Graphene-coated lithium-ion battery positive electrode material and preparation method thereof

Abstract

This invention belongs to the field of lithium battery cathode material technology, specifically relating to graphene-coated lithium-ion battery cathode materials and their preparation methods. The invention first prepares an iron phosphate precursor, then modifies it with nickel doping, adds graphene coating, and finally adds a lithium source, ascorbic acid, and carbon-forming materials for calcination. The resulting graphene-coated lithium-ion battery cathode material exhibits excellent charge-discharge performance and cycle efficiency. Nickel doping induces lattice distortion, promoting lithium-ion diffusion; the calcination and carbonization of the carbon-forming materials and graphene constructs a nitrogen-doped carbon-coated lithium iron phosphate network, improving the charge-discharge specific capacity and cycle stability of the lithium battery; the composite binder of this invention contains nitrile groups, which improve the mechanical strength of the cathode sheet by forming strong hydrogen bonds; the cathode sheet prepared by mixing the graphene-coated lithium-ion battery cathode material, conductive carbon black, and composite binder to obtain a cathode slurry, and then coating it onto a current collector, exhibits excellent stability and conductivity.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery cathode material technology, specifically relating to graphene-coated lithium-ion battery cathode materials and their preparation methods. Background Technology

[0002] In recent years, graphene has been favored for its excellent properties, such as high electron mobility, large specific surface area, and good electrical conductivity.

[0003] It has a two-dimensional nanolayer structure, good flexibility, high mechanical strength, and excellent chemical and thermodynamic stability, and is therefore widely used in the fields of storage and energy conversion.

[0004] Chinese invention patent CN110311113B discloses a graphene-coated lithium-ion battery cathode material. The first aspect of this invention provides a graphene-coated lithium-ion battery cathode material, the raw materials of which include graphene, a lithium source, and a metal hydroxide precursor; wherein the metal hydroxide precursor is a nickel-cobalt-manganese hydroxide precursor and / or a cobalt hydroxide precursor. The coin cell battery made from the graphene-coated cathode material provided by this invention exhibits lower DC internal resistance compared to ordinary cathode materials, and its discharge specific capacity, rate performance, and cycle performance are all improved to a certain extent, demonstrating superior electrochemical performance. However, the existing technology has a technical problem: it has not further improved the graphene coating structure and the composition of the carbonized coating layer of the lithium-ion battery cathode material to enhance the charge-discharge cycle and electrochemical performance of the lithium-ion battery cathode material. Summary of the Invention

[0005] The purpose of this invention is to provide graphene-coated lithium-ion battery cathode materials and their preparation methods, in order to solve the technical problem in the prior art that the charge-discharge cycle and electrochemical performance of lithium-ion battery cathode materials have not been improved through further modifications.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] The preparation method of graphene-coated lithium-ion battery cathode material includes the following steps:

[0008] S1. Preparation of ferric phosphate precursor: Dissolve phosphorus source and iron source in deionized water according to mass parts to prepare phosphorus source solution and iron source solution respectively. Add phosphorus source solution dropwise to iron source solution, add 0.5~2 parts of surfactant, adjust pH to 2~2.5 with ammonia water, stir for 1~2 h, and react at 150~180℃ for 10~12 h. After the reaction is completed, cool to room temperature, filter and collect filter cake, wash with deionized water and ethanol, and dry at 80~90℃ to obtain ferric phosphate precursor;

[0009] S2. Preparation of graphene-coated modified iron phosphate precursor: Add 1-5 parts by mass of iron phosphate precursor to graphene oxide dispersion, add 0.02-0.04 parts by mass of nickel chloride, stir for 1-2 h, react at 160-180℃ for 10-12 h, wash the gel with deionized water, freeze dry at 0-5℃ for 48-72 h, grind, and calcine at 500-600℃ for 4-6 h in an argon atmosphere to obtain graphene-coated modified iron phosphate precursor;

[0010] S3. Preparation of graphene-coated cathode material: By mass, 1-10 parts of lithium source and 1-1.5 parts of ascorbic acid are added to 2-5 parts of graphene-coated modified iron phosphate precursor, anhydrous ethanol is added and the mixture is ground. Under an argon atmosphere, the mixture is heated to 350-400℃ and pre-calcined for 3-4 hours. After cooling to room temperature, 1-2 parts of carbon-forming material are added and ground. Under an argon atmosphere, the mixture is heated to 600-700℃ and calcined for 10-12 hours to obtain graphene-coated cathode material.

