CN110459755B

CN110459755B - Sulfur/polypyrrole/graphene/carbon nanotube composite film, preparation method and application thereof

Info

Publication number: CN110459755B
Application number: CN201910842232.4A
Authority: CN
Inventors: 苏岳锋; 姚金雨; 卢赟; 陈来; 包丽颖; 赵双义; 赵晨颖; 史宏娟; 陈实; 吴锋
Original assignee: Beijing Institute of Technology BIT
Current assignee: Beijing Institute of Technology BIT
Priority date: 2019-09-06
Filing date: 2019-09-06
Publication date: 2020-12-29
Anticipated expiration: 2039-09-06
Also published as: CN110459755A

Abstract

The invention provides a sulfur/polypyrrole/graphene/carbon nanotube composite film, a preparation method and application thereof, and belongs to the technical field of chemical energy storage batteries. Polypyrrole in the film is grafted on reduced graphene oxide; the reduced graphene oxide and the functionalized carbon nanotube are interwoven to form a three-dimensional carbon skeleton; elemental sulfur is supported in the three-dimensional carbon skeleton. According to the method, self-assembly is initiated by using an oxidation-reduction reaction between pyrrole and graphene oxide, meanwhile, a functionalized multi-walled carbon nanotube is used as a second carbon skeleton to provide an ion/electron rapid transmission channel, and a flexible self-supporting composite film is constructed through one-step vacuum filtration and post-sulfur loading. By utilizing the strong adsorbability of polypyrrole to polysulfide and the synergistic effect of the interwoven three-dimensional conductive framework constructed by graphene and carbon nano tubes, the problems of inherent non-conductivity of elemental sulfur, shuttle effect of polysulfide and the like can be solved, and the electrochemical performance of the lithium-sulfur battery is further improved.

Description

Sulfur/polypyrrole/graphene/carbon nanotube composite film, preparation method and application thereof

Technical Field

The invention provides a sulfur/polypyrrole/graphene/carbon nanotube composite film, a preparation method and application thereof, and belongs to the technical field of chemical energy storage batteries.

Background

In order to alleviate the environmental pollution problem and reduce the dependence on fossil fuels, the development and utilization of renewable alternative energy technologies are urgently needed. Secondary batteries, represented by lithium ion batteries, are one of the most viable options in electrochemical energy storage devices. With the rapid development of mobile electronic devices, electric automobiles, emerging wearable electric technologies and other related fields, commercial batteries on the market cannot meet the requirements, and people also put forward higher requirements on the research and development of battery technologies. Therefore, environmentally friendly secondary batteries having high volumetric energy density, high safety, long cycle life, and low cost have been the focus of research in recent years. Lithium sulfur batteries, as a promising competitor in the next generation of rechargeable battery systems, rely on the unique advantages of sulfur positive electrodes: the ultrahigh theoretical energy density is 2600W h/kg or 2800W h/L (3-5 times of that of the traditional intercalation anode); the relatively safe low working voltage is 2.15V; meanwhile, the elemental sulfur is one of the most abundant elements on the earth, the theoretical specific capacity of the elemental sulfur can reach 1675mA h/g, the cost is low, and the elemental sulfur is environment-friendly.

However, the commercial application and mass production of lithium-sulfur battery positive electrode materials are still hindered by various aspects, such as (1) elemental sulfur and its final discharge product are electron/ion insulators, resulting in low utilization of active materials; (2) the dissolution of the intermediate product lithium polysulfide and the shuttle effect bring about side reaction circulation, which leads to the irreversible loss of active substances, reduces the coulombic efficiency of the battery and leads the capacity of the battery to be rapidly attenuated; (3) volume expansion/contraction of sulfur during charge and discharge causes pulverization and structural destruction of the electrode material, which further results in poor interface contact between the active material and the current collector and deterioration of battery cycle stability.

In order to solve the above problems, researchers at home and abroad have taken a series of measures to design a high-performance lithium sulfur battery having a complicated composition and a reasonable structure. The most common strategy is to combine elemental sulfur with highly conductive nanostructured composites to enhance reaction kinetics and improve electrochemical performance. Carbon-based materials have received a great deal of attention because of their desirable physicochemical properties, including: (1) the rich porosity can realize high sulfur load and relieve volume expansion in the discharging process; (2) the large specific surface area can physically block polysulfide, and the controllable surface chemistry can anchor and adsorb polysulfide; (3) high conductivity ensures fast ion/electron transport and improves sulfur utilization. Based on these advantages, the functional carbon-based material plays an important role in improving the capacity, cycle performance, and the like of the battery. In addition, the conductive polymer is also widely concerned due to the characteristics of rich appearance, high conductivity, easy synthesis, good environmental stability and the like. In recent years, there have been reports on the use of carbon-based material/conductive polymer composite positive electrode material in lithium sulfur batteries: patent application CN105070887A reports a lithium-sulfur battery positive electrode material, which is characterized in that the material comprises a sulfur/graphene oxide/CNTs composite, the sulfur/graphene oxide/CNTs composite is coated with a conductive polymer layer, and the conductive polymer layer is coated with an adhesive layer. Patent application CN106450245A reports a preparation method of a flexible chargeable and dischargeable lithium-sulfur battery positive electrode material, wherein ammonium persulfate is used for oxidizing and polymerizing pyrrole, hydrofluoric acid is used for reducing graphene oxide, and a graphene-polypyrrole/sulfur-graphene sandwich-shaped composite film is obtained by a layer-by-layer suction filtration preparation method.

