Preparation method of hard carbon

Abstract

The invention belongs to the technical field of sodium battery material preparation, and particularly relates to a preparation method of hard carbon. Bamboo is used as a biomass raw material, powder is obtained through grinding, and it is ensured that all internal structures can fully react. A two-step calcination process is used, metal organic complexes in plants can be converted into metal nitrides or metal carbides after primary calcination treatment, the metal nitrides or metal carbides can react with acid more easily to form a metal ion solution, and the metal ion solution is cleaned up. Compared with the prior art, the method has the advantages that the bamboos are directly treated by the strong base, the enzyme preparation or the pore-forming agent, the efficiency is higher, the cost is lower, and the content of metal impurities in the obtained product is lower. The hard carbon prepared by the method is relatively large in interlayer spacing of 0.35-0.4 nm, is extremely low in pure impurity content, is suitable for a sodium-ion battery negative electrode, and has relatively good electrochemical performance. And the production process is small in pollution, low in equipment loss and suitable for large-scale production of the sodium-ion battery negative electrode material.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sodium battery material preparation, and particularly relates to a preparation method of hard carbon. BACKGROUND

[0002] Sodium battery is the main development direction of future energy storage batteries, and the negative electrode material of sodium ion battery is preferred due to the lower cost of hard carbon. At present, sodium ion compounds are used as positive electrode materials of batteries, and in the process of charging and discharging, compared with the charging and discharging of traditional lithium ion batteries (the atomic radius of lithium ion is 0.076 nanometers, and the radius of sodium ion is 0.102 nanometers), the negative electrode material of sodium ion battery repeatedly adsorbs and desorbs in the process of charging and discharging, and the structure changes more than that of lithium ion battery, and the structure is easy to collapse and unstable. Therefore, the preparation of suitable hard carbon negative electrode material to solve the predicament of sodium ion battery negative electrode material is the current research direction.

[0003] Hard carbon is difficult to be graphitized, and is a thermal decomposition product of a high molecular polymer, and there are various methods for preparing hard carbon material, such as the preparation method of hard carbon material disclosed in patent No. 202011078447.2, which comprises the following steps: firstly, mixing coal-based material with hard carbon precursor and then pressing to obtain a hard sheet; the hard carbon precursor is a carbohydrate and / or gelatin; then, high-temperature carbonization is performed on the obtained hard sheet to obtain hard carbon material. The hard carbon material is prepared by the method of pressing the sheet to make the hard carbon precursor and the coal-based material closely contact, so as to improve the reactivity of the two and increase the carbonization yield, while reducing the specific surface area, increasing the disorder of the carbon layer and the distance between the carbon layers in the carbonization process; however, the hard carbon prepared by the method has low hard carbonization degree, and as a sodium battery negative electrode material, the structure is unstable and easy to collapse.

[0004] Patent No. 2022108082771 discloses a preparation method of biomass sodium ion battery negative electrode, and alcohol is used in the preparation process, which can dissolve part of the hard carbon raw material, resulting in a decrease in yield, and tar is generated in the formation process of the prepared hard carbon, and the purity of the product is not high. Patent No. 2022101204087 discloses a preparation method of hard carbon negative electrode material for sodium ion battery, and the hard carbon negative electrode material provided by the invention can significantly improve the first charge-discharge efficiency and cycle life; however, the product cycle of the invention is long, and the cost is high.

[0005] In addition, the preparation process of hard carbon also faces challenges such as inability to scale production and easy graphitization of raw materials.

[0006] Therefore, it is necessary to further study the preparation process of hard carbon negative electrode material to solve the deficiencies of the prior art, reduce the cost, and optimize the performance. SUMMARY

[0007] The technical problem to be solved by the present application is to provide a preparation method of hard carbon, which uses bamboo as a biomass raw material, efficiently removes metal ion impurities through a two-step calcination method, and prepares hard carbon for battery negative electrodes.

