Vulcanized polyacrylonitrile-based potassium-sulfur battery positive electrode material, preparation method thereof and potassium-sulfur battery

Abstract

The invention belongs to the technical field of battery positive electrode materials, and particularly relates to a sulfurized polyacrylonitrile-based potassium-sulfur battery positive electrode material and a preparation method thereof as well as a potassium-sulfur battery and a preparation method of the positive electrode material, and the preparation method comprises the following steps: mixing polyacrylonitrile and sulfur powder according to a ratio, performing ball milling, performing heat preservation for 1-3 hours at 150-165 DEG C in an inert atmosphere, and performing cooling to room temperature to obtain a polyacrylonitrile-based potassium-sulfur battery positive electrode material; then raising the temperature to 400-500 DEG C, keeping the temperature for 0.5-1.5 hours, and vulcanizing to obtain vulcanized polyacrylonitrile; uniformly mixing vulcanized polyacrylonitrile, a conductive agent, a binder, polyethylene glycol and a solvent to obtain a membrane casting solution; coating the surface of a glass plate with the membrane casting solution, dipping the glass plate in water for 30-60 seconds to separate a membrane from the glass plate, and performing solvent exchange on the membrane in water for 5-8 hours; and after the solvent exchange is finished, drying the film to obtain the electrode material. The invention aims to solve the problems that the sulfur content is low, the conductivity is poor and the redox kinetics is delayed when the existing vulcanized polyacrylonitrile is used as the positive electrode material of the potassium-sulfur battery, the traditional polyacrylonitrile phase conversion film becomes brittle after being vulcanized, the microstructure is difficult to regulate and control in the preparation process of an electrode plate and the like.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of battery positive electrode materials, and particularly relates to a sulfidized polyacrylonitrile-based potassium-sulfur battery positive electrode material, a preparation method thereof and a potassium-sulfur battery. BACKGROUND

[0002] The statements herein are provided only to enhance understanding of the present application and are not necessarily intended to constitute the prior art.

[0003] Potassium-sulfur batteries are a kind of alkali metal-sulfur battery systems, which are based on the reversible redox reaction between a sulfur positive electrode and a potassium metal negative electrode. During discharging, sulfur is reduced to potassium polysulfide, and during charging, potassium polysulfide is oxidized to regenerate sulfur. Potassium ions migrate directionally between the positive electrode and the negative electrode. However, potassium-sulfur batteries face many challenges in practical applications, one of which is the performance of the positive electrode material.

[0004] Sulfidized polyacrylonitrile (SPAN) has unique advantages as a potential positive electrode material for potassium-sulfur batteries. However, there are still some deficiencies in using SPAN as a positive electrode material. On the one hand, the number of sulfur binding sites in the molecular structure of SPAN is limited, resulting in a low sulfur content, which severely limits the energy density of the battery. On the other hand, traditional SPAN membranes are prepared by electrospinning, which has low production efficiency and requires a long time, resulting in high battery costs. Traditional phase inversion membranes become brittle after sulfidization, leading to poor battery performance.

[0005] In addition, when constructing the electrode structure of traditional potassium-sulfur battery positive electrode sheets, solid powders of SPAN active material, conductive agent, and polymer binder are preliminarily dry-mixed in a certain mass ratio, the components are roughly dispersed, then a solvent is added, and a slurry is obtained by dispersion. The slurry is uniformly coated on an aluminum foil current collector using a doctor blade or a coating machine, and after drying, the electrode is obtained. The conductive network, ion channel, and pore structure formed in this way are randomly distributed products, making it difficult to accurately control the microstructure and performance of the electrode and unable to fully utilize the advantages of SPAN materials. SUMMARY

[0006] In view of the deficiencies in the prior art, the purpose of the present application is to provide a sulfidized polyacrylonitrile-based potassium-sulfur battery positive electrode material, a preparation method thereof, and a potassium-sulfur battery, in order to solve the problems of low sulfur content, poor conductivity, and redox kinetics hysteresis of existing sulfidized polyacrylonitrile as a positive electrode material for potassium-sulfur batteries, the brittleness of traditional polyacrylonitrile phase inversion membranes after sulfidization, and the difficulty in controlling the microstructure during the preparation process of electrode sheets.