Although the above prior art improves the performance of lithium sulfur batteries to some extent, there are still some significant drawbacks. The ternary composite positive electrode as in patent application CN105070887A was prepared by a conventional slurry coating method, wherein the lower mass density of the sulfur positive electrode, the use of metal current collector and binder inevitably hindered the further increase of the volumetric energy density of the lithium sulfur battery. The oxidation of pyrrole and the reduction of graphene oxide in the preparation process of patent application CN106450245A involve conventional reagents and methods, however, the use of conventional redox agents often causes environmental burden and introduces additional functional groups, so each experimental step requires several times of washing and drying to remove the excessive oxidizing agent or reducing agent, and the process is complex and time-consuming.

Disclosure of Invention

In view of the above, the present invention aims to overcome the above disadvantages of the sulfur positive electrode, improve the structural design and surface chemical properties of the carbon-based composite material, and provide a sulfur/polypyrrole/graphene/carbon nanotube composite film, a preparation method and applications thereof. According to the method, pyrrole and graphene oxide are mixed and aged by virtue of a simple self-assembly strategy, and the oxidation-reduction reaction between pyrrole and pyrrole leads to partial reduction of graphene oxide and low polymerization of pyrrole without other oxidation or reducing agents. In addition, the functionalized multi-walled carbon nanotube is added as a second carbon skeleton component, so that on one hand, the conductivity and porosity of the material can be improved, and on the other hand, the graphene sheet layer can be well isolated, and the agglomeration is reduced. Finally, obtaining the light self-supporting polypyrrole/carbon nanotube/graphene composite film serving as a sulfur carrier through one-step suction filtration. The lithium-sulfur battery using the composite film as the positive electrode remarkably inhibits the shuttle effect of polysulfide, effectively reduces the self-discharge rate of the battery, relieves the volume change in the charge-discharge process, and further shows good cycling stability and high specific discharge capacity.

The invention realizes the aim through the following technical scheme:

a sulfur/polypyrrole/graphene/carbon nanotube composite film is composed of elemental sulfur, polypyrrole, reduced graphene oxide and functionalized multi-walled carbon nanotubes; the method comprises the following steps of mixing pyrrole with graphene oxide, polymerizing the pyrrole in situ to obtain polypyrrole, and reducing the graphene oxide into reduced graphene oxide; the polypyrrole is grafted on the reduced graphene oxide; the reduced graphene oxide and the functionalized multi-walled carbon nanotube are interwoven to form a three-dimensional carbon skeleton; the elemental sulfur is loaded on the surface and inside of the three-dimensional carbon skeleton; based on the total mass of the film being 100%, the mass fraction of elemental sulfur is 55% -60%, the mass fraction of polypyrrole is 1% -10%, the mass fraction of functionalized multi-walled carbon nanotubes is 20% -25%, and the balance is reduced graphene oxide.

A preparation method of a sulfur/polypyrrole/graphene/carbon nanotube composite film comprises the following steps:

(1) preparing a polypyrrole/graphene composite aqueous solution:

adding a graphene oxide aqueous solution into an ethanol aqueous solution, performing ultrasonic dispersion for 0.5-2 h, adding pyrrole, stirring uniformly, standing and aging at room temperature for 2-3 days, adding sodium polystyrene sulfonate, heating to 80-90 ℃ under a stirring condition, reacting for 2-4 h, cooling to room temperature after the reaction is finished, performing centrifugal washing, and dispersing into water with the purity of deionized water to obtain a polypyrrole/graphene composite aqueous solution; wherein the mass ratio of the graphene oxide to the pyrrole to the sodium polystyrene sulfonate is 1: 10-100: 5-10;

(2) preparing a polypyrrole/graphene/carbon nanotube composite film:

adding the polypyrrole/graphene composite aqueous solution into a functionalized multi-walled carbon nanotube, uniformly stirring, adding polyethylene glycol octyl phenyl ether (TritonX-100), and performing ultrasonic dispersion for 10-60 min to obtain a polypyrrole/graphene/carbon nanotube composite aqueous solution; sucking the composite aqueous solution, performing vacuum filtration by using a polypropylene film, drying, and stripping from the polypropylene film to obtain a polypyrrole/graphene/carbon nanotube composite film; the functionalized multi-walled carbon nanotube is a multi-walled carbon nanotube containing polar functional groups, and the mass of the functionalized multi-walled carbon nanotube is 1-3 times that of the graphene oxide in the step (1); the thickness of the polypyrrole/graphene/carbon nanotube composite film is 10-30 mu m;

(3) preparing a sulfur/polypyrrole/graphene/carbon nanotube composite film:

uniformly dripping a sulfur toluene solution on the polypyrrole/graphene/carbon nano tube composite film, heating to 150-160 ℃ under the protection of inert gas, preserving heat for 8-12 hours, then heating to 240-300 ℃, and preserving heat for 0.5-1 hour to obtain a sulfur/polypyrrole/graphene/carbon nano tube composite film; wherein the mass of sulfur dripped onto the polypyrrole/graphene/carbon nano tube composite film is 1-30 mg.

