CN112103477A - Single-layer reduced graphene oxide lithium cobaltate composite material and preparation method and application thereof - Google Patents
Single-layer reduced graphene oxide lithium cobaltate composite material and preparation method and application thereof Download PDFInfo
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- H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
- H01M4/52—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron
- H01M4/525—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron of mixed oxides or hydroxides containing iron, cobalt or nickel for inserting or intercalating light metals, e.g. LiNiO2, LiCoO2 or LiCoOxFy
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Abstract
The application discloses a single-layer reduced graphene oxide lithium cobaltate composite material and a preparation method and application thereof. The preparation method comprises the following steps: preparing an aqueous solution of single-layer graphene oxide; adding lithium cobaltate into the aqueous solution of the single-layer graphene oxide; and after uniformly mixing, carrying out spray drying to obtain the composite material. The composite material has a continuous three-dimensional conductive structure, wherein the single-layer reduced graphene oxide is coated on the surface of lithium cobaltate, and bridging connection is formed among the lithium cobaltate. The preparation method combines the mixing process of the monolayer graphene oxide and the lithium cobaltate with an efficient spray drying technology, has a simple process, is suitable for mass production, does not need to add various additives, and has low production cost; a complete, continuous and well-conductive single-layer reduced graphene oxide coating layer can be formed on the surface of lithium cobaltate, and the rate discharge performance and the cycle stability of the lithium ion battery are improved.
Description
Technical Field
The application relates to but is not limited to the field of electrochemistry, in particular to but not limited to a single-layer reduced graphene oxide lithium cobaltate composite material and a preparation method and application thereof.
Background
The lithium ion battery is used as an energy storage device with excellent performance, and is widely applied to the fields of civil products such as electric automobiles, daily electronic products, standing type energy storage power stations and the like. However, most of the lithium ion batteries at present cannot fully satisfy the practical needs of the above-mentioned various devices and apparatuses due to the limitation of the critical materials of the batteries. The main problems of the lithium cobaltate cathode material, which is one of the key materials of the lithium ion battery, in practical application are as follows: low specific energy density and poor charge-discharge performance of large current density.
Due to excellent chemical stability, good electron/ion conductivity, and lithium ion storage capacity, applications of graphene in lithium ion battery electrode active materials have been widely studied in recent years. At present, graphene mostly participates in electrochemical reaction in the form of a conductive additive, and the simple physical mixing causes the weak binding force between the graphene and an electrode active material, so that the excellent physical and chemical properties of the graphene are difficult to be fully exerted.
Disclosure of Invention
The method is simple in process, and in the prepared single-layer reduced graphene oxide lithium cobaltate composite material, the single-layer reduced graphene oxide lithium cobaltate is high in combination degree and good in uniformity.
The application provides a method for preparing a single-layer reduced graphene oxide lithium cobaltate composite material, which comprises the following steps: preparing an aqueous solution of single-layer graphene oxide; adding lithium cobaltate into the aqueous solution of the single-layer graphene oxide; and after uniformly mixing, carrying out spray drying to obtain the composite material.
The application also provides a single-layer reduced graphene oxide lithium cobaltate composite material, and the composite material is prepared by the method.
The application also provides an application of the single-layer reduced graphene oxide lithium cobaltate composite material as a lithium ion battery anode active material.
The application also provides a lithium ion battery, and the positive electrode of the lithium ion battery can comprise a binder, a conductive agent and the single-layer reduced graphene oxide lithium cobaltate composite material.
The application has the beneficial effects that:
(1) according to the preparation method, a complete, continuous and good-conductivity single-layer reduced graphene oxide coating layer can be formed on the surface of the lithium cobaltate active material through one-time operation, and the process is simple;
(2) various additives such as organic solvents, surfactants, reducing agents, oxidizing agents and the like are not required to be added in the generation process of the single-layer reduced graphene oxide coating layer, so that the production cost is low;
(3) the preparation method skillfully combines the mixing process of the single-layer graphene oxide and the lithium cobaltate with an efficient spray drying technology, and is suitable for mass production and manufacture of the single-layer reduced graphene oxide lithium cobaltate composite material;
(4) the lithium ion battery prepared by the method has excellent rate discharge performance and cycle stability.
