CN105161724B - Porous carbon sphere for lithium-sulfur battery and preparation method and application of porous carbon sphere - Google Patents

Abstract

The invention discloses a porous carbon sphere for a lithium-sulfur battery, a preparation method and an application thereof. The porous carbon sphere for a lithium-sulfur battery is formed by disorderly winding ribbon graphite. The preparation method comprises the following steps: (1) preparing Si-C-O particles; (2) preparing porous carbon spheres. The porous carbon sphere for the lithium-sulfur battery is suitable for preparing the positive electrode material of the lithium-sulfur battery, and can improve the capacity and cycle stability of the lithium-sulfur battery and the conductivity of the sulfur electrode. The preparation method is simple, the yield is high and it can be produced in batches.

Description

Porous carbon spheres for lithium-sulfur battery and its preparation method and application

technical field

The invention belongs to the field of secondary power lithium batteries, and in particular relates to a positive electrode material for a lithium-sulfur battery and a preparation method and application thereof.

Background technique

The development of society has an increasing demand for fossil fuels such as oil and natural gas. However, with the increase in the consumption of fossil fuels, the depletion trend of these non-renewable fossil fuels has attracted more and more attention. At the same time, the burning of fossil fuels caused Air pollution and the greenhouse effect are also becoming global problems. In order to solve the energy shortage and environmental pollution problems caused by fuel-powered vehicles, power batteries have gradually become a focus of attention. Power batteries include fuel cells, lead-acid batteries, nickel metal hydride batteries and lithium-ion batteries. Fuel cells need to use the scarce precious metal platinum as a catalyst and expensive proton exchange membranes, and at the same time, it is difficult to prepare, store and transport fuel hydrogen. The mass specific energy and specific power of lead-acid batteries are lower, and the cycle life is shorter. At the same time, the manufacture of lead-acid batteries is more polluting. Although nickel-hydrogen batteries have been commercialized, they have disadvantages such as low charge and discharge efficiency, poor cycle life, and low specific energy. In contrast, lithium-ion batteries have the characteristics of high energy density (2 to 3 times that of lead-acid batteries and nickel-metal hydride batteries), long cycle life, high energy density, small self-discharge, no memory effect, and wide operating temperature range. It is the battery with the best comprehensive performance at present. Therefore, lithium-ion batteries are also considered to be one of the best power battery solutions for a long time to come.

At present, the positive electrode materials of commercial lithium-ion batteries are mainly transition metal oxides such as LiCoO ₂ , LiMnO ₂ , LiFePO ₄ , etc., whose specific capacity cannot meet the needs of power batteries. Elemental sulfur has the advantages of low cost, low toxicity, and environmental friendliness because of its theoretical specific capacity (1672 mAh/g) and high specific energy (2600 Wh/kg). Secondary power lithium battery cathode material. However, elemental sulfur also has some problems as a positive electrode material. On the one hand, the inherent electronic insulation of elemental sulfur makes it electrochemically inactive; on the other hand, the discharge intermediate product of sulfur electrode, polysulfide, has high solubility , it is easy to cause the loss of active materials and destroy the cycle stability of the battery system.

In order to improve the electrochemical performance of elemental sulfur in the battery, most researchers currently try to compound sulfur with porous carbon materials with high surface area and high conductivity as the carrier of sulfur and elemental sulfur, thereby increasing the conductivity of the positive electrode material, providing Electrochemical reaction active sites and inhibit the dissolution and diffusion of discharge products Li ₂ Sx (x>2). This can not only improve the electron ion transfer ability of the cathode, but also keep the intermediate polysulfide produced in the reduction process in the cathode, avoiding its dissolution in the organic electrolyte, thereby improving the capacity retention rate of the lithium-sulfur battery. At present, the commonly used porous carbon materials are mostly carbon nanotubes, graphene, etc. After these nanomaterials are compounded with sulfur to make batteries, they are prone to damage during the electrode cycle charge and discharge process, so the cycle stability is poor.