[0008] The technical scheme adopted is:

[0009] A preparation method of hard carbon, comprising the following steps:

[0010] (1) Selecting bamboo stems of 2-6 years of age, grinding them mechanically to break the cell walls, and obtaining bamboo powder;

[0011] (2) Pre-treating the powder by drying it and performing a first calcination under an inert gas atmosphere, to obtain a carbonization product after pre-treatment;

[0012] (3) Washing the carbonization product after acid treatment to effectively remove metal elements in the biomass, to obtain a carbonization precursor;

[0013] (4) Drying the carbonization precursor and performing a second calcination under an inert gas atmosphere, to obtain the hard carbon.

[0014] Preferably, the bamboo stems are selected from any one of Phyllostachys edulis, Dendrocalamus giganteus, Phyllostachys pubescens, and Phyllostachys edulis. If the cost is to be further reduced, recycled waste primary bamboo products can be used, including but not limited to recycled waste chopsticks, bamboo sticks, landscape decorations, handicrafts, and the like.

[0015] The bamboo is stripped of the skin and leaves, mechanically ground to break the cell walls, and sieved; the sieving range is 200-3000 mesh. Depending on the equipment conditions, 200-1000 mesh sieving can be used, and the final hard carbon product needs to be ball-milled to prepare electrode slurry and improve dispersibility; or 1000 mesh or more sieving can be used, and ball-milling is not needed, and electrode slurry can be directly prepared. Grinding the bamboo into powder is a necessary step, because the bamboo wall is thick, and the bamboo powder can ensure that the internal structure is fully exposed and fully participates in the reaction.

[0016] Preferably, during the first calcination, the heating rate is 1-10℃ / min under an inert gas atmosphere, and the temperature is raised to 400-500℃.

[0017] As a further preferred, the temperature is kept for 3-8 hours after the temperature is raised. The reaction needs to be carried out in inert gas to avoid exposure to air to reduce the reaction of oxygen and carbon in the bamboo powder to produce CO, CO2, etc., thereby avoiding reducing the product yield. At the same time, the reaction in inert gas can avoid the reaction of metal complexes in the bamboo biomass with oxygen to produce inert oxides such as MgO, Cr2O3, Fe3O4, etc., which are difficult to clean up by pickling. 8 hours can ensure that all metal organic complexes in the raw material are converted into metal carbide and metal nitride, which are easier to react with acid and then cleaned up. If the production cost and efficiency are considered, the holding time of 5 hours is preferred.

[0018] Preferably, the acid used is any one of hydrochloric acid, acetic acid, sulfuric acid and nitric acid with pH < 6.0. The reaction of the carbonization product after pretreatment with acid can convert the metal carbide and nitride into a metal ion solution. Considering volatility and safety, hydrochloric acid and nitric acid are preferred.

[0019] As a further preferred, the carbon precursor after acid treatment is washed with water for 2-5 times until the pH of the waste liquid is neutral. The metal ions remaining in the sample can be washed away.

[0020] Preferably, the temperature is raised to 1000-1500℃ at a rate of 1-10℃ / min in the secondary calcination. The reaction needs to be carried out in inert gas to avoid exposure to air to reduce the reaction of oxygen and carbon in the bamboo powder to produce CO, CO2, etc., thereby avoiding reducing the product yield.

[0021] Preferably, the temperature is kept for 1-5 hours after the temperature is raised in the secondary calcination.

[0022] Preferably, the inert atmosphere in the primary calcination and the secondary calcination is nitrogen, argon or any proportion of the mixture of the two.

[0023] Preferably, the interlayer spacing of the hard carbon is 0.35-0.4 nm, the battery capacity is 320 mAh g -1 , and the initial charging efficiency is greater than 80%.