[0007] In order to achieve the above-mentioned purpose, the present application is realized by the following technical solutions: In a first aspect, the present application provides a preparation method of a sulfidized polyacrylonitrile-based positive electrode material for potassium-sulfur batteries, comprising the following steps: The polyacrylonitrile and sulfur powder are mixed in a certain proportion and ball milled, and then, in an inert atmosphere, the mixture is first kept at 150-165℃ for 1-3h, and then heated to 400-500℃ for 0.5-1.5h to perform sulfidization, thereby obtaining sulfidized polyacrylonitrile; The sulfidized polyacrylonitrile, conductive agent, binder, polyethylene glycol and solvent are mixed uniformly to obtain a casting solution; The casting solution is coated on the surface of a glass plate, and then immersed in water for 30-60s, after which the film is separated from the glass plate, and the film is subjected to solvent exchange in water for 5-8h; After the solvent exchange is completed, the film is dried, thereby obtaining the electrode material.

[0008] In a second aspect, the present application provides a sulfidized polyacrylonitrile-based positive electrode material for potassium-sulfur batteries, which is prepared by the above preparation method.

[0009] In a third aspect, the present application provides a potassium-sulfur battery, wherein the positive electrode sheet is prepared from the sulfidized polyacrylonitrile-based positive electrode material.

[0010] The beneficial effects achieved by one or more embodiments of the present application are as follows: The improved sulfidization reaction process adopted in the present application optimizes the molecular structure of the sulfidized polyacrylonitrile (SPAN), increases the combination sites of sulfur, and effectively increases the sulfur content by performing sulfidization before the reaction, thereby avoiding the situation that the electrode sheet is fragile due to phase inversion and then sulfidization, and improving the energy density of the battery.

[0011] The high-conductivity conductive agent is added to the phase inversion film precursor solution, and these conductive agents form a continuous conductive network in the phase inversion film, thereby significantly improving the conductivity of the electrode sheet and reducing the battery polarization.

[0012] By precisely controlling the microstructure of the phase inversion film, such as porosity and pore size distribution, and the thickness of the phase inversion film, the number and proportion of micropores can be effectively controlled, thereby providing a fast channel for the transmission of potassium ions, improving the overall performance of the battery, promoting the redox reaction kinetics of the sulfidized polyacrylonitrile (SPAN) during the charging and discharging process, and improving the charging and discharging efficiency of the battery.

[0013] The phase inversion film prepared by the present application has a unique microstructure, which can effectively buffer the volume change of the sulfidized polyacrylonitrile (SPAN) during the charging and discharging process, enhance the structural stability of the electrode sheet, and improve the cycle life of the battery, and the battery can still maintain excellent capacity during a long cycle.

[0014] The phase inversion method can accurately control the microstructure parameters of the phase inversion film, so that the prepared electrode sheet can better adapt to the working requirements of the potassium-sulfur battery. The preparation process can be adjusted according to the actual needs of the battery to obtain electrode sheets with different performances, and the performance of the battery is further improved. BRIEF DESCRIPTION OF DRAWINGS

[0015] The drawings accompanying the specification of this application form a part thereof, serve to provide further understanding of the application, and together with the description, explain the application. The specific embodiments of the application and its explanation are used to explain the application, and do not constitute an improper limitation on the application.

[0016] Figure 1 is a SPAN powder image with a magnification of 100 times in Example 1 of the application; Figure 2 is a SPAN powder image with a magnification of 1k times in Example 1 of the application; Figure 3 is a SPAN film front surface state diagram after phase inversion in Example 1 of the application; Figure 4 is a SPAN film side surface state diagram after phase inversion in Example 1 of the application; Figure 5 is a SPAN film folded 180° state diagram after phase inversion in Example 1 of the application; still maintains the original state, indicating that the phase inversion film has good flexibility; Figure 6 is a SPAN film after cutting state diagram after phase inversion in Example 1 of the application; Figure 7 is a SPAN film after cutting state diagram after phase inversion in Example 2 of the application; still can restore the original state, indicating that the phase inversion electrode sheet has good flexibility; Figure 8 is a SPAN film front surface state diagram after phase inversion in Example 2 of the application; Figure 9 is a SPAN film side surface state diagram after phase inversion in Example 2 of the application; compared with Example 1, the thickness of the film is obviously increased; Figure 10 is a SPAN film after cutting state diagram after phase inversion in Example 2 of the application; Figure 11 is a SPAN film after cutting state diagram after phase inversion in Example 3 of the application; still can restore the original state, indicating that the phase inversion electrode sheet has good flexibility; Figure 12 is a SPAN phase inversion film product 100µm micro-morphology diagram in Example 2 of the application; Figure 13 is an infrared absorption spectrum of five different ratio films in the application; Figure 14 is the Raman spectrum of the SPAN phase inversion film product in Example 2 of the present application; Figure 15 is the CV curve of the SPAN phase inversion film product 20 pm at different scanning rates in Example 1 of the present application; Figure 16 is the comparison chart of the charge-discharge curves of the SPAN phase inversion film products 20 pm and 100 pm in Example 1 and Example 2 of the present application. DETAILED DESCRIPTION