Contents of the invention

The technical problem to be solved in the present invention is to overcome the deficiencies of the prior art, and provide a porous carbon sphere for lithium-sulfur batteries suitable for preparing lithium-sulfur battery cathode materials, which can improve the capacity and cycle stability of lithium-sulfur batteries, and improve the conductivity of sulfur electrodes. , also provides a method for preparing porous carbon spheres for lithium-sulfur batteries with simple method, high yield and mass production, and correspondingly provides the application of the porous carbon spheres in the preparation of lithium-sulfur batteries.

In order to solve the problems of the technologies described above, the present invention adopts the following technical solutions:

A porous carbon sphere for a lithium-sulfur battery, wherein the porous carbon sphere is formed by disorderly winding ribbon graphite.

In the aforementioned porous carbon spheres for lithium-sulfur batteries, preferably, the ribbon-shaped graphite is composed of multilayer graphite sheets, and the number of layers of the multilayer graphite sheets is 10 to 20 layers.

As a general technical concept, the present invention also provides a preparation method of porous carbon spheres for lithium-sulfur batteries, comprising the following steps:

(1) Preparation of Si-C-O particles: heat and vaporize low-molecular polycarbosilane, and then prepare Si-C-O particles by low-pressure chemical vapor deposition under the protection of inert gas;

(2) Preparation of porous carbon spheres: the Si-C-O particles obtained in step (1) are subjected to high-temperature heat treatment under the protection of an inert gas to remove the oxygen and silicon elements in the Si-C-O particles to obtain porous carbon for lithium-sulfur batteries ball.

In the above preparation method, preferably, in the step (1), the atomic ratio of silicon and carbon in the low-molecular polycarbosilane is 1:2, and the number-average molecular weight of the low-molecular polycarbosilane is 500-2000.

In the above preparation method, preferably, in the step (2), the temperature of the high-temperature heat treatment is 2600°C-2700°C, and the holding time is 1h-3h.

In the above preparation method, preferably, in the step (1), the temperature of the low-pressure chemical vapor deposition is 1000°C-1200°C, the deposition time is 0.5h-3h, and the pressure of the low-pressure chemical vapor deposition is not high at 1000 Pa.

In the above preparation method, preferably, in the step (1), the heating and gasification temperature of the low-molecular polycarbosilane is 120°C-180°C.

As a general technical idea, the present invention also provides an application of the above-mentioned porous carbon spheres for lithium-sulfur batteries or the porous carbon spheres for lithium-sulfur batteries prepared by the above preparation method in lithium-sulfur batteries.

In the above application, preferably, the application includes mixing the porous carbon spheres for the lithium-sulfur battery with sulfur powder to prepare a positive electrode material for the lithium-sulfur battery.

In the above application, preferably, the mass ratio of the porous carbon spheres for the lithium-sulfur battery to the sulfur powder is 3:2.

In step (1) of the preparation method of the present invention, the inert gas may preferably be nitrogen, and in step (2), the protective medium for high-temperature heat treatment may preferably be argon.

Compared with the prior art, the present invention has the advantages of:

(1) The porous carbon spheres for lithium-sulfur batteries of the present invention are structurally a porous structure in which graphite ribbons composed of 10 to 20 layers of graphite flakes are wound in disorder. High, high stability, so when it is applied to the battery, it can maintain integrity for a long time during the charging and discharging process of the battery, so the cycle stability is high.

(2) The preparation method of the porous carbon spheres for lithium-sulfur batteries of the present invention has the advantages of small investment in equipment, simple operation, low preparation cost, high yield, and easy realization of mass production.

(3) In the method for preparing porous carbon spheres for lithium-sulfur batteries of the present invention, since the porous carbon spheres are prepared at a higher temperature and have fewer structural defects, the subsequently prepared lithium-sulfur batteries have better performance.

Description of drawings

Fig. 1 is a schematic diagram of the principle of preparing Si-C-O particles by low-pressure chemical vapor deposition of the present invention.

Fig. 2 is a SEM microscopic image of Si-C-O particles prepared in step (1) of Example 1 of the present invention.

Fig. 3 is an XRD phase analysis diagram of Si-C-O particles prepared in step (1) of Example 1 of the present invention.

