Negative electrode material and preparation method therefor, and lithium battery

Abstract

The present invention provides a negative electrode material and a preparation method therefor, and a lithium battery. The present invention further provides a negative electrode material prepared on the basis of phosphorus-doped coating pitch, and a lithium battery. In the present invention, by modifying a coating layer on a graphite surface, and introducing phosphorus into an amorphous carbon coating layer, the number of lithium storage sites of a graphite negative electrode material can be increased, enhancing the specific capacity of the material; additionally, the structure of the coating layer can be improved, the short-range ordered structure of the doped coating layer is increased, so that the interlayer spacing is expanded, improving the rate performance of the graphite negative electrode material.

Description

A negative electrode material and its preparation method and lithium battery Technical Field

[0001] This invention relates to a negative electrode material, its preparation method, and a lithium battery, belonging to the field of lithium-ion battery technology. Background Technology

[0002] Graphite has seen rapid development in many fields due to its excellent properties, especially in semiconductors and new energy materials.

[0003] Most commercially available anode materials currently use graphite. Graphite has a theoretical specific capacity of 372 mAh / g, but its reversible capacity is low, and graphite sheets are prone to cracking or peeling during cycling. Therefore, introducing a suitable coating layer can effectively stabilize the structure and surface stability of graphite particles. Currently, the market often 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 interlayer spacing of graphite, thereby accelerating the diffusion of lithium ions between graphite layers and improving the rate performance of the graphite anode. Summary of the Invention

[0005] To address the aforementioned technical problems, the present invention aims to provide a phosphorus-doped coated asphalt and its preparation method. By doping phosphorus into the coated asphalt, the specific capacity of the material can be increased, thereby improving the material's rate performance.

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

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

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

[0009] When the heat treatment temperature is controlled at 400℃, the effective doping ratio and doping success rate are relatively high, and coated asphalt with good doping effect can be prepared.

[0010] In the above-mentioned method for preparing phosphorus-doped coated pitch, preferably, the phosphorus source is selected from one or more combinations of triphenylphosphine, phenylphosphonic acid, diphenylphosphine oxide, phenylphosphine trioxide, etc.

[0011] In the above-mentioned method for preparing phosphorus-doped coated asphalt, preferably, the first carbon source is selected from one or more combinations of petroleum asphalt, liquid asphalt, coal tar pitch, etc.

[0012] In the above-described method for preparing phosphorus-doped coated asphalt, preferably, the mixing of the phosphorus source and the first carbon source is carried out in a VC mixer; and an inert gas is introduced during the mixing process, the VC mixer rotates at 800-1500 r / min, and the mixing time is 10-40 min. Preferably, the inert atmosphere includes nitrogen atmosphere, helium atmosphere, neon atmosphere, etc.

[0013] In the above-mentioned method for preparing phosphorus-doped coated pitch, preferably, the heat treatment time is 2-6 hours and the heating rate is 1-10℃ / min.

[0014] In the above-mentioned method for preparing phosphorus-doped coated pitch, preferably, the heat treatment is carried out in a vertical reactor furnace or a box furnace.

[0015] In the above-mentioned method for preparing phosphorus-doped coated pitch, preferably, the protective atmosphere includes an inert atmosphere, more preferably, the inert atmosphere includes a nitrogen atmosphere, a helium atmosphere, a neon atmosphere, etc.

[0016] The present invention also provides a phosphorus-doped coated bitumen, which is prepared by the above-described method for preparing phosphorus-doped coated bitumen.

[0017] 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 pitch provided by the present invention, and specifically includes the following steps:

[0018] 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 coating on its surface; the mass ratio of the spherical graphite to the second carbon source is 100:(1-2);

[0019] The spherical graphite with amorphous carbon coating on its surface is mixed with the phosphorus-doped coated pitch and subjected to a second carbonization treatment to obtain the surface-modified spherical natural graphite anode material; the mass ratio of the spherical graphite with amorphous carbon coating on its surface to the phosphorus-doped coated pitch is 100:(1-5).

[0020] In the above-mentioned method for preparing the negative electrode material, preferably, the mixing of spherical graphite with the second carbon source and the mixing of spherical graphite with amorphous carbon coating with phosphorus-doped coated pitch are carried out in a VC mixer; and an inert gas is introduced during the mixing process, the VC mixer speed 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.

