Reduced and oxidized porous graphene, preparation method thereof, porous graphene coating current collector and application

Abstract

The invention provides reduced-oxidized porous graphene, a preparation method thereof, a porous graphene coating current collector and application. The preparation method of the reduced-oxidized porous graphene comprises the following steps: under the action of a pore-forming agent, performing chemical pore-forming on graphene oxide with abundant defects on the surface, and finally performing high-temperature treatment to obtain the powdery reduced-oxidized porous graphene. According to the technical scheme, abundant defects generated in the process of reducing graphene oxide and graphene subjected to chemical pore forming are introduced into the current collector conductive slurry and / or the electrode conductive slurry, so that ions and electrons can be directly and quickly transmitted between the electrode material and the current collector; and the charge-discharge performance of the battery under a high-rate condition is remarkably improved, so that the problem of steric effect generated by the unique two-dimensional plane structure of the graphene nanosheet on the transmission of lithium ions in the electrode is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of current collector materials, and particularly relates to a reduced graphene oxide porous graphene and a preparation method thereof, a porous graphene-coated current collector, and an application thereof. Background Art

[0002] With the development of modern consumer electronics and new energy vehicle industries, higher requirements have been put forward for the performance of lithium-ion batteries. As an important component of lithium-ion batteries, the current collector is used to carry the electrode material, collect the current in the electrode material, and realize the external output of large current, which plays a crucial role in the electrochemical performance of lithium-ion batteries. However, the current collector still faces various challenges. The limited contact area and weak adhesion force between the electrode material and the current collector will lead to an increase in the interface contact resistance, further increasing the internal resistance of the lithium battery. In addition, local corrosion of the current collector during long-term cycling is also a serious problem, causing an increase in resistance, shedding of the electrode material, and the like.

[0003] Due to the presence of the conductive carbon coating layer, the carbon-coated current collector not only increases the contact area between the electrode material and the current collector, but also strengthens the adhesion between them as a buffer layer, effectively reducing the resistance, suppressing the increase in resistance during the charge and discharge process of the battery, and improving the comprehensive performance of the lithium-ion battery. Currently, the mainstream carbon coating materials are mainly carbon black (CB) and graphite. However, their thick coatings (2 - 5 μm) occupy too much electrode weight and volume, which greatly reduces the specific capacity and energy density.

[0004] To solve the above problems, in the prior art, graphene nanosheets with a smaller thickness (0.2 - 2 μm), stronger adhesion, and higher conductivity are coated on the current collector surface. Due to the small contact internal resistance between the graphene nanosheets and the battery active material, the battery internal resistance is reduced; moreover, it can also prevent the direct contact between the current collector and the electrolyte, improving the cycle stability and lifespan of the battery. For example, Chinese Invention Patent CN112366097A provides a preparation method of an all-carbon electrode for improving the performance of graphene-based supercapacitors. To solve the problem of easy stacking of graphene, a porous graphene oxide dispersion is used as the base solution, and carbon nanotubes and carbon onions are added, and a mixed solution of the three is prepared by ultrasonic dispersion. Then, the mixed solution is subjected to a hydrothermal reaction to obtain a porous graphene hydrogel composite with carbon nanotubes and carbon onions, and the porous graphene hydrogel is pressed onto the current collector. This patent assembles two-dimensional graphene into a three-dimensional hydrogel, reducing the stacking of graphene itself. At the same time, the graphene hydrogel has a three-dimensional porous structure, which can provide more electrolyte contact areas and electron transport channels. Another example is Chinese Invention Patent CN103633334A, which provides a graphene / aluminum foil composite current collector, its preparation method, an electrochemical electrode, and an electrochemical battery or capacitor. To solve the problems of direct contact between the aluminum foil and the electrolyte and the lithium intercalation reaction during the charge and discharge process of the battery, resulting in the corrosion of the aluminum foil and the reduction of its lifespan, a graphene suspension is coated on the aluminum foil with surface roughness treatment and then subjected to multiple rollings to prepare a graphene / aluminum foil composite current collector. Then, granular conductive carbon black is added to the electrode conductive paste for electrochemical battery testing. The "surface-point" contact mode and "surface-surface" contact mode between graphene and conductive carbon black effectively improve their electrical contact and charge transfer, significantly reducing the electrode sheet resistance and battery internal resistance. However, as a conductive agent, graphene can build a good conductive network through the lap joints between the sheets with a small usage amount, greatly improving the conductivity of the entire electrode. However, specifically for each active material particle, the graphene sheets cannot completely cover the entire particle surface, and the electron transport on the exposed surface outside the "surface-point" contact will obviously be relatively lagging, thus affecting the charge transfer speed. In particular, the charge must cross the interlayer direction of the graphene nanosheets to complete the electrochemical process, but the low interlayer conductivity of the graphene nanosheets limits the charge transfer between the current collector and the electrode material, further seriously affecting the battery rate charge and discharge performance.

[0005] Therefore, based on the above analysis of the prior art, to solve the technical problem that the unique two-dimensional planar structure of graphene nanosheets generates a "steric hindrance effect" on the transport of lithium ions inside the electrode, restricting the charge transfer between the current collector and the electrode material, the present invention provides a graphene with abundant defects generated during the reduction of graphene oxide and chemical pore formation, and applies it to the current collector conductive paste and the electrode conductive paste to solve the problems existing in the prior art. Summary of the Invention

[0006] To solve the above problems, the present invention provides a reduced graphene oxide porous material, a preparation method thereof, a porous graphene-coated current collector, and an application thereof.