Fig. 4 is a SEM microscopic image of the porous carbon spheres for lithium-sulfur batteries prepared in step (2) of Example 1 of the present invention.

Fig. 5 is an XRD phase analysis diagram of the porous carbon spheres for lithium-sulfur batteries prepared in step (2) of Example 1 of the present invention.

Fig. 6 is a SEM microscopic image of Si-C-O particles prepared in step (1) of Example 2 of the present invention.

Fig. 7 is an XRD phase analysis diagram of Si-C-O particles prepared in step (1) of Example 2 of the present invention.

Fig. 8 is a SEM microscopic image of the porous carbon spheres for lithium-sulfur batteries prepared in step (2) of Example 2 of the present invention.

Fig. 9 is an XRD phase analysis chart of the porous carbon spheres for lithium-sulfur batteries prepared in step (2) of Example 2 of the present invention.

Fig. 10 is a SEM microscopic image of Si-C-O particles prepared in step (1) of Example 3 of the present invention.

Fig. 11 is an XRD phase analysis diagram of Si-C-O particles prepared in step (1) of Example 3 of the present invention.

Fig. 12 is a SEM microscopic image of the porous carbon spheres for lithium-sulfur batteries prepared in step (2) of Example 3 of the present invention.

Fig. 13 is an XRD phase analysis diagram of porous carbon spheres for lithium-sulfur batteries prepared in step (2) of Example 3 of the present invention.

Figure 14 is a transmission electron microscope image of porous carbon spheres for lithium-sulfur batteries prepared in step (2) of Example 3 of the present invention; where a is a transmission electron microscope image of multiple particles, and b is a high-resolution transmission electron microscope image of a single particle , c is a high-resolution transmission electron microscope image of a single particle.

Fig. 15 is the charge-discharge curve of the lithium-sulfur battery prepared in Example 3 of the present invention.

Fig. 16 is a diagram of the cycle life of the lithium-sulfur battery prepared in Example 3 of the present invention.

Figure 17 shows the rate performance of the lithium-sulfur battery prepared in Example 3 of the present invention.

illustration:

1. Low molecular polycarbosilane; 2. Three-neck flask; 3. Thermometer; 4. Heating mantle; 5. Tube furnace; 6. Graphite boat; 7. Nitrogen tank; 8. Vacuum pump.

detailed description

The present invention will be further described below in conjunction with the accompanying drawings and specific preferred embodiments, but the protection scope of the present invention is not limited thereby.

Example 1:

A porous carbon sphere for a lithium-sulfur battery of the present invention is formed by disorderly winding strip-shaped graphite. Ribbon graphite is composed of 10-20 graphite flakes.

A preparation method of the porous carbon spheres for lithium-sulfur batteries of the above-mentioned present embodiment, comprising the following steps:

(1) Preparation of Si-CO particles: use low-molecular polycarbosilane (the number-average molecular weight Mn is about 700, and the atomic ratio of Si and C is 1:2) as raw material for heating and gasification, and under the protection of _N2 atmosphere, it is carried out by low-pressure chemical Si-CO particles were prepared by vapor deposition process. The schematic diagram of the equipment used in low-pressure chemical vapor deposition is shown in Figure 1, and the specific method is as follows: put low-molecular polycarbosilane 1 (special resin) in a three-necked flask 2, and the three ports of the three-necked flask 2 are respectively connected to a nitrogen tank 7 One end of the air inlet pipe, the tube furnace 5 and the thermometer 3, the other end of the tube furnace 5 is connected with a vacuum pump 8, and a graphite boat 6 is placed in the tube furnace 5. Turn on the vacuum pump 8, and heat the tube furnace 5. During the heating process of the tube furnace 5, the vacuum pump 8 keeps running. After the temperature of the tube furnace 5 rises to the deposition temperature, turn on nitrogen, and turn on the three-necked flask heating jacket 4 , the low molecular weight polycarbosilane 1 in the three-necked flask 2 was heated to the set temperature. The pressure difference causes the gaseous low-molecular-weight polycarbosilane volatilized in the three-necked flask 2 to enter the tube furnace 5. These gaseous low-molecular-weight polycarbosilanes are cracked in the high-temperature zone of the tube furnace 5 to generate Si-CO particles and deposited on the graphite boat 6. superior. In the low-pressure chemical vapor deposition process, the purpose of introducing nitrogen gas mainly has two aspects, one is to provide the protective atmosphere required for the cracking of special resin, and the other is to transport the gaseous special resin to the tube furnace 5 . Therefore, the flow rate of nitrogen can also be adjusted according to the amount of Si-CO particles to be prepared. At the end of the LPCVD process, turn off the nitrogen and vacuum pumps, stop heating the tube furnace, and immediately stop heating the specialty resin.

