A lignin-based sulfur-supporting substrate for lithium-sulfur batteries and a preparation method and application thereof

Abstract

The application provides a lignin-based sulfur-supporting material for a lithium-sulfur battery and a preparation method and application thereof. The preparation method of the lignin-based sulfur-supporting material for the lithium-sulfur battery is as follows: first, grafting an amine compound with high nitrogen content into a lignin molecule through a Mannich reaction to obtain an aminated lignin; then, compounding the aminated lignin with CNTs and a transition metal salt, and carrying out high-temperature activation treatment on the obtained composite material to obtain a hierarchical porous transition metal / nitrogen co-doped lignin carbon-carbon nanotube composite material. The obtained composite material is applied to a lithium-sulfur battery, so that the cycle performance and rate performance of the battery are both significantly improved.

Description

TECHNICAL FIELD

[0001] The present application relates to a lignin-based sulfur-supporting substrate for lithium-sulfur batteries and a preparation method and application thereof, and belongs to the technical field of high-value utilization of waste biomass and preparation of carbon material carriers. BACKGROUND

[0002] With the continuous development of society, people's consumption of energy is increasing, and traditional energy is becoming more and more scarce, and mankind urgently needs a new type of energy to replace traditional energy. At the same time, with the continuous expansion of the industrial scale, environmental pollution is becoming more and more serious, and environmental governance has become a major problem that people urgently need to solve at the moment. Lithium-sulfur batteries have a theoretical energy density of 2600wh·kg -1 , which is about 3-5 times that of commercial lithium-ion batteries, has the advantages of low cost and high specific capacity, and has the potential to become a substitute for the next generation of high-energy-density rechargeable batteries. As a new type of energy storage device, its development has great significance for solving the current energy shortage. However, lithium-sulfur batteries also have many technical problems, mainly in three aspects: first, the insulating nature of sulfur and polysulfides makes it a poor conductor as a positive electrode material; second, the intermediate product polysulfide in the sulfur discharge process is prone to "shuttle effect" between electrodes, resulting in a decrease in active material utilization; third, the volume of sulfur will expand after chemical reaction, with a volume expansion rate of about 80% during discharge, which will damage the original structure of the electrode material. These will lead to a decrease in the capacity of lithium-sulfur batteries, poor rate performance and cycle life.

[0003] In order to solve these problems, current research on the positive electrode mainly includes the following aspects: carbon material / sulfur composite, transition metal compound / sulfur composite, and conductive polymer / sulfur composite. Porous carbon has become one of the main development directions of lithium-sulfur battery positive electrode materials in recent years due to its high porosity, high electrical conductivity, ideal ion transmission channel, and physical adsorption capacity.

[0004] However, the physical interaction between non-polar carbon materials and polar polysulfides is weak, and the polysulfides cannot be effectively anchored and quickly converted during the cycle process, resulting in the dissolution of polysulfides in the electrolyte. At the same time, high porosity is usually accompanied by a less effective long-range conductive carbon network, resulting in poor overall conductivity of the sulfur / carbon composite. Studies have shown that the addition of non-metallic heteroatoms (including N, P, S, B, etc.), transition metals and transition metal compounds can form a strong adsorption effect on polysulfides, significantly improving the cycle performance and rate performance of lithium-sulfur batteries. Non-metallic heteroatoms, especially nitrogen doping, have been widely studied and applied. Nitrogen adsorption of polysulfides mainly occurs in soluble Li2S8 and high-order polysulfides (Li2S x,4≤x≤7) and the liquid-liquid single-phase reaction stage and the liquid-solid two-phase reaction stage of high-order polysulfides and Li2S2 or Li2S. The addition of nitrogen has three main effects. First, it promotes the chemical bonding between the carbon support and the sulfur chain during the sulfur loading process, allowing the sulfur to be uniformly distributed in the carbon carrier. Second, nitrogen itself has a lone electron, which can be considered as a Lewis base that strongly interacts with the Li + of polysulfides, thereby strongly adsorbing polysulfides. At the same time, the doping of nitrogen atoms contributes more electrons to the delocalized π bond, thereby improving the overall conductivity of the carbon material. In addition to metal heteroatoms, transition metals and transition metal compounds (such as Mn, Fe, Ni, Zn, Co, etc.) not only have strong chemical adsorption on polysulfides, but also have a certain catalytic effect on the conversion of polysulfides, accelerating the chemical reaction rate and thereby reducing the diffusion of high-concentration polysulfides.

