Preparation method of modified graphene electrode material for lithium ion battery and related products

Abstract

The present application belongs to the technical field of graphene preparation, and discloses a preparation method of modified graphene electrode material for lithium ion battery and related products, the method comprising: premixing graphite powder with concentrated sulfuric acid, adding oxidant in batches, and filtering after oxidation, washing to neutrality with dilute hydrochloric acid and deionized water; adding deionized water to the washed graphite oxide, and obtaining graphene oxide dispersion by centrifugal separation after multi-stage ultrasonic treatment; mixing the graphene oxide dispersion with a predetermined amount of urea, and centrifugally washing and freeze-drying after multi-step hydrothermal reaction to obtain nitrogen-doped modified graphene powder; grinding and mixing the nitrogen-doped modified graphene powder with anhydrous ethanol, and coating it on the surface of pretreated nickel foam collector by impregnation, and compacting it to obtain modified electrode material. Through the scheme of the present application, the conductivity and cycle stability of graphene electrodes in lithium ion batteries can be improved.

Description

Technical Field

[0001] The present application relates to the field of graphene preparation, and in particular to a method for preparing a modified graphene electrode material for a lithium-ion battery and related products. Background Art

[0002] As the most promising chemical energy storage device, lithium-ion batteries play an important role in portable electronic devices, electric vehicles, and large-scale energy storage. The performance of electrode materials directly determines the comprehensive performance of lithium-ion batteries. Among them, graphene is considered to be a new electrode material with great potential due to its excellent conductivity, large specific surface area, and excellent mechanical properties. However, graphene still faces many challenges in practical applications: first, the two-dimensional planar structure composed of sp2 hybridized carbon atoms causes strong van der Waals forces between graphene sheets, which is easy to agglomerate, reducing its specific surface area and electrochemical active sites; second, the energy band structure of graphene determines that there is an upper limit to its intrinsic conductivity, and the conductivity fluctuates greatly during the charge and discharge process, affecting the rate performance and cycle stability of the battery; third, the graphene surface lacks active functional groups, and the interface wettability with the electrolyte is poor, which is not conducive to the rapid transmission of lithium ions.

[0003] In order to solve the above problems, researchers have carried out a lot of research on graphene modification. Among them, regulating the electronic structure of graphene by heteroatom doping has been proven to be an effective modification strategy. In particular, nitrogen atom doping can effectively regulate the electronic structure and surface properties of graphene because nitrogen atoms have similar atomic radius to carbon atoms but stronger electronegativity. Nitrogen doping can not only introduce more active sites, but also improve the intrinsic conductivity of graphene and improve its electrochemical properties. At present, the preparation methods of nitrogen-doped graphene mainly include in-situ doping and post-treatment doping. In-situ doping is to directly introduce nitrogen-containing precursors during the growth of graphene to achieve doping, such as chemical vapor deposition (CVD) method, pyrolysis method, etc.; post-treatment doping is to react the prepared graphene with nitrogen-containing compounds at high temperature to achieve doping, such as ammonia heat treatment method, plasma treatment method, etc. These methods have their own advantages and disadvantages: in-situ doping can obtain a more uniform doping structure, but the process conditions are harsh and the cost is high; post-treatment doping operation is relatively simple, but the doping uniformity and controllability are poor. In addition, the doping configuration (pyridinic nitrogen, pyrrolic nitrogen, graphitic nitrogen, etc.) and doping content of nitrogen-doped graphene have an important influence on its electrochemical performance. Studies have shown that different doping configurations have different electronic structures and catalytic activities, among which pyridinic nitrogen and graphitic nitrogen are most effective in improving conductivity and electrochemical performance. However, how to precisely control the doping position and doping form of nitrogen atoms remains a challenge.

[0004] Therefore, there is an urgent need for a technical solution to improve the conductivity and cycle stability of graphene electrodes in lithium-ion batteries. Summary of the invention

[0005] One purpose of the present application is to provide a method for preparing a modified graphene electrode material for a lithium-ion battery and related products, at least to solve the technical problems in the prior art that the conductivity of the graphene electrode needs to be improved and the conductivity is unstable.

[0006] To achieve the above objectives, some embodiments of the present application provide the following aspects:

[0007] In a first aspect, an embodiment of the present application provides a method for preparing a modified graphene electrode material for a lithium-ion battery, comprising: premixing graphite powder with concentrated sulfuric acid, adding an oxidant in batches, and filtering after oxidation is completed, and washing to neutrality with dilute hydrochloric acid and deionized water; adding the washed graphite oxide to deionized water, and obtaining a graphene oxide dispersion by centrifugal separation after multi-stage ultrasonic treatment; mixing the graphene oxide dispersion with a predetermined amount of urea, and performing a multi-step hydrothermal reaction, centrifugal washing and freeze-drying to obtain a nitrogen-doped modified graphene powder; grinding and mixing the nitrogen-doped modified graphene powder with anhydrous ethanol, and coating it on the surface of a pretreated nickel foam collector by impregnation, and performing a compaction treatment to obtain a modified electrode material.

[0008] In a possible implementation, graphite powder is premixed with concentrated sulfuric acid and then an oxidant is added in batches, and after oxidation is completed, suction filtration is performed to wash to neutrality using dilute hydrochloric acid and deionized water, including: slowly adding a predetermined amount of graphite powder to concentrated sulfuric acid for premixing, and using an external ice bath to control the temperature of the reaction system; adding potassium dichromate oxidant in batches under continuous stirring conditions, and the amount added in each batch is a preset proportion of the total amount; adding predetermined amounts of deionized water and hydrogen peroxide to the reaction mixture in sequence under stirring, and immediately performing suction filtration and separation treatment; using a dilute hydrochloric acid solution with a predetermined volume ratio to preliminarily clean the filter cake, and then repeatedly rinsing with deionized water until the filtrate is neutral to obtain oxidized graphite.

