WO2017028301A1 - Sulfur-carbon composite comprising carbon substrate and sulfur for lithium-sulfur batteries and process for preparing the same - Google Patents

Sulfur-carbon composite comprising carbon substrate and sulfur for lithium-sulfur batteries and process for preparing the same Download PDF

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WO2017028301A1
WO2017028301A1 PCT/CN2015/087635 CN2015087635W WO2017028301A1 WO 2017028301 A1 WO2017028301 A1 WO 2017028301A1 CN 2015087635 W CN2015087635 W CN 2015087635W WO 2017028301 A1 WO2017028301 A1 WO 2017028301A1
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sulfur
carbon
carbon composite
substrate
carbon substrate
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PCT/CN2015/087635
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French (fr)
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Yuguo GUO
Yaxia YIN
Nianwu LI
Yunhua Chen
Nahong ZHAO
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Robert Bosch Gmbh
Institute Of Chemistry Chinese Academy Of Sciences
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/139—Processes of manufacture
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/362—Composites
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
    • H01M4/583—Carbonaceous material, e.g. graphite-intercalation compounds or CFx
    • H01M4/587—Carbonaceous material, e.g. graphite-intercalation compounds or CFx for inserting or intercalating light metals
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Definitions

  • the present invention relates to a sulfur-carbon composite comprising carbon substrate and sulfur, wherein the carbon substrate consists of microporous carbon layers and graphene sheets, in which the microporous carbon layers are interbedded into the interlayer of the graphene sheets, and wherein sulfur is encapsulated into the microporous structure of the carbon substrate, an electrode and a lithium-sulfur battery comprising said sulfur-carbon composite as well as a process for preparing said sulfur-carbon composite.
  • Li-S batteries The commercial applications of lithium-sulfur (Li-S) batteries are hindered by several problems, such as the poor electronic/ionic conductivity of sulfur and discharged products, the dissolution of lithium polysulfides in an organic electrolyte, and the volume change during charge/discharge processes.
  • Various strategies have been attempted and developed to solve the above problems, for example, fabrication of porous carbon-sulfur composites, preparation of polymer-sulfur composites, optimization of an organic electrolyte, and use of coating layers or carbon interlayers.
  • micropores were proven to be the most effective pore structure in confining the polysulfides diffusion for good cycling performance.
  • the present invention provides a sulfur-carbon composite comprising carbon substrate and sulfur, wherein the carbon substrate consists of microporous carbon layers and graphene sheets, in which the microporous carbon layers are interbedded into the interlayer of the graphene sheets, and wherein sulfur is encapsulated into the microporous structure of the carbon substrate.
  • said carbon substrate is abbreviated as G@MC, wherein G stands for graphene sheets and MC stands for microporous carbon layers.
  • the symbol “@” denotes that the substance used before the symbol is stacked, coated or interbedded by the substance used after the symbol. Therefore, the expression “G@MC” , abbreviated for the inventive carbon substrate composed of graphene sheets and microporous carbon layers, denotes that the graphene sheets are stacked, coated or interbedded by the microporous carbon layers, and in addition, the abbreviation “G@MC-S” indicates that G@MC is further loaded with sulfur.
  • micropourous carbon layers and “graphene sheets” are those generally known in the art and respectively denote carbon layers which comprises of micropores, and an allotrope of carbon in the form of a two-dimensional, atomic-scale, hexagonal lattice in which one atom forms each vertex.
  • the present invention provides an electrode, which comprises the sulfur-carbon composite of the present invention.
  • the present invention further provides a lithium-sulfur battery, which comprises the sulfur-carbon composite of the present invention.
  • a process for preparing the above sulfur-carbon composite comprises the steps of: (1) preparation of graphene oxides; (2) adding carbonaceous precursor into an aqueous solution of the prepared graphene oxides, together with sulfuric acid and refluxing, then filtering and drying; (3) infiltrating the resulting material from step (2) into an solution containing a chemical activating agent, followed by heating at 700-900°C in an inert atmosphere; or alternatively (3’ ) directly heating the resulting material from step (2) by gas etching method at 700-900°C; or alternatively (3” ) infiltrating the resulting material from step (2) into an solution containing a chemical activating agent, followed by heating at 700-900°C in an etching gas atmosphere, wherein either step (3) , or step (3’ ) , or step (3”) is preformed, with step (3) is specially preferred over step (3’ ) or (3” ) ; (4) the material obtained in either step (3) , or step (3’ ) , or step (3”
  • the specific structure of the carbon substrate (G@MC) that is, the microporous carbon layers are interbedded into the interlayer of the graphene sheets, which forms a sandwich type-like laminated structure, can obtain the following advantages: 1) by combining the stacked graphene sheets that are interbedded by the microporous carbon layers, the G@MC achieves large pore volume, such as 2.65 cm 3 /g, which can encapsulate high content of sulfur and polysulfides in the stacked microporous structure, 2) the G@MC with a lot of sp 2 hybrid carbon ensures high conductivity, can provide 3-D electron transfer pathways for sulfur and discharge products, 3) the G@MC with the stacked structure can absorb a lot of polysulfides and confine the polysulfides diffusion, and provide adequate nanospace for sulfur expansion ensuring the structural integrity during the cycling. Based on the above, higher sulfur loading, better capacity retention and rate capability improvement can be realized in the present invention.
  • Figure 1 is a schematic view of carbon-sulfur cathode material based on the inventive G@MC substrate and G@MC-S composite.
