Method for preparing nitrogen-doped biomass hard carbon by PVP (Polyvinyl Pyrrolidone) thermal cracking

Abstract

The invention discloses a method for preparing nitrogen-doped biomass hard carbon by PVP (Polyvinyl Pyrrolidone) thermal cracking. The method comprises the following steps: S1, roasting a biomass precursor at 450-550 DEG C to prepare pre-carbonized biomass; s2, washing the pre-carbonized biomass with an alkali solution until the pH value is 7, and then washing with an acid solution until the pH value is 7; s3, calcining the pre-carbonized biomass subjected to acid and alkali washing at the temperature of 1200-1600 DEG C to prepare biomass hard carbon; and S4, respectively placing the biomass hard carbon and PVP in different containers, placing the containers in the same closed space, and carrying out chemical vapor deposition at 600-900 DEG C for 2-12 hours. Nitrogen is doped in a chemical vapor deposition manner, so that the spacing of hard carbon layers can be enlarged, more sodium storage sites are provided in the presence of pyridine nitrogen and pyrrole nitrogen, and the material capacity is improved; the sodium ion binding energy can be changed, the ion transmission rate of the material can be improved, and the material with high capacity, high rate performance and relatively high first-week coulombic efficiency can be obtained.

Description

Technical Field

[0001] The present invention relates to the field of sodium-ion batteries, and specifically to a method for preparing nitrogen-doped biomass hard carbon by pyrolysis of PVP, which is a method for preparing a hard carbon negative electrode of a sodium-ion battery from a biomass precursor. Background Art

[0002] In the field of sodium-ion batteries, currently, the negative electrode material is a key issue hindering the large-scale commercial application of sodium-ion batteries. Among various negative electrode materials, carbon-based materials have advantages such as rich raw materials, low cost, and simple synthesis steps. Especially for hard carbon materials in carbon-based materials, their technical route is similar to that of artificial graphite in lithium-ion batteries, and they are currently the materials closest to large-scale commercial application.

[0003] Hard carbon belongs to amorphous carbon and is difficult to graphitize even at temperatures above 2500°C. In contrast, amorphous carbon that can be graphitized at temperatures above 2500°C is soft carbon, such as petroleum coke, the precursor of artificial graphite. Hard carbon is mainly composed of short-range ordered microcrystalline graphite structures, which usually bend and stack and link with each other to form a highly distorted state, while surrounding many nanopores. In addition, the pyrolysis of the original precursor molecules and the overflow of heteroatoms during the carbonization process will form a large number of vacancies, edges, and defects. Nanopores, vacancies, edges, and defects can all provide more active sites for sodium ions, but the vacancies, edges, and defects are prone to form "dead sodium" after binding with sodium, which will greatly reduce the first-cycle Coulombic efficiency; in addition, limited by the hard carbon structure, hard carbon does not have freely moving electrons like graphite, resulting in poor conductivity and poor rate performance of the material. Therefore, only by solving these two problems brought by the hard carbon structure itself can the electrochemical performance be improved and commercialization be achieved. Summary of the Invention

[0004] To solve the rate problem and the first-cycle efficiency problem of hard carbon materials, the present invention provides a method for preparing nitrogen-doped biomass hard carbon by pyrolysis of PVP. After preparing the biomass hard carbon, the small-molecule carbon source and nitrogen source obtained by pyrolysis of PVP are deposited on the surface of the biomass hard carbon by chemical vapor deposition, and then different hard carbons are modified into nitrogen-doped hard carbons, and the obtained nitrogen-doped hard carbons have high specific capacity, relatively high first-cycle Coulombic efficiency, and good rate performance.

[0005] Another object of the present invention is to provide a biomass hard carbon prepared by the above method of preparing nitrogen-doped biomass hard carbon from a biomass precursor.

[0006] Another object of the present invention is to provide the application of the above biomass hard carbon in the negative electrode material of a sodium-ion battery.