[0009] In a possible implementation, the washed graphite oxide is added to deionized water, subjected to multi-stage ultrasonic treatment, and then centrifuged to obtain a graphene oxide dispersion, comprising: adding graphite oxide to deionized water at a preset mass concentration; placing an aqueous solution containing graphite oxide in a cylindrical container of a preset volume, and adjusting the distance between the bottom of the container and the ultrasonic probe before performing multi-stage ultrasonic treatment; centrifuging the dispersion after ultrasonic treatment and collecting the supernatant to obtain a graphene oxide dispersion.

[0010] In a possible implementation, the graphene oxide dispersion is mixed with a predetermined amount of urea, and after a multi-step hydrothermal reaction, centrifugal washing and freeze-drying are performed to obtain nitrogen-doped modified graphene powder, including: measuring the graphene oxide dispersion that has been centrifuged; adding urea in batches according to a preset mass fraction and fully stirring and dissolving; placing the mixed solution on a magnetic stirrer for stirring and dispersing; transferring the mixed solution to a hydrothermal reactor and sealing it; performing a multi-step hydrothermal reaction at a preset temperature and naturally cooling to room temperature; centrifuging the reaction product and washing it alternately; pre-freezing the washed product and freeze-drying it at a preset vacuum degree to obtain nitrogen-doped modified graphene powder.

[0011] In a possible implementation, the nitrogen-doped modified graphene powder is ground and mixed with anhydrous ethanol, impregnated and coated on the surface of a pretreated nickel foam collector, and compacted to obtain a modified electrode material, including: placing the nitrogen-doped modified graphene powder in a mortar, and adding anhydrous ethanol dropwise for grinding; cleaning the nickel foam collector; immersing the treated collector in the mixture for coating; and drying and compacting the coated electrode to obtain a modified electrode material.

[0012] In a possible implementation, an aqueous solution containing graphite oxide is placed in a cylindrical container of a preset volume, and a multi-stage ultrasonic treatment is performed after adjusting the distance between the bottom of the container and the ultrasonic probe, including: performing continuous ultrasonic treatment on the solution at a preset temperature in the first stage, followed by pausing for a predetermined time to allow the temperature to drop to an initially set temperature; performing continuous ultrasonic treatment on the solution at a preset temperature in the second stage, and pausing for a predetermined time to cool the solution after the treatment is completed; and completing the remaining ultrasonic treatment after adjusting the solution temperature to a preset value in the third stage.

[0013] In a possible implementation, a multi-step hydrothermal reaction is carried out at a preset temperature and naturally cooled to room temperature, comprising: heating the mixed liquid in the reactor from room temperature to a first preset temperature at a first preset heating rate and maintaining it for a predetermined time; raising the temperature to a second preset temperature at a second preset heating rate for a hydrothermal reaction; after the hydrothermal reaction is completed, a segmented cooling strategy is adopted, first cooling at a first preset cooling rate for a predetermined time, and then cooling to room temperature at a second preset cooling rate.

[0014] In the second aspect, an embodiment of the present application also discloses a control device for preparing modified graphene electrode materials for lithium-ion batteries, comprising: a processor, a memory, and a system bus; wherein the processor and the memory are connected via the system bus; the memory is used to store one or more programs, and the one or more programs include instructions, which, when executed by the processor, enable the processor to execute the method described in any one of the above embodiments.

[0015] Compared with the prior art, this application achieves the following beneficial effects:

[0016] In the preparation method and related products of a modified graphene electrode material for a lithium-ion battery of the present application, nitrogen doping modification is carried out by multi-stage ultrasound and multi-step temperature control, more active sites are introduced, and the intrinsic conductivity of graphene is improved, thereby significantly improving the conductivity of the graphene electrode in the lithium-ion battery. Nitrogen doping and optimized process reduce the conductivity fluctuation of the graphene electrode during charging and discharging, and improve the cycle stability of the battery. Nitrogen doping changes the electronic structure of graphene, increases electrochemical active sites, and is conducive to the rapid transmission of lithium ions, thereby improving the overall electrochemical performance of lithium-ion batteries. Through segmented ultrasonic treatment and multi-step temperature control, the exfoliation and nitrogen doping process of graphite oxide are more uniform, which improves the utilization rate of materials and the uniformity of products. The process parameters of the preparation method of the present application are controllable, with high repeatability and industrial application prospects. The prepared nitrogen-doped modified graphene material has a large specific surface area, a developed pore structure and a uniform nitrogen doping distribution. These characteristics work together to make the material exhibit excellent electrochemical properties in lithium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] One or more embodiments are exemplarily described by pictures in the corresponding drawings, and these exemplified descriptions do not constitute limitations on the embodiments. Elements with the same reference numerals in the drawings represent similar elements, and unless otherwise stated, the figures in the drawings do not constitute proportional limitations.

