A graphene conductive agent and its preparation method

Abstract

The present invention discloses a graphene conductive agent and a preparation method thereof. Through predispersion, the present invention realizes the preliminary dispersion of graphene and carbon materials, and then combines processes such as shear emulsification, high-pressure homogenization, and sanding to achieve the step-by-step peeling and mixing of graphene, resulting in a more uniform size of the prepared graphene conductive agent. By modifying graphene, the modified graphene has good dispersibility and strong conductivity, overcoming the defect of its easy agglomeration, enabling the conductive agent to be evenly distributed, forming a continuous conductive network, accelerating the electron transfer speed during the charge and discharge process of the battery, significantly improving the electrochemical performance of the lithium-ion battery, and increasing its service life.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery conductive agents, and particularly to a graphene conductive agent and a preparation method thereof. Background Art

[0002] Currently, the international energy structure is shifting from traditional fossil fuels to clean, safe, and low-carbon energy sources such as wind energy and solar energy, and there is an urgent need for the support of efficient energy storage systems. Electrochemical energy storage, especially rechargeable batteries, is considered the most successful energy storage technology. Among all alternatives, lithium-ion batteries (LIBs) are recognized for their portability, lack of memory effect, and high specific energy density. Currently, lithium iron phosphate batteries are the main type of lithium batteries, and ternary batteries are also being developed. With the increasing market demand for longer battery life and the high-density characteristics of ternary batteries themselves, ternary material lithium-ion batteries have become a research hotspot. At present, the positive electrode active materials of lithium-ion batteries generally use transition metal oxides. However, the conductivity of transition metal oxides is generally low, and the capacity is often difficult to fully exert under large current discharge conditions; the negative electrode active materials of lithium-ion batteries mostly use carbon materials with good conductivity. However, due to the volume expansion and contraction of carbon materials during the insertion and extraction of lithium, the conductivity of the electrode will rapidly decrease after several cycles. Appropriately adding a conductive agent to the positive and negative electrode active materials can improve the above problems and enhance the performance of lithium-ion batteries under high-power working conditions.

[0003] The functions of the conductive agent include: collecting the microcurrent between the active material and the current collector, reducing the resistance, and increasing the electron conductivity; promoting the infiltration of the electrolyte into the electrode sheet, increasing the migration rate of lithium ions in the electrode material, and reducing polarization; optimizing the mechanical properties of the electrode sheet, facilitating processing, and avoiding peeling. Traditional conductive agents have disadvantages such as low rate performance and capacity.

[0004] Currently, commonly used lithium-ion battery conductive agents include acetylene black, carbon black, artificial graphite, natural graphite, and graphene. Acetylene black is a chain-like substance composed of spherical amorphous carbon particles and is the most widely used conductive agent at present. It has a low price, but to enhance the mutual contact between the electrode active materials, a large amount of addition is required, resulting in a decrease in the electrode capacity; carbon nanotubes are linear one-dimensional carbonaceous materials. Compared with acetylene black, carbon nanotubes have better conductivity and a smaller addition amount, but currently, the price of carbon nanotubes is expensive, and there are disadvantages such as difficult dispersion when used as a conductive agent. Graphene nanosheets are two-dimensional flexible materials with high conductivity. Their electronic conductivity and thermal conductivity are very good. The contact with the active material is surface-point contact, which can maximize the role of the conductive agent, reduce the amount of the conductive agent used, and thus more active material can be used to increase the capacity of the lithium battery. It has been widely studied in the energy field.

[0005] CN110957494A discloses a graphene lithium-ion battery conductive agent and a preparation method thereof, including the following steps: (1) preparing graphene materials; (2) preparing a binder / solvent mixture; (3) adding graphene materials to the binder / solvent mixture to prepare graphene slurry; (4) adding other conductive components to the prepared graphene slurry, fully stirring and mixing, and then transferring the slurry to a homogenizing emulsifier for dispersion to obtain a graphene lithium-ion battery conductive agent. This conductive agent can be used as a positive electrode conductive additive in lithium-ion batteries, making the resulting batteries have characteristics such as high capacity, high Coulomb efficiency, and good cycle stability. However, the graphene used in this invention is not modified and is prone to agglomeration, resulting in a reduction in the effective contact area between graphene and lithium ions, making it difficult for lithium ions to be inserted and extracted, increasing the internal resistance of the battery core, and reducing the service life of the battery. Summary of the Invention

[0006] In view of the above-mentioned defects of the prior art, the technical problem to be solved by the present invention is to provide a graphene conductive agent with good dispersibility.

