Carbon quantum dot composite positive electrode conductive agent and preparation method thereof

Abstract

The present application relates to carbon quantum dots material preparation technical field, especially to a kind of carbon quantum dots composite positive electrode conductive agent and its preparation method.The carbon quantum dots composite positive electrode conductive agent by weight parts, comprising the following raw materials:carbon quantum dots 80-180 parts, carbon fiber 1-5 parts;The carbon quantum dots, its preparation raw material includes:organic carbon 80-160 parts, strong acid 8-18 parts, auxiliary agent 0.1-3 parts, first solvent 370-470 parts, dispersing agent 0.1-0.8 parts;The carbon fiber, its preparation raw material includes:acrylonitrile 5-15 parts, comonomer 0.01-0.5 parts, initiator 0.01-0.5 parts, second solvent 50-80 parts.The positive electrode material and lithium ion battery prepared by the carbon quantum dots composite positive electrode conductive agent, can improve the conductivity of positive electrode material and the electrochemical activity of lithium ion battery, also can maintain good cycle stability.

Description

Technical Field

[0001] The present invention relates to the technical field of carbon quantum dot material preparation, in particular to the field of IPC H01M4, and more specifically to a carbon quantum dot composite positive electrode conductive agent and a preparation method thereof. Background Art

[0002] With the development of science and technology, people's demand for lithium-ion batteries is increasing. As a part of lithium-ion batteries, the performance of positive electrode materials directly affects the performance of lithium-ion batteries. In order to improve the performance of positive electrode materials for lithium-ion batteries, conductive agents are added during the production process of positive electrode materials. Although conductive agents can be dispersed in the positive electrode materials to improve the conductivity of the materials, traditional conductive agents such as conductive graphite and carbon black have limited conductivity and are difficult to evenly disperse in the electrodes. Moreover, conductive agents themselves are not electrochemically active, and adding too much will also affect the performance of the electrodes and lithium-ion batteries.

[0003] Carbon quantum dots, as a new type of 0D spherical carbon nanomaterial, not only share the characteristics of traditional semiconductor metal quantum dots, but also offer advantages such as simple preparation and ease of functionalization. By combining carbon fibers with carbon quantum dots, a high-performance carbon quantum dot composite positive electrode conductive agent is prepared. This not only increases the specific surface area and mechanical strength of the carbon fibers, but also increases the binding sites of the electroactive substances, facilitating the flow of electrolytes and ion transport, resulting in high electrochemical activity.

[0004] Prior art patent application CN113471427A discloses a carbon quantum dot and graphene composite material, its preparation method, and its application. The composite material is synthesized using anhydrous citric acid as the organic carbon and ferric chloride hexahydrate as the catalyst via a simple catalytic graphite method. Capacitors assembled from this composite material exhibit good cycling stability, but their low specific capacity limits their application in lithium-ion batteries.

[0005] Patent application publication number CN109244422A discloses a carbon quantum dot / CNT composite cathode conductive agent and its preparation method. The cathode conductive agent is obtained by dispersing CNT slurry, citric acid, and ethylenediamine in water, followed by dialysis after a hydrothermal reaction. A lithium-ion battery prepared with this cathode conductive agent exhibited low capacity retention after 400 cycles, indicating poor cycling stability, limiting its application in lithium-ion batteries.

[0006] Applicants have discovered that conventional conductive agents have limited conductivity and are difficult to evenly disperse in electrodes. Furthermore, conductive agents themselves lack electrochemical activity, and excessive addition can affect the performance of both the electrode and the lithium-ion battery. Therefore, a need exists for a positive electrode conductive agent that, when added in small amounts to the positive electrode material, can both improve the conductivity of the positive electrode material and the electrochemical activity of the lithium-ion battery while maintaining good cycling stability. Summary of the Invention

[0007] In order to solve the above problems, the first aspect of the present invention provides a carbon quantum dot composite positive electrode conductive agent, the raw materials for preparing the carbon quantum dot composite positive electrode conductive agent include: carbon quantum dots and carbon fibers.

