CN115207326A

CN115207326A - Low-expansion silicon-carbon composite material and preparation method thereof

Info

Publication number: CN115207326A
Application number: CN202210983290.0A
Authority: CN
Inventors: 宋志涛; 陈佐川; 陈飞; 宋会英; 高永静
Original assignee: Sichuan Kuntian New Energy Technology Co ltd
Current assignee: Sichuan Kuntian New Energy Technology Co ltd
Priority date: 2022-08-16
Filing date: 2022-08-16
Publication date: 2022-10-18

Abstract

The invention relates to the technical field of lithium ion battery material preparation, and provides a low-expansion silicon-carbon composite material and a preparation method thereof, wherein the composite material is of a core-shell structure, an inner core comprises nano silicon, a carbon matrix and a metal dopant, an outer shell comprises amorphous carbon doped with nitrogen, and the mass of the outer shell is 1-10% of that of the composite material. According to the invention, the silicon-rare earth co-doped graphite paper composite material is prepared by embedding nano silicon on the surface of graphite paper by an oxygen plasma technology to reduce expansion and reducing impedance by rare earth doping. Through the technical scheme, the problems of large expansion and large impedance of the silicon-carbon material in the prior art are solved.

Description

Low-expansion silicon-carbon composite material and preparation method thereof

Technical Field

The invention relates to the technical field of preparation of lithium ion battery materials, in particular to a low-expansion silicon-carbon composite material and a preparation method thereof.

Background

Silicon carbon materials are applied to high-energy-density lithium ion batteries due to the advantages of high energy density, wide material sources and the like, but low-temperature performance deviation and cycle performance deviation of the materials are caused by poor electron conductivity of silicon and large full-current expansion of the silicon. In addition, the great stress generated by the volume change of the silicon material is easy to cause the separation of the active material and the conductive agent, greatly damages an electron transmission path inside the electrode, and even causes the peeling of an electrode coating from a current collector, so that the capacity of the battery is continuously reduced until the battery is completely damaged.

Another disadvantage of silicon cathodes is the low electron conductivity of the silicon material itself, about 10 ^-3 S·cm ^-1 And the rate of movement of lithium ions in the silicon negative electrode is low. And the electronic conductivity of the silicon-based material is improved mainly by doping, and the expansion is reduced by reducing the grain size of silicon. The existing silicon doping technology is mainly used for doping by a solid phase method/a liquid phase method, and has the problems of poor consistency, easy agglomeration and the like, so that the expansion is not improved, and the technical problem which is difficult to overcome by technical personnel is solved.

Disclosure of Invention

The 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 a silicon-carbon material in the prior art.

The technical scheme of the invention is as follows:

a low-expansion silicon-carbon composite material is of a core-shell structure, wherein a core comprises nano silicon, a carbon matrix and a metal dopant, a shell is composed of amorphous carbon doped with nitrogen, and the mass of the shell is 1% -10% of that of the composite material.

As a further technical scheme, the inner core consists of 10-50% of nano silicon, 1-10% of rare earth metal dopant and the balance of carbon matrix.

The invention also provides a preparation method of the low-expansion silicon-carbon composite material, which comprises the following steps:

s1, introducing oxygen-containing gas by taking a graphite paper substrate as a substrate and silane as a target material, and carrying out plasma treatment to obtain a silicon-doped graphite paper composite material;

s2, introducing oxygen-containing gas by taking the silicon-doped graphite paper composite material as a matrix and a rare earth metal dopant as a target material, and carrying out 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 mixed gas, wherein the mixed gas is mixed gas of a carbon source and an ammonia source;

and S4, cooling and post-treating to obtain the low-expansion silicon-carbon composite material.

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

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

As a further technical solution, 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.

As a further technical scheme, in the step S3, the volume ratio of the nitrogen source to the carbon source is 1.

As a further technical scheme, the carbon source in the step S3 comprises one or more of methane, acetylene, ethylene and ethane.

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

As a further technical scheme, in the step S4, the calcining specifically comprises: the temperature is raised to 700-1100 ℃ at the heating rate of 1-10 ℃/min and is preserved for 10-120min.

