WO2009153051A1 - Use of a superfine expanded graphite and preparation thereof - Google Patents

Use of a superfine expanded graphite and preparation thereof Download PDF

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Publication number
WO2009153051A1
WO2009153051A1 PCT/EP2009/004439 EP2009004439W WO2009153051A1 WO 2009153051 A1 WO2009153051 A1 WO 2009153051A1 EP 2009004439 W EP2009004439 W EP 2009004439W WO 2009153051 A1 WO2009153051 A1 WO 2009153051A1
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layers
gas
range
solid
expanded graphite
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PCT/EP2009/004439
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French (fr)
Inventor
Nitin Kaskhedikar
Joachim Maier
Arndt Simon
Vladimir Fedorov
Victor Makotchenko
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MAX-PLANCK-Gesellschaft zur Förderung der Wissenschaften e.V.
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Priority to EP09765631A priority Critical patent/EP2304827A1/en
Publication of WO2009153051A1 publication Critical patent/WO2009153051A1/en

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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
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/20—Graphite
    • C01B32/21—After-treatment
    • C01B32/22—Intercalation
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/20—Graphite
    • C01B32/21—After-treatment
    • C01B32/22—Intercalation
    • C01B32/225—Expansion; Exfoliation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES OR LIGHT-SENSITIVE DEVICES, OF THE ELECTROLYTIC TYPE
    • H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
    • H01G11/22—Electrodes
    • H01G11/24—Electrodes characterised by structural features of the materials making up or comprised in the electrodes, e.g. form, surface area or porosity; characterised by the structural features of powders or particles used therefor
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES OR LIGHT-SENSITIVE DEVICES, OF THE ELECTROLYTIC TYPE
    • H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
    • H01G11/22—Electrodes
    • H01G11/30—Electrodes characterised by their material
    • H01G11/32—Carbon-based
    • H01G11/34—Carbon-based characterised by carbonisation or activation of carbon
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES OR LIGHT-SENSITIVE DEVICES, OF THE ELECTROLYTIC TYPE
    • H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
    • H01G11/22—Electrodes
    • H01G11/30—Electrodes characterised by their material
    • H01G11/32—Carbon-based
    • H01G11/36—Nanostructures, e.g. nanofibres, nanotubes or fullerenes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES OR LIGHT-SENSITIVE DEVICES, OF THE ELECTROLYTIC TYPE
    • H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
    • H01G11/22—Electrodes
    • H01G11/30—Electrodes characterised by their material
    • H01G11/50—Electrodes characterised by their material specially adapted for lithium-ion capacitors, e.g. for lithium-doping or for intercalation
    • 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
    • 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 uses of superfine expanded graphite and to methods for the preparation thereof.
  • the use of the superfine expanded graphite material preferably takes place in an electrochemical device, for example in secondary batteries, such as in a lithium battery, in a super- capacitor, in an electrochromic device or in a solar energy cell.
  • lithium batteries are known in rechargeable form. Such batteries comprise positive and negative electrodes with a non- aqueous electrolyte disposed between them.
  • the positive electrode of the battery can, for example, be LiCo ⁇ 2 (referred to as the "cathode” in the Li-battery community) and the negative electrode can for ex- ample be carbon (referred to as the "anode” in Li-battery community).
  • the positive electrode can for example be Mn ⁇ 2 and the negative electrode can be lithium metal.
  • ionically conducting salt such as Li(TFSI), i.e. lithium bis(trifluorosulphonyl)imide, LiPF ⁇ , i.e.
  • lithium hexafluorophosphate LiBOB (lithium bis(oxaltoborate) or LiClO4, i.e. lithium perchlorate, which are present, with a low degree of dissociation within a non-aqueous solvent, such as a mixture of DME (dimethylethane) and EC (ethylene carbonate), a mixture of DEC (diethylene carbonate) and EC, or a mixture of DMC (dimethyl carbonate) and EC or PC (propylene carbonate) or combinations thereof.
  • a useful range for the degree of disso- ciation lies in the range from 1 x 1O 1 to 10 8 HmoH.
  • dry polymer electrolytes there are so-called dry polymer electrolytes.
  • the salt is selected as before (i.e. for example from Li(TFSI), LiPFe, LiBOB or LiClO 4 ) and is dispersed in a polymer or mixture of polymers.
  • Suitable polymers comprise PEO (polyethylene oxide), PVDF (polyvinylene di-fluoride), PAN (poly- acrylonitrile) , and PMMA (polymethyl methyl acrylate).
  • graphite As far as lithium batteries are concerned, various materials have been investigated as anodes, of which graphite is one of the most relevent candidates.
  • One of the major advantages of graphite is the high chemical poten- tial of lithium in its intercalate resulting in a low voltage, e.g. 0.0 IV for LiC6 versus lithium.
  • the most important drawback of graphite is that its specific capacity is limited to 372 mAh g- 1 .
  • the object of the present invention is to provide a novel material in the form of an expanded graphite having a smaller particle size and a higher specific surface than previous expanded graphites which is well- suited for use in such electrochemical devices, for example as an electrode material, and also to propose a method for the preparation of such "superfine" expanded graphite which is beneficial for a large number of applications.
  • the present invention proposes the use of superfine expanded graphite having a bulk density in the range 1.3 to 1.8 kg/ ⁇ r 3 and comprising ribbon-like particles having a thickness of 2 - 3 nm and a length of 13 nm to 100 nm and comprising a plurality of spaced apart graphene layers with a spacing of from 0.2 to 0.4 nm, the ribbon-like par- tides being randomly orientated relative to one another with pores therebetween, and a specific surface in the range from 150 m 2 g" 1 to 500 m 2 g 1 , particularly in the range from 250 m 2 g" 1 to 280 m 2 g" 1 .
  • Each ribbon-like particle typically comprises a plurality of spaced apart graphene layers.
