A flexible composite carbon fiber positive electrode carrier material for lithium-sulfur batteries, and a preparation method and applications thereof

Abstract

The application relates to a lithium-sulfur battery flexible composite carbon fiber positive electrode carrier material and a preparation method and application thereof, and belongs to the technical field of new materials. The preparation process of the material is as follows: cobalt-based metal organic framework (ZIF-67) particles are grown on the surface of cotton fibers to introduce cobalt elements and form a precursor of the flexible composite carbon fiber material. Then, the lithium-sulfur battery flexible composite carbon fiber positive electrode carrier material is obtained through pyrolysis. The material can be directly used as a lithium-sulfur battery positive electrode after loading sulfur, and compared with a traditional lithium-sulfur battery positive electrode synthesis process, the material avoids the use of organic solvents and binders and finally realizes the effect of a flexible electrode. Compared with a single carbon fiber flexible positive electrode, cobalt-carbon catalyst particles are formed after ZIF-67 is carbonized, the conversion of lithium polysulfide is promoted, the shuttle effect of lithium polysulfide is significantly weakened, and good initial specific capacity and long cycle stability are exhibited.

Description

Technical Field

[0001] This invention relates to a flexible composite carbon fiber cathode carrier material for lithium-sulfur batteries, its preparation method, and its application, belonging to the technical field of lithium-sulfur battery material preparation. Background Technology

[0002] Lithium-sulfur batteries, as a novel, efficient, and environmentally friendly energy storage device, are crucial for improving energy utilization and achieving green and low-carbon goals. This is mainly because, compared to other energy storage devices, lithium-sulfur batteries have a higher theoretical specific capacity (1675 mAh·g). -1 Lithium-sulfur batteries are inexpensive, environmentally friendly, and rich in sulfur reserves, making them promising candidates for energy storage. Flexible cathode materials, as a key component of batteries, can endow lithium-sulfur batteries with bendable properties, thereby expanding the application scenarios of energy storage devices and extending their application to wearable devices.

[0003] However, lithium-sulfur batteries still have some drawbacks hindering their further development. First, the low conductivity of elemental sulfur and its discharge products (Li₂S and Li₂S₂) slows down electron transfer. Second, the "shuttle effect" of lithium polysulfides, a reaction intermediate, between the positive and negative electrodes leads to a continuous decrease in battery capacity. Finally, the preparation method of the positive electrode in lithium-sulfur batteries not only requires the composite of a carrier material and elemental sulfur, but also, like traditional lithium-ion batteries, the combined action of binders and solvents to form a slurry, which is then coated onto the current collector. Therefore, the preparation process is complex and the material cost is high.

[0004] To address the poor conductivity of positive electrode active materials and the shuttling of reaction intermediates in lithium-sulfur batteries, researchers typically use carbon materials as sulfur carriers to improve the electrode's conductivity. Flexible carbon fiber, as a novel positive electrode carrier material for lithium-sulfur batteries, can be directly composited with sulfur and used as an electrode. Its preparation process is simple, reducing material costs, and it imparts the characteristics of a flexible electrode to the battery, making it highly valuable for applications. However, single flexible carbon fiber only physically adsorbs lithium polysulfides and cannot effectively suppress their shuttling. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a flexible composite carbon fiber cathode support material for lithium-sulfur batteries, its preparation method, and its application. Cobalt is introduced by growing cobalt-based metal-organic framework (ZIF-67) particles coated on the surface of cotton fibers, forming a precursor for the flexible composite carbon fiber material. The flexible composite carbon fiber cathode support material for lithium-sulfur batteries is then obtained through pyrolysis. The inherent porous structure of the cotton fibers physically confines the shuttle of lithium polysulfides, and the growth of cobalt-coated carbon catalyst particles catalyzes the conversion reaction of lithium polysulfides, significantly reducing the shuttle effect and improving the performance of lithium-sulfur batteries.

