Hard carbon coated with graphene in situ, preparation method of hard carbon, negative plate and sodion hybrid capacitor

Abstract

The invention relates to hard carbon coated with graphene in situ, a preparation method of the hard carbon, a negative plate and a sodium ion hybrid capacitor. The preparation method of the in-situ graphene-coated hard carbon comprises the following steps: mixing a phenolic resin precursor solution with a surfactant, carrying out a condensation polymerization reaction under the shearing action, and drying the product of the condensation polymerization reaction to prepare spherical phenolic resin particles; wherein the phenolic resin precursor solution comprises a phenolic resin precursor and a solvent; performing first carbonization treatment on the spherical phenolic resin particles to obtain a hard carbon precursor; and performing second carbonization treatment on the hard carbon precursor in the presence of a carbon source to prepare the hard carbon with in-situ coated graphene. By adding the surfactant and carrying out condensation polymerization under the shearing action, the prepared hard carbon can effectively reduce the volume expansion rate in the charge-discharge process, and the structural stability of the electrode is improved; the graphene shell layer is formed in situ by the hard carbon material, so that the first coulombic efficiency of the hard carbon material is remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion hybrid capacitor technology, and in particular to hard carbon with in-situ graphene coating, its preparation method, negative electrode sheet, and sodium-ion hybrid capacitor. Background Technology

[0002] With the surge in demand for renewable energy storage, sodium-ion hybrid capacitors, due to their abundant sodium resources, low cost, and excellent low-temperature performance, are considered an ideal alternative to lithium-ion hybrid capacitors in the field of electrochemical energy storage. Hard carbon materials, with their unique layered disordered structure and abundant porosity, can provide diverse storage sites for sodium ions, and their microstructure can be controlled through precursors and carbonization processes, making them a research hotspot for anode materials in sodium-ion hybrid capacitors.

[0003] However, the current performance bottlenecks of hard carbon materials severely restrict the large-scale application of sodium-ion hybrid capacitors. From a conductivity perspective, the low crystallinity of hard carbon results in a tortuous electron transport path, numerous grain boundaries and defects, leading to an intrinsic conductivity generally lower than that of graphite-based anode materials. This not only increases the internal resistance of the hybrid capacitor and exacerbates energy loss during charging and discharging, but also limits the power output capability of the hybrid capacitor at high rates. Regarding initial coulombic efficiency, the abundant unsaturated carbon atoms, dangling bonds, and oxygen-containing functional groups on the surface of hard carbon readily undergo reduction decomposition reactions with the electrolyte during the first charge and discharge process, forming an unstable solid electrolyte interface film. This process irreversibly consumes a large amount of active sodium ions and electrolyte, resulting in low initial coulombic efficiency, directly reducing the initial energy density of the hybrid capacitor, and causing irreversible capacity loss, affecting the overall lifecycle performance of the hybrid capacitor.

[0004] Therefore, there is an urgent need to develop a preparation method that can improve the first coulombic efficiency of hard carbon materials while taking into account the structural stability of hard carbon materials. Summary of the Invention

[0005] Therefore, it is necessary to provide in-situ graphene-coated hard carbon and its preparation method, as well as negative electrode sheets and sodium-ion hybrid capacitors, which can improve the first coulombic efficiency of hard carbon materials while taking into account structural stability.

[0006] One aspect of this application provides a method for preparing hard carbon with in-situ graphene coating, comprising the following steps: mixing a phenolic resin precursor solution with a surfactant, performing a polycondensation reaction under shear, and drying the product of the polycondensation reaction to obtain spherical phenolic resin particles; wherein the phenolic resin precursor solution comprises a phenolic resin precursor and a solvent, and the surfactant is at least one of a nonionic surfactant and anionic surfactant; subjecting the spherical phenolic resin particles to a first carbonization treatment to obtain a hard carbon precursor; subjecting the hard carbon precursor to a second carbonization treatment in the presence of a carbon source to obtain hard carbon with in-situ graphene coating; wherein the temperature of the second carbonization treatment is higher than the temperature of the first carbonization treatment.

[0007] By adding surfactants and initiating a condensation reaction under shear, the surface tension of the phenolic resin droplets formed by condensation is reduced, making it easier for the droplets to form spherical shapes. Furthermore, the surfactant forms a molecular layer on the surface of the phenolic resin droplets, acting as a steric hindrance to prevent droplet aggregation and merging, thus maintaining the dispersion stability of the phenolic resin droplets. This allows the prepared phenolic resin particles to self-assemble into a uniform spherical structure, which is easily maintained after subsequent first and second carbonization treatments. This enables the prepared hard carbon to effectively reduce the volume expansion rate during charge and discharge, improve electrode structural stability, and thus extend its service life. Further, the first carbonization process promotes the volatilization and removal of oxygen heteroatoms in the precursor, leading to the formation of an ordered graphite-like layer inside the hard carbon and the formation of numerous microporous structures on the material surface. In the subsequent second carbonization process at a higher temperature, the long-range ordered graphite-like layer surrounds and shrinks the active sites to form closed pores. The surface micropores, after high-temperature carbonization, promote carbon layer rearrangement, gradually closing the microporous structure to form closed pores. The gases produced by the decomposition of carbon source (such as H2, CO, etc.) further etch the internal pores of hard carbon, increasing the proportion of closed pores. With the increase of the proportion of closed pores, the specific surface area and unsaturated sites on the surface of hard carbon material increase. Using it as a negative electrode material can improve the first-cycle efficiency of sodium-ion hybrid capacitors or batteries.

