Method for improving mesoporous of porous carbon for lignin-based lithium battery silicon-carbon

Abstract

The application discloses a method for improving mesopores of porous carbon for lignin-based lithium battery silicon-carbon, and belongs to the technical field of lithium ion battery materials. The method comprises the following steps: adding 0.5-5 wt% of nano carbon black (particle size 10-30 nm) into lignin with water content less than or equal to 5%, uniformly and efficiently dispersing the lignin, mixing the lignin with a potassium carbonate activator and a urea auxiliary activator according to a mass ratio of 1:(1.2-2):(0.6-1), hot plasticizing and granulating at 130 DEG C, carbonizing and activating by programmed temperature rising to 850-880 DEG C, and then separating alkali and carbon, high-temperature alkali steaming (pH greater than or equal to 12, 120-130 DEG C), acid boiling (2M HCl, 80-90 DEG C), water washing and fine grinding (D 50 =5-9 mu m), and finally obtaining porous carbon with a mesopore (2-5 nm) ratio greater than 30%. The method has the advantages of simple operation, low raw material cost, good ecological, environmental protection, economic and social benefits, and wide application prospect.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion battery materials, and particularly relates to a method for improving mesopores in a porous carbon for a lignin-based lithium-silicon-carbon. BACKGROUND

[0002] Currently, lithium ion batteries widely use graphite negative electrodes, and the theoretical capacity thereof is 372 mAh / g. The energy density of the lithium ion battery has approached the limit of the graphite negative electrode, and there is little room for improvement. On the other hand, the theoretical capacity of a silicon-carbon negative electrode is 4200 mAh / g, which is more than 10 times that of the graphite negative electrode. Therefore, to further improve the energy density of the lithium ion battery, the silicon-carbon negative electrode needs to be used, and the current is in a window period from the graphite negative electrode to the silicon-carbon negative electrode. The preparation method of the silicon-carbon negative electrode is relatively complex. Currently, the CVD technology line of Group-14 in the United States is generally used, which needs to first prepare a porous carbon and silane, then deposit the silane into the porous carbon by using a CVD method, and then coat a layer of carbon outside the silicon particles. The CVD method has extremely strict requirements for the porous carbon. In addition to the requirement for a certain specific surface area and total pore volume, the number and proportion of micropores (0.5-2 nm) and mesopores (2-5 nm) also have strict requirements. Since nanosilicon needs to be deposited in the porous carbon, the number and proportion of mesopores are particularly important.

[0003] Unfortunately, the porous carbon manufactured by using various carbon source precursors (coconut shell carbon, phenolic resin, petroleum coke, and biomass) and different activation methods (physical activation and chemical activation) is microporous carbon, the proportion of micropores is more than 90%, and the proportion of mesopores is generally less than 20%, which cannot meet the process requirements of the CVD silicon deposition. At the same time, there is a serious bottleneck at the raw material end: 1) resource shortage: the global annual resource of coconut shell carbon is less than 1 million tons, and the dependence on import is high, which cannot meet the demand of rapid development. 2) high cost: the price of coconut shell carbonized material has broken through 7000 yuan / ton; the price of phenolic resin is as high as 12000-15000 yuan / ton.

[0004] The applicant has previously filed and authorized ZL201911088470.7: a method for preparing microcrystalline graphene capacitor carbon by using lignin as a raw material, which focuses on the preparation of capacitor carbon. The microcrystalline graphene capacitor carbon prepared by using lignin as a raw material has excellent mechanical and electrical properties, and has a very broad application prospect. However, it is mainly microporous, and the proportion of mesopores is only about 11%, which is difficult to adapt to the demand of depositing silicon in the porous carbon of the lithium-silicon-carbon negative electrode.

[0005] Therefore, developing a lignin-based porous carbon that solves the raw material bottleneck of new energy carbon materials and improves the number and proportion of mesopores is a problem that needs to be solved at present, and the present application arises at the historic moment. SUMMARY

[0006] In view of the above, it is necessary to provide a method for improving mesopores in lignin-based porous carbon for lithium battery silicon-carbon, which realizes generational leap in mesopore ratio, conductivity, cost and environmental protection through the dual-core innovation of carbon black stress pore forming and green conversion of lignin, and removes the obstacles of key materials and technologies for industrialization of lithium battery silicon-carbon negative electrode.

[0007] To achieve the above purpose, the technical scheme adopted by the present application is:

[0008] A method for improving mesopores in lignin-based porous carbon for lithium battery silicon-carbon, the method comprising adding nano-carbon black with a particle size of 10-30 nm in lignin raw material, and sequentially mixing lignin and carbon black, adding activator and activator aid, hot plasticizing, carbonizing and activating, and post-treatment to obtain a porous carbon material with a mesopore ratio of >30%.

