A nitrogen-doped carbon nanosphere anode, its preparation method and application

Abstract

This invention belongs to the field of battery anode material technology, and discloses a nitrogen-doped carbon nanosphere anode, its preparation method, and its application. It includes a current collector and an active layer coated on the surface of the current collector. The active layer comprises nitrogen-doped porous carbon nanospheres, a three-dimensional composite conductive network, and a binder. Carbon nanotubes are interspersed between the layers of graphene-like materials, forming one-dimensional bridging conductive paths between the nitrogen-doped porous carbon nanosphere particles, thus constructing a continuous and interconnected three-dimensional electron transport network. This invention constructs a three-dimensional conductive agent slurry using graphene and carbon nanotubes, forming a continuous electron transport network in the anode, solving the problem of discontinuous conductive paths in traditional point-like conductive agents. The three-dimensional conductive network enhances the contact stability between carbon nanosphere particles, improving the rate capability and long-cycle performance of the anode. This method is compatible with existing aqueous anode slurry preparation and coating processes, and has good potential for industrial application.

Description

Technical Field

[0001] This invention belongs to the field of battery anode material technology, specifically relating to a nitrogen-doped carbon nanosphere anode, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries, with their advantages of high operating voltage, high energy density, long cycle life, low self-discharge rate, and good environmental adaptability, have been widely used in consumer electronics, electric vehicles, smart grids, and large-scale energy storage. With the rapid development of new energy vehicles, fast-charging equipment, and high-power energy storage systems, lithium-ion batteries are placing higher demands on anode materials, especially requiring them to possess high reversible capacity, excellent rate performance, good cycle stability, and low manufacturing cost.

[0003] Currently, commercially available lithium-ion battery anode materials are mainly graphite-based carbon materials. Although graphite has a stable lithium intercalation platform and a relatively high initial coulombic efficiency, its theoretical specific capacity is only 372 mAh / g, and the lithium-ion diffusion kinetics are limited under high-rate charge and discharge, making it difficult to meet the development requirements of high-energy-density and fast-charging batteries. At the same time, during high-current charging, graphite anodes are prone to problems such as increased concentration polarization and increased risk of lithium plating, affecting battery safety and cycle life. Therefore, developing new high-capacity, high-rate, and structurally stable carbon-based anode materials is of great practical significance.

[0004] Phenolic resins have become an important precursor for the preparation of carbon anodes due to their advantages such as wide availability of raw materials, low cost, mild reaction conditions, and high carbon yield. However, existing phenolic resin-based carbon nanosphere anodes still have significant shortcomings: First, conventional phenolic resins have a limited structural control method, resulting in insufficient pore structure after carbonization and limited internal ion transport channels, which is not conducive to electrolyte wetting and rapid lithium-ion diffusion. Second, after carbonization, conventional phenolic resins are mainly composed of amorphous carbon or low-graphitized carbon, resulting in poor intrinsic electronic conductivity, increased polarization, and significant capacity decay under high-rate charge and discharge. Finally, the conductive pathways formed by point contacts between carbon particles are prone to loosening during electrode rolling and cycling, leading to insufficient stability of the internal electron transport network of the electrode.

[0005] To overcome the aforementioned problems, existing technologies typically add traditional particulate conductive agents such as acetylene black, conductive carbon black, and Super P to the electrode slurry. These conductive agents are mainly dispersed between the active materials through point-to-point contact. Although they can improve conductivity to some extent, they suffer from drawbacks such as easy agglomeration, short conductive paths, and a single contact mode, making it difficult to form a continuous and interconnected three-dimensional conductive network inside the electrode. For spherical carbon materials, traditional conductive agents cannot effectively bridge the gaps between carbon nanospheres, especially under high compaction density and long cycling conditions, where the problem of electrode conductivity degradation becomes more prominent.

[0006] Graphene, as a two-dimensional sheet carbon material, possesses excellent electron transport capabilities, high specific surface area, and good flexibility, enabling it to form surface contacts with active materials and construct two-dimensional electron transport channels. Carbon nanotubes, as one-dimensional conductive carbon materials, have high aspect ratios, high conductivity, and mechanical strength, enabling long-range bridging between active material particles. Combining these two materials can construct a multi-dimensional continuous three-dimensional conductive network, significantly improving the electron transport efficiency and structural stability of the electrode.

[0007] In summary, existing phenolic resin-based carbon anode materials still face technical bottlenecks in pore structure control, intrinsic conductivity, and conductive network construction, making it impossible to simultaneously achieve high capacity, high rate capability, and long cycling performance. To overcome these shortcomings, there is an urgent need to develop a carbon anode material and its preparation method based on three-dimensional conductive agent control, in order to improve the overall electrochemical performance of the electrode and meet the needs of large-scale applications. Summary of the Invention

[0008] The purpose of this invention is to provide a nitrogen-doped carbon nanosphere anode, its preparation method, and its applications, to address the problems of insufficient conductivity, underdeveloped pore structure, poor rate performance, and unstable conductive contact during cycling in existing phenolic resin-based carbon nanosphere anode materials. This invention achieves synergistic optimization of the carbon nanosphere material structure and the electrode conductive network by regulating the structure of the carbon nanosphere precursor and introducing a three-dimensional conductive agent constructed from graphene-like materials and carbon nanotubes into the electrode slurry.