[0005] Single-component materials often cannot meet the actual application requirements, and composite materials make up for the shortcomings of single materials due to the synergistic effect of multiple components, and have better performance. Carbon nanotubes have better long-range conductivity, and the overall conductivity of the material is better than that of porous carbon materials, and can form a long-distance continuous conductive network. At the same time, carbon nanotubes have high mechanical strength, which can effectively reduce the structure collapse of the positive electrode material caused by the volume expansion of sulfur, and as an additive added to the porous carbon can achieve good results.

[0006] Lignin is the only renewable biomass resource with aromatic structure in nature, and its reserves are abundant. It is mainly derived from black liquor produced in the papermaking process of the pulp and paper industry, which has a great pollution to the ecological environment. Due to its unique structural characteristics and abundant reserves, lignin has great development potential in the preparation of low-cost and environmentally friendly energy storage porous carbon materials. Using lignin as raw material to prepare high-performance electrode materials for energy storage devices provides a direction for solving the current energy crisis and environmental pollution, and realizes the high-value conversion of waste biomass. Lignin molecules contain active groups such as ketone groups, carboxyl groups, phenolic hydroxyl groups, methoxyl groups, aldehyde groups, and alcohol hydroxyl groups. These groups can be used for modification treatments such as nitration, etherification, carboxymethylation, sulfonation, quaternary ammonium modification, and amination modification, thereby expanding the application range of lignin.

[0007] Therefore, it is of great significance to develop high-performance positive electrode materials for lithium-sulfur batteries. SUMMARY

[0008] In view of the deficiencies of the prior art, the present application provides a lignin-based sulfur-supporting substrate for lithium-sulfur batteries and a preparation method and application thereof.The present application grafts an amine compound with high nitrogen content into a lignin molecule through a Mannich reaction to increase the content of nitrogen elements in the lignin molecule, increase the nitrogen content and uniform distribution in the porous carbon; meanwhile, the lignin carbon-carbon nanotube composite material is obtained through high-temperature activation treatment after being compounded with CNTs and transition metal salt, and the obtained composite material is applied to lithium-sulfur batteries, so that the cycle performance and rate performance of the batteries are significantly improved.

[0009] The present application is specifically realized through the following schemes:

[0010] A preparation method of a lignin-based sulfur-supporting substrate for lithium-sulfur batteries comprises the following steps:

[0011] (1) lignin is dissolved in an alkaline solution and heated to a reaction temperature; then an aqueous formaldehyde solution in which an amine compound is dissolved is continuously added under stirring to perform a reaction; after the reaction is completed, the pH of the system is adjusted to 5-7, and filtration, washing and freeze-drying are performed to obtain amine-modified lignin;

[0012] (2) a transition metal salt aqueous solution and a carbon nanotube aqueous dispersion are added to the amine-modified lignin aqueous dispersion, and after stirring, the solvent is removed by evaporation and freeze-drying is performed to obtain an amine-modified lignin / CNTs / transition metal salt composite material;

[0013] (3) the amine-modified lignin / CNTs / transition metal salt composite material obtained in step (2) is subjected to high-temperature calcination treatment, and then subjected to acid washing, deionized water washing and drying to obtain a transition metal / nitrogen co-doped lignin carbon-carbon nanotube composite material, which is the lignin-based sulfur-supporting substrate for lithium-sulfur batteries.

[0014] According to the present application, preferably, the lignin in step (1) is industrial lignin, including at least one of pine alkaline lignin, broad-leaved wood alkaline lignin, wheat straw alkaline lignin, bamboo pulp alkaline lignin, reed alkaline lignin, wood pulp alkaline lignin, cotton pulp alkaline lignin, corncob enzymatic hydrolysis lignin, sodium lignosulfonate and bagasse alkaline lignin, which are common commercially available products.

[0015] According to the present application, preferably, the pH of the alkaline solution in step (1) is 10-12, more preferably 11; the alkali is NaOH or KOH; and the ratio of the volume of the alkaline solution to the mass of the lignin is 10-30 mL:1 g.

[0016] According to the application, preferably, the amine compound in step (1) is melamine, dicyandiamide, urea or ethylenediamine; the volume-to-mass ratio of the aqueous formaldehyde solution to the amine compound is 10-20 mL:1 g; and the aqueous formaldehyde solution containing the amine compound is prepared by adding the amine compound into the aqueous formaldehyde solution at 80-90°C and stirring for 5-15 min.