[0009] In the present application, further, the addition ratio of the nano-carbon black is 0.5-5 wt%.

[0010] In the present application, further, the activator is potassium carbonate as the activator, and urea is added as the activator aid, wherein the mass ratio of the lignin, potassium carbonate and urea is 1:1.2-2:0.6-1.

[0011] In the present application, further, the nano-carbon black is conductive carbon black, which acts as an in-situ template to induce mesopore formation in the carbonization process.

[0012] In the present application, further, the specific steps include: (1) mixing and dispersing the nano-carbon black and lignin powder with a water content of ≤5% by a high-efficiency homogenizer; (2) adding the activator potassium carbonate and the activator aid urea, grinding and mixing uniformly; (3) hot plasticizing the obtained mixed raw material at a lignin glass transition temperature of 130℃; (4) in an inert atmosphere electric furnace, programming the temperature to 850-880℃ for carbonization and activation for 1-2 hours; (5) after alkali-carbon separation, sequentially performing high-temperature alkali steaming, acid boiling and water washing to neutral; (6) after drying, grinding to D 50 =7-9μm to obtain the product.

[0013] In the present application, further, the temperature programming of the step (4) is: 5℃ / min to 250℃→4℃ / min to 650℃→2℃ / min to 850℃→3℃ / min to 880℃→holding for 60 minutes.

[0014] In the present application, further, in the step (5), the alkali-carbon separation adopts colloidal mill beating to D 50 =15-35μm, filtering, and separating carbon mud and alkali liquor.

[0015] In the present application, further, the high-temperature alkali steaming condition of step (5) is: 120-130℃, pH≥12, for 4-6 hours; the acid boiling is: adding 2M HCl, 80-90℃, for 4-6 hours.

[0016] The present application uses nano carbon black to improve the quantity and proportion of mesopores in lignin-based porous carbon, which is essentially to use the good affinity between carbon black and lignin powder, and to highly disperse and uniformly mix a small amount of carbon black powder with lignin powder. Lignin has the properties of thermosetting resin, and when the temperature reaches the glass point of lignin, lignin melts and softens, shrinks in volume, and then solidifies, wrapping nano carbon black particles therein. During high-temperature carbonization, lignin decomposes and carbonizes, shrinks in volume, and generates stress, causing cracks in the lignin-derived carbon around the carbon black particles, forming a large number of mesopores.

[0017] The quantity and pore size of the mesopores initiated by carbon black are related to the particle size and type of carbon black particles, and various types of carbon black can be used, such as conductive carbon black, reinforcing carbon black, printing carbon black, and dye carbon black, with particle sizes of D 50 =15-35nm, and conductive carbon black should be used preferentially. Obviously, this technical route is only suitable for lignin carbon source precursor porous carbon and cannot be used for the technical route of various carbonized materials to make porous carbon.

[0018] The dispersity of nano carbon black particles in lignin powder is very important, and various methods should be used to disperse and add nano carbon black particles into lignin to avoid concentrated addition, and high-efficiency homogenizing machines should be used for mixing and stirring to achieve the best homogenization.

[0019] The present application has the following beneficial effects.

[0020] (1) Innovative "carbon black stress pore forming" mechanism: traditional mesopore improvement relies on template method (such as SiO2), which requires complex post-treatment to remove residual materials. The present application first creates an in-situ pore forming principle based on nano carbon black: a. Melting and wrapping: lignin melts and softens at the glass transition temperature (130℃), uniformly mixes with highly compatible carbon black, and then solidifies to form a "carbon black-lignin" composite; b. Stress cracking: lignin shrinks dramatically (volume reduction >40%) during carbonization, while the volume of carbon black remains unchanged, generating directional tensile stress around it, inducing micro-cracks to form uniform mesopores (2-5 nm). Revolutionary breakthrough: carbon black has dual functions of template and conductive agent, completely avoiding the template removal step, solving the "pollution" and "inefficiency" problems.

[0021] (2) Raw materials and process innovation: The present invention replaces imported coconut shell charcoal (global annual resources are only 1 million tons) or fossil-based phenolic resin with papermaking / biomass waste (annual supply > 10 million tons), reducing the raw material cost by > 50%. Lignin is used as the carbon source raw material. Lignin is a green, renewable and sustainable biomass resource. It is a waste of the biomass industry (papermaking and pulping industry and bagasse / straw hydrolysis sugar industry). In my country, the amount of lignin that can be extracted from papermaking black liquor alone is more than 10 million tons per year. The use of lignin raw materials solves the raw material bottleneck of new energy carbon materials. In addition, the present invention is compatible with existing production lines and only adds carbon black in the first step of the traditional capacitor carbon process (such as ZL201911088470.7); the high-efficiency homogenizer ensures the dispersion of nano carbon black and avoids local agglomeration ( Figure 3 SEM images in the middle verify uniform mesopores).

