阅读说明：本技术 一种多孔石墨硅碳复合材料及其制备方法、应用 (Porous graphite silicon carbon composite material and preparation method and application thereof ) 是由 徐军红 陈和平 陈玉 于 2021-09-08 设计创作，主要内容包括：本发明属于锂离子电池技术领域,具体涉及一种多孔石墨硅碳复合材料及其制备方法、应用。本发明的多孔石墨硅碳复合材料的制备方法,包括如下步骤：1)将酸化石墨、催化剂、分散剂、溶剂混合均匀,喷雾干燥,得到前驱体；2)将得到的前驱体置于碳源气体中,在600-900℃下保温1-6h,然后降温,酸洗,得到石墨复合材料；碳源气体为甲烷、乙炔中的至少一种；3)将制得的石墨复合材料与正硅酸乙酯、氨水、异丙醇混合均匀,然后在50-120℃下反应1-6h,过滤,干燥,然后在还原性气氛下,于700-1200℃保温1-6h,即得。本发明的多孔石墨硅碳复合材料结构稳定、导电率强、低膨胀,能够大幅图提高电池的循环性能。(The invention belongs to the technical field of lithium ion batteries, and particularly relates to a porous graphite silicon carbon composite material, and a preparation method and application thereof. The preparation method of the porous graphite silicon carbon composite material comprises the following steps: 1) uniformly mixing acidified graphite, a catalyst, a dispersing agent and a solvent, and spray-drying to obtain a precursor; 2) placing the obtained precursor in a carbon source gas, preserving heat for 1-6h at the temperature of 600-900 ℃, then cooling and pickling to obtain a graphite composite material; the carbon source gas is at least one of methane and acetylene; 3) the prepared graphite composite material is uniformly mixed with tetraethoxysilane, ammonia water and isopropanol, then the mixture is reacted for 1 to 6 hours at the temperature of between 50 and 120 ℃, filtered and dried, and then the mixture is subjected to heat preservation for 1 to 6 hours at the temperature of between 700 and 1200 ℃ in a reducing atmosphere, so that the graphite composite material is obtained. The porous graphite silicon carbon composite material has the advantages of stable structure, strong conductivity and low expansion, and can greatly improve the cycle performance of the battery.)

1. The preparation method of the porous graphite silicon carbon composite material is characterized by comprising the following steps:

1) uniformly mixing acidified graphite, a catalyst, a dispersing agent and a solvent, and spray-drying to obtain a precursor; the catalyst is at least one of nickel chloride, cobalt chloride and ferric chloride; the solvent is N-methyl pyrrolidone;

2) placing the precursor obtained in the step 1) in a carbon source gas, preserving heat for 1-6h at the temperature of 900 ℃ with 600-; the carbon source gas is at least one of methane and acetylene;

3) uniformly mixing the graphite composite material prepared in the step 2) with tetraethoxysilane, ammonia water and isopropanol, then reacting for 1-6h at 50-120 ℃, filtering, drying, and then preserving heat for 1-6h at 700-1200 ℃ in a reducing atmosphere to obtain the graphite composite material.

2. The method of preparing a porous graphite silicon carbon composite material according to claim 1, wherein the acidified graphite of step 1) is prepared by a method comprising the steps of: adding graphite into mixed acid of concentrated nitric acid and concentrated sulfuric acid, soaking for 40-60h, filtering, and washing.

3. The method for preparing the porous graphite silicon-carbon composite material according to claim 2, wherein the mixed acid of the concentrated nitric acid and the concentrated sulfuric acid is obtained by mixing the concentrated nitric acid and the concentrated sulfuric acid in a volume ratio of 1: 1.

4. The preparation method of the porous graphite silicon-carbon composite material according to claim 1, wherein the mass ratio of the acidified graphite, the catalyst and the dispersing agent in the step 1) is 100:1-5: 1-5.

