CN110158200B - Porous carbon nanofiber, preparation method thereof and lithium-sulfur battery - Google Patents

Abstract

The invention relates to the field of lithium-sulfur batteries, and discloses porous carbon nanofibers, a preparation method thereof, and a lithium-sulfur battery. A preparation method of porous carbon nanofibers, the method comprises the following steps: (1) mixing a pore-forming agent, a surface dispersant, a carbon source and an organic solvent to obtain a spinning solution; (2) mixing the spinning solution Electrospinning is performed to obtain carbon fibers; (3) the carbon fibers are carbonized, the pore-forming agent is removed, washed and dried. The porous carbon nanofibers prepared by the invention have good mechanical strength, high electrical conductivity and a crisscross network structure, and can be directly used as a good carrier for sulfur without the need for conductive agents and binders. At the same time, the porous carbon nanofibers have a large specific surface area and strong adsorption capacity, which can alleviate the dissolution of polysulfides in the electrolyte. The lithium-sulfur battery prepared by using the porous carbon nanofiber of the present invention has excellent cycle performance and rate performance.

Description

Porous carbon nanofibers, preparation method thereof, and lithium-sulfur battery

technical field

The invention relates to the field of lithium-sulfur battery materials, in particular to a porous carbon nanofiber and a preparation method thereof, and a lithium-sulfur battery prepared by using the above-mentioned porous carbon nanofiber.

Background technique

With the consumption of a large amount of fossil energy and the pollution to the environment, the battery plays an important role as a sustainable clean energy. At present, lithium-ion battery cathode materials cannot meet practical needs due to their low capacity. The theoretical specific capacity of lithium-sulfur battery cathode materials is as high as 1675mAh/g, so it is a promising cathode material. However, lithium-sulfur batteries also face several problems: (1) The intermediate products generated during the charging and discharging process, that is, polysulfides, are easily soluble in organic electrolytes, which reduces the utilization rate of the positive active material sulfur and reduces the battery cycle performance; (2) The ionic conductivity and electronic conductivity of sulfur are extremely low; (3) The volume change of sulfur in the process of ion de-intercalation causes the structure of the electrode material to be destroyed and the capacity to decline. The production process of traditional lithium-sulfur battery cathode materials is complex and requires precise control. Inactive substances such as conductive agents and binders required in the coating process reduce the relative content of active material sulfur, thereby reducing the energy density. At the same time, the binder is easy to fail during the charging and discharging process of the battery, causing the active material to separate from the current collector, resulting in poor battery rate performance and limiting the development of lithium-sulfur batteries.

SUMMARY OF THE INVENTION

The purpose of the present invention is to overcome the problem of poor performance of lithium-sulfur batteries in the prior art, to provide porous carbon nanofibers and a preparation method thereof, and a lithium-sulfur battery prepared by using the porous carbon nanofibers. Carbon nanofibers have good mechanical strength, high electrical conductivity, and a criss-cross network structure, and can directly serve as a good carrier for sulfur without the need for conductive agents and binders. At the same time, the porous carbon nanofibers have a large specific surface area and strong adsorption capacity, which can alleviate the dissolution of polysulfides in the electrolyte. The lithium-sulfur battery prepared by using the porous carbon nanofiber of the present invention has excellent cycle performance and rate performance.

In order to achieve the above purpose, a first aspect of the present invention provides a method for preparing porous carbon nanofibers, the method comprising the following steps:

(1) mixing pore former, surface dispersant, carbon source and organic solvent to obtain spinning solution;

(2) electrospinning the spinning solution to obtain carbon fibers;

(3) carbonizing the carbon fiber, removing the pore-forming agent, washing and drying.

Preferably, the pore-forming agent is silica microspheres, zinc oxide or calcium carbonate; preferably, the particle size of the pore-forming agent is 7-30 nm.

Preferably, the surface dispersant is one or more of sodium dodecylbenzenesulfonate, sodium dodecyl sulfate and polyoxyethylene polyoxypropylene ether block copolymer.

Preferably, the carbon source is one or more of polyacrylonitrile, polyvinylpyrrolidone, polyimide and phenolic resin.

Preferably, the organic solvent is one or more of N,N-dimethylformamide, N-methylpyrrolidone, N,N-dimethylacetamide and dimethylsulfoxide.

Preferably, the weight ratio of the pore-forming agent, surface dispersant, carbon source and organic solvent is (0.3-0.9):(0.9-2.7):1:(5-15).

Preferably, the mixing conditions include: the mixing temperature is 40-80° C., and the mixing time is 2-16 h.

Preferably, the conditions of the electrospinning include: the voltage is 15-30 kV, the distance between the collecting plate and the needle is 15-25 cm, and the advancing speed of the propelling pump is 0.8-1.2 mL/h.

Preferably, the carbonization conditions include: the carbonization gas is argon or nitrogen, the carbonization temperature is 800-1200°C, the heating rate is 2-5°C/min, and the carbonization time is 6-14h.

Preferably, the method for removing the pore-forming agent is to soak the carbonized carbon fibers in a hydrofluoric acid solution or a sodium hydroxide solution.

Preferably, after step (2) and before step (3), pre-oxidation is also performed.