The invention also provides a negative electrode, which comprises the low-expansion silicon-carbon composite material prepared by the preparation method.

The invention has the beneficial effects that:

1. according to the invention, graphite paper is used as a matrix, and the oxygen plasma is used for depositing nano-silicon on the surface and in the matrix, so that the effect of restricting the expansion of silicon in the charging and discharging process can be realized, and the problems of poor consistency and agglomeration caused by solid-phase or liquid-phase doping can be avoided by the oxygen plasma treatment technology. In addition, the plasma technology of the invention takes the graphite paper as the matrix, the graphite paper has high electronic conductivity and strong flexibility, and the full electric expansion and impedance of the silicon can be reduced.

2. The invention also utilizes rare earth element doping and leads the rare earth element to be coated on the surface of the silicon-doped graphite paper composite material, and the rare earth element coating does not directly contact with electrolyte, thereby reducing the occurrence of side reaction of nano silicon and further improving the cycle performance.

3. The surface of the coated nano silicon is further deposited by carbon and nitrogen, so that the expansion of the pole piece in the charging and discharging process can be restrained, and on the other hand, the nitrogen has lower electronic impedance than the carbon, the impedance of the composite material is reduced, the electrolyte is isolated, the side reaction is reduced, and the storage and cycle performance is improved.

Drawings

The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.

Fig. 1 is an SEM image of a low expansion silicon carbon composite prepared in example 1.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall relate to the scope of protection of the present invention.

Example 1

S1, taking a graphite paper substrate as a substrate and methyl silane as a target, introducing oxygen (with the flow rate of 50 mL/min), and carrying out plasma treatment for 60min under the conditions that the frequency is 2MHz and the power is 100W to obtain a silicon-doped graphite paper composite material;

s2, taking the silicon-doped graphite paper composite material as a substrate and cerium chloride as a target material, introducing oxygen, and carrying out plasma treatment for 60min under the conditions that the frequency is 2MHz and the power is 100W to obtain a silicon-rare earth co-doped graphite paper composite material;

and S3, transferring the silicon-rare earth co-doped graphite paper composite material into a tubular furnace, firstly introducing argon inert gas to remove air in the tube, then introducing carbon-nitrogen mixed gas (ammonia gas: methane =1 10), with the flow rate of 10mL/min, heating to 950 ℃ at the heating rate of 5 ℃/min, keeping the temperature for 60min, then cooling to room temperature under the argon inert atmosphere, and crushing to obtain the low-expansion silicon-carbon composite material.

Example 2

S1, taking a graphite paper substrate as a substrate and dimethylsilane as a target, introducing oxygen (flow rate is 50 mL/min), and carrying out plasma treatment for 120min under the conditions of frequency of 1MHz and power of 50W to obtain a silicon-doped graphite paper composite material;

s2, taking the silicon-doped graphite paper composite material as a matrix, taking lanthanum sulfate as a target material, introducing oxygen, and carrying out plasma treatment for 120min under the conditions that the frequency is 1MHz and the power is 50W to obtain a silicon-rare earth co-doped graphite paper composite material;

and S3, transferring the silicon-rare earth co-doped graphite paper composite material into a tubular furnace, firstly introducing argon inert gas to remove air in the tube, then introducing carbon-nitrogen mixed gas (ammonia gas: acetylene =1 10), with the flow rate of 10mL/min, heating to 700 ℃ at the speed of 1 ℃/min, preserving the temperature for 120min, then cooling to room temperature under the inert atmosphere of argon, and crushing to obtain the low-expansion silicon-carbon composite material.

Example 3

S1, taking a graphite paper substrate as a substrate and hexamethyldisilane as a target, introducing oxygen (flow rate is 50 mL/min), and carrying out plasma treatment for 10min under the conditions of frequency of 5MHz and power of 200W to obtain a silicon-doped graphite paper composite material;

s2, taking the silicon-doped graphite paper composite material as a matrix, taking europium chloride as a target material, introducing oxygen, and carrying out plasma treatment for 10min under the conditions that the frequency is 5MHz and the power is 200W to obtain a silicon-rare earth co-doped graphite paper composite material;

and S3, transferring the silicon-rare earth co-doped graphite paper composite material into a tubular furnace, firstly introducing argon inert gas to remove air in the tube, then introducing carbon-nitrogen mixed gas (ammonia gas: ethylene =1 10), heating to 1100 ℃ at a heating rate of 10 ℃/min at a flow rate of 10mL/min, preserving heat for 10min, then cooling to room temperature under an argon inert atmosphere, and crushing to obtain the low-expansion silicon-carbon composite material.

