Phosphorus-doped coated asphalt and preparation method thereof, negative electrode material and lithium battery

Abstract

The present invention provides a phosphorus-doped coated asphalt and a preparation method thereof, a negative electrode material and a lithium battery. The preparation method of the phosphorus-doped coated asphalt comprises: mixing a phosphorus source with a first carbon source and performing a heat treatment to obtain a phosphorus-doped coated asphalt; the mass ratio of the first carbon source to the phosphorus source is 20: (1-4), and the temperature of the heat treatment is 100-400°C. The present invention also provides a negative electrode material and a lithium battery prepared based on the above-mentioned phosphorus-doped coated asphalt. The present invention modifies the coating layer on the surface of graphite and introduces phosphorus elements into the amorphous carbon coating layer, thereby increasing the lithium storage sites of the graphite negative electrode material and improving the material specific capacity; at the same time, the coating layer structure can be improved, the short-range ordered structure of the coating layer after doping is increased, the interlayer spacing is enlarged, and the rate performance of the graphite negative electrode material is improved.

Description

Technical Field

[0001] The invention relates to phosphorus-doped coated asphalt and a preparation method thereof, a negative electrode material and a lithium battery, belonging to the technical field of lithium ion batteries. Background Art

[0002] Graphite has developed rapidly in many fields due to its excellent properties, and is particularly widely used in semiconductors and new energy materials.

[0003] Currently, most commercial negative electrode materials are made of graphite, which has a theoretical specific capacity of 372 mAh / g, a low reversible capacity, and the graphite flakes are prone to cracking or peeling during the cycle. Based on this, the introduction of a suitable coating layer can effectively stabilize the structure and surface stability of the graphite particles. Currently, the market mostly uses amorphous carbon to coat spherical graphite to improve its structural stability, but this coating has limited improvement effect and cannot improve the rate performance of graphite.

[0004] Doping heteroatoms into the graphite lattice can widen the distance between graphite layers, thereby accelerating the diffusion of lithium ions between graphite layers and improving the rate performance of the graphite negative electrode. Summary of the invention

[0005] In order to solve the above technical problems, the purpose of the present invention is to provide a phosphorus-doped coated asphalt and a preparation method thereof. By doping phosphorus into the coated asphalt, the specific capacity of the lifting material can be increased and the material rate performance can be improved.

[0006] The present invention also aims to provide a negative electrode material prepared by using the above-mentioned phosphorus-doped coated asphalt and a lithium battery containing a negative electrode prepared by using the negative electrode material.

[0007] To achieve the above object, the present invention first provides a method for preparing phosphorus-doped coated asphalt, wherein the method for preparing phosphorus-doped coated asphalt comprises the following steps:

[0008] A phosphorus source is mixed with a first carbon source, and heat-treated in a protective atmosphere to obtain a phosphorus-doped coated asphalt; the mass ratio of the first carbon source to the phosphorus source is 20:(1-4), and the heat treatment temperature is 100-400°C.

[0009] In the above-mentioned method for preparing phosphorus-doped coated asphalt, preferably, the phosphorus source is selected from one or a combination of two or more of triphenylphosphine, phenylphosphonic acid, diphenylphosphine oxide, phenylphosphinic acid, etc.

[0010] In the above-mentioned method for preparing phosphorus-doped coated asphalt, preferably, the first carbon source is selected from one or a combination of two or more of petroleum asphalt, liquid asphalt, coal tar asphalt, etc.

[0011] In the above method for preparing phosphorus-doped coated asphalt, preferably, the phosphorus source and the first carbon source are mixed in a VC mixer; and an inert gas is introduced during the mixing process, the rotation speed of the VC mixer is 800-1500 r / min, and the mixing time is 10-40 min. Preferably, the inert atmosphere includes nitrogen atmosphere, helium atmosphere, neon atmosphere, etc.

[0012] In the above-mentioned method for preparing phosphorus-doped coated asphalt, preferably, the heat treatment time is 2-6 hours, and the heating rate is 1-10°C / min.

[0013] In the above-mentioned method for preparing phosphorus-doped coated asphalt, preferably, the heat treatment is carried out in a vertical reactor furnace or a box furnace.

[0014] In the above-mentioned method for preparing phosphorus-doped coated asphalt, preferably, the protective atmosphere includes an inert atmosphere, and more preferably, the inert atmosphere includes a nitrogen atmosphere, a helium atmosphere, a neon atmosphere, or the like.