[0007] The above objects of the present invention are achieved by the following technical solutions:

[0008] A method for preparing nitrogen-doped biomass hard carbon by pyrolysis of PVP, comprising the following steps:

[0009] S1. Roast the biomass precursor at 450 - 550 °C for more than 2 h to obtain pre-carbonized biomass.

[0010] S2. Wash the pre-carbonized biomass with an alkali solution, wash until pH = 7, and then wash with an acid solution until pH = 7.

[0011] S3. Calcinate the pre-carbonized biomass after acid and alkali washing at 1200 - 1600 °C for more than 2 h to obtain biomass hard carbon.

[0012] S4. Place the biomass hard carbon and PVP in different containers and then put them in the same closed space, and carry out chemical vapor deposition at 600 - 900 °C for 2 - 12 h.

[0013] The pre-carbonization in step S1 of the present invention is to better realize the acid and alkali washing in step S2. The purpose of the acid and alkali washing in step S2 is to remove the ash in the bamboo. Ash will hinder the current transmission, resulting in a decrease in the conductivity of the electrode and affecting the rate performance of the material. In addition, ash cannot store sodium and does not provide capacity, which will reduce the specific capacity of the material. The main components of bamboo ash are metal ions, silicon dioxide and silicate. These substances exist in the cell wall, cell gap and epidermal cells of bamboo; they will also exist inside or between the long-chain structures of cellulose, hemicellulose and lignin. The purpose of alkali washing is to remove silicon-containing substances, and the purpose of acid washing is to remove metal ions. If only the bamboo is acid and alkali washed, the effect of removing ash will be very poor. Therefore, an airtight pre-carbonization operation is required to destroy the cell structure and at the same time cause thermal cracking of the long-chain molecules, converting the metal elements inside and between the molecules into metal salts.

[0014] The high-temperature calcination in step S3 is to reduce the defects and large surface pores of the hard carbon and improve the first-cycle Coulomb efficiency of the material. Under the condition of high temperature and air isolation, some short-range ordered graphene will undergo carbon rearrangement, fusion and growth, expanding the ordered part of the material.

[0015] Step S4 is the key step to obtain high-performance hard carbon. First, PVP and the prepared hard carbon need to be placed in the same closed space without contact. Then, the atmosphere in the closed space where the chemical vapor deposition process occurs is replaced with a protective atmosphere or maintained in a vacuum state, and the closed space is heated to 600 - 900 °C at a heating rate of 5 °C / min and held for 2 - 12 h. At this time, PVP will undergo thermal cracking, cracking out small molecule gases such as water, nitrogen oxides, aldehydes and ketones, ammonia nitrogen compounds, nitrite nitrogen compounds, alkenynes, etc. These small molecule gases diffuse in the closed space and produce active carbon atoms and active nitrogen-containing groups when the temperature reaches certain conditions. When these substances come into contact with the surface of the hard carbon, they will react on the surface of the hard carbon and grow into a short-range ordered soft carbon layer. As the deposition time increases, the soft carbon layer continues to grow, and the nitrogen-containing groups will also be continuously converted into pyrrole nitrogen, pyridine nitrogen and graphitic nitrogen bound to the carbon layer. Among them, pyrrole nitrogen and pyridine nitrogen help to improve the material capacity and expand the interlayer spacing of microcrystalline graphite; graphitic nitrogen helps to improve the electrical conductivity of the material. Expanding the interlayer spacing and improving the electrical conductivity both contribute to improving the rate performance of the material. The final obtained material microstructure is as shown in Figure 1 shown, and the deposition situation of nitrogen element is as shown in Figure 2 shown.

[0016] Preferably, in the step S1, the pre-carbonization temperature is 450 - 550 °C.

[0017] Preferably, in the step S1, the pre-carbonization time is 2 h.

