Method for preparing sodium ion hard carbon negative electrode material by lignin depolymerization separation and application

Abstract

The application discloses a method for preparing sodium-ion hard carbon negative materials by lignin depolymerization and separation and application, and belongs to the technical field of lignin depolymerization and separation and sodium-ion negative hard carbon material production. After lignin is oxidatively depolymerized into small molecules, the lignin is separated by a membrane separation instrument, and lignin with a hierarchical molecular weight range of 3K Da or above, 3K-2K Da, 2K-1K Da, 1K-500 Da, 500-200 Da and 200 Da or below is obtained, and the lignin is subjected to pre-carbonization and high-temperature carbonization treatment to prepare lignin-based hard carbon. In the carbonization and high-temperature reforming process of lignin with different molecular weight segments, the lignin-based hard carbon microcrystalline structure is directionally regulated due to different molecular structures, crosslinking structures and active site exposure degrees of the lignin itself, and the problems of poor energy storage performance of the lignin-based hard carbon, unclear sodium storage mechanism and the like caused by uneven precursor molecular structure and unclear structure are solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lignin depolymerization and separation and sodium ion negative electrode hard carbon material production, and particularly relates to a method for preparing a sodium ion hard carbon negative electrode material by lignin depolymerization and separation and application thereof. BACKGROUND

[0002] Hard carbon as a negative electrode material has attracted extensive attention in sodium ion batteries. Its unique long-range ordered and short-range disordered carbon microcrystalline structure makes it an excellent choice for sodium ion battery negative electrode materials. Hard carbon has high capacity, supports fast charging, and is highly safe, and these characteristics enable it to meet the demand for high energy density in mobile power sources and home energy storage applications. Although the diversity and complexity of the precursor structure lead to controversy over the specific mechanism of sodium storage in hard carbon materials. Moreover, in practical applications, there are relatively few reports on sodium ion batteries that can support large current fast charging and maintain good cycle stability, but the application prospect of hard carbon has been recognized.

[0003] Lignin is mostly an industrial waste produced by paper mills and cellulosic ethanol plants, and if it can be used as a sustainable precursor for hard carbon, it will greatly benefit the environment and sustainable development. Lignin-based hard carbon has a rich closed pore structure and external pore structure with different pore sizes, which are beneficial to the storage of sodium ions, thus providing high sodium storage capacity. On the one hand, the internal crystal arrangement of lignin-based hard carbon is disordered and the interlayer spacing is large, which makes the expansion and contraction of the hard carbon negative electrode more uniform during discharge, increasing its cycle stability, charge and discharge performance, and prolonging the cycle life of sodium ion batteries; on the other hand, the volume change of lignin-based hard carbon after sodium intercalation is small, with good low temperature and fast charging performance, which makes it an excellent choice for sodium ion battery negative electrode materials. Lignin-based hard carbon has high sodium storage capacity, excellent cycle stability, good sodium intercalation performance and sustainability, but also has challenges in economy and first coulombic efficiency. Therefore, through appropriate treatment and improvement, lignin-based hard carbon is expected to become one of the preferred choices for sodium ion battery negative electrode materials.

[0004] The advantages of lignin with uniform molecular weight mainly lie in improving the application value and stability of lignin, and optimizing its chemical and physical properties. Through fractionation, the heterogeneity of lignin, including the polydispersity of molecular weight and the complexity of structure, can be reduced, thereby improving the performance stability of lignin in downstream applications. The polydispersity of lignin molecular weight leads to uneven structure performance, causing unstable product performance and reducing the application value of the product. Using the fractionation method, lignin with similar reactivity and application performance can be prepared according to the molecular weight, which is an effective means to solve the above problems. In this way, lignin with different molecular weight ranges can be obtained. By comparing and analyzing the structure and product performance of lignin obtained by different lignin fractionation methods, the lignin of each fraction can be used according to its characteristics and performance, providing guidance for the fractionation of lignin and the development of subsequent products under different extraction methods and performance requirements, and promoting the development of lignin products in terms of stability and high value. In addition, fractionated lignin can also help to improve the uniformity of lignin and improve the reactivity of its unique groups, further expanding the application of lignin in the fields of material science and chemical production. In turn, the application value of lignin can be further improved, and the chemical and physical properties can be optimized. These advantages make fractionated lignin a hot topic in research and industrial applications. SUMMARY

