An organic sulfur small molecule compound for a lithium-sulfur battery cathode material, a preparation method thereof, and an application thereof

Abstract

The present invention discloses an organic sulfur small molecule compound for a lithium-sulfur battery cathode material, a preparation method thereof, and an application. The general structural formula of the compound is: Ar-S n -Ar; wherein, Ar is a fluorobenzaldehyde group; n is the number of sulfur atoms, taking a value of 3 or 4, and bis(4-fluorobenzaldehyde) tetrasulfide (BFBTS) is preferably used. The present invention is prepared by one-step synthesis, with a clear structure and easy to scale up. This material is applied to the cathode of all-solid-state lithium-sulfur batteries, effectively improving the electrochemical performance. The aldehyde group and halogen substituents enhance the redox activity and conductivity, stabilize the sulfur chain, and inhibit the polysulfide shuttle effect. The initial capacity of the BFBTS cathode battery reaches 500 mAh / g at a 1C rate, and the capacity retention rate is greater than 80% after 500 cycles, showing high capacity and excellent cycle stability. This material provides an innovative strategy for high-performance lithium-sulfur battery cathodes, is suitable for fields such as electric vehicles and energy storage systems, and has both high energy density and stable cycle performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of cathode materials for lithium-sulfur batteries, and particularly relates to an organic sulfur small molecule compound for cathode materials of lithium-sulfur batteries, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium-ion batteries are widely used in fields such as portable electronic devices and electric vehicles due to their high energy density. However, the energy density of traditional lithium-ion batteries has approached the theoretical limit and it is difficult to meet the growing energy demand. Lithium-sulfur batteries use elemental sulfur (S8) as the cathode material, with a theoretical specific capacity as high as 1675 mAh / g and great potential for energy density, and are considered an ideal choice for the next generation of high-energy density energy storage devices.

[0003] However, the traditional lithium-sulfur battery cathode using elemental sulfur (S8) has the following inherent defects, which seriously hinder its practical application:

[0004] Poor conductivity of sulfur: Elemental sulfur is essentially an insulator, resulting in low electronic conductivity of the electrode, limited utilization rate of active substances, and poor rate performance.

[0005] "Shuttle effect" of polysulfides: During charge and discharge, soluble polysulfide intermediates (Li2S x , 4≤x≤8) will be generated at the sulfur cathode. These polysulfides are easily soluble in common electrolytes and shuttle between the positive and negative electrodes, resulting in problems such as loss of active substances, decreased utilization rate of sulfur at the positive electrode, corrosion of the negative electrode, reduction of Coulomb efficiency, and attenuation of cycle life, that is, the "shuttle effect".

[0006] Volume expansion: Sulfur will undergo significant volume changes during charge and discharge, resulting in unstable electrode structure, shedding of active substances, and affecting the cycle life of the battery.

[0007] To solve the above problems, researchers are committed to developing new cathode materials for lithium-sulfur batteries. As an important sulfur cathode material, organic sulfur compounds have attracted much attention because they can fix sulfur atoms in the organic molecular framework through covalent bonds, thereby inhibiting the dissolution and "shuttle effect" of polysulfides to a certain extent. However, existing organic sulfur small molecule cathode materials still face problems such as low capacity and insufficient cycle stability, and their performance is still limited by aspects such as sulfur chain length, molecular conductivity, redox activity, and compatibility with electrolytes.

[0008] Therefore, there is an urgent need to develop new organic sulfur small molecule cathode materials with controllable structures and excellent performances to improve the comprehensive electrochemical performance of lithium-sulfur batteries. Summary of the Invention

[0009] In view of this, the object of the present invention is to overcome the problems existing in the prior art, and to provide an organic sulfur small molecule compound for a lithium-sulfur battery cathode material, a preparation method thereof, and an application thereof. This compound is a novel organic sulfur small molecule compound designed based on aldehyde group-containing and halogen-substituted benzene derivatives, and the application of this compound as a cathode material for all-solid-state lithium-sulfur batteries. The present invention aims to optimize the structure of organic sulfur small molecules through a molecular engineering strategy, improve their redox activity, electronic conductivity, and structural stability, thereby enhancing the energy density, cycle stability, and rate performance of lithium-sulfur batteries.