Preferably, the volume ratio of ethanol to water in the ethanol aqueous solution in the step (1) is 1: 0.5-2, wherein the water is water with the purity of deionized water or more.

Preferably, the concentration of the TritonX-100 in the composite aqueous solution in the step (2) is 0.1 wt% to 0.5 wt%.

Preferably, the functionalized multi-walled carbon nanotubes obtained in step (2) are obtained by acidifying multi-walled carbon nanotubes. The specific method comprises the following steps: ultrasonically soaking a multi-walled carbon nanotube prepared by a CVD method for 20-40 min by using concentrated hydrochloric acid, centrifugally washing, and then treating at 480-520 ℃ for 0.5-1.5 h to obtain a purified multi-walled carbon nanotube; and adding the purified multi-walled carbon nano-tube into dilute nitric acid, heating and refluxing for 6-12 h at constant temperature of 100-120 ℃, centrifugally washing, and drying in vacuum to obtain the functionalized carbon nano-tube.

Preferably, the ultrasonic frequency used for the ultrasonic dispersion in the step (2) is 80kW to 100 kW.

Preferably, the pore diameter of the polypropylene membrane in the step (2) is 0.22-0.45 μm.

Preferably, the vacuum degree in the vacuum filtration in the step (2) is-0.08 MPa to-0.1 MPa.

Preferably, the concentration of the toluene solution of sulfur in the step (3) is 10-30 mg/mL.

The application of the sulfur/polypyrrole/graphene/carbon nanotube composite film is that the film is used as the positive electrode of a lithium-sulfur battery.

Has the advantages that:

(1) according to the invention, pyrrole and graphene oxide are directly mixed, the partial reduction of the graphene oxide and the low polymerization of the pyrrole are promoted by controlling reaction conditions and utilizing the oxidation-reduction reaction between the pyrrole and the graphene oxide, and meanwhile, the self-assembly of a product can be realized without adding other oxidation or reducing agents.

(2) The graphene oxide nanosheets and the multi-walled carbon nanotubes with high specific surface area and pore structures provide enough contact sites for sulfur growth, and can solve the problems of low active substance loading capacity, low active substance utilization rate and the like in the conventional lithium-sulfur battery cathode material; the functionalized multi-walled carbon nanotube and the residual oxygen-containing functional group on the surface of the partially reduced graphene oxide have good anchoring effect on sulfur and a soluble polysulfide intermediate generated in situ through chemical interaction, and the cycling stability and the rate capability of the battery can be obviously improved.

(3) The polypyrrole rich in heteroatom nitrogen can form a chemical bond with sulfur, and the strong affinity between the polypyrrole and polysulfide can effectively reduce the diffusion of polysulfide into an electrolyte, thereby further inhibiting a shuttle effect.

(4) The polypyrrole/graphene/carbon nanotube three-dimensional network with high conductivity provides a rapid channel for ion/charge transmission, and reaction kinetics are further improved, so that the charge and discharge speed, capacity and cycling stability of the battery are increased. In addition, the light flexible film carrier can adapt to the volume change of active substances in the charge and discharge process, effectively maintains the structural integrity of the anode material in the circulation process, and prolongs the service life of the lithium-sulfur battery.

(5) The preparation method is simple to operate, easy to realize the process and the technology, green and environment-friendly, and convenient for large-scale commercial production. The suction filtration of the composite aqueous solution under the condition of high vacuum degree ensures that the product has extremely thin thickness, which is beneficial to improving the volume specific capacity and the volume energy density of the battery. In addition, the polypropylene membrane with the aperture of 0.22-0.45 mu m is used for suction filtration, so that a large amount of loss of a product in the suction filtration process can be avoided, and the polypyrrole/graphene/carbon nanotube composite material is easy to peel off from the filter membrane after being dried to obtain the flexible self-supporting film carrier.

(6) The invention adopts a post-sulfur loading method to load sulfur, and firstly, a toluene solution of sulfur is dripped on a composite film to load nano sulfur particles. The subsequent heating process is carried out in two stages, in the first stage, the mixture is heated to 150-160 ℃ under the protection of inert gas, and the temperature is kept for 8-12 hours, so that elemental sulfur can be ensured to be completely and uniformly permeated into the composite film; and in the second stage, heating to 240-300 ℃, preserving the temperature for 0.5-2 h, and then carrying out ring-opening reaction on the elemental sulfur in the mixture so as to fix the sulfur on the composite film.

Drawings

In FIG. 1, (a) is a TG map of PG, CG and PCG in comparative example 1, comparative example 2 and example 1, and (b) is a TG map of PG-S, CG-S and PCG-S in comparative example 1, comparative example 2 and example 1.

