Carbon-coated nickel nano-carbon cage composite material and preparation method thereof, lithium-sulfur battery positive electrode active material and preparation method and application thereof

Abstract

The present invention relates to the technical field of positive electrode materials for lithium-sulfur batteries, and discloses a carbon-coated nickel nano-carbon cage composite material and a preparation method thereof, a lithium-sulfur battery positive electrode active material and a preparation method and application thereof. The composite material comprises carbon-coated nickel nanoparticles and nano-carbon cages, wherein the carbon-coated nickel nanoparticles contain a metallic nickel core and a graphitized carbon layer shell coated on the surface of the metallic nickel core, and the nano-carbon cage contains a nano-carbon cage empty core; and there is at least one mesoporous distribution peak in the pore size distribution curve of the composite material. The carbon-coated nickel nano-carbon cage composite material has a rich mesoporous structure, which is conducive to the mass transfer and diffusion of reactants and products in the battery reaction. It can "anchor" the intermediate product polysulfide through physical confinement, and can also catalyze the electrochemical conversion of sulfur-containing substances, provide active sites for electrochemical reactions, improve the utilization rate of active sulfur, and improve the charge and discharge specific capacity and cycle stability of the lithium-sulfur battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-sulfur battery positive electrode materials, and in particular to a carbon-coated nickel nano-carbon cage composite material and a preparation method thereof, a lithium-sulfur battery positive electrode active material and a preparation method and application thereof. Background Art

[0002] With the rapid development of electric vehicles and mobile electronic devices in recent years, the energy density of lithium-ion batteries has been difficult to meet the needs. Lithium-sulfur batteries have a high theoretical specific capacity (1675mAh·g -1 ) and theoretical energy density (2600Wh·kg -1 ), environmentally friendly, safe, non-toxic, and low-cost, it has attracted considerable attention as the most promising next-generation high-energy secondary battery system. However, lithium-sulfur batteries also have numerous drawbacks. Sulfur's electronic insulation at room temperature, up to 80% volume expansion during charge and discharge, and the "shuttle effect" caused by the dissolution of sulfur discharge intermediates, polysulfides, into the electrolyte, all contribute to the relatively low specific capacity and poor cycle performance of current lithium-sulfur batteries, severely restricting their practical application.

[0003] In order to solve these problems, researchers mainly use two methods to improve the performance of lithium-sulfur batteries. The first is to suppress the "shuttle effect" of polysulfide ion intermediates in the battery reaction process through physical confinement. For example, CN112661137A discloses a porous carbon sphere and its preparation method, and applies the porous carbon sphere in lithium-sulfur batteries. The porous carbon spheres prepared by this preparation method are assembled into secondary particles from hollow primary particles. Although they have a certain limiting effect on polysulfides, the kinetics of the electrochemical conversion reaction of polysulfides are slow, which affects the performance of lithium-sulfur batteries. The second is to adsorb the intermediate product lithium polysulfide through the synergistic effect of transition metal chemical adsorption and electrocatalysis, and catalyze its electrochemical conversion, providing active sites for electrochemical reactions and improving the utilization rate of active sulfur. However, due to the high activity of transition metal nanoparticles, they are prone to agglomeration or side reactions during the electrochemical reaction, which affects the performance and application of such materials. For example, CN110931752A discloses a lithium-sulfur battery positive electrode material composed of nitrogen-doped porous carbon loaded with metallic nickel. In this material, the metal particles are only dispersed on the carbon material and are in an exposed state. They are very prone to agglomeration or oxidation, and even spontaneous combustion in the air. At the same time, side reactions are prone to occur during the electrochemical reaction, leading to failure. Summary of the Invention

[0004] The purpose of the present invention is to overcome the above-mentioned problems existing in the prior art and provide a carbon-coated nickel nano-carbon cage composite material and its preparation method, a lithium-sulfur battery positive electrode active material and its preparation method and application, so as to improve the specific capacity and cycle stability of lithium-sulfur batteries.

[0005] In order to achieve the above-mentioned objectives, the first aspect of the present invention provides a carbon-coated nickel nano-carbon cage composite material, which comprises carbon-coated nickel nanoparticles and nano-carbon cages, wherein the carbon-coated nickel nanoparticles contain a metallic nickel core and a graphitized carbon layer shell coated on the surface of the metallic nickel core, and the nano-carbon cage contains a nano-carbon cage empty core; and the pore size distribution curve of the composite material has at least one mesopore distribution peak.

[0006] A second aspect of the present invention provides a method for preparing a carbon-coated nickel nano-carbon cage composite material, the method comprising the following steps:

[0007] (1) providing a homogeneous solution containing a nickel source, a polybasic organic carboxylic acid, and a solvent, and then removing the solvent from the homogeneous solution to obtain a precursor, wherein the molar ratio of the nickel source to the polybasic organic carboxylic acid is 1:0.5-1.5, calculated as nickel element;

[0008] (2) calcining the precursor at a constant temperature under an inert atmosphere or a reducing atmosphere to obtain a calcined product;

[0009] (3) contacting the calcined product with an acid for reaction, and then performing solid-liquid separation.

[0010] The third aspect of the present invention provides a lithium-sulfur battery positive electrode active material based on a carbon-coated nickel nano-carbon cage composite material. The lithium-sulfur battery positive electrode active material contains a carbon-coated nickel nano-carbon cage composite material, graphene and elemental sulfur. The carbon-coated nickel nano-carbon cage composite material is the carbon-coated nickel nano-carbon cage composite material described in the first aspect or the carbon-coated nickel nano-carbon cage composite material prepared according to the method described in the second aspect.

[0011] A fourth aspect of the present invention provides a method for preparing a positive electrode active material for a lithium-sulfur battery, the method comprising:

[0012] (a) providing a mixture containing a carbon-coated nickel nano-carbon cage composite material, graphene and elemental sulfur;

[0013] (b) subjecting the mixture to high-temperature treatment under the protection of an inert atmosphere and at a temperature not lower than the melting point of sulfur; the carbon-coated nickel nano-carbon cage composite material is the carbon-coated nickel nano-carbon cage composite material described in the first aspect or the carbon-coated nickel nano-carbon cage composite material prepared according to the method described in the second aspect.

[0014] The fifth aspect of the present invention provides an application of the carbon-coated nickel nano-carbon cage composite material described in the first aspect or the lithium-sulfur battery positive electrode active material based on the carbon-coated nickel nano-carbon cage composite material described in the third aspect in a lithium-sulfur battery.