[0015] The present invention also provides a phosphorus-doped coated asphalt, which is prepared by the above-mentioned method for preparing the phosphorus-doped coated asphalt.

[0016] The present invention also provides a method for preparing a negative electrode material, wherein the method for preparing the negative electrode material is carried out using the phosphorus-doped coated asphalt provided by the present invention, and specifically comprises the following steps:

[0017] The spherical graphite is mixed with a second carbon source and subjected to a first carbonization treatment in a protective atmosphere to obtain spherical graphite with amorphous carbon coated on the surface; the mass ratio of the spherical graphite to the second carbon source is 100:(1-2);

[0018] The spherical graphite with amorphous carbon coated on the surface is mixed with the phosphorus-doped coated asphalt and subjected to a second carbonization treatment to obtain the surface-modified spherical natural graphite negative electrode material; the mass ratio of the spherical graphite with amorphous carbon coated on the surface to the phosphorus-doped coated asphalt is 100:(1-5).

[0019] In the above-mentioned method for preparing the negative electrode material, preferably, the mixing of the spherical graphite and the second carbon source, and the mixing of the spherical graphite with amorphous carbon coated on the surface and the phosphorus-doped coated asphalt are respectively carried out in a VC mixer; and an inert gas is introduced during the mixing process, the rotation speed of the VC mixer is 800-1500r / min, and the mixing time is 10-40min. Preferably, the inert atmosphere includes nitrogen atmosphere, helium atmosphere, neon atmosphere, etc.

[0020] In the above-mentioned method for preparing the negative electrode material, preferably, the protective atmosphere includes an inert atmosphere, and more preferably, the inert atmosphere includes a nitrogen atmosphere, a helium atmosphere, a neon atmosphere, or the like.

[0021] In the above method for preparing the negative electrode material, preferably, the spherical graphite is natural graphite with a particle size of 10-20 μm.

[0022] In the above method for preparing the negative electrode material, preferably, the second carbon source is selected from one or a combination of two or more of petroleum asphalt, liquid asphalt, coal tar asphalt, etc.

[0023] In the above method for preparing the negative electrode material, preferably, the temperature of the first carbonization is 900-1500° C., the time is 2-10 h, and the heating rate is 1-10° C. / min.

[0024] In the above method for preparing the negative electrode material, preferably, the temperature of the second carbonization is 900-1500° C., the time is 2-10 h, and the heating rate is 1-10° C. / min.

[0025] The present invention modifies the coating layer on the surface of graphite and introduces phosphorus into the amorphous carbon coating layer, thereby increasing the lithium storage sites of the graphite negative electrode material and improving the material specific capacity; at the same time, the coating layer structure can be improved, the short-range ordered structure of the coating layer after doping is increased, the interlayer spacing is expanded, and the rate performance of the graphite negative electrode material is improved.

[0026] The present invention also provides a negative electrode material, wherein the negative electrode material is prepared by the negative electrode material preparation method mentioned above.

[0027] The negative electrode material is a surface-modified spherical natural graphite negative electrode material, the inner core of which is spherical graphite, the middle layer of which is thinner amorphous carbon, and the outer shell of which is phosphorus-doped amorphous carbon.

[0028] The present invention also provides a lithium battery, wherein the negative electrode of the lithium ion battery is made of the negative electrode material mentioned above.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] The negative electrode material of the present invention is a modified coating layer on the surface of spherical graphite. During high-temperature calcination, phosphorus atoms in a phosphorus source are in-situ doped into the coating layer, incorporated into the carbon matrix skeleton in the form of chemical bonds, and uniformly coated on the surface of the carbonized spherical graphite.

[0031] After being doped with phosphorus atoms, the conductive network on the surface of the material can be improved, the interlayer spacing of the carbon material can be widened, the diffusion of lithium ions can be accelerated, and a buffer can be provided for lithium ions to enter the interlayer of spherical graphite. In addition to improving the cycle stability of the material, the negative electrode material of the present invention also improves the rate performance of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a cycle performance diagram of a battery made of the negative electrode materials provided in the embodiments and comparative examples;

[0033] Figure 2 It is a rate performance diagram of batteries made of materials provided in the embodiments and comparative examples;

[0034] Figure 3 is a high-resolution transmission electron microscopy image of the negative electrode materials provided in Comparative Example 1 and Example 3;

[0035] Figure 4 It is a graph showing the electrochemical test results of a 18650 full battery system made of the negative electrode materials provided in Example 3 and Comparative Examples 2-4. DETAILED DESCRIPTION

[0036] In order to have a clearer understanding of the technical features, purposes and beneficial effects of the present invention, the technical solution of the present invention is now described in detail below, but it should not be construed as limiting the applicable scope of the present invention.