Additional features and advantages of the application will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the application. The objectives and other advantages of the application may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
Drawings
The accompanying drawings are included to provide a further understanding of the claimed subject matter and are incorporated in and constitute a part of this specification, illustrate embodiments of the subject matter and together with the description serve to explain the principles of the subject matter and not to limit the subject matter.
Fig. 1 is a field emission scanning electron microscope image of a conventional lithium cobaltate positive electrode active material;
fig. 2 is a field emission scanning electron microscope image of a single-layer reduced graphene oxide lithium cobaltate composite material according to an embodiment of the present application;
FIG. 3 is a graph of cycle performance of a lithium ion battery of a comparative example of the present application;
FIG. 4 is a graph of the cycling performance of a lithium ion battery of an embodiment of the present application;
FIG. 5 is a graph showing the rate charge and discharge curves of a lithium ion battery according to a comparative example of the present application;
fig. 6 is a rate charge and discharge curve of the lithium ion battery according to the embodiment of the present application.
Detailed Description
To make the objects, technical solutions and advantages of the present application more apparent, embodiments of the present application will be described in detail below with reference to the accompanying drawings. It should be noted that the embodiments and features of the embodiments in the present application may be arbitrarily combined with each other without conflict.
In order to solve the problems of low specific energy density and poor high-current density charge-discharge performance of lithium cobaltate, a means of introducing a new functional material to coat the lithium cobaltate can be adopted. However, the method for coating the lithium cobaltate active material by using the new functionalized material not only has complex operation steps, but also needs to add various additives such as an organic solvent, a surfactant, a reducing agent, an oxidizing agent and the like, so that the treatment cost is high.
The embodiment of the application provides a method for preparing a single-layer reduced graphene oxide lithium cobaltate composite material, which comprises the following steps: preparing an aqueous solution of single-layer graphene oxide; adding lithium cobaltate into the aqueous solution of the single-layer graphene oxide; and after uniformly mixing, carrying out spray drying to obtain the composite material.
The term "reduced graphene oxide" as used herein refers to: and carrying out partial thermal reduction reaction on the single-layer graphene oxide to obtain reduced graphene oxide.
According to the embodiment of the application, the good conductive anisotropy and the structural characteristic of controllable curvature of the single-layer reduced graphene oxide are utilized, the single-layer reduced graphene oxide is coated on the surface of the lithium cobaltate active material, and then partial thermal reduction is carried out on the lithium cobaltate active material, so that the single-layer reduced graphene oxide lithium cobaltate composite material with a three-dimensional conductive network structure and good conductivity is constructed.
The preparation method provided by the embodiment of the application can form the single-layer reduced graphene oxide coating layer through one-time operation, the process is simple, and various additives such as an organic solvent, a surfactant, a reducing agent and an oxidizing agent are not required to be added in the generation process of the single-layer reduced graphene oxide coating layer, so that the production cost is low.
In addition, the preparation method disclosed by the embodiment of the application combines the mixing process of the single-layer graphene oxide and the lithium cobaltate with a spray drying technology, is simple in process, and is suitable for mass production and manufacturing of the lithium cobaltate composite material of the single-layer reduced graphene oxide lithium ion battery.
In the embodiment of the present application, the mass ratio of the single-layer graphene oxide to the water may be 1 × 10-5:1～50×10-51, for example, may be 10X 10-5:1、20×10-5:1、30×10-5:1、40×10-5:1、45×10-51, and the like. When the mass ratio of the single-layer graphene oxide to the water is 1 multiplied by 10-5:1～50×10-5When the content is in the range of 1, the conductivity of lithium cobaltate can be greatly improved, and the monolayer graphene oxide can be uniformly dispersed in water, which is favorable for the full progress of the modification reaction.
In the embodiment of the present application, the mass ratio of the lithium cobaltate to the aqueous solution of the single-layer graphene oxide may be 0.01:1 to 0.5:1, and may be, for example, 0.05:1, 0.1:1, 0.2:1, 0.3:1, 0.4:1, or the like. When the mass ratio of the lithium cobaltate to the aqueous solution of the single-layer graphene oxide is within the range of 0.01: 1-0.5: 1, a coating layer can be well formed, the situation that the coating layer is too thick is not easy to occur, and the formation of the coating layer of the single-layer reduced graphene oxide is facilitated.