[0015] Through the above technical solution, the present invention has the following advantages:

[0016] (1) The carbon-coated nickel nano-carbon cage composite material provided by the present invention includes carbon-coated nickel nanoparticles and nano-carbon cages, wherein the carbon-coated nickel nanoparticles contain a metallic nickel core and a graphitized carbon layer shell coated on the surface of the metallic nickel core, and the nano-carbon cage contains a nano-carbon cage empty core; there is at least one mesoporous distribution peak in the pore size distribution curve of the composite material. The carbon-coated nickel nano-carbon cage composite material has a rich mesoporous structure, which is beneficial to the mass transfer and diffusion of reactants and products in the battery reaction; preferably, it has a multi-level mesoporous structure, thereby being able to give the composite material more diverse functions, making it suitable for more application fields;

[0017] (2) The carbon-coated nickel nano-carbon cage composite material provided by the present invention can not only "anchor" the intermediate polysulfide through physical confinement, but also catalyze the electrochemical conversion of sulfur-containing substances, provide active sites for electrochemical reactions, improve the utilization rate of active sulfur, and improve the charge and discharge specific capacity and cycle stability of the lithium-sulfur battery prepared from the carbon-coated nickel nano-carbon cage composite material;

[0018] (3) The method for preparing a positive electrode active material for a lithium-sulfur battery provided by the present invention comprises mixing a carbon-coated nickel nano-carbon cage composite material with graphene and elemental sulfur, and then subjecting the mixture to a high-temperature melting process to obtain a positive electrode active material for a lithium-sulfur battery. This method is simple and low-cost. The prepared positive electrode active material for a lithium-sulfur battery effectively inhibits the "shuttle effect" of polysulfide ion intermediates during the battery reaction process, while catalyzing the electrochemical conversion of sulfur-containing substances, providing active sites for electrochemical reactions, and improving the utilization rate of active sulfur. The lithium-sulfur battery prepared using this positive electrode active material for a lithium-sulfur battery has a high specific capacity and good cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a TEM image of the carbon-coated nickel nano-carbon cage composite material prepared in Preparation Example 1;

[0020] Figure 2 is the XRD pattern of the carbon-coated nickel nano-carbon cage composite material prepared in Preparation Example 1;

[0021] Figure 3 is the N2 adsorption-desorption isotherm of the carbon-coated nickel nano-carbon cage composite material prepared in Preparation Example 1;

[0022] Figure 4 is the BJH pore size distribution curve of the carbon-coated nickel nano-carbon cage composite material prepared in Preparation Example 1;

[0023] Figure 5 is a Raman spectrum of the carbon-coated nickel nano-carbon cage composite material prepared in Preparation Example 1;

[0024] Figure 6 is a TEM image of the carbon-coated nickel nano-carbon cage composite material prepared in Preparation Example 3;

[0025] Figure 7 is the XRD pattern of the carbon-coated nickel nano-carbon cage composite material prepared in Preparation Example 3;

[0026] Figure 8 is the N2 adsorption-desorption isotherm of the carbon-coated nickel nano-carbon cage composite material prepared in Preparation Example 3;

[0027] Figure 9 is the BJH pore size distribution curve of the carbon-coated nickel nano-carbon cage composite material prepared in Preparation Example 3;

[0028] Figure 10 is a Raman spectrum of the carbon-coated nickel nano-carbon cage composite material prepared in Preparation Example 3;

[0029] Figure 11 1 is a cycle performance diagram of a lithium-sulfur battery prepared using the positive electrode active material of Example 1;

[0030] Figure 12 This is a cycle performance diagram of a lithium-sulfur battery prepared using the positive electrode active material of Example 3. DETAILED DESCRIPTION

[0031] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0032] The first aspect of the present invention provides a carbon-coated nickel nano-carbon cage composite material, which includes carbon-coated nickel nanoparticles and nano-carbon cages, wherein the carbon-coated nickel nanoparticles contain a metallic nickel core and a graphitized carbon layer shell coated on the surface of the metallic nickel core, and the nano-carbon cage contains a nano-carbon cage empty core; the pore size distribution curve of the composite material has at least one mesopore distribution peak.

[0033] In the present invention, the term "nanocarbon cage" has the conventional meaning in the art, specifically referring to a hollow cage-shaped nanocarbon cage surrounded by graphitized carbon layers; "nanocarbon cage hollow core" refers to a hollow core formed by graphitized carbon layers in a nanocarbon cage.

[0034] In the present invention, the term "metallic nickel" means that the valence state of nickel is zero.

[0035] In the present invention, the term "graphitized carbon layer" refers to a carbon structure having a layered structure that can be clearly observed under a high-resolution transmission electron microscope, rather than an amorphous structure.

[0036] The inventors of the present invention found in the course of research that although the chemical adsorption and electrocatalysis of transition metal nanomaterials have a certain limiting effect on polysulfides, the chemical stability of the material is poor and it lacks a confining and fixing effect on polysulfides; while the chemical stability of nanocarbon materials is better, and they inhibit polysulfides by physical confinement, but the kinetics of the electrochemical conversion reaction of polysulfides are slow. If the two are combined in an appropriate manner, a new synergistic effect may be produced, so that the material exhibits new and unique properties. The carbon-coated nickel nano-carbon cage composite material provided by the present invention includes carbon-coated nickel nanoparticles and nano-carbon cages, wherein the carbon-coated nickel nanoparticles contain a metallic nickel core and a graphitized carbon layer shell coated on the surface of the metallic nickel core, and the nano-carbon cage contains a nano-carbon cage empty core; the composite material can "anchor" the intermediate polysulfide by physical confinement, and can also catalyze the electrochemical conversion of sulfur-containing substances, provide active sites for electrochemical reactions, and improve the utilization rate of active sulfur. In addition, the carbon-coated nickel nano-carbon cage composite material also has a rich mesoporous structure, which is beneficial to the mass transfer and diffusion of reactants and products in the battery reaction.

[0037] According to some embodiments of the present invention, preferably, based on the total weight of the composite material, the nickel content is 2-12% by weight, preferably 5-10% by weight; and the carbon content is 85-95% by weight, preferably 87-93% by weight. In this preferred embodiment, the moderate nickel content can effectively catalyze the electrochemical reaction of polysulfides while ensuring the overall conductivity of the electrode material.

[0038] According to some embodiments of the present invention, the carbon-coated nickel nano-carbon cage composite material may further contain oxygen, which may be oxygen in various forms formed in the graphitized carbon layer during the preparation of the composite material. More preferably, the content of oxygen is 0.2-10% by weight, preferably 1-3% by weight, and more preferably 1-1.8% by weight, based on the total amount of the composite material.

[0039] According to some embodiments of the present invention, preferably, based on the total amount of the composite material, the content of the carbon-coated nickel nanoparticles is 3-15% by weight, preferably 4-14% by weight, and more preferably 7-13% by weight; the content of the nanocarbon cage is 85-97% by weight, preferably 86-96% by weight, and more preferably 87-93% by weight. In this preferred embodiment, the carbon-coated nickel nanoparticles can effectively catalyze the electrochemical reaction of the intermediate polysulfide, and the nanocarbon cage can both improve the conductivity and play a certain physical confinement role, which is conducive to further improving the specific capacity and cycle stability of the lithium-sulfur battery. In the composite material, the content of the carbon-coated nickel nanoparticles and the nanocarbon cage is obtained by measuring the content of nickel in the calcined product (carbon-coated nickel nanomaterial) obtained by preparing the composite material and the content of nickel in the composite material, and calculated according to the following formula:

[0040] Content of carbon-coated nickel nanoparticles = [content of nickel in carbon-coated nickel nano-carbon cage composite material ÷ content of nickel in calcined product (carbon-coated nickel nanomaterial)] × 100%;

[0041] The content of carbon nanocages = 100% - the content of carbon-coated nickel nanoparticles.

[0042] According to some embodiments of the present invention, the carbon-coated nickel nano-carbon cage composite material may contain various doping elements that are known to those skilled in the art and can be applied to carbon materials. Preferably, the composite material does not contain nitrogen, sulfur, boron, phosphorus, fluorine, chlorine, bromine, iodine and other elements.

[0043] In the present invention, the carbon and oxygen contents in the carbon-coated nickel nano-carbon cage composite material are determined by elemental analysis, and the nickel content is the normalized content after deducting the carbon and oxygen contents.