[0017] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0018] In order to solve the technical problems described in the background art, the present application provides a preparation method of a sulfidized polyacrylonitrile-based potassium-sulfur battery positive electrode material, comprising the following steps: After the polyacrylonitrile and sulfur powder are mixed and ball milled in proportion, the sulfidized polyacrylonitrile is obtained by first being kept at 150-165℃ for 1-3h, then being heated to 400-500℃ for 0.5-1.5h for sulfidization in an inert atmosphere; The sulfidized polyacrylonitrile, conductive agent, binder, polyethylene glycol and solvent are uniformly mixed to obtain a casting solution; The casting solution is coated on the surface of a glass plate, then immersed in water for 30-60s, after which the film is separated from the glass plate, and the film is subjected to solvent exchange in water for 5-8h; After the solvent exchange is completed, the film is dried to obtain the electrode material.

[0019] In the preparation of the sulfidized polyacrylonitrile, the low-temperature stage of 150-165℃ is first kept, at which temperature the sulfur is in a liquid state and can fully melt and penetrate into the molecular structure of the polyacrylonitrile (PAN), achieving uniform dispersion of the sulfur and PAN and increasing the contact sites between them; if direct high-temperature is used, the sulfur is easily lost by rapid volatilization, or local reaction is intense, leading to uneven dispersion. The high-temperature stage of 400-500℃ is then kept for the chemical reaction of PAN and sulfur (such as the formation of C-S bond between the cyano group of PAN and sulfur) to form a stable SPAN structure; the reaction rate is faster at high temperature, which can complete the deep sulfidization in a short time and avoid decomposition of the material caused by excessive heating.

[0020] The low-temperature stage (150-165℃) of the application is lower than 200℃: the sulfur melting is more moderate, the loss of sulfur due to premature volatilization at high temperature is reduced, the combination sites of sulfur and PAN are increased, thereby the sulfur content of SPAN is increased; the high-temperature stage (400-500℃) of the application is higher than 350℃, and the holding time is relatively short, so that the reaction can be more complete in a short time, a more stable C-S bond structure is formed, and structural defects caused by insufficient reaction at 350℃ are avoided; at the same time, the short holding time (0.5-1.5h) reduces the side reaction, improves the structural integrity of SPAN, and is also conducive to improving the production efficiency.

[0021] The sulfuration of the application is completed in the powder stage, and the flexibility of the film can be maintained by combining with the subsequent phase inversion process.

[0022] Polyethylene glycol (PEG) is used as a pore-forming agent. PEG has good water solubility. When the casting solution is coated and placed in water for solvent exchange, PEG will dissolve into water and be eluted, thereby leaving uniform distributed pores in the film. This method can accurately control the size and distribution of the pores, form a through porous structure, provide a fast channel for potassium ion transport, and solve the problem of random distribution of ion channels in traditional electrodes.

[0023] Through the porous structure regulated by PEG, a more effective conductive network can be formed in cooperation with the conductive agent, which promotes the redox reaction kinetics of sulfurized polyacrylonitrile (SPAN) in the charging and discharging process, and improves the charging and discharging efficiency and rate performance of the battery.

[0024] PEG has good compatibility with the binder (such as polyacrylonitrile), and will not damage the overall structure of the film while forming pores, which helps to maintain the flexibility of the phase inversion film, avoids the problem of brittle phase inversion film after sulfuration, enhances the structural stability of the electrode sheet, and improves the cycle life of the battery.

[0025] During the solvent exchange process, non-solvent water penetrates into the film, reducing the solubility of the polymer (sulfurized polyacrylonitrile, binder, etc.) in the mixed solution, promoting the aggregation and precipitation of the polymer molecular chain, and forming a stable three-dimensional skeleton structure; the organic solvent (such as DMF) in the film gradually diffuses into water and is completely removed; during the solvent exchange process, PEG gradually dissolves and elutes from the film with the penetration of water, leaving uniform distributed pores in the polymer skeleton; an adequate time of 5-8h ensures that PEG is completely removed, forming a through porous network to provide a fast channel for potassium ion transport.