[0024] The application uses bamboo as raw material, has the advantages of wide source, sufficient yield, low cost and less damage to equipment, and is suitable for large-scale production. According to the chemical structure and characteristics of bamboo, two calcination methods are created to efficiently remove metal ions in bamboo. Bamboo, as a common plant, contains metal organic complexes such as sodium, magnesium, potassium, iron and chromium in its stem. Direct preparation of hard carbon has too many metal impurities and poor electrochemical performance. Therefore, before processing, traditional methods need to use strong alkali, enzyme preparation, pore forming agent and other methods to treat bamboo to remove metal organic complex impurities. However, these methods have long processing time, low efficiency and high cost. Compared with the existing process of pretreatment and cleaning in air, the application optimizes the reaction of inert gas to avoid the generation of inert metal oxides which are difficult to clean, and to avoid the reaction of carbon in biomass with oxygen to reduce product yield. The application uses a two-step calcination method to convert cellulose and other organic matter in bamboo into carbon in an inert atmosphere, and sodium, magnesium, potassium, iron and chromium metal complexes are converted into metal carbide or nitride, which is easy to react with acid to form a metal ion solution. After acid treatment and cleaning, metal impurities can be effectively removed to form a pure carbon precursor. After secondary calcination in an inert atmosphere, hard carbon is formed. The product produced by this process has less impurities, good performance, high capacity and high first charge efficiency.

[0025] Compared with the prior art, the application has the following advantages:

[0026] The bamboo used in the application is used as raw material, which has sufficient supply, low storage and transportation cost, less damage to equipment, and is suitable for large-scale production, thereby reducing production cost compared with coconut shell, pitch and phenolic resin.

[0027] The application is carried out under the protection of inert gas throughout the process to avoid exposure to air, thereby reducing the reaction of oxygen with carbon structure in biomass to produce CO, CO2 and other gases, and further avoiding the reduction of product yield. At the same time, the reaction of metal complexes in bamboo biomass with oxygen in inert gas can avoid the generation of inert oxides such as MgO, Cr2O3 and Fe3O4, so as to ensure that metal ions can be cleaned by acid washing.

[0028] The application uses a two-step calcination process to convert metal organic complexes in plants into metal nitride or metal carbide after one calcination treatment, which is easier to react with acid to form a metal ion solution and be cleaned. Compared with other existing technologies, the use of strong alkali, enzyme preparation or pore forming agent directly treats bamboo, which has higher efficiency, lower cost and lower metal impurity content of the obtained product.

[0029] The hard carbon prepared by the application has a large interlayer spacing and a very low impurity content, and is suitable for sodium ion battery negative electrode and has good electrochemical performance. The production process has little pollution and low equipment wear, and is suitable for large-scale production of sodium ion battery negative electrode material. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 Flow chart of the preparation method of the present application.

[0031] Figure 2 Hard carbon battery test chart prepared for Example One of the present application.

[0032] Figure 3 Hard carbon battery test chart prepared for Example Two of the present application.

[0033] Figure 4 Hard carbon battery test chart prepared for Example Three of the present application.

[0034] Figure 5 Hard carbon battery side view for Comparative Example One.

[0035] Figure 6 Transmission electron microscope chart of hard carbon prepared for Example One of the present application.

[0036] Figure 7 Transmission electron microscope chart of hard carbon prepared for Example Two of the present application.

[0037] Figure 8 Transmission electron microscope chart of hard carbon prepared for Example Three of the present application. DETAILED DESCRIPTION

[0038] The drawings are only for example illustration; the reagents or instruments used in the present application are not specified by the specific manufacturer; they are all conventional products purchased from the market general sales channel.

[0039] The battery test conditions used in the examples comply with the national standard GB / T 43114-2023.

[0040] Any changes, modifications, equivalent replacements, improvements, etc. within the principles of the present application shall be included in the protection scope of the present application.

[0041] Example One

[0042] A preparation method of hard carbon, comprising the following steps:

[0043] (1) Selecting 4-year-old bamboo culm of Phyllostachys pubescens, grinding by mechanical wall breaking, and passing through a 400-mesh sieve to obtain bamboo powder.

[0044] (2) The product obtained in step (1) is heated to 480℃ at a heating rate of 2℃ / min in a nitrogen environment for 2 hours to obtain a carbonized product.

[0045] (3) Adding hydrochloric acid with pH=5.0 to the product obtained in step (2) and fully reacting, then washing with clean water until the washing liquid is neutral to obtain a precursor.