[0008] On the other hand, by using a segmented carbonization process involving the first and second carbonization treatments, combined with a carbon source, graphene can be grown in situ on the surface of the hard carbon precursor. This not only increases the proportion of closed pores in the hard carbon and optimizes ion storage sites, but also forms a uniform coating layer, improving the material's conductivity and significantly enhancing electron transport efficiency. Furthermore, the graphene shell grown in situ during the second carbonization process tightly coats the surface of the hard carbon core, forming a physical barrier that blocks direct contact between the electrolyte and the unsaturated bonds on the hard carbon surface. This passivates the active sites on the hard carbon surface, allowing it to be used to prepare negative electrode sheets. When applied to sodium-ion hybrid capacitors, this significantly improves the first-turn coulombic efficiency of the sodium-ion hybrid capacitors.

[0009] In some embodiments, the phenolic resin precursor includes one or more of phenolic precursors and aldehyde precursors; the phenolic resin precursor solution satisfies at least one of the following conditions:

[0010] (1) The aldehyde precursors include any one or more of formaldehyde, acetaldehyde, furfural, and trioxymethylene;

[0011] (2) The phenolic precursors include any one or more of cresol, phenol, and resorcinol;

[0012] (3) The phenolic resin precursor solution contains 10wt%-15wt% of phenolic resin precursor by mass.

[0013] (4) The molar ratio of the phenolic precursor to the aldehyde precursor is 1:(1~1.3);

[0014] (5) The solvent is an alcohol-water mixture, which is a mixture of ethanol and water in a volume ratio of (1~5):1.

[0015] In some embodiments, the surfactant is any one or more of polyvinyl alcohol, sodium dodecyl sulfate, sodium tetradecyl sulfate, and sodium dodecylbenzene sulfonate; and / or,

[0016] In the step of mixing the phenolic resin precursor solution with the surfactant, the mass ratio of the surfactant to the phenolic resin precursor is (0.005~0.05):1, based on the amount of material added.

[0017] In some embodiments, the step of mixing a phenolic resin precursor solution with a surfactant and carrying out a polycondensation reaction under shear, followed by drying the polycondensation product to obtain spherical phenolic resin particles, satisfies at least one of the following conditions:

[0018] (1) Under stirring conditions, an alkaline solution is added to the phenolic resin precursor solution to make the pH value of the phenolic resin precursor solution 8.0-9.0;

[0019] (2) Under stirring conditions, an alkaline solution is added to the phenolic resin precursor solution, wherein the alkaline solution is selected from any one or more of ammonia water, sodium hydroxide solution, and sodium carbonate solution;

[0020] (3) The temperature of the polycondensation reaction is 40℃-60℃;

[0021] (4) The polycondensation reaction takes 40 min - 70 min;

[0022] (5) The polycondensation reaction is carried out under stirring at a speed of 900 rpm to 1200 rpm;

[0023] (6) The drying is carried out under vacuum conditions, the drying temperature is 60℃-80℃, and the drying time is 12 h-18 h;

[0024] (7) The particle size D50 of the spherical phenolic resin particles is 3μm-30μm.

[0025] In some embodiments, the step of subjecting the spherical phenolic resin particles to a first carbonization treatment to obtain a hard carbon precursor satisfies at least one of the following conditions:

[0026] (1) The temperature of the first carbonization treatment is 300℃-450℃;

[0027] (2) The time for the first carbonization treatment is 1h-3h;

[0028] (3) Before performing the first carbonization treatment, the spherical phenolic resin particles are heated to the temperature of the first carbonization treatment at a rate of 3℃ / min - 5℃ / min.

[0029] (4) The first carbonization process is carried out in a first protective gas atmosphere, wherein the first protective gas includes any one or more of nitrogen, argon, helium, neon and krypton;

[0030] (5) The hard carbon precursor is a spherical particle.

[0031] In some embodiments, the step of subjecting the hard carbon precursor to a second carbonization treatment in the presence of a carbon source to obtain hard carbon with in-situ graphene coating satisfies at least one of the following conditions:

[0032] (1) The temperature of the second carbonization treatment is 800 ℃-1100 ℃;

[0033] (2) The second carbonization treatment takes 2 h to 5 h;

[0034] (3) Before performing the second carbonization treatment, the hard carbon precursor is heated to the temperature of the second carbonization treatment at a rate of 5℃ / min - 10℃ / min.

[0035] (4) The carbon source includes any one or more of methane, ethane, carbon dioxide, methanol, ethanol and acetone;

[0036] (5) The carbon source includes any one or more of methane, ethane, carbon dioxide, methanol, ethanol and acetone, and the carbon source is introduced in gaseous form with a flow rate of 20 sccm-200 sccm;

[0037] (6) The second carbonization process is carried out in a second protective gas atmosphere, wherein the second protective gas includes any one or more of nitrogen, argon, helium, neon and krypton;

[0038] (7) The hard carbon with in-situ graphene coating obtained is spherical particles.

[0039] In a second aspect, this application provides an in-situ graphene-coated hard carbon material, prepared using any of the above-described methods for preparing in-situ graphene-coated hard carbon.

[0040] In some embodiments, the system includes a hard carbon core and a graphene coating layer located on at least a portion of the surface of the hard carbon core; at least a portion of the graphene in the graphene coating layer is covalently bonded to the hard carbon core.

[0041] In a third aspect, this application provides a negative electrode sheet comprising a hard carbon material in situ coated with graphene, as described above.

[0042] In a fourth aspect, this application provides a sodium-ion hybrid capacitor, including the aforementioned negative electrode. Attached Figure Description

[0043] Figure 1 These are scanning electron microscope (SEM) images of the graphene-coated hard carbon materials prepared in Example 1 and Comparative Example 1 of this application. Detailed Implementation

[0044] To facilitate understanding of this application, a more complete description will be provided below with reference to the accompanying drawings. Preferred embodiments of this application are shown in the drawings. However, this application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of this application.