[0037] Example 1

[0038] This embodiment provides a surface-modified spherical natural graphite negative electrode material, which is prepared by the following steps:

[0039] 1. Preparation of coated asphalt:

[0040] 7.5 g of phenylphosphonic acid and 50 g of petroleum asphalt were mixed in a VC mixer for 30 min, inert gas was introduced into the mixer, and the rotation speed was 500 r / min to obtain a first mixture;

[0041] The first mixture is placed in a vertical reactor furnace under a protective atmosphere and heat treated at 200° C. for 3 hours, wherein the stirring speed of the vertical reactor is 30 rpm and the heating rate is 5° C. / min, to obtain a phosphorus-doped coated asphalt;

[0042] 2. Preparation of primary carbonized spherical graphite:

[0043] 20 g of petroleum asphalt and 1000 g of spherical graphite (particle size of 16-18 μm) were mixed in a VC mixer for 30 min, inert gas was introduced into the mixer, and the speed was 1000 r / min to obtain a second mixture;

[0044] The second mixed material is placed in a box furnace under a protective atmosphere, heated to 1100° C. at a heating rate of 5° C. / min, then kept at this temperature for 4 hours, and finally cooled to room temperature at a cooling rate of 5° C. / min to obtain primary carbonized spherical graphite;

[0045] 3. Preparation of modified negative electrode materials:

[0046] 40 g of the phosphorus-doped coated asphalt and 1000 g of primary carbonized spherical graphite were placed in a VC mixer and mixed for 30 min, with inert gas introduced into the mixer at a mixing speed of 1000 r / min to obtain a third mixture;

[0047] The third mixture was placed in a box furnace under a protective atmosphere and heated to 1000°C at a heating rate of 5°C / min, then kept at this temperature for 4 hours, and finally cooled to room temperature at a cooling rate of 5°C / min to obtain a modified material. The material was taken out and sieved to obtain a surface-modified spherical natural graphite negative electrode material, named Example 1.

[0048] Example 2

[0049] This embodiment provides a surface-modified spherical natural graphite negative electrode material, which is prepared by the following steps:

[0050] 1. Preparation of modified coating layer:

[0051] 7.5 g of phenylphosphonic acid and 50 g of petroleum asphalt were mixed in a VC mixer for 30 min, inert gas was introduced into the mixer, and the rotation speed was 500 r / min to obtain a first mixture;

[0052] The first mixture is placed in a vertical reactor furnace under a protective atmosphere and heat treated at 200° C. for 3 hours, wherein the stirring speed of the vertical reactor is 30 rpm and the heating rate is 5° C. / min, to obtain a phosphorus-doped coated asphalt;

[0053] 2. Preparation of primary carbonized spherical graphite:

[0054] 20 g of petroleum asphalt and 1000 g of spherical graphite (particle size of 16-18 μm) were mixed in a VC mixer for 30 min, inert gas was introduced into the mixer, and the speed was 1000 r / min to obtain a second mixture;

[0055] The second mixed material is placed in a box furnace under a protective atmosphere, heated to 1100° C. at a heating rate of 5° C. / min, then kept at this temperature for 4 hours, and finally cooled to room temperature at a cooling rate of 5° C. / min to obtain primary carbonized spherical graphite;

[0056] 3. Preparation of modified negative electrode materials:

[0057] 40 g of the phosphorus-doped coated asphalt and 1000 g of primary carbonized spherical graphite were placed in a VC mixer and mixed for 30 min, with inert gas introduced into the mixer at a mixing speed of 1000 r / min to obtain a third mixture;

[0058] The third mixture was placed in a box furnace under a protective atmosphere and heated to 1100°C at a heating rate of 5°C / min, then kept at this temperature for 4 hours, and finally cooled to room temperature at a cooling rate of 5°C / min to obtain a modified material. The material was taken out and sieved to obtain a surface-modified spherical natural graphite negative electrode material, named Example 2.