Preferably, the pre-oxidation conditions include: a temperature of 200-220° C., a heating rate of 2-5° C./min, and a time of 1-2 h.

The second aspect of the present invention provides the porous carbon nanofibers prepared by the above method, wherein the porous carbon nanofibers have an average diameter of 500 nm-2 μm, have a porous structure, a specific surface area of 150-350 m ² /g, and an average pore diameter of 8-35nm.

A third aspect of the present invention provides a lithium-sulfur battery, wherein the battery includes a positive electrode, a negative electrode and a separator, wherein the positive electrode contains the above-mentioned porous carbon nanofibers.

In the present invention, by using a combination of pore-forming agent, surface dispersant, carbon source and organic solvent, and defining the content relationship among them, the prepared porous carbon nanofibers are shown in Figures 1 and 2, with uniform distribution and crisscrossing The network structure has the advantages of no agglomeration; each carbon nanofiber has a porous structure with an average pore size of 8-35nm, which can reach a specific surface area of 150-350m ² /g. As a result, carbon nanofibers have good mechanical properties and high electrical conductivity, and can be directly used as a good carrier for sulfur without the need for conductive agents and binders. Therefore, no coating process is required, and the mechanical strength is high. Compared with the use of binders A lithium-sulfur battery positive electrode material of a conductive agent and a current collector, the lithium-sulfur battery positive electrode material of the present invention has low cost, simple and convenient preparation method, and can be industrially produced on a large scale.

At the same time, porous carbon nanofibers have large specific surface area and strong adsorption capacity, which can adsorb polysulfides, inhibit the shuttle effect of polysulfides, relieve the dissolution of polysulfides in electrolyte, and can be used as a good carrier for sulfur. , the prepared flexible porous carbon fibers can be used as a carrier for the positive active material of lithium-sulfur batteries, exhibiting excellent electrochemical properties, such as excellent cycle performance and rate capability.

Description of drawings

Fig. 1 is the scanning electron microscope picture of porous carbon nanofiber of the present invention;

Fig. 2 is the enlarged porous carbon nanofiber scanning electron microscope image of the present invention;

Fig. 3 is the nitrogen adsorption and desorption curve of porous carbon fiber of the present invention;

Fig. 4 is the scanning electron microscope picture of the carbon nanofiber of comparative example 1;

Figure 5 is a graph of the 100-cycle cycle performance of a lithium-sulfur battery assembled with a porous carbon nanofiber flexible cathode material at a current density of 0.25C (1C=1672mA/g);

Figure 6 is a graph of the 100-cycle cycle performance of a lithium-sulfur battery assembled with a porous carbon nanofiber flexible cathode material at a current density of 1C (1C=1672 mA/g);

Figure 7 is a graph showing the rate performance of a lithium-sulfur battery assembled with a porous carbon nanofiber flexible cathode material.

Detailed ways

The endpoints of ranges and any values disclosed herein are not limited to the precise ranges or values, which are to be understood to encompass values proximate to those ranges or values. For ranges of values, the endpoints of each range, the endpoints of each range and the individual point values, and the individual point values can be combined with each other to yield one or more new ranges of values that Ranges should be considered as specifically disclosed herein.

A first aspect of the present invention provides a method for preparing porous carbon nanofibers, the method comprising the following steps:

(1) mixing pore former, surface dispersant, carbon source and organic solvent to obtain spinning solution;

(2) electrospinning the spinning solution to obtain carbon fibers;

(3) carbonizing the carbon fiber, removing the pore-forming agent, washing and drying.

According to the method of the present invention, the pore-forming agent may be silica microspheres, zinc oxide or calcium carbonate. Wherein, the particle size of the pore-forming agent may be 7-30 nm, and the porous carbon nanofibers prepared by the pore-forming agent within this particle size range have a better porous structure and a relatively uniform pore size.

According to the method of the present invention, the silica microspheres can be commercially available monodisperse amino silica microspheres or core-shell silica magnetic microspheres and the like.

According to the method of the present invention, the surface dispersant may be one or more of sodium dodecylbenzenesulfonate, sodium dodecyl sulfate and polyoxyethylene polyoxypropylene ether block copolymer. The synergistic effect of the surface dispersant, the pore-forming agent, the carbon source and the organic solvent makes the prepared porous carbon nanofibers evenly distributed and have a crisscross network structure.

According to the method of the present invention, the carbon source may be one or more of polyacrylonitrile, polyvinylpyrrolidone, polyimide and phenolic resin. Wherein, the number-average molecular weight of the polyacrylonitrile is 500,000 to 3 million, preferably 1.5 million to 2 million; the number-average molecular weight of the polyvinylpyrrolidone is 500,000 to 3 million, preferably 1.5 million to 2 million; The number average molecular weight of the polyimide is 500,000 to 3 million, preferably 1.5 million to 2 million; the number average molecular weight of the phenolic resin is 500,000 to 3 million, preferably 1.5 million to 2 million.

According to the method of the present invention, the organic solvent may be one or more of N,N-dimethylformamide, N-methylpyrrolidone, N,N-dimethylacetamide and dimethylsulfoxide .

