Negative electrode material and preparation method therefor and use thereof, and lithium-ion battery

Abstract

A negative electrode material and a preparation method therefor and the use thereof, and a lithium-ion battery. The preparation method for the negative electrode material comprises the following steps: (1) mixing graphitized needle coke, supercritical-fluid-stripped graphene and chitosan to obtain a mixture; and (2) carbonizing the mixture to obtain a negative electrode material. In the method, the graphitized needle coke and the supercritical-fluid-stripped graphene are used in combination with the chitosan, and a graphitized needle coke negative electrode material modified by a graphene two-dimensional laminated structure is obtained by means of solid-phase physical stirring and mixing and carbonization treatment, thereby achieving rapid transport of lithium ions, and providing excellent rate capability and cycle performance.

Description

A negative electrode material, preparation method and application thereof, and lithium-ion battery

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This disclosure is based on and claims the priority of Chinese patent application with application number 202410034799.X and application date January 9, 2024. The entire content of the Chinese patent application is hereby incorporated into this disclosure by reference. Technical Field

[0003] The present disclosure relates to the field of batteries, and in particular, to a negative electrode material, a preparation method and application thereof, and a lithium-ion battery. Background Art

[0004] Lithium-ion batteries (LIBs) are currently the most widely used energy storage system in electric vehicles. With the rapidly increasing demand for shorter charging times in electric vehicles, fast charging of high-energy-density lithium batteries has become a crucial requirement for practical applications. To provide a better user experience, charging times for electric vehicle lithium batteries should be similar to the 8-10 minute refueling time of internal combustion engine vehicles while maintaining high energy density. This would reduce user waiting time, overcome range anxiety, promote mainstream adoption of electric vehicles, and achieve sustainable development. However, most electric vehicles on the market require 2-6 hours to fully charge, far from providing a "full tank" experience. This phenomenon is primarily due to the slow lithium ion intercalation rate caused by the anisotropic properties of commercial graphite anodes. The ultimate goal of fast charging is extreme fast charging (XFC), which aims to provide an 80% charge in electric vehicles within 15 minutes (at ~4°C), a level not achievable with untreated commercial graphite. Therefore, improving the fast-charging performance of anode materials is urgently needed. Summary of the Invention

[0005] The present disclosure is based on the inventors' discovery and understanding of the following facts and problems: the anisotropy of commercial graphite negative electrodes leads to a slow rate of lithium ion insertion. Therefore, there is an urgent need to improve the fast charging performance by modifying the negative electrode material.

[0006] The present disclosure aims to at least partially address one of the technical problems in the related art. To this end, embodiments of the present disclosure provide a negative electrode material, preparation method, and application thereof, as well as a lithium-ion battery. The negative electrode material comprises graphitized needle coke, supercritical fluid exfoliated graphene, and chitosan, and is modified by solid-phase physical mixing and carbonization to obtain a modified graphene two-dimensional sheet structure. The material achieves rapid lithium ion transport and exhibits excellent rate and cycling performance.

[0007] The present disclosure provides a method for preparing a negative electrode material, comprising the following steps:

[0008] (1) mixing graphitized needle coke, supercritical fluid exfoliated graphene, and chitosan to obtain a mixture;

[0009] (2) The mixture is carbonized to obtain a negative electrode material.

[0010] The advantages and technical effects brought by the preparation method of the negative electrode material of the embodiment of the present disclosure are as follows: supercritical fluid exfoliated graphene has high conductivity, and its introduction can significantly improve the rate performance and fast charging performance of the negative electrode material. The coating effect of chitosan improves the coulombic efficiency of the negative electrode material. Graphitized needle coke, supercritical fluid exfoliated graphene are combined with chitosan, and a graphitized needle coke negative electrode material modified with a two-dimensional sheet structure of graphene is obtained through solid-phase physical stirring and mixing and carbonization treatment. The carbonization coating effect of chitosan in the carbonization process tightly bonds the supercritical fluid exfoliated graphene and the graphitized needle coke, giving full play to the high conductivity of the supercritical fluid exfoliated graphene, which helps to greatly improve the rate performance of the negative electrode material. In addition, chitosan forms a thin and uniform carbon shell on the surface of the material, which is conducive to the formation of SEI film, and can also improve the long cycle performance of the negative electrode material, realize the rapid transmission of lithium ions, and has excellent rate performance and cycle performance. The performance of the negative electrode material is better than that of commercial graphite negative electrode. The method disclosed herein avoids the need for using environmentally unfriendly solutions such as hydrochloric acid, formic acid, and phosphoric acid in the traditional liquid phase method for preparing negative electrode materials. It is simple to operate, requires simple equipment, and is easy to implement industrial production.