[0059] Example 3

[0060] This embodiment provides a surface-modified spherical natural graphite negative electrode material, which is prepared by the following steps:

[0061] 1. Preparation of modified coating layer:

[0062] 7.5 g of phenylphosphonic acid and 50 g of petroleum asphalt were mixed in a VC mixer for 30 min, inert gas was introduced into the mixer, and the rotation speed was 500 r / min to obtain a first mixture;

[0063] The first mixture is placed in a vertical reactor furnace under a protective atmosphere and heat treated at 400° C. for 3 hours, wherein the stirring speed of the vertical reactor is 30 rpm and the heating rate is 5° C. / min, to obtain a phosphorus-doped coated asphalt;

[0064] 2. Preparation of primary carbonized spherical graphite:

[0065] 20 g of petroleum asphalt and 1000 g of spherical graphite (particle size of 16-18 μm) were mixed in a VC mixer for 30 min, inert gas was introduced into the mixer, and the speed was 1000 r / min to obtain a second mixture;

[0066] The second mixed material is placed in a box furnace under a protective atmosphere, heated to 1100° C. at a heating rate of 5° C. / min, then kept at this temperature for 4 hours, and finally cooled to room temperature at a cooling rate of 5° C. / min to obtain primary carbonized spherical graphite;

[0067] 3. Preparation of modified negative electrode materials:

[0068] 40 g of the phosphorus-doped coated asphalt and 1000 g of primary carbonized spherical graphite were placed in a VC mixer and mixed for 30 min, with inert gas introduced into the mixer at a mixing speed of 1000 r / min to obtain a third mixture;

[0069] The third mixture was placed in a box furnace under a protective atmosphere and heated to 1100°C at a heating rate of 5°C / min, then kept at this temperature for 5 hours, and finally cooled to room temperature at a cooling rate of 5°C / min to obtain a modified material. The material was taken out and sieved to obtain a surface-modified spherical natural graphite negative electrode material, named Example 3.

[0070] Example 4

[0071] This embodiment provides a surface-modified spherical natural graphite negative electrode material, which is prepared by the following steps:

[0072] 1. Preparation of modified coating layer:

[0073] 10 g of phenylphosphonic acid and 50 g of petroleum asphalt were mixed in a VC mixer for 30 min, an inert gas was introduced into the mixer, and the rotation speed was 500 r / min to obtain a first mixture;

[0074] The first mixture is placed in a vertical reactor furnace under a protective atmosphere and heat treated at 400° C. for 3 hours, wherein the stirring speed of the vertical reactor is 30 rpm and the heating rate is 5° C. / min, to obtain a phosphorus-doped coated asphalt;

[0075] 2. Preparation of primary carbonized spherical graphite:

[0076] 20 g of petroleum asphalt and 1000 g of spherical graphite (particle size of 16-18 μm) were mixed in a VC mixer for 30 min, inert gas was introduced into the mixer, and the speed was 1000 r / min to obtain a second mixture;

[0077] The second mixed material is placed in a box furnace under a protective atmosphere, heated to 1100° C. at a heating rate of 5° C. / min, then kept at this temperature for 4 hours, and finally cooled to room temperature at a cooling rate of 5° C. / min to obtain primary carbonized spherical graphite;

[0078] 3. Preparation of modified negative electrode materials:

[0079] 40 g of the phosphorus-doped coated asphalt and 1000 g of primary carbonized spherical graphite were placed in a VC mixer and mixed for 30 min, with inert gas introduced into the mixer at a mixing speed of 1000 r / min to obtain a third mixture;

[0080] The third mixture was placed in a box furnace under a protective atmosphere and heated to 1100°C at a heating rate of 5°C / min, then kept at this temperature for 5 hours, and finally cooled to room temperature at a cooling rate of 5°C / min to obtain a modified material. The material was taken out and sieved to obtain a surface-modified spherical natural graphite negative electrode material, named Example 4.

