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

Abstract

The invention belongs to the technical field of lithium ion battery positive electrode materials, and particularly relates to a graphene-coated lithium ion battery positive electrode material and a preparation method thereof. The preparation method comprises the following steps: firstly preparing an iron phosphate precursor, then doping nickel for modification, adding graphene for coating, and finally adding a lithium source, ascorbic acid and a carbon forming material for calcining, thereby obtaining the graphene-coated lithium ion battery positive electrode material which has excellent charge-discharge performance and cycle efficiency. Lattice distortion can be caused by nickel doping, and diffusion of lithium ions is promoted; the carbon-forming material and graphene are calcined and carbonized to construct a nitrogen-doped carbon-coated lithium iron phosphate network, so that the charge-discharge specific capacity and the cycle stability of the lithium battery are improved; the composite binder contains a nitrile group, and the mechanical strength of the positive plate is improved by forming a strong hydrogen bond; after the graphene-coated lithium ion battery positive electrode material, the conductive carbon black and the composite binder are mixed to prepare the positive electrode slurry, the current collector is coated with the positive electrode slurry, and the prepared positive plate has excellent stability and conductivity.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium battery cathode materials, and particularly relates to a graphene-coated lithium ion battery cathode material and a preparation method thereof. Background Art

[0002] In recent years, graphene has been widely used in the fields of storage and energy conversion due to its excellent properties such as fast electron mobility, large specific surface area, good electrical conductivity, having a two-dimensional nano-layered structure, good flexibility, high mechanical strength, excellent chemical and thermodynamic stability, etc.

[0003] Chinese invention patent with publication number CN110311113B discloses a graphene-coated lithium ion battery cathode material. In the first aspect of this invention, a graphene-coated lithium ion battery cathode material is provided, and the preparation raw materials include graphene, a lithium source, and a precursor of metal hydroxide; wherein, the precursor of metal hydroxide is a precursor of nickel cobalt manganese hydroxide and / or a precursor of cobalt hydroxide. The button battery prepared from the graphene-coated cathode material provided by this invention has a lower DC internal resistance compared with ordinary cathode materials, and its discharge specific capacity, rate performance, and cycle performance are all improved to a certain extent, showing more excellent electrochemical performance. However, the existing technology has the technical problem that the graphene coating structure and the composition of the coated carbonized layer of the lithium ion battery cathode material are not further improved to improve the charge and discharge cycle and electrochemical performance of the lithium battery cathode material. Summary of the Invention

[0004] The purpose of the present invention is to provide a graphene-coated lithium ion battery cathode material and a preparation method thereof, which are used to solve the technical problem that in the prior art, the lithium ion battery cathode material is not further improved to improve the charge and discharge cycle and electrochemical performance of the lithium battery cathode material.

[0005] To achieve the above purpose, the present invention adopts the following technical solutions: A preparation method of a graphene-coated lithium ion battery cathode material includes the following steps: S1. Prepare a ferric phosphate precursor: By mass, dissolve a phosphorus source and an iron source in deionized water respectively to obtain a phosphorus source solution and an iron source solution. Drop the phosphorus source solution into the iron source solution, add 0.5 - 2 parts of a surfactant, adjust the pH to 2 - 2.5 with ammonia water, stir for 1 - 2 h, then react at 150 - 180 °C for 10 - 12 h. After the reaction ends, cool to room temperature, filter and collect the filter cake, wash it with deionized water and ethanol, and dry it at 80 - 90 °C to obtain the ferric phosphate precursor; S2. Preparation of graphene-coated modified iron phosphate precursor: By mass, add 1-5 parts of iron phosphate precursor into the graphene oxide dispersion, add 0.02-0.04 parts of nickel chloride, stir for 1-2 h, react at 160-180 °C for 10-12 h, wash the gel with deionized water, freeze-dry at 0-5 °C for 48-72 h, grind, and then calcine at 500-600 °C for 4-6 h in an argon atmosphere to obtain the graphene-coated modified iron phosphate precursor; S3. Preparation of graphene-coated cathode material: By mass, add 1-10 parts of lithium source and 1-1.5 parts of ascorbic acid into 2-5 parts of graphene-coated modified iron phosphate precursor, add anhydrous ethanol and grind, pre-calcine at 350-400 °C for 3-4 h in an argon atmosphere, cool to room temperature, add 1-2 parts of carbon-forming material and grind, and then calcine at 600-700 °C for 10-12 h in an argon atmosphere to obtain the graphene-coated cathode material.