[0017] According to the application, preferably, the mass ratio of the lignin, the amine compound and the formaldehyde in step (1) is 1:0.5-1.5:0.5-1.5.

[0018] According to the application, preferably, the reaction temperature in step (1) is 75-95°C; and the reaction time is 3-6 h, and more preferably 4 h.

[0019] According to the application, preferably, in step (1), 0.5-1 mol / L hydrochloric acid aqueous solution is used to adjust the pH of the system to 5-7.

[0020] According to the application, preferably, the washing in step (1) is washing with deionized water until the filtrate is neutral.

[0021] According to the application, preferably, the concentration of the aqueous amineated lignin dispersion in step (2) is 10-30 mg / mL.

[0022] According to the application, preferably, the transition metal salt in step (2) is at least one of FeCl3, NiCl2·6H2O and CoCl2·6H2O; the concentration of the transition metal salt aqueous solution is 0.1-0.3 g / mL; and the mass ratio of the amineated lignin to the transition metal salt is 1:0.5-2.

[0023] According to the application, preferably, the concentration of the aqueous carbon nanotube dispersion in step (2) is 1-3 mg / mL; the mass of the carbon nanotubes is 5-15% of the mass of the amineated lignin; and the aqueous carbon nanotube dispersion is prepared by adding the carbon nanotubes into deionized water and ultrasonic treatment for 3-6 h; the carbon nanotubes are single-walled or multi-walled carbon nanotubes, which are commercially available; the amount of the carbon nanotubes in the application should not be too much or too little; too little carbon nanotubes cannot play a good conductive and supporting role, and the compounding effect is not obvious; and too much carbon nanotubes will reduce the sulfur loading due to the small inner diameter of the carbon nanotubes.

[0024] According to the application, preferably, the stirring time in step (2) is 10-15 h; and the evaporation of the solvent is constant-temperature stirring evaporation, and the evaporation temperature is 70-90°C.

[0025] According to the application, preferably, the high-temperature calcination treatment in step (3) is carried out under a nitrogen atmosphere, and the nitrogen flow rate is 100-300 mL / min.

[0026] According to the application, preferably, the high-temperature calcination treatment in step (3) is carried out by first increasing the temperature to 200-300℃ at a temperature increasing rate of 5-10℃ / min, and then keeping the temperature for 30-40 min, and then continuing to increase the temperature to a carbonization temperature at a temperature increasing rate of 5-10℃ / min, and keeping the temperature for 2-3 h, wherein the carbonization temperature is 700-900℃.

[0027] According to the application, preferably, the acid pickling in step (3) is carried out by using a dilute hydrochloric acid aqueous solution with a concentration of 0.5-1 mol / L, and the acid pickling time is 5-7 h. The acid pickling time should not be too long or too short, and the concentration of the hydrochloric acid should not be too high or too low. If the acid pickling time is too short or the concentration of the hydrochloric acid is too low, a large amount of metal will remain and not be washed away, which will reduce the porosity of the carbon material. If the acid pickling time is too long or the concentration of the hydrochloric acid is too high, the transition metal anchored in the carbon layer will be washed away, so that the transition metal cannot play a role in adsorbing and catalyzing polysulfides. The deionized water washing is carried out until the filtrate is neutral. The drying is carried out at 55-65℃ for 10-15 h.

[0028] The application provides a lignin-based sulfur-supporting substrate for lithium-sulfur batteries, which is prepared by the above preparation method.

[0029] According to the application, the above lignin-based sulfur-supporting substrate for lithium-sulfur batteries is applied in a positive electrode of a lithium-sulfur battery.

[0030] Compared with the prior art, the application has the following advantages and beneficial effects:

[0031] 1. Common methods for preparing nitrogen-doped porous carbon materials generally involve physically mixing carbon and nitrogen sources through grinding or in a solvent system. However, physical methods often fail to achieve a fully uniform mixture, leading to uneven nitrogen distribution within the porous carbon. Furthermore, simple physical mixing can easily result in significant nitrogen loss during pyrolysis. This invention utilizes a chemical method to link the carbon and nitrogen sources through chemical bonds, resulting in a better uniform distribution of nitrogen. The chemical bonds also significantly reduce nitrogen loss during calcination. This invention grafts high-nitrogen-content amine compounds onto lignin molecules via the Mannich reaction, amination modifying the lignin and increasing its nitrogen content. This chemical method achieves uniform nitrogen distribution in the carbon material, enabling in-situ doping of carbon materials with heteroatoms. The nitrogen doping provides a strong adsorption effect on polysulfides, effectively reducing the impact of polysulfide shuttle effects. Using lignin as the main carbon source and activating it with transition metal salt activators, hierarchical porous carbon with high porosity and high specific surface area is obtained. At the same time, the addition of CNTs improves the overall conductivity and overall structural stability of the material, which significantly improves the cycle performance and rate performance of lithium-sulfur batteries.