According to the method of the present invention, the weight ratio of the pore-forming agent, the surface dispersant, the carbon source and the organic solvent is (0.3-0.9): (0.9-2.7): 1: (5-15). The porous carbon nanofibers prepared within the range have large specific surface area and strong adsorption capacity, and the porous carbon nanofibers are uniformly distributed.

According to the method of the present invention, the mixing conditions are for the purpose of fully contacting the pore-forming agent, the surface dispersant, the carbon source and the organic solvent. For example, the mixing conditions may include but are not limited to: the mixing temperature is 40-80° C., The time is 2-16h.

According to the method of the present invention, the conditions of the electrospinning are aimed at making the final solidification into fibers, and the conditions of electrospinning include but are not limited to: the voltage is 15-30kV, the distance between the collecting plate and the needle is 15-25cm, The advance rate of the propelling pump was 0.8-1.2 mL/h. In the present invention, the electrospinning method is convenient for industrialized large-scale production, and the prepared carbon fiber has a large area and good flexibility, and can be used as an unsupported electrode carrier for a lithium-sulfur battery.

According to the method of the present invention, the carbonization conditions include but are not limited to: the carbonization gas is argon or nitrogen, the carbonization temperature is 800-1200°C, the heating rate is 2-5°C/min, and the carbonization time is 6-14h. In the present invention, if pre-oxidation is performed before carbonization, the heating rate during carbonization is preferably 4-5 °C/min; if pre-oxidation is not performed before carbonization, the heating rate during carbonization is preferably 2-3 °C/min.

According to the method of the present invention, the method for removing the pore-forming agent may be to soak the carbonized carbon fibers in a hydrofluoric acid solution or a sodium hydroxide solution. The soaking conditions, such as soaking time, are not particularly limited, and the purpose is to be able to remove the pore-forming agent.

According to the method of the present invention, after step (2) and before step (3), pre-oxidation can also be performed. Preferably, the pre-oxidation conditions may include, but are not limited to: a temperature of 200-220° C., a heating rate of 2-5° C./min, and a time of 1-2 h. In the present invention, the pre-oxidation step can stabilize the carbon fiber structure collected by spinning.

According to the method of the present invention, the drying conditions are aimed at obtaining porous carbon nanofibers, and the drying conditions may include, but are not limited to: the drying temperature is 100-120° C., and the drying time is 8-16 h.

According to the method of the present invention, the preparation method of porous carbon nanofibers may comprise the following steps:

(1) mixing the pore-forming agent and the surface dispersing agent in an organic solvent, ultrasonicating with a cell crusher after stirring, repeating multiple times, then adding a carbon source, and stirring to obtain a spinning solution;

(2) electrospinning the spinning solution to obtain carbon fibers;

(3) carbonizing the carbon fiber, removing the pore-forming agent, washing and drying, wherein, before carbonization, pre-oxidation is optionally performed.

The second aspect of the present invention provides the porous carbon nanofibers prepared by the above method, wherein the porous carbon nanofibers have an average diameter of 500 nm-2 μm, have a porous structure, a specific surface area of 150-350 m ² /g, and an average pore diameter of 8-35nm.

In the present invention, the porous carbon nanofibers can be shown in Figures 1 and 2. It can be seen from the figures that the porous carbon nanofibers of the present invention have a relatively uniform diameter and rough surface, and can be used as a three-dimensional conductive carrier of sulfur, which is beneficial to The transport of lithium ions and electrons in the electrochemical reaction process, and due to the large three-dimensional space, can better absorb the poly-liquid sulfur electrolyte and increase the quality of the active material. Moreover, the porous carbon nanofiber of the present invention has excellent flexibility, can be used as a positive electrode material carrier for soft pack batteries, and is a good choice as a positive electrode carrier for lithium liquid sulfur batteries.

A third aspect of the present invention provides a lithium-sulfur battery, wherein the battery includes a positive electrode, a negative electrode and a separator, wherein the positive electrode contains the above-mentioned porous carbon nanofibers.

Specifically, the porous carbon nanofibers of the present invention are used as the carrier of the positive electrode material.

The present invention will be described in detail below by means of examples.

In the following examples, silica microspheres were purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd., and the batch was SLBP7956V;

Sodium dodecylbenzenesulfonate was purchased from Beijing Taize Jiaye Technology Development Co., Ltd., batch D1401036;

N,N-dimethylformamide was purchased from Beijing Chemical Reagent Factory, batch E1514039;

The cell crusher was purchased from Ningbo Xinzhi Biotechnology Co., Ltd., model SCIENTZ-IID;

Polyacrylonitrile was purchased from Beijing Bailingwei Technology Co., Ltd., the batch is LH80Q67, and the number average molecular weight is 1.5 million;

Zinc oxide was purchased from Shanghai Tengzhun Biotechnology Co., Ltd., batch W12A051;

Polyoxyethylene polyoxypropylene ether block copolymer was purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd., batch SLBL1780V;

N-methylpyrrolidone was purchased from Sinopharm Chemical Reagent Co., Ltd., batch 20170803;

Polyvinylpyrrolidone was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., the batch is D1613027, and the number-average molecular weight and weight-average molecular weight are 1.5 million;

The spinning machine was purchased from Beijing Jite Pullen Biotechnology Co., Ltd., model ET-1334H;

Scanning electron microscope was purchased from Hitachi High-tech Co., Ltd., model Hitachi SU8020;

The specific surface area analyzer was purchased from MacMeretic Equipment Co., Ltd., model number ASAP2020.

