Composite conductive agent, method for preparing the same, and use thereof

Abstract

The application provides a composite conductive agent and a preparation method and application thereof, comprising a carbon matrix and a coating layer coated on the surface of the carbon matrix, wherein the coating layer comprises an amidoxime group-containing polymer. The composite conductive agent can not only increase the conductivity between the positive electrode material and the current collector and between the positive electrode material particles, but also the coating layer of the composite conductive agent is the amidoxime group-containing polymer, the polymer has strong chelation capacity to transition metal ions, so that the risk of transition metal element dissolution can be greatly reduced, and the battery has good cycle performance, rate performance and safety performance.

Description

Technical Field

[0001] This application relates to the field of secondary battery technology, specifically to a composite conductive agent, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries are widely used in portable communication and electronic products such as mobile phones and laptops due to their advantages such as high specific energy, high operating voltage, light weight, low self-discharge, long cycle life, stable discharge performance, no memory effect, and low environmental pollution.

[0003] Transition metal elements in existing lithium secondary battery cathode materials such as lithium manganese oxide, lithium nickel manganese oxide, and lithium cobalt oxide are easily dissolved during battery cycling. This leads to side reactions in the electrolyte under the catalysis of transition metal ions, resulting in gas production. Transition metal ions can also accumulate in the separator or negative electrode and be reduced to metal particles, hindering the transport of lithium ions. This results in increased internal impedance, increased battery polarization, and a sharp decline in battery capacity, which is detrimental to the battery's cycle performance, rate performance, and safety performance. Summary of the Invention

[0004] In view of this, this application provides a composite conductive agent, wherein the coating layer of the composite conductive agent is a polymer containing a amine oxime group. This polymer has a strong chelating ability for transition metal ions, thereby greatly reducing the risk of transition metal element dissolution and enabling the battery to have high capacity and good cycle performance, rate performance and safety performance.

[0005] The first aspect of this application provides a composite conductive agent comprising a carbon matrix and a coating layer disposed on the surface of the carbon matrix, the coating layer comprising a polymer containing a amine oxime group.

[0006] In the composite conductive agent of this application, the carbon matrix is ​​encapsulated in a polymer containing a metallo-oxime groups. On the one hand, the polymer coating layer makes the composite conductive agent easy to wet with electrolyte and increases the specific surface area of ​​the composite conductive agent, giving the conductive agent a higher electron transport rate, thereby promoting the capacity of the cathode material and improving the rate performance of the battery. On the other hand, the metallo-oxime groups have a strong chelating ability for transition metal ions, which can rapidly complex the transition metal ions generated by the decomposition of the cathode material and inhibit the diffusion of transition metal ions in the electrolyte, thereby improving the cycle performance, rate performance and safety performance of the battery.

[0007] Optionally, the oximation rate of the polymer containing the amylopyridine oxime group is 50% to 60%.

[0008] Optionally, the carbon matrix includes one or more of one-dimensional carbon materials, two-dimensional carbon materials, and particulate carbon materials.

[0009] Optionally, the radial dimension of the one-dimensional carbon material is 2nm-30nm, and the length of the one-dimensional carbon material is 1μm-20μm.

[0010] Optionally, the lateral dimension of the two-dimensional carbon material is 0.5 μm to 3 μm, and the thickness of the two-dimensional carbon material is 1 nm to 10 nm.

[0011] Optionally, the particle size of the particulate carbon material is 0.5 μm to 2 μm.

[0012] Optionally, the average thickness of the coating layer is 80 nm to 90 nm.

[0013] Optionally, the coating layer has a porous structure with an average pore size of 2nm-50nm.

[0014] Optionally, the mass ratio of the polymer containing the amylopyridine group to the carbon matrix is ​​(30-60):1.

[0015] Optionally, the surface of the carbon matrix has oxygen-containing groups, which form chemical bonds with the polymer containing a amine oxime group.

[0016] Optionally, the conductive agent includes one or more of carbon black, graphite, carbon nanotubes, carbon nanofibers, C60, graphene, and graphene oxide.

[0017] Optionally, the polymer containing a methylamine oxime group has an electrical conductivity of 3 S / m to 3.6 S / m.

[0018] Optionally, the molecular weight of the polymer containing the amine oxime group is 50,000 to 150,000.

[0019] Optionally, the polymer containing a metallo-oxime group includes polyacrylonitrile containing a metallo-oxime group.

[0020] Optionally, the specific surface area of ​​the composite conductive agent is greater than or equal to 300 m². 2 / g.

[0021] In a second aspect, a method for preparing a composite conductive agent includes: reacting a carbon matrix with a polymer monomer and an initiator to obtain a pre-coated carbon matrix, wherein the polymer monomer contains a cyano group;

[0022] The pre-coated carbon matrix is ​​reacted with an oxime agent to obtain a composite conductive agent; the oxime agent is a compound having hydroxyl and amino groups; it includes a carbon matrix and a coating layer disposed on the surface of the carbon matrix, the coating layer comprising a polymer containing a amine oxime group.

[0023] Optionally, the mass ratio of the oxime agent to the pre-coated carbon matrix is ​​1:(5-20).

[0024] Optionally, the oxime agent includes one or more of hydroxylamine, hydroxylamine hydrochloride, hydroxyurea sulfate, and 2-amino-3-hydroxypyridine.

[0025] Optionally, the mass ratio of the carbon matrix to the polymer monomer is 1:(30-60).

[0026] Optionally, the step of reacting the pre-coated carbon matrix with an oxime agent to obtain a composite conductive agent includes: placing the pre-coated carbon matrix in a solution containing an oxime agent, adding an alkaline substance, and mixing to obtain a composite conductive agent.

[0027] Thirdly, this application provides an electrode sheet, the electrode sheet comprising a current collector and an electrode material layer disposed on the current collector, the electrode material layer comprising a composite conductive agent as described in the first aspect.

[0028] Fourthly, this application provides a secondary battery comprising a composite conductive agent as described in the third aspect of this application.