[0044] According to some embodiments of the present invention, preferably, the pore size distribution curve of the composite material has a dual mesopore distribution peak, and the dual mesopore distribution peaks correspond to the first most probable pore size and the second most probable pore size, respectively; more preferably, the first most probable pore size is smaller than the second most probable pore size; further preferably, the first most probable pore size is 2-5 nm, and the second most probable pore size is 8-40 nm. In this preferred embodiment, the small pore size can further provide a large specific surface area for the composite material, increasing active sites, while the large pore size can provide diffusion channels for molecules or ions, accelerating mass transfer while providing higher stability.

[0045] According to some embodiments of the present invention, preferably, the specific surface area of ​​the composite material is 200-450m 2 / g, preferably 240-400m 2 / g.

[0046] According to some embodiments of the present invention, preferably, the mesopore volume of the composite material is 0.5-1.5 cm 3 / g, preferably 1-1.4cm 3 / g.

[0047] According to some embodiments of the present invention, preferably, the ratio of the mesopore volume of the composite material to the total pore volume is 90-100%, preferably 95-100%, more preferably 98-100%.

[0048] In the present invention, the term "mesopore" is defined as a pore with a pore diameter in the range of 2-50 nm.

[0049] In the present invention, the term "mesopore distribution peak" refers to a mesopore distribution peak on a pore distribution curve obtained by calculating a desorption curve according to the Barrett-Joyner-Halenda (BJH) method.

[0050] In the present invention, the pore structure properties of the carbon-coated nickel nano-carbon cage composite material were examined using the BET test method. Specifically, a Quantachrome AS-6B analyzer was used for measurement. The BET specific surface area and pore volume of the material were obtained using the Brunauer-Emmett-Taller (BET) method, and the mesopore distribution curve was calculated from the desorption curve using the Barrett-Joyner-Halenda (BJH) method.

[0051] According to some embodiments of the present invention, preferably, the average particle size of the metallic nickel core is 1-200 nm, preferably 5-40 nm.

[0052] According to some embodiments of the present invention, preferably, the average thickness of the graphitized carbon layer shell is 0.3-6 nm, preferably 0.3-4 nm.

[0053] According to some embodiments of the present invention, preferably, the average particle size of the empty core of the nanocarbon cage is 1-200 nm, preferably 5-40 nm.

[0054] In the present invention, the average particle size of the metallic nickel core, the average thickness of the graphitized carbon layer shell, and the average particle size of the nanocarbon cage empty core are all measured by transmission electron microscopy.

[0055] According to some embodiments of the present invention, preferably, the metallic nickel core comprises a face-centered cubic lattice structure and / or a hexagonal close-packed lattice structure, preferably a face-centered cubic lattice structure. The lattice structure of the metallic nickel core can be determined by XRD characterization.

[0056] According to some embodiments of the present invention, preferably, in the Raman curve of the composite material, ID / I G The range is 0.7-1.5, preferably 0.8-0.9.

[0057] In the present invention, the graphitization degree of the carbon-coated nickel nano-carbon cage composite material is characterized by Raman spectroscopy, 1355 cm -1 The peak at 1585 cm (D peak) is attributed to structural defects and is amorphous carbon. -1 The peak (G peak) is attributed to the carbon in the planar structure. Usually I D / I G (D peak and G peak intensity ratio) to characterize the degree of graphitization of the material. D / I G The higher the value, the more defects there are and the lower the degree of graphitization. The Raman spectrum of the material was obtained using a RM2000 micro-confocal Raman spectrometer (product of Reinshaw Company). Technical specifications: The excitation source used was a He-Ne laser with a wavelength of 525 nm.

[0058] A second aspect of the present invention provides a method for preparing a carbon-coated nickel nano-carbon cage composite material, the method comprising the following steps:

[0059] (1) providing a homogeneous solution containing a nickel source, a polybasic organic carboxylic acid, and a solvent, and then removing the solvent from the homogeneous solution to obtain a precursor, wherein the molar ratio of the nickel source to the polybasic organic carboxylic acid is 1:0.5-1.5, calculated as nickel element;

[0060] (2) calcining the precursor at a constant temperature under an inert atmosphere or a reducing atmosphere to obtain a calcined product;

[0061] (3) contacting the calcined product with an acid for reaction, and then performing solid-liquid separation.

[0062] According to some embodiments of the present invention, the method for preparing a carbon-coated nickel nano-carbon cage composite material prepares a precursor by controlling a specific ratio of a nickel source and a polybasic organic carboxylic acid, and the precursor is subjected to a constant temperature calcination and then contacted with an acid for reaction. The obtained carbon-coated nickel nano-carbon cage composite material includes carbon-coated nickel nanoparticles and nano-carbon cages, wherein the carbon-coated nickel nanoparticles contain a metallic nickel core and a graphitized carbon layer shell coated on the surface of the metallic nickel core, and the nano-carbon cage contains a nano-carbon cage empty core; and there is at least one mesopore distribution peak in the pore size distribution curve of the composite material.

[0063] According to some embodiments of the present invention, in step (1), the molar ratio of the nickel source to the polybasic organic carboxylic acid, calculated as nickel element, is 1:0.5-1.5, preferably 1:0.6-1.2, more preferably 1:0.6-1, and even more preferably 1:0.6-0.84. When the molar ratio of the nickel source to the polybasic organic carboxylic acid, calculated as nickel element, is within the above-defined range, the proportion of carbon-coated nickel nanoparticles in the resulting composite material is moderate. However, when the molar ratio of the nickel source to the polybasic organic carboxylic acid is too high, the overall conductivity of the composite material decreases; when the molar ratio of the nickel source to the polybasic organic carboxylic acid is too low, the proportion of carbon-coated nickel nanoparticles in the composite material also decreases, thereby affecting the electrocatalytic effect thereof and hindering the improvement of the specific capacity and cycle stability of the lithium-sulfur battery.

[0064] According to some embodiments of the present invention, in step (1), there is no particular limitation on the method for forming the homogeneous solution. For example, the homogeneous solution may be formed by heating, more preferably by heating and stirring. The present invention also does not particularly limit the heating temperature and stirring rate, as long as the solution can be formed.

[0065] According to some embodiments of the present invention, preferably, in step (1), the precursor is a precursor obtained by dissolving a nickel source and a polybasic organic carboxylic acid in a solvent to form a homogeneous solution, and then removing the solvent from the homogeneous solution. The present invention does not particularly limit the type of the solvent, and the type of solvent is subject to the ability to form a homogeneous solution. Preferably, the solvent is water and / or ethanol, more preferably water; the present invention does not particularly limit the amount of the solvent, and the amount of the solvent is also subject to the ability to form a homogeneous solution. The solvent in the homogeneous solution can be removed by direct evaporation, and the temperature and process of evaporation can adopt existing technologies known to those skilled in the art. For example, the solvent in the homogeneous solution can be removed by heating and evaporating to dryness.

[0066] According to some embodiments of the present invention, preferably, in step (1), the nickel source is selected from at least one of nickel acetate, Ni(OH)2, NiO, NiCO3 and basic nickel carbonate, preferably nickel acetate.

[0067] According to some embodiments of the present invention, in step (1), the polybasic organic carboxylic acid has a wide selection range. As long as it can form a complex with the nickel source, the purpose of the present invention can be achieved. In order to further improve the specific capacity and cycle stability of the lithium-sulfur battery, preferably, the polybasic organic carboxylic acid is selected from at least one of citric acid, terephthalic acid, 2,5-pyridinedicarboxylic acid, maleic acid, fumaric acid, succinic acid, tartaric acid, malic acid, gluconic acid and trimesic acid. More preferably, the polybasic organic carboxylic acid is citric acid.

