A nano-composite material for lithium-sulfur batteries, a preparation method thereof, and applications thereof

Abstract

The present invention discloses a nano-composite material for lithium-sulfur batteries, a preparation method thereof and an application thereof, belonging to the technical field of battery materials. The nano-composite material for lithium-sulfur batteries has a network structure formed by the interweaving of a number of nanofibers. The nanofibers have a multi-nanoporous tube shell layer, the tube shell layer is a hollow nano-carbonitride tube, the inside of the tube shell layer is a nanowire, the nanowire is a carbon-supported transition metal carbide, and there is a cavity for accommodating sulfur between the tube shell layer and the nanowire. The lithium-sulfur battery prepared from the material of the present invention has an initial discharge specific capacity higher than 1300 mAh·g-1, an initial Coulomb efficiency of more than 90%, and after 500 cycles, the capacity retention rate can reach more than 91%.

Description

Technical Field

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

[0002] A chemical battery, also known as a chemical power source, is a device that directly converts the energy generated by a chemical reaction into electrical energy. With the progress of science and technology and the rapid development of society, people's demand for chemical power sources is increasing day by day.

[0003] A lithium-sulfur battery is a type of lithium battery. A lithium-sulfur battery is a lithium battery with sulfur element as the positive electrode of the battery and metallic lithium as the negative electrode. Elementary sulfur is rich in reserves on the earth and has the characteristics of low price and environmental friendliness. For a lithium-sulfur battery using sulfur as the positive electrode material, its material theoretical specific capacity and battery theoretical specific energy are relatively high, reaching 1675 mAh / g and 2600 Wh / kg respectively, which are much higher than the capacity of lithium cobalt oxide batteries (<150 mAh / g) widely used commercially. And sulfur is an environmentally friendly element with basically no pollution to the environment, so it is a very promising lithium battery.

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

[0005] S1. The electronic conductivity and ionic conductivity of elementary sulfur are poor. The conductivity of sulfur materials at room temperature is extremely low, and the final products of the discharge reaction, Li2S and Li2S2, are also electronic insulators with poor conductivity. Therefore, the utilization rate of active substances is low and the rate performance is not good.

[0006] S2. The intermediate product during the charge and discharge process, soluble polysulfide, dissolves in the electrolyte and moves back and forth between the positive and negative electrodes, forming the so-called shuttle effect, resulting in capacity attenuation.

[0007] S3. When elementary sulfur is completely discharged to form Li2S, the volume expands by 80%, which is extremely likely to cause the collapse of the structure of the positive electrode material and reduce the cycle stability of the material.

[0008] In order to solve the problem of rapid capacity attenuation of the above lithium-sulfur batteries, 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 part of the core solution, and performs electrospinning 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 carbides, which improves the specific capacity and initial Coulomb efficiency of the lithium-sulfur battery. However, it does not make relevant improvements to improve the cycle capacity retention rate of the lithium-sulfur battery. 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, and to provide a nano-composite material for lithium-sulfur batteries, which effectively combines the advantages of metal carbide anchoring catalysis and carbon shell conductive network and physical confinement, has multi-site adsorption and catalytic effects on polysulfides, can be used as a positive electrode carrier and positive electrode side interlayer of a lithium-sulfur battery, and can exhibit high capacity and long cycle stability.

[0010] Another object of the present invention is to provide a method for preparing a nanocomposite material for lithium-sulfur batteries.

[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 nanocomposite material for lithium-sulfur batteries, the nanocomposite material for lithium-sulfur batteries having a mesh structure interwoven by a plurality of nanofibers, the nanofibers having a tube shell layer with multiple nanopores, the tube shell layer being a hollow nano carbon nitride tube, the interior of the tube shell layer being a nanowire, the nanowire being a carbon-loaded transition metal carbide, and a cavity for accommodating sulfur being provided between the tube shell layer and the nanowire.

[0016] The nanocomposite material for lithium-sulfur battery of the present invention, the carbon-loaded transition metal carbide in the form of nanowires can enhance the strong chemical adsorption and catalytic effect on polysulfides, and improve the reaction kinetics; the cavity between the tube shell layer and the nanowire can store a large amount of sulfur, and can accommodate the volume change of sulfur, playing a confining role to inhibit the dissolution and shuttle effect of sulfides. The tube shell layer with multiple nanopores plays a physical encapsulation role on sulfur, and the tube shell layer is a hollow nano carbon nitride tube with conductivity, which can ensure good electrical contact of the composite material inside the electrode, and improve the electrochemical performance of the sulfur-containing composite material when used as the positive electrode of lithium-sulfur battery. The carbon-sulfur composite material of the present invention effectively combines the advantages of metal carbide anchoring catalysis and carbon shell conductive network and physical confinement, has multi-site adsorption and catalytic effect on polysulfides, can be used as a positive electrode carrier for lithium-sulfur battery loaded sulfur element and inhibit polysulfide shuttle positive electrode side interlayer, and can show high capacity and long cycle stability.

[0017] Preferably, the nanowires are one or more of iron carbide, cobalt carbide, molybdenum carbide, nickel carbide or manganese carbide.

[0018] Further preferably, the nanowires are one or more of iron carbide, cobalt carbide or nickel carbide.

[0019] Preferably, the diameter of the nanowires is less than 100 nm.

[0020] Preferably, the diameter of the nanofibers is 100 nm to 700 nm, and the thickness of the tube shell layer is 20 to 80 nm.