[0007] To achieve the above object, the present invention provides a preparation method of reduced graphene oxide porous material. Graphene oxide with abundant defects on the surface is chemically pore-formed and then heat-treated at high temperature to obtain powdery reduced graphene oxide porous material.

[0008] In some specific embodiments, the graphene oxide is prepared by an improved Hummers method.

[0009] In some specific embodiments, the high-temperature treatment includes treating at 2200-2400 °C for 1-3 h.

[0010] As a preferred embodiment, the method of chemical pore formation includes but is not limited to any one of photolithography, carbothermal reduction, template method, solvothermal method, chemical vapor deposition method, etc.

[0011] In some specific embodiments, the solvothermal method includes ultrasonically dispersing graphene oxide in a pore-forming agent, performing a solvothermal reaction under sealed conditions, freeze-drying, and finally performing the high-temperature treatment to obtain powdery reduced graphene oxide porous material.

[0012] In some specific embodiments, the pore-forming agent includes but is not limited to any one of hydrogen peroxide solution, potassium permanganate solution, potassium hydroxide solution, etc.

[0013] In some specific embodiments, the ultrasonic dispersion includes ultrasonically dispersing a mixed solution formed by mixing a graphene oxide suspension with a hydrogen peroxide solution for 0.5-1 h.

[0014] In some specific embodiments, the conditions of the solvothermal reaction are 170-190 °C and the reaction time is 6-8 hours.

[0015] In some specific embodiments, the freeze-drying includes freeze-drying in a -40 °C vacuum freeze-drying oven for 6-24 h.

[0016] In some specific embodiments, in the graphene oxide suspension, the content of graphene oxide is 1-3 mg·mL -1 。

[0017] In some specific embodiments, the volume ratio of the graphene oxide suspension to the solution of the pore-forming agent is 9:1.

[0018] As one of the objectives of the invention, the invention also provides a reduced graphene oxide porous graphene, which is prepared by the preparation method described above.

[0019] As one of the objectives of the invention, the invention also provides a porous graphene concentrated slurry, which is obtained by mixing the reduced graphene oxide porous graphene prepared by the preparation method described above, a dispersant and an organic solvent, stirring and dispersing uniformly, and then grinding; wherein, the mass percentage content of the reduced graphene oxide porous graphene is 25-30%.

[0020] In some specific embodiments, the dispersant is one or a combination of polyvinylpyrrolidone, polyacrylamide, sodium dodecylbenzenesulfonate, polyvinyl alcohol, sodium polyacrylate, etc., but is not limited thereto.

[0021] As a preferred embodiment, the dispersant is polyvinylpyrrolidone.

[0022] As a preferred embodiment, the organic solvent is N-methylpyrrolidone.

[0023] In some specific embodiments, the mass ratio of the reduced graphene oxide porous graphene, the dispersant and the organic solvent is (25-30):3:(67-72).

[0024] As one of the objectives of the invention, the invention also provides a porous graphene-coated current collector, which at least includes the reduced graphene oxide porous graphene prepared by the preparation method described above.

[0025] Or, the porous graphene-coated current collector includes the porous graphene concentrated slurry as described above.

[0026] As one of the objectives of the invention, the invention also provides a preparation method of a porous graphene-coated current collector, which includes the following steps:

[0027] S1. Mix the aqueous polypropylene resin solution with the porous graphene concentrated slurry described above, and form a first porous graphene resin solution through an emulsification process;

[0028] S2. Add deionized water to the porous graphene resin solution to obtain a second porous graphene resin solution;

[0029] S3. Coat the second porous graphene resin solution on a metal pole piece to obtain the porous graphene-coated current collector.

[0030] In some specific embodiments, in S1, in the aqueous polypropylene resin solution, the concentration of the polypropylene resin is 35-45 wt%.

[0031] In some specific embodiments, the mass ratio of the aqueous polypropylene resin solution to the porous graphene concentrated slurry is 1:6.

[0032] In some specific embodiments, the mass of deionized water added in S2 is one-third of the mass of water in the aqueous polypropylene resin solution in S1.

[0033] In some specific embodiments, in S3, the coating thickness of the second porous graphene resin solution is 1 - 5 μm.

[0034] In some specific embodiments, the metal current collector is aluminum foil or copper foil.

[0035] Preferably, the thickness of the metal current collector is 5 - 50 μm.

[0036] In some specific embodiments, the thickness of the aluminum foil is 12 μm.

[0037] In some specific embodiments, the thickness of the copper foil is 6 μm.

[0038] As one of the objects of the invention, the present invention also provides an application of the porous graphene-coated current collector described above as an electrode material in a capacitor or an electrochemical cell.

[0039] As one of the objects of the invention, the present invention also provides a positive electrode sheet, comprising a current collector and a positive electrode active material; wherein the current collector and / or the positive electrode active material contains the reduced graphene oxide with pores described above.

[0040] As one of the objects of the invention, the present invention also provides a positive electrode sheet, wherein the current collector is the porous graphene-coated current collector described above.

[0041] The beneficial technical effects obtained by the present invention:

[0042] 1. By adopting the technical solution of the present invention, the abundant defects generated during the reduction of graphene oxide and the graphene with pores after chemical pore formation are introduced into the current collector conductive paste and / or the electrode conductive paste, enabling ions and electrons to be directly and rapidly transmitted between the electrode material and the current collector, significantly improving the charge and discharge performance of the battery under high-rate conditions, and thus solving the "steric hindrance effect" caused by the unique two-dimensional planar structure of graphene nanosheets to the transmission of lithium ions inside the electrode.

