A fast-charging phosphorus-doped hard carbon-coated graphite negative electrode material and its preparation method and application

Abstract

The present invention provides a fast-charging phosphorus-doped hard carbon-coated graphite negative electrode material, and a preparation method and application thereof. The preparation method of the fast-charging graphite negative electrode material comprises the following steps: 1) placing microcrystalline graphite powder in a dispersion containing polyacrylonitrile, ultrasonicating, and obtaining a mixed solution; 2) heating and stirring the mixed solution to obtain polyacrylonitrile-coated microcrystalline graphite powder; 3) mixing the polyacrylonitrile-coated microcrystalline graphite powder with a phytic acid solution, performing a hydrothermal reaction, filtering, and freeze-drying to obtain phosphorus-doped polyacrylonitrile-coated microcrystalline graphite powder; 4) heat-treating the phosphorus-doped polyacrylonitrile-coated microcrystalline graphite powder to obtain a fast-charging graphite negative electrode material. The present invention constructs a stable and uniform phosphorus-doped hard carbon coating layer on the surface of microcrystalline graphite through a simple liquid phase coating, hydrothermal, and high-temperature carbonization method, thereby improving the ion and electron transmission performance and having important application prospects in fast-charging batteries.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to a fast-charging phosphorus-doped hard carbon-coated graphite negative electrode material, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium-ion batteries are rechargeable batteries that store energy through reversible lithium ion intercalation. Compared to other commercially available rechargeable batteries, lithium-ion batteries offer higher specific energy, higher energy density, and longer cycle life. However, a significant disadvantage of electric vehicles compared to gasoline vehicles is the time required to charge them. The "fast charging anxiety" and high-power energy storage requirements of electric vehicles place higher demands on the charging speed of lithium-ion battery systems.

[0003] Graphite anodes dominate the lithium-ion battery market due to their three lower charge-discharge platforms (0.01-0.2V), good cycle stability, and low cost. The charging rate and low-temperature performance of lithium-ion batteries are mainly limited by the poor lithiation-delithiation kinetics of graphite anodes. At high current densities, the slow reaction kinetics of graphite anodes lead to electrode polarization, reducing their operating potential to 0V vs Li / Li. + As a result, undesirable lithium deposition occurs on the negative electrode surface, leading to battery capacity degradation and safety issues.

[0004] Lithium intercalation into graphite electrodes mainly involves four steps: (1) diffusion of solvated lithium ions in the electrolyte; (2) desolvation of solvated lithium ions at the solid electrolyte interface (SEI); (3) lithium ions entering the graphite interior through the SEI membrane; and (4) lithium ion diffusion within the graphite bulk phase. Among them, the desolvation step of solvated lithium ions has been shown to be the rate-limiting step, especially under fast charging and low temperature conditions. Therefore, accelerating the desolvation process is the most effective way to improve the fast charging capability and low-temperature performance of lithium-ion batteries. Recent theoretical calculations have shown that inorganic SEI components including Li2CO3, LiF and Li3PO4 have the potential to reduce the lithium ion desolvation barrier.

[0005] Microcrystalline graphite is a natural graphite with abundant resources and low cost. It has a medium degree of graphitization (>92%) and is composed of many differently oriented microcrystals (less than 1μm in size). When used as negative electrode materials, it helps to increase its contact area with the electrolyte, increase the insertion and extraction speed of lithium ions, and increase the fast charging performance of the battery. However, microcrystalline graphite is an isotropic polycrystalline formed by the aggregation of fine microcrystals. It has low mechanical strength and is very easy to break during the production process, resulting in the production of graphite particles with difficult to control particle size and irregular morphology. Therefore, there are problems such as low initial efficiency and poor cycle stability, which limit its application. Coating a hard carbon layer on the surface of microcrystalline graphite can provide more lithium ion transfer sites and reduce the generation of lithium plating under high current charging. It is one of the effective methods to improve the electrochemical performance of microcrystalline graphite negative electrode materials.