[0021] In the above-mentioned method for preparing the negative electrode material, preferably, the protective atmosphere includes an inert atmosphere, more preferably, the inert atmosphere includes a nitrogen atmosphere, a helium atmosphere, a neon atmosphere, etc.

[0022] In the above-mentioned method for preparing the negative electrode material, preferably, the spherical graphite is natural graphite with a particle size of 10-20 μm.

[0023] In the above-mentioned method for preparing the negative electrode material, preferably, the second carbon source is selected from one or more combinations of petroleum asphalt, liquid asphalt, coal tar pitch, etc.

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

[0025] In the above-mentioned method for preparing the negative electrode material, preferably, the temperature of the second carbonization is 900-1500℃, the time is 2-10h, and the heating rate is 1-10℃ / min.

[0026] This invention modifies the coating layer on the surface of graphite by introducing phosphorus into the amorphous carbon coating layer, which can increase the lithium storage sites of the graphite anode material and improve the specific capacity of the material. At the same time, it can improve the coating layer structure. After doping, the short-range ordered structure of the coating layer increases and the interlayer spacing expands, thereby improving the rate performance of the graphite anode material.

[0027] The present invention also provides a negative electrode material, wherein the negative electrode material is prepared by the above-described method for preparing a negative electrode material.

[0028] This anode material is a surface-modified spherical natural graphite anode material with a core of spherical graphite, a middle layer of relatively thin amorphous carbon, and a shell of phosphorus-doped amorphous carbon.

[0029] The present invention also provides a lithium battery, wherein the negative electrode of the lithium-ion battery is made of the aforementioned negative electrode material.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 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 from the phosphorus source are in situ doped into the coating layer and incorporated into the carbon matrix skeleton in the form of chemical bonds, and uniformly coated on the surface of the carbonized spherical graphite.

[0032] Phosphorus doping improves the conductive network on the material surface, widens the interlayer spacing of carbon materials, accelerates lithium-ion diffusion, and provides a buffer 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 this invention also improves the rate performance of the material. Attached Figure Description

[0033] Figure 1 is a graph showing the cycle performance of batteries made from the negative electrode materials provided in the embodiments and comparative examples;

[0034] Figure 2 is a rate performance diagram of batteries made from the materials provided in the embodiments and comparative examples;

[0035] Figure 3 is a high-resolution transmission electron microscope image of the negative electrode materials provided in Comparative Example 1 and Example 3;

[0036] Figure 4 shows the electrochemical test results of the 18650 full cell system made with the negative electrode materials provided in Example 3 and Comparative Examples 2-4;

[0037] Figure 5 is the XPS full scan spectrum of the phosphorus-doped coated pitch provided in Example 1;

[0038] Figure 6 is a high-resolution XPS spectrum of carbon elements in the phosphorus-doped coated pitch provided in Example 1;

[0039] Figure 7 is a high-resolution XPS spectrum of oxygen in the phosphorus-doped coated pitch provided in Example 1;

[0040] Figure 8 is a high-resolution XPS spectrum of phosphorus in the phosphorus-doped coated pitch provided in Example 1;

[0041] Figure 9 is a diagram showing the first-principles calculation results of the negative electrode material provided in Example 1. Detailed Implementation

[0042] In order to provide a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention will now be described in detail below, but it should not be construed as limiting the scope of implementation of the present invention.

[0043] Example 1

[0044] This embodiment provides a surface-modified spherical natural graphite anode material, which is prepared through the following steps:

[0045] 1. Preparation of coated asphalt:

[0046] 7.5g of phenylphosphonic acid and 50g of petroleum asphalt were mixed in a VC mixer for 30 minutes. Inert gas was introduced into the mixer and the rotation speed was 500r / min to obtain the first mixture.

[0047] The first mixture was placed in a vertical reactor furnace under a protective atmosphere and heat-treated at 200°C for 3 hours. The stirring speed of the vertical reactor was 30 rpm and the heating rate was 5°C / min, to obtain phosphorus-doped coated asphalt.