In the present embodiment, the mixing may be performed by stirring or the like.
In the embodiment of the present application, the stirring speed may be 60 rpm/min to 240 rpm/min, for example, 80 rpm/min, 100 rpm/min, 120 rpm/min, 150 rpm/min, 180 rpm/min, 200 rpm/min, and the like; the stirring time may be 10min to 120min, for example, 20min, 30min, 60min, 90min, or the like.
In the present embodiment, the spray drying may be performed at an outlet temperature of 150 to 200 ℃, for example, 160 ℃, 170 ℃, 180 ℃, 190 ℃ or the like; the feed rate may be 300mL/min to 800mL/min, for example, 400mL/min, 500mL/min, 600mL/min, 700mL/min, or the like. When the outlet temperature of the spray drying process is 150-200 ℃, the monolayer graphene oxide can be subjected to proper thermal reduction, so that the prepared lithium cobaltate composite material has good conductivity.
In the embodiment of the present application, the single-layer graphene oxide may be commercially available single-layer graphene oxide, and for example, the single-layer graphene oxide may be single-layer graphene oxide purchased from shanghai carbon source sink new energy technology limited.
The single-layer reduced graphene oxide lithium cobaltate composite material prepared by the embodiment of the application has a continuous three-dimensional conductive structure, and the single-layer reduced graphene oxide is coated on the surface of lithium cobaltate, so that a complete, continuous and good-conductivity single-layer reduced graphene oxide coating layer is formed on the surface of a positive electrode active material of a lithium cobaltate positive electrode, and bridging connection is formed among the lithium cobaltate active materials.
In the single-layer reduced graphene oxide lithium cobaltate composite material prepared in the embodiment of the application, the thickness of the coated single-layer reduced graphene oxide layer may be, for example, 0.34 nm.
The embodiment of the application provides application of a single-layer reduced graphene oxide lithium cobaltate composite material as a positive electrode active material of a lithium ion battery.
The embodiment of the application provides a lithium ion battery, including: the anode comprises a binder, a conductive agent and the single-layer reduced graphene oxide lithium cobaltate composite material; the negative electrode may be a carbon material, a metal oxide, a metal or an alloy, and may be, for example, a metallic lithium sheet. The lithium ion battery has excellent rate discharge performance and cycle stability.
The binder can be any one or more of various lithium ion battery binders such as polyvinylidene fluoride, sodium carboxymethyl cellulose, styrene butadiene rubber and the like.
The conductive agent can be selected from any one or more of various lithium ion battery conductive agents such as acetylene black, carbon black, graphite, carbon nanotubes, ketjen black and the like.
The mass ratio of the single-layer reduced graphene oxide lithium cobaltate composite material to the binder to the conductive agent can be 80:10: 10.
The positive electrode of the lithium ion battery can be prepared by the following method: and uniformly stirring the single-layer reduced graphene oxide lithium cobaltate composite material, the binder and the conductive agent in a solvent according to a ratio to form slurry, coating the slurry on the surface of the current collector, drying in vacuum, and tabletting to obtain the anode.
The solvent may be any one or more selected from various lithium ion battery solvents such as N-methylpyrrolidone, ethylene carbonate, diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate.
The current collector can be selected from various lithium ion battery current collectors such as aluminum foil.
The vacuum drying time may be 8 hours, 10 hours, 12 hours, 15 hours, etc.
The diaphragm can be selected from various lithium ion battery diaphragms such as microporous polypropylene (Celgard2300) diaphragms.
The electrolyte can be selected from any one or more of various lithium ion battery electrolytes such as liquid electrolyte, solid electrolyte, gel electrolyte and the like, and is preferably lithium hexafluorophosphate (LiPF)6) LiPF prepared by mixing Ethylene Carbonate (EC), diethyl carbonate (DEC) and methyl ethyl carbonate (EMC) and having a content of preferably 1mol/L6Body of/EC, DEC, EMCMixing at volume ratio of 1:1: 1.
The lithium ion battery can be assembled in a glove box filled with high-purity argon.
The single-layer graphene oxide used in the following examples was purchased from Shanghai carbon Source Cui New energy science, Inc.