[0045] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0046] Common hard carbon materials, due to their abundant unsaturated carbon atoms, dangling bonds, and oxygen-containing functional groups on their surface, are prone to reduction and decomposition reactions with the electrolyte during the first charge and discharge process, forming an unstable solid electrolyte interface film, which easily leads to low initial coulombic efficiency.

[0047] To address this issue, this application provides at least one in-situ graphene-coated hard carbon and its preparation method, a negative electrode sheet, and a sodium-ion hybrid capacitor.

[0048] According to a typical embodiment of this application, a method for preparing hard carbon with in-situ graphene coating is provided, comprising the following steps: mixing a phenolic resin precursor solution with a surfactant, subjecting it to a condensation reaction under shear, and drying it to obtain spherical phenolic resin particles; subjecting the spherical phenolic resin particles to a first carbonization treatment and a second carbonization treatment to obtain hard carbon with in-situ graphene coating; wherein the temperature of the second carbonization treatment is higher than the temperature of the first carbonization treatment. The surfactant can be at least one of a nonionic surfactant and anionic surfactant.

[0049] In some embodiments, a method for preparing hard carbon with in-situ graphene coating is provided, comprising the following steps: mixing a phenolic resin precursor solution with a surfactant, performing a polycondensation reaction under shear, and drying the product of the polycondensation reaction to obtain spherical phenolic resin particles; wherein the phenolic resin precursor solution includes a phenolic resin precursor and a solvent, and the surfactant is at least one of a nonionic surfactant and anionic surfactant; subjecting the spherical phenolic resin particles to a first carbonization treatment to obtain a hard carbon precursor; subjecting the hard carbon precursor to a second carbonization treatment in the presence of a carbon source to obtain hard carbon with in-situ graphene coating; wherein the temperature of the second carbonization treatment is higher than the temperature of the first carbonization treatment.

[0050] By adding surfactants and initiating a condensation reaction under shear, the surface tension of the phenolic resin droplets formed by condensation is reduced, making it easier for the droplets to form spherical shapes. Furthermore, the surfactant forms a molecular layer on the surface of the phenolic resin droplets, acting as a steric hindrance to prevent droplet aggregation and merging, thus maintaining the dispersion stability of the phenolic resin droplets. This allows the prepared phenolic resin particles to self-assemble into a uniform spherical structure, which is easily maintained after subsequent first and second carbonization treatments. This enables the prepared hard carbon to effectively reduce the volume expansion rate during charge and discharge, improve electrode structural stability, and thus extend its service life. Further, the first carbonization process promotes the volatilization and removal of oxygen heteroatoms in the precursor, leading to the formation of an ordered graphite-like layer inside the hard carbon and the formation of numerous microporous structures on the material surface. In the subsequent second carbonization process at a higher temperature, the long-range ordered graphite-like layer surrounds and shrinks the active sites to form closed pores. The surface micropores, after high-temperature carbonization, promote carbon layer rearrangement, gradually closing the microporous structure to form closed pores. The gases produced by the decomposition of carbon source (such as H2, CO, etc.) further etch the internal pores of hard carbon, increasing the proportion of closed pores. With the increase of the proportion of closed pores, the specific surface area and unsaturated sites on the surface of hard carbon material increase. Using it as a negative electrode material can improve the first-cycle efficiency of sodium-ion hybrid capacitors or batteries.

[0051] On the other hand, by using a segmented carbonization process involving the first and second carbonization treatments, combined with a carbon source, graphene can be grown in situ on the surface of the hard carbon precursor. This not only increases the proportion of closed pores in the hard carbon and optimizes ion storage sites, but also forms a uniform coating layer, improving the material's conductivity and significantly enhancing electron transport efficiency. Furthermore, the graphene shell grown in situ during the second carbonization process tightly coats the surface of the hard carbon core, forming a physical barrier that blocks direct contact between the electrolyte and the unsaturated bonds on the hard carbon surface. This passivates the active sites on the hard carbon surface, allowing it to be used to prepare negative electrode sheets. When applied to sodium-ion hybrid capacitors, this significantly improves the first-turn coulombic efficiency of the sodium-ion hybrid capacitors.

[0052] In some embodiments, the phenolic resin precursor includes one or more of phenolic precursors and aldehyde precursors. The aldehyde precursor includes, but is not limited to, any one or more of formaldehyde, acetaldehyde, furfural, and trioxymethylene; the phenolic precursor includes, but is not limited to, any one or more of cresol, phenol, and resorcinol.

[0053] In some embodiments, the molar ratio of the phenolic precursor to the aldehyde precursor is 1:(1~1.3). Non-limitingly, the molar ratio can be 1:1.0, 1:1.1, 1:1.2, 1:1.3, etc. It should be noted that the above molar ratio refers to the molar proportion of the phenolic hydroxyl groups in the phenolic precursor and the aldehyde groups in the aldehyde precursor. When the phenolic or aldehyde precursor contains multiple corresponding functional groups, those skilled in the art can perform corresponding conversions to determine the amount of phenolic or aldehyde precursor used. Maintaining the ratio of the phenolic to aldehyde precursor within the above range is beneficial for increasing the reaction rate and obtaining spherical phenolic resin particles.

[0054] In some embodiments, the phenolic resin precursor solution contains 10wt%-15wt% of phenolic resin precursor by mass, specifically 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, etc., and is not limited thereto. A phenolic resin precursor solution with this concentration is beneficial for efficient polycondensation to obtain spherical phenolic resin particles with relatively uniform particle size.

[0055] In some embodiments, the solvent is an alcohol-water mixture. Optionally, the alcohol-water mixture is a mixture of ethanol and water in a volume ratio of (1~5):1, which has a better dissolution and dispersion effect on phenolic resin precursors and is conducive to promoting the polycondensation reaction.