Example 1

(1) Preparation of porous carbon nanofibers

Mix 0.69 g of silica microspheres with a particle size of 7 nm and 2.07 g of sodium dodecylbenzenesulfonate in 18 mL of N,N-dimethylformamide organic solvent, stir the solution for 10 min, and use The cell crusher was sonicated for 30 min, and the solution was dispersed evenly by doing this twice, that is, stirring again for 10 min, and sonicating for 30 min. Then, 1.8 g of polyacrylonitrile was added, and the solution was heated and stirred at 70° C. for 12 h until the solution became a homogeneous state, and there was no viscous spinning solution of insoluble matter.

The above spinning solution was transferred into a 20 mL disposable syringe, the model of the spinning needle of the spinning machine was 21, and the syringe needle was connected to the positive high pressure of the spinning machine. The aluminum foil is a spinning collection plate, and the aluminum foil is connected to the negative high voltage. The distance between the collecting plate and the needle was 20 cm, the spinning voltage was 15 kV, and the advancing speed of the propelling pump was 1.0 mL/h to obtain carbon fibers.

The carbon fibers collected by spinning were peeled off from the collection plate, transferred to a muffle furnace for pre-oxidation, raised to 200°C at a heating rate of 5°C/min, and kept for 1 h. The pre-oxidized carbon fibers were subjected to high-temperature carbonization treatment. The carbonization treatment conditions included: the flow rate of argon gas was 100 sccm, and then the temperature was raised to 1100 °C at a heating rate of 5 °C/min, and the temperature was naturally cooled after holding for 12 h. Finally, the carbonized carbon fibers were soaked in 10 wt% hydrofluoric acid to remove the silica pore-forming agent. The carbon fibers were washed with deionized water by suction filtration, and dried at 110 °C for 12 h under vacuum conditions to obtain porous carbon nanomaterials. fiber.

(2) The obtained porous carbon nanofibers were characterized and tested for nitrogen adsorption and desorption:

The obtained porous carbon nanofibers are observed under a scanning electron microscope, and the scanning electron microscope image as shown in FIG. 1 and the enlarged scanning electron microscope image as shown in FIG. 2 are obtained. It can be seen from FIG. 1 that the porous carbon nanofibers of the present invention are distributed in layers and intertwined, and the diameter of the carbon fibers is relatively uniform and the surface is relatively rough. It can be seen from FIG. 2 that the average diameter of the carbon nanofibers is 900 nm, and the fibers have a porous structure.

Nitrogen adsorption and desorption curve of porous carbon nanofibers: after weighing the mass of the sample, vacuum heating and degassing at 250 °C for 4 h, and weighing again to determine the lost mass. The vacuum condition is 500 μmHg (about 0.67 bar), and the analysis test During the process, the sample is placed in a liquid nitrogen environment. Using a specific surface area analyzer to calculate the specific surface area, the nitrogen adsorption and desorption curve as shown in Figure 3 is obtained. It can be seen from Figure 3 that the nitrogen adsorption and desorption curve of the prepared porous carbon fiber belongs to a typical IV type curve. There are obvious H3 hysteresis loops in the figure, indicating the existence of mesopores (2-50nm) (IUPAC classification) in the hierarchical porous carbon, and in the higher relative pressure range (P/P ₀ =0.7-1.0), The adsorption-desorption curve has an obvious upward trend, which is caused by capillary condensation. According to the BJH (the Barrett-Joyner-Halenda) pore size distribution calculation, the specific surface area of the porous carbon fiber is 275.5 m ² /g, and the average pore size is 8.8 nm.

Example 2

(1) Preparation of porous carbon nanofibers

Mix 0.69 g of zinc oxide with a particle size of 7 nm and 2.07 g of polyoxyethylene polyoxypropylene ether block copolymer in 18 mL of N-methylpyrrolidone organic solvent, stir the solution for 10 min, and use a cell crusher to sonicate 30min, do this twice to make the solution evenly dispersed, that is, stir again for 10min, and sonicate for 30min. Then, 1.8 g of polyvinylpyrrolidone was added, and the solution was heated and stirred at 70° C. for 12 h until the solution became a homogeneous, viscous spinning solution with no insoluble matter.

The above spinning solution was transferred into a 20 mL disposable syringe, the model of the spinning needle of the spinning machine was 21, and the syringe needle was connected to the positive high pressure of the spinning machine. The aluminum foil is a spinning collection plate, and the aluminum foil is connected to the negative high voltage. The distance between the collecting plate and the needle was 25 cm, the spinning voltage was 30 kV, and the advancing speed of the propelling pump was 1.2 mL/h to obtain carbon fibers.

The carbon fibers collected by spinning were peeled off from the collection plate, transferred to a muffle furnace for pre-oxidation, raised to 200°C at a heating rate of 5°C/min, and kept for 1 h. The pre-oxidized carbon fibers were subjected to high-temperature carbonization treatment. The carbonization treatment conditions included: the flow rate of argon gas was 100 sccm, and then the temperature was raised to 1100 °C at a heating rate of 5 °C/min, and the temperature was naturally cooled after holding for 12 h. Finally, the carbonized carbon fibers were soaked in hydrochloric acid solution to remove the zinc oxide pore-forming agent, and the carbon fibers were washed with deionized water by suction filtration, and dried at 110 °C for 12 h under vacuum conditions to obtain porous carbon nanofibers.

