A low-expansion silicon-carbon composite material and a preparation method thereof

Abstract

The present invention relates to the technical field of the preparation of lithium-ion battery materials, and provides a low-expansion silicon-carbon composite material and a preparation method thereof. The composite material has a core-shell structure. The core includes nano-silicon, a carbon matrix, and a metal dopant. The outer shell is composed of nitrogen-doped amorphous carbon, and the mass of the outer shell is 1%-10% of the mass of the composite material. The present invention embeds nano-silicon on the surface of graphite paper through an oxygen plasma technique to reduce expansion and reduces impedance through rare earth doping to prepare a silicon-rare earth co-doped graphite paper composite material. Through the above technical solution, the problems of large expansion and large impedance of silicon-carbon materials in the prior art are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery material preparation, and specifically, to a low-expansion silicon-carbon composite material and a preparation method thereof. Background Art

[0002] Silicon-carbon materials are applied to high-energy-density lithium-ion batteries due to their advantages such as high energy density and wide material sources. However, due to the poor electronic conductivity of silicon itself and its large expansion at full charge, the low-temperature performance and cycling performance of the materials are poor. In addition, the huge stress generated by the volume change of silicon materials is also likely to cause the detachment of active substances and conductive agents, greatly damaging the electron transport path inside the electrode, and even causing the electrode coating to peel off from the current collector, resulting in a continuous decrease in battery capacity until the battery is completely damaged.

[0003] Another disadvantage of silicon anodes is the low electronic conductivity of silicon materials themselves, which is about 10 -3 S·cm -1 or so, and the migration rate of lithium ions in silicon anodes is low. Improving the electronic conductivity of silicon-based materials mainly involves doping to increase the electronic conductivity and reducing the particle size of silicon to reduce expansion. The current silicon doping technology mainly uses solid-phase method / liquid-phase method for doping, which has problems such as poor consistency and easy agglomeration, resulting in no improvement in expansion, which is a technical problem difficult for technicians to overcome. Summary of the Invention

[0004] The present invention provides a low-expansion silicon-carbon composite material and a preparation method thereof, which solve the problems of large expansion and large impedance of silicon-carbon materials in the prior art.

[0005] The technical solution of the present invention is as follows:

[0006] A low-expansion silicon-carbon composite material has a core-shell structure. The core includes nano-silicon, a carbon matrix, and a metal dopant. The shell is composed of nitrogen-doped amorphous carbon, and the mass of the shell is 1%-10% of the mass of the composite material.

[0007] As a further technical solution, the core is composed of 10%-50% nano-silicon, 1%-10% rare earth metal dopant, and the balance carbon matrix.

[0008] The present invention also provides a preparation method of a low-expansion silicon-carbon composite material, including the following steps:

[0009] S1. Using a graphite paper matrix as the substrate, silane as the target, introducing a gas containing oxygen, and performing plasma treatment to obtain a silicon-doped graphite paper composite material;

[0010] S2. Using the silicon-doped graphite paper composite material as the matrix and the rare earth metal dopant as the target, introducing a gas containing oxygen, and performing plasma treatment to obtain a silicon-rare earth co-doped graphite paper composite material;

[0011] S3. Calcining the silicon-rare earth co-doped graphite paper composite material in a mixed gas, where the mixed gas is a mixed gas of a carbon source and a nitrogen source;

[0012] S4. Cooling and performing post-treatment to obtain the low-expansion silicon-carbon composite material.

[0013] As a further technical solution, in steps S1 and S2, the parameters of the plasma treatment are: the frequency is 1 - 5 MHz, the power is 50 - 200 W, and the treatment time is 10 - 120 min.

[0014] As a further technical solution, in step S1, the silane includes one or more of methylsilane, dimethylsilane, ethynyltrimethylsilane, and hexamethyldisilane.

[0015] As a further technical solution, in step S2, the rare earth metal dopant includes one or more of chlorides, sulfates, and nitrates of cerium, lanthanum, europium, neodymium, and yttrium.

[0016] As a further technical solution, in step S3, the volume ratio of the nitrogen source to the carbon source is 1:10.

[0017] As a further technical solution, in step S3, the carbon source includes one or more of methane, acetylene, ethylene, and ethane.

[0018] As a further technical solution, in step S3, the nitrogen source includes one of ammonia, ammonia water, ammonium bicarbonate, and ammonium carbonate.

[0019] As a further technical solution, in step S4, the calcination specifically is: heating at a rate of 1 - 10 °C / min to 700 - 1100 °C and holding for 10 - 120 min.

[0020] The present invention also provides a negative electrode, which includes the low-expansion silicon-carbon composite material prepared by the above preparation method.

