Porous carbon for positive electrode of lithium-sulfur battery

Abstract

The invention provides porous carbon for a positive electrode of a lithium-sulfur battery, which is prepared by the following steps: firstly, taking polyurethane as a carbon precursor, dipping in an ionic liquid, and activating by a one-step method, the porous carbon is good in conductivity, large in specific surface area, uniform in pore size distribution, capable of constructing an efficient conductive network and high in sulfur loading capacity, simple in preparation process, low in cost and suitable for industrial production. And uniformly mixing the obtained porous carbon with sublimed sulfur according to a mass ratio, and carrying out melt diffusion to obtain the porous carbon for the positive electrode of the lithium-sulfur battery. According to the preparation method, the problems of serious pollution to links, serious corrosion to equipment and the like caused by adoption of activating agents such as strong base KOH and NaOH in a traditional process are solved, the obtained porous carbon is compounded with sulfur, high sulfur loading capacity can be realized, abundant defect sites on the surface can also store charges, and high specific capacity can be obtained when the porous carbon is used as a positive electrode material of a lithium ion battery. The porous carbon can be subjected to element doping by controlling the type and impregnation ratio of the added ionic liquid; the pore volume and the specific surface area of the porous carbon can be regulated and controlled by controlling the activation temperature and time, the used polyurethane carbon precursor contains nitrogen, the nitrogen-doped high-specific-surface-area porous carbon can be obtained through ionic liquid activation, nitrogen exists in the form of C-N bonds, electron conduction can be enhanced, an efficient conductive network can be constructed, and the nitrogen-doped high-specific-surface-area porous carbon can be obtained. And the prepared nitrogen-doped high-specific-surface-area porous carbon and sulfur are subjected to a melt diffusion method to obtain the high-sulfur-loading sulfur-carbon composite material, and the high-sulfur-loading sulfur-carbon composite material is used for the lithium-sulfur battery, so that the electrochemical performance of the lithium-sulfur battery can be improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and particularly relates to a porous carbon for the positive electrode of a lithium-sulfur battery. Background Art

[0002] The global energy crisis and environmental problems are becoming increasingly serious. Mankind needs to reduce its dependence on fossil fuels. Therefore, it is urgent to vigorously develop environmentally friendly new energy and efficient energy storage systems. The theoretical specific energy of a lithium-sulfur battery is as high as 2500 Wh / kg. Sulfur is a positive electrode material based on a "conversion reaction", and its theoretical specific capacity of the reaction reaches 1672 mAh / g. It has a high cost performance and is environmentally friendly, just meeting this demand. Although lithium-sulfur batteries have extremely high theoretical specific capacity and energy density, the conductivity of elemental sulfur and its final products is extremely poor. At room temperature, elemental sulfur is an electron and ion insulator, and its conductivity is only 5×10 -30 S / cm. When used as an electrode material, it is difficult to activate, has a low utilization rate, and expands and contracts during the charge and discharge process. After complete lithiation, the volume expands by about 80%, resulting in the separation of sulfur from the conductive framework, serious damage to the battery structure, and severe capacity attenuation. Usually, the solution to the above problems is to prepare porous carbon to load sulfur, thereby restricting the volume expansion phenomenon of sulfur during the charge and discharge process. However, its poor conductivity and low sulfur loading are still the key problems restricting the development of lithium-sulfur batteries.

[0003] Therefore, how to obtain a porous carbon for the positive electrode of a lithium-sulfur battery with a high sulfur loading and good conductivity is an urgent problem to be solved by people at present. Summary of the Invention

[0004] The purpose of the present invention is to provide a porous carbon for the positive electrode of a lithium-sulfur battery, which is obtained by using polyurethane as a carbon precursor and through carbonization and ionic liquid activation. The porous carbon for the positive electrode of a lithium-sulfur battery has good conductivity, a large specific surface area, and a uniform pore size distribution. It can construct an efficient conductive network and a high sulfur loading, has a simple preparation process and low cost, and can improve the electrochemical performance of lithium-sulfur batteries.

[0005] The above object of the present invention is achieved through the following technical solutions: A porous carbon for the positive electrode of a lithium-sulfur battery, where polyurethane is crushed to a particle size of 1-5 cm, impregnated into an ionic liquid for 1-24 h, and then calcined in a nitrogen atmosphere. The calcination temperature is 600-1000 °C and the calcination time is 30-60 min. After cooling, it is ground and sieved to obtain the porous carbon for the positive electrode of a lithium-sulfur battery.

[0006] Compared with the prior art, in view of the problems of poor electrical conductivity and underdeveloped pore structure of the current-carrying porous carbon in the electrodes of lithium-sulfur batteries, the above technical solution is adopted. The added ionic liquid is activated to form pores during the calcination process, and multi-element doping is carried out at the same time. The doped elements enhance the short-layer electron conduction and construct a good conductive network. This method of using ionic liquid for pore activation and element doping to prepare porous carbon for the positive electrode of lithium-sulfur batteries is simple, less polluting to the environment, and easy to promote.

[0007] Further, the crushing of the polyurethane is to crush it to a particle size of 1-3 cm.

[0008] Adopting the above technical solution and controlling the particle size of the polyurethane particles within this range can better absorb the ionic liquid during the impregnation process.

[0009] Further, the impregnation time is 12-24 h℃.

[0010] Further, calcination is carried out under a nitrogen atmosphere, and the calcination temperature is 700-900 °C.

[0011] In summary, the present invention has the following beneficial effects: 1. Using polyurethane as a carbon precursor, nitrogen-doped porous carbon can be directly obtained through activation without the need to additionally add a nitrogen source, and the process is simple.