[0056] In some embodiments, the surfactant is any one or more of polyvinyl alcohol (PVA), sodium dodecyl sulfate, sodium tetradecyl sulfate, and sodium dodecylbenzene sulfonate. These surfactants can reduce the surface tension of water, effectively promoting the formation of spherical emulsions from the phenolic resin precursor, which is beneficial for forming spherical phenolic resin particles with more uniform particle size. This results in the prepared in-situ graphene-coated hard carbon exhibiting better stability and electrochemical performance. In some embodiments, the weight-average molecular weight of the polyvinyl alcohol is 70,000 to 140,000, for example, 70,000, 80,000, 90,000, 100,000, 120,000, 140,000, etc.

[0057] In some embodiments, in the step of mixing the phenolic resin precursor solution with the surfactant, the mass ratio of the surfactant to the phenolic resin precursor is (0.005~0.05):1, based on the amount of feed. This better maintains the dispersibility of the generated phenolic resin liquid, resulting in more uniform particle size of the generated spherical phenolic resin particles and improving the yield of spherical phenolic resin particles. Non-limitingly, the mass ratio of the surfactant to the phenolic resin precursor can be 0.0005:1, 0.01:1, 0.02:1, 0.03:1, 0.04:1, 0.05:1, etc., and is not limited thereto.

[0058] In some embodiments, the step of mixing a phenolic resin precursor solution with a surfactant and carrying out a polycondensation reaction under shear, followed by drying the polycondensation product to obtain spherical phenolic resin particles, includes: adding an alkaline solution to the phenolic resin precursor solution under stirring conditions to adjust the pH of the phenolic resin precursor solution to 8.0-9.0, such as 8.0, 8.2, 8.4, 8.6, 8.8, or 9.0, which helps to improve the efficiency of the polycondensation reaction and results in more uniform spherical phenolic resin particles. Optionally, the alkaline solution is selected from any one or more of ammonia, sodium hydroxide solution, and sodium carbonate solution.

[0059] In some embodiments, the temperature of the polycondensation reaction is 40℃-60℃, such as 40℃, 45℃, 50℃, 55℃, 60℃, etc., and is not limited here. Optionally, the time of the polycondensation reaction is 40 min-70 min, specifically 40 min, 45 min, 50 min, 55 min, 60 min, 65 min, 70 min, etc., and is not limited here. By performing the polycondensation reaction through the above process, the polycondensation proceeds at a suitable rate, which is beneficial for forming spherical phenolic resin particles with relatively uniform particle size and suitable particle shape.

[0060] In some embodiments, the above-mentioned polycondensation reaction is carried out under stirring at a speed of 900 rpm to 1200 rpm, such as 900 rpm, 1000 rpm, 1100 rpm, 1200 rpm, etc., which is not limited here. By controlling the above stirring speed, the synergistic effect with the surfactant can be better achieved, resulting in more uniform particle size and better sphericality of the prepared spherical phenolic resin particles.

[0061] In some embodiments, the particle size D50 of the prepared spherical phenolic resin particles is 3 μm-30 μm, specifically 3 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, etc., which are not limited here.

[0062] In some embodiments, the product of the polycondensation reaction is centrifuged to remove the liquid, and then the solid product is washed with a detergent, which can be an alcohol-water mixture, such as a 1:1 volume ratio of ethanol and water. Optionally, the washing is performed more than once. The washed product can then be dried.

[0063] In some embodiments, drying is carried out under vacuum conditions at a temperature of 60 ℃-80 ℃ for 12 h-18 h, which effectively removes the solvent mixed in the spherical phenolic resin particles and prevents the disorder of the phenolic resin from being increased due to excessively high temperature, which would reduce the pore structure of the subsequent hard carbon formation.

[0064] In some embodiments, the steps of mixing a phenolic resin precursor solution with a surfactant and performing a polycondensation reaction under shear, followed by drying the polycondensation product to obtain spherical phenolic resin particles, include: dissolving the phenolic resin precursor in a mixture of ethanol and water at a volume ratio of (1~5):1, stirring to form a phenolic resin precursor solution with a concentration of 10 wt%-15 wt%; adjusting the pH to 8-9.0 by adding ammonia water to the solution under stirring at 900 rpm-1200 rpm, and simultaneously adding 0.5%~5% of the surfactant by mass of the phenolic resin precursor, and performing a polycondensation reaction at 40℃-60℃ for 40 min-70 min to obtain a polycondensation product system containing spherical phenolic resin particles; washing the polycondensation product system 2-5 times by centrifugation with a water and ethanol mixture at a volume ratio of 1:1; and drying the washed product under vacuum at 60℃-80℃ for 12 h-18 h to obtain spherical phenolic resin particles.

[0065] In some embodiments, the temperature of the first carbonization treatment is 300℃-450℃, specifically 300℃, 325℃, 350℃, 375℃, 400℃, 425℃, 450℃, etc., and is not limited here. Performing the first carbonization treatment at the above temperature can effectively remove and volatilize small molecules formed by oxygen heteroatoms in the spherical phenolic resin particles, and more effectively promote the formation of an ordered graphite-like layer inside the hard carbon and the formation of a large number of microporous structures on the material surface. In some embodiments, the time of the first carbonization treatment is 1h-3h, specifically 1h, 1.5h, 2h, 2.5h, 3h, etc., and is not limited here.

[0066] In some embodiments, before performing the first carbonization treatment, the spherical phenolic resin particles are heated to the temperature of the first carbonization treatment at a rate of 3°C / min - 5°C / min, which helps to further improve the effect of the first carbonization treatment.

[0067] In some embodiments, the first carbonization process is carried out in a first protective gas atmosphere, the first protective gas including any one or more of nitrogen, argon, helium, neon and krypton.

[0068] In some embodiments, the hard carbon precursor is a spherical particle, which helps to prepare in-situ graphene-coated hard carbon materials with better stability and electrochemical performance through a second carbonization process.