[0029] Fifthly, this application provides an electronic device comprising a secondary battery as described in the fourth aspect. Attached Figure Description

[0030] Figure 1 A schematic diagram illustrating the reaction between a polymer containing a amine oxime group and transition metal ions, provided in an embodiment of this application.

[0031] Figure 2 This is a schematic diagram of the structure of a composite conductive agent provided in an embodiment of this application;

[0032] Figure 3 This is a schematic diagram of the structure of a composite conductive agent provided in an embodiment of this application;

[0033] Figure 4 This is a schematic diagram of the structure of a composite conductive agent provided in an embodiment of this application;

[0034] Figure 5 A schematic diagram of a polymerization reaction provided in an embodiment of this application;

[0035] Figure 6 This is a mapping diagram of the full cell of Embodiment 1 of this application after 100 cycles;

[0036] Figure 7 For the reason Figure 6 The elemental composition diagram of the negative electrode surface of the full cell of Example 1 after 100 cycles;

[0037] Figure 8 The mapping diagram of the full cell of Comparative Example 1 of this application after 100 cycles;

[0038] Figure 9 For the reason Figure 8 The resulting diagram shows the elemental composition of the negative electrode surface of the full cell in Comparative Example 1 after 100 cycles. Detailed Implementation

[0039] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0040] The positive electrode active material is a crucial component of the battery cathode, possessing specific sites for lithium ion insertion and extraction. Currently, widely used positive electrode active materials mainly include lithium iron phosphate (olivine structure), lithium manganese oxide, lithium nickel manganese oxide (spinel structure), and lithium cobalt oxide (layered structure). However, under actual charge-discharge environments, especially high-temperature conditions, these materials exhibit transition metal leaching. Free transition metals catalyze the decomposition of the electrolyte solvent, and the leached metals are reduced and continuously accumulate on the separator or negative electrode surface, even forming metal particles. This hinders lithium-ion transport, leading to increased internal impedance, increased battery polarization, and a sharp decline in battery capacity. Furthermore, transition metals can damage the SEI film, causing continuous loss of active lithium and a decrease in battery energy density. To address this, this application provides a composite conductive agent. The polymer coating layer in this composite conductive agent can inhibit the diffusion of transition metal ions in the electrolyte, improving electrode conductivity while reducing the risk of transition metal leaching, ensuring good cycle performance, rate performance, and safety performance of the battery.

[0041] In this application, the composite conductive agent includes a carbon matrix and a coating layer on the surface of the carbon matrix. The coating layer includes a polymer containing a metallo-oxime group. The polymer containing a metallo-oxime group refers to a polymer containing the functional group metallo-oxime. In some embodiments of this application, the structural formula of the polymer containing a metallo-oxime group is shown in formula (I):

[0042]

[0043] In formula (I), R1 and R2 are independently selected from substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, substituted or unsubstituted cycloalkyl, and substituted or unsubstituted aryl groups. Modifying the polymer in the skin layer with a metallo-oxime group (—C(NOH)NH2) in this application can enable the composite conductive agent to complex transition metal ions. Please refer to [link to relevant documentation]. Figure 1 , Figure 1This is a schematic diagram illustrating the reaction principle of a polymer containing a metallo-oxime group with transition metal ions according to an embodiment of this application. The metallo-oxime group has a chelating effect and can react with transition metal ions to form a complex, thereby inhibiting the release of transition metal ions into the electrolyte.

[0044] In this application, the amylopyramid group in the polymer is obtained by oximation treatment of the cyano group in the raw polymer. The oximation rate refers to the conversion rate of the cyano group to the amylopyramid group, that is, the molar ratio of the amylopyramid group in the polymer after the reaction to the cyano group in the polymer before the reaction. In the embodiments of this application, the oximation rate of the polymer containing amylopyramid groups is 50% to 60%, and the specific oximation rate of the polymer containing amylopyramid groups can be, but is not limited to, 50%, 52%, 54%, 55%, 57%, 59%, or 60%. When the content of amylopyramid groups in the polymer is controlled at 50% to 60%, the composite conductive agent can fully complex the transition metal elements dissolved after the phase change of the positive electrode active material during the charging and discharging process, inhibit the transfer of transition metal elements to the negative electrode side, and ensure that the cell has a high capacity; moreover, the polymer containing amylopyramid groups has high conductivity, which is beneficial to the decomposition of the composite conductive agent. In some embodiments of this application, the polymer containing a methylamine oxime group adsorbs metal elements at a rate greater than or equal to 80 ppm.

[0045] In some embodiments of this application, the polymer containing a metallo-oxime group includes polyacrylonitrile containing a metallo-oxime group. Polyacrylonitrile has a stable long-chain structure and good conductivity. When used as a coating layer, the metallo-oxime-containing polyacrylonitrile can act as an electron transfer medium to promote electron transfer between the electrolyte and the carbon matrix, thereby ensuring that the carbon matrix can decompose rapidly and fully to provide active lithium. Moreover, polyacrylonitrile contains cyano groups, so the cyano groups can be directly converted into metallo-oxime groups through oximation treatment, making the preparation method simpler. In addition, polyacrylonitrile is abundant, inexpensive, and environmentally friendly, not only having the advantages of environmental friendliness but also achieving uniform coating of the carbon matrix surface at a lower cost. In some embodiments of this application, the average molecular weight of the metallo-oxime-containing polyacrylonitrile is 50,000 to 150,000. The average molecular weight of the metallo-oxime-containing polyacrylonitrile can specifically be, but is not limited to, 50,000, 80,000, 100,000, 120,000, or 150,000. In some embodiments of this application, the polymer containing a metallo-oxime group has an electrical conductivity of 3 S / m to 3.6 S / m. Specifically, but not limited to, the electrical conductivity of the polymer containing a metallo-oxime group may be 3 S / m, 3.2 S / m, 3.3 S / m, 3.5 S / m, or 3.6 S / m. The higher electrical conductivity of the polymer containing a metallo-oxime group in the coating layer is beneficial for promoting the decomposition of the carbon matrix.