[0068] According to some embodiments of the present invention, preferably, in step (2), the inert atmosphere is provided by at least one of nitrogen, argon, neon and helium; and / or

[0069] The reducing atmosphere is provided by hydrogen and, optionally, an inert gas, which is at least one of nitrogen, argon, neon, and helium.

[0070] According to some embodiments of the present invention, preferably, in step (2), the constant temperature calcination conditions include: a temperature of 500-1200°C, preferably 600-1000°C; and a time of 0.5-10 hours, preferably 1-5 hours. The constant temperature calcination temperature and time within the above preferred ranges are conducive to increasing the degree of graphitization of the carbon layer shell in the carbon-coated nickel nano-carbon cage composite material and improving the electrical conductivity of the composite material.

[0071] According to some embodiments of the present invention, preferably, in step (2), the constant temperature calcination step includes: first heating to 500-800°C, preferably 600-700°C, at a rate of 1-20°C / min, preferably 5-10°C / min, and holding the temperature for 0.5-10h, preferably 1-5h; then heating to 800-1200°C, preferably 900-1000°C, at a rate of 1-20°C / min, preferably 5-10°C / min, and holding the temperature for 0.5-10h, preferably 1-5h. In the present invention, the two-stage heating method is conducive to forming a complete carbon-coated nickel core-shell structure and improving the degree of graphitization of the composite material.

[0072] According to some embodiments of the present invention, in step (3), the step of contacting the calcined product with an acid is preferably: mixing the calcined product with an acid. The present invention does not particularly limit the mixing method, and the mixing can be carried out by ultrasonic or stirring. The acid can be an acid commonly used in the art, as long as it can form a carbon-coated nickel nano-carbon cage composite material including carbon-coated nickel nanoparticles and nano-carbon cages.

[0073] According to some embodiments of the present invention, preferably, in step (3), the acid is an inorganic acid and / or an organic acid, preferably at least one of hydrochloric acid, sulfuric acid, nitric acid, and citric acid, more preferably hydrochloric acid. More preferably, the acid is provided in the form of an aqueous solution; further preferably, the concentration of the acid in the aqueous solution is 0.1-10 mol / L, preferably 1-3 mol / L.

[0074] According to some embodiments of the present invention, in order to further promote the formation of a carbon-coated nickel nano-carbon cage composite material comprising carbon-coated nickel nanoparticles and nano-carbon cages, preferably, in step (3), the calcined product is contacted with an excess acid for reaction, and more preferably, the molar ratio of the calcined product to the acid calculated as nickel element is 1:1.03-10.

[0075] According to some embodiments of the present invention, preferably, in step (3), the contact reaction conditions include: temperature of 95-110° C., preferably 100-105° C.; time of 5-18 h, preferably 8-12 h.

[0076] According to some embodiments of the present invention, in step (3), there is no particular limitation on the solid-liquid separation method, and the solid-liquid separation can be performed by any solid-liquid separation method known in the art, for example, solid-liquid separation can be performed by filtration.

[0077] According to some embodiments of the present invention, preferably, in step (3), the steps of washing and drying are also included after the solid-liquid separation. The washing is used to remove the acid remaining on the carbon-coated nickel nano-carbon cage composite material. Therefore, various water washing methods that can make the carbon-coated nickel nano-carbon cage composite material washed to neutrality are all applicable to the present invention. The drying is used to remove the water on the carbon-coated nickel nano-carbon cage composite material. Drying can adopt normal pressure drying or reduced pressure drying, and drying conditions can adopt any feasible prior art.

[0078] According to some embodiments of the present invention, the carbon-coated nickel nano-carbon cage composite material may contain various doping elements that are known to those skilled in the art and can be applied to carbon materials. Preferably, the composite material does not contain nitrogen, sulfur, boron, phosphorus, fluorine, chlorine, bromine, iodine and other elements.

[0079] The third aspect of the present invention provides a lithium-sulfur battery positive electrode active material based on a carbon-coated nickel nano-carbon cage composite material. The lithium-sulfur battery positive electrode active material contains a carbon-coated nickel nano-carbon cage composite material, graphene and elemental sulfur. The carbon-coated nickel nano-carbon cage composite material is the carbon-coated nickel nano-carbon cage composite material described in the first aspect or the carbon-coated nickel nano-carbon cage composite material prepared according to the method described in the second aspect.

[0080] According to some embodiments of the present invention, by combining a carbon-coated nickel nano-carbon cage composite material including carbon-coated nickel nanoparticles and nano-carbon cages with graphene and elemental sulfur, the three act synergistically, so that a lithium-sulfur battery prepared using the positive electrode active material has high specific capacity and long cycle stability.

[0081] According to some embodiments of the present invention, preferably, based on the total amount of the positive electrode active material of the lithium-sulfur battery, the content of nickel element is 0.1-0.6 weight %, the content of graphene is 5-20 weight %, and the content of elemental sulfur is 60-90 weight %;

[0082] More preferably, based on the total amount of the positive electrode active material of the lithium-sulfur battery, the content of nickel element is 0.2-0.5 weight %, the content of graphene is 8-15 weight %, and the content of elemental sulfur is 70-80 weight %.

[0083] According to some embodiments of the present invention, preferably, based on the total amount of the lithium-sulfur battery positive electrode active material, the content of the carbon-coated nickel nano-carbon cage composite material is 1-20 weight %, the content of the graphene is 5-20 weight %, and the content of the elemental sulfur is 60-90 weight %;

[0084] More preferably, based on the total amount of the lithium-sulfur battery positive electrode active material, the content of the carbon-coated nickel nano-carbon cage composite material is 5-10 weight %, the content of the graphene is 8-15 weight %, and the content of the elemental sulfur is 70-80 weight %.

[0085] A fourth aspect of the present invention provides a method for preparing a positive electrode active material for a lithium-sulfur battery, the method comprising:

[0086] (a) providing a mixture containing a carbon-coated nickel nano-carbon cage composite material, graphene and elemental sulfur;

[0087] (b) subjecting the mixture to high-temperature treatment under the protection of an inert atmosphere and at a temperature not lower than the melting point of sulfur; the carbon-coated nickel nano-carbon cage composite material is the carbon-coated nickel nano-carbon cage composite material described in the first aspect or the carbon-coated nickel nano-carbon cage composite material prepared according to the method described in the second aspect.

[0088] According to some embodiments of the present invention, preferably, in step (a), the mixture is obtained by mixing and grinding the carbon-coated nickel nano-carbon cage composite material with graphene and elemental sulfur.

[0089] According to some embodiments of the present invention, preferably, in step (a), the carbon-coated nickel nano-carbon cage composite material, the graphene and the elemental sulfur are used in such an amount that, based on the total weight of the mixture, the content of graphene is 5-20% by weight, the content of elemental sulfur is 60-90% by weight, and the content of the carbon-coated nickel nano-carbon cage composite material is 1-20% by weight;

[0090] More preferably, the amounts of the carbon-coated nickel nano-carbon cage composite material, graphene and elemental sulfur are such that, based on the total weight of the mixture, the graphene content is 8-15% by weight, the elemental sulfur content is 70-80% by weight, and the carbon-coated nickel nano-carbon cage composite material content is 5-10% by weight.

[0091] According to some embodiments of the present invention, in step (b), the high-temperature treatment is performed at a temperature not lower than the melting temperature of sulfur, so that the elemental sulfur is in a molten state and can be evenly dispersed on the graphene conductive network. Preferably, the high-temperature treatment conditions include: a temperature of 100-200°C for 1-48 hours, preferably 8-40 hours, more preferably 10-30 hours, and even more preferably 12-20 hours. Preferably, the high-temperature treatment is performed in a closed reactor, for example, a conventional autoclave.