[0011] In some embodiments, in step (1), the method for preparing supercritical fluid exfoliated graphene includes: mixing graphite and supercritical CO2, performing fluid shearing, and then discharging CO2 through pressure relief to obtain supercritical fluid exfoliated graphene.

[0012] In some embodiments, the fluid shear temperature is 44-62°C;

[0013] and / or, the shear pressure of the fluid is 8.6-11.3 MPa;

[0014] And / or, the fluid shearing speed is 1600-2400 r / min;

[0015] And / or, the fluid shearing time is 15-60 minutes.

[0016] In some embodiments, in step (1), the D of the graphitized needle coke is 50 10-19μm;

[0017] And / or, the tap density of the graphitized needle coke is greater than or equal to 1.15 g / cm 3 ;

[0018] And / or, the specific surface area of ​​the graphitized needle coke is less than or equal to 1.6 m 2 / g;

[0019] And / or, the compacted density of the needle coke after graphitization is 1.45-1.55 g / cm 3 .

[0020] In some embodiments, in step (1), the chitosan is water-soluble chitosan;

[0021] and / or, the degree of polymerization of the chitosan is greater than 400;

[0022] And / or, the carbon content of the chitosan is greater than 40%.

[0023] In some embodiments, in step (1), the mass ratio of the graphitized needle coke, supercritical fluid exfoliated graphene, and chitosan is 70-90:0.01-15:2-10;

[0024] And / or, the mixing is performed by stirring.

[0025] In some embodiments, in step (2), the carbonization treatment includes: heating to 300-400°C at a heating rate of 1-5°C / min, keeping warm for 0.5-3h, then heating to 1300-1800°C at a heating rate of 1-5°C / min, keeping warm for 0.5-3h, and finally cooling.

[0026] In some embodiments, the particle size of the negative electrode material is 20-70 microns.

[0027] The present disclosure provides a negative electrode material prepared using the preparation method described in the present disclosure. In the present disclosure, the graphitized needle coke negative electrode material modified with a two-dimensional graphene sheet structure, obtained by combining graphitized needle coke, supercritical fluid exfoliated graphene, and chitosan, achieves rapid lithium ion transport and exhibits excellent rate and cycling performance.

[0028] The present disclosure provides an application of a negative electrode material for a lithium-ion battery. The present disclosure has all the advantages of the negative electrode material, which will not be described in detail here.

[0029] The present disclosure provides a lithium-ion battery comprising the negative electrode material of the present disclosure. The present disclosure has all the advantages of the negative electrode material, which will not be described in detail here. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG1 shows the impedance performance of the embodiment of the present disclosure and the comparative example. DETAILED DESCRIPTION

[0031] The embodiments of the present disclosure are described in detail below, and examples of the embodiments are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to be used to explain the present disclosure, but should not be understood as limiting the present disclosure.

[0032] A method for preparing a negative electrode material according to an embodiment of the present disclosure includes the following steps:

[0033] (1) mixing graphitized needle coke, supercritical fluid exfoliated graphene, and chitosan to obtain a mixture;

[0034] (2) The mixture is carbonized to obtain a negative electrode material.

