Modified vulcanized polyacrylonitrile as well as preparation method and application thereof

Abstract

The invention provides modified vulcanized polyacrylonitrile as well as a preparation method and application thereof. The preparation method of the modified sulfurized polyacrylonitrile comprises the following steps: S1, mixing hydroxylated carbon nanotubes, acrylonitrile and a first catalyst, and carrying out a cyano hydrolysis reaction to obtain a first product; s2, mixing the first product obtained in S1 with a solvent and a second catalyst, then carrying out a first polymerization reaction, and then adding acrylonitrile to carry out a second polymerization reaction, so as to obtain a second product; and S3, mixing the second product obtained in S2 with elemental sulfur, and carrying out a modification reaction to obtain the modified vulcanized polyacrylonitrile. Compared with the prior art, the preparation method provided by the invention has the advantages that the process is simple, the grafting position is adjustable, and the capacity of the modified vulcanized polyacrylonitrile can be better improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of positive electrode materials, in particular to a modified sulfurized polyacrylonitrile and a preparation method and application thereof. BACKGROUND

[0002] Sulfurized polyacrylonitrile (SPAN) is considered as a high potential cathode material for lithium-sulfur batteries, which can inhibit the dissolution of polysulfides in ether-based electrolyte, and has high safety and long cycle performance. In addition, the wide raw material source, low cost and high theoretical specific capacity make it more competitive. However, the electronic conductivity of SPAN is low, and a large amount of conductive agent is often added in the actual use process to improve the rate performance of the battery, which makes the active material content in the electrode sheet low, and the battery energy density is seriously affected. SUMMARY

[0003] Therefore, the purpose of the present application is to provide a modified sulfurized polyacrylonitrile and a preparation method and application thereof. The high-conductivity carbon nanotubes are grafted on acrylonitrile, and the carbon nanotube grafted acrylonitrile and acrylonitrile monomer are block polymerized by anionic polymerization to obtain a high-regularity polyacrylonitrile segment, which is beneficial to the subsequent carbonization to form a pyridine ring, and a high-capacity sulfurized polyacrylonitrile is obtained.

[0004] The present application provides a preparation method of a modified sulfurized polyacrylonitrile, comprising the following steps: S1, hydroxylated carbon nanotubes, acrylonitrile and a first catalyst are mixed to perform a cyano hydrolysis reaction to obtain a first product; S2, the first product obtained in S1 is mixed with a solvent and a second catalyst to perform a first polymerization reaction, and then acrylonitrile is added to perform a second polymerization reaction to obtain a second product; S3, the second product obtained in S2 is mixed with elemental sulfur to perform a modification reaction to obtain a modified sulfurized polyacrylonitrile.

[0005] In some embodiments, in step S1, the first catalyst comprises concentrated sulfuric acid.

[0006] In some embodiments, in step S1, the mass ratio of the hydroxylated carbon nanotubes to the acrylonitrile is 1:50-100.

[0007] In some embodiments, in step S1, the mass ratio of the first catalyst to the acrylonitrile is 1:5-10.

[0008] In some embodiments, in step S1, the temperature of the cyano hydrolysis reaction is 50-150°C, and the time of the cyano hydrolysis reaction is 2-8h.

[0009] In some embodiments, after the cyano hydrolysis reaction in step S1, the mixture solution after the cyano hydrolysis reaction is subjected to extraction and reduced pressure distillation.

[0010] In some embodiments, in step S2, the solvent includes one or more of cyclohexane, n-hexane, and tetrahydrofuran.

[0011] In some embodiments, in step S2, the second catalyst includes one or more of butyllithium and sodium naphthalene.

[0012] In some embodiments, in step S2, the mass ratio of the first product to the acrylonitrile is 1:10-20.

[0013] In some embodiments, in step S2, the solvent and the acrylonitrile are used in a ratio of (15 mL-45 mL):1 g.

[0014] In some embodiments, in step S2, the mass ratio of the second catalyst to the acrylonitrile is 1:100-2000.

[0015] In some embodiments, in step S2, the first polymerization reaction is carried out at a temperature of -80℃ to -50℃ for 20 min to 60 min.

[0016] In some embodiments, in step S2, the second polymerization reaction is carried out at a temperature of -80℃ to -50℃ for 120 min to 180 min.

[0017] In some embodiments, in step S2, the second polymerization reaction is terminated by adding ethanol, and the amount of the ethanol added is >400 mL.

[0018] In some embodiments, after the second polymerization reaction in step S2, the mixture solution is washed with deionized water for 3-5 times to remove un-polymerized products and products with low polymerization degree.