[0007] To achieve the above object, the present invention provides a graphene conductive agent with good dispersibility. By modifying graphene, the modified graphene has good dispersibility and strong conductivity, overcomes the defect of its easy agglomeration, makes the conductive agent evenly distributed, can form a continuous conductive network, speeds up the electron transfer speed during the charge and discharge process of the battery, significantly improves the electrochemical performance of the lithium-ion battery, and increases its service life.

[0008] To achieve the above invention object, the present invention adopts the following technical scheme:

[0009] A graphene conductive agent includes the following components: graphene oxide, carbon materials, polyvinylpyrrolidone, organic solvents, and adhesives.

[0010] Preferably, the graphene conductive agent, by weight, includes the following components: 5-10 parts of graphene oxide, 3-5 parts of carbon materials, 10-20 parts of polyvinylpyrrolidone, 80-100 parts of organic solvents, and 3-5 parts of adhesives.

[0011] Preferably, the graphene oxide is modified graphene oxide, and its preparation method is as follows:

[0012] 1) Add methyl trifluoromethanesulfonate to a dichloromethane solution containing N,N-dimethylthiocarboxamide and stir to react. Successively add anhydrous ether and ethyl acetate to obtain a mixed solution. Place the mixed solution at -30 to -20 °C, and a solid will precipitate. Filter and collect the solid matter, recrystallize, and dry to obtain a colorless liquid;

[0013] 2) Add lithium bis(trifluoromethanesulfonyl)imide and a colorless liquid to water, stir and react. After the reaction is completed, collect the oil phase. The oil phase is washed with an aqueous solution of lithium bis(trifluoromethanesulfonyl)imide and water respectively, and then concentrated under reduced pressure to obtain a pale yellow solid;

[0014] 3) Ultrasonically disperse graphene oxide in an ethanol aqueous solution, add the pale yellow solid prepared in step 2), and obtain a reaction solution after ultrasonic treatment. React the reaction solution, centrifuge, filter and collect the precipitate. The precipitate is washed with an ethanol aqueous solution, dried and ground to obtain intercalated modified graphene oxide;

[0015] 4) Mix the intercalated modified graphene oxide and tetrahydrofuran and ultrasonicate, add triethylamine, add 2-bromoisobutyryl bromide and stir to react. Filter, collect the filter cake, wash and dry it, then mix it with a solvent and copper chloride and ultrasonicate. Add methyl methacrylate and pentamethyldiethylenetriamine, stir to react to obtain a mixture, add tetrahydrofuran and stir, filter, collect the filter residue, wash and dry it to obtain modified graphene oxide.

[0016] Further preferably, the preparation method of the modified graphene oxide is as follows:

[0017] 1) Under a nitrogen atmosphere, add 20 - 30 mL of methyl trifluoromethanesulfonate to a 50 - 80 mL dichloromethane solution containing 15 - 19 g of N,N-dimethylthiocarboxamide at 0 - 5 °C. After the dropping is completed, stir and react at 20 - 40 °C for 10 - 12 h. After the reaction is completed, add 50 - 150 mL of anhydrous ether and 100 - 200 mL of ethyl acetate in sequence to obtain a mixed solution. Then place the mixed solution at -30 to -20 °C for 1 - 2 h, and a solid will precipitate. Filter, collect the solid, recrystallize with ethyl acetate, and evaporate the solvent under reduced pressure to obtain a colorless liquid;

[0018] 2) Add 28 - 32 g of lithium bis(trifluoromethanesulfonyl)imide and 20 - 25 g of the colorless liquid to 40 - 60 mL of water, stir and react at 20 - 40 °C for 3 - 5 h. After the reaction is completed, collect the oil phase. The oil phase is washed 2 - 3 times with an aqueous solution of 0.05 - 0.3 mol / L lithium bis(trifluoromethanesulfonyl)imide and water respectively, and then concentrated under reduced pressure to obtain a pale yellow solid;