[0006] Chinese invention patent application CN113889596A discloses a method for preparing a nitrogen-doped hard carbon-coated artificial graphite composite material. The resulting nitrogen-doped hard carbon-coated artificial graphite composite material has a core-shell structure, with an artificial graphite core and a nitrogen-doped hard carbon coating. The material's rate capability, cycling performance, and consistency are significantly improved. However, this method has limited potential for improving the electrochemical performance of graphite anode materials, and no clear solution has been proposed for reducing the interfacial desolvation energy and increasing the lithium ion transmission rate within the graphite particles. Summary of the Invention

[0007] The main purpose of the present invention is to address the problems and shortcomings of the existing technology and provide a fast-charging phosphorus-doped hard carbon-coated graphite negative electrode material, its preparation method, and its application. This negative electrode material is a composite material of phosphorus-doped hard carbon-coated microcrystalline graphite. The phosphorus-doped hard carbon layer can promote the desolvation of solvated lithium ions at the interface, enhancing interfacial ion and electron transport. At the same time, the "isotropic" nature of the internal microcrystalline graphite reduces the solid-phase ion diffusion path, enhancing the lithium ion transmission rate, and further improving the electrochemical performance and fast-charging performance of the negative electrode material.

[0008] In order to solve the above technical problems, the technical solutions of the present invention are as follows:

[0009] A fast-charging phosphorus-doped hard carbon-coated graphite negative electrode material, wherein the negative electrode material is a core-shell structure, the core is a microcrystalline graphite material, and the shell is a phosphorus-doped amorphous hard carbon material.

[0010] Preferably, the shell has a thickness of 3-7 nm.

[0011] A method for preparing a fast-charging phosphorus-doped hard carbon-coated graphite negative electrode material comprises the following steps:

[0012] 1) placing microcrystalline graphite powder in a dispersion containing polyacrylonitrile and ultrasonicating to obtain a mixed solution;

[0013] 2) heating and stirring the mixed solution to obtain polyacrylonitrile-coated microcrystalline graphite powder;

[0014] 3) mixing the polyacrylonitrile-coated microcrystalline graphite powder with a phytic acid solution and performing a hydrothermal reaction, filtering and freeze-drying to obtain a phosphorus-doped polyacrylonitrile-coated microcrystalline graphite powder;

[0015] 4) heat-treating the phosphorus-doped polyacrylonitrile-coated microcrystalline graphite powder to obtain the negative electrode material.

[0016] Preferably, the mass ratio of polyacrylonitrile to microcrystalline graphite in step 1) is 2-10:100.

[0017] Preferably, the ultrasonic treatment in step 1) is carried out at room temperature, with a power of 80-120 W and a time of 20-40 min.

[0018] Preferably, the heating temperature in step 2) is 60-80° C., the stirring is magnetic stirring, and the rotation speed is 100-300 rpm.

[0019] Preferably, the hydrothermal reaction temperature in step 3) is 120-180° C. and the reaction time is 8-10 h.

[0020] Preferably, the heat treatment in step 3) is carried out in an argon atmosphere at a temperature of 800-1200° C. for 1-3 hours.

[0021] Preferably, the heating rate of the heat treatment is 5-10°C min -1 .

[0022] Preferably, the argon gas flow rate is 100-150 sccm.

[0023] The negative electrode material is used in high energy density lithium ion batteries.

[0024] Compared with the prior art, the present invention has the following beneficial effects: the phosphorus-doped hard carbon-coated microcrystalline graphite composite negative electrode material and its preparation method provided by the present invention construct a stable and uniform phosphorus-doped hard carbon coating layer on the surface of the microcrystalline graphite through a simple liquid phase coating, hydrothermal and high-temperature carbonization method. The hard carbon layer rapidly stores lithium while providing more effective lithium ion reaction sites. The phosphorus doping promotes the desolvation of solvated lithium ions at the interface, effectively reducing the desolvation energy and enhancing the diffusion rate of lithium ions through the SEI. The "isotropic" characteristics of the internal microcrystalline graphite shorten the lithium ion transmission path, enhance the lithium ion transmission rate, and significantly enhance the fast charging performance. In addition, the preparation method is simple, the raw materials are widely available and the cost is low, making it very suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic diagram of the preparation process of the present invention;

