Preparation method of modified porous carbon based on photocatalytic halogen atom transfer and silicon-carbon negative electrode

Abstract

The invention relates to a preparation method of modified porous carbon based on photocatalytic halogen atom transfer and a silicon-carbon negative electrode, and the preparation method comprises the following steps: (S1) dipping porous carbon in a C1-C5 halogenated hydrocarbon solution containing a photosensitizer to obtain halogenated hydrocarbon-loaded porous carbon; (S2) carrying out photocatalytic reaction on the halohydrocarbon-loaded porous carbon in an inert atmosphere, washing and drying to obtain a modified porous carbon precursor; and (S3) carrying out heat treatment on the modified porous carbon precursor in an inert atmosphere to obtain the modified porous carbon based on photocatalytic halogen atom transfer. Carbon free radicals are introduced into pores of porous carbon through a halogen atom transfer (XAT) reaction by adopting C1-C5 halogenated hydrocarbon, and then amorphous carbon is formed through heat treatment and is filled in ultra-micropores, so that the problems of low effective deposition rate of silicon and irreversible lithium intercalation caused by the fact that the ultra-micropores cannot be utilized are solved; and further, the electrochemical performance of the silicon-carbon negative electrode is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of porous carbon, and particularly relates to a preparation method of modified porous carbon based on photocatalytic halogen atom transfer and a silicon-carbon negative electrode. Background Art

[0002] In recent years, with the continuous development of lithium-ion batteries, silicon-carbon composite materials, as a new type of negative electrode material, have gradually received extensive attention. Due to its high theoretical specific capacity (about 3579 mAh / g), silicon materials have great application potential in lithium-ion batteries. However, during the charge and discharge process, silicon will undergo significant volume expansion (up to about 300%), resulting in the structural damage and capacity decline of silicon-carbon composite materials, which limits their practical application in batteries. To solve this problem, researchers have adopted various methods, and among them, the preparation of silicon-carbon composite materials by chemical vapor deposition technology has become an effective solution. In this preparation method, silane gas (such as silane SiH4) is introduced into the reaction chamber of the porous carbon material. Under high-temperature conditions (usually between 300°C and 600°C), the Si-H bonds of silane molecules break, generating highly active silicon atoms and hydrogen gas; with the migration and adsorption of silicon atoms, these active silicon atoms will deposit in the pores of the porous carbon material to form amorphous nano-silicon.

[0003] However, the structure of the porous carbon skeleton has an important influence on the performance of the silicon-carbon composite material prepared by chemical vapor deposition of silicon. Especially when the proportion of ultra-micropores (pore diameter less than 0.7 nm) in the porous carbon is relatively large, the following problems will occur: (1) Silane cannot be cracked and deposited: Due to the too small pore diameter of ultra-micropores, silane molecules cannot enter and crack and deposit inside them, resulting in the inability to utilize ultra-micropores and reducing the effective deposition rate of silicon. (2) Irreversible lithium intercalation: During the lithium intercalation process, ultra-micropores will capture lithium ions and form irreversible lithium intercalation, leading to battery capacity attenuation and decreased cycle performance.

[0004] Therefore, reducing the number of ultra-micropores in the porous carbon material is the key to improving the performance of the chemical vapor deposition silicon-carbon negative electrode, and traditional methods (such as chemical activation, physical activation) are difficult to precisely control the proportion of ultra-micropores and pore volume. Summary of the Invention

[0005] In view of the problems that ultra-micropores in porous carbon in the prior art cannot effectively deposit silicon and there is irreversible lithium intercalation, and traditional methods are difficult to precisely control the proportion of ultra-micropores, the present invention provides a preparation method of modified porous carbon based on photocatalytic halogen atom transfer. By using halogen atom transfer (XAT) reaction to introduce carbon free radicals into the ultra-micropores of porous carbon, and then forming amorphous carbon by heat treatment to fill the ultra-micropores, the problems of ineffective silicon deposition in ultra-micropores and irreversible lithium intercalation are solved, thereby improving the electrochemical performance of the silicon-carbon negative electrode material.

[0006] To achieve the above object, the present invention adopts the following technical solutions: A preparation method of modified porous carbon based on photocatalytic halogen atom transfer, comprising the following steps: (S1) Immerse the porous carbon in a C1-C5 halogenated hydrocarbon solution containing a photosensitizer, filter, wash, and dry after immersion to obtain porous carbon loaded with halogenated hydrocarbon; (S2) Carry out a photocatalytic reaction on the porous carbon loaded with halogenated hydrocarbon under an inert atmosphere, and obtain a modified porous carbon precursor after washing and drying; (S3) Heat-treat the modified porous carbon precursor under an inert atmosphere to obtain modified porous carbon based on photocatalytic halogen atom transfer.

