A carbon fiber composite material for lithium-sulfur battery positive electrode and its preparation method and application

Abstract

The present invention discloses a carbon fiber composite material for a lithium-sulfur battery cathode, its preparation method and application, belonging to the technical field of battery materials. The preparation method of the carbon fiber composite material for the lithium-sulfur battery cathode comprises the following steps: uniformly mixing a carbon source and a solvent to obtain a core solution; uniformly mixing a transition metal salt, a carbon-nitrogen source and a solvent to obtain a shell solution; coaxial electrospinning the core solution and the shell solution to obtain core-shell structure nanofibers; pre-oxidizing the core-shell structure nanofibers and carbonizing them under a non-oxidizing atmosphere to obtain a carbon fiber composite material; calcining the carbon fiber composite material of S3 under a modified atmosphere to obtain a carbon fiber composite material for the lithium-sulfur battery cathode; the modified atmosphere is at least one of phosphine, sulfur vapor, ammonia or selenium vapor. The lithium-sulfur battery prepared from the material of the present invention has an initial discharge specific capacity higher than 1300 mAh·g-1, and after 500 cycles, the capacity retention rate can reach more than 90%.

Description

Technical Field

[0001] The present invention relates to the technical field of battery materials, and more specifically, to a carbon fiber composite material for a lithium-sulfur battery positive electrode, and a preparation method and application thereof. Background Art

[0002] Chemical batteries, also known as chemical power sources, are devices that convert the energy generated by chemical reactions directly into electrical energy. With the advancement of science and technology and the rapid development of society, people's demand for chemical power sources is increasing.

[0003] Lithium-sulfur battery is a type of lithium battery. Lithium-sulfur battery is a lithium battery that uses sulfur as the positive electrode and metallic lithium as the negative electrode. Elemental sulfur is abundant in the earth and has the characteristics of low price and environmental friendliness. Lithium-sulfur batteries that use sulfur as the positive electrode material have high material theoretical specific capacity and battery theoretical specific energy, reaching 1675mAh / g and 2600Wh / kg respectively, which are much higher than the capacity of lithium cobalt oxide batteries widely used in commercial applications (<150mAh / g). In addition, sulfur is an environmentally friendly element that has little pollution to the environment, making it a very promising lithium battery.

[0004] Although lithium-sulfur batteries have the great advantage of high energy density, they also have some problems that need to be solved. The commercial application of lithium-sulfur batteries is limited mainly due to the following problems:

[0005] S1. The electronic conductivity and ionic conductivity of elemental sulfur are poor. The conductivity of sulfur materials at room temperature is extremely low. The final products of the discharge reaction, Li2S and Li2S2, are also electronic insulators with poor conductivity. Therefore, the utilization rate of active materials is low and the rate performance is poor.

[0006] S2. Soluble polysulfides are intermediate products in the charge and discharge process. They dissolve in the electrolyte and move back and forth between the positive and negative electrodes, forming the so-called shuttle effect, which causes capacity decay.

[0007] S3. When elemental sulfur is discharged and completely generates Li2S, its volume expands by 80%, which can easily cause the collapse of the positive electrode material structure and reduce the cycle stability of the material.

[0008] In order to solve the problem of fast capacity decay of the above-mentioned lithium-sulfur battery, the prior art discloses a nano-carbon fiber composite material, which divides the core solution into three parts, adds a transition metal acetate to each core solution, and electrospins with the shell solution. The prepared nano-carbon fiber composite material has a three-channel core-shell structure, and the three-channel hollow carbon fiber tube is filled with transition metal carbide, which improves the specific capacity and first coulomb efficiency of the lithium-sulfur battery. However, it does not make relevant improvements for improving the cycle capacity retention rate of lithium-sulfur batteries. Summary of the invention

[0009] The technical problem to be solved by the present invention is to overcome the defects and shortcomings of existing nano-carbon fiber composite materials that cannot improve the cycle capacity retention rate of lithium-sulfur batteries. A method for preparing a carbon fiber composite material for a lithium-sulfur battery positive electrode is provided. A coaxial electrospinning method is adopted, a polymer solution with a high carbon yield is used as a shell solution, a polymer solution with a low carbon yield is used as a core solution, a metal salt is dissolved and dispersed in the shell solution, and a carbon fiber composite material with a villi cavity shape is prepared through coaxial electrospinning. A transition metal compound is formed in the carbon fiber composite material, which has a strong adsorption-catalytic effect on polysulfides. While being able to give full play to the theoretical capacity of sulfur, the cycle capacity retention rate of the lithium-sulfur battery is significantly improved, and the sulfur loading amount is further increased.

[0010] Another object of the present invention is to provide a carbon fiber composite material for a lithium-sulfur battery positive electrode.

[0011] Another object of the present invention is to provide a positive electrode plate for a lithium-sulfur battery.

[0012] Another object of the present invention is to provide a positive electrode side interlayer of a lithium-sulfur battery.

[0013] Another object of the present invention is to provide a lithium-sulfur battery.