Example 1
Preparing a single-layer reduced graphene oxide lithium cobaltate composite material:
preparing 200ml of aqueous solution containing 0.01g of monolayer graphene oxide; adding 2g of lithium cobaltate particles into the aqueous solution of the single-layer graphene oxide, and stirring for 20min at the stirring speed of 240 revolutions per minute; and (3) carrying out spray drying treatment on the reacted mixed solution at the outlet temperature of 180 ℃ under the conditions of a dispersion state and a feeding flow rate of 400mL/min to obtain the single-layer reduced graphene oxide lithium cobaltate composite material.
The prepared single-layer reduced graphene oxide lithium cobaltate composite material is used as a lithium ion battery anode active material to prepare a lithium ion battery:
mixing a single-layer reduced graphene oxide lithium cobaltate composite material, a conductive agent carbon black and a binder polyvinylidene fluoride according to a mass ratio of 80:10:10 by taking N-methyl pyrrolidone as a solvent, uniformly stirring to form slurry, coating the slurry on the surface of an aluminum foil, performing vacuum drying for 12 hours, and tabletting to obtain a positive plate with the diameter of 10 mm.
Metallic lithium is used as a negative electrode, a microporous polypropylene (Celgard2300) membrane is used as a diaphragm, and 1mol/L LiPF6The electrolyte is prepared from the following components of/EC + DEC + EMC (the volume ratio is 1:1: 1).
The CR2025 button cell was assembled in a glove box filled with high purity argon. And standing for 12 hours, and then carrying out electrochemical performance test.
Example 2
Preparing a single-layer reduced graphene oxide lithium cobaltate composite material:
preparing 200ml of aqueous solution containing 0.01g of monolayer graphene oxide; adding 100g of lithium cobaltate particles into the aqueous solution of the single-layer graphene oxide, and stirring for 120min at the stirring speed of 120 revolutions per minute; and (3) carrying out spray drying treatment on the reacted mixed solution at the outlet temperature of 150 ℃ under the conditions of a dispersion state and a feeding flow rate of 300mL/min to obtain the single-layer reduced graphene oxide lithium cobaltate composite material.
The prepared single-layer reduced graphene oxide lithium cobaltate composite material is used as a lithium ion battery anode active material to prepare a lithium ion battery:
mixing a single-layer reduced graphene oxide lithium cobaltate composite material, a conductive agent carbon black and a binder polyvinylidene fluoride according to a mass ratio of 80:10:10 by taking N-methyl pyrrolidone as a solvent, uniformly stirring to form slurry, coating the slurry on the surface of an aluminum foil, then performing vacuum drying for 12 hours, and tabletting to obtain a positive plate with the diameter of 10 mm.
Metallic lithium is used as a negative electrode, a microporous polypropylene (Celgard2300) membrane is used as a diaphragm, and 1mol/L LiPF6The electrolyte is prepared from the following components of/EC + DEC + EMC (the volume ratio is 1:1: 1).
The CR2025 button cell was assembled in a glove box filled with high purity argon. And standing for 12 hours, and then carrying out electrochemical performance test.
Example 3
Preparing a single-layer reduced graphene oxide lithium cobaltate composite material:
preparing 200ml of aqueous solution containing 0.003g of single-layer graphene oxide; adding 20g of lithium cobaltate particles into the aqueous solution of the single-layer graphene oxide, and stirring for 10min at the stirring speed of 60 revolutions per minute; and (3) carrying out spray drying treatment on the reacted mixed solution at the outlet temperature of 200 ℃ under the conditions of a dispersion state and a feeding flow rate of 800mL/min to obtain the single-layer reduced graphene oxide lithium cobaltate composite material.
The prepared single-layer reduced graphene oxide lithium cobaltate composite material is used as a lithium ion battery anode active material to prepare a lithium ion battery:
mixing a single-layer reduced graphene oxide lithium cobaltate composite material, a conductive agent carbon black and a binder polyvinylidene fluoride according to a mass ratio of 80:10:10 by taking N-methyl pyrrolidone as a solvent, uniformly stirring to form slurry, coating the slurry on the surface of an aluminum foil, then performing vacuum drying for 12 hours, and tabletting to obtain a positive plate with the diameter of 10 mm.