In this embodiment, the low-molecular polycarbosilane 1 is heated at a set temperature of 120° C. (ie, the heating vaporization temperature), the low-pressure chemical vapor deposition temperature is 1000° C., the pressure is 1000 Pa, and the deposition time is 1 h. The SEM microscopic morphology of the Si-C-O particles prepared in this example is shown in Figure 2. It can be seen that the surface of the Si-C-O particles prepared by low-pressure chemical vapor deposition at 1000°C is spherical, the surface of the particles is smooth, and the particles are connected to each other. The element content of the prepared Si-C-O particles is shown in Table 1. It can be seen that the particles prepared by LPCVD at 1000°C are mainly composed of three elements: silicon, carbon and oxygen. The XRD phase analysis of the prepared Si-C-O particles is shown in Figure 3. In the XRD spectrum, only the diffraction peaks corresponding to graphite can be observed, but the diffraction peaks corresponding to SiC cannot be observed. This is mainly due to the preparation The temperature is low, the SiC phase is difficult to crystallize, and the silicon atoms can only bond with carbon atoms and/or bond with oxygen atoms locally, which is not enough to form a separate phase.

Table 1: Element content table of Si-C-O particles prepared in step (1) of Example 1

element % by weight Atomic % C 82.78 89.74 o 06.49 05.28 Si 10.73 04.97

(2) Preparation of porous carbon spheres: place the graphite crucible containing the Si-CO particles obtained in step (1) in a graphite heating furnace, and perform high-temperature heat treatment under the protection of argon to remove silicon and oxygen element to obtain porous carbon spheres for lithium-sulfur batteries. In this embodiment, the high temperature heat treatment temperature is 2700° C., and the holding time is 1 h. The SEM microscopic morphology of the prepared porous carbon spheres for lithium-sulfur batteries is shown in Figure 4. It can be seen that the surface of Si-CO particles after heat treatment at 2700 °C is still smooth and spherical. The phase analysis of the prepared porous carbon spheres for lithium-sulfur batteries is shown in Figure 5. The XRD diffraction results of the particles obtained after heat treatment at 2700°C show that they only contain graphite phase. The content of the prepared porous carbon spheres for lithium-sulfur batteries is shown in Table 2. It can be seen that the particles obtained after heat treatment at 2700°C only contain carbon elements. This shows that during the high-temperature heat treatment of Si-CO particles at 2700 °C, the Si-C and Si-O bonds are broken, and silicon and oxygen elements escape in the form of gaseous SiO, CO, CO ₂ , etc., and then produce atomic-scale holes, and the remaining The C element crystallizes to form ribbon-shaped graphite and presents an intertwined porous structure.

Table 2: Element content table of porous carbon spheres for lithium-sulfur batteries prepared in step (2) of Example 1

element % by weight Atomic % C 100.00 100.00

Example 2:

A porous carbon sphere for a lithium-sulfur battery of the present invention is formed by disorderly winding strip-shaped graphite. Ribbon graphite is composed of 10-20 graphite flakes.