[0019] In some embodiments, in step S3, the mass ratio of the second product to the elemental sulfur is 1:2-6.

[0020] In some embodiments, in step S3, the second product and the elemental sulfur are mixed by a ball milling method.

[0021] In some embodiments, in step S3, the mixing time is 2 h-10 h.

[0022] In some embodiments, in step S3, the specific operation of the modification reaction is: placing the powder obtained after mixing the second product and elemental sulfur into a reaction kettle, introducing an inert atmosphere, and performing heat treatment. Optionally, the inert atmosphere comprises one or more of nitrogen, argon, and helium. Optionally, the temperature of the heat treatment is 200-500 DEG C, and the time of the heat treatment is 2-8 hours.

[0023] According to another aspect of the present application, the present application provides a modified sulfurized polyacrylonitrile, which is prepared by the preparation method described in the above technical solution.

[0024] According to another aspect of the present application, the present application provides a positive electrode sheet, which comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one side surface of the positive electrode current collector in the thickness direction, and the positive electrode active material layer comprises a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder. The positive electrode active material comprises the modified sulfurized polyacrylonitrile prepared by the preparation method described in the above technical solution or the modified sulfurized polyacrylonitrile described in the above technical solution.

[0025] According to another aspect of the present application, the present application provides a lithium-sulfur battery, which comprises a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode comprises the positive electrode sheet described in the above technical solution.

[0026] The present application provides a modified sulfurized polyacrylonitrile, a preparation method and application thereof, and has the following beneficial effects: 1. The hydroxylated carbon nanotube is grafted on the acrylonitrile through a cyano hydrolysis reaction, which has better uniformity and controllability compared with simple mixing or coating. The carbon nanotube grafted acrylonitrile is block polymerized with acrylonitrile monomers by using an anionic polymerization method, so that the ratio of the modified unit to the acrylonitrile monomer is adjustable, which is more conducive to the formation of a pyridine ring through subsequent carbonization, thereby obtaining a modified sulfurized polyacrylonitrile with high capacity.

[0027] 2. The preparation method provided by the present application is simple in process and adjustable in grafting position, and can better improve the capacity of the modified sulfurized polyacrylonitrile. DETAILED DESCRIPTION

[0028] In order to enable personnel in the technical field to better understand the present application, the technical solutions of the present application will be described clearly and completely in combination with the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0029] In the description of the present application, "the same chemical composition" should be understood broadly, that is, the main components of both have consistent chemical composition, or the chemical composition of both is basically consistent, which can have errors within the range allowed by those skilled in the art or contain impurities within the allowable range.

[0030] In the description of the present application, "A and / or B" can include any one of the cases of A alone, the case of B alone, the case of A and B, wherein A and B are only used for example, which can be any technical feature connected by "and / or" in the present application.

[0031] Unless otherwise specified, all technical terms used in the present application have the same meaning as generally understood by those skilled in the art to which the present application belongs. All patents and publications referred to in the present application are incorporated herein by reference in their entirety. The terms "comprising" or "including" are open-ended expressions, that is, they include the contents indicated in the present application, but do not exclude other contents.

[0032] At present, sulfidized polyacrylonitrile (SPAN) is considered as a high potential lithium-sulfur battery cathode material, which can inhibit the dissolution of polysulfides in ether-based electrolyte in lithium-sulfur battery, and also has high safety and long cycle performance. In addition, the wide raw material source, low cost and high theoretical specific capacity make it more competitive. However, the electronic conductivity of SPAN is low, and a large amount of conductive agent is often added in the actual use process to improve the rate performance of the battery, which makes the active material content in the electrode sheet low, and the battery energy density is seriously affected. Therefore, the key to improve the electronic conductivity of SPAN material is to reduce the electronic transmission resistance through structural modification, composite conductive material or optimization of preparation process.

[0033] On this basis, the inventors of the present application graft hydroxylated carbon nanotubes on acrylonitrile through cyano hydrolysis reaction, which has better uniformity and controllability compared with simple mixing or coating; block polymerization of carbon nanotube grafted acrylonitrile and acrylonitrile monomer is carried out by anionic polymerization method, so that the ratio of modified unit to acrylonitrile monomer can be adjusted, which is more conducive to the formation of pyridine ring in subsequent carbonization, thereby obtaining modified sulfidized polyacrylonitrile with high capacity. Specifically, the present application adopts the following technical solutions: According to one aspect of the present application, the present application provides a preparation method of modified sulfidized polyacrylonitrile, comprising the following steps: S1, hydroxylated carbon nanotubes, acrylonitrile and a first catalyst are mixed to carry out cyano hydrolysis reaction to obtain a first product; S2, the first product obtained in S1 is mixed with a solvent and a second catalyst to carry out a first polymerization reaction, and then acrylonitrile is added to carry out a second polymerization reaction to obtain a second product; S3. The second product obtained in S2 is mixed with elemental sulfur and subjected to a modification reaction to obtain modified vulcanized polyacrylonitrile.