[0092] According to some embodiments of the present invention, preferably, in step (b), the high temperature treatment is performed under the protection of an inert atmosphere, and the inert atmosphere is an atmosphere formed by argon and / or nitrogen.

[0093] According to some embodiments of the present invention, preferably, in step (b), after subjecting the mixture to high-temperature treatment, the step further includes naturally cooling the obtained reaction product to room temperature and grinding it into powder, thereby obtaining the lithium-sulfur battery positive electrode active material.

[0094] The fifth aspect of the present invention provides an application of the carbon-coated nickel nano-carbon cage composite material described in the first aspect or the lithium-sulfur battery positive electrode active material based on the carbon-coated nickel nano-carbon cage composite material described in the third aspect in a lithium-sulfur battery.

[0095] According to some embodiments of the present invention, preferably, the lithium-sulfur battery positive electrode active material can be combined with a conductive agent and a binder to prepare a lithium-sulfur battery positive electrode material.

[0096] According to some embodiments of the present invention, preferably, the positive electrode of the lithium-sulfur battery includes a current collector and a positive electrode material coated and / or filled on the current collector, the positive electrode material contains a positive electrode active material, a conductive agent and a binder, and the positive electrode active material is the lithium-sulfur battery positive electrode active material described in the fourth aspect.

[0097] According to some embodiments of the present invention, the lithium-sulfur battery includes an electrode group and a non-aqueous electrolyte, which is sealed in a battery casing. The electrode group includes a positive electrode, a negative electrode and a separator, and the separator is located between the positive electrode and the negative electrode. The positive electrode is the positive electrode of the lithium-sulfur battery provided by the present invention.

[0098] Since the present invention only involves the improvement of the positive electrode active material in the prior art lithium-sulfur positive electrode material, there is no special limitation on other components and structures of the lithium-sulfur battery.

[0099] For example, for the positive electrode of a lithium-sulfur battery, the binder of the positive electrode material of the present invention can be any binder known in the art that can be used for lithium-sulfur batteries. It can be selected from fluorine-containing resins and / or polyolefin compounds, such as one or more of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and styrene-butadiene rubber. The content of the binder can be 0.01-15% by weight based on the weight of the positive electrode active material. The content and type of the conductive agent of the positive electrode material of the present invention are well known to those skilled in the art. For example, the content of the conductive agent is generally 0-30% by weight based on the positive electrode active material. The conductive agent can be selected from one or more of conductive carbon black (Super-P), acetylene black, nickel powder, copper powder, and conductive graphite. The current collector can be any current collector known to those skilled in the art, such as aluminum foil, copper foil, and nickel-plated steel strip. In the present invention, aluminum foil is selected as the current collector.

[0100] According to some embodiments of the present invention, a metal lithium sheet may be directly used as the negative electrode in the lithium-sulfur battery.

[0101] According to some embodiments of the present invention, the electrolyte used in the lithium-sulfur battery can be an electrolyte conventionally used in the art, the mass ratio of the injected volume of the electrolyte to the active material sulfur is generally 1-40 μL / mg, and the concentration of the electrolyte is generally 0.2-8.0 mol / L.

[0102] According to some embodiments of the present invention, the separator has electrical insulation and liquid retention properties, is disposed between the positive electrode and the negative electrode, and is sealed in the battery case together with the positive electrode, the negative electrode, and the electrolyte. The separator can be any of various separators commonly used in the art, such as a composite membrane formed by welding or bonding various grades of polyethylene, polypropylene, modified polyethylene felt, modified polypropylene felt, ultrafine glass fiber felt, vinylon felt, or nylon felt produced by various manufacturers known to those skilled in the art with a wettable polyolefin microporous membrane.

[0103] The present invention will be described in detail below through examples.

[0104] XRD is used to obtain information such as the composition of the material and the structure or morphology of the atoms or molecules within the material. The XRD diffractometer used was an XRD-6000 X-ray powder diffractometer (Shimadzu, Japan). The XRD test conditions were: Cu target, Kα radiation (wavelength λ = 0.154 nm), tube voltage of 40 kV, tube current of 200 mA, and a scanning speed of 10° (2θ) / min.

[0105] The surface morphology of the material was characterized using high-resolution transmission electron microscopy (HRTEM). The microscope used was a JEM-2100 (JEOL Ltd.) at an accelerating voltage of 200 kV. The particle size of the nanoparticles in the sample was measured using HRTEM images.

[0106] The pore structure properties of the material were determined using the BET test method. Specifically, a Quantachrome AS-6B analyzer was used for measurement. The specific surface area of ​​the material was determined using the Brunauer-Emmett-Taller (BET) method, and the pore distribution curve was calculated from the desorption curve using the Barrett-Joyner-Halenda (BJH) method.

[0107] Preparation Examples 1-3 are used to illustrate carbon-coated nickel nano-carbon cage composite materials and preparation methods thereof.

[0108] Preparation Example 1

[0109] (1) 20 g of nickel acetate and 10 g of citric acid were weighed in a molar ratio of 1:0.6 and added to a beaker containing 40 mL of deionized water. The mixture was stirred at 80 °C to obtain a homogeneous solution, which was then heated and evaporated to dryness to obtain a precursor (nickel citrate).

[0110] (2) The precursor obtained in step (1) was placed in a porcelain boat, and then the porcelain boat was placed in the constant temperature zone of a tube furnace, nitrogen was introduced at a flow rate of 100 mL / min, and the temperature was raised to 600°C at a rate of 5°C / min. After the constant temperature was maintained for 2 hours, the temperature was raised to 1000°C at a rate of 5°C / min. After the constant temperature was maintained for 2 hours, the heating was stopped, and the mixture was cooled to room temperature under a nitrogen atmosphere to obtain a calcined product (carbon-coated nickel nanomaterial).

[0111] (3) The calcined product obtained in step (2) was added to an aqueous solution containing 1M hydrochloric acid (the molar ratio of the calcined product to the acid, calculated as nickel element, was 1:2), and then the temperature was raised to 105° C. and stirred for 12 hours. After cooling to room temperature, the mixture was filtered, and the filter cake was washed with water until neutral, placed in an oven, and dried overnight to obtain a carbon-coated nickel nano-carbon cage composite material.

[0112] The transmission electron microscope (TEM) image of the composite material is as follows: Figure 1 As shown in the figure, the composite material is a large-particle carbon-coated nickel nano-carbon cage composite material, comprising carbon-coated nickel nanoparticles and nano-carbon cages. The carbon-coated nickel nanoparticles contain a metallic nickel core and a graphitized carbon layer covering the metallic nickel core, while the nano-carbon cages contain an empty nano-carbon cage core. The average thickness of the graphitized carbon layer is 0.3-4 nm, and the average particle size of the metallic nickel core or the empty nano-carbon cage core is 10-40 nm.

[0113] The mass percentages of nickel in the calcined product (carbon-coated nickel nanomaterial) were determined by an elemental analyzer and an X-ray fluorescence spectrometer (XRF). The mass percentages of the elements contained in the carbon-coated nickel nano-carbon cage composite material were: 88.27% carbon, 0.91% hydrogen, 1.14% oxygen, and 9.68% nickel.

[0114] Calculation shows that in the carbon-coated nickel nano-carbon cage composite material, the content of carbon-coated nickel nanoparticles is 12.3% by weight, and the content of nano-carbon cages is 87.7% by weight.