[0021] The beneficial effects of the present invention are as follows:

[0022] 1. In the present invention, using graphite paper as the matrix, depositing nano-silicon on the surface and inside through oxygen plasma can play a role in restraining the expansion of silicon during charge and discharge. Through the oxygen plasma treatment technology, problems such as poor consistency and agglomeration caused by solid-phase or liquid-phase doping can be avoided. In addition, the plasma technology of the present invention uses graphite paper as the matrix, and graphite paper has high electronic conductivity and strong flexibility, which can also reduce the full-charge expansion and impedance of silicon itself.

[0023] 2. In the present invention, rare earth elements are also doped and coated on the surface of the silicon-doped graphite paper composite material. After being coated with rare earth elements, it does not directly contact the electrolyte, reducing the occurrence of side reactions of nano-silicon and further improving the cycling performance.

[0024] 3. The surface of the coated nano-silicon is further deposited with carbon and nitrogen. On the one hand, it can restrain the expansion of the electrode during charge and discharge. On the other hand, nitrogen has a lower electron impedance than carbon, reducing the impedance of the composite material, isolating the electrolyte to reduce side reactions, and enhancing the storage and cycling performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present invention will be further described in detail below in conjunction with the drawings and specific embodiments.

[0026] Figure 1 SEM image of the low-expansion silicon-carbon composite material prepared for Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of the present invention.

[0028] Example 1

[0029] S1. Using a graphite paper substrate as the matrix, methylsilane as the target, introducing oxygen (flow rate 50 mL / min), and performing plasma treatment for 60 min under the conditions of a frequency of 2 MHz and a power of 100 W to obtain a silicon-doped graphite paper composite material;

[0030] S2. Using the silicon-doped graphite paper composite material as the matrix, cerium chloride as the target, introducing oxygen, and performing plasma treatment for 60 min under the conditions of a frequency of 2 MHz and a power of 100 W to obtain a silicon-rare earth co-doped graphite paper composite material;

[0031] S3. Transfer the silicon-rare earth co-doped graphite paper composite material to a tubular furnace. First, introduce argon inert gas to remove the air in the tube, then introduce a carbon-nitrogen mixed gas (ammonia: methane = 1:10), with a flow rate of 10 mL / min, and heat it to 950 °C at a heating rate of 5 °C / min and hold for 60 min. Then cool it to room temperature in an argon inert atmosphere and pulverize to obtain a low-expansion silicon-carbon composite material.

[0032] Example 2

[0033] S1. Using the graphite paper matrix as the substrate, dimethylsilane as the target, introducing oxygen (flow rate: 50 mL / min), and performing plasma treatment for 120 min under the conditions of a frequency of 1 MHz and a power of 50 W to obtain a silicon-doped graphite paper composite material;

[0034] S2. Using the silicon-doped graphite paper composite material as the substrate, lanthanum sulfate as the target, introducing oxygen, and performing plasma treatment for 120 min under the conditions of a frequency of 1 MHz and a power of 50 W to obtain a silicon-rare earth co-doped graphite paper composite material;

[0035] S3. Transfer the silicon-rare earth co-doped graphite paper composite material to a tubular furnace. First, introduce argon inert gas to remove the air in the tube. Then, introduce a carbon-nitrogen mixed gas (ammonia: acetylene = 1:10) with a flow rate of 10 mL / min, and heat it to 700 °C at a rate of 1 °C / min and hold for 120 min. Then, cool it to room temperature in an argon inert atmosphere and crush it to obtain a low-expansion silicon-carbon composite material.

[0036] Example 3

[0037] S1. Using the graphite paper matrix as the substrate, hexamethyldisilane as the target, introducing oxygen (flow rate: 50 mL / min), and performing plasma treatment for 10 min under the conditions of a frequency of 5 MHz and a power of 200 W to obtain a silicon-doped graphite paper composite material;

[0038] S2. Using the silicon-doped graphite paper composite material as the substrate, europium chloride as the target, introducing oxygen, and performing plasma treatment for 10 min under the conditions of a frequency of 5 MHz and a power of 200 W to obtain a silicon-rare earth co-doped graphite paper composite material;

[0039] S3. Transfer the silicon-rare earth co-doped graphite paper composite material to a tubular furnace. First, introduce argon inert gas to remove the air in the tube. Then, introduce a carbon-nitrogen mixed gas (ammonia: ethylene = 1:10) with a flow rate of 10 mL / min, heat it to 1100 °C at a heating rate of 10 °C / min and hold for 10 min. Then, cool it to room temperature in an argon inert atmosphere and crush it to obtain a low-expansion silicon-carbon composite material.

[0040] Comparative Example 1

[0041] Transfer the silicon-doped graphite paper composite material prepared in step S1 of Example 1 to a tubular furnace. First, introduce argon inert gas to remove the air in the tube. Then, introduce a carbon-nitrogen mixed gas (ammonia: methane = 1:10), and heat it to 950 °C at a heating rate of 5 °C / min and hold for 3 h. Then, cool it to room temperature in an argon inert atmosphere and crush it to obtain a low-expansion silicon-carbon composite material.