[0046] (4) The precursor obtained in step (3) is placed in a nitrogen environment and heated to 1200°C at a heating rate of 2°C / min for 2 hours to obtain hard carbon.

[0047] (5) The hard carbon obtained in step (4) is ball milled with steel beads at 650 rpm for one hour (alternating clockwise and counterclockwise) to obtain a hard carbon product with finer particles.

[0048] Example Two

[0049] A method for preparing hard carbon, comprising the following steps:

[0050] (1) Selecting 4-year-old bamboo culm of Phyllostachys pubescens, grinding by mechanical wall breaking, and passing through a 1000-mesh sieve to obtain bamboo powder.

[0051] (2) The product obtained in step (1) is heated to 450°C at a heating rate of 5°C / min in a nitrogen environment for 3 hours to obtain a carbonized product.

[0052] (3) Adding pH=5.0 nitric acid to the product obtained in step (2) and reacting thoroughly, then washing with clean water until the washing liquid is neutral to obtain a precursor.

[0053] (4) The precursor obtained in step (3) is placed in a nitrogen environment and heated to 1100°C at a heating rate of 5°C / min for 3 hours to obtain hard carbon.

[0054] Example Three

[0055] A method for preparing hard carbon, comprising the following steps:

[0056] (1) Purchasing 600-mesh sieved bamboo powder from the market.

[0057] (2) The product obtained in step (1) is heated to 450°C at a heating rate of 2°C / min in an argon environment for 3 hours to obtain a carbonized product.

[0058] (3) Adding pH=5.0 hydrochloric acid to the product obtained in step (2) and reacting thoroughly, then washing with clean water until the washing liquid is neutral to obtain a precursor.

[0059] (4) The precursor obtained in step (3) is placed in an argon environment and heated to 1300°C at a heating rate of 2°C / min for 2 hours to obtain hard carbon.

[0060] (5) The hard carbon obtained in step (4) is ball milled with steel beads at 650 rpm for one hour (alternating clockwise and counterclockwise) to obtain a hard carbon product with finer particles.

[0061] Comparative Example 1

[0062] The hard carbon fiber was purchased from a well-known Japanese company; product number Type 2.

[0063] For the three sets of embodiments described above, sodium sheets were used as the negative electrode, NaPF6 / DC+DMC (EC:DMC = 1:1 vol%) was used as the electrolyte, and glass fiber was used as the separator to assemble coin cells. Electrochemical tests were conducted according to the national standard GB / T 43114-2023. The battery performance test results are as follows: Figure 2 , 3 As shown in Figure 4, the initial charge capacity of the hard carbon fiber obtained using this method is consistently around 320 mAh g. -1 The discharge capacity is around 260mAh g. -1 Around 80%, the initial recharge effect can reach over 80%. From Figure 6 , 7 As shown in the transmission electron microscope (TEM) images (Figure 8), the hard carbon product produced by this invention consists of short-range disordered carbon chains that coil to form a dense porous structure, providing sodium storage space. The interlayer spacing is 0.35 nm or more, consistent with the structural characteristics of hard carbon. This invention utilizes low-cost, widely available raw materials, making it suitable for large-scale production of hard carbon for use as anodes in sodium-ion batteries.

[0064] Currently, the best-selling hard carbon battery product manufacturer on the market is a well-known Japanese company. Batteries assembled using the same process were tested according to the national standard GB / T 43114-2023, and the results were as follows... Figure 5 As shown, its initial charge capacity is 305.4 mAh g. -1 The discharge capacity is 245.6 mAh g. -1 All of them are lower than the product of this invention.

[0065] Example 4

[0066] A method for preparing hard carbon includes the following steps:

[0067] (1) Select bamboo stems of 3-year-old moso bamboo, grind them by mechanical wall breaking, and pass them through a 1500-mesh sieve to obtain bamboo powder.

[0068] (2) The product obtained in step (1) is heated to 480°C at a heating rate of 5°C / min and stored for 5 hours in a nitrogen atmosphere to obtain the carbonized product.