[0011] Preferably, the phosphorus-iron molar ratio of the phosphorus source and the iron source in S1 is 0.9~1.1:0.9~1.1, the phosphorus source is any one of phosphoric acid and ammonium dihydrogen phosphate, and the iron source is any one of ferrous sulfate and ferrous nitrate.

[0012] Preferably, the surfactant in S1 is one or more of citric acid, hexadecyltrimethylammonium bromide, and sodium dodecylbenzenesulfonate.

[0013] Preferably, the graphene oxide in S2 has 1 to 5 layers, an average particle size of 4 to 5 μm, and a concentration of 5 to 10 mg / mL.

[0014] Preferably, the amount of graphene oxide added in S2 is 10-15% of the mass of the iron phosphate precursor.

[0015] Preferably, the lithium source in S3 is any one of lithium hydroxide, lithium carbonate, or lithium oxide.

[0016] Preferably, the method for preparing the carbon-forming material in S3 includes the following steps:

[0017] S11. According to the mass fraction, dry the tea seed shells at 75~85℃, pulverize and grind them through a 40~60 mesh sieve to obtain tea seed shell powder. Add 50~60 parts of tea seed shell powder to 200~300 parts of deionized water, soak at room temperature for 10~12 hours, filter and collect the solid, add 500~1000 parts of deionized water, extract at 80~90℃ for 4~6 hours, and collect the extract by suction filtration.

[0018] S12. Concentrate 500-1000 parts of the extract under reduced pressure to 200-300 parts by weight, add 78-80 parts of chloroform and 15-20 parts of n-butanol, stir at room temperature for 30-40 minutes to remove protein, centrifuge and filter to collect the supernatant, dialyze for 4-6 hours, add 500-1000 parts of anhydrous ethanol after dialysis, precipitate for 10-12 hours, centrifuge to collect the precipitate, freeze dry at -5-0℃ to obtain tea seed shell polysaccharide extract;

[0019] S13. By weight, 2-3 parts of tea seed shell polysaccharide extract, 1-2 parts of glucose and 0.5-0.6 parts of citric acid are mixed to prepare carbon material.

[0020] Preferably, the molecular weight cutoff for dialysis in S12 is 3500~5000 Da.

[0021] This invention provides a method for preparing the graphene-coated lithium-ion battery cathode material described above, resulting in a graphene-coated lithium-ion battery cathode material.

[0022] This invention also provides the application of graphene-coated lithium-ion battery cathode material for preparing lithium battery cathode sheets. The preparation method of the lithium battery cathode sheet includes the following steps:

[0023] S21. By weight, add 7-8 parts of graphene-coated lithium-ion battery cathode material, 1-2 parts of composite binder and 1-2 parts of conductive carbon black into a mixer, dilute with organic solvent to a solid content of 30-40 wt%, stir at 700-800 rpm for 2-3 min, then stir at 2000-2500 rpm for 15-20 min, and finally stir at 3000-4000 rpm for 2-3 min to obtain cathode slurry;

[0024] S22. The positive electrode slurry is coated on the current collector and dried at 60~80℃ for 12~18h. The slurry is then sliced ​​and pressed to obtain the positive electrode sheet for lithium batteries.

[0025] Preferably, the organic solvent in S21 is any one of N-methylpyrrolidone, n-hexane, or tetrahydrofuran.

[0026] Preferably, the thickness of the positive electrode slurry coating in S22 is 150~200μm, and the pressing pressure is 5~7t.

[0027] Preferably, the method for preparing the composite adhesive in S21 includes the following steps:

[0028] S31. By mass, add 40-45 parts of styrene, 3-4 parts of methyl acrylate, 8-10 parts of acrylonitrile, 1-1.5 parts of itaconic acid and 150-200 parts of deionized water to a reaction vessel, purge with nitrogen, add 1-2 parts of sodium dodecylbenzenesulfonate and 1-2 parts of ammonium persulfate, stir at 300-400 rpm for 1-2 hours, heat to 60-70℃ and react for 2-3 hours, then heat to 80-90℃ and react for 2-3 hours to obtain an emulsion;

[0029] S32. Add 20-30 parts by weight of lithium sulfate to the emulsion, stir at 1000-2000 rpm for 10-20 min to break the emulsion, filter to collect the solid, dry at 60-70℃ for 12-18 h, wash with deionized water, dry at 60-70℃ for 48-60 h, and grind to obtain latex powder.