[0018] Figure 1 A schematic diagram of a process for preparing a modified graphene electrode material for a lithium-ion battery disclosed in an embodiment of the present application;

[0019] Figure 2 This is a diagram of experimental results of a multi-stage ultrasonic treatment method disclosed in an embodiment of the present application;

[0020] Figure 3 This is a diagram of experimental results of a multi-step hydrothermal reaction method disclosed in an embodiment of the present application. DETAILED DESCRIPTION

[0021] Various exemplary embodiments of the present disclosure will now be described in detail with reference to the accompanying drawings. It should be noted that the relative arrangement of components and steps, numerical expressions and numerical values ​​set forth in these embodiments do not limit the scope of the present disclosure unless otherwise specifically stated.

[0022] Those skilled in the art can understand that the terms "first", "second" and the like in the embodiments of the present disclosure are only used to distinguish different steps, devices or modules, etc., and neither represent any specific technical meaning nor represent the necessary logical order between them. It should also be understood that in the embodiments of the present disclosure, "multiple" can refer to two or more, and "at least one" can refer to one, two or more. It should also be understood that for any component, data or structure mentioned in the embodiments of the present disclosure, in the absence of explicit limitation or contrary revelation given in the context, it can generally be understood as one or more. In addition, the term "and / or" in the present disclosure is only a description of the association relationship of the associated objects, indicating that there can be three relationships, for example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in the present disclosure generally indicates that the associated objects before and after are an "or" relationship. It should also be understood that the description of each embodiment in the present disclosure emphasizes the differences between the embodiments, and the same or similar parts can refer to each other. For the sake of brevity, they will not be repeated one by one.

[0023] At the same time, it should be understood that, for ease of description, the sizes of the various parts shown in the drawings are not drawn according to the actual proportional relationship. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present disclosure and its application or use. The techniques, methods and devices known to ordinary technicians in the relevant fields may not be discussed in detail, but where appropriate, the techniques, methods and devices should be considered part of the specification. It should be noted that similar numbers and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further discussed in subsequent drawings.

[0024] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0025] First embodiment

[0026] Figure 1 The following is a schematic diagram of a method for preparing a modified graphene electrode material for a lithium-ion battery disclosed in an embodiment of the present application. Figure 1As shown, at step S101, graphite powder is pre-mixed with concentrated sulfuric acid, and then an oxidant is added in batches, and after the oxidation is completed, suction filtration is performed to wash to neutrality with dilute hydrochloric acid and deionized water. This includes: slowly adding a predetermined amount of graphite powder to concentrated sulfuric acid for pre-mixing, and using an external ice bath to control the temperature of the reaction system; adding potassium dichromate oxidant in batches under continuous stirring conditions, and the amount added in each batch is a preset proportion of the total amount; adding a predetermined amount of deionized water and hydrogen peroxide to the reaction mixture in sequence under stirring, and immediately performing suction filtration and separation treatment; using a predetermined volume ratio of dilute hydrochloric acid solution to preliminarily wash the filter cake, and then repeatedly washing with deionized water until the filtrate is neutral to obtain oxidized graphite.

[0027] Specifically, in the process of preparing graphite oxide, a quantitative amount of high-quality graphite powder can be pre-mixed with concentrated sulfuric acid under low temperature conditions. Specifically, in one embodiment, 2.35 grams of graphite powder are slowly added to 46.8 milliliters of concentrated sulfuric acid and fully mixed using a mechanical stirring device. During this process, an external ice bath device is used to cool the reaction system to ensure that the temperature of the mixture is maintained at a low level. After being mixed evenly, 6.15 grams of potassium dichromate oxidant are slowly added in batches under continuous stirring, and the amount added each time should be controlled between 15-25% of the total amount. During the addition of the oxidant, the temperature of the reaction system is stably controlled at about 34.5°C by adjusting the feeding rate and the external ice bath temperature. After the addition of the oxidant is completed, the stirring reaction is continued at this temperature for about 2 hours.

[0028] After the oxidation reaction is completed, in order to dilute the reaction system and terminate the oxidation reaction, first slowly add 92.6 ml of deionized water to the reaction mixture. During this process, stirring should be continued to ensure uniform mixing, and the reaction temperature should be closely monitored. In the following 15 minutes, 282 ml of deionized water is added in batches, and 5.2 ml of hydrogen peroxide is added to reduce the residual oxidant. After the dilution process is completed, the mixture is immediately filtered and separated while the temperature is high.

[0029] After obtaining the filter cake, it needs to be fully purified. First, use a dilute hydrochloric acid solution with a volume ratio of 1:10 to wash the filter cake to remove residual metal ions. Then rinse it repeatedly with deionized water until the pH value of the filtrate reaches a neutral range of 6.8-7.2. During the cleaning process, each rinse should ensure that the cleaning liquid is in full contact with the filter cake. Stirring or ultrasound-assisted methods can be used to improve the cleaning efficiency. The final graphite oxide should have a typical brown appearance and have obvious layered structural characteristics.

[0030] In step S102, the washed graphite oxide is added to deionized water, and after multi-stage ultrasonic treatment, a graphene oxide dispersion is obtained by centrifugal separation. The process includes: adding graphite oxide to deionized water at a preset mass concentration; placing an aqueous solution containing graphite oxide in a cylindrical container of a preset volume, and adjusting the distance between the bottom of the container and the ultrasonic probe to perform multi-stage ultrasonic treatment; centrifuging the dispersion after ultrasonic treatment and collecting the supernatant to obtain a graphene oxide dispersion.

