CN116040627A - Porous carbon, composite material, diaphragm modification material and application - Google Patents

Abstract

The invention belongs to the field of energy storage materials, and in particular relates to porous carbon, a composite material, a diaphragm modification material and application thereof, wherein the porous carbon is prepared from agricultural wastes through a carbonization-activation method, and metal sulfide particles are compounded with the porous carbon through a load-annealing mode. The membrane modification layer prepared by coating the obtained material on the surface of a commercial membrane can inhibit the shuttle effect of polysulfide in a lithium sulfur and sodium sulfur battery through a capture-catalytic conversion mechanism, so that the service life of the battery is prolonged; in lithium ion batteries, lithium metal batteries and sodium ion batteries, the wettability of the electrolyte can be improved, the ion flux is uniform, and the generation of dendrites to pierce through a diaphragm is effectively prevented. The invention has simple operation, can realize the disposal and high-value utilization of agricultural wastes, and provides a new idea for the disposal of organic solid wastes and the research and development of high-performance secondary batteries.

Description

A kind of porous carbon, composite material, diaphragm modification material and application

technical field

The invention belongs to the field of energy storage materials, and specifically relates to a porous carbon, a composite material, a diaphragm modification material and an application thereof.

Background technique

The organic materials discarded in the process of agricultural production are called agricultural waste. It is estimated that the amount of agricultural waste in China is increasing at a rate of 5-10% every year, and the annual output of straw-like agricultural waste is about 900 million tons, of which about 200 million tons are not used; the annual production of livestock and poultry waste The yields of nitrogen and phosphorus were 1.229×10 ⁷ t and 2.046×10 ⁶ t, respectively, but the comprehensive utilization rate was lower than 60%, and most of them were directly discarded and burned on site, destroying the ecological environment.

Alkali metal-sulfur batteries (lithium-sulfur batteries, sodium-sulfur batteries) have attracted much attention due to their high theoretical capacity and high energy density. However, the shuttle effect of long-chain polysulfide ions and poor redox reaction kinetics during charge and discharge hinder the commercial development of their batteries. In addition, the pore size (50-500nm) of the current commercial polypropylene separator is much larger than the kinetic diameter of polysulfide ions (>1.6nm), which cannot effectively suppress the "shuttle effect", which in turn affects the long-term cycle stability and rate performance of the sulfur cathode. .

Contents of the invention

According to the deficiencies of the prior art above, the present invention provides a porous carbon, a composite material, a diaphragm modification material and its application, which effectively opens up a new approach and solution for the subsequent treatment of agricultural waste, and can be expanded to produce The porous carbon has rich surface active functional groups and has the characteristics of metal-heteroatom co-doping. By further loading metal sulfides and using them as separator modification materials, the performance of various types of secondary batteries can be generally improved. In addition, using the scheme of constructing a diaphragm modification layer can improve the problem of rapid capacity fading caused by the "shuttle effect" in lithium-sulfur and sodium-sulfur batteries, as well as the battery safety problems caused by dendrite growth in other secondary batteries, which has a good ecological benefits and economic benefits.

Through a lot of research, 1) the inventors found that by selecting different activators and optimizing technical parameters such as treatment atmosphere, treatment temperature, and feed ratio, the carbonization-activation strategy can be used to convert agricultural waste into a specific surface area of 800-3500m ² / g's porous carbon, whose hierarchical porous structure can promote the rapid transport of electrolytes and ions, and use physical interactions such as electrostatic adsorption to block the shuttle of polysulfide ions. In addition, it has a large specific surface area and is an ideal host for highly catalytically active catalysts, which can effectively inhibit the deactivation of the catalyst due to agglomeration or adsorption of a large amount of polysulfide ions, and provide a large reaction interface for the conversion of polysulfide ions, effectively Anchoring dissolved polysulfide ions to accelerate their catalytic conversion efficiency. And it has a wide application value in the fields of catalysis, energy storage, and environmental restoration. 2) Not only that, the inventors found that after heteroatom doping and transition metal sulfide loading on the obtained porous carbon material, the high-value utilization of resources can be realized at the same time, and the performance of the alkali metal sulfur battery can be improved. It has been further improved and has great development advantages. It can be used to prepare separator modification layer materials with good electrochemical properties, accelerate the charge and discharge reaction kinetics, and increase the battery capacity. 3) The diaphragm modification layer formed after the application of the diaphragm modification material is rich in surface pores and functional groups, has good wettability with the electrolyte, and has good ion conduction on the interface, which can effectively uniform the ion flux and reduce the formation of dendrites. generate. At the same time, because the diaphragm modification layer has a certain rigidity compared with the polymer diaphragm, it can effectively avoid the battery short circuit caused by the dendrite piercing the diaphragm, so it is used in other types of secondary batteries (such as lithium-ion batteries, lithium metal batteries, Sodium-ion batteries) also have great application potential in the field.