[0006] This material can be directly loaded with sulfur and used as a cathode in lithium-sulfur batteries. Its synthesis process is simpler than traditional lithium-sulfur battery cathodes, avoiding the use of organic solvents and binders, ultimately achieving a flexible electrode effect. Compared to single carbon fiber flexible cathodes, ZIF-67, after carbonization, forms cobalt-carbon catalyst particles, promoting the conversion of lithium polysulfides and significantly weakening the shuttle effect of lithium polysulfides, exhibiting good initial specific capacity and long-term cycle stability.

[0007] To achieve the above objectives, the present invention provides a flexible composite carbon fiber cathode carrier material for lithium-sulfur batteries, its preparation method, and its application.

[0008] A flexible composite carbon fiber cathode carrier material for lithium-sulfur batteries, characterized in that,

[0009] (1) Cobalt is introduced by growing cobalt-based metal-organic framework (ZIF-67) particles on the surface of cotton fibers to form a precursor of flexible composite carbon fiber material.

[0010] (2) Through high-temperature pyrolysis, cotton fibers are transformed into a three-dimensional conductive network with cobalt carbon particles uniformly coated on their surface. The prepared material has good flexibility and can be directly loaded with sulfur and applied to the positive electrode of lithium-sulfur batteries without the need for binders and conductive carbon black.

[0011] (3) The cathode material can effectively promote the conversion of lithium polysulfides and suppress the shuttle effect during the charging and discharging process of lithium-sulfur batteries.

[0012] The above-mentioned objective of this invention is achieved through the following technical solution:

[0013] The first aspect of this invention provides a flexible composite carbon fiber cathode carrier material for lithium-sulfur batteries and a method for preparing the same, comprising the following steps:

[0014] (1) Cobalt salt and 2-methylimidazole were added to a certain amount of methanol solution in a certain proportion to prepare ZIF-67 precursor solution.

[0015] (2) Add the cleaned and dried cotton fiber cloth to the above precursor solution, soak for a certain period of time, and then take it out and dry it.

[0016] (3) The dried cotton fiber cloth is added to a mixed solution of methanol and ammonia and left to stand for 24 hours to allow ZIF-67 to grow and coat the surface of the cotton fiber, thus obtaining a composite cotton fiber material.

[0017] (4) The composite cotton fiber material was pyrolyzed at high temperature in a tube furnace to obtain a flexible composite carbon fiber cathode carrier material for lithium-sulfur batteries.

[0018] In the flexible composite carbon fiber cathode carrier material for lithium-sulfur batteries and its preparation method described in this invention, the cobalt salt includes cobalt nitrate hexahydrate, cobalt acetate tetrahydrate, cobalt chloride hexahydrate, etc. Preferably, it is cobalt nitrate hexahydrate.

[0019] In the flexible composite carbon fiber cathode carrier material for lithium-sulfur batteries and its preparation method described in this invention, the molar ratio of cobalt salt to 2-methylimidazole is 1:3 to 8. Preferably, it is 1:3.

[0020] In the flexible composite carbon fiber cathode carrier material for lithium-sulfur batteries and its preparation method described in this invention, the amount of methanol required for the ZIF-67 precursor solution is directly related to the concentration of cobalt salt, ranging from 0.6 to 2.8 mol / L. Preferably, it is 2.8 mol / L.

[0021] In the flexible composite carbon fiber cathode carrier material for lithium-sulfur batteries and its preparation method described in this invention, the volume ratio of ammonia in the mixed solution of methanol and ammonia is 33% to 75%. Preferably, it is 75%.

[0022] In the flexible composite carbon fiber cathode carrier material for lithium-sulfur batteries and its preparation method described in this invention, the cotton fiber cloth used in step (2) is 10cm×10cm in size.

[0023] Furthermore, the diameter of cotton fibers is 10–30 μm.

[0024] Preferably, the diameter is 10 μm.

[0025] In the flexible composite carbon fiber cathode carrier material for lithium-sulfur batteries and its preparation method described in this invention, in step (3), the ZIF-67 particles grown on the surface of cotton fibers have a particle size range of 0–2 μm. Preferably, the particle size range is 500–1000 nm.

[0026] In the flexible composite carbon fiber cathode carrier material for lithium-sulfur batteries and its preparation method described in this invention, the high-temperature pyrolysis temperature in step (4) is 600–900°C. Preferably, it is 900°C.