[0021] The present invention also protects a preparation method of the above-mentioned nanocomposite material for lithium-sulfur batteries, comprising the following steps:

[0022] S1. Respectively prepare a shell layer carbon and nitrogen source solution, an outer core layer carbon source solution and an inner core layer carbon and nitrogen source solution, and add a carbon-containing transition metal salt to the inner core layer carbon and nitrogen source solution;

[0023] S2. Electrospinning the shell layer carbon and nitrogen source solution, the outer core layer carbon source solution and the inner core layer carbon and nitrogen source solution coaxially in three layers;

[0024] S3. Pre-oxidize the material in S2 and carbonize it in a non-oxidizing atmosphere to obtain a nanocomposite material for lithium-sulfur batteries;

[0025] Wherein, in S1, the molecular weights of the shell layer carbon and nitrogen source and the inner core layer carbon and nitrogen source are both greater than the molecular weight of the outer core layer carbon source, and the molecular weight of the outer core layer carbon source is 5000 to 10000;

[0026] The mass ratio of the sum of the mass of the carbon-containing transition metal salt and the inner core layer carbon and nitrogen source to the mass of the carbon-containing transition metal salt is 100:(5 - 20);

[0027] In S2, the liquid output ratio of the shell layer carbon and nitrogen source solution, the outer core layer carbon source solution and the inner core layer carbon and nitrogen source solution is 1:(1.5 - 2):1;

[0028] In S3, the pre-oxidation temperature is 220 - 310 °C, and the pre-oxidation time is 1 - 6 h; the carbonization temperature is 600 - 1000 °C, and the carbonization time is 2 - 6 h.

[0029] The present invention uses a three - layer coaxial electrospinning method to prepare a shell layer solution and a core layer solution with a carbon - nitrogen source of relatively large molecular weight, and uses a carbon source of relatively small molecular weight as the outer core layer solution. A carbon - containing transition metal salt is dispersed in the core layer carbon - nitrogen source solution. After three - layer coaxial electrospinning, a composite fiber with a core - shell structure is formed. After pre - oxidation and carbonization treatment, nanopores formed during the shrinkage and shaping process of the carbonization sintering process are obtained, and a core - shell tube - shaped nanofiber with a conductive carbon axis anchored with metal carbide is obtained. The nanofibers are intertwined into a network to form a nanocomposite material for lithium - sulfur batteries. The nanocomposite material for lithium - sulfur batteries prepared by the method of the present invention can have a multi - site adsorption and catalytic effect on polysulfides, can be used as a positive electrode carrier and an intermediate layer in lithium - sulfur batteries, and can exhibit high capacity and long - cycle stability.

[0030] Preferably, in S1, the outer core layer carbon source is one or more of polymethyl methacrylate, poly(vinylidene fluoride), or polylactic acid.

[0031] Preferably, in S2, the liquid discharge volume ratio of the shell layer carbon - nitrogen source solution, the outer core layer carbon source solution, and the core layer carbon - nitrogen source solution is 1:2:1.

[0032] Preferably, in S1, the shell layer carbon - nitrogen source is one or more of polyacrylonitrile, polyacrylamide, or polyimide.

[0033] Preferably, the molecular weight of the shell layer carbon - nitrogen source is 80,000 - 150,000.

[0034] More preferably, the molecular weight of the shell layer carbon - nitrogen source is 120,000 - 150,000.

[0035] Preferably, the mass concentration of the shell layer carbon - nitrogen source is 45% - 85%.

[0036] More preferably, the mass concentration of the shell layer carbon - nitrogen source is 55% - 85%.

[0037] Preferably, the core layer carbon - nitrogen source is one or more of polyacrylonitrile, polyacrylamide, or polyimide.

[0038] Preferably, the molecular weight of the core layer carbon - nitrogen source is 80,000 - 150,000.

[0039] More preferably, the molecular weight of the core layer carbon - nitrogen source is 100,000 - 120,000.

[0040] Preferably, the mass concentration of the core layer carbon - nitrogen source is 45% - 55%.

[0041] Preferably, the mass concentration percentage of the outer core layer carbon source and the shell layer carbon - nitrogen source solution is 15% - 85%.

[0042] Preferably, the preparation methods of the shell layer carbon and nitrogen source solution and the outer core layer carbon source solution may be adding the carbon and nitrogen source / carbon source into a solvent, sealing and stirring for 3 to 6 h in an environment of 40 to 60 °C.

[0043] The preparation method of the inner core layer carbon and nitrogen source solution may be adding the carbon and nitrogen source into a solvent, sealing and stirring for 3 to 6 h in an environment of 40 to 60 °C, and then adding a carbon-containing transition metal salt and ultrasonically treating for 0.5 to 3 h.

[0044] Preferably, in S2, the spinning speeds of the shell layer carbon and nitrogen source solution and the inner core layer carbon and nitrogen source solution are 0.1 to 3 mL / h, and the spinning speed of the outer core layer carbon source solution is 0.2 to 3 mL / h.

[0045] Preferably, in S2, the spinning voltage is 9 to 15 kV, and the distance from the spinneret to the collector is 8 to 15 cm.

[0046] Preferably, the carbon-containing transition metal salt is at least one of iron salt, cobalt salt, nickel salt, molybdenum salt or manganese salt.

[0047] Preferably, the carbon-containing transition metal salt is acetylacetonate or carbonate.

[0048] The carbon-containing transition metal salt is at least one of iron acetylacetonate, cobalt acetylacetonate, nickel acetylacetonate, molybdenum acetylacetonate and manganese acetylacetonate.

[0049] Preferably, in S3, the non-oxidizing atmosphere is one or more of nitrogen, hydrogen or argon.

[0050] Preferably, after the electrospinning in S2, the obtained material is soaked and dried in a phenolic resin solution; then the step S3 is carried out.

[0051] The present invention also protects a positive electrode sheet of a lithium-sulfur battery, which comprises the above-mentioned nano-composite material for a lithium-sulfur battery.

[0052] The above-mentioned nano-composite material for a lithium-sulfur battery, sulfur element and conductive agent powder are dry-mixed to obtain a mixed powder; a binder is added to the mixed powder and stirred evenly to obtain a slurry and prepare a positive electrode sheet of a lithium-sulfur battery.