[0026] Figure 2 (a), (b), and (c) are the SEM images of MG, MG@PAN-5%, and MG@PAN(P)-5%, respectively;

[0027] Figure 3 (a), (b) and (c) are XRD patterns of Comparative Example 2, Comparative Example 1 and Example 1, wherein (b) is a partial enlarged view of the (004) crystal plane, and (c) is a partial enlarged view of the (110) crystal plane; (d) is a Raman pattern of Comparative Example 2, Comparative Example 1 and Example 1; (e) and (f) are Fourier transform infrared spectra of Comparative Example 2, Comparative Example 1 and Example 1, wherein (f) is a partial enlarged view of the PC bond;

[0028] Figure 4 The XPS results of Example 1 are shown in Figure 1, where (a) is the overall spectrum and (b) is the P 2p fine spectrum.

[0029] Figure 5 The first-cycle cyclic voltammetry curves of different graphite negative electrodes in Comparative Example 2, Comparative Example 1, and Example 1 are shown, wherein (a) is a comparison of the first-cycle cyclic voltammetry curves of different graphite negative electrodes, and (b), (c), and (d) are detailed views of the cyclic voltammetry curves of Comparative Example 2, Comparative Example 1, and Example 1, respectively;

[0030] Figure 6 Figure 2 is the EIS analysis of the negative electrode before and after cycling for Comparative Example 2, Comparative Example 1, and Example 1, where (a) and (b) are the electrochemical impedance spectra and fitting data before cycling, respectively, and (c) and (d) are the electrochemical impedance spectra and fitting data after 100 cycles, respectively;

[0031] Figure 7 It is a schematic diagram of the principle of the present invention. DETAILED DESCRIPTION

[0032] The principles and features of the present invention are described below in conjunction with examples, which are only used to explain the present invention and are not intended to limit the scope of the present invention. In addition, it is worth noting that the raw materials involved in the present invention are all common commercially available products unless otherwise specified.

[0033] Example 1

[0034] like Figure 1 As shown, this embodiment provides a method for preparing a negative electrode material, wherein the negative electrode material is a phosphorus-doped hard carbon-coated microcrystalline graphite composite material, comprising the following steps:

[0035] (1) Place 50 mg of polyacrylonitrile (PAN) in 40 ml of nitrogen-dimethylformamide (DMF) solvent and ultrasonicate at room temperature for 30 min until the polyacrylonitrile is evenly dispersed in the DMF solvent;

[0036] (2) Add 1 g of microcrystalline graphite (MG) to the DMF mixture obtained in step (1), ultrasonicate for 30 min, and stir at room temperature for 1 h until the microcrystalline graphite is evenly dispersed in the mixture. Place the mixed solution on a magnetic stirrer and stir at 70°C until the DMF solvent is completely mixed to obtain a 5 wt% MG@PAN precursor powder with a simple surface coating.

[0037] (3) The dry powder obtained in step (2) was added to 50 ml of 50% (w / v, unit g / L) phytic acid aqueous solution in a reactor and hydrothermally reacted at 150°C for 9 h. The solution was then filtered and freeze-dried to obtain MG@PAN(P)-5% precursor powder;

[0038] (4) The dried powder obtained in step (3) was placed in an argon atmosphere in a tube furnace and heat treated at 1000°C for 2 h, with a heating rate of 5°C min -1 , the argon flow rate is 100 sccm, and high-temperature carbonization is performed to form a phosphorus-doped hard carbon coating layer, and finally a microcrystalline graphite material with a surface phosphorus-doped hard carbon chemically stable coating of 5wt% (MG@PAN(P)-5%) is obtained.