[0009] In a first aspect, the present invention provides a method for preparing a nitrogen-doped carbon nanosphere anode, comprising the following steps: Preparation of phenolic resin nanosphere precursors using S1: m-Aminophenol was dissolved in a mixed solvent of water and ethanol, followed by the addition of polyvinylpyrrolidone and urea to ensure thorough dispersion. The mass ratio of polyvinylpyrrolidone to m-aminophenol was 0.05:1, and the mass ratio of urea to m-aminophenol was 0.025:1. Formaldehyde was then added, with a molar ratio of m-aminophenol to formaldehyde of 1:3.5, and 25 wt% ammonia solution was added dropwise. The mixture was stirred at 35°C for 24 h to form a nitrogen-containing phenolic resin nanosphere suspension. After the reaction was completed, the suspension was centrifuged, washed, and dried to obtain the phenolic resin nanosphere precursor. S2 preparation of nitrogen-doped carbon nanospheres: The obtained phenolic resin nanosphere precursor was placed in an inert atmosphere and first heated to 200℃ at a heating rate of 1℃ / min for low-temperature stabilization treatment and held for 2 h; then heated to 800℃ at a heating rate of 2℃ / min for high-temperature carbonization treatment and held for 2 h. After natural cooling, nitrogen-doped porous carbon nanospheres were obtained. S3 preparation of three-dimensional conductive agent slurry: Graphene oxide and carboxylated carbon nanotubes in a mass ratio of 1:1 were dispersed in deionized water, and sodium carboxymethyl cellulose was added, with the amount of sodium carboxymethyl cellulose being 5 wt% of the total mass of graphene oxide and carboxylated carbon nanotubes; then, the mixture was ultrasonically dispersed for 60 min and subjected to high-speed shearing for 30 min to obtain an aqueous three-dimensional conductive agent slurry with a solid content of 2.5 wt%. S4 preparation of nitrogen-doped carbon nanosphere anode sheets: Nitrogen-doped carbon nanospheres were premixed in a three-dimensional conductive agent slurry, followed by the addition of sodium carboxymethyl cellulose / styrene-butadiene rubber composite binder and deionized water. After thorough stirring and degassing, a uniformly dispersed nitrogen-doped carbon nanosphere anode slurry was obtained. This slurry was coated onto the surface of a copper foil current collector, and after drying, rolling and cutting, a nitrogen-doped carbon nanosphere anode sheet based on the three-dimensional conductive agent slurry was obtained. The mass ratio of the nitrogen-doped carbon nanospheres, the solid components in the three-dimensional conductive agent slurry, and the binder, based on dry weight, is 94:4:4.

[0010] Appropriate nitrogen doping can provide additional active sites while ensuring framework stability. If the nitrogen content is insufficient, the carbon nanospheres become "electron-insulating islands," and the three-dimensional conductive network only improves inter-particle transport but cannot solve the internal polarization of the particles; while if the nitrogen content exceeds the limit, it can easily lead to the deterioration of the carbon nanosphere framework.

[0011] The synergistic effect of m-aminophenol and urea as dual nitrogen sources: m-Aminophenol, as a phenolic monomer and an endogenous nitrogen source, contains both phenolic hydroxyl and amino groups in its molecular structure. It can undergo a condensation reaction with formaldehyde, directly embedding nitrogen atoms into the phenolic resin skeleton to form a highly cross-linked nitrogen-containing polymer network. This ensures that nitrogen atoms are evenly distributed in the carbon skeleton after carbonization, rather than being adsorbed only on the surface.

[0012] Urea, as an exogenous auxiliary nitrogen source and pore-forming agent, decomposes to produce gases (NH3, CO2, etc.) during low-temperature stabilization and carbonization, which on the one hand replenishes the nitrogen content, and on the other hand creates a rich mesoporous / microporous structure inside the carbon spheres.

[0013] Synergistic effect: If only m-aminophenol is used without urea, the nitrogen content and pore richness are insufficient; if only urea is used without m-aminophenol, nitrogen is difficult to stably embed into the carbon framework, and the spherical morphology is difficult to maintain. Only through the synergy of the two can carbon nanospheres with high nitrogen doping, high porosity, and uniform morphology be obtained.

[0014] Synergistic effect of polyvinylpyrrolidone (PVP) structure modifier and dual nitrogen source: PVP forms a micellar template in an ethanol / water mixed solvent, regulating the nucleation and growth of phenolic oligomers. The carbonyl and pyrrolidone groups of PVP can form hydrogen bonds or coordination interactions with the amino groups of m-aminophenol and the amide groups of urea, "anchoring" the nitrogen source at the micellar interface. This ensures that nitrogen is uniformly doped synchronously during the spherical growth process, rather than being locally enriched later. This guarantees the structural uniformity and electrochemical consistency of the carbonized carbon nanospheres.