[0027] Furthermore, the high-temperature pyrolysis time is 1–3 hours.

[0028] Preferably, the high-temperature pyrolysis time is 2 hours.

[0029] Another aspect of the present invention is to provide a lithium-sulfur battery, comprising the lithium-sulfur battery flexible composite carbon fiber positive electrode carrier material described in the first aspect, which is loaded with sulfur and used as the positive electrode material.

[0030] Furthermore, this positive electrode material is cut into circular electrode sheets with a diameter of 12mm, and assembled with a separator, electrolyte, negative electrode sheet, etc. to form a lithium-sulfur battery.

[0031] Furthermore, each square centimeter of flexible composite carbon fiber cathode carrier material is uniformly loaded with 2 mg of elemental sulfur.

[0032] Furthermore, the electrolyte used is a mixed solution of DME:DOL (volume ratio of 1:1) with 1M LiTFSI dissolved and 1.0% LiNO3 added by mass, and the amount of electrolyte added to each battery is 30μL.

[0033] The beneficial effects of this invention are as follows: This invention uses common cotton fiber cloth as a carbon source, introduces the transition metal cobalt element by growing and coating ZIF-67 on the cotton fiber, and then pyrolyzes the flexible composite carbon fiber cathode carrier material at high temperature, applying it to the cathode of a lithium-sulfur battery. After carbonization, the cotton fiber cloth becomes a three-dimensional carbon fiber conductive network, which can enhance the electron transport efficiency of the cathode. At the same time, the carbon fiber has a large number of porous structures, which can physically adsorb lithium polysulfides; while ZIF-67, after pyrolysis, becomes a cobalt-carbon catalyst, which is coated on the surface of the carbon fiber. This unique structure can fully expose the cobalt active sites, thereby participating in the catalytic conversion of lithium polysulfides, and because it is coated on the outside of the carbon fiber, it can also act as a barrier to further block the diffusion of lithium polysulfides. Relying on the synergistic effect of the two, the lithium-sulfur battery has good charge and discharge performance. Moreover, the flexible composite carbon fiber cathode material differs from traditional cathode materials, avoiding the use of binders and organic solvents, and does not need to be attached to the current collector, thus greatly simplifying the manufacturing process, reducing material costs, and possessing the characteristic of flexibility.

[0034] Leveraging the abundant hydroxyl active sites on the surface of cotton fibers, cobalt-based metal-organic framework (ZIF-67) particles can grow and coat the cotton fiber surface more uniformly, thus introducing cobalt before carbonization and forming a precursor for flexible composite carbon fiber materials. Through a one-step high-temperature pyrolysis, the cotton fibers transform into a three-dimensional conductive network with cobalt carbon particles uniformly coated on their surface. Compared to ordinary cobalt oxide particles, ZIF-67-derived carbon materials have a higher specific surface area and better conductivity, providing abundant active sites and accelerating electron transfer during the cathode reaction. The prepared material exhibits excellent flexibility and can be directly applied to the cathode of lithium-sulfur batteries after sulfur loading, without the need for binders or conductive carbon black. This cathode material can effectively promote the conversion of lithium polysulfides and suppress the shuttle effect during the charge and discharge process of lithium-sulfur batteries.

[0035] Thanks to the introduction of cobalt-carbon catalyst particles, lithium-sulfur batteries using the flexible composite carbon fiber cathode material of this invention have high specific capacity and cycle stability. They exhibit an initial discharge specific capacity of 1352 mAh / g at a current of 0.2C and a remaining capacity of 876 mAh / g after 150 cycles. Attached Figure Description

[0036] Figure 1 These are digital photographs of the materials used in Example 1 and Comparative Example 1. Wherein, a represents cotton, and b represents ZIF-67@cotton.

[0037] Figure 2 This is a scanning electron microscope image of the cotton fiber cloth in Example 1.

[0038] Figure 3 The images shown are scanning electron microscope (SEM) images of the materials used in Example 1 and Comparative Example 1. In the images, a represents cotton, and b represents ZIF-67@cotton.