[0039] The MG@PAN(P)-5% obtained in Example 1 was used as the active material to prepare an electrode. The active material, PAA binder, CMC dispersant, and Super P were weighed in a mass ratio of 95.5:1.5:1.5:1.5 to prepare a slurry. After mixing evenly, the slurry was coated on the rough surface of the copper foil using a coater. After air drying until the solvent evaporated, the electrode was transferred to an 80°C vacuum oven and dried overnight. A tablet press was used for tableting at 10 MPa, and finally a circular electrode with a diameter of 12 mm was obtained by a punching machine. The prepared MG@PAN(P)-5% electrode was matched with lithium metal to assemble a battery, and the electrochemical performance was tested in a lithium-ion battery electrolyte (1M LiPF6 dissolved in a mixed solvent of EC / DEC / DMC with a volume ratio of 1:1:1).

[0040] Comparative Example 1

[0041] This embodiment uses the same experimental materials and conditions as in embodiment 1, except that the phosphorus doping process in step (3) is omitted. Specifically,

[0042] (1) The steps are the same as those in Example 1 and will not be repeated here;

[0043] (2) The steps are the same as those in Example 1 and will not be repeated here. Finally, a 5 wt% MG@PAN precursor powder with a simple surface coating is obtained.

[0044] (3) The dried powder obtained in step (2) was placed in an argon atmosphere in a tube furnace and heat treated at 1000°C for 2 h, with a heating rate of 5°C min -1 , the argon flow rate is 100 sccm, and high-temperature carbonization is performed to form a hard carbon coating layer, and finally a microcrystalline graphite material with a surface hard carbon chemically stable coating of 5 wt% (MG@PAN-5%) is obtained.

[0045] The MG@PAN-5% obtained in Comparative Example 1 was used as the active material to prepare an electrode, and the steps were the same as those in Example 1, which will not be repeated here.

[0046] Comparative Example 2

[0047] This comparative example uses conventional microcrystalline graphite material as the electrode active material, specifically:

[0048] The active material, PAA binder, CMC dispersant, and Super P slurry were weighed in a mass ratio of 95.5:1.5:1.5:1.5. After mixing evenly, the slurry was applied to the rough surface of the copper foil using a coater. After air drying until the solvent evaporated, the electrode was transferred to an 80°C vacuum oven and dried overnight. A tablet press was used for tableting at 10MPa, and finally a circular electrode with a diameter of 12mm was obtained by a punching machine. The prepared microcrystalline graphite electrode was matched with lithium metal to assemble a battery, and the electrochemical performance was tested in a lithium-ion battery electrolyte (1M LiPF6 dissolved in a mixed solvent of EC / DEC / DMC with a volume ratio of 1:1:1).

[0049] Example 2

[0050] In this example, a 2 wt% MG@PAN(P) electrode was prepared to explore the effect of the coating content on the fast charging performance of the battery. Specifically:

[0051] This example uses the same experimental raw materials and conditions as Example 1, and only changes the amount of polyacrylonitrile in step (1), specifically:

[0052] (1) 20 mg of polyacrylonitrile was placed in 40 ml of nitrogen-dimethylformamide (DMF) solvent and ultrasonicated at room temperature for 30 min until the polyacrylonitrile was evenly dispersed in the DMF solvent;

[0053] (2) The steps are the same as those in Example 1 and will not be repeated here. Finally, a 2 wt% MG@PAN precursor powder with a simple surface coating is obtained.

[0054] Steps (3) and (4) are the same as those in Example 1 and are not described again here. Finally, a microcrystalline graphite material with a 2 wt% surface hard carbon chemically stably coated thereon (MG@PAN(P)-2%) is obtained.

[0055] The MG@PAN(P)-2% obtained in Example 2 was used as the active material to prepare an electrode. The steps were the same as those in Example 1 and will not be repeated here.

[0056] Example 3

[0057] In this example, a 10 wt% MG@PAN(P) electrode was prepared to investigate the effect of the coating content on the fast charging performance of the battery. Specifically:

[0058] This example uses the same experimental raw materials and conditions as Example 1, and only changes the amount of polyacrylonitrile in step (1), specifically:

[0059] (1) Place 100 mg of polyacrylonitrile in 40 ml of nitrogen-dimethylformamide (DMF) solvent and ultrasonicate at room temperature for 30 min until the polyacrylonitrile is evenly dispersed in the DMF solvent;

[0060] (2) The steps are the same as those in Example 1 and will not be repeated here. Finally, a 10 wt% MG@PAN precursor powder with a simple surface coating is obtained.