[0010] Based on the organic sulfur small molecule cathode material containing aldehyde group and halogen-substituted benzene derivatives and its application in all-solid-state lithium-sulfur batteries, the discharge specific capacity at 1C can reach 500 mAh / g, and the capacity retention rate is 80%.

[0011] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0012] <The first aspect>

[0013] The present invention designs an organic sulfur small molecule compound for a lithium-sulfur battery cathode material, and its structural general formula is: Ar-S n -Ar;

[0014] Among them, Ar is fluorobenzaldehyde group; n is the number of sulfur atoms, and the value is 3 or 4.

[0015] Preferably, the fluorobenzaldehyde group includes one of 2-fluorobenzaldehyde group (o-fluorobenzaldehyde), 3-fluorobenzaldehyde group (m-fluorobenzaldehyde), or 4-fluorobenzaldehyde group (p-fluorobenzaldehyde). In the organic sulfur small molecule compound, Ar is a substituted phenyl group, and there is one and only one aldehyde group (-CHO) and one fluorine atom (-F) connected to the substituted phenyl group as substituents.

[0016] Ar is preferably 4-fluorobenzaldehyde group (p-fluorobenzaldehyde). When n takes the value of 4, the organic sulfur small molecule compound is bis(4-fluorobenzaldehyde) tetrasulfide (abbreviated as BFBTS), and its structural formula is F-C6H3(CHO)-S4-C6H3(CHO)-F.

[0017] In the present invention, the value of the number of sulfur atoms n is 3 or 4, and the energy density of the sulfur battery is relatively high. The energy storage capacity of the sulfur-based cathode material mainly comes from the redox reaction of sulfur elements, and the length of the sulfur chain directly determines the number of sulfur atoms in each organic sulfur molecule. When the value of n is too small (less than 3), it means that the number of sulfur atoms contained in each small organic sulfur molecule is significantly reduced. The reduction in the number of atoms directly leads to a decrease in the content of the active substance sulfur, thereby reducing the theoretical specific capacity of the cathode material. Although the molecular weight of the organic sulfur compound also decreases with the decrease in the sulfur chain length, the reduction in capacity usually exceeds the reduction in molecular weight, so the overall energy density will be adversely affected. In addition, the length of the sulfur chain affects the redox reaction activity of sulfur to a certain extent. Although organic sulfur compounds are designed to improve the insulation of elemental sulfur, too short a sulfur chain may not be able to fully exert the redox potential of sulfur, resulting in insufficient redox reaction of sulfur or a change in the reaction mechanism, reducing the utilization rate of sulfur and further reducing the actual discharge capacity. The present invention emphasizes enhancing the redox activity by introducing aldehyde groups and halogen substituents, but if the sulfur chain itself is too short, it may be difficult to reach the ideal capacity level even with the assistance of substituents.

[0018] The present invention preferably controls the sulfur chain length to be 3 - 4, aiming to balance the capacity and the solubility of polysulfides. Too short a sulfur chain length (n < 3) may change the electronic structure and chemical properties of the sulfur chain, and may instead affect the stability of the sulfur chain. Although short sulfur chains may theoretically reduce the formation of long-chain polysulfides (the main substance causing the shuttle effect), too short a sulfur chain may form other types of soluble sulfur species, and there may still be problems of dissolution and migration.

[0019] <Second aspect>

[0020] The present invention also provides a preparation method of the organic sulfur small molecule compound, including the following steps:

[0021] Mix fluorobenzaldehyde with Na2S n Add them to a polar solvent, heat to 80 - 100 °C, stir and react for 12 - 24 h, pour the reaction mixture into an anti-solvent, and the obtained precipitate product is the organic sulfur small molecule compound.

[0022] Preferably, the fluorobenzaldehyde is one of 2-fluorobenzaldehyde group (o-fluorobenzaldehyde), 3-fluorobenzaldehyde group (m-fluorobenzaldehyde), or 4-fluorobenzaldehyde group (p-fluorobenzaldehyde).

[0023] Preferably, the value of n in Na2S n is 3 or 4.

[0024] Preferably, fluorobenzaldehyde and Na2S nThe molar ratio is 1:1.1 - 1.5, preferably 1:1.2. For fluorobenzaldehyde and Na2S n , the dosage ratio of polar solvent is 1 mol:1.1 - 1.5 mol:80 - 120 ml.

[0025] Preferably, the polar solvent includes N,N-dimethylformamide (DMF).