[0030] S33. By weight, 5-6 parts of carboxymethyl cellulose, 50-60 parts of polyvinylidene fluoride and 40-50 parts of latex powder are mixed to prepare a composite adhesive.

[0031] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0032] 1. This invention first prepares an iron phosphate precursor, then modifies it by doping with nickel, and adds graphene coating to obtain a graphene-coated modified iron phosphate precursor. Finally, a lithium source, ascorbic acid, and carbon-forming materials are added and calcined to obtain a graphene-coated lithium-ion battery cathode material with excellent charge-discharge performance and cycle efficiency. Nickel doping can induce lattice distortion and promote lithium-ion diffusion. The polysaccharides in the carbon-forming materials suppress the particle size of lithium iron phosphate particles during calcination, making the cathode material easier to wet with electrolyte. The carbonization of carbon-forming materials and graphene constructs a nitrogen-doped carbon-coated lithium iron phosphate network, which improves the charge-discharge specific capacity and cycle stability of the lithium battery. The composite binder of this invention contains nitrile groups, which improve the mechanical strength of the cathode sheet by forming strong hydrogen bonds. The cathode sheet obtained by mixing the graphene-coated lithium-ion battery cathode material, conductive carbon black, and composite binder to prepare a cathode slurry and coating it on a current collector has excellent stability and conductivity.

[0033] 2. This invention involves extracting polysaccharide from tea seed shells, then mixing it with glucose and citric acid to prepare a carbon-forming material. The long-chain polysaccharides and monosaccharides in the tea seed shell polysaccharide extract have good adsorption capacity and can form a strong steric barrier effect, inhibiting the particle size growth of lithium iron phosphate during calcination and making the cathode material easier to wet with electrolyte. The tea seed shell polysaccharide extract contains nitrogen, which can construct a nitrogen-doped carbon-coated lithium iron phosphate network, facilitating rapid electron transfer and improving electronic conductivity. The carboxyl and hydroxyl groups in the citric acid molecules can react violently with the raw material molecules during calcination, releasing a large amount of gas and forming a porous structure, shortening the electron and lithium ion transport distance. The coating structure formed after calcination of the carbon-forming material and graphene improves the conductivity of the cathode material.

[0034] 3. The latex powder obtained by polymerizing styrene, methyl acrylate, acrylonitrile, and itaconic acid contains nitrile groups, which can form strong hydrogen bonds with the hydroxyl groups on the surface of the lithium-ion battery positive electrode material, thereby improving the mechanical strength of the positive electrode sheet. The presence of ester groups can improve the compatibility between the positive electrode sheet and the electrolyte. Carboxymethyl cellulose can adjust the pH value, reduce the oxidation degree of ferrous iron, reduce the formation of lithium phosphate on the electrode surface, and prevent the electrode structure from being damaged. The composite binder prepared by mixing latex powder, carboxymethyl cellulose, and polyvinylidene fluoride can improve the stability of the positive electrode sheet. Detailed Implementation

[0035] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] Example 1: The preparation method of the graphene-coated lithium-ion battery cathode material in this example includes the following steps:

[0037] S1. Preparation of ferric phosphate precursor: Phosphoric acid and ferrous sulfate were dissolved in deionized water according to mass parts to prepare a phosphoric acid solution with a concentration of 20 g / L and a ferrous sulfate solution with a concentration of 25 g / L. 50 parts of the phosphoric acid solution were added dropwise to 60 parts of the ferrous sulfate solution, 0.5 parts of citric acid were added, and the pH was adjusted to 2.1 with ammonia. After stirring for 1 h, the mixture was reacted at 150 °C for 12 h. After the reaction was completed, the mixture was cooled to room temperature, filtered to collect the filter cake, washed with deionized water and ethanol, and dried at 80 °C to obtain the ferric phosphate precursor.

[0038] S2. Preparation of graphene-coated modified iron phosphate precursor: By mass, 2 parts of iron phosphate precursor were added to 60 parts of 5 mg / mL graphene oxide dispersion, and 0.04 parts of nickel chloride were added. After stirring for 1-2 h, the mixture was reacted at 180℃ for 12 h. The gel was washed with deionized water, freeze-dried at 0℃ for 48 h, ground, and then calcined at 600℃ for 6 h in an argon atmosphere to obtain graphene-coated modified iron phosphate precursor.