[0069] (3) Add nitric acid with pH=5.5 to the product obtained in step (2) and react fully. After filtration, rinse with water until the pH of the washing solution is neutral to obtain the precursor.

[0070] (4) The precursor obtained in step (3) is placed in a nitrogen environment, and heated to 1300°C at a heating rate of 5°C / min for 3 hours to obtain hard carbon.

[0071] Example Five

[0072] A method for preparing hard carbon, comprising the following steps:

[0073] (1) Selecting 3-year-old bamboo culm of Phyllostachys pubescens, grinding by mechanical wall breaking, and passing through a 2000-mesh sieve to obtain bamboo powder.

[0074] (2) The product obtained in step (1) is placed in a nitrogen environment, and heated to 450°C at a heating rate of 5°C / min for 8 hours to obtain a carbonized product.

[0075] (3) Adding nitric acid with pH=4.5 to the product obtained in step (2) and fully reacting, then washing with clean water until the washing liquid is neutral to obtain a precursor.

[0076] (4) The precursor obtained in step (3) is placed in a nitrogen environment, and heated to 1300°C at a heating rate of 5°C / min for 3 hours to obtain hard carbon.

[0077] Of course, the above description is not a limitation of the present application, and the present application is not limited to the above examples. Changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present application should also be within the scope of the present application.

Claims

1. A method for producing hard carbon, characterized by, The method comprises the following steps: (1) selecting bamboo stems with an age of 2-6 years, grinding the bamboo stems by mechanical wall breaking to obtain bamboo powder, and ensuring that the internal structure of the bamboo powder fully participates in the reaction; (2) pre-treating the bamboo powder by drying and performing a first calcination under an inert atmosphere to obtain a carbonization product; (3) cleaning the carbonization product after acid treatment to effectively remove metal elements in the biomass, and obtaining a carbonization precursor; (4) drying the carbonization precursor and performing a second calcination under an inert gas atmosphere to obtain the hard carbon.

2. The method for preparing hard carbon according to claim 1, characterized in that, The bamboo stems are selected from any one of Phyllostachys edulis, Dendrocalamopsis oldhami, Phyllostachys pubescens and Phyllostachys edulis, the stems are stripped of the skin and leaves, and are ground by mechanical wall breaking and sieved; the sieving range is 200-3000 mesh, and the bamboo powder is completely in a powder form, so that the internal structure of the bamboo powder can fully participate in the reaction.

3. The method for preparing hard carbon according to claim 1, characterized in that, During the first calcination, the temperature is raised at a rate of 1-10 ℃ / min to 400-500 ℃, and the first calcination is performed under an inert gas to avoid oxidation to produce CO and CO2, thereby avoiding a reduction in the product yield.

4. The method for preparing hard carbon according to claim 3, characterized in that, During the first calcination, the temperature is raised and then maintained for 3-8 hours.

5. The method for preparing hard carbon according to claim 1, characterized in that, The acid used is any one of hydrochloric acid, acetic acid, sulfuric acid and nitric acid with a pH less than 6.

0.

6. The method for preparing hard carbon according to claim 5, characterized in that, The carbon precursor after acid treatment is cleaned with water for 2-5 times until the pH of the cleaning waste liquid is neutral, so that the metal oxides and nitrides are cleaned.

7. The method for preparing hard carbon according to claim 1, characterized in that, During the second calcination, the temperature is raised at a rate of 1-10 ℃ / min to 1000-1500 ℃, and the second calcination is performed under an inert gas to avoid oxidation to produce CO and CO2, thereby avoiding a reduction in the product yield.

8. The method for preparing hard carbon according to claim 7, characterized in that, During the second calcination, the temperature is raised and then maintained for 1-5 hours.

9. The method for preparing hard carbon according to claim 1, characterized in that, The inert gas atmosphere during the first and second calcinations is nitrogen, argon or a mixture of the two gases in any ratio.

10. The method for preparing hard carbon according to claim 1, characterized in that, The interlayer distance of the hard carbon is 0.35-0.4 nm, and the battery capacity reaches 320 mAh g -1 , and the initial charging efficiency is greater than 80%.

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