[0031] Specifically, after the preparation of graphite oxide is completed, a graphene oxide dispersion needs to be prepared by dispersion treatment. First, the graphite oxide prepared in the above steps is accurately weighed according to a mass concentration of 0.52 mg / mL and added to an appropriate amount of deionized water. To ensure the accuracy of measurement, an analytical balance can be used for weighing, and a volumetric flask can be used for constant volume.

[0032] During the dispersion process, ultrasonic treatment is preferred for exfoliation. In the specific operation, the aqueous solution containing graphite oxide is placed in an ultrasonic processor, the ultrasonic power is set in the range of 300-400W, and the treatment time is maintained for 3-4 hours. In order to avoid the solution temperature being too high during the ultrasonic process and affecting the dispersion effect, intermittent ultrasonic treatment can be used, that is, pausing for a preset time after continuous treatment, or configuring a cooling water bath system outside the ultrasonic processor.

[0033] The method comprises placing an aqueous solution containing graphite oxide in a cylindrical container of a preset volume, and performing multi-stage ultrasonic treatment after adjusting the distance between the bottom of the container and the ultrasonic probe, including: performing continuous ultrasonic treatment on the solution under preset temperature conditions in the first stage, and then pausing for a predetermined time to allow the temperature to drop to the initial set temperature; performing continuous ultrasonic treatment on the solution at a preset temperature in the second stage, and pausing for a predetermined time to cool the solution after the treatment is completed; and completing the remaining ultrasonic treatment after adjusting the solution temperature to a preset value in the third stage.

[0034] In one embodiment, preferably, for the ultrasonic dispersion process, the above-mentioned graphite oxide is first mixed with an appropriate amount of deionized water, and a segmented ultrasonic treatment scheme is adopted. Specifically, the mixed solution is placed in a cylindrical glass container with a volume of 232.6 ml, and the bottom of the container is 23.4 mm away from the ultrasonic probe. The frequency of the ultrasonic wave is set to 37.8 kHz and the power density is 0.627 W / cm2. The entire ultrasonic process is divided into three stages: the first stage is 72 minutes of continuous ultrasonication at 22.3°C, followed by a 17.3-minute pause to allow the temperature to drop to the original temperature; the second stage continues ultrasonication at 23.1°C for 68 minutes, pauses for 15.7 minutes; the third stage completes the remaining 52 minutes of ultrasonic treatment at 22.8°C. The change in solution temperature during the ultrasonic process is controlled by the following formula: T(t) = + (P × t) / (m × c), where T(t) is the temperature at time t, is the initial temperature, P is the ultrasonic power, m is the mass of the solution, and c is the specific heat capacity. When the measured temperature exceeds the theoretical set value by 3-5°C, the ultrasound can be immediately stopped and cooled to within the range of ±2°C of the theoretical temperature.

[0035] In another embodiment, the distance between the container and the probe can be adjusted to compensate for the drop in the liquid level due to evaporation, thereby maintaining the transmission efficiency of the ultrasonic wave. Specifically, during the ultrasonic treatment process, due to the evaporation loss of the solution, the probe position needs to be dynamically adjusted. Combining the evaporation rate of the solution with the speed of the liquid level drop, a probe position compensation model is established: h(t) = - vt + α·sin(2πt / τ), where h(t) is the distance from the probe to the liquid surface at time t, is the initial position, v is the average liquid level drop rate, τ is the ultrasonic period, α is the fluctuation correction coefficient, where the fluctuation correction coefficient α ranges from 0.15 to 0.25 mm. The average liquid level drop rate v can be calculated by the formula v = ( ·P + ·A)·exp(-E / RT) is calculated, where and are the power and area influence coefficients, which are 0.0023 and 0.0047, respectively, A is the liquid surface area, E is the evaporation activation energy, R is the gas constant, and T is the current operating temperature, that is, the temperature of the reactants in the current operation. In the first stage of ultrasonic process, when the temperature is 22.3℃, the calculated liquid level drop rate is about 0.0368 mm / min.

[0036] To ensure the stable transmission of ultrasonic energy, combined with the correlation between the probe position and the solution temperature change, when the solution temperature reaches 92.6% of the set value, the position fine-tuning is started, and the adjustment step is calculated by the formula Δh = β·(T - )·(h - ), where β is the position adjustment coefficient, which is 0.0826, and T is the current operating temperature. The target temperature. When the position deviation exceeds 1.73 mm, a segmented adjustment method is adopted, and each adjustment amount does not exceed 37.8% of the total deviation, and the adjustment interval is not less than 8.2 seconds. In the second and third stages of the ultrasonic process, due to the relatively small temperature fluctuation, the position adjustment is mainly based on the liquid level drop, and the distance from the probe tip to the liquid surface is always maintained within the range of 12.7±1.6 mm.

[0037] After the ultrasonic treatment is completed, the dispersion is centrifuged using a centrifuge. The dispersion is transferred to a centrifuge tube and centrifuged at 4200 rpm for 30-35 minutes, for example, 32 minutes. During the centrifugation process, it should be ensured that the centrifuge tubes are placed symmetrically in pairs to keep the centrifuge balanced. After the centrifugation is completed, the supernatant is carefully collected using a pipette to avoid touching the precipitate. The resulting supernatant is the graphene oxide dispersion, which is light brown to dark brown and translucent.

[0038] During the preparation of the dispersion, the dispersion effect can be optimized by adjusting the initial concentration of graphite oxide, ultrasonic power, treatment time and other parameters. For example, when the concentration of the dispersion is found to be too high, the initial feed amount can be appropriately reduced; when insufficient dispersion is observed, the ultrasonic treatment time can be appropriately extended or the ultrasonic power can be increased. The prepared dispersion should avoid violent shaking and can be stored at 4°C for later use.