[0046] In some embodiments of this application, the carbon matrix includes one or more of one-dimensional carbon materials, two-dimensional carbon materials, and particulate carbon materials. In some embodiments, the radial dimension of the one-dimensional carbon material is 2nm-30nm, and the length of the one-dimensional carbon material is 1μm-20μm; the lateral dimension of the two-dimensional carbon material is 0.5μm-3μm, and the thickness of the two-dimensional carbon material is 1nm-10nm; the particle size of the particulate carbon material is 0.5μm-2μm. Carbon matrices within the above size range have a large surface area, which is beneficial for increasing the contact area between the cathode material and the conductive agent, thereby improving the conductivity of the electrode. Furthermore, this size of carbon matrix is ​​more conducive to the polymer fully coating the carbon matrix, forming a structurally stable composite conductive agent. In some embodiments of this application, the carbon matrix in the composite conductive agent can specifically be one or more of carbon black, graphite, carbon nanotubes, carbon nanofibers, C60, graphene, and graphene oxide. These carbon materials have a large specific surface area and high electrical conductivity; adding them to the electrode can greatly improve the conductivity of the electrode.

[0047] In some embodiments of this application, the conductive agent includes two-dimensional carbon materials. Compared with one-dimensional carbon materials and particulate carbon materials, two-dimensional carbon materials have a more complete conductive and thermally conductive network, and their larger specific surface area is more conducive to electrolyte wetting. In some embodiments, the conductive agent includes one or more of graphene and graphene oxide. Graphene and / or graphene oxide have excellent conductivity, which can better improve the conductivity of the electrode and promote the battery capacity. In some embodiments of this application, the surface of the carbon matrix has oxygen-containing groups. Polymers containing amylopectin groups can form chemical bonds with oxygen-containing groups, thereby improving the stability of the coating layer. In some embodiments, the conductive agent includes graphene oxide. Graphene oxide contains abundant oxygen-containing active groups, which is beneficial to polymer adhesion, thereby improving the structural stability of the composite conductive agent. In some embodiments of this application, the mass ratio of the polymer containing amylopectin groups to the carbon matrix is ​​(30-60):1. The mass ratio of the polymer containing the amylopyridine group to the carbon matrix may be, but is not limited to, 30:1, 35:1, 40:1, 45:1, 50:1, 55:1 or 60:1.

[0048] Please see Figure 2 , Figure 2 This is a schematic diagram of the structure of a composite conductive agent provided in an embodiment of this application. Figure 2 In this structure, the carbon matrix 10 has a granular structure, and a coating layer 20 is provided on the surface of the carbon matrix 10. (See also...) Figure 3 , Figure 3 This is a schematic diagram of the structure of a composite conductive agent provided in an embodiment of this application. Figure 3 In this structure, the carbon matrix 10 is a one-dimensional structure, and a coating layer 20 is provided on the surface of the carbon matrix 10. (See also...) Figure 4 , Figure 4This is a schematic diagram of the structure of a composite conductive agent provided in an embodiment of this application. Figure 4 In this embodiment, the carbon matrix 10 has a two-dimensional structure, and a coating layer 20 is provided on the surface of the carbon matrix 10. In some embodiments of this application, the average thickness of the coating layer is 80 nm to 90 nm. The average thickness of the coating layer can be, but is not limited to, 80 nm, 82 nm, 85 nm, 87 nm, 88 nm, or 90 nm. Controlling the thickness of the coating layer can ensure that the composite conductive agent has a high specific surface area, promote the wetting of the composite conductive agent by the electrolyte, and the coating layer can effectively inhibit the dissolution of transition metals in the carbon matrix.

[0049] In some embodiments of this application, the specific surface area of ​​the composite conductive agent is greater than or equal to 300 m². 2 / g, the specific surface area of ​​the composite conductive agent can be, but is not limited to, 300m². 2 / g、320m 2 / g、330m 2 / g or 350m 2 / g. In some embodiments of this application, the coating layer has a porous structure, which can further increase the specific surface area, thereby promoting electrolyte wetting. In some embodiments of this application, the pore size of the porous structure is 2nm-50nm, and the pore size can be, but is not limited to, 2nm, 5nm, 10nm, 15nm, 20nm, 30nm, 40nm or 50nm. The average pore size of the porous structure of the coating layer can be measured by scanning electron microscopy.

[0050] In the composite conductive agent provided in this application, the carbon matrix is ​​coated with a conductive polymer, thereby improving the wettability of the electrolyte to the material; the polymer is modified with a amine oxime group, which makes the composite conductive agent have a strong chelating ability for transition metal ions, thereby inhibiting the dissolution of transition metal ions. This composite conductive agent has a good metal element dissolution inhibition effect on ternary cathode active materials (NCM, nickel cobalt manganese), binary cathode active materials (nickel cobalt, nickel manganese), lithium cobalt oxide (LCO), lithium iron phosphate (LFP), lithium nickel oxide (LNO), lithium iron ferrite (LFO), etc., and has a very wide range of applications. Its application in batteries can improve the cycle performance, rate performance and safety performance of batteries.

[0051] This application also provides a method for preparing the above-mentioned composite conductive agent, comprising the following steps:

[0052] S1: Prepare a carbon matrix solution, add polymer monomers and initiators to it to obtain a pre-coated carbon matrix;

[0053] S2: A composite conductive agent is obtained by reacting a pre-coated carbon matrix with an oxime agent, wherein the oxime agent is a compound having hydroxyl and amino groups.