[0069] In some embodiments, the temperature of the second carbonization treatment is 800℃-1100℃, which is beneficial for the formation of the graphene coating layer. Non-limitingly, the temperature of the second carbonization treatment can be 800℃, 85℃, 900℃, 950℃, 1000℃, 1050℃, 1100℃, etc., and is not limited herein. Optionally, the time of the second carbonization treatment is 2 h-5 h, non-limitingly, it can be 2 h, 3 h, 4 h, 5 h, etc., and is not limited herein.

[0070] In some embodiments, the temperature of the first carbonization treatment is 300 ℃-450 ℃, and the temperature of the second carbonization treatment is 800 ℃-1100 ℃. The first carbonization temperature mainly affects the internal pore structure of the material. Controlling the temperature of the first carbonization treatment within the above range is beneficial to forming suitable internal pores. Too low a temperature may result in fewer internal pores or more micropores, while too high a temperature may result in a reduction of closed pores in the material.

[0071] In some embodiments, before performing the second carbonization treatment, the hard carbon precursor is heated to the temperature of the second carbonization treatment at a rate of 5°C / min - 10°C / min, which helps to further improve the effect of the second carbonization treatment. It can be understood that after performing the first carbonization treatment, the second carbonization treatment can be performed directly by heating, or it can be performed by cooling and then heating again for the second carbonization treatment.

[0072] In some embodiments, the carbon source includes any one or more of methane, ethane, carbon dioxide, methanol, ethanol, and acetone. Optionally, the carbon source is introduced in gaseous form at a flow rate of 20 sccm to 200 sccm.

[0073] In some embodiments, the second carbonization process is carried out in a second protective gas atmosphere, which includes any one or more of nitrogen, argon, helium, neon and krypton.

[0074] In some embodiments, the hard carbon with in-situ graphene coating obtained after the second carbonization treatment is in the form of spherical particles.

[0075] In some embodiments, after the second carbonization treatment, the product is cooled in the furnace and then ground to obtain hard carbon coated with graphene in situ.

[0076] In another typical embodiment of this application, an in-situ graphene-coated hard carbon material is provided, which is prepared by any of the above-described methods for preparing in-situ graphene-coated hard carbon.

[0077] The in-situ graphene-coated hard carbon material prepared by the above method exhibits high intrinsic conductivity. This is achieved through the addition of a surfactant and a condensation reaction under shear conditions, resulting in the self-assembly of phenolic resin particles into uniform spherical structures. Furthermore, the first and second carbonization treatments effectively reduce the volume expansion rate during charge and discharge, improving electrode structural stability. When used to prepare negative electrodes in sodium-ion hybrid capacitors, it significantly improves the first-turn coulombic efficiency.

[0078] In some embodiments, the material includes a hard carbon core and a graphene coating layer located on at least a portion of the surface of the hard carbon core; at least a portion of the graphene in the graphene coating layer is covalently bonded to the hard carbon core. The covalent bond between the graphene coating layer and the hard carbon core enhances the interfacial bonding force, allowing the graphene coating layer to more tightly coat the surface of the hard carbon core, effectively passivating the active sites on the hard carbon surface and resulting in a more significant improvement in the first-turn coulombic efficiency of the sodium ion mixed capacitor. Simultaneously, the covalent bond provides electron channels, reducing the resistance of the hard carbon material with in-situ graphene coating, increasing the conductivity of the graphene-coated hard carbon material, and significantly improving the electron transport efficiency of the material.

[0079] According to another typical embodiment of this application, a negative electrode is provided, which comprises any of the above-described in-situ graphene-coated hard carbon materials.

[0080] The negative electrode of this application uses the aforementioned in-situ graphene-coated hard carbon material, which has good stability, is not prone to expansion, and has good conductivity, which is beneficial to improving the first-cycle coulombic efficiency of sodium-ion hybrid capacitors or secondary batteries containing it.

[0081] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer located on the surface of the negative electrode current collector, wherein the negative electrode active material layer contains the aforementioned hard carbon material in situ coated with graphene.

[0082] Understandably, the negative electrode active material layer may also contain binders (such as polyvinylidene fluoride) and conductive agents (such as conductive carbon black), which will not be described in detail here.

[0083] It should be noted that the above-mentioned negative electrode sheet can be used not only in sodium-ion hybrid capacitors, but also in secondary batteries, such as lithium-ion batteries.

[0084] According to another typical embodiment of this application, a sodium-ion hybrid capacitor is provided, which includes the above-described negative electrode.

[0085] The sodium-ion hybrid capacitor of this application has excellent initial coulombic efficiency, good stability, and long service life due to the use of the above-mentioned negative electrode, and has good prospects for large-scale application.

[0086] Understandably, a sodium-ion hybrid capacitor is a type of sodium-ion capacitor that includes a battery-type (Faraday process) electrode and a capacitor-type (non-Faraday process) electrode, combining the double-layer energy storage of a capacitor with the redox reaction energy storage of a battery.

[0087] In some embodiments, the sodium-ion mixed capacitor includes a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the negative electrode is the aforementioned negative electrode.

[0088] In some embodiments, the positive electrode of the sodium-ion hybrid capacitor includes a positive current collector and a positive active material layer located on the surface of the positive current collector. The positive active material of the positive electrode layer can be any one of layered oxide, polyanion, or Prussian blue / white.

[0089] Understandably, the electrolyte is the medium for ion transport, providing mobile sodium ions, and can be in liquid, solid, or quasi-solid state; the role of the diaphragm is to physically isolate the positive and negative electrodes to prevent short circuits, while allowing ions to pass freely. For example, the diaphragm material can be a polyolefin porous membrane or a cellulose membrane.

[0090] The following are specific examples.