[0043] 2. By adopting the technical solution of the present invention, the graphene after pore formation is introduced into the current collector conductive paste and / or the electrode conductive paste at the same time. The hydrogen bonds, van der Waals forces and spin interactions between the graphene make the combination of the current collector and the active material closer. Different from the current common "surface - point" contact mode between graphene and conductive carbon black and the "point - point" contact mode between conductive carbon black and conductive carbon black, the present invention effectively improves the electrical contact and charge transfer between the electrode material and the current collector through the "surface - surface" contact mode, and significantly reduces the resistance of the electrode sheet and the internal resistance of the battery.

[0044] 3. In the present invention, after pore formation is carried out on graphene oxide, and then the obtained porous graphene is ground into a concentrated paste and directly coated on the current collector (aluminum foil / copper foil), tight adhesion with the current collector can be achieved without subsequent treatment steps, which has the advantages of simple preparation method and easy operation. Brief Description of the Drawings

[0045] Figure 1 It is a comparison chart of 3C rate charging for the examples and comparative examples of the present invention.

[0046] Figure 2 It is a comparison chart of 3C rate discharging for the examples and comparative examples of the present invention.

[0047] Figure 3 It is a comparison chart of 5C rate charging for the examples and comparative examples of the present invention.

[0048] Figure 4 It is a comparison chart of 5C rate discharging for the examples and comparative examples of the present invention. Detailed Embodiments

[0049] To make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are some, but not all, of the embodiments of the present application.

[0050] In the present invention, after pore formation is carried out on graphene oxide, and then the obtained reduced graphene oxide porous graphene is ground into a concentrated paste and directly coated on the current collector (aluminum foil / copper foil), tight adhesion with the current collector can be achieved without subsequent treatment steps.

[0051] Furthermore, the reduced graphene oxide porous graphene is used as the active material of the electrode conductive paste, and the porous graphene is used as the conductive agent to replace the conductive carbon black, so that both the current collector conductive paste and the electrode conductive paste contain porous graphene.

[0052] The present invention utilizes the abundant defects generated during the production process of graphene oxide and the porous structure formed by creating pores in graphene oxide, enabling ions and electrons to be directly and rapidly transported between the electrode material and the current collector, thereby improving the charge and discharge performance of the battery under high-rate conditions.

[0053] Based on the above analysis, the porous structure on the surface of graphene oxide can be achieved according to the pore creation methods in the prior art, such as physical pore creation methods, chemical pore creation methods, etc.

[0054] As one of the specific implementation manners, the chemical pore creation method includes, but is not limited to, any one of photolithography etching method, carbothermal reduction method, template method, solvothermal method, chemical vapor deposition method.

[0055] As a preferred implementation manner, the solvothermal method includes ultrasonically dispersing reduced graphene oxide in the pore-forming agent, carrying out a solvothermal reaction under sealed conditions, freeze-drying, and then performing high-temperature treatment to obtain powdery reduced porous graphene oxide.

[0056] As a preferred implementation manner, the graphene oxide with abundant defects on the surface is prepared by the improved Hummers method or by the method provided in Chinese Patent Application No. CN107226468A.

[0057] As a preferred implementation manner, the improved Hummers method includes: mixing graphite and potassium permanganate evenly, first adding preheated concentrated sulfuric acid to the mixture under stirring conditions to obtain a reaction system, and stirring for reaction for 10 - 40 min; then, second adding preheated concentrated sulfuric acid to the reaction system, continuing to stir for reaction for 20 - 40 min, slowly pouring the obtained reaction product into ice-cold deionized water for dilution, adding hydrogen peroxide after stabilization, standing still and washing the precipitate to obtain the graphene oxide, and the surface of the obtained graphene oxide contains abundant defects.

[0058] As a preferred implementation manner, in the improved Hummers method, the mass ratio of graphite to potassium permanganate is 1:6 - 8.

[0059] As a preferred implementation manner, the temperature of the reaction is 60 - 80 °C.

[0060] As a preferred implementation manner, the preheating temperature is 60 - 80 °C.

[0061] As a preferred implementation manner, the volume of the first addition of preheated concentrated sulfuric acid is 15 - 30 mL / g of graphene oxide.

[0062] As a preferred implementation manner, the volume of the first addition of preheated concentrated sulfuric acid is 10 - 20 mL / g of graphene oxide.

[0063] As a preferred embodiment, the cleaning includes using 1 mol / L dilute hydrochloric acid and deionized water for the precipitate.

[0064] The graphene oxide prepared by adopting the above technical solution has rich defects (a large number of oxygen-containing functional groups) on the surface, and at the same time, the local carbon atom structure on the graphene surface is changed, which is convenient for the subsequent pore-forming process, and then a porous structure is formed.

[0065] Pore formation can be achieved on the surface of graphene oxide with rich defects by any of the existing chemical pore-forming methods in the prior art, including but not limited to lithography etching method, carbothermal reduction method, template method, solvothermal method, chemical vapor deposition method, etc.

[0066] In some specific embodiments, the solvothermal method is adopted to perform pore formation on the surface of graphene oxide with rich defects to prepare porous graphene, and the specific method includes:

[0067] Disperse the graphene oxide with rich defects in deionized water, and perform ultrasonic stirring to form a uniform graphene oxide; preferably, the content of graphene oxide is 1 - 3 mg·mL -1 .