[0054] In step 100 of this application, the carbon matrix is ​​dispersed in a solvent, which can be water or an organic solvent, including at least one selected from dimethyl sulfoxide, N,N-dimethylformamide, and N,N-dimethylacetamide (DMAC). In some embodiments of this application, the carbon matrix solution is ultrasonically treated to promote sufficient dispersion of the carbon matrix in the solution, and then a polymer monomer and an initiator are added, wherein the polymer monomer contains a cyano group. In some embodiments of this application, the initiator includes one or more selected from azobisisobutyronitrile (AIB) and ammonium persulfate (APS). In some embodiments of this application, the mass ratio of carbon matrix to polymer monomer is 1:(30-60), and the mass ratio of polymer monomer to initiator is (2-5):1.

[0055] In this application, the polymer in the polymer solution is a cyano-containing polymer. In some embodiments, the polymer monomer includes acrylonitrile (AN), which contains cyano groups. The polyacrylonitrile obtained after polymerization can be directly subjected to subsequent oxime treatment, thus simplifying the preparation process. Moreover, acrylonitrile has good solubility in organic solvents, which is beneficial for the in-situ polymerization to uniformly coat the carbon matrix. The generated polyacrylonitrile contains polymer chains and has good conductivity. In addition, acrylonitrile has low cost, which helps to reduce production costs. In some embodiments of this application, the surface of the carbon matrix has oxygen-containing groups, which can provide reaction sites for monomer polymerization, thereby accelerating the polymerization efficiency. A polymer layer containing cyano groups (-CN) is formed on the surface of the carbon matrix. Furthermore, the oxygen-containing groups can form chemical bonds, such as hydrogen bonds, with polymers containing amine oxime groups. The presence of these chemical bonds gives the coating layer good structural stability, enabling it to stably coat the carbon matrix surface. In some embodiments, the conductive agent includes graphene oxide. The surface of graphene oxide has abundant oxygen-containing groups and structural defects that can serve as active sites for polymerization reactions, which is beneficial to improving polymerization efficiency, and the resulting coating layer has better bonding with the carbon matrix.

[0056] Please see Figure 5 , Figure 5 This is a schematic diagram of a polymerization reaction provided in one embodiment of this application. Figure 5 The carbon matrix is ​​graphene oxide. Cyano-containing polymer monomers are polymerized on the surface of graphene oxide to form a cyano-containing polymer layer on the carbon matrix. The cyano-containing polymer layer is then oxime-treated to obtain a polymer modified with a cyano-amine oxime group (—C(NOH)NH2).

[0057] In some embodiments of this application, step 100 specifically includes: ultrasonically dispersing a carbon matrix in a solvent, adding an initiator and polymer monomers, mixing and stirring the mixture at 60°C–70°C for 40–60 hours, collecting the solid product after centrifugation and vacuum drying to obtain a pre-coated carbon matrix. This application employs in-situ polymerization to form a polymer coating layer on the surface of a carbon matrix. The coating layer formed by this method exhibits good uniformity and density; furthermore, in-situ polymerization is beneficial for improving the bonding force between the carbon matrix and the coating layer, ensuring superior overall conductivity; in addition, the in-situ polymerization method allows for easy control of the reactant ratio and reaction time, thereby enabling flexible control of the coating layer thickness, making the method simpler and more effective.

[0058] In step 200 of this application, the pre-coated carbon matrix is ​​reacted with an oxime agent to generate a metallo-oxime group from the cyano group in the polymer, thus obtaining a composite conductive agent. The oxime agent is a compound having hydroxyl and amino groups. In some embodiments of this application, the oxime agent includes one or more of hydroxylamine, hydroxylamine hydrochloride, hydroxyurea sulfate, and 2-amino-3-hydroxypyridine. In some embodiments of this application, the mass ratio of the oxime agent to the pre-coated carbon matrix is ​​1:(5-20), and the specific mass ratio may be, but is not limited to, 1:5, 1:8, 1:10, 1:12, 1:15, or 1:20.

[0059] In some embodiments of this application, the reaction between the pre-coated carbon matrix and the oximating agent includes: dissolving the oximating agent in an organic solvent to obtain an oximating agent solution, wherein the organic solvent includes one or more of dimethyl sulfoxide, N,N-dimethylformamide, and N,N-dimethylacetamide; placing the pre-coated carbon matrix in the oximating agent solution; adding sodium bicarbonate and sodium hydroxide; reacting at 55°C-65°C while simultaneously stirring and mixing; and centrifuging and drying the mixture after the reaction to obtain the composite conductive agent. In some embodiments of this application, hydroxylamine hydrochloride is used as the oximating agent, and the reaction equation between the polymer and hydroxylamine hydrochloride is shown in formula (II):

[0060]

[0061] The composite conductive agent prepared by the method provided in this application not only has good conductivity but also effectively inhibits the dissolution of transition metals. Moreover, the equipment used in this preparation method is simple, the preparation conditions are easy to control, and it is cost-effective. The processing environment of this preparation method is at room temperature and pressure, without the need to build an additional closed system, resulting in low production costs. In addition, this preparation method uses in-situ polymerization, which can achieve rapid coating of carbon matrix, with uniform and controllable coating thickness, resulting in a composite conductive agent with a large specific surface area and a high product yield.

[0062] This application also provides an electrode sheet comprising an active material and the composite conductive agent of this application. The composite conductive agent of this application can be added to the positive electrode sheet and / or the negative electrode sheet to improve the overall conductivity of the electrode sheet. In some embodiments of this application, the electrode sheet comprises a current collector and an active material layer disposed on the current collector, the active material layer comprising the active material and the composite conductive agent. In some embodiments of this application, the active material layer further comprises a binder, which may be one or more of polyvinylidene fluoride, polytetrafluoroethylene, polyhexafluoropropylene, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polyamide, polyacrylonitrile, and polyacrylate.

[0063] For the positive electrode, in some embodiments of this application, the active material includes one or more of lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, ternary materials, lithium manganese phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxide phosphate, lithium vanadium oxide, and lithium-rich manganese-based materials. In some embodiments, the active material includes lithium iron phosphate, and the amine oxime group of the polymer in the composite conductive agent has good complexing properties for iron ions, thus more effectively inhibiting the dissolution of iron ions in the active material. In some embodiments of this application, the current collector of the positive electrode is aluminum foil.