[0006] Preferably, in S1, the molar ratio of phosphorus to iron of the phosphorus source and iron source 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.

[0007] Preferably, in S1, the surfactant is one or a combination of citric acid, cetyltrimethylammonium bromide, and sodium dodecylbenzenesulfonate.

[0008] Preferably, in S2, the number of layers of graphene oxide is 1-5 layers, the average particle size of graphene oxide is 4-5 μm, and the concentration of the graphene oxide dispersion is 5-10 mg / mL.

[0009] Preferably, in S2, the addition amount of graphene oxide is 10-15% of the mass of the iron phosphate precursor.

[0010] Preferably, in S3, the lithium source is any one of lithium hydroxide, lithium carbonate, and lithium oxide.

[0011] Preferably, the preparation method of the carbon-forming material in S3 includes the following steps: S11. By mass, dry the tea seed husk at 75-85 °C, crush and grind it through a 40-60 mesh sieve to obtain tea seed husk powder. Add 50-60 parts of tea seed husk powder into 200-300 parts of deionized water, soak at room temperature for 10-12 h, filter and collect the solid, add it into 500-1000 parts of deionized water, extract at 80-90 °C for 4-6 h, and collect the extract by suction filtration; S12. By mass parts, concentrate 500 - 1000 parts of the extract under reduced pressure to 200 - 300 parts, add 78 - 80 parts of chloroform and 15 - 20 parts of n-butanol, stir for 30 - 40 min at room temperature for protein removal treatment, centrifuge and filter to collect the supernatant, perform dialysis treatment for 4 - 6 h. After dialysis is completed, add 500 - 1000 parts of absolute ethanol, perform alcohol precipitation for 10 - 12 h, then centrifuge to collect the precipitate, and freeze-dry at -5 - 0 °C to obtain the tea seed shell polysaccharide extract; S13. By mass parts, mix 2 - 3 parts of the tea seed shell polysaccharide extract, 1 - 2 parts of glucose and 0.5 - 0.6 parts of citric acid to obtain the carbon material.

[0012] Preferably, the dialysis cut-off molecular weight in S12 is 3500 - 5000 Da.

[0013] The present invention provides the graphene-coated lithium-ion battery cathode material prepared by the preparation method of the graphene-coated lithium-ion battery cathode material described above.

[0014] The present invention also provides the application of the graphene-coated lithium-ion battery cathode material, which is used for preparing a lithium battery cathode plate. The preparation method of the lithium battery cathode plate includes the following steps: S21. By mass parts, add 7 - 8 parts of the graphene-coated lithium-ion battery cathode material, 1 - 2 parts of the composite binder and 1 - 2 parts of conductive carbon black into a blender, dilute with an organic solvent to a solid content of 30 - 40 wt%, stir at a speed of 700 - 800 rpm for 2 - 3 min, then stir at a speed of 2000 - 2500 rpm for 15 - 20 min, and finally stir at a speed of 3000 - 4000 rpm for 2 - 3 min to obtain the cathode slurry; S22. Coat the cathode slurry on the current collector, dry at 60 - 80 °C for 12 - 18 h, slice with a slicing machine, and press to obtain the lithium battery cathode plate.

[0015] Preferably, the organic solvent in S21 is any one of N-methylpyrrolidone, n-hexane, and tetrahydrofuran.

[0016] Preferably, the coating thickness of the cathode slurry in S22 is 150 - 200 μm, and the pressure for pressing is 5 - 7 t.

[0017] Preferably, the preparation method of the composite binder 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 into a reaction kettle. Introduce nitrogen gas, add 1 - 2 parts of sodium dodecylbenzenesulfonate and 1 - 2 parts of ammonium persulfate, then stir at a speed of 300 - 400 rpm for 1 - 2 h. Heat up to 60 - 70 °C and react for 2 - 3 h, then heat up to 80 - 90 °C and react for 2 - 3 h to obtain an emulsion. S32. By mass, add 20 - 30 parts of lithium sulfate into the emulsion, stir at a speed of 1000 - 2000 rpm for 10 - 20 min to demulsify, filter and collect the solid, dry it at 60 - 70 °C for 12 - 18 h, wash it with deionized water, dry it at 60 - 70 °C for 48 - 60 h, and grind it to obtain latex powder. S33. By mass, mix 5 - 6 parts of carboxymethyl cellulose, 50 - 60 parts of polyvinylidene fluoride and 40 - 50 parts of latex powder to obtain a composite binder.

