Lithium-sulfur battery composite cathode material and preparation method thereof

Abstract

The application belongs to the technical field of secondary battery positive electrode materials. More particularly, it relates to a lithium-sulfur battery composite positive electrode material and a preparation method thereof. The product comprises: hollow porous carbon microspheres, and elemental sulfur loaded in the hollow porous carbon microspheres; the elemental sulfur is distributed in the internal cavities and the outer surfaces of the hollow porous carbon microspheres; the mass percentage of the elemental sulfur is 75-80% based on the total mass of the lithium-sulfur battery composite positive electrode material; the porosity of the hollow porous carbon microspheres is 50-55%; the surface of the hollow porous carbon microspheres has mesopores and macropores with a pore size distribution of 2-50 nm; and the particle size distribution thereof is 0.5-15 mu m; the shell thickness distribution is 50-500 nm; wherein the mass ratio of the sulfur content in the internal cavities to the outer surfaces is 8-10:1; in addition, the carbon microspheres are further doped with nitrogen elements and silicon elements.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of secondary battery cathode materials. More particularly, it relates to a lithium-sulfur battery composite cathode material and a preparation method thereof. BACKGROUND

[0002] A lithium-sulfur battery uses metallic lithium as the negative electrode and elemental sulfur as the positive active material. Its working principle is based on the reversible multi-step electrochemical reaction between sulfur and lithium ions (generating a series of lithium polysulfides Li2S X However, this reaction process also leads to some severe technical challenges for lithium-sulfur batteries, which seriously restricts their commercialization process. These challenges mainly focus on the sulfur cathode side:

[0003] Insulation of sulfur and its discharge products (Li2S2 / Li2S): The electronic conductivity of elemental sulfur and the final discharge product lithium sulfide is extremely low, which seriously hinders the transmission of electrons, leading to low active material utilization, poor rate performance, and high polarization.

[0004] Large volume change during charging and discharging: The volume expansion rate of sulfur is as high as 80% when it is completely discharged into Li2S. This huge volume change will destroy the integrity of the positive electrode structure, leading to electrode pulverization, separation of active materials from the conductive network, and thus exacerbating capacity decay.

[0005] Therefore, developing a new type of composite cathode material and its preparation method, which aims to maintain high sulfur content and high loading under the premise of reasonable structural design to strengthen the synergistic confinement of polysulfides, effectively buffer the volume change, and maintain its own excellent electronic conductivity, is of great significance for promoting the practical application of high-performance lithium-sulfur batteries. SUMMARY

[0006] The technical problem to be solved by the present application is that the existing lithium-sulfur battery cathode material needs to maintain high conductivity during battery use and buffer the material pulverization caused by volume change. Based on the above-mentioned difficulties, the present application provides a lithium-sulfur battery composite cathode material and a preparation method thereof.

[0007] The purpose of the present application is to provide a lithium-sulfur battery composite cathode material.

[0008] Another purpose of the present application is to provide a preparation method of a lithium-sulfur battery composite cathode material.

[0009] The above-mentioned purposes of the present application are achieved by the following technical solutions:

[0010] A lithium-sulfur battery composite cathode material, comprising:

[0011] Hollow porous carbon microspheres, and elemental sulfur loaded in the hollow porous carbon microspheres;

[0012] The elemental sulfur is distributed in the internal cavity and the outer surface of the hollow porous carbon microspheres;

[0013] The mass percentage of the elemental sulfur is 75-80% based on the total mass of the lithium-sulfur battery composite positive electrode material;

[0014] The porosity of the hollow porous carbon microspheres is 50-55%.

[0015] The technical solution has the following beneficial effects:

[0016] The technical solution uses the hollow porous carbon microspheres to construct an electron transmission network, and loads the elemental sulfur in the hollow porous carbon microspheres, so that the electrons can be quickly transmitted to each sulfur particle through the carbon skeleton, greatly overcoming the obstacle caused by the insulation of sulfur, and improving the utilization rate and rate performance of the active material.

[0017] The hollow structure provides important buffer space for the volume expansion of sulfur in the charging and discharging process, effectively absorbs stress, prevents the electrode material from being pulverized and the structure from collapsing due to the dramatic change in volume, and thus ensures the structural integrity and long cycle life of the electrode.