[0019] 3) Ultrasonically disperse 5 - 10 g of graphene oxide in 300 - 500 mL of 70 - 99 wt% ethanol aqueous solution, add 2 - 3 g of the pale yellow solid prepared in step 2), heat to 30 - 40 °C and react for 10 - 12 h, centrifuge, filter and collect the precipitate. The precipitate is washed 2 - 3 times with 70 - 99 wt% ethanol aqueous solution and then dried to obtain intercalated modified graphene oxide;

[0020] 4) Mix 1 - 3 g of intercalated modified graphene and 200 - 300 mL of tetrahydrofuran, then ultrasonically treat for 30 - 60 min. Add 10 - 20 mL of triethylamine, and dropwise add 3 - 5 mL of 2 - bromoisobutyryl bromide at a rate of 1 - 2 drops / second under an argon atmosphere and stir for reaction for 10 - 12 h. After the reaction is completed, filter, collect the filter cake, wash it successively with chloroform, tetrahydrofuran, and water, dry it at 30 - 50 °C for 4 - 6 h, then mix it with 100 - 200 mL of a mixed solvent of N,N - dimethylformamide and anhydrous methanol with a volume ratio of 3 - 5:1 - 2 and 1 - 2 g of copper chloride, ultrasonically treat for 30 - 60 min, add 2 - 3 g / mL of methyl methacrylate and 0.5 - 1.5 g of pentamethyldiethylenetriamine, irradiate and stir for reaction for 20 - 24 h under an ultraviolet lamp with a wavelength of 256 - 365 nm to obtain a mixture, add 50 - 100 mL of tetrahydrofuran, stir for 10 - 20 min, filter, collect the filter residue, wash it 2 - 3 times with tetrahydrofuran, ethanol, and water respectively, and then dry it at 40 - 60 °C to obtain modified graphene oxide.

[0021] Preferably, the carbon material is one or a mixture of two of carbon nanotubes and acetylene black; further preferably, the carbon material is a mixture of carbon nanotubes and acetylene black with a mass ratio of 1 - 3:2 - 4.

[0022] Preferably, the organic solvent is one of N - methylpyrrolidone, N,N - dimethylformamide, water, methanol, ethanol, ethylene glycol, acetone, methyl ethyl ketone, tetrahydrofuran, toluene, xylene, dichlorotoluene, or chloroform.

[0023] Preferably, the binder is one of polyvinylidene fluoride and styrene - butadiene rubber.

[0024] The present invention also discloses a preparation method of the above - mentioned graphene conductive agent, which includes the following steps:

[0025] S1 Weigh each component according to the formula, and mix graphene oxide, carbon material, polyvinylpyrrolidone, and organic solvent evenly to obtain a conductive agent pre - dispersion liquid;

[0026] S2 Subject the conductive agent pre - dispersion liquid to high - speed shearing, high - pressure homogenization, and then sand grinding, add the binder, stir, filter, and dry to obtain the graphene conductive agent.

[0027] Preferably, in step S2, the shearing speed is 5000 - 6000 rpm and the shearing time is 5 - 10 min.

[0028] Preferably, in step S2, the high - pressure homogenization pressure is 10 - 100 MPa and the homogenization time is 1 - 2 h.

[0029] Preferably, in step S2, the temperature of the sand mill bin is ≤55 °C, and the cyclic sand grinding is carried out for 1.5 - 2 h.

[0030] Preferably, in the step S2, the drying temperature is 100-150°C and the drying time is 6-8h.

[0031] In the present invention, the preliminary dispersion of graphene and carbon materials is achieved through pre-dispersion, and then through the combination of processes such as shear emulsification, high-pressure homogenization, and sand grinding, the step-by-step peeling and mixing of graphene are realized. The prepared graphene conductive agent has a more uniform size. By comprehensively utilizing the advantages of the "surface-point" contact formed by the coating of graphene and active materials, the conductive network formed by carbon nanotubes, and the good dispersion of acetylene black conductive agent, a multi-level conductive structure is constructed, effectively reducing the dosage ratio of the conductive agent, reducing the internal resistance and temperature rise, and improving performance such as capacity, charge-discharge rate, and cycle life.