[0022] (3) Conductivity-pore formation synergy: Conductive carbon black can be regarded as a template for generating mesopores. When using other pore-forming templates, the template residue must be cleaned after the carbonization-activation reaction is completed. Conductive carbon black can not only effectively increase the number and proportion of mesopores, but also does not need to be removed after the reaction is completed (traditional templates require acid / alkali etching, which produces highly polluted wastewater). It can also increase the conductivity of porous carbon, achieving multiple goals at one stroke. In addition, the amount of conductive carbon black used is very small, only 0.5-5 wt%, and the cost per ton of porous carbon is only increased by less than 500 yuan, but the product performance premium is significant (the market price of mesoporous carbon is greater than 150,000 yuan / ton, which is more than 30,000 yuan / ton higher than traditional microporous carbon); combined with the reduction of lignin raw material cost by more than 50% (vs coconut shell carbon / phenolic resin), a "high performance-low cost" closed loop is achieved.

[0023] In summary, the present invention solves the problem of mesopore regulation through the "stress pore formation" mechanism, relies on the resource utilization of lignin, breaks the shackles of raw materials for porous carbon, extends from material mechanism innovation to industrial implementation adaptation, and achieves a generational leap in mesoporosity, conductivity, cost and environmental protection, removing key material and technical barriers for the industrialization of lithium battery silicon-carbon negative electrodes. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The technical roadmap for lignin-based porous carbon on silicon-carbon substrate for lithium battery anode.

[0025] Figure 2 This is a photo of hot-molded pellets of mixed raw materials (lignin, carbon black, potassium carbonate, and urea).

[0026] Figure 3 This is a scanning electron microscope (SEM) photograph of the porous carbon fired in Example 1 with 1% nano-carbon black added.

[0027] Figure 4 This is the BET pore size distribution diagram of the lignin-based porous carbon fired by adding 1% nano-carbon black in Example 1.

[0028] Figure 5 BET pore size distribution of lignin-based porous carbon without nano-carbon black calcination for Comparative Example 1. DETAILED DESCRIPTION

[0029] In order to make the above objectives, features and advantages of the present application more apparent, specific embodiments of the present application will be described in detail below with reference to the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present application. However, it will be apparent to one skilled in the art that the present application can be practiced without the specific details given herein, and that the present application can be practiced with other systems, methods, components, materials, etc. in addition to or other than those described herein. Therefore, the scope of the present application is not intended to be limited to the following specific embodiments disclosed below.

[0030] EMBODIMENTS

[0031] 1. Materials and equipment

[0032] The raw materials are as follows.

[0033] Lignin powder (moisture content ≤5%, source: "Zhejiang Jiefa Technology Co., Ltd." separated and purified from papermaking black liquor).

[0034] Nano-conductive carbon black (purchased on the market, particle size 10-30 nm, Super P conductive carbon black is selected).

[0035] Activator: potassium carbonate (industrial grade, purity ≥99%).

[0036] Co-activator: urea (agricultural grade, nitrogen content ≥46%).

[0037] The equipment includes: high-efficiency homogenizer (rotational speed ≥3000 rpm), thermoplastic kneader, program-controlled high-temperature electric furnace (inert atmosphere), colloid mill, jet mill (D 50 Control range 1-50 μm).

[0038] 2. Implementation steps (technical route map as Figure 1 ).

[0039] Step 1: Nano-carbon black homogenization and dispersion: add nano-carbon black (1 wt% by mass ratio) to the dried lignin powder, mix for 30 min at 3000 rpm by high-efficiency homogenizer, so that the carbon black is embedded in the lignin network in the form of single particles, achieving high dispersion and close combination.

[0040] Step 2: Complex addition of activator: Add activator K2CO3 and co-activator urea into the carbon black-containing lignin in a proportion of 1:1.5:0.75 by mass, grind and mix uniformly until the mixture is a uniform off-white powder (urea breaks the hydrogen bond network of lignin, promoting the penetration of K2CO3 into the structure of lignin).

[0041] Step 3: Thermoplastic granulation: Put the mixed powder into a thermoplastic kneader, melt at the glass transition temperature of lignin (130°C), solidify and granulate, and the product obtained after granulation is as shown in Figure 2

[0042] Step 4: Carbonization-activation: Load the granular raw material into a silicon carbide crucible, introduce nitrogen gas (flow rate 1 L / min), and heat according to the following program (initial temperature 50°C).

[0043] 5°C / min → 250°C (remove volatile matter).

[0044] 4°C / min → 650°C (primary carbonization).

[0045] 2°C / min → 850°C (activation start).

[0046] 3°C / min → 880°C (peak temperature).

[0047] Hold for 60 min (key stage for mesopore formation).

[0048] Cool to room temperature.