[0061] Steps (3) and (4) are the same as those in Example 1 and are not repeated here. Finally, a microcrystalline graphite material with a surface hard carbon chemically stably coated with 10 wt% (MG@PAN(P)-10%) is obtained.

[0062] The MG@PAN(P)-10% obtained in Example 3 was used as the active material to prepare an electrode. The steps were the same as those in Example 1 and will not be repeated here.

[0063] Example 4

[0064] In this example, a 5 wt% MG@PAN(P)-4 electrode was prepared to investigate the effect of carbonization temperature on the fast charging performance of the battery. Specifically:

[0065] This embodiment uses the same experimental raw materials and conditions as in embodiment 1, and only changes the temperature of high-temperature carbonization in step (4), specifically:

[0066] (1) The steps are the same as those in Example 1 and will not be repeated here;

[0067] Steps (2) and (3) are the same as those in Example 1 and are not described here again. Finally, a 5 wt% MG@PAN(P) precursor powder with a simple surface coating is obtained.

[0068] (4) The dried powder obtained in step (3) was placed in an argon atmosphere in a tube furnace and heat treated at 800°C for 2 h at a heating rate of 5°C min -1 , the argon flow rate is 100 sccm, and high-temperature carbonization is performed to form a hard carbon coating layer, and finally a microcrystalline graphite material with a surface hard carbon chemically stable coating of 5 wt% (MG@PAN(P)-5%-4) is obtained.

[0069] The MG@PAN(P)-5%-4 obtained in Example 4 was used as the active material to prepare an electrode. The steps were the same as those in Example 1 and will not be repeated here.

[0070] Example 5

[0071] In this example, a 5 wt% MG@PAN(P)-5 electrode was prepared to investigate the effect of carbonization temperature on the fast charging performance of the battery. Specifically:

[0072] This comparative example uses the same experimental raw materials and conditions as Example 1, except that the temperature of high-temperature carbonization is changed in step (4), specifically:

[0073] (1) The steps are the same as those in Example 1 and will not be repeated here;

[0074] Steps (2) and (3) are the same as those in Example 1 and are not described here again. Finally, a 5 wt% MG@PAN(P) precursor powder with a simple surface coating is obtained.

[0075] (4) The dried powder obtained in step (3) was placed in an argon atmosphere in a tube furnace and heat treated at 1200°C for 2 h at a heating rate of 5°C min -1 , the argon flow rate is 100 sccm, and high-temperature carbonization is performed to form a hard carbon coating layer, and finally a microcrystalline graphite material with a surface hard carbon chemically stable coating of 5 wt% (MG@PAN(P)-5%-5) is obtained.

[0076] The MG@PAN(P)-5%-5 obtained in Example 5 was used as the active material to prepare an electrode. The steps were the same as those in Example 1 and will not be repeated here.

[0077] Example 6

[0078] In this example, a 5 wt% MG@PAN(P)-6 electrode was prepared to investigate the effect of the heating rate on the fast charging performance of the battery. Specifically:

[0079] This comparative example uses the same experimental raw materials and conditions as Example 1, and only changes the heating rate in step (4), specifically:

[0080] (1) The steps are the same as those in Example 1 and will not be repeated here;

[0081] Steps (2) and (3) are the same as those in Example 1 and are not described here again. Finally, a 5 wt% MG@PAN(P) precursor powder with a simple surface coating is obtained.

[0082] (4) The dried powder obtained in step (3) was placed in an argon atmosphere in a tube furnace and heat treated at 1000°C for 2 h at a heating rate of 10°C min -1 , the argon flow rate is 100 sccm, and high-temperature carbonization is performed to form a hard carbon coating layer, and finally a microcrystalline graphite material with a surface hard carbon chemically stable coating of 5 wt% (MG@PAN(P)-5%-6) is obtained.

