Method for improving specific capacity and first cycle efficiency of pyrolytic carbon coated silicon negative electrode material, and negative electrode plate prepared from pyrolytic carbon coated silicon negative electrode material

Abstract

The invention provides a method for improving the specific capacity and the first cycle efficiency of a pyrolytic carbon coated silicon negative electrode material, and a negative electrode plate prepared by the method. Different from the existing gas-phase and liquid-phase coating technology, the method comprises the following steps: uniformly mixing a pure silicon material with a liquid-phase carbon source to obtain a silicon-containing mixed solution; then spraying the mixed solution into a gas-phase pyrolysis reaction furnace at a certain target temperature to obtain precursor particles; the temperature pyrolysis avoids the self-nucleation of pyrolytic carbon and the formation of by-product carbon particles. And finally carrying out high-temperature carbonization on the precursor particles to obtain the final silicon-carbon composite material. A silicon negative electrode material is subjected to carbon coating by using a liquid-phase carbon source twice gas-phase pyrolysis coating technology to obtain the silicon-carbon composite material. The prepared material can realize carbon coating of a silicon raw material with high carbonization yield, almost no carbon byproduct is generated, and the purity and the silicon content of the product are improved. When the silicon-carbon composite material is used as a negative electrode of a lithium ion battery, high specific capacity and first coulombic efficiency are achieved.

Description

Technical Field

[0001] The present disclosure relates to the technical field of lithium-ion battery material preparation, and particularly relates to a silicon-carbon composite material, a process improvement method thereof, and a negative electrode sheet prepared therefrom. Technical Background

[0002] Experimental exploration has found that silicon materials can form a Li 22 Si5 configuration. According to calculations, the specific capacity at this state is 4200 mAh / g, which is the highest specific capacity known at present. In terms of mass specific capacity, silicon is much higher than the graphite-based negative electrodes currently in use. In addition, the lithium deintercalation voltage of silicon is low (0.4V), and the lithium intercalation voltage is much higher than that of graphite, which can effectively inhibit the formation of lithium dendrites and reduce the risk of diaphragm piercing, and has higher safety than traditional carbon negative electrodes. Moreover, silicon ranks second in the earth's crust (26.4%), and is widely present in substances such as ore soil and sand. Therefore, silicon materials are widely sourced, relatively inexpensive, and pollution-free, and thus have been the focus of development. However, silicon undergoes a huge volume expansion (about 300%) during charge and discharge in the battery, which in turn brings problems such as material structure damage, loss of electrical contact, and continuous fragmentation of the SEI film to consume lithium ions.

[0003] To solve the problems of volume expansion and poor conductivity of silicon materials during the cycle of lithium-ion batteries, numerous methods have been proposed in different aspects. For example, in terms of the material itself, methods such as nanosizing, carbon coating, and composite with other metals have been applied to silicon materials. In addition, efforts have also been made in modifying the binder and electrolyte. Carbon materials have certain mechanical strength and high conductivity. Therefore, compounding silicon with carbon can not only relieve the expansion of silicon materials but also increase the conductivity of the composite material. The preparation of core-shell structure materials by carbon coating has thus been widely studied, especially gas-phase coating [such as CN202210177732.2, CN202311296879.4] and liquid-phase coating [such as CN202310941729.8, CN202310871403.2]. However, gas-phase coating takes a long time, has low raw material utilization rate, and high cost; liquid-phase coating uses a large amount of organic solvents, most of the carbon sources are toxic, and tail gas treatment increases the cost. The present invention uses a liquid-phase carbon source as the dispersion medium to make the raw materials mix more evenly and the mixing process simpler. In addition, gas-phase pyrolysis coating makes the carbon coating uniform, has a high yield, a simple process, and is easy to scale up production.

[0004] [CN201811077874.1] First, uniformly mix pure silicon material with a liquid-phase carbon source to obtain a silicon-containing mixed solution; then, drop the mixed solution into a vapor-phase pyrolysis reaction furnace that has been preheated to a relatively high target temperature in an inert gas atmosphere to obtain a carbon-coated silicon anode material. Although this one-step method can achieve a good modification effect on silicon raw materials, pyrolytic carbon spheres will inevitably form during the vapor-phase pyrolysis process, which will significantly reduce the specific capacity and the first charge-discharge efficiency of the product. Therefore, a reasonable process flow design to reduce the impact of by-products on the silicon-carbon composite material prepared by the vapor-phase pyrolysis method is particularly important. Summary of the Invention

[0005] To solve the above-mentioned technical problems, the present disclosure provides a method for improving the specific capacity and the first-cycle efficiency of a pyrolytic carbon-coated silicon anode material, including its anode sheet.