[0053] The present invention also protects a positive electrode side interlayer of a lithium-sulfur battery, which comprises the above-mentioned nano-composite material for a lithium-sulfur battery.

[0054] The present invention also protects a lithium-sulfur battery, which comprises a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte, and the positive electrode sheet is the above-mentioned positive electrode sheet of a lithium-sulfur battery.

[0055] The present invention also protects a lithium-sulfur battery, which comprises a positive electrode sheet, an interlayer, a separator, a negative electrode sheet and an electrolyte, the interlayer is arranged between the positive electrode sheet and the separator, and the interlayer is the above-mentioned positive electrode side interlayer of a lithium-sulfur battery.

[0056] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0057] The present invention discloses a nanocomposite material for lithium-sulfur batteries, which effectively combines the advantages of metal carbide anchoring catalysis, carbon shell conductive network and physical confinement, has multi-site adsorption and catalysis effects on polysulfides, can be used as a carrier for sulfur elemental positive electrode of lithium-sulfur batteries and an interlayer on the positive electrode side to inhibit polysulfide shuttling, and can exhibit high capacity and long cycle stability. The lithium-sulfur battery prepared from the material of the present invention has a first discharge specific capacity higher than 1300 mAh·g -1 The first discharge specific capacity, the first Coulombic efficiency is more than 90%, and after 500 cycles, the capacity retention rate can reach more than 91%.

[0058] The preparation method of the nanocomposite material for lithium-sulfur batteries of the present invention has simple process, low cost and easy operation. Brief Description of the Drawings

[0059] Figure 1 It is a scanning electron microscope (SEM) image of the cross-section of the nanocomposite material for lithium-sulfur batteries in Example 1 of the present invention.

[0060] Figure 2 It is a transmission electron microscope (TEM) image of the nanocomposite material for lithium-sulfur batteries in Example 1 of the present invention.

[0061] Figure 3 It is the cycle performance graph of Examples 15 and 16 under different sulfur loadings of the nanocomposite material for lithium-sulfur batteries in Example 1 of the present invention;

[0062] Figure 4 It is the thermogravimetric (TGA) graph of the nanocomposite material for lithium-sulfur batteries in Example 1 of the present invention.

[0063] Figure 5 It is the X-ray diffraction (XRD) graph of the nanocomposite material for lithium-sulfur batteries in Example 1 of the present invention. Detailed Embodiments

[0064] The present invention will be 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 material reagents used in the embodiments of the present invention are conventional raw material reagents purchased.

[0065] Example 1

[0066] A nanocomposite material for lithium-sulfur batteries, the nanocomposite material for lithium-sulfur batteries has a network structure formed by interweaving a number of nanofibers, the nanofibers have a multi-nanoporous tube shell layer, the tube shell layer is a hollow carbon nitride nanotube, the inside of the tube shell layer is a nanowire, the nanowire is a carbon-supported transition metal carbide, and there is a cavity for accommodating sulfur between the tube shell layer and the nanowire.

[0067] Among them, the nanowire is Fe3C, and the diameter of the nanowire is 80 nm.

[0068] The diameter of the nanofiber is 600 nm, and the thickness of the tube shell layer is 55 nm.

[0069] The preparation method of the above-mentioned nanocomposite for lithium-sulfur batteries comprises the following steps:

[0070] S1. Dissolve polyacrylonitrile powder with a molecular weight of 150,000 in DMF with a mass concentration of 55% as the shell layer carbon and nitrogen source solution;

[0071] Dissolve polylactic acid with a molecular weight of 8,000 in DMF with a mass concentration of 15% as the outer core layer carbon source solution;

[0072] Dissolve polyimide with a molecular weight of 100,000 in DMF with a mass concentration of 55%. Then add iron acetylacetonate with a particle size less than 100 nm to the solution and disperse it by ultrasonic waves as the inner core layer carbon and nitrogen source solution; the amount of iron acetylacetonate accounts for 11.5% of the total solute mass.

[0073] S2. Respectively introduce the shell layer carbon and nitrogen source solution, the outer core layer carbon source solution and the inner core layer carbon and nitrogen source solution into a three-layer coaxial needle. The thickness ratio of the inner and outer layers of the coaxial needle is 1:1.2, and the liquid discharge ratio of the inner core layer carbon and nitrogen source solution, the outer core layer carbon source solution and the shell layer carbon and nitrogen source solution is 1:2:1. Apply a voltage of 15 kV and a receiving distance of 14 cm to obtain a composite nanofiber layer with a core structure and a shell structure.

[0074] S3. Heat the composite nanofiber layer with a core structure and a shell structure in a muffle furnace from room temperature to 290 °C at a rate of 2 °C / min and keep it at a constant temperature for 2 h. Then heat it from room temperature to 900 °C at a rate of 2 °C / min in an N2 atmosphere and keep it at a constant temperature for 5 h to fully carbonize the fiber shell layer and the inner core layer and fully thermally decompose the outer core layer to optimize the morphology and obtain a core-shell tube-axis nanofiber nanocomposite for lithium-sulfur batteries with an interwoven network structure.

[0075] Example 2

[0076] A nanocomposite for a lithium-sulfur battery, the nanocomposite for a lithium-sulfur battery has a network structure formed by interweaving a plurality of nanofibers. The nanofibers have a tube shell layer with multiple nanopores. The tube shell layer is a hollow nano carbon and nitrogen tube. Inside the tube shell layer is a nanowire. The nanowire is a carbon-supported transition metal carbide. There is a cavity for accommodating sulfur between the tube shell layer and the nanowire.

[0077] Among them, the nanowire is Co2C, and the diameter of the nanowire is 76 nm.

[0078] The diameter of the nanofiber is 620 nm, and the thickness of the tube shell layer is 55 nm.