A preparation method of the porous carbon spheres for lithium-sulfur batteries of the above-mentioned present embodiment, comprising the following steps:

(1) Preparation of Si-C-O particles: The raw materials and method for preparing Si-C-O particles in this example are basically the same as in Example 1, the only difference being that in this example, the set temperature for heating low-molecular polycarbosilane is 150 °C, the low pressure chemical vapor deposition temperature is 1100 °C, the pressure is 1000 Pa, and the deposition time is 1 h. The SEM microscopic morphology of the prepared Si-C-O particles is shown in Figure 6. It can be seen that the surface of the Si-C-O particles prepared by low-pressure chemical vapor deposition at 1100 °C is spherical, with a small amount of granular protrusions distributed on the surface of the particles, and the spherical particles are connected to each other. The element content of the prepared Si-C-O particles is shown in Table 3. It can be seen that the particles prepared by low-pressure chemical vapor deposition at 1100°C are mainly composed of three elements: silicon, carbon and oxygen. The phase analysis of the prepared Si-C-O particles is shown in Figure 7. In the XRD spectrum, only the diffraction peaks corresponding to graphite can be observed, but the diffraction peaks corresponding to SiC cannot be observed. This is mainly due to the preparation temperature Low, the SiC phase is difficult to crystallize, so it is difficult to observe the corresponding diffraction peaks of SiC.

Table 3: Element content table of Si-C-O particles prepared in step (1) of Example 2

element % by weight Atomic % C 88.29 92.26 o 07.41 05.82 Si 04.30 01.92

(2) Preparation of porous carbon spheres: place the graphite crucible containing the Si-CO particles obtained in step (1) in a graphite heating furnace, and perform high-temperature heat treatment under the protection of argon to remove silicon and oxygen element to obtain porous carbon spheres for lithium-sulfur batteries. In this embodiment, the high temperature heat treatment temperature is 2600° C., and the holding time is 1 h. The SEM microscopic morphology of the prepared porous carbon spheres for lithium-sulfur batteries is shown in Figure 8. It can be seen that the Si-CO particles after heat treatment at 2600 °C have a smooth surface and a spherical shape. The phase analysis of the prepared porous carbon spheres for lithium-sulfur batteries is shown in Figure 9. The XRD diffraction results of the particles obtained after heat treatment at 2600°C show that they only contain diffraction peaks corresponding to graphite. The content of the prepared porous carbon spheres for lithium-sulfur batteries is shown in Table 4. It can be seen that the particles obtained after heat treatment at 2600°C only contain carbon elements. This shows that during the high-temperature heat treatment of Si-CO particles at 2700 °C, the Si-C and Si-O bonds are broken, and silicon and oxygen elements escape in the form of gaseous SiO, CO, CO ₂ , etc., and then produce atomic-scale holes, and the remaining The C element crystallizes to form ribbon-shaped graphite and presents an intertwined porous structure.

Table 4: Element content table of porous carbon spheres for lithium-sulfur batteries prepared in step (2) of Example 2

element % by weight Atomic % C 100.00 100.00

Example 3:

A porous carbon sphere for a lithium-sulfur battery of the present invention is formed by disorderly winding strip-shaped graphite. Ribbon graphite is composed of 10-20 graphite flakes.

A preparation method of the porous carbon spheres for lithium-sulfur batteries of the above-mentioned present embodiment, comprising the following steps:

(1) Preparation of Si-C-O particles: The raw materials and method for preparing Si-C-O particles in this example are basically the same as in Example 1, the only difference is that in this example, the set temperature for heating low-molecular polycarbosilane is 180 °C, the low pressure chemical vapor deposition temperature is 1200 °C, the pressure is 1000 Pa, and the deposition time is 1 h. The SEM microscopic morphology of the prepared Si-C-O particles is shown in Figure 10. It can be seen that the Si-C-O particles synthesized by low-pressure chemical vapor deposition at 1200 °C are still spherical, but their surface is flaky and convex. The element content of the prepared Si-C-O particles is shown in Table 5. It can be seen that the particles prepared by LPCVD at 1200°C are mainly composed of three elements: silicon, carbon and oxygen. The phase analysis of the prepared Si-C-O particles is shown in Figure 11. The diffraction peaks of the graphite phase and the SiC phase can be clearly observed in the XRD spectrum of the Si-C-O particles prepared by low-pressure chemical deposition at 1200 °C.