[0083] The MG@PAN(P)-5%-6 obtained in Example 6 was used as the active material to prepare an electrode. The steps were the same as those in Example 1 and will not be repeated here.

[0084] Example 7

[0085] In this example, a 5 wt% MG@PAN(P)-7 electrode was prepared to investigate the effect of argon flow rate on the fast charging performance of the battery. Specifically:

[0086] This example uses the same experimental materials and conditions as Example 1, except that the argon gas flow rate is changed in step (4), specifically:

[0087] (1) The steps are the same as those in Example 1 and will not be repeated here;

[0088] Steps (2) and (3) are the same as those in Example 1 and are not described here again. Finally, a 5 wt% MG@PAN(P) precursor powder with a simple surface coating is obtained.

[0089] (4) The dried powder obtained in step (3) was placed in an argon atmosphere in a tube furnace and heat treated at 1000°C for 2 h at a heating rate of 5°C min -1 , the argon flow rate is 150 sccm, and high-temperature carbonization is performed to form a hard carbon coating layer, and finally a microcrystalline graphite material with a surface hard carbon chemically stable coating of 5 wt% (MG@PAN(P)-5%-7) is obtained.

[0090] The MG@PAN(P)-5%-7 obtained in Example 7 was used as the active material to prepare an electrode. The steps were the same as those in Example 1 and will not be repeated here.

[0091] The following is a description of the detection and analysis of the above embodiments and comparative examples:

[0092] Table 1 Electrochemical properties of graphite-based negative electrode materials obtained in each group of comparative examples and examples

[0093]

[0094] The MG@PAN(P)-5% material prepared in Example 1 of the present invention, the MG@PAN-5% material prepared in Comparative Example 1, and the MG material prepared in Comparative Example 2 were characterized by SEM morphology. Figure 2 As shown in the figure, the MG particles before and after coating are uneven and blocky, and the overall size of the micron particles is 2 to 3 μm. After PAN coating, the MG micron particles present a relatively uniform surface, with some particle deposition layers. The irregular MG particles are significantly reduced after coating, and phosphorus doping does not change the overall morphology of the material. This preliminarily indicates the existence of a surface coating layer and that the coating does not destroy the crystal structure of the MG.

[0095] The MG@PAN(P)-5% material prepared in Example 1 of the present invention, the MG@PAN-5% material prepared in Comparative Example 1, and the MG material prepared in Comparative Example 2 were subjected to XRD composition analysis, Raman analysis, Fourier transform infrared spectroscopy analysis, and XPS analysis. The results are as follows: Figure 3 、 Figure 4 and as shown in Table 2.

[0096] Table 2

[0097]

[0098] Phosphating and coating do not destroy the crystal structure of MG, but expand the distance between graphite layers, which is beneficial to the rapid transmission of lithium ions. Orientation (OI) is the peak intensity of the material (004) crystal plane I (004) and (110) crystal plane peak intensity I (110) The ratio of OI to MgO can be used to indicate the orientation of the material. The smaller the OI, the better the isotropy of the material, which is more conducive to the diffusion of lithium ions in the negative electrode material. Therefore, this characteristic will directly affect the impedance of the negative electrode and high-rate charging performance. The calculation shows that the OI value of MG@PAN(P)-5% material is the smallest, so theoretically, the rate performance of MG@PAN(P)-5% material is the best. R value (I D and I G The ratio of α to β (α) can characterize the degree of disorder near the surface of the carbon material. Calculations show that the R value of the material increases after coating, indicating increased disorder. MG@PAN(P)-5% has the highest R value, with a shift in peak position, due to the abundant edge defects created by phosphorus doping, which facilitates lithium ion insertion. IR and XPS characterization confirm the presence of P-C and P-O bonds.