[0026] Preferably, the reaction is carried out under nitrogen protection. The reaction system is heated to 80 - 100 °C and stirred for 12 - 24 hours. After the reaction, it is cooled to room temperature. The main purpose of cooling to room temperature is to terminate the reaction and prepare for the subsequent precipitation step. At this time, the system mainly exists in the form of a solution or suspension, and a large amount of precipitate product will not be obtained.

[0027] Preferably, the anti-solvent is deionized water. As an anti-solvent, deionized water can effectively induce the large-scale and efficient precipitation of BFBTS from the DMF solution and improve the product purity.

[0028] Preferably, the obtained precipitate product is centrifuged to collect the solid product, which is the small organic sulfur compound. The solid product after centrifugation is washed 2 - 3 times with deionized water and ethanol respectively to remove the residual reactants and solvents. The washed product is placed in a vacuum drying oven and vacuum dried at 50 - 70 °C for 10 - 15 hours to obtain the target product.

[0029] The fluorobenzaldehyde is 4-fluorobenzaldehyde, and Na2S n is Na2S4, and the obtained product is bis(4-fluorobenzaldehyde) tetrasulfide (BFBTS).

[0030] <Third aspect>

[0031] The present invention also provides an application of the small organic sulfur compound in the preparation of a positive electrode of a lithium-ion battery.

[0032] The positive electrode of the lithium-ion battery includes a small organic sulfur compound, a conductive additive, and a binder. The mass ratio of the small organic sulfur compound, the conductive additive, and the binder is 7:2:1.

[0033] The conductive additive is selected from one or more of carbon black, acetylene black, graphene, and carbon nanotubes.

[0034] The binder is selected from one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and sodium carboxymethyl cellulose (CMC).

[0035] Preparation method of the positive electrode of a lithium-ion battery: Grind and mix an organic sulfur small molecule compound and a conductive additive evenly, add them to a binder / NMP solution, continue to grind and stir to make a uniform positive electrode paste, and then evenly coat and vacuum dry the positive electrode paste.

[0036] The present invention also provides an application of the organic sulfur small molecule compound in a all-solid-state lithium-sulfur secondary battery.

[0037] The present invention also provides a all-solid-state lithium-sulfur secondary battery, including a sulfide solid electrolyte, a negative electrode, and a positive electrode of a lithium-ion battery prepared from the above-mentioned organic sulfur small molecule compound.

[0038] The negative electrode includes a lithium metal negative electrode or a metal lithium composite negative electrode.

[0039] The sulfide solid electrolyte includes a sulfide electrolyte including Li6PS5Cl, Li3PS4, Li 10 GeP2S 12 , one of Li6PS5Br and Li6PS5I.

[0040] Through molecular structure design, the present invention realizes high energy density and more stable cycling performance, and has more excellent battery performance. The capacity of traditional organic sulfur positive electrode materials is limited. However, in this application, by introducing aldehyde groups and halogen substituents, the redox activity and utilization rate of sulfur are effectively improved, thus making it possible to achieve higher energy density and meet high energy requirements. The more stable molecular structure formed by the introduction of halogen substituents may inhibit the dissolution of polysulfides and the occurrence of the shuttle effect, and enhance the structural stability of the electrode material, thereby improving the cycle life of the battery.

[0041] Compared with the all-solid-state lithium metal battery in the prior art, the beneficial effects of the present invention are as follows:

[0042] (1) High capacity and stable sulfur chains: By precisely controlling the sulfur chain length (n = 3, 4), the generation of long-chain polysulfides is effectively reduced, thereby suppressing the "shuttle effect" of polysulfides to a certain extent and improving the stability of sulfur chains. At the same time, the tetrathio chain structure can provide a relatively high theoretical capacity. After calculation, the theoretical specific capacity of BFBTS can reach more than 714 mAh / g.

[0043] (2) Enhancement of the redox activity of aldehyde groups: The aldehyde group (-CHO) introduced into the molecular structure as an electron-withdrawing group can effectively adjust the overall electronic structure of the molecule, enhance the redox activity of the molecule, lower the energy barrier of the sulfur redox reaction, thereby improving the utilization rate of sulfur and the actual capacity of the battery.