[0081] Example 5

[0082] This embodiment provides a surface-modified spherical natural graphite negative electrode material, which is prepared by the following steps:

[0083] 1. Preparation of modified coating layer:

[0084] 5 g of phenylphosphonic acid and 50 g of petroleum asphalt were mixed in a VC mixer for 30 min, an inert gas was introduced into the mixer, and the rotation speed was 500 r / min to obtain a first mixture;

[0085] The first mixture is placed in a vertical reactor furnace under a protective atmosphere and heat treated at 400° C. for 3 hours, wherein the stirring speed of the vertical reactor is 30 rpm and the heating rate is 5° C. / min, to obtain a phosphorus-doped coated asphalt;

[0086] 2. Preparation of primary carbonized spherical graphite:

[0087] 20 g of petroleum asphalt and 1000 g of spherical graphite (particle size of 16-18 μm) were mixed in a VC mixer for 30 min, inert gas was introduced into the mixer, and the speed was 1000 r / min to obtain a second mixture;

[0088] The second mixed material is placed in a box furnace under a protective atmosphere, heated to 1100° C. at a heating rate of 5° C. / min, then kept at this temperature for 4 hours, and finally cooled to room temperature at a cooling rate of 5° C. / min to obtain primary carbonized spherical graphite;

[0089] 3. Preparation of modified negative electrode materials:

[0090] 40 g of the phosphorus-doped coated asphalt and 1000 g of primary carbonized spherical graphite were placed in a VC mixer and mixed for 30 min, with inert gas introduced into the mixer at a mixing speed of 1000 r / min to obtain a third mixture;

[0091] The third mixture was placed in a box furnace under a protective atmosphere and heated to 1100°C at a heating rate of 5°C / min, then kept at this temperature for 5 hours, and finally cooled to room temperature at a cooling rate of 5°C / min to obtain a modified material. The material was taken out and sieved to obtain a surface-modified spherical natural graphite negative electrode material, named Example 5.

[0092] Example 6

[0093] This embodiment provides a surface-modified spherical natural graphite negative electrode material, which is prepared by the following steps:

[0094] 1. Preparation of modified coating layer:

[0095] 7.5 g of phenylphosphonic acid and 50 g of petroleum asphalt were mixed in a VC mixer for 30 min, inert gas was introduced into the mixer, and the rotation speed was 500 r / min to obtain a first mixture;

[0096] The first mixture is placed in a vertical reactor furnace under a protective atmosphere and heat treated at 400° C. for 3 hours, wherein the stirring speed of the vertical reactor is 30 rpm and the heating rate is 5° C. / min, to obtain a phosphorus-doped coated asphalt;

[0097] 2. Preparation of primary carbonized spherical graphite:

[0098] 20 g of petroleum asphalt and 1000 g of spherical graphite (particle size of 16-18 μm) were mixed in a VC mixer for 30 min, inert gas was introduced into the mixer, and the speed was 1000 r / min to obtain a second mixture;

[0099] The second mixed material is placed in a box furnace under a protective atmosphere, heated to 1100° C. at a heating rate of 5° C. / min, then kept at this temperature for 4 hours, and finally cooled to room temperature at a cooling rate of 5° C. / min to obtain primary carbonized spherical graphite;

[0100] 3. Preparation of modified negative electrode materials:

[0101] 30 g of the phosphorus-doped coated asphalt and 1000 g of primary carbonized spherical graphite were placed in a VC mixer and mixed for 30 min, with inert gas introduced into the mixer at a mixing speed of 1000 r / min to obtain a third mixture;

[0102] The third mixture was placed in a box furnace under a protective atmosphere and heated to 1100°C at a heating rate of 5°C / min, then kept at this temperature for 5 hours, and finally cooled to room temperature at a cooling rate of 5°C / min to obtain a modified material. The material was taken out and sieved to obtain a surface-modified spherical natural graphite negative electrode material, named Example 6.

[0103] Comparative Example 1

[0104] This comparative example provides a modified graphite negative electrode material coated with asphalt, which is prepared by the following steps:

[0105] 1. Preparation of modified coating layer:

[0106] 1 g of phenylphosphonic acid and 50 g of petroleum asphalt were mixed in a VC mixer for 30 min, an inert gas was introduced into the mixer, and the rotation speed was 500 r / min to obtain a first mixture;

[0107] The first mixture is placed in a vertical reactor furnace under a protective atmosphere and heat treated at 400° C. for 3 hours, wherein the stirring speed of the vertical reactor is 30 rpm and the heating rate is 5° C. / min, to obtain a phosphorus-doped coated asphalt;