[0015] Low-temperature stabilization and high-temperature carbonization require careful control of heating rates and temperatures. Heating rates and temperatures exceeding the above ranges will affect the conductivity of the final product, thus impacting its application in lithium-ion batteries.

[0016] The "step-by-step synergy" of low-temperature stabilization and high-temperature carbonization involves several steps. Low-temperature stabilization (increasing temperature by 1℃ / min to 200℃): At a lower temperature, the phenolic resin undergoes further cross-linking and curing, releasing small molecules (water, formaldehyde, urea decomposition products) to form a rigid framework. If this step is omitted or performed too quickly, the spheres will melt, collapse, or stick together at subsequent high temperatures, losing their porous spherical morphology. High-temperature carbonization (increasing temperature by 2℃ / min to 800℃): Deep carbonization is performed on the stabilized rigid framework, increasing carbon content and conductivity, and retaining nitrogen atoms in stable configurations (graphitic nitrogen, pyridine nitrogen).

[0017] Sodium carboxymethyl cellulose, graphene oxide, and carboxylated carbon nanotubes can be uniformly intercalated between graphene sheets by ultrasonic dispersion and high-speed shearing to obtain a graphene / carbon nanotube three-dimensional conductive agent slurry.

[0018] In some embodiments, sodium carboxymethyl cellulose, graphene oxide, and carboxylated carbon nanotubes can also be dispersed using other dispersion methods, such as ultrasonic dispersion or mechanical stirring combined with ultrasonic dispersion.

[0019] In some implementations, step S4 involves mixing for 4 hours using planetary stirring, high-speed shearing, or vacuum stirring.

[0020] In some embodiments, the degassing is performed under vacuum for 20 minutes.

[0021] In a second aspect, the present invention provides a nitrogen-doped carbon nanosphere anode, comprising a current collector and an active layer coated on the surface of the current collector, wherein the active layer comprises the following components: nitrogen-doped porous carbon nanospheres, a three-dimensional composite conductive network and a binder; The nitrogen-doped porous carbon nanospheres are prepared by steps S1 and S2 in the preparation method; In the three-dimensional composite conductive network, two-dimensional graphene-like material sheets are coated on the surface of the nitrogen-doped porous carbon nanospheres, and one-dimensional carbon nanotubes are interspersed between the graphene-like material sheets and form one-dimensional bridging conductive paths between the nitrogen-doped porous carbon nanosphere particles.

[0022] Through the above design, nitrogen source introduction, spherical morphology control and pore structure preconstruction can be realized simultaneously during the formation of phenolic resin nanospheres. In this three-dimensional conductive agent slurry, graphene sheets provide two-dimensional planar conductive channels, and carbon nanotubes are interspersed between graphene sheets and form one-dimensional bridging conductive paths between carbon nanosphere particles, thereby constructing a continuous and interconnected three-dimensional electron transport network.

[0023] Thirdly, the present invention provides an application of nitrogen-doped carbon nanosphere anode in lithium-ion batteries. The nitrogen-doped carbon nanosphere anode prepared by the above method can exhibit high initial capacity, excellent rate performance and stable cycle life.

[0024] The beneficial effects of this invention are: 1. The present invention provides a nitrogen-doped carbon nanosphere anode based on three-dimensional conductive agent slurry regulation and its preparation method. Nitrogen-containing functional monomers and structure regulators synergistically participate in phenolic condensation polymerization to obtain nitrogen-doped, porous, and structurally stable carbon nanospheres. A three-dimensional conductive agent slurry is constructed using graphene-like materials and carbon nanotubes, forming a continuous electron transport network in the anode. The surface contact of graphene-like materials and the linear bridging effect of carbon nanotubes improve the problem of discontinuous conductive paths in traditional point-like conductive agents. The three-dimensional conductive network enhances the contact stability between carbon nanosphere particles, improving the rate performance and long-cycle performance of the anode. This method is compatible with existing aqueous anode slurry preparation and coating processes and has good potential for industrial application.

[0025] 2. The application of the nitrogen-doped carbon nanosphere anode based on three-dimensional conductive agent slurry regulation provided by this invention in lithium-ion batteries shows that even at a high current density of 5.0 A / g, the electrode can still maintain a high reversible capacity. At the same time, the electrode can still maintain a relatively complete structure and stable electrochemical activity after experiencing high-rate charge and discharge. It has good rate response capability and capacity recovery characteristics, and can exhibit high initial capacity, excellent rate performance and stable cycle life.