[0032] Graphene is a novel two-dimensional atomic crystal composed of a single atomic layer with carbon atoms connected by sp 2 hybridization. Due to its excellent electrical conductivity, thermal conductivity, and electron transport channels, graphene is widely used in conductive agents for lithium batteries. However, due to the high specific surface area of graphene, there is a van der Waals force between the layers of graphene materials, and it is extremely easy for the graphene sheets to stack and agglomerate, resulting in difficult dispersion in conductive slurries, which greatly limits the exertion and application of its conductive performance. The inventor first reacts N,N-dimethylthiocarboxamide with the strong alkylating agent methyl trifluoromethanesulfonate and then performs ion exchange with lithium bis(trifluoromethane)sulfonimide. Through hydrogen bond action and electrostatic adsorption action, it is inserted into the interlayer structure of graphene oxide, increasing the distance between graphene layers. Then, a polymer is grafted onto the surface of the intercalated and modified graphene. This polymer enhances the flexibility of graphene, increases the active sites on the surface of graphene, improves the transport of electrons between the polymer and graphene, greatly improves the specific capacitance of the material. The modified graphene has good dispersion and strong conductivity, overcomes the defect of easy agglomeration, better exerts its excellent properties, enables the prepared conductive agent to fully fill the pores between battery active materials, can form an efficient three-dimensional conductive network in the electrode sheet, speeds up the electron transport speed during the charge-discharge process of the battery, significantly improves the electrochemical performance of the lithium-ion battery, and improves its service life.

[0033] Compared with the prior art, the present invention has the following beneficial effects: The process of the present invention is stable, easy to operate, and has low cost, and has good commercial prospects; by modifying graphene, through hydrogen bond action and electrostatic adsorption action, it is inserted into the interlayer structure of graphene oxide, increasing the distance between graphene layers. Then, a polymer is grafted onto the surface of the intercalated and modified graphene. This polymer enhances the flexibility of graphene, increases the active sites on the surface of graphene, speeds up the electron transport speed during the charge-discharge process of the battery, significantly improves the electrochemical performance of the lithium-ion battery, and improves its service life. Detailed implementation mode

[0034] The following will clearly and completely describe the concept of the present invention and the technical effects produced in combination with the embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative labor shall fall within the scope of protection of the present invention.

[0035] To avoid unnecessary repetition, unless otherwise specified, the items used in the following embodiments are all commercially available products, and the methods used are all conventional methods unless otherwise specified.

[0036] The sources of some raw materials used in the present invention are as follows:

[0037] Graphene oxide, with a thickness of 0.55 - 1.2 nm, a diameter of 0.5 - 3 μm, and 2 - 5 layers.

[0038] Carbon nanotubes, which are multi-walled carbon nanotubes, with a purity of 99% and a diameter of 10 - 100 nm.

[0039] Acetylene black, with a particle size of 36 nm, a resistivity of 0.2 Ω·m, and a specific surface area of 65 m 2 / g.

[0040] Polyvinylidene fluoride, with a bulk density of 0.961 g / cm 3 , a heat distortion temperature of 125 - 140 °C, a melting temperature of 171 °C, and a melt flow rate of 16 - 40 g / 10 min.

[0041] Polyvinylpyrrolidone, BASF Luvitec K30.

[0042] Example 1

[0043] A preparation method of a graphene conductive agent includes the following steps:

[0044] S1 Mix 10 g of modified graphene oxide, 1 g of carbon nanotubes, 4 g of acetylene black, 20 g of polyvinylpyrrolidone, and 100 mL of N-methylpyrrolidone, and stir at a speed of 700 rpm for 10 min to obtain a conductive agent pre-dispersion liquid;

[0045] S2 Shear the conductive agent pre-dispersion liquid at a speed of 6000 rpm for 10 min, perform high-pressure homogenization at 50 Mpa for 2 h, then sand mill, with the temperature in the sand mill bin ≤ 55 °C, perform cyclic sand milling for 1.5 h, add 5 g of polyvinylidene fluoride and stir for 5 min, filter and collect the filter cake, and dry the filter cake at 120 °C for 8 h to obtain the graphene conductive agent.