[0026] In some embodiments, the mass ratio of polyacrylonitrile to sulfur powder is 1:0.5-1.5, preferably 1:0.7-1.2, and further preferably 1:0.9-1.1.

[0027] In some embodiments, the vulcanization is performed in an inert atmosphere, first at 150-160℃ for 1.5-2.5h, and then at 400-500℃ for 1.5-2.5h.

[0028] Preferably, the vulcanization is performed in an inert atmosphere, first at 153-157℃ for 1.7-2.3h, and then at 430-470℃ for 0.5-1.5h.

[0029] Further preferably, the vulcanization is performed in an inert atmosphere, first at 154-156℃ for 1.7-2.3h, and then at 440-460℃ for 0.7-1.3h.

[0030] In some embodiments, the mass ratio of the vulcanized polyacrylonitrile, the conductive agent, the binder, and the polyethylene glycol is 60-80:3-10:10-20:10-20.

[0031] Preferably, the conductive agent is carbon black, conductive graphite, or Ketjen black.

[0032] Preferably, the binder is polyacrylonitrile or polyvinylidene fluoride.

[0033] In some embodiments, the solvent is N-methylpyrrolidone (NMP) or N,N-dimethylformamide (DMF).

[0034] In some embodiments, the casting solution is coated on the surface of a glass plate to obtain a coating layer with a thickness of 20-300μm.

[0035] The film in this range can maintain sufficient mechanical strength (avoiding damage during cutting and assembly) and good flexibility. When the thickness is moderate, the active material loading (energy density) and the potassium ion transport path length (kinetic efficiency) are balanced. When the coating is too thin, the film is prone to breakage, and the risk of breaking during cutting and assembling the battery is high, resulting in a decrease in the yield of finished products. The active material loading is too low to meet the energy demand of the potassium-sulfur battery in practical applications. The uneven distribution of the pore-forming agent and the fewer ion transport channels caused by the thin coating result in poor kinetic performance.

[0036] When the coating is too thick, the increased thickness increases the potassium ion transport path length and the polarization, significantly reducing the charge and discharge efficiency and the rate performance. Insufficient solvent exchange in the thick film leaves residual organic solvents or pore-forming agents, affecting the electrical conductivity and structural stability of the film. The thick film is prone to cracking when folded or bent, reducing the cycle life of the electrode. The long drying time of the thick film makes it difficult to coat, increasing the production cost.

[0037] In some embodiments, after the solvent exchange is completed, the thin film is wrapped with a silicon oil paper to absorb water, then laid flat and pressed with a glass sheet before drying.

[0038] The silicone oil paper has certain water absorption, can absorb the residual moisture on the surface of the film, and accelerates the drying process; meanwhile, its non-stick property avoids the adhesion of the film and the wrapping material, facilitating subsequent separation. Pressing the flat film with a glass sheet can fix its shape, avoid edge curling, wrinkling or shrinkage deformation caused by uneven evaporation of moisture during the drying process, and ensure that the film remains flat. Pressing can also make the film thickness uniformly distributed, avoid local over-thickness or under-thickness, maintain the consistency of the microstructure (such as pore distribution), and improve the performance stability of the electrode. Flat drying reduces the stress concentration in the film, which helps to maintain its flexibility.

[0039] In a second aspect, the present application provides a sulfidized polyacrylonitrile-based potassium-sulfur battery positive electrode material prepared by the preparation method, having a through-hole porous structure.

[0040] In a third aspect, the present application provides a potassium-sulfur battery, whose positive electrode sheet is prepared from the sulfidized polyacrylonitrile-based potassium-sulfur battery positive electrode material.

[0041] The present application will be further described in detail below in conjunction with specific examples, but the scope of protection of the present application is not limited to these examples.

[0042] Example 1 A preparation method of a sulfidized polyacrylonitrile-based potassium-sulfur battery positive electrode material, comprising the following steps: (1) Preparation of sulfidized polyacrylonitrile (SPAN): Polyacrylonitrile (PAN) with a molecular weight of 150,000 and sulfur powder were mixed in a mass ratio of 1:1 and ball-milled for 2 h to obtain a light yellow powder; the light yellow powder mixture was uniformly placed in a crucible and transferred to a tube furnace for sulfidization operation, with a temperature curve set at 155℃ for 2 h, 450℃ for 1 h, and a heating rate of 5℃ / min, and sulfidized in a nitrogen atmosphere for 8 h under the set temperature curve, to obtain black SPAN powder.