[0091] Example 1

[0092] S1. Dissolve 3g of resorcinol and 2.4mL of formaldehyde aqueous solution (formaldehyde mass concentration is 35%) in 50mL of a mixture of water and ethanol with a volume ratio of 2:1, and stir to form a homogeneous solution. During the stirring process, first stir at a low speed of 300rpm for 10min to initially disperse the precursor, and then stir at a high speed of 600rpm for 20min to ensure complete dissolution.

[0093] S2. Preparation of spherical phenolic resin: Under a stirring speed of 1000 rpm, ammonia water was added dropwise to the above solution to adjust the pH to 9.0, and 0.15 g of PVA (weight average molecular weight 80000) surfactant was added at the same time. The polycondensation reaction was carried out in a water bath at 40°C for 45 min to obtain the spherical phenolic resin product system.

[0094] S3. Centrifugation and washing: Transfer the spherical phenolic resin product system to a centrifuge tube and centrifuge at 8000 rpm for 10 min to separate the precipitate. After discarding the supernatant, add a mixture of water and ethanol in a 1:1 volume ratio, and gently stir to redisperse the precipitate. Repeat the centrifugation-washing operation 3 times to remove unreacted monomers and impurities.

[0095] S4. Drying treatment: Place the washed spherical phenolic resin particles in a vacuum drying oven, set the temperature to 70℃, and dry for 16 hours to obtain dried spherical phenolic resin particles with a D50 particle size of 5μm.

[0096] S5. Low-temperature pre-carbonization: The dried precursor is spread evenly in a quartz boat in a tube furnace, the air is removed by evacuation and argon is introduced as a protective gas. Then, it is heated to 350°C at a heating rate of 5°C / min and held at this temperature for 1.5 hours to complete the low-temperature pre-carbonization, remove small molecule volatiles, and form a preliminary carbon skeleton.

[0097] S6. High-Temperature Carbonization and Graphene Coating: After pre-carbonization and natural cooling to room temperature, the system was evacuated again and purged with argon gas. The temperature was then increased to 1200℃ at a rate of 10℃ / min, while methane at a flow rate of 80 sccm was introduced as a gaseous carbon source. At this high temperature, the precursor further carbonized to form hard carbon, and the carbon atoms produced by the decomposition of methane grew in situ on the surface of the hard carbon to form a graphene layer. The coating process lasted for 2.5 hours.

[0098] S7. Cooling and Post-processing: After the reaction is complete, turn off the heating and carbon source, and continue to purge with argon gas to allow the sample to cool to room temperature with the furnace. After removing the sample, lightly grind it using an agate mortar to disperse the agglomerated particles, thus obtaining graphene-coated spherical hard carbon material. Its scanning electron microscope (SEM) image is shown below. Figure 1 As shown in Figure a on the left.

[0099] Example 2

[0100] The only difference from Example 1 is that in step S2, 0.15g of sodium dodecyl sulfate was used instead of 0.15g of PVA as the surfactant. In step S4, the D50 particle size of the obtained spherical phenolic resin particles was 5.2μm.

[0101] Example 3

[0102] The only difference from Example 1 is that in step S2, the amount of PVA surfactant used is 1g. In step S4, the D50 particle size of the obtained spherical phenolic resin particles is 3μm.

[0103] Example 4

[0104] The only difference from Example 1 is that the stirring rate in step S2 is 900 rpm. In step S4, the D50 particle size of the obtained spherical phenolic resin particles is 10 μm.

[0105] Example 5

[0106] The only difference from Example 1 is that the stirring rate in step S2 is 700 rpm. In step S4, the D50 particle size of the obtained spherical phenolic resin particles is 13 μm.

[0107] Example 6

[0108] The only difference from Example 1 is that in step S5, the temperature is kept at 350°C for 5 hours to complete the low-temperature pre-carbonization.

[0109] Comparative Example 1

[0110] The difference from Example 1 is that the stirring rate in step S2 is reduced to 200 rpm, and no PVA surfactant is added, resulting in irregular block-shaped phenolic resin particles.

[0111] The SEM image of the hard carbon material prepared in this comparative example is as follows: Figure 1 As shown in Figure b on the right.

[0112] Comparative Example 2

[0113] The difference from Example 1 is that the carbon source in step S6 is replaced with argon protective gas at a corresponding flow rate. All other conditions remain unchanged.

[0114] Comparative Example 3

[0115] The difference from Example 1 is that the carbonization temperature in S5 is changed to 1200°C, while other conditions remain the same.

[0116] Comparative Example 4

[0117] The difference from Example 1 is that step S5 is omitted, and the spherical phenolic resin particles prepared in step S4 are directly subjected to high-temperature carbonization and graphene coating in step S6. This step and subsequent steps are the same as in Example 1.

[0118] Comparative Example 5

[0119] The only difference from Example 1 is that the surfactant used in step S2 is hexadecyltrimethylammonium chloride, and the resulting phenolic resin particles are spherical with an uneven surface.

[0120] Comparative Example 6

[0121] The difference from Example 1 lies in steps S6 and S7, which are detailed below:

[0122] S6. After pre-carbonization is completed and the mixture is allowed to cool naturally to room temperature, a vacuum is drawn again and argon gas is introduced for purging. The temperature is then increased to 1200℃ at a rate of 10℃ / min and maintained at this high temperature for 2.5 hours.

[0123] S7. After the reaction is complete, turn off the heating, keep argon gas flowing through, and allow the sample to cool to room temperature with the furnace. After removing the sample, lightly grind it with an agate mortar to disperse the agglomerated particles and obtain hard carbon precursor particles. Take commercial graphene powder (Xianfeng Nano XF001W type graphene powder) and hard carbon precursor particles at a mass ratio of 0.1:1, mix them evenly in deionized water, and then freeze-dry to obtain non-in-situ coated hard carbon material.