(2) The obtained porous carbon nanofibers were characterized and tested for nitrogen adsorption and desorption:

Characterization was carried out according to the method of Example 1, and porous carbon nanofibers similar to those shown in FIG. 1 and FIG. 2 were obtained, and the average diameter of the porous carbon nanofibers was 900 nm.

The nitrogen adsorption and desorption test was carried out according to the method of Example 1, and it was calculated that the specific surface area of the porous carbon fiber was 190.3 m ² /g, and the average pore diameter was 8.2 nm.

Example 3

Mix 0.3 g of silica microspheres with a particle size of 7 nm and 0.9 g of sodium dodecylbenzenesulfonate in 5 mL of N,N-dimethylformamide organic solvent, stir the solution for 10 min, and use The cell crusher was sonicated for 30 min, and the solution was dispersed evenly by doing this twice, that is, stirring again for 10 min, and sonicating for 30 min. Then, 1 g of polyacrylonitrile was added, and the solution was heated and stirred at 70° C. for 12 h until the solution became a homogeneous, viscous spinning solution with no insoluble matter.

The above spinning solution was transferred into a 20 mL disposable syringe, the model of the spinning needle of the spinning machine was 21, and the syringe needle was connected to the positive high pressure of the spinning machine. The aluminum foil is a spinning collection plate, and the aluminum foil is connected to the negative high voltage. The distance between the collecting plate and the needle was 15 cm, the spinning voltage was 25 kV, and the advancing speed of the propelling pump was 0.8 mL/h to obtain carbon fibers.

The carbon fibers collected by spinning were peeled off from the collection plate, transferred to a muffle furnace for pre-oxidation, raised to 200°C at a heating rate of 5°C/min, and kept for 1 h. The pre-oxidized carbon fibers were subjected to high-temperature carbonization treatment. The carbonization treatment conditions included: nitrogen gas flow was 100 sccm, then the temperature was raised to 1100 °C at a heating rate of 5 °C/min, and the temperature was naturally cooled after holding for 12 h. Finally, the carbonized carbon fibers were soaked in 10 wt% hydrofluoric acid to remove the silica pore-forming agent, the carbon fibers were washed with deionized water by suction filtration, and dried at 110 °C for 12 h under vacuum conditions to obtain porous carbon nanomaterials. fiber.

(2) The obtained porous carbon nanofibers were characterized and tested for nitrogen adsorption and desorption:

Characterization was carried out according to the method of Example 1, and a structure similar to that shown in Figure 1 and Figure 2 was obtained, and the average diameter of the porous carbon nanofibers was 500 nm.

The nitrogen adsorption and desorption test was carried out according to the method of Example 1, and it was calculated that the specific surface area of the porous carbon fiber was 166.7 m ² /g, and the average pore diameter was 8.2 nm.

Example 4

Mix 0.9 g of silica microspheres with a particle size of 7 nm and 2.7 g of sodium dodecylbenzenesulfonate in 15 mL of N,N-dimethylformamide organic solvent, stir the solution for 10 min, and use The cell crusher was sonicated for 30 min, and the solution was dispersed evenly by doing this twice, that is, stirring again for 10 min, and sonicating for 30 min. Then, 1 g of polyacrylonitrile was added, and the solution was heated and stirred at 70° C. for 12 h until the solution became a homogeneous, viscous spinning solution with no insoluble matter.

The above spinning solution was transferred into a 20 mL disposable syringe, the model of the spinning needle of the spinning machine was 21, and the syringe needle was connected to the positive high pressure of the spinning machine. The aluminum foil is a spinning collection plate, and the aluminum foil is connected to the negative high voltage. The distance between the collecting plate and the needle was 20 cm, the spinning voltage was 15 kV, and the advancing speed of the propelling pump was 1.0 mL/h to obtain carbon fibers.

The carbon fibers collected by spinning are peeled off from the collecting plate and subjected to high temperature carbonization treatment. The carbonization treatment conditions include: the flow rate of argon gas is 100sccm, and then the temperature rises to 1100°C at a heating rate of 2°C/min, and the temperature is naturally cooled after holding for 12 hours. . Finally, the carbonized carbon fibers were soaked in 10 wt% hydrofluoric acid to remove the silica pore-forming agent, the carbon fibers were washed with deionized water by suction filtration, and dried at 110 °C for 12 h under vacuum conditions to obtain porous carbon nanomaterials. fiber.

(2) The obtained porous carbon nanofibers were characterized and tested for nitrogen adsorption and desorption:

Characterization was carried out according to the method of Example 1, and a structure similar to that shown in Figure 1 and Figure 2 was obtained, and the average diameter of the porous carbon nanofibers was 1 μm.

The nitrogen adsorption and desorption test was carried out according to the method of Example 1, and it was calculated that the specific surface area of the porous carbon fiber was 189.6 m ² /g, and the average pore diameter was 8.8 nm.