[0035] The preparation method of the negative electrode material of the embodiment of the present disclosure, supercritical fluid exfoliated graphene has high conductivity, and its introduction can significantly improve the rate performance and fast charging performance of the negative electrode material. The coating effect of chitosan improves the coulombic efficiency of the negative electrode material. Graphitized needle coke, supercritical fluid exfoliated graphene are combined with chitosan, and a graphitized needle coke negative electrode material modified with a two-dimensional sheet structure of graphene is obtained through solid-phase physical stirring and mixing and carbonization treatment. The carbonization coating effect of chitosan in the carbonization process tightly bonds the supercritical fluid exfoliated graphene and the graphitized needle coke, giving full play to the high conductivity of the supercritical fluid exfoliated graphene, which helps to greatly improve the rate performance of the negative electrode material. In addition, chitosan forms a thin and uniform carbon shell on the surface of the material, which is conducive to the formation of SEI film, and can also improve the long cycle performance of the negative electrode material, realize the rapid transmission of lithium ions, and has excellent rate performance and cycle performance. The performance of the negative electrode material is better than that of commercial graphite negative electrode. The method disclosed herein avoids the need for using environmentally unfriendly solutions such as hydrochloric acid, formic acid, and phosphoric acid in the traditional liquid phase method for preparing negative electrode materials. It is simple to operate, requires simple equipment, and is easy to implement industrial production.

[0036] In some embodiments, in step (1), the method for preparing supercritical fluid exfoliated graphene includes: mixing graphite and supercritical CO2, performing fluid shearing, and then discharging CO2 by pressure relief to obtain supercritical fluid exfoliated graphene;

[0037] Optionally, the graphite is pre-treated by ball milling to obtain graphite powder, and then the pre-treated graphite powder is added to an autoclave and supercritical CO2 is introduced for mixing; optionally, the ball milling is performed using a ball mill;

[0038] Optionally, the temperature of the fluid shearing is 44-62°C, specifically, for example, 44°C, 50°C, 54°C, 60°C, 62°C; and / or the pressure of the fluid shearing is 8.6-11.3MPa, specifically, for example, 8.6MPa, 9MPa, 9.8MPa, 10MPa, 11MPa, 11.3MPa; and / or the speed of the fluid shearing is 1600-2400r / min, specifically, for example, 1600r / min, 2000r / min, 2100r / min, 2400r / min; and / or the time of the fluid shearing is 15-60min, specifically, for example, 15min, 30min, 35min, 45min, 60min.

[0039] In the disclosed embodiments, the method for preparing supercritical fluid exfoliated graphene utilizes the excellent diffusivity, permeability and solubility of supercritical CO2, and its low viscosity and surface tension, which are conducive to intercalation into the graphite interlayers; and the high-speed rotation generates a strong shear force between the CO2 fluids, which not only strengthens the diffusion process of the molecules, but also promotes the destruction of the van der Waals forces between the graphite layers by the molecules, and combines the rapid pressure relief and expansion of supercritical CO2 to exfoliate the graphite to prepare supercritical fluid exfoliated graphene. Compared with other methods of widening the graphite interlayer spacing by introducing strong acids or strong bases to achieve the effect of rapid embedding of lithium ions into graphite, this method not only greatly reduces costs, but also does not produce a large amount of waste acid, and is environmentally friendly. Compared with other graphenes, supercritical fluid exfoliated graphene is conducive to further improving fast charging performance, rate performance and cycle performance.

[0040] In the disclosed embodiments, the graphite raw material is pre-treated by ball milling to reduce the particle size and number of layers, facilitating supercritical exfoliation. In a supercritical state, temperature, pressure, fluid shear rate, and time significantly affect graphene. By selecting temperature and pressure to achieve a supercritical fluid suitable for the anode material, graphene exfoliation can accelerate electron transfer, thereby improving the rate capability of the anode material.

[0041] In some embodiments, graphite is pre-treated by ball milling to obtain graphite powder, and then the pre-treated graphite powder is added to an autoclave and packaged, and the temperature of the autoclave is set. When the temperature reaches the set value, CO2 gas is introduced, and the pressure pump is turned on. When the pressure reaches a predetermined value, the valve is closed to stop the introduction of CO2 gas, and fluid shearing is performed. Then, CO2 is discharged by pressure relief to obtain supercritical fluid exfoliated graphene; optionally, the ball mill is used.