The present invention specifically provides a porous carbon, which uses agricultural waste as a biomass material, mixes the biomass material with an activator, puts it in a reaction vessel, and conducts activation heat treatment under the protection of an inert gas to obtain the porous carbon;

Wherein, the agricultural waste is at least one of silkworm excrement, basswood, fungus residue, orange peel, orange peel, wheat straw, rice straw, corn straw, miscanthus, and ramie, preferably silkworm excrement;

The activator is at least one of zinc chloride, phosphoric acid, potassium ferrate, potassium hydroxide, ferric chloride, ferric nitrate, and water vapor, preferably potassium hydroxide.

As a preferred solution, the mass ratio of the biomass material to the activator is 1:0.1-6, preferably 1:4; in addition, when the activator is potassium ferrate, the concentration of the potassium ferrate solution is 0.01-0.4M , preferably 0.3M; when the activator is zinc chloride, the concentration of the zinc chloride solution is 1-10M, preferably 5M; when the activator is phosphoric acid, the concentration of phosphoric acid is 1-85wt%, preferably 50wt% .

The inert gas is nitrogen, argon or hydrogen-argon mixed gas, preferably argon, and the flow rate of the inert gas is 0.1-2L/min, preferably 1L/min; the temperature of the activation heat treatment is 200-1500°C, preferably 800°C, the heating rate is 1-20°C/min, preferably 3°C/min, and the activation heat treatment time is 0.5-10h, preferably 3h.

As a preferred solution, the agricultural waste needs to be pretreated, and the pretreatment steps are cleaning and drying;

The cleaning is at least one of water washing, hydrochloric acid washing, sulfuric acid washing, nitric acid washing, hydrofluoric acid pickling, sodium hydroxide washing, potassium hydroxide alkali washing, ethanol washing, preferably pickling, more preferably hydrochloric acid washing ;

The drying temperature is 40-150°C, preferably 80°C, and the drying time is 6-72h, preferably 48h.

As a preferred solution, depending on the type of agricultural waste, for some larger particle sizes, the dried agricultural waste needs to be further pulverized and sieved, and the particle size of the pulverized powder is 50-400 mesh, preferably 100 head. And for some agricultural wastes with smaller particle size, such as silkworm excrement, fungal residue, etc., there is no need to crush and sieve.

As a preferred solution, according to the selection of the type of activator, when the activator is easy to react with the impurities (calcium, magnesium, silicon, etc.) coexisting in the biomass material, such as potassium hydroxide, potassium ferrate, etc. The pretreated agricultural waste also needs to be pre-carbonized and heat-treated under the protection of an inert gas before being mixed with the activator; wherein, the inert gas is nitrogen, argon or hydrogen-argon mixed gas, preferably argon; inert gas The gas flow rate is 0.1-1L/min, preferably 1L/min; the temperature of the pre-carbonization heat treatment is 200-1500°C, preferably 500°C, and the heating rate is 1-20°C/min, preferably 5°C/min , the pre-carbonization heat treatment time is 0.5-10h, preferably 1h.

The porous carbon provided by the present invention, the carbonization-activation process, and further synergistic control of the heat treatment process (such as atmosphere, heating rate, treatment temperature, treatment time) and the type and proportion of the activator can be adjusted and controlled. The morphology of the obtained precursor and the specific surface area of the material can be improved to improve its electrochemical performance. Biomass porous carbon with different physicochemical characteristics can be obtained by changing the type of biomass to be treated by utilizing its natural morphology and elemental composition. The present inventors found that in the system of the present invention, the specific surface area of the obtained porous carbon can be significantly increased by using an activator, and the specific surface area of the obtained porous carbon is distributed between 800-3500m ² /g. Reaction characteristics can convert biomass into oriented carbon lattices that can be combined with various endogenous non-carbon elements in biomass during heat treatment or modified on the surface of porous carbon in the form of functional groups, which can be used for catalysis in subsequent applications , The adsorption process provides abundant active sites. By further loading sulfide on porous carbon, its application potential in lithium-sulfur battery catalytic materials can be enhanced.