[0014] The above-mentioned purpose of the present invention is achieved through the following technical solutions:

[0015] A method for preparing a carbon fiber composite material for a lithium-sulfur battery positive electrode comprises the following steps:

[0016] S1. Mixing the carbon source and the solvent evenly to obtain a core solution;

[0017] S2. mixing the transition metal salt, the carbon-nitrogen source and the solvent to obtain a shell solution;

[0018] S3. Coaxially electrospinning the core solution and the shell solution to obtain core-shell structured nanofibers; pre-oxidizing the core-shell structured nanofibers and carbonizing them in a non-oxidizing atmosphere to obtain a carbon fiber composite material;

[0019] S4. The carbon fiber composite material of S3 is calcined under a modified atmosphere to obtain a carbon fiber composite material for a lithium-sulfur battery positive electrode;

[0020] Among them, in S1, the mass fraction of the carbon source in the core solution is 45% to 80%;

[0021] In S2, in the shell solution, the mass fraction of the transition metal salt is 5% to 45%, and the mass fraction of the carbon-nitrogen source is 30% to 85%;

[0022] The number average molecular weight of the carbon-nitrogen source is greater than the number average molecular weight of the carbon source;

[0023] In S3, the electrospinning voltage was 8-30 kV, and the mass ratio of the core solution to the shell solution was 1:(1.4-2);

[0024] The oxidation temperature is 120-350°C, and the pre-oxidation time is 0.5-4h;

[0025] The carbonization temperature is 750-1300°C and the carbonization time is 1-6h;

[0026] In S4, the modified atmosphere is at least one of phosphine, sulfur vapor, ammonia or selenium vapor, the calcination temperature is 300-900° C., and the calcination time is 1-12 hours.

[0027] Among them, in S1, the carbon source can be one or more of polystyrene, polyvinyl pyrrolidone or polymethyl methacrylate;

[0028] In S2, the transition metal salt may be one or more of metal chloride, oxalate, nitrate, acetylacetonate or carbonate;

[0029] The carbon-nitrogen source may be one or more of polyacrylonitrile, polypyrrole, polyamide, urea or melamine.

[0030] In the preparation method of the present invention, a polymer solution with a low carbon yield is used as a core solution, and a polymer solution with a high carbon yield is used as a shell solution. A transition metal salt is dissolved and dispersed in the shell solution, and a core-shell structure nanofiber is obtained by coaxial spinning. The core-shell structure nanofiber is subjected to pre-oxidation treatment to shape and carbonization, and the core carbon source of the fiber is decomposed during the carbonization process. The metal salt is thermally decomposed to form a corresponding metal element and catalyze the carbon source to produce carbon nanotubes, so as to prepare a carbon fiber composite material with a fluff cavity shape, so that the conductivity of the material is improved, and the nano-fluff cavity structure can make the sulfur element have a larger contact area inside the carbon material, promoting the reaction kinetics of polysulfide. In addition, this fluff carbon nanotube structure can prevent structural damage caused by the "volume effect". S4 is calcined in a modified atmosphere to form a transition metal compound inside the carbon fiber composite material, correspondingly forming a transition metal compound-carbon nanotube. The transition metal compound-carbon nanotube can have a strong adsorption-catalytic effect on polysulfide, which plays a role in improving the reaction kinetics of polysulfide. The carbon fiber composite material for the positive electrode of a lithium-sulfur battery prepared by the method of the present invention has a shell layer containing "carbon-nitrogen" elements and a tube-shell shape with a cavity structure, which can play a "confinement-adsorption-conductivity" role on polysulfides. The carbon-sulfur composite material obtained by applying the material to the positive electrode sheet of a lithium-sulfur battery and melting sulfur with sulfur element effectively combines the advantages of metal compound particle anchoring catalysis and carbon shell conductive confinement, so that while being able to give full play to the theoretical capacity of sulfur, it significantly improves the cycle capacity retention rate of the lithium-sulfur battery and further increases the sulfur loading.

[0031] The metal compound-carbon nanotube@nitrogen-doped hollow carbon nanofiber designed by the present invention is a nitrogen-doped carbon fiber multi-level "core-shell" nano-lithium-sulfur battery positive electrode carbon fiber composite material with a cavity structure, which can be used as a sulfur carrier, can provide a large specific surface area, and can load a large amount of positive electrode active material sulfur inside the conductive-catalytic material; it not only enables the lithium-sulfur battery to exert a specific capacity close to the theory, but also has a significant improvement in cycle stability.

[0032] The method of the present invention can be industrially prepared, is simple and environmentally friendly, and the obtained carbon fiber composite material in the shape of a villi cavity can be used as the positive electrode of a lithium-sulfur battery. Compared with traditional lithium-sulfur batteries, the obtained lithium-sulfur battery exhibits high load capacity and long cycle stability.

[0033] Preferably, in S4, the modified atmosphere is at least one of phosphine and sulfur vapor.

[0034] After calcination in an S4 modified atmosphere, at least one metal compound selected from the group consisting of metal phosphide, metal sulfide, metal nitride and metal selenide can be formed.

[0035] Preferably, in S2, the transition metal in the transition metal salt is at least one of iron, nickel and cobalt.

[0036] Among them, the transition metal salt can be at least one of nickel chloride, cobalt chloride, iron chloride, nickel oxalate, cobalt oxalate, iron oxalate, nickel nitrate, cobalt nitrate, iron nitrate, nickel carbonate, cobalt carbonate, and iron carbonate.

[0037] Preferably, in S2, the mass fraction of the transition metal salt in the shell solution is 10% to 25%.

[0038] If too little transition metal salt is added, the catalyst metal compound content is low, and there are fewer active sites for the catalytic conversion of polysulfides, which will cause the battery cycle capacity to remain low; if too much transition metal salt is added, the degree of metal agglomeration will increase and the structure of the material will be destroyed, which will also cause the battery cycle capacity to remain low.