[0124] Experimental group 1

[0125] The powder resistance of the materials prepared in the above embodiments and comparative examples was tested, and the results are shown in Table 1.

[0126] Powder resistivity test: The Suzhou Jinglü ST2742B powder resistivity tester was used for testing; 1g of the powder material of the example and comparative example was weighed and put into the measuring fixture. The fixture was put into the pre-vibration tester. After the pre-vibration tester was started and vibrated for 15s, the fixture was put back into the powder resistivity tester and the software was started to start the test. The powder resistivity test results are shown in Table 1.

[0127] Table 1

[0128]

[0129] As can be seen from the test results in Table 1 above, the graphene-coated hard carbon particles prepared in the examples are all spherical. In Comparative Example 1, no surfactant was added during the preparation of the phenolic resin, and neither the prepared phenolic resin particles nor the final hard carbon product formed a spherical structure. In Comparative Example 5, a cationic surfactant was added, and neither the prepared phenolic resin particles nor the final hard carbon product formed a spherical structure. On the other hand, in Comparative Example 2, no carbon source was introduced during the second carbonization treatment (i.e., step S6), and the prepared hard carbon did not form a graphene coating layer, resulting in a high powder resistivity.

[0130] Experimental group 2

[0131] The hard carbon materials prepared in the above examples and comparative examples were subjected to electrochemical performance tests according to the following methods.

[0132] A slurry was prepared by thoroughly mixing hard carbon material, conductive carbon black, and polyvinylidene fluoride in a 90:5:5 mass ratio, and then uniformly coated onto the surface of copper foil. After drying in a vacuum oven at 80°C for 12 hours, the electrode was cut into uniform circular pieces with a diameter of 10 mm to serve as the positive electrode. Each circular electrode contained approximately 0.8–1.2 mg·cm³ of [acid / substance not specified in the original text]. -2The active material components are as follows: The negative electrode uses metallic sodium sheets, and the electrolyte is a 1:1 volume mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) and a 1 mol / L sodium hexafluorophosphate (NaPF6). The cut electrodes are assembled into a CR2032 type button-type semi-hybrid capacitor in an argon-atmosphere glove box.

[0133] The samples were charged and discharged using a LANDCT2001B test system with a current of 100 mA / g, a discharge cutoff voltage of 0 V, and a charging cutoff voltage of 2 V. The initial coulombic efficiency and the capacity retention rate after 500 cycles were tested. The test results are shown in Table 2.

[0134] Table 2

[0135]

[0136] As can be seen from the test results in Table 2, compared with the comparative example, the in-situ graphene-coated hard carbon material prepared in the examples exhibits both superior initial coulombic efficiency and 500-cycle capacity retention. Specifically, in Example 3, compared to Example 1, the amount of surfactant was higher, resulting in a decrease in both the initial coulombic efficiency and 500-cycle capacity retention of the prepared in-situ graphene-coated hard carbon material. This indicates that the amount of surfactant affects the cycling performance of the prepared in-situ graphene-coated hard carbon, and an appropriate amount will further improve the cycling performance. In Example 5, compared to Example 1, the stirring speed in the polycondensation reaction of step S2 was lower, resulting in a decrease in both the initial coulombic efficiency and 500-cycle capacity retention of the prepared in-situ graphene-coated hard carbon material. In Example 6, the first carbonization treatment (i.e., the low-temperature pre-carbonization in step S5) was too long, also leading to a decrease in both the initial coulombic efficiency and 500-cycle capacity retention of the prepared in-situ graphene-coated hard carbon material. It is evident that the in-situ graphene-coated hard carbon material with a spherical structure prepared by the method of this application has good structural stability. As a negative electrode material, it can improve the first coulombic efficiency and cycle stability of the battery.

[0137] Experimental group 3

[0138] Super P conductive carbon black was used as the conductive agent, and a mixture of styrene-butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC) was used as the water-based binder. The ratio of SBR to CMC was 4:1. The solid content of the SBR used was 40%, and the solid content of the CMC used was 2.5%.

[0139] The active material (hard carbon composite material prepared according to each embodiment and comparative example), activated carbon, conductive agent and water-based binder were mixed in a mass ratio of 8:0.5:05:1. In a planetary mixer, deionized water was added dropwise while stirring until the material was uniformly mixed into a black paste. Then, stirring was continued for 15 minutes to obtain the negative electrode slurry. The negative electrode slurry was coated on a smooth and flat aluminum foil and placed in a vacuum drying oven to dry at 80°C for 12 hours to obtain the negative electrode sheet.

[0140] A positive electrode slurry was prepared by mixing NFPP (composite sodium iron pyrophosphate):SP (conductive carbon black):AC (activated carbon) in a ratio of 91%:4%:5% with solvent NMP. The positive electrode slurry was then coated onto aluminum foil to obtain the positive electrode sheet. This sodium ion mixed capacitor adopts a polyanionic system. A cellulose membrane was selected as the separator. The electrolyte was a 1:1 (volume ratio) mixture of ethylene carbonate and dimethyl carbonate as the solvent, and 1 mol / L sodium hexafluorophosphate was used as the solute. A 1.2Ah sodium ion mixed capacitor was prepared by a stacking process.

[0141] The samples were charged and discharged using the Blue Electric test system. The voltage range for charging and discharging was 1.5V-3.5V. The test results are shown in Table 3.

[0142] Table 3

[0143]

[0144] As can be seen from the test data in Table 3, when applied to sodium-ion hybrid capacitors, the negative electrode made of graphene-coated hard carbon material prepared in the examples exhibits high initial coulombic efficiency and 500-cycle capacity retention. Among them, the surfactant content and stirring speed during the preparation of phenolic resin in the preparation methods of Examples 1, 2, and 4 are more suitable, and the initial coulombic efficiency and 500-cycle capacity retention of the sodium-ion hybrid capacitors prepared are particularly excellent.