[0068] Mix the hydrogen peroxide (H 2 O 2 ) solution with the graphene oxide suspension, and after ultrasonic dispersion, seal and heat the mixture for reaction to obtain a mixed solution; preferably, the mass fraction of the hydrogen peroxide (H 2 O 2 ) solution is 0.5%; the volume ratio of the hydrogen peroxide (H 2 O 2 ) solution to the graphene oxide suspension is 9:1; the conditions for the sealed heating reaction are heating at 170 - 190 °C for 7 - 9 hours.

[0069] Place the obtained mixed solution in vacuum freeze-drying and then perform high-temperature treatment to obtain powdery porous reduced graphene oxide (h-rGO).

[0070] Preferably, the temperature of the vacuum freeze-drying is -40 °C, and the drying time is 6 - 24 h.

[0071] Preferably, the temperature of the high-temperature treatment is 2200 - 2400 °C, and the time of the high-temperature treatment is 1 - 3 h.

[0072] The technical solution of the present invention will be described in detail below through specific examples.

[0073] Example 1

[0074] First, this embodiment provides a porous graphene-coated current collector, and its specific steps include:

[0075] (1) Prepare graphene oxide (GO) by the improved Hummers method

[0076] Mix 1 g of graphite with a particle size of 0.5 - 1 μm and 6 g of solid potassium permanganate evenly, add 20 mL of concentrated sulfuric acid preheated to 60 °C, heat the mixed solution to 80 °C and stir continuously for 2 min, then continue to add 10 mL of concentrated sulfuric acid preheated to 60 °C, and continue to stir at 80 °C for 3 min. After the reaction is completed, slowly pour the obtained reaction product into deionized water with ice for dilution. After stabilization, add 3 mL of hydrogen peroxide (30%) solution, let it stand for a period of time, first wash the precipitate with 1 mol / L dilute hydrochloric acid solution, and then wash it with deionized water until the solution is neutral. Centrifuge to obtain graphene oxide.

[0077] (2) Preparation of porous graphene

[0078] ① Add the graphene oxide (GO) provided in step (1) to a container filled with deionized water solution. The oxygen-containing functional groups of GO improve the stability and dispersibility of GO in water. Stir ultrasonically for 1 hour to form a uniform suspension for standby. In the graphene oxide suspension, the content of graphene oxide is 2 mg·mL -1 .

[0079] ② Mix the hydrogen peroxide (H 2 O 2 ) solution with a mass fraction of 0.5% and the graphene oxide suspension in a volume ratio of 9:1. After ultrasonic dispersion for 0.5 hour, seal the mixture in a high-pressure reactor and heat it at 180 °C for 7 hours.

[0080] ③ Place the obtained mixed solution in a -40 °C vacuum freeze-drying oven for freeze-drying for 12 h, and then place it in a tube furnace for high-temperature treatment at 2300 °C for 2 h to obtain porous reduced graphene oxide (h-rGO) powder.

[0081] (3) Preparation of porous graphene concentrated slurry

[0082] Disperse and mix h-rGO powder, dispersant polyvinylpyrrolidone (PVP), and solvent N-methylpyrrolidone in a mass fraction ratio of 28:3:69 in a batching tank and stir for 30 minutes to obtain porous graphene concentrated slurry.

[0083] (4) Preparation of porous graphene-coated current collector

[0084] ① Weigh polyacrylic acid resin and deionized water in a mass ratio of 24:35 in a batching tank and stir until evenly mixed to prepare Solution No. 1.

[0085] ②Add porous graphene concentrated slurry to Solution No. 1. The mass ratio of Solution No. 1 to the porous graphene concentrated slurry is 1:6, and emulsify for 10 minutes to make Solution No. 2.

[0086] ⑧Grind Solution No. 2 for 60 minutes, and add 1 / 3 of the weight of the deionized water added when preparing Solution No. 1 in step ① to make Solution No. 3.

[0087] ④After corona is turned on, coat Solution No. 3 on a 12-μm aluminized foil using a coater, with a coating thickness of 1 μm on both the front and back sides to obtain a porous graphene-coated current collector.

[0088] Based on the above-mentioned porous graphene-coated current collector, this example further prepares a soft-pack battery. The specific steps include:

[0089] Positive electrode sheet: The positive electrode active material lithium iron phosphate: conductive agent (using the porous graphene provided in this example): binder (polyvinylidene fluoride) are mixed according to the mass fraction ratio of 97.5:0.5:2. Use N-methylpyrrolidone (NMP) solvent to add the above mixture to a double planetary mixer and shear and disperse it evenly to prepare a positive electrode slurry. The viscosity of the slurry is 8000 - 12000 mPa·s.

[0090] Use a transfer coater to coat the positive electrode slurry on the porous graphene-coated current collector prepared in this example, with a coating thickness of 1 μm on both the front and back sides. After rolling, take a sample to test the peel strength and the resistance of the electrode sheet.

[0091] Negative electrode sheet: Mix the negative electrode active material graphite: conductive carbon black: binder carboxymethyl cellulose + styrene-butadiene rubber (CMC + SBR) according to the mass fraction ratio of 95:2:3. Use deionized water solvent to coat the evenly dispersed slurry on a 6-μm copper foil, with a coating thickness of 1 μm on both the front and back sides. After rolling and slitting, obtain a graphite negative electrode sheet.