[0044] (3)Synergistic effect of halogen substituents: The fluorine atoms (F) introduced into the molecular structure have strong electronegativity, which can stabilize the sulfur chain, enhance the strength of the intramolecular covalent bond, thereby reducing the solubility of small organic sulfur molecules in the electrolyte and further inhibiting the generation and diffusion of polysulfides.

[0045] (4)Optimizing molecular design to improve conductivity: The conjugated structure of the benzene ring and the introduction of aldehyde groups and halogen substituents improve the electronic conductivity of the material to a certain extent, reduce the electrode polarization, and are beneficial to improving the rate performance and energy efficiency of the battery. Description of the Drawings

[0046] Other features, objects, and advantages of the present invention will become more apparent by reading the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0047] Figure 1 Cycling performance curve of BFBTS prepared for Example 1 in a lithium-sulfur battery;

[0048] Figure 2 Cycling performance comparison chart of Example 1 and Comparative Example 2 in a lithium-sulfur battery;

[0049] Figure 3 Mass spectrum of Example 1;

[0050] Figure 4 Mass spectrum of Example 2. Detailed Description of the Invention

[0051] The present invention will be described in detail below with reference to the embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that those of ordinary skill in the art can make several adjustments and improvements without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0052] Example 1

[0053] This example provides the preparation of a positive electrode sheet of bis(4-fluorobenzaldehyde) tetrasulfide (BFBTS).

[0054] The preparation method of the bis(4-fluorobenzaldehyde) tetrasulfide (BFBTS) includes the following steps:

[0055] S1. Raw materials and reagents:

[0056] Reagent preparation: 4-fluorobenzaldehyde (99%, Sigma-Aldrich), N,N-dimethylformamide (DMF, anhydrous, analytical grade), deionized water, absolute ethanol, Na2S4 (99%, Ron reagent).

[0057] S2. Synthesis process:

[0058] Add 4-fluorobenzaldehyde (12.4 g, 0.1 mol) and Na2S4 (17.4 g, 0.12 mol) into a 250 mL three-necked flask equipped with a magnetic stir bar. Add 100 mL of anhydrous N,N-dimethylformamide (DMF) as the solvent into the flask. Place the flask in an oil bath, under the protection of a nitrogen atmosphere, heat it up to 90 °C, and keep stirring and reacting for 18 hours. During the reaction, the color of the system gradually changes from light yellow to dark red. After the reaction is completed, stop heating and cool it to room temperature. Pour the reaction mixture into 500 mL of deionized water, stir vigorously, and a red solid precipitate will form. Collect the red solid precipitate by centrifugation, set the rotation speed to 4000 rpm, and the centrifugation time to 10 minutes. Wash the collected red solid precipitate three times with 200 mL of deionized water and 100 mL of anhydrous ethanol respectively to fully remove the residual reactants and solvents. Place the washed red solid product in a vacuum drying oven and dry it under vacuum at 60 °C for 12 hours to obtain the red solid product bis(4-fluorobenzaldehyde) tetrasulfide (BFBTS), and the structural formula is as follows:

[0059] .

[0060] Figure 3 It is the mass spectrum of the product of Example 1. The proportion of S is 34.25%, and it is confirmed to be BFBTS.

[0061] S3. Preparation of the positive electrode sheet:

[0062] Weigh the BFBTS material, conductive carbon black (Super P, Timcal), and polyvinylidene fluoride (PVDF, Solef5130) according to a mass ratio of 7:2:1.

[0063] Add the weighed BFBTS material and conductive carbon black into an agate mortar and grind and mix them evenly.

[0064] Dissolve the PVDF binder in 20 mL of N-methylpyrrolidone (NMP, anhydrous, 99%, Sigma-Aldrich) solvent to prepare a PVDF / NMP solution.

[0065] Gradually add the evenly ground BFBTS and carbon black mixture into the PVDF / NMP solution, continue to grind and stir, and adjust the slurry viscosity to 500 mPa·s to make a uniform black positive electrode slurry.

[0066] Coat the prepared positive electrode slurry evenly on an aluminum foil current collector with a thickness of 20 μm.

[0067] The coated aluminum foil was placed in a vacuum oven and vacuum dried at 60 °C for 12 hours to fully remove the solvent NMP.

[0068] The dried electrode sheet was rolled to 50 μm using a roll press to improve the density of the electrode sheet and the contact of the materials.

[0069] The rolled electrode sheet was cut into circular electrode sheets with a diameter of 10 mm.