[0042] In some embodiments, in step (1), the D of the graphitized needle coke is 50 10-19 μm; and / or the tap density of the graphitized needle coke is greater than or equal to 1.15 g / cm3 ; and / or, the specific surface area of ​​the needle coke after graphitization is less than or equal to 1.6m 2 / g; and / or, the compacted density of the needle coke after graphitization is 1.45-1.55 g / cm 3 ; and / or, the graphitized needle coke is obtained by subjecting needle coke to graphitization heat treatment; optionally, the temperature of the graphitization heat treatment is 2400-2800° C., specifically, for example, 2400° C., 2500° C., 2600° C., 2700° C., 2800° C., and the time of the graphitization heat treatment is 15-25 hours, specifically, for example, 15 hours, 18 hours, 20 hours, 22 hours, 25 hours;

[0043] And / or, in step (1), the chitosan is water-soluble chitosan; and / or, the degree of polymerization of the chitosan is greater than 400; and / or, the carbon content of the chitosan is greater than 40%.

[0044] In the disclosed embodiments, the types of graphitized needle coke and chitosan were selected to further improve the fast-charging performance, rate capability, and cycle performance of the negative electrode material. In the disclosed embodiments, if the carbon content of chitosan is too high, the economic cost will be high, while if it is too low, it will not be conducive to the formation of a stable carbon layer. By selecting the carbon content of chitosan, graphene can be better attached to the surface of the graphitized needle coke, further facilitating the faster passage of electrons through the graphene to the graphitized needle coke.

[0045] In some embodiments, in the step (1), the mass ratio of the graphitized needle coke, supercritical fluid exfoliated graphene, and chitosan is 70-90: 0.01-15: 2-10, specifically, 70-90 (for example, 70, 75, 80, 85, 90): 0.01-15 (for example, 0.01, 0.1, 1, 2, 3, 4, 5, 6, 7, 8, 10, 15): 2-10 (for example, 2, 4, 6, 8, 10). In the disclosed embodiments, by selecting the mass ratio of graphitized needle coke, supercritical fluid exfoliated graphene, and chitosan, it is beneficial to further improve the fast charging performance, rate performance, and cycle performance of the negative electrode material. When the amount of supercritical fluid exfoliated graphene is relatively too high, it is not conducive to the formation of a complete coating layer of chitosan, and thus is not conducive to improving the performance of the negative electrode material; when it is relatively too low, the high conductivity characteristics of the supercritical fluid exfoliated graphene cannot be fully utilized. A relatively high chitosan dosage can lead to an excessively thick carbon layer, hindering the passage of electrons and ions through the carbon layer and instead creating a barrier, reducing the fast-charging performance of the negative electrode material. A relatively low chitosan dosage prevents the graphene from being coated and attached to the surface of the graphite needle coke, preventing the highly conductive supercritical fluid-exfoliated graphene from fully functioning. The interaction between the graphitized needle coke, supercritical fluid-exfoliated graphene, and chitosan further improves the rapid transport of lithium ions, enhancing rate capability and cycling performance.

[0046] In some embodiments, in step (1), the mass ratio of the graphitized needle coke to the supercritical fluid exfoliated graphene is 92-98:2-8, specifically, 92-98 (e.g., 92, 94, 95, 96, 98):2-8 (e.g., 2, 4, 5, 6, 8).

[0047] In some embodiments, in step (1), the mixing is performed by stirring; optionally, the mixing is performed in a soymilk maker; the mixing time is 60-200 seconds, specifically, for example, 60 seconds, 80 seconds, 100 seconds, 120 seconds, 150 seconds, or 200 seconds. In the disclosed embodiments, a soymilk maker is more conducive to uniformly mixing the powdered materials than conventional stirring.