[0064] For the negative electrode sheet, in some embodiments of this application, the active material includes one or more of the following: natural graphite, artificial graphite, hard carbon, soft carbon, lithium titanate, iron oxide, lithium titanium phosphate, titanium dioxide, silicon, silicon suboxide, tin and its oxides, and antimony and its oxides. In some embodiments of this application, the current collector of the negative electrode sheet is copper foil.

[0065] In some embodiments of this application, the mass ratio of the composite conductive agent to the active material is 1:(10-100), meaning the amount of the composite conductive agent added relative to the active material is 1%-10%. Specifically, the mass ratio of the composite conductive agent to the active material can be, but is not limited to, 1:10, 1:15, 1:20, 1:50, 1:80, or 1:100. In some embodiments of this application, the mass ratio of the composite conductive agent to the active material is 1:(20-50), meaning the amount of the composite conductive agent added relative to the active material is 2%-5%.

[0066] In some embodiments of this application, the mass percentage of the active material in the active material layer is 80%-99%. Specifically, the mass percentage of the active material in the active material layer may be, but is not limited to, 80%, 85%, 89%, 90%, 93%, 95%, 98%, or 99%. In some embodiments of this application, the mass percentage of the composite conductive agent in the active material layer is 0.1%-15%. Specifically, the mass percentage of the composite conductive agent in the active material layer may be, but is not limited to, 0.1%, 1%, 3%, 5%, 10%, or 15%. In some embodiments of this application, the mass percentage of the binder in the active material layer is 0.1%-15%, and in some embodiments, the mass percentage of the binder in the active material layer is 1%-7%.

[0067] This application also provides a method for preparing the above-mentioned electrode sheet. In some embodiments, the method for preparing the positive electrode sheet includes a direct mixing method, which includes: mixing a composite conductive agent, an active material, a binder, and a solvent to form an electrode sheet slurry; coating the electrode sheet slurry onto the surface of a current collector; and drying to obtain the electrode sheet. The solvent can be any solvent available in the prior art, such as one or more selected from N-methylpyrrolidone (NMP), dimethylformamide (DMF), diethylformamide (DEF), dimethyl sulfoxide (DMSO), and tetrahydrofuran (THF). The amount of solvent used is such that the content of the active material in the slurry is 20-90 wt%, and further, the amount of solvent used is such that the content of the active material in the slurry is 40-85 wt%. In some embodiments of this application, the electrode sheet drying temperature is 60℃~150℃, preferably 80℃~130℃, and the drying time is 0.5h-5h.

[0068] This application also provides a secondary battery, which includes the electrode provided in this application. Because the secondary battery uses the electrode provided in this application, it has higher capacity and cycle performance.

[0069] In this application, the separator of the secondary battery can be any separator known to those skilled in the art. For example, the separator can be one or more of polyolefin microporous membrane, polyethylene terephthalate, polyethylene felt, glass fiber felt or ultrafine glass fiber paper.

[0070] In this application, the electrolyte of the secondary battery comprises a solution of an electrolyte lithium salt in a non-aqueous solvent. In embodiments of this application, the electrolyte lithium salt includes one or more of lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium hexafluorosilicate (Li2SiF6), lithium tetraphenylborate (LiB(C6H5)4), lithium chloride (LiCl), lithium bromide (LiBr), lithium chloroaluminate (LiAlCl4), lithium fluorocarbon sulfonate (LiC(SO2CF3)3), LiCH3SO3, LiN(SO2CF3)2, and LiN(SO2C2F5)2. In some embodiments of this application, the non-aqueous solvent includes one or more of chain esters and cyclic esters. In some embodiments of this application, the chain ester includes one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), methyl propyl carbonate (MPC), and dipropyl carbonate (DPC). In some embodiments of this application, the chain esters include fluorinated, sulfur-containing, or unsaturated chain organic esters. In some embodiments of this application, the cyclic esters include one or more of ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), γ-butyrolactone (γ-BL), and sulpholactone. In some embodiments of this application, the cyclic esters include fluorinated, sulfur-containing, or unsaturated cyclic organic esters. In some embodiments of this application, the non-aqueous solvent includes one or more of chain ethers and cyclic ether solutions. In some embodiments of this application, the cyclic ethers include one or more of tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), 1,3-dioxolane (DOL), and 4-methyl-1,3-dioxolane (4-MeDOL). In some embodiments of this application, the cyclic ethers include fluorinated, sulfur-containing, or unsaturated cyclic organic ethers. In some embodiments of this application, the chain ether includes one or more of dimethoxymethane (DMM), 1,2-dimethoxyethane (DME), 1,2-dimethoxypropane (DMP), and diethylene glycol dimethyl ether (DG). In some embodiments of this application, the chain ether includes fluorine-containing, sulfur-containing, or unsaturated chain organic ethers. In some embodiments of this application, the concentration of the electrolyte lithium salt in the electrolyte is 0.1 mol / L to 15 mol / L. In some embodiments of this application, the concentration of the electrolyte lithium salt is 1 mol / L to 10 mol / L.

[0071] In the embodiments of this application, the battery can be fabricated using either a lamination process or a winding process. In some embodiments of this application, the battery is fabricated using a lamination process.

[0072] This application also provides an electronic device that includes a secondary battery provided in this application, which powers the electronic device.

[0073] The technical solution of this application will be further described below with reference to several embodiments.

[0074] Example 1

[0075] 1) Preparation of composite conductive agents

[0076] Preparation of the pre-coated carbon matrix: 100 mL of commercially available graphene oxide solution (concentration 2 mg / mL, graphene average side length 1 μm, thickness 1 nm) and 200 mL of water were ultrasonically mixed in a 500 mL flask for 0.5 h to separate the stacked graphene oxide. 15 mL (12 g) of acrylonitrile monomer was added to the graphene oxide solution, the flask was sealed with a rubber stopper, and ultrasonication continued for 10 min. Nitrogen gas was bubbled into the flask for about 30 min, and 100 mg of ammonium persulfate (APS) initiator was added, followed by bubbling for another 30 min. The system was heated to 65 °C and stirred magnetically to carry out the polymerization reaction. After 20 h, the reaction was stopped. Subsequently, after centrifugation, washing three times with ethanol, and drying in a 60 °C vacuum oven, a two-dimensional composite material of polyacrylonitrile-coated graphene oxide, GO-PAN, was obtained, in which the electrical conductivity of polyacrylonitrile PAN was 20 S / m.