[0005] The technical problem solved by the present application is to provide a method for preparing a sodium-ion hard carbon negative electrode material by depolymerizing and separating lignin. Another technical problem to be solved by the present application is to provide a sodium-ion hard carbon negative electrode material prepared by the above method. Still another technical problem to be solved by the present application is to provide an application of the sodium-ion hard carbon negative electrode material prepared by depolymerizing and separating lignin.

[0006] To solve the above technical problems, the technical scheme adopted by the present application is as follows:

[0007] A method for preparing a sodium-ion hard carbon negative electrode material by depolymerizing and separating lignin, after oxidatively depolymerizing lignin, membrane separation is used to obtain lignin with a molecular weight range of 3K Da or more, 3K-2K Da, 2K-1K Da, 1K-500 Da, 500-200 Da, and 200 Da or less. The lignin in each molecular weight segment is subjected to pre-carbonization and high-temperature carbonization treatment to prepare corresponding lignin hard carbon for preparing a sodium-ion hard carbon negative electrode material.

[0008] The preparation process of the depolymerized lignin is as follows: lignin is dispersed in deionized water, H2O2 and a composite base are added, and a depolymerized lignin solution is prepared by depolymerization.

[0009] The type of lignin is selected from any one of sulfite lignin, sulfonate lignin, alkali lignin, and hydrolyzed lignin. The initial lignin has a molecular weight of 6K-8K Da and a purity of ≥93%.

[0010] The method for precipitating the lignin of different molecular weight from the retentate liquid is rotary evaporation, and the temperature is 50-80 DEG C.

[0011] The method for preparing the sodium ion hard carbon negative electrode material by depolymerization and separation of lignin has the following specific steps:

[0012] (1) dispersing lignin into deionized water, adding H2O2 and composite alkali, depolymerizing to obtain a depolymerized lignin solution;

[0013] (2) separating the depolymerized lignin obtained in step (1) according to the size of the molecular weight cut-off by using a membrane separation instrument, drying and weighing the lignin of different molecular weight ranges, and obtaining small molecule lignin powder after crushing, ball milling and screening;

[0014] (3) carbonizing the small molecule lignin powder prepared in step (2), washing with HCl aqueous solution and distilled water until neutral, drying, and placing in a tube furnace in an argon atmosphere to perform high temperature structure reforming to obtain lignin-based hard carbon.

[0015] The carbonization temperature in step (2) is 600-800 DEG C, and the carbonization instrument is a muffle furnace without any atmosphere or a tube furnace in an argon atmosphere.

[0016] The high temperature reforming temperature in step (3) is 1200-1600 DEG C.

[0017] The method for preparing the sodium ion hard carbon negative electrode material by depolymerization and separation of lignin prepares small molecule lignin-based hard carbon.

[0018] The application of the depolymerized and separated lignin-based hard carbon in sodium ion battery negative electrode material.

[0019] Compared with the prior art, the present application has the following advantages:

[0020] (1) The present application uses a lignin membrane separator to separate the depolymerized lignin by ceramic membranes and organic membranes of different molecular weight cut-off, to regulate the molecular weight and its distribution range and the reactive functional groups of lignin, and to realize the directional regulation of the structure of lignin and its carbonization process. The problems of complex lignin molecular structure, few reactive sites, low carbon yield of hard carbon, poor thermal carbon storage performance of untreated lignin, etc. are solved.

[0021] (2) The present application uses fractionated lignin as raw material to prepare hard carbon. Compared with traditional macromolecular lignin raw material, the lignin separated by the present application has small molecular weight and smaller and smaller PDI distribution, which means that the molecular weight distribution of lignin is more uniform, which helps to improve the processability of the material.