[0048] In some embodiments, in step (1), the graphitized needle coke is first mixed with supercritical fluid exfoliated graphene, optionally for 30-100 seconds, and then mixed with chitosan, optionally for 30-100 seconds, to obtain a mixture; optionally, the mixing is carried out in a soymilk maker.

[0049] In some embodiments, in step (2), the carbonization treatment includes: heating to 300-400°C (specifically, for example, 300°C, 350°C, 380°C, 400°C) at a heating rate of 1-5°C / min (specifically, for example, 1°C / min, 2°C / min, 3°C / min, 4°C / min, 5°C / min), keeping warm for 0.5-3h (specifically, for example, 0.5h, 1h, 1.5h, 2h, 3h), and then heating at a heating rate of 1-5°C / min (specifically, for example, 1°C / min, 2°C / min, 3°C / min, 4°C / min, 5°C / min). The temperature is raised at a rate to 1300-1800°C (specifically, for example, 1300°C, 1400°C, 1500°C, 1600°C, 1700°C, 1800°C), kept warm for 0.5-3h (specifically, for example, 0.5h, 1h, 1.5h, 2h, 3h), and finally cooled; optionally, the temperature is lowered to 20-30°C, specifically, for example, 20°C, 25°C, 30°C; optionally, the temperature is raised from room temperature; optionally, the carbonization treatment is performed by placing the mixture into an alumina porcelain boat and carrying out the carbonization treatment in a tube furnace; the atmosphere of the carbonization treatment is an Ar atmosphere, and optionally, the flow rate is 20-40sccm. In the embodiment of the present disclosure, a carbonization treatment process is selected, which first stays at a low temperature of 300-400°C and then rises to a high temperature to stay. This can make the chitosan reach a molten state first, so that it can be better dispersed in the graphene and graphitized needle coke. Then carbonization is carried out to evenly attach the graphene to the surface of the graphitized needle coke. Chitosan forms a thin and uniform carbon shell on the surface of the material, forming a graphitized needle coke negative electrode material modified with a two-dimensional graphene sheet structure, further improving the rapid transmission performance of lithium ions, and improving the rate performance and cycle performance.

[0050] In some embodiments, the carbonization process further includes grinding and screening.

[0051] In some embodiments, the particle size of the negative electrode material is 20-70 microns, specifically, for example, 20 microns, 30 microns, 40 microns, 50 microns, 55 microns, 60 microns, 70 microns.

[0052] The present invention discloses a negative electrode material prepared using the preparation method described in the present invention. In the present invention, the graphitized needle coke negative electrode material modified with a two-dimensional graphene sheet structure, obtained by combining graphitized needle coke, supercritical fluid exfoliated graphene, and chitosan, achieves rapid lithium ion transport and exhibits excellent rate and cycling performance.

[0053] The present invention discloses an application of a negative electrode material for a lithium-ion battery. ...

[0054] A lithium-ion battery according to an embodiment of the present disclosure includes the negative electrode material according to the embodiment of the present disclosure. The embodiment of the present disclosure has all the advantages of the negative electrode material, which will not be described in detail here.

[0055] The present disclosure is described below with reference to specific embodiments. It should be noted that these embodiments are merely illustrative and do not limit the present disclosure in any way.

[0056] Example 1

[0057] The method for preparing the negative electrode material comprises the following steps:

[0058] (1) Graphite is pre-milled to obtain graphite powder, and then the pre-treated graphite powder is added to an autoclave, supercritical CO2 is introduced for mixing, and fluid shearing is performed, the fluid shearing temperature is 54°C, the fluid shearing pressure is 9.8 MPa, the fluid shearing speed is 2100 r / min, and the fluid shearing time is 35 min, and then the CO2 is discharged by pressure relief to obtain supercritical fluid exfoliated graphene;

[0059] (2) 19.0 g of graphitized needle coke (purchased from B&T) and 1.0 g of supercritical fluid exfoliated graphene were placed in a soymilk maker and stirred for 100 s, and then 2 g of chitosan (purchased from Shandong Haiyihua Co., Ltd., with a carbon content greater than 40%) was added and stirred for 80 s to obtain a mixture.