[0079] The preparation method of the above-mentioned nano-composite material for lithium-sulfur batteries comprises the following steps:

[0080] S1. Dissolve polyacrylamide powder with a molecular weight of 150,000 in DMF with a mass concentration of 45% as the shell layer carbon and nitrogen source solution;

[0081] Dissolve polymethyl methacrylate with a molecular weight of 8,000 in DMF with a mass concentration of 25% as the outer core layer carbon source solution;

[0082] Dissolve polyacrylonitrile with a molecular weight of 100,000 in DMF with a mass concentration of 45%. Then add cobalt acetylacetonate with a particle size less than 100 nm to the solution and ultrasonically disperse it as the inner core layer carbon and nitrogen source solution; the amount of cobalt acetylacetonate accounts for 12.5% of the total solute mass.

[0083] S2. Respectively introduce the shell layer carbon and nitrogen source solution, the outer core layer carbon source solution and the inner core layer carbon and nitrogen source solution into a three-layer coaxial needle. The thickness ratio of the inner and outer layers of the coaxial needle is 1:1.2, and the ratio of the liquid discharge amounts of the inner core layer carbon and nitrogen source solution, the outer core layer carbon source solution and the shell layer carbon and nitrogen source solution is 1:2:1. Apply a voltage of 15 kV and a receiving distance of 14 cm to obtain a composite nanofiber layer with a core structure and a shell structure.

[0084] S3. Heat the composite nanofiber layer with a core structure and a shell structure in a muffle furnace from room temperature to 280 °C at a rate of 2 °C / min and keep it at a constant temperature for 2 h. Then heat it from room temperature to 1000 °C at a rate of 2 °C / min in an N2 atmosphere and keep it at a constant temperature for 5 h to fully carbonize the fiber shell layer and the inner core layer and fully thermally decompose the outer core layer to optimize the morphology and obtain a core-shell tube-shaped nanofiber nano-composite material for lithium-sulfur batteries interwoven into a network structure.

[0085] Example 3

[0086] A nano-composite material for lithium-sulfur batteries, the nano-composite material for lithium-sulfur batteries has a network structure interwoven by a number of nanofibers. The nanofibers have a multi-nanoporous tube shell layer, the tube shell layer is a hollow nano-carbon and nitrogen tube, the inside of the tube shell layer is a nanowire, the nanowire is a carbon-supported transition metal carbide, and there is a cavity for accommodating sulfur between the tube shell layer and the nanowire.

[0087] Among them, the nanowire is Ni3C and the diameter of the nanowire is 77 nm.

[0088] The diameter of the nanofiber is 610 nm and the thickness of the tube shell layer is 55 nm.

[0089] The preparation method of the above-mentioned nano-composite material for lithium-sulfur batteries comprises the following steps:

[0090] S1. Dissolve polyimide powder with a molecular weight of 120,000 in DMF at a mass concentration of 45% as the shell carbon and nitrogen source solution;

[0091] Dissolve polyvinylidene fluoride with a molecular weight of 6,000 in DMF at a mass concentration of 35% as the outer core layer carbon source solution;

[0092] Dissolve polyacrylamide with a molecular weight of 120,000 in DMF at a mass concentration of 45%. Then add nickel acetylacetonate with a particle size less than 100 nm to the solution and disperse it by ultrasonic wave as the inner core layer carbon and nitrogen source solution; the amount of nickel acetylacetonate accounts for 10.5% of the total solute mass.

[0093] S2. Respectively introduce the shell carbon and nitrogen source solution, the outer core layer carbon source solution, and the inner core layer carbon and nitrogen source solution into a three-layer coaxial needle. The thickness ratio of the inner and outer layers of the coaxial needle is 1:1.1, and the ratio of the liquid discharge amounts of the inner core layer carbon and nitrogen source solution, the outer core layer carbon source solution, and the shell carbon and nitrogen source solution is 1:1.5:1. Apply a voltage of 15 kV and a receiving distance of 14 cm to obtain a composite nanofiber layer with a core structure and a shell structure.

[0094] S3. Heat the composite nanofiber layer with a core structure and a shell structure in a muffle furnace from room temperature to 280 °C at a rate of 2 °C / min and keep it at a constant temperature for 1.5 h. Then heat it from room temperature to 1000 °C at a rate of 2 °C / min in an N2 atmosphere and keep it at a constant temperature for 3 h to fully carbonize the fiber shell layer and the inner core layer and fully thermally decompose the outer core layer to optimize the morphology and obtain a core-shell tubular nanofiber lithium-sulfur battery nanocomposite material intertwined into a network structure.

[0095] Example 4

[0096] A nanocomposite material for lithium-sulfur batteries, the nanocomposite material for lithium-sulfur batteries has a network structure intertwined by a number of nanofibers. The nanofibers have a multi-nanoporous tube shell layer, the tube shell layer is a hollow nano carbon and nitrogen tube, the inside of the tube shell layer is a nanowire, the nanowire is a carbon-supported transition metal carbide, and there is a cavity for accommodating sulfur between the tube shell layer and the nanowire.

[0097] Among them, the nanowire is Mo2C and the diameter of the nanowire is 78 nm.

[0098] The diameter of the nanofiber is 615 nm and the thickness of the tube shell layer is 55 nm.

[0099] The preparation method of the above-mentioned nanocomposite material for lithium-sulfur batteries includes the following steps:

[0100] S1. Dissolve polyimide powder with a molecular weight of 120,000 in DMF at a mass concentration of 55% as the shell carbon and nitrogen source solution;

[0101] Dissolve polyvinylidene fluoride with a molecular weight of 9,000 in DMF at a mass concentration of 15% as the outer core layer carbon source solution;

[0102] Dissolve polyacrylonitrile with a molecular weight of 100,000 in DMF at a mass concentration of 55%. Then add molybdenum acetylacetonate with a particle size less than 100 nm to the solution and disperse it by ultrasonic treatment as the inner core layer carbon and nitrogen source solution; the amount of molybdenum acetylacetonate accounts for 11.5% of the total solute mass.