[0018] Under the premise of ensuring high energy density of the product, a considerable amount of elemental sulfur needs to be realized; and under this high loading condition, part of the elemental sulfur is distributed on the surface and part is distributed inside the carbon microspheres. In this way, the elemental sulfur distributed on the surface can quickly provide initial capacity and serve as the "frontline position" for internal sulfur reaction, and the control of the porosity plays a key balancing role. If the porosity is too low, although the strength of the carbon layer structure can be significantly improved, it is not conducive to ion transmission, thereby increasing ion impedance. Therefore, the appropriate lower limit of the porosity is the basis for ensuring sufficient ion conductivity of the product under the condition of loading elemental sulfur inside and outside. However, if the porosity is too high, the strength of the carbon layer will be significantly reduced, and the stress generated by the expansion of the corresponding amount of elemental sulfur inside the electrode material during the expansion process cannot be effectively limited.

[0019] Further, the surface of the hollow porous carbon microspheres has mesopores and macropores with a pore size distribution of 2-50 nm.

[0020] The above-sized channels provide unobstructed channels for electrolyte infiltration and rapid diffusion of lithium ions, further reducing the ion transmission resistance at the interface, thereby improving the large-current charge and discharge rate; in particular, it can be beneficial for the elemental sulfur to fully and uniformly penetrate into the internal cavity of the carbon microspheres under capillary action during the processing of the product, thereby avoiding local aggregation on the surface.

[0021] Further, the hollow porous carbon microspheres have a particle size distribution of 0.5-15 μm.

[0022] If the particle size is too small, the particles are prone to agglomeration, which is not conducive to uniform coating of the electrode slurry; and if the particle size is too large, the diffusion path of lithium ions will be too long, and more importantly, the mechanical strength of the large particles will be significantly reduced, and the buffering effect for volume expansion will be weakened.

[0023] Further, the hollow porous carbon microspheres have a shell thickness distribution of 50-500 nm.

[0024] The shell thickness of 50-500 nm ensures that the carbon spheres have sufficient mechanical strength to withstand the volume expansion stress of sulfur in the cycle and prevent breakage. At the same time, the carbon wall has small electronic transmission resistance at this thickness, ensuring excellent overall conductivity.

[0025] Further, the portion of the elemental sulfur distributed in the internal cavity is a first portion; and the portion of the elemental sulfur distributed on the outer surface is a second portion.

[0026] The mass ratio of the first portion to the second portion is 8-10:1.

[0027] The above technical solution has the following beneficial effects:

[0028] By mainly confining the sulfur inside the carbon microspheres, sufficient contact between the sulfur and the conductive carbon substrate is ensured, and the active material is prevented from being disconnected from the conductive network, thereby ensuring high active material utilization;

[0029] The elemental sulfur inside is physically limited by the carbon microspheres, which greatly suppresses the dissolution and shuttle effect of polysulfides, and is a fundamental guarantee for long cycle life;

[0030] The elemental sulfur outside is directly exposed to the electrolyte, providing a fast reaction interface, which helps to reduce polarization and contributes to high initial capacity and excellent rate performance.

[0031] Further, the hollow porous carbon microspheres are doped with nitrogen and silicon elements.

[0032] The introduced nitrogen-containing functional groups have polarity and can form strong chemical bonding with polysulfides, firmly "anchoring" them on the carbon skeleton, thereby strongly suppressing the shuttle effect; and the doping of nitrogen elements can enhance the effect and further improve the physical strength of the carbon skeleton.

[0033] A preparation method of a lithium-sulfur battery composite positive electrode material, the specific preparation steps comprising:

[0034] providing hollow porous carbon microspheres;

[0035] Mixing the hollow porous carbon microspheres and elemental sulfur, heating to 180-200℃ in inert atmosphere, and keeping for 10-12h to load elemental sulfur on the hollow porous carbon microspheres;

[0036] Cooling, and the lithium-sulfur battery composite positive electrode material is obtained.

[0037] Further, the specific preparation step further comprises:

[0038] Mixing the hollow porous carbon microspheres and elemental sulfur, heating to 180-200℃ in inert atmosphere at a rate of 0.3-0.5℃ / min, and keeping for 10-12h under a pressure of 0.2-0.3MPa to load elemental sulfur on the hollow porous carbon microspheres.

[0039] By slow heating, the surface sulfur is prevented from melting too fast and blocking the pore channels, ensuring that the internal pores have enough time to be filled with sulfur, which is beneficial to achieve the ideal internal and external sulfur distribution ratio; the external pressure can overcome the capillary resistance, forcibly push the molten sulfur into smaller pores and deep cavities, further ensure the loading rate and uniformity of sulfur, and reduce the residual external sulfur, so that more sulfur is protected inside the carbon spheres.