Using metallic lithium as negative electrode and microporous polymerPropylene (Celgard2300) membrane as a diaphragm and 1mol/L LiPF6The electrolyte is prepared from the following components of/EC + DEC + EMC (the volume ratio is 1:1: 1).
The CR2025 button cell was assembled in a glove box filled with high purity argon. And standing for 12 hours, and then carrying out electrochemical performance test.
Comparative example 1
Mixing lithium cobaltate positive electrode material, conductive agent carbon black and adhesive polyvinylidene fluoride according to the mass ratio of 80:10:10 by taking N-methyl pyrrolidone as a solvent, uniformly stirring to form slurry, coating the slurry on the surface of an aluminum foil, then carrying out vacuum drying for 12 hours, and tabletting to obtain a positive electrode plate with the diameter of 10 mm.
Metallic lithium is used as a negative electrode, a microporous polypropylene (Celgard2300) membrane is used as a diaphragm, and 1mol/L LiPF6The electrolyte is prepared from the following components of/EC + DEC + EMC (the volume ratio is 1:1: 1).
The CR2025 button cell was assembled in a glove box filled with high purity argon. And standing for 12 hours, and then carrying out electrochemical performance test.
Comparative example 2
200mL of a mixed aqueous solution containing 0.01g of graphene oxide and 0.1g of magnesium chloride was prepared. 2g of lithium cobaltate was added to the above mixed aqueous solution, and the mixture was stirred at a stirring speed of 500 rpm for 8 minutes. And (3) washing the reacted composite material with water, performing suction filtration, and placing the composite material in a vacuum drying oven with the pressure of less than-0.08 Mpa for vacuum drying to obtain the lithium cobaltate composite material.
Mixing the lithium cobaltate composite material, the conductive agent carbon black and the adhesive polyvinylidene fluoride according to the mass ratio of 80:10:10 by taking N-methyl pyrrolidone as a solvent, uniformly stirring to form slurry, coating the slurry on the surface of an aluminum foil, then carrying out vacuum drying for 12 hours, and tabletting to obtain a positive plate with the diameter of 10 mm.
Metallic lithium is used as a negative electrode, a microporous polypropylene (Celgard2300) membrane is used as a diaphragm, and 1mol/L LiPF6The electrolyte is prepared from the following components of/EC + DEC + EMC (the volume ratio is 1:1: 1).
The CR2025 button cell was assembled in a glove box filled with high purity argon. And standing for 12 hours, and then carrying out electrochemical performance test.
Comparative example 3
200mL of a mixed aqueous solution containing 0.005g of graphene oxide, 1.3g of manganese sulfate, 3.5g of sodium hypophosphite, and 5.8g of sodium citrate was prepared. 2g of lithium cobaltate was added to the above mixed aqueous solution, and the mixture was stirred at a stirring speed of 450 rpm for 15 minutes. And (3) washing the reacted composite material with water, performing suction filtration, and performing vacuum drying in a vacuum drying oven with the pressure of less than-0.08 Mpa. And obtaining the lithium cobaltate composite material.
Mixing the lithium cobaltate composite material, the conductive agent carbon black and the adhesive polyvinylidene fluoride according to the mass ratio of 80:10:10 by taking N-methyl pyrrolidone as a solvent, uniformly stirring to form slurry, coating the slurry on the surface of an aluminum foil, then carrying out vacuum drying for 12 hours, and tabletting to obtain a positive plate with the diameter of 10 mm.
Metallic lithium is used as a negative electrode, a microporous polypropylene (Celgard2300) membrane is used as a diaphragm, and 1mol/L LiPF6The electrolyte is prepared from the following components of/EC + DEC + EMC (the volume ratio is 1:1: 1).
And assembling the CR2032 button cell in a glove box filled with high-purity argon. And standing for 12 hours, and then carrying out electrochemical performance test.
Performance testing and results:
(1) the microscopic morphology of the existing lithium cobaltate positive electrode active material and the single-layer reduced graphene oxide lithium cobaltate composite material of example 1 was tested by using a field emission scanning electron microscope (Zeiss Ultra 55, germany), and the test results are shown in fig. 1 and 2.