[0103] S2. Feed the shell layer carbon and nitrogen source solution, the outer core layer carbon source solution, and the inner core layer carbon and nitrogen source solution into a three-layer coaxial needle respectively. The thickness ratio of the inner and outer layers of the coaxial needle is 1:1.5, and the ratio of the liquid discharge amounts of the inner core layer carbon and nitrogen source solution, the outer core layer carbon source solution, and the shell layer carbon and nitrogen source solution is 1:1.5:1. Apply a voltage of 15 kV and a receiving distance of 14 cm to obtain a composite nanofiber layer with a core structure and a shell structure.

[0104] S3. Heat the composite nanofiber layer with a core structure and a shell structure in a muffle furnace from room temperature to 280 °C at a rate of 2 °C / min and keep it at a constant temperature for 3 h. Then heat it from room temperature to 1000 °C at a rate of 2 °C / min in an N2 atmosphere and keep it at a constant temperature for 4 h to fully carbonize the fiber shell layer and the inner core layer and fully thermally decompose the outer core layer to optimize the morphology, and obtain a core-shell tubular nanofiber lithium-sulfur battery nanocomposite material interwoven into a network structure.

[0105] Example 5

[0106] A nanocomposite material for lithium-sulfur batteries. The nanocomposite material for lithium-sulfur batteries has a network structure interwoven by a number of nanofibers. The nanofibers have a multi-nanoporous tube shell layer. The tube shell layer is a hollow nano carbon and nitrogen tube. Inside the tube shell layer is a nanowire. The nanowire is a carbon-supported transition metal carbide. There is a cavity for accommodating sulfur between the tube shell layer and the nanowire.

[0107] Among them, the nanowire is Mn3C, and the diameter of the nanowire is 74 nm.

[0108] The diameter of the nanofiber is 615 nm, and the thickness of the tube shell layer is 53 nm.

[0109] The preparation method of the above-mentioned nanocomposite material for lithium-sulfur batteries includes the following steps:

[0110] S1. Dissolve polyimide powder with a molecular weight of 150,000 in DMF at a mass concentration of 55% as the shell layer carbon and nitrogen source solution;

[0111] Dissolve polymethyl methacrylate with a molecular weight of 8,000 in DMF at a mass concentration of 25% as the outer core layer carbon source solution;

[0112] Dissolve polyacrylamide with a molecular weight of 100,000 in DMF at a mass concentration of 55%. Then add manganese acetylacetonate with a particle size less than 100 nm to the solution and ultrasonically disperse it to obtain the inner core layer carbon and nitrogen source solution; the amount of manganese acetylacetonate accounts for 11.5% of the total solute mass.

[0113] S2. Introduce the shell layer carbon and nitrogen source solution, the outer core layer carbon source solution, and the inner core layer carbon and nitrogen source solution into a three-layer coaxial needle respectively. The thickness ratio of the inner and outer layers of the coaxial needle is 1:1.5, and the ratio of the liquid discharge amounts of the inner core layer carbon and nitrogen source solution, the outer core layer carbon source solution, and the shell layer carbon and nitrogen source solution is 1:1.5:1. Apply a voltage of 15 kV and a receiving distance of 14 cm to obtain a composite nanofiber layer with a core structure and a shell structure.

[0114] S3. Heat the composite nanofiber layer with a core structure and a shell structure in a muffle furnace from room temperature to 285 °C at a rate of 2 °C / min and keep it at a constant temperature for 2 h. Then heat it from room temperature to 950 °C at a rate of 2 °C / min in an N2 atmosphere and keep it at a constant temperature for 5 h to fully carbonize the fiber shell layer and the inner core layer and fully thermally decompose the outer core layer to optimize the morphology and obtain a core-shell tubular nanofiber lithium-sulfur battery nano-composite material interwoven into a network structure.

[0115] Example 6

[0116] A nano-composite material for lithium-sulfur batteries, the nano-composite material for lithium-sulfur batteries has a network structure interwoven by a number of nanofibers. The nanofibers have a multi-nanoporous tube shell layer, the tube shell layer is a hollow nano-carbon and nitrogen tube, the inside of the tube shell layer is a nanowire, the nanowire is a carbon-supported transition metal carbide, and there is a cavity for accommodating sulfur between the tube shell layer and the nanowire.

[0117] Among them, the nanowire is Fe3C, and the diameter of the nanowire is 80 nm.

[0118] The diameter of the nanofiber is 617 nm, and the thickness of the tube shell layer is 53 nm.

[0119] The preparation method of the above nano-composite material for lithium-sulfur batteries includes the following steps:

[0120] S1. Dissolve polyacrylamide powder with a molecular weight of 140,000 in DMF at a mass concentration of 55% as the shell layer carbon and nitrogen source solution;

[0121] Dissolve polylactic acid with a molecular weight of 6,000 in DMF at a mass concentration of 15% as the outer core layer carbon source solution;

[0122] Dissolve polyimide with a molecular weight of 120,000 in DMF at a mass concentration of 55%. Then add iron acetate with a particle size less than 100 nm to the solution and ultrasonically disperse it to obtain the inner core layer carbon and nitrogen source solution; the amount of iron acetate accounts for 11.5% of the total solute mass.

[0123] S2. The shell carbon and nitrogen source solution, the outer core layer carbon source solution, and the inner core layer carbon and nitrogen source solution are respectively introduced into a three-layer coaxial needle. The thickness ratio of the inner and outer layers of the coaxial needle is 1:1.2, and the ratio of the liquid discharge amounts of the inner core layer carbon and nitrogen source solution, the outer core layer carbon source solution, and the shell carbon and nitrogen source solution is 1:2:1. The applied voltage is 15 kV, and the receiving distance is 14 cm, obtaining a composite nanofiber layer with a core structure and a shell structure.