[0040] Further, the hollow porous carbon microspheres are provided by:

[0041] Using silica microspheres as a hard template;

[0042] Coating a carbon-containing organic polymer layer on the surface of the hard template to obtain a precursor;

[0043] Carbonizing the precursor at high temperature, and then cooling to room temperature to obtain carbonized material;

[0044] Chemically etching the carbonized material to remove the hard template to obtain hollow carbon microspheres;

[0045] Treating the hollow carbon microspheres with an activating agent to obtain hollow porous carbon microspheres.

[0046] Further, the hollow porous carbon microspheres are provided by:

[0047] Using silica microspheres as a hard template;

[0048] Coating a carbon-containing organic polymer layer on the surface of the hard template to obtain a precursor;

[0049] The organic polymer layer is a polyacrylonitrile coating layer;

[0050] Heating to 800-1000℃ at a rate of 2-4℃ / min under inert gas protection, carbonizing the precursor for 3-4h, and then cooling to room temperature to obtain carbonized material;

[0051] The carbonized material is chemically etched to remove the hard template, to obtain hollow carbon microspheres;

[0052] The hollow carbon microspheres and the sodium silicate-potassium hydroxide composite solution are mixed and impregnated, dried, and then heated and activated in an inert atmosphere at a temperature of 780-850°C, and cooled to obtain the hollow porous carbon microspheres;

[0053] In the sodium silicate-potassium hydroxide composite solution, the mass ratio of sodium silicate to potassium hydroxide is 1:8-10.

[0054] The hard template method is the most reliable method for preparing hollow microspheres with highly controllable morphology and size. By controlling the particle size of the silica template and the thickness of the polymer coating, the particle size and shell thickness of the final carbon microspheres can be accurately controlled, ensuring the consistency of the product batch and the designability of the structure.

[0055] Sodium silicate decomposes at high temperature, and the sodium component may assist in activation, and more importantly, the Si element is doped; the combination of potassium hydroxide and sodium silicate realizes "one-step" simultaneous completion of "deep pore making" and "N / Si co-doping". DETAILED DESCRIPTION

[0056] The application will be further described below in conjunction with specific examples, but the examples do not limit the application in any form. Unless otherwise specified, the reagents, methods and equipment used in the application are conventional reagents, methods and equipment in the technical field.

[0057] Unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0058] Example 1

[0059] Preparation of hollow porous carbon microspheres:

[0060] Dissolve the polyacrylonitrile in the DMF solvent to obtain a polyacrylonitrile solution with a concentration of 8 mg / mL;

[0061] Add the silica microspheres to the polyacrylonitrile solution, and ultrasonically disperse for 20 min at a frequency of 80 kHz to obtain a silica dispersion; wherein the amount of the silica microspheres is 3% of the mass of the polyacrylonitrile solution;

[0062] After the silica dispersion is incubated and stirred at a temperature of 60°C and a stirring speed of 60 r / min for 24 h, centrifugal separation is performed, the filter cake is collected, the filter cake is washed with anhydrous ethanol for 3 times, and the washed filter cake is dried to obtain the precursor;

[0063] The precursor is heated to 800℃ at a rate of 2℃ / min in an argon atmosphere, the precursor is carbonized for 3h, and then cooled to room temperature to obtain a carbonized material;

[0064] The carbonized material and excess 5% by mass hydrofluoric acid are mixed, and then subjected to ultrasonic reaction for 24h at an ultrasonic frequency of 60kHz and a temperature of 60℃ to remove the silicon dioxide by etching with hydrofluoric acid, and then filtered, washed, dried, and sieved to obtain hollow carbon microspheres;

[0065] The hollow carbon microspheres and a sodium silicate-potassium hydroxide composite solution are mixed at a mass ratio of 1:5, and then immersed at room temperature for 12h, and then filtered, dried, heated to 780℃ at a rate of 5℃ / min in an argon atmosphere, and then subjected to activation reaction for 80min, cooled to room temperature, washed with deionized water, dried, dispersed, and sieved to obtain hollow porous carbon microspheres;

[0066] In the composite solution of sodium silicate-potassium hydroxide, the mass ratio of sodium silicate to potassium hydroxide is 1:8, and the amount of water in the solution is 80% of the total mass of the composite solution;