[0039] In step S103, the graphene oxide dispersion is mixed with a predetermined amount of urea, and after a multi-step hydrothermal reaction, centrifugal washing and freeze drying are performed to obtain nitrogen-doped modified graphene powder. The steps include: measuring the graphene oxide dispersion that has been centrifuged; adding urea in batches according to a preset mass fraction and stirring and dissolving it fully; placing the mixed solution on a magnetic stirrer for stirring and dispersion; transferring the mixed solution to a hydrothermal reactor and sealing it; performing a multi-step hydrothermal reaction at a preset temperature and naturally cooling it to room temperature; centrifuging the reaction product and washing it alternately; pre-freezing the washed product and freeze drying it at a preset vacuum degree to obtain nitrogen-doped modified graphene powder.

[0040] When performing nitrogen doping modification, you first need to accurately measure 30.5 ml of centrifuged graphene oxide dispersion. Considering that the mass ratio of graphene oxide to urea has an important impact on product performance, it is necessary to accurately calculate and weigh the appropriate amount of urea based on the actual content of graphene oxide in the dispersion to maintain the mass ratio of the two at 1:28.5. When adding urea, it can be added in batches, adding about 25-30% of the total amount each time, and stirring thoroughly to ensure that the urea is completely dissolved.

[0041] Place the mixed liquid on a magnetic stirrer for stirring and dispersion. The stirring rate can be set within the range of 400-600 rpm and continue stirring for about half an hour. During the stirring process, the uniformity of the mixed liquid should be observed to ensure that there is no obvious agglomeration. After the stirring is completed, the mixed liquid is transferred to a hydrothermal reactor lined with polytetrafluoroethylene. The transfer process should be as complete as possible. A small amount of deionized water can be used to rinse the container to ensure the transfer rate of the material.

[0042] Before sealing the reactor, check the integrity of the sealing ring and ensure that the bolts are tightened. Place the sealed reactor in an oven preheated to 162°C for hydrothermal reaction, and the reaction time should be controlled within about 3 hours. During the reaction, ensure that the oven temperature is stable to avoid excessive temperature fluctuations. After the reaction is completed, cool the reactor naturally to room temperature, and the cooling time usually takes 3-4 hours.

[0043] Among them, a multi-step hydrothermal reaction is carried out at a preset temperature and naturally cooled to room temperature, including: heating the mixed liquid in the reactor from room temperature to a first preset temperature at a first preset heating rate and maintaining it for a predetermined time; raising the temperature to a second preset temperature at a second preset heating rate for a hydrothermal reaction; after the hydrothermal reaction is completed, a segmented cooling strategy is adopted, first cooling at a first preset cooling rate for a predetermined time, and then cooling to room temperature at a second preset cooling rate.

[0044] In one embodiment, preferably, for the nitrogen doping modification process, a multi-step temperature control scheme can be used to achieve uniform doping of nitrogen atoms. First, the prepared mixed solution is stirred at a speed of 273 revolutions per minute, and the temperature is raised from room temperature to 87.3°C at a heating rate of 2.37°C / minute, and this temperature is maintained for 28.4 minutes. At this stage, urea gradually decomposes to produce ammonia and carbon dioxide, and the release rate of ammonia can be calculated by the following formula: ,in is the pre-exponential factor, Ea is the activation energy, R is the gas constant, T is the current operating temperature, that is, the temperature of the reactants currently operating, is the urea concentration. Subsequently, the temperature was raised to 162°C at a rate of 1.82°C / min for hydrothermal reaction. After the hydrothermal reaction, a segmented cooling strategy was adopted: the temperature was cooled at a rate of 3.27°C / min for the first 42 minutes, and then cooled to room temperature at a rate of 1.86°C / min. This cooling method can reduce the agglomeration of nitrogen-doped graphene sheets.

[0045] After the reactor is completely cooled, open the reactor and transfer the reaction product to a centrifuge tube for centrifugal separation. The centrifugal conditions can be set to 5000-6000 rpm for 15-20 minutes. After obtaining the precipitate, alternately wash it with deionized water and anhydrous ethanol. The number of washes with each solvent is not less than 5-6 times, and a total of 11 complete washing processes are completed. During the washing process, sufficient dispersion and centrifugal separation are required each time.

[0046] Finally, the washed product is transferred to a freeze drying bottle, pre-frozen at -50°C to -60°C for 2-3 hours, and then freeze-dried at a vacuum degree of no more than 10 Pa. The drying time usually takes 24-36 hours. After drying, nitrogen-doped modified graphene is obtained, and the product should be in a loose black powder state.

[0047] In step S104, the nitrogen-doped modified graphene powder is ground and mixed with anhydrous ethanol, and then coated on the surface of the pretreated nickel foam current collector by immersion, and compacted to obtain a modified electrode material. The steps include: placing the nitrogen-doped modified graphene powder in a mortar, and adding anhydrous ethanol dropwise to grind; cleaning the nickel foam current collector; immersing the treated current collector in the mixture for coating; and drying and compacting the coated electrode to obtain a modified electrode material.

[0048] Specifically, in the electrode material preparation stage, the nitrogen-doped modified graphene powder prepared above is first transferred to an agate mortar. According to the electrode area and the target loading amount, the required amount of powder is accurately weighed. Subsequently, an appropriate amount of anhydrous ethanol is added dropwise to the mortar while grinding. The grinding process should use moderate force to ensure that the material is fully wetted and evenly dispersed until a mixture of moderate viscosity is formed. The consistency of the mixture should be able to evenly adhere to the surface of the current collector, but there should be no obvious flow.