[0048] 2. Preparation of primary carbonized spherical graphite:

[0049] 20g of petroleum asphalt and 1000g of spherical graphite (particle size 16-18μm) were mixed in a VC mixer for 30min. Inert gas was introduced into the mixer and the rotation speed was 1000r / min to obtain a second mixture.

[0050] The second mixture was placed in a box furnace under a protective atmosphere and heated to 1100°C at a heating rate of 5°C / min. It was then held 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.

[0051] 3. Preparation of modified negative electrode materials:

[0052] 40g of the phosphorus-doped coated pitch and 1000g of primary carbonized spherical graphite were placed in a VC mixer and mixed for 30min. Inert gas was introduced into the mixer and the mixing speed was 1000r / min to obtain a third mixture.

[0053] 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. It was then held at this temperature for 4 hours and finally cooled to room temperature at a cooling rate of 5°C / min to obtain the modified material. The modified material was then removed, sieved, and surface-modified spherical natural graphite anode material was obtained, which was named Example 1.

[0054] Example 2

[0055] This embodiment provides a surface-modified spherical natural graphite anode material, which is prepared through the following steps:

[0056] 1. Preparation of the modified coating layer:

[0057] 7.5g of phenylphosphonic acid and 50g of petroleum asphalt were mixed in a VC mixer for 30 minutes. Inert gas was introduced into the mixer and the rotation speed was 500r / min to obtain the first mixture.

[0058] The first mixture was placed in a vertical reactor furnace under a protective atmosphere and heat-treated at 200°C for 3 hours. The stirring speed of the vertical reactor was 30 rpm and the heating rate was 5°C / min, to obtain phosphorus-doped coated asphalt.

[0059] 2. Preparation of primary carbonized spherical graphite:

[0060] 20g of petroleum asphalt and 1000g of spherical graphite (particle size 16-18μm) were mixed in a VC mixer for 30min. Inert gas was introduced into the mixer and the rotation speed was 1000r / min to obtain a second mixture.

[0061] The second mixture was placed in a box furnace under a protective atmosphere and heated to 1100°C at a heating rate of 5°C / min. It was then held 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.

[0062] 3. Preparation of modified negative electrode materials:

[0063] 40g of the phosphorus-doped coated pitch and 1000g of primary carbonized spherical graphite were placed in a VC mixer and mixed for 30min. Inert gas was introduced into the mixer and the mixing speed was 1000r / min to obtain a third mixture.

[0064] 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. It was then held at this temperature for 4 hours and finally cooled to room temperature at a cooling rate of 5°C / min to obtain the modified material. The modified material was then removed, sieved, and surface-modified spherical natural graphite anode material was obtained, which was named Example 2.

[0065] Example 3

[0066] This embodiment provides a surface-modified spherical natural graphite anode material, which is prepared through the following steps:

[0067] 1. Preparation of the modified coating layer:

[0068] 7.5g of phenylphosphonic acid and 50g of petroleum asphalt were mixed in a VC mixer for 30 minutes. Inert gas was introduced into the mixer and the rotation speed was 500r / min to obtain the first mixture.

[0069] The first mixture was placed in a vertical reactor furnace under a protective atmosphere and heat-treated at 400°C for 3 hours. The stirring speed of the vertical reactor was 30 rpm and the heating rate was 5°C / min, to obtain phosphorus-doped coated asphalt.

[0070] 2. Preparation of primary carbonized spherical graphite:

[0071] 20g of petroleum asphalt and 1000g of spherical graphite (particle size 16-18μm) were mixed in a VC mixer for 30min. Inert gas was introduced into the mixer and the rotation speed was 1000r / min to obtain a second mixture.

[0072] The second mixture was placed in a box furnace under a protective atmosphere and heated to 1100°C at a heating rate of 5°C / min. It was then held 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.

[0073] 3. Preparation of modified negative electrode materials:

[0074] 40g of the phosphorus-doped coated pitch and 1000g of primary carbonized spherical graphite were placed in a VC mixer and mixed for 30min. Inert gas was introduced into the mixer and the mixing speed was 1000r / min to obtain a third mixture.