[0067] Through the control of the above process conditions, hollow porous carbon microspheres with a porosity of 50%, a surface pore size distribution of 2-32nm, and a shell thickness distribution of 50-400nm are obtained, and particles with a particle size distribution of 0.5-15μm are obtained through sieving;

[0068] Preparation of a composite positive electrode material:

[0069] The total mass of the composite positive electrode material is used as a basis, and the mass percentage of elemental sulfur is 75%, and the balance is hollow porous carbon microspheres, and elemental sulfur and hollow porous carbon microspheres are prepared;

[0070] The two are mixed and added to a high-pressure reactor, heated to 180℃ at a rate of 0.3℃ / min under nitrogen protection, and then subjected to heat preservation and pressure treatment at a pressure of 0.2MPa for 10h to load elemental sulfur on the hollow porous carbon microspheres, and then naturally cooled to room temperature to discharge, and thus a composite positive electrode material is obtained;

[0071] Through the control of the above preparation process, the heating rate, the heat preservation temperature, and the pressure, elemental sulfur can be distributed on the outer surface and in the internal cavity, and the part of the elemental sulfur distributed in the internal cavity is a first part, and the part of the elemental sulfur distributed on the outer surface is a second part;

[0072] The mass ratio of the first part to the second part is 8:1.

[0073] Example 2

[0074] Preparation of hollow porous carbon microspheres:

[0075] Dissolve the polyacrylonitrile in the DMF solvent to obtain a polyacrylonitrile solution with a concentration of 9 mg / mL;

[0076] Add the silica microspheres to the polyacrylonitrile solution, and ultrasonically disperse at a frequency of 80 kHz for 20 min to obtain a silica dispersion; the amount of the silica microspheres is 4% of the mass of the polyacrylonitrile solution;

[0077] Centrifugalize the silica dispersion after low-speed stirring at 60℃ and a stirring speed of 70 r / min for 24 h, collect the filter cake, wash the filter cake with anhydrous ethanol for 3 times, dry the washed filter cake, and obtain a precursor;

[0078] Heat the precursor in an argon atmosphere at a rate of 3℃ / min to 900℃, carbonize the precursor for 3.5 h, and then cool to room temperature to obtain a carbonized material;

[0079] Mix the carbonized material and excess hydrofluoric acid with a mass fraction of 5%, and continuously ultrasonically react at an ultrasonic frequency of 60 kHz and a temperature of 60℃ for 24 h to remove the silica by etching with the hydrofluoric acid, and then filter, wash, dry, and sieve to obtain hollow carbon microspheres;

[0080] Mix the hollow carbon microspheres and a sodium silicate-potassium hydroxide composite solution at a mass ratio of 1:5, and immerse at room temperature for 12 h, and then filter, dry, heat to 810℃ at a rate of 5℃ / min in an argon atmosphere, and activate for 90 min, and then cool to room temperature, wash with deionized water, dry, disperse, and sieve to obtain hollow porous carbon microspheres;

[0081] The mass ratio of sodium silicate to potassium hydroxide in the composite solution is 1:9, and the amount of water in the solution is 80% of the total mass of the composite solution;

[0082] By controlling the above process conditions, hollow porous carbon microspheres with a porosity of 52%, a surface pore size distribution of 2-46 nm, and a shell thickness distribution of 50-440 nm are obtained, and particles with a particle size distribution of 0.5-15 μm are obtained by sieving;

[0083] Preparation of a composite positive electrode material:

[0084] Prepare elemental sulfur and hollow porous carbon microspheres according to a mass percentage of elemental sulfur of 78% and the balance being hollow porous carbon microspheres, based on the total mass of the composite positive electrode material;

[0085] The two are mixed and added to a high-pressure reactor, heated to 190℃ at a rate of 0.4℃ / min under nitrogen protection, and treated at a pressure of 0.25 MPa for 11 h to load elemental sulfur on the hollow porous carbon microspheres, and then naturally cooled to room temperature to obtain the composite positive electrode material;

[0086] Through the above preparation process, the heating rate, holding temperature, and pressure are controlled to make elemental sulfur distributed on the outer surface and in the internal cavity. The part of the elemental sulfur distributed in the internal cavity is the first part, and the part of the elemental sulfur distributed on the outer surface is the second part.

[0087] The mass ratio of the first part to the second part is 9.2:1.