[0049] When preparing the current collector, you can select a nickel foam sheet with a size of 2.1cm×1.2cm. First, use acetone and anhydrous ethanol to ultrasonically clean it for about 5 minutes in sequence to remove surface oil and impurities. The cleaned current collector is naturally dried at room temperature or dried in a vacuum drying oven for a short time. Immerse the treated nickel foam current collector in the above mixture, and the immersion time is controlled to be about 3 seconds. During the immersion process, a clamping tool can be used to fix one end of the current collector to ensure uniform immersion.

[0050] After impregnation, the current collector is immediately lifted up slowly at a uniform speed to ensure that the mixture forms a uniform coating on the current collector surface. The coated electrode is placed in a vacuum drying oven at 60-65°C for 24 hours. During the drying process, the electrode should be placed horizontally to avoid material loss due to gravity.

[0051] The dried electrode needs to be compacted. Place the electrode in the mold of the tablet press and apply a pressure of 10.5 MPa. The holding time is 30-40 seconds, for example, 32 seconds. During the pressing process, attention should be paid to the uniform application of pressure to avoid electrode deformation or material peeling caused by local uneven force. The actual loading amount of the active substance on each working electrode is confirmed by weighing and controlled within the range of 2.1 ± 0.1 mg. If the loading amount is found to be insufficient or excessive, the coating process needs to be repeated.

[0052] After the electrode is prepared, the appearance of the electrode should be inspected to ensure that the coating is uniform and there are no obvious cracks or shedding. The surface morphology of the electrode can be observed under a microscope to check the distribution of the material. The prepared electrode should avoid direct contact, and can be temporarily encapsulated with polytetrafluoroethylene film and stored in a dry environment for future use.

[0053] Figure 2 This is a graph showing the experimental results of a multi-stage ultrasonic treatment method disclosed in the embodiments of this application. Figure 2 As shown in the figure, the horizontal axis is the ultrasonic treatment temperature (20-25°C), the vertical axis is the cumulative ultrasonic time (0-240min), and the vertical axis represents the dispersion efficiency (%). The red asterisks in the figure correspond to the optimal process parameters in the three stages: (22.3°C, 72min), (23.1°C, 140min) and (22.8°C, 192min).

[0054] From the surface morphology, it can be observed that within the temperature range of 22-23°C, as the ultrasonic time increases, the dispersion efficiency shows a trend of first rising rapidly and then flattening. This is because the ultrasonic energy can effectively overcome the van der Waals force between graphite layers in the initial stage and promote peeling; while in the later stage, due to the shielding effect of the peeled sheets, the energy transfer efficiency decreases. Temperature also has a significant effect on the dispersion effect. Too low a temperature will increase the viscosity of the solution and limit peeling, while too high a temperature will cause violent convection of the solution and affect energy transfer.

[0055] Through the analysis of experimental data, it was found that the use of segmented ultrasonic treatment can effectively control the solution temperature while maintaining a high dispersion efficiency. The first stage (72 minutes) is mainly to achieve the initial peeling of large-sized flakes; the second stage (68 minutes) further improves the dispersion uniformity; the third stage (52 minutes) is used to optimize the flake size distribution. A cooling time of 15-17 minutes is set between each stage to allow the temperature to fall back to the optimal range.

[0056] Figure 3 This is a diagram showing the experimental results of a multi-step hydrothermal reaction method disclosed in the examples of this application. Figure 3As shown, the experimental data collection covers the range of heating rate 1.5-3.5℃ / min and cooling rate 1.5-3.5℃ / min. A total of 421 experimental data points were collected to form a process parameter space. The experimental process achieves regulation of the reaction process through programmed heating and cooling strategies. In the heating stage, experimental data show that when the heating rate is 2.37℃ / min, the reaction kinetics of graphene oxide and urea reaches the best balance. This rate makes the thermal decomposition process of urea well matched with the doping process of nitrogen atoms, avoiding uneven reactions caused by too fast heating and efficiency losses caused by too slow heating. During the experiment, the constant temperature platform at 87.3℃ was maintained for 28.4 minutes. Based on the kinetic characteristics of urea decomposition, the release rate of ammonia and the defect formation rate of the graphene lattice reached a dynamic balance. Experimental data show that at this temperature, the decomposition reaction rate constant of urea is , which is consistent with the optimal reaction rate calculated theoretically.

[0057] In the second stage, the temperature was raised to 162°C at a rate of 1.82°C / min. Experimental data showed that this heating rate ensured the uniform distribution of nitrogen atoms in the graphene sheets and avoided agglomeration caused by local overheating. At this heating rate, the distribution of the three forms of pyridinic nitrogen, pyrrolic nitrogen and graphitic nitrogen in the product was relatively uniform.

[0058] The design of the cooling strategy also shows obvious regularity. The cooling rate of 3.27℃ / min was used in the first 42 minutes. The rapid cooling in this stage helps to "freeze" the formed nitrogen-doped structure and prevent atomic rearrangement at high temperature. Experimental data show that at this cooling rate, the nitrogen content of the product remains at around 7.45%, and the chemical environment of the nitrogen atoms is most evenly distributed.