[0060] (3) The mixture was placed in an alumina porcelain boat and carbonized in a tube furnace in an Ar atmosphere with a flow rate of about 30 sccm. The temperature of the tube furnace was raised from room temperature to 380°C at a heating rate of 2.5°C / min and then held for 1 hour. The temperature was then raised to 1600°C at a heating rate of 5°C / min and then held for 2 hours. The sample was then cooled to room temperature. The carbonized sample was ground and sieved (250 mesh) to obtain the negative electrode material.

[0061] Performance testing method:

[0062] 1) The negative electrode material, acetylene black and PVDF were mixed in a mass ratio of 95:1.5:3.5, an appropriate amount of NMP was added, and the mixture was stirred at room temperature for 6 h. The mixture was coated on a copper foil and dried at 120 ° C for 12 h. The negative electrode sheet with a diameter of 13 mm was pressed using a tablet press.

[0063] 2) Assemble CR2032 button batteries in the glove box.

[0064] 3) Carry out charge and discharge tests in the Xinwei battery testing system to test the battery's rate performance, cycle performance, and impedance performance at different current densities.

[0065] Example 2

[0066] The preparation method and performance testing method are the same as those in Example 1, except that, in step (2), the amount of graphitized needle coke used is 19.6 g, and the amount of supercritical fluid exfoliated graphene used is 0.4 g.

[0067] Example 3

[0068] The preparation method and performance testing method are the same as those in Example 1, except that in step (2), the amount of graphitized needle coke used is 18.4 g, and the amount of supercritical fluid exfoliated graphene used is 1.6 g.

[0069] Comparative Example 1

[0070] The performance test method is the same as that of Example 1, except that the negative electrode material uses a commercial graphite negative electrode.

[0071] Comparative Example 2

[0072] The performance test method is the same as that of Example 1, except that the negative electrode material is graphitized needle coke.

[0073] Comparative Example 3

[0074] The preparation method and performance testing method are the same as those in Example 1, except that in step (2), the amount of graphitized needle coke used is 20.0 g, and no supercritical fluid is added to exfoliate the graphene.

[0075] Comparative Example 4

[0076] The preparation method and performance testing method are the same as those in Example 1, except that in step (2), the supercritical fluid exfoliated graphene is replaced by commercial graphene (purchased from Pioneer Nano).

[0077] Comparative Example 5

[0078] The preparation method and performance testing method are the same as those in Example 1, except that in step (2), the supercritical fluid exfoliated graphene is replaced by few-layer graphene.

[0079] Comparative Example 6

[0080] The preparation method and performance testing method are the same as those in Example 1, except that in step (2), the supercritical fluid exfoliated graphene is replaced by graphene oxide.

[0081] Comparative Example 7

[0082] The preparation method and performance testing method are the same as those in Example 1, except that in step (2), the supercritical fluid exfoliated graphene is replaced by hydrogenated graphene.

[0083] Comparative Example 8

[0084] The preparation method and performance test method are the same as those in Example 1, except that, in step (2), no chitosan is added, 19.0 g of graphitized needle coke and 1.0 g of supercritical fluid exfoliated graphene are placed in a soymilk machine and stirred for 100 s to obtain a mixture, which is then ground and sieved (250 mesh) to obtain a negative electrode material.

[0085] Comparative Example 9

[0086] The preparation method and performance testing method are the same as those in Example 1, except that in step (2), chitosan is replaced by asphalt.

[0087] Comparative Example 10

[0088] The preparation method and performance testing method are the same as those in Example 1, except that in step (2), chitosan is replaced by epoxy resin.

[0089] Comparative Example 11

[0090] The preparation method and performance testing method are the same as those in Example 1, except that in step (2), chitosan is replaced by glucose.