[0043] (2) Preparation of phase inversion film: 3g of prepared sulfidized polyacrylonitrile (SPAN), 0.5g of PAN, 0.2g of CB, 0.5g of PEG, and 9g of DMF were mixed in a beaker and heated to 80℃ in a magnetic stirrer for 15 min to obtain a casting solution for standby; After cooling to room temperature, a 20μm film applicator was used to coat the film on a glass plate, and then the glass plate was placed in a deionized water tank together with the glass plate, and after waiting for 30s, the glass plate was slightly shaken to make the film separate from the glass plate. The film was taken out after solvent exchange in deionized water for at least 5h, and the film was dried at room temperature wrapped with silicone oil paper, and a glass sheet was pressed to prevent edge curling, and dried for more than 8h to obtain a phase inversion film.

[0044] (3) Preparation of potassium-sulfur battery The dried phase inversion film prepared in step (2) is wrapped with silicone oil paper on both sides, and then placed in a cutting machine to cut into a circular SPAN electrode piece of appropriate size. During this process, attention should be paid to not crushing the phase inversion film.

[0045] The prepared SPAN electrode piece is assembled into a half battery: first, a block of metallic potassium is cut into a small piece with a knife, and the metallic potassium is rolled flat, and then punched into a circular potassium piece with a punch, with a thickness of about 0.1 mm and a diameter of about 8 mm.

[0046] First, the positive shell of the battery is placed, then the SPAN electrode piece is placed on the positive electrode, 50 μL of electrolyte is added (if the electrode piece is folded during this process, it needs to be unfolded with tweezers, and if it is accidentally damaged, a new electrode piece is replaced), then a glass fiber separator is placed, then the previously prepared potassium piece is placed, then 50 μL of electrolyte is added, then a thick gasket, a thin gasket, and a spring are sequentially placed, and finally the battery negative shell is covered. The assembled battery is quickly turned over so that the positive electrode is on top and placed in a hydraulic machine to compress it.

[0047] The batteries are labeled and placed in a plastic bag for use (the batteries used for detecting electrochemical performance are all CR2032 type button batteries). The entire process is carried out in an Ar-filled glove box.

[0048] Figure 1 is a SPAN powder image in Example 1 with a magnification of 100 times; Figure 2 is a SPAN powder image in Example 1 of the present application with a magnification of 1k times, which is Figure 1 and Figure 2 It can be seen that the sulfurized SPAN powder presents a dispersed elliptical granular shape, and the particle size is not uniform, with some SPAN particles agglomerating together to form large particles, so a suitable solvent is needed to dissolve the SPAN to ensure the uniform thickness of the subsequent phase inversion film.

[0049] Figure 3 is a SPAN film after phase inversion in Example 1, which is Figure 4 is a SPAN film after phase inversion in Example 1 of the present application, which is Figure 3 and Figure 4 It can be seen from the front view that the surface of the phase inversion film is flat and smooth, but the edge of the film is curled and the phase inversion film is slightly wrinkled due to shrinkage during the drying process. It can be seen from the side view that the thickness of the phase inversion film using a fixed thickness applicator is uniform.

[0050] Figure 5is a state diagram of the SPAN membrane after phase inversion in Example 1 folded 180°. It can be seen that after folding 180°, the SPAN membrane still maintains its original state, indicating that the phase inversion membrane has good flexibility.

[0051] Figure 6 is a state diagram of the SPAN membrane after phase inversion in Example 1 after cutting. It can be seen that after cutting, the electrode sheet presents a regular circular shape and is black in color due to the removal of the curled edges of the phase inversion membrane. Figure 6

[0052] Figure 15 is a CV curve of the SPAN phase inversion membrane product in Example 1 at different scanning rates of 20µm. It can be seen that, Figure 15 Figure 15 reflects the process of redox reaction in the battery. By observing the shape of the curve, the position and intensity of the peaks, and other characteristics, the electrochemical performance of the battery can be understood. As the scanning rate increases from 0.3mV / s to 10mV / s, the overall value of the current density increases. This is because a higher scanning rate means that the electrode undergoes a greater potential change in a shorter time, resulting in more charge transfer and thus a greater current. The CV curve usually has oxidation and reduction peaks, and in the figure, it can be observed that there are obvious peaks at different potentials. At scanning rates of 0.3mV / s, 3mV / s, 5mV / s, and 7mV / s, the curves are well closed, indicating that the thin film structure is relatively stable. However, at a scanning rate of 10mV / s, the curve closure is poor, leading to an increase in overpotential and a certain damage to the stability of the thin film structure. At high potential change rates, the electrode reaction system rapidly deviates from the thermodynamic equilibrium state, and the peaks at different scanning speeds have a certain potential shift, indicating that the reaction system is more likely to be a quasi-reversible system, not a completely reversible reaction. However, at the same scanning rate, especially at low and medium scanning speeds, the curve closure is better, and the reaction system has a certain degree of reversibility. Overall, the battery has certain cycle stability.