[0022] (3) The modified lignin hard carbon structure is more uniform, the molecular weight is smaller, the carbon microcrystalline structure with higher disorder degree can be obtained, and the electrochemical performance of the sodium ion battery negative material is more improved.

[0023] (4) The method is simple to operate and low in energy consumption, and has reference significance for preparing biomass-based hard carbon from other types of fractionated lignin. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 GPC molar mass distribution diagram of lignin not depolymerized in Comparative Example 1 and lignin depolymerized and separated in Examples 1-4, 3K or more, 3K-1K, 1K-500, 500 or less. DETAILED DESCRIPTION

[0025] The application will be further illustrated below in combination with specific examples, which are implemented on the premise of the technical scheme of the application, and it should be understood that the examples are only used to illustrate the application and not used to limit the scope of the application.

[0026] In the application, lignin of 3K Da or more is referred to as 3K or more, lignin of 3K-1K Da is referred to as 3K-1K, lignin of 1K-500 Da is referred to as 1K-500, and lignin of 500 Da or less is referred to as 500 or less.

[0027] The test methods used in the application are as follows:

[0028] 1. Molecular structure property test

[0029] Agilent 1260 was used to test the molecular weight (GPC) of the sample: the detector was Agilent RID G1362A; the chromatographic column was Waters Ultrahydrogel, 300*7.8mm, 500-250-120A; the mobile phase was 0.1mol / L NaNO3 aqueous solution; the flow rate was 1mL / min; the temperature was 40℃; the standard was PEG; and the solvent was water.

[0030] 2. Battery performance test (discharge specific capacity (mAh g -1 ) and initial efficiency)

[0031] 2.1 Battery production

[0032] Preparation of the electrode: hard carbon material, conductive agent (super-P) and adhesive (PVDF) were mixed in a mass ratio of 7:1:1, and a slurry was prepared with NMP as a dispersant, the slurry was coated on a copper foil and dried in a forced air drying oven at 80℃ for 12h. The dried electrode sheet was punched into a circular electrode sheet with a diameter of 15mm for standby.

[0033] Assembling of button cell: The assembling of the button cell was carried out in an argon glove box with the moisture and oxygen content less than 0.01 ppm. The CR2032 specified cell shell was used, and the electrolyte was 1 mol / L NaPF6(solute: ethylene carbonate: dimethyl carbonate: methyl ethyl carbonate = 1:1:1 mixed solution). The metal sodium sheet was used as the counter electrode and reference electrode, and the separator was GF / D glass fiber separator. The cell was assembled in the order of positive shell, electrode sheet, separator, sodium sheet, gasket, spring and negative shell from bottom to top.

[0034] 2.2 Performance test

[0035] Constant current charge and discharge test (GCD) was tested on CT-6008Tn battery detection system.

[0036] Example 1

[0037] (1) Preparation of depolymerized lignin

[0038] 20 g of sodium lignosulfonate was dispersed in 20 mL of deionized water, and then 8 g of 30 wt% H2O2, 10 g of composite alkali (sodium hydroxide: ammonia water = 1:1, mass ratio) was added, and then depolymerized at 40°C for 0.5 h to obtain a depolymerized lignin solution.

[0039] (2) Fractionation of depolymerized lignin

[0040] The obtained depolymerized lignin solution was diluted and added to a membrane separation instrument, and passed through a ceramic membrane with a cut-off of 3K Da to obtain two kinds of lignin above and below 3K Da. The lignin above 3K Da was dried after rotary evaporation at 60°C to obtain a sample.

[0041] (3) Preparation of lignin-based hard carbon

[0042] A certain amount of lignin powder above 3K Da was first carbonized (600°C for 2h in a muffle furnace), and then boiled and washed with 1 mol / L HCl aqueous solution, and then washed with distilled water until neutral and dried. Finally, it was placed in a tube furnace in an argon atmosphere for high-temperature structural reforming at 1300°C to obtain lignin-based hard carbon.