[0115] The X-ray diffraction (XRD) pattern of the composite material is shown in Figure 2 As shown in the figure, the diffraction peaks at 2θ angles of 44.3°, 51.6°, and 76.1° correspond to the (111), (200), and (220) planes of the nickel core, which is a face-centered cubic lattice structure. The diffraction spectrum of the composite material also contains a diffraction peak corresponding to graphene (2θ angle of 26.0°).

[0116] The N2 adsorption-desorption isotherm of the composite material is shown in Figure 3 As shown, the BJH pore size distribution curve is as follows Figure 4 As shown in the figure, the specific surface area of ​​the composite material is 242.38m 2 / g, and the mesopore volume is 1.22cm 3 / g, accounting for 100% of the total pore volume, and there are two mesopore distribution peaks at 3.77nm and 33.68nm in the pore size distribution curve of the composite material.

[0117] Figure 5 The Raman spectrum of the composite material shows that the composite material has obvious D peak and G peak. D / I G =0.80, indicating that the composite material has a certain degree of graphitization.

[0118] Preparation Example 2

[0119] (1) 20 g of nickel acetate and 14 g of citric acid were weighed in a molar ratio of 1:0.84 and added to a beaker containing 40 mL of deionized water. The mixture was stirred at 80 °C to obtain a homogeneous solution, which was then heated and evaporated to dryness to obtain a precursor (nickel citrate).

[0120] (2) The precursor obtained in step (1) was placed in a porcelain boat, and then the porcelain boat was placed in the constant temperature zone of a tube furnace, nitrogen was introduced at a flow rate of 100 mL / min, and the temperature was raised to 600°C at a rate of 5°C / min. After the constant temperature was maintained for 2 hours, the temperature was raised to 1000°C at a rate of 5°C / min. After the constant temperature was maintained for 2 hours, the heating was stopped, and the mixture was cooled to room temperature under a nitrogen atmosphere to obtain a calcined product (carbon-coated nickel nanomaterial).

[0121] (3) The calcined product obtained in step (2) was added to an aqueous solution containing 1M hydrochloric acid (the molar ratio of the calcined product to the acid, calculated as nickel element, was 1:2), and then the temperature was raised to 105° C. and stirred for 12 hours. After cooling to room temperature, the mixture was filtered, and the filter cake was washed with water until neutral, placed in an oven, and dried overnight to obtain a carbon-coated nickel nano-carbon cage composite material.

[0122] Transmission electron microscopy revealed that the composite material is a medium-sized carbon-coated nickel nano-carbon cage composite material comprising carbon-coated nickel nanoparticles and carbon nanocages. The carbon-coated nickel nanoparticles contain a metallic nickel core surrounded by a graphitized carbon shell, while the carbon nanocages contain an empty carbon nanocage core. The graphitized carbon layer has an average thickness of 0.3-4 nm, and the average particle size of the metallic nickel core or the empty carbon nanocage core is 10-30 nm.

[0123] The elemental analyzer and X-ray fluorescence spectrometer (XRF) determined that the mass percentage of nickel in the calcined product (carbon-coated nickel nanomaterial) was approximately 77.26%, and the mass percentages of the elements contained in the carbon-coated nickel nano-carbon cage composite material were: carbon 91.35%, hydrogen 0.98%, oxygen 1.56%, and nickel 6.11%.

[0124] Calculation shows that in the carbon-coated nickel nano-carbon cage composite material, the content of carbon-coated nickel nanoparticles is 7.9% by weight, and the content of nano-carbon cages is 92.1% by weight.

[0125] The BET test method shows that the specific surface area of ​​the composite material is 358.64m 2 / g, and the mesopore volume is 1.07 cm 3 / g, accounting for 100% of the total pore volume, and there are two mesopore distribution peaks at 3.69nm and 16.37nm in the pore size distribution curve of the composite material.

[0126] Raman testing shows that the composite material has obvious D peak and G peak, I D / I G =0.88, indicating that the composite material has a certain degree of graphitization.

[0127] Preparation Example 3

[0128] (1) 20 g of nickel acetate and 20 g of citric acid were weighed in a molar ratio of 1:1.2 and added to a beaker containing 40 mL of deionized water. The mixture was stirred at 80 °C to obtain a homogeneous solution, which was then heated and evaporated to dryness to obtain a precursor (nickel citrate).

[0129] (2) The precursor obtained in step (1) was placed in a porcelain boat, and then the porcelain boat was placed in the constant temperature zone of a tube furnace, nitrogen was introduced at a flow rate of 100 mL / min, and the temperature was raised to 600°C at a rate of 5°C / min. After the constant temperature was maintained for 2 hours, the temperature was raised to 1000°C at a rate of 5°C / min. After the constant temperature was maintained for 2 hours, the heating was stopped, and the mixture was cooled to room temperature under a nitrogen atmosphere to obtain a calcined product (carbon-coated nickel nanomaterial).

[0130] (3) The calcined product obtained in step (2) is added to an aqueous solution containing 1M hydrochloric acid (the molar ratio of the calcined product to the acid is 1:2 based on the nickel element). The temperature is then raised to 105°C and stirred for 12 hours. After cooling to room temperature, the mixture is filtered and the filter cake is washed with water until neutral, placed in an oven, and dried overnight to obtain a carbon-coated nickel nano-carbon cage composite material.

[0131] The transmission electron microscope (TEM) image of the carbon-coated nickel nano-carbon cage composite material is as follows: Figure 6 As shown in the figure, the composite material is mainly a small-particle carbon-coated nickel nano-carbon cage composite material, including carbon-coated nickel nanoparticles and nanocarbon cages. The carbon-coated nickel nanoparticles contain a metallic nickel core and a graphitized carbon layer covering the surface of the metallic nickel core, and the nanocarbon cages contain a nanocarbon cage empty core. The average thickness of the graphitized carbon layer is 0.3-4nm, and the average particle size of the metallic nickel core or the nanocarbon cage empty core is 5-15nm.

[0132] The mass percentage of nickel in the calcined product (carbon-coated nickel nanomaterial) was determined by an elemental analyzer and an X-ray fluorescence spectrometer (XRF). The mass percentages of the elements contained in the composite material were: 94.07% carbon, 1.00% hydrogen, 1.95% oxygen, and 2.98% nickel.

[0133] Calculation shows that in the carbon-coated nickel nano-carbon cage composite material, the content of carbon-coated nickel nanoparticles is 3.9% by weight, and the content of nano-carbon cages is 96.1% by weight.

[0134] The X-ray diffraction (XRD) pattern of the composite material is shown in Figure 7 As shown in the figure, it can be seen that the diffraction peaks with 2θ angles of 25.9° and 42.8° correspond to the (002) and (100 / 101) planes of the graphitized carbon layer, respectively, and it can be seen that the material is mainly nanocarbon cages.

[0135] The N2 adsorption-desorption isotherm of the composite material is shown in Figure 8 As shown, the BJH pore size distribution curve is as follows Figure 9 As shown in the figure, the specific surface area of ​​the nanocarbon cage material is 432.86 m 2 / g, and the mesopore volume is 0.93 cm 3 / g, accounting for 100% of the total pore volume, and there are two mesopore distribution peaks at 3.67nm and 9.05nm in the pore size distribution curve of the composite material.

[0136] Figure 10 The Raman spectrum of the composite material shows that the composite material has obvious D peak and G peak. D / I G =1.02, indicating that the composite material has a certain degree of graphitization.

[0137] Comparative Preparation Example 1

[0138] Carbon-coated nickel nanomaterials were prepared according to the method disclosed in Example 1 of CN109304195A. The specific physical and chemical characterization results are detailed in CN109304195A.