[0049] Step 5: Post-treatment: alkali-carbon separation: Soak the alkali-carbon block in 4 times the amount of clean water, pulp to D50=25μm with a colloid mill, filter to separate carbon mud and alkali solution, and the alkali solution is used for potassium carbonate recovery and reuse.

[0050] Alkali steam washing: The carbon mud still contains a large amount of potassium carbonate, load the carbon mud into an Erlenmeyer flask, add water to adjust the pH to ≥12, cover with gauze, cook at 130°C for 4 h, and wash with water until the pH is 8.5 (remove alkali-soluble impurities and residual K2CO3).

[0051] Acid cooking purification: Load the carbon mud after alkali washing into an Erlenmeyer flask, add 2M HCl (solid-liquid ratio 1:2.5), stir at 90°C for 5 h, and wash with deionized water several times until the pH is 7 (remove acid-soluble impurities).

[0052] Step 6: Fine grinding treatment: Dry the carbon mud at 120°C for 8 h, grind to D 50 =8μm with an air flow grinder to obtain the final product.

[0053] The scanning electron microscope (SEM) photo of the porous carbon fired with 1% nano carbon black in the above example is as shown in Figure 3 ​As shown, the figure directly proves the uniform dispersibility of nano carbon black in the lignin matrix and the stress pore-forming effect (micro-cracks → mesopores).

[0054] Comparative example:

[0055] The comparative example is that no conductive carbon black is added in the lignin raw material, and the rest of the operations are exactly the same as those in Example 1.

[0056] Product testing:

[0057] The lignin-based porous carbon fired in the example and the comparative example is sent to the “Tianhe Scientific Analysis and Test Center” for testing the specific surface area, total pore volume, average pore size and pore size distribution of the porous carbon by the gas adsorption BET method. The pore size distribution of the lignin-based porous carbon with carbon black added in the example is shown in Figure 4 The pore size distribution of the porous carbon without carbon black added in the comparative example is shown in Figure 5 The BET report of Tianhe Company uses 1.7 nm as the dividing point between micropores and mesopores, instead of the commonly used dividing point of 2 nm. From Figure 4 and Figure 5 It can be seen that the addition of 1% conductive carbon black (20 nm particle size) in the example of the present application significantly improves the number and proportion of mesopores, from 11.6% of the comparative example without carbon black to 32.15% after the addition of carbon black, and the effect is very obvious. The average pore size is also increased from 1.833 nm to 2.136 nm.

[0058] The above-described examples only express several embodiments of the present application, and the description is more specific and detailed, but it cannot be understood as a limitation on the scope of the present application.

Claims

1. A method for increasing the mesopores in porous carbon for lignin-based lithium battery silicon carbon, characterized in that: The method includes adding nano carbon black with a particle size of 10-30 nm to the lignin raw material, and then uniformly mixing the lignin and carbon black, adding an activator and a co-activator, uniformly mixing, thermally granulating, carbonizing and activating, and post-treating to obtain a porous carbon material with a mesopore ratio of more than 30%.

2. The method according to claim 1, characterized in that The addition ratio of the nano carbon black to the lignin is 0.5-5 wt%.

3. The method according to claim 1, characterized in that The activator is potassium carbonate as an activator and urea as a co-activator, wherein the mass ratio of the lignin, potassium carbonate and urea is 1:(1.2-2):(0.6-1).

4. The method according to claim 1, wherein The nano carbon black is conductive carbon black, which acts as an in-situ template to induce mesopore formation during the carbonization process.

5. The method according to any one of claims 1 to 4, characterized in that The specific steps of the method include: (1) Mix and disperse nano carbon black and lignin powder with a moisture content of ≤5% through a high-efficiency homogenizer; (2) Add activator potassium carbonate and co-activator urea, grind and mix evenly; (3) The mixed raw materials are thermally pelletized at the lignin glass transition temperature of 130°C; (4) In an inert atmosphere electric furnace, program the temperature to 850-880°C for carbonization activation for 1-2 hours; (5) After alkali-carbon separation, high-temperature alkali steaming, acid boiling and water washing are carried out in sequence until neutral; (6) After drying, grind to D 50 =7-9μm, and the product was obtained.

6. The method according to claim 5, characterized in that The temperature rise program in step (4) is as follows: 5°C / min to 250°C → 4°C / min to 650°C → 2°C / min to 850°C → 3°C / min to 880°C → keep warm for 60 minutes.

7. The method according to claim 5, characterized in that In the step (5), the alkali carbon separation is performed by colloid grinding to D 50 =15-35μm, filter to separate carbon mud and alkali solution.

8. The method according to claim 5, wherein The high-temperature alkali steaming conditions of step (5) are: treating at a temperature of 120-130°C and a pH of ≥12 for 4-6 hours; the acid boiling conditions are: adding 2M HCl and treating at 80-90°C for 4-6 hours.

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