[0075] 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. It was then held at this temperature for 5 hours and finally cooled to room temperature at a cooling rate of 5°C / min to obtain the modified material. The material was then removed, sieved, and surface-modified spherical natural graphite anode material was obtained, which was named Example 3.

[0076] Example 4

[0077] This embodiment provides a surface-modified spherical natural graphite anode material, which is prepared through the following steps:

[0078] 1. Preparation of the modified coating layer:

[0079] 10g of phenylphosphonic acid and 50g of petroleum asphalt were mixed in a VC mixer for 30 minutes. Inert gas was introduced into the mixer and the rotation speed was 500r / min to obtain the first mixture.

[0080] The first mixture was placed in a vertical reactor furnace under a protective atmosphere and heat-treated at 400°C for 3 hours. The stirring speed of the vertical reactor was 30 rpm and the heating rate was 5°C / min, to obtain phosphorus-doped coated asphalt.

[0081] 2. Preparation of primary carbonized spherical graphite:

[0082] 20g of petroleum asphalt and 1000g of spherical graphite (particle size 16-18μm) were mixed in a VC mixer for 30min. Inert gas was introduced into the mixer and the rotation speed was 1000r / min to obtain a second mixture.

[0083] The second mixture was placed in a box furnace under a protective atmosphere and heated to 1100°C at a heating rate of 5°C / min. It was then held 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.

[0084] 3. Preparation of modified negative electrode materials:

[0085] 40g of the phosphorus-doped coated pitch and 1000g of primary carbonized spherical graphite were placed in a VC mixer and mixed for 30min. Inert gas was introduced into the mixer and the mixing speed was 1000r / min to obtain a third mixture.

[0086] 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. It was then held at this temperature for 5 hours and finally cooled to room temperature at a cooling rate of 5°C / min to obtain the modified material. The material was then removed, sieved, and surface-modified spherical natural graphite anode material was obtained, which was named Example 4.

[0087] Example 5

[0088] This embodiment provides a surface-modified spherical natural graphite anode material, which is prepared through the following steps:

[0089] 1. Preparation of the modified coating layer:

[0090] 5g of phenylphosphonic acid and 50g of petroleum asphalt were mixed in a VC mixer for 30 minutes. Inert gas was introduced into the mixer and the rotation speed was 500r / min to obtain the first mixture.

[0091] The first mixture was placed in a vertical reactor furnace under a protective atmosphere and heat-treated at 400°C for 3 hours. The stirring speed of the vertical reactor was 30 rpm and the heating rate was 5°C / min, to obtain phosphorus-doped coated asphalt.

[0092] 2. Preparation of primary carbonized spherical graphite:

[0093] 20g of petroleum asphalt and 1000g of spherical graphite (particle size 16-18μm) were mixed in a VC mixer for 30min. Inert gas was introduced into the mixer and the rotation speed was 1000r / min to obtain a second mixture.

[0094] The second mixture was placed in a box furnace under a protective atmosphere and heated to 1100°C at a heating rate of 5°C / min. It was then held 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.

[0095] 3. Preparation of modified negative electrode materials:

[0096] 40g of the phosphorus-doped coated pitch and 1000g of primary carbonized spherical graphite were placed in a VC mixer and mixed for 30min. Inert gas was introduced into the mixer and the mixing speed was 1000r / min to obtain a third mixture.

[0097] 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. It was then held at this temperature for 5 hours and finally cooled to room temperature at a cooling rate of 5°C / min to obtain the modified material. The modified material was then removed, sieved, and surface-modified spherical natural graphite anode material was obtained, which was named Example 5.

[0098] Example 6

[0099] This embodiment provides a surface-modified spherical natural graphite anode material, which is prepared through the following steps:

[0100] 1. Preparation of the modified coating layer:

[0101] 7.5g of phenylphosphonic acid and 50g of petroleum asphalt were mixed in a VC mixer for 30 minutes. Inert gas was introduced into the mixer and the rotation speed was 500r / min to obtain the first mixture.

[0102] The first mixture was placed in a vertical reactor furnace under a protective atmosphere and heat-treated at 400°C for 3 hours. The stirring speed of the vertical reactor was 30 rpm and the heating rate was 5°C / min, to obtain phosphorus-doped coated asphalt.