[0053] Example 2 A method for preparing a sulfidized polyacrylonitrile-based potassium-sulfur battery positive electrode material, comprising the following steps: (1) Preparation of sulfidized polyacrylonitrile (SPAN): Mix polyacrylonitrile (PAN) with a molecular weight of 150,000 and sulfur powder in a mass ratio of 1:1 and ball mill for 2h to obtain a light yellow powder; evenly place the light yellow powder mixture in a crucible and transfer it to a tube furnace for sulfidation operation. The temperature curve is set to 155℃ for 2 hours and 450℃ for 1 hour with a heating rate of 5℃ / min. Sulfurize for 8h in a nitrogen atmosphere with the set temperature curve to obtain black SPAN powder.

[0054] ​​(2) Preparation of phase inversion film: 3g of prepared sulfided polyacrylonitrile (SPAN), 0.5g of PAN, 0.2g of CB, 0.5g of PEG, and 9g of DMF were mixed in a beaker and heated to 80°C in a magnetic stirrer for 15min to obtain a casting solution for standby; After cooling to room temperature, a 100μm film applicator was used to coat the film on a glass plate, and then the glass plate was placed in a deionized water tank, and the glass plate was slightly shaken to make the film separate from the glass plate. After solvent exchange in deionized water for at least 5h, the film was taken out, dried at room temperature with silicon oil paper, pressed with a glass sheet to prevent curling, and dried for more than 8h to obtain a phase inversion film.

[0055] (3) Preparation of potassium-sulfur battery The dried phase inversion film prepared in step (2) was wrapped with silicon oil paper on both sides and placed in a cutting machine to cut into a suitable size of circular electrode sheet. This process should pay attention to not crushing the phase inversion film.

[0056] The prepared SPAN electrode sheet was assembled into a half-cell: First, a small piece of bulk metal potassium was cut off with a knife, and the metal potassium was rolled flat and punched into a circular sheet with a punch, with a thickness of about 0.1mm and a diameter of about 8mm.

[0057] First, the positive shell of the battery was placed, then the SPAN electrode sheet was placed on the positive electrode, 50μL of electrolyte was added (if the electrode sheet is folded during this process, it needs to be unfolded with tweezers, and if it is accidentally damaged, a new electrode sheet is replaced), then a glass fiber separator was placed, followed by the previously prepared potassium sheet, then 50μL of electrolyte was added, and then thick and thin gaskets and springs were placed in turn, and finally the negative shell of the battery was covered.

[0058] The assembled battery was quickly turned over so that the positive electrode was on top and placed in a hydraulic machine to press tightly.

[0059] After marking the battery, it was placed in a plastic bag for standby (the type of battery used for detecting electrochemical performance was CR2032 type button cell). The whole process was carried out in an Ar-filled glove box.

[0060] Figure 7 is a state diagram of the SPAN film after phase inversion of Example 1 after cutting and folding 180°. The film can still be restored to its original state after folding 180°, indicating that the phase inversion electrode sheet has good toughness.

[0061] Figure 8 is a front view of the SPAN film after phase inversion of Example 2, which shows Figure 8It can be seen that the surface of the phase inversion film is flat and the thickness is uniform. However, the drying process can induce shrinkage of the phase inversion film, resulting in curling of the film edge and wrinkles on the film surface.

[0062] Figure 9 is the side view of the SPAN film after phase inversion in Example 2. Compared with Example 1, the thickness of the film is significantly increased.

[0063] Figure 10 is the state diagram of the SPAN film after cutting in Example 2 after phase inversion; Figure 12 is the micro-morphology diagram of the SPAN phase inversion film product of 100 pm in Example 2. Figure 12 It can be seen that the surface of the phase inversion film is uniformly distributed with large pores, and the connecting parts around the large pores are distributed with fine small pores. The small pores are uniformly distributed, showing a three-dimensional network structure or a sponge-like structure, and part of the SPAN can be seen through the pores.