[0039] Preferably, in S2, the mass fraction of the carbon-nitrogen source in the shell solution is 40% to 70%.

[0040] Preferably, in S3, the core solution and the shell solution are coaxially electrospun to obtain core-shell structured nanofibers; after the core-shell structured nanofibers are soaked in a phenolic resin solution and dried, the core-shell structured nanofibers are pre-oxidized and carbonized in a non-oxidizing atmosphere to obtain a carbon fiber composite material.

[0041] The core-shell structure nanofibers after coaxial electrospinning are immersed in a phenolic resin solution, dried, pre-oxidized and carbonized; and the composite material obtained through the S4 step is in a film shape and can be used as an interlayer on the positive electrode side of a lithium-sulfur battery.

[0042] Preferably, the solvent in S1 and S2 is at least one of N,N-dimethylformamide, N,N-dimethylacetamide and dimethyl sulfoxide.

[0043] Preferably, the S3 carbonization treatment atmosphere is a non-oxidizing atmosphere, and the non-oxidizing atmosphere is a nitrogen atmosphere, an argon atmosphere, or an argon-hydrogen mixed atmosphere.

[0044] Preferably, the number average molecular weight of the carbon-nitrogen source is greater than the number average molecular weight of the carbon source, because the viscosity of the solution with a large molecular weight will be high, and the prepared shell layer will be more stable in structure after pre-oxidation and carbonization, and will not be easily broken during subsequent processing; and from the preparation process, the molecular weight of the carbon-nitrogen source as a shell solution is greater than that of the carbon source core solution, and the filamentation efficiency is better during the electrospinning preparation process.

[0045] The present invention also protects the carbon fiber composite material for lithium-sulfur battery positive electrode prepared by the method for preparing the carbon fiber composite material for lithium-sulfur battery positive electrode described in any one of the above items.

[0046] The carbon fiber composite material for lithium-sulfur battery positive electrode prepared by the present invention comprises a core layer and a shell layer, wherein the core layer is a metal single substance precipitated in the carbonization process, and a plurality of "fluffy" metal-carbon nanotubes grow during the decomposition of the carbon source; the "fluffy" metal-carbon nanotubes are modified into "fluffy" metal compound-carbon nanotubes, and the metal compound-carbon nanotubes are connected to the shell; and the shell layer is a shell material with a cavity inside generated during the carbonization process of the nitrogen-carbon source.

[0047] The present invention also protects a lithium-sulfur battery positive electrode sheet, comprising the above-mentioned carbon fiber composite material for lithium-sulfur battery positive electrode.

[0048] The carbon fiber composite material for lithium-sulfur battery positive electrode and conductive agent powder are dry-mixed to obtain mixed powder; a binder is added to the mixed powder, and the mixture is stirred evenly to obtain slurry and prepare a lithium-sulfur battery positive electrode sheet.

[0049] The above-mentioned positive electrode sheet of the lithium-sulfur battery can be prepared by a conventional preparation method: the above-mentioned carbon fiber composite material for the positive electrode of the lithium-sulfur battery and elemental sulfur are mixed and heated for sulfur melting treatment to obtain a nanofiber sulfur carrier composite material loaded with sulfur; the slurry is prepared by grinding it, mixing it with a conductive agent and a binder, and then coating the electrode sheet or directly using it as the electrode sheet to obtain a lithium-sulfur battery positive electrode sheet.

[0050] The present invention also protects a lithium-sulfur battery positive electrode side interlayer, comprising the above-mentioned carbon fiber composite material for lithium-sulfur battery positive electrode.

[0051] The present invention also protects a lithium-sulfur battery, comprising a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte, wherein the positive electrode sheet is the positive electrode sheet of the lithium-sulfur battery described above.

[0052] The present invention also protects a lithium-sulfur battery, comprising a positive electrode sheet, an interlayer, a separator, a negative electrode sheet and an electrolyte, wherein the interlayer is arranged between the positive electrode sheet and the separator, and the interlayer is the positive electrode side interlayer of the lithium-sulfur battery described above.

[0053] Compared with the prior art, the present invention has the following beneficial effects:

[0054] The invention discloses a method for preparing a carbon fiber composite material for a lithium-sulfur battery positive electrode. The invention adopts a coaxial electrostatic spinning method, a polymer solution with a high carbon yield is used as a shell solution, a polymer solution with a low carbon yield is used as a core solution, and a transition metal salt is dissolved and dispersed in the shell solution. A carbon fiber composite material with a villi cavity shape is prepared through coaxial electrostatic spinning, and a transition metal compound is formed in the carbon fiber composite material by calcining in a modified atmosphere. The transition metal compound has a strong adsorption-catalytic effect on polysulfide. Therefore, while being able to give full play to the theoretical capacity of sulfur, the cycle capacity retention rate of the lithium-sulfur battery is significantly improved, and the sulfur loading amount is further increased.

[0055] The lithium-sulfur battery prepared by the carbon fiber composite material for the lithium-sulfur battery positive electrode of the present invention has a capacity higher than 1300 mAh·g -1 The first discharge specific capacity is high, and after 500 cycles, it still has a high discharge specific capacity, and the capacity retention rate can reach more than 90%. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 This is a scanning electron microscope (SEM) image of the cross section of the carbon fiber composite material for the lithium-sulfur battery positive electrode of Example 2 of the present invention.

[0057] Figure 2 This is a transmission electron microscope (TEM) image of the carbon fiber composite material for the lithium-sulfur battery positive electrode of Example 1 of the present invention.