5. The method for preparing porous graphite silicon carbon composite material according to claim 1, wherein the hydrochloric acid concentration for acid washing in step 2) is 0.1 mol/L.

6. The method for preparing the porous graphite silicon-carbon composite material according to claim 1, wherein the step of uniformly mixing the graphite composite material with the tetraethoxysilane, the ammonia water and the isopropanol in the step 3) is to add the graphite composite material into a mixed solution prepared from the tetraethoxysilane, the ammonia water and the isopropanol.

7. The preparation method of the porous graphite silicon-carbon composite material according to claim 6, wherein the mass ratio of the ethyl orthosilicate, the ammonia water, the isopropanol and the graphite composite material in the step 3) is 1:3:2: 10.

8. The method for preparing the porous graphite silicon-carbon composite material according to claim 1, wherein the reducing atmosphere in the step 3) is a mixed atmosphere of hydrogen and argon.

9. A porous graphite silicon carbon composite material prepared by the method for preparing the porous graphite silicon carbon composite material as claimed in claim 1.

10. Use of the porous graphite silicon carbon composite material of claim 9 in a lithium ion battery.

Technical Field

The invention belongs to the technical field of lithium ion batteries, and particularly relates to a porous graphite silicon carbon composite material, and a preparation method and application thereof.

Background

With the continuous expansion of the application range of lithium ion batteries, the requirements of users on the lithium ion batteries are more diversified, and the lithium ion batteries are required to have high capacity, long cycle life and high safety performance, and also have the functions of small volume, light weight and the like.

Currently, improvements to lithium ion batteries are mostly focused on improvements in the electrode materials. For example, the negative electrode material is required to have high energy density, good quick charging performance, and low expansion ratio. The negative electrode material in the current market mainly takes a graphite material as a main material, the theoretical specific capacity of the negative electrode material is 372Ah/g, and the energy density is low.

The silicon carbon material serving as a novel negative electrode material has a specific capacity of about 2000mAh/g, but has a high expansion rate of 200%, so that the problem that the cycle performance of a lithium ion battery is seriously deteriorated and the high-temperature storage performance is poor is caused.

On one hand, the silicon and graphite compounding can reduce the material expansion, so that a synergistic effect is generated between the silicon-based material and the carbon-based material, respective advantages of the silicon-based material and the carbon-based material are exerted, the cycle performance is improved, and the quick charging performance of the material is improved

The invention patent with publication number CN109638269A discloses a silicon/expanded graphite/amorphous carbon composite material and a preparation method thereof, firstly preparing SiO₂Mixing aerogel massive solids with magnesium powder, soaking in HF solution, cleaning and drying to obtain nano porous silicon; and then mixing, stirring, soaking and filtering the nano porous silicon and the expanded graphite in ethanol, and finally coating in a protective atmosphere to obtain the silicon/expanded graphite/amorphous carbon composite material. Such asThe mode of combining silicon and graphite is physical adsorption, the uniformity and the conductivity of the material are poor, and the cycle performance is still to be improved.

Disclosure of Invention

The invention aims to provide a porous graphite silicon carbon composite material, and simultaneously provides a preparation method of the porous graphite silicon carbon composite material, and finally, the invention further provides application of the porous graphite silicon carbon composite material.

In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:

a preparation method of a porous graphite silicon carbon composite material comprises the following steps:

1) uniformly mixing acidified graphite, a catalyst, a dispersing agent and a solvent, and spray-drying to obtain a precursor; the catalyst is at least one of nickel chloride, cobalt chloride and ferric chloride; the solvent is N-methyl pyrrolidone;

2) placing the precursor obtained in the step 1) in a carbon source gas, preserving heat for 1-6h at the temperature of 900 ℃ with 600-; the carbon source gas is at least one of methane and acetylene;

3) uniformly mixing the graphite composite material prepared in the step 2) with tetraethoxysilane, ammonia water and isopropanol, then reacting for 1-6h at 50-120 ℃, filtering, drying, and then preserving heat for 1-6h at 700-1200 ℃ in a reducing atmosphere to obtain the graphite composite material.