[0018] The pre-carbonization temperature of step S1 is 450 °C - 550 °C. The main basis is that moso bamboo will be completely carbonized at 450 °C. If the pre-carbonization temperature is lower than 450 °C, the carbonization reaction is incomplete, and some ash will still be fixed in or between molecules, which will affect the effect of removing ash by acid-base washing. If the pre-carbonization temperature exceeds 550 °C, it has basically no influence on the hard carbon obtained subsequently. Limiting it to 550 °C mainly considers the energy consumption problem. The pre-carbonization time of step S2 is two hours. If the pre-carbonization time is too short, like the above-mentioned too low pre-carbonization temperature, it will cause incomplete carbonization reaction and affect ash removal. If the pre-carbonization time is too long, it has no influence on the material itself. The energy consumption problem is also considered.

[0019] It should be noted that the biomass precursor itself in step S1 has a great influence on the material. Different biomass precursors have a large difference in the capacity of the final obtained materials after passing through steps S1 - S4, but the rate performance difference is not significant. This is mainly affected by the composition and structure of the biomass precursor. The ash content of the biomass hard carbon obtained from different biomass precursors is different (some biomass materials still have relatively high ash residue after acid-base washing), the interlayer spacing of graphite microcrystals is different, the order degree of the material is different, the size, distribution and morphology of the pore size are different, etc. These will all affect the final performance of the material.

[0020] Preferably, the concentration of the alkali solution in step S2 is 1 mol / L.

[0021] Preferably, the alkali washing in step S2 is carried out under a water bath condition, the water bath time is more than 5 h, and the water bath temperature is 40 - 80 °C.

[0022] Preferably, the concentration of the acid solution in step S2 is 2 mol / L.

[0023] Preferably, the acid washing in step S2 is carried out at room temperature, and the acid washing time is more than 12 h.

[0024] The types of acid and alkali required for acid-base washing in step S2 have little influence on the material, and finally they can all be removed by washing with deionized water. Among them, the concentrations of the acid solution and the alkali solution have little influence on the material, and it can be implemented as long as the concentration is not particularly large or particularly small, because when the concentration is particularly large, the reaction will change, such as the dehydration reaction of concentrated sulfuric acid; while when the concentration is particularly small, the ash reaction will be incomplete. The alkali washing needs to be carried out under a water bath condition to increase the reaction rate between the alkali solution and the silicon-containing substance.

[0025] Preferably, the calcination temperature in step S3 is 1400 °C.

[0026] Preferably, the calcination time in step S3 is 2 h.

[0027] The calcination temperature in step S3 has a greater influence on the material. When the precursor is carbonized at a low temperature (about 600 - 800 °C), its microscopic carbon layer structure usually shows a relatively low degree of graphitization and contains abundant defects and porosity. Therefore, the carbon material obtained by low-temperature carbonization has a large specific surface area, which leads to the formation of a large amount of SEI film on the surface of the material during the first cycle, consuming too many sodium ions and resulting in a low first-cycle Coulombic efficiency; in addition, due to the strong binding energy between the defects and Na + and the obvious diffusion barrier, it causes Na + to be unable to continue to be removed during the first-cycle charging, which also results in a low first-cycle Coulombic efficiency. However, as the carbonization temperature increases, the graphite microcrystalline structure will merge and grow, and the graphitization degree of the hard carbon material will increase to some extent. The pores existing in the material will gradually collapse and close, resulting in a lower surface area and fewer defects, which can improve the first-cycle Coulombic efficiency. But too high a temperature will promote the excessive graphitization of the hard carbon material, thus bringing adverse effects on the specific capacity and the sodium ion diffusion barrier.

[0028] Preferably, the chemical vapor deposition temperature in step S4 is 800 °C.

[0029] Preferably, the chemical vapor deposition time in step S4 is 2 h.