[0088] Example 3

[0089] Preparation of hollow porous carbon microspheres:

[0090] Polyacrylonitrile is dissolved in DMF solvent to obtain a polyacrylonitrile solution with a concentration of 10 mg / mL;

[0091] Silica microspheres are added to the polyacrylonitrile solution, and ultrasonic dispersion is performed at a frequency of 80 kHz for 20 min to obtain a silica dispersion liquid. The amount of silica microspheres is 5% of the mass of the polyacrylonitrile solution.

[0092] The silica dispersion liquid is incubated and stirred at a temperature of 60℃ and a stirring speed of 80 r / min for 24 h, then centrifuged, the filter cake is collected, washed with anhydrous ethanol for 3 times, and then dried to obtain a precursor.

[0093] The precursor is heated to 1000℃ at a rate of 4℃ / min in an argon atmosphere, and the precursor is incubated and carbonized for 4 h, and then cooled to room temperature to obtain a carbonized material.

[0094] The carbonized material and excess hydrofluoric acid with a mass fraction of 5% are mixed, and then ultrasonic reaction is performed at an ultrasonic frequency of 60 kHz and a temperature of 60℃ for 24 h to remove silica by etching with hydrofluoric acid, and then filtered, washed, dried, and sieved to obtain hollow carbon microspheres.

[0095] The hollow carbon microspheres and the sodium silicate-potassium hydroxide composite solution are mixed at a mass ratio of 1:5, and then immersed at room temperature for 12 h, and then filtered, dried, and then heated to 850℃ at a rate of 5℃ / min in an argon atmosphere, and then activated for 100 min, and then cooled to room temperature, washed with deionized water, dried, dispersed, and sieved to obtain the hollow porous carbon microspheres.

[0096] The mass ratio of sodium silicate to potassium hydroxide in the composite solution is 1:10, and the amount of water in the solution is 80% of the total mass of the composite solution.

[0097] Through the control of the above process conditions, the hollow porous carbon microspheres with a porosity of 55%, a surface pore size distribution of 2-50 nm, and a shell thickness distribution of 50-500 nm are obtained. By sieving, the particles with a particle size distribution of 0.5-15 μm are obtained.

[0098] Preparation of the composite cathode material:

[0099] Based on the total mass of the composite cathode material, the mass percentage of elemental sulfur is 80%, and the balance is hollow porous carbon microspheres. Elemental sulfur and hollow porous carbon microspheres are prepared.

[0100] Mix them and add them to a high-pressure reactor. Under nitrogen protection, heat at a rate of 0.5°C / min to 200°C. Under a pressure of 0.3 MPa, heat and press for 12 h to load elemental sulfur on the hollow porous carbon microspheres. After natural cooling to room temperature, discharge, and the composite cathode material is obtained.

[0101] Through the control of the above process conditions, the hollow porous carbon microspheres with a porosity of 55%, a surface pore size distribution of 2-50 nm, and a shell thickness distribution of 50-500 nm are obtained. By sieving, the particles with a particle size distribution of 0.5-15 μm are obtained.

[0102] The mass ratio of the first part to the second part is 10:1.

[0103] Example 4

[0104] The difference between this example and Example 1 is:

[0105] No sodium silicate is added, and the rest of the conditions remain unchanged.

[0106] Example 5

[0107] The difference between this example and Example 1 is:

[0108] By sieving, select hollow porous carbon microspheres with a particle size distribution of 0.1-20 μm, and the rest of the conditions remain unchanged.

[0109] Example 6

[0110] The difference between this example and Example 1 is:

[0111] Preparation of hollow porous carbon microspheres:

[0112] polyacrylonitrile is dissolved in DMF solvent to obtain a polyacrylonitrile solution with a concentration of 8 mg / mL;

[0113] The silica microspheres are added to the polyacrylonitrile solution, and ultrasonic dispersion is performed at a frequency of 80 kHz for 20 min to obtain a silica dispersion liquid; wherein the amount of the silica microspheres is 3% of the mass of the polyacrylonitrile solution;

[0114] The silica dispersion liquid is incubated and stirred at a temperature of 60℃ and a stirring speed of 60 r / min for 24 h, then centrifugal separation is performed, the filter cake is collected, the filter cake is washed with anhydrous ethanol for 3 times, and then the washed filter cake is dried to obtain a precursor;

[0115] The precursor is heated to 800℃ at a rate of 2℃ / min in an argon atmosphere, the precursor is incubated and carbonized for 3 h, and then cooled to room temperature to obtain a carbonized material;