[0070] Example 2

[0071] This example provides the preparation of a bis(4-fluorobenzaldehyde) trisulfide positive electrode sheet.

[0072] The preparation method of the bis(4-fluorobenzaldehyde) trisulfide includes the following steps:

[0073] S1. Raw materials and reagents:

[0074] Reagent preparation: 4-fluorobenzaldehyde (99%, Sigma-Aldrich), N,N-dimethylformamide (DMF, anhydrous, analytical grade), deionized water, absolute ethanol, Na2S3 (99%, Tianfu Chemical Industry).

[0075] S2. Synthesis process:

[0076] 4-Fluorobenzaldehyde (12.4 g, 0.1 mol) and Na2S3 (15.0 g, 0.12 mol) were added to a 250 mL three-necked flask equipped with a magnetic stirrer. 100 mL of anhydrous N,N-dimethylformamide (DMF) was added to the flask as a solvent. The flask was placed in an oil bath and heated to 90 °C under a nitrogen atmosphere, and stirring was maintained for 18 hours. During the reaction, the color of the system gradually changed from light yellow to orange-red. After the reaction, heating was stopped and the mixture was cooled to room temperature. The reaction mixture was poured into 500 mL of deionized water and stirred vigorously to precipitate an orange-red solid precipitate. The orange-red solid precipitate was collected by centrifugation at a rotation speed of 4000 rpm for 10 minutes. The collected orange-red solid precipitate was washed three times with 200 mL of deionized water and 100 mL of absolute ethanol respectively to fully remove the residual reactants and solvents. The washed orange-red solid product was placed in a vacuum drying oven and vacuum dried at 60 °C for 12 hours to obtain an orange-red solid product bis(4-fluorobenzaldehyde) trisulfide, and the structural formula is as follows:

[0077] .

[0078] Figure 4Mass spectrum of Example 2, the proportion of S is 20.66%, confirmed as bis(4-fluorobenzaldehyde) trisulfide.

[0079] S3. The preparation of the positive electrode sheet is the same as that in Example 1.

[0080] Example 3

[0081] This example provides the preparation of a positive electrode sheet of bis(2-fluorobenzaldehyde) tetrasulfide. The preparation method steps and parameters of the bis(2-fluorobenzaldehyde) tetrasulfide (BFBTS) are exactly the same as those in Example 1, except that the reagent raw material 4-fluorobenzaldehyde is replaced with 2-fluorobenzaldehyde, and the structural formula is as follows:

[0082] 。

[0083] Example 4

[0084] This example provides the preparation of a positive electrode sheet of bis(3-fluorobenzaldehyde) tetrasulfide. The preparation method steps and parameters of the bis(3-fluorobenzaldehyde) tetrasulfide (BFBTS) are exactly the same as those in Example 1, except that the reagent raw material 4-fluorobenzaldehyde is replaced with 3-fluorobenzaldehyde, and the structural formula is as follows:

[0085] 。

[0086] Comparative Example 1

[0087] This comparative example provides the preparation of bis(benzaldehyde) tetrasulfide (BBTS) as a comparative material, whose structure is similar to BFBTS but does not contain fluorine substituents.

[0088] The preparation method of the bis(benzaldehyde) tetrasulfide (BBTS) includes the following steps:

[0089] S1. Raw materials and reagents:

[0090] Reagent preparation: Benzaldehyde (99%, Sigma-Aldrich), N,N-dimethylformamide (DMF, anhydrous, 99%, Sigma-Aldrich), deionized water, absolute ethanol, Na2S4 (99%, Ron reagent)

[0091] S2. Synthesis process:

[0092] Benzaldehyde (10.6 g, 0.1 mol) and Na2S4 (17.4 g, 0.12 mol) were added to a 250 mL three-necked flask equipped with a magnetic stir bar. 100 mL of anhydrous N,N-dimethylformamide (DMF) was added to the flask as a solvent. The flask was placed in an oil bath and heated to 90 °C under a nitrogen atmosphere while stirring for 18 hours. During the reaction, the color of the system gradually changed from light yellow to brownish red. After the reaction, heating was stopped and the mixture was cooled to room temperature. The reaction mixture was poured into 500 mL of deionized water and stirred vigorously to precipitate a brownish red solid. The red solid precipitate was collected by centrifugation at a speed of 4000 rpm for 10 minutes. The collected brownish red solid precipitate was washed three times with 200 mL of deionized water and 100 mL of anhydrous ethanol respectively to fully remove the residual reactants and solvents. The washed red solid product was placed in a vacuum drying oven and dried under vacuum at 60 °C for 12 hours to obtain a brownish red solid product bis(benzaldehyde) tetrasulfide (BBTS), and the structural formula is as follows:

[0093] .