[0034] In step S1 of this invention, hydroxylated carbon nanotubes, acrylonitrile, and a first catalyst are mixed and subjected to cyanohydrolysis to obtain a first product, which is carbon nanotube-grafted acrylonitrile. Compared with ordinary carbon nanotubes, hydroxylated carbon nanotubes have a higher hydroxyl content, which can react with the cyano groups in acrylonitrile to generate ester groups, thereby grafting hydroxylated carbon nanotubes onto the acrylonitrile monomer. Furthermore, this invention uses acrylonitrile instead of polyacrylonitrile for grafting, avoiding the inability to control the grafting position during the cyanohydrolysis reaction of polyacrylonitrile and hydroxylated carbon nanotubes, which could cause the hydroxylated carbon nanotubes to graft into the middle of the polyacrylonitrile molecular chain, preventing the formation of a pyridine ring structure during the carbonization of the second product and thus affecting the capacity of the modified vulcanized polyacrylonitrile.

[0035] In a specific embodiment of the present invention, the first catalyst preferably comprises concentrated sulfuric acid. The present invention uses the above-mentioned first catalyst to create an acidic environment, activate the hydroxyl groups on the surface of the hydroxylated carbon nanotubes, promote their hydrolysis reaction with cyano groups, and form stable covalent bonds, ensuring the subsequent acquisition of the desired grafted product. The present invention does not have any special restrictions on the source of the first catalyst; commercially available products well known to those skilled in the art can be used.

[0036] In a specific embodiment of the present invention, the preferred mass ratio of the hydroxylated carbon nanotubes to the acrylonitrile is 1:50~100, specifically 1:50, 1:60, 1:70, 1:80, 1:90, 1:100, etc. When the mass of acrylonitrile is too high, the amount of acrylonitrile grafted without undergoing cyano hydrolysis increases, failing to achieve the desired modification effect; when the mass of acrylonitrile is too low, the amount of cyano groups provided by acrylonitrile decreases, resulting in fewer grafting sites and a lower content of the first product, thus failing to improve conductivity. The present invention does not have any special restrictions on the source of the hydroxylated carbon nanotubes and acrylonitrile; commercially available products well known to those skilled in the art can be used.

[0037] In a specific embodiment of the present invention, the mass ratio of the first catalyst to the acrylonitrile is preferably 1:5 to 10, specifically 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, etc. When the content of the first catalyst is too high, the amount of acrylonitrile limits the reaction process, resulting in waste of the first catalyst; when the content of the first catalyst is too low, the catalytic effect decreases, and the reaction rate is slow.

[0038] In a specific embodiment of the present invention, the mixing method preferably includes at least one of stirring, ultrasonication, and oscillation. Through the above mixing operation, the hydroxylated carbon nanotubes are uniformly dispersed in acrylonitrile and grafted onto the acrylonitrile monomer under the action of a first catalyst, thereby obtaining the expected first product.

[0039] In a specific embodiment of the present invention, the cyano hydrolysis reaction is a reaction in which the carboxyl group of the hydroxylated carbon nanotube reacts with the cyano group of the acrylonitrile monomer to form an ester group. The preferred temperature for the cyano hydrolysis reaction is 50℃~150℃, specifically 50℃, 80℃, 110℃, 130℃, 150℃, etc. The preferred reaction time is 2h~8h, specifically 2h, 3h, 4h, 5h, 6h, 7h, 8h, etc. By selecting the above-mentioned cyano hydrolysis reaction conditions, the present invention avoids the destruction of the structure of the hydroxylated carbon nanotube and acrylonitrile, accelerates the hydrolysis reaction kinetics, ensures sufficient contact between the hydroxylated carbon nanotube and acrylonitrile, and guarantees the performance of the subsequently obtained grafted product.

[0040] In a specific embodiment of the present invention, the cyano hydrolysis reaction preferably further includes the following steps: extraction and vacuum distillation of the mixed solution after the cyano hydrolysis reaction. The vacuum distillation temperature is preferably 35℃~45℃, specifically 35℃, 40℃, 45℃, etc., and the vacuum distillation temperature depends on the vacuum degree of the vacuum distillation apparatus. Specifically, the present invention removes unreacted acrylonitrile, the first catalyst, and small molecule byproducts through extraction, and further concentrates the grafted product by vacuum distillation, thereby improving the purity of the first product.