The acidified graphite in step 1) is prepared by a method comprising the following steps: adding graphite into mixed acid of concentrated nitric acid and concentrated sulfuric acid, soaking for 40-60h, filtering, and washing.

Preferably, the mixed acid of the concentrated nitric acid and the concentrated sulfuric acid is obtained by mixing the concentrated nitric acid and the concentrated sulfuric acid in a volume ratio of 1: 1.

In the step 1), the mass ratio of the acidified graphite to the catalyst to the dispersant is 100:1-5: 1-5.

Preferably, the dispersant is polyvinylpyrrolidone.

Uniformly mixing acidified graphite, a catalyst, a dispersing agent and a solvent to obtain a dispersion liquid, wherein the mass fraction of the acidified graphite is 1-5%.

And 2) placing the precursor in a carbon source gas, namely placing the precursor in a tubular furnace, removing air in the tubular furnace by using argon, and then introducing the carbon source gas. And in the step 2), cooling is carried out after the carbon source gas is stopped being introduced. Further, the carbon source gas is stopped and then the argon gas is introduced for natural cooling.

The hydrochloric acid concentration for the acid washing was 0.1 mol/L. And washing with deionized water after acid washing.

The graphite composite material is uniformly mixed with tetraethoxysilane, ammonia water and isopropanol, namely the graphite composite material is added into a mixed solution prepared from tetraethoxysilane, ammonia water and isopropanol. The mass fraction of the tetraethoxysilane in the mixed solution is 1-5%.

The reducing atmosphere is a mixed atmosphere of hydrogen and argon. Preferably, the volume ratio of hydrogen to argon is 1: 1.

The mass ratio of the ethyl orthosilicate to the ammonia water to the isopropanol to the graphite composite material is 1:3:2: 10.

The porous graphite silicon carbon composite material is prepared by the preparation method of the porous graphite silicon carbon composite material.

An application of the porous graphite silicon carbon composite material in a lithium ion battery.

The invention has the beneficial effects that:

the porous graphite silicon carbon composite material of the invention is uniformly doped with the metal catalyst in the porous graphite, the carbon nano tube grows on the surface of the porous graphite by taking the active point of the catalyst as the base point, and the carbon nano tube is connected with the porous graphite through a chemical bond, so that the porous graphite silicon carbon composite material has the advantages of stable structure, strong conductivity and the like. The tetraethoxysilane is decomposed under the alkaline condition to generate a silicon-oxygen compound, and the silicon-oxygen compound is doped between layers of graphite through a hydrothermal reaction and can be adsorbed on the surface, so that the material has the characteristics of low expansion and high conductivity. Furthermore, the isopropanol additive is favorable for accelerating the decomposition of the tetraethoxysilane and promoting the reaction process.

Drawings

Fig. 1 is an SEM image of the porous graphite silicon carbon composite material prepared in example 1.

Detailed Description

In order to make the technical problems to be solved, the technical solutions adopted and the technical effects achieved by the present invention easier to understand, the technical solutions of the present invention are clearly and completely described below with reference to specific embodiments.

Preparation example 1

The preparation method of the acidified graphite of the preparation example comprises the following steps: adding 100g of artificial graphite into 200mL of mixed acid, and soaking for 6h, wherein the mixed acid is formed by mixing concentrated nitric acid and concentrated sulfuric acid in a volume ratio of 1: 1; (1800 mL of deionized water is added to prepare a solution with the mass fraction of 5 wt%. The solution is soaked for 48 hours, filtered, washed by the deionized water and dried in vacuum at 80 ℃ for 48 hours to obtain the acidified graphite.