[0058] Figure 3 This is the (TGA) thermogravimetric diagram of the carbon fiber composite material for the lithium-sulfur battery positive electrode of Example 1 of the present invention.

[0059] Figure 4 The X-ray diffraction (XRD) diagrams of the carbon fiber composite materials for lithium-sulfur battery positive electrodes of Example 1, Comparative Example 1 and Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0060] The present invention is further described below in conjunction with specific embodiments, but the embodiments do not limit the present invention in any form. Unless otherwise specified, the raw materials and reagents used in the embodiments of the present invention are conventionally purchased raw materials and reagents.

[0061] Example 1

[0062] A method for preparing a carbon fiber composite material for a lithium-sulfur battery positive electrode comprises the following steps:

[0063] S1. dissolving polystyrene in N,N-dimethylformamide to obtain a core solution; the mass percentage of polystyrene in the core solution is 55%;

[0064] S2. The nickel chloride is dissolved in N,N-dimethylformamide, and then polyacrylonitrile is added and stirred to dissolve to obtain a shell solution; the mass percentage of nickel chloride in the shell solution is 15%, and the mass percentage of polyacrylonitrile in the shell solution is 30%;

[0065] S3. The shell solution and the core solution are coaxially electrospun at a voltage of 10 kV, a receiving distance of 20 cm, an iron metal plate as a receiving plate, a spinning rate of 0.5 mL / h for the core solution, and a spinning rate of 0.8 mL / h for the shell solution; core-shell structured nanofibers are obtained; the core-shell structured nanofibers are then pre-oxidized at 120°C for 1 h and carbonized at 850°C for 6 h in a nitrogen atmosphere to obtain a carbon fiber composite material.

[0066] S4. The above materials are calcined in a phosphine atmosphere at a temperature of 300°C for 2 hours to obtain a carbon fiber composite material for a positive electrode of a lithium-sulfur battery with a nano-villi cavity having a multi-level "core-shell" structure of Ni2P-CNTs@NHCF.

[0067] Example 2

[0068] A method for preparing a carbon fiber composite material for a lithium-sulfur battery positive electrode comprises the following steps:

[0069] S1. dissolving polyvinyl pyrrolidone in N,N-dimethylacetamide to obtain a core solution; the mass percentage of polyvinyl pyrrolidone in the core solution is 45%;

[0070] S2. Dissolving cobalt oxalate in N,N-dimethylacetamide, and then adding polypyrrole to dissolve with stirring to obtain a shell solution; the mass percentage of cobalt oxalate in the shell solution is 10% and the mass percentage of polypyrrole in the shell solution is 40%;

[0071] S3. The shell solution and the core solution are coaxially electrospun at a voltage of 20 kV, a receiving distance of 20 cm, an iron metal plate as a receiving plate, a spinning rate of the core solution of 2 mL / h, and a spinning rate of the shell solution of 4 mL / h; core-shell structured nanofibers are obtained; the core-shell structured nanofibers are then pre-oxidized at 130°C for 2 h and carbonized at 950°C in a nitrogen atmosphere for 4 h to obtain a carbon fiber composite material.

[0072] S4. The above materials are calcined in an ammonia atmosphere at a temperature of 600°C for 10 hours to obtain a carbon fiber composite material for a positive electrode of a nano-villus cavity lithium-sulfur battery having a Co3N4-CNTs@NHCF multi-level "core-shell" structure.

[0073] Example 3

[0074] A method for preparing a carbon fiber composite material for a lithium-sulfur battery positive electrode comprises the following steps:

[0075] S1. dissolving polymethyl methacrylate in dimethyl sulfoxide to obtain a core solution; the mass percentage of polymethyl methacrylate in the core solution is 55%;

[0076] S2. The ferric nitrate is dissolved in dimethyl sulfoxide, and then the polyamide is added and stirred to dissolve to obtain a shell solution; the mass percentage of ferric nitrate in the shell solution is 15% and the mass percentage of polyamide in the shell solution is 50%;

[0077] S3. The shell solution and the core solution are coaxially electrospun at a voltage of 18 kV, a receiving distance of 20 cm, an iron metal plate as a receiving plate, a spinning rate of the core solution of 4 mL / h, and a spinning rate of the shell solution of 6 mL / h; core-shell structured nanofibers are obtained; the core-shell structured nanofibers are then pre-oxidized at 150°C for 3 h and carbonized at 1050°C in a nitrogen atmosphere for 4 h to obtain a carbon fiber composite material.

[0078] S4. The above materials are calcined in a phosphine atmosphere at a temperature of 300°C for 2 hours to obtain a carbon fiber composite material for positive electrode of a nano-villus cavity lithium-sulfur battery having a Fe3P-CNTs@NHCF multi-level "core-shell" structure.

[0079] Example 4

[0080] A method for preparing a carbon fiber composite material for a lithium-sulfur battery positive electrode comprises the following steps:

[0081] S1. dissolving polymethyl methacrylate in N,N-dimethylformamide to obtain a core solution; the mass percentage of polymethyl methacrylate in the core solution is 65%;

[0082] S2. nickel carbonate was dissolved in N, N-dimethylformamide, and then (polyacrylonitrile: melamine = 6:1) was added and stirred to dissolve to obtain a shell solution; the mass percentage of nickel carbonate in the shell solution was 25% and the mass percentage of the mixture of polyacrylonitrile and melamine in the shell solution was 70%;

[0083] S3. The shell solution and the core solution are coaxially electrospun at a voltage of 25 kV, a receiving distance of 20 cm, an iron metal plate as a receiving plate, a spinning liquid outlet rate of the core solution of 3 mL / h, and a spinning liquid outlet rate of the shell solution of 4.5 mL / h; core-shell structured nanofibers are obtained; the core-shell structured nanofibers are then pre-oxidized at 200°C for 2 h and carbonized at 950°C for 3 h in a nitrogen atmosphere to obtain a carbon fiber composite material.