The present invention also provides a composite material. The above-mentioned porous carbon and sulfide are mixed in a solvent, then the mixture is vacuum filtered, the filter residue is taken and dried, and annealing is performed under the protection of an inert gas to obtain porous carbon. - Sulfide composites.

As a preferred version, the sulfide is cobalt disulfide, sodium sulfide, potassium sulfide, zinc sulfide, magnesium sulfide, ferrous sulfide, manganese sulfide, lead sulfide, cadmium sulfide, antimony sulfide, bismuth sulfide, molybdenum sulfide, stannous sulfide , silver sulfide, copper sulfide, nickel sulfide, calcium sulfide, strontium sulfide, barium sulfide, preferably cobalt disulfide; the solvent is ethanol, ethylene glycol, dimethylformamide, ethylene glycol monomethyl ether, butanol , octanol or octyl acetate; the inert gas is nitrogen, argon or hydrogen-argon mixed gas, preferably argon, and the gas flow rate is 0.1-1L/min, preferably 1L/min; the temperature of the annealing treatment is 200-1500°C, preferably 600°C, the heating rate is 1-20°C/min, preferably 5°C/min, and the annealing treatment time is 0.5-10h, preferably 2h.

In the process of mixing the sulfide with the porous carbon, the sulfide can be directly selected from the above-mentioned sulfide, or the sulfide can be prepared by mixing and heat-treating a sulfur source and a transition metal salt in a high-boiling solvent. The sulfur source is sulfur powder, sulfur Urea or cysteine, preferably thiourea; the transition metal salt is a cobalt source, and the cobalt source is cobalt chloride, cobalt nitrate or cobalt acetate, preferably cobalt chloride; the high boiling point solvent is ethylene dichloride Alcohol, dimethylformamide, ethylene glycol monomethyl ether, butanol, octanol or octyl acetate, preferably ethylene glycol; the heat treatment temperature is 80-300°C, preferably 150°C, and the reaction time is 12 -24h, preferably 12h, the heating method is hydrothermal, water bath or oil bath, preferably oil bath; the stirring method is magnetic stirring, and the stirring speed is 0-1500rpm, preferably 300rpm.

The invention also provides a diaphragm modification material, which is prepared by mixing the composite material with a conductive agent, a binder, and a solvent, and grinding it into a slurry to obtain the diaphragm modification material.

As a preferred version, the conductive agent is at least one of Super-P, superconducting carbon black, acetylene black, Ketjen black, preferably Super-P; the binder is polyvinylidene fluoride (PVDF), At least one of sodium carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), preferably polyvinylidene fluoride; the solvent is at least one of ultrapure water, N-methylpyrrolidone (NMP), ethanol One, preferably N-methylpyrrolidone; the mass ratio of the composite material to the conductive agent and the binding agent is 8:1:1, and the quality of the solvent is the total of the composite material, the conductive agent and the binding agent. 1-10 times of mass, preferably 5 times.

The present invention also provides an application of a diaphragm modification material. The diaphragm modification material is compounded on the diaphragm surface and dried to obtain a diaphragm modification layer. The diaphragm modification layer is applied to a secondary battery, and the secondary battery is mainly a lithium-sulfur battery, a sodium-sulfur battery Batteries, and other types of lithium ion batteries, lithium metal batteries, sodium ion batteries, etc.; the operating temperature of the secondary battery is -20-80°C, preferably 25°C.

Wherein, the material of the diaphragm is polyethylene, polypropylene, glass fiber or cellulose, preferably polypropylene, the number of layers of the diaphragm is 1-3 layers, preferably 1 layer, and the diameter of the diaphragm is 10-50mm, preferably 12-19mm, most preferably 19mm; the composite process of the diaphragm modification material and the diaphragm is scraping coating, spray coating, spin coating, preferably scraping coating; the drying method is vacuum drying, freeze drying or blast drying, preferably blast drying Drying, the drying temperature is 25-200°C, preferably 50°C; the thickness of the membrane modification material after drying is 1-100 μm, preferably 15 μm.