Comparative Example 1

According to the method of the embodiment, the difference is that without using sodium dodecylbenzene sulfonate, agglomeration easily occurs in the solvent, so that the needle is easily blocked during solution spinning, and the spinning cannot be carried out for a long time, and in the obtained carbon fiber The surface is prone to lumps.

The characterization was carried out according to the method of Example 1, and the results are shown in Figure 4. It can be seen from Figure 4 that the distribution of the pore structure on the carbon fiber is very uneven, and the diameter is uneven.

Comparative Example 2

Mix 0.2 g of silica microspheres with a particle size of 7 nm and 0.9 g of sodium dodecylbenzenesulfonate in 5 mL of N,N-dimethylformamide organic solvent, stir the solution for 10 min, and use The cell crusher was sonicated for 30 min, and the solution was dispersed evenly by doing this twice, that is, stirring again for 10 min, and sonicating for 30 min. Then, 1 g of polyacrylonitrile was added, and the solution was heated and stirred at 70° C. for 12 h until the solution became a homogeneous, viscous spinning solution with no insoluble matter.

The above spinning solution was transferred into a 20 mL disposable syringe, the model of the spinning needle of the spinning machine was 21, and the syringe needle was connected to the positive high pressure of the spinning machine. The aluminum foil is a spinning collection plate, and the aluminum foil is connected to the negative high voltage. The distance between the collecting plate and the needle was 20 cm, the spinning voltage was 15 kV, and the advancing speed of the propelling pump was 1.0 mL/h to obtain carbon fibers.

The carbon fibers collected by spinning were peeled off from the collection plate, transferred to a muffle furnace for pre-oxidation, raised to 200°C at a heating rate of 5°C/min, and kept for 1 h. The pre-oxidized carbon fibers were subjected to high-temperature carbonization treatment. The carbonization treatment conditions included: the flow rate of argon gas was 100 sccm, then the temperature was raised to 1100 °C at a heating rate of 5 °C/min, and the temperature was naturally cooled after holding for 12 h. Finally, the carbonized carbon fibers were soaked in 10 wt% hydrofluoric acid to remove the silica pore-forming agent. The carbon fibers were washed with deionized water by suction filtration, and dried at 110 °C for 12 h under vacuum conditions to obtain porous carbon nanomaterials. fiber.

(2) The obtained porous carbon nanofibers were characterized and tested for nitrogen adsorption and desorption:

Characterized according to the method of Example 1, the average diameter of the porous carbon nanofibers is 400 nm.

The nitrogen adsorption and desorption test was carried out according to the method of Example 1, and it was calculated that the specific surface area of the porous carbon fiber was 66.7 m ² /g, and the average pore diameter was 8.5 nm.

Comparative Example 3

Mix 1 g of silica microspheres with a particle size of 7 nm and 2.7 g of sodium dodecylbenzenesulfonate in 15 mL of N,N-dimethylformamide organic solvent. The crusher was sonicated for 30 min, and the solution was dispersed evenly by doing this twice, that is, stirring again for 10 min and sonicating for 30 min. Then, 1 g of polyacrylonitrile was added, and the solution was heated and stirred at 70° C. for 12 h until the solution became a homogeneous, viscous spinning solution with no insoluble matter.

The above spinning solution was transferred into a 20 mL disposable syringe, the model of the spinning needle of the spinning machine was 21, and the syringe needle was connected to the positive high pressure of the spinning machine. The aluminum foil is a spinning collection plate, and the aluminum foil is connected to the negative high voltage. The distance between the collecting plate and the needle was 20 cm, the spinning voltage was 15 kV, and the advancing speed of the propelling pump was 1.0 mL/h to obtain carbon fibers. The needle is prone to clogging when spinning.

The carbon fibers collected by spinning were peeled off from the collection plate, transferred to a muffle furnace for pre-oxidation, raised to 200°C at a heating rate of 5°C/min, and kept for 1 h. The pre-oxidized carbon fibers were subjected to high-temperature carbonization treatment. The carbonization treatment conditions included: the flow rate of argon gas was 100 sccm, and then the temperature was raised to 1100 °C at a heating rate of 5 °C/min, and the temperature was naturally cooled after holding for 12 h. Finally, the carbonized carbon fibers were soaked in 10 wt% hydrofluoric acid to remove the silica pore-forming agent, the carbon fibers were washed with deionized water by suction filtration, and dried at 110 °C for 12 h under vacuum conditions to obtain porous carbon nanomaterials. fiber.

(2) The obtained porous carbon nanofibers were characterized and tested for nitrogen adsorption and desorption:

Characterized according to the method of Example 1, the diameters of the porous carbon nanofibers are not uniform, and the diameters are distributed in the range of 300 nm-1 μm.

The nitrogen adsorption and desorption test was carried out according to the method of Example 1, and it was calculated that the specific surface area of the porous carbon fiber was 50.8 m ² /g, and the average pore diameter was 8.9 nm.

Test Example 1

(1) Preparation of polysulfide electrolyte

1 mol/L lithium bis(trifluoromethanesulfonyl)imide and 0.5 mol lithium nitrate were added to the mixed solution of 1,3-dioxolane and ethylene glycol dimethyl ether (volume ratio was 1:1). Pour sulfur and lithium sulfide with a molar ratio of 5:1 into the above solution, and then stir in an oil bath for 24 hours at a temperature of 60 °C until the solution is stirred uniformly to obtain a Li ₂ S ₆ polysulfide electrolyte with a sulfur concentration of 2 mol/L.