[0046] The preparation method of the modified graphene oxide is as follows:

[0047] 1) Under a nitrogen atmosphere, 24.9 mL of methyl trifluoromethanesulfonate was added to a 60 mL dichloromethane solution containing 17.8 g of N,N-dimethylthiocarboxamide at 5 °C. The mixture was stirred at 30 °C for 12 h. After the reaction, 100 mL of anhydrous ether and 150 mL of ethyl acetate were added successively to obtain a mixed solution. Then, the mixed solution was reacted at -20 °C for 2 h, and a solid precipitated. The solid was filtered, collected, recrystallized with ethyl acetate, and the solvent was removed by distillation under reduced pressure. After drying at 50 °C for 8 h, a colorless liquid was obtained;

[0048] 2) 31.6 g of lithium bis(trifluoromethane)sulfonimide and 23.1 g of the colorless liquid obtained in step 1) were added to 50 mL of water. The mixture was stirred at 30 °C for 4 h. After the reaction, the oil phase was collected. The oil phase was washed three times with a 0.1 mol / L aqueous solution of lithium bis(trifluoromethane)sulfonimide and water respectively, and then concentrated under reduced pressure to obtain a pale yellow solid;

[0049] 3) 5 g of graphene oxide was placed in 300 mL of a 99 wt% ethanol aqueous solution and ultrasonically dispersed at 80 kHz for 1 h. 2.8 g of the pale yellow solid prepared in step 2) was added, and the mixture was heated to 35 °C and reacted for 12 h. After centrifugation, the precipitate was filtered and collected. The precipitate was washed three times with a 99 wt% ethanol aqueous solution and then dried at 60 °C for 8 h to obtain intercalated modified graphene oxide;

[0050] 4) 2.5 g of intercalated modified graphene oxide and 250 mL of tetrahydrofuran were mixed and ultrasonically treated at 80 kHz for 40 min. 15 mL of triethylamine was added. Under an argon atmosphere, 4.5 mL of 2-bromoisobutyryl bromide was added dropwise at a rate of 2 drops per second and stirred for 12 h. After the reaction was completed, the mixture was filtered, and the filter cake was washed three times with chloroform, tetrahydrofuran, and water respectively. After drying at 40 °C for 5 h, it was mixed with 200 mL of a mixed solvent of N,N-dimethylformamide and anhydrous methanol with a volume ratio of 4:1 and 1.5 g of copper chloride and ultrasonically treated for 40 min. 2.6 g of methyl methacrylate and 1.2 g of pentamethyldiethylenetriamine were added, and the mixture was irradiated and stirred under ultraviolet light with a wavelength of 365 nm for 24 h to obtain a mixture. 80 mL of tetrahydrofuran was added, and the mixture was stirred for 150 min. After filtration, the filter residue was washed three times with tetrahydrofuran, ethanol, and water respectively and then dried at 50 °C to obtain modified graphene oxide.

[0051] Comparative Example 1

[0052] A preparation method of a graphene conductive agent, comprising the following steps:

[0053] S1 Mix 10 g of modified graphene oxide, 1 g of carbon nanotubes, 4 g of acetylene black, 20 g of polyvinylpyrrolidone, and 100 mL of N-methylpyrrolidone, and stir at 700 rpm for 10 min to obtain a conductive agent pre-dispersion solution;

[0054] S2 Shear the conductive agent pre-dispersion solution at 6000 rpm for 10 min, perform high-pressure homogenization at 50 Mpa for 2 h, and then perform sand milling. The temperature of the sand mill bin is ≤ 55 °C, cycle sand milling for 1.5 h, add 5 g of polyvinylidene fluoride and stir for 5 min, filter and collect the filter cake, and dry the filter cake at 120 °C for 8 h to obtain a graphene conductive agent.

[0055] The preparation method of the modified graphene oxide is as follows:

[0056] 1) Under a nitrogen atmosphere, add 24.9 mL of methyl trifluoromethanesulfonate to a dichloromethane solution containing 17.8 g of N,N-dimethylthiocarboxamide at 5 °C and 60 mL. After the addition is complete, stir and react at 30 °C for 12 h. After the reaction is completed, add 100 mL of anhydrous ether and 150 mL of ethyl acetate to obtain a mixed solution. Then, place the mixed solution at -20 °C and react for 2 h. Solids will precipitate. Filter and collect the solid matter, recrystallize with ethyl acetate, remove the solvent by reduced pressure distillation, and dry at 50 °C for 8 h to obtain a colorless liquid;