[0059] The cooling rate was then adjusted to 1.86℃ / min until room temperature. This slow cooling process is crucial to inhibit product agglomeration and maintain the characteristics of large specific surface area. The experimental results show that by using this nonlinear cooling strategy, the specific surface area of ​​the final product can reach 172.5m² / g, and the pore size distribution is concentrated in the range of 3.2-4.1nm.

[0060] The three-dimensional surface diagram shows the relationship between process parameters and product performance. At the optimal process point (heating rate 2.37℃ / min, cooling rate first stage 3.27℃ / min), the uniformity of nitrogen doping reached the highest value of 92.6%. From the contour distribution, it can be seen that a relatively gentle "plateau area" is formed around the process point, which has a certain fault tolerance. Scanning electron microscopy observations found that the samples prepared under this process condition showed a uniform lamellar structure with moderate wrinkles and the interlayer spacing maintained at about 0.368nm. Transmission electron microscopy analysis further confirmed the uniform distribution of nitrogen atoms on the graphene plane, without obvious agglomeration.

[0061] In summary, segmented ultrasonic treatment can gradually weaken the van der Waals forces between graphene oxide sheets, promoting the exfoliation of large-sized graphene oxide sheets. Temperature control during the ultrasonic process ensures the integrity of the graphene oxide sheet structure and avoids structural damage caused by excessive temperature. In the nitrogen doping modification process, the multi-step temperature control scheme adopted makes the release process of ammonia more uniform, which is conducive to the uniform distribution of nitrogen atoms in the graphene lattice. By controlling the decomposition rate and reaction pressure of urea, the nitrogen content and chemical state in the product can be adjusted. The adoption of a nonlinear cooling strategy effectively inhibits the agglomeration of graphene sheets during the modification process and maintains the large specific surface area characteristics of the product. These process improvements make the final product have a uniform nitrogen doping distribution and a developed pore structure. After urea reduction and nitrogen doping, the graphene sheets are disordered, transparent, and wrinkled gauze-like structures, and some flakes are stacked to form a multilayer structure. Nitrogen atoms are doped into the graphene lattice in the form of pyridinic nitrogen (pyridinic N), pyrrolic nitrogen (pyrrolic N) and graphitic nitrogen (graphitic N). The obtained nitrogen-doped modified graphene material has a typical mesoporous structure, with a pore size distribution concentrated in the range of 3.2-4.1nm and a specific surface area of ​​up to 172.5m2 / g. The nitrogen content is about 7.45% (atomic fraction). The doping of nitrogen atoms changes the electronic structure of graphene and reveals more electrochemical active sites. The material has a high specific surface area and a developed pore structure, which is conducive to the rapid transmission and storage of electrolyte ions and can significantly improve the electrochemical properties of graphene.

[0062] Second embodiment

[0063] In addition, Example 2 of the present application also provides a control device for preparing modified graphene electrode materials for lithium-ion batteries, including: a processor, a memory, and a system bus; the processor and the memory are connected via the system bus; the memory is used to store one or more programs, and the one or more programs include instructions, which, when executed by the processor, enable the processor to execute any of the above methods.

[0064] Furthermore, an embodiment of the present application also discloses a computer program product, which, when executed on a terminal device, enables the terminal device to execute any of the above methods.

[0065] The computer program product provided in the embodiment of the present application includes one or more computer programs / instructions, and when the computer program / instructions are executed by the processor, the process or function described in the embodiment of the present application is generated in whole or in part. The computer may be a general-purpose computer, a special-purpose computer, a computer network, or other programmable devices. The computer instructions may be stored in a computer-readable storage medium, or transmitted from one computer-readable storage medium to another computer-readable storage medium. For example, the computer instructions may be transmitted from a website site, a computer, a server, or a data center to another website site, a computer, a server, or a data center by wired (e.g., coaxial cable, optical fiber, digital subscriber line (DSL)) or wireless (e.g., infrared, wireless, microwave, etc.) mode. The computer-readable storage medium may be any available medium that a computer can access or a data storage device such as a server or a data center that includes one or more available media integrations. The available medium may be a magnetic medium, (e.g., a floppy disk, a hard disk, a tape), an optical medium (e.g., a DVD), or a semiconductor medium (e.g., a solid state disk (SSD)), etc.

[0066] The flow chart or block diagram in the accompanying drawings shows the possible architecture, function and operation of the equipment, method and computer program product according to various embodiments of the present application. In this regard, each box in the flow chart or block diagram can represent a module, a program segment or a part of a code, and the module, the program segment or a part of the code contains one or more executable instructions for realizing the specified logical function. It should also be noted that in some alternative implementations, the functions marked in the box can also occur in a different order from the order marked in the accompanying drawings. For example, two boxes represented in succession can actually be executed substantially in parallel, and they can sometimes be executed in the opposite order, depending on the functions involved. It should also be noted that each box in the block diagram and / or flow chart, and the combination of the boxes in the block diagram and / or flow chart can be implemented with a dedicated system for hardware that performs a specified function or operation, or can be implemented with a combination of dedicated hardware and computer instructions.

[0067] The scope of the present application is defined by the appended claims rather than the above description, and it is intended that all changes that fall within the meaning and scope of the equivalent elements of the claims are included in the present application. Any figure mark in the claims should not be regarded as limiting the claims involved. In addition, it is clear that the word "comprising" does not exclude other units or steps, and the singular does not exclude the plural. Multiple units or devices stated in the device claim may also be implemented by one unit or device through software or hardware. The words "first", "second", etc. are only used to distinguish the description, and do not indicate any particular order, nor can they be understood as indicating or implying relative importance.