[0008] The carbon quantum dots are prepared from the following raw materials, calculated by weight: 80-160 parts of organic carbon, 8-18 parts of strong acid, 0.1-3 parts of additive, 370-470 parts of first solvent, and 0.1-0.8 parts of dispersant; the carbon fibers are prepared from the following raw materials, calculated by weight: 5-15 parts of acrylonitrile, 0.01-0.5 parts of comonomer, 0.01-0.5 parts of initiator, and 50-80 parts of second solvent.

[0009] Preferably, the carbon quantum dot composite positive electrode conductive agent comprises the following raw materials: carbon quantum dots and carbon fibers; the carbon quantum dots, by weight, are prepared from raw materials including: 80-120 parts of organic carbon, 8-14 parts of strong acid, 1-3 parts of additive, 370-450 parts of first solvent, and 0.1-0.5 parts of dispersant; the carbon fibers, by weight, are prepared from raw materials including: 8-13 parts of acrylonitrile, 0.01-0.3 parts of comonomer, 0.01-0.3 parts of initiator, and 50-70 parts of second solvent.

[0010] Preferably, the organic carbon is selected from one or more of citric acid and its esters, glucosamine, small molecule alcohols, oils, urea, amino acids, sugarcane (SACCHARUM OFFICINARUM L.) extract, and gluconic acid esters; further, it is sugarcane (SACCHARUM OFFICINARUM L.) extract.

[0011] Preferably, the sugarcane (SACCHARUM OFFICINARUM L.) extract can be commercially available, such as sugarcane (SACCHARUM OFFICINARUM L.) extract 10-1 produced by Xi'an Ruierli Bioengineering Co., Ltd.

[0012] Preferably, the strong acid is selected from one or more of concentrated sulfuric acid, concentrated nitric acid, perchloric acid, selenic acid, chloric acid, chlorous acid, concentrated perchloric acid, permanganic acid, and perxenic acid; further, it is concentrated sulfuric acid and concentrated nitric acid; further, the mass ratio of concentrated sulfuric acid to concentrated nitric acid is (2-5):1; further, it is 3:1.

[0013] The concentrated sulfuric acid is a concentrated sulfuric acid aqueous solution with a content of 98%; the concentrated nitric acid is a concentrated nitric acid aqueous solution with a content of 68%.

[0014] Preferably, the auxiliary agent is selected from one or more of concentrated ammonia solution, sodium hydroxide, sodium bicarbonate, and phosphate buffer; further, it is sodium hydroxide.

[0015] Preferably, the first solvent is selected from one or more of propanol, ethanol, isopropanol, acetone, methanol, isopropanone, propylene carbonate, methyl butyl ketone, and methyl isobutyl ketone; further, it is ethanol and isopropanol; further, the mass ratio of ethanol and isopropanol is (2-5):1; further, the mass ratio of ethanol and isopropanol is 3:1.

[0016] The mass ratio of the organic carbon to the first solvent is 1:(1-5); preferably, 1:4.

[0017] Preferably, the dispersant has a hydroxyl value of 70-120 mgKOH / g and an average molecular weight of 600-3000.

[0018] Preferably, the dispersant is selected from one or more of polyethylene glycol, sodium polyphosphate, and sodium hexametaphosphate; further, polyethylene glycol; and further, PEG-1500.

[0019] Preferably, the PEG-1500 can be commercially available, such as PEG-1000 produced by Sanyo Corporation of Japan.

[0020] Preferably, the comonomer is selected from one or more of methyl acrylate, methyl methacrylate, methacrylic acid and itaconic acid; preferably, it is methyl acrylate.

[0021] Preferably, the initiator is selected from one or more of azobisisobutyronitrile, dimethyl azobisisobutyrate, benzoyl peroxide, lauroyl peroxide, potassium persulfate, sodium persulfate, and ammonium persulfate; preferably, dimethyl azobisisobutyrate.

[0022] Preferably, the second solvent is selected from one or more of sodium thiocyanate, dimethyl sulfoxide, nitric acid, acetone, N,N-dimethylformamide, and deionized water; preferably, it is a mixed solution of dimethyl sulfoxide, N,N-dimethylformamide and deionized water; further, the mass ratio of dimethyl sulfoxide, N,N-dimethylformamide and deionized water is (1-10):1:(1-4); further, it is 3:1:2.

[0023] Preferably, the mass ratio of acrylonitrile to the comonomer is (55-75):1; preferably, it is 50:1.

