Double-layer coated graphite negative electrode material, preparation method thereof and lithium ion battery

Abstract

The invention discloses a double-layer coated graphite negative electrode material, a preparation method thereof and a lithium ion battery. The graphite negative electrode material comprises an inner core and a coating layer, the inner core is graphite, and the coating layer is sequentially provided with a soft carbon coating layer and an organic polymer coating layer. The preparation method comprises the following steps: firstly, dynamically coating the surface of graphite to form a soft carbon coating layer, then growing on the soft carbon coating layer by an in-situ polymerization method to form an organic polymer coating layer, and finally performing high-temperature carbonization treatment to obtain the double-layer coated graphite negative electrode material. The preparation method solves the problems of non-uniform coating of single-layer soft carbon, poor conductivity and insufficient lithium ion embedding channels. The material provided by the invention has high specific capacity and excellent rate capability, is suitable for a lithium ion battery negative electrode, and is especially suitable for a high-power charge-discharge scene.

Description

Technical Field

[0001] This invention relates to a double-layer coated graphite anode material, its preparation method, and a lithium-ion battery, belonging to the field of battery material technology. Background Technology

[0002] Graphite anode materials have long been widely used as the preferred anode material for commercial lithium-ion batteries due to their high theoretical specific capacity, excellent cycle stability, wide availability of raw materials, relatively low cost, and significant advantages of being environmentally friendly, safe, and non-toxic. However, as electric vehicles, large-scale energy storage systems, and other new energy applications place increasingly stringent demands on battery energy density and power density, the intrinsic structural defects of conventional artificial graphite materials are gradually becoming a technological bottleneck. Although its highly ordered crystal structure is conducive to the stable storage of lithium ions, its inherently narrow interlayer spacing severely limits the solid-phase diffusion rate of lithium ions within the graphite lattice. This results in lithium ions being unable to be inserted or extracted in time under high-rate charge and discharge conditions, leading to severe electrode polarization. This not only reduces voltage efficiency and usable capacity, but more importantly, it causes the precipitation of metallic lithium on the surface of graphite particles, especially at edges and defects. The precipitated dendrites may pierce the separator, causing internal short circuits in the battery and posing serious safety hazards. At the same time, the uneven insertion / extraction of lithium ions during repeated charge and discharge processes also induces volume expansion and contraction of graphite particles. Long-term accumulation leads to electrode structure damage, separation of active material and current collector, and consequently, accelerated capacity decay and shortened battery life. To overcome these bottlenecks, the industry commonly uses soft carbon materials to modify the surface of graphite, as soft carbon has a larger interlayer spacing and higher structural disorder, which theoretically can broaden lithium-ion transport channels and buffer volume changes. However, existing single-layer soft carbon coating technology has significant limitations: First, traditional solid-state mixing or static heat treatment processes are difficult to achieve complete, uniform, and dense coating on the complex graphite particle surface, leaving many uncoated or excessively thin areas, which remain the starting point for side reactions and lithium plating; second, the electronic conductivity of soft carbon materials themselves is usually not ideal, limiting the rapid charge transport within the electrode; third, a single soft carbon coating layer fails to construct gradient channels conducive to rapid ion migration at the microscopic level, and its effect on repairing active sites on the graphite surface is limited, resulting in the sacrifice of too much reversible capacity to improve rate performance. This leads to a trade-off between capacity and rate performance, making it impossible to achieve both high capacity and high rate performance simultaneously, and fundamentally failing to meet the application requirements of high capacity and high rate. Therefore, developing a novel composite coating structure that can achieve uniform sealing, possess excellent electron-ion conductivity, and synergistically construct efficient ion embedding channels with the core has become a key technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this invention provides a double-layer coated graphite anode material, comprising a core and a coating layer, wherein the core is graphite, and the coating layer is sequentially composed of a soft carbon coating layer and an organic polymer coating layer.

[0004] Preferably, the soft carbon coating layer includes a soft carbon coating agent, which is one of the asphalts with a softening point of less than 200°C and a carbon residue ratio of 0.5% to 1.5%; the core is artificial graphite.

[0005] Preferably, the organic polymer coating layer includes an organic polymer coating agent, which includes one of polyaniline, polypyrrole, and polydopamine, and its mass accounts for 0.5% to 1.5% of the total mass of the graphite and soft carbon coating layers.

[0006] Preferably, the interlayer spacing D002 of the soft carbon coating layer is 0.35 nm to 0.36 nm, and the interlayer spacing D002 of the organic polymer coating layer is 0.37 nm to 0.38 nm, thereby forming a gradually increasing spacing channel from the inside to the outside of the graphite core; and the organic polymer coating layer is a nitrogen-doped carbon framework structure.