[0039] S3. Preparation of graphene-coated cathode material: By mass, 2.4 parts of lithium hydroxide and 1 part of ascorbic acid were added to 2 parts of graphene-coated modified iron phosphate precursor, anhydrous ethanol was added and the mixture was ground. The mixture was pre-calcined at 400℃ for 4 hours in an argon atmosphere, cooled to room temperature, and then 1.5 parts of carbon-forming material were added and ground. The mixture was then calcined at 600℃ for 12 hours in an argon atmosphere to obtain graphene-coated cathode material.

[0040] The method for preparing the carbon-forming material in this embodiment includes the following steps:

[0041] S11. According to the mass fraction, dry the tea seed shells at 80℃, crush and grind them through a 40-mesh sieve to obtain tea seed shell powder. Add 60 parts of tea seed shell powder to 300 parts of deionized water, soak at room temperature for 12 hours, filter and collect the solid, add 1000 parts of deionized water, extract at 80℃ for 6 hours, and filter to obtain the extract.

[0042] S12. According to the mass fraction, 1000 parts of the extract were concentrated to 300 parts under reduced pressure, 80 parts of chloroform and 20 parts of n-butanol were added, and the mixture was stirred at room temperature for 40 min to remove protein. The supernatant was collected by centrifugation and filtration, and dialyzed for 6 h. After dialysis, 500 parts of anhydrous ethanol were added, and the precipitate was collected by centrifugation after 12 h. The precipitate was then freeze-dried at -5℃ to obtain tea seed shell polysaccharide extract.

[0043] S13. By weight, 3 parts of tea seed shell polysaccharide extract, 1 part of glucose and 0.5 parts of citric acid are mixed to prepare carbon material.

[0044] This embodiment describes the preparation of a graphene-coated lithium-ion battery cathode material according to the above-described method for preparing graphene-coated lithium-ion battery cathode materials.

[0045] The graphene-coated lithium-ion battery cathode material of this embodiment is used to prepare lithium battery cathode sheets. The preparation method of the lithium battery cathode sheet includes the following steps:

[0046] S21. According to the mass parts, add 7 parts of graphene-coated lithium-ion battery cathode material, 1 part of composite binder and 2 parts of conductive carbon black into a mixer, add N-methylpyrrolidone, dilute to a solid content of 30wt%, stir at 800rpm for 2min, then stir at 2000rpm for 15min, and finally stir at 4000rpm for 2min to obtain cathode slurry.

[0047] S22. The positive electrode slurry is coated on the current collector with a coating thickness of 150μm. The pressing pressure is 5t. The slurry is dried at 60℃ for 12h. The slurry is sliced ​​by a slicer and pressed to obtain the positive electrode sheet of lithium battery.

[0048] The method for preparing the composite adhesive in this embodiment includes the following steps:

[0049] S31. According to the mass parts, 45 parts of styrene, 4 parts of methyl acrylate, 8 parts of acrylonitrile, 1 part of itaconic acid and 150 parts of deionized water are added to a reaction vessel, nitrogen gas is introduced, 1 part of sodium dodecylbenzenesulfonate and 1 part of ammonium persulfate are added, and the mixture is stirred at 300 rpm for 1 hour, heated to 60°C and reacted for 2 hours, and then heated to 80°C and reacted for 2 hours to obtain an emulsion.

[0050] S32. By mass, 20 parts of lithium sulfate were added to the emulsion, stirred at 1000 rpm for 10 min to break the emulsion, the solid was collected by filtration, dried at 60°C for 12 h, washed with deionized water, dried at 60°C for 48 h, and ground to obtain latex powder.

[0051] S33. By weight, 5 parts of carboxymethyl cellulose, 50 parts of polyvinylidene fluoride and 45 parts of latex powder are mixed to prepare a composite adhesive.

[0052] Example 2, the preparation method of the graphene-coated lithium-ion battery cathode material of this example includes the following steps:

[0053] S1. Preparation of ferric phosphate precursor: According to the mass parts, ammonium dihydrogen phosphate and ferrous nitrate were dissolved in deionized water to prepare ammonium dihydrogen phosphate solution with a concentration of 20 g / L and ferrous nitrate solution with a concentration of 8 g / L. 50 parts of ammonium dihydrogen phosphate solution were added dropwise to 50 parts of ferrous nitrate solution, and 0.5 parts of hexadecyltrimethylammonium bromide were added. The pH was adjusted to 2.3 with ammonia water. After stirring for 2 h, the reaction was carried out at 180℃ for 12 h. After the reaction was completed, the mixture was cooled to room temperature, filtered and the filter cake was collected. The filter cake was washed with deionized water and ethanol and dried at 90℃ to obtain ferric phosphate precursor.