  • Figure 2a and 2b are respectively Scanning Electron Microscopy (SEM) image of the G@MC substrate and the G@MC-S composite prepared according to the present invention.
  • Figure 2c and 2d are Transmission Electron Microscopy (TEM) image of the G@MC substrate prepared according to the present invention.
  • Figure 2e and 2f are respectively Transmission Electron Microscopy (TEM) image of the sulfur-carbon composite prepared according to the present invention and the corresponding elemental mapping images for carbon and sulfur.
  • TEM Transmission Electron Microscopy
  • Figure 3a and 3b are respectively plots showing the corresponding pore size distribution curve calculated by the Density Functional Theory (DFT) method and the N 2 adsorption-desorption isotherm of the G@MC substrates prepared in the present Examples, .
  • DFT Density Functional Theory
  • Figure 4a and 4b are respectively a plot showing the thermal analysis curves of sulfur-carbon composites prepared in the present Examples and a picture showing the sealed Li 2 S 4 /DME solution without and with G@MC addition.
  • Figures 5a-5f are respectively discharge-charge curves of the G@MC-S composites prepared in the present Examples.
  • Figures 6a and 6b are plots showing the cycling performance of the G@MC-S composites prepared in the present Examples.
  • Figures 7a and 7b are plots showing the rate capability of the G@MC5-S nanocomposite prepared in Example 4.
  • Figure 8 is a plot showing the cycling performance of the G@MC5-S and G@MC5-S-H nanocomposites prepared in Examples 4 and 5.
  • micropores shall be understood as referring to the pores having a pore diameter equal to or less than 2.0 nm.
  • the inventive carbon substrate only possesses micropores and does not have any mesopores or larger pores.
  • the present invention relates to a sulfur-carbon composite, comprising carbon substrate (G@MC) and sulfur, wherein sulfur is encapsulated into the microporous structure of the G@MC substrate, and the G@MC is composed of microporous carbon layers and graphene sheets, in which the microporous carbon layers are interbedded into the interlayer of the graphene sheets, as shown in Figure 1.
  • the carbon substrate (G@MC) is characterized by wrinkled multilayer nanosheets with the particle size of sever micrometers, and the stacked structure are preserved after sulfur encapsulation.
  • the TEM images of the G@MC substrate in Figures 2c and 2d reveal the graphene sheets structure of G@MC substrate, which is stemmed from the graphene oxides and retained after the activation process.
  • the microporous structure derived from the carbonaceous precursor is uniformly distributed on the graphene sheets with clear boundary.
  • sulfur is well dispersed into the microporous carbon layers of G@MC-S composite ( Figure 2f) and the graphene sheets structure can also be retained (Figure 2e) .
  • the G@MC substrate has a hierarchical microporous structure with microporous diameter less than 2.0 nm, preferably from 0.4 nm to 2.0 nm.
  • the G@MC substrate prepared according to the present invention has a diameter of less than 2.0 nm, wherein said hierarchical structure is particularly reflected by two peaks, one of which is around the pore size of about 0.4 nm and the other one of which is around the pore size of about 2.0 nm.
  • the G@MC substrate has a BET specific surface area in the range of from 1200 to 4000 m 2 /g, preferably from 1600 to 3600 m 2 /g, more preferably from 2000 to 3600 m 2 /g.
  • the microporous carbon has a pore volume of 0.5-3 cm 3 /g, preferably 1-3 cm 3 /g, more preferably 1-2.7 cm 3 /g.
  • the G@MC-S composite has a sulfur load amount of 60-90 wt%, preferably 60-80 wt% based on the total weight of the sulfur-carbon composite. From the curves in Figure 4a, the sulfur contents of each G@MC-S composites prepared in the present Examples can be calculated. As seen from Figure 4b, with the addition of G@MC substrate, the polysulfide Li 2 S 4 /DME can be dissolved and the solution turns to be clear.
  • the present invention further relates to a process for preparing the above sulfur-carbon composite and the process specifically comprises the steps of:
  • step (2) adding carbonaceous precursor to an aqueous solution of the graphene oxides prepared in step (1) , together with sulfuric acid and refluxing, then filtering and drying;
  • step (3) infiltrating the material obtained in step (2) into a solution containing a chemical activating agent, followed by heating at 700-900°C in an inert atmosphere; or alternatively (3’ ) directly heating the material obtained in step (2) by gas etching method at 700-900°C; or alternatively (3” ) infiltrating the material obtained in step (2) into a solution containing a chemical activating agent, followed by heating at 700-900°C in an etching gas atmosphere; wherein either step (3) , or step (3’ ) , or step (3” ) is preformed;
  • step (3) the material obtained in either step (3) , or step (3’ ) , or step (3” ) is neutralized and purified to obtain the G@MC substrate;
  • step (3) is especially preferred over step (3’ ) or step (3” ) .
  • the graphene oxides in the above step (1) can be prepared according to the ordinary method in the art, for example, modified hummers method.
  • the carbonaceous precursor in the above step (2) is one or more selected from the group consisting of sucrose, glucose, fructose, and any combination thereof.
  • the refluxing in step (2) can be carried out at 100-120°C for 10 h.
  • the chemical activating agent in the step (3) is one or more selected from the group consisting of hydroxides or carbonates of alkali metals, preferably KOH and NaOH.
  • the carbon material obtained from step (2) is in a mass ratio of 1: 0.5 to 1: 12, preferably 1: 1 to 1: 8, more preferably 1: 2 to 1: 6 relative to the mass of KOH.