[0084] S4. The above materials are calcined in a sulfur atmosphere at a temperature of 550°C for 3 hours to obtain a carbon fiber composite material for positive electrode of a nano-villus cavity lithium-sulfur battery having a Ni3S2-CNTs@NHCF multi-level "core-shell" structure.

[0085] Example 5

[0086] A method for preparing a carbon fiber composite material for a lithium-sulfur battery positive electrode comprises the following steps:

[0087] S1. dissolving polystyrene in N,N-dimethylacetamide to obtain a core solution; the mass percentage of polystyrene in the core solution is 70%;

[0088] S2. Dissolve nickel nitrate in N,N-dimethylacetamide, and then add (polyacrylonitrile: urea = 8:1) and stir to dissolve to obtain a shell solution; the mass percentage of nickel nitrate in the shell solution is 15% and the mass percentage of the mixture of polyacrylonitrile and urea in the shell solution is 63%;

[0089] S3. The shell solution and the core solution are coaxially electrospun at a voltage of 20 kV, a receiving distance of 20 cm, an iron metal plate as a receiving plate, a spinning rate of the core solution of 2.5 mL / h, and a spinning rate of the shell solution of 3.5 mL / h; core-shell structured nanofibers are obtained; the core-shell structured nanofibers are then pre-oxidized at 260°C for 2.5 h and carbonized at 850°C in a nitrogen atmosphere for 4 h to obtain a carbon fiber composite material.

[0090] S4. The above materials are calcined in a selenium atmosphere at a temperature of 500°C for 4 hours to obtain a carbon fiber composite material for positive electrode of a nano-villi cavity lithium-sulfur battery having a multi-level "core-shell" structure of NiSe2-CNTs@NHCF.

[0091] Example 6

[0092] A method for preparing a carbon fiber composite material for a lithium-sulfur battery positive electrode comprises the following steps:

[0093] S1. dissolving polyvinyl pyrrolidone in N,N-dimethylacetamide to obtain a core solution; the mass percentage of polyvinyl pyrrolidone in the core solution is 70%;

[0094] S2. Dissolve cobalt carbonate in N,N-dimethylacetamide, and then add polypyrrole to dissolve with stirring to obtain a shell solution; the mass percentage of cobalt carbonate in the shell solution is 13% and the mass percentage of polypyrrole in the shell solution is 85%;

[0095] S3. The shell solution and the core solution are coaxially electrospun at a voltage of 25 kV, a receiving distance of 20 cm, an iron metal plate as a receiving plate, a spinning liquid outlet rate of the core solution of 3.5 mL / h, and a spinning liquid outlet rate of the shell solution of 5.5 mL / h; core-shell structured nanofibers are obtained; the core-shell structured nanofibers are then pre-oxidized at 280°C for 3.5 h and carbonized at 950°C in a nitrogen atmosphere for 6 h to obtain a carbon fiber composite material.

[0096] S4. The above materials are calcined in a selenium atmosphere at a temperature of 500°C for 4 hours to obtain a carbon fiber composite material for a positive electrode of a nano-villi cavity lithium-sulfur battery having a CoSe2-CNTs@NHCF multi-level "core-shell" structure.

[0097] Example 7

[0098] A method for preparing a carbon fiber composite material for a lithium-sulfur battery positive electrode comprises the following steps:

[0099] S1. dissolving polymethyl methacrylate in N,N-dimethylacetamide to obtain a core solution; the mass percentage of polymethyl methacrylate in the core solution is 60%;

[0100] S2. Dissolving cobalt nitrate in N,N-dimethylacetamide, and then adding polyamide to dissolve with stirring to obtain a shell solution; the mass percentage of cobalt nitrate in the shell solution is 30% and the mass percentage of polyamide in the shell solution is 75%;

[0101] S3. The shell solution and the core solution are coaxially electrospun at a voltage of 30 kV, a receiving distance of 20 cm, an iron metal plate as a receiving plate, a spinning liquid outlet rate of the core solution of 2.5 mL / h, and a spinning liquid outlet rate of the shell solution of 3.5 mL / h; core-shell structured nanofibers are obtained; the core-shell structured nanofibers are then pre-oxidized at 240°C for 2.5 h and carbonized at 1050°C in a nitrogen atmosphere for 5 h to obtain a carbon fiber composite material.

[0102] S4. The above materials are calcined in a sulfur vapor atmosphere at a temperature of 500°C for 3 hours to obtain a carbon fiber composite material for positive electrode of a nano-villi cavity lithium-sulfur battery having a Co9S8-CNTs@NHCF multi-level "core-shell" structure.

[0103] Embodiments 8 to 14

[0104] A lithium-sulfur battery positive electrode sheet comprises the carbon fiber composite material for lithium-sulfur battery positive electrode of embodiments 1 to 7.