[0032] 2. In this invention, transition metal salts are added. The transition metal salts act as both activators and dopants, achieving the dual functions of pore formation and transition metal doping. The hierarchical porous structure effectively stores sulfur molecules and has a certain physical adsorption effect on polysulfides. The loading of metal nanoparticles plays a strong role in the chemical adsorption and catalytic conversion of polysulfides, greatly improving the cycle performance and rate performance of the battery.

[0033] 3. The preparation process of this invention is simple and low in cost, which can help the development of the new energy and new materials field. At the same time, using lignin as the main carbon source, it realizes the high-value-added conversion of waste biomass. Attached Figure Description

[0034] Figure 1 The images show the FTIR spectra of the aminated lignin and unmodified lignin obtained in Example 1.

[0035] Figure 2 The image shows the XRD pattern of the lignin-based sulfur-supported substrate for lithium-sulfur batteries prepared in Example 1.

[0036] Figure 3 This is a SEM image of the lignin-based sulfur-supported substrate for lithium-sulfur batteries prepared in Example 1.

[0037] Figure 4 This is a TEM image of the lignin-based sulfur-supported substrate for lithium-sulfur batteries prepared in Example 1.

[0038] Figure 5The sulfur-supporting substrate prepared in Example 1 and Comparative Example 1 was applied to lithium-sulfur batteries for comparison of rate performance. DETAILED DESCRIPTION

[0039] The application will be further described in conjunction with the following examples without limiting the application to the examples.

[0040] The materials involved in the following examples and comparative examples can be obtained from commercial channels.

[0041] The lignin-based sulfur-supporting substrate prepared in the application can effectively alleviate the main problems of lithium-sulfur batteries and improve the cycle performance and rate performance of lithium-sulfur batteries. The prepared composite carbon material is named X / N-LPC / CNTs-Y-Z, X represents a transition metal, Y is m(CNTs) / m(aminated lignin), and Z represents the calcination temperature.

[0042] The lignin used in the examples and comparative examples is pine alkaline lignin.

[0043] Example 1

[0044] A preparation method of a lignin-based sulfur-supporting substrate for lithium-sulfur batteries, comprising the following steps:

[0045] (1) 2 g of lignin was taken in a 250 mL three-necked flask, 40 mL of NaOH aqueous solution with pH = 11 was added, and stirring was performed to obtain a lignin solution. The lignin solution was heated to 90℃ under stirring. 2 g of melamine was added to 24 mL of water containing 1.64 g of formaldehyde, and stirring was performed at 90℃ for 10 min. The obtained formaldehyde aqueous solution containing amine compounds was added to the lignin solution, and stirring reaction was performed at 90℃ for 4 h. After the reaction was completed, 0.5 mol / L dilute hydrochloric acid aqueous solution was added to adjust the pH of the system to 6. The precipitate was obtained by filtration, and the obtained precipitate was washed with deionized water until the filtrate was neutral. Then, the precipitate was freeze-dried at -40℃ for 24 h to obtain aminated lignin.

[0046] (2) 2 g of aminated lignin was taken in a 250 mL beaker, 100 mL of deionized water was added, and stirring was performed to obtain aminated lignin aqueous dispersion. 0.1 g of multi-walled carbon nanotubes was taken in a 100 mL beaker, 50 mL of deionized water was added, and ultrasonic treatment was performed for 3 h to obtain a CNTs dispersion. NiCl2 solution (2 g of NiCl2·6H2O was dissolved in 10 mL of deionized water) and the CNTs dispersion were sequentially added dropwise to the aminated lignin dispersion, and the dropwise addition time was 3 min. Then, stirring was performed for 12 h. The solvent was evaporated by water bath heating at 80℃, and freeze-drying was performed at -40℃ for 24 h to obtain aminated lignin / CNTs / NiCl2·6H2O composite material.