In FIG. 2, (a) is an SEM photograph of PG in comparative example 1, (b) is a partially enlarged SEM photograph of PG in comparative example 1, and (c) is a side-view SEM photograph of PG in comparative example 1.

In fig. 3, (a) is an SEM image of CG in comparative example 2, (b) is a partially enlarged SEM image of CG in comparative example 2, and (c) is a side SEM image of CG in comparative example 2.

FIG. 4 (a) is an SEM photograph of the PCG in example 1, (b) is a partially enlarged SEM photograph of the PCG in example 1, and (c) is a side SEM photograph of the PCG in example 1.

In fig. 5, (a) is an Electrochemical Impedance (EIS) chart before cycling of the cells assembled in comparative example 1, comparative example 2 and example 1, and (b) is an EIS chart after cycling at 0.5C (1C 1675mA/g) for 50 weeks of the cells assembled in comparative example 1, comparative example 2 and example 1.

Fig. 6 is a graph of cycle tests performed at different current densities for the assembled batteries of comparative example 1, comparative example 2, and example 1.

Fig. 7 is a graph of the cycle test performed at a current density of 0.2C for the assembled battery of example 1.

Fig. 8 is a graph of the cycle test performed at a current density of 0.2C for the assembled battery of example 2.

Fig. 9 is a graph of the cycling test performed at a current density of 0.2C for the assembled cell of example 3.

Fig. 10 is an X-ray diffraction (XRD) pattern of functionalized multi-walled carbon nanotubes (MWCNTs), Graphene Oxide (GO), polypyrrole (PPy) and PCG in example 1.

FIG. 11 is a view showing the flexible structure of the PCG-S in example 1.

Detailed Description

In order to more clearly and clearly describe the objects, technical solutions and advantages of the present invention, the present invention will be further described in detail with reference to specific embodiments. It should be understood that the following described examples are only for illustrating the present invention and should not be construed as limiting the scope of the present invention. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products available commercially. Additionally, the endpoints of the ranges and any values disclosed herein are not limited to the precise range or value, and such ranges or values should be understood to encompass values close to those ranges or values.

In the following examples:

thermogravimetric analysis (TG) test: model STA449F3 synchronous thermal analyzer, german Navy (NETZSCH).

Scanning Electron Microscope (SEM) testing: HITACHI model S-4800 field emission scanning Electron microscope, Japan.

X-ray diffraction (XRD) test: rigaku UltimaIV-185X-ray diffractometer, Japan.

Sodium polystyrene sulfonate was purchased from aladdin, Mw 70000.

Pyrrole was purchased from the national pharmaceutical group chemical agents, ltd, and the purchased pyrrole was distilled under reduced pressure in a water bath at 60 c to further purify the polymerization inhibitor before use.

Assembly and testing of CR2025 button cells: the composite films prepared in the comparative examples or examples were cut into small 1cm diameter disks using a hand-held pole-piece cutter for use as the positive electrode, a metallic lithium disk as the negative electrode, a Celgard2300 porous membrane as the separator, and 1, 3-dioxolane and ethylene glycol dimethyl ether (DOL: DME: 1v/v) and 1.0M lithium bistrifluoromethylsulfonyl imide (LiTFSI) and 0.2M LiNO₃The mixture as an additive was finally assembled into a 2025 button cell in an argon filled glove box.

Constant current charge and discharge test: CT2001A LAND battery test system, wuhan; the charging and discharging voltage interval is 1.7-2.8V (vs. Li/Li)⁺) (ii) a The test temperature was 33 ℃.

And (3) testing alternating current impedance: CHI660E electrochemical workstation, shanghai; the test voltage is 2.4V, the test frequency range is 0.01 Hz-100 kHz, the amplitude is 5mV, and the counter electrode is taken as a reference electrode.

Comparative example 1

A positive electrode material of a lithium-sulfur battery comprises a sulfur/polypyrrole/graphene composite, and the technical scheme adopted by the comparative example comprises the following steps:

1) preparing graphene oxide:

preparing graphene oxide by adopting an improved Hummers method: under the conditions of ice bath and stirring, 1.5g of expanded graphite powder with the granularity of 150 mu m is mixed and soaked for 0.5h with 180mL of concentrated sulfuric acid with the mass fraction of 98% and 20mL of phosphoric acid; then slowly adding 9g of potassium permanganate, and controlling the temperature to be below 5 ℃; after the reaction is carried out for 0.5h, the reaction system is continuously stirred for 24h under the condition of 50 ℃ water bath; after the reaction is finished, pouring the reaction liquid into 220g of ice cubes, and slowly adding 5mL of hydrogen peroxide with the mass fraction of 30% into the ice cubes until the color of the solution becomes beige; then, carrying out suction filtration to remove strong acid, and washing with 5% hydrochloric acid and deionized water for three times respectively; and obtaining graphene oxide mother liquor after centrifugal dialysis treatment. And carrying out ultrasonic treatment on the mother liquor for 2 hours and preparing a 10mg/mL graphene oxide aqueous solution.

