslinking agent and ethylene glycol diglycidyl ether crosslinking agent, adjust the pH to 8.5-10.0, and carry out the double crosslinking reaction at 70-85℃ for 1.5-2.5 hours; (4) Add graphene oxide to the system, wherein the mass ratio of graphene oxide to carbon nanotubes is 0.1-0.3:

1. After ultrasonic dispersion, reduce it in situ with hydrazine hydrate at 85-95℃ for 1-2 hours. (5) The conductive agent is obtained by spray drying.

9. The preparation method according to claim 8, characterized in that, The free radical polymerization reaction in step (2) is a stepwise grafting reaction: Step 1: Polyethyleneimine and carbon nanotubes are pre-reacted at 65-75℃ for 1.5-2 hours; Step 2: The acrylic monomer continues to polymerize in the pores of the anchoring layer for 3-4 hours.

10. The preparation method according to claim 9, characterized in that, The amount of acrylic monomer added in the pre-reaction is 5-10 wt% of the total amount of acrylic monomer; the remaining 90-95 wt% of acrylic monomer is added in the second step.

11. The preparation method according to claim 7 or 8, characterized in that, The double crosslinking reaction in step (3) is as follows: The amino group of polyethyleneimine is crosslinked with glutaraldehyde, and the mass ratio of glutaraldehyde to polyethyleneimine is 0.15-0.25:

1. The carboxyl groups of polyacrylic acid are crosslinked using ethylene glycol diglycidyl ether, with a mass ratio of ethylene glycol diglycidyl ether to polyacrylic acid of 0.10-0.15:1.

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