[0092] After the positive electrode sheet is die-cut, it is combined with the negative electrode sheet to form a soft-pack battery with a capacity of 3 Ah, and cycle and rate charge-discharge tests are carried out.

[0093] Example 2

[0094] First, this example provides a porous graphene-coated current collector. The specific steps include:

[0095] A graphene-coated current collector. The specific steps include:

[0096] (1) Preparation of graphene concentrated slurry

[0097] Disperse graphene powder (h-rGO powder prepared in Example 1), dispersant polyvinylpyrrolidone (PVP), and solvent N-methylpyrrolidone in a mixing tank in a mass fraction ratio of 28:3:69, and stir and disperse them. After grinding for 30 minutes, a porous graphene concentrated slurry is obtained.

[0098] (2) Preparation of graphene-coated current collector

[0099] ① Weigh polyacrylic resin and deionized water in a mass ratio of 24:35 in a mixing tank, and stir until evenly mixed to prepare Solution No. 1.

[0100] ② Add the graphene concentrated slurry (solid content 6% wt) to Solution No. 1. The mass ratio of Solution No. 1 to the porous graphene concentrated slurry is 1:6, and emulsify for 10 minutes to prepare Solution No. 2.

[0101] ⑧ Grind Solution No. 2 for 60 minutes, and supplement 1 / 3 of the weight of the deionized water added when preparing Solution No. 1 in step ① to prepare Solution No. 3.

[0102] ④ After corona is turned on, coat Solution No. 3 on a 12 μm aluminum foil using a coater, with a coating thickness of 1 μm on both sides to obtain a graphene-coated current collector.

[0103] In this example, a soft-pack battery was further prepared on the basis of the above-mentioned porous graphene-coated current collector. The specific steps include:

[0104] Positive electrode sheet: The positive electrode active material lithium iron phosphate, conductive agent (porous graphene provided in Example 1), and binder polyvinylidene fluoride are mixed in a mass fraction ratio of 97.5:0.5:2. Use N-methylpyrrolidone (NMP) solvent to add the above mixture to a double planetary mixer and shear and disperse it evenly. The viscosity of the prepared positive electrode slurry is 8000 - 12000 mPa·s. Use a transfer coater to coat the positive electrode slurry on the graphene-coated current collector prepared in this example, with a coating thickness of 1 μm on both sides. After rolling, take samples to test the peel strength and pole piece resistance.

[0105] Negative electrode sheet: Mix the negative electrode active material graphite, conductive carbon black, and binder carboxymethyl cellulose + styrene-butadiene rubber (CMC + SBR) in a mass fraction ratio of 95:2:3. Use deionized water solvent to evenly disperse the mixture of slurry and coat it on a 6 μm copper foil, with a coating thickness of 1 μm on both sides. After rolling and slitting, a graphite negative electrode sheet is obtained.

[0106] After the positive electrode sheet is die-cut, it is combined with the negative electrode sheet to form a soft-pack battery with a capacity of 3 Ah, and cycle and rate charge and discharge tests are carried out.

[0107] Example 3

[0108] This embodiment provides a soft-pack battery, and its specific steps include:

[0109] Positive electrode sheet: The positive electrode active material lithium iron phosphate: graphene: binder (polyvinylidene fluoride) are mixed according to a mass fraction ratio of 97.5:0.5:2. Using N-methylpyrrolidone (NMP) solvent, the above mixture is added into a double planetary mixer and uniformly shear-dispersed to prepare a positive electrode slurry. The positive electrode slurry is coated on the porous graphene-coated current collector prepared in Example 1 using a transfer coater. After rolling, samples are taken to test the peel strength and the resistance of the electrode sheet.

[0110] Negative electrode sheet: The negative electrode active material graphite: conductive carbon black: binder carboxymethyl cellulose + styrene-butadiene rubber (CMC + SBR) are mixed according to a mass fraction ratio of 95:2:3. Using deionized water solvent, the slurry obtained by uniformly dispersing the mixture is coated on a 6-μm-thick copper foil. After rolling and slitting, a graphite negative electrode sheet is obtained.

[0111] After die-cutting the positive electrode sheet, it is combined with the negative electrode sheet to form a soft-pack battery with a capacity of 3 Ah, and cycle and rate charge-discharge tests are carried out.

[0112] Example 4

[0113] This embodiment provides a soft-pack battery, and its specific steps include:

[0114] Positive electrode sheet: The positive electrode active material lithium iron phosphate: conductive carbon black: binder (polyvinylidene fluoride) are mixed according to a mass fraction ratio of 97.5:0.5:2. Using N-methylpyrrolidone (NMP) solvent, the above mixture is added into a double planetary mixer and uniformly shear-dispersed to prepare a positive electrode slurry. The positive electrode slurry is coated on the porous graphene-coated current collector prepared in Example 1 using a transfer coater. After rolling, samples are taken to test the peel strength and the resistance of the electrode sheet.

[0115] Negative electrode sheet: The negative electrode active material graphite: conductive carbon black: binder carboxymethyl cellulose + styrene-butadiene rubber (CMC + SBR) are mixed according to a mass fraction ratio of 95:2:3. Using deionized water solvent, the slurry obtained by uniformly dispersing the mixture is coated on a 6-μm-thick copper foil. After rolling and slitting, a graphite negative electrode sheet is obtained.