[0103] 2. Preparation of primary carbonized spherical graphite:

[0104] 20g of petroleum asphalt and 1000g of spherical graphite (particle size 16-18μm) were mixed in a VC mixer for 30min. Inert gas was introduced into the mixer and the rotation speed was 1000r / min to obtain a second mixture.

[0105] The second mixture was placed in a box furnace under a protective atmosphere and heated to 1100°C at a heating rate of 5°C / min. It was then held 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.

[0106] 3. Preparation of modified negative electrode materials:

[0107] 30g of the phosphorus-doped coated pitch and 1000g of primary carbonized spherical graphite were placed in a VC mixer and mixed for 30min. Inert gas was introduced into the mixer and the mixing speed was 1000r / min to obtain a third mixture.

[0108] 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. It was then held at this temperature for 5 hours and finally cooled to room temperature at a cooling rate of 5°C / min to obtain the modified material. The modified material was then removed, sieved, and surface-modified spherical natural graphite anode material was obtained, which was named Example 6.

[0109] Comparative Example 1

[0110] This comparative example provides a modified graphite anode material coated with asphalt, which is prepared through the following steps:

[0111] 1. Preparation of the modified coating layer:

[0112] 1g of phenylphosphonic acid and 50g of petroleum asphalt were mixed in a VC mixer for 30 minutes. Inert gas was introduced into the mixer and the rotation speed was 500r / min to obtain the first mixture.

[0113] The first mixture was placed in a vertical reactor furnace under a protective atmosphere and heat-treated at 400°C for 3 hours. The stirring speed of the vertical reactor was 30 rpm and the heating rate was 5°C / min, to obtain phosphorus-doped coated asphalt.

[0114] 2. Preparation of primary carbonized spherical graphite:

[0115] 20g of petroleum asphalt and 1000g of spherical graphite (particle size 16-18μm) were mixed in a VC mixer for 30min. Inert gas was introduced into the mixer and the rotation speed was 1000r / min to obtain a second mixture.

[0116] The second mixture was placed in a box furnace under a protective atmosphere and heated to 1100°C at a heating rate of 5°C / min. It was then held 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.

[0117] 3. Preparation of modified negative electrode materials:

[0118] 40g of the phosphorus-doped coated pitch and 1000g of primary carbonized spherical graphite were placed in a VC mixer and mixed for 30min. Inert gas was introduced into the mixer and the mixing speed was 1000r / min to obtain a third mixture.

[0119] 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. It was then carbonized at this temperature for 5 hours and then cooled to room temperature at a cooling rate of 5°C / min to obtain the modified material. The modified material was then removed, sieved, and the modified graphite anode material coated with asphalt was obtained, which was named Comparative Example 1.

[0120] Comparative Example 2

[0121] Purchase commercially available natural graphite product 1, named Comparative Example 2, and its specific specifications are shown in Table 1.

[0122] Comparative Example 3

[0123] Purchase commercially available natural graphite product 2, named Comparative Example 3, with specific specifications as shown in Table 1.

[0124] Comparative Example 4

[0125] Purchase commercially available natural graphite product 3, named Comparative Example 4, and its specific specifications are shown in Table 1.

[0126] Comparative Example 5

[0127] This comparative example provides a surface-modified spherical natural graphite anode material, which is prepared through the following steps:

[0128] 1. Preparation of the modified coating layer:

[0129] 7.5g of phenylphosphonic acid and 50g of petroleum asphalt were mixed in a VC mixer for 30 minutes. Inert gas was introduced into the mixer and the rotation speed was 500r / min to obtain the first mixture.

[0130] The first mixture was placed in a vertical reactor furnace under a protective atmosphere and heat-treated at 700°C for 3 hours. The stirring speed of the vertical reactor was 30 rpm and the heating rate was 5°C / min, to obtain phosphorus-doped coated asphalt.

[0131] 2. Preparation of primary carbonized spherical graphite:

[0132] 20g of petroleum asphalt and 1000g of spherical graphite (particle size 16-18μm) were mixed in a VC mixer for 30min. Inert gas was introduced into the mixer and the rotation speed was 1000r / min to obtain a second mixture.