[0124] S3. The composite nanofiber layer with a core structure and a shell structure is heated in a muffle furnace to 280 °C at a rate of 2 °C / min and kept at a constant temperature for 2 h. Then, it is heated to 900 °C at a rate of 2 °C / min under a N2 atmosphere and kept at a constant temperature for 5 h to fully carbonize the fiber shell layer and the inner core layer and fully thermally decompose the outer core layer to optimize the morphology, obtaining a core-shell tube-shaped nanofiber nanocomposite for a lithium-sulfur battery interwoven into a network structure.

[0125] Example 7

[0126] A nanocomposite for a lithium-sulfur battery, the nanocomposite for a lithium-sulfur battery has a network structure interwoven by a number of nanofibers. The nanofibers have a multi-nanoporous tube shell layer, the tube shell layer is a hollow nano carbon and nitrogen tube, the inside of the tube shell layer is a nanowire, the nanowire is a carbon-supported transition metal carbide, and there is a cavity for accommodating sulfur between the tube shell layer and the nanowire.

[0127] Among them, the nanowire is Ni3C, and the diameter of the nanowire is 76 nm.

[0128] The diameter of the nanofiber is 611 nm, and the thickness of the tube shell layer is 56 nm.

[0129] The preparation method of the above-mentioned nanocomposite for a lithium-sulfur battery includes the following steps:

[0130] S1. Dissolve polyacrylonitrile powder with a molecular weight of 140,000 in DMF with a mass concentration of 55% as the shell carbon and nitrogen source solution;

[0131] Dissolve polylactic acid with a molecular weight of 7,000 in DMF with a mass concentration of 25% as the outer core layer carbon source solution;

[0132] Dissolve polyimide with a molecular weight of 110,000 in DMF with a mass concentration of 55%. Then, add nickel acetate with a particle size less than 100 nm to the solution and ultrasonically disperse it as the inner core layer carbon and nitrogen source solution; the amount of nickel acetate accounts for 12.5% of the total solute mass.

[0133] S2. The shell layer carbon and nitrogen source solution, the outer core layer carbon source solution, and the inner core layer carbon and nitrogen source solution are respectively introduced into a three-layer coaxial needle. The thickness ratio of the inner and outer layers of the coaxial needle is 1:1.2, and the ratio of the liquid discharge amounts of the inner core layer carbon and nitrogen source solution, the outer core layer carbon source solution, and the shell layer carbon and nitrogen source solution is 1:2:1. A voltage of 15 kV is applied, and the receiving distance is 14 cm to obtain a composite nanofiber layer with a core structure and a shell structure.

[0134] S3. The composite nanofiber layer with a core structure and a shell structure is heated in a muffle furnace to 280 °C at a rate of 2 °C / min and kept at a constant temperature for 3 h. Then, it is heated to 1000 °C at a rate of 2 °C / min in an N2 atmosphere and kept at a constant temperature for 4 h to fully carbonize the fiber shell layer and the inner core layer and fully thermally decompose the outer core layer to optimize the morphology, obtaining a core-shell tube-axis nanofiber nanocomposite for a lithium-sulfur battery interwoven into a network structure.

[0135] Examples 8 - 14

[0136] A lithium-sulfur battery positive electrode sheet includes the nanocomposite for a lithium-sulfur battery of Examples 1 - 7.

[0137] The preparation method of the above lithium-sulfur battery positive electrode sheet includes the following steps:

[0138] Weigh sulfur powder and the nanocomposite for a lithium-sulfur battery of Examples 1 - 7 according to a mass ratio of 4:1, conduct sufficient mixing, heat to 155 °C in a reaction kettle to melt sulfur for 8 hours to obtain a core-shell lithium-sulfur battery nanocomposite loaded with sulfur.

[0139] Mix the core-shell lithium-sulfur battery nanocomposite loaded with sulfur, the conductive agent carbon nanotubes, and the binder polyvinylidene fluoride (PVDF) binder in a ratio of 8:1:1, apply the slurry by coating on the current collector aluminum foil, then place it in a vacuum drying oven at 60 °C for 8 h, and cut it to prepare the lithium-sulfur battery positive electrode sheet.

[0140] The sulfur loading is 2.2 mg / cm 2 .

[0141] Example 15

[0142] A lithium-sulfur battery positive electrode sheet includes the nanocomposite for a lithium-sulfur battery of Example 1.

[0143] The preparation method of the above lithium-sulfur battery positive electrode sheet includes the following steps:

[0144] Weigh sulfur powder and the nanocomposite for a lithium-sulfur battery of Example 1 according to a mass ratio of 6:1, conduct sufficient mixing, heat to 155 °C in a reaction kettle to melt sulfur for 8 hours to obtain a core-shell lithium-sulfur battery nanocomposite loaded with sulfur.

[0145] The sulfur-loaded core-shell lithium-sulfur battery nanocomposite material, the conductive agent carbon nanotubes and the binder polyvinylidene fluoride (PVDF) binder are mixed in the ratio of 8:1:1, applied 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 the lithium-sulfur battery positive electrode sheet.

[0146] Sulfur loading is 6 mg / cm 2 .

[0147] Example 16

[0148] A lithium-sulfur battery positive electrode sheet comprises the nanocomposite material for lithium-sulfur battery of embodiment 1.

[0149] The method for preparing the above-mentioned lithium-sulfur battery positive electrode sheet comprises the following steps:

[0150] Sulfur powder and the nanocomposite material for lithium-sulfur batteries of Example 1 were weighed in a mass ratio of 9:1, mixed thoroughly, and heated to 155° C. in a reaction kettle to melt sulfur for 8 hours to obtain a sulfur-loaded core-shell nanocomposite material for lithium-sulfur batteries.