[0077] Oxime treatment: 20 mg of hydroxylamine hydrochloride was added to 200 mL of N,N dimethylformamide. The mixture was heated to 45 °C and stirred for 30 min to obtain a hydroxylamine hydrochloride solution. 10 mg of sodium bicarbonate and 4 mg of sodium hydroxide were added, and the mixture was stirred at 60 °C for 1 h. Then, 200 mg of pre-coated carbon matrix was added to the reactor. After dissolution, the mixture was heated to 80 °C and reacted for 12 h. 5 mg of sodium bicarbonate and 2 mg of sodium hydroxide were added and reacted for 6 h. After cooling, the mixture was centrifuged at 8000 rpm and dried to obtain the composite conductive agent, namely GO-PAO.

[0078] 2) Preparation of lithium secondary batteries

[0079] Lithium cobalt oxide, composite conductive agent, and binder (polyvinylidene fluoride, PVDF) were mixed in a weight ratio of 94:4:2. N-methylpyrrolidone was added and stirred into a homogeneous slurry. The slurry was coated onto aluminum foil and dried at 100°C for 4 hours. The positive electrode sheet was obtained by rolling and slitting.

[0080] A negative electrode slurry is prepared by mixing commercial graphite, conductive agent (Super P), and binder (sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR)) in a weight ratio of 90:5:5. The slurry is coated on a copper foil current collector, and after drying, rolling, and slitting, a negative electrode sheet is obtained.

[0081] Preparation of the full cell: The positive electrode, separator, and negative electrode prepared above were sequentially stacked to form a cell. The cell was housed in a battery casing, and an electrolyte (a 1 mol / L LiPF6 EC+DEC mixture (EC:DEC volume ratio 1:1)) was injected. The battery casing was then sealed to obtain the full cell. The cycle life and energy density of the full cell were tested.

[0082] Preparation of half-cell: The positive electrode sheet prepared above is assembled with a lithium metal sheet to form a coin cell.

[0083] Example 2

[0084] The preparation of the composite conductive agent in Example 2 is the same as in Example 1. The difference between Example 2 and Example 1 is that the positive electrode active material in Example 2 is lithium iron phosphate (LFP), and the lithium secondary battery is prepared by the same method as in Example 1.

[0085] Example 3

[0086] The preparation of the composite conductive agent in Example 3 is the same as in Example 1. The difference between Example 3 and Example 1 is that the positive electrode active material in Example 3 is ternary lithium (Li(NiCoMn)O2), and the lithium secondary battery is prepared by the same method as in Example 1.

[0087] Example 4

[0088] Example 4 uses single-walled carbon nanotubes (SWCNTs) as the carbon matrix, i.e., one-dimensional carbon material as the carbon matrix. The SWCNTs have a length of 500 nm and a diameter of 10 nm. The composite conductive agent and lithium secondary battery are prepared using the same method as in Example 1.

[0089] Example 5

[0090] Example 5 uses the same method as Example 1 to prepare a two-dimensional composite material GO-PAN of polyacrylonitrile-coated graphene oxide. The difference between Example 5 and Example 1 is that when the pre-coated carbon matrix is ​​reacted with hydroxylamine hydrochloride, the mass of hydroxylamine hydrochloride is 10 mg, that is, the mass ratio of hydroxylamine hydrochloride to polymer is 1:20.

[0091] Example 6

[0092] Example 6 prepared a two-dimensional composite material GO-PAN of polyacrylonitrile-encapsulated graphene oxide using the same method as in Example 1. The difference between Example 6 and Example 1 is that when the pre-coated carbon matrix was reacted with hydroxylamine hydrochloride, the mass of hydroxylamine hydrochloride was 40 mg, that is, the mass ratio of hydroxylamine hydrochloride to polymer was 1:5.

[0093] Example 7

[0094] The difference between Example 7 and Example 1 is that when the carbon matrix reacts with the polymer monomer, the polymer monomer acrylonitrile is 25 mL (20 g), that is, the mass ratio of carbon matrix to polymer monomer is 1:100.

[0095] Example 8

[0096] The difference between Example 8 and Example 1 is that when the carbon matrix reacts with the polymer monomer, the polymer monomer acrylonitrile is 5 mL (4 g), that is, the mass ratio of carbon matrix to polymer monomer is 1:20.

[0097] Example 9

[0098] In Example 9, the carbon matrix used was graphene, and a composite conductive agent and a lithium secondary battery were prepared using the same method as in Example 1.

[0099] Example 10

[0100] In Example 10, the polymer monomer used was benzonitrile, and the initiator was diaminodiphenyl sulfone (DDS). The conductivity of the polymer formed was 5 S / m. The composite conductive agent and lithium secondary battery were prepared using the same method as in Example 1.

[0101] To highlight the beneficial effects of this application, the following comparative examples are provided.

[0102] Comparative Example 1

[0103] Lithium cobalt oxide, a conductive agent (graphene oxide, GO), and a binder (polyvinylidene fluoride, PVDF) were mixed in a weight ratio of 94:4:2. N-methylpyrrolidone was added and stirred to form a homogeneous slurry. The slurry was coated onto aluminum foil and dried at 100°C for 4 hours. The resulting positive electrode sheet was obtained by rolling and slitting. A lithium secondary battery was prepared using the same method as in Example 1.