(2) Assembling the lithium-sulfur button battery

The C2032 model coin cell battery was assembled in a glove box filled with argon gas with both water and oxygen content below 0.1 ppm. The porous carbon nanofibers obtained in Example 1 were punched into discs with a diameter of 1.2 cm and a mass of 2 mg, and the above-mentioned polysulfide electrolyte was added dropwise at a ratio of 15 μL/mg as the positive electrode of the lithium-sulfur battery, and the porous membrane Celgard 2400 was used as the separator. , metal lithium is used as the negative electrode of the battery, and a lithium-sulfur button battery is assembled.

(3) Charge and discharge cycle test

The lithium-sulfur coin cell was first discharged to 1.6V, and then cycled between 1.6V and 2.8V. The discharge specific capacity of lithium-sulfur batteries is calculated based on the mass of the positive active material sulfur.

(a) Under the current density of 0.25C (1C=1672mA/g), the first discharge specific capacity of the battery and the discharge specific capacity after 100 cycles were tested. The results are shown in Figure 5. It can be seen from Figure 5 that the first discharge specific capacity It is 924.6mAh/g, the specific capacity after 100 cycles is 709.5mAh/g, and the capacity retention rate is 84.3% (compared with the discharge specific capacity in the second cycle).

(b) Under the high current density of 1C (1C=1672mA/g), the first discharge specific capacity of the battery and the discharge specific capacity after 100 cycles were detected. The results are shown in Figure 6. It can be seen from Figure 6 that the first discharge ratio The capacity was 715.6 mAh/g, the specific capacity after 100 cycles was 510.2 mAh/g, and the capacity retention rate was 82% (compared with the second-cycle discharge specific capacity).

(c) After 24 cycles of rate discharge at room temperature, the capacity remains at 641.1mAh/g at a high rate of 1C, and after 30 cycles of rate discharge, the capacity remains at 572.8mAh/g at a high rate of 2C. When the current density returns to small When the rate is 0.25C, the capacity is 761.5mAh/g, as shown in Figure 7.

Test Example 2-4

According to the method of Test Example 1, except that the porous carbon nanofibers of Example 1 were replaced with the porous carbon nanofibers of Examples 2-4, the results were similar to those of Test Example 1.

Test Comparative Example 1

The method of Test Example 1 was followed, except that the porous carbon nanofibers of Example 1 were replaced with the porous carbon nanofibers of Comparative Example 1.

Under the current density of 0.25C (1C=1672mA/g), the specific capacity of the first discharge is 734.7mAh/g, the specific capacity after 100 cycles is 513.5mAh/g, and the capacity retention rate is 89% (compared to the specific capacity of the second cycle discharge) compared to).

Under the high current density of 1C (1C=1672mA/g), the specific capacity of the first discharge is 583.8mAh/g, the specific capacity after 100 cycles is 412.4mAh/g, and the capacity retention rate is 73.4% (compared to the second cycle discharge ratio) capacity compared).

Test Comparative Example 2

The method of Test Example 1 was followed, except that the porous carbon nanofibers of Example 1 were replaced with the porous carbon nanofibers of Comparative Example 2.

Under the current density of 0.25C (1C=1672mA/g), the specific capacity of the first discharge was 741.7mAh/g, the specific capacity after 100 cycles was 534.9mAh/g, and the capacity retention rate was 90.6% (compared to the specific capacity of the second cycle of discharge) compared to).

Under the high current density of 1C (1C=1672mA/g), the specific capacity of the first discharge is 608.2mAh/g, the specific capacity after 100 cycles is 440.8mAh/g, and the capacity retention rate is 76.3% (compared to the second cycle discharge ratio) capacity compared).

Test Comparative Example 3

The method of Test Example 1 was followed, except that the porous carbon nanofibers of Example 1 were replaced with the porous carbon nanofibers of Comparative Example 3.

Under the current density of 0.25C (1C=1672mA/g), the specific capacity of the first discharge is 820.4mAh/g, the specific capacity after 100 cycles is 618.5mAh/g, and the capacity retention rate is 81.2% (compared to the specific capacity of the second cycle discharge) compared to).

Under the high current density of 1C (1C=1672mA/g), the specific capacity of the first discharge is 663.9mAh/g, the specific capacity after 100 cycles is 406.2mAh/g, and the capacity retention rate is 73.3% (compared to the second cycle discharge ratio) capacity compared).

It can be seen from Examples 1-4 and Comparative Examples 1-3 that the porous carbon nanofibers of the present invention are uniformly distributed, have a crisscross network structure, and have no agglomeration phenomenon; each carbon nanofiber has a porous structure with an average pore size It is 8-35nm and can reach a specific surface area of 150-350m ² /g, that is, the porous carbon nanofibers of the present invention have a large specific surface area and strong adsorption capacity. In contrast, document 1 (without using surfactant) is prone to agglomeration in the solvent, which makes the needle easily blocked during solution spinning, making it impossible to spin for a long time, and the surface of the obtained carbon fiber is prone to agglomeration, and the carbon fiber is prone to lumps. The pore structure distribution is very uneven. Comparative examples 2 and 3 (the feed weight ratio of pore-forming agent, surface dispersant, carbon source and organic solvent is not within the scope of the present invention) exists that the needle is prone to plugging and the diameter of the porous carbon nanofibers is not uniform when spinning. , the surface is not smooth and prone to bulge and block, and the specific surface area is small.