[0133] The second mixture was placed in a box furnace under a protective atmosphere and heated to 1100°C at a heating rate of 5°C / min. It was then held 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.

[0134] 3. Preparation of modified negative electrode materials:

[0135] 40g of the phosphorus-doped coated pitch and 1000g of primary carbonized spherical graphite were placed in a VC mixer and mixed for 30min. Inert gas was introduced into the mixer and the mixing speed was 1000r / min to obtain a third mixture.

[0136] 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. It was then held at this temperature for 5 hours and finally cooled to room temperature at a cooling rate of 5°C / min to obtain the modified material. The modified material was then removed, sieved, and surface-modified spherical natural graphite anode material was obtained, which was named Comparative Example 5.

[0137] Table 1. Specifications of the comparative natural graphite products

[0138] Electrodes were fabricated using the negative electrode materials of Examples 1-6 and Comparative Examples 1-5, and 2032 coin cells were assembled for electrochemical testing. The electrochemical testing was carried out according to the following steps:

[0139] The test material was mixed evenly according to the 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 dried in a vacuum oven at 80°C for 12 h to obtain the negative electrode sheet.

[0140] CR2032 coin cells were assembled in a glove box (water and oxygen were both less than 0.01 ppm), with lithium foil as the counter electrode and 1 mol / L LiF6 dissolved in DMC:DEC:EC (volume ratio 1:1:1).

[0141] The assembled coin cells were subjected to charge-discharge cycle tests at a current density of 1C and rate tests at 0.1C / 0.3C / 0.5C / 1C / 3C / 5C on the Blue Electric system. The test results are shown in Table 2 and Figures 1-4.

[0142] Table 2 Electrochemical performance data of each embodiment and comparative example

[0143] As shown in Table 2, after modification, the reversible capacity in the first cycle and the capacity retention rate after 100 cycles of the anode material in the examples were significantly improved. The improvement in reversible capacity is due to 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 lithium ions on the material, thereby increasing the capacity. Appropriately increasing the carbonization temperature and time can better exert the electrochemical performance of the modified material. However, more doping will result in more surface defects on the material, consuming more electrolyte during the first charge and discharge cycle, leading to a decrease in reversible capacity. The electrolyte will also be continuously consumed during the cycling process. Although the capacity retention rate is still relatively high, it shows a decreasing trend with the increase of doping amount. In addition, the strong adsorption energy of phosphorus for lithium ions also means that a more stable lithium intercalation structure is formed during the charge and discharge process, which can improve the cycle performance.

[0144] As can be seen from Table 2, Examples 1-6 of the present invention exhibit high reversible capacity in the first week and high capacity retention after 100 cycles, especially Examples 3, 5, and 6, which show high reversible capacity (mAh·g) in the first week. -1 The values ​​were 387.6, 380.2, and 381.4, respectively, with corresponding capacity retention rates of 0.98, 0.977, and 0.971 after 100 cycles. Comparative Examples 5 and 1 are not included within the scope of this invention, and their corresponding reversible capacities in the first cycle (mAh·g) were... -1 The values ​​were 360.3 and 361.6 respectively, and the corresponding capacity retention rates after 100 cycles were 0.927 and 0.921 respectively.

[0145] Based on the results of Examples 1 and 3, Example 1, by using a lower heat treatment temperature during the coating of asphalt with a phosphorus source and a lower second carbonization temperature during the preparation of the modified anode material, both reduced the first-cycle reversible capacity (mAh·g) of the final prepared anode compared to Example 3. -1 ), Capacity retention rate after 100 cycles;

[0146] A comparison of Examples 2 and 3 shows that in Example 3, using a higher heat treatment temperature during the coating of asphalt with a phosphorus source can increase the first-cycle reversible capacity (mAh·g) of the final prepared negative electrode. -1 ), Capacity retention rate after 100 cycles;

[0147] Comparative Example 5 used a higher heat treatment temperature of 700℃, and the ratio of phenylphosphonic acid to petroleum asphalt in Comparative Example 1 was 20:0.4, corresponding to the first-week reversible capacity (mAh·g). -1 The values ​​were 360.3 and 361.6, respectively, corresponding to capacity retention rates of 0.927 and 0.921 after 100 cycles.