[0116] The carbonized material and excess hydrofluoric acid with a mass fraction of 5% are mixed, and then ultrasonic reaction is performed at an ultrasonic frequency of 60 kHz and a temperature of 60℃ for 24 h to remove the silica by etching with the hydrofluoric acid, and then filtration, washing and drying are performed, and screening is performed to obtain hollow carbon microspheres;

[0117] The hollow carbon microspheres and a sodium silicate-potassium hydroxide composite solution are mixed at a mass ratio of 1:5, and then immersed at room temperature for 12 h, and then filtration, drying, heating and activation reaction are performed at a rate of 5℃ / min to 880℃ in an argon atmosphere for 90 min, and then cooled to room temperature, washed with deionized water, dried, dispersed and screened to obtain hollow porous carbon microspheres;

[0118] In the composite solution of sodium silicate and potassium hydroxide, the mass ratio of sodium silicate to potassium hydroxide is 1:9, and the amount of water in the solution is 80% of the total mass of the composite solution;

[0119] Through the control of the above process conditions, hollow porous carbon microspheres with a porosity of 60%, a surface pore size distribution of 2-75 nm and a shell thickness distribution of 50-400 nm are obtained, and particles with a particle size distribution of 0.5-15 μm are obtained through screening;

[0120] The remaining conditions are basically unchanged.

[0121] Example 7

[0122] Compared with Example 1, the difference is that:

[0123] Preparation of hollow porous carbon microspheres:

[0124] polyacrylonitrile is dissolved in DMF solvent to obtain a polyacrylonitrile solution with a concentration of 7 mg / mL;

[0125] The silica microspheres are added to the polyacrylonitrile solution, and ultrasonic dispersion is performed at a frequency of 80 kHz for 20 min to obtain a silica dispersion liquid; wherein the amount of the silica microspheres is 3% of the mass of the polyacrylonitrile solution;

[0126] The silica dispersion liquid is kept at a temperature of 60℃ and stirred at a speed of 60 r / min for 18 h, and then centrifugal separation is performed, the filter cake is collected, the filter cake is washed with anhydrous ethanol for 3 times, and then the washed filter cake is dried to obtain a precursor;

[0127] The precursor is heated to 780℃ at a rate of 2℃ / min in an argon atmosphere, the precursor is kept for carbonization for 3 h, and then cooled to room temperature to obtain a carbonized material;

[0128] The carbonized material and excess hydrofluoric acid with a mass fraction of 5% are mixed, and then ultrasonic reaction is performed at an ultrasonic frequency of 60 kHz and a temperature of 60℃ for 24 h to remove the silica by etching with the hydrofluoric acid, and then filtration, washing and drying are performed, and screening is performed to obtain hollow carbon microspheres;

[0129] The hollow carbon microspheres and a sodium silicate-potassium hydroxide composite solution are mixed at a mass ratio of 1:5, and then immersed at room temperature for 12 h, and then filtration, drying, heating and activation reaction are performed at a rate of 5℃ / min to 780℃ in an argon atmosphere for 80 min, and then cooled to room temperature, washed with deionized water, dried, dispersed and screened to obtain hollow porous carbon microspheres;

[0130] In the sodium silicate-potassium hydroxide composite solution, the mass ratio of sodium silicate to potassium hydroxide is 1:8, and the amount of water in the solution is 80% of the total mass of the composite solution;

[0131] Through control of the above process conditions, hollow porous carbon microspheres with a porosity of 50%, a surface pore size distribution of 2-32 nm and a shell thickness distribution of 45-360 nm are obtained, and particles with a particle size distribution of 0.5-15 μm are obtained through screening;

[0132] The other conditions remain basically unchanged.

[0133] Comparative Example 1

[0134] Compared with Example 1, the difference is that:

[0135] Preparation of hollow porous carbon microspheres:

[0136] The polyacrylonitrile is dissolved in a DMF solvent to obtain a polyacrylonitrile solution with a concentration of 8 mg / mL;

[0137] The silica microspheres are added to the polyacrylonitrile solution, and ultrasonic dispersion is carried out at a frequency of 80 kHz for 20 min to obtain a silica dispersion liquid; wherein the amount of the silica microspheres is 3% of the mass of the polyacrylonitrile solution;

[0138] The silica dispersion liquid is kept at a temperature of 60℃ and stirred at a speed of 60 r / min for 24 h, and then centrifugal separation is carried out, the filter cake is collected, the filter cake is washed with anhydrous ethanol for 3 times, and then the washed filter cake is dried to obtain a precursor;