  • the heating at 700-900°C in an inert atmosphere in the above step (3) can be carried out with a heating rate of 1-10°C/min.
  • the etching gas used in step (3) and (3” ) is one or more selected from the group consisting of water steam, carbon dioxide and air.
  • the G@MC substrate obtained in step (4) can be further dried before performing step (5) , i.e., loading with sulfur.
  • the loading of sulfur in the above step (5) comprises mixing homogenously the G@MC substrate with sulfur, preferably with a mass ratio of 1: 1 to 1: 5, and then heated at 150-160°C, and subsequently heated at 250-500°C, preferably 400°C.
  • the loading of sulfur can be carried out in a sealed glass tube filled with inert gas such as nitrogen, argon etc. or in vacuo.
  • the G@MC substrate is mixed with sublimed sulfur, preferably with a mass ratio of 1: 1 to 1: 5, and is ground for 10-30 min in an agate mortar. Afterwards, the mixture is sealed in an evacuated quartz tube and heated at 150-160°C for 1-20 h. Then, the temperature is increased to 400°C and kept at this temperature for 5-20 h and cooled down to room temperature so as to obtain G@MC-S nanocomposite.
  • the present invention further relates to an electrode, which comprises the sulfur-carbon composite according to the present invention.
  • the present invention further relates to a lithium-sulfur battery, which comprises the sulfur-carbon composite according to the present invention.
  • the conductive G@MC carbon substrate according to the present invention has both favorable electric conductivity and high volume of micropores for sulfur encapsulation, thus is very promising in use as the substrate material for sulfur to form the sulfur-carbon composite for Li-S battery. Moreover, the preparation process is simple to implement, and all raw materials are low in price, all these merits make the composite very promising for Li-S batteries.
  • Potential applications of the composite according to the present invention include high-energy-density lithium ion batteries with acceptable high power density for energy storage applications, such as power tools, photovoltaic cells and electric vehicles.
  • Graphene oxides were synthesized by oxidation of graphite using modified hummers method. Specifically, 10 g of natural graphite and 5 g of NaNO 3 were placed in a beaker. Then 250 ml H 2 SO 4 and 30 g of KMnO 4 were added with stirring in an ice water bath. The mixture was then stirred at 35°C for 2h, and 500 ml distilled water was added. After stirring for 15 min, the reaction was terminated by addition of 1.5 L distilled water and 1000 ml of 30% H 2 O 2 solution. The mixture was washed with 5% HCl solution in order to remove metal ions, and centrifuged at 15000rpm for several times in order to remove acid.
  • sucrose 15 g was added to 200 ml an aqueous solution of GO (0.75 mg/ml) prepared as above under sonication, which was placed in a round bottom flask. Then, 100 ml sulfuric acid was added to the solution and refluxed at 120°C for 10 h. The resulting black suspension was then filtered several times and dried at 100°C for 12 h. The obtained material was infiltrated with KOH with a mass ratio of 1: 2 via soaking in a solution of KOH followed by stirring for 12 h. The impregnated slurry was dried at 110°C for 12 h and then heated at 900°C for 2 h in a tube furnace under Ar atmosphere with a heating rate of 5°C/min.
  • the obtained product was neutralized with 1 M HCl solution and washed with distilled water for several times. Finally, the carbon was dried at 100°C for 20 h.
  • the obtained product was nominated as G@MC 2 (2 denotes the mass ratio of the KOH to the material obtained after filtering and drying in the active process) .
  • G@MC 2 material 150 mg was mixed with 350 mg sublimed sulfur and was ground for 30 min in an agate mortar. Afterwards, the mixture was sealed in an evacuated quartz tube and heated at 155°C for 6 h. Then the temperature was increased to 400°C and kept at this temperature for 10 h and cooled down to room temperature. The obtained product was nominated as G@MC2-S nanocomposite.
  • the microstructure of the samples was examined with a JEOL 6710F field-emission scanning electron microscope (FE-SEM) and a Tecnai G2 F20 U-TWIN field-emission transmission electron microscope (FE-TEM) .
  • the N 2 adsorption–desorption analysis was performed using an Autosorb-1 analyzer from Quantachrome Instruments.
  • Raman spectra were obtained using a DXR from Thermo Scientific with a laser wavelength of 532 nm.
  • Thermogravimetric (TG) analysis was measured on a TG/DTA 6300 instrument, in which the sample was heated in alumina crucible under N 2 flow to 500°C at a heating rate of 10°C min -1 to obtain the S content in the composite.
  • X-ray photoelectron spectroscopy (XPS) was performed on the Thermo Scientific ESCALab 250Xi using 200 W monochromated Al K ⁇ radiation.
  • the cathode slurry was prepared by mixing 75 wt% G@MC-S nanocomposite, 15 wt% Ketjen black, and 10 wt% of polyvinylidene difluoride (PVDF, Alfa Aesar) dissolved in N-methyl-2-pyrrolidone (NMP, Aldrich) .
  • PVDF polyvinylidene difluoride
  • NMP N-methyl-2-pyrrolidone
  • the sulfur cathodes were produced by coating the slurry on aluminum foil and drying at 60°C for 12 h.
  • the cell tests were evaluated using Swagelok-type cells cycled at room temperature between 1.8 V and 2.7 V, which were fabricated in an argon-filled glove box using lithium metal as the counter electrode and a microporous polyethylene separator.