Preparation example 2

The preparation method of the acidified graphite of the preparation example comprises the following steps: adding 100g of artificial graphite into 200mL of mixed acid, and soaking for 6h, wherein the mixed acid is formed by mixing concentrated nitric acid and concentrated sulfuric acid in a volume ratio of 1: 1; then 4800mL of deionized water was added to prepare a solution with a mass fraction of 2 wt%. Then soaking for 52h, filtering, washing by deionized water, and vacuum drying for 48h at 85 ℃ to obtain the acidified graphite.

Example 1

The preparation method of the porous graphite silicon carbon composite material comprises the following steps:

1) preparation of graphite catalytic precursor

Adding 100g of acidified graphite into 3000mL of N-methyl pyrrolidone, uniformly mixing to obtain a mixed solution with the mass fraction of 3.2%, then adding 3g of polyvinylpyrrolidone serving as a dispersing agent, adding 3g of nickel chloride, performing ultrasonic dispersion, and performing spray drying to obtain a graphite catalytic precursor;

2) preparation of graphite carbon nano tube composite material

Transferring the graphite catalysis precursor prepared in the step 1) to a tubular furnace, introducing argon to remove air in the tube, introducing methane serving as a carbon source gas, heating to 750 ℃, preserving heat for 3 hours, stopping introducing methane, naturally cooling to room temperature, carrying out acid washing for 5 times by using hydrochloric acid with the concentration of 0.1mol/L, washing for 5 times by using deionized water, and drying to obtain a graphite carbon nanotube composite material;

3) preparation of porous graphite silicon carbon composite material

Adding 1g of tetraethoxysilane into a mixed solvent to prepare a dispersion liquid with the mass fraction of 3%, wherein the mixed solvent is formed by mixing 3mL of ammonia water, 2mL of isopropanol and 30mL of deionized water, and the mass fraction of the ammonia water is 25%;

then adding 10g of the graphite carbon nanotube composite material prepared in the step 2) into the dispersion liquid, stirring and mixing uniformly, then transferring the mixture into a high-pressure reaction kettle, reacting for 3 hours at the temperature of 60 ℃, filtering and drying after the reaction;

transferring the dried substance into a tube furnace, introducing argon to remove air in the tube, and then introducing hydrogen and argon mixed gas, wherein the volume ratio of the hydrogen and argon mixed gas is 1:1, hydrogen and argon are mixed; and then heating to 900 ℃, preserving heat for 3 hours, then cooling to room temperature in the atmosphere of introducing argon, taking out, and crushing to obtain the catalyst.

Example 2

The preparation method of the porous graphite silicon carbon composite material comprises the following steps:

1) preparation of graphite catalytic precursor

Adding 100g of acidified graphite into 10L of N-methyl pyrrolidone, uniformly mixing to obtain a mixed solution with the mass fraction of 1%, then adding 1g of polyvinylpyrrolidone serving as a dispersing agent, adding 1g of cobalt chloride, performing ultrasonic dispersion, and performing spray drying to obtain a graphite catalytic precursor;

2) preparation of graphite carbon nano tube composite material

Transferring the graphite catalytic precursor prepared in the step 1) to a tubular furnace, introducing argon to remove air in the tube, introducing acetylene serving as a carbon source gas, heating to 600 ℃, preserving heat for 6 hours, stopping introducing the acetylene, naturally cooling to room temperature, carrying out acid washing for 5 times by using hydrochloric acid with the concentration of 0.1mol/L, washing for 5 times by using deionized water, and drying to obtain a graphite carbon nanotube composite material;

3) preparation of porous graphite silicon carbon composite material

Adding 1g of tetraethoxysilane into a mixed solvent to prepare a dispersion liquid with the mass fraction of 1%, wherein the mixed solvent is formed by mixing 3mL of ammonia water, 3mL of isopropanol and 100mL of deionized water, and the mass fraction of the ammonia water is 25%;

then adding 10g of the graphite carbon nanotube composite material prepared in the step 2) into the dispersion liquid, stirring and mixing uniformly, then transferring the mixture into a high-pressure reaction kettle, reacting for 6 hours at the temperature of 50 ℃, filtering and drying after the reaction;

transferring the dried substance into a tube furnace, introducing argon to remove air in the tube, and then introducing hydrogen and argon mixed gas, wherein the volume ratio of the hydrogen and argon mixed gas is 1:1, hydrogen and argon are mixed; and then heating to 700 ℃, preserving heat for 6 hours, then cooling to room temperature under the atmosphere of introducing argon, taking out, and crushing to obtain the catalyst.