[0007] Preferably, the gradient-spacing channel can guide lithium ions to be embedded in the inner layer in an orderly and rapid manner.

[0008] Preferably, the nitrogen-doped carbon framework structure can provide additional electrochemical capacity through the pseudocapacitive effect.

[0009] This invention also provides a method for preparing the above-mentioned double-layer coated graphite anode material, comprising the following steps:

[0010] (A) Soft carbon coating: Graphite and pitch are mixed and a soft carbon coating layer is formed on the graphite surface through a dynamic coating process to obtain graphite@C material;

[0011] (B) Monomer solution preparation: Prepare monomer solutions under the corresponding pH conditions;

[0012] (C) In-situ polymerization coating: The graphite@C material obtained in step A is mixed with the monomer solution prepared in step B, and an oxidant is added to carry out an in-situ polymerization reaction to form an organic polymer coating layer on the surface of the soft carbon coating layer, thereby obtaining graphite@C@polymer material;

[0013] (D) High-temperature carbonization: The graphite@C@polymer material obtained in step C is subjected to high-temperature treatment under an inert atmosphere to carbonize the organic polymer coating layer, and finally the graphite@C@polymer-C double-layer coated graphite anode material is obtained.

[0014] Preferably, in step A, the residual carbon ratio of the asphalt is 0.5% to 1.5%; the mixing is carried out in a VC mixing device, the mixing speed is 300 r / min to 500 r / min, and the mixing time is 30 min to 50 min.

[0015] Preferably, in step A, the dynamic coating process is a vertical reactor coating process, wherein the reactor body rotation speed is 25Hz to 35Hz, the temperature is 550℃ to 650℃, and the heat preservation time is 3h to 5h before cooling to room temperature; the asphalt is a low softening point asphalt, that is, one type of asphalt with a softening point of less than 200℃.

[0016] Preferably, in step B, the monomer solution has a mass concentration of 0.5% to 1.5%; the monomer is one of aniline, pyrrole, and dopamine hydrochloride.

[0017] When the monomer is aniline, it needs to be prepared in a 1.5 mol / L hydrochloric acid solution, wherein the hydrochloric acid solution is prepared by diluting concentrated hydrochloric acid with a mass fraction of 36% with distilled water;

[0018] When the monomer is pyrrole, it needs to be prepared in a 0.1 mol / L hydrochloric acid solution, wherein the hydrochloric acid solution is prepared by diluting concentrated hydrochloric acid with a mass fraction of 36% with distilled water;

[0019] When the monomer is dopamine hydrochloride, it needs to be prepared in a Tris buffer solution at pH 8.5, wherein the PBS buffer solution at pH 8.5 is prepared by mixing 1 mol / L Tris alkaline solution with 36% concentrated hydrochloric acid.

[0020] Preferably, in step B, the preparation of the monomer solution and the in-situ polymerization reaction temperature conditions in step C are as follows:

[0021] When the monomer is aniline, the in-situ polymerization reaction needs to be carried out at a temperature of 0°C to 5°C for 6 hours. The oxidant is ammonium persulfate, which is pre-dissolved in hydrochloric acid to prepare an oxidant solution. The mass concentration of ammonium persulfate in the oxidant solution is 1.25% to 3.75%.

[0022] When the monomer is pyrrole, the in-situ polymerization reaction needs to be carried out at a temperature of 0°C to 5°C for 24 hours. The oxidant is ferric chloride, which is pre-dissolved in hydrochloric acid solution to prepare an oxidant solution with a mass concentration of 1% to 3%.

[0023] When the monomer is dopamine hydrochloride, the in-situ polymerization reaction needs to be carried out at a temperature of 20°C to 25°C for 24 hours, and the oxidant is oxygen in the air.

[0024] Preferably, in step C, the volume ratio of the monomer solution to the oxidant solution is 1:1.

[0025] Preferably, in step C, the mass ratio of the graphite@C material to the target polymer is 100:0.5 to 100:1.5.

[0026] Preferably, in step C, after the reaction is completed, the obtained graphite@C@polymer material is post-treated, including: filtration to remove the solution, washing with methanol, ethanol and deionized water alternately until the filtrate is colorless and neutral, and then drying in a vacuum drying oven at 55°C to 65°C for 24 hours.

[0027] Preferably, in step D, the high-temperature treatment is carried out under an inert atmosphere of nitrogen or argon, the heat treatment temperature is 1100°C to 1200°C, the heat treatment time is 3 to 5 hours, and then it is cooled to room temperature.