[0006] In a first aspect, the present disclosure provides a method for improving the specific capacity and the first-cycle efficiency of a pyrolytic carbon-coated silicon anode material, characterized in that the preparation method includes the following steps:

[0007] (1) Uniformly mix pure silicon material with a liquid-phase carbon source to obtain a silicon-containing mixed solution for standby;

[0008] (2) Spray the mixed solution into a vapor-phase pyrolysis reaction furnace that has been preheated to a relatively low target temperature to obtain precursor-coated particles;

[0009] (3) Perform secondary high-temperature carbonization on the precursor particles in the reaction furnace to obtain the final silicon-carbon composite material.

[0010] As a preferred technical solution of the present disclosure, the silicon anode material described in step (1) of the present disclosure is selected from any one or a combination of two or more of silicon particles, silicon wires, silicon wafers, and porous silicon particles. Based on the technical advantages of vapor-phase pyrolytic carbonization coating, this method has strong applicability to the shape of the material to be coated.

[0011] As a preferred technical solution of the present disclosure, the size of the silicon is 5-2500 nm, such as 50 nm, 200 nm, 600 nm, etc., preferably 50-1500 nm. Based on the technical advantages of vapor-phase pyrolytic carbonization coating, this method also has strong applicability to the size of the material to be coated.

[0012] As a preferred technical solution of the present disclosure, the liquid-phase carbon source described in step (1) is selected from any one or a combination of at least two of pyridine, pyrrole, cyclohexane, n-heptane, piperidine, aniline, pyrimidine, or wash oil, preferably selected from any one or a combination of at least two of pyridine, pyrrole, cyclohexane, aniline, or wash oil.

[0013] As a preferred technical solution of the present disclosure, the mass-volume ratio of the silicon anode material and the liquid carbon source described in step (1) is 1 g:(2 - 20) mL, such as 1 g:2 mL, 1 g:6 mL, 1 g:10 mL, etc., and preferably 1 g:(2 - 10) mL. The mass-volume ratio of the silicon anode material to the liquid carbon source refers to the proportional relationship between the silicon anode material and the liquid carbon source, where 1 g of silicon corresponds to 2 - 10 mL of the liquid carbon source.

[0014] The present disclosure can control the thickness of the carbon layer of the prepared silicon-carbon composite material by controlling the addition amount of the liquid carbon source.

[0015] The present disclosure has no special restrictions on the mixing method, and any mixing method that can uniformly mix the modified silicon and the liquid carbon source is applicable to the present disclosure. Preferably, it is ultrasonic dispersion for 15 - 60 min, such as 15 min, 20 min, 35 min, 45 min, etc., and then stirring for 6 - 12 h, such as 6 h, 8 h, 10 h, 12 h, etc.

[0016] In the present disclosure, by optimizing the steps of the gas-phase pyrolytic carbonization coating process, the generation of by-products (pyrolytic carbon spheres) during the carbonization process is greatly reduced. While purifying the product, carbon layer coating on the silicon raw material also improves the cycling performance of the product, achieving an increase in the specific capacity and the first-cycle Coulombic efficiency of the product.

[0017] As a preferred technical solution of the present disclosure, the appropriate program temperature design principle described in step (2) is that the carbon source can undergo a gas-phase pyrolytic coating reaction and will not form carbon sphere by-products by self-nucleation. In addition, the described temperature can be a temperature range with a program change rather than a single fixed temperature. For example, a uniform temperature increase of 700 °C, 750 °C, 800 °C, or 700 - 800 °C. Preferably, according to different ratios of the carbon source, its approximate range is 400 - 800 °C. A lower pyrolysis temperature makes the energy of the cracked molecules lower, and it can only undergo reactions with lower activation energies (depositing carbonization on the silicon surface) and cannot form carbon sphere by-products by self-nucleation.

[0018] As a preferred technical solution of the present disclosure, the time experienced in the gas-phase pyrolytic carbonization process described in step (2) is determined by process design to be 0.25 - 3 h, and preferably 0.5 - 2 h. A longer holding time can make the growth of the carbon layer as regular as possible, but there is an upper limit restricted by the temperature.

[0019] The present disclosure can also control the thickness of the carbon layer of the prepared silicon-carbon composite material by controlling the temperature in step (2).

[0020] As a preferred technical solution of the present disclosure, the method of adding the mixed solution described in step (2) into the gas-phase pyrolysis reaction furnace includes, but is not limited to, spraying, injection, dropping, etc., and spraying is preferred. The spraying method can make the raw materials heat more evenly and have a larger heating area after entering the reaction furnace, so that the heating rate is faster.

