Preparation method of weak graphene oxide for anode doping of lithium ion battery

Abstract

The invention discloses a preparation method of weak graphene oxide for anode doping of a lithium ion battery, which comprises the following steps of: performing intercalation treatment on graphite by using sulfuric acid and nitric acid, and oxidizing the graphite by using chromium trioxide and manganese dioxide as oxidants; and after manganese dioxide is destroyed by hydrogen peroxide and converted into manganese ions, Fenton oxidation is continuously adopted to obtain weak graphene oxide. According to the method, intercalation energy is provided based on an oxidation reaction, so that graphite is slightly oxidized but does not generate a large amount of oxygen-containing structures, acid is intercalated on the graphite based on the oxidation reaction, the interlayer spacing is increased, then Fenton oxidation is carried out, a slightly oxidized area is partially damaged, and then liquid phase stripping is carried out, so that the graphene is obtained. The ethylene carbonate dispersion liquid can be directly used for doping and preparing an anode material of a lithium battery of an electrode.

Description

Technical Field

[0001] The invention mainly relates to a method for preparing a few-layer graphene for weak oxidation-assisted liquid phase exfoliation for lithium ion battery anode doping. Background Art

[0002] As a form of chemical power source, lithium-ion battery has the advantages of high operating voltage, high energy density, light weight, small size, good safety, and green environmental protection. It has become the leader among new high-energy green batteries and the most mature power source in the field of pure electric vehicles and hybrid electric vehicles. Especially with the rapid rise of the new energy vehicle industry, higher requirements are placed on its battery performance indicators such as long battery life, fast charging and discharging, high safety, and long life. Therefore, researching and developing new lithium-ion battery electrode materials with better performance is one of its important development directions.

[0003] The anode material of the battery plays a vital role in the overall performance of the battery. Among many anode materials, lithium iron phosphate has good safety, stable structure, high operating voltage, high theoretical discharge capacity, long cycle life, abundant raw material resources, low price, and is green and environmentally friendly. It is considered by many researchers to be one of the most promising new generation lithium-ion positive electrode materials for electric vehicles and hybrid electric vehicles. However, due to its own structural characteristics, the material itself has poor electrical conductivity and large current discharge performance, which limits the application and development of the material. This problem can be effectively solved by anode doping with graphene, carbon nanotubes, etc.

[0004] The six-ring structure of graphene is the most stable structure among the known organic materials. The two-dimensional nanostructure composed of this structure gives it excellent mechanical, electrical and thermal properties: it has the highest strength among the known materials, with a Young's modulus of up to 1000 GPa and a strength of 125 GPa. It has extremely high conductivity and an electron migration rate of up to 1.5×10 4 cm 2 ·V -1 ·S -1, which is much higher than metal conductors or semiconductors, and is very suitable for use in battery anode materials to improve the conductivity of lithium iron phosphate. The positive electrode material of lithium iron phosphate is a metal compound with general conductivity, and it needs to be doped with a certain conductive material to form a conductive network (pathway). The conventional process uses the introduction of carbon materials, such as graphene, but graphene is a sheet-like film material with a size of about 100 square nanometers. Although it has good conductivity, its lamellar structure will form a maze effect, affecting the embedding and extraction of lithium ions. Compared with the general hummers method, the present invention uses weak oxidants CrO3 and MnO2 to oxidize graphite, reduce its embedding of lamellar heteroatoms, such as oxygen atoms, and then destroy the oxygen-containing functional groups of oxidized graphite through Fenton oxidation, so that it carries heteroatoms out of the carbon layer in the form of CO2, resulting in holes in the carbon layer, forming channels for lithium ions, and facilitating the entry and exit of ions.

[0005] Graphene and graphene oxide can be obtained through liquid phase exfoliation and redox method respectively. Liquid phase exfoliation can directly obtain graphene with high conductivity. However, due to the small spacing between graphite layers, which is 0.335nm, the liquid phase exfoliation cycle is long and the solvent requirements are high, such as NMP, which has a high boiling point and is difficult to remove, making industrialization difficult. The redox method can not only directly form corresponding holes on the graphene surface through oxidation to form a three-dimensional structure and ion channels, but also form a 3D structure through self-assembly. However, due to the corresponding oxidation damage, even if the sheet is deoxygenated by the reducing agent, it is difficult for the product to obtain good conductivity. Summary of the invention

[0006] Purpose of the invention: In view of the above existing problems and shortcomings, the purpose of the present invention is to provide a method for preparing weakly oxidized graphene for lithium-ion battery anode doping.