Example 3

The preparation method of the porous graphite silicon carbon composite material comprises the following steps:

1) preparation of graphite catalytic precursor

Adding 100g of acidified graphite into 2000mL of N-methyl pyrrolidone, uniformly mixing to obtain a mixed solution with the mass fraction of 5%, then adding 5g of polyvinylpyrrolidone as a dispersing agent, adding 5g of nickel chloride as a catalyst, performing ultrasonic dispersion, and performing spray drying to obtain a graphite catalysis precursor;

2) preparation of graphite carbon nano tube composite material

Transferring the graphite catalysis precursor prepared in the step 1) to a tubular furnace, introducing argon to remove air in the tube, introducing methane serving as a carbon source gas, heating to 900 ℃, preserving heat for 1h, stopping introducing methane, naturally cooling to room temperature, carrying out acid washing for 5 times by using hydrochloric acid with the concentration of 0.1mol/L, washing for 5 times by using deionized water, and drying to obtain a graphite carbon nanotube composite material;

3) preparation of porous graphite silicon carbon composite material

Adding 1g of tetraethoxysilane into a mixed solvent to prepare a dispersion liquid with the mass fraction of 5%, wherein the mixed solvent is formed by mixing 3mL of ammonia water, 2mL of isopropanol and 20mL of deionized water, and the mass fraction of the ammonia water is 25%;

then adding 10g of the graphite carbon nanotube composite material prepared in the step 2) into the dispersion liquid, stirring and mixing uniformly, then transferring the mixture into a high-pressure reaction kettle, reacting at the temperature of 120 ℃ for 1h, filtering and drying after the reaction;

transferring the dried substance into a tube furnace, introducing argon to remove air in the tube, and then introducing hydrogen and argon mixed gas, wherein the volume ratio of the hydrogen and argon mixed gas is 1:1, hydrogen and argon are mixed; and then heating to 1200 ℃, preserving heat for 1h, then cooling to room temperature under the atmosphere of introducing argon, taking out, and crushing to obtain the product.

Example 4

The preparation method of the porous graphite silicon carbon composite material comprises the following steps:

1) preparation of graphite catalytic precursor

Adding 100g of acidified graphite into 2000mL of N-methyl pyrrolidone, uniformly mixing to obtain a mixed solution with the mass fraction of 5%, then adding 3g of polyvinylpyrrolidone serving as a dispersing agent, adding 3.5g of ferric chloride serving as a catalyst, performing ultrasonic dispersion, and performing spray drying to obtain a graphite catalysis precursor;

2) preparation of graphite carbon nano tube composite material

Transferring the graphite catalytic precursor prepared in the step 1) to a tubular furnace, introducing argon to remove air in the tube, introducing methane serving as a carbon source gas, heating to 780 ℃, preserving heat for 1h, stopping introducing methane, naturally cooling to room temperature, carrying out acid washing for 5 times by using hydrochloric acid with the concentration of 0.1mol/L, washing for 5 times by using deionized water, and drying to obtain a graphite carbon nanotube composite material;