[0018] In summary, due to the adoption of the above technical solutions, the beneficial effects of the present invention are as follows: 1. The present invention first prepares a lithium iron phosphate precursor, then dopes nickel for modification, adds graphene coating to obtain a modified lithium iron phosphate precursor coated with graphene, and finally adds a lithium source, ascorbic acid and a carbon - forming material to calcine to obtain a lithium - ion battery cathode material coated with graphene, which has excellent charge - discharge performance and cycle efficiency. Nickel doping can cause lattice distortion and promote the diffusion of lithium ions; the polysaccharide substances in the carbon - forming material inhibit the particle size of lithium iron phosphate particles during the calcination process, making the cathode material easily wetted by the electrolyte; the carbon - forming material and graphene are calcined and carbonized to construct 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 the present invention contains nitrile groups and improves the mechanical strength of the cathode sheet by forming strong hydrogen bonds; after mixing the lithium - ion battery cathode material coated with graphene, conductive carbon black and the composite binder to obtain a cathode slurry and coating it on a current collector, the obtained cathode sheet has excellent stability and conductivity.

[0019] 2. The present invention extracts polysaccharide extract from tea seed hulls, then adds glucose and citric acid and mixes them to prepare a carbon material. The long-chain polysaccharides and monosaccharides in the polysaccharide extract of tea seed hulls have good adsorption capacity, can form a strong steric hindrance barrier effect, inhibit the particle size growth of lithium iron phosphate during the calcination process, and make the cathode material easily infiltrated by the electrolyte; the polysaccharide extract of tea seed hulls contains nitrogen elements, can construct a nitrogen-doped carbon-coated lithium iron phosphate network, helps the rapid transfer of electrons, and improves the electronic conductivity; the carboxyl and hydroxyl groups in the citric acid molecule can react violently with the raw material molecules during the calcination process, release a large amount of gas, form a pore structure, and shorten the transmission distance of electrons and lithium ions; the coating structure formed by the carbon material and graphene after calcination improves the conductivity of the cathode material.

[0020] 3. The latex powder prepared by polymerizing styrene, methyl acrylate, acrylonitrile and itaconic acid in the present invention contains nitrile groups, can form strong hydrogen bonds with the hydroxyl groups on the surface of the lithium ion battery cathode material, improves the mechanical strength of the cathode sheet, and contains ester groups that can improve the compatibility between the cathode sheet and the electrolyte; carboxymethyl cellulose can adjust the pH value, reduce the degree of oxidation of ferrous iron, reduce the formation of lithium phosphate on the electrode surface, and prevent the destruction of the electrode structure; the composite binder prepared by mixing the latex powder, carboxymethyl cellulose and polyvinylidene fluoride can improve the stability of the cathode sheet. Detailed implementation mode

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0022] Example 1. The preparation method of the graphene-coated lithium ion battery cathode material in this example includes the following steps: S1. Prepare a ferric phosphate precursor: By mass, dissolve phosphoric acid and ferrous sulfate in deionized water respectively to obtain a phosphoric acid solution with a concentration of 20 g / L and a ferrous sulfate solution with a concentration of 25 g / L. Drop 50 parts of the phosphoric acid solution into 60 parts of the ferrous sulfate solution, add 0.5 part of citric acid, adjust the pH to 2.1 with ammonia water, stir for 1 h, then react at 150 °C for 12 h. After the reaction is completed, cool to room temperature, filter and collect the filter cake, wash it with deionized water and ethanol, and dry it at 80 °C to obtain the ferric phosphate precursor; S2. Preparation of modified iron phosphate precursor coated with graphene: By mass, add 2 parts of the iron phosphate precursor to 60 parts of a 5 mg / mL graphene oxide dispersion, add 0.04 part of nickel chloride, stir for 1 - 2 h, then react at 180 °C for 12 h. Wash the gel with deionized water, freeze-dry at 0 °C for 48 h, grind, and then calcine in an argon atmosphere by heating to 600 °C for 6 h to obtain the modified iron phosphate precursor coated with graphene; S3. Preparation of cathode material coated with graphene: By mass, add 2.4 parts of lithium hydroxide and 1 part of ascorbic acid to 2 parts of the modified iron phosphate precursor coated with graphene, add anhydrous ethanol and grind. In an argon atmosphere, heat to 400 °C and pre-calcine for 4 h. Cool to room temperature, then add 1.5 parts of carbon-forming material and grind. In an argon atmosphere, heat to 600 °C and calcine for 12 h to obtain the cathode material coated with graphene.