[0043] Example 2

[0044] (1) Preparation of depolymerized lignin: same as step (1) of Example 1.

[0045] (2) Fractionation of depolymerized lignin

[0046] The obtained depolymerized lignin solution was diluted and added into a membrane separation instrument, and two kinds of lignin with a molecular weight of 3K-1K Da and below 1K Da were obtained by passing through a ceramic membrane with a cut-off of 1K Da. The lignin in the range of 3K-1K Da was dried after rotary evaporation at 60°C to obtain a sample.

[0047] (3) Preparation of lignin-based hard carbon

[0048] The 3K-1K Da lignin powder was weighed and carbonized (600°C for 2h in a muffle furnace), then washed with 1 mol / L HCl aqueous solution by boiling, and then washed with distilled water until neutral and dried. Finally, it was placed in a tube furnace in an argon atmosphere for high-temperature structural reforming at 1300°C to obtain lignin-based hard carbon.

[0049] Example 3

[0050] (1) Preparation of depolymerized lignin: the same as step (1) of Example 1.

[0051] (2) Fractionation of depolymerized lignin

[0052] The obtained depolymerized lignin solution was diluted and added into a membrane separation instrument, and two kinds of lignin with a molecular weight of 3K-1K Da and below 1K Da were obtained by passing through a ceramic membrane with a cut-off of 1K Da. The lignin in the range of 3K-1K Da was dried after rotary evaporation at 60°C to obtain a sample.

[0053] (3) Preparation of lignin-based hard carbon

[0054] The 3K-1K Da lignin powder was weighed and carbonized (600°C for 2h in a muffle furnace), then washed with 1 mol / L HCl aqueous solution by boiling, and then washed with distilled water until neutral and dried. Finally, it was placed in a tube furnace in an argon atmosphere for high-temperature structural reforming at 1300°C to obtain lignin-based hard carbon.

[0055] Comparative Example 1

[0056] The commercially purchased sodium lignosulfonate powder without depolymerization treatment was carbonized at 600°C, washed with HCl aqueous solution and distilled water until neutral and dried, and placed in a tube furnace in an argon atmosphere for high-temperature structural reforming at 1300°C to obtain lignin-based hard carbon.

[0057] The molecular structure properties and battery performance of the samples prepared in Examples 1-3 and Comparative Example 1 above were tested, and the test results are shown in Table 1, and the optimal range of lignin molecular weight is 3K-1K.

[0058] Table 1 Sodium storage performance of lignin-based hard carbon in different molecular weight ranges

[0059]

[0060] Example 4

[0061] (1) Preparation of depolymerized lignin

[0062] 20 g of alkali lignin was dispersed in 20 mL of deionized water, 8 g of 30 wt% H2O2, 10 g of composite alkali (sodium hydroxide: ammonia water = 1 : 1, mass ratio) was added, and then depolymerized at 40°C for 0.5 h to obtain a depolymerized lignin solution.

[0063] (2) Fractionation of depolymerized lignin

[0064] The obtained depolymerized lignin solution was diluted and added to a membrane separation instrument, and passed through a ceramic membrane with a cut-off of 3 K Da to obtain two kinds of lignin above and below 3 K Da. The lignin above 3 K Da was dried by rotary evaporation at 60°C to obtain a sample.

[0065] (3) Preparation of lignin-based hard carbon

[0066] A certain amount of lignin powder above 3 K Da was first carbonized (600°C for 2 h in a muffle furnace), then boiled and washed with 1 mol / L HCl aqueous solution, and then washed with distilled water until neutral and dried. Finally, it was placed in a tube furnace under argon atmosphere for high-temperature structural reforming at 1300°C to obtain lignin hard carbon.

[0067] Example 5

[0068] (1) Preparation of depolymerized lignin: same as step (1) of Example 1.

[0069] (2) Fractionation of depolymerized lignin

[0070] The obtained depolymerized lignin solution was diluted and added to a membrane separation instrument, and passed through a ceramic membrane with a cut-off of 3 K Da to obtain two kinds of lignin above and below 3 K Da. The lignin above 3 K Da was dried by rotary evaporation at 60°C to obtain a sample.