[0116] After die-cutting the positive electrode sheet, it is combined with the negative electrode sheet to form a soft-pack battery with a capacity of 3 Ah, and cycle and rate charge-discharge tests are carried out.

[0117] Example 5

[0118] This embodiment provides a soft-pack battery, and its specific steps include:

[0119] (1) Preparation of the carbon-coated current collector:

[0120] ① Weigh polyacrylic resin and deionized water with a mass ratio of 24:35 in a batching tank, and stir until evenly mixed to prepare Solution No. 1.

[0121] ② Add conductive carbon black concentrated slurry (solid content 6% wt) to Solution No. 1. The mass ratio of Solution No. 1 to the conductive carbon black concentrated slurry is 1:6, and emulsify for 10 minutes to prepare Solution No. 2.

[0122] ③ Grind Solution No. 2 for 60 minutes, and add 1 / 3 of the weight of the deionized water added when preparing Solution No. 1 in step ① to prepare Solution No. 3.

[0123] ④ After starting the corona, coat Solution No. 3 on a 12-μm aluminized foil using a coater, with a coating thickness of 1 μm on both the front and back sides to obtain a carbon-coated current collector.

[0124] (2) Preparation of soft-pack battery:

[0125] Positive electrode sheet: The positive electrode active material lithium iron phosphate: conductive agent (porous graphene provided in Example 1): binder (polyvinylidene fluoride) are mixed according to a mass fraction ratio of 97.5:0.5:2. Use N-methylpyrrolidone (NMP) solvent to add the above mixture into a double planetary mixer and shear and disperse it evenly to prepare a positive electrode slurry. Use a transfer coater to coat the positive electrode slurry on the carbon-coated current collector, and after rolling, take a sheet to test the peel strength and sheet resistance.

[0126] Negative electrode sheet: Mix the negative electrode active material graphite: conductive carbon black: binder carboxymethyl cellulose + styrene-butadiene rubber (CMC + SBR) according to a mass fraction ratio of 95:2:3. Use deionized water solvent to coat the evenly dispersed slurry on a 6-μm copper foil, and roll and slit to obtain a graphite negative electrode sheet.

[0127] After die-cutting the positive electrode sheet, it is combined with the negative electrode sheet to form a soft-pack battery with a capacity of 3 Ah, and cycle and rate charge-discharge tests are carried out.

[0128] Comparative Example 1

[0129] This comparative example provides a soft-pack battery, and its specific steps include:

[0130] Positive electrode sheet: The positive electrode active material lithium iron phosphate: conductive carbon black: binder polyvinylidene fluoride are mixed according to a mass fraction ratio of 97.5:0.5:2. Use N-methylpyrrolidone (NMP) solvent to add the above mixture into a double planetary mixer and shear and disperse it evenly to prepare a positive electrode slurry. Use a transfer coater to coat the positive electrode slurry on the carbon-coated current collector (refer to Example 5), and after rolling, take a sheet to test the peel strength and sheet resistance.

[0131] Negative electrode sheet: Graphite as the negative active material, conductive carbon black, and binder carboxymethyl cellulose + styrene-butadiene rubber (CMC + SBR) are mixed in a mass fraction ratio of 95:2:3. Deionized water is used as the solvent to uniformly disperse the mixture into a slurry, which is then coated on a 6-μm-thick copper foil. The coating thickness is 1 μm on both the front and back sides, and after rolling and slitting, a graphite negative electrode sheet is obtained.

[0132] Comparative Example 2

[0133] This example provides a soft-pack battery, and its specific steps include:

[0134] Positive electrode sheet: Lithium iron phosphate as the positive active material, conductive carbon black, and binder polyvinylidene fluoride are mixed in a mass fraction ratio of 97.5:0.5:2. N-methylpyrrolidone (NMP) solvent is used to add the above mixture into a double planetary mixer for uniform shear dispersion to prepare a positive electrode slurry. The positive electrode slurry is coated on a graphene-coated current collector (see Example 2) using a transfer coater, and after rolling, samples are taken to test the peel strength and electrode resistance.

[0135] Negative electrode sheet: Graphite as the negative active material, conductive carbon black, and binder carboxymethyl cellulose + styrene-butadiene rubber (CMC + SBR) are mixed in a mass fraction ratio of 95:2:3. Deionized water solvent is used to uniformly disperse the mixture into a slurry, which is then coated on a 6-μm-thick copper foil, and after rolling and slitting, a graphite negative electrode sheet is obtained.

[0136] Comparative Example 3

[0137] This example provides a soft-pack battery, and its specific steps include:

[0138] Positive electrode sheet: Lithium iron phosphate as the positive active material, graphene, and binder polyvinylidene fluoride are mixed in a mass fraction ratio of 97.5:0.5:2. N-methylpyrrolidone (NMP) solvent is used to add the above mixture into a double planetary mixer for uniform shear dispersion to prepare a positive electrode slurry. The positive electrode slurry is coated on a graphene-coated current collector, and after rolling, samples are taken to test the peel strength and electrode resistance.

[0139] Negative electrode sheet: Graphite as the negative active material, conductive carbon black, and binder carboxymethyl cellulose + styrene-butadiene rubber (CMC + SBR) are mixed in a mass fraction ratio of 95:2:3. Deionized water solvent is used to uniformly disperse the mixture into a slurry, which is then coated on a 6-μm-thick copper foil, and after rolling and slitting, a graphite negative electrode sheet is obtained.