[0108] 2. Preparation of primary carbonized spherical graphite:

[0109] 20 g of petroleum asphalt and 1000 g of spherical graphite (particle size of 16-18 μm) were mixed in a VC mixer for 30 min, inert gas was introduced into the mixer, and the speed was 1000 r / min to obtain a second mixture;

[0110] The second mixed material is placed in a box furnace under a protective atmosphere, heated to 1100° C. at a heating rate of 5° C. / min, then kept at this temperature for 4 hours, and finally cooled to room temperature at a cooling rate of 5° C. / min to obtain primary carbonized spherical graphite;

[0111] 3. Preparation of modified negative electrode materials:

[0112] 40 g of the phosphorus-doped coated asphalt and 1000 g of primary carbonized spherical graphite were placed in a VC mixer and mixed for 30 min, with inert gas introduced into the mixer at a mixing speed of 1000 r / min to obtain a third mixture;

[0113] The third mixture was placed in a box furnace under a protective atmosphere and heated to 1100°C at a heating rate of 5°C / min, followed by carbonization treatment at this temperature for 5 hours, and then cooled to room temperature at a cooling rate of 5°C / min to obtain a modified material. The modified material was taken out and sieved to obtain a modified graphite negative electrode material coated with asphalt, which was named Comparative Example 1.

[0114] Comparative Example 2

[0115] A natural graphite product 1 available on the market was purchased and named as comparative example 2. The specific specification parameters are shown in Table 1.

[0116] Comparative Example 3

[0117] A natural graphite product 2 sold on the market was purchased and named as comparative example 3. The specific specification parameters are shown in Table 1.

[0118] Comparative Example 4

[0119] A natural graphite product 3 available on the market was purchased and named as comparative example 4. The specific specification parameters are shown in Table 1.

[0120] Table 1 Specifications of natural graphite products in comparative examples

[0121]

[0122] The negative electrode materials of Examples 1-6 and Comparative Examples 1-4 were made into electrodes, and 2032 button cells were assembled for electrochemical testing. The electrochemical testing was performed according to the following steps:

[0123] The materials to be tested were mixed evenly in a mass ratio of negative electrode material: conductive agent (Super P): binder (LA133) = 94:3:3, coated on copper foil with a coating thickness of 200 μm, and placed in a vacuum oven at 80°C for 12 hours to obtain a negative electrode sheet;

[0124] CR2032 button cells were assembled in a glove box (water and oxygen were less than 0.01 ppm), where lithium sheets were used as counter electrodes and the electrolyte was 1 mol / L LiF 6 Dissolved in DMC:DEC:EC (volume ratio 1:1:1);

[0125] The assembled button cell was tested on the BlueDian system at a current density of 1C for charge and discharge cycles and at rates of 0.1C / 0.3C / 0.5C / 1C / 3C / 5C. The test results are shown in Table 2. Figure 1-Figure 4 shown.

[0126] Table 2 Electrochemical performance data of various embodiments and comparative examples

[0127]

[0128] It can be seen from Table 2 that after modification, the first-week reversible capacity and the capacity retention rate after 100 cycles corresponding to the negative electrode material of the embodiment are greatly improved. Among them, the improvement in reversible capacity comes from the formation of chemical bonds such as P=O / PC in the surface coating layer after phosphorus doping. The formation of these chemical bonds helps to enhance the adsorption energy of the material to lithium ions, thereby improving the capacity. Properly increasing the carbonization temperature and time can better exert the electrochemical properties of the modified material, but more added amounts of doping will cause too many surface defects of the material, consume more electrolyte during the first week of charge and discharge, resulting in a decrease in reversible capacity, and the cycle process will continue to consume electrolyte. Although the value of the capacity retention rate is still high, it shows a trend of decreasing with the increase of the doping amount; in addition, the strong adsorption energy of phosphorus to lithium ions also means that a more stable lithium insertion structure is formed during the charge and discharge process, which can improve the cycle performance.