[0007] Further, in step (S1), the particle size D50 of the porous carbon is 1-10 μm, the specific surface area is 1000-3000 m² / g, preferably 2000-3000 m² / g, the total pore volume is 0.5-2.0 cm³ / g, and the proportion of ultramicropores is 15%-30%; preferably, the porous carbon is dried before immersion, and the drying is vacuum drying at 130-160 °C for 5-8 h.

[0008] Further, the photosensitizer is at least one of tris(2-phenylpyridine)iridium (Ir(ppy)3), ruthenium(III) tris(bipyridine) dichloride (Ru(bpy)3Cl2), 2,4,5,6-tetrakis(9-carbazolyl) isophthalonitrile (4CzlPN), and its dosage is 0.5-1.5 wt% of the porous carbon.

[0009] Further, in step (S1), the C1-C5 halogenated hydrocarbon is at least one of dichloromethane, chloroform, chloroethane, dichloroethane, chloropropane, chloroisopropane, chlorobutane, chloroisobutane, chloropentane, bromoethane, bromopropane, bromoisopropane, bromobutane, iodoethane, iodopropane, preferably at least one of dichloromethane, chloroethane, chloropropane, bromoethane. The molecular size of the halogenated hydrocarbon in the present invention is small and matches the kinetic diameter of the ultramicropores, so that the halogenated hydrocarbon can penetrate into the pore channels during the immersion process. The preferably halogenated hydrocarbon has a smaller molecular size and better effect; at the same time, the bond energy between halogen and carbon is low, and it is easy to be excited and broken to form carbon radicals during the subsequent photocatalytic reaction.

[0010] Further, in step (S1), the concentration of the C1-C5 haloalkane solution is 0.05 - 0.5 mol / L, and the solvent is at least one of ethanol, acetone, and tetrahydrofuran; the dosage of the porous carbon and the C1-C5 haloalkane solution is 1 g : (10 - 20) mL; the impregnation conditions are: impregnation at 20 - 30 °C for 12 - 48 h, and preferably, ultrasonic treatment is applied during impregnation, with an ultrasonic frequency of 30 - 40 kHz, a power of 50 - 100 W, and a time of 10 - 30 min.

[0011] Further, in step (S1), the washing is ethanol washing 3 - 5 times to remove the physically adsorbed haloalkanes on the surface of the porous carbon; the drying is vacuum drying at 60 - 80 °C for 6 - 12 h.

[0012] Further, in step (S2), the conditions of the photocatalytic reaction are: light source with a wavelength of 200 - 800 nm and a light intensity of 50 - 150 mW / cm² irradiates for 1 - 5 h at 20 - 50 °C; the wavelength is preferably 350 - 500 nm. Under photoexcitation, the C-X bond of the haloalkane (R-X) breaks to generate a carbon radical (R·) and a halogen radical (X·), and the carbon radical is fixed on the pore wall surface through physical adsorption or chemical bonding to form a uniform coating layer.

[0013] Further, in step (S2), the inert atmosphere is nitrogen and / or argon; the washing is soaking in acetone or ethanol for 1 - 3 h and then washing with water until neutral to remove the unreacted halogen by-products; the drying is vacuum drying at 80 - 120 °C for 4 - 12 h.

[0014] Further, in step (S3), the conditions of the heat treatment are: heat preservation at 400 - 600 °C for 1 - 5 h; the inert atmosphere is nitrogen and / or argon. During the heat treatment process, the carbon radicals dehydrogenate and carbonize to form amorphous carbon filling in the ultramicropores. The heat treatment temperature of 400 - 600 °C is lower than the graphitization temperature of the porous carbon skeleton, which can effectively avoid the destruction of the main structure.

[0015] The schematic diagram of the principle of the present invention is as Figure 1 shown.

[0016] In the second aspect, the present invention provides a modified porous carbon based on photocatalytic halogen atom transfer, which is prepared by the aforementioned preparation method.

[0017] In a third aspect, the present invention also provides a silicon-carbon negative electrode, which is prepared by subjecting the modified porous carbon obtained by the foregoing preparation method to vapor deposition of silicon and carbon coating. The processes of vapor deposition of silicon and carbon coating are well-known to those skilled in the art. For example, organosilicon silicon source gas is used for vapor deposition of silicon, and the organosilicon source gas is selected from at least one of silane, dichlorosilane, trichlorosilane, silicon tetrachloride, silicon tetrafluoride, disilane, etc.; for example, carbon source gas is used for carbon coating, and the carbon source gas is selected from at least one of C1-4 alkanes, C2-4 alkenes, and C2-4 alkynes.