[0007] Technical solution: To achieve the above-mentioned invention object, the present invention adopts the following technical solution: A method for preparing weakly oxidized graphene for lithium ion battery anode doping, comprising the following steps: S1, adding an oxidant to flake graphite with a mesh size of 500 or more to carry out an acid intercalation oxidation reaction on the flake graphite, thereby forming limited oxidation to increase the interlayer spacing of the flake graphite; S2, performing oxidation etching on the reaction product obtained in step S1 by Fenton oxidation method to form ion channel gaps; S3, the reaction product of step S2 is subjected to solid-liquid centrifugal separation and deionized water washing to obtain a solid product, which is dispersed in ethyl acetate and subjected to ultrasonic stripping to obtain a weakly oxidized graphene target product.

[0008] Furthermore, in step S1, for every 1 mass part of flake graphite, the added oxidant consists of the following components: 30-100 parts of sulfuric acid, 0.5-5 parts of nitric acid, 1-5 parts of manganese dioxide, and 0.2-3 parts of chromium trioxide.

[0009] Furthermore, the temperature of the acid intercalation oxidation of the flake graphite is 40-50° C., and the reaction time is 6-12 hours.

[0010] Further, the Fenton oxidation process in step S2 is as follows: The product obtained in step S1 is poured into ice water, and hydrogen peroxide with a concentration of 30% is added until the material is converted into a dark green or black slurry; then the solid and liquid are separated by centrifugation, and the solid is washed with deionized water to obtain a solid, and a mixture of 10% hydrogen peroxide and 0.5% ferrous sulfate is added to carry out Fenton oxidation reaction for more than 12 hours. After the reaction is overnight, the solid and liquid are separated by centrifugation, and the solid product is washed with deionized water; the Fenton oxidation reaction temperature is 10-40°C.

[0011] Furthermore, the ultrasonic peeling in step S3 is performed for 2-10 hours at a frequency of 2-10 MHz.

[0012] Beneficial effects: Compared with the prior art, the present invention provides intercalation energy based on an oxidation reaction, so that graphite is weakly oxidized but does not produce a large amount of oxygen-containing structures. Based on the oxidation reaction, acid is intercalated into graphite to increase the interlayer spacing, and then Fenton oxidation is performed to partially destroy the weakly oxidized area. Then, through liquid phase stripping, a dispersion of ethylene carbonate of a single layer or at least a layer of porous graphene is obtained, which can be directly used for doping to prepare the anode material of a lithium battery for an electrode. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is the Raman spectrum of Example 1 of the present invention.

[0014] Figure 2 This is the Raman spectrum of Example 2 of the present invention.

[0015] Figure 3 This is the Raman spectrum of Example 3 of the present invention.

[0016] Figure 4 The graphene XRD patterns obtained by oxidation with different oxidants in the present invention. DETAILED DESCRIPTION

[0017] The present invention is further explained below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention. After reading the present invention, various equivalent forms of modifications to the present invention by those skilled in the art all fall within the scope defined by the claims attached to this application. Example

[0018] (1) In 1 part of 5000 mesh flake graphite, add 50g of sulfuric acid as intercalation acid, 1g of nitric acid as auxiliary intercalation acid, 3 parts of manganese dioxide and 0.5 parts of chromium trioxide as oxidant. The reaction temperature is 40°C and the reaction time is 6 hours.

[0019] (2) The product obtained in step 1 was poured into ice water, and 30% hydrogen peroxide was added until the material was converted into a dark green to black slurry. The solid was separated by centrifugation and washed with deionized water. Subsequently, a 10% hydrogen peroxide-0.5% FeSO4 mixture was added for Fenton oxidation. The reaction temperature was 10°C and the reaction time was 2 hours. After the reaction, the solid and liquid were separated by centrifugation, and the solid was washed with deionized water.