[0023] The preparation method of the carbon-forming material in this example includes the following steps: S11. By mass, dry the tea seed shells at 80 °C, crush and grind them through a 40-mesh sieve to obtain tea seed shell powder. Add 60 parts of the tea seed shell powder to 300 parts of deionized water, soak at room temperature for 12 h, filter and collect the solid, add it to 1000 parts of deionized water, extract at 80 °C for 6 h, and filter with suction to collect the extract; S12. By mass, concentrate 1000 parts of the extract under reduced pressure to 300 parts, add 80 parts of chloroform and 20 parts of n-butanol, stir at room temperature for 40 min for protein removal treatment, centrifuge and filter to collect the supernatant, perform dialysis treatment for 6 h. After dialysis is completed, add 500 parts of anhydrous ethanol, perform alcohol precipitation for 12 h, centrifuge to collect the precipitate, and freeze-dry at -5 °C to obtain the tea seed shell polysaccharide extract; S13. By mass, mix 3 parts of the tea seed shell polysaccharide extract, 1 part of glucose and 0.5 part of citric acid to obtain the carbon-forming material.

[0024] The lithium-ion battery cathode material coated with graphene prepared in this example according to the above preparation method of the lithium-ion battery cathode material coated with graphene.

[0025] The application of the lithium-ion battery cathode material coated with graphene in this example is used to prepare a lithium battery cathode sheet. The preparation method of the lithium battery cathode sheet includes the following steps: S21. By mass, add 7 parts of the lithium-ion battery cathode material coated with graphene, 1 part of the composite binder and 2 parts of conductive carbon black to a blender, add N-methylpyrrolidone, dilute to a solid content of 30 wt%, stir at a speed of 800 rpm for 2 min, then stir at a speed of 2000 rpm for 15 min, and finally stir at a speed of 4000 rpm for 2 min to obtain the cathode slurry; S22. Coating the positive electrode slurry on the current collector, with the coating thickness of the positive electrode slurry being 150 μm, the pressure of tablet pressing being 5 t, drying at 60 °C for 12 h, slicing with a slicing machine, and tablet pressing to obtain the positive electrode sheet of the lithium battery.

[0026] The preparation method of the composite binder in this embodiment includes the following steps: S31. By mass, adding 45 parts of styrene, 4 parts of methyl acrylate, 8 parts of acrylonitrile, 1 part of itaconic acid, and 150 parts of deionized water into a reaction kettle, introducing nitrogen, adding 1 part of sodium dodecylbenzenesulfonate and 1 part of ammonium persulfate, and then stirring at a speed of 300 rpm for 1 h, heating to 60 °C and reacting for 2 h, and then heating to 80 °C and reacting for 2 h to obtain an emulsion; S32. By mass, adding 20 parts of lithium sulfate into the emulsion, stirring at a speed of 1000 rpm for 10 min for demulsification, filtering to collect the solid, drying at 60 °C for 12 h, washing with deionized water, drying at 60 °C for 48 h, and grinding to obtain the latex powder; S33. By mass, mixing 5 parts of carboxymethyl cellulose, 50 parts of polyvinylidene fluoride, and 45 parts of the latex powder to obtain the composite binder.