[0139] Examples 1-3 are used to illustrate the positive electrode active material of lithium-sulfur batteries and the preparation method thereof.

[0140] Example 1

[0141] Preparation of the positive active material for lithium-sulfur batteries: The carbon-coated nickel nano-carbon cage composite material prepared in Preparation Example 1, commercially available graphene material (purchased from Zhongke Times Nano, brand 170104), and elemental sulfur powder were weighed in a mass ratio of 5%:15%:80%, ground, and mixed thoroughly. The resulting mixture was placed in a reactor under an argon atmosphere and heat-treated at 160°C for 12 hours.

[0142] Example 2

[0143] Preparation of the positive active material for lithium-sulfur batteries: The carbon-coated nickel nano-carbon cage composite material prepared in Preparation Example 2, commercially available graphene material (purchased from Zhongke Times Nano, brand 170104), and elemental sulfur powder were weighed in a mass ratio of 5%:15%:80%, ground, and mixed thoroughly. The resulting mixture was placed in a reactor under an argon atmosphere and heat-treated at 160°C for 12 hours.

[0144] Example 3

[0145] Preparation of the positive active material for lithium-sulfur batteries: The carbon-coated nickel nano-carbon cage composite material prepared in Preparation Example 3, commercially available graphene material (purchased from Zhongke Times Nano, brand 170104), and elemental sulfur powder were weighed in a mass ratio of 5%:15%:80%, ground, and mixed thoroughly. The resulting mixture was placed in a reactor under an argon atmosphere and heat-treated at 160°C for 12 hours.

[0146] Comparative Example 1

[0147] Preparation of the positive active material for lithium-sulfur batteries: The carbon-coated nickel nanomaterial prepared in Comparative Preparation Example 1, commercially available graphene material (purchased from Zhongke Times Nano, brand 170104), and elemental sulfur powder were weighed in a mass ratio of 5%:15%:80%, ground, and mixed thoroughly. The resulting mixture was placed in a reactor under an argon atmosphere and heat-treated at 160°C for 12 hours.

[0148] Comparative Example 2

[0149] Preparation of the lithium-sulfur battery positive electrode active material: Commercially available graphene material (purchased from Zhongke Times Nano, brand 170104) and elemental sulfur powder were weighed in a mass ratio of 20%:80%, ground, and mixed thoroughly. The resulting mixture was placed in a reactor under an argon atmosphere and heated at 160°C for 12 hours.

[0150] Test Case

[0151] This test example is used to illustrate a lithium-sulfur battery and its preparation method.

[0152] Preparation of positive electrode and battery assembly:

[0153] 1) Preparation of the Positive Electrode: The lithium-sulfur battery positive electrode active material obtained in Examples 1-3, the conductive agent Super-P, and the NMP solution of the binder PVDF were mixed at a mass ratio of 8:1:1. The specific method was as follows: the dried lithium-sulfur battery positive electrode active material and the conductive agent were ground in a mortar for 15 minutes. After grinding, the PVDF solution (5% by mass) was added according to the proportion and stirred on a magnetic stirrer for 6 hours. The resulting paste slurry was evenly coated on a current collector aluminum foil, carbon cloth, or carbon paper, and then dried in a vacuum drying oven at 60°C for 20 hours before use.

[0154] 2) Battery assembly: The dried electrode is formed into a circular electrode with a diameter of 12 mm; and maintained at a pressure of 8 MPa for 30-180 seconds to obtain the positive electrode of the lithium-sulfur battery. Metallic lithium is used as the negative electrode, Celgard membrane is used as the separator, 1 mol / L lithium bis(trifluoromethylsulfonyl)imide is selected as the electrolyte, 1% lithium nitrate is used as the additive, and the solvent volume ratio DOL (1,3-dioxolane): DME (ethylene glycol dimethyl ether) = 1:1 is assembled into a CR2032 button cell. The amount of electrolyte added is 20 times that of elemental sulfur (mass ratio). The entire battery assembly is completed in a glove box to prepare lithium-sulfur batteries A1-A3.

[0155] Conventional battery performance testing: The battery was charged and discharged using a LAND CT2001A charge and discharge instrument from Wuhan Lanbo Electronics Co., Ltd. over a voltage range of 1.7 V to 2.8 V. The assembled lithium-sulfur battery was tested for specific capacity at rates of 0.2C, 0.5C, and 1C, and for cycling performance at a rate of 1C. Figure 11 The graph is a cycle performance diagram of a lithium-sulfur battery prepared using the positive electrode active material of Example 1. Figure 12 This is a cycle performance diagram of a lithium-sulfur battery prepared using the positive electrode active material of Example 3.

[0156] The lithium-sulfur battery A1 prepared using the lithium-sulfur battery positive electrode active material of Example 1 has a discharge specific capacity of 812 mAh / g at a rate of 0.5C and a discharge specific capacity of 726 mAh / g at a rate of 1C, with a capacity loss of 0.12% per 1C cycle.

[0157] The lithium-sulfur battery A2 prepared using the lithium-sulfur battery positive electrode active material of Example 2 has a discharge specific capacity of 798 mAh / g at a rate of 0.5C and 714 mAh / g at a rate of 1C, with a capacity loss of 0.13% per 1C cycle.

[0158] The lithium-sulfur battery A3 prepared using the lithium-sulfur battery positive electrode active material of Example 3 has a discharge specific capacity of 777 mAh / g at a rate of 0.5C and a discharge specific capacity of 684 mAh / g at a rate of 1C, with a capacity loss of 0.19% per 1C cycle.

[0159] Comparative test example

[0160] The lithium-sulfur battery positive electrode was prepared, the battery was assembled, and the battery performance was tested according to the test example method.

[0161] The lithium-sulfur battery prepared by using the lithium-sulfur battery positive electrode active material of Comparative Example 1 has a discharge capacity of only 783 mAh / g at a rate of 0.5C; a discharge capacity of only 699 mAh / g at a rate of 1C, and a capacity loss of 0.15% per 1C cycle.

[0162] The lithium-sulfur battery prepared using the lithium-sulfur battery positive electrode active material of Comparative Example 2 (i.e., conventional graphene-sulfur positive electrode material) has a discharge specific capacity of only 691 mAh / g at a rate of 0.5C; the discharge specific capacity at a rate of 1C is only 602 mAh / g, and the capacity loss per 1C cycle is 0.18%.

[0163] From the above data analysis, it can be concluded that the lithium-sulfur battery prepared using the carbon-coated nickel nano-carbon cage composite material provided by the present invention has both high charge and discharge specific capacity and good cycle stability.

[0164] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection of the present invention.

Claims

1. A lithium-sulfur battery positive electrode active material based on a carbon-coated nickel nano-carbon cage composite material, characterized in that: The lithium-sulfur battery positive electrode active material contains a carbon-coated nickel nano-carbon cage composite material, graphene and elemental sulfur. The carbon-coated nickel nano-carbon cage composite material includes carbon-coated nickel nanoparticles and nano-carbon cages, wherein the carbon-coated nickel nanoparticles contain a metallic nickel core and a graphitized carbon layer shell coated on the surface of the metallic nickel core, and the nano-carbon cage is a nano-carbon cage empty core; the pore size distribution curve of the composite material has a double mesopore distribution peak, the first most probable pore size is 2-5 nm, and the second most probable pore size is 16.37-40 nm; Based on the total amount of the composite material, the content of the carbon-coated nickel nanoparticles is 4-14% by weight; the content of the nanocarbon cages is 86-96% by weight; The mesopore volume of the composite material is 0.5-1.5 cm 3 / g.