[0042] Comparative Example 2

[0043] 5 g of nano - silicon, 5 g of cerium chloride and 100 g of graphite paper were added to 500 mL of ethanol solution. After ball - milling and mixing evenly, it was spray - dried, and then transferred to a tubular furnace. First, argon inert gas was introduced to remove the air in the tube, and then a carbon - nitrogen mixed gas (ammonia: methane = 1:10) was introduced, and it was heated to 950 °C at a heating rate of 5 °C / min and kept warm for 3 h. Then it was cooled to room temperature in an argon inert atmosphere and pulverized to obtain a silicon - cerium - doped graphite paper composite material.

[0044] Experimental Example 1: Morphology test

[0045] The silicon - carbon composite material in Example 1 was subjected to SEM test, and the test results are as Figure 1 shown. It can be seen from Figure 1 that the material presents a granular structure, and the particle size of the particles is between 5 - 15 μm.

[0046] Experimental Example 2: Coin - type battery test

[0047] The silicon - carbon composite materials in Examples 1 - 3 and Comparative Examples 1 - 2 were used as the anode materials of lithium - ion batteries to assemble coin - type batteries, which were denoted as A1, A2, A3, B1, and B2 respectively.

[0048] The specific preparation method is as follows: A binder, a conductive agent and a solvent were added to the anode material of the lithium - ion battery, stirred to make a slurry, coated on a copper foil, and dried and rolled to obtain an anode sheet; the binder used was LA132, the conductive agent was SP, and the solvent was NMP (N - methylpyrrolidone). The dosage ratios of the anode material, SP, PVDF, and NMP were 95 g: 1 g: 4 g: 220 mL; LiPF 6 was used as the electrolyte, and a mixture of EC and DEC with a volume ratio of 1:1 was used as the solvent; a lithium metal sheet was used as the counter electrode, and a polypropylene (PP) membrane was used as the separator. The coin - type battery was assembled in a glove box filled with hydrogen. The electrochemical performance was tested on a Wuhan Blue - Electric CT2001A battery tester, and the charge - discharge voltage range was 0.005 V to 2.0 V, and the charge - discharge rate was 0.1 C. The test results are shown in Table 1.

[0049] Table 1 Comparison of the performance of coin - type batteries in Examples and Comparative Examples

[0050]

[0051] As can be seen from the data in Table 1, in Comparative Example 1, oxygen plasma treatment was not used, and rare earth element doping and coating were not carried out. In Comparative Example 2, the same raw materials as in the examples were used, but the conventional ball milling method was adopted, resulting in the specific capacity and the first efficiency of the silicon-carbon composite material prepared in the examples of the present invention being significantly better than those of the comparative examples. The reason may be that: in the examples, the powder conductivity of the material was improved and the impedance was reduced by doping rare earth compounds in the nano-silicon, thereby improving the first efficiency; at the same time, the oxygen plasma technology was adopted to improve the density and the tapped density of the material, and the specific capacity of the material was improved.

[0052] Experimental Example 3: Soft-pack battery test

[0053] The silicon-carbon composite materials in Examples 1-3 and Comparative Examples 1-2 were doped with 90% artificial graphite as the negative electrode material to prepare negative electrode sheets, and NCM532 was used as the positive electrode material; LiPF 6 was used as the electrolyte, and a mixture of EC and DEC with a volume ratio of 1:1 was used as the solvent; Celgard 2400 membrane was used as the separator to prepare 5 Ah soft-pack batteries, labeled as C1, C2, C3, D1, and D2. The liquid absorption and retention capacity, the resilience of the electrode sheet, and the cycle performance of the negative electrode sheets were tested respectively.

[0054] a. Liquid absorption capacity test

[0055] A 1 mL burette was used, and V mL of the electrolyte was sucked. A drop was added to the surface of the electrode sheet and timed until the electrolyte was completely absorbed. The time t was recorded, and the liquid absorption speed V / t of the electrode sheet was calculated. The test results are shown in Table 2.

[0056] b. Liquid retention rate test

[0057] The theoretical liquid absorption amount m of the electrode sheet was calculated according to the electrode sheet parameters 1 , and the weight m of the electrode sheet was weighed 2 . Then the electrode sheet was placed in the electrolyte and soaked for 24 h, and the weight of the electrode sheet was weighed as m 3 . The liquid absorption amount m of the electrode sheet was calculated 3 -m 2 , and calculated according to the following formula: Liquid retention rate = (m 3 -m 2 ) * 100% / m 1 . The test results are shown in Table 2.