[0021] As a preferred technical solution of the present disclosure, the secondary carbonization temperature described in step (3) is 850 - 1200 °C, and the time is 0.25 - 5 h. The preferred temperature is 900 - 1100 °C, and the preferred time is 0.5 - 3 h. Performing secondary carbonization at high temperature makes the carbon layer more regular to improve its conductivity and structural strength.

[0022] In a second aspect, the present disclosure provides a silicon-carbon composite material prepared by the preparation method described in the first aspect.

[0023] The silicon-carbon composite material provided by the present disclosure can effectively reduce the generation of by-products during the gas-phase pyrolysis coating of the silicon negative electrode material, and at the same time inhibit the volume expansion of silicon during the lithiation reaction process, so that the subsequently assembled battery has stable cycle performance and higher specific capacity and charge-discharge efficiency.

[0024] In a third aspect, the present disclosure provides an application of the silicon-carbon composite material described in the second aspect in a negative electrode material for a lithium-ion battery.

[0025] In a fourth aspect, the present disclosure provides a negative electrode sheet including the silicon-carbon composite material described in the second aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Attached Figure 1 is a performance comparison between Example 1 and Comparative Example 1 at a current density of 500 mA / g.

[0027] Attached Figure 2 is a performance comparison between Example 1 and Comparative Example 2 at a current density of 2 A / g. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] In order to more clearly understand the above objects, features, and advantages of the present disclosure, the solutions of the present disclosure will be further described below. It should be noted that, without conflict, the embodiments of the present disclosure and the features in the embodiments can be combined with each other.

[0029] Many specific details are set forth in the following description to facilitate a thorough understanding of the present disclosure, but the present disclosure may be practiced in other ways different from those described herein; obviously, the embodiments in the specification are only a part of the embodiments of the present disclosure, rather than all the embodiments.

[0030] The present disclosure provides a preparation method for a negative electrode sheet for a lithium-ion battery:

[0031] The silicon-carbon composite materials, sodium carboxymethyl cellulose, acetylene black, and single-walled carbon nanotube aqueous slurry obtained in Examples 1-6 and Comparative Examples 1-2 were mixed at a mass ratio of 91:6:2:1, coated on a copper foil, and vacuum-dried at 100°C for 12 h. The copper foil coated with the electrode material was cut into circular pole pieces with a diameter of 12 mm and pressed at a pressure of 9 MPa for 120 s to obtain the pole pieces as the negative electrode;

[0032] Example 1

[0033] This example provides a preparation method of a silicon-carbon composite material, including the following steps:

[0034] (1) Weigh 0.5 g of flaky silicon particles with an average diameter of 600 nm, place them in an elemental bottle equipped with magnetic stirring, add 3 mL of pyrrole, and mix evenly. Then, perform ultrasonic treatment for 30 min and stir on a magnetic stirrer for 12 h to obtain a mixed solution for standby.

[0035] (2) Gradually spray the mixed solution into a carbonization furnace that is under a nitrogen atmosphere and has been preheated to 700°C, and then keep it warm for 1 h to obtain a silicon-carbon composite material precursor.

[0036] (3) Sinter the precursor in a horizontal carbonization furnace at 900°C under a nitrogen atmosphere for 2 h with a heating rate of 10°C / min to obtain the silicon-carbon composite material.

[0037] Example 2

[0038] This example provides a preparation method of a silicon-carbon composite material, including the following steps:

[0039] (1) Weigh 0.5 g of flaky silicon particles with an average diameter of 600 nm, place them in an elemental bottle equipped with magnetic stirring, add 3 mL of pyrrole, and mix evenly. Then, perform ultrasonic treatment for 30 min and stir on a magnetic stirrer for 12 h to obtain a mixed solution for standby.

[0040] (2) Gradually spray the mixed solution into a carbonization furnace that is under a nitrogen atmosphere and has been preheated to 750°C, and then keep it warm for 1 h to obtain a silicon-carbon composite material precursor.

[0041] (3) Immediately raise the furnace temperature of the precursor to 1000°C with a heating rate of 10°C / min, and keep it warm for 2 h to obtain the silicon-carbon composite material.

[0042] Example 3

[0043] This example provides a preparation method of a silicon-carbon composite material.

[0044] The difference from Example 1 is that the silicon source in step (1) is nano-silicon particles (average size of 200 nm), and the liquid carbon source is aniline.

[0045] Example 4

[0046] This example provides a method for preparing a silicon-carbon composite material.

[0047] The difference from Example 1 is that in step (2), after spraying for 0.5 h at 750 °C, the temperature is raised to 800 °C and kept for 0.5 h.