[0041] In step S2 of this invention, the first product obtained in S1 is mixed with a solvent and a second catalyst to carry out a first polymerization reaction, and then acrylonitrile is added to carry out a second polymerization reaction to obtain a second product, which is carbon nanotube-grafted polyacrylonitrile. Specifically, the first polymerization reaction and the second polymerization reaction are anionic polymerization reactions. The initiation of anionic polymerization can be achieved by the negative ions in the second catalyst molecule forming anionic active centers with the first product and acrylonitrile monomer, thereby carrying out a chain initiation reaction to obtain a block polymer of the first product and acrylonitrile. This avoids the situation where the polymerization product of the first product alone cannot form a pyridine ring after carbonization, thus making it impossible to synthesize modified vulcanized polyacrylonitrile.

[0042] In a specific embodiment of the present invention, the solvent preferably includes one or more of cyclohexane, n-hexane, and tetrahydrofuran; the preferred amounts of the solvent and acrylonitrile are (15mL~45mL): 1g, specifically 15mL / g, 20mL / g, 25mL / g, 30mL / g, 35mL / g, 40mL / g, 45mL / g, etc. The present invention uses the above-mentioned solvent and its amounts to dissolve the first product and acrylonitrile, ensuring the smooth progress of the subsequent first and second polymerization reactions, thereby obtaining a second product that meets the performance requirements. The present invention does not have any special restrictions on the source of the solvent; commercially available products well known to those skilled in the art can be used.

[0043] In a specific embodiment of the present invention, the second catalyst preferably comprises one or more of butyllithium and sodium naphthalene. The use of the above-mentioned second catalyst in the present invention provides an active initiation site for anion formation, ensuring the subsequent chain initiation reaction. The present invention does not have any special restrictions on the source of the second catalyst; commercially available products well known to those skilled in the art can be used.

[0044] In a specific embodiment of the present invention, the mass ratio of the first product to the acrylonitrile is preferably 1:10 to 20, specifically 1:10, 1:12, 1:14, 1:16, 1:18, 1:20, etc. If the acrylonitrile content is too low, the molecular chain will be too short; if the acrylonitrile content is too high, the excess acrylonitrile monomer may self-polymerize to form free polyacrylonitrile chains, failing to achieve the effect of improving conductivity.

[0045] In a specific embodiment of the present invention, the mass ratio of the second catalyst to the acrylonitrile is preferably 1:100 to 2000, specifically 1:100, 1:500, 1:1000, 1:1500, 1:2000, etc. The present invention selects the above-mentioned mass ratio of the second catalyst to acrylonitrile to provide sufficient active initiation sites, ensuring that the subsequent obtained second product meets the expected requirements.

[0046] In a specific embodiment of the present invention, the preferred temperature for the first polymerization reaction is -80℃ to -50℃, specifically -80℃, -75℃, -70℃, -65℃, -60℃, -55℃, -50℃, etc.; the preferred time for the first polymerization reaction is 20 min to 60 min, specifically 20 min, 30 min, 40 min, 50 min, 60 min, etc. By selecting the above-mentioned reaction conditions for the first polymerization reaction, the present invention ensures that the first product polymerizes first, thereby guaranteeing the acquisition of the expected second product, and further obtaining modified vulcanized polyacrylonitrile that meets the performance requirements of the present invention.

[0047] In a specific embodiment of the present invention, the preferred temperature for the second polymerization reaction is -80℃ to -50℃, specifically -80℃, -75℃, -70℃, -65℃, -60℃, -55℃, -50℃, etc.; the preferred time for the second polymerization reaction is 120 min to 180 min, specifically 120 min, 135 min, 150 min, 165 min, 180 min, etc. The present invention selects the above-mentioned reaction conditions for the second polymerization reaction to ensure that the subsequent block copolymer molecular chains of the first product and acrylonitrile are well-defined, forming pyridine rings during carbonization, increasing the material capacity, and thus further obtaining modified vulcanized polyacrylonitrile that meets the performance requirements of the present invention.

[0048] In a specific embodiment of the present invention, the second polymerization reaction is terminated by adding ethanol. The amount of ethanol added is preferably >400 mL, specifically 400 mL, 500 mL, 600 mL, 700 mL, etc. The present invention does not impose any special restrictions on the source of the ethanol; commercially available products well-known to those skilled in the art can be used.