[0099] The electrochemical behaviors of the MG@PAN(P)-5% material prepared in Example 1 of the present invention, the MG@PAN-5% material prepared in Comparative Example 1, and the MG material prepared in Comparative Example 2 were analyzed. Figure 5 、 Figure 6As shown. From the cyclic voltammetry curves of the first cycle of the three graphite negative electrodes, it can be observed that the redox peak potential difference of the MG@PAN(P)-5% negative electrode is the smallest, the smaller the battery reversibility, the smaller the polarization. Since SEI has not yet formed on the surface of the fresh electrode before activation, there is only an incomplete semicircle in the high-frequency region. After the electrode is cycled 100 times, according to the fitted equivalent circuit diagram, it can be seen that Rs is the internal resistance of the battery, and the first semicircle R in the high-frequency part is SEI is the resistance of lithium ions passing through the SEI film, and the second semicircle R ct is the resistance during electron transfer. The MG@PAN(P)-5% negative electrode exhibits the lowest R SEI and R ct , proving that the MG@PAN(P)-5% electrode has lower ion and electron transport resistance.

[0100] The electrochemical performance of all the embodiments and comparative examples of the present invention was analyzed, as shown in Table 1. It can be seen that 5wt% is the optimal coating content, 1000℃ is the optimal high temperature carbonization temperature, and 5℃min -1 is the optimal heating rate, 100 sccm is the optimal argon flow rate, and MG@PAN(P)-5% has the best rate performance.

[0101] It can be seen that the present invention constructs a stable and uniform phosphorus-doped hard carbon coating layer on the surface of microcrystalline graphite through a simple liquid phase coating, hydrothermal and high temperature carbonization method. The phosphorus-doped hard carbon layer can promote the desolvation of solvated lithium ions at the interface and enhance the interfacial ion and electron transport. At the same time, the "isotropic" characteristic of the internal microcrystalline graphite reduces the solid phase ion diffusion path and enhances the lithium ion transmission rate (such as Figure 7 As shown), the electrochemical performance and fast charging performance of the negative electrode material are further improved. In addition, the preparation method is simple in process, the raw materials are widely available and the cost is low, making it very suitable for industrial production.

[0102] It should be noted that those skilled in the art may make various modifications and improvements without departing from the scope of the present invention, and these modifications and improvements fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be based on the appended claims.

Claims

1. A method for preparing a fast-charging phosphorus-doped hard carbon-coated graphite negative electrode, characterized in that: The steps include: 1) placing microcrystalline graphite powder in a dispersion containing polyacrylonitrile and ultrasonicating to obtain a mixed solution; 2) heating and stirring the mixed solution to obtain polyacrylonitrile-coated microcrystalline graphite powder; 3) mixing the polyacrylonitrile-coated microcrystalline graphite powder with a phytic acid solution and subjecting the mixture to a hydrothermal reaction, followed by filtration and freeze-drying to obtain phosphorus-doped polyacrylonitrile-coated microcrystalline graphite powder; 4) heat-treating the phosphorus-doped polyacrylonitrile-coated microcrystalline graphite powder to obtain the phosphorus-doped hard carbon-coated graphite negative electrode material; The mass ratio of polyacrylonitrile to microcrystalline graphite in step 1) is 2-10:100; the hydrothermal reaction temperature in step 3) is 120-180°C and the time is 4-10 h; the heat treatment in step 4) is carried out under an argon atmosphere at a temperature of 800-1200°C and a time of 1-3 h.

2. The preparation method according to claim 1, wherein The heating temperature in step 2) is 60-80° C., the stirring is magnetic stirring, and the rotation speed is 100-300 rpm.

3. The preparation method according to claim 1, wherein The heating rate of the heat treatment is 5-10 ℃ min -1 .

4. The preparation method according to claim 1, wherein The argon gas flow rate is 100-150 sccm.

5. The fast-charging phosphorus-doped hard carbon-coated graphite negative electrode material prepared by the preparation method according to claim 1, characterized in that: The negative electrode material is a core-shell structure, the core is a microcrystalline graphite material, and the shell is a phosphorus-doped amorphous hard carbon material.

6. Use of the negative electrode material as claimed in claim 5 in a fast-charge lithium-ion battery.

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