[0105] The method for preparing the above-mentioned lithium-sulfur battery positive electrode sheet comprises the following steps:

[0106] Weigh sulfur powder and the carbon fiber composite material for lithium-sulfur battery positive electrode of Examples 1 to 7 in a mass ratio of 4:1, mix them thoroughly, heat them to 155° C. and melt the sulfur in a reactor for 8 hours to obtain a sulfur-loaded nano-hair cavity carbon fiber composite material;

[0107] The obtained sulfur-loaded nano-villi cavity carbon fiber composite material, conductive agent carbon nanotubes and polyvinylidene fluoride (PVDF) binder are mixed in the ratio of 8:1:1, smeared on the current collector aluminum foil through slurry preparation, and then placed in a vacuum drying oven at 60°C for 8 hours, and cut to prepare lithium-sulfur battery positive electrode sheets.

[0108] Examples 15 to 21

[0109] A lithium-sulfur battery comprises a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte, wherein the positive electrode sheet is the positive electrode sheet of the lithium-sulfur battery of embodiments 8 to 14.

[0110] The preparation method of the above lithium-sulfur battery comprises the following steps:

[0111] Preparation of electrolyte: Ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and 1 mol / L LiPF6 in a volume ratio of 1:1:1 were uniformly mixed to obtain an electrolyte;

[0112] Using a metal lithium sheet as a counter electrode and a polypropylene microporous membrane Celgard 2400 as a separator, the positive electrode sheets of the lithium-sulfur batteries of Examples 8 to 14 were transferred to a glove box filled with argon gas for assembly of button batteries to obtain button batteries of model CR2032.

[0113] Embodiments 22 to 28

[0114] A lithium-sulfur battery positive electrode side interlayer comprises the carbon fiber composite material for lithium-sulfur battery positive electrode of embodiments 1 to 7.

[0115] The preparation method of the interlayer on the positive electrode side of the lithium-sulfur battery is basically the same as the preparation method of the carbon fiber composite material for the positive electrode of the lithium-sulfur battery in Examples 1 to 7, except that:

[0116] S3. The core solution and the shell solution are coaxially electrospun to obtain core-shell structured nanofibers; the core-shell structured nanofibers are immersed in a 5% phenolic resin solution for 20 minutes and dried, and then the core-shell structured nanofibers are pre-oxidized and carbonized in a non-oxidizing atmosphere to obtain a carbon fiber composite material.

[0117] Embodiments 29 to 35

[0118] A lithium-sulfur battery comprises a positive electrode sheet, an interlayer, a separator, a negative electrode sheet and an electrolyte, wherein the interlayer is arranged between the positive electrode sheet and the separator, and the interlayer is the positive electrode side interlayer of the lithium-sulfur battery of Examples 22 to 28.

[0119] The preparation method of the above lithium-sulfur battery comprises the following steps:

[0120] Preparation of electrolyte: Ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and 1 mol / L LiPF6 in a volume ratio of 1:1:1 were uniformly mixed to obtain an electrolyte;

[0121] Preparation of positive electrode sheet: sulfur powder, conductive agent carbon nanotubes and polyvinylidene fluoride (PVDF) binder are mixed in a ratio of 8:1:1, smeared on the current collector aluminum foil through slurry preparation, and then placed in a 60°C vacuum drying oven for 8 hours, and cut to prepare lithium-sulfur battery positive electrode sheet.

[0122] Using a metal lithium sheet as a counter electrode and a polypropylene microporous membrane Celgard 2400 as a separator, the positive electrode side interlayer of the lithium-sulfur battery of Examples 22 to 28 was transferred to a glove box filled with argon gas for assembly of button batteries to obtain button batteries of model CR2032.

[0123] Comparative Example 1

[0124] A method for preparing a carbon fiber composite material comprises the following steps:

[0125] S1. dissolving polystyrene in N,N-dimethylformamide to obtain a core solution; the mass percentage of polystyrene in the core solution is 55%;

[0126] S2. The nickel chloride is dissolved in N,N-dimethylformamide, and then polyacrylonitrile is added and stirred to dissolve to obtain a shell solution; the mass percentage of nickel chloride in the shell solution is 5%, and the mass percentage of polyacrylonitrile in the shell solution is 30%;

[0127] S3. The shell solution and the core solution were coaxially electrospun at a voltage of 10 kV, a receiving distance of 20 cm, an iron metal plate as a receiving plate, a spinning rate of 0.5 mL / h for the core solution, and a spinning rate of 0.8 mL / h for the shell solution; core-shell structured nanofibers were obtained; the core-shell structured nanofibers were then pre-oxidized at 120 °C for 1 h and carbonized at 850 °C for 6 h in a nitrogen atmosphere to obtain Ni-CNTs@NHCF material.

[0128] Different from Example 1, step S4 is not included.

[0129] Comparative Example 2

[0130] A method for preparing a carbon fiber composite material comprises the following steps:

[0131] S1. dissolving polystyrene in N,N-dimethylformamide to obtain a core solution; the mass percentage of polystyrene in the core solution is 55%;

[0132] S2. Add N,N-dimethylformamide to polyacrylonitrile and stir to dissolve to obtain a shell solution; the mass percentage of polyacrylonitrile in the shell solution is 30%;

[0133] S3. The shell solution and the core solution were coaxially electrospun at a voltage of 10 kV, a receiving distance of 20 cm, an iron metal plate as a receiving plate, a spinning liquid outlet rate of the core solution of 0.5 mL / h, and a spinning liquid outlet rate of the shell solution of 0.8 mL / h; the core-shell structured nanofibers were obtained; and the core-shell structured nanofibers were pre-oxidized at 120°C for 1 h and carbonized at 850°C for 6 h in a nitrogen atmosphere;

[0134] S4. The above materials are calcined in a phosphine atmosphere at a temperature of 300° C. for 2 h to obtain a NHCF carbon fiber composite material.