[0104] Comparative Example 2

[0105] Add 15 mL (12 g) of acrylonitrile monomer to 200 mL of water, seal the flask with a rubber stopper, and continue sonication for 10 min. Purge the flask with nitrogen gas for approximately 30 min, then add 100 mg of ammonium persulfate (APS) initiator and continue bubbling for another 30 min. Heat the system to 65 °C and stir with a magnetic stirrer to carry out the polymerization reaction. After 20 h, stop the reaction. Subsequently, after centrifugation, washing three times with ethanol, and drying in a vacuum oven at 60 °C, polyacrylonitrile is obtained. Polyacrylonitrile is directly added to the positive electrode, with a weight ratio of lithium cobalt oxide, polyacrylonitrile, and binder (polyvinylidene fluoride, PVDF) of 94:4:2. The positive electrode and lithium secondary battery are prepared using the same method as in Example 1.

[0106] Comparative Example 3

[0107] Comparative Example 3 prepared a polyacrylonitrile-encapsulated graphene oxide two-dimensional composite material GO-PAN using the same method as Example 1. The polyacrylonitrile-encapsulated graphene oxide two-dimensional composite material GO-PAN was directly added to the positive electrode. The weight ratio of lithium cobalt oxide, GO-PAN, and binder (polyvinylidene fluoride, PVDF) was 94:4:2. The positive electrode and lithium secondary battery were prepared using the same method as Example 1.

[0108] Effect Example

[0109] To strongly support the beneficial effects of the technical solutions in the embodiments of this application, the following tests are provided:

[0110] 1) The morphology of the composite conductive agents prepared in each embodiment was characterized to obtain the thickness of the coating layer in the composite conductive agent. The mass ratio of the polymer to the carbon matrix was obtained by ICP testing. The ICP testing specifically included: determining the mass ratio of C, N, and O elements in the composite conductive agent. Since the atomic ratio of the amine oxime group is C:N:O = 1:2:1, the oxime group content can be obtained from the amount of O element. Since the N element belongs to the unoximated cyano group (-CN), the number of cyano groups can be obtained. The mass of the main chain C element can be obtained from the number of branched cyano groups and amine oxime groups. The mass of the remaining C element is the mass of the carbon matrix, thus obtaining the mass ratio of the carbon matrix to the polymer coating layer. The specific surface area of ​​the composite conductive agent was obtained by BET specific surface area testing. The test results are shown in Table 1.

[0111] Table 1. Structural parameters of the composite conductive agents in Examples 1-10 and Comparative Examples 2 and 3.

[0112]

[0113]

[0114] 2) The oxime effect of the composite conductive agents prepared in Examples 1-10 was tested. The test method was as follows: the composite conductive agents were characterized by Fourier transform infrared spectroscopy (FT-IR). As can be seen from the infrared spectrum, at 2242 cm⁻¹... -1 The characteristic absorption peak at 1647 cm⁻¹ is a cyano group (-CN), indicating that the composite conductive agent contains polyacrylonitrile (PAN); -1 and 937cm -1 The absorption peak appearing at this point is a characteristic absorption peak of the C=N, N-O bond in the oxime group, which proves the success of the oxime reaction, that is, the polyacrylonitrile contains a geminal oxime group.

[0115] 3) The oximation rate of the composite conductive agents prepared in Examples 1-10 was tested. The test method was as follows: the mass ratio of N and O elements in the sample was tested by ICP, and then the molar ratio of N and O atoms was calculated. Since the N:O ratio in the amyl oxime group is 2:1 and there is no O element in the cyano group, the oximation treatment introduces the same number of N and O atoms. Therefore, the number of O atoms is the number of amyl oxime groups. The N atoms in the amyl oxime group that are proportional to O are subtracted from the total number of N atoms. The remaining N atoms are the N atoms in the cyano group. Thus, the number of oximated and unoximated cyano groups in polypropylene can be calculated. Oxidation rate = number of moles of oximated cyano groups / total number of moles of cyano groups. The experimental results are shown in Table 2.

[0116] Table 2. Oximation rate parameters of the composite conductive agents in Examples 1-10

[0117]

[0118]

[0119] As shown in Table 2, the oximation rate of the composite conductive agent obtained in Example 5 of this application is 42.6%, which is relatively low. This is because the content of added hydroxylamine hydrochloride is small, and the oximation reaction is insufficient. The oximation rate of the composite conductive agent obtained in Example 6 is 62.6%, which is relatively high. This is because the content of added hydroxylamine hydrochloride is too large, and the cyano group can be fully oximated. However, the excessively high oximation rate will reduce the conductivity of the coating layer on the carbon matrix surface and affect the overall conductivity of the composite conductive agent. In Example 10, polystyrene is used as the coating layer. The cyano group in polystyrene is located in the side chain. The content of cyano group is small and it is not easy to undergo oximation reaction, so the oximation rate is low.

[0120] 4) Cycle life and discharge capacity tests were conducted on the full cells prepared in Examples 1-10 and Comparative Examples 1-3. The specific test conditions were as follows: the charge / discharge voltage range was 3V-4.5V; the charge / discharge regime was 0.7C constant current to the upper voltage limit and 0.2C constant current to the lower voltage limit. The initial discharge capacity was recorded. After 400 charge / discharge cycles, the discharge capacity of the 400th cycle was recorded. The capacity retention rate after 400 cycles was calculated using the formula: Capacity retention rate after 400 cycles (%) = Discharge capacity after 400 cycles / Discharge capacity of the first cycle × 100%. After disassembling the cycled full cells, the negative electrode was removed, and the total content of transition metal elements in the negative electrode was tested using an inductively coupled plasma (ICP) instrument. The dissolution amount of transition metal elements in the negative electrode was obtained. The performance test results of the full cells of Examples 1-10 and Comparative Examples 1-3 are shown in Table 3. In the table, "-" indicates that the content of transition metal elements in the sample is below the detection limit.