[0151] The sulfur-loaded core-shell lithium-sulfur battery nanocomposite material, the conductive agent carbon nanotubes and the binder polyvinylidene fluoride (PVDF) binder are mixed in the ratio of 8:1:1, applied 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 the lithium-sulfur battery positive electrode sheet.

[0152] Sulfur loading is 10 mg / cm 2 .

[0153] Examples 17 to 25

[0154] 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 16.

[0155] The preparation method of the above lithium-sulfur battery comprises the following steps:

[0156] Preparation of electrolyte: 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), 2% LiNO3 and 1 mol / L lithium bistrifluoromethanesulfonyl imide (LiTFSI) were mixed uniformly in a volume ratio of 1:1 to obtain an electrolyte;

[0157] 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 16 were transferred to a glove box filled with argon gas for assembly of button batteries to obtain button batteries of model CR2032.

[0158] Embodiments 26 to 32

[0159] A lithium-sulfur battery positive electrode side interlayer, comprising the nanocomposites for lithium-sulfur batteries of Examples 1 to 7.

[0160] The preparation method of the lithium-sulfur battery positive electrode side interlayer is basically the same as that of the nanocomposites for lithium-sulfur batteries of Examples 1 to 7, except that:

[0161] After S2 electrospinning, the obtained material is soaked in a 10% phenolic resin solution for 30 min and dried; then the S3 step is carried out.

[0162] Examples 33 to 39

[0163] A lithium-sulfur battery, comprising a positive electrode sheet, an interlayer, a separator, a negative electrode sheet and an electrolyte. The interlayer is arranged between the positive electrode sheet and the separator, and the interlayer is the lithium-sulfur battery positive electrode side interlayer of Examples 26 to 32.

[0164] The preparation method of the above lithium-sulfur battery comprises the following steps:

[0165] Preparation of the electrolyte: Mix 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) with a volume ratio of 1:1, 2% LiNO3 and 1 mol / L lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) evenly to obtain the electrolyte;

[0166] Preparation of the positive electrode sheet: Mix sulfur powder, conductive agent carbon nanotubes and polyvinylidene fluoride (PVDF) binder in a ratio of 8:1:1, apply the slurry on the current collector aluminum foil by mixing, and then place it in a vacuum drying oven at 60 °C for 8 h and cut to prepare the lithium-sulfur battery positive electrode sheet.

[0167] Using a lithium metal sheet as the counter electrode and a polypropylene microporous membrane Celgard 2400 as the separator, transfer the lithium-sulfur battery positive electrode side interlayer of Examples 22 to 28 to a glove box filled with argon for the assembly of a button battery, and assemble a button battery of model CR2032.

[0168] Comparative Example 1

[0169] A preparation method of a carbon fiber composite material is basically the same as that of Example 1. Different from Example 1, a carbon-containing transition metal salt is not added to the inner core layer carbon source solution.

[0170] The rest is the same as that of Example 1 and will not be elaborated here.

[0171] Comparative Example 2

[0172] A lithium-sulfur battery positive electrode side interlayer, comprising the nanocomposites for lithium-sulfur batteries of Comparative Example 1, and the preparation method is the same as that of Example 22.

[0173] Result detection

[0174] The cross-section of the nano-composite material for the lithium-sulfur battery of Example 1 of the present invention was characterized by a scanning electron microscope (SEM), and the results are as Figure 1 shown. The nano-composite material for the lithium-sulfur battery of Example 1 of the present invention was characterized by a transmission electron microscope (TEM) image, and the results are as Figure 2 shown. The nano-composite material for the lithium-sulfur battery of Example 1 of the present invention was made into the positive electrode sheets of Examples 15 and 16 and then made into a lithium-sulfur battery, and the cycle performance was tested. The test results are as Figure 3 shown. The nano-composite material for the lithium-sulfur battery of Example 1 of the present invention was tested by a thermogravimetric analyzer, and the test results are as Figure 4 shown. The nano-composite material for the lithium-sulfur battery of Example 1 of the present invention was tested by an X-ray diffractometer, and the test results are as Figure 5 shown.

[0175] The lithium-sulfur battery was prepared from the carbon fiber composite material of Comparative Example 1 according to the same preparation method as that of Examples 15 to 21.

[0176] The lithium-sulfur battery positive electrode side interlayer of Comparative Example 2 was prepared into a lithium-sulfur battery according to the same preparation method as that of Examples 29 to 35.

[0177] The assembled lithium-sulfur batteries of each example and comparative example were subjected to electrochemical performance testing on a Neware test system, with a voltage range of 1.7 to 2.8 V and a discharge rate of 0.5 C.

[0178] The specific test results are as described in Table 1 below:

[0179]

[0180] Continued Table 1

[0181]

[0182] Continued Table 1

[0183]

[0184] Continued Table 1

[0185]

[0186] The above Figures 1 to 4 and the analysis of Table 1 are as follows:

[0187] Figure 1 is the scanning electron microscope (SEM) image of the cross-section of the nano-composite material for the lithium-sulfur battery of Example 1 of the present invention. Figure 2 is the transmission electron microscope (TEM) image of the nano-composite material for the lithium-sulfur battery of Example 1 of the present invention. FromFigure 1 and Figure 2 It can be seen that the nano-composite material for lithium-sulfur batteries prepared by the present invention has a network structure formed by the interweaving of a number of nanofibers. The nanofibers have a multi-nanoporous tube shell layer, and the tube shell layer is a hollow nano-carbon nitride tube. Inside the tube shell layer are nanowires, and the nanowires are carbon-supported transition metal carbides. There is a cavity for accommodating sulfur between the tube shell layer and the nanowires.