[0024] Preferably, the mass ratio of the carbon quantum dots to the carbon fibers is (40-70):1; preferably, it is 60:1.

[0025] The applicant unexpectedly discovered that by selecting ethanol and isopropanol in a mass ratio of (2-5):1 as the first solvent, when the mass ratio of organic carbon to solvent is 1:(1-5), the obtained carbon quantum dots have uniform particle size and an average particle size of 1-2nm. This is because, on the one hand, the surface tension of the solvent after the mixture of ethanol and isopropanol in a mass ratio of (2-5):1 is small, which can reduce the aggregation of particles. On the other hand, the synergistic effect of ethanol and isopropanol can greatly increase the solubility of organic carbon in the system, so that the carbon quantum dots prepared therefrom are evenly distributed in the system. When the mass ratio of organic carbon to solvent is 1:(1-5), the prepared carbon quantum dots have good dispersion, uniform size distribution, and an average particle size of 1-2nm. The carbon quantum dots with good dispersion and uniform size distribution are attached to the surface of carbon fibers, increasing the specific surface area of ​​the carbon quantum dot composite positive electrode conductive agent, making the contact area between the carbon quantum dot composite positive electrode conductive agent and the positive electrode material larger, thereby increasing the electrochemical performance of the positive electrode material, and thus improving the specific capacity of the lithium-ion battery.

[0026] The second aspect of the present invention provides a method for preparing the carbon quantum dot composite positive electrode conductive agent as described above, comprising the following steps:

[0027] S1. Mix organic carbon and strong acid in a flask, heat in an oil bath to 80-150°C, and stir evenly.

[0028] S2. The solution in the flask was centrifuged at 9000-12000 r / min for 40-50 min, the supernatant was taken, the additive was added to the supernatant and stirred evenly;

[0029] S3. The first solvent was added to the solution in step (2), mixed thoroughly with stirring, and centrifuged at a speed of 9000-12000 r / min for 40-50 min, and the supernatant was removed to obtain carbon quantum dots;

[0030] S4 acrylonitrile, comonomer, initiator, second solvent placed in a nitrogen atmosphere with stirring, heated to 50-80 ℃ in an oil bath, and kept warm for 5-7h to obtain a substance;

[0031] S5. The substance was rinsed with hot water and filtered, vacuum dried and ground to obtain organic fibers;

[0032] S6. The organic fiber is placed in an oxidizing atmosphere at 220-280°C for 50-150 min, and then placed in an inert atmosphere at 1200-1600°C for 5-10 min to obtain substance 2;

[0033] S7. Place the second substance into a sealed device filled with an inert atmosphere and maintain a temperature of 2500-2800°C for 5-20s to obtain carbon fiber;

[0034] S8. Ultrasonic dispersion of carbon quantum dots and carbon fibers in water at an ultrasonic power of 400-600 W for 10-20 min.

[0035] S9. The solution in S8 was then transferred to a hydrothermal reactor for a hydrothermal reaction. The reaction temperature of the hydrothermal reaction was 150-250 ° C. The reaction was kept warm for 4-6h and cooled to room temperature.

[0036] S10. Take out the solution from step S9 and dialyze it using a dialysis bag with a molecular weight cut-off of 1-3 kD for 24-48 hours to obtain the product.