2. The positive electrode active material for lithium-sulfur batteries according to claim 1, wherein Based on the total weight of the composite material, the content of nickel is 2-12% by weight; and the content of carbon is 85-95% by weight.

3. The positive electrode active material for lithium-sulfur batteries according to claim 2, wherein: Based on the total weight of the composite material, the content of nickel is 5-10% by weight; the content of carbon is 87-93% by weight; and / or Based on the total amount of the composite material, the content of the carbon-coated nickel nanoparticles is 7-13% by weight; and the content of the nanocarbon cages is 87-93% by weight.

4. The positive electrode active material for lithium-sulfur batteries according to claim 1, wherein The composite material has a specific surface area of ​​200-450 m² / g; and / or The ratio of the mesopore volume of the composite material to the total pore volume is 90-100%.

5. The positive electrode active material for lithium-sulfur batteries according to claim 4, wherein The composite material has a specific surface area of ​​240-400 m² / g; and / or The mesopore volume of the composite material is 1-1.4 cm 3 / g; and / or The ratio of the mesopore volume of the composite material to the total pore volume is 95-100%.

6. The positive electrode active material for lithium-sulfur batteries according to claim 5, wherein The ratio of the mesopore volume of the composite material to the total pore volume is 98-100%.

7. The positive electrode active material for lithium-sulfur batteries according to claim 1, wherein The average particle size of the metallic nickel core is 1-200 nm; and / or The average thickness of the graphitized carbon layer shell is 0.3-6 nm; and / or The average particle size of the empty core of the nanocarbon cage is 1-200 nm.

8. The positive electrode active material for lithium-sulfur batteries according to claim 7, wherein: The average particle size of the metallic nickel core is 5-40 nm; and / or The average thickness of the graphitized carbon layer shell is 0.3-4 nm; and / or The average particle size of the empty core of the nanocarbon cage is 5-40 nm.

9. The positive electrode active material for lithium-sulfur batteries according to claim 1, wherein The metallic nickel core comprises a face-centered cubic lattice structure and / or a hexagonal close-packed lattice structure; and / or In the Raman curve of the composite material, I D / I G The range is 0.7-1.

5.

10. The positive electrode active material for lithium-sulfur batteries according to claim 9, wherein: The metallic nickel core comprises a face-centered cubic lattice structure; and / or In the Raman curve of the composite material, I D / I G The range is 0.8-0.

9.

11. The positive electrode active material for lithium-sulfur batteries according to claim 1, wherein The method for carbon-coated nickel nano-carbon cage composite material comprises the following steps: (1) providing a homogeneous solution containing a nickel source, a polybasic organic carboxylic acid, and a solvent, and then removing the solvent from the homogeneous solution to obtain a precursor, wherein the molar ratio of the nickel source to the polybasic organic carboxylic acid is 1:0.6-0.84, calculated as nickel element; (2) calcining the precursor at a constant temperature under an inert atmosphere or a reducing atmosphere to obtain a calcined product; The constant temperature calcination step comprises: firstly heating to 500-800°C, holding the temperature constant for 0.5-10 hours, then heating to 900-1000°C, holding the temperature constant for 0.5-10 hours; (3) contacting the calcined product with an acid for reaction, and then performing solid-liquid separation.

12. The positive electrode active material for lithium-sulfur batteries according to claim 11, wherein The nickel source is selected from at least one of nickel acetate, Ni(OH)2, NiO, NiCO3 and basic nickel carbonate; and / or The polybasic organic carboxylic acid is at least one selected from citric acid, terephthalic acid, 2,5-pyridinedicarboxylic acid, maleic acid, fumaric acid, succinic acid, tartaric acid, malic acid, gluconic acid and trimesic acid.

13. The positive electrode active material for lithium-sulfur batteries according to claim 12, wherein: In step (1), the molar ratio of the nickel source to the polybasic organic carboxylic acid is 1:0.6-1, calculated as nickel element; and / or The nickel source is nickel acetate; and / or The polybasic organic carboxylic acid is citric acid.

14. The positive electrode active material for lithium-sulfur batteries according to claim 11, wherein The constant temperature calcination step includes: firstly heating to 500-800°C at a rate of 1-20°C / min, maintaining the temperature for 0.5-10 h, then heating to 800-1200°C at a rate of 1-20°C / min, maintaining the temperature for 0.5-10 h.

15. The positive electrode active material for lithium-sulfur batteries according to claim 14, wherein: The constant temperature calcination step includes: firstly heating the temperature to 600-700°C at a rate of 5-10°C / min, maintaining the temperature for 1-5 hours, then heating the temperature to 900-1000°C at a rate of 5-10°C / min, and maintaining the temperature for 1-5 hours.

16. The positive electrode active material for lithium-sulfur batteries according to claim 11, wherein: In step (3), the contact reaction conditions include: temperature of 95-110°C; time of 5-18h; and / or The molar ratio of the calcined product to the acid, calculated on the basis of nickel element, is 1:1.03-10.

17. The positive electrode active material for lithium-sulfur batteries according to claim 16, wherein: In step (3), the contact reaction conditions include: temperature of 100-105°C; time of 8-12 hours.

18. The positive electrode active material for lithium-sulfur batteries according to claim 1, wherein: Based on the total amount of the positive electrode active material of the lithium-sulfur battery, the content of nickel element is 0.1-0.6% by weight; the content of graphene is 5-20% by weight; and the content of elemental sulfur is 60-90% by weight.

19. The positive electrode active material for lithium-sulfur batteries according to claim 18, wherein: Based on the total amount of the positive electrode active material of the lithium-sulfur battery, the content of nickel element is 0.2-0.5% by weight; the content of graphene is 8-15% by weight; and the content of elemental sulfur is 70-80% by weight.

20. A method for preparing the positive electrode active material for lithium-sulfur batteries according to any one of claims 1 to 19, characterized in that: The method includes: (a) providing a mixture containing a carbon-coated nickel nano-carbon cage composite material, graphene and elemental sulfur; (b) subjecting the mixture to high-temperature treatment under the protection of an inert atmosphere and at a temperature not lower than the melting point of sulfur.

21. The method according to claim 20, wherein In step (a), the amounts of the carbon-coated nickel nano-carbon cage composite material, graphene, and elemental sulfur are such that, based on the total weight of the mixture, the graphene content is 5-20% by weight, the elemental sulfur content is 60-90% by weight, and the carbon-coated nickel nano-carbon cage composite material content is 1-20% by weight.

22. The method according to claim 21, wherein The amounts of the carbon-coated nickel nano-carbon cage composite material, graphene and elemental sulfur are such that, based on the total weight of the mixture, the graphene content in the obtained mixture is 8-15% by weight, the elemental sulfur content is 70-80% by weight, and the carbon-coated nickel nano-carbon cage composite material content is 5-10% by weight.

23. The method according to claim 20, wherein In step (b), the conditions of the high temperature treatment include: a temperature of 100-200°C and a time of 1-48 hours; and / or The inert atmosphere is an atmosphere formed of argon and / or nitrogen.

24. The method according to claim 23, wherein In step (b), the high temperature treatment time is 8-40 hours.

25. The method according to claim 24, wherein In step (b), the high temperature treatment time is 10-30 hours.

26. The method according to claim 25, wherein In step (b), the high temperature treatment time is 12-20 hours.

27. Use of the lithium-sulfur battery positive electrode active material according to any one of claims 1 to 19 in a lithium-sulfur battery.

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