[0027] Example 2. The preparation method of the graphene-coated positive electrode material for lithium-ion batteries in this embodiment includes the following steps: S1. Preparing the iron phosphate precursor: By mass, dissolving ammonium dihydrogen phosphate and ferrous nitrate in deionized water respectively to obtain an ammonium dihydrogen phosphate solution with a concentration of 20 g / L and a ferrous nitrate solution with a concentration of 8 g / L, dropping 50 parts of the ammonium dihydrogen phosphate solution into 50 parts of the ferrous nitrate solution, adding 0.5 part of cetyltrimethylammonium bromide, adjusting the pH to 2.3 with ammonia water, stirring for 2 h, reacting at 180 °C for 12 h, cooling to room temperature after the reaction, filtering to collect the filter cake, washing with deionized water and ethanol, and drying at 90 °C to obtain the iron phosphate precursor; S2. Preparing the graphene-coated modified iron phosphate precursor: By mass, adding 3 parts of the iron phosphate precursor into 50 parts of a 6 mg / mL graphene oxide dispersion liquid, adding 0.02 part of nickel chloride, stirring for 2 h, reacting at 160 °C for 12 h, washing the gel with deionized water, freeze-drying at 4 °C for 72 h, grinding, and then calcining at 500 °C for 4 h in an argon atmosphere to obtain the graphene-coated modified iron phosphate precursor; S3. Preparation of graphene-coated cathode material: By mass, add 7 parts of lithium carbonate and 1.5 parts of ascorbic acid to 3 parts of the modified iron phosphate precursor coated with graphene, add anhydrous ethanol and grind. In an argon atmosphere, heat to 350 °C and pre-calcine for 4 h. Cool to room temperature and then add 1.5 parts of carbon-forming material and grind. In an argon atmosphere, heat to 650 °C and calcine for 10 h to obtain the graphene-coated cathode material.

[0028] The graphene-coated lithium-ion battery cathode material prepared in this example according to the above preparation method of the graphene-coated lithium-ion battery cathode material.

[0029] Application of the graphene-coated lithium-ion battery cathode material of this example, used for preparing a lithium battery cathode sheet. The preparation method of the lithium battery cathode sheet includes the following steps: S21. By mass, add 7 parts of the graphene-coated lithium-ion battery cathode material, 2 parts of the composite binder and 1 part of conductive carbon black to a blender, add tetrahydrofuran to dilute to a solid content of 35 wt%, stir at a speed of 700 rpm for 2 min, then stir at a speed of 2000 rpm for 15 min, and finally stir at a speed of 3000 rpm for 2 min to obtain the positive electrode slurry. S22. Coat the positive electrode slurry on the current collector. The coating thickness of the positive electrode slurry is 160 μm, the pressure of pressing is 6 t, dry at 80 °C for 12 h, slice with a slicing machine, and press to obtain the lithium battery cathode sheet.

[0030] The difference between the carbon-forming material of this example and that of Example 1 is that the composition content is replaced with 2 parts of tea seed shell polysaccharide extract, 2 parts of glucose and 0.6 part of citric acid.

[0031] The preparation method of the composite binder of this example is the same as that of the composite binder in Example 1.

[0032] Example 3. The preparation method of the graphene-coated lithium-ion battery cathode material of this example includes the following steps: S1. Preparation of iron phosphate precursor: By mass, dissolve phosphoric acid and ferrous nitrate in deionized water respectively to obtain a phosphoric acid solution with a concentration of 40 g / L and a ferrous nitrate solution with a concentration of 70 g / L. Drop 50 parts of the phosphoric acid solution into 50 parts of the ferrous nitrate solution, add 2 parts of sodium dodecylbenzenesulfonate, adjust the pH to 2.2 with ammonia water, stir for 1 h, then react at 180 °C for 10 h. After the reaction, cool to room temperature, filter and collect the filter cake, wash with deionized water and ethanol, and dry at 80 °C to obtain the iron phosphate precursor. S2. Preparation of modified iron phosphate precursor coated with graphene: By mass, add 5 parts of the iron phosphate precursor to 50 parts of a 6 mg / mL graphene oxide dispersion, add 0.04 parts of nickel chloride, stir for 2 h, react at 180 °C for 12 h, wash the gel with deionized water, freeze-dry at 0 °C for 72 h, grind, and then calcine in an argon atmosphere by heating to 500 °C for 6 h to obtain the modified iron phosphate precursor coated with graphene; S3. Preparation of cathode material coated with graphene: By mass, add 1 part of lithium oxide and 1.5 parts of ascorbic acid to 5 parts of the modified iron phosphate precursor coated with graphene, add anhydrous ethanol and grind. In an argon atmosphere, heat to 350 °C and pre-calcine for 3 h. Cool to room temperature and then add 1.3 parts of carbon-forming material and grind. In an argon atmosphere, heat to 700 °C and calcine for 12 h to obtain the cathode material coated with graphene.

[0033] The lithium-ion battery cathode material coated with graphene prepared in this example is prepared according to the above preparation method of the lithium-ion battery cathode material coated with graphene.