  • the electrolyte was 1 M bis- (trifluoromethane) sulfonamide lithium (LiTFSI) in a mixed solvent of 1, 2-dimethoxyethane and 1, 3-dioxolane (1: 1, v/v) with 0.1 M LiNO 3 additives.
  • LiTFSI bis- (trifluoromethane) sulfonamide lithium
  • the performance of the cells was tested using an Arbin BT2000 system. The cells was tested at 0.2C for the initial 5 cycles, and then at 0.5C for the following cycles ( Figure 5, 6 and 8) .
  • the specific capacity was calculated on the mass of elemental sulfur.
  • the prepared sulfur-carbon composite G@MC2-S delivered an initial discharge capacity of 1,245 mAh/g at a current rate of 0.2 C, maintained the capacity of 786.6 mAh/g after 100 cycles at 0.5C, and the Coulombic efficiency is around 101-102%.
  • the sulfur-carbon composite (G@MC 3-S) was prepared by repeating the procedures in Example 1 except that the KOH was used in a mass ratio of 3: 1 to the carbon material obtained after filtering and drying in the active process.
  • the sulfur-carbon composite (G@MC 4-S) was prepared by repeating the procedures in Example 1 except that the KOH was used in a mass ratio of 4: 1 to the carbon material obtained after filtering and drying in the active process.
  • the sulfur-carbon composite (G@MC 5-S) was prepared by repeating the procedures in Example 1 except that the KOH was used in a mass ratio of 5: 1 to the carbon material obtained after filtering and drying in the active process.
  • the sulfur-carbon composite (G@MC 5-S-H, wherein “H” denotes a high S content) was prepared by repeating the procedures in Example 1 except that the KOH was used in a mass ratio of 5: 1 relative to the carbon material obtained after filtering and drying in the active process and that the amount of the sublimed sulfur was changed to 500 mg.
  • the sulfur-carbon composite (G@MC 6-S) was prepared by repeating the procedures in Example 1 except that the KOH was used in a mass ratio of 6: 1 to the carbon material obtained after filtering and drying in the active process.
  • the specific surface area (S BET ) was calculated by the Brunauer-Emmet-Teller (BET) method for G@MC2 to G@MC6;
  • the coulombic efficiency of the G@MC2-S to G@MC 6-S and of the G@MC5-S-H is around 101-102%.
  • the G@MC5-S composite can acquire a capacity of 785.7 mAh/g.
  • G@MC5-S-H has a sulfur content of 75.4%, which is higher than 68.8% of G@MC5-S, as seen from Figure 8, the G@MC5-S and G@CM5-S-H composites deliver a similar capacity of 545.3 mAh/g and 541.3 mAh/g after 500 cycles, respectively.

Abstract

A sulfur-carbon composite comprises carbon substrate and sulfur, wherein carbon s-ubstrate consists of microporous carbon layers and graphene sheets, in which the mi-croporous carbon layers are interbedded into the interlayer of the graphene sheets, a-nd wherein sulfur is encapsulated into the microporous structure of the carbon subst-rate, as well as an electrode and a lithium-sulfur battery comprising said sulfur-carb-on composite and a process for preparing said sulfur-carbon composite.

Description

Sulfur-Carbon Composite Comprising Carbon Substrate and Sulfur For Lithium-Sulfur Batteries And Process For Preparing The Same Technical field
The present invention relates to a sulfur-carbon composite comprising carbon substrate and sulfur, wherein the carbon substrate consists of microporous carbon layers and graphene sheets, in which the microporous carbon layers are interbedded into the interlayer of the graphene sheets, and wherein sulfur is encapsulated into the microporous structure of the carbon substrate, an electrode and a lithium-sulfur battery comprising said sulfur-carbon composite as well as a process for preparing said sulfur-carbon composite.
Background Art
The commercial applications of lithium-sulfur (Li-S) batteries are hindered by several problems, such as the poor electronic/ionic conductivity of sulfur and discharged products, the dissolution of lithium polysulfides in an organic electrolyte, and the volume change during charge/discharge processes. Various strategies have been attempted and developed to solve the above problems, for example, fabrication of porous carbon-sulfur composites, preparation of polymer-sulfur composites, optimization of an organic electrolyte, and use of coating layers or carbon interlayers. Among them, micropores were proven to be the most effective pore structure in confining the polysulfides diffusion for good cycling performance.
However, the low pore volume leads to the low sulfur loading in the microporous carbon, and the microporous carbon constituted of amorphous carbon results in poor rate performance. Thus, there are still needs on how to simultaneously improve the pore volume and the conductivity of microporous carbon, which are crucial to the development of carbon-sulfur composites.
Summary of Invention
It is therefore an object of the present invention to provide a novel sulfur-carbon composite with a specific interlayed or sandwiched structure and a corresponding preparation process, which makes it possible to solve the above problems.
The present invention provides a sulfur-carbon composite comprising carbon substrate and sulfur, wherein the carbon substrate consists of microporous carbon layers and graphene sheets, in which the microporous carbon layers are interbedded into the interlayer of the graphene sheets, and wherein sulfur is encapsulated into the microporous structure of the carbon substrate. Hereinafter, said carbon substrate is  abbreviated as G@MC, wherein G stands for graphene sheets and MC stands for microporous carbon layers.