[0094] S3 is the same as in Example 1.

[0095] Comparative Example 2

[0096] This comparative example provided the preparation of elemental S (99%, Adamas) as a comparative material, and the preparation of the positive electrode sheet was the same as in Example 1 and Comparative Example 1.

[0097] Comparative Example 3

[0098] This comparative example provided the preparation of a bis(fluorobenzene) tetrasulfide positive electrode sheet. The preparation method steps and parameters of the bis(fluorobenzene) tetrasulfide (molar ratio of fluorobenzene:Na2S4 is 1:1.2) were exactly the same as in Example 1, except that the reagent raw material 4-fluorobenzaldehyde was replaced with fluorobenzene, and the structural formula is as follows:

[0099] .

[0100] Comparative Example 4

[0101] This comparative example provided the preparation of a bis(4-aminobenzaldehyde) tetrasulfide positive electrode sheet. The preparation method steps and parameters of the bis(4-aminobenzaldehyde) tetrasulfide (molar ratio of 4-aminobenzaldehyde:Na2S4 is 1:1.2) were exactly the same as in the example, except that the reagent raw material 4-fluorobenzaldehyde was replaced with 4-aminobenzaldehyde, and the structural formula is as follows:

[0102] .

[0103] The amino group (-NH2), as a substituent on the phenyl group, may have insufficient redox activity or an unfavorable direction compared to the preferred aldehyde group (-CHO) and fluorine atom (-F) of the present invention in the application of organic sulfur cathode materials: Electron-donating property of the amino group: The amino group (-NH2) is a typical electron-donating group. Compared with the electron-withdrawing aldehyde group (-CHO) and fluorine atom (-F), the amino group will increase the electron cloud density of the benzene ring. Reduction of the redox potential: Electron-donating groups usually lower the oxidation potential of the molecule, making it more difficult to be oxidized. And the cathode material needs to undergo an oxidation reaction to release electrons. The introduction of the amino group may cause the redox potential of the organic sulfur molecule to be too low, resulting in insufficient activity within the positive electrode potential window of the lithium-sulfur battery and making it difficult to effectively participate in the electrochemical reaction, thereby reducing the sulfur utilization rate and battery capacity. Poor chemical stability and compatibility issues with the electrolyte: The amino group may react adversely with the sulfide electrolyte, leading to an increase in interfacial side reactions and affecting battery performance. Limited or unfavorable improvement in conductivity, and the effect of the amino group on conductivity is uncertain: Although the amino group contains a nitrogen atom and may introduce a certain electron conjugation effect, its effect on improving the conductivity of small organic sulfur molecules may not be as obvious as the synergistic effect of the benzene ring conjugation system and electron-withdrawing groups. It may even reduce the conductivity: If the amino group causes the molecular structure to be more crowded or the electronic structure to change unfavorably, it may instead reduce the conductivity of the material.

[0104] Comparative Example 5

[0105] This comparative example provides the preparation of a bis(aniline) tetrasulfide cathode sheet. The preparation method steps and parameters of the bis(phenylamine) tetrasulfide (the molar ratio of aniline to Na2S4 is 1:1.2) are exactly the same as those in the example, except that the reagent raw material 4-fluorobenzaldehyde is replaced with aniline, and the structural formula is as follows:

[0106] 。

[0107] Comparative Example 6

[0108] This comparative example provides the preparation of a bis(4-fluorobenzaldehyde) disulfide cathode sheet. This comparative example provides the preparation of a bis(4-fluorobenzaldehyde) disulfide cathode sheet.

[0109] The preparation method of the bis(4-fluorobenzaldehyde) disulfide includes the following steps:

[0110] S1. Raw materials and reagents:

[0111] Reagent preparation: 4-fluorobenzaldehyde (99%, Sigma-Aldrich), N,N-dimethylformamide (DMF, anhydrous, analytical grade), deionized water, absolute ethanol, Na2S2 (99%, Tianfu Chemical Industry).