[0048] Examples 5 - 6

[0049] This example provides a method for preparing a silicon-carbon composite material.

[0050] The difference from Example 1 is that in step (1), the mass-to-volume ratio of flaky silicon particles to pyrrole is 0.5 g:1 mL (Example 5) and 1 g:10 mL (Example 6), respectively.

[0051] Comparative Example 1

[0052] This comparative example provides a method for preparing a silicon-carbon composite material. The difference from Example 1 is that in step (2), the temperature is 900 °C and the third step is not carried out anymore.

[0053] Comparative Example 2

[0054] This comparative example provides a method for preparing a silicon-carbon composite material. The difference from Example 1 is that step (3) is not carried out anymore.

[0055] Performance Test

[0056] Table 1 Test data of Examples 1 - 6 and Comparative Examples 1 - 2

[0057]

[0058] It can be seen from Figure 1 that the low-temperature gas-phase carbonization coating process plays an important role in improving the specific capacity and initial efficiency of the pyrolysis-coated silicon-carbon composite material. The specific capacity increases from 1518 mAh·g –1 to 2389 mAh·g –1 , and the initial charge-discharge efficiency increases from 86.4% to 89.4%. And Figure 2It is further shown that subsequent secondary high-temperature sintering is indispensable for ensuring the role of carbon layer coating in improving the cycle stability of the material (at a current density of 2 A / g, the capacity retention rate of the material without secondary high-temperature sintering after 1000 cycles is only 18.25%, while that of the material after high-temperature sintering is 81.34%). Thus, it can be seen that the material prepared by the method (high-low temperature multiple gas-phase carbonization coating technology) proposed in the present invention for improving the specific capacity and initial efficiency of the pyrolytically coated silicon-carbon anode material can achieve efficient carbon coating of silicon raw materials with almost no carbon by-products. After the negative electrode sheet prepared by this method is made into a button battery, a higher specific capacity and a higher initial Coulomb efficiency can be achieved.

Claims

1. A method for improving the specific capacity and the first cycle efficiency of a pyrolytic carbon-coated silicon anode material, characterized in that the preparation method comprises the following steps: (1) Uniformly mix pure silicon material with a liquid-phase carbon source to obtain a silicon-containing mixed solution for standby; (2) Spray the mixed solution into a gas-phase pyrolysis reaction furnace preheated to a relatively low target temperature to obtain precursor-coated particles; (3) Perform secondary high-temperature carbonization on the precursor-coated particles in a carbonization furnace to obtain a final silicon-carbon composite material.

2. The preparation method according to claim 1, characterized in that the silicon anode material described in step (1) is selected from any one or a combination of two or more of silicon particles, silicon wires, silicon wafers, and porous silicon particles; the size of the described silicon is 5-2500 nm, and the preferred nanosize is 50-1500 nm.

3. The preparation method according to claim 1, characterized in that the liquid-phase carbon source described in step (1) is selected from any one or a combination of at least two of pyridine, pyrrole, cyclohexane, n-heptane, piperidine, aniline, pyrimidine, or wash oil, preferably selected from any one or a combination of at least two of pyridine, pyrrole, cyclohexane, aniline, or wash oil.

4. The preparation method according to claims 1 to 3, characterized in that the mass-volume ratio of the silicon material and the liquid-phase carbon source described in step (1) is 1 g:(2-20) mL, preferably 1 g:(2-10) mL.

5. The preparation method according to claim 1, characterized in that the appropriate temperature design principle described in step (2) is that the carbon source can undergo a chemical vapor deposition reaction and will not form carbon sphere by-products by self-nucleation; in addition, the described temperature can be a non-single fixed temperature and can be a programmed temperature range; according to the different carbon sources and ratios, the temperature range is preferably 400-800 °C.

6. The preparation method according to claim 1, characterized in that the time experienced in the gas-phase pyrolysis process described in step (2) is determined by process design to be 0.25-3 h, preferably 0.5-2 h.

7. The preparation method according to claim 1, characterized in that the method of adding the mixed solution into the gas-phase pyrolysis reaction furnace described in step (2) includes but is not limited to spraying, injection, dropping, etc., preferably spraying.

8. The preparation method according to claim 1, characterized in that the temperature of the secondary carbonization described in step (3) is 850-1200 °C and the time is 0.25-5 h; the preferred temperature is 900-1100 °C, and the preferred time is 0.5-3 h.

9. A silicon-carbon composite material prepared by the preparation method according to any one of claims 1 to 8.

10. Application of the silicon-carbon composite material according to claim 9 in a negative electrode material of a lithium-ion battery.

11. A negative electrode sheet, characterized in that, Comprising the silicon-carbon composite material according to claim 10.

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