[0039] Figure 4 The images are scanning electron microscope (SEM) images of ZIF-67@cotton prepared under different methanol-ammonia-water ratios in Examples 5-10.

[0040] Where a represents methanol:ammonia = 5 mL:15 mL; b represents methanol:ammonia = 10 mL:10 mL; and c represents methanol:ammonia = 20 mL:10 mL.

[0041] Figure 5 Characterization of CNTS obtained after carbonization in Comparative Example 1. Where a is a digital photograph of CNTS and b is a scanning electron microscope image of CNTS.

[0042] Figure 6 The images shown are scanning electron microscope (SEM) images of the materials involved in Example 1 and Comparative Example 1. In the images, a represents CNTS and b represents Co@CNTS.

[0043] Figure 7 The image shows the XRD pattern of the cotton fibers in Example 1.

[0044] Figure 8 This is a standard ZIF-67 XRD plot.

[0045] Figure 9 The image shows the XRD pattern of ZIF-67@cotton in Example 1.

[0046] Figure 10 The image shown is Co@CNTS from Example 1.

[0047] Figure 11 The image shows the charge-discharge test results of CNTS as the sulfur-supporting material in the positive electrode of a lithium-sulfur battery in Comparative Example 1 at a current of 0.5C.

[0048] Figure 12 The image shows the charge-discharge test results of Co@CNTS as the sulfur-supporting material for the positive electrode of a lithium-sulfur battery in Example 1 at a current of 0.5C.

[0049] Figure 13 The image shows the charge-discharge test results of Co@CNTS as the sulfur-supporting material for the positive electrode of a lithium-sulfur battery in Example 1 at a current of 0.2C. Detailed Implementation

[0050] The following non-limiting embodiments are intended to enable those skilled in the art to more fully understand the invention, but do not limit the invention in any way.

[0051] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; the reagents and materials described are commercially available unless otherwise specified.

[0052] Example 1

[0053] (1) Cut the cotton fiber cloth into 10cm×10cm sizes, wash it with water and ethanol, and dry it for later use.

[0054] (2) Add 3.98g of cobalt nitrate hexahydrate and 3.38g of 2-methylimidazole to 5ml of methanol to prepare a homogeneous solution.

[0055] (3) Immerse the cotton cloth in the above solution and evacuate it in a vacuum oven for 2 hours. Then take out the cotton cloth and dry it in an oven at 60°C for 4 hours to obtain cotton cloth loaded with MOF precursor.

[0056] (4) Add the cotton cloth loaded with MOF precursor to a mixed solution of 45 mL ammonia and 15 mL methanol, and let it stand for 24 h. After the ZIF-67 particles have grown fully on the surface of the cotton fibers, remove them and dry them to obtain ZIF-67@cotton.

[0057] (5) Cotton fibers with ZIF-67 growth were placed in a tube furnace and heated to 900℃ at a rate of 5℃ / min under an argon atmosphere, and held for 2 hours. A flexible composite carbon fiber cathode carrier material was obtained and named Co@CNTS.

[0058] (6) The flexible composite carbon fiber positive electrode carrier material is cut and mixed with sulfur powder, placed in an oven, and the sulfur is melted at 155°C to obtain a flexible composite carbon fiber positive electrode sheet, which is then assembled into a lithium-sulfur battery.

[0059] Example 2

[0060] 3.98 g of cobalt nitrate hexahydrate and 3.38 g of 2-methylimidazole were added to 20 ml of methanol. Other conditions were the same as in Example 1.

[0061] Examples 3-4

[0062] In step (1) of Example 1, cobalt nitrate hexahydrate was replaced with cobalt acetate tetrahydrate and cobalt chloride hexahydrate. Other conditions were the same as in Example 1.

[0063] Examples 5-10

[0064] The amounts of ammonia and methanol in step (4) of Example 1 were adjusted to: 15 mL ammonia and 5 mL methanol; 30 mL ammonia and 10 mL methanol; 10 mL ammonia and 10 mL methanol; 20 mL ammonia and 20 mL methanol; 10 mL ammonia and 20 mL methanol; and 15 mL ammonia and 30 mL methanol. Other conditions were the same as in Example 1.