Table 5: Element content table of Si-C-O particles prepared in step (1) of Example 3

element % by weight Atomic % C 71.20 83.34 o 5.95 5.22 Si 22.85 11.44

(2) Preparation of porous carbon spheres: place the graphite crucible containing the Si-C-O particles obtained in step (1) in a graphite heating furnace, and perform high-temperature heat treatment under the protection of argon to remove silicon and oxygen element to obtain porous carbon spheres for lithium-sulfur batteries. In this embodiment, the high temperature heat treatment temperature is 2700° C., and the holding time is 1 h. The SEM microscopic morphology of the prepared porous carbon spheres for lithium-sulfur batteries is shown in Figure 12. It can be seen that after heat treatment at 2700 °C, the particles are still spherical, and the lamellar protrusions on the surface of the particles still exist. Figure 14 is a transmission electron microscope image of the prepared porous carbon spheres for lithium-sulfur batteries, in which b and c can clearly see that the obtained porous carbon spheres for lithium-sulfur batteries are formed by the disordered winding of ribbon graphite The porous structure of the graphite ribbons constitutes the pores of the porous carbon spheres. The content of the prepared porous carbon spheres for lithium-sulfur batteries is shown in Table 6. It can be seen that the particles obtained after heat treatment at 2700°C only contain carbon elements. The phase analysis of the prepared porous carbon spheres for lithium-sulfur batteries is shown in Figure 13. In the XRD spectrum of the particles obtained after heat treatment at 2700 °C, there are only diffraction peaks corresponding to graphite, indicating that Si-C-O particles are exposed to high temperatures at 2700 °C. During the heat treatment, the Si-C and Si-O bonds are broken, silicon and oxygen elements escape, and then atomic-scale pores are generated, and the remaining C elements crystallize to form ribbon graphite and present an intertwined porous structure.

Table 6: Element content table of porous carbon spheres for lithium-sulfur batteries prepared in step (2) of Example 3

element % by weight Atomic % C 100.0 100.0

An application of the porous carbon spheres for lithium-sulfur batteries of this embodiment in the preparation of lithium-sulfur batteries, which refers to mixing the porous carbon spheres for lithium-sulfur batteries of this embodiment with sulfur powder to prepare lithium-sulfur batteries The positive electrode material, the mass ratio of porous carbon spheres for lithium-sulfur batteries to sulfur powder is 3:2. The preparation method of the positive electrode material is as follows: firstly, the porous carbon spheres are pulverized, and mixed with the sulfur powder at a mass ratio of 3:2; subsequently, the mixed powder is heated to 155° C., and then kept for 10 hours to obtain the porous carbon spheres- Sulfur composite material; finally, 80wt% porous carbon sphere-sulfur composite material, 10wt% superconducting carbon and 10wt% PVDF were mixed uniformly to form a slurry, coated on aluminum foil, and dried under vacuum at 60°C for 24h to obtain the Cathode material.