[0091] Comparative Example 12

[0092] The preparation method and performance test method are the same as those in Example 1, except that in step (3), the carbonization temperature is 1200°C, that is, the tubular furnace heating program is: from room temperature to 380°C at a heating rate of 2.5°C / min, stay for 1 hour, then heat to 1200°C at a heating rate of 5°C / min, stay for 2 hours, and cool to room temperature.

[0093] Comparative Example 13

[0094] The preparation method and performance test method are the same as those in Example 1, except that in step (3), the temperature rise program of the tubular furnace is: from room temperature to 1600°C at a heating rate of 2.5°C / min, stay for 2 hours, and then cool to room temperature.

[0095] Table 1

[0096] The conductivity data of the supercritical fluid exfoliated graphene in Example 1 and the graphene in Comparative Examples 4-7 are shown in Table 1. Among them, the conductivity of the supercritical fluid exfoliated graphene is as high as 4.5×10 6 S / m, the excellent conductivity can accelerate the transmission of electrons, which is beneficial to improving the rate performance of the negative electrode material, so that the negative electrode material still has a higher capacity at a high rate.

[0097] Table 2 Rate performance (mAh / g)

[0098] As can be seen from Table 2, the introduction of supercritical fluid exfoliated graphene in Examples 1-3 makes the rate performance of the negative electrode material better than that of the commercial graphite negative electrode, the graphitized needle coke raw material and the negative electrode material without adding supercritical fluid exfoliated graphene in Comparative Examples 1-3, indicating that supercritical fluid exfoliated graphene improves the rate performance of the negative electrode material, and the performance is better than that of the commercial graphite negative electrode.

[0099] In Example 2, the amount of supercritical fluid exfoliated graphene added is 2% of the total mass of supercritical fluid exfoliated graphene and graphitized needle coke, and the capacity is 53.6 mAh g at 3C rate. -1 The performance of the negative electrode material at high rate is poor compared with that of Example 1. When the amount of supercritical fluid exfoliated graphene added is 8%, the performance is lower than that of Example 1. This is because, although the amount of graphene is increased in Example 3, the amount of graphene added is relatively large, resulting in incomplete chitosan coating, which is not conducive to fully and tightly adhering graphene to the needle coke after graphitization, and is not conducive to high conductivity, thereby affecting the high rate performance of the negative electrode material.

[0100] By comparing Example 1 with Comparative Examples 4-7, it can be seen that the negative electrode material using supercritical fluid exfoliated graphene in Example 1 has a significantly improved capacity at high rate compared to the negative electrode materials using other graphenes.

[0101] Comparing Example 1 with Comparative Examples 8-11, it can be seen that compared with other coating materials, the negative electrode material of Example 1 still has 59.9 mA g at a 3C rate. -1 This is not only due to the high conductivity of supercritical fluid exfoliated graphene, but also due to the excellent effect of chitosan coating. After graphitization, needle coke, supercritical fluid exfoliated graphene and chitosan cooperate with each other, which is beneficial to further improve the conductivity of the negative electrode material and enhance the capacity of the negative electrode material at high rates.

[0102] By comparing Example 1 with Comparative Examples 12-13, it can be seen that the optimization of the carbonization temperature and the heating process is conducive to further improving the rate performance of the negative electrode material. The carbonization temperature in Example 1 is relatively high, and the carbon layer conductive network formed by chitosan has strong conductivity and excellent performance. In the carbonization process, the temperature is first kept at a low temperature of 300-400°C and then raised to a high temperature. This allows the chitosan to reach a molten state first, thereby better dispersing it in the graphene and graphitized needle coke. Carbonization is then performed to evenly attach the graphene to the surface of the graphitized needle coke. Chitosan forms a thin and uniform carbon shell on the surface of the material, forming a modified graphitized needle coke negative electrode material modified with a two-dimensional graphene sheet structure, further improving the rapid transmission performance of lithium ions, and improving the rate performance and cycle performance.