[0057] 2) Add 31.6 g of lithium bis(trifluoromethanesulfonyl)imide salt and 23.1 g of the colorless liquid obtained in step 1) to 50 mL of water, stir and react at 30 °C for 4 h. After the reaction is completed, collect the oil phase. Wash the oil phase three times with a 0.1 mol / L aqueous solution of lithium bis(trifluoromethanesulfonyl)imide and water, and then concentrate by reduced pressure to obtain a pale yellow solid;

[0058] 3) Place 5 g of graphene oxide in 300 mL of a 99 wt% ethanol aqueous solution, ultrasonically disperse at 80 kHz for 1 h, add 2.8 g of the pale yellow solid prepared in step 2), heat to 35 °C and react for 12 h, centrifuge, filter and collect the precipitate. Wash the precipitate three times with a 99 wt% ethanol aqueous solution and dry at 60 °C for 8 h to obtain modified graphene oxide.

[0059] Comparative Example 2

[0060] A preparation method of a graphene conductive agent, comprising the following steps:

[0061] S1 Mix 10 g of modified graphene oxide, 1 g of carbon nanotubes, 4 g of acetylene black, 20 g of polyvinylpyrrolidone, and 100 mL of N-methylpyrrolidone, and stir at 700 rpm for 10 min to obtain a conductive agent pre-dispersion solution;

[0062] S2 Shear the conductive agent pre-dispersion liquid at a speed of 6000 rpm for 10 min, perform high-pressure homogenization at 50 Mpa for 2 h, then grind it. The temperature of the grinding machine hopper is ≤ 55 °C, perform cyclic grinding for 1.5 h, add 5 g of polyvinylidene fluoride and stir for 5 min, filter and collect the filter cake. Dry the filter cake at 120 °C for 8 h to obtain the graphene conductive agent.

[0063] The preparation method of the modified graphene oxide is as follows:

[0064] Mix 2.5 g of graphene oxide and 250 mL of tetrahydrofuran, then perform ultrasonic treatment for 40 min. Add 15 mL of triethylamine, and dropwise add 4.5 mL of 2-bromo-2-methylpropionyl bromide at a rate of 2 drops per second under an argon atmosphere and stir for 12 h. After the reaction is completed, filter and collect the filter cake, and wash it 3 times with chloroform, tetrahydrofuran, and water respectively. Dry it at 40 °C for 5 h, then mix it with 200 mL of a mixed solvent of N,N-dimethylformamide and anhydrous methanol with a volume ratio of 4:1 and 1.5 g of copper chloride, and perform ultrasonic treatment at 80 kHz for 40 min. Add 2.6 g of methyl methacrylate and 1.2 g of pentamethyldiethylenetriamine, irradiate and stir the reaction under an ultraviolet lamp with a wavelength of 365 nm for 24 h to obtain a mixture. Add 80 mL of tetrahydrofuran, stir for 150 min, filter, and collect the filter residue. Wash it 3 times with tetrahydrofuran, ethanol, and water respectively, and then dry it at 50 °C to obtain the modified graphene oxide.

[0065] Comparative Example 3

[0066] A preparation method of a graphene conductive agent includes the following steps:

[0067] S1 Mix 10 g of graphene oxide, 1 g of carbon nanotubes, 4 g of acetylene black, 20 g of polyvinylpyrrolidone, and 100 mL of N-methylpyrrolidone, and stir at a speed of 700 rpm for 10 min to obtain a conductive agent pre-dispersion liquid;

[0068] S2 Shear the conductive agent pre-dispersion liquid at a speed of 6000 rpm for 10 min, perform high-pressure homogenization at 50 Mpa for 2 h, then grind it. The temperature of the grinding machine hopper is ≤ 55 °C, perform cyclic grinding for 1.5 h, add 5 g of polyvinylidene fluoride and stir for 5 min, filter and collect the filter cake. Dry the filter cake at 120 °C for 8 h to obtain the graphene conductive agent.