[0148] The comparison shows that only by controlling the ratio of phenylphosphonic acid to petroleum asphalt and the heat treatment temperature within the range claimed in this application can a higher first-week reversible capacity (mAh·g) be obtained. -1 After 100 cycles, the capacity retention rate, whether above or below the above range, cannot achieve good performance.

[0149] As shown in Figures 1 and 2, the cycling performance and rate performance of Examples 1-6 with phosphorus-doped coatings are superior to those of Comparative Example 1. This performance difference stems from the lower phosphorus doping content in Comparative Example 1, where the coating structure is still dominated by a large amount of amorphous carbon with low structural order, thus failing to effectively improve the material's cycling performance. Furthermore, the low phosphorus doping content did not effectively expand the interlayer spacing of the coating, failing to significantly improve lithium-ion transport kinetics. As shown in Figure 3, effective phosphorus doping increases the structural order of the coating, making it more stable and improving the material's cycling stability. Simultaneously, the increased interlayer spacing after doping facilitates lithium-ion diffusion and provides a buffer for lithium-ion entry into the graphite layer, thereby improving rate performance.

[0150] Electrochemical tests were conducted on 18650 full-cell systems using Example 3 (with better performance) and Comparative Examples 2-4. The tests were performed according to the following steps: NCM811 was used as the positive electrode, and the materials from Example 3 and Comparative Examples 2-4 were used as the negative electrode. Assembly and testing were carried out according to the national standard GB / T 18287-2013. The test results are shown in Figure 4. It can be seen that the 18650 full-cell system made with the negative electrode material of Example 3 still maintained a capacity retention rate of up to 90.2% after 800 cycles. The 18650 full-cell system made with the graphite product of Comparative Example 2 had a capacity retention rate below 90% after 500 cycles, while the 18650 full-cell systems made with the graphite products of Comparative Examples 3 and 4 had a capacity retention rate below 90% after 250 cycles. These comparative results show that the modified material of this invention has excellent cycle performance and has met market demand.

[0151] The negative electrode materials of Example 3 and Comparative Example 5 were subjected to ICP testing, and the results are shown in Table 3:

[0152] Table 3

[0153] As can be seen from Figure 1 and Table 3, both Example 3 and Comparative Example 5 had the same proportion of phosphorus source added. After ICP testing of the obtained negative electrode materials, it was found that the content of P element in the sample of Comparative Example 5 was very low, accounting for only 1 / 4 of that in Example 3. This indicates that the P element loss in Comparative Example 5 was severe and the effective doping amount was low. Therefore, the electrochemical performance of Comparative Example 5 was also average.

[0154] This invention significantly improves the cycle performance of the negative electrode by using phosphorus-doped modified asphalt. Since the asphalt surface has a large number of oxygen-containing functional groups, industrially, high-temperature graphitization is commonly used to eliminate them. However, this method requires maintaining a temperature above 2500℃ for a certain period, resulting in high energy consumption and cost. As shown in the XPS experimental results in Figures 5-8, this invention, through phosphorus-doping modification of the coated asphalt, can regulate the oxygen-containing functional groups on the asphalt surface, replacing some of them with phosphorus-containing functional groups. Figure 7 shows that the CO and C=O bonds on the asphalt surface are replaced by PO and P=O bonds, indicating that phosphorus-containing functional groups replace some of the oxygen-containing functional groups on the asphalt surface.