2) Preparation of polypyrrole/graphene composite film (PG):

adding 5.6mL of the graphene oxide aqueous solution prepared in the step 1) into 200mL of mixed solution of ethanol and deionized water (the volume ratio of the ethanol to the deionized water is 1: 1), and ultrasonically dispersing for 1h under the frequency of 100 kW; adding 2.8mL of pyrrole, stirring uniformly, aging at room temperature for three days, and changing the color of the solution from brown yellow to brown black to obtain a polypyrrole/graphene mixed suspension; adding 0.28g of sodium polystyrene sulfonate serving as a dispersing agent into the suspension; heating the mixed solution in a water bath at 80 ℃ for 4 hours under the condition of magnetic stirring to further reduce the graphene oxide, wherein the color of the solution is further blackened from brown black; after naturally cooling to room temperature, centrifugally washing the obtained suspension for three times, and then dispersing the suspension into 240mL of deionized water to obtain a polypyrrole/graphene composite solution; sucking 30mL of the composite solution, performing suction filtration by using a polypropylene film with the aperture of 0.22 mu m under the condition that the vacuum degree is minus 0.1MPa, drying, and peeling from the filter paper to obtain the polypyrrole/graphene composite film.

3) Preparation of a sulfur/polypyrrole/graphene composite film (PG-S):

preparing a 20mg/mL sulfur toluene solution: 300mg of sublimed sulfur powder stored in a vacuum drying oven at 60 ℃ is taken out, 15mL of toluene is added, the opening is sealed, the vacuum drying oven is placed on a heat collection type constant temperature heating magnetic stirrer, and the heating and stirring are carried out at the temperature of 60 ℃ until the sulfur powder is completely dissolved in the toluene solution.

And (3) uniformly dropwise adding 0.5mL of the sulfur toluene solution on the polypyrrole/graphene composite film, heating to 155 ℃ in a closed container filled with argon, preserving heat for 10 hours, and then continuously heating to 240 ℃ and preserving heat for 0.5 hour to obtain the sulfur/polypyrrole/graphene composite film.

The results of the TG tests on PG and PG-S are shown in FIG. 1, and the calculated polypyrrole content was 6.8 wt%, the corresponding sulfur loading was 41.8 wt%, and the active content was low.

The SEM test results of PG are shown in fig. 2, from which it can be seen that PG has a relatively flat flocculent surface, a low surface roughness, and a low porosity; the side view shows that the PG thickness is about 5.8 μm and the dense layered structure is due to irreversible agglomeration and stacking of graphene oxide during reduction and suction filtration.

PG-S electrochemical impedance results are shown in FIG. 5, which shows a very large charge transfer impedance before and after PG-S cycling due to poor conductivity.

The electrochemical performance of the battery assembled by PG-S is shown in figure 6, and due to poor conductivity, slow reaction kinetics and low specific discharge capacity of PG-S; specific results are shown in table 1.

Comparative example 2

A positive electrode material of a lithium-sulfur battery comprises a sulfur/graphene/carbon nanotube composite, and the technical scheme adopted by the comparative example comprises the following steps:

1) preparing graphene oxide in the same way as in step 1) of comparative example 1;

2) and (3) purifying and acidifying the carbon nano tube:

carrying out ultrasonic soaking treatment on a multi-walled carbon nanotube prepared by a commercial CVD method for 30min by using 12mol/L concentrated hydrochloric acid, carrying out centrifugal washing for three times, and then carrying out high-temperature heating treatment for 1h at 480 ℃; and then adding 1g of the purified multi-walled carbon nano-tube into 100mL of dilute nitric acid with the concentration of 2.6mol/L, stirring and refluxing at constant temperature of 100 ℃ for 12h, centrifuging and washing to be neutral, and finally drying in vacuum at 60 ℃ for 24h to obtain the functionalized carbon nano-tube.

3) Preparing a graphene/carbon nanotube composite film (CG):

dispersing 5.6mL of the graphene oxide aqueous solution prepared in the step 1) in 240mL of deionized water, and performing ultrasonic dispersion for 1h under the frequency of 100 kW; adding 112mg of the purified functionalized carbon nano tube, and uniformly stirring to obtain the graphene oxide/carbon nano tube mixed suspension. To the above suspension, 0.56g of sodium polystyrene sulfonate and 560. mu.L of Triton-X as a dispersant were added, and ultrasonic dispersion was carried out at a frequency of 100kW for 30min, and 1400. mu.L of a sodium ascorbate solution (1M) was further added and heated under magnetic stirring at 80 ℃ for 3 hours to reduce graphene oxide. And cooling to room temperature, sucking 30mL of the composite solution, performing suction filtration by using a polypropylene film with the aperture of 0.45 mu m under the condition that the vacuum degree is-0.08 MPa, drying, and peeling from the filter paper to obtain the graphene/carbon nanotube composite film.

4) Preparing a sulfur/graphene/carbon nanotube composite film (CG-S):

And (3) uniformly dropwise adding 0.5mL of the toluene solution of the sulfur on the graphene/carbon nano tube composite film, heating to 155 ℃ in a closed container filled with argon, preserving heat for 10 hours, and then continuously heating to 240 ℃ and preserving heat for 0.5 hour to obtain the sulfur/graphene/carbon nano tube composite film.