The present invention has the following advantages:

(1) The present invention can realize high-value utilization of agricultural waste, solve environmental pollution problems derived from improper disposal methods, and generate considerable economic benefits. The existing agricultural waste conversion products have disadvantages such as low added value and poor quality. However, the present invention converts agricultural waste into porous carbon with a high specific surface area and uses it as a catalyst carrier, so that the performance advantages of the biomass itself can be effectively utilized and have greater economic benefits.

(2) The preparation process of the porous carbon of the present invention is simple. Compared with catalyst supports such as traditional graphene and carbon nanotubes, the porous carbon prepared by this method has a significant cost advantage. The one-step activation method of the substance can be obtained directly, which has the potential to expand the production. After loading sulfide, its catalytic performance is further improved.

(3) The porous carbon prepared by carbonization-activation two-step method or direct activation of biomass is characterized by high specific surface area, and the doping of original non-carbon elements improves its intrinsic adsorption capacity and catalytic activity. During the heat treatment of loaded sulfide, its high specific surface area provides a good place for crystal growth, improving its dispersion and catalytic activity. Effectively inhibit catalyst deactivation due to agglomeration or adsorption of a large amount of polysulfide ions, provide a larger reaction interface for the conversion of polysulfide ions, and effectively anchor dissolved polysulfide ions to accelerate their catalytic conversion efficiency.

(4) Compared with other types of supports, the porous carbon also has a greater contribution to the anchoring and transformation of polysulfides. First, during the pyrolysis process, the degree of graphitization of porous carbon increases, which is beneficial to Improve the defect of poor conductivity of elemental sulfur and its discharge products. Second, biomass itself has a large amount of non-carbon elements, which can assist in the construction of the beneficial structure of porous carbon during the carbonization-activation process. Polar heteroatoms are mainly conducive to improving the catalytic performance of porous carbon, and metal salts are mainly helpful to improve the catalytic properties of porous carbon. degree of graphitization. Third, during the activation process, a large number of micropores, mesopores, and macropores are formed on the surface of the carbon material, which is conducive to the physical fixation of polysulfides and the rapid transport of ions. Fourth, the modified layer of the separator has a large specific surface area, rich active sites, light weight, and will not significantly increase the total mass of the battery.

(5) After the porous carbon loaded with metal sulfide prepared by the present invention is mixed with a conductive agent, a binder, and a solvent, a diaphragm modification layer can be obtained by simply grinding and coating a commercial diaphragm, which is suitable for large quantities Production, combined with its stabilizing effect on the negative electrode of the battery, the application field can be further extended to lithium-ion batteries, sodium-ion batteries, other types of lithium metal batteries, etc.

Description of drawings

Fig. 1 is the transmission electron micrograph of the bacterium residue derived porous carbon prepared by carbonization-potassium hydroxide activation method;

Fig. 2 is the transmission electron micrograph of the silkworm excrement derived porous carbon prepared by carbonization-potassium hydroxide activation method;

Figure 3 is a scanning electron microscope image of silkworm excrement derived porous carbon loaded with cobalt sulfide;

Figure 4 shows the charge and discharge curves of lithium-sulfur batteries assembled with different diaphragms at 0.2C;

Figure 5 is a long-term cycle diagram of lithium-sulfur batteries assembled with different separators at 0.2C;

Figure 6 is a constant current charge and discharge curve of lithium || lithium symmetric batteries assembled with different separators at a current density of 0.5 mA/cm ² .

Fig. 7 is a nitrogen adsorption-desorption isotherm diagram of fungal residue carbon activated by 0.3M potassium ferrate.

Detailed ways

The present invention is further described below in conjunction with embodiment and accompanying drawing.

Example 1:

The agricultural and forestry waste described in this embodiment is silkworm excrement, and the selected silkworm excrement is only one kind of agricultural waste. If there are requirements for the structure and elemental composition of the biomass itself, the corresponding agricultural waste can be selected. The cobalt dichloride described in this example is only one of the supported substances, and other types of catalysts can be supported on the porous carbon carrier according to the characteristics of the reaction to be catalyzed. The method of this embodiment can also be applied to the disposal of other organic solid wastes, and the application fields of its products can also be extended to the development of other types of secondary battery devices and non-energy storage fields that require carbon materials.