[0037] The applicant unexpectedly discovered that after the carbon fiber raw material is placed in an oxidizing atmosphere at 220-280°C for 50-150 minutes, the prepared carbon fiber has high density and good conductivity, and the prepared lithium-ion battery has high cycle stability. Placing the carbon fiber raw material in an oxidizing atmosphere at 220-280°C for reaction for 50-150 minutes can not only form hydroxyl groups and carbonyl groups in the carbon fiber molecules, so that hydrogen bonds can be formed between and within the molecules, thereby improving the stability of the carbon fiber, but also improve the density and compactness of the carbon fiber, forming a strong conductive network, so that the positive electrode conductive agent prepared from the carbon fiber and carbon quantum dots is in closer contact with the positive electrode material, reducing the probability of insufficient contact between the conductive agent and the active material caused by the volume expansion and contraction changes of the positive electrode material during the charge and discharge process, and suppressing the increase in resistance caused by insufficient contact between the positive electrode conductive agent and the active material, thereby improving the cycle performance of the lithium-ion battery. If the reaction time is too long, the production cost will increase. If the reaction time is too short, the reaction with oxygen will not be sufficient, and hydroxyl and carbonyl groups cannot be formed in the carbon fiber molecules. The stability and density of the carbon fiber are low, and it cannot be in close contact with the positive electrode material, thus failing to improve the cycle performance of the lithium-ion battery. The density of the prepared carbon fiber is 1.8-2.2g / cm 3 The resistivity is 700 μΩ / cm at 25°C, indicating good conductivity and high density. The carbon quantum dot positive electrode conductive agent is applied to lithium-ion batteries, and the resulting battery has a low internal resistance of 27.5 mΩ. After 1,000 discharges at a 3C rate, the battery capacity still reaches more than 95%, indicating that the composite material has high electrochemical activity and reversibility, and good stability.

[0038] The applicant unexpectedly discovered that the carbon quantum dot positive electrode conductive agent prepared by selecting the mass ratio of the prepared carbon quantum dots to carbon fibers is (40-70): 1, which can not only increase the specific surface area and mechanical strength of the carbon fibers, but also increase the binding sites of the electroactive substances. The prepared carbon quantum dot positive electrode conductive agent is applied to the positive electrode material, which can load a large amount of active substances, so that the positive electrode material has high electrochemical activity and high cycle stability. This is because if there are too many carbon fibers, the carbon quantum dots will not play a role in enhancing the electron transfer rate, and the effect of improving the detection rate and sensitivity is limited. When there are too few carbon fibers, there are too many carbon quantum dots, which may hinder the transmission of electrons and reduce the electrochemical sensing efficiency of the positive electrode material. Therefore, based on the carbon quantum dots and carbon fibers prepared by the present invention, a carbon quantum dot positive electrode conductive agent is prepared according to a mass ratio of (40-70): 1. The carbon quantum dots are uniformly loaded on the surface of the carbon fibers, improving the surface defects of the carbon fibers, increasing their specific surface area and stability, increasing the binding sites with the positive electrode material, and adopting a "line-point" mode of contact. It is easy to form a good conductive network in the electrode, which not only shows good conductivity and stability, but also can reduce the amount of conductive agent used and increase the battery capacity. When the amount of carbon quantum dot positive electrode conductive agent introduced is 0.1wt%, the charge transfer impedance of the prepared positive electrode material is small. At a scan rate of 10mV / s, the peak potential difference between the cathode and anode of the positive electrode material is about 20mV, indicating that the positive electrode material prepared by the carbon quantum dot positive electrode conductive agent has high electrochemical activity and reversibility. When the carbon quantum dot positive electrode material is used in a lithium-ion battery, the specific capacity is 1400 mAh / g at a current density of 100 mA / g at 25°C; when discharged 1000 times at a 3C rate, the battery capacity still reaches more than 95%, indicating that the composite material has high electrochemical activity and reversibility and good stability.

[0039] The reaction temperature of the hydrothermal reaction of the mixed liquid of carbon fiber and carbon quantum dots is 150-250°C, and the holding time is 4-6h. The obtained carbon quantum dot positive electrode conductive agent has high reversibility and stability. This is because when the temperature is too high, the carbon quantum dots themselves are prone to a series of reactions at high temperatures, affecting the stability of the composite material. When the temperature is too low, the carbon quantum dots do not obtain enough energy and cannot be evenly attached to the surface of the carbon fiber, and the carbon quantum dot positive electrode conductive agent cannot be obtained. Therefore, the carbon quantum dot positive electrode conductive agent obtained at a temperature of 150-250°C and a holding reaction time of 4-6h has a small charge transfer impedance for the prepared positive electrode material, which makes it easier for electrons to diffuse in it. The carbon fibers and carbon quantum dots, as positive electrode conductive agents, form a good conductive network, providing a convenient channel for the transport of electrons in the electrode, thereby improving the specific capacity of the lithium-ion battery. When the carbon quantum dot positive electrode conductive agent is used as the negative electrode material of the lithium-ion battery, the specific capacity is 1400mAh / g at a current density of 100mA / g at 25°C.