[0026] 3. Nitrogen-doped carbon nanospheres and three-dimensional conductive slurry exhibit complementary and synergistic effects in both lithium storage activity and electron transport, thus resolving the contradiction of "high capacity and high rate capability being mutually exclusive." If relying solely on the electron conduction of the nitrogen-doped nanospheres, severe ohmic and concentration polarization will occur inside the electrode at high rates (e.g., 5 A / g), preventing the full utilization of active sites. The three-dimensional conductive slurry (graphene oxide + carboxylated carbon nanotubes) of this invention precisely compensates for this shortcoming: graphene sheets coat the carbon nanospheres through surface contact, providing a low-impedance two-dimensional electron collection interface; carbon nanotubes form one-dimensional long-range bridges between the nanospheres, reducing interparticle contact resistance. With the synergistic effect of both, a reversible capacity of 364 mAh / g can still be maintained after 1000 cycles at an extreme rate of 5 A / g, with a retention rate consistently above 93%. Attached Figure Description

[0027] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained from these drawings without creative effort.

[0028] Figure 1 This is an X-ray diffraction pattern of the nitrogen-doped carbon nanospheres prepared in Example 1 of this invention.

[0029] Figure 2 This is a field emission scanning electron microscope image of the nitrogen-doped carbon nanospheres prepared in Example 1 of this invention.

[0030] Figure 3 This is the full X-ray photoelectron spectrum of the nitrogen-doped carbon nanospheres prepared in Example 1 of this invention.

[0031] Figure 4 This is a scanning electron microscope image of the surface of the carbon negative electrode sheet prepared in Example 1 of the present invention.

[0032] Figure 5 This is a constant current charge-discharge curve of the nitrogen-doped carbon nanosphere negative electrode prepared in Example 1 of the present invention.

[0033] Figure 6 This is a graph showing the electrochemical rate performance of the nitrogen-doped carbon nanosphere anode prepared in Example 1 of this invention.

[0034] Figure 7 This is a graph showing the electrochemical cycling performance of nitrogen-doped carbon nanospheres and pure carbon nanospheres prepared in Example 1 of this invention.

[0035] Figure 8 This is a graph showing the electrochemical cycling performance of nitrogen-doped carbon nanospheres prepared in Example 1 of this invention in different conductive agent slurries. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention.

[0037] The following explanations are provided for some of the terms or nouns that appear in the description of this invention: In this invention, "room temperature" refers to ambient temperature, ranging from approximately 10°C to approximately 40°C. In some embodiments, "room temperature" refers to a temperature ranging from approximately 20°C to approximately 30°C; in other embodiments, "room temperature" refers to a temperature ranging from approximately 25°C to approximately 30°C; and in still other embodiments, "room temperature" refers to 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, 40°C, etc.

[0038] This invention utilizes nitrogen-containing functional monomers and structure regulators in the synergistic participation of phenolic polymerization to obtain nitrogen-doped, porous, and structurally stable nitrogen-doped carbon nanospheres. Aqueous three-dimensional conductive agent slurry is constructed using graphene-like materials and carbon nanotubes, forming a continuous electron transport network in the negative electrode. The surface contact of graphene and the linear bridging effect of carbon nanotubes improve the discontinuous conductive path of traditional point-like conductive agents. Finally, the three-dimensional conductive network enhances the contact stability between carbon nanosphere particles, improving the rate performance and long-cycle performance of the negative electrode. Through these interconnected steps, a nitrogen-doped carbon nanosphere negative electrode sheet exhibiting high initial capacity, excellent rate performance, and stable cycle life is obtained and applied to lithium-ion batteries.

[0039] Example 1 Preparation of S1 phenolic resin nanosphere precursor: 4.0 g of m-aminophenol was weighed and added to a mixed solvent consisting of 80 mL of deionized water and 40 mL of anhydrous ethanol. The mixture was magnetically stirred for 30 min at room temperature to ensure complete dissolution of the m-aminophenol and the formation of a homogeneous solution. Subsequently, 0.2 g of polyvinylpyrrolidone was added to the solution to regulate the nucleation, growth, and dispersion stability of the phenolic resin nanospheres. Stirring was continued for another 30 min to ensure complete dispersion of the polyvinylpyrrolidone. To further improve the nitrogen doping degree and the number of defect sites in the carbon nanospheres, 0.1 g of urea was added, followed by 9.6 mL of a 37 wt% formaldehyde solution. Stirring was continued for another 30 min to ensure thorough mixing of the formaldehyde and m-aminophenol. Then, 1.0 mL of 25 wt% ammonia was slowly added dropwise as an alkaline catalyst. The reaction was continued at 35 °C with stirring for 24 h to induce a condensation reaction between the m-aminophenol and formaldehyde, gradually forming nitrogen-containing phenolic resin nanospheres with uniform particle size. Finally, the obtained brownish-red suspension was centrifuged, and the washed product was dried in a vacuum drying oven for 12 h to obtain a well-dispersed phenolic resin nanosphere precursor.