[0018] Compared with the prior art, the present invention has the following beneficial effects: 1. The halogen atom transfer (XAT) reaction is a method for generating carbon radicals widely used in photochemistry and photocatalysis. Through photoexcitation or photocatalysis, a halogen atom (such as Cl, Br, I) is transferred from an organic halide (R-X) to a radical initiator or catalyst, while generating a carbon-centered radical (R·). The present invention uses C1-C5 halogenated hydrocarbons to introduce carbon radicals into the pores of porous carbon through the halogen atom transfer (XAT) reaction. The molecular size of the C1-C5 halogenated hydrocarbons selected in the present invention matches the kinetic diameter of the ultramicropores. The carbon radicals formed after impregnation and photocatalytic reaction can effectively adhere to the pore walls of the ultramicropores; then, amorphous carbon is formed by heat treatment and filled in the ultramicropores, thus solving the problems of low effective deposition rate of silicon caused by the inability to utilize ultramicropores and irreversible lithium insertion, and further improving the electrochemical performance of the silicon-carbon negative electrode, especially improving the first Coulomb efficiency and cycling performance significantly.

[0019] 2. Compared with the traditional chemical activation or physical activation methods, the present invention provides a new technology for precisely regulating the number and pore volume of ultramicropores. This technology can generate carbon radicals under mild conditions and fill the ultramicropores by heat treatment, avoiding the problems of non-uniformity and difficulty in control in the traditional methods. Description of the Drawings

[0020] Figure 1 is a schematic diagram of the principle of the present invention; Figure 2 are the pore size distribution diagrams of the porous carbon in the implementation and the pore size distribution diagram of the modified porous carbon prepared in Example 1; Figure 3 is the first charge-discharge curve of the lithium battery in Application Example 1. Detailed Embodiments

[0021] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be described in detail below. The following examples facilitate a better understanding of the present invention, but do not limit the present invention. In the following examples, unless otherwise specified, the parts are by mass.

[0022] The porous carbon is selected from Shandong Shengquan New Energy Technology Co., Ltd., with a D50 particle size of 2.0 μm, a specific surface area of 2056 m² / g, a total pore volume of 0.87 cm³ / g, a proportion of ultramicropores of 28.2%, and an average pore diameter of 1.7 nm.

[0023] Example 1 (S1) Dissolve 0.5 g of the photosensitizer Ru(bpy)3Cl2 in 1000 mL of an ethanol solution (concentration 0.1 mol / L) of bromoethane (C2H5Br). Then, put 50 g of the dried (dried in a vacuum drying oven at 150 °C for 6 h) porous carbon into the aforementioned ethanol solution of bromoethane to form a mixture. The mixture is placed in a constant temperature oscillator (25 °C, 120 rpm) and impregnated for 24 hours; after the impregnation is completed, it is filtered, washed three times with ethanol, and vacuum dried at 70 °C for 6 hours to obtain porous carbon loaded with bromoethane; (S2) Spread the porous carbon loaded with bromoethane on a quartz reactor, introduce nitrogen at a flow rate of 50 mL / min, and use a UV LED light source with a wavelength of 365 nm (light intensity 100 mW / cm²) to vertically irradiate the material surface at 30 °C for 2 hours for photocatalytic reaction (the reaction process is as follows); the reacted porous carbon is successively soaked in acetone for 1 h, washed with pure water until neutral, and vacuum dried at 80 °C for 6 hours to obtain a modified porous carbon precursor; (Carbon radicals are adsorbed on the pore walls) (S3) Transfer the modified porous carbon precursor to a tube furnace, heat it to 550 °C at a rate of 5 °C / min in a nitrogen atmosphere (flow rate 200 mL / min), hold for 3 hours, and cool to room temperature to obtain modified porous carbon based on photocatalytic halogen atom transfer.

[0024] Example 2 The rest is the same as in Example 1, except that in step (S1), chloropropane (C3H7Cl) is used instead of bromoethane.

[0025] Example 3 The rest is the same as in Example 1, except that in step (S1), iodopropane (C3H7I) is used instead of bromoethane.

[0026] Example 4 The rest is the same as in Example 1, except that in step (S1), 1-chloropentane (C5H 11 Cl) is used instead of bromoethane, and the concentration of the 1-chloropentane solution is 0.3 mol / L.