[0040] Beneficial effects:

[0041] 1. Ethanol and isopropanol are selected as the first solvent in a mass ratio of (2-5):1. On the one hand, the solvent has a low surface tension, which can reduce the aggregation of particles and make the carbon quantum dots prepared therefrom evenly distributed in the system. On the other hand, the synergistic effect of ethanol and isopropanol can greatly increase the solubility of organic carbon in the system. When the mass ratio of organic carbon to solvent is 1:(1-5), the prepared carbon quantum dots have good dispersion in the solution, uniform size distribution, and an average particle size of 1-2nm. The well-dispersed and uniformly distributed carbon quantum dots adhere to the surface of the carbon fiber, increasing the specific surface area of ​​the carbon quantum dot composite positive electrode conductive agent, resulting in a larger contact area between the carbon quantum dot composite positive electrode conductive agent and the positive electrode material, thereby increasing the electrochemical performance of the positive electrode material and further improving the specific capacity of the lithium-ion battery.

[0042] 2. Placing the carbon fiber raw material in an oxidizing atmosphere at 220-280°C for reaction for 50-150 minutes can not only form hydroxyl and carbonyl groups in the carbon fiber molecules, but also form hydrogen bonds between and within the molecules, thereby improving the stability of the carbon fiber. It can also improve the density and compactness of the carbon fiber, forming a strong conductive network. This makes the positive electrode conductive agent prepared by carbon fiber and carbon quantum dots more closely in contact with the positive electrode material, reducing the probability of insufficient contact between the conductive agent and the active material caused by the volume expansion and contraction changes of the positive electrode material during the charge and discharge process, and inhibiting the increase in resistance caused by insufficient contact between the positive electrode conductive agent and the active material, thereby improving the conductivity of the positive electrode material and improving the electrochemical performance and cycle stability of the lithium-ion battery.

[0043] 3. The mass ratio of carbon quantum dots to carbon fibers is selected to be (40-70):1 to prepare carbon quantum dots composite positive electrode conductive agent, which can not only increase the specific surface area and mechanical strength of carbon fibers, but also increase the binding sites of electroactive substances. The carbon quantum dots are evenly loaded on the surface of carbon fibers, which improves the surface defects of carbon fibers, increases their specific surface area and stability, increases the binding sites with positive electrode materials, and adopts "line-point" mode contact, which easily forms a good conductive network in the electrode, not only showing good conductivity and stability, but also reducing the amount of conductive agent used, and improving the electrochemical activity and cycle stability of the battery.

[0044] 4. When the reaction temperature of the hydrothermal reaction of the mixed liquid of carbon fiber and carbon quantum dots is 150-250°C and the holding time is 4-6h, the obtained carbon quantum dot composite positive electrode conductive agent has high reversibility and stability.

[0045] 5. The carbon quantum dot composite positive electrode conductive agent prepared by the present invention is not only suitable for lithium-ion batteries, but can also be used in supercapacitors, electrochemical sensors, dye-sensitized solar cells and other fields. DETAILED DESCRIPTION

[0046] Example

[0047] Example 1

[0048] Example 1 provides a carbon quantum dot composite positive electrode conductive agent, the raw materials for preparing the carbon quantum dot composite positive electrode conductive agent include: carbon quantum dots and carbon fibers.

[0049] The carbon quantum dots are prepared from the following raw materials, calculated by weight: 100 parts of organic carbon, 10 parts of strong acid, 2.5 parts of additives, 400 parts of a first solvent, and 0.3 parts of a dispersant; the carbon fibers are prepared from the following raw materials, calculated by weight: 10 parts of acrylonitrile, 0.2 parts of a comonomer, 0.15 parts of an initiator, and 60 parts of a second solvent.

[0050] The mass ratio of the carbon quantum dots to the carbon fibers is 60:1.

[0051] The organic carbon is selected from sugarcane (SACCHARUM OFFICINARUM L.) extract.

[0052] The sugarcane (SACCHARUM OFFICINARUM L.) extract was purchased from Xi'an Ruierli Bioengineering Co., Ltd., a supplier of sugarcane (SACCHARUM OFFICINARUM L.) extract 10-1.