Constructing a separator to modify the interlayer is considered to be an effective way to suppress the shuttle effect, and cobalt disulfide is a catalytic species widely used in the interlayer, which has a strong affinity with polysulfides. However, the poor conductivity of cobalt disulfide (6.7×10 ³ S/cm, 300K) makes it unable to effectively mediate the electron transfer process during the redox reaction of the sulfur cathode, and cobalt disulfide is easy to agglomerate during charge and discharge. Phenomenon, its catalytic activity decreases significantly with the increase of the number of cycles.

In order to improve the above problems, the present embodiment takes the following measures: the silkworm excrement biomass raw material is cleaned with ultrapure water to remove excess impurities on the surface, the dried biomass raw material is placed in a tube furnace under an argon atmosphere, Heat to 500°C at a heating rate of 5°C/min, and hold for 1h. After the material is cooled to room temperature, take the carbonized silkworm sand and potassium hydroxide powder (mass ratio = 1:4) and grind them evenly in a mortar to mix them well, then place them in a tube furnace at 3°C/min Under Ar atmosphere, the temperature was raised to 800°C and kept for 3h. Transfer the cooled product to a beaker, and slowly add dilute hydrochloric acid dropwise while stirring until the solution becomes acidic. After stirring at room temperature for 12 hours, use distilled water to filter the activated material to neutrality, and then place it at 80°C for blasting. Carry out drying treatment in the oven, and the structural characteristics of the material are shown in accompanying drawing 2.

Take 242mg of CoCl ₂ ·6H ₂ O, 170mg of thiourea and 150mg of Cansha carbon, add it to 70mL of ethylene glycol solution, put it in an oil bath at 150°C, keep magnetic stirring at 300rpm, heat for 12h, then cool, filter and dry. The dried sample was annealed at 600°C for 2 hours at a heating rate of 5°C/min in an argon atmosphere to prepare a composite material. The structural characteristics of the material are shown in Figure 3.

Weigh the composite material, acetylene black and polyvinylidene fluoride according to the mass ratio of 8:1:1, add an appropriate amount of N-methylpyrrolidone and grind them evenly in a mortar. Subsequently, the slurry was coated on the surface of a commercial separator with a doctor blade, and the coated separator was placed in an oven at 50° C. for drying, and sliced into discs with a diameter of 19 mm. After matching the sulfur positive electrode and lithium negative electrode loaded on CMK-3, it is assembled into a button lithium-sulfur battery. See Figure 4-6 for battery performance.

Example 2:

Wash the orange peel biomass raw material with ultra-pure water to remove excess impurities on the surface and pulverize it. Place the dried biomass raw material in a tube furnace and heat it to 500 °C at a heating rate of 5 °C/min in an argon atmosphere. , keep warm for 1h. After the material was cooled to room temperature, the carbonized orange peel and potassium hydroxide powder (mass ratio = 1:4) were ground in a mortar to make the two fully mixed, and then placed in a tube furnace at 5°C/min Under Ar atmosphere, the temperature was raised to 800°C and kept for 2h. Transfer the cooled product to a beaker, and slowly add dilute hydrochloric acid dropwise while stirring until the solution becomes acidic. After stirring at room temperature for 12 hours, use distilled water to filter the activated material to neutrality, and then place it at 80°C for blasting. Drying in an oven.

Other steps are with embodiment 1.

Example 3:

The raw material of fungus residue biomass (solid waste after cultivating mushrooms) is cleaned with ultrapure water to remove excess impurities on the surface, soaked in dilute hydrochloric acid with a mass concentration of 10wt% for 24 hours to remove impurities, and then rinsed with ultrapure water to medium After drying, the dried biomass raw material was placed in a tube furnace under an argon atmosphere, heated to 500 °C at a heating rate of 3 °C/min, and kept for 1 h. After the material was cooled to room temperature, the carbonized slag and potassium hydroxide powder (mass ratio = 1:4) were fully mixed in a container, and then placed in a tube furnace to heat up to 800°C, keep warm for 3h. Transfer the cooled product to a beaker, and slowly add dilute hydrochloric acid dropwise while stirring until the solution becomes acidic. After stirring at room temperature for 12 hours, use distilled water to filter the activated material to neutrality, and then place it at 80°C for blasting. Drying process is carried out in the oven, and the structural characteristics of the material are shown in Figure 1.