[0140] Comparative Example 4

[0141] This example provides a soft-pack battery, and its specific steps include:

[0142] Positive electrode sheet: The positive electrode active material lithium iron phosphate: graphene: binder polyvinylidene fluoride are mixed according to a mass fraction ratio of 97.5:0.5:2. The above mixture is added to a double planetary mixer using N-methylpyrrolidone (NMP) solvent and uniformly sheared and dispersed to prepare a positive electrode slurry. The positive electrode slurry is coated on a carbon-coated current collector using a transfer coater, and after rolling, a sheet is taken for testing the peel strength and the electrode sheet resistance.

[0143] Negative electrode sheet: The negative electrode active material graphite: conductive carbon black: binder carboxymethyl cellulose + styrene-butadiene rubber (CMC + SBR) are mixed according to a mass fraction ratio of 95:2:3. The slurry obtained by uniformly dispersing the mixture using deionized water solvent is coated on a 6 μm thick copper foil, and after rolling and slitting, a graphite negative electrode sheet is obtained.

[0144] After die-cutting the positive electrode sheet, it is combined with the negative electrode sheet to form a soft-pack battery with a capacity of 3 Ah, and cycle and rate charge-discharge tests are carried out.

[0145] After die-cutting the positive electrode sheet, it is combined with the negative electrode sheet to form a soft-pack battery with a capacity of 3 Ah, and cycle and rate charge-discharge tests are carried out.

[0146] After die-cutting the positive electrode sheet, it is combined with the negative electrode sheet to form a soft-pack battery with a capacity of 3 Ah, and cycle and rate charge-discharge tests are carried out.

[0147] Table 1 Comparison of the electrochemical performance of the batteries provided by the examples and the comparative examples

[0148] Group Rolling and peeling strength (N / m) Rolled electrode resistance (Ω) DC internal resistance (mΩ) Example 1 104 0.0763 2.264 Example 2 106 0.1592 3.355 Example 3 107 0.1653 3.421 Example 4 90 0.2657 4.136 Example 5 86 0.3722 4.032 Comparative Example 1 84 0.4795 5.543 Comparative Example 2 87 0.4022 4.684 Comparative Example 3 103 0.3814 4.026 Comparative Example 4 88 0.4205 3.998

[0149] Table 1 shows the test results of various electrochemical performances of the examples and the comparative examples. Figure 1 - Figure 4 It is a comparison chart of high-rate (3C and 5C) charge and discharge for the examples and the comparative examples. From the data analysis in Table 1 and Figure 1 - Figure 4 it can be seen that in Example 1, porous graphene is introduced simultaneously in the current collector conductive slurry and the electrode conductive slurry, which has a higher peel strength, enabling ions and electrons to be directly and rapidly transmitted between the electrode material and the current collector. The combined effect of the porous graphene shows a lower electrode sheet resistance, DC internal resistance, and optimal high-rate (3C and 5C) charge-discharge performance.

[0150] Secondly, in Examples 2 - 3, the conductive agent or current collector is a porous graphene material, and the other is graphene, which is also a combination of surface and surface, so it has a higher binding force. However, due to the interlayer conductivity of graphene nanosheets, the charge transfer between the current collector and the electrode material is restricted, and the unique two-dimensional planar structure produces a "steric effect" on the lithium-ion transport inside the electrode. Therefore, the graphene without pores leads to an increase in resistance. Similar to the "surface - surface" combination method of porous graphene and porous graphene provided in Example 1, Examples 2 and 3 also belong to the "surface - surface" combination method. However, due to the interlayer conductivity of the non-porous graphene, the sheet resistance of the electrode and the internal resistance of the battery are significantly higher than those in Example 1.

[0151] In Examples 4 and 5, the combination of conductive carbon black and graphene is in the form of "point - surface". However, when Example 4 is compared with Comparative Example 2, and Example 5 is compared with Comparative Example 4 respectively, the results show that the peeling strength of the combination of porous graphene and conductive carbon black is better than that of the non-porous graphene, and the internal resistance is also relatively reduced.

[0152] In Comparative Example 1, the conductive carbon black and the carbon-coated current collector are in point - point contact. In Comparative Example 2, the conductive carbon black and the graphene-coated current collector are in "point - surface" contact between the carbon black and the graphene. In Comparative Example 3, the graphene and the graphene-coated current collector are in surface - surface contact. Obviously, compared with the "point - surface" combination of carbon black and graphene and the "point - point" combination of carbon black and carbon black, the "surface - surface" combination method of graphene and graphene has a greater peeling strength through comparing the results of the peeling strength of the examples and comparative examples, indicating that the hydrogen bonds, van der Waals forces and spin interactions between graphene make the combination of the current collector and the active material more compact, which can effectively improve the electrical contact and charge transfer between the current collector and the active material. However, the low interlayer conductivity of graphene nanosheets restricts the charge transfer between the current collector and the electrode material, and the sheet resistance of the electrode and the internal resistance of the battery are still relatively large.