[0053] The strong acid is selected from concentrated sulfuric acid and concentrated nitric acid; the mass ratio of the concentrated sulfuric acid to the concentrated nitric acid is 3:1.

[0054] The concentrated sulfuric acid is a concentrated sulfuric acid aqueous solution with a content of 98%; the concentrated nitric acid is a concentrated nitric acid aqueous solution with a content of 68%.

[0055] The auxiliary agent is selected from sodium bicarbonate.

[0056] The first solvent is selected from ethanol and isopropanol, and the mass ratio of the ethanol to isopropanol is 3:1.

[0057] The dispersant has a hydroxyl value of 75-80 mgKOH / g and an average molecular weight of 1450-1550.

[0058] The dispersant is selected from polyethylene glycol, and the polyethylene glycol is purchased from PEG-1500 produced by Sanyo Company of Japan.

[0059] The comonomer is selected from methyl acrylate.

[0060] The initiator is selected from dimethyl azobisisobutyrate;

[0061] The second solvent is a mixed solution of dimethyl sulfoxide, N,N-dimethylformamide and deionized water.

[0062] The mass ratio of the dimethyl sulfoxide, N,N-dimethylformamide and deionized water is 3:1:2.

[0063] A method for preparing a carbon quantum dot composite positive electrode conductive agent comprises the following steps:

[0064] (1) Mix the sugarcane (SACCHARUM OFFICINARUM L.) extract with concentrated sulfuric acid and concentrated nitric acid in a flask, heat in an oil bath to 140°C, and stir evenly.

[0065] (2) Centrifuge the solution in the flask at 10,000 rpm for 48 minutes, collect the supernatant, and add sodium bicarbonate to the supernatant;

[0066] (3) adding ethanol, isopropanol and polyethylene glycol to the solution of step (2), mixing and stirring thoroughly, centrifuging at a speed of 10000 r / min for 48 min, and removing the supernatant to obtain carbon quantum dots;

[0067] (4) Place acrylonitrile, comonomer, initiator, and second solvent in a nitrogen atmosphere, stir, heat to 65°C in an oil bath, and keep warm for 6 hours to obtain substance 1;

[0068] (5) washing and filtering the substance with hot water, vacuum drying and grinding to obtain organic fiber;

[0069] (6) The organic fiber was placed in an oxidizing atmosphere at 270°C for 120 min and then placed in an inert atmosphere at 1500°C for 7 min to obtain substance 2;

[0070] (7) placing the second substance into a sealed device with an inert atmosphere and maintaining the temperature at 2600°C for 10 seconds to obtain carbon fiber;

[0071] (8) Ultrasonic dispersion of carbon quantum dots and carbon fibers in water with an ultrasonic power of 450 W and an ultrasonic time of 15 min;

[0072] (9) The solution in step (8) was then transferred to a hydrothermal reactor for a hydrothermal reaction at a temperature of 180° C. The reaction was kept at this temperature for 5 h and then cooled to room temperature;

[0073] (10) The solution in step (9) was taken out and dialyzed using a dialysis bag with a molecular weight cut-off of 2 kD. The dialysis time was 30 h to obtain the product.

[0074] Example 2

[0075] Example 2 provides a carbon quantum dot composite positive electrode conductive agent, the raw materials for preparing the agent include: carbon quantum dots and carbon fibers.

[0076] The carbon quantum dots are prepared from the following raw materials, calculated by weight: 100 parts of organic carbon, 10 parts of strong acid, 2.5 parts of additives, 400 parts of a first solvent, and 0.3 parts of a dispersant; the carbon fibers are prepared from the following raw materials, calculated by weight: 10 parts of acrylonitrile, 0.5 parts of a comonomer, 0.15 parts of an initiator, and 60 parts of a second solvent.

[0077] The mass ratio of the carbon quantum dots to the carbon fibers is 25:1.

[0078] The organic carbon is selected from sugarcane (SACCHARUM OFFICINARUM L.) extract.

[0079] The sugarcane (SACCHARUM OFFICINARUM L.) extract was purchased from Xi'an Ruierli Bioengineering Co., Ltd., a supplier of sugarcane (SACCHARUM OFFICINARUM L.) extract 10-1.

[0080] The strong acid is selected from concentrated sulfuric acid and concentrated nitric acid; the mass ratio of the concentrated sulfuric acid to the concentrated nitric acid is 3:1.