Take 291mg of cobalt nitrate hexahydrate, 64mg of sulfur powder and 150mg of bacterial residue carbon, add it to 70mL of ethylene glycol solution, put the oil bath at 150°C, keep magnetic stirring at 300rpm, heat for 12h, then cool, filter and dry. The dried sample was annealed at 600°C for 2h at a heating rate of 5°C/min under an argon atmosphere to prepare a composite material.

Other steps are with embodiment 1.

Example 4:

Same as Example 1, the difference is that the sulfide supported by porous carbon is manganese sulfide.

Dissolve 170mg MnSO ₄ ·5H ₂ O in a mixed solution of 8mL ethanol and 12mL water, then add 170mg silkworm excrement porous carbon, keep magnetic stirring at 300rpm at room temperature for 10h, and dry. The dried material was annealed at 600° C. for 2 h at a heating rate of 5° C./min in an argon atmosphere to prepare a porous carbon composite material loaded with manganese sulfide. Other steps are with embodiment 1.

Example 5:

The same as in Example 1, the difference is that the obtained diaphragm modification layer is applied to other lithium metal batteries, and the positive electrode material is selenium, iodine and other elements in the sixth or seventh main group of the periodic table. Taking lithium-selenide batteries and lithium-iodine batteries as examples, it mainly solves the shuttle problem of polyselenide and polyiodide in the discharge intermediate products, improves the overall conductivity of the positive electrode material, and prevents its side reaction with the lithium metal negative electrode, causing rapid capacity decay. . At the same time, due to the rigidity of the material, it can prevent dendrites from piercing the separator and causing short circuit of the battery. The membrane modification layer is located on one or both sides of the membrane.

Other steps are with embodiment 1.

Embodiment 6:

Same as Example 1, the difference is that the obtained diaphragm modification layer is applied to lithium ion batteries, and the positive electrode materials are lithium iron phosphate, lithium cobaltate, lithium nickelate, lithium manganate (lithium aluminate) and their ternary composite materials. Its main function is to improve the wettability of the diaphragm and the electrolyte, uniform lithium ion flux, prevent the phenomenon of lithium precipitation at the negative electrode, inhibit the growth of lithium dendrites, and prevent the dendrites from piercing the diaphragm. The membrane modification layer is located on one or both sides of the membrane.

Other steps are with embodiment 1.

Embodiment 7:

Same as Example 1, the difference is that the obtained separator modification layer is applied to a sodium ion battery. The positive electrode material is Na _x MO ₂ (M is Fe, Co, Ni, Mn, Cr, Ti and other transition metal elements), Na _x M[M'(CN) ₆ ]y·zH ₂ O, (M and M' are Fe, Co, Ni, Mn, Cu, Zn and other transition metals), polyanion positive electrodes (sodium ferric sulfate, sodium vanadium fluorophosphate and sodium vanadium phosphate), etc., its main function is to improve the wettability of the separator and electrolyte, uniform The flux of sodium ions prevents the phenomenon of sodium precipitation in the negative electrode, promotes the reversible storage of sodium ions, inhibits the growth of sodium dendrites, and prevents dendrites from piercing the diaphragm. The membrane modification layer is located on one or both sides of the membrane.

Other steps are with embodiment 1.

Embodiment 8:

Same as Example 1, the difference is that the obtained separator modification layer is applied to (room temperature) sodium-sulfur battery, sodium-selenium battery or sodium-iodide battery. It mainly solves the shuttle problem of polysulfide, polyselenide, and polyiodide, which are intermediate products of discharge, and prevents them from side-reacting with the sodium metal anode, causing rapid capacity decay. When applied to high-temperature sodium-sulfur batteries, due to the good thermal stability of carbon materials, the heat resistance of the separator material can be improved and the stable operation of the battery can be maintained. The membrane modification layer is located on one or both sides of the membrane.

Other steps are with embodiment 1.

Embodiment 9:

Same as Example 3, except that the resulting porous carbon was activated by potassium ferrate. Other steps are with embodiment 3.