[0139] The precursor is heated to 800℃ at a rate of 2℃ / min in an argon atmosphere, the precursor is kept for carbonization for 3 h, and then cooled to room temperature to obtain a carbonized material;

[0140] The carbonized material and excess hydrofluoric acid with a mass fraction of 5% are mixed, and then ultrasonic reaction is carried out at an ultrasonic frequency of 60 kHz and a temperature of 60℃ for 24 h to remove the silica by etching with hydrofluoric acid, and then filtration, washing and drying are carried out, and screening is carried out to obtain hollow carbon microspheres;

[0141] The hollow carbon microspheres and a sodium silicate-potassium hydroxide composite solution are mixed at a mass ratio of 1:5, and then immersed at room temperature for 12 h, and then filtration, drying, heating and activation reaction are carried out at a rate of 5℃ / min to 880℃ in an argon atmosphere for 90 min, and then cooled to room temperature, washed with deionized water, dried, dispersed and screened to obtain hollow porous carbon microspheres;

[0142] In the sodium silicate-potassium hydroxide composite solution, the mass ratio of sodium silicate to potassium hydroxide is 1:3, and the amount of water in the solution is 80% of the total mass of the composite solution;

[0143] Through the control of the above process conditions, hollow porous carbon microspheres with a porosity of 32%, a surface pore size distribution of 0.5-25 nm and a shell thickness distribution of 50-400 nm are obtained, and particles with a particle size distribution of 0.5-15 μm are obtained through screening;

[0144] The remaining conditions are basically unchanged.

[0145] Of course, since the porosity or pore size distribution of the carbon microspheres is adjusted in the above examples or comparative examples, the difficulty in the process of infiltrating elemental sulfur will increase, and it can be understood that in order to achieve the corresponding distribution of elemental sulfur, the infiltration time and pressure can be reasonably adjusted to achieve the same, which will not be described in detail here.

[0146] The products obtained in the above examples or comparative examples are subjected to performance testing, and the specific testing methods and testing results are as follows:

[0147] The composite cathode material, conductive agent Super P, and binder PVDF were mixed in a mass ratio of 8:1:1 and dispersed into a cathode slurry under the action of solvent NMP. The slurry was uniformly coated on an aluminum foil using a doctor blade coater. The coated electrode sheet was transferred to a vacuum oven at 80°C and dried for 12 hours to completely remove the solvent and moisture.

[0148] The dried electrode sheet was punched into a small round sheet with a diameter of 14 mm using a puncher to obtain a cathode electrode sheet;

[0149] The mass of each electrode sheet was accurately measured, and the actual mass of the active material sulfur (S) in each electrode sheet was calculated;

[0150] A lithium metal sheet was used as the anode electrode sheet, and a Celgard 2325 separator was used as the separator;

[0151] The electrolyte formula was: 1.0 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in a mixed solvent of 1,3-dioxolane (DOL) and dimethoxyethane (DME) (volume ratio 1:1), with the addition of 2 wt% LiNO3 additive;

[0152] The cathode electrode sheet, separator, anode electrode sheet, electrolyte, cathode shell, anode shell, gasket, and other components were assembled into a CR2032 button cell;

[0153] The battery was left to stand for 24 hours to allow the electrolyte to fully soak;

[0154] First charge-discharge capacity test:

[0155] At a temperature of 25°C, at a rate of 0.1C (1C calculated based on the mass of the active material S calculated above), and at a voltage window of 1.7-2.8V, the battery was charged and discharged, and the first discharge and charge specific capacity was recorded in mAh / g. The first coulombic efficiency was calculated based on the formula: first coulombic efficiency = (first charge capacity / first discharge capacity)*100%, and the test results are shown in Table 1;

[0156] Table 1: First charge-discharge test results

[0157]

[0158] Rate and cycle performance test:

[0159] At a temperature of 25°C and a voltage window of 1.7-2.8V, the battery was subjected to constant current charge-discharge cycles at 0.1C and 0.3C, respectively. After 100 cycles, the capacity retention rate of the battery was tested, and the detailed test results are shown in Table 2;

[0160] Table 2: Capacity retention rate test results

[0161]

[0162] From the test results of Table 1 and Table 2, it can be seen that the product obtained by the present application has excellent initial coulomb efficiency and stable performance during long-term cycling.

[0163] The above examples are the preferred embodiments of the present application, but the embodiments of the present application are not limited to the above examples, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application should be equivalent replacement methods, and are all included in the protection scope of the present application.