[0071] (3) Preparation of lignin-based hard carbon

[0072] A certain amount of lignin powder above 3 K Da was first carbonized (600°C for 2 h in a tube furnace under argon atmosphere), then boiled and washed with 1 mol / L HCl aqueous solution, and then washed with distilled water until neutral and dried. Finally, it was placed in a tube furnace under argon atmosphere for high-temperature structural reforming at 1300°C to obtain lignin-based hard carbon.

[0073] Example 6

[0074] (1) Preparation of depolymerized lignin: same as step (1) of Example 1.

[0075] (2) Fractionation of depolymerized lignin

[0076] The obtained depolymerized lignin solution was diluted and added into a membrane separation instrument, and two kinds of lignin above and below 3K Da were obtained by passing through a ceramic membrane with a cut-off of 3K Da. The lignin above 3K Da was dried after rotary evaporation at 60°C to obtain a sample.

[0077] (3) Preparation of lignin-based hard carbon

[0078] A certain amount of lignin powder above 3K Da was weighed and carbonized (600°C for 2h in a muffle furnace), then boiled and washed with 1 mol / L HCl aqueous solution, and then washed with distilled water until neutral and dried. Finally, it was placed in a tube furnace in an argon atmosphere for high-temperature structural reforming at 1400°C to obtain lignin-based hard carbon.

[0079] The molecular structure properties and battery performance of the samples prepared in the above Examples 2, 5-6 and Comparative Example 1 were tested, and the test results are shown in Table 2. The influence of the selection of lignin species in the carbonization process of small molecule lignin and hard carbon, the pre-carbonization atmosphere, and the high-temperature reforming temperature on the sodium storage performance of hard carbon was mainly illustrated.

[0080] Table 2 Sodium storage performance of lignin-based hard carbon under different lignin species, carbonization atmosphere, and high-temperature reforming temperature conditions

[0081]

[0082] The condition screening of Table 2 mainly involves the reaction conditions in the entire reaction process, and the selection of lignin species and high-temperature reforming temperature have no significant effect on the performance of the prepared lignin-based hard carbon. However, after isolating oxygen during pre-carbonization, the reversible capacity and initial efficiency of the lignin-based hard carbon refined at 1300°C are improved.

[0083] The molecular weight distribution of the samples prepared in the above comparative examples and examples was tested, and the test results are shown in Table 3.

[0084] Table 3 GPC data of lignin in different molecular weight ranges after depolymerization and separation

[0085]

[0086] A smaller and smaller molecular weight distribution PDI (polydispersity index) means that the molecular weight distribution of the polymer is more and more uniform. As can be seen from Table 3, the molecular weight of the original lignin (not depolymerized) is 1656, 3733 g.mol -1)max, mainly because the molecular weight of lignin itself is large and has multiple molecular weight ranges, and its molecular weight distribution coefficient PDI is also the largest 2.25, indicating that its molecular weight distribution is the most uneven. Combined with the molecular weight distribution diagram of Figure 1 It can be clearly seen from the molecular weight distribution diagram that the molecular weight and PDI of lignin after fractionation are getting narrower and narrower. The molecular weight distribution of lignin below 500 Da is the narrowest, that is, the molecular weight of the molecules in the mixture is relatively close, and the distribution is the most uniform. However, the sodium storage performance of the hard carbon prepared from the lignin with the molecular weight below 500 is not the best, mainly because the above lignin has no part of the oxygen-containing functional group which is beneficial to the construction of the hard carbon microcrystalline structure. In addition to the lignin below 500, the lignin distribution of 1K-500 is also relatively uniform, but its molecular weight distribution is very similar to that of the lignin in the range of 3K-1K, but the latter has more active functional groups, which is beneficial to the subsequent carbonization and the improvement of sodium storage performance. Compared with the undegraded lignin, the sodium storage performance of the lignin which is gradually close to uniformization by depolymerization, low molecularization and regulation of molecular weight distribution is greatly improved, which can prove that the smaller and smaller PDI means that the molecular weight distribution of lignin is more uniform, which will help to improve the physical properties and processing performance of the material, thereby optimizing the product quality and production efficiency (Comparative Example 1, Examples 1-3). After comparing the sodium storage performance of lignin-based hard carbons regulated by different lignin types, carbonization atmosphere, high-temperature reforming temperature and other conditions, it can be established that the lignin with a molecular weight range of 3K-1K and a PDI of 1.20 is the best hard carbon precursor in the experimental system, and the best regulation scheme of lignin-based hard carbon is Example 2.