The results of the TG tests on CG and CG-S are shown in FIG. 1, and the corresponding sulfur loading is calculated to be 53.8 wt%, and the active substance content is improved compared with PG-S.

The result of the CG SEM test is shown in FIG. 3, and it can be seen that the CG surface has obvious protrusions, and the surface roughness and the porosity are high; the side view shows that the CG thickness was about 16.2 μm and significant delamination of the structure was observed.

The CG-S electrochemical impedance results are shown in FIG. 5, the conductivity of the electrode is greatly improved by using the carbon nanotube as the second component of the carbon skeleton, and the charge transfer impedance before and after CG-S cycle is lower than that of PG-S.

The electrochemical performance of the battery assembled by CG-S is shown in figure 6, the discharge specific capacity is improved to a certain extent by improving the conductivity, but the cycle performance of CG-S is poorer at higher multiplying power due to the lack of the chemical adsorption effect of polypyrrole on polysulfide; specific results are shown in table 1.

Example 1

A high-performance lithium-sulfur battery positive electrode material comprises a sulfur/polypyrrole/graphene/carbon nanotube composite, and the technical scheme adopted by the embodiment comprises the following steps:

2) purification and acidification of carbon nanotubes are the same as step 2) in comparative example 2;

3) preparation of polypyrrole/graphene/carbon nanotube composite film (PCG):

adding 5.6mL of the graphene oxide aqueous solution prepared in the step 1) into 200mL of mixed solution of ethanol and deionized water (the volume ratio of the ethanol to the deionized water is 1: 1), and ultrasonically dispersing for 1h under the frequency of 100 kW; adding 2.8mL of pyrrole, stirring uniformly, aging at room temperature for three days, and changing the color of the solution from brown yellow to brown black to obtain a polypyrrole/graphene mixed suspension; adding 0.28g of sodium polystyrene sulfonate serving as a dispersing agent into the suspension; heating the mixed solution in a water bath at 80 ℃ for 4 hours under the condition of magnetic stirring to further reduce the graphene oxide, wherein the color of the solution is further blackened from brown black; after naturally cooling to room temperature, centrifugally washing the precursor suspension for three times, and then dispersing the precursor suspension into 240mL of deionized water to obtain a polypyrrole/graphene composite solution; adding 112mg of purified carboxyl carbon nano tube into the polypyrrole/graphene composite solution, mixing and stirring uniformly, then adding 560 mu L of TritonX-100, and performing ultrasonic dispersion for 30min at the frequency of 80kW to obtain the polypyrrole/graphene/carbon nano tube composite aqueous solution. Sucking 30mL of composite aqueous solution, performing suction filtration by using a polypropylene film with the aperture of 0.45 mu m under the condition that the vacuum degree is minus 0.08MPa, drying, and peeling from the polypropylene film to obtain a compact polypyrrole/graphene/carbon nanotube composite film serving as an active substance carrier.

4) Preparing a sulfur/polypyrrole/graphene/carbon nanotube composite film (PCG-S):

And (3) uniformly dropwise adding 0.5mL of the sulfur toluene solution on the polypyrrole/graphene/carbon nanotube composite film, heating the polypyrrole/graphene/carbon nanotube composite film in a closed container filled with argon to 155 ℃, preserving heat for 10 hours, and then continuously heating the polypyrrole/graphene/carbon nanotube composite film to 240 ℃ and preserving heat for 0.5 hour to obtain the sulfur/polypyrrole/graphene/carbon nanotube composite film.

The results of the TG test on PCG-S are shown in FIG. 1, and it was calculated that the polypyrrole content was 3.5 wt%, the corresponding sulfur loading was 57 wt%, and the active material content was high.

The SEM test result of the PCG is shown in FIG. 4, and it can be seen that the surface structure of the PCG is similar to that of CG, the surface roughness is high, and the large number of folds and woven protrusions improve the specific surface area and the porosity of the material; the side view shows that the PCG is about 14.6 μm thick and the PCG is observed to be a three-dimensional hierarchical structure.

The PCG-S electrochemical impedance results are shown in fig. 5, and it can be seen from the figure that the charge transfer impedance before and after the PCG-S cycle is small, because the polypyrrole/graphene/carbon nanotube three-dimensional network promotes the rapid ion/charge transfer, limits the "shuttle effect" of polysulfides, improves the wettability of the electrode, and further improves the reaction kinetics.

The electrochemical performance of the battery assembled by the PCG-S is shown in FIGS. 6-7, and due to the synergistic effect of the components of sulfur, polypyrrole, graphene and the carbon nano tube, the discharge specific capacity of the PCG-S at 0.2 ℃ can reach 1201.9mAh/g, and the capacity still remains 790.2mAh/g after 300-week circulation; the coulombic efficiency under 1C is over 98 percent, the capacity retention rate after 200 cycles is up to more than 88 percent, and the electrochemical stability under higher magnification is obviously improved. Specific results are shown in table 1.