Compared with the single-layer reduced graphene oxide lithium cobaltate composite material in fig. 2, the lithium cobaltate positive active material in fig. 1 has the advantages that on the surface of the single-layer reduced graphene oxide lithium cobaltate composite material in fig. 2, the single-layer reduced graphene oxide not only covers the surface of lithium cobaltate, but also forms bridging connection among lithium cobaltate powder to form a continuous and complete three-dimensional conductive structure, and the structure can effectively improve the conductivity of the lithium ion positive material.
(2) The lithium ion batteries of the examples and the comparative examples were tested for charge and discharge performance at different charge and discharge rates (0.2C, 0.5C, 1C, 2C, 5C) and at a voltage range of 2.5 to 4.6V using a blue cell test system (china, LAND CT-2001A).
The test results of the cycle performance and the rate charge and discharge performance of the lithium ion battery in the comparative example 1 are shown in fig. 3 and fig. 5, and it can be seen that the capacity of the lithium ion battery in the comparative example 1 is seriously attenuated about 370 times of cycle, and the specific capacities of the lithium ion battery in the comparative example 1 at 0.2C and 5C discharge are about 140mAh/g and 0mAh/g, respectively.
The lithium ion battery of comparative example 2 can be cycled about 400 times; the specific discharge capacities at 0.2C and 5C were 170mAh/g and 60 mAh/g.
The lithium ion battery of comparative example 3 can be cycled approximately 450 times; the specific discharge capacity at 0.2C and 5C was 177mAh/g and 71 mAh/g.
The test results of the cycle performance and the rate charge and discharge performance of the lithium ion battery in example 1 are shown in fig. 4 and fig. 6, and it can be seen that the lithium ion battery in example 1 can be stably cycled for more than 500 times, and the specific capacities at 0.2C and 5C discharge are respectively about 228mAh/g and 105.7 mAh/g.
The lithium ion battery of example 2 can be stably cycled for more than 500 cycles with specific capacities of about 200.2mAh/g and 89.5mAh/g at 0.2C and 5C discharge, respectively.
The lithium ion battery of example 3 can be stably cycled for more than 500 times with specific capacities of about 218.8mAh/g and 96.3mAh/g at 0.2C and 5C discharge, respectively.
Compared with the comparative example, the preparation method has the advantages that the uneven preparation process is simple, the cycle performance of the prepared battery is greatly improved and can reach more than 500 times, and the small-rate discharge performance and the large-rate discharge performance are improved.
Although the embodiments disclosed in the present application are described above, the descriptions are only for the convenience of understanding the present application, and are not intended to limit the present application. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the disclosure as defined by the appended claims.
Claims (9)
1. A method of preparing a single layer reduced graphene oxide lithium cobaltate composite material, comprising:
preparing an aqueous solution of single-layer graphene oxide;
adding lithium cobaltate into the aqueous solution of the single-layer graphene oxide;
and after uniformly mixing, carrying out spray drying to obtain the composite material.
2. The method of claim 1, wherein the monolayer graphene oxide to water mass ratio is 1 x 10-5:1～50×10-5:1。
3. The method according to claim 1 or 2, wherein the mass ratio of the lithium cobaltate to the aqueous solution of the single-layer graphene oxide is 0.01:1 to 0.5: 1.
4. The method according to claim 1 or 2, wherein the mixing is performed by stirring, the stirring speed is 60-240 rpm, and the stirring time is 10-120 min.
5. The process according to claim 1 or 2, wherein the spray drying is carried out at an outlet temperature of 150 ℃ to 200 ℃ and a feed flow rate of 300mL/min to 800 mL/min.
6. A single layer reduced graphene oxide lithium cobaltate composite material prepared by the method of any one of claims 1 to 5.
7. The composite material according to claim 6, which has a continuous three-dimensional conductive structure, wherein a single layer of reduced graphene oxide is coated on the surface of lithium cobaltate, and bridging connections are formed between the lithium cobaltate.
8. Use of the single-layer reduced graphene oxide lithium cobaltate composite material according to claim 6 or 7 as a positive electrode active material for a lithium ion battery.
9. A lithium ion battery, a positive electrode of which comprises a binder, a conductive agent and the single-layer reduced graphene oxide lithium cobaltate composite material of claim 6 or 7.
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