[0034] Application of the lithium-ion battery cathode material coated with graphene in this example, used for preparing a lithium battery cathode sheet. The preparation method of the lithium battery cathode sheet includes the following steps: S21. By mass, add 8 parts of the lithium-ion battery cathode material coated with graphene, 1 part of the composite binder, and 1 part of conductive carbon black to a blender, dilute with n-hexane to a solid content of 40 wt%, stir at a speed of 800 rpm for 3 min, then stir at a speed of 2500 rpm for 15 min, and finally stir at a speed of 4000 rpm for 2 min to obtain the cathode slurry; S22. Coat the cathode slurry on the current collector. The coating thickness of the cathode slurry is 200 μm, the pressure for pressing is 7 t, dry at 80 °C for 12 h, slice with a slicing machine, and press to obtain the lithium battery cathode sheet.

[0035] The difference between the composite binder in this example and the composite binder in Example 1 is that the composition content is replaced with 6 parts of carboxymethyl cellulose, 60 parts of polyvinylidene fluoride, and 40 parts of latex powder.

[0036] The preparation method of the carbon-forming material in this example is the same as that of the carbon-forming material in Example 1.

[0037] Example 4. The preparation method of the lithium-ion battery cathode material coated with graphene in this example includes the following steps: S1. Preparation of iron phosphate precursor: By mass, ammonium dihydrogen phosphate and ferrous sulfate are respectively dissolved in deionized water to obtain an ammonium dihydrogen phosphate solution with a concentration of 40 g / L and a ferrous sulfate solution with a concentration of 40 g / L. The ammonium dihydrogen phosphate solution is dropped into the ferrous sulfate solution, 1 part of cetyltrimethylammonium bromide is added, the pH is adjusted to 2.3 with ammonia water, and after stirring for 2 h, the reaction is carried out at 160 °C for 10 h. After the reaction is completed, it is cooled to room temperature, the filter cake is collected by filtration, washed with deionized water and ethanol, and dried at 80 °C to obtain the iron phosphate precursor; S2. Preparation of graphene-coated modified iron phosphate precursor: By mass, 5 parts of the iron phosphate precursor are added to 75 parts of a 10 mg / mL graphene oxide dispersion, 0.03 part of nickel chloride is added, and after stirring for 1 h, the reaction is carried out at 170 °C for 12 h. The gel is 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 the graphene-coated modified iron phosphate precursor; S3. Preparation of graphene-coated cathode material: By mass, 1.2 parts of lithium hydroxide and 1.4 parts of ascorbic acid are added to 5 parts of the graphene-coated modified iron phosphate precursor, ground with absolute ethanol, pre-calcined at 350 °C for 4 h in an argon atmosphere, cooled to room temperature, then 1.8 parts of carbon-forming material are added and ground, and then calcined at 650 °C for 12 h in an argon atmosphere to obtain the graphene-coated cathode material.

[0038] The graphene-coated lithium-ion battery cathode material prepared in this example is prepared according to the above preparation method of the graphene-coated lithium-ion battery cathode material.

[0039] The application of the graphene-coated lithium-ion battery cathode material in this example is used to prepare a lithium battery cathode plate. The preparation method of the lithium battery cathode plate includes the following steps: S21. By mass, 8 parts of the graphene-coated lithium-ion battery cathode material, 1 part of the composite binder, and 1 part of conductive carbon black are added to a blender, diluted with N-methylpyrrolidone to a solid content of 30 wt%, stirred at a speed of 800 rpm for 3 min, then stirred at a speed of 2500 rpm for 20 min, and finally stirred at a speed of 4000 rpm for 2 min to obtain the cathode slurry; S22. The cathode slurry is coated on the current collector, the coating thickness of the cathode slurry is 190 μm, the pressure of pressing is 5 t, dried at 60 °C for 18 h, sliced by a slicing machine, and pressed to obtain the lithium battery cathode plate.

[0040] The difference between the composite binder in this example and that in Example 1 is that the raw material dosage of the latex powder is replaced with 40 parts of styrene, 3 parts of methyl acrylate, 10 parts of acrylonitrile, 1.5 parts of itaconic acid, and 200 parts of deionized water.

[0041] The preparation method of the carbonaceous material in this example is the same as that of the carbonaceous material in Example 1.