The above-mentioned positive electrode material is assembled on a lithium-sulfur battery as a positive electrode sheet. The charge-discharge curves of the lithium-sulfur battery at 0.2C (1C=1672mA/g) for the first, second, third and 50th times are shown in Figure 15 It can be seen from the figure that the first discharge curve shows two stable discharge plateaus, corresponding to the conversion of S _s to polysulfide ions Li ₂ S _x and the further reduction of polysulfide ions to Li ₂ S, respectively. The initial discharge capacity of the lithium-sulfur battery using the porous carbon sphere-sulfur composite material as the positive electrode in this embodiment is as high as 950mAh/g, while the initial discharge capacity of current commercial lithium-sulfur batteries is generally only 700-800mAh/g. Therefore, the lithium-sulfur battery of this embodiment Lithium-sulfur batteries have relatively good initial discharge capacity. The cycle life of the lithium-sulfur battery at 0.2C (1C=1672mA/g) is shown in Figure 16. It can be seen from the figure that the capacity of the battery can be maintained at 93.9% after 100 cycles (relative to the 5th charge and discharge Capacity, the same below), after 200 cycles, the capacity of the battery can be maintained at 83%, after 300 cycles, the capacity of the battery can be maintained at 71.1%, after 400 cycles, the capacity of the battery can be maintained at 65%, after 500 cycles The capacity can be maintained at 55%. However, the current commercial lithium-sulfur battery has a capacity retention rate of only 60% to 70% after 100 cycles, indicating that the lithium-sulfur battery in this example with the porous carbon sphere-sulfur composite material as the positive electrode has better cycle stability. In the porous carbon sphere-sulfur composite, the porous carbon sphere can not only provide an effective conductive channel for the sulfur electrode, but also buffer the volume change caused by the charging and discharging process of the sulfur electrode, and the microporous structure in the porous carbon sphere can effectively Inhibit the dissolution of polysulfides in the electrolyte during charging and discharging, so the battery capacity retention rate is greatly improved. The porous carbon spheres for lithium-sulfur batteries in this embodiment are porous structures formed by disorderly winding of graphite ribbons, and the graphite ribbons are composed of multi-layer (10-20 layers) graphite sheets. There are more layers and higher stability, so it is not easy to be damaged during the charging and discharging process of the battery, and can remain intact for a long time, so the cycle stability is higher than that of the current commercial lithium-sulfur battery. Figure 17 is the rate performance of the positive electrode composed of porous carbon spheres-sulfur composites. It can be seen from the figure that the positive electrode composed of porous carbon spheres-sulfur composites has good rate performance, with a capacity of 750 mAh/g at 0.1C , after 30 cycles at different rates, the discharge capacity can still reach 500 mAh/g at 4C, which shows that the porous carbon spheres can greatly improve the conductivity of the sulfur electrode.

The above descriptions are only preferred implementations of the present invention, and the scope of protection of the present invention is not limited to the above examples. All technical solutions under the idea of the present invention belong to the protection scope of the present invention. It should be pointed out that for those skilled in the art, improvements and modifications without departing from the principle of the present invention should also be regarded as the protection scope of the present invention.

Claims (8)

1. A porous carbon sphere for a lithium-sulfur battery, characterized in that, the porous carbon sphere is formed by disorderly winding of ribbon-shaped graphite, and the ribbon-shaped graphite is composed of multilayer graphite sheets, and the multilayer graphite sheets The number of layers is 10-20 layers. 2. A preparation method of porous carbon spheres for lithium-sulfur batteries, comprising the following steps: (1) Preparation of Si-C-O particles: heat and vaporize low-molecular polycarbosilane, and then prepare Si-C-O particles by low-pressure chemical vapor deposition under the protection of inert gas; (2) Preparation of porous carbon spheres: the Si-C-O particles obtained in step (1) are subjected to high-temperature heat treatment under the protection of an inert gas to remove the oxygen and silicon elements in the Si-C-O particles to obtain porous carbon for lithium-sulfur batteries ball; In the step (2), the temperature of the high temperature heat treatment is 2600° C. to 2700° C., and the holding time is 1 h to 3 h. 3. The preparation method according to claim 2, characterized in that, in the step (1), the atomic ratio of silicon and carbon in the low-molecular polycarbosilane is 1:2, and the low-molecular polycarbosilane The number average molecular weight of carbosilane is 500-2000. 4. The preparation method according to claim 2 or 3, characterized in that, in the step (1), the temperature of the low-pressure chemical vapor deposition is 1000°C-1200°C, and the deposition time is 0.5h-3h. The pressure of the low-pressure chemical vapor deposition mentioned above is not higher than 1000 Pa. 5 . The preparation method according to claim 2 or 3 , characterized in that, in the step (1), the heating and gasification temperature of the low-molecular polycarbosilane is 120° C. to 180° C. 6 . 6. a porous carbon sphere for lithium-sulfur battery as claimed in claim 1 or the porous carbon sphere for lithium-sulfur battery that the preparation method according to any one of claims 2 to 5 makes in the lithium-sulfur battery application. 7. The application according to claim 6, characterized in that the application comprises mixing the porous carbon spheres for the lithium-sulfur battery with sulfur powder to prepare the positive electrode material for the lithium-sulfur battery. 8. The application according to claim 7, characterized in that the mass ratio of the porous carbon spheres for the lithium-sulfur battery to the sulfur powder is 3:2.