Comparative example 1

The silicon-doped graphite paper composite material prepared in the step S1 of example 1 is transferred to a tubular furnace, argon inert gas is firstly introduced to remove air in the tube, then carbon-nitrogen mixed gas (ammonia gas: methane = 1.

Comparative example 2

Adding 5g of nano silicon, 5g of cerium chloride and 100g of graphite paper into 500mL of ethanol solution, performing ball milling and uniform mixing, performing spray drying, transferring to a tube furnace, firstly introducing an argon inert gas to remove air in the tube, then introducing a carbon-nitrogen mixed gas (ammonia gas: methane =1 10), heating to 950 ℃ at a heating rate of 5 ℃/min, performing heat preservation for 3 hours, then cooling to room temperature under an argon inert atmosphere, and crushing to obtain the silicon-cerium doped graphite paper composite material.

Experimental example 1: topography testing

SEM tests were performed on the silicon carbon composite material of example 1, and the test results are shown in fig. 1. As can be seen from FIG. 1, the material has a granular structure, and the particle size of the granules is between 5 and 15 μm.

Experimental example 2: button cell test

The silicon-carbon composite materials in examples 1-3 and comparative examples 1-2 are used as negative electrode materials of lithium ion batteries to assemble button batteries, which are respectively marked as A1, A2, A3, B1 and B2.

The preparation method comprises the following steps:adding a binder, a conductive agent and a solvent into a lithium ion battery 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 LA132, the conductive agent is SP, the solvent is NMP (N-methylpyrrolidone), and the dosage proportion of the negative electrode material, SP, PVDF and NMP is 95g:1g:4g:220mL; liPF in electrolyte ₆ As electrolyte, a mixture of EC and DEC with the volume ratio of 1:1 is solvent; the metal lithium sheet is used as a counter electrode, and the diaphragm is a polypropylene (PP) film. The button cell was assembled in a hydrogen-filled glove box. The electrochemical performance is carried out on a battery tester of Wuhan blue electricity CT2001A type, the charging and discharging voltage range is 0.005V to 2.0V, and the charging and discharging speed is 0.1C. The test results are shown in table 1.

TABLE 1 comparison of button cell performance of examples and comparative examples

As can be seen from the data in table 1, the oxygen plasma treatment is not adopted in comparative example 1, the rare earth element doping and cladding are not performed, and the raw materials identical to those in the example are adopted in comparative example 2, but the conventional ball milling method is adopted, so that the specific capacity and the first efficiency of the silicon-carbon composite material prepared in the example of the invention are obviously superior to those of the comparative example. The reasons for this may be: in the embodiment, the rare earth compound is doped in the nano silicon to improve the powder conductivity of the material, reduce the impedance and improve the first efficiency; meanwhile, the oxygen plasma technology is adopted, so that the density and the tap density of the material are improved, and the specific capacity of the material is improved.

Experimental example 3: pouch cell testing

The silicon-carbon composite materials in examples 1-3 and comparative examples 1-2 were doped with 90% artificial graphite as a negative electrode material to prepare a negative electrode sheet, and NCM532 was used as a positive electrode material; liPF in electrolyte ₆ As electrolyte, a mixture of EC and DEC with the volume ratio of 1:1 is solvent; and (3) preparing 5Ah soft package batteries by using the Celgard 2400 membrane as a diaphragm, wherein the labels are C1, C2, C3, D1 and D2. Respectively testing the liquid absorption and retention capacity of the negative plate and the rebound of the pole plateSex and cycle performance.

a. Imbibition ability test

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

b. Liquid retention test

Calculating the theoretical liquid absorption amount m of the pole piece according to the pole piece parameters ₁ And weighing the weight m of the pole piece ₂ Then, the pole piece is placed in electrolyte to be soaked for 24 hours, and the weight of the pole piece is weighed to be m ₃ Calculating the amount m of the pole piece liquid absorption 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.