In the context of the present specification, the symbol “@” denotes that the substance used before the symbol is stacked, coated or interbedded by the substance used after the symbol. Therefore, the expression “G@MC” , abbreviated for the inventive carbon substrate composed of graphene sheets and microporous carbon layers, denotes that the graphene sheets are stacked, coated or interbedded by the microporous carbon layers, and in addition, the abbreviation “G@MC-S” indicates that G@MC is further loaded with sulfur.
In the context of the present specification, the meanings of “micropourous carbon layers” and “graphene sheets” are those generally known in the art and respectively denote carbon layers which comprises of micropores, and an allotrope of carbon in the form of a two-dimensional, atomic-scale, hexagonal lattice in which one atom forms each vertex.
The present invention provides an electrode, which comprises the sulfur-carbon composite of the present invention.
The present invention further provides a lithium-sulfur battery, which comprises the sulfur-carbon composite of the present invention.
According to the present invention, a process for preparing the above sulfur-carbon composite is provided, which comprises the steps of: (1) preparation of graphene oxides； (2) adding carbonaceous precursor into an aqueous solution of the prepared graphene oxides, together with sulfuric acid and refluxing, then filtering and drying； (3) infiltrating the resulting material from step (2) into an solution containing a chemical activating agent, followed by heating at 700-900℃ in an inert atmosphere； or alternatively (3’ ) directly heating the resulting material from step (2) by gas etching method at 700-900℃； or alternatively (3” ) infiltrating the resulting material from step (2) into an solution containing a chemical activating agent, followed by heating at 700-900℃ in an etching gas atmosphere, wherein either step (3) , or step (3’ ) , or step (3”) is preformed, with step (3) is specially preferred over step (3’ ) or (3” ) ； (4) the material obtained in either step (3) , or step (3’ ) , or step (3” ) is neutralized and purified to obtain the carbon substrate； and (5) loading of sulfur into the carbon substrate obtained in the above step (4) .
According to the present invention, the specific structure of the carbon substrate (G@MC) , that is, the microporous carbon layers are interbedded into the interlayer of the graphene sheets, which forms a sandwich type-like laminated structure, can obtain the following advantages: 1) by combining the stacked graphene sheets that are interbedded by the microporous carbon layers, the G@MC achieves large pore volume, such as 2.65 cm3/g, which can encapsulate high content of sulfur and  polysulfides in the stacked microporous structure, 2) the G@MC with a lot of sp2 hybrid carbon ensures high conductivity, can provide 3-D electron transfer pathways for sulfur and discharge products, 3) the G@MC with the stacked structure can absorb a lot of polysulfides and confine the polysulfides diffusion, and provide adequate nanospace for sulfur expansion ensuring the structural integrity during the cycling. Based on the above, higher sulfur loading, better capacity retention and rate capability improvement can be realized in the present invention.
Brief Description of Drawings
Figure 1 is a schematic view of carbon-sulfur cathode material based on the inventive G@MC substrate and G@MC-S composite.
Figure 2a and 2b are respectively Scanning Electron Microscopy (SEM) image of the G@MC substrate and the G@MC-S composite prepared according to the present invention.
Figure 2c and 2d are Transmission Electron Microscopy (TEM) image of the G@MC substrate prepared according to the present invention.
Figure 2e and 2f are respectively Transmission Electron Microscopy (TEM) image of the sulfur-carbon composite prepared according to the present invention and the corresponding elemental mapping images for carbon and sulfur.
Figure 3a and 3b are respectively plots showing the corresponding pore size distribution curve calculated by the Density Functional Theory (DFT) method and the N2 adsorption-desorption isotherm of the G@MC substrates prepared in the present Examples, .
Figure 4a and 4b are respectively a plot showing the thermal analysis curves of sulfur-carbon composites prepared in the present Examples and a picture showing the sealed Li2S4/DME solution without and with G@MC addition.
Figures 5a-5f are respectively discharge-charge curves of the G@MC-S composites prepared in the present Examples.
Figures 6a and 6b are plots showing the cycling performance of the G@MC-S composites prepared in the present Examples.
Figures 7a and 7b are plots showing the rate capability of the G@MC5-S nanocomposite prepared in Example 4.
Figure 8 is a plot showing the cycling performance of the G@MC5-S and  G@MC5-S-H nanocomposites prepared in Examples 4 and 5.
Detailed Description of Preferred Embodiments
In the context of the present invention, the term “micropores” shall be understood as referring to the pores having a pore diameter equal to or less than 2.0 nm. The inventive carbon substrate only possesses micropores and does not have any mesopores or larger pores.
The present invention relates to a sulfur-carbon composite, comprising carbon substrate (G@MC) and sulfur, wherein sulfur is encapsulated into the microporous structure of the G@MC substrate, and the G@MC is composed of microporous carbon layers and graphene sheets, in which the microporous carbon layers are interbedded into the interlayer of the graphene sheets, as shown in Figure 1.
The detailed images can be further seen from Figures 2a-2f, according to the SEM images in Figures 2a and 2b, the carbon substrate (G@MC) is characterized by wrinkled multilayer nanosheets with the particle size of sever micrometers, and the stacked structure are preserved after sulfur encapsulation. The TEM images of the G@MC substrate in Figures 2c and 2d reveal the graphene sheets structure of G@MC substrate, which is stemmed from the graphene oxides and retained after the activation process. Moreover, the microporous structure derived from the carbonaceous precursor is uniformly distributed on the graphene sheets with clear boundary. Furthermore, sulfur is well dispersed into the microporous carbon layers of G@MC-S composite (Figure 2f) and the graphene sheets structure can also be retained (Figure 2e) .