[0047] (3) The amine-modified lignin / CNTs / NiCl2·6H2O composite obtained in step (2) is placed in a porcelain boat, and subjected to calcination treatment in a tube furnace under a N2 atmosphere, with a N2 flow rate of 200 mL / min, and the temperature is raised at a rate of 10°C / min to 250°C, maintained for 30 min, then raised at a rate of 10°C / min to 900°C, maintained for 2 h, and then naturally cooled to room temperature. The calcination product is subjected to acid washing treatment with a 1 mol / L hydrochloric acid solution for 6 h, filtered, washed with deionized water until the filtrate is neutral, and dried at 60°C for 12 h to obtain a hierarchical porous Ni / N-LPC / CNTs-5%-900 composite, which is a lignin-based sulfur carrier for lithium-sulfur batteries.

[0048] The FTIR spectra of the amine-modified lignin and unmodified lignin obtained in this example are shown in Figure 1 From Figure 1 it can be seen that the modified lignin has a clear vibration peak at 3380 cm -1 , which belongs to the N-H stretching vibration peak, and is significantly stronger than that of unmodified lignin. There is a clear vibration peak at 1565 cm -1 , which belongs to the C=N stretching vibration peak, indicating that melamine is well grafted into lignin. The elemental analysis test results show that the nitrogen content of Ni / N-LPC / CNTs-5%-900 is 6.54%, which has a relatively high nitrogen content.

[0049] The XRD spectrum of the lignin-based sulfur carrier for lithium-sulfur batteries obtained in this example is shown in Figure 2 From Figure 2 it can be seen that Ni / N-LPC / CNTs-5%-900 clearly observes the characteristic peaks of metallic nickel at 44.57° and 52.87°, 76.41°, indicating that nickel metal is successfully doped into the porous carbon.

[0050] The SEM spectrum of the lignin-based sulfur carrier for lithium-sulfur batteries obtained in this example is shown in Figure 3 It can be seen that the material prepared has a rich porous structure, which is beneficial to the storage of sulfur and plays a certain hindering effect on the dissolution of polysulfides.

[0051] The TEM spectrum of the lignin-based sulfur carrier for lithium-sulfur batteries obtained in this example is shown in Figure 4 It can be seen that the carbon nanotubes are uniformly distributed inside the porous carbon, forming a stable three-dimensional conductive network structure.

[0052] Example 2

[0053] A method for preparing a lignin-based sulfur carrier for lithium-sulfur batteries, comprising the following steps:

[0054] (1) Take 2 g of lignin into a 250 mL three-necked flask, add 40 mL of NaOH aqueous solution with pH = 11, stir to obtain a lignin solution, and heat to 90°C under stirring; add 2 g of dicyandiamide to 23.5 mL of water containing 1.43 g of formaldehyde, stir at 90°C for 10 min, add the obtained formaldehyde aqueous solution containing amine compounds to the lignin solution, and stir at 90°C for 4 h; after the reaction is completed, add 0.5 mol / L dilute hydrochloric acid aqueous solution to adjust the pH of the system to 6, filter to obtain a precipitate, wash the obtained precipitate with deionized water until the filtrate is neutral, and freeze-dry at -40°C for 24 h to obtain amine-modified lignin.

[0055] (2) Take 2 g of amine-modified lignin into a 250 mL beaker, add 100 mL of deionized water and stir to obtain an amine-modified lignin aqueous dispersion; take 0.2 g of multi-walled carbon nanotubes into a 100 mL beaker, add 100 mL of deionized water, and ultrasonically treat for 3 h to obtain a CNTs dispersion; slowly add FeCl3 solution (2 g of FeCl3 dissolved in 10 mL of deionized water) and the CNTs dispersion to the lignin dispersion in sequence, and the dropping time is 3 min; then stir for 12 h; evaporate the solvent in a water bath at 80°C, and freeze-dry at -40°C for 24 h to obtain an amine-modified lignin / CNTs / FeCl3 composite material.

[0056] (3) Place the amine-modified lignin / CNTs / FeCl3 composite material obtained in step (2) in a porcelain boat, and perform calcination treatment in a tube furnace under N2 atmosphere, with an N2 flow rate of 200 mL / min, and the temperature rising steps are as follows: rise to 250°C at a rate of 10°C / min from room temperature, keep for 30 min, then rise to 800°C at a rate of 10°C / min, keep for 2 h, then naturally cool to room temperature; use 1 mol / L hydrochloric acid solution to perform acid washing treatment on the calcined product for 6 h, filter, wash the obtained solid with deionized water until the filtrate is neutral, and dry at 60°C for 12 h to obtain a hierarchical porous Fe / N-LPC / CNTs-10%-800 composite material, which is a lignin-based sulfur-supporting substrate for lithium-sulfur batteries.