[0040] S2 stabilization and high-temperature carbonization treatment: The dried phenolic resin nanosphere precursor was placed in a tube furnace and subjected to low-temperature stabilization treatment under an inert atmosphere. The stabilization treatment involved heating to 200°C at a rate of 1°C / min under a nitrogen atmosphere and holding at that temperature for 2 hours. This low-temperature stabilization treatment promoted further cross-linking and curing of the phenolic resin nanospheres, improved their thermal stability, and reduced spherical structure collapse, particle fusion, or breakage during subsequent carbonization. Subsequently, the stabilized phenolic resin nanospheres were further subjected to high-temperature carbonization treatment in the same tube furnace. The temperature was increased to 800°C at a rate of 2°C / min under a nitrogen atmosphere, held at that temperature for 2 hours, and then naturally cooled to room temperature to obtain nitrogen-doped carbon nanospheres.

[0041] Preparation of S3 three-dimensional conductive agent slurry: Graphene oxide and carboxylated carbon nanotubes in a mass ratio of 1:1 were weighed and added to deionized water. Sodium carboxymethyl cellulose (5 wt% of the mass of graphene oxide and carboxylated carbon nanotubes) was added to the system as a dispersant. After ultrasonic dispersion for 1 h, the mixture was sheared at high speed for 30 min to obtain a three-dimensional conductive agent slurry with a solid content of 2.5%.

[0042] In this three-dimensional conductive agent slurry, graphene sheets provide two-dimensional planar conductive channels, and carbon nanotubes form one-dimensional bridging conductive paths. Together, they construct a continuous three-dimensional conductive network.

[0043] Preparation of S4 nitrogen-doped carbon nanosphere anode sheet: 94 g of nitrogen-doped carbon nanospheres were weighed and added to the three-dimensional conductive agent slurry containing 4 g of solid components (total weight of graphene and carbon nanotubes) for premixing to ensure the three-dimensional conductive agent fully coats or bridges the carbon nanosphere particles. Then, 4 g of sodium carboxymethyl cellulose / styrene-butadiene rubber composite binder was added, and an appropriate amount of deionized water was added to adjust to a suitable viscosity (i.e., no significant particle sedimentation in the flowing state) and the required solid content. The mixture was stirred for 4 h using planetary stirring, high-speed shearing, or vacuum stirring, followed by vacuum degassing for 20 min to obtain a uniformly dispersed nitrogen-doped carbon nanosphere anode slurry with suitable rheological properties. Finally, the obtained nitrogen-doped carbon nanosphere anode slurry was uniformly coated onto the surface of a copper foil current collector, with the coating thickness adjusted according to the target areal density. The dried electrode sheet was then rolled to ensure a tight bond between the electrode active layer and the copper foil current collector and to increase the electrode sheet compaction density. It was then cut to the required size to obtain a nitrogen-doped carbon nanosphere anode sheet controlled by the three-dimensional conductive agent slurry.

[0044] The comparative example was prepared according to the preparation method of Example 1: Table 1 Preparation of comparative examples

[0045] 1. Morphology and phase-free analysis were performed on the nitrogen-doped carbon nanosphere anode sheet obtained in Example 1 above, which was controlled by a three-dimensional conductive agent slurry: Figure 1 This is the X-ray diffraction (XRD) spectrum of nitrogen-doped carbon nanospheres from Example 1. As can be seen from the figure, the sample exhibits a relatively broad diffraction peak in the range of approximately 22°-25° at 2θ, corresponding to the (002) crystal plane of the carbon material; and a weaker and broader diffraction peak in the range of approximately 42°-45° at 2θ, corresponding to the (101) crystal plane of the carbon material. These two characteristic peaks indicate that after high-temperature carbonization, the phenolic resin nanospheres have been successfully transformed into carbon materials, forming a carbon framework with a certain short-range ordered structure. No obvious impurity crystalline phase diffraction peaks were observed in the figure, indicating that after washing, drying, and carbonization, no obvious crystalline impurities remained in the sample, and the resulting product has high carbon phase purity. Furthermore, the (002) peak position is shifted to a smaller angle (approximately 26.5°) compared to the (002) peak position of ideal graphite, indicating that the interlayer spacing of the carbon nanospheres is larger than that of graphite. This may be due to the nitrogen element and structure regulator introduced during the carbonization process. A larger carbon interlayer spacing is beneficial for the insertion / extraction of lithium ions between carbon layers, and can also shorten the lithium ion diffusion path, thereby improving the lithium storage activity of the material.

[0046] Figure 2This is a field emission scanning electron microscope (FESEM) image of the nitrogen-doped carbon nanospheres prepared in Example 1 of this invention. As can be seen from the image, the obtained carbon material exhibits a regular spherical or near-spherical morphology with clear particle boundaries and a relatively smooth surface. This indicates that the spherical structure of the phenolic resin nanosphere precursor can be well maintained after polycondensation and carbonization treatment using m-aminophenol and formaldehyde as raw materials. The particle size distribution of each carbon nanosphere is relatively uniform, with a particle size of approximately 200-300 nm.