[0049] In a specific embodiment of the present invention, the second polymerization reaction is further followed by the following operation: washing with deionized water 3 to 5 times to remove unpolymerized products and products with low degree of polymerization, so as to ensure that a high-purity second product is obtained.

[0050] In step S3 of this invention, the second product obtained in S2 is mixed with elemental sulfur and subjected to a modification reaction to obtain modified vulcanized polyacrylonitrile.

[0051] In a specific embodiment of the present invention, the preferred mass ratio of the second product to the elemental sulfur is 1:2 to 6, specifically 1:2, 1:3, 1:4, 1:5, 1:6, etc. If the elemental sulfur content is too high, it is detrimental to improving the conductivity of the modified vulcanized polyacrylonitrile; if the elemental sulfur content is too low, the degree of vulcanization is insufficient, and modified vulcanized polyacrylonitrile with high cycle life and high specific capacity cannot be prepared. The present invention does not impose any special restrictions on the source of the elemental sulfur; commercially available products well known to those skilled in the art can be used.

[0052] In a specific embodiment of the present invention, the preferred operation for mixing the second product and elemental sulfur is by ball milling; the preferred mixing time is 2h to 10h, specifically 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, etc. The present invention selects the above mixing method and mixing time to ensure that the second product and elemental sulfur are fully contacted and uniformly mixed.

[0053] In a specific embodiment of the present invention, the modification reaction includes a carbonization reaction and a sulfidation reaction. The specific operation of the modification reaction is as follows: the powder after mixing the second product and elemental sulfur is placed in a reaction vessel, an inert atmosphere is introduced, and heat treatment is performed. The inert atmosphere preferably includes one or more of nitrogen, argon, and helium; the heat treatment temperature is preferably 200℃~500℃, specifically 200℃, 250℃, 300℃, 350℃, 400℃, 450℃, 500℃, etc.; the heat treatment time is preferably 2h~8h, specifically 2h, 3h, 4h, 5h, 6h, 7h, 8h, etc. The present invention selects the above modification reaction conditions to ensure that the subsequently obtained modified sulfidated polyacrylonitrile has good performance.

[0054] This invention grafts hydroxylated carbon nanotubes onto acrylonitrile via cyano hydrolysis, offering better uniformity and controllability compared to simple mixing or coating. Anionic polymerization is used to synthesize polyacrylonitrile, yielding highly regular polyacrylonitrile fragments, which are more conducive to subsequent carbonization to form pyridine rings, resulting in high-capacity vulcanized polyacrylonitrile. Anionic polymerization is also used to block polymerize the carbon nanotube grafted acrylonitrile with acrylonitrile monomers, allowing for an adjustable ratio of modified units to acrylonitrile monomers without affecting polymer regularity. Compared to existing technologies, the preparation method provided by this invention is simple, the grafting position is adjustable, and it can better improve the capacity of modified vulcanized polyacrylonitrile.

[0055] According to another aspect of the present invention, a modified vulcanized polyacrylonitrile is provided, comprising the modified vulcanized polyacrylonitrile prepared by the preparation method described in the above-described technical solution. Therefore, this modified vulcanized polyacrylonitrile possesses all the features and advantages of the modified vulcanized polyacrylonitrile described in the above-described technical solution, which will not be repeated here.

[0056] According to another aspect of the present invention, a positive electrode sheet is provided, comprising a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector along the thickness direction, the positive active material layer comprising a positive active material, a positive conductive agent and a positive binder; The positive electrode active material includes the modified vulcanized polyacrylonitrile prepared by the preparation method described in the above technical solution, or the modified vulcanized polyacrylonitrile described in the above technical solution. Therefore, the positive electrode sheet possesses all the characteristics and advantages of the modified vulcanized polyacrylonitrile described in the above technical solution, which will not be elaborated further here.

[0057] In a specific embodiment of the present invention, the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector along the thickness direction; wherein, the positive current collector may be a metal foil, a foamed metal, or a composite current collector. For example, as a metal foil, silver-surfaced aluminum or stainless steel, stainless steel, copper, aluminum, nickel, carbon electrode, carbon, or titanium, etc., may be used; the foamed metal may be foamed nickel, foamed copper, foamed aluminum, foamed alloy, etc.; the composite current collector may include a polymer material base layer and a metal layer, and the composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polyethylene, etc.).