[0068] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art can easily mention changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims, and the above embodiments should be regarded as exemplary and non-restrictive.

Claims

1. A method for preparing a modified graphene electrode material for a lithium ion battery, characterized in that: include: After premixing graphite powder with concentrated sulfuric acid, an oxidant is added in batches, and after oxidation is completed, the mixture is filtered and washed with dilute hydrochloric acid and deionized water until it is neutral; The washed graphite oxide is added to deionized water, and after multi-stage ultrasonic treatment, a graphene oxide dispersion is obtained by centrifugal separation; wherein the method comprises: adding graphite oxide to deionized water at a preset mass concentration; placing the aqueous solution containing graphite oxide in a cylindrical container of a preset volume, and adjusting the distance between the bottom of the container and the ultrasonic probe to perform multi-stage ultrasonic treatment; centrifuging the dispersion after ultrasonic treatment and collecting the supernatant to obtain the graphene oxide dispersion; The aqueous solution containing graphite oxide is placed in a cylindrical container of a preset volume, and a multi-stage ultrasonic treatment is performed after adjusting the distance between the bottom of the container and the ultrasonic probe, including: performing continuous ultrasonic treatment on the solution at a preset temperature in the first stage, and then pausing for a predetermined time to reduce the temperature to the initial set temperature; performing continuous ultrasonic treatment on the solution at a preset temperature in the second stage, and pausing for a predetermined time to cool the solution after the treatment is completed; and completing the remaining ultrasonic treatment after adjusting the solution temperature to a preset value in the third stage; Wherein, adjusting the distance between the bottom of the container and the ultrasonic probe also includes: establishing a probe position compensation model in combination with the evaporation rate of the solution and the liquid level drop speed; The model includes: , in, express The distance from the probe to the liquid surface at any time, represents the initial distance, represents the average liquid level drop rate, represents the ultrasonic cycle, Indicates the fluctuation correction coefficient, the value range is 0.15-0.25 mm; The graphene oxide dispersion is mixed with a predetermined amount of urea, and after a multi-step hydrothermal reaction, centrifugal washing and freeze drying are performed to obtain nitrogen-doped modified graphene powder; The nitrogen-doped modified graphene powder is ground and mixed with anhydrous ethanol, coated on the surface of the pretreated nickel foam current collector by immersion, and compacted to obtain a modified electrode material.

2. The preparation method according to claim 1, characterized in that: in, After premixing graphite powder with concentrated sulfuric acid, an oxidant is added in batches, and after oxidation is completed, the graphite powder is filtered and washed with dilute hydrochloric acid and deionized water until it is neutral, including: A predetermined amount of graphite powder is slowly added to concentrated sulfuric acid for premixing, and an external ice bath is used to control the temperature of the reaction system; Add potassium dichromate oxidant in batches under continuous stirring, with each batch adding a preset proportion of the total amount; Adding predetermined amounts of deionized water and hydrogen peroxide to the reaction mixture in sequence under stirring, and immediately performing suction filtration and separation treatment; The filter cake is preliminarily cleaned with a dilute hydrochloric acid solution of a predetermined volume ratio, and then repeatedly rinsed with deionized water until the filtrate is in a neutral state to obtain graphite oxide.

3. The preparation method according to claim 1, characterized in that: in, The graphene oxide dispersion is mixed with a predetermined amount of urea, and after a multi-step hydrothermal reaction, centrifugal washing and freeze drying are performed to obtain nitrogen-doped modified graphene powder, including: measuring the graphene oxide dispersion that has been subjected to centrifugal treatment; Add urea in batches according to the preset mass ratio and stir thoroughly to dissolve; The mixed solution was placed on a magnetic stirrer for stirring and dispersion; The mixed solution is transferred into a hydrothermal reactor and sealed; Perform a multi-step hydrothermal reaction at a preset temperature and cool naturally to room temperature; The reaction products were centrifuged and washed alternately; The cleaned product is pre-frozen and freeze-dried under a preset vacuum degree to obtain nitrogen-doped modified graphene powder.

4. The preparation method according to claim 1, characterized in that: in, The nitrogen-doped modified graphene powder is ground and mixed with anhydrous ethanol, and then coated on the surface of the pretreated nickel foam current collector by immersion, and compacted to obtain a modified electrode material, including: The nitrogen-doped modified graphene powder was placed in a mortar and ground by adding anhydrous ethanol dropwise; Cleaning the nickel foam current collector; immersing the treated current collector into the mixture for coating; The coated electrode is dried and compacted to obtain a modified electrode material.

5. The preparation method according to claim 3, characterized in that: in, A multi-step hydrothermal reaction was performed at a preset temperature and cooled naturally to room temperature, including: The mixed liquid in the reaction kettle is heated from room temperature to a first preset temperature at a first preset heating rate and maintained for a predetermined time; Raising the temperature to a second preset temperature at a second preset heating rate to perform a hydrothermal reaction; After the hydrothermal reaction is completed, a staged cooling strategy is adopted, first cooling at a first preset cooling rate for a predetermined time, and then cooling to room temperature at a second preset cooling rate.

6. A control device for preparing a modified graphene electrode material for a lithium-ion battery, characterized in that: include: A processor, a memory, and a system bus; wherein the processor and the memory are connected via the system bus; The memory is used to store one or more programs, wherein the one or more programs include instructions, and when the instructions are executed by the processor, the processor executes the method according to any one of claims 1 to 5.

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