[0054] S2. Preparation of graphene-coated modified iron phosphate precursor: According to the mass ratio, 3 parts of iron phosphate precursor were added to 50 parts of 6 mg / mL graphene oxide dispersion, 0.02 parts of nickel chloride were added, stirred for 2 h, reacted at 160℃ for 12 h, the gel was washed with deionized water, freeze-dried at 4℃ for 72 h, ground, and calcined at 500℃ for 4 h in an argon atmosphere to obtain graphene-coated modified iron phosphate precursor.

[0055] S3. Preparation of graphene-coated cathode material: According to the mass ratio, 7 parts of lithium carbonate and 1.5 parts of ascorbic acid are added to 3 parts of graphene-coated modified iron phosphate precursor, anhydrous ethanol is added and ground, and the mixture is pre-calcined at 350℃ for 4 hours in an argon atmosphere. After cooling to room temperature, 1.5 parts of carbon-forming material are added and ground, and the mixture is calcined at 650℃ for 10 hours in an argon atmosphere to obtain graphene-coated cathode material.

[0056] This embodiment describes the preparation of a graphene-coated lithium-ion battery cathode material according to the above-described method for preparing graphene-coated lithium-ion battery cathode materials.

[0057] The graphene-coated lithium-ion battery cathode material of this embodiment is used to prepare lithium battery cathode sheets. The preparation method of the lithium battery cathode sheet includes the following steps:

[0058] S21. According to the mass parts, add 7 parts of graphene-coated lithium-ion battery cathode material, 2 parts of composite binder and 1 part of conductive carbon black into a mixer, add tetrahydrofuran to dilute to a solid content of 35wt%, stir at 700rpm for 2min, then stir at 2000rpm for 15min, and finally stir at 3000rpm for 2min to obtain cathode slurry.

[0059] S22. The positive electrode slurry is coated on the current collector with a coating thickness of 160μm. The pressing pressure is 6t. The slurry is dried at 80℃ for 12h. The slurry is sliced ​​by a slicer and pressed to obtain the positive electrode sheet of lithium battery.

[0060] The difference between the carbon-forming material in this embodiment and that in Example 1 is that the composition is replaced with 2 parts of tea seed shell polysaccharide extract, 2 parts of glucose and 0.6 parts of citric acid.

[0061] The composite adhesive in this embodiment is prepared using the same method as the composite adhesive in Example 1.

[0062] Example 3, the preparation method of the graphene-coated lithium-ion battery cathode material of this example includes the following steps:

[0063] S1. Preparation of ferric phosphate precursor: Phosphoric acid and ferrous nitrate were dissolved in deionized water according to mass parts to prepare a phosphoric acid solution with a concentration of 40 g / L and a ferrous nitrate solution with a concentration of 70 g / L. 50 parts of phosphoric acid solution were added dropwise to 50 parts of ferrous nitrate solution, and 2 parts of sodium dodecylbenzenesulfonate were added. The pH was adjusted to 2.2 with ammonia water. After stirring for 1 h, the mixture was reacted at 180 °C for 10 h. After the reaction was completed, the mixture was cooled to room temperature, filtered to collect the filter cake, washed with deionized water and ethanol, and dried at 80 °C to obtain the ferric phosphate precursor.

[0064] S2. Preparation of graphene-coated modified iron phosphate precursor: According to the mass ratio, 5 parts of iron phosphate precursor were added to 50 parts of 6 mg / mL graphene oxide dispersion, 0.04 parts of nickel chloride were added, stirred for 2 h, reacted at 180℃ for 12 h, the gel was washed with deionized water, freeze-dried at 0℃ for 72 h, ground, and calcined at 500℃ for 6 h in an argon atmosphere to obtain graphene-coated modified iron phosphate precursor.

[0065] S3. Preparation of graphene-coated cathode material: According to the mass ratio, 1 part of lithium oxide and 1.5 parts of ascorbic acid are added to 5 parts of graphene-coated modified iron phosphate precursor, anhydrous ethanol is added and ground, and the mixture is pre-calcined at 350℃ for 3 hours in an argon atmosphere. After cooling to room temperature, 1.3 parts of carbon-forming material are added and ground, and the mixture is calcined at 700℃ for 12 hours in an argon atmosphere to obtain graphene-coated cathode material.

[0066] This embodiment describes the preparation of a graphene-coated lithium-ion battery cathode material according to the above-described method for preparing graphene-coated lithium-ion battery cathode materials.