[0135] The difference from Example 1 is that no transition metal salt is added to the S2 shell solution.

[0136] Comparative Example 3

[0137] A method for preparing a carbon fiber composite material comprises the following steps:

[0138] S1. dissolving polyacrylonitrile in N,N-dimethylformamide to obtain a core solution; the mass percentage of polyacrylonitrile in the core solution is 55%;

[0139] S2. The nickel chloride is dissolved in N,N-dimethylformamide, and then polyacrylonitrile is added and stirred to dissolve to obtain a shell solution; the mass percentage of nickel chloride in the shell solution is 5%, and the mass percentage of polyacrylonitrile in the shell solution is 30%;

[0140] S3. The solution was electrospun at a voltage of 10 kV, a receiving distance of 20 cm, an iron metal plate as a receiving plate, and a spinning rate of the shell solution of 0.8 mL / h; nanofibers were prepared; and the nanofibers were pre-oxidized at 120° C. for 1 h and carbonized at 850° C. for 6 h in a nitrogen atmosphere;

[0141] S4. calcining the above materials in a phosphine atmosphere at a temperature of 300° C. for 2 h to obtain a carbon fiber composite material.

[0142] The difference from Example 1 is that the core solution does not contain a carbon source.

[0143] Comparative Example 4

[0144] A lithium-sulfur battery positive electrode side interlayer comprises the carbon fiber composite material for the lithium-sulfur battery positive electrode of comparative example 1, and the preparation method is the same as that of example 22.

[0145] Comparative Example 5

[0146] A positive electrode side interlayer of a lithium-sulfur battery comprises the carbon fiber composite material for the positive electrode of a lithium-sulfur battery of comparative example 2, and the preparation method is the same as that of example 22.

[0147] Comparative Example 6

[0148] A positive electrode side interlayer of a lithium-sulfur battery comprises the carbon fiber composite material for a positive electrode of a lithium-sulfur battery of comparative example 3, and the preparation method is the same as that of example 22.

[0149] Results

[0150] The cross section of the carbon fiber composite material for the lithium-sulfur battery positive electrode of Example 2 of the present invention was characterized by scanning electron microscopy (SEM). The results are as follows: Figure 1 The carbon fiber composite material for the positive electrode of the lithium-sulfur battery of Example 1 of the present invention was characterized by transmission electron microscopy (TEM), and the results are as follows Figure 2 The carbon fiber composite material for the positive electrode of the lithium-sulfur battery of Example 1 of the present invention was tested by a thermogravimeter, and the test results are as follows Figure 3 The carbon fiber composite material for the positive electrode of the lithium-sulfur battery of Example 1 of the present invention and the materials of Comparative Example 1 and Comparative Example 2 were characterized by an X-ray diffractometer. The results are as follows: Figure 4 shown.

[0151] The carbon fiber composite materials of Comparative Example 1, Comparative Example 2 and Comparative Example 3 were prepared into lithium-sulfur batteries according to the same preparation method as Examples 15 to 21.

[0152] The positive electrode side interlayers of the lithium-sulfur batteries of Comparative Examples 4, 5 and 6 were prepared by the same preparation method as in Examples 29 to 35 to obtain lithium-sulfur batteries.

[0153] The lithium-sulfur batteries assembled in the embodiments and comparative examples were subjected to electrochemical performance tests on a Xinwei test system, with a voltage range of 1.7 to 2.8 V and a discharge rate of 0.5C.

[0154] The specific test results are shown in Table 1 below:

[0155]

[0156] Table 1

[0157]

[0158] Table 1

[0159]

[0160]

[0161] Above Figures 1 to 4The analysis of Table 1 is as follows:

[0162] Figure 1 This is a scanning electron microscope (SEM) image of the cross section of the carbon fiber composite material for the lithium-sulfur battery positive electrode of Example 2 of the present invention. Figure 2 This is a transmission electron microscope (TEM) image of the carbon fiber composite material for the positive electrode of a lithium-sulfur battery according to Example 1 of the present invention. Figure 1 and Figure 2 It can be seen that the carbon fiber composite material for the positive electrode of a lithium-sulfur battery prepared by the present invention has a core layer and a shell layer. The core layer is a metal element precipitated during the carbonization process, and a plurality of "fluffy" metal-carbon nanotubes grow during the decomposition of the carbon source; the "fluffy" metal-carbon nanotubes are modified "fluffy" metal compound-carbon nanotubes, and the metal compound-carbon nanotubes are connected to the shell; the shell layer is a shell material with a cavity inside that is generated during the carbonization process of the nitrogen-carbon source.

[0163] Figure 3 This is a thermogravimetric diagram of the carbon fiber composite material for the positive electrode of a lithium-sulfur battery according to Example 1 of the present invention. Figure 3 It can be seen that Ni2P accounts for 21.8% of the material and is able to maintain the cycling capacity up to 91.6%.

[0164] Figure 4 The X-ray diffraction (XRD) diagrams of the carbon fiber composite material for lithium-sulfur battery positive electrode of Example 1 of the present invention and the materials of Comparative Examples 1 and 2 are shown. Figure 4 It can be seen that the carbon fiber composite material for the positive electrode of a lithium-sulfur battery in Example 1 of the present invention has a diffraction peak of nickel phosphide.