[0087] The sodium storage performance of the undegraded lignin of Comparative Example 1 and the lignin-based hard carbons obtained in Examples 1-6 was tested, and the results in Table 1 showed that as the degree of fractionation of lignin deepened, the molecular weight of lignin gradually decreased, and the sodium storage performance of the obtained lignin-based hard carbons appeared a great difference. Among these lignin-based hard carbon materials, the lignin-based hard carbon prepared from the lignin with a molecular weight range of 3K-1K in the fractionation showed the highest reversible capacity of 358.07 mAh g -1 at 0.05 A g -1 , and the initial coulombic efficiency was 76%.

[0088] The above only describes the preferred embodiments of the present application, and it should be noted that for ordinary skilled persons in the art, some improvements and refinements can be made without departing from the principles of the present application, and these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. A method for preparing sodium ion hard carbon negative electrode materials by depolymerization and separation of lignin, characterized by: After oxidative depolymerization of lignin, membrane separation is used to obtain lignin with graded molecular weight ranges of above 3K Da, 3K-2K Da, 2K-1K Da, 1K-500Da, 500-200Da, and below 200Da. The lignin in each molecular weight range is subjected to pre-carbonization and high-temperature carbonization treatment to prepare corresponding lignin hard carbon, which is used to prepare sodium ion hard carbon negative electrode materials; the preparation process of the depolymerized lignin is as follows: lignin is dispersed in deionized water, H2O2 and a composite alkali are added, and depolymerization is performed to prepare a depolymerized lignin solution; the type of the lignin is selected from any one of sulfate lignin, sulfonate lignin, alkali lignin, and hydrolyzed lignin, the initial lignin molecular weight is 6K~8KDa, and the purity is ≥93%; the method for precipitating lignin of different molecular weights from the retained liquid is rotary evaporation at a temperature of 50~80°C.

2. The method for preparing sodium ion hard carbon negative electrode material by depolymerization and separation of lignin according to claim 1, characterized in that: The specific steps are as follows: (1) Dispersing lignin in deionized water, adding H2O2 and complex alkali, depolymerizing to obtain a depolymerized lignin solution; (2) The depolymerized lignin solution obtained in step (1) is separated by a membrane separation instrument according to the molecular weight cut-off size, and the lignin in different molecular weight ranges is dried and weighed, and then crushed, ball-milled and sieved to obtain a small molecular weight lignin powder; (3) The small molecular lignin powder obtained in step (2) is carbonized, washed with HCl aqueous solution and distilled water until neutral and dry, and placed in a tubular furnace in an argon atmosphere for high-temperature structural reforming to obtain lignin-based hard carbon.

3. The method for preparing sodium ion hard carbon negative electrode material by depolymerization and separation of lignin according to claim 2, characterized in that: The carbonization temperature in step (2) is 600-800° C., and the carbonization apparatus is a muffle furnace without any atmosphere and a tubular furnace with argon atmosphere.

4. The method for preparing a sodium ion hard carbon negative electrode material by depolymerization and separation of lignin according to claim 2, characterized in that: The high-temperature reforming temperature of step (3) is 1200-1600°C.

5. The method for preparing sodium ion hard carbon negative electrode material by depolymerization and separation of lignin according to claim 2, wherein small molecule lignin-based hard carbon is prepared.

6. Use of the depolymerized and separated lignin-based hard carbon according to claim 5 in sodium ion battery negative electrode materials.

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