[0081] The concentrated sulfuric acid is a concentrated sulfuric acid aqueous solution with a content of 98%; the concentrated nitric acid is a concentrated nitric acid aqueous solution with a content of 68%.

[0082] The auxiliary agent is selected from sodium bicarbonate.

[0083] The first solvent is selected from methanol.

[0084] The dispersant has a hydroxyl value of 75-80 mgKOH / g and an average molecular weight of 1450-1550.

[0085] The dispersant is selected from polyethylene glycol, and the polyethylene glycol is purchased from PEG-1500 produced by Sanyo Company of Japan.

[0086] The comonomer is selected from methyl acrylate.

[0087] The initiator is selected from dimethyl azobisisobutyrate;

[0088] The second solvent is a mixed solution of dimethyl sulfoxide, N,N-dimethylformamide and deionized water.

[0089] The mass ratio of the dimethyl sulfoxide, N,N-dimethylformamide and deionized water is 3:1:2.

[0090] A method for preparing a carbon quantum dot composite positive electrode conductive agent comprises the following steps:

[0091] (1) Mix the sugarcane (SACCHARUM OFFICINARUM L.) extract with concentrated sulfuric acid and concentrated nitric acid in a flask, heat in an oil bath to 140°C, and stir evenly.

[0092] (2) Centrifuge the solution in the flask at 10,000 rpm for 48 minutes, collect the supernatant, and add sodium bicarbonate to the supernatant;

[0093] (3) adding ethanol, isopropanol and polyethylene glycol to the solution of step (2), mixing and stirring thoroughly, centrifuging at a speed of 10000 r / min for 48 min, and removing the supernatant to obtain carbon quantum dots;

[0094] (4) Place acrylonitrile, comonomer, initiator, and second solvent in a nitrogen atmosphere, stir, heat to 65°C in an oil bath, and keep warm for 6 hours to obtain substance 1;

[0095] (5) washing and filtering the substance with hot water, vacuum drying and grinding to obtain organic fiber;

[0096] (6) The organic fiber was placed in an oxidizing atmosphere at 270°C for 20 min and then placed in an inert atmosphere at 1500°C for 7 min to obtain substance 2;

[0097] (7) placing the second substance into a sealed device with an inert atmosphere and maintaining the temperature at 2600°C for 10 seconds to obtain carbon fiber;

[0098] (8) Ultrasonic dispersion of carbon quantum dots and carbon fibers in water with an ultrasonic power of 450 W and an ultrasonic time of 15 min;

[0099] (9) The solution in step (8) was then transferred to a hydrothermal reactor for a hydrothermal reaction at a temperature of 180° C. The reaction was kept at this temperature for 5 h and then cooled to room temperature;

[0100] (10) The solution in step (9) was taken out and dialyzed using a dialysis bag with a molecular weight cut-off of 2 kD. The dialysis time was 30 h to obtain the product.

[0101] Performance testing methods

[0102] 1. Particle size test

[0103] For Example 1-2, 15 μm carbon quantum dots were taken with a microsyringe and dropped onto a copper mesh several times. The mesh was allowed to dry naturally. The dispersion and particle size of the carbon quantum dots were observed using a transmission electron microscope. The measured data are recorded in Table 1.

[0104] 2. Carbon fiber density test

[0105] For Examples 1-2, the density of the carbon fiber was measured using GB / T 30019-2013 “Determination of Density of Carbon Fibers”, and the measured data are recorded in Table 1.

[0106] 3. Carbon fiber resistivity test

[0107] For Examples 1-2, the resistivity of the carbon fibers was measured using GBT 32993-2016 “Determination of Volume Resistivity of Carbon Fibers,” and the measured data are recorded in Table 1.

[0108] 4. Cyclic voltammetry test

[0109] For Examples 1-2, electrodes were prepared with LiCoO2 positive electrode materials, and the cyclic voltammetry curves of the electrodes were measured in a three-electrode system, wherein a platinum sheet was used as a counter electrode, a silver wire was used as a reference electrode, the prepared electrode was used as a working electrode, and a 1 mol / L LiTFSI solution was used as an electrolyte. The scan rate was 10 mV / s, and the cathode and anode peak potential differences of the electrodes were calculated from the cyclic voltammetry curves. The measured data are recorded in Table 1.