[0028] Preferably, in step C, the pH and temperature of the reaction system are controlled to ensure that the polymer generated by polymerization is in an intermediate oxidation state.

[0029] The present invention also provides a lithium-ion battery, wherein the above-mentioned double-layer coated graphite anode material or the double-layer coated graphite anode material prepared by the above-mentioned preparation method is used.

[0030] Preferably, the lithium-ion battery is a coin cell.

[0031] The beneficial effects of this invention are:

[0032] This invention achieves multiple synergistic effects by constructing a bilayer coating structure with a specific interlayer spacing gradient and chemical composition, solving the industry challenge of simultaneously achieving high capacity and high rate performance. Firstly, in terms of structural design, through precise process control, a unique combination of a soft carbon coating layer (D002) of 0.35 nm to 0.36 nm and an organic polymer coating layer (D002) of 0.37 nm to 0.38 nm is formed. This gradually decreasing interlayer spacing channel from the outer layer to the graphite core provides unprecedented kinetic advantages for lithium-ion intercalation: the wider interlayer spacing of the outer layer acts as a high-speed entry point, significantly reducing the initial intercalation resistance of ions and guiding them rapidly into the coating layer; subsequently, the gradually narrowing channel spacing orderly guides ions to the graphite core, effectively avoiding the risk of electrode polarization and lithium plating caused by local ion congestion, thereby greatly improving the rate and orderliness of ion intercalation. This is the fundamental reason for achieving superior fast-charging performance. Secondly, regarding the integrity of the coating, the organic polymer, with its excellent in-situ polymerization film-forming ability, can effectively fill the microscopic defects and discontinuous regions that may exist in the first layer of soft carbon coating, achieving a more uniform and dense secondary encapsulation of graphite particles. This repair effect not only further suppresses side reactions and volume expansion, but also ensures the continuity and effectiveness of the aforementioned gradient channels. Thirdly, in terms of electrochemical performance, the nitrogen-doped carbon framework formed after polymer carbonization brings dual benefits: firstly, the introduction of nitrogen significantly improves the intrinsic electronic conductivity of the coating layer and reduces charge transfer impedance, building a high-speed highway for electron transport; secondly, this doped structure can generate pseudocapacitance through reversible surface redox reactions. The capacity contribution of this non-Radidatic process provides the battery with additional, rapid, and reversible capacity without excessively relying on lithium-ion intercalation between graphite layers. This allows the material of this invention to achieve a further increase in total capacity while maintaining the high specific capacity of graphite. Finally, in terms of interface characteristics, this composite coating layer also optimizes the wettability with the electrolyte. In summary, this invention is not a simple superposition of features, but rather a systematic innovation at the levels of material structure, conductivity, and interface chemistry achieved through the synergistic effect of multiple mechanisms such as gradient spacing channels, defect repair mechanisms, and nitrogen-doped pseudocapacitance. Ultimately, it achieves the outstanding effect of significantly improving 1.5C rate performance while maintaining high specific capacitance. Attached Figure Description

[0033] Figure 1 SEM image of the graphite@C material prepared in Example 1;

[0034] Figure 2 SEM image of the graphite@C@polymer material prepared in Example 1;

[0035] Figure 3 This is a comparison chart of direct charging tests of battery rates for Example 1, Comparative Example 1, and Comparative Example 2.

[0036] Figure 4 Comparison chart of AC impedance tests of batteries in Example 1, Comparative Example 1, and Comparative Example 2;

[0037] Figure 5 The graph shows a comparison of the conductivity tests of materials in Example 1, Comparative Example 1, and Comparative Example 2. Detailed Implementation

[0038] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those familiar to those skilled in the art. Furthermore, any methods and materials similar to or equivalent to those described herein may be applied to this invention. The preferred embodiments and materials described herein are for illustrative purposes only and do not limit the scope of this application.

[0040] Unless otherwise specified, the experimental methods used in the following examples are conventional methods, and the experimental materials used in the following examples are all purchased from commercial channels.

[0041] Example 1: Preparation of a novel soft carbon in-situ polymerized bilayer composite coated graphite anode material

[0042] Step A: Add 20 kg of graphite and 0.4 kg of pitch (pitch with a coking value of 50%, a softening point of 150℃, and a carbon residue ratio of about 1%) to a VC mixer and mix at 400 rpm for 40 minutes to obtain a homogeneous mixture. Put the mixture into a vertical reactor, set the speed to 30 Hz, heat to 600℃ and keep it at that temperature for 4 hours, and then cool to room temperature to obtain graphite@C material.