[0103] Table 3 Cycling performance (mAh / g, 0.1C)

[0104] As can be seen in Table 3, the chitosan carbon coating protects the graphitized needle coke, preventing structural damage caused by long-term immersion of the negative electrode material in the electrolyte during long-term cycling, which helps improve long-term cycling stability. Furthermore, a good coating also facilitates the formation of the SEI film, which in turn improves the cycling performance of the negative electrode material. The coating materials of Comparative Examples 9-11 have relatively low carbon contents, and a complete coating cannot be formed after carbonization. Furthermore, the conductivity of the carbon layer formed after carbonization is inferior to that of the chitosan carbon layer.

[0105] Figure 1 compares the impedance performance of lithium batteries from Examples 1-3 and Comparative Examples 1-2. In Examples 1-3, the introduction of chitosan into the negative electrode materials and the exfoliation of graphene by supercritical fluid significantly reduced the battery impedance. Increasing the graphene addition from 2% in Example 2 to 5% in Example 1 significantly reduced the impedance; further increasing it to 8% in Example 3 revealed a less pronounced impedance reduction. This is primarily due to the increased surface area of ​​the carbon coating caused by the excessive addition of supercritical fluid-exfoliated graphene.

[0106] In the present disclosure, the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present disclosure. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification and features of different embodiments or examples without contradiction.

[0107] Although the above embodiments have been shown and described, it is understood that the above embodiments are illustrative and are not to be construed as limitations on the present disclosure. Changes, modifications, substitutions and variations of the above embodiments by those skilled in the art are all within the scope of protection of the present disclosure.

Claims

1. A method for preparing a negative electrode material, comprising the following steps: (1) Mix graphitized needle coke, supercritical fluid exfoliated graphene, and chitosan to obtain a mixture; (2) Carbonize the mixture to obtain a negative electrode material.

2. The preparation method of the negative electrode material according to claim 1, wherein in the step (1), the preparation method of the supercritical fluid exfoliated graphene comprises: Mix graphite and supercritical CO2, perform fluid shearing, and then discharge CO2 by pressure relief to obtain supercritical fluid exfoliated graphene.

3. The method for preparing a negative electrode material according to claim 2, wherein the temperature of the fluid shearing is 44 - 62 °C; and / or, the pressure of the fluid shearing is 8.6 - 11.3 MPa; and / or, the speed of the fluid shearing is 1600 - 2400 r / min; and / or, the time of the fluid shearing is 15 - 60 min.

4. The preparation method of the negative electrode material according to any one of claims 1 to 3, wherein in the step (1), the D of the graphitized needle coke 50 is 10-19 μm; And / or, the tapped density of the graphitized needle coke is greater than or equal to 1.15 g / cm 3 ; And / or, the specific surface area of the graphitized needle coke is less than or equal to 1.6 m 2 / g; And / or, the compacted density of the graphitized needle coke is 1.45 - 1.55 g / cm 3 .

5. The method for preparing a negative electrode material according to any one of claims 1 to 4, wherein in the step (1), the chitosan is water-soluble chitosan; and / or, the degree of polymerization of the chitosan is greater than 400; and / or, the carbon content of the chitosan is greater than 40%.

6. The method for preparing a negative electrode material according to any one of claims 1 to 5, wherein in the step (1), the mass ratio of the graphitized needle coke, supercritical fluid exfoliated graphene, and chitosan is 70 - 90:0.01 - 15:2 - 10; and / or, the mixing is performed by stirring.

7. The preparation method of the negative electrode material according to any one of claims 1 to 6, wherein in the step (2), the carbonization treatment comprises: Heat at a heating rate of 1 - 5 °C / min to 300 - 400 °C, hold for 0.5 - 3 h, then heat at a heating rate of 1 - 5 °C / min to 1300 - 1800 °C, hold for 0.5 - 3 h, and finally cool down; and / or, the particle size of the negative electrode material is 20 - 70 microns.

8. A negative electrode material, which is prepared by the method according to any one of claims 1 to 7.

9. An application of the negative electrode material according to claim 8, wherein the negative electrode material is used in a lithium-ion battery.

10. A lithium-ion battery, comprising the negative electrode material according to claim 8.

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