Table 2 comparison of liquid absorbing and retaining capacities of pouch batteries prepared in examples and comparative examples

As can be seen from Table 2, the liquid-absorbing and liquid-retaining abilities of the silicon composite materials obtained in examples 1 to 3 were significantly higher than those of the comparative example. Experimental results show that the silicon-carbon composite material has high liquid absorption and retention capacity.

c. Pole piece rebound rate test

Firstly, testing the average thickness of a pole piece of the lithium ion battery by using a thickness tester to be D1, then placing the pole piece in a vacuum drying oven at 80 ℃ for drying for 48 hours, testing the thickness of the pole piece to be D2, and calculating according to the following formula: rebound rate = (D2-D1) × 100%/D1. The test results are shown in table 3.

d. Pole piece resistivity testing

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

TABLE 3 pole piece rebound and pole piece resistivity for the examples and comparative examples

As can be seen from the data in table 3, the negative electrode sheets made of the silicon-carbon composites obtained in examples 1 to 3 have significantly lower rebound ratios and electrical resistivities than those of the comparative examples, i.e., the negative electrode sheets made of the silicon-carbon composites of the present invention have lower rebound ratios and electrical resistivities. The reason for this may be: the material prepared by adopting the oxygen plasma technology can reduce the expansion, and meanwhile, the rare earth compound is doped to improve the electronic conductivity of the material and reduce the resistivity of the pole piece.

e. Cycle performance test

The charge and discharge multiplying power is 1C/1C, the voltage range is 2.8V-4.2V, and the cycle performance of the battery is tested at the temperature of 25 +/-3 ℃. The test results are shown in table 4.

TABLE 4 cyclability of the examples and comparative examples

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, which may be due to: the silicon-carbon composite material has a more stable pole piece structure in the charging and discharging processes, and the cycle performance of the silicon-carbon composite material is improved; in addition, the rare earth elements are doped in the plasma, so that the impedance is reduced, and the cycle performance of the material is improved.

The present invention is not limited to the above preferred embodiments, and any modifications, equivalent substitutions, improvements, etc. within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. The low-expansion silicon-carbon composite material is characterized in that the composite material is of a core-shell structure, the core comprises nano silicon, a carbon matrix and a metal dopant, the shell is made of amorphous carbon doped with nitrogen, and the mass of the shell is 1% -10% of that of the composite material.

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

3. The method for preparing a low expansion silicon carbon composite material according to claim 1 or 2, comprising the steps of:

4. The method for preparing a low expansion silicon carbon composite material according to claim 3, wherein in the steps S1 and S2, the parameters of the plasma treatment are as follows: the frequency is 1-5MHz, the power is 50-200W, and the treatment time is 10-120min.

5. The method for preparing a low expansion silicon carbon composite material as claimed in claim 3, wherein in the step S1, the silane comprises one or more of methylsilane, dimethylsilane, ethynyltrimethylsilane and hexamethyldisilane.

6. The method of claim 3, wherein in the step S2, the rare earth dopant comprises one or more of chlorides, sulfates and nitrates of cerium, lanthanum, europium, neodymium and yttrium.

7. The method for preparing a low expansion silicon-carbon composite material according to claim 3, wherein the volume ratio of the nitrogen source to the carbon source in step S3 is 1.

8. The method for preparing a low expansion silicon-carbon composite material according to claim 3, wherein the nitrogen source in step S3 comprises one of ammonia gas, ammonia water, ammonium bicarbonate and ammonium carbonate.

9. The method for preparing the low expansion silicon-carbon composite material according to claim 3, wherein the calcination in the step S4 is specifically: heating to 700-1100 deg.C at a heating rate of 1-10 deg.C/min, and maintaining for 10-120min.

10. A negative electrode comprising the low expansion silicon carbon composite material obtained by the production method according to any one of claims 3 to 9.