[0020] (3) The product obtained in step 2 was dispersed in ethylene carbonate and subjected to ultrasonic exfoliation under stirring. The ultrasonic time was 4 h and the ultrasonic frequency was 3 MHz. Example

[0021] (1) 1 part of 500 mesh flake graphite, 100 g of sulfuric acid as intercalation acid, 2 g of nitric acid as intercalation aid acid, 3 parts of manganese dioxide and 0.5 parts of chromium trioxide as oxidant, the reaction temperature is 40°C, and the reaction time is 12 hours.

[0022] (2) The product obtained in step 1 was poured into ice water, and 30% hydrogen peroxide was added until the material was converted into a dark green to black slurry. The solid was separated by centrifugation and washed with deionized water. Subsequently, a 10% hydrogen peroxide-0.5% FeSO4 mixture was added for Fenton oxidation. The reaction temperature was 40°C and the reaction time was 2 hours. After the reaction was completed, the solid and liquid were separated by centrifugation, and the solid was washed with deionized water.

[0023] (3) The product obtained in step 2 was dispersed in ethylene carbonate and subjected to ultrasonic exfoliation under stirring. The ultrasonic time was 4 h and the ultrasonic frequency was 5 MHz. Example

[0024] (1) 1 part of 1000 mesh flake graphite, 100 g of sulfuric acid as intercalation acid, 1.5 g of nitric acid as intercalation aid acid, 3 parts of manganese dioxide and 0.5 parts of chromium trioxide as oxidant, reaction temperature 40 ° C, reaction time 12 hours.

[0025] (2) The product obtained in step 1 was poured into ice water, and 30% hydrogen peroxide was added until the material was converted into a dark green to black slurry. The solid was separated by centrifugation and washed with deionized water. Subsequently, a 10% hydrogen peroxide-0.5% FeSO4 mixture was added for Fenton oxidation. The reaction temperature was 30°C and the reaction time was 12 hours. After the reaction was completed, the solid and liquid were separated by centrifugation, and the solid was washed with deionized water.

[0026] (3) The product obtained in step 2 was dispersed in ethylene carbonate and subjected to ultrasonic exfoliation under stirring. The ultrasonic time was 4 h and the ultrasonic frequency was 3 MHz.

[0027] Performance data test By testing the graphene samples prepared in the examples, the following test results were obtained, as shown in Table 1. The specific surface area was measured by the BET nitrogen adsorption method, and the conductivity was measured by four-probe test after tableting. The four-probe test can reflect the conductivity of the product, and the BET test can reflect the porosity of the product.

[0028] (All samples were freeze-dried after centrifugation, and the specific surface area was tested by BET nitrogen adsorption. The test showed that the samples were loose and uncompacted sponge-like solids, and the conductivity was a dense solid after tableting. The Raman spectrum showed a slightly compacted powder, and the Raman spectrum test wavelength was 532nm) from Figure 1-3 From the results in Table 1, it can be seen that when graphite is converted into graphite oxide under the action of concentrated sulfuric acid and an oxidant, a d peak is formed in the Raman spectrum, which represents that oxygen-containing functional groups are introduced into the sp2-hybridized carbon layer to form sp3-hybridized carbon atoms. This will occur regardless of the proportion of MnO2 and CrO3 added to the sulfur-nitric acid mixed acid system in the first step. However, in the second step, the proportion of sp3-hybridized defective carbon atoms will decrease through different degrees of Fenton oxidation. In specific embodiments 1-3, the ratio of Id / Ig decreases successively, and the d peak decays successively. This is because the hydroxyl radicals in the Fenton oxidation destroy the oxygen-containing functional groups, causing some of the oxygen-containing functional groups to leave the carbon layer in the form of CO2. This process will cause the carbon layer to form voids and defects.

[0029] As the intensity of Fenton oxidation increases, by comparing Example 1 with Examples 2-3, the experiment found that extending the time of Fenton oxidation will significantly increase the damage of the carbon layer caused by Fenton oxidation, increase the loss of sp3 hybridized carbon, increase the proportion of sp2 hybridized carbon in the carbon layer, increase the specific surface area, and enhance the conductivity of carbon atoms.