In one embodiment of the sulfur-carbon composite according to the present invention, the G@MC substrate has a hierarchical microporous structure with microporous diameter less than 2.0 nm, preferably from 0.4 nm to 2.0 nm. As can be seen from Figure 3a, the G@MC substrate prepared according to the present invention has a diameter of less than 2.0 nm, wherein said hierarchical structure is particularly reflected by two peaks, one of which is around the pore size of about 0.4 nm and the other one of which is around the pore size of about 2.0 nm.
In another embodiment of the sulfur-carbon composite according to the present invention, as shown in Figure 3b, the G@MC substrate has a BET specific surface area in the range of from 1200 to 4000 m2/g, preferably from 1600 to 3600 m2/g, more preferably from 2000 to 3600 m2/g.
In a further embodiment of the sulfur-carbon composite according to the present invention, the microporous carbon has a pore volume of 0.5-3 cm3/g, preferably 1-3 cm3/g, more preferably 1-2.7 cm3/g.
In a still further embodiment of the sulfur-carbon composite according to the present invention, the G@MC-S composite has a sulfur load amount of 60-90 wt％, preferably 60-80 wt％ based on the total weight of the sulfur-carbon composite. From the curves in Figure 4a, the sulfur contents of each G@MC-S composites prepared in the present Examples can be calculated. As seen from Figure 4b, with the addition of G@MC substrate, the polysulfide Li2S4/DME can be dissolved and the solution turns to be clear.
The present invention further relates to a process for preparing the above sulfur-carbon composite and the process specifically comprises the steps of:
(1) preparation of graphene oxides；
(2) adding carbonaceous precursor to an aqueous solution of the graphene oxides prepared in step (1) , together with sulfuric acid and refluxing, then filtering and drying；
(3) infiltrating the material obtained in step (2) into a solution containing a chemical activating agent, followed by heating at 700-900℃ in an inert atmosphere； or alternatively (3’ ) directly heating the material obtained in step (2) by gas etching method at 700-900℃； or alternatively (3” ) infiltrating the material obtained in step (2) into a solution containing a chemical activating agent, followed by heating at 700-900℃ in an etching gas atmosphere； wherein either step (3) , or step (3’ ) , or step (3” ) is preformed；
(4) the material obtained in either step (3) , or step (3’ ) , or step (3” ) is neutralized and purified to obtain the G@MC substrate；
(5) loading of sulfur into the G@MC substrate obtained in step (4) .
In the above inventive process, step (3) is especially preferred over step (3’ ) or step (3” ) .
In an embodiment of the present invention, the graphene oxides in the above step (1) can be prepared according to the ordinary method in the art, for example, modified hummers method.
In another embodiment of the present invention, the carbonaceous precursor in the above step (2) is one or more selected from the group consisting of sucrose, glucose, fructose, and any combination thereof. Preferably, the refluxing in step (2) can be carried out at 100-120℃ for 10 h.
In a further embodiment of the present invention, the chemical activating agent in the step (3) is one or more selected from the group consisting of hydroxides or carbonates of alkali metals, preferably KOH and NaOH. Preferably, the carbon material obtained from step (2) , is in a mass ratio of 1: 0.5 to 1: 12, preferably 1: 1 to 1: 8, more preferably 1: 2 to 1: 6 relative to the mass of KOH. Still preferably, the heating at 700-900℃ in an inert atmosphere in the above step (3) can be carried out with a heating rate of  1-10℃/min.
In a further embodiment of the present invention, the etching gas used in step (3) and (3” ) , independently from each other, is one or more selected from the group consisting of water steam, carbon dioxide and air.
In a further embodiment of the present invention, the G@MC substrate obtained in step (4) can be further dried before performing step (5) , i.e., loading with sulfur.
In a further embodiment of the present invention, the loading of sulfur in the above step (5) comprises mixing homogenously the G@MC substrate with sulfur, preferably with a mass ratio of 1: 1 to 1: 5, and then heated at 150-160℃, and subsequently heated at 250-500℃, preferably 400℃.
In a specific embodiment, the loading of sulfur can be carried out in a sealed glass tube filled with inert gas such as nitrogen, argon etc. or in vacuo. The G@MC substrate is mixed with sublimed sulfur, preferably with a mass ratio of 1: 1 to 1: 5, and is ground for 10-30 min in an agate mortar. Afterwards, the mixture is sealed in an evacuated quartz tube and heated at 150-160℃ for 1-20 h. Then, the temperature is increased to 400℃ and kept at this temperature for 5-20 h and cooled down to room temperature so as to obtain G@MC-S nanocomposite.
The present invention further relates to an electrode, which comprises the sulfur-carbon composite according to the present invention.
The present invention further relates to a lithium-sulfur battery, which comprises the sulfur-carbon composite according to the present invention.
The conductive G@MC carbon substrate according to the present invention has both favorable electric conductivity and high volume of micropores for sulfur encapsulation, thus is very promising in use as the substrate material for sulfur to form the sulfur-carbon composite for Li-S battery. Moreover, the preparation process is simple to implement, and all raw materials are low in price, all these merits make the composite very promising for Li-S batteries.
Potential applications of the composite according to the present invention include high-energy-density lithium ion batteries with acceptable high power density for energy storage applications, such as power tools, photovoltaic cells and electric vehicles.