[0188] Figure 3 This is the cyclic performance graph of the nano-composite material for lithium-sulfur batteries in Example 1 of the present invention under different sulfur loadings, for Examples 15 and 16. From Figure 3 It can be seen that when the nano-composite material for lithium-sulfur batteries of Example 1 is used as the carrier of the positive electrode sheet of the lithium-sulfur battery, when the sulfur loading is 6 mg·cm -2 , the discharge specific capacity in the first cycle after activation is 1310 mAh·g -1 , and after 50 cycles, the discharge specific capacity is 920 mAh·g -1 , and the capacity retention rate is 70.23%; when the nano-composite material for lithium-sulfur batteries of Example 1 is used as the carrier of the positive electrode sheet of the lithium-sulfur battery in Example 1, when the sulfur loading is 10 mg·cm -2 , the discharge specific capacity in the first cycle after activation is 1160 mAh·g -1 , and after 50 cycles, the discharge specific capacity is 908 mAh·g -1 , and the capacity retention rate is 78.28%. It shows that the material of the present invention can load more active substance sulfur and can exhibit a relatively stable capacity at the same time.

[0189] Figure 4 This is the thermogravimetric (TGA) graph of the nano-composite material for lithium-sulfur batteries in Example 1 of the present invention. From Figure 4 It can be seen that during the calcination process at 0 - 300 °C, the mass fraction of the nano-composite material for lithium-sulfur batteries decreased by 78.2%, indicating that the shell layer structure composed of carbon-nitrogen in the material was lost within the range of 0 - 300 °C under air sintering, and only Fe3C with adsorption-catalysis function remained. The thermogravimetric data can prove that the amount of the catalyst is small, only accounting for 21.8%, and can keep the cyclic capacity as high as 92.3%.

[0190] Figure 5 This is the X-ray diffraction (XRD) graph of the nano-composite material for lithium-sulfur batteries in Example 1 of the present invention. From Figure 5 It can be seen that the nano-composite material for lithium-sulfur batteries in Example 1 of the present invention has a diffraction peak of iron carbide.

[0191] As can be seen from Table 1, for the lithium-sulfur battery prepared from the nano-composite material for lithium-sulfur batteries of the present invention, when the sulfur loading is 2.2 mg·cm -2When it has a specific discharge capacity higher than 1300 mAh·g -1 for the first discharge, the first Coulombic efficiency is above 90%, and after 500 cycles, the capacity retention rate can reach above 91%.

[0192] It can be seen from Example 1 and Comparative Example 1 that without adding a carbon-containing transition metal salt, the metal carbide cannot be formed in the inner core layer of the material, and the absence of the adsorption-catalytic layer will cause a significant reduction in the capacity retention rate of the lithium-sulfur battery.

[0193] Obviously, the above-mentioned embodiments of the present invention are merely examples for clearly illustrating the present invention, rather than limiting the implementation manners of the present invention. For those of ordinary skill 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 enumerate all the implementation manners here. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the claims of the present invention.

Claims

1. A nanocomposite material for the positive electrode or the interlayer on the positive electrode side of a lithium-sulfur battery, characterized in that, The nano-composite material for the lithium-sulfur battery has a network structure formed by the interweaving of a number of nanofibers. The nanofibers have a multi-nanoporous tube shell layer, the tube shell layer is a hollow nano-carbon nitride tube, the interior of the tube shell layer is a nanowire, the nanowire is a carbon-supported transition metal carbide, and there is a cavity for accommodating sulfur between the tube shell layer and the nanowire; The preparation method of the nano-composite material includes the following steps: S1. Respectively prepare a shell layer carbon nitride source solution, an outer core layer carbon source solution, and an inner core layer carbon nitride source solution, and add a carbon-containing transition metal salt to the inner core layer carbon nitride source solution; S2. Electrospin the shell layer carbon nitride source solution, the outer core layer carbon source solution, and the inner core layer carbon nitride source solution coaxially in three layers; S3. Pre-oxidize the material in S2 and carbonize it in a non-oxidizing atmosphere to obtain the nano-composite material for the lithium-sulfur battery; Among them, in S1, the molecular weight of the shell layer carbon nitride source and the molecular weight of the inner core layer carbon nitride source are both greater than the molecular weight of the outer core layer carbon source, and the molecular weight of the outer core layer carbon source is 5000-10000; The mass ratio of the sum of the mass of the carbon-containing transition metal salt and the inner core layer carbon nitride source to the mass of the carbon-containing transition metal salt is 100:(5-20); In S2, the liquid output ratio of the shell layer carbon nitride source solution, the outer core layer carbon source solution, and the inner core layer carbon nitride source solution is 1:(1.5-2):1; In S3, the pre-oxidation temperature is 220-310°C, and the pre-oxidation time is 1-6 h; the carbonization temperature is 600-1000°C, and the carbonization time is 2-6 h.

2. The nanocomposite material according to claim 1, wherein The nanowire is one or more of iron carbide, cobalt carbide, molybdenum carbide, nickel carbide, or manganese carbide.

3. The nanocomposite material according to claim 1, characterized in that, The diameter of the nanowire is less than 100 nm.

4. The nanocomposite material according to claim 1, wherein In S1, the outer core layer carbon source is one or several of polymethyl methacrylate, poly(vinylidene fluoride), or polylactic acid.

5. The nanocomposite material according to claim 1, wherein The mass concentration percentage of the outer core layer carbon source and the shell layer carbon nitride source solution is 15%-85%.

6. A lithium-sulfur battery, characterized in that, It includes a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte. The positive electrode sheet includes the nano-composite material according to any one of claims 1-5.

7. A lithium-sulfur battery, characterized in that, It includes a positive electrode sheet, an interlayer, a separator, a negative electrode sheet, and an electrolyte. The interlayer is arranged between the positive electrode sheet and the separator, and the interlayer includes the nano-composite material according to any one of claims 1-5.

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