[0121] Table 3 Performance parameters of the full cells in Examples 1-10 and Comparative Examples 1-3

[0122]

[0123]

[0124] Table 3 shows that, compared to the comparative examples, the full cells of Examples 1-10 of this application exhibit lower dissolution of transition metal elements and higher capacity retention after cycling. This indicates that the composite conductive agent of this application can effectively improve the capacity retention of the battery. Specifically, Comparative Example 1 uses lithium cobalt oxide as the positive electrode material. The cobalt element in the positive electrode material dissolves during cycling, resulting in a high content of cobalt element detected on the negative electrode side. However, the battery of this application contains a polymer with a metallo-oxime group in the composite conductive agent, which can effectively inhibit the dissolution of transition metals and reduce the content of transition metals on the negative electrode side. The battery of Comparative Example 2 incorporates polyacrylonitrile particles, while the polymer layer of the conductive agent in Comparative Example 3 is not oxime-treated. The experimental results show that the unoxime-treated polymer cannot inhibit the dissolution of transition metals.

[0125] For each embodiment, Examples 1-3 used different types of cathode materials. As shown in Table 3, the method of this application has a good effect on inhibiting the dissolution of transition metal elements for different types of cathode materials. Example 4 uses single-walled carbon nanotubes as the carbon matrix, resulting in a smaller specific surface area of ​​the composite conductive agent and a certain reduction in the complexation performance of the coating layer. In Example 5, the composite conductive agent has a low oxime content during preparation, resulting in a low oxime rate. Some transition metals in the cathode material will be released into the electrolyte, leading to a decrease in battery performance. In Example 6, the composite conductive agent has a high oxime content during preparation, resulting in a high oxime rate and reduced conductivity, leading to a decrease in battery performance. In Example 7, the composite conductive agent has a high oxime content during preparation, resulting in a high oxime rate and reduced conductivity, leading to a decrease in battery performance. During the preparation process, the content of carbon matrix is ​​relatively low compared to polymer, while the content of polymer is relatively high. The specific surface area of ​​the composite conductive agent is reduced to a certain extent, resulting in decreased battery conductivity, poor electrode conductivity, and a certain decrease in battery performance compared to Example 1. In Example 8, the content of carbon matrix is ​​relatively high compared to polymer, and its coating layer is thinner. The coating layer has a weaker effect on inhibiting the dissolution of metal elements, and there is a certain amount of transition metal dissolution on the negative electrode side. In Example 9, graphene is used as the carbon matrix. Compared to graphene oxide, the bonding strength between its coating layer and the carbon matrix is ​​poor, and the complexation performance of the coating layer is reduced to a certain extent. In Example 10, the polymer in the composite conductive agent is polyphenylene nitrile. The polymer has low conductivity, resulting in poor conductivity of the composite conductive agent, low cycle retention rate of the battery, and relatively poor rate performance.

[0126] The negative electrode sheets of the full cells from Example 1 and Comparative Example 1 were disassembled after 100 cycles. The microstructure of the negative electrode surface was observed using a scanning electron microscope (SEM), and the elemental composition of the negative electrode surface was analyzed using EDS mapping. (See [link to relevant documentation]). Figure 6and Figure 7 , Figure 6 This is a mapping diagram of the full cell of Embodiment 1 of this application after 100 cycles. Figure 7 For the reason Figure 6 The resulting elemental composition diagram of the negative electrode surface of the full cell in Example 1 after 100 cycles is shown in the figure. Figure 6 and Figure 7 As can be seen, in the full cell containing the composite conductive agent in Example 1 of this application, no cobalt was detected in the negative electrode. Please refer to... Figure 8 and Figure 9 , Figure 8 This is a mapping diagram of the full cell of Comparative Example 1 of this application after 100 cycles. Figure 9 For the reason Figure 8 The resulting elemental composition diagram of the negative electrode surface of the full cell in Comparative Example 1 after 100 cycles is derived from... Figure 8 and Figure 9 It can be seen that cobalt can be detected in the negative electrode of the full cell in Comparative Example 1, which uses a conventional conductive agent.

[0127] The above description represents the preferred embodiments of this application, but should not be construed as limiting the scope of this application. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this application, and these improvements and modifications are also considered to be within the scope of protection of this application.

Claims

1. A composite conductive agent, characterized by, The composite conductive agent comprises a carbon matrix and a coating layer arranged on the surface of the carbon matrix, wherein the coating layer comprises an amidoxime group-containing polymer; the amidoxime group-containing polymer has an oximation rate of 50% to 60%; the amidoxime group-containing polymer has an electrical conductivity of 3 S / m to 3.6 S / m; the average thickness of the coating layer is 80 nm to 90 nm; the specific surface area of the composite conductive agent is greater than or equal to 300 m 2 / g; the coating layer has a porous structure, the average pore size of the porous structure is 2 nm to 50 nm; the mass ratio of the amidoxime group-containing polymer to the carbon matrix is (30 to 60):1; the surface of the carbon matrix has oxygen-containing groups, and the oxygen-containing groups form chemical bonds with the amidoxime group-containing polymer.

2. The composite conductive agent of claim 1, wherein The conductive agent includes one or more of carbon black, graphite, carbon nanotube, carbon nanofiber, C60, graphene, and graphene oxide.

3. A method of producing the composite conductive agent according to any one of claims 1 to 2, characterized by, Comprise: reacting the carbon substrate with a polymer monomer and an initiator to obtain a pre-coated carbon substrate, the polymer monomer containing a cyano group; reacting the pre-coated carbon substrate with an oximation agent to obtain a composite conductive agent; the oximation agent is a compound having a hydroxyl group and an amino group; comprising a carbon substrate and a coating layer provided on the surface of the carbon substrate, the coating layer comprising an amine oxime group-containing polymer.

4. A pole piece characterized by, The pole piece comprises a current collector and an electrode material layer provided on the current collector, and the electrode material layer comprises the composite conductive agent according to any one of claims 1-2.

5. A secondary battery characterized by comprising: The secondary battery comprises the pole piece according to claim 4.

6. An electronic device, comprising: The electronic device comprises the secondary battery according to claim 5.

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