[0030] It should be noted that the influence of the chemical vapor deposition temperature in step S4 on the material is mainly reflected in the nitrogen element content and the contents of pyridine nitrogen, pyrrole nitrogen, and graphitic nitrogen. As the temperature increases, the nitrogen element content shows a trend of first increasing and then decreasing; while pyridine nitrogen and pyrrole nitrogen will transform into graphitic nitrogen. The higher the nitrogen element content, the higher the material capacity; the higher the graphitic nitrogen content, the better the material conductivity, and the better the corresponding material rate performance. The influence of the chemical vapor deposition time in step S4 on the material is mainly reflected in the thickness of the soft carbon layer. The longer the deposition time, the thicker the soft carbon layer. If the soft carbon layer is too thick, it will affect the entry of sodium ions into the hard carbon material, resulting in a decrease in the material capacity; if the deposition time is too short, the soft carbon layer will be too thin to play a role in repairing defects, and the first-cycle Coulombic efficiency of the material will decrease. In addition, the mass ratio of PVP to the biomass hard carbon in step S4 should be greater than 2:1 mainly to ensure that the small molecule gas generated by the thermal cracking of PVP is in excess and the chemical vapor deposition will not terminate due to insufficient reactants.

[0031] The present invention also protects a hard carbon prepared by the method for preparing nitrogen-doped biomass hard carbon by thermal cracking of PVP.

[0032] The present invention also protects the application of the above hard carbon material in the preparation of the negative electrode of a sodium-ion battery.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] By using the method provided by the present invention, the biomass hard carbon prepared has a first-cycle discharge specific capacity as high as 739.91 mAh·g -1 , a reversible capacity of up to 623.25 mAh·g -1 , a first-cycle Coulombic efficiency as high as 84.23%, a reversible capacity of up to 379.96 mAh·g at a large current rate of 1C -1 , and a capacity retention rate of 96.46% after 300 cycles at a current density of 1C. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a high-resolution transmission electron microscope image of the hard carbon prepared according to Example 1 in the present invention.

[0036] Figure 2 It is an energy-dispersive X-ray spectrometer image of the N element of the hard carbon prepared according to Example 1 in the present invention.

[0037] Figure 3 It is a photograph of the experimental operation of step S4 of the present invention, where the left corundum boat contains PVP and the right corundum boat contains hard carbon. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] The present invention will be further described below in conjunction with specific embodiments, but the embodiments do not limit the present invention in any form. Unless otherwise specified, the raw material reagents used in the embodiments of the present invention are conventional raw material reagents purchased commercially.

[0039] Example 1

[0040] A method for preparing nitrogen-doped biomass hard carbon by pyrolysis of PVP, comprising the following steps:

[0041] S1. Bake the moso bamboo at 500 °C for 2 h to obtain pre-carbonized moso bamboo.

[0042] S2. Wash the pre-carbonized moso bamboo with 1 M sodium hydroxide solution, wash until the pH = 7, and then wash with 2 M hydrochloric acid until the pH = 7.

[0043] S3. Calcinate the pre-carbonized moso bamboo washed with acid and alkali at 1400 °C for 2 h to obtain moso bamboo hard carbon.

[0044] S4. Place the moso bamboo hard carbon and PVP in different containers and then place them in the same closed space. The mass ratio of PVP to moso bamboo hard carbon is 5:1, and carry out chemical vapor deposition at 800 °C for 2 h.

[0045] Example 2

[0046] A method for preparing nitrogen-doped biomass hard carbon by pyrolysis of PVP, which is different from Example 1 in that: S4. Place the moso bamboo hard carbon and PVP in different containers and then place them in the same closed space. The mass ratio of PVP to moso bamboo hard carbon is 5:1, and carry out chemical vapor deposition at 600 °C for 2 h.

[0047] Example 3

[0048] A method for preparing nitrogen-doped biomass hard carbon by pyrolysis of PVP, which is different from Example 1 in that: S4. Place the moso bamboo hard carbon and PVP in different containers and then place them in the same closed space. The mass ratio of PVP to moso bamboo hard carbon is 5:1, and carry out chemical vapor deposition at 700 °C for 2 h.

[0049] Example 4

[0050] A method for preparing nitrogen-doped biomass hard carbon by pyrolysis of PVP, which is different from Example 1 in that: S4. Place the moso bamboo hard carbon and PVP in different containers and then place them in the same closed space. The mass ratio of PVP to moso bamboo hard carbon is 5:1, and carry out chemical vapor deposition at 900 °C for 2 h.