[0058] In a specific embodiment of the present invention, the positive electrode active material layer includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder. The positive electrode active material includes modified vulcanized polyacrylonitrile prepared by the preparation method described in the above technical solution, or the modified vulcanized polyacrylonitrile described in the above technical solution. As an example, the positive electrode binder may include at least one of styrene-butadiene rubber (SBR), waterborne acrylic resin, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), and polyvinyl butyral (PVB); the positive electrode conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, conductive carbon black (SP), graphene, and carbon nanofibers. These materials are all commercially available.

[0059] In a specific embodiment of the present invention, the preparation method of the positive electrode sheet adopts a method well known to those skilled in the art: first, the raw materials are mixed in a solvent in a certain proportion to form a slurry, and then the positive electrode slurry is coated on the positive electrode current collector. After drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0060] According to another aspect of the present invention, a lithium-sulfur battery is provided, comprising a positive electrode, a negative electrode, a separator, and an electrolyte; The positive electrode includes the positive electrode sheet described in the above technical solution. Therefore, this lithium-sulfur battery possesses all the features and advantages of the positive electrode sheet described in the above technical solution, which will not be repeated here.

[0061] In a specific embodiment of the present invention, the negative electrode is specifically a negative electrode sheet, which may be a metal foil, such as lithium foil.

[0062] In a specific embodiment of the present invention, the electrolyte acts as a conductor of ions between the positive and negative electrodes, and includes an electrolyte salt and a solvent; wherein, the electrolyte salt preferably includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate, more preferably lithium hexafluorophosphate and lithium difluorophosphate. Lithium carbonate; the solvent preferably includes one or more of the following: ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, fluoroethylene carbonate, vinylene sulfate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0063] In a specific embodiment of the present invention, the electrolyte preferably further includes film-forming additives, such as negative electrode film-forming additives and positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance and additives that improve battery high-temperature or low-temperature performance.

[0064] In a specific embodiment of the present invention, the separator preferably includes a glass fiber membrane or a Celgard separator; the present invention places the separator between the positive and negative electrodes, assembles them, and then presses them to obtain an electrode assembly. The electrode assembly can be a wound electrode assembly or a stacked electrode assembly; the shape of the electrode assembly can be cylindrical, flat, or polygonal, etc.; the electrode assembly preferably also has tabs, which can conduct current from the electrode assembly; the tabs include a positive tab and a negative tab.

[0065] In a specific embodiment of the present invention, the number of the electrode components contained in the battery can be one or more, and those skilled in the art can select according to specific actual needs.

[0066] In a specific embodiment of the present invention, the battery may include an outer packaging, which can be used to encapsulate the aforementioned electrode assembly and electrolyte. The outer packaging of the battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. Specifically, the hard shell may include a housing and a cover plate, wherein the housing may include a bottom plate and a side plate connected to the bottom plate, the bottom plate and the side plate forming a receiving cavity, the housing having an opening communicating with the receiving cavity, and the cover plate being able to cover the opening to close the receiving cavity, the electrode assembly being encapsulated in the aforementioned receiving cavity, and the electrolyte being immersed in the electrode assembly; it may also be a soft pack, such as a pouch-type soft pack; the material of the soft pack may be plastic, and examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0067] The present invention does not impose any particular restrictions on the shape of the battery; it can be cylindrical, square, or any other arbitrary shape, and those skilled in the art can choose according to specific practical needs.

[0068] In specific embodiments of the present invention, the present invention does not impose any special restrictions on the preparation method of the battery. The battery can be obtained by assembling the above-mentioned battery components and then sequentially performing conventional technical solutions such as formation, aging, secondary sealing and degassing, and capacity testing, which are well known to those skilled in the art.

[0069] To further illustrate the present invention, the following embodiments are provided for detailed description. All raw materials used in the following embodiments of the present invention are commercially available.

[0070] Example 1 S1. Add 0.5g of hydroxylated carbon nanotubes to 25g of acrylonitrile and mix thoroughly. Add 3g of concentrated sulfuric acid, stir and heat to 80℃ for cyano hydrolysis reaction for 3h. Extract the resulting mixed solution and distill under reduced pressure at 45℃ to obtain the high-purity first product, namely carbon nanotube-grafted acrylonitrile, denoted as CNT-AN.

[0071] S2. Disperse 2g of the first product evenly in 800mL of cyclohexane, add 20mg of butyllithium, stir, keep the reaction system temperature at -50℃, carry out the first polymerization reaction for 40min, then add 20g of acrylonitrile to carry out the second polymerization reaction for 150min, add 500mL of ethanol to terminate the reaction, wash 3 times to obtain the second product, namely carbon nanotube grafted polyacrylonitrile, denoted as CNT-PAN.