The biomass raw material of fungus residue is cleaned with ultrapure water to remove excess impurities on the surface, soaked in dilute hydrochloric acid with a mass concentration of 10wt% for 24 hours to remove impurities, then rinsed with ultrapure water until neutral, and then dried. The biomass raw material was placed in a tube furnace under an argon atmosphere, heated to 500 °C at a heating rate of 5 °C/min, and kept for 1 h. After the material is cooled to room temperature, mix the carbonized slag and potassium ferrate solution (0.3M) in a container, add an appropriate amount of distilled water and stir for 12 hours, dry it at 80°C and place it in a tube furnace at 5°C /min in an Ar atmosphere to raise the temperature to 800°C and keep it warm for 2h. Transfer the cooled product to a beaker, slowly add dilute hydrochloric acid dropwise while stirring until the solution becomes acidic, stir at room temperature for 12 hours, wash alternately with hydrochloric acid and sodium hydroxide solution until the filtrate is colorless, and extract the material with distilled water Filter until neutral, and then place it in a blast oven at 80°C for drying treatment. The nitrogen adsorption-desorption isotherm diagram of the obtained material is shown in Figure 7. After the test, the instrument calculated the specific surface area of the material according to the BET formula to be 1911.8m ² /g.

Comparative example 1:

Same as Example 1, the difference is that the biomass is not activated with potassium hydroxide. Other steps are with embodiment 1. The pore-forming effect of potassium hydroxide is mainly achieved by converting carbonaceous substances (amorphous carbon, methyl, methine, etc.) into carbon monoxide by potassium hydroxide under high temperature conditions. The specific reaction is as follows:

6KOH+2C＝2K+3H ₂ +2K ₂ CO ₃

K ₂ CO ₃ ＝K ₂ O+CO ₂

CO ₂ +C=2CO

K ₂ CO ₃ +2C＝2K+3CO

K ₂ O+C=2K+CO

Therefore, the porosity and specific surface area of the carbon material obtained by direct pyrolysis without adding an activator are low, and cannot provide a large adsorption reaction interface, so its ability to adsorb other substances and support metal catalysts is reduced.

Comparative example 2:

Same as Example 1, the difference is that the obtained porous carbon is directly applied to the modified layer of the lithium-sulfur battery separator without cobalt sulfide loading, and other steps are the same as in Example 1. See Figure 5 for battery performance.

In summary, the present invention uses the carbonization-activation method to prepare activated carbon with high specific surface area, which is a good barrier to block the shuttle of polysulfides. Compared with other methods for preparing activated carbon, the activated carbon produced by this method is partially graphitized amorphous Carbon with a specific surface area up to >3000m ² /g. This method is a universal method for the production of porous carbon with high specific surface area from agricultural wastes, which is of great significance for promotion. After loading about 10% of sulfide, the specific surface area can be maintained at about 2000m ² /g, which ensures the efficient transport of lithium ions in the electrolyte and effectively anchors polysulfides (Fig. 4). At the same time, it can also expand its application in the field of supported catalysts by loading other types of transition metal salt nanoparticles.

The invention prepares porous carbon from agricultural waste and loads cobalt sulfide and applies it to the diaphragm modification layer of the lithium-sulfur battery, which can effectively suppress the shuttle effect in the charge-discharge process, accelerate the redox kinetics of the charge-discharge reaction, thereby improving the performance of the lithium-sulfur battery. specific capacity and cycle stability; in previous studies, the initial capacity of lithium-sulfur batteries using ordinary polypropylene separators was about 1200mAh/g, and after applying the porous carbon loaded with cobalt sulfide in the present invention, the sulfur loading was about 1.6mg/cm At ² o'clock, the capacity of the first cycle at 0.2C rate can reach more than 1500mAh/g (Figure 4-5). In addition, because the diaphragm modification layer has the advantages of improving electrolyte wettability, promoting ion conduction, and uniform ion flux, it can improve the problem of dendrite growth in metal batteries. It has certain application value (figure 6).