[0057] Example 3

[0058] A preparation method of a lignin-based sulfur-supporting substrate for lithium-sulfur batteries, comprising the following steps:

[0059] (1) Take 2 g of lignin into a 250 mL three-necked flask, add 40 mL of NaOH aqueous solution with pH = 11, stir to obtain a lignin solution, and heat to 90°C under stirring; add 2 g of urea to 25 mL of water containing 2.05 g of formaldehyde, stir at 90°C for 10 min, add the obtained formaldehyde aqueous solution containing amine compounds to the lignin solution, and stir at 90°C for 4 h; after the reaction is completed, 0.5 mol / L dilute hydrochloric acid aqueous solution is added to adjust the pH of the system to 6, and the precipitate is obtained by filtration, and the obtained precipitate is washed with deionized water until the filtrate is neutral, and then freeze-dried at -40°C for 24 h to obtain amine-modified lignin.

[0060] (2) Take 2 g of amine-modified lignin into a 250 mL beaker, add 100 mL of deionized water and stir to obtain an amine-modified lignin aqueous dispersion; take 0.1 g of multi-walled carbon nanotubes into a 100 mL beaker, add 50 mL of deionized water, and ultrasonic treat for 3 h to obtain a CNTs dispersion; add CoCl2 solution (2 g of CoCl2·6H2O dissolved in 10 mL of deionized water) and CNTs dispersion to the lignin dispersion in sequence, and drop for 3 min, and then stir for 12 h; evaporate the solvent in a water bath at 80°C, and freeze-dry at -40°C for 24 h to obtain an amine-modified lignin / CNTs / CoCl2·6H2O composite material.

[0061] (3) Place the amine-modified lignin / CNTs / CoCl2·6H2O composite material obtained in step (2) in a porcelain boat, and perform calcination treatment in a tube furnace under N2 atmosphere, with a N2 flow rate of 200 mL / min, and a temperature rising step of: rising to 250°C at a rate of 10°C / min from room temperature, holding for 30 min, then rising to 900°C at a rate of 10°C / min, holding for 2 h, and then naturally cooling to room temperature; perform acid washing treatment on the calcined product using 1 mol / L hydrochloric acid solution for 6 h, filter, wash the obtained solid with deionized water until the filtrate is neutral, and dry at 60°C for 12 h to obtain a hierarchical porous Co / N-LPC / CNTs-5%-900 composite material, which is a lignin-based sulfur-supporting substrate for lithium-sulfur batteries.

[0062] Comparative Example 1

[0063] A preparation method of a lignin-based sulfur-supporting substrate for lithium-sulfur batteries, comprising the following steps:

[0064] (1) Take 2 g of lignin in a 250 mL beaker, add 100 mL of deionized water and stir to obtain a lignin water dispersion; take 0.1 g of multi-walled carbon nanotubes in a 100 mL beaker, add 50 mL of deionized water and ultrasonic treatment for 3 h to obtain a CNTs dispersion; add NiCl2 solution (2 g of NiCl2·6H2O dissolved in 10 mL of deionized water) and CNTs dispersion to the lignin dispersion in sequence, and drop for 3 min each time, then stir for 12 h; evaporate the solvent in a water bath at 80°C, and freeze-dry at -40°C for 24 h to obtain a lignin / CNTs / NiCl2·6H2O composite material.

[0065] (2) Place the lignin / CNTs / NiCl2·6H2O composite material obtained in step (1) in a porcelain boat, and perform calcination treatment in a tube furnace under N2 atmosphere, with an N2 flow rate of 200 mL / min, and the temperature rising steps being: rising to 250°C at 10°C / min from room temperature, keeping for 30 min, then rising to 900°C at 10°C / min, keeping for 2 h, then naturally cooling to room temperature; use 1 mol / L hydrochloric acid solution to perform acid washing treatment on the calcined product for 6 h, filter, wash the obtained solid with deionized water until the filtrate is neutral, and dry at 60°C for 12 h to obtain a hierarchical porous Ni-LPC / CNTs-5%-900 composite material, which is a lignin-based sulfur carrier for lithium-sulfur batteries.