[0067] The graphene-coated lithium-ion battery cathode material of this embodiment is used to prepare lithium battery cathode sheets. The preparation method of the lithium battery cathode sheet includes the following steps:

[0068] S21. By mass, add 8 parts of graphene-coated lithium-ion battery cathode material, 1 part of composite binder and 1 part of conductive carbon black into a mixer, dilute with n-hexane to a solid content of 40 wt%, stir at 800 rpm for 3 min, then stir at 2500 rpm for 15 min, and finally stir at 4000 rpm for 2 min to obtain cathode slurry.

[0069] S22. The positive electrode slurry is coated on the current collector with a coating thickness of 200μm. The pressing pressure is 7t. The slurry is dried at 80℃ for 12h. The slurry is sliced ​​by a slicer and pressed to obtain the lithium battery positive electrode sheet.

[0070] The difference between the composite adhesive in this embodiment and the composite adhesive in Example 1 is that the composition is replaced with 6 parts of carboxymethyl cellulose, 60 parts of polyvinylidene fluoride and 40 parts of latex powder.

[0071] The carbon-forming material in this embodiment is prepared using the same method as the carbon-forming material in Example 1.

[0072] Example 4: The preparation method of the graphene-coated lithium-ion battery cathode material in this example includes the following steps:

[0073] S1. Preparation of ferric phosphate precursor: According to the mass parts, ammonium dihydrogen phosphate and ferrous sulfate were dissolved in deionized water to prepare ammonium dihydrogen phosphate solution with a concentration of 40 g / L and ferrous sulfate solution with a concentration of 40 g / L respectively. The ammonium dihydrogen phosphate solution was added dropwise to the ferrous sulfate solution, and 1 part of hexadecyltrimethylammonium bromide was added. The pH was adjusted to 2.3 with ammonia water. After stirring for 2 h, the reaction was carried out at 160℃ for 10 h. After the reaction was completed, the mixture was cooled to room temperature, filtered and the filter cake was collected. The filter cake was washed with deionized water and ethanol and dried at 80℃ to obtain ferric phosphate precursor.

[0074] S2. Preparation of graphene-coated modified iron phosphate precursor: 5 parts by mass of iron phosphate precursor were added to 75 parts by mass of 10 mg / mL graphene oxide dispersion, 0.03 parts by mass of nickel chloride were added, and the mixture was stirred for 1 h. The mixture was then reacted at 170 °C for 12 h. The gel was washed with deionized water, freeze-dried at 0 °C for 72 h, ground, and then calcined at 500 °C for 6 h in an argon atmosphere to obtain graphene-coated modified iron phosphate precursor.

[0075] S3. Preparation of graphene-coated cathode material: According to the mass parts, 1.2 parts of lithium hydroxide and 1.4 parts of ascorbic acid are added to 5 parts of graphene-coated modified iron phosphate precursor, anhydrous ethanol is added and ground, and the mixture is pre-calcined at 350℃ for 4 hours in an argon atmosphere. After cooling to room temperature, 1.8 parts of carbon-forming material are added and ground, and the mixture is calcined at 650℃ for 12 hours in an argon atmosphere to obtain graphene-coated cathode material.

[0076] This embodiment describes the preparation of a graphene-coated lithium-ion battery cathode material according to the above-described method for preparing graphene-coated lithium-ion battery cathode materials.

[0077] The graphene-coated lithium-ion battery cathode material of this embodiment is used to prepare lithium battery cathode sheets. The preparation method of the lithium battery cathode sheet includes the following steps:

[0078] S21. By weight, add 8 parts of graphene-coated lithium-ion battery cathode material, 1 part of composite binder and 1 part of conductive carbon black into a mixer, dilute with N-methylpyrrolidone to a solid content of 30 wt%, stir at 800 rpm for 3 min, then stir at 2500 rpm for 20 min, and finally stir at 4000 rpm for 2 min to obtain cathode slurry.

[0079] S22. The positive electrode slurry is coated on the current collector with a coating thickness of 190μm. The pressing pressure is 5t. The slurry is dried at 60℃ for 18h. The slurry is sliced ​​by a slicer and pressed to obtain the positive electrode sheet of lithium battery.

[0080] The difference between the composite adhesive in this embodiment and that in Example 1 is that the amount of latex powder used is replaced with 40 parts styrene, 3 parts methyl acrylate, 10 parts acrylonitrile, 1.5 parts itaconic acid and 200 parts deionized water.

[0081] The carbon-forming material in this embodiment is prepared using the same method as the carbon-forming material in Example 1.