3) preparation of porous graphite silicon carbon composite material

Adding 1g of tetraethoxysilane into a mixed solvent to prepare a dispersion liquid with the mass fraction of 5%, wherein the mixed solvent is formed by mixing 3mL of ammonia water, 2mL of isopropanol and 20mL of deionized water, and the mass fraction of the ammonia water is 25%;

then adding 10g of the graphite carbon nanotube composite material prepared in the step 2) into the dispersion liquid, stirring and mixing uniformly, then transferring the mixture into a high-pressure reaction kettle, reacting at the temperature of 110 ℃ for 1.5h, filtering and drying after reaction;

transferring the dried substance into a tube furnace, introducing argon to remove air in the tube, and then introducing hydrogen and argon mixed gas, wherein the volume ratio of the hydrogen and argon mixed gas is 1:1, hydrogen and argon are mixed; and then heating to 1080 ℃, preserving heat for 2 hours, then cooling to room temperature under the atmosphere of introducing argon, taking out, and crushing to obtain the product.

Comparative example

The preparation method of the composite anode material of the comparative example includes the steps of: adding 1g of nano silicon, 10g of artificial graphite and 1g of PVP into deionized water, uniformly stirring, transferring to a high-pressure reaction kettle, reacting at 120 ℃ for 1h, filtering, drying, transferring to a tubular furnace, introducing argon to remove air, introducing a mixed gas of hydrogen and argon in a volume ratio of 1:1, heating to 800 ℃, preserving heat for 1h, and cooling to room temperature under an inert atmosphere to obtain the nano-silicon/PVP composite material.

Test examples

(1) SEM test

The porous graphite silicon carbon composite material of example 1 was taken for SEM test, and the test results are shown in fig. 1.

As can be seen from FIG. 1, the silicon-carbon composite material prepared by the present invention is in an aggregated state of particles, and the particle size of the particles is between 10 and 30 μm.

(2) Physical and chemical testing

The silicon-carbon composite materials prepared in examples 1 to 4 and comparative example were used as negative electrode materials, and the specific surface area and powder conductivity of the materials were measured according to the methods in GB/T243339-2009 "standard for graphite-based negative electrode materials for lithium ion batteries", and the results are shown in the following table.

(3) Electricity withholding test

The silicon-carbon composite materials prepared in examples 1 to 4 and comparative example were used as negative electrode materials, and the following tests were carried out:

adding a binder, a conductive agent and a solvent into the negative electrode material, stirring and pulping, coating the mixture on copper foil, and drying and rolling to prepare a negative electrode plate; the binder is LA136D, the conductive agent is conductive carbon black (SP), the solvent is N-methylpyrrolidone (NMP), and the using ratio of the negative electrode material, SP, LA136D and NMP is 95 g: 1 g: 4 g: 220 mL.

LiPF in the electrolyte used₆The electrolyte is 1.3mol/L, and the solvent is a mixture of EC and DEC with the volume ratio of 1: 1; the metal lithium sheet is a counter electrode, the diaphragm adopts a polypropylene (PP) film, and the button cell is assembled in an argon-filled glove box.

The electrochemical performance is tested on a Wuhan blue current CT2001A type battery tester for the charge and discharge performance, the charge and discharge voltage range is 0.005V-2.0V, and the charge and discharge rate is 0.1C. The test results are shown in the following table.

TABLE 1 comparison of Material Properties in examples 1-4 and comparative examples

As can be seen from the above table, the specific capacity and the first efficiency of the porous graphite/silicon carbon composite material of the present invention are significantly better than those of the comparative examples, because the carbon nanotubes are coated on the nano silicon surface of the core material of the composite material, which improves the conductivity of the material, and the gram volume advantage of the material is exerted, thereby improving the first efficiency of the material. Meanwhile, the carbon nano tube with the network structure has a large specific surface area, so that the powder conductivity of the material is improved.