[0165] It can be seen from Table 1 that the lithium-sulfur battery prepared by the carbon fiber composite material for the lithium-sulfur battery positive electrode of the present invention has a capacity higher than 1300 mAh·g -1 The first discharge specific capacity is high, and after 500 cycles, it still has a high discharge specific capacity, and the capacity retention rate can reach more than 90%.

[0166] It can be seen from Example 1 and Comparative Example 1 that the material of Comparative Example 1 has not been treated with S4 modified atmosphere. Since it has not been treated with phosphine in modified atmosphere, the interior of the material is composed of only metallic nickel element-carbon nanotubes; due to the single crystal plane structure of metallic nickel element, although it can improve the conductivity of the material and make the initial capacity higher, the adsorption-catalytic effect on polysulfides is relatively poor, resulting in relatively low polysulfide conversion efficiency during the charge and discharge cycle, and a small number of polysulfides will produce a shuttle effect, resulting in a relatively low capacity retention rate.

[0167] It can be seen from Example 1 and Comparative Example 2 that the capacity of the prepared lithium-sulfur battery is very low without adding transition metal salts to S2, and the capacity retention rate after 500 cycles is only 50%. This is because without the addition of transition metal salts, metal-carbon nanotubes with a "fluffy" shape cannot be grown in the carbonization process, and the conductivity of the material is poor compared to Example 1, resulting in the inability to fully exert the specific capacity of sulfur; at the same time, the material does not have a transition metal compound to play an adsorption-catalytic role for polysulfides, resulting in slow reaction kinetics during charging and discharging, and some polysulfides dissolve in the electrolyte and shuttle to the negative electrode side, causing sulfur loss, thereby resulting in poor capacity retention.

[0168] It can be seen from Example 1 and Comparative Example 3 that compared with Example 1, Comparative Example 3 does not contain a core solution, and the prepared material has no core-shell structure, which cannot play a good confined loading role for polysulfides, resulting in low capacity utilization, poor battery cycle stability, and high capacity attenuation.

[0169] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A method for preparing a carbon fiber composite material for a lithium-sulfur battery positive electrode, characterized in that: The steps include: S1. Mixing the carbon source and the solvent evenly to obtain a core solution; S2. mixing the transition metal salt, the carbon-nitrogen source and the solvent to obtain a shell solution; S3. Coaxially electrospinning the core solution and the shell solution to obtain core-shell structured nanofibers; pre-oxidizing the core-shell structured nanofibers and carbonizing them in a non-oxidizing atmosphere to obtain a carbon fiber composite material; S4. The carbon fiber composite material of S3 is calcined under a modified atmosphere to obtain a carbon fiber composite material for a lithium-sulfur battery positive electrode; Among them, in S1, the mass fraction of the carbon source in the core solution is 45% to 80%; In S2, in the shell solution, the mass fraction of the transition metal salt is 5% to 45%, and the mass fraction of the carbon-nitrogen source is 30% to 85%; In S3, the electrospinning voltage was 8-30 kV, and the mass ratio of the core solution to the shell solution was 1:(1.4-2); The pre-oxidation temperature is 120-350°C, and the pre-oxidation time is 0.5-4h; The carbonization temperature is 750-1300°C and the carbonization time is 1-6h; In S4, the modified atmosphere is at least one of phosphine, sulfur vapor, ammonia or selenium vapor, the calcination temperature is 300-900° C., and the calcination time is 1-12 hours.

2. The method for preparing a carbon fiber composite material for a lithium-sulfur battery positive electrode according to claim 1, characterized in that: In S4, the modified atmosphere is at least one of phosphine and sulfur vapor.

3. The method for preparing a carbon fiber composite material for a lithium-sulfur battery positive electrode according to claim 1, characterized in that: In S2, the transition metal in the transition metal salt is at least one of iron, nickel and cobalt.

4. The method for preparing a carbon fiber composite material for a lithium-sulfur battery positive electrode according to claim 1, characterized in that: In S2, the mass fraction of the transition metal salt in the shell solution is 10% to 25%.

5. The method for preparing a carbon fiber composite material for a lithium-sulfur battery positive electrode according to claim 1, characterized in that: In S2, the mass fraction of the carbon-nitrogen source in the shell solution is 40% to 70%.

6. The carbon fiber composite material for lithium-sulfur battery positive electrode prepared by the method for preparing the carbon fiber composite material for lithium-sulfur battery positive electrode according to any one of claims 1 to 5.

7. A lithium-sulfur battery positive electrode sheet, characterized in that: It includes the carbon fiber composite material for lithium-sulfur battery positive electrode as described in claim 6.

8. A lithium-sulfur battery positive electrode side interlayer, characterized in that: It includes the carbon fiber composite material for lithium-sulfur battery positive electrode as described in claim 6.

9. A lithium-sulfur battery, characterized in that: It comprises a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte, wherein the positive electrode sheet is the positive electrode sheet of the lithium-sulfur battery as claimed in claim 7.

10. A lithium-sulfur battery, characterized in that: It comprises a positive electrode sheet, an interlayer, a separator, a negative electrode sheet and an electrolyte, wherein the interlayer is arranged between the positive electrode sheet and the separator, and the interlayer is the positive electrode side interlayer of the lithium-sulfur battery according to claim 8.

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