[0069] Test Example 1

[0070] Positive electrode sheet resistivity test:

[0071] Preparation of the positive electrode sheet The positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O 2, the graphene conductive agent prepared in Example 1 or Comparative Examples 1-3, and sodium carboxymethylcellulose were mixed at a mass ratio of 95:4:1 and stirred evenly to obtain a positive electrode paste. After coating, pre-drying, rolling, and drying, a positive electrode sheet was obtained. The volume resistivity of the positive electrode sheet was measured on the above-mentioned coated PET separator using a Guangzhou four-probe RTS-9 type four-probe tester. The test results are shown in Table 1:

[0072] Table 1 Test Results of Resistivity of Positive Electrode Sheets

[0073] Positive electrode sheet resistivity (Ω·cm) Example 1 9.68 Comparative Example 1 11.75 Comparative Example 2 12.31 Comparative Example 3 14.56

[0074] It can be seen from the experimental results in Table 1 that the positive electrode sheet prepared with the graphene conductive agent obtained in Example 1 has the smallest resistance. The difference between Example 1 and other comparative examples lies in the addition of modified graphene grafted with polymers on intercalated modified graphene. The modified graphene has good dispersibility and strong conductivity, overcomes the defect of easy agglomeration, makes the conductive agent evenly distributed, can form a continuous conductive network, reduces the internal resistance, and thus reduces the resistivity.

[0075] Test Example 2

[0076] Electrochemical performance test:

[0077] Preparation of the negative electrode sheet: Artificial graphite, conductive agent carbon black, thickening agent sodium carboxymethylcellulose, and binder polyvinylidene fluoride were uniformly dispersed in water at a mass ratio of 96:1:1:2 to obtain a negative electrode paste. After coating, rolling, and drying, a negative electrode sheet was obtained.

[0078] Assembly of the battery: The positive electrode sheet, negative electrode sheet, and ceramic-coated separator prepared with the graphene conductive agent obtained in Example 1 and Comparative Examples 1-3 according to the method of Test Example 1 were stacked, injected with electrolyte, and then encapsulated. After steps such as formation and grading, a lithium-ion soft-pack battery was obtained.

[0079] Room temperature cycle performance test: At 25 °C, constant current charging at 1C to 4.2V, constant voltage charging until cut-off at 0.05C, and discharging at 1C to 3V were counted as one cycle. After 500 charge-discharge cycles, the discharge capacity of the 500th cycle was recorded and the capacity retention rate of the 500th cycle was calculated. The calculation formula is:

[0080] Capacity retention rate of the 500th cycle = (Discharge capacity of the 500th cycle / Discharge capacity of the first cycle) × 100%

[0081] High temperature cycle performance test: At 60 °C, constant current charging at 1C to 4.2V, constant voltage charging until cut-off at 0.05C, and discharging at 1C to 3V were counted as one cycle. After 500 charge-discharge cycles, the discharge capacity of the 500th cycle was recorded and the capacity retention rate of the 500th cycle was calculated. The calculation formula is the same as the above formula. The test results are shown in Table 2.

[0082] Table 2 Cycling performance test results of lithium-ion batteries

[0083]

[0084] It can be seen from the experimental data in Table 2 that the graphene conductive agent prepared in Example 1 has the best electrochemical performance. The possible reason for this phenomenon is that after N,N-dimethylthiocarboxamide reacts with the strong alkylating agent methyl trifluoromethanesulfonate and then undergoes ion exchange with lithium bis(trifluoromethane)sulfonimide, it is inserted into the interlayer structure of graphene oxide through hydrogen bonding and electrostatic adsorption, increasing the distance between graphene layers. Then, a polymer is grafted onto the surface of the intercalated and modified graphene. This polymer enhances the flexibility of graphene, increases the active sites on the surface of graphene, improves the transport of electrons between the polymer and graphene, greatly increases the specific capacitance of the material. The modified graphene has good dispersibility and strong conductivity, overcomes the defect of easy agglomeration, and better exerts its excellent properties, enabling the prepared conductive agent to fully fill the pores between battery active materials, form an efficient three-dimensional conductive network in the electrode sheet, accelerate the electron transport speed during the charge and discharge process of the battery, significantly improve the electrochemical performance of the lithium-ion battery, and extend its service life.