[0027] Example 5 The rest is the same as in Example 1, except that in step (S2): a visible light LED light source with a wavelength of 450 nm is used to replace the ultraviolet LED light source with a wavelength of 365 nm, and the irradiation time is 4 hours.

[0028] Comparative Example 1 The rest is the same as in Example 1, except that step (S2) is omitted, and a porous carbon loaded with halogenated hydrocarbons is used to replace the modified porous carbon precursor in step (S3).

[0029] Comparative Example 2 The rest is the same as in Example 1, except that n-decyl bromide (C 10 H 21 Br) is used to replace bromoethane.

[0030] Application Example 1 30 g of the modified porous carbon prepared in Example 1 was placed in a tubular rotary furnace with a rotary furnace speed of 50 r / min. Nitrogen was introduced at a flow rate of 5 L / min. After heating to 460 °C at a heating rate of 5 °C / min in a nitrogen environment, the nitrogen flow rate was maintained and silane (SiH4) gas was introduced at a flow rate of 0.2 L / min for chemical vapor deposition for 1 h. The silane cracked to form nano-silicon deposited in the pores of the porous carbon. After the deposition was completed, the silane gas was stopped, and helium gas was continuously introduced at a flow rate of 5 L / min to remove the excess silane gas. Then, acetylene gas was introduced at a flow rate of 0.2 L / min and maintained at 500 °C for gas phase deposition for 1 h. After the acetylene gas cracked, a carbon coating layer was formed to obtain a silicon-carbon negative electrode material.

[0031] Application Examples 2-5 Other conditions were the same as in Application Example 1, except that the modified porous carbon was prepared in Examples 2-5.

[0032] Comparative Application Examples 1-2 Other conditions were the same as in Application Example 1, except that the modified porous carbon was prepared in Comparative Examples 1-2.

[0033] Testing and Analysis 1) Performance testing of the modified porous carbon Particle size distribution and specific surface area testing: According to GB / T 19587—2017 Gas Adsorption BET Method, a Tristar II 3020 type fully automatic specific surface and pore size analyzer produced by Micromeritics Instrument Corporation of the United States was used to conduct a low-temperature nitrogen adsorption experiment on the modified porous carbon prepared in the examples and comparative examples, and its specific surface area, pore size distribution, and pore volume were measured. The test results are shown in Table 1. The pore size distribution diagrams of the porous carbon in the examples and the pore size distribution diagram of the modified porous carbon prepared in Example 1 are as Figure 2 shown.

[0034] Table 1 Pore size distribution, specific surface area and pore volume of modified porous carbon 。

[0035] It can be seen from Table 1 that the proportion of ultramicropores in the modified porous carbon prepared by the preparation method of the present invention is significantly reduced, and the specific surface area and pore volume are correspondingly slightly reduced; at the same time, combined with Figure 2 the pore size distribution diagram in, the significant reduction of ultramicropores results in a corresponding increase in the proportion of micropores. The reduction of pore volume and specific surface area is mainly due to the significant reduction of the proportion of ultramicropores, verifying the effect of carbon free radicals filling ultramicropores by carbonization. In Comparative Example 1, the photocatalytic halogen transfer reaction was not carried out, and the reduction of the proportion of ultramicropores in the prepared modified porous carbon was limited. In Comparative Example 2, a halogenated hydrocarbon with a larger molecular ruler was used, and the reduction of the proportion of ultramicropores in the prepared modified porous carbon was also limited. The possible reason is that the halogenated hydrocarbon with a larger molecular ruler does not match the kinetic diameter of ultramicropores and cannot effectively penetrate into ultramicropores. Therefore, the carbon free radicals formed after the photocatalytic reaction cannot effectively adhere to the pore wall.

[0036] 2) Performance test of silicon-carbon anode materials prepared in application examples and comparative application examples The silicon-carbon anode materials prepared in the above application examples and comparative application examples were used as the anode in lithium-ion batteries, assembled into lithium batteries and their electrochemical performance was tested. Specifically: the silicon-carbon composite anode material was mixed with polyacrylic acid (PAA) and conductive carbon black (Super P) at a mass ratio of 8:1:1, ground into a slurry with deionized water as the solvent, and then evenly coated on a copper foil, controlling the wet film thickness to be 200 μm, and then vacuum dried at 80 °C for 12 hours. The dried material was punched into a 12 mm electrode sheet. In an argon glove box, a lithium metal sheet was used as the counter electrode, Celgard 2600 was used as the separator, and the electrolyte was a 1 M LiPF6 solution, in which the solvent was prepared according to the volume ratio of ethylene carbonate (EC): diethyl carbonate (DEC) = 1:1, and 25% fluoroethylene carbonate (FEC) was added as an additive. Finally, a CR2032 coin cell was assembled. The charge-discharge test was carried out using a BlueTEC CT2001A test system in the voltage range of 0.01~1.5 V and the current density of 100 mA / g. The test results are shown in Table 2, and the first charge-discharge curve of the lithium battery in Application Example 1 is as Figure 3 shown.