[0082] Comparative Example 1 differs from Example 1 in that the cathode material is replaced with lithium iron phosphate without graphene coating.

[0083] Comparative Example 2 differs from Example 1 in that the carbon-forming material is replaced with glucose.

[0084] Comparative Example 3 differs from Example 1 in that the composite adhesive is replaced with polyvinylidene fluoride.

[0085] Performance testing

[0086] Preparation of lithium batteries for testing: The lithium battery positive electrode sheets prepared in each example and comparative example were assembled and pressed in an argon atmosphere using polyethylene as the battery separator and 1 mol / L lithium hexafluorophosphate added to a solvent made of ethylene carbonate, diethyl carbonate and dimethyl carbonate in a volume ratio of 1:1:1 as the electrolyte.

[0087] The Newway Battery Testing System was used to test the initial discharge capacity of the test lithium batteries prepared in each embodiment and comparative example at 0.5C and 1C rates, and the capacity retention rate after 100 cycles at 1C rate.

[0088] The test results are shown in Table 1:

[0089] Table 1 Test Results

[0090] Serial number Initial discharge capacity at 0.5C rate (mAh / g) First discharge capacity at 1C rate (mAh / g) Capacity retention rate (%) after 100 cycles Example 1 176.9 156.9 90.1 Example 2 177.2 157.2 89.6 Example 3 175.6 155.6 89.4 Example 4 177.5 157.4 88.5 Comparative Example 1 149.5 134.3 80.3 Comparative Example 2 158.2 142.9 86.9 Comparative Example 3 168.4 156.5 83.7

[0091] As shown in Table 1, the lithium batteries prepared in Examples 1-4 had initial discharge capacities of 175.6-177.5 mAh / g and 155.6-157.4 mAh / g at 0.5C and 1C rates, respectively, and a capacity retention rate of 88.5-90.1% after 100 cycles. In Comparative Example 1, the cathode material was replaced with uncoated lithium iron phosphate. Due to the poor conductivity and low lithium-ion diffusion coefficient of lithium iron phosphate, the carbon-forming material could not form a conductive network to coat the cathode material, resulting in initial discharge capacities of only 149.5 mAh / g and 134.3 mAh / g at 0.5C and 1C rates, respectively. This indicates that the graphene-coated lithium-ion battery cathode material prepared in this invention has excellent charge / discharge specific capacity and cycle efficiency when used to prepare lithium battery cathode sheets. Furthermore, this demonstrates that the graphene-coated lithium-ion battery cathode material prepared in this invention has excellent conductivity and cycle stability.

[0092] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

[0093] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to specific implementations. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for preparing graphene-coated lithium-ion battery cathode material, characterized in that, The steps include: S1. According to the mass parts, dissolve the phosphorus source and iron source in deionized water to prepare phosphorus source solution and iron source solution respectively. Add the phosphorus source solution dropwise to the iron source solution, add 0.5~2 parts of surfactant, adjust the pH to 2~2.5 with ammonia water, stir for 1~2 h, and react at 150~180℃ for 10~12 h. After the reaction is completed, cool to room temperature, filter and collect the filter cake, wash with deionized water and ethanol, and dry at 80~90℃ to obtain the iron phosphate precursor. S2. According to the mass ratio, 1-5 parts of iron phosphate precursor are added to graphene oxide dispersion, 0.02-0.04 parts of nickel chloride are added, and the mixture is stirred for 1-2 hours. Then, it is reacted at 160-180℃ for 10-12 hours. The gel is washed with deionized water, freeze-dried at 0-5℃ for 48-72 hours, ground, and then calcined at 500-600℃ for 4-6 hours in an argon atmosphere to obtain graphene-coated modified iron phosphate precursor. S3. By mass, 1-10 parts of lithium source and 1-1.5 parts of ascorbic acid are added to 2-5 parts of graphene-coated modified iron phosphate precursor, anhydrous ethanol is added and the mixture is ground. Under an argon atmosphere, the mixture is heated to 350-400℃ and pre-calcined for 3-4 hours. After cooling to room temperature, 1-2 parts of carbon-forming material are added and ground. Under an argon atmosphere, the mixture is heated to 600-700℃ and calcined for 10-12 hours to obtain graphene-coated cathode material. The method for preparing the carbon-forming material in S3 includes the following steps: S11. According to the mass fraction, dry the tea seed shells at 75~85℃, pulverize and grind them through a 40~60 mesh sieve to obtain tea seed shell powder. Add 50~60 parts of tea seed shell powder to 200~300 parts of deionized water, soak at room temperature for 10~12 hours, filter and collect the solid, add 500~1000 parts of deionized water, extract at 80~90℃ for 4~6 hours, and collect the extract by suction filtration. S12. Concentrate 500-1000 parts of the extract under reduced pressure to 200-300 parts by weight, add 78-80 parts of chloroform and 15-20 parts of n-butanol, stir at room temperature for 30-40 minutes to remove protein, centrifuge and filter to collect the supernatant, dialyze for 4-6 hours, add 500-1000 parts of anhydrous ethanol after dialysis, precipitate for 10-12 hours, centrifuge to collect the precipitate, freeze dry at -5-0℃ to obtain tea seed shell polysaccharide extract; S13. By weight, 2-3 parts of tea seed shell polysaccharide extract, 1-2 parts of glucose and 0.5-0.6 parts of citric acid are mixed to prepare carbon material.