[0110] 5. Specific capacity and cycle stability

[0111] To evaluate the electrochemical performance and cycling stability of Examples 1-2 in lithium-ion batteries, the specific capacity was measured using a battery tester at a charge and discharge current density of 100 mA / g and a potential range of 1.0-2.5 V. The specific capacity was recorded after the first charge and discharge cycle and after 1000 charge and discharge cycles, and the capacity retention was calculated. Batteries based on Examples 1-2 were assembled in a glove box: the positive electrode material was a mixture of LiCoO2 and 0.1 wt% carbon quantum dot composite positive electrode conductive agent, and the negative electrode was graphite.

[0112] Table 1

[0113]

Claims

1. A carbon quantum dot composite positive electrode conductive agent, characterized in that The invention comprises the following raw materials: carbon quantum dots and carbon fibers; the carbon quantum dots are prepared from the following raw materials, calculated by weight: 80-120 parts of organic carbon, 8-14 parts of a strong acid, 1-3 parts of an additive, 370-450 parts of a first solvent, and 0.1-0.5 parts of a dispersant; the carbon fibers are prepared from the following raw materials, calculated by weight: 8-13 parts of acrylonitrile, 0.01-0.3 parts of a comonomer, 0.01-0.3 parts of an initiator, and 50-70 parts of a second solvent; The first solvent is ethanol and isopropanol in a mass ratio of 3:1; The mass ratio of the organic carbon to the first solvent is 1:4; The mass ratio of the carbon quantum dots to the carbon fibers is 60:1; The preparation method of the carbon quantum dot composite positive electrode conductive agent comprises the following steps: S1. The organic carbon and the strong acid are mixed and stirred in a flask, the supernatant is centrifuged, and then the first solvent, the additive, and the dispersant are added and stirred continuously. The supernatant is removed by centrifugation to obtain carbon quantum dots; S2 acrylonitrile, comonomer, initiator, and second solvent are placed in a nitrogen atmosphere with stirring, heated in an oil bath to 50-80 ° C, and kept warm for 5-7h, and then pre-oxidized, carbonized, and graphitized to obtain carbon fibers; S3. The graphene solid was added to the carbon quantum dots and ultrasonically mixed, the mixed liquid was charged into a reactor, placed in a hydrothermal reactor at a constant temperature, cooled to room temperature and dialyzed to obtain a carbon quantum dot composite cathode conductive agent; The pre-oxidation temperature in step S2 is 220-280° C. and maintained in an oxidizing atmosphere for 50-150 minutes; In step S3, the reaction temperature in the hydrothermal reactor is 150-250° C., and the holding time is 4-6 hours.

2. A carbon quantum dot composite positive electrode conductive agent according to claim 1, characterized in that The dispersant is selected from one or more of polyethylene glycol, sodium polyphosphate, and sodium hexametaphosphate.

3. The carbon quantum dot composite positive electrode conductive agent according to claim 1, characterized in that: The mass ratio of acrylonitrile to the comonomer is (55-75):

1.

4. A method for preparing a carbon quantum dot composite positive electrode conductive agent according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1. The organic carbon and the strong acid are mixed and stirred in a flask, the supernatant is centrifuged, and then the first solvent, the additive, and the dispersant are added and stirred continuously. The supernatant is removed by centrifugation to obtain carbon quantum dots; S2 acrylonitrile, comonomer, initiator, and second solvent are placed in a nitrogen atmosphere with stirring, heated in an oil bath to 50-80 ° C, and kept warm for 5-7h, and then pre-oxidized, carbonized, and graphitized to obtain carbon fibers; S3. The graphene solid was added to the carbon quantum dots and ultrasonically mixed, the mixed liquid was charged into a reactor, placed in a hydrothermal reactor at a constant temperature, cooled to room temperature and dialyzed to obtain a carbon quantum dot composite cathode conductive agent; The pre-oxidation temperature in step S2 is 220-280° C. and maintained in an oxidizing atmosphere for 50-150 minutes; In step S3, the reaction temperature in the hydrothermal reactor is 150-250° C., and the holding time is 4-6 hours.

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