[0043] Step B: Take 127.77 mL of 36% concentrated hydrochloric acid and slowly add it to 872.23 mL of distilled water. Stir to prepare a 1.5 mol / L hydrochloric acid solution. Take 500 mL of this hydrochloric acid solution and add 4.89 mL of aniline. Stir continuously at 0°C in an ice-water bath to obtain a 1% aniline hydrochloric acid solution.

[0044] Step C: Take another 500 mL of 1.5 mol / L hydrochloric acid solution and add 12.5 g of ammonium persulfate. After dissolving, an ammonium persulfate hydrochloric acid solution is obtained. At 0℃, 420 g of graphite@C is added to the aniline hydrochloric acid solution. After dispersing evenly, the ammonium persulfate hydrochloric acid solution (volume ratio 1:1) is slowly sprayed on. After the addition is complete, the reaction is continuously stirred for 6 hours. After the reaction, the solution is removed by filtration. The product is washed 3 times with ethanol and then washed with water until the filtrate is colorless and neutral. The solid product is placed in a vacuum drying oven and dried at 60℃ for 24 hours to obtain the graphite@C@polymer material.

[0045] Step D: The graphite@C@polymer material is treated at 1150℃ for 4 hours under nitrogen protection and then cooled to room temperature to obtain a graphite@C@polymer-C bilayer composite coating material.

[0046] Example 2: Preparation of a novel soft carbon in-situ polymerized bilayer composite coated graphite anode material

[0047] Step A: Add 15 kg of graphite and 0.125 kg of pitch (pitch with a coking value of 60%, a softening point of 130℃, and a carbon residue ratio of about 0.5%) to a VC mixer and mix at 350 rpm for 30 minutes to obtain a homogeneous mixture. Put the mixture into a vertical reactor, set the speed to 25 Hz, heat to 550℃ and hold at that temperature for 3 hours, and cool to room temperature to obtain graphite@C material.

[0048] Step B: Take 9.67 mL of 36% concentrated hydrochloric acid and slowly add it to 990.33 mL of distilled water. Stir to prepare a 0.1 mol / L hydrochloric acid solution. Take 500 mL of this hydrochloric acid solution and add 2.5 g of pyrrole. Stir continuously in an ice-water bath at 3°C ​​to obtain a 0.5% pyrrole hydrochloric acid solution.

[0049] Step C: Take another 500 mL of 0.1 mol / L hydrochloric acid solution and add 5 g of ferric chloride. After dissolving, a ferric chloride hydrochloric acid solution is obtained. At 0℃, 425 g of graphite@C is added to the pyrrole hydrochloric acid solution. After dispersing evenly, the ferric chloride hydrochloric acid solution (volume ratio 1:1) is slowly sprayed on. After the addition is complete, the reaction is continuously stirred for 24 hours. After the reaction, the solution is removed by filtration. The product is washed three times with methanol and then washed with water until the filtrate is colorless and neutral. The solid product is placed in a vacuum drying oven and dried at 55℃ for 24 hours to obtain the graphite@C@polymer material.

[0050] Step D: The graphite@C@polymer material is treated at 1100℃ for 3 hours under argon protection and then cooled to room temperature to obtain a graphite@C@polymer-C bilayer composite coating material.

[0051] Example 3: Preparation of a novel soft carbon in-situ polymerized bilayer composite coated graphite anode material

[0052] Step A: Add 25 kg of graphite and 0.60 kg of pitch (62% coking value, 200℃ softening point pitch, and carbon residue ratio of approximately 1.5%) to a VC mixer and mix at 450 rpm for 50 minutes to obtain a homogeneous mixture. Transfer the mixture to a vertical reactor, set the rotation speed to 35 Hz, heat to 650℃ and hold at that temperature for 5 hours, then cool to room temperature to obtain graphite@C material.

[0053] Step B: Take 15.14 g of Tris base and slowly add it to 1000 mL of distilled water. Stir to prepare a 1 mol / L Tris base solution. Adjust the pH to 8.5 using 36% hydrochloric acid solution to obtain a Tris buffer solution. Take 1000 mL of this buffer solution and add 15 g of dopamine hydrochloride. Stir continuously in an ice-water bath at 25°C to obtain a 1.5% dopamine hydrochloride solution.

[0054] Step C: Add 850g of graphite@C to an aniline hydrochloric acid solution at 25℃ and stir continuously for 24 hours. After the reaction, filter to remove the solution, wash three times with ethanol and wash with water until the filtrate is colorless and neutral, and place the solid product in a vacuum drying oven at 65℃ for 24 hours to obtain graphite@C@polymer material.