[0129] from Figure 1-Figure 2 It can be seen that the cycle performance and rate performance of Examples 1-6 in which the coating layer is doped with phosphorus are better than those of Comparative Example 1. Such performance comparison is due to the fact that the phosphorus content in Comparative Example 1 is relatively low, and the coating layer structure is still mainly composed of a large amount of amorphous carbon, and the structural order is relatively low, so the cycle performance of the material cannot be effectively improved. At the same time, the low content of phosphorus doping fails to effectively expand the interlayer spacing of the coating layer, and cannot well improve the lithium ion transmission kinetics. Figure 3As shown, after effective phosphorus doping, the orderliness of the coating layer structure increases, the coating layer structure is more stable, and the cycle stability of the material is improved. At the same time, the interlayer spacing of the coating layer is enlarged after doping, which is conducive to the diffusion of lithium ions and provides a buffer for lithium ions to enter the graphite layer, thereby improving the rate performance.

[0130] The embodiment 3 with better performance and the comparative examples 2-4 were subjected to 18650 full battery system electrochemical test. The test was carried out according to the following steps: NCM811 was used as the positive electrode, and the materials of the embodiment 3 and the comparative examples 2-4 were used to make the negative electrode. The assembly and test were carried out according to the national standard GB / T18287-2013. The test results are shown in FIG. Figure 4 As shown, it can be seen that the 18650 full battery made of the negative electrode material of Example 3 still has a capacity retention rate of up to 90.2% after 800 cycles, the capacity retention rate of the 18650 full battery made of the graphite product of Comparative Example 2 drops below 90% at 500 cycles, and the capacity retention rate of the 18650 full battery made of the graphite products of Comparative Examples 3 and 4 drops below 90% after 250 cycles. These comparison results show that the modified material of the present invention has excellent cycle performance and has met market demand.

[0131] According to the above results, it can be seen that the present invention modifies the coating layer of spherical graphite by phosphorus doping, which, on the one hand, provides more lithium storage sites after doping, improves the specific capacity of the material, and improves the structural orderliness of the coating layer after doping ( Figure 3 ), thereby improving the cycle life of the material; on the other hand, doping improves the conductive network on the surface of the material, widens the interlayer spacing of the carbon material, accelerates the diffusion of lithium ions, and provides a buffer for lithium ions to enter the spherical graphite interlayer. In addition to improving the cycle stability of the material, the modified material also improves the rate performance of the material.

Claims

1. A method for preparing a negative electrode material, characterized in that: The method for preparing the negative electrode material comprises the following steps: Phenylphosphonic acid and a first carbon source are mixed in a VC mixer at a speed of 500 r / min for 30 min and an inert gas is introduced during the mixing process, and then heat-treated at 400° C. for 3 h in a protective atmosphere to obtain phosphorus-doped coated asphalt; wherein the first carbon source is petroleum asphalt, the mass ratio of the first carbon source to the phenylphosphonic acid is 20:(2-3), and the heating rate of the heat treatment is 5° C. / min; Spherical graphite is mixed with a second carbon source, and subjected to a first carbonization treatment in a protective atmosphere to obtain spherical graphite with amorphous carbon coated on the surface; the mass ratio of the spherical graphite to the second carbon source is 100:(1-2), the spherical graphite is natural graphite with a particle size of 16-18 μm; the second carbon source is petroleum asphalt; The spherical graphite with amorphous carbon coated on the surface is mixed with the phosphorus-doped coated asphalt and subjected to a second carbonization treatment to obtain the surface-modified spherical natural graphite negative electrode material; the mass ratio of the spherical graphite with amorphous carbon coated on the surface to the phosphorus-doped coated asphalt is 100:(1-5); wherein the temperature of the second carbonization is 900-1500°C.

2. The method for preparing the negative electrode material according to claim 1, characterized in that: The mixing of the spherical graphite and petroleum asphalt, and the mixing of the spherical graphite with amorphous carbon coated on the surface and the phosphorus-doped coated asphalt are respectively carried out in a VC mixer; Inert gas is introduced during the mixing process, the rotation speed of the VC mixer is 800-1500 r / min, and the mixing time is 10-40 min.

3. The method for preparing the negative electrode material according to claim 1, characterized in that: The temperature of the first carbonization is 900-1500°C, the time is 2-10h, and the heating rate is 1-10°C / min; The second carbonization time is 2-10 hours, and the heating rate is 1-10°C / min.

4. A negative electrode material, characterized in that: The negative electrode material is prepared by the method for preparing the negative electrode material according to any one of claims 1 to 3.

5. A lithium battery, characterized in that: The negative electrode of the lithium battery is made of the negative electrode material according to claim 4.

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