The XRD measurement result of PCG is shown in fig. 10, and the peak around 12.7 ° in the spectrum of PCG disappears, but a large envelope peak appears at 23.5 °. This demonstrates the redox reaction between pyrrole and graphene oxide: the pyrrole intercalates into the graphene oxide layer by in situ polymerization, further leading to reduction of the graphene oxide and expansion of the interlayer spacing.

The flexibility of the PCG-S is demonstrated in fig. 11, where it can be seen that the structural integrity of the material remains good even when bent in half.

Example 2

3) preparation of polypyrrole/graphene/carbon nanotube composite film (PCG):

adding 5.6mL of the graphene oxide aqueous solution prepared in the step 1) into 300mL of mixed solution of ethanol and deionized water (the volume ratio of the ethanol to the deionized water is 1: 2), and carrying out ultrasonic treatment at the frequency of 100kW for 2 h; adding 5.6mL of pyrrole, stirring uniformly, aging at room temperature for 48h, and changing the color of the solution from brown yellow to brown black to obtain a polypyrrole/graphene mixed suspension; adding 0.56g of sodium polystyrene sulfonate as a dispersing agent into the suspension; heating the mixed solution in a water bath at 90 ℃ for 2 hours under the condition of magnetic stirring to further reduce the graphene oxide, wherein the color of the solution is further blackened from brown black; after naturally cooling to room temperature, centrifugally washing the precursor suspension for three times, and then dispersing the precursor suspension into 240mL of deionized water to obtain a polypyrrole/graphene composite solution; adding 168mg of purified carboxyl carbon nano tube into the polypyrrole/graphene composite solution, uniformly mixing and stirring, adding 1.2mL of TritonX-100, and performing ultrasonic dispersion for 60min at the frequency of 100kW to obtain the polypyrrole/graphene/carbon nano tube composite aqueous solution. Absorbing 60mL of composite aqueous solution, performing suction filtration by using a polypropylene film with the aperture of 0.45 mu m under the condition that the vacuum degree is minus 0.08MPa, drying, and peeling from the polypropylene film to obtain a compact polypyrrole/graphene/carbon nanotube composite film serving as an active substance carrier.

preparing a 30mg/mL sulfur toluene solution: taking out 450mg of sublimed sulfur powder stored in a vacuum drying oven at 60 ℃, adding 15mL of toluene, sealing, placing on a heat-collecting constant-temperature heating magnetic stirrer, and heating and stirring at the temperature of 60 ℃ until the sulfur powder is completely dissolved in the toluene solution.

And uniformly dropwise adding 1mL of the sulfur toluene solution on the polypyrrole/graphene/carbon nanotube composite film, heating the polypyrrole/graphene/carbon nanotube composite film in a closed container filled with argon to 155 ℃, preserving heat for 12 hours, and then continuously heating the polypyrrole/graphene/carbon nanotube composite film to 300 ℃ and preserving heat for 1 hour to obtain the sulfur/polypyrrole/graphene/carbon nanotube composite film.

Electrochemical performance of the battery assembled by PCG-S is shown in FIG. 8, and specific discharge capacity results at a current density of 0.2C are shown in Table 1.

Example 3

3) preparation of polypyrrole/graphene/carbon nanotube composite film (PCG):

adding 5.6mL of the graphene oxide aqueous solution prepared in the step 1) into 300mL of mixed solution of ethanol and deionized water (the volume ratio of the ethanol to the deionized water is 2: 1), and ultrasonically dispersing for 0.5h under the frequency of 100 kW; adding 0.56mL of pyrrole, stirring uniformly, aging at room temperature for three days, and changing the color of the solution from brown yellow to brown black to obtain a polypyrrole/graphene mixed suspension; adding 0.28g of sodium polystyrene sulfonate serving as a dispersing agent into the suspension; heating the mixed solution in a water bath at 90 ℃ for 3 hours under the condition of magnetic stirring to further reduce the graphene oxide, wherein the color of the solution is further blackened from brown black; after naturally cooling to room temperature, centrifugally washing the precursor suspension for three times, and then dispersing the precursor suspension into 240mL of deionized water to obtain a polypyrrole/graphene composite solution; adding 56mg of purified carboxyl carbon nano tube into the polypyrrole/graphene composite solution, uniformly mixing and stirring, then adding 280 mu L of TritonX-100, and performing ultrasonic dispersion for 30min under the frequency of 80kW to obtain the polypyrrole/graphene/carbon nano tube composite aqueous solution. Sucking 30mL of composite aqueous solution, performing suction filtration by using a polypropylene film with the aperture of 0.22 mu m under the condition that the vacuum degree is-0.1 MPa, drying, and peeling from the polypropylene film to obtain a compact polypyrrole/graphene/carbon nanotube composite film serving as an active substance carrier.

preparing 10mg/mL sulfur in toluene: 150mg of sublimed sulfur powder stored in a vacuum drying oven at 60 ℃ is taken out, 15mL of toluene is added, the opening is sealed, the vacuum drying oven is placed on a heat collection type constant temperature heating magnetic stirrer, and the heating and stirring are carried out at the temperature of 60 ℃ until the sulfur powder is completely dissolved in the toluene solution.