[0065] Comparative Example 1

[0066] (1) Cut the cotton fiber cloth into 10cm×10cm sizes, wash it with water and ethanol, dry it and set it aside. Name it cotton.

[0067] (2) The cotton fibers were arranged in a tube furnace and heated to 900℃ at a rate of 5℃ / min under an argon atmosphere, and held for 2 hours. The flexible carbon fiber cathode carrier material was obtained and named CNTS.

[0068] (3) After cutting the flexible carbon fiber cathode carrier material, sulfur is loaded to obtain a flexible carbon fiber cathode sheet, which is then assembled into a lithium-sulfur battery.

[0069] Figure 1 These are digital photographs comparing the cotton fiber fabric before and after ZIF-67 coating in Example 1. After ZIF-67 coating, the cotton fiber fabric exhibits a distinct purple color with uniform coloring, indicating good uniformity of the prepared ZIF-67@cotton material and uniform adhesion of ZIF-67 to the cotton fiber fabric. The cotton fiber fabric was then characterized using scanning electron microscopy (SEM). Figure 2 It can be observed that the cotton fiber fabric has a uniform three-dimensional network structure. For example... Figure 3 As shown, the average diameter of a single fiber is 20μm and the surface is smooth. When ZIF-67 grows on its surface, the surface of the cotton fiber is uniformly covered with polyhedral particles of about 700nm.

[0070] Examples 5-10 discuss the effect of different ammonia concentrations on the growth of ZIF-67 on cotton fiber surfaces, such as... Figure 4 As shown, when the ammonia concentration is high (e.g. Figure 4 In section a), a large number of ZIF-67 particles crystallize and grow outside the cotton fibers, failing to form a coating on the fibers. This means that, after subsequent carbonization, they cannot provide a continuous conductive network as a positive electrode material for batteries. When the ammonia concentration is low (e.g., Figure 4 In section c), only a small amount of ZIF-67 particles grow on the surface of the cotton fibers. As the main source of the positive electrode catalytic system in the battery, this small amount of ZIF-67 particles will weaken the catalytic effect. Therefore, a 1:1 ratio of ammonia to methanol is preferred, as this provides the best coating effect of ZIF-67 on the cotton fibers (e.g., ...). Figure 4 (b)

[0071] Figure 5The image shows the characterization of CNTS obtained after carbonization in Comparative Example 1. In the image, a is a digital photograph of the CNTS, and b is a scanning electron microscope image of the CNTS. As can be seen from the images, the carbonized CNTS exhibits a complete three-dimensional network structure with regularly shaped carbon fibers. Figure 6 The images show scanning electron microscope (SEM) images of the materials involved in Example 1 and Comparative Example 1. In the images, a represents CNTS and b represents Co@CNTS. The figures show that Co@CNTS maintains a complete coating structure. XRD analysis reveals obvious signal peaks for cobalt metal and its oxides. Simultaneously, the carbon fiber shows significant graphitization, with a sharp peak at 26°. According to the XRD patterns, (…) Figure 7 , 8 9) The ZIF-67 signal peak was superimposed on the XRD peak of cotton by ZIF-67, which also proved the successful preparation of ZIF-67@cotton. After carbonization, Co@CNTS was obtained (its XRD is as shown in Figure 9). Figure 10 (As shown).

[0072] The carbonized three-dimensional carbon fiber substrate in Comparative Example 1 still exhibits good flexibility. It was used as a sulfur-carrying material for the positive electrode of a lithium-sulfur battery and subjected to cyclic charge-discharge tests at 0.5C. (See attached image.) Figure 11 As shown, the initial discharge specific capacity at 0.5C is only 512mAh / g, and the remaining capacity after 200 cycles is 375mAh / g.