[0145] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0146] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims, and the specification and drawings can be used to interpret the content of the claims.

Claims

1. A method for producing a hard carbon coated with graphene in situ, characterized by, The steps include the following: The phenolic resin precursor solution is mixed with a surfactant, and a polycondensation reaction is carried out under shearing action, and the product of the polycondensation reaction is dried to prepare spherical phenolic resin particles; wherein the phenolic resin precursor solution includes a phenolic resin precursor and a solvent, and the surfactant is at least one of a non-ionic surfactant and an anionic surfactant; The spherical phenolic resin particles are subjected to a first carbonization treatment to obtain a hard carbon precursor; The hard carbon precursor is subjected to a second carbonization treatment in the presence of a carbon source to obtain hard carbon with in-situ coated graphene; wherein the temperature of the second carbonization treatment is higher than the temperature of the first carbonization treatment.

2. The method for preparing hard carbon with in-situ graphene coating as described in claim 1, characterized in that, The phenolic resin precursor includes one or more of a phenolic precursor and an aldehyde precursor; The phenolic resin precursor solution satisfies at least one of the following conditions: (1) The aldehyde precursor includes any one or more of formaldehyde, acetaldehyde, furfural, and trioxane; (2) The phenolic precursor includes any one or more of cresol, phenol, and m-dihydroxybenzene; (3) The mass percentage content of the phenolic resin precursor in the phenolic resin precursor solution is 10wt%-15wt%; (4) The molar ratio of the phenolic precursor to the aldehyde precursor is 1:(1-1.3); (5) The solvent is an alcohol-water mixed solvent, which is a mixture of ethanol and water in a volume ratio of (1-5):

1.

3. The method for preparing hard carbon with in-situ graphene coating as described in claim 1, characterized in that, The surfactant is any one or more of polyvinyl alcohol, sodium dodecyl sulfate, sodium tetradecyl sulfate, and sodium dodecylbenzenesulfonate; and / or, In the step of mixing the phenolic resin precursor solution with the surfactant, the mass ratio of the surfactant to the phenolic resin precursor is (0.005-0.05):1 based on the amount of the materials.

4. The method of claim 1-3, wherein, The step of mixing the phenolic resin precursor solution with the surfactant, and carrying out a polycondensation reaction under shearing action, and drying the product of the polycondensation reaction to obtain spherical phenolic resin particles satisfies at least one of the following conditions: (1) Under stirring conditions, an alkali solution is added to the phenolic resin precursor solution to make the pH value of the phenolic resin precursor solution 8.0-9.0; (2) Under stirring conditions, an alkali solution is added to the phenolic resin precursor solution, and the alkali solution is selected from any one or more of ammonia, sodium hydroxide solution, and sodium carbonate solution; (3) The temperature of the polycondensation reaction is 40°C-60°C; (4) The time of the polycondensation reaction is 40min-70min; (5) The polycondensation reaction is carried out under stirring at a rotation speed of 900rpm-1200rpm; (6) The drying is carried out under vacuum conditions, and the temperature of the drying is 60°C-80°C, and the time is 12h-18h; (7) The particle size D50 of the spherical phenolic resin particles is 3μm-30μm.

5. The method of claim 1-3, wherein the in-situ coated graphene hard carbon is prepared by the steps of: The step of subjecting the spherical phenolic resin particles to a first carbonization treatment to obtain a hard carbon precursor satisfies at least one of the following conditions: (1) The temperature of the first carbonization treatment is 300°C-450°C; (2) the first carbonization treatment is performed for 1 h-3 h; (3) before the first carbonization treatment, the spherical phenolic resin particles are heated to the temperature of the first carbonization treatment at a rate of 3 ℃ / min-5 ℃ / min; (4) the first carbonization treatment is performed in a first protective gas atmosphere, the first protective gas comprising any one or more of nitrogen, argon, helium, neon and krypton; (5) the hard carbon precursor is a spherical particle.

6. The method of claim 1-3, wherein the in-situ coated graphene hard carbon is prepared by the steps of: The second carbonization treatment is performed on the hard carbon precursor in the presence of a carbon source to obtain hard carbon with in-situ coated graphene, and the step satisfies at least one of the following conditions: (1) the temperature of the second carbonization treatment is 800 ℃-1100 ℃; (2) the second carbonization treatment is performed for 2 h-5 h; (3) before the second carbonization treatment, the hard carbon precursor is heated to the temperature of the second carbonization treatment at a rate of 5 ℃ / min-10 ℃ / min; (4) the carbon source comprises any one or more of methane, ethane, carbon dioxide, methanol, ethanol and acetone; (5) the carbon source comprises any one or more of methane, ethane, carbon dioxide, methanol, ethanol and acetone, and the carbon source is introduced in a gaseous manner at a flow rate of 20 sccm-200 sccm; (6) the second carbonization treatment is performed in a second protective gas atmosphere, the second protective gas comprising any one or more of nitrogen, argon, helium, neon and krypton; (7) the hard carbon with in-situ coated graphene obtained is a spherical particle.

7. A hard carbon material coated in situ with graphene, characterized in that, The method for preparing hard carbon with in-situ coated graphene is prepared by using any one of the methods according to claims 1-6.

8. The in-situ graphene-coated hard carbon material of claim 7, wherein, comprising a hard carbon core and a graphene coating layer located on at least part of the surface of the hard carbon core; at least part of the graphene in the graphene coating layer is covalently bonded to the hard carbon core.

9. A negative electrode sheet characterized by comprising: The hard carbon material with in-situ coated graphene comprises the hard carbon material with in-situ coated graphene according to claim 7 or 8.

10. A sodium-ion hybrid capacitor characterized by, The negative electrode sheet comprises the negative electrode sheet according to claim 9.

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