And (3) uniformly dropwise adding 0.1mL of the sulfur toluene solution on the polypyrrole/graphene/carbon nanotube composite film, heating to 155 ℃ in a closed container filled with argon, preserving heat for 8 hours, and then continuously heating to 240 ℃ and preserving heat for 0.5 hours to obtain the sulfur/polypyrrole/graphene/carbon nanotube composite film.

Electrochemical performance of the battery assembled by PCG-S is shown in FIG. 9, and specific discharge capacity at a current density of 0.2C is shown in Table 1.

TABLE 1 specific discharge capacity (mA h/g) results at different current densities

According to the test results, the composite film provided by the invention can be used as the lithium-sulfur battery anode to obviously improve the content and utilization rate of active substances of the lithium-sulfur battery anode material, so that the discharge capacity and the cycle performance are obviously improved, and especially the electrochemical performance under higher multiplying power is obviously improved. The method disclosed by the invention has the advantages of environment-friendly raw materials, simple and efficient process flow and wide experimental conditions, and provides an alternative approach for developing high-performance flexible lithium-sulfur batteries.

The present invention includes, but is not limited to, the above embodiments, and any equivalent substitutions or partial modifications made under the principle of the spirit of the present invention should be considered as being within the scope of the present invention.

Claims

1. A sulfur/polypyrrole/graphene/carbon nanotube composite film is characterized in that: the film consists of elemental sulfur, polypyrrole, reduced graphene oxide and functionalized multi-walled carbon nanotubes; the method comprises the following steps of mixing pyrrole with graphene oxide, polymerizing the pyrrole in situ to obtain polypyrrole, and reducing the graphene oxide into reduced graphene oxide; the polypyrrole is grafted on the reduced graphene oxide; the reduced graphene oxide and the functionalized multi-walled carbon nanotube are interwoven to form a three-dimensional carbon skeleton; the elemental sulfur is loaded on the surface and inside of the three-dimensional carbon skeleton; based on 100% of the total mass of the film, the mass fraction of elemental sulfur is 55% -60%, the mass fraction of polypyrrole is 1% -10%, the mass fraction of functionalized multi-walled carbon nanotubes is 20% -25%, and the balance is reduced graphene oxide;

the film is prepared by the following method, and the method comprises the following steps:

(1) preparing a polypyrrole/graphene composite aqueous solution:

(2) preparing a polypyrrole/graphene/carbon nanotube composite film:

adding a functionalized multi-walled carbon nanotube into the polypyrrole/graphene composite aqueous solution, uniformly stirring, adding TritonX-100, and performing ultrasonic dispersion for 10-60 min to obtain a polypyrrole/graphene/carbon nanotube composite aqueous solution; sucking the composite aqueous solution, performing vacuum filtration by using a polypropylene film, drying, and stripping from the polypropylene film to obtain a polypyrrole/graphene/carbon nanotube composite film; the functionalized multi-walled carbon nanotube is a multi-walled carbon nanotube containing polar functional groups, and the mass of the functionalized multi-walled carbon nanotube is 1-3 times that of the graphene oxide in the step (1); the thickness of the polypyrrole/graphene/carbon nanotube composite film is 10-30 mu m;

(3) preparing a sulfur/polypyrrole/graphene/carbon nanotube composite film:

2. The sulfur/polypyrrole/graphene/carbon nanotube composite film according to claim 1, wherein: the functionalized multi-walled carbon nanotube is obtained by acidizing the multi-walled carbon nanotube.

3. The sulfur/polypyrrole/graphene/carbon nanotube composite film according to claim 1, wherein: in the step (1), the volume ratio of ethanol to water in the ethanol aqueous solution is 1: 0.5-2, wherein the water is water with the purity of deionized water or more.

4. The sulfur/polypyrrole/graphene/carbon nanotube composite film according to claim 1, wherein: in the step (2), the concentration of the TritonX-100 in the composite aqueous solution is 0.1 wt% -0.5 wt%.

5. The sulfur/polypyrrole/graphene/carbon nanotube composite film according to claim 1, wherein: the ultrasonic frequency adopted by the ultrasonic dispersion in the step (2) is 80 kW-100 kW.

6. The sulfur/polypyrrole/graphene/carbon nanotube composite film according to claim 1, wherein: the aperture of the polypropylene film in the step (2) is 0.22-0.45 μm.

7. The sulfur/polypyrrole/graphene/carbon nanotube composite film according to claim 1, wherein: the vacuum degree in the vacuum filtration in the step (2) is-0.08 MPa to-0.1 MPa.

8. The sulfur/polypyrrole/graphene/carbon nanotube composite film according to claim 1, wherein: the concentration of the toluene solution of sulfur in the step (3) is 10-30 mg/mL.

9. Use of the sulfur/polypyrrole/graphene/carbon nanotube composite film according to claim 1 or 2, wherein: the composite film is used as the positive electrode of the lithium-sulfur battery.