[0073] Similarly, the material from Example 1 was subjected to a cyclic charge-discharge test at 0.5C, and the results are as follows. Figure 12 As shown, this material exhibits an initial discharge specific capacity of 868 mAh / g, and after 200 cycles, the remaining capacity is 547 mAh / g, significantly better than CNTS in the comparative example. Cyclic charge-discharge tests were also conducted at 0.2C; the results are as follows. Figure 13 As shown, the material exhibits an initial discharge specific capacity of 1352 mAh / g, reaching the theoretical specific capacity of lithium-sulfur batteries (1675 mAh / g). -1 The remaining capacity after 150 cycles was 876 mAh / g, which is 80.7% of the total capacity of the lithium polysulfide. This is significantly better than that of CNTS in the comparative example. This indicates that the flexible composite carbon fiber cathode support material plays a significant role in promoting the conversion of lithium polysulfides in the presence of cobalt and its metal oxides, thus exhibiting superior electrochemical performance compared to CNTS.

[0074] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a flexible composite carbon fiber cathode carrier material for lithium-sulfur batteries, characterized in that, The method comprises the following steps: (1) a ZIF-67 precursor solution is prepared by adding a cobalt salt and 2-methylimidazole into a methanol solution, wherein the molar ratio of the cobalt salt to 2-methylimidazole is 1:3-8; (2) a clean and dried cotton fiber cloth is added into the ZIF-67 precursor solution, taken out after soaking and dried, wherein the concentration of the cobalt salt in the ZIF-67 precursor solution is 0.6-2.8 mol / L; (3) the dried cotton fiber cloth is added into a mixed solution of methanol and ammonia water and left to stand, so that ZIF-67 particles grow and coat on the surface of the cotton fiber to obtain a composite cotton fiber material; (4) the composite cotton fiber material is pyrolyzed at high temperature in a tube furnace to obtain a flexible composite carbon fiber positive electrode carrier material for lithium-sulfur batteries.

2. The preparation method of the flexible composite carbon fiber positive electrode carrier material of the lithium-sulfur battery according to claim 1, characterized in that: The cobalt salt is one or more of cobalt nitrate hexahydrate, cobalt acetate tetrahydrate and cobalt chloride hexahydrate.

3. The preparation method of the flexible composite carbon fiber positive electrode carrier material of the lithium-sulfur battery according to claim 1, characterized in that: In step (2), the diameter of the surface fibers in the cotton fiber cloth is 10-30 μm.

4. The preparation method of the flexible composite carbon fiber positive electrode carrier material of the lithium-sulfur battery according to claim 1, characterized in that: In step (3), the volume ratio of ammonia water to methanol in the mixed solution of methanol and ammonia water is 3:1-1:

2.

5. The method of claim 1, wherein the method further comprises the steps of: mixing the carbon fiber with a lithium ion conductive material; and coating the carbon fiber with the lithium ion conductive material. In step (3), the particle size of the ZIF-67 particles is less than 2 μm.

6. The method of claim 1, wherein the method further comprises the step of: 5 heating the mixture to a temperature of about 300°C to about 400°C for about 1 hour to about 5 hours. In step (4), the pyrolysis temperature is 600-900 ℃, and the pyrolysis time is 1-3 h.

7. A flexible composite carbon fiber cathode support material for lithium-sulfur batteries, characterized in that The preparation method is prepared by any one of claims 1-6.

8. A positive electrode sheet of a lithium-sulfur battery, characterized by: The positive electrode sheet is prepared by using the composite carbon fiber positive electrode carrier material of claim 7.

9. The positive electrode sheet of lithium-sulfur battery according to claim 8, characterized in that: The positive electrode sheet is prepared after loading sulfur on the composite carbon fiber positive electrode carrier material, and each square centimeter of the flexible composite carbon fiber positive electrode carrier material uniformly loads 2 mg of sulfur.

10. A lithium-sulfur battery, characterized by: The battery comprises a positive electrode sheet, a separator, an electrolyte and a negative electrode sheet, and the positive electrode is the positive electrode sheet of claim 8 or 9; The electrolyte is a mixed solution of ethylene glycol dimethyl ether and 1,3-dioxolane in which lithium bis(trifluoromethanesulfonyl)imide and LiNO3 are dissolved, and the volume ratio of ethylene glycol dimethyl ether to 1,3-dioxolane in the mixed solution is 1:1.

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