[0047] Figure 3 This is the full X-ray photoelectron spectroscopy (XPS) spectrum of the nitrogen-doped carbon nanospheres prepared in Example 1 of this invention. As shown in the figure, the sample exhibits distinct C 1s, N 1s, and O 1s characteristic peaks at approximately 284.6 eV, 399.1 eV, and 531.7 eV, respectively, indicating that the material is mainly composed of carbon, nitrogen, and oxygen. Besides these elements, no other significant impurity element characteristic peaks were detected in the spectrum, indicating that the obtained material has high purity and that nitrogen has been successfully doped into the carbon material framework. Nitrogen doping can modulate the electronic structure of carbon materials, improve their electronic conductivity, and introduce more defect sites and lithium storage active centers into the carbon framework, thereby enhancing the material's ability to adsorb and store lithium ions.

[0048] Figure 4 This is a field emission scanning electron microscope (FESEM) image of the carbon anode sheet prepared in Example 1 of this invention. As can be seen from the image, the carbon nanospheres maintain a relatively intact spherical morphology on the electrode surface, indicating good structural stability during the slurry preparation, coating, drying, and rolling processes. Simultaneously, flocculent, network, or sheet-like conductive structures are clearly observed between the carbon nanosphere particles. These structures mainly originate from the graphene / carbon nanotube composite three-dimensional conductive agent slurry. The carbon nanotubes are interwoven in a one-dimensional fibrous manner, while the graphene sheets are attached to or spread on the particle surface and in the interparticle spaces, together forming a continuous three-dimensional conductive network. This structure helps to improve the problem of traditional particulate conductive agents relying solely on point contact for conductivity, enabling rapid electron transport between carbon nanospheres, graphene, and carbon nanotubes.

[0049] 2. Performance testing process: The obtained carbon anode sheet was assembled into a lithium-ion coin cell in a glove box. The working electrode was the nitrogen-doped carbon nanosphere anode sheet prepared in the example, the lithium metal sheet was used as the counter electrode, the separator was a polypropylene membrane, and the electrolyte was 1 mol / L LiPF6 ethylene carbonate / diethyl carbonate.

[0050] Figure 5This is a galvanostatic charge-discharge curve of the nitrogen-doped carbon nanosphere anode in Example 1 at a current density of 0.1 A / g. As shown in the figure, the initial discharge specific capacity reaches 1230 mAh / g, and the initial charge specific capacity is 884 mAh / g, exhibiting a high initial lithium storage capacity. Some irreversible capacity loss occurs during the first charge-discharge cycle, mainly related to the reduction and decomposition of the electrolyte on the electrode surface, the formation of a solid electrolyte interfacial film, and the irreversible reactions involving defect sites and nitrogen-containing functional groups on the carbon material surface. After the first activation cycle, the charge-discharge curves of the second and third cycles largely overlap, indicating that the electrode interface gradually stabilizes, and the lithium-ion insertion / extraction process has good reversibility. This also shows that the carbon nanosphere anode can maintain a relatively stable structure in the early stages of cycling.

[0051] Figure 6 The figure shows the rate performance curves of the nitrogen-doped carbon nanosphere anode in Example 1. As the current density gradually increased from 0.1 A / g to 0.2, 0.5, 1.0, 2.0, and 5.0 A / g, the reversible capacity of the electrode decreased in a stepwise manner, with average charge specific capacities of approximately 829, 677, 579, 505, 433, and 371 mAh / g, respectively. Even at the high current density of 5.0 A / g, the electrode maintained a high reversible capacity, indicating that the material has fast charge transport capability and good lithium-ion diffusion kinetics. When the current density returned to 0.1 A / g, the reversible specific capacity rebounded to 724 mAh / g, indicating that the electrode maintained a relatively intact structure and stable electrochemical activity after high-rate charge-discharge, exhibiting good rate response and capacity recovery characteristics. This is mainly attributed to the nanoscale structure of the carbon nanosphere material and the effective construction of electron transport pathways by the three-dimensional conductive agent network.

[0052] Figure 7This is a comparison of the cycling performance of the nitrogen-doped carbon nanosphere anode and the pure carbon nanosphere anode in Example 1 at a current density of 0.2 A / g. It can be seen that the nitrogen-doped carbon nanosphere electrode initially has a reversible capacity of 678 mAh / g, which decreases slightly in the initial cycling stage and then gradually stabilizes. After 300 cycles, it still maintains a reversible capacity of approximately 591 mAh / g, with a capacity retention rate as high as 87.1%, demonstrating excellent long-term cycling stability. Simultaneously, the coulombic efficiency remains close to 100% throughout the cycling process, indicating fewer side reactions during repeated charge-discharge cycles, a stable electrode / electrolyte interface, and high reversibility of lithium-ion insertion and extraction. In contrast, the pure carbon nanosphere electrode without nitrogen doping exhibits significant capacity decay, with its discharge capacity gradually decreasing from approximately 529 mAh / g initially to approximately 280 mAh / g after 300 cycles, with a capacity retention rate of only about 52.9%. The results show that pure carbon nanospheres lack structural stability and lithium storage activity during long-term cycling, leading to the gradual failure of active sites and a decrease in charge transport capacity, thus causing continuous capacity decay. The capacity of the nitrogen-doped carbon nanosphere electrode after 300 cycles is approximately 2.1 times that of the pure carbon nanosphere electrode, fully demonstrating that nitrogen doping significantly promotes the improvement of lithium storage performance. On the one hand, the introduction of nitrogen atoms can modulate the electronic structure of carbon materials and improve their intrinsic electronic conductivity; on the other hand, nitrogen doping can introduce more defect sites and lithium storage active centers into the carbon framework, increasing the adsorption and storage capacity of lithium ions.