[0064] Figure 14 is the Raman spectrum diagram of the SPAN phase inversion film product in Example 2. Figure 14 It can be seen that there is a characteristic peak of S-S bond at about 475 cm -1 , and a characteristic peak of C-S bond at about 940 cm -1 , which proves the existence of SPAN and the successful introduction of disulfide bond. The D peak and G peak near 1330 cm -1 and 1560 cm -1 are two important characteristic peaks in the spectrum of graphene and carbon materials (such as graphite, carbon nanotubes, etc.). Their position, intensity and shape can provide a lot of information. The D peak represents the defect mode in graphene or carbon material, and the G peak represents the graphite mode. The intensity of the D peak is proportional to the defect density in the material and is also related to the degree of amorphization of the material. In a completely amorphous carbon material, the D peak will be very significant. The intensity of the G peak is proportional to the degree of graphitization of the material, and its width and shape can also reflect the order of the material. Higher crystallinity and order usually correspond to a narrower G peak.

[0065] Figure 16is the contrast chart of charge-discharge curves of SPAN phase inversion film products 20 µm and 100 µm in Example 1 and Example 2, in which, 1 refers to the first cycle of charge-discharge curve, 5 refers to the fifth cycle of charge-discharge curve, and 10 refers to the tenth cycle of charge-discharge curve, and it can be seen from the charts a and b that the film thickness is closely related to the charge-discharge performance. The first cycle discharge specific capacity of 20 µm film and 100 µm film is all above 400 mAh / g, and in the subsequent cycles, the specific capacity of 100 µm film is generally lower than that of 20 µm film. It is found that the charge-discharge performance of 20 µm film generally exceeds that of the film with thickness above 100 µm, and the performance of 100 µm film in most products is less decreased compared with that of 20 µm film.

[0066] Example 3 The difference from Example 1 is that the mass ratio of polyacrylonitrile (PAN) to sulfur powder is 1:6, and the others are the same as those in Example 1.

[0067] Figure 11 is the state chart of the SPAN film of Example 3 after phase inversion and folding 180°, and the film can still restore the original state after folding 180°, indicating that the phase inversion electrode sheet has good toughness.

[0068] Example 4 The difference from Example 1 is that the mass ratio of polyacrylonitrile (PAN) to sulfur powder is 1:9, and the others are the same as those in Example 1.

[0069] Example 5 The difference from Example 1 is that the mass ratio of polyacrylonitrile (PAN) to sulfur powder is 1:10, and the others are the same as those in Example 1.

[0070] Example 6 The difference from Example 1 is that the mass ratio of polyacrylonitrile (PAN) to sulfur powder is 1:12, and the others are the same as those in Example 1.

[0071] Figure 13 is the infrared absorption spectrum of the five different ratio films in Example 1, Example 3-6 of the present application, in which Figure 13 It can be seen that the infrared absorption spectrum of 1-6 film has one more C-O doublet than that of 1-1 film in 1260-1140 cm -1 , and the difference between the raw materials of the two films is that a small amount of PMMA is added in 1-6 film. When there is one more C-O doublet in 1260-1140 cm -1 , it is the fingerprint region of PMMA methyl acrylate structure. When the strong C=O peak near 1730 cm -1 is observed, it is confirmed that the ester bond exists, and then the C-H stretching vibration in 3000-2800 cm -1 and 1400-1300 cm -1flexural vibration of PMMA can be determined. In combination with the above figure, it can be seen that there is a stretching vibration absorption peak of S-S bond at 550 m -1 cm-1, and a C-S bond at 660~670 cm -1 -1. Meanwhile, there is C≡N at 2240-2241 cm -1 -1, which is a characteristic peak of PAN binder. There is a stretching vibration absorption peak of C=N double bond at about 1345 cm -1 -1360 cm -1 -1. This indicates that during the vulcanization process, the cleavage of nitrile group occurs while S-S and C-S bonds are introduced by the reaction of PAN and S.

[0072] Comparative Example 1 The difference from Example 1 is that polyethylene glycol is omitted, and the others are the same as Example 1.

[0073] Comparative Example 2 The difference from Example 1 is that during vulcanization, first heat at 220℃ for 2 h in an inert atmosphere, then heat at 350℃ for 4 h, and the others are the same as Example 1.

[0074] Comparative Example 3 The difference from Example 1 is that polyethylene glycol is omitted, and the obtained slurry is directly coated on aluminum foil, and the coating amount is the same as Example 1, and the others are the same as Example 1.