[0072] S3. 2g of the second product and 6g of elemental sulfur were ball-milled for 5 hours. The mixed powder was placed in a sealed reaction vessel, argon gas was introduced, and the modification reaction was carried out at 350℃ for 4 hours. After natural cooling, modified vulcanized polyacrylonitrile was obtained.

[0073] Example 2 The difference between Example 2 and Example 1 is that the amount of concentrated sulfuric acid added in step S1 of this example is 5g.

[0074] Example 3 The difference between Example 3 and Example 1 is that the amount of butyllithium added in step S2 of this example is 10 mg.

[0075] Example 4 The difference between Example 4 and Example 1 is that the amount of elemental sulfur added in step S3 of this example is 4g.

[0076] Example 5 The difference between Example 5 and Example 1 is that the modification reaction temperature in step S3 of this example is 250°C and the time is 2 hours.

[0077] Example 6 The difference between Example 6 and Example 1 is that the amount of hydroxylated carbon nanotubes added in step S1 of this example is 0.1g.

[0078] Example 7 The difference between Example 7 and Example 1 is that the amount of acrylonitrile added in step S2 of this example is 50g.

[0079] Comparative Example 1 S1. Disperse 20g of acrylonitrile evenly in 800mL of cyclohexane, add 20mg of butyllithium, stir, keep the reaction system temperature at -50℃, carry out the second polymerization reaction for 150min, add 500mL of ethanol to terminate the reaction, and wash 3 times to obtain polyacrylonitrile.

[0080] S2. 2g of polyacrylonitrile and 6g of elemental sulfur were ball-milled for 5 hours. The mixed powder was then placed in a sealed reaction vessel, argon gas was introduced, and the modification reaction was carried out at 350℃ for 4 hours. After natural cooling, vulcanized polyacrylonitrile was obtained.

[0081] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that ordinary carbon nanotubes are used instead of hydroxylated carbon nanotubes in step S1 of this comparative example.

[0082] Battery performance test: Using polyvinylidene fluoride (PVDF) as a binder and conductive carbon black (SP) as a conductive agent, and using the modified vulcanized polyacrylonitrile prepared in Examples 1-7 and Comparative Example 2, or the vulcanized polyacrylonitrile prepared in Comparative Example 1, as the positive electrode active material, the preparation process of the positive electrode sheet is as follows: A positive electrode slurry is prepared according to a mass ratio of modified vulcanized polyacrylonitrile:PVDF:SP of 85:10:5 and a mass ratio of vulcanized polyacrylonitrile:PVDF:SP of 80:10:10. This slurry is then coated onto aluminum foil using a doctor blade, dried at room temperature, and cut into sheets by rolling to obtain the positive electrode sheet. A coin cell is assembled using lithium foil as the negative electrode sheet, LiPF6 as the solute, and ethylene carbonate and dimethyl carbonate as solvents to prepare the electrolyte.

[0083] The button batteries prepared in the above embodiments and comparative examples were tested using a blue-light testing device. The specific operation is as follows: (1) Charge capacity test: Charge the battery to 3V at a rate of 0.1C. The charge capacity at this time is recorded as the first charge capacity.

[0084] (2) Discharge specific capacity test: Discharge the battery to 1V at a rate of 0.1C. The discharge capacity at this time is recorded as the first discharge specific capacity.

[0085] (3) First-cycle efficiency: First-cycle efficiency = (first charge specific capacity / first discharge specific capacity) × 100%.

[0086] (4) Capacity retention rate after 100 cycles: Discharge the battery to 1V at a rate of 0.33C, and then charge the battery to 3V at a rate of 0.33C. This is one cycle, recorded as the first cycle. The charging capacity at this time is recorded as the first cycle charging capacity. Repeat the above operation 100 times, end the test, and record the charging capacity of the 100th cycle. The capacity retention rate after 100 cycles = (charging capacity of the 100th cycle / charging capacity of the first cycle) × 100%.

[0087] The test results are shown in Table 1 below.