[0053] Figure 8 The cycling performance of the nitrogen-doped carbon nanosphere anode prepared in Example 1 at a high current density of 5 A / g was compared using different conductive slurry systems. While maintaining consistent active material, binder dosage, and battery testing conditions, the electrochemical performance differences between the three-dimensional conductive slurry of this invention and different conductive agent combinations were compared. Figure 8It can be seen that the electrode prepared using the three-dimensional conductive slurry of this invention exhibits the best cycling stability and capacity retention. After 1000 cycles, the capacity decay is minimal, and the electrode still maintains a reversible capacity of approximately 364 mAh / g, indicating that it can construct a stable and continuous electron transport network under high-rate conditions. In contrast, Comparative Example 1, which uses conventional conductive carbon black Super P as the conductive agent, shows the most significant cycling capacity decay, with a capacity of only about 207 mAh / g after 1000 cycles, and a capacity retention rate significantly lower than that of the system of this invention. This indicates that the conductive network formed by a single zero-dimensional carbon black particle is limited and cannot meet the requirements of rapid electron transport during high-rate cycling. Comparative Example 2 uses carboxylated carbon nanotubes instead of graphene to construct a conductive system, with a capacity of approximately 197 mAh / g after 1000 cycles; Comparative Example 3 uses graphene oxide instead of carbon nanotubes, with a capacity of approximately 254 mAh / g after 1000 cycles. Both exhibit superior performance compared to the traditional Super P system, but are still significantly lower than the three-dimensional conductive paste system of this invention, indicating that it is difficult to simultaneously achieve electron transport efficiency, structural stability, and electrode integrity using only one-dimensional or two-dimensional conductive materials.

[0054] Based on the above data, it can be concluded that there is a synergistic effect between nitrogen-doped carbon nanospheres and three-dimensional conductive paste: Figure 7 At a current density of 0.2 A / g, the nitrogen-doped carbon nanosphere electrode of this invention exhibits an initial reversible capacity of 678 mAh / g, which remains at 591 mAh / g after 300 cycles, representing a capacity retention of 87.1%. In contrast, the pure carbon nanosphere electrode without nitrogen doping has an initial capacity of only 529 mAh / g, which drops sharply to 280 mAh / g after 300 cycles, with a retention rate of only 52.9%. This demonstrates that nitrogen doping significantly improves lithium storage capacity and structural stability.

[0055] Figure 8 After 1000 cycles at a high rate of 5 A / g, the electrode using the three-dimensional conductive paste (graphene oxide + carboxylated carbon nanotubes) of this invention still maintains a reversible capacity of 364 mAh / g; while the capacity of pure graphene oxide (Comparative Example 3) is 254 mAh / g, pure carboxylated carbon nanotubes (Comparative Example 2) is only 197 mAh / g, and the traditional Super P (Comparative Example 1) is 207 mAh / g.

[0056] Figure 4 Field emission scanning electron microscopy revealed that carbon nanotubes are interspersed between graphene sheets and attached / spread on the surface and interstices of carbon nanospheres, forming a continuous three-dimensional conductive network.

[0057] Nitrogen-doped porous carbon nanospheres introduce defect sites and lithium storage active centers through nitrogen atoms. Figure 3 XPS confirmed successful N doping, and with the help of porous structure and expanded carbon interlayer spacing ( Figure 1 XRD shortens the lithium-ion diffusion path, imparting high reversible capacity to the electrode. Figure 7 (678 mAh / g). However, nitrogen doping itself can disrupt the conjugated structure of the carbon framework to some extent, leading to a decrease in the intrinsic electronic conductivity of the material. If relying solely on the electron conduction of the nitrogen-doped spheres, severe ohmic and concentration polarization will occur inside the electrode at high rates (e.g., 5 A / g), and the active sites cannot be fully utilized. The three-dimensional conductive agent slurry (graphene oxide + carboxylated carbon nanotubes) of this invention precisely overcomes this shortcoming.