2. The method for preparing the lithium-ion battery cathode material according to claim 1, characterized in that, The molecular weight cutoff for dialysis in S12 is 3500~5000 Da.

3. The method for preparing the graphene-coated lithium-ion battery cathode material according to claim 1, characterized in that, The phosphorus-iron molar ratio of the phosphorus source and the iron source in S1 is 0.9~1.1:0.9~1.

1. The phosphorus source is any one of phosphoric acid and ammonium dihydrogen phosphate, the iron source is any one of ferrous sulfate and ferrous nitrate, and the surfactant is one or more combinations of citric acid, hexadecyltrimethylammonium bromide and sodium dodecylbenzenesulfonate.

4. The method for preparing the graphene-coated lithium-ion battery cathode material according to claim 1, characterized in that, In S2, the number of graphene oxide layers is 1 to 5, the average particle size of graphene oxide is 4 to 5 μm, the concentration of graphene oxide dispersion is 5 to 10 mg / mL, and the amount of graphene oxide added is 10 to 15% of the mass of the iron phosphate precursor; in S3, the lithium source is any one of lithium hydroxide, lithium carbonate, or lithium oxide.

5. The lithium-ion battery cathode material prepared by the method for preparing graphene-coated lithium-ion battery cathode material according to any one of claims 1-4.

6. The application of the graphene-coated lithium-ion battery cathode material according to claim 5, characterized in that, The process for preparing lithium-ion battery cathode sheets includes the following steps: S21. By weight, add 70-80 parts of graphene-coated lithium-ion battery cathode material, 10-20 parts of composite binder and 10-20 parts of conductive carbon black into a mixer, dilute with organic solvent to a solid content of 30-40 wt%, stir at 700-800 rpm for 2-3 min, then stir at 2000-2500 rpm for 15-20 min, and finally stir at 3000-4000 rpm for 2-3 min to obtain cathode slurry; S22. The positive electrode slurry is coated on the current collector and dried at 60~80℃ for 12~18h. The slurry is then sliced ​​and pressed to obtain the positive electrode sheet for lithium batteries.

7. The application of the graphene-coated lithium-ion battery cathode material according to claim 6, characterized in that, The preparation method of the composite adhesive in S21 includes the following steps: S31. By mass, add 40-45 parts of styrene, 3-4 parts of methyl acrylate, 8-10 parts of acrylonitrile, 1-1.5 parts of itaconic acid and 150-200 parts of deionized water to a reaction vessel, purge with nitrogen, add 1-2 parts of sodium dodecylbenzenesulfonate and 1-2 parts of ammonium persulfate, stir at 300-400 rpm for 1-2 hours, heat to 60-70℃ and react for 2-3 hours, then heat to 80-90℃ and react for 2-3 hours to obtain an emulsion; S32. Add 20-30 parts by weight of lithium sulfate to the emulsion, stir at 1000-2000 rpm for 10-20 min to break the emulsion, filter to collect the solid, dry at 60-70℃ for 12-18 h, wash with deionized water, dry at 60-70℃ for 48-60 h, and grind to obtain latex powder. S33. By weight, 5-6 parts of carboxymethyl cellulose, 50-60 parts of polyvinylidene fluoride and 40-50 parts of latex powder are mixed to prepare a composite adhesive.

8. The application of the graphene-coated lithium-ion battery cathode material according to claim 6, characterized in that, The organic solvent in S21 is any one of N-methylpyrrolidone, n-hexane, or tetrahydrofuran; the thickness of the positive electrode slurry coating in S22 is 150~200μm, and the pressing pressure is 5~7t.