[0055] Step D: The graphite@C@polymer material is treated at 1200℃ for 5 hours under nitrogen protection and then cooled to room temperature to obtain a graphite@C@polymer-C bilayer composite coating material.

[0056] Comparative Example 1: Preparation of a Graphite Anode Material

[0057] 20 kg of graphite and 0.80 kg of pitch (50% coking value, 150℃ softening point, and approximately 2% carbon residue) were added to a VC mixer and mixed at 400 rpm for 40 minutes to obtain a homogeneous mixture. This mixture was then transferred to a vertical reactor, heated to 600℃ at 30 Hz and held at that temperature for 4 hours. After cooling to room temperature, graphite@C material was obtained. This material was used as the negative electrode active material and mixed with conductive agent acetylene black and binder PVDF at a mass ratio of 90:5:5. NMP solvent was added, and the mixture was ground to form a slurry. This slurry was coated onto copper foil, vacuum dried at 120℃ for 12 hours, and then cut into discs as negative electrode sheets. Lithium metal was used as the counter electrode, Celgard 2400 as the separator, and 1 mol / L... A three-component LiPF6 electrolyte solution was prepared by mixing EC:DMC:EMC in a ratio of 1:1:1 (v / v). CR2032 coin cells were assembled in an argon-protected glove box. Cell testing was conducted at 25°C. The cells were first discharged at 0.02C for 300 minutes, then discharged at 0.05C to 0.005V, allowed to stand for 10 minutes, discharged at 0.05mA to 0.005V, allowed to stand for 5 minutes, discharged at 0.01mA to 0.005V, and charged at 0.1C to 2V. The specific capacity and the ratio of current capacity to specific capacity at a 1.5C rate were measured.

[0058] Comparative Example 2: Preparation of a Graphite Anode Material

[0059] Step A: Add 20 kg of graphite and 0.4 kg of pitch (50% coking value, 150℃ softening point pitch, and approximately 1% carbon residue) to a VC mixer and mix at 400 rpm for 40 minutes to obtain a homogeneous mixture. Transfer the mixture to a vertical reactor, set the rotation speed to 30 Hz, heat to 600℃ and hold for 4 hours, then cool to room temperature to obtain graphite@C material.

[0060] Step B: Take 127.77 mL of 36% concentrated hydrochloric acid and slowly add it to 872.23 mL of distilled water. Stir to prepare a 1.5 mol / L hydrochloric acid solution. Take 500 mL of this hydrochloric acid solution and add 4.89 mL of aniline. Stir continuously at room temperature (25°C) to obtain a 1% aniline hydrochloric acid solution.

[0061] Step C: Take another 500 mL of 1.5 mol / L hydrochloric acid solution and add 12.5 g of ammonium persulfate. After dissolving, a 0.1% ammonium persulfate hydrochloric acid solution is obtained. At room temperature (25℃), 420 g of graphite@C is added to the aniline hydrochloric acid solution. After dispersing evenly, the ammonium persulfate hydrochloric acid solution (volume ratio 1:1) is slowly sprayed on. After the addition is complete, the reaction is continuously stirred for 6 hours. After the reaction, the solution is removed by filtration. The product is washed three times with ethanol and then washed with water until the filtrate is colorless and neutral. The solid product is placed in a vacuum drying oven and dried at 60℃ for 24 hours to obtain the graphite@C@polymer material.

[0062] Step D: The graphite@C@polymer material is treated at 1150℃ for 4 hours under nitrogen protection and then cooled to room temperature to obtain graphite@C@polymer-C material. This material is used as the negative electrode active material and mixed with conductive agent acetylene black and binder PVDF at a mass ratio of 90:5:5. NMP solvent is added, and the mixture is ground to form a slurry. This slurry is then coated onto copper foil, vacuum dried at 120℃ for 12 hours, and cut into discs to serve as the negative electrode sheet. Lithium metal is used as the counter electrode. Using a 2400 diaphragm, and a CR2032 coin cell half-cell was assembled in an argon-protected glove box using a 1 mol / L LiPF6 three-component mixed solvent with an EC:DMC:EMC ratio of 1:1:1 (v / v) as the electrolyte. The cell was tested at 25°C. The cell was first discharged at 0.02C for 300 minutes, then discharged at 0.05C to 0.005V, allowed to stand for 10 minutes, discharged at 0.05mA to 0.005V, allowed to stand for 5 minutes, discharged at 0.01mA to 0.005V, and charged at 0.1C to 2V. The specific capacity and the ratio of the current capacity to the specific capacity at a 1.5C rate were measured.