[0051] Example 5

[0052] A method for preparing nitrogen-doped biomass hard carbon by pyrolysis of PVP, which is different from Example 1 in that: S4. The moso bamboo hard carbon and PVP are respectively placed in different containers and then placed in the same closed space. The mass ratio of PVP to moso bamboo hard carbon is 5:1, and chemical vapor deposition is carried out at 800 °C for 4 h..

[0053] Example 6

[0054] A method for preparing nitrogen-doped biomass hard carbon by pyrolysis of PVP, which is different from Example 1 in that: S4. The moso bamboo hard carbon and PVP are respectively placed in different containers and then placed in the same closed space. The mass ratio of PVP to moso bamboo hard carbon is 5:1, and chemical vapor deposition is carried out at 800 °C for 8 h.

[0055] Example 7

[0056] A method for preparing nitrogen-doped biomass hard carbon by pyrolysis of PVP, which is different from Example 1 in that: S4. The moso bamboo hard carbon and PVP are respectively placed in different containers and then placed in the same closed space. The mass ratio of PVP to moso bamboo hard carbon is 5:1, and chemical vapor deposition is carried out at 800 °C for 12 h.

[0057] Example 8

[0058] A method for preparing nitrogen-doped biomass hard carbon by pyrolysis of PVP, which is different from Example 1 in that: S3. The pre-carbonized moso bamboo after acid-base washing is calcined at 1200 °C for 2 h to obtain moso bamboo hard carbon.

[0059] Example 9

[0060] A method for preparing nitrogen-doped biomass hard carbon by pyrolysis of PVP, which is different from Example 1 in that: S3. The pre-carbonized moso bamboo after acid-base washing is calcined at 1600 °C for 2 h to obtain moso bamboo hard carbon.

[0061] Comparative Example 1

[0062] A method for preparing biomass hard carbon, which is different from Example 1 in that: Step S4 is not carried out.

[0063] Comparative Example 2

[0064] A method for preparing biomass hard carbon, which is different from Example 1 in that: S3. The pre-carbonized moso bamboo after acid-base washing is calcined at 1200 °C for 2 h to obtain moso bamboo hard carbon.

[0065] Step S4 is not carried out.

[0066] Comparative Example 3

[0067] A method for preparing biomass hard carbon, which is different from Example 1 in that: S3. calcining the pre-carbonized bamboo after acid and alkali washing at 1600° C. for 2 hours to obtain bamboo hard carbon.

[0068] Step S4 is not performed.

[0069] Comparative Example 4

[0070] A method for preparing biomass hard carbon, which is different from Example 1 in that: S1. the selected biomass is coconut shell.

[0071] Comparative Example 5

[0072] A method for preparing biomass hard carbon, which is different from Example 1 in that: S1. the selected biomass is corn platycodon.

[0073] Performance Testing

[0074] Preparation of negative electrode material: Weigh 0.08g of hard carbon, 0.01g of acetylene black (conductive agent), and 0.01g of PVDF (HSV900, binder), grind them thoroughly, add 0.33ml of NMP to disperse and mix. After slurrying, draw the slurry on copper foil to make sheets, dry them at 85℃ with air blast, and cut them into discs with a diameter of 12mm.

[0075] Button cell assembly: Assembled in a glove box with argon atmosphere (H 2 O<0.01ppm,O 2 <0.01ppm), with sodium metal sheet as the counter electrode, 1M NaPF 6 The solution (solvent EC:DEC volume ratio of 1:1) was used as the electrolyte, and glass fiber (Grade GF / D) was used as the separator to assemble into a CR2032 button cell.

[0076] Electrochemical performance test: The battery was tested for constant current charge and discharge at 0.1C at 0-3V at 25°C. A long cycle test was performed at 1C at 0-3V, with 300 cycles. 1C = 300mAh g -1 .