It can be seen from Test Examples 1-4 and Test Comparative Examples 1-3 that the lithium-sulfur battery prepared by using the porous carbon nanofibers of the present invention has a first discharge specific capacity of 0.25C (1C=1672mA/g) at a current density of 924.6mAh/g, and the specific capacity after 100 cycles is 709.5mAh/g. Under the high current density of 1C (1C=1672mA/g), the specific capacity of the first discharge is 715.6mAh/g, and the specific capacity after 100 cycles is 510.2mAh/g. After 24 cycles of rate discharge at room temperature, the capacity remains at 641.1mAh/g at a high rate of 1C, and after 30 cycles of rate discharge, the capacity remains at 572.8mAh/g at a high rate of 2C, when the current density returns to a small rate of 0.25C again , the capacity was 761.5 mAh/g, and both showed excellent rate performance. It can be seen that the lithium-sulfur batteries prepared by using the porous carbon nanofibers of the present invention obviously have excellent cycle performance and rate performance compared with the lithium-sulfur batteries prepared by using the porous carbon nanofibers of Comparative Examples 1-3.

The preferred embodiments of the present invention have been described above in detail with reference to the accompanying drawings, however, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, a variety of simple modifications can be made to the technical solutions of the present invention, including combining various technical features in any other suitable manner. These simple modifications and combinations should also be regarded as the content disclosed in the present invention. All belong to the protection scope of the present invention.

Claims (12)

1. A preparation method of porous carbon nanofiber comprises the following steps:

(1) mixing a pore-forming agent, a surface dispersant, a carbon source and an organic solvent to obtain a spinning solution;

(2) carrying out electrostatic spinning on the spinning solution to obtain carbon fibers;

(3) carbonizing the carbon fiber, removing a pore-forming agent, washing and drying;

wherein the charging weight ratio of the pore-forming agent, the surface dispersant, the carbon source and the organic solvent is (0.3-0.9): (0.9-2.7): 1: (5-15);

wherein the pore-forming agent is silicon dioxide microspheres, zinc oxide or calcium carbonate;

the surface dispersant is one or more of sodium dodecyl benzene sulfonate, sodium dodecyl sulfate and polyoxyethylene polyoxypropylene ether block copolymer;

the organic solvent is one or more of N, N-dimethylformamide, N-methylpyrrolidone, N-dimethylacetamide and dimethyl sulfoxide;

the carbon source is one or more of polyacrylonitrile, polyvinylpyrrolidone, polyimide and phenolic resin.

2. The method of claim 1, wherein the pore former has a particle size of 7-30 nm.

3. The method according to claim 1, wherein the polyacrylonitrile has a number average molecular weight of 50 to 300 ten thousand, and the polyvinylpyrrolidone has a number average molecular weight of 50 to 300 ten thousand; the number average molecular weight of the polyimide is 50 to 300 ten thousand; the phenolic resin has a number average molecular weight of 50 to 300 ten thousand.

4. The method according to claim 3, wherein the polyacrylonitrile has a number average molecular weight of 150 to 200 ten thousand, and the polyvinylpyrrolidone has a number average molecular weight of 150 to 200 ten thousand; the number average molecular weight of the polyimide is 150 to 200 ten thousand; the phenolic resin has a number average molecular weight of 150 to 200 ten thousand.

5. The method of claim 1, wherein the conditions of mixing comprise: the mixing temperature is 40-80 ℃, and the mixing time is 2-16 h.

6. The method of claim 1, wherein the electrospinning conditions comprise: the voltage is 15-30kV, the distance between the collecting plate and the needle is 15-25cm, and the advancing speed of the propulsion pump is 0.8-1.2 mL/h.

7. The method of claim 1, wherein the carbonizing conditions comprise: the carbonization gas is argon or nitrogen, the carbonization temperature is 800-1200 ℃, the heating rate is 2-5 ℃/min, and the carbonization time is 6-14 h.

8. The method according to claim 1, wherein the pore former is removed by immersing the carbonized carbon fiber in a hydrofluoric acid solution or a sodium hydroxide solution.

9. The method of claim 1, wherein after step (2) and before step (3), further performing a pre-oxidation.

10. The method of claim 9, wherein the pre-oxidation conditions comprise: the temperature is 200 ℃ and 220 ℃, the heating rate is 2-5 ℃/min, and the time is 1-2 h.

11. Porous carbon nanofiber prepared by the method as claimed in any one of claims 1 to 10, wherein the porous carbon nanofiber has an average diameter of 500nm to 2 μm, a porous structure, a specific surface area of 150-350m²(ii)/g, the average pore diameter is 8-35 nm.

12. A lithium-sulfur battery comprising a positive electrode, a negative electrode and a separator, wherein the positive electrode contains the porous carbon nanofiber according to claim 11.

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