[0063] Comparative Example 3: Preparation of a Graphite Anode Material

[0064] Step A: Add 20 kg of graphite and 0.4 kg of pitch (50% coking value, 150℃ softening point pitch, and approximately 1% carbon residue) to a VC mixer and mix at 400 rpm for 40 minutes to obtain a homogeneous mixture. Transfer the mixture to a vertical reactor, set the rotation speed to 30 Hz, heat to 600℃ and hold for 4 hours, then cool to room temperature to obtain graphite@C material.

[0065] Step B: Take 127.77 mL of 36% concentrated hydrochloric acid and slowly add it to 872.23 mL of distilled water. Stir to prepare a 1.5 mol / L hydrochloric acid solution. Take 500 mL of this hydrochloric acid solution and add 4.89 mL of aniline. Stir continuously at 0°C in an ice-water bath to obtain a 1% aniline hydrochloric acid solution.

[0066] Step C: Take another 500 mL of 1.5 mol / L hydrochloric acid solution and add 12.5 g of ammonium persulfate. After dissolving, an ammonium persulfate hydrochloric acid solution is obtained. At 0℃, 420 g of graphite@C is added to the aniline hydrochloric acid solution. After dispersing evenly, the ammonium persulfate hydrochloric acid solution (volume ratio 1:1) is slowly sprayed on. After the addition is complete, the reaction is continuously stirred for 6 hours. After the reaction, the solution is removed by filtration. The product is washed 3 times with ethanol and then washed with water until the filtrate is colorless and neutral. The solid product is placed in a vacuum drying oven and dried at 60℃ for 24 hours to obtain the graphite@C@polymer material.

[0067] Step D is omitted; graphite@C@polymer material is used directly.

[0068] Using this material as the negative electrode active material, it was mixed with conductive agent acetylene black and binder PVDF at a mass ratio of 90:5:5, NMP solvent was added, and the mixture was ground to form a slurry. This slurry was then coated onto copper foil, vacuum dried at 120°C for 12 hours, and cut into discs to serve as the negative electrode. Lithium metal was used as the counter electrode. Using a 2400 diaphragm, and a CR2032 coin cell half-cell was assembled in an argon-protected glove box using a 1 mol / L LiPF6 three-component mixed solvent with an EC:DMC:EMC ratio of 1:1:1 (v / v) as the electrolyte. The cell was tested at 25°C. The cell was first discharged at 0.02C for 300 minutes, then discharged at 0.05C to 0.005V, allowed to stand for 10 minutes, discharged at 0.05mA to 0.005V, allowed to stand for 5 minutes, discharged at 0.01mA to 0.005V, and charged at 0.1C to 2V. The specific capacity and the ratio of the current capacity to the specific capacity at a 1.5C rate were measured.

[0069] As described in the comparative examples, the materials obtained in all the above embodiments were tested using the following method: The obtained material was used as the negative electrode active material, mixed with conductive agent acetylene black and binder polyvinylidene fluoride at a mass ratio of 90:5:5, N-methylpyrrolidone was added as a solvent, and the mixture was ground to form a slurry. This slurry was then coated onto a copper foil current collector, vacuum dried at 120°C for 12 hours, and cut into round sheets as the negative electrode sheet; lithium metal was used as the counter electrode, Celgard... Using a 2400 diaphragm and an electrolyte solution of 1 mol / L LiPF6 three-component mixed solvent mixed at EC:DMC:EMC = 1:1:1 (v / v), CR2032 coin cells were assembled in an argon-protected glove box. Battery testing was conducted in a 25℃ constant-temperature chamber using a battery testing system. First, the cells were discharged at a constant current rate of 0.02C for 300 minutes, then discharged at a rate of 0.05C to 0.005V, allowed to stand for 10 minutes, then discharged at a constant current rate of 0.05mA to 0.005V, allowed to stand for 5 minutes, then discharged at a constant current rate of 0.01mA to 0.005V, and finally charged at a rate of 0.1C to 2V. The discharge capacity was recorded, and the specific capacity was calculated. Then, the cells were discharged at a rate of 1.5C to 0.005V, the discharge capacity was recorded, and the ratio of the discharge capacity to the specific capacity was calculated. All test results are recorded in Table 1.