(4) Pouch cell testing

The silicon-carbon composite materials prepared in examples 1 to 4 and comparative example and artificial graphite were mixed in a mass ratio of 2:1 to prepare a negative electrode sheet as a negative electrode material, and a ternary material (Li (Ni)_0.6Co_0.2Mn_0.2)O₂) Preparing a positive plate for a positive material; the electrolyte is LiPF₆Solution of electrolyte LiPF₆In a concentration of 1.3mol/L, and the solvent is Ethylene Carbonate (EC) and diethyl carbonate in a volume ratio of 1:1(DEC) mixtures; and (3) preparing the 5Ah flexible package battery by using the Celgard 2400 membrane as a diaphragm.

1) Testing of liquid absorption capacity and liquid retention rate

And (3) adopting a 1mL burette, sucking the electrolyte VmL, dripping one drop on the surface of the negative pole piece, timing until the electrolyte is completely absorbed, recording the time t, and calculating the liquid absorption speed V/t of the pole piece. The test results are shown in table 2.

Calculating the theoretical liquid absorption amount m of the negative pole piece according to the pole piece parameters₁And weighing the weight m of the negative plate₂Then placing the negative plate into the electrolyte to be soaked for 24 hours, and weighing the weight of the negative plate as m₃Calculating the liquid absorption m of the negative pole piece₃-m₂And calculated according to the following formula: retention rate ═ m₃-m₂)*100％/m₁. The test results are shown in table 2.

2) Testing resistivity and rebound rate of pole piece

The resistivity of the negative plate was measured using a resistivity tester, and the results are shown in table 3.

Firstly, testing the average thickness of the negative plate to be D1 by using a thickness gauge, then placing the negative plate in a vacuum drying oven at 80 ℃ for drying for 48h, testing the thickness of the negative plate to be D2, and calculating according to the following formula: the rebound rate was (D2-D1) × 100%/D1. The test results are shown in table 2.

TABLE 2 comparison of pole piece Properties made with the materials of examples 1-4 and comparative example

As can be seen from table 2, the liquid absorbing and retaining capabilities of the porous graphite/silicon-carbon composite negative electrode material of the present invention are significantly higher than those of the comparative example, which is mainly because the porous graphite and the carbon nanotubes with high specific surface area of the porous graphite/silicon-carbon composite negative electrode material provided by the present invention improve the liquid absorbing and retaining capabilities of the material.

The rebound rate of the negative plate prepared by the porous graphite/silicon-carbon composite negative electrode material is obviously lower than that of a comparative example, probably because the nano-silicon material is uniformly doped among the porous graphite/carbon nano tubes by a hydrothermal reaction method, the expansion can be reduced, the electronic conductivity of the material is improved by the actually connected carbon nano tubes, and the resistivity of the plate is reduced.

3) Cycle performance test

The cycle performance of the battery is tested at the temperature of 25 +/-3 ℃ with the charge-discharge multiplying power of 1C/1C and the voltage range of 2.8V-4.2V. The test results are shown in table 3.

And (4) carrying out constant current and constant voltage charging at a rate of 2C, and calculating the constant current ratio of the material, namely the constant current charging electric quantity/(constant current and constant voltage charging electric quantity).

The test results are shown in table 3.

Table 3 comparison of performance of batteries made with the materials of examples 1-4 and comparative example

	Capacity retention (%) after 500 cycles	2C constant current ratio
			Example 1	86.62	93.5％
Example 2	85.78	92.8％
			Examples3	84.39	91.9％
Example 4	83.91	91.5％
			Comparative example	76.76	84.3％

It can be seen from table 3 that the cycle performance and rate capability of the battery prepared from the porous graphite/silicon carbon composite negative electrode material of the present invention are significantly better than those of the comparative example, which is probably because the electrode plate prepared from the porous graphite/silicon carbon composite negative electrode material of the present invention has a lower expansion rate and electrolyte solution retention capacity, and the structure of the electrode plate is more stable during the charging and discharging processes, thereby improving the cycle performance. The carbon nano tube is connected with the graphite through a chemical bond, so that the impedance is lower, the multiplying power is better, and a cathode material system has high constant current ratio and good multiplying power performance.