Examples
The following non-limiting examples illustrate various features and characteristics of  the present invention, which is not to be construed as limited thereto:
Example 1
The preparation of sulfur-carbon composite (G@MC-S) according to the present invention
(1) Preparation of graphene oxides (briefed as GO)
Graphene oxides (GO) were synthesized by oxidation of graphite using modified hummers method. Specifically, 10 g of natural graphite and 5 g of NaNO3 were placed in a beaker. Then 250 ml H2SO4 and 30 g of KMnO4 were added with stirring in an ice water bath. The mixture was then stirred at 35℃ for 2h, and 500 ml distilled water was added. After stirring for 15 min, the reaction was terminated by addition of 1.5 L distilled water and 1000 ml of 30％ H2O2 solution. The mixture was washed with 5％ HCl solution in order to remove metal ions, and centrifuged at 15000rpm for several times in order to remove acid.
(2) Preparation of G@MC
15 g of sucrose was added to 200 ml an aqueous solution of GO (0.75 mg/ml) prepared as above under sonication, which was placed in a round bottom flask. Then, 100 ml sulfuric acid was added to the solution and refluxed at 120℃ for 10 h. The resulting black suspension was then filtered several times and dried at 100℃ for 12 h. The obtained material was infiltrated with KOH with a mass ratio of 1: 2 via soaking in a solution of KOH followed by stirring for 12 h. The impregnated slurry was dried at 110℃ for 12 h and then heated at 900℃ for 2 h in a tube furnace under Ar atmosphere with a heating rate of 5℃/min. Subsequently, the obtained product was neutralized with 1 M HCl solution and washed with distilled water for several times. Finally, the carbon was dried at 100℃ for 20 h. The obtained product was nominated as G@MC 2 (2 denotes the mass ratio of the KOH to the material obtained after filtering and drying in the active process) .
(3) Preparation of sulfur-carbon composite (G@MC-S) containing the above G@MC 2
150 mg of G@MC 2 material was mixed with 350 mg sublimed sulfur and was ground for 30 min in an agate mortar. Afterwards, the mixture was sealed in an evacuated quartz tube and heated at 155℃ for 6 h. Then the temperature was increased to 400℃ and kept at this temperature for 10 h and cooled down to room temperature. The obtained product was nominated as G@MC2-S nanocomposite.
Material characterization
X-ray powder diffraction (XRD) patterns were obtained on a Rigaku D/max-2500 (Cu Kα radiation, λ ＝ 0.15405 nm) operating at 5° min-1. The microstructure of the samples was examined with a JEOL 6710F field-emission scanning electron microscope (FE-SEM) and a Tecnai G2 F20 U-TWIN field-emission transmission electron microscope (FE-TEM) . The N2 adsorption–desorption analysis was  performed using an Autosorb-1 analyzer from Quantachrome Instruments. Raman spectra were obtained using a DXR from Thermo Scientific with a laser wavelength of 532 nm. Thermogravimetric (TG) analysis was measured on a TG/DTA 6300 instrument, in which the sample was heated in alumina crucible under N2 flow to 500℃ at a heating rate of 10℃ min-1 to obtain the S content in the composite. X-ray photoelectron spectroscopy (XPS) was performed on the Thermo Scientific ESCALab 250Xi using 200 W monochromated Al Kα radiation.
Electrochemical test
The cathode slurry was prepared by mixing 75 wt％ G@MC-S nanocomposite, 15 wt％ Ketjen black, and 10 wt％ of polyvinylidene difluoride (PVDF, Alfa Aesar) dissolved in N-methyl-2-pyrrolidone (NMP, Aldrich) . The sulfur cathodes were produced by coating the slurry on aluminum foil and drying at 60℃ for 12 h. The cell tests were evaluated using Swagelok-type cells cycled at room temperature between 1.8 V and 2.7 V, which were fabricated in an argon-filled glove box using lithium metal as the counter electrode and a microporous polyethylene separator. The electrolyte was 1 M bis- (trifluoromethane) sulfonamide lithium (LiTFSI) in a mixed solvent of 1, 2-dimethoxyethane and 1, 3-dioxolane (1: 1, v/v) with 0.1 M LiNO3 additives. The performance of the cells was tested using an Arbin BT2000 system. The cells was tested at 0.2C for the initial 5 cycles, and then at 0.5C for the following cycles (Figure 5, 6 and 8) . The specific capacity was calculated on the mass of elemental sulfur.
As shown in Figure 5a, 6a and 6b, the prepared sulfur-carbon composite G@MC2-S delivered an initial discharge capacity of 1,245 mAh/g at a current rate of 0.2 C, maintained the capacity of 786.6 mAh/g after 100 cycles at 0.5C, and the Coulombic efficiency is around 101-102％.
Example 2
The sulfur-carbon composite (G@MC 3-S) was prepared by repeating the procedures in Example 1 except that the KOH was used in a mass ratio of 3: 1 to the carbon material obtained after filtering and drying in the active process.
Example 3
The sulfur-carbon composite (G@MC 4-S) was prepared by repeating the procedures in Example 1 except that the KOH was used in a mass ratio of 4: 1 to the carbon material obtained after filtering and drying in the active process.
Example 4
The sulfur-carbon composite (G@MC 5-S) was prepared by repeating the procedures in Example 1 except that the KOH was used in a mass ratio of 5: 1 to the carbon material obtained after filtering and drying in the active process.