Claims

1. A lithium-sulfur battery composite cathode material, characterized in that, The lithium-sulfur battery composite positive electrode material comprises: Hollow porous carbon microspheres and elemental sulfur loaded in the hollow porous carbon microspheres; The elemental sulfur is distributed in the internal cavities and the outer surfaces of the hollow porous carbon microspheres; The portion of the elemental sulfur distributed in the internal cavities is a first portion; the portion of the elemental sulfur distributed on the outer surfaces is a second portion; The mass ratio of the first portion to the second portion is 8-10:1; The mass percentage of the elemental sulfur is 75-80% based on the total mass of the lithium-sulfur battery composite positive electrode material; The porosity of the hollow porous carbon microspheres is 50-55%.

2. The lithium-sulfur battery composite cathode material of claim 1, wherein, The hollow porous carbon microspheres have mesopores and macropores with a pore size distribution of 2-50 nm on the surfaces.

3. The lithium-sulfur battery composite cathode material of claim 2, wherein, The particle size distribution of the hollow porous carbon microspheres is 0.5-15 μm.

4. The lithium-sulfur battery composite cathode material of claim 3, wherein the lithium-sulfur battery composite cathode material is characterized by: The shell thickness distribution of the hollow porous carbon microspheres is 50-500 nm.

5. The lithium-sulfur battery composite cathode material of claim 1, wherein, The hollow porous carbon microspheres are doped with nitrogen and silicon elements.

6. A method of preparing a lithium-sulfur battery composite cathode material as claimed in any one of claims 1 to 5, characterized in that, The specific preparation steps comprise: Providing hollow porous carbon microspheres; Mixing the hollow porous carbon microspheres and elemental sulfur, heating to 180-200 ℃ in an inert atmosphere, and holding for 10-12 h to load the elemental sulfur in the hollow porous carbon microspheres; Cooling to obtain the lithium-sulfur battery composite positive electrode material.

7. The method of claim 6, wherein the lithium-sulfur battery composite cathode material is prepared by the steps of: mixing sulfur and a lithium source to form a mixture; adding a carbon source to the mixture; and heating the mixture to form the lithium-sulfur battery composite cathode material. The specific preparation steps further comprise: Mixing the hollow porous carbon microspheres and elemental sulfur, heating to 180-200 ℃ at a rate of 0.3-0.5 ℃ / min in an inert atmosphere, and holding for 10-12 h under a pressure of 0.2-0.3 MPa to load the elemental sulfur in the hollow porous carbon microspheres.

8. The method of claim 6, wherein the lithium-sulfur battery composite cathode material is prepared by the steps of: mixing sulfur and a lithium source to form a mixture; adding a carbon source to the mixture; and heating the mixture to form the lithium-sulfur battery composite cathode material. The provision of the hollow porous carbon microspheres comprises: Using silica microspheres as a hard template; Coating a carbon-containing organic polymer layer on the surface of the hard template to obtain a precursor; High-temperature carbonization of the precursor, followed by cooling to room temperature to obtain carbonized material; Chemical etching of the carbonized material to remove the hard template to obtain hollow carbon microspheres; Treatment of the hollow carbon microspheres with an activating agent to obtain hollow porous carbon microspheres.

9. The method of claim 8, wherein the lithium-sulfur battery composite cathode material is prepared by the steps of: mixing sulfur and a lithium source to form a mixture; adding a carbon source to the mixture; and heating the mixture to form the lithium-sulfur battery composite cathode material. The provision of the hollow porous carbon microspheres further comprises: Using silica microspheres as a hard template; Coating a carbon-containing organic polymer layer on the surface of the hard template to obtain a precursor; The organic polymer layer is a polyacrylonitrile coating layer; Heating to 800-1000 ℃ at a rate of 2-4 ℃ / min under inert gas protection, carbonizing the precursor for 3-4 h, and then cooling to room temperature to obtain carbonized material; Chemical etching of the carbonized material to remove the hard template to obtain hollow carbon microspheres; Mixing and impregnating the hollow carbon microspheres with a sodium silicate-potassium hydroxide composite solution, drying, heating in an inert atmosphere at a temperature of 780-850 ℃ to activate, and cooling to obtain the hollow porous carbon microspheres; In the sodium silicate-potassium hydroxide composite solution, the mass ratio of sodium silicate to potassium hydroxide is 1:8-10.

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