[0042] Comparative Example 1. The difference between this comparative example and Example 1 is that the cathode material is replaced by lithium iron phosphate without graphene coating.

[0043] Comparative Example 2. The difference between this comparative example and Example 1 is that the carbonaceous material is replaced by glucose.

[0044] Comparative Example 3. The difference between this comparative example and Example 1 is that the composite binder is replaced by polyvinylidene fluoride.

[0045] Performance Test Preparation of the test lithium battery: The positive electrode sheets of the lithium batteries prepared in each example and comparative example, with polyethylene as the battery separator, and 1 mol / L lithium hexafluorophosphate added to a solvent composed of ethylene carbonate, diethyl carbonate, and dimethyl carbonate in a volume ratio of 1:1:1 as the electrolyte, were assembled in an argon atmosphere and pressed to obtain the test lithium battery.

[0046] The test was carried out using a Neware battery test system to test the initial discharge capacity of the test lithium batteries prepared in each example and comparative example at 0.5C and 1C rates and the capacity retention rate after 100 cycles at 1C rate.

[0047] The test results are shown in Table 1: Table 1 Test Results Serial number Initial discharge capacity at 0.5 C rate (mAh / g) Initial discharge capacity at 1 C 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 It can be seen from the data in Table 1 that the initial discharge capacities of the test lithium batteries prepared in Examples 1 to 4 at 0.5C and 1C rates are 175.6 - 177.5 mAh / g and 155.6 - 157.4 mAh / g respectively, and the capacity retention rate after 100 cycles is 88.5 - 90.1%. In Comparative Example 1, the cathode material is replaced by lithium iron phosphate without graphene coating. Lithium iron phosphate has poor self-conductivity and a small lithium ion diffusion coefficient, and only the carbonaceous material cannot form a conductive network covering the cathode material, resulting in its initial discharge capacities at 0.5C and 1C rates being only 149.5 mAh / g and 134.3 mAh / g. This shows that the graphene-coated lithium ion battery cathode material prepared by the present invention has excellent charge-discharge specific capacity and cycle efficiency when applied to the preparation of the positive electrode sheet of a lithium battery, further indicating that the graphene-coated lithium ion battery cathode material prepared by the present invention has excellent conductivity and cycle stability.

[0048] As described above, only the preferred specific embodiments of the present invention are provided, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, making equivalent substitutions or changes, should be covered by the protection scope of the present invention.

[0049] The preferred embodiments of the present invention disclosed above are only used to help illustrate the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the present invention to only the specific embodiments. Obviously, many modifications and changes can be made according to the content of this specification. This specification selects and specifically describes these embodiments to better explain the principle and practical application of the present invention, so that those skilled in the art can well understand and utilize the present invention. The present invention is only limited by the claims and their full scope and equivalents.

Claims

1. A method for preparing a graphene-coated lithium-ion battery positive electrode material, characterized in that: The steps include: S1. Preparation of an iron phosphate precursor: dissolving a phosphorus source and an iron source in deionized water to prepare a phosphorus source solution and an iron source solution, respectively, by mass, dropping the phosphorus source solution into the iron source solution, adding 0.5 to 2 parts of a surfactant, adjusting the pH to 2 to 2.5 with aqueous ammonia, stirring for 1 to 2 hours, reacting at 150 to 180° C. for 10 to 12 hours, cooling to room temperature after the reaction, collecting the filter cake by filtration, washing with deionized water and ethanol, and drying at 80 to 90° C. to obtain an iron phosphate precursor; S2. Preparation of a graphene-coated modified iron phosphate precursor: Add 1 to 5 parts of an iron phosphate precursor to a graphene oxide dispersion by mass, add 0.02 to 0.04 parts of nickel chloride, stir for 1 to 2 hours, react at 160 to 180° C. for 10 to 12 hours, wash the gel with deionized water, freeze-dry at 0 to 5° C. for 48 to 72 hours, grind, heat to 500 to 600° C. in an argon atmosphere, and calcine for 4 to 6 hours to obtain a graphene-coated modified iron phosphate precursor; S3. Preparation of graphene-coated positive electrode material: Add 1-10 parts of lithium source and 1-1.5 parts of ascorbic acid to 2-5 parts of graphene-coated modified iron phosphate precursor by mass, add anhydrous ethanol and grind, heat to 350-400°C in an argon atmosphere and pre-calcine for 3-4 hours, cool to room temperature, add 1-2 parts of carbon-forming material and grind, heat to 600-700°C in an argon atmosphere and calcine for 10-12 hours to obtain a graphene-coated positive electrode material.