[0012] 2. Using ionic liquid as an activator, element-doped porous carbon with developed pore diameters can be obtained, avoiding problems such as large pollution to the environment and serious corrosion of equipment by traditional activators such as strong alkalis KOH and NaOH.

[0013] 3. When the element-doped porous carbon prepared by the present invention is used for the positive electrode of a lithium-sulfur battery, the sulfur loading is higher than 70%, and the rate performance is excellent. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a scanning electron microscope (SEM) image of the porous carbon for the positive electrode of the lithium-sulfur battery prepared in Example 1 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0015] The following further elaborates on the present invention in conjunction with embodiments.

[0016] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges. For numerical ranges, between the endpoint values of each range, between the endpoint values and individual point values of each range, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0017] Example 1: A porous carbon for the positive electrode of a lithium-sulfur battery is prepared according to the following steps: Step 1: Put 10 g of polyurethane into a crusher for crushing, then impregnate it into 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid. The impregnation time is 12 h, and then it is placed in a high-temperature furnace for activation. The activation temperature is 800 °C and the activation time is 30 min to obtain phosphorus and nitrogen-doped porous carbon. Mix the prepared phosphorus and nitrogen-doped porous carbon with sublimed sulfur in a mass ratio of 1:3. Under a nitrogen atmosphere, heat it to 100 °C at a rate of 10 °C / min, then heat it to 300 °C at a rate of 5 °C / min. After maintaining at 300 °C for 10 min, continue to heat it to 350 °C at a rate of 5 °C / min and maintain for 5 min. After cooling, the positive electrode active sulfur-carbon composite material C@S is obtained.

[0018] Example 2: Put 10 g of polyurethane into a crusher for crushing, then impregnate it into 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid. The impregnation time is 12 h, and then it is placed in a high-temperature furnace for activation. The activation temperature is 900 °C and the activation time is 30 min to obtain phosphorus and nitrogen-doped porous carbon. Mix the prepared phosphorus and nitrogen-doped porous carbon with sublimed sulfur in a mass ratio of 1:2. Under a nitrogen atmosphere, heat it to 100 °C at a rate of 10 °C / min, then heat it to 300 °C at a rate of 5 °C / min. After maintaining at 300 °C for 10 min, continue to heat it to 350 °C at a rate of 5 °C / min and maintain for 5 min. After cooling, the positive electrode active sulfur-carbon composite material C@S is obtained.

[0019] Preparation of the electrode: Mix the positive electrode activated carbon material (C@S), conductive agent (Ketjen black, model ECP-600JD), and binder (PVDF) in a mass ratio of 8:1:1, add an appropriate amount of DMF as a solvent, stir evenly, and then coat it on an aluminum current collector with a carbon film. Then bake it in a vacuum oven at 40 °C for 6 h. After the electrode sheet is dried, use a mold to cut the electrode sheet and assemble it into a soft-pack battery.

[0020] After the composite of Example 1, Example 2 and commercially available activated carbon with sublimed sulfur is assembled into a soft-pack battery for electrochemical performance testing, the test results are shown in the following table: Table 1 Electrochemical performance test of different samples Sample Specific capacity at 0.5C current density F / g Specific capacity retention rate after 40 cycles at 0.5C current density % Specific capacity at 0.1C current density F / g Specific capacity retention rate after 60 cycles at 0.1C current density % Example 1 1050 85% 1350 82% Example 2 980 84% 1305 81% Composite of commercially available activated carbon and sulfur 700 75% 860 69% After testing, the specific capacity of the sulfur-carbon composite electrode prepared in Example 2 is the largest. When cycling 40 and 60 times, the specific capacity retention rate of the electrodes prepared in Example 1 and Example 2 is higher than that of the commercially available activated carbon / sulfur composite electrode. It can be seen that the porous carbon prepared by the method of the present invention has excellent performance as a lithium-sulfur battery electrode.

[0021] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A porous carbon for positive electrode of lithium-sulfur battery, It is characterized in that Firstly, the polyurethane is crushed and then immersed in an ionic liquid, and porous carbon is obtained by a one-step activation method. After the obtained porous carbon and sublimated sulfur are evenly mixed according to a mass ratio, the porous carbon for the positive electrode of the lithium-sulfur battery is obtained by melt diffusion.

2. The porous carbon for lithium-sulfur battery positive electrode according to claim 1, It is characterized in that The polyurethane is crushed, wherein the crushing refers to using a crusher to crush the polyurethane into particles with a particle size of 1-5 cm.

3. The porous carbon for lithium-sulfur battery positive electrode according to claim 1, It is characterized in that The ionic liquid is one or more of 1-butyl-3-methylimidazolium hexafluorophosphate and 1-ethyl-3-methylimidazolium tetrafluoroborate.

4. The porous carbon for lithium-sulfur battery positive electrode according to claim 1, It is characterized in that The immersion time is 1-24h.

5. The porous carbon for lithium-sulfur battery positive electrode according to claim 1, It is characterized in that During the screening process, the activation is carried out at a temperature of 500-900°C and a time of 30-90 minutes.

6. The porous carbon for lithium-sulfur battery positive electrode according to claim 1, It is characterized in that The mass ratio of porous carbon to sublimated sulfur is 1:1~1:

5.

7. The porous carbon for lithium-sulfur battery positive electrode according to claim 1, It is characterized in that The melt diffusion process was carried out in a nitrogen atmosphere, and the temperature was first raised to 100°C at a rate of 10°C / min, then raised to 300°C at a rate of 5°C / min, maintained at 300°C for 10 min, and then continued to be raised to 350°C at a rate of 5°C / min and maintained for 5 min.

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