Example 5
The sulfur-carbon composite (G@MC 5-S-H, wherein “H” denotes a high S content) was prepared by repeating the procedures in Example 1 except that the KOH was used in a mass ratio of 5: 1 relative to the carbon material obtained after filtering and drying in the active process and that the amount of the sublimed sulfur was changed to 500 mg.
Example 6
The sulfur-carbon composite (G@MC 6-S) was prepared by repeating the procedures in Example 1 except that the KOH was used in a mass ratio of 6: 1 to the carbon material obtained after filtering and drying in the active process.
The structure properties of the above prepared G@MC2 to G@MC6, such as the specific surface area, the total pore volume and so on and the electrochemical results of the above carbon-sulfur composites, were summarized in the following Table 1.
Table 1
Figure PCTCN2015087635-appb-000001
a: the specific surface area (SBET) was calculated by the Brunauer-Emmet-Teller (BET) method for G@MC2 to G@MC6；
b: VT represents the total pore volume (at relative pressure of p/p0＝0.99) for G@MC2 to G@MC6；
c: the specific capacity was calculated based on the mass of elemental sulfur and at rate of 0.2C；
d: the specific capacity was calculated based on the mass of elemental sulfur and at rate of 0.5C.
As shown in Figure 6b, the coulombic efficiency of the G@MC2-S to G@MC 6-S and of the G@MC5-S-H is around 101-102％.
From Figures 7a and 7b, even at a current rate of 2 C, the G@MC5-S composite can acquire a capacity of 785.7 mAh/g.
From Figure 4a, although G@MC5-S-H has a sulfur content of 75.4％, which is higher than 68.8％ of G@MC5-S, as seen from Figure 8, the G@MC5-S and G@CM5-S-H composites deliver a similar capacity of 545.3 mAh/g and 541.3 mAh/g after 500 cycles, respectively.

Claims (13)

  1. A sulfur-carbon composite, comprising carbon substrate and sulfur, wherein said carbon substrate consists of microporous carbon layers and graphene sheets, in which the microporous carbon layers are interbedded into the interlayer of the graphene sheets, and wherein sulfur is encapsulated into the microporous structure of the carbon substrate.
  2. The sulfur-carbon composite according to claim 1, wherein the carbon substrate has a hierarchical microporous structure with pore diameter less than 2.0 nm, preferably from 0.4 nm to 2.0 nm.
  3. The sulfur-carbon composite according to any one of claims 1 to 3, wherein the carbon substrate has a BET specific surface area ranging from 1200 to 4000 m2/g, preferably from 1600 to 3600 m2/g, more preferably from 2000 to 3600 m2/g.
  4. The sulfur-carbon composite according to any one of claims 1 to 4, wherein the carbon substrate has a pore volume of 0.5-3 cm3/g, preferably 1-3 cm3/g, more preferably 1-2.7 cm3/g.
  5. The sulfur-carbon composite according to any one of claims 1 to 5, wherein the sulfur-carbon composite has a sulfur content of 60-90 wt％, preferably 60-80 wt％based on the total weight of the sulfur-carbon composite.
  6. An electrode, comprising the sulfur-carbon composite of any one of claims 1 to 5.
  7. A lithium-sulfur battery, comprising the sulfur-carbon composite of any one of claims 1 to 5.
  8. A process for preparing the sulfur-carbon composite of any one of claims 1 to 5, comprising the steps of:
    (1) Preparation of graphene oxides；
    (2) Adding carbonaceous precursor into an aqueous solution of graphene oxides prepared from step (1) , together with sulfuric acid and refluxing, then filtering and drying；
    (3) infiltrating the material obtained in step (2) into a solution containing a chemical activating agent, followed by heating at 700-900℃ in an inert atmosphere； or alternatively (3’ ) directly heating the material obtained in step (2) by gas etching method at 700-900℃； or alternatively (3” ) infiltrating the material obtained in step (2) into a solution containing a chemical activating agent, followed by heating at 700-900℃ in an etching gas atmosphere； wherein either steps (3) , or step (3’ ) , or step (3” ) is performed；
    (4) the material obtained in either step (3) , or step (3’ ) , or step (3” ) is neutralized and purified to obtain the carbon substrate；
    (5) Loading of sulfur into the carbon substrate obtained in step (4) .
  9. The process according to claim 8, wherein said carbonaceous precursor in step (2) is one or more selected from the group consisting of sucrose, glucose, fructose, and any combinations thereof.
  10. The process according to claim 8 or 9, wherein said chemical activating agent in step (3) is one or more selected from the group consisting of hydroxides or carbonates of alkali metals, preferably KOH and NaOH.
  11. The process according to claim 10, wherein the weight ratio of the material obtained in step (2) to KOH (solid phase) is from 1∶0.5 to 1∶12, preferably 1∶1 to 1∶8, more preferably 1∶2 to 1∶6.
  12. The process according to claim 8 or 9, wherein the etching gas in steps (3’ ) and in step (3” ) , independently from each other, is one or more selected from the group consisting of water steam, carbon dioxide and air.
  13. The process according to claim 8 or 9, wherein the sulfur loading step (5) comprises mixing homogeneously the carbon substrate obtained in step (4) with sulfur, and then heating at 150-160℃, and subsequently heating at 250-500℃, preferably 400℃.
PCT/CN2015/087635 2015-08-20 2015-08-20 Sulfur-carbon composite comprising carbon substrate and sulfur for lithium-sulfur batteries and process for preparing the same WO2017028301A1 (en)

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