[0088] Table 1. Performance test results of button batteries As shown in Table 1, under the same positive electrode formulation, higher electronic conductivity leads to higher capacity. Comparative Example 2 uses ordinary carbon nanotubes for grafting. Due to the low hydroxyl content on the surface of ordinary carbon nanotubes, the resulting modified polyacrylonitrile does not significantly improve electronic conductivity. Furthermore, less conductive agent is added during the positive electrode fabrication process. Therefore, Comparative Example 1 performs slightly better than Comparative Example 2. Examples 1-7 use hydroxylated carbon nanotubes for grafting, significantly improving the electronic conductivity of the modified polyacrylonitrile. Therefore, although the electrodes prepared in Examples 1-7 contain less conductive agent compared to Comparative Example 1, their charge / discharge capacity, first-cycle efficiency, and cycle performance are all superior to Comparative Example 1. Example 4 reduced the sulfur content, and Example 5 reduced the modification reaction temperature and time, leading to a decrease in lithium-ion binding sites. However, because the grafting of hydroxylated carbon nanotubes improves electronic conductivity, the capacity of the modified sulfurized polyacrylonitrile only decreases slightly.

[0089] In the foregoing description of this specification, references to terms such as "one embodiment," "another embodiment," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment is included in at least one embodiment of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples, without contradiction. Additionally, it should be noted that in this specification, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features.

[0090] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing modified vulcanized polyacrylonitrile, characterized in that, Includes the following steps: S1, hydroxylated carbon nanotubes, acrylonitrile, and the first catalyst were mixed and subjected to cyano hydrolysis to obtain the first product. S2. The first product obtained in S1 is mixed with solvent and second catalyst to carry out the first polymerization reaction, and then acrylonitrile is added to carry out the second polymerization reaction to obtain the second product. S3. The second product obtained in S2 is mixed with elemental sulfur and subjected to a modification reaction to obtain modified vulcanized polyacrylonitrile.

2. The preparation method according to claim 1, characterized in that, In step S1, the first catalyst includes concentrated sulfuric acid; And / or, the mass ratio of the hydroxylated carbon nanotubes to the acrylonitrile is 1:50~100; And / or, the mass ratio of the first catalyst to the acrylonitrile is 1:5~10; And / or, the temperature of the cyano hydrolysis reaction is 50℃~150℃, and the time of the cyano hydrolysis reaction is 2h~8h; And / or, the cyano hydrolysis reaction further includes the following operations: extraction and vacuum distillation of the mixed solution after the cyano hydrolysis reaction.

3. The preparation method according to claim 1, characterized in that, In step S2, the solvent includes one or more of cyclohexane, n-hexane, and tetrahydrofuran; And / or, the second catalyst comprises one or more of butyllithium and sodium naphthalene; And / or, the mass ratio of the first product to the acrylonitrile is 1:10~20; And / or, the amount of solvent used is (15mL~45mL):1g; And / or, the mass ratio of the second catalyst to the acrylonitrile is 1:100~2000.

4. The preparation method according to claim 1, characterized in that, In step S2, the temperature of the first polymerization reaction is -80℃ to -50℃, and the time of the first polymerization reaction is 20 min to 60 min; And / or, the temperature of the second polymerization reaction is -80℃ to -50℃, and the time of the second polymerization reaction is 120 min to 180 min.

5. The preparation method according to claim 1, characterized in that, In step S2, the second polymerization reaction is terminated by adding ethanol, wherein the amount of ethanol added is >400 mL; And / or, the second polymerization reaction may further include the following steps: washing with deionized water 3 to 5 times to remove unpolymerized products and products with low degree of polymerization.

6. The preparation method according to claim 1, characterized in that, In step S3, the mass ratio of the second product to the elemental sulfur is 1:2~6; And / or, the specific operation of mixing the second product and elemental sulfur is: mixing by ball milling; And / or, the mixing time is 2h to 10h.

7. The preparation method according to claim 1, characterized in that, In step S3, the specific operation of the modification reaction is as follows: the powder after mixing the second product and elemental sulfur is placed in a reaction vessel, an inert atmosphere is introduced, and heat treatment is performed. Optionally, the inert atmosphere includes one or more of nitrogen, argon, and helium; Optionally, the heat treatment temperature is 200℃~500℃, and the heat treatment time is 2h~8h.

8. A modified vulcanized polyacrylonitrile, characterized in that, The modified vulcanized polyacrylonitrile prepared by the preparation method according to any one of claims 1 to 7 above.

9. A positive electrode sheet, characterized in that, It includes a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector along the thickness direction, wherein the positive active material layer includes a positive active material, a positive conductive agent and a positive binder; The positive electrode active material includes the modified vulcanized polyacrylonitrile prepared by the preparation method according to any one of claims 1 to 7 or the modified vulcanized polyacrylonitrile according to claim 8.

10. A lithium-sulfur battery, characterized in that, Includes positive electrode, negative electrode, membrane, and electrolyte; The positive electrode comprises the positive electrode sheet as described in claim 9.

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