[0077] The specific test data are shown in Table 1 below:

[0078] Table 1. Performance test data of examples and comparative examples

[0079]

[0080]

[0081] The present invention discloses a method for preparing nitrogen-doped biomass hard carbon. Specifically, by means of pyrolysis, PVP (polyvinylpyrrolidone) generates small molecule gases as carbon source and nitrogen source, and then is deposited in the prepared biomass hard carbon by chemical vapor deposition. This method dopes nitrogen by chemical vapor deposition, which can expand the interlayer spacing of hard carbon, provide more sodium storage sites in the form of pyridine nitrogen and pyrrole nitrogen, and improve the material capacity; it can change the sodium ion binding energy, improve the ion transport rate of the material, and improve the rate performance of the material. Compared with doping nitrogen in the biomass precursor state, it can avoid the damage to the structure of the hard carbon material caused by the overflow of nitrogen elements during the heating process and reduce the generation of defects; compared with simple doping by mixing with a nitrogen source after sintering into hard carbon, it can avoid the influence of the residues of the nitrogen source on the material. At the same time, the chemical vapor deposition of the carbon source can form a layer of soft carbon film on the surface of the hard carbon, which can block the surface defects of the hard carbon, reduce the specific surface area of the material, and improve the first-cycle Coulombic efficiency of the material. Finally, a material with high capacity, high rate performance, and relatively high first-cycle Coulombic efficiency is obtained.

[0082] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing nitrogen-doped biomass hard carbon by thermal cracking of PVP, characterized in that: The steps include: S1. calcining the biomass precursor at 450-550° C. to prepare pre-carbonized biomass; S2. The pre-carbonized biomass is washed with an alkaline solution until the pH is 7 and then washed with an acid solution until the pH is 7; S3. The pre-carbonized biomass after acid and alkali washing is calcined at 1200°C to 1600°C to prepare biomass hard carbon; S4. Place the biomass hard carbon and PVP in different containers and place them in the same closed space for chemical vapor deposition at 600°C to 900°C for 2 to 12 hours.

2. The method for preparing nitrogen-doped biomass hard carbon by thermal cracking of PVP as claimed in claim 1, characterized in that: In the step S1, the biomass precursor is derived from one or more organisms rich in cellulose, hemicellulose, and lignin, including bamboo, coconut shells, rice shells, straw, and wood, biological waste, and domestic production waste.

3. The method for preparing nitrogen-doped biomass hard carbon by thermal cracking of PVP as claimed in claim 1, characterized in that: The mass ratio of PVP to biomass hard carbon in S4 is greater than 2:

1.

4. The method for preparing nitrogen-doped biomass hard carbon by thermal cracking of PVP as claimed in claim 1, characterized in that: In step S2, the alkali in the alkaline solution includes one or more of sodium hydroxide and potassium hydroxide, and the acid in the acid solution includes one or more of hydrochloric acid, sulfuric acid, and phosphoric acid.

5. The method for preparing nitrogen-doped biomass hard carbon by thermal cracking of PVP as claimed in claim 1, characterized in that: In S2, the alkali washing is carried out in a water bath for more than 5 hours at a water bath temperature of 40-80° C., and the acid washing is carried out at room temperature for more than 12 hours.

6. The method for preparing nitrogen-doped biomass hard carbon by thermal cracking of PVP as claimed in claim 1, characterized in that: The calcination atmosphere in step S1 is one or more of an inert gas atmosphere selected from nitrogen, argon, helium and neon; the calcination atmosphere in step S3 is an inert gas atmosphere selected from one or more of nitrogen, argon, helium and neon.

7. The method for preparing nitrogen-doped biomass hard carbon by thermal cracking of PVP as claimed in claim 1, characterized in that: The gas in the enclosed space in S4 needs to be kept in a vacuum state or filled with an inert atmosphere in advance.

8. The method for preparing nitrogen-doped biomass hard carbon by thermal cracking of PVP as claimed in claim 1, characterized in that: In the above S4, the heating rate to reach 600°C to 900°C is 3-8°C / min.

9. Biomass hard carbon prepared by the method for preparing nitrogen-doped biomass hard carbon by thermal cracking of PVP as claimed in any one of claims 1 to 8.

10. Application of the biomass hard carbon according to claim 9 in sodium ion batteries.

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