[0030] By comparing Examples 2 and 3, it is found that increasing the reaction temperature of Fenton oxidation does not significantly improve the conductivity and defectivity. This may be because when a large number of impurity ions are present, the excessively high temperature leads to the accelerated decomposition of hydrogen peroxide in the system. Although the reaction rate is increased, the final progress is reduced, resulting in insufficient removal of the proportion of sp3 hybridized carbon atoms when the reaction endpoint is reached.

[0031] In the first step of the oxidation process in the sulfuric acid system, a small change in the ratio of acid to oxidant and the mesh size of graphite cannot significantly change the properties of the oxidation product. One reason is that exfoliation and fragmentation will occur during the oxidation of graphite, so the mesh size has little effect on graphite oxidation. The ratio of the amount of oxidant added to the acid cannot significantly change the properties of graphite oxidation. This is because the oxidation of graphite is a process in which an oxidant diffuses in the graphite flakes with the help of acid, and its kinetics are restricted by the diffusion rate. Therefore, increasing or decreasing the oxidant and acid-base ratio will only affect the progress of graphite diffusion oxidation from the edge to the center. Even if the oxidant is insufficient, the final result is that some graphite cannot be converted into oxidized graphite, and finally remains in the form of graphite blocks / powder during the exfoliation process. This part of the residue can still be reused as a raw material for oxidation after drying.

[0032] In addition, the scheme of oxidizing graphite by CrO3 as an oxidative change is an important content of the present invention. In practice, we have tried a large number of oxidant combinations and found that CrO3 has a characteristic oxidation for graphite, which can significantly destroy the crystal structure of graphite and directly convert graphite into an amorphous state in the oxidation stage (that is, the graphite oxide formed is different from other oxidants, including potassium permanganate. The oxidation products of other oxidants are ordered crystal structures, which are difficult to peel off. During Fenton oxidation, the diffusion of hydroxyl radicals is also difficult. The system with hexavalent chromium in the present invention can directly form amorphous graphite oxide, making it easier for the oxidant to enter and exit the sheet and diffuse in the sheet, such as Figure 4 The XRD diagrams under different oxidants are shown as follows: the 2-theta angle corresponds to the product: 11.2 is the normal peak of graphite oxide with a crystal structure, 26.5° is the graphite peak, 22.5° is the graphite-sulfuric acid GIC peak, and the mixed acid of sulfuric acid and nitric acid can form a normal peak of graphite oxide with a crystal structure of 11.2. The process is similar to the hummer oxidation method. Sulfuric acid-potassium persulfate and sulfuric acid-nitric acid are mixed as conventional peaks for forming intercalation compounds with oxidation. They are all sharp peaks, and the lattice integrity state exceeds 70%. After the introduction of CrO3, the product forms a huge package peak, which is an embodiment of the amorphous state. This is also the important reason why this patent uses CrO3 as the core oxidant to synthesize graphite oxide in sulfuric acid even though the hexavalent chromium is very toxic. In addition, the frequency of ultrasonic stripping in the present invention is preferably 5MHz. Compared with conventional medium and low frequency ultrasonic stripping, the high frequency ultrasonic stripping used in the present invention effectively improves the efficiency of layered stripping and can be directly used for lithium battery anode materials doped with active conductive materials.

[0033] It should be noted here that during the weak oxidation of graphite, oxidation areas will be formed (the oxidation of graphite is not uniform, just like a Dalmatian, some areas are oxidized to form non-conductive areas mainly composed of sp3-hybridized carbon atoms with oxygen functional groups), and some areas are complete sp2-hybridized conductive carbon atom areas. Therefore, when oxidizing in sulfuric acid in the first step, it is necessary to minimize the content of sp3-hybridized oxidation areas, that is, reduce the embedding of oxygen atoms. As long as the oxidation process is sufficient to increase the spacing between graphite layers and allow them to be peeled off, the sp3-hybridized oxidation areas formed at this time will be in the Fenton oxidation process (similar to the process of digesting sewage, reducing cod by destroying alcohols, aldehydes and ketones with hydrogen peroxide), directly oxidizing the hydroxyl and aldehyde-ketone functional groups formed in the first step of sulfuric acid oxidation in the graphene oxide into carbon dioxide, and ion channels can be etched out at this time.