Claims

1. A graphene conductive agent, characterized in that, by weight, it comprises the following components: 5-10 parts of modified graphene oxide, 3-5 parts of carbon material, 10-20 parts of polyvinylpyrrolidone, 80-100 parts of organic solvent, 3-5 parts of binder; the carbon material is one or a mixture of two of carbon nanotubes and acetylene black; The preparation method of the modified graphene oxide is as follows: 1) Under a nitrogen atmosphere, add 20-30 mL of methyl trifluoromethanesulfonate to a 50-80 mL dichloromethane solution containing 15-19 g of N,N-dimethylthiocarboxamide at 0-5 °C, stir and react at 20-40 °C for 10-12 h. After the reaction is completed, add 50-150 mL of anhydrous ether and 100-200 mL of ethyl acetate to obtain a mixed solution. Then place the mixed solution at -30~-20 °C for 1-2 h, solids will precipitate, filter, collect the solid matter, recrystallize with ethyl acetate, and remove the solvent under reduced pressure to obtain a colorless liquid; 2) Add 28-32 g of lithium bis(trifluoromethane)sulfonimide salt and 20-25 g of the colorless liquid to 40-60 mL of water, stir and react at 20-40 °C for 3-5 h. After the reaction is completed, collect the oil phase. The oil phase is washed 2-3 times with a 0.05-0.3 mol / L aqueous solution of lithium bis(trifluoromethane)sulfonimide and water respectively, and then concentrated under reduced pressure to obtain a pale yellow solid; 3) Place 5-10 g of graphene oxide in 300-500 mL of a 70-99 wt% ethanol aqueous solution for ultrasonic dispersion, add 2-3 g of the pale yellow solid prepared in step 2), heat to 30-40 °C and react for 10-12 h, centrifuge, filter and collect the precipitate. The precipitate is washed 2-3 times with a 70-99 wt% ethanol aqueous solution and then dried to obtain intercalated modified graphene oxide; 4) Mix 1-3 g of intercalated modified graphene oxide and 200-300 mL of tetrahydrofuran, ultrasonically treat for 30-60 min, add 10-20 mL of triethylamine, and dropwise add 3-5 mL of 2-bromo-2-methylpropionyl bromide at a rate of 1-2 drops / second under an argon atmosphere and stir and react for 10-12 h. After the reaction is completed, filter, collect the filter cake and wash it with chloroform, tetrahydrofuran, and water respectively, dry it at 30-50 °C for 4-6 h, then mix it with 100-200 mL of a mixed solvent of N,N-dimethylformamide and anhydrous methanol with a volume ratio of 3-5:1-2 and 1-2 g of copper chloride, ultrasonically treat for 30-60 min, add 2-3 g of methyl methacrylate and 0.5-1.5 g of pentamethyldiethylenetriamine, irradiate and stir and react under an ultraviolet lamp with a wavelength of 256-365 nm for 20-24 h to obtain a mixture, add 50-100 mL of tetrahydrofuran, stir for 10-20 min, filter, collect the filter residue and wash it with tetrahydrofuran, ethanol, and water 2-3 times respectively, and then dry it at 40-60 °C to obtain modified graphene oxide.

2. The graphene conductive agent according to claim 1, characterized in that: The organic solvent is one of N-methylpyrrolidone, N,N-dimethylformamide, methanol, ethanol, ethylene glycol, acetone, methyl ethyl ketone, tetrahydrofuran, toluene, xylene, dichlorotoluene or chloroform.

3. The graphene conductive agent according to claim 1, characterized in that: the binder is one of polyvinylidene fluoride and styrene-butadiene rubber.

4. The preparation method of the graphene conductive agent according to any one of claims 1-3, characterized in that it comprises the following steps: S1 Weigh each component according to the formula, and uniformly mix the modified graphene oxide, carbon material, polyvinylpyrrolidone and organic solvent to obtain a conductive agent pre-dispersion liquid; S2 Subject the conductive agent pre-dispersion liquid to high-speed shearing, high-pressure homogenization and then sanding, add the binder, stir, filter and dry to obtain the graphene conductive agent.

5. The preparation method of the graphene conductive agent according to claim 4, characterized in that: in step S2, the shearing speed is 5000-6000 rpm and the shearing time is 5-10 min.

6. The preparation method of the graphene conductive agent according to claim 4, characterized in that: in step S2, the high-pressure homogenization pressure is 10-100 MPa and the homogenization time is 1-2 h.

Images