2. The method for preparing a graphene-coated lithium-ion battery positive electrode material according to claim 1, characterized in that: The method for preparing the carbon-forming material in S3 comprises the following steps: S11, drying tea seed husk at 75-85° C., grinding and passing through a 40-60 mesh sieve to obtain tea seed husk powder, adding 50-60 parts of the tea seed husk powder to 200-300 parts of deionized water, soaking at room temperature for 10-12 hours, filtering and collecting the solid, adding it to 500-1000 parts of deionized water, extracting at 80-90° C. for 4-6 hours, and collecting by suction to obtain an extract; S12, by mass, 500-1000 parts of the extract were concentrated under reduced pressure to 200-300 parts, 78-80 parts of chloroform and 15-20 parts of n-butanol were added, and the mixture was stirred at room temperature for 30-40 minutes to remove protein, and the supernatant was collected by centrifugal filtration, and dialyzed for 4-6 hours. After the dialysis was completed, 500-1000 parts of anhydrous ethanol were added, and after alcohol precipitation for 10-12 hours, the precipitate was collected by centrifugation, and freeze-dried at -5-0°C to obtain a tea seed chitosan extract; S13. Mix 2-3 parts of tea seed chitosan extract, 1-2 parts of glucose and 0.5-0.6 parts of citric acid by mass to prepare a carbon material.

3. The method for preparing a positive electrode material for a lithium ion battery according to claim 2, characterized in that: The dialysis cut-off molecular weight of the S12 is 3500-5000Da.

4. The method for preparing a graphene-coated lithium-ion battery positive electrode material according to claim 1, characterized in that: The phosphorus-iron molar ratio of the phosphorus source to the iron source in S1 is 0.9-1.1:0.9-1.1, the phosphorus source is any one of phosphoric acid and diammonium 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.

5. The method for preparing a graphene-coated lithium-ion battery positive electrode material according to claim 1, characterized in that: The number of graphene oxide layers in S2 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; the lithium source in S3 is any one of lithium hydroxide, lithium carbonate, and lithium oxide.

6. According to the method for preparing a graphene-coated lithium-ion battery positive electrode material according to any one of claims 1 to 5, a graphene-coated lithium-ion battery positive electrode material is prepared.

7. The graphene-coated lithium-ion battery positive electrode material according to claim 6, characterized in that: For preparing a positive electrode sheet for a lithium battery, the preparation method of the positive electrode sheet for a lithium battery comprises the following steps: S21. Add 70-80 parts of graphene-coated lithium-ion battery positive electrode material, 10-20 parts of composite binder and 10-20 parts of conductive carbon black into a mixer, add an organic solvent to dilute to a solid content of 30-40wt%, stir at a speed of 700-800rpm for 2-3min, then stir at a speed of 2000-2500rpm for 15-20min, and finally stir at a speed of 3000-4000rpm for 2-3min to obtain a positive electrode slurry; S22, coating the positive electrode slurry on the current collector, drying at 60-80°C for 12-18h, slicing with a slicer, and pressing to obtain a positive electrode sheet for a lithium battery.

8. The graphene-coated lithium-ion battery positive electrode material according to claim 7, characterized in that: The method for preparing the composite binder in S21 comprises the following steps: S31. 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 into a reactor by mass, introduce nitrogen, add 1-2 parts of sodium dodecylbenzene sulfonate and 1-2 parts of ammonium persulfate, stir at a speed of 300-400 rpm for 1-2 hours, heat to 60-70° C. for reaction for 2-3 hours, and then heat to 80-90° C. for reaction for 2-3 hours to obtain an emulsion; S32, adding 20-30 parts of lithium sulfate to the emulsion by mass, stirring at a speed of 1000-2000 rpm for 10-20 min to break the emulsion, filtering and collecting the solid, drying at 60-70° C. for 12-18 h, washing with deionized water, drying at 60-70° C. for 48-60 h, and grinding to obtain a latex powder; S33. Mix 5-6 parts of carboxymethyl cellulose, 50-60 parts of polyvinylidene fluoride and 40-50 parts of latex powder by mass to prepare a composite adhesive.

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