[0058] Table 2 Comparison of liquid absorption and retention capacity of soft-pack batteries prepared in examples and comparative examples

[0059]

[0060] As can be seen from Table 2, the liquid absorption and retention capacity of the silicon composite materials obtained in Examples 1-3 is significantly higher than that of the comparative examples. The experimental results show that the silicon-carbon composite material of the present invention has a high liquid absorption and retention capacity.

[0061] c. Test of the pole piece rebound rate

[0062] First, use a thickness gauge to measure the average thickness of the pole piece as D1. Then, place the pole piece in a vacuum drying oven at 80 °C for 48 h, measure the thickness of the pole piece as D2, and calculate according to the following formula: Rebound rate = (D2 - D1) * 100% / D1. The test results are shown in Table 3.

[0063] d. Test of the pole piece resistivity

[0064] Use a resistivity tester to measure the resistivity of the pole piece. The test results are shown in Table 3.

[0065] Table 3 Pole piece rebound rate and pole piece resistivity of examples and comparative preparations

[0066]

[0067] As can be seen from the data in Table 3, the rebound rate and resistivity of the negative pole pieces made of the silicon-carbon composite materials obtained in Examples 1-3 are significantly lower than those of the comparative examples, that is, the negative pole pieces made of the silicon-carbon composite material of the present invention have a lower rebound rate and resistivity. The reason may be that: the materials prepared by the oxygen plasma technology can reduce the expansion, and at the same time, the doping of rare earth compounds improves the electronic conductivity of the materials and reduces the resistivity of their pole pieces.

[0068] e. Test of the cycle performance

[0069] The charge-discharge rate is 1C / 1C, and the voltage range is 2.8V - 4.2V. Test the cycle performance of the battery at a temperature of 25 ± 3 °C. The test results are shown in Table 4.

[0070] Table 4 Cycle performance of examples and comparative examples

[0071]

[0072] As can be seen from Table 4, the cycle performance of the battery made of the silicon-carbon composite material of the present invention is significantly better than that of the comparative example. The reason may be that: the structure of the pole piece of the silicon-carbon composite material of the present invention is more stable during the charge-discharge process, improving its cycle performance; in addition, by doping rare earth elements by plasma, the impedance is reduced and the cycle performance of the material is improved.

[0073] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A low-expansion silicon-carbon composite material, characterized in that, the composite material has a core-shell structure, the core includes nano-silicon, a carbon matrix, and a metal dopant, and the shell is composed of nitrogen-doped amorphous carbon, and the mass of the shell is 1%-10% of the mass of the composite material; The preparation method of the low-expansion silicon-carbon composite material includes the following steps: S1. Using a graphite paper substrate as the substrate, silane as the target, introducing a gas containing oxygen, and performing plasma treatment to obtain a silicon-doped graphite paper composite material; S2. Using the silicon-doped graphite paper composite material as the substrate, a rare earth metal dopant as the target, introducing a gas containing oxygen, and performing plasma treatment to obtain a silicon-rare earth co-doped graphite paper composite material; S3. Calcining the silicon-rare earth co-doped graphite paper composite material in a mixed gas, and the mixed gas is a mixed gas of a carbon source and a nitrogen source; S4. Cooling and post-treating to obtain the low-expansion silicon-carbon composite material.

2. The low-expansion silicon-carbon composite material according to claim 1, characterized in that, the core is composed of 10%-50% nano-silicon, 1%-10% rare earth metal dopant, and the balance carbon matrix.

3. The low-expansion silicon-carbon composite material according to claim 1, characterized in that, in the steps S1 and S2, the parameters of the plasma treatment are: the frequency is 1-5 MHz, the power is 50-200 W, and the treatment time is 10-120 min.

4. The low-expansion silicon-carbon composite material according to claim 1, characterized in that, in the step S1, the silane includes one or more of methylsilane, dimethylsilane, ethynyltrimethylsilane, and hexamethyldisilane.

5. The low-expansion silicon-carbon composite material according to claim 1, characterized in that, in the step S2, the rare earth metal dopant includes one or more of chlorides, sulfates, and nitrates of cerium, lanthanum, europium, neodymium, and yttrium.

6. The low-expansion silicon-carbon composite material according to claim 1, characterized in that, in the step S3, the volume ratio of the nitrogen source to the carbon source is 1:

10.

7. The low-expansion silicon-carbon composite material according to claim 1, characterized in that, in the step S3, the nitrogen source includes one of ammonia gas, ammonia water, ammonium bicarbonate, and ammonium carbonate.

8. The low-expansion silicon-carbon composite material according to claim 1, characterized in that, in the step S4, the calcination is specifically: heating at a heating rate of 1-10 °C / min to 700-1100 °C and holding for 10-120 min.

9. A negative electrode, characterized in that, it includes the low-expansion silicon-carbon composite material according to any one of claims 1-8.

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