[0070] Table 1 Test Results

[0071] An analysis of comprehensive test data from Examples 1-3 and Comparative Examples 1-3 of this invention reveals the advantages of the bilayer composite coating structure based on a specific interlayer spacing gradient and nitrogen doping chemistry. Firstly, the ratio of current capacity to specific capacity at 1.5C rate in Comparative Example 1 demonstrates the fundamental deficiency of single-layer soft carbon coating in high-rate fast charging scenarios. Although the specific capacity of Comparative Example 1 is similar to that of the embodiments of this invention, proving that its basic lithium intercalation capability is retained, this precisely highlights its core problem: severely insufficient ion transport kinetics. Under high-pressure charging and discharging conditions, lithium ions cannot effectively and rapidly complete the insertion / extraction process within the electrode solid phase within a limited time. This directly confirms the problems pointed out in the background art, such as slow ion diffusion rate, easy electrode polarization, and lithium plating caused by the inherent narrow interlayer spacing of graphite and the limitations of single soft carbon coating. All embodiments of this invention achieve a leap in rate performance, with an improvement of over 70% compared to Comparative Example 1. This qualitative leap is rooted in the unique gradient-spacing channel structure of this invention. The interlayer spacing D002 of the soft carbon coating layer in this invention is 0.35 nm to 0.36 nm, while the interlayer spacing D002 of the outer organic polymer coating layer is further expanded to 0.37 nm to 0.38 nm. This precisely controllable interlayer spacing gradient from the outer layer of the material to the graphite core creates a high-speed channel with decreasing resistance for lithium-ion intercalation. The wider polymer-C interlayer spacing of the outer layer acts as an efficient ion collection and pre-distribution region, significantly reducing the energy barrier for initial ion intercalation; ions are then orderly guided to the graphite core with the smallest interlayer spacing for storage via the soft carbon layer with intermediate spacing. This structural design fundamentally optimizes the ion transport path and dynamics within the electrode particles, which is the core physical mechanism for the breakthrough improvement in rate performance.

[0072] Most importantly, this remarkable rate performance improvement did not come at the expense of energy density. The specific capacity of the examples remained consistently high, ranging from 357.8 mAh / g to 359.5 mAh / g, even slightly higher than Comparative Example 1. This demonstrates the contribution of the nitrogen-doped pseudocapacitive effect in the present invention. The polymer-C layer formed after polymer carbonization is a nitrogen-doped carbon framework. The introduction of nitrogen not only significantly improves the electronic conductivity of the coating layer, but also provides additional pseudocapacitive capacity through a reversible surface redox reaction. This non-Radidatic process contributes capacity rapidly and stably, providing a beneficial increment to the overall capacity without over-reliance on or blocking the bulk lithium intercalation of graphite, thus achieving both extremely high rate performance and excellent specific capacity.

[0073] Furthermore, Comparative Example 2 was carried out under non-optimized polymerization conditions. Although a bilayer coating was macroscopically formed, its specific capacity and rate performance were inferior to those of the Example. This confirms that strict control of the pH and temperature of the polymerization reaction is crucial to obtain polyaniline with intermediate oxidation state and high intrinsic conductivity. Non-optimal conditions resulted in poor conductivity of the coating layer and a significant increase in interfacial impedance, failing to effectively utilize the electron transport network. Comparative Example 3, due to the omission of the crucial high-temperature carbonization step, showed better rate performance than the single-layer soft carbon but was far inferior to the fully treated Example. This indicates that the uncarbonized polymer cannot form a stable nitrogen-doped carbon framework and an ideal wide interlayer spacing structure. Its electronic conductivity and structural stability are insufficient to synergistically construct an efficient ion-electron dual continuous transport network with the soft carbon layer.

[0074] In summary, this invention achieves a bilayer composite coating structure that combines the advantages of gradient spacing channels, uniform and tight packaging, and nitrogen-doped pseudocapacitance through a series of indispensable process steps: forming a soft carbon bottom layer with a specific interlayer spacing by dynamic coating of liquid-phase asphalt, in-situ polymerization of monomers with precise control of their oxidation state, and high-temperature carbonization to obtain a nitrogen-doped wide-interlayer-spacing polymer-C outer layer. At the same time, it solves the long-standing problems in the field of ion transport kinetics, interface stability, and capacity-rate contradiction, and successfully overcomes the core bottleneck of high-capacity graphite anode materials in high-rate application scenarios.

[0075] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

[0076] The present invention and its embodiments have been described above. This description is not restrictive, and the accompanying drawings are only one embodiment of the present invention. The actual application is not limited to this. In conclusion, if those skilled in the art are inspired by this description and design similar methods and embodiments without departing from the spirit of the present invention, they should all fall within the protection scope of the present invention.