[0034] Therefore, the process of this patent is completely different from the reaction of sulfuric acid + ammonium persulfate. Our persulfate is added as a digestion step, and it does not participate in the oxidation reaction in the concentrated sulfuric acid system. From the perspective of the oxidation using the sulfuric acid + persulfate system in conventional technology, persulfate is used to insert oxygen-containing functional groups into graphite oxide, that is, it replaces the scheme of chromium trioxide and manganese dioxide. Ammonium persulfate will undergo violent splitting in sulfuric acid to produce peroxydisulfuric acid with an electrode potential much higher than that of chromium trioxide, which will cause graphite to undergo violent oxidation and digestion in the first stage. Therefore, conventionally, a small amount of ammonium persulfate (usually not more than 5% of the mass of graphite) is usually used to simply intercalate graphite, that is, to form a graphite-sulfuric acid intercalation compound, and then other oxidants, such as potassium permanganate, are added for oxidation. Our oxidation of sulfuric acid and chromium trioxide replaces potassium permanganate instead of ammonium persulfate.

[0035] In short, the normal steps of graphite oxidation are divided into: 1. Intercalation (graphite + concentrated sulfuric acid + ammonium persulfate or potassium persulfate or potassium nitrate or sodium nitrate); 2. Oxidation (directly adding potassium permanganate to the above system) to form graphite oxide.

[0036] The oxidation process of the present invention includes: 1. intercalation (graphite + sulfuric acid + nitric acid); 2. oxidation (directly adding CrO3 + MnO2 to the system); 3. hydrogen peroxide destroys manganese dioxide and converts it into manganese ions; 4. Fenton oxidation (hydrogen peroxide + iron salt). The difference is that the conventional technology increases the interlayer spacing by potassium persulfate and then uses the most violent potassium permanganate oxidation method to destroy graphite, and finally forms highly oxygenated graphite. This kind of graphite will be completely destroyed by the Fenton reagent during Fenton oxidation, and no product can be obtained. However, we use a relatively mild method to oxidize, and the obtained oxidized graphite can be subjected to Fenton oxidation to selectively destroy the oxygen-containing functional groups, and finally obtain a damaged graphene with a large specific surface area, high conductivity, and ion channels formed through the damaged area.

Claims

1. A method for preparing weakly oxidized graphene for lithium-ion battery anode doping, Features The following steps are involved: S1, adding an oxidant to flake graphite with a mesh size of 500 or more to carry out an acid intercalation oxidation reaction on the flake graphite, thereby forming limited oxidation to increase the interlayer spacing of the flake graphite; S2, performing oxidation etching on the reaction product obtained in step S1 by Fenton oxidation method to form ion channel gaps; S3, the reaction product of step S2 is subjected to solid-liquid centrifugal separation and deionized water washing to obtain a solid product, which is dispersed in ethyl acetate and subjected to ultrasonic stripping to obtain a weakly oxidized graphene target product.

2. The method for preparing weakly oxidized graphene for lithium ion battery anode doping according to claim 1, Features: In step S1, for every 1 mass part of flake graphite, the added oxidant consists of the following components: 30-100 parts of sulfuric acid, 0.5-5 parts of nitric acid, 1-5 parts of manganese dioxide, and 0.2-3 parts of chromium trioxide.

3. The method for preparing weakly oxidized graphene for lithium ion battery anode doping according to claim 2, Features: The temperature of the acid intercalation oxidation of the flake graphite is 40-50° C., and the reaction time is 6-12 hours.

4. The method for preparing weakly oxidized graphene for lithium ion battery anode doping according to claim 1, Features: The Fenton oxidation process in step S2 is as follows: The product obtained in step S1 is poured into ice water, and hydrogen peroxide with a concentration of 30% is added until the material is converted into a dark green or black slurry; then the solid and liquid are separated by centrifugation, and the solid is washed with deionized water to obtain a solid, and a mixture of 10% hydrogen peroxide and 0.5% ferrous sulfate is added to carry out Fenton oxidation reaction for more than 12 hours. After the reaction is overnight, the solid and liquid are separated by centrifugation, and the solid product is washed with deionized water; the Fenton oxidation reaction temperature is 10-40°C.

5. The method for preparing weakly oxidized graphene for lithium ion battery anode doping according to claim 1, Features: The ultrasonic peeling time in step S3 is 2-10 hours, and the ultrasonic frequency is 2-10 MHz.

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