[0066] In the present comparative example, the lignin is not modified by amination.

[0067] Comparative Example 2

[0068] A preparation method of a lignin-based sulfur carrier for lithium-sulfur batteries, comprising the following steps:

[0069] (1) Take 2 g of lignin in a 250 mL three-necked flask, add 40 mL of NaOH aqueous solution with pH = 11, and stir to obtain a lignin solution, and heat to 90°C under stirring; add 2 g of melamine to 24 mL of water containing 1.64 g of formaldehyde, stir at 90°C for 10 min, add the obtained formaldehyde aqueous solution containing amine compounds to the lignin solution, and stir at 90°C for 4 h; after the reaction is completed, add 0.5 mol / L dilute hydrochloric acid aqueous solution to adjust the pH of the system to 6, filter to obtain a precipitate, wash the obtained precipitate with deionized water until the filtrate is neutral, and then freeze-dry at -40°C for 24 h to obtain aminated lignin.

[0070] (2) Take 2 g of aminated lignin in a 250 mL beaker, add 100 mL of deionized water and stir to obtain an aqueous dispersion of aminated lignin; add a NiCl2solution (2 g of NiCl2·6H2O dissolved in 10 mL of deionized water) dropwise to the aminated lignin dispersion, the dropwise addition time being 3 min, and then stir for 12 h; evaporate the solvent in a water bath at 80°C, and freeze-dry at -40°C for 24 h to obtain an aminated lignin / NiCl2·6H2O composite material.

[0071] (3) Place the aminated lignin / NiCl2·6H2O composite material obtained in step (2) in a porcelain boat, and perform calcination treatment in a tube furnace under a N2atmosphere, the N2flow rate being 200 mL / min, and the temperature rising steps being: rising to 250°C at a rate of 10°C / min from room temperature, holding for 30 min, then rising to 900°C at a rate of 10°C / min, holding for 2 h, and then naturally cooling to room temperature; perform acid washing of the calcination product using a 1 mol / L hydrochloric acid solution for 6 h, filter, wash the obtained solid with deionized water until the filtrate is neutral, and dry at 60°C for 12 h to obtain a hierarchical porous Ni / N-LPC-900 composite material, which is a lignin-based sulfur-supporting substrate for lithium-sulfur batteries.

[0072] No CNTs are added as additives in the present comparative example.

[0073] Test Example 1

[0074] The lignin-based sulfur-supporting substrates prepared in Examples 1-3 and Comparative Examples 1-2 are used to prepare lithium-sulfur battery positive electrode materials, and the method used is one of the conventional methods for preparing lithium-sulfur battery positive electrode sheets, which is described simply here.

[0075] The prepared sulfur-supporting substrate is ground and mixed with sublimed sulfur at a mass ratio of 3:7, and placed in a 25 mL reaction kettle (the gas in the reaction kettle is replaced with argon gas in a glove box), and heated at 155°C for 12 h to obtain a lithium-sulfur battery positive electrode material; the positive electrode sheet is prepared according to the conventional method in the art.

[0076] CR2032 batteries are assembled in a glove box, and the assembled CR2032 batteries are subjected to constant current charge / discharge performance tests in a Neware battery performance test system at a current density of 1C in a voltage range of 1.7-2.8 V, and the cycle performance of the test batteries at different current densities is tested, and the results are shown in Table 1.

[0077] Table 1 Comparison of cycle performance of lithium-sulfur batteries using the sulfur-supporting substrates prepared in Examples 1-3 and Comparative Examples 1-2

[0078]

[0079] As can be seen from Table 1, the sulfur-loaded substrate prepared in the embodiment of the present application is applied to a lithium-sulfur battery, and the obtained lithium-sulfur battery has a higher initial specific discharge capacity, and the discharge capacity retention rate is above 77% after 400 cycles at a 1C discharge density.

[0080] By Figure 5 As can be seen from the comparison of the rate performance of Example 1 and Comparative Example 1, the discharge specific capacity of Example 1 is better than that of Comparative Example 1 at different discharge currents, and the coulombic efficiency is above 97%.

[0081] The above embodiments are preferred embodiments of the present application, but the embodiments of the present application are not limited to the above embodiments, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application shall be equivalent replacement modes and shall be included in the protection scope of the present application.