Claims

1. A double-layer coated graphite anode material, characterized in that, The graphite anode material includes a core and a coating layer, wherein the core is graphite, and the coating layer is provided with a soft carbon coating layer and an organic polymer coating layer in sequence.

2. The double-layer coated graphite anode material according to claim 1, characterized in that, The soft carbon coating layer includes a soft carbon coating agent, which is one of the asphalts with a softening point of less than 200°C and a carbon residue ratio of 0.5% to 1.5%; the core is artificial graphite.

3. The double-layer coated graphite anode material according to claim 1, characterized in that, The organic polymer coating layer includes an organic polymer coating agent, which includes one of polyaniline, polypyrrole, and polydopamine, and its mass accounts for 0.5% to 1.5% of the total mass of the graphite and soft carbon coating layers.

4. The double-layer coated graphite anode material according to claim 1, characterized in that, The interlayer spacing D002 of the soft carbon coating layer is 0.35 nm to 0.36 nm, and the interlayer spacing D002 of the organic polymer coating layer is 0.37 nm to 0.38 nm.

5. A method for preparing the double-layer coated graphite anode material according to any one of claims 1-4, characterized in that, Includes the following steps: (A) Soft carbon coating: Graphite and pitch are mixed and a soft carbon coating layer is formed on the graphite surface through a dynamic coating process to obtain graphite@C material; (B) Monomer solution preparation: Prepare monomer solutions under the corresponding pH conditions; (C) In-situ polymerization coating: The graphite@C material obtained in step A is mixed with the monomer solution prepared in step B, and an oxidant is added to carry out an in-situ polymerization reaction to form an organic polymer coating layer on the surface of the soft carbon coating layer, thereby obtaining graphite@C@polymer material; (D) High-temperature carbonization: The graphite@C@polymer material obtained in step C is subjected to high-temperature treatment in an inert atmosphere to carbonize the organic polymer coating layer, and finally the graphite@C@polymer-C double-layer coated graphite anode material is obtained.

6. The method for preparing the double-layer coated graphite anode material according to claim 5, characterized in that, In step A, the carbon residue ratio of the asphalt is 0.5% to 1.5%; the mixing is carried out in a VC mixing equipment, with a mixing speed of 300 r / min to 500 r / min and a mixing time of 30 min to 50 min.

7. The method for preparing the double-layer coated graphite anode material according to claim 5, characterized in that, In step A, the dynamic coating process is a vertical reactor coating process. The reactor body rotation speed is 25Hz to 35Hz, the temperature is 550℃ to 650℃, and the holding time is 3h to 5h before cooling to room temperature. The asphalt is one type of asphalt with a softening point of less than 200℃.

8. The method for preparing the double-layer coated graphite anode material according to claim 5, characterized in that, In step B, the monomer solution has a mass concentration of 0.5% to 1.5%; the monomer is one of aniline, pyrrole, and dopamine hydrochloride. When the monomer is aniline, it needs to be prepared in a 1.5 mol / L hydrochloric acid solution, wherein the hydrochloric acid solution is prepared by diluting concentrated hydrochloric acid with a mass fraction of 36% with distilled water; When the monomer is pyrrole, it needs to be prepared in a 0.1 mol / L hydrochloric acid solution, wherein the hydrochloric acid solution is prepared by diluting concentrated hydrochloric acid with a mass fraction of 36% with distilled water; When the monomer is dopamine hydrochloride, it needs to be prepared in a Tris buffer solution at pH 8.5, wherein the PBS buffer solution at pH 8.5 is prepared by mixing 1 mol / L Tris alkaline solution with 36% concentrated hydrochloric acid.

9. The method for preparing the double-layer coated graphite anode material according to claim 5, characterized in that, In step B, the preparation of the monomer solution and the temperature conditions for the in-situ polymerization reaction in step C are as follows: When the monomer is aniline, the in-situ polymerization reaction needs to be carried out at a temperature of 0°C to 5°C for a reaction time of 6 hours, wherein the oxidant is ammonium persulfate; When the monomer is pyrrole, the in-situ polymerization reaction needs to be carried out at a temperature of 0°C to 5°C for 24 hours, and the oxidant is ferric chloride. When the monomer is dopamine hydrochloride, the in-situ polymerization reaction needs to be carried out at a temperature of 20°C to 25°C for 24 hours, and the oxidant is oxygen in the air.

10. A lithium-ion battery, characterized in that, The double-layer coated graphite anode material includes any one of claims 1-4 or the double-layer coated graphite anode material prepared by the preparation method according to any one of claims 5-9.

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