Claims (10)

1. A porous carbon, characterized by: the agricultural waste is used as biomass material, the biomass material is mixed with an activating agent and then placed in a reaction vessel, and activated heat treatment is carried out under the protection of inert gas to obtain porous carbon;

wherein the agricultural waste is at least one of silkworm excrement, basswood, fungus residues, orange peel, wheat straw, rice straw, corn straw, miscanthus and ramie;

the activator is at least one of zinc chloride, phosphoric acid, potassium ferrate, potassium hydroxide, ferric chloride, ferric nitrate and water vapor.

2. A porous carbon according to claim 1, wherein: the mass ratio of the biomass material to the activating agent is 1:0.1-6; the inert gas is nitrogen, argon or hydrogen-argon mixed gas, and the flow rate of the inert gas is 0.1-2L/min; the temperature of the activation heat treatment is 200-1500 ℃, the heating rate is 1-20 ℃/min, and the time of the activation heat treatment is 0.5-10h.

3. A porous carbon according to claim 1, wherein: the agricultural waste is subjected to pretreatment, and the pretreatment step is cleaning and drying;

the cleaning is at least one of water washing, hydrochloric acid washing, sulfuric acid washing, nitric acid washing, hydrofluoric acid washing, sodium hydroxide washing, potassium hydroxide alkaline washing and ethanol washing;

the drying temperature is 40-150 ℃ and the drying time is 6-72h.

4. A porous carbon according to claim 4, wherein: the dried agricultural waste can be further crushed and sieved, and the granularity of the crushed powder is 50-400 meshes.

5. A porous carbon according to claim 3 or 4, characterized in that: the pretreated agricultural waste is subjected to pre-carbonization heat treatment under the protection of inert gas before being mixed with an activating agent;

wherein the inert gas is nitrogen, argon or hydrogen-argon mixed gas; the flow rate of the inert gas is 0.1-1L/min; the temperature of the pre-carbonization heat treatment is 200-1500 ℃, the heating rate is 1-20 ℃/min, and the pre-carbonization heat treatment time is 0.5-10h.

6. A composite material, characterized in that the porous carbon of claim 1 and sulfide are mixed in a solvent, then the mixture is filtered in vacuum, filter residues are taken and dried, and annealing treatment is carried out under the protection of inert gas, thus obtaining the porous carbon-sulfide composite material.

7. The composite material of claim 6, wherein the sulfide is cobalt disulfide, sodium sulfide, potassium sulfide, zinc sulfide, magnesium sulfide, ferrous sulfide, manganese sulfide, lead sulfide, cadmium sulfide, antimony sulfide, bismuth sulfide, molybdenum sulfide, stannous sulfide, silver sulfide, copper sulfide, nickel sulfide, calcium sulfide, strontium sulfide, barium sulfide; the solvent is ethanol, glycol, dimethylformamide, ethylene glycol monomethyl ether, butanol, octanol or octyl acetate; the inert gas is nitrogen, argon or hydrogen-argon mixed gas, and the gas flow rate is 0.1-1L/min; the annealing treatment temperature is 200-1500 ℃, the heating rate is 1-20 ℃/min, and the annealing treatment time is 0.5-10h.

8. A membrane modification material, characterized in that the membrane modification material is obtained by mixing the composite material according to claim 6 with a conductive agent, a binder and a solvent and grinding the mixture into slurry.

9. The separator-modifying material of claim 8, wherein the conductive agent is at least one of Super-P, superconducting carbon black, acetylene black, ketjen black; the binder is at least one of polyvinylidene fluoride, sodium carboxymethyl cellulose and styrene-butadiene rubber; the solvent is at least one of ultrapure water, N-methyl pyrrolidone and ethanol; the mass ratio of the composite material to the conductive agent to the binder is 8:1:1, and the mass of the solvent is 1-10 times of the total mass of the composite material, the conductive agent and the binder.

10. The use of the membrane modification material of claim 8, wherein the membrane modification material is compounded on the surface of a membrane and dried to obtain a membrane modification layer, and the membrane modification layer is used in a secondary battery;

wherein the diaphragm is made of polyethylene, polypropylene, glass fiber or cellulose, the number of layers of the diaphragm is 1-3, and the diameter of the diaphragm is 10-50mm; the process of compounding the diaphragm decorative material and the diaphragm is knife coating, spraying and spin coating; the drying mode is vacuum drying, freeze drying or air drying, and the drying temperature is 25-200 ℃; the thickness of the membrane modification material after drying is 1-100 μm.

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