[0155] Based on the first-principles calculations in Figure 9, it can be found that the substituted functional group has a stronger adsorption capacity for lithium, resulting in a stronger Li adsorption capacity. + Adsorption capacity can increase the Li stored per unit mass of material + The quantity increases the battery's energy density, resulting in higher first-cycle reversible capacity, while also reducing Li... + Irreversible losses during charge and discharge mitigate capacity decay, resulting in better cycle performance, such as cycle stability. In first-principles calculations, E... b E represents the adsorption energy. b =E 体系 -(E 表面 +E 吸附分子 ); where E 体系 E represents the total energy after the adsorbed molecules bind to the surface. 表面 The energy of the solid surface; E 吸附分子 This represents the energy of the adsorbed molecules (typically under gas phase or isolated conditions). Lower E values... b A higher negative adsorption energy usually indicates stronger adsorption: molecules or atoms are more firmly adsorbed on the surface, resulting in a more stable adsorption system. A lower adsorption energy means the system has difficulty removing adsorbed molecules from the surface. Therefore, stronger interactions between molecules and the solid surface are desirable characteristics for anode materials. Based on the first-property calculations above, it can be seen that the Eo of Li-PO3 (P=O) and Li-PO4 (POo) is... b The values ​​are -3.87676 eV and -4.60069 eV, respectively, while the E values ​​for Li-C=O (C=O) and Li-CO (CO) are... b The values ​​are -2.19921 eV and -2.84214 eV respectively. It can be seen that after asphalt modification, P=O and PO have lower adsorption energies for Li, indicating stronger adsorption capacity and thus stronger Li adsorption capacity. + Adsorption capacity.

[0156] Based on the above results, it can be seen that the present invention modifies the coating layer of spherical graphite by phosphorus doping. On the one hand, doping provides more lithium storage sites, improves the specific capacity of the material, and enhances the structural order of the coating layer (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 lithium ion diffusion, and provides a buffer for lithium ions to enter the interlayer of spherical graphite. 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 preparation method of this negative electrode material includes the following steps: A phosphorus source is mixed with a first carbon source and heat-treated in a protective atmosphere to obtain phosphorus-doped coated pitch; wherein the mass ratio of the first carbon source to the phosphorus source is 20:(1-4), and the heat treatment temperature is 100-400℃. 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 coating on its surface; the mass ratio of the spherical graphite to the second carbon source is 100:(1-2); The spherical graphite with amorphous carbon coating on its surface is mixed with the phosphorus-doped coated asphalt and subjected to a second carbonization treatment to obtain the surface-modified spherical natural graphite anode material; the mass ratio of the spherical graphite with amorphous carbon coating on its surface to the phosphorus-doped coated asphalt is 100:(1-5). The second carbon source is selected from one or more of petroleum asphalt, liquid asphalt, and coal tar pitch.

2. The method for preparing the negative electrode material according to claim 1, characterized in that, The phosphorus source is phenylphosphonic acid.

3. The method of claim 1, wherein the method is characterized by: The first carbon source is petroleum bitumen.

4. The method for preparing the negative electrode material according to claim 1, characterized in that, The heat treatment temperature is 400℃.

5. The method for preparing the negative electrode material according to claim 1, characterized in that, The mixing of the phosphorus source and the first carbon source is carried out in a VC mixer; Furthermore, inert gas is introduced during the mixing process, and the rotation speed of the VC mixer is 800-1500 r / min, with a mixing time of 10-40 min.

6. The method of claim 5, wherein the method further comprises a step of mixing the carbon material and the binder. The VC mixer mixes at a speed of 500 r / min for 30 minutes.

7. The method for preparing the negative electrode material according to claim 1, characterized in that, The heat treatment time is 2-6 hours, and the heating rate is 1-10℃ / min.

8. The method for preparing the negative electrode material according to claim 7, characterized in that, The heat treatment lasted for 3 hours, with a heating rate of 5°C / min.

9. The method for preparing the negative electrode material according to claim 1, characterized in that, The mixing of spherical graphite with the second carbon source and the mixing of spherical graphite with amorphous carbon coating with phosphorus-doped coated pitch are carried out in a VC mixer, respectively. Inert gas is introduced during the mixing process, and the VC mixer rotates at 800-1500 r / min for 10-40 min.

10. The method for preparing the negative electrode material according to claim 1, characterized in that, The temperature of the first carbonization is 900-1500℃, the time is 2-10h, and the heating rate is 1-10℃ / min. The second carbonization is carried out at a temperature of 900-1500℃ for 2-10 hours, with a heating rate of 1-10℃ / min.

11. The method for preparing the negative electrode material according to claim 1, characterized in that, The spherical graphite is natural graphite with a particle size of 10-20 μm.

12. The method for preparing the negative electrode material according to claim 1, characterized in that, The second carbon source is petroleum bitumen.

13. 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 1-12.

14. A lithium battery, characterized in that, The negative electrode of the lithium battery is made of the negative electrode material as described in claim 13.

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