[0058] The three-dimensional conductive network successfully translates the high lithium storage activity of nitrogen-doped carbon spheres into high capacity under high-rate long-cycle conditions. Figure 8 (364 mAh / g after 1000 cycles). This effect is not a simple summation—if pure GO or pure CNT were used alone, the capacity would only reach 254 mAh / g and 197 mAh / g respectively, both far lower than the system of this invention; the traditional point-contact conductive agent Super P is only 207 mAh / g. This shows that only a three-dimensional network can match the charge transport requirements of nitrogen-doped spheres at high rates, achieving a balance between "high capacity" and "high rate".

[0059] In summary, this nitrogen-doped carbon nanosphere anode exhibits high initial capacity, excellent rate performance, and stable cycle life. Its superior electrochemical performance can be attributed to several factors: first, the nitrogen-doped carbon nanospheres possess abundant defect sites and active lithium storage sites, which enhance the material's specific capacity; second, the nanosphere structure helps shorten the lithium-ion diffusion path and alleviate structural stress during cycling; and third, the graphene / carbon nanotube three-dimensional conductive network enhances the electron transport capability within the electrode and improves the contact stability between active particles, thereby improving the material's electrochemical performance under high-rate and long-cycle conditions.

[0060] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the invention by those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the invention should be included within the scope of protection of the invention.

Claims

1. A method for preparing a nitrogen-doped carbon nanosphere anode, characterized in that, Includes the following steps: Preparation of phenolic resin nanosphere precursors using S1: m-Aminophenol was dissolved in a mixed solvent of water and ethanol, followed by the addition of polyvinylpyrrolidone and urea to ensure thorough dispersion. The mass ratio of polyvinylpyrrolidone to m-aminophenol was 0.05:1, and the mass ratio of urea to m-aminophenol was 0.025:

1. Formaldehyde was then added, with a molar ratio of m-aminophenol to formaldehyde of 1:3.5, and 25 wt% ammonia solution was added dropwise. The mixture was stirred at 35°C for 24 h to form a nitrogen-containing phenolic resin nanosphere suspension. After the reaction was completed, the suspension was centrifuged, washed, and dried to obtain the phenolic resin nanosphere precursor. S2 preparation of nitrogen-doped carbon nanospheres: The obtained phenolic resin nanosphere precursor was placed in an inert atmosphere and first heated to 200℃ at a heating rate of 1℃ / min for low-temperature stabilization treatment and held for 2 h; then heated to 800℃ at a heating rate of 2℃ / min for high-temperature carbonization treatment and held for 2 h. After natural cooling, nitrogen-doped porous carbon nanospheres were obtained. S3 preparation of three-dimensional conductive agent slurry: Graphene oxide and carboxylated carbon nanotubes in a mass ratio of 1:1 were dispersed in deionized water, and sodium carboxymethyl cellulose was added, with the amount of sodium carboxymethyl cellulose being 5 wt% of the total mass of graphene oxide and carboxylated carbon nanotubes; then, the mixture was ultrasonically dispersed for 60 min and subjected to high-speed shearing for 30 min to obtain an aqueous three-dimensional conductive agent slurry with a solid content of 2.5 wt%. S4 preparation of nitrogen-doped carbon nanosphere anode sheets: Nitrogen-doped carbon nanospheres were premixed in a three-dimensional conductive agent slurry, followed by the addition of sodium carboxymethyl cellulose / styrene-butadiene rubber composite binder and deionized water. After thorough stirring and degassing, a uniformly dispersed nitrogen-doped carbon nanosphere anode slurry was obtained. This slurry was coated onto the surface of a copper foil current collector, and after drying, rolling and cutting, a nitrogen-doped carbon nanosphere anode sheet based on the three-dimensional conductive agent slurry was obtained. The mass ratio of the nitrogen-doped carbon nanospheres, the solid components in the three-dimensional conductive agent slurry, and the binder, based on dry weight, is 94:4:

4.

2. The method for preparing nitrogen-doped carbon nanosphere anodes as described in claim 1, characterized in that, In step S4, the mixture is stirred for 4 hours using planetary stirring, high-speed shearing, or vacuum stirring.

3. The method for preparing nitrogen-doped carbon nanosphere anodes as described in claim 1, characterized in that, The degassing process was performed under vacuum for 20 minutes.

4. A nitrogen-doped carbon nanosphere anode, comprising a current collector and an active layer coated on the surface of the current collector, characterized in that, The active layer comprises the following components: nitrogen-doped porous carbon nanospheres, a three-dimensional composite conductive network, and a binder; The nitrogen-doped porous carbon nanospheres are prepared by steps S1 and S2 of the preparation method described in claim 1; In the three-dimensional composite conductive network, two-dimensional graphene-like material sheets are coated on the surface of the nitrogen-doped porous carbon nanospheres, and one-dimensional carbon nanotubes are interspersed between the graphene-like material sheets and form one-dimensional bridging conductive paths between the nitrogen-doped porous carbon nanosphere particles.

5. The application of a nitrogen-doped carbon nanosphere anode in lithium-ion batteries, characterized in that, The negative electrode of the lithium-ion battery comprises the nitrogen-doped carbon nanosphere negative electrode as described in claim 4.

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