[0037] Table 2 Performance test 。

[0038] As can be seen from Table 2, when the silicon-carbon negative electrode material prepared by silicon deposition and carbon coating of the modified porous carbon based on photocatalytic halogen atom transfer prepared in the embodiment of the present invention is used as the negative electrode of a lithium battery, the initial Coulomb efficiency is significantly improved, and it can reach more than 86% even at a low voltage (0.8V), and the capacity retention rate after 100 cycles is also as high as more than 95%.

[0039] The above detailed description is a specific description of one of the feasible embodiments of the present invention. This embodiment is not intended to limit the patent scope of the present invention. Any equivalent implementation or change without departing from the present invention should be included within the scope of the technical solution of the present invention.

Claims

1. A preparation method of modified porous carbon based on photocatalytic halogen atom transfer, characterized in that, It includes the following steps: (S1) Immerse porous carbon into a C1-C5 halogenated hydrocarbon solution containing a photosensitizer. After the immersion ends, filter, wash, and dry to obtain porous carbon loaded with halogenated hydrocarbon; (S2) Carry out a photocatalytic reaction on the porous carbon loaded with halogenated hydrocarbon under an inert atmosphere. After washing and drying, obtain a modified porous carbon precursor; (S3) Carry out heat treatment on the modified porous carbon precursor under an inert atmosphere to obtain modified porous carbon based on photocatalytic halogen atom transfer.

2. The preparation method according to claim 1, wherein In step (S1), the particle size D50 of the porous carbon is 1-10 μm, the specific surface area is 1000-3000 m² / g, the total pore volume is 0.5-2.0 cm³ / g, and the proportion of ultra-micropores is 15%-30%.

3. The preparation method according to claim 1, characterized in that, In step (S1), the photosensitizer is at least one of tris(2-phenylpyridine)iridium, ruthenium(III) tris(bipyridine) chloride, and 2,4,5,6-tetrakis(9-carbazolyl) isophthalonitrile, and its dosage is 0.5-1.5 wt% of the porous carbon.

4. The preparation method according to claim 1, characterized in that, In step (S1), the C1-C5 halogenated hydrocarbon is at least one of dichloromethane, chloroform, chloroethane, dichloroethane, chloropropane, chloroisopropane, chlorobutane, chloroisobutane, chloropentane, bromoethane, bromopropane, bromoisopropane, bromobutane, iodoethane, and iodopropane.

5. The preparation method according to claim 4, characterized in that, In step (S1), the C1-C5 halogenated hydrocarbon is at least one of dichloromethane, chloroethane, chloropropane, and bromoethane.

6. The preparation method according to claim 1, characterized in that, In step (S1), the concentration of the C1-C5 halogenated hydrocarbon solution is 0.05-0.5 mol / L, and the solvent is at least one of ethanol, acetone, and tetrahydrofuran; the dosage of the porous carbon and the C1-C5 halogenated hydrocarbon solution is 1 g:(10-20) mL; the conditions for the immersion are: immerse at 20-30 °C for 12-48 h; and / or The washing is carried out by washing with ethanol 3-5 times; the drying is carried out by vacuum drying at 60-80 °C for 6-12 h.

7. The preparation method according to claim 1, wherein In step (S2), the conditions for the photocatalytic reaction are: irradiate with a light source with a wavelength of 200-800 nm and a light intensity of 50-150 mW / cm² at 20-50 °C for 1-5 h.

8. The preparation method according to claim 1, characterized in that, In step (S2), the inert atmosphere is nitrogen and / or argon; the washing is carried out by soaking in acetone or ethanol for 1-3 h and then washing with water until neutral; the drying is carried out by vacuum drying at 80-120 °C for 4-12 h.

9. The preparation method according to claim 1, characterized in that, In step (S3), the conditions for the heat treatment are: keep the temperature at 400-600 °C for 1-5 h; the inert atmosphere is nitrogen and / or argon.

10. A silicon-carbon negative electrode, characterized in that, It is prepared by gas-phase deposition of silicon and carbon coating on the modified porous carbon prepared by the preparation method according to any one of claims 1-9.

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