[0112] S2. Synthesis Process:

[0113] Add 4-fluorobenzaldehyde (12.4 g, 0.1 mol) and Na2S2 (9.4 g, 0.12 mol) into a 250 mL three-necked flask equipped with a magnetic stir bar. Add 100 mL of anhydrous N,N-dimethylformamide (DMF) to the flask as a solvent.

[0114] Place the flask in an oil bath. Under the protection of a nitrogen atmosphere, heat it to 90 °C and keep stirring for 18 hours. During the reaction, the color of the system gradually changes from light yellow to yellow.

[0115] After the reaction is completed, stop heating and cool to room temperature. Pour the reaction mixture into 500 mL of deionized water and stir vigorously to precipitate a yellow solid.

[0116] Collect the yellow solid precipitate by centrifugation at a rotational speed of 4000 rpm for 10 minutes.

[0117] Wash the collected yellow solid precipitate three times with 200 mL of deionized water and 100 mL of anhydrous ethanol respectively to fully remove the residual reactants and solvents.

[0118] Place the washed yellow solid product in a vacuum drying oven and dry it under vacuum at 60 °C for 12 hours to obtain the yellow solid product bis(4-fluorobenzaldehyde) disulfide, the structural formula is as follows:

[0119] .

[0120] S3 is the same as Example 1.

[0121] Performance Test Example

[0122] Assemble the products obtained in the above examples and comparative examples: In an inert gas glove box with a water and oxygen content ≤ 0.01 ppm, use the corresponding electrode sheet as the positive electrode, weigh 120 mg of Li6PS5Cl and press it into a tablet, which has been pressed into a thin sheet with an area of 0.785 cm 2 and a thickness of 1 mm as the electrolyte layer. The positive electrode is a pole piece with a unit area loading of 5 mg·cm -2 . The lithium metal is the negative electrode layer, and an all-solid-state lithium-sulfur battery is assembled. Finally, perform cyclic charge and discharge. First, charge and discharge at 0.1C for two cycles, and then perform a long cycle test at 1C. Test its initial efficiency and the capacity retention rate after 500 cycles. The results are shown in Table 1 below:

[0123] Table 1 Cyclic Performance Test

[0124]

[0125] As can be seen from Table 1, the experimental results fully prove that the BFBTS material has high capacity, excellent cycle stability, good rate performance and high redox activity, and exhibits excellent comprehensive electrochemical performance as a cathode material for lithium-sulfur batteries. The comparative experiment further verifies the synergistic effect of halogen substituents on improving the performance of organic sulfur cathode materials. The cycle performance curve of BFBTS prepared in Example 1 in the lithium-sulfur battery is as Figure 1 shown, and the comparison of the cycle performance between Example 1 and Comparative Example 2 in the lithium-sulfur battery is as Figure 2 shown.

[0126] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various deformations or modifications within the scope of the claims, which does not affect the essence of the present invention.

Claims

1. A method for preparing a positive electrode of a lithium ion battery, characterized in that: The method comprises the following steps: grinding and mixing an organic sulfur small molecule compound and a conductive additive uniformly, adding the mixture to a binder / NMP solution, continuing to grind and stir to prepare a positive electrode slurry, and then coating the positive electrode slurry and vacuum drying the mixture; The organic sulfur small molecule compound is prepared by the following steps: 4-Fluorobenzaldehyde and Na2S4 were added to the polar solvent DMF, heated to 90°C, stirred and reacted for 18 h, and the reaction mixture was poured into the anti-solvent deionized water. The resulting precipitated product was an organic sulfur small molecule compound; the molar ratio of 4-fluorobenzaldehyde to Na2S4 was 1:1.

2.

2. The method for preparing a positive electrode of a lithium ion battery according to claim 1, characterized in that: The conductive additive is selected from one or more of carbon black, acetylene black, graphene, and carbon nanotubes.

3. The method for preparing a positive electrode of a lithium ion battery according to claim 1, characterized in that: The binder is selected from one or more of polyvinylidene fluoride, polytetrafluoroethylene, and sodium carboxymethyl cellulose.

4. An all-solid-state lithium-sulfur secondary battery, characterized in that: The invention comprises a sulfide solid electrolyte, a negative electrode and a lithium ion battery positive electrode obtained by the preparation method as claimed in claim 1.

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