[0153] Based on the above analysis, the present invention applies porous graphene in the coated current collector, which synergistically acts with the porous graphene added to the active material. The combination of hydrogen bonds, van der Waals forces and spin interactions is tighter, strengthening the adhesion between the electrode material and the current collector, thereby improving the electrical contact and charge transfer between them, reducing the internal resistance of the electrode sheet, and also inhibiting the corrosion of the current collector during long-term cycling. At the same time, aiming at the low interlayer conductivity of graphene nanosheets in the prior art, which limits the charge transfer between the current collector and the electrode material, and the unique two-dimensional planar structure produces a "steric effect" on the transport of lithium ions inside the electrode. The present invention creates pores on the surface of graphene, and the graphene after pore creation is introduced into both the current collector conductive paste and the electrode conductive paste. The hydrogen bonds, van der Waals forces and spin interactions between graphene make the combination of the current collector and the active material tighter. Different from the "surface-point" contact mode of the commonly used graphene and conductive carbon black and the "point-point" contact mode of conductive carbon black and conductive carbon black, the "surface-surface" contact mode effectively improves the electrical contact and charge transfer between them, significantly reducing the electrode sheet resistance and the internal resistance of the battery, enabling ions to directly and quickly transfer between the electrode material and the current collector, and significantly improving the charge and discharge performance of the battery under high-rate conditions.

[0154] The above are only the preferred embodiments of the present invention, which do not limit the protection scope of the present invention. For those skilled in the art, various changes and modifications can be made to the present invention. Any changes, modifications, substitutions, integrations and parameter changes made to these embodiments by means of conventional substitutions or capable of achieving the same functions without departing from the principle and spirit of the present invention fall within the protection scope of the present invention.

Claims

1. A method for preparing reduced-oxidation porous graphene, characterized in that: The graphene oxide with abundant defects on the surface is chemically porous and finally treated at high temperature to obtain powdery reduced-oxidized porous graphene.

2. The method for preparing reduced-oxidation porous graphene according to claim 1, characterized in that: The chemical pore-forming method includes any one of photolithography, carbon thermal reduction, template method, solvent thermal method, and chemical vapor deposition method; And / or, the graphene oxide is prepared by using an improved Hummers method; The high temperature treatment includes treating at 2200-2400° C. for 1-3 hours.

3. The method for preparing reduced-oxidation porous graphene according to claim 2, characterized in that: The solvothermal method comprises ultrasonically dispersing graphene oxide in a pore-forming agent, performing a solvothermal reaction under sealed conditions, freeze-drying the graphene oxide, and finally subjecting the graphene oxide to high temperature treatment to obtain powdered reduced-oxidized porous graphene.

4. The method for preparing reduced-oxidation porous graphene according to claim 3, characterized in that: The pore-forming agent includes any one of hydrogen peroxide solution, potassium permanganate solution, and potassium hydroxide solution; The ultrasonic dispersion comprises mixing the redox graphene suspension with the pore-forming agent to form a mixed solution, and ultrasonicating for 0.5 to 1 hour; The conditions of the solvent thermal reaction include a reaction temperature of 170 to 190° C. and a reaction time of 6 to 8 hours; The freeze drying comprises freeze drying in a -40°C vacuum freeze drying oven for 6 to 24 hours; And / or, in the redox graphene suspension, the content of graphene oxide is 1 to 3 mg·mL -1 ; The volume ratio of the redox graphene suspension to the pore-forming agent is 9:

1.

5. A reduced-oxidation porous graphene prepared by the preparation method according to any one of claims 1 to 4.

6. A porous graphene concentrated slurry, comprising mixing the reduced-oxidized porous graphene prepared by the preparation method according to any one of claims 1 to 4, a dispersant and an organic solvent, stirring and dispersing them uniformly, and grinding them to obtain the slurry; wherein, The mass percentage content of the reduced-oxidized porous graphene is 25-30%; And / or, the dispersant is one or a combination of polyvinyl pyrrolidone, polyacrylamide, sodium dodecylbenzene sulfonate, polyvinyl alcohol, sodium polyacrylate; preferably, polyvinyl pyrrolidone; The organic solvent is N-methylpyrrolidone; And / or, the mass ratio of the reduced-oxidation porous graphene, the dispersant and the organic solvent is (25-30):3:(67-72).

7. A porous graphene-coated current collector, comprising at least the reduced-oxidized porous graphene as claimed in claim 5, or the porous graphene concentrated slurry as claimed in claim 6.

8. A method for preparing a porous graphene coating current collector, characterized in that: The following steps are involved: S1. mixing the polypropylene resin aqueous solution with the porous graphene concentrated slurry as claimed in claim 6, and forming a first porous graphene resin solution through an emulsification process; S2. adding deionized water to the porous graphene resin solution to obtain a second porous graphene resin solution; S3. Coating the second porous graphene resin solution on the metal pole piece to obtain the porous graphene coating current collector.

9. The method for preparing a porous graphene coating current collector according to claim 8, characterized in that: In S1, the concentration of the polypropylene resin in the polypropylene resin aqueous solution is 35-45wt%; The mass ratio of the polypropylene resin aqueous solution to the porous graphene concentrated slurry is 1:6; and / or, in S2, the mass of the deionized water added is one third of the mass of the water in the polypropylene resin aqueous solution in S1; And / or, in S3, the coating thickness of the second porous graphene resin solution is 1 to 5 μm; The thickness of the metal pole piece is 5-50 μm.

10. Use of the porous graphene coating current collector as claimed in claim 7 as an electrode material in a dry electrode, a capacitor or an electrochemical cell.

11. A positive electrode sheet comprising a current collector and a positive electrode active material; The current collector and / or the positive electrode active material contains the reduced-oxidation porous graphene as claimed in claim 5; Or, the current collector is the porous graphene coating current collector as described in claim 7.

12. A lithium ion battery, comprising at least the positive electrode sheet according to claim 11.

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