[0075] The constant current charge-discharge data of the electrode films prepared by using PAN itself as the binder of SPAN in Example 1 and Comparative Examples 1-3 are as follows (current density 0.05 A / g). The first circle discharge specific capacity of Example 1 is 370 mAh / g, and the discharge specific capacity after 100 cycles is 80 mAh / g, and the CV curve is in a closed shape as a whole, proving that the battery can be charged and discharged with high specific capacity for multiple cycles. The first circle discharge specific capacity of Comparative Example 2 is 251 mAh / g, and the discharge specific capacity after 100 cycles is 60 mAh / g, and the capacity of the battery decreases obviously after multiple charge-discharge cycles. Comparative Examples 1 and 3 have poor performance due to the lack of pore-forming agent, and the first circle discharge specific capacity is 240 mAh / g and 230 mAh / g, respectively, and the discharge specific capacity after 100 cycles is 45 mAh / g and 40 mAh / g, respectively, and the cycle performance of the battery is poor.

[0076] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Those skilled in the art can make various modifications and changes to the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a potassium-sulfur battery cathode material based on sulfurized polyacrylonitrile, characterized in that: Includes the following steps: Polyacrylonitrile and sulfur powder are mixed in a certain proportion and ball-milled. Then, in an inert atmosphere, the mixture is first kept at 150-165℃ for 1-3 hours, and then heated to 400-500℃ and kept for 0.5-1.5 hours to vulcanize, thus obtaining vulcanized polyacrylonitrile. Vulcanized polyacrylonitrile, conductive agent, binder, polyethylene glycol and solvent are mixed evenly to obtain casting solution; The casting solution is coated onto the surface of a glass plate, and then immersed in water for 30-60 seconds to separate the film from the glass plate. The film is then subjected to solvent exchange in water for 5-8 hours. After solvent exchange is complete, the film is dried to obtain the electrode material.

2. The method for preparing the sulfurized polyacrylonitrile-based potassium-sulfur battery cathode material according to claim 1, characterized in that: The mass ratio of polyacrylonitrile to sulfur powder is 1:0.5-1.

5.

3. The method for preparing the sulfurized polyacrylonitrile-based potassium-sulfur battery cathode material according to claim 1, characterized in that: During vulcanization, in an inert atmosphere, first hold at 150-160℃ for 1.5-2.5 hours, then hold at 400-500℃ for 1.5-2.5 hours. Preferably, during vulcanization, in an inert atmosphere, the temperature is first maintained at 153-157℃ for 1.7-2.3h, and then maintained at 430-470℃ for 0.5-1.5h. Preferably, during vulcanization, the temperature is first maintained at 154-156℃ for 1.7-2.3h in an inert atmosphere, and then maintained at 440-460℃ for 0.7-1.3h.

4. The method for preparing the sulfurized polyacrylonitrile-based potassium-sulfur battery cathode material according to claim 1, characterized in that: The mass ratio of vulcanized polyacrylonitrile, conductive agent, binder and polyethylene glycol is 60-80:3-10:10-20:10-20.

5. The method for preparing the sulfurized polyacrylonitrile-based potassium-sulfur battery cathode material according to claim 1, characterized in that: The conductive agent is carbon black, conductive graphite, or Ketjen black. Alternatively, the adhesive may be polyacrylonitrile or polyvinylidene fluoride.

6. The method for preparing the sulfurized polyacrylonitrile-based potassium-sulfur battery cathode material according to claim 1, characterized in that: The solvent is N-methylpyrrolidone or N,N-dimethylformamide.

7. The method for preparing the sulfurized polyacrylonitrile-based potassium-sulfur battery cathode material according to claim 1, characterized in that: The casting solution is applied to the surface of a glass plate, and the resulting coating has a thickness of 20-300 μm.

8. The method for preparing the sulfurized polyacrylonitrile-based potassium-sulfur battery cathode material according to claim 1, characterized in that: After solvent exchange is complete, the film is wrapped in silicone paper to absorb water, then laid flat, pressed with a glass slide, and dried to obtain the final product.

9. A sulfurized polyacrylonitrile-based potassium-sulfur battery cathode material, characterized in that: Prepared by any one of the preparation methods described in claims 1-8, it has a through-porous structure.

10. A potassium-sulfur battery, characterized in that: Its positive electrode is made from the sulfurized polyacrylonitrile-based potassium-sulfur battery positive electrode material as described in claim 9.

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