Claims

1. A method for preparing a lignin-based sulfur-supporting substrate for lithium-sulfur batteries, comprising the following steps: (1) dissolving lignin in an alkaline solution and heating to a reaction temperature; then adding an aqueous formaldehyde solution in which an amine compound is dissolved under stirring to carry out a reaction; after the reaction is completed, adjusting the pH of the system to 5-7, and then filtering, washing, and freeze-drying to obtain an aminated lignin; the amine compound is melamine, dicyandiamide, urea, or ethylenediamine; the mass ratio of lignin, amine compound, and formaldehyde is 1:0.5-1.5:0.5-1.5; (2) adding an aqueous transition metal salt solution and an aqueous carbon nanotube dispersion to the aqueous dispersion of the aminated lignin, stirring, then evaporating the solvent, and freeze-drying to obtain an aminated lignin / CNTs / transition metal salt composite material; the transition metal salt is at least one of FeCl3, NiCl2·6H2O, and CoCl2·6H2O; the mass ratio of the aminated lignin to the transition metal salt is 1:0.5-2; the mass of the carbon nanotubes is 5-15% of the mass of the aminated lignin; (3) subjecting the aminated lignin / CNTs / transition metal salt composite material obtained in step (2) to high-temperature calcination treatment, then acid washing, deionized water washing, and drying to obtain a transition metal / nitrogen co-doped lignin carbon-carbon nanotube composite material, which is the lignin-based sulfur-supporting substrate for lithium-sulfur batteries; the high-temperature calcination treatment process is as follows: first, heating at a heating rate of 5-10℃ / min to 200-300℃, and maintaining the temperature for 30-40 min; then, continuing to heat at a heating rate of 5-10℃ / min to a carbonization temperature, and maintaining the temperature for 2-3 h, the carbonization temperature being 700-900℃. In step (1), the pH of the alkaline solution is 10-12; the base is NaOH or KOH; the volume ratio of the alkaline solution to the mass of lignin is 10-30 mL:1 g. In step (1), the volume ratio of the aqueous formaldehyde solution to the mass of the amine compound is 10-20 mL:1 g; the aqueous formaldehyde solution in which the amine compound is dissolved is prepared by the following method: adding the amine compound to the aqueous formaldehyde solution at 80-90℃, and stirring for 5-15 min. In step (1), the reaction temperature is 75-95℃; the reaction time is 3-6 h; in step (1), the pH of the system is adjusted to 5-7 using a 0.5-1 mol / L aqueous hydrochloric acid solution; the washing is deionized water washing until the filtrate is neutral. In step (2), the concentration of the aqueous dispersion of the aminated lignin is 10-30 mg / mL; the concentration of the aqueous transition metal salt solution is 0.1-0.3 g / mL. In step (2), the concentration of the aqueous carbon nanotube dispersion is 1-3 mg / mL; the stirring time is 10-15 h; the evaporation of the solvent is constant-temperature stirring evaporation, and the evaporation temperature is 70-90℃. In step (3), the high-temperature calcination treatment is carried out under a nitrogen atmosphere, and the nitrogen flow rate is 100-300 mL / min. ​ ​ ​ 2. The method for preparing the lignin-based sulfur-supported substrate for lithium-sulfur batteries according to claim 1, characterized in that, ​ 3. The method for preparing the lignin-based sulfur-supported substrate for lithium-sulfur batteries according to claim 1, characterized in that, ​ 4. The method for preparing the lignin-based sulfur-supported substrate for lithium-sulfur batteries according to claim 1, characterized in that, ​ 5. The method for preparing the lignin-based sulfur-supported substrate for lithium-sulfur batteries according to claim 1, characterized in that, ​ 6. The method for preparing the lignin-based sulfur-supported substrate for lithium-sulfur batteries according to claim 1, characterized in that, ​ 7. The method for preparing the lignin-based sulfur-supported substrate for lithium-sulfur batteries according to claim 1, characterized in that, ​ The pickling is carried out by using a dilute hydrochloric acid aqueous solution with a concentration of 0.5-1 mol / L, and the pickling time is 5-7 h; the deionized water washing is carried out until the filtrate is neutral; and the drying is carried out at 55-65 ℃ for 10-15 h.

8. A lignin-based sulfur-supporting substrate for lithium-sulfur batteries, characterized in that, The lithium-sulfur battery is prepared by using the preparation method of claim 1.

9. The lithium-sulfur battery lignin-based sulfur-supporting material of claim 8 is applied in a lithium-sulfur battery positive electrode.

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