  • the use of the use of the porous superfine expanded graphite as a negative electrode in lithium- based electrochemical storage devices leads to high reversible capacities greater than 700 mAh/g and sufficient cycling performance over a wide voltage range (0 to 3 V). It is important for the present invention that the material is porous with interconnected pores and with the pore sizes ranging from typically about 0.5 nm to 50 nm and with a majority of pores preferably being below 10 nm in size. Another important feature of the material of the present invention is that there is some fluorine present in the carbon material and this fluorine increases the capacity of the structure which is beneficial for lithium intercalation, i.e. lithium storage properties.
  • the fluorine content is normally from 0.5 to 5 % and more preferably in the range from 2 to 3 %.
  • the invention particularly relates to the use of a material prepared by adding CIF3 in liquid form to solid carbon, heating the mixture at a reaction temperature between 22 to 100 0 C to form intercalated compounds of the formula C2F»xClF3 and ClF gas with the C2F being a solid material present in layers and the CIF3 being a gaseous material present between adjacent layers of C2F, according to the reaction
  • the method further comprising the step of subsequently heating the intercalated compound to expel the CIF3 gas and to simultaneously form CF4 gas, with the gas formation and expulsion serving to expand the structure formed by the C2F layers, with the C2F layers changing composition to carbon layers with a percentage of fluorine in the range up to 5 %, said heating temperature lying in the range from 400 to 500 0 C.
  • x is an arbitrary integer, i.e. 1, 2, 3...n where n can be a large number.
  • the invention relates to an expanded graphite structure prepared by adding CIF3 in liquid form to solid carbon, heating the mixture at a reaction temperature between 22 to 100 0 C to form intercalated compounds of the formula C 2 F»xClF3 and ClF gas with the C2F being a solid material present in layers and the CIF3 being a gaseous material present be- tween adjacent layers of C2F, according to the reaction
  • the method further comprising the step of subsequently heating the inter- calated compound to expel the CIF3 gas and to simultaneously form CF4 gas, with the gas formation and expulsion serving to expand the structure formed by the C 2 F layers, with the C2F layers changing composition to carbon layers with a percentage of fluorine in the range up to 5 %, said heating temperature lying in the range from 400 to 500 0 C.
  • x is an arbitrary integer, i.e. 1, 2, 3...n where n can be a large number.
  • the solid carbon is initially present in the form of graphite, either as chunks of graphite or graphite powder or graphite flakes. Preferred uses of the material are set forth in the dependent claim.
  • the expanded graphite is specifically useful as an electrode material of the present invention especially as a negative electrode material (anode material in the lithium battery community) due to its high storage capacity and excellent electronic conductivity.
  • porous expanded graphite when used as anode in Li-based electrochemical storage device show high reversible capacities (> 70OmAh/ g) and sufficient cycling performance over wide voltage range (0-3V).
  • Fig. 1 the nitrogen adsorption/ desorption isotherms of the as pre- pared expanded graphite sample
  • Fig. 2 the pore size distribution of the prepared expanded graphite sample
  • Fig. 3 a scanning electron micrograph of the expanded graphite sample
  • Fig. 4 a low resolution transmission electron micrograph of the expanded graphite sample
  • Fig. 5 a high resolution transmission electron micrograph of the expanded graphite sample
  • Fig. 6 a graph showing the specific capacity of the expanded graph- ite material of the present invention when used in a lithium battery as the negative electrode material at a charge rate of C/5,
  • Fig. 7 a graph showing that the specific capacity of the expanded graphite material of the present invention when used in a lithium battery as the negative electrode remains substantially constant over a large number of charge/ discharge cycles at a charge rate of C, and
  • Fig. 8 the specific capacity of the expanded graphite material of the present invention when used in a lithium battery as the negative electrode showing that the specific capacity remains substantially constant over a large number of charge/ discharge cycles at charge rates from 10 C to 60 C.
  • the graph shows a typical galvostatic charge/ discharge curve for material produced in accordance with the above method.
  • the specific capacity of the electrode on the first discharge is considerably higher than after repeated charge /discharge cycles, which is why the limb of the curve which extends to about 2400 mAh/g and which is labeled "first discharge" differs from the limb which finishes at about 700 to 1000 mAh/g which shows the specific capacity for repeated charge/ discharge cycles.
  • the curve which extends upwardly to the right from the origin shows the charging property of the electrode material.
  • the inset shows the specific capacity of the electrode material as a result of the cycle number and it can be seen that the drop-off in specific capacity for up to 10 charge cycles is only minimal and indeed the curve flattens off and thus further cyclic charging and discharging of the electrode does not result in a significant change of the specific capacity of the electrode material.
  • Fig. 8 shows the specific capacity of the electrode material for significantly higher charge rates of 10, 20, 30, 40, 50 and 60 C, with 10 C meaning that the charge cycle lasts 1/ 10 of an hour, and 60 C meaning that it lasts one 1/60 of an hour, i.e. one minute (60 C). It can be seen that even at very high charge rates, such as one minute, the electrode material still has a specific capacity of around 75 mAh/g and this is a very good value when compared with the prior art.
  • Expanded graphite is known per se.
  • the known EG is prepared by thermal decomposition of some graphite intercalation compounds (GIC) in the regime of "thermal shock".
  • GIC graphite intercalation compounds
  • This method allows to prepare EG with a bulk density of ⁇ 3 kg/m 3 , a particle size of -40 nm, and specific surface up to 150 m 2 /g.
  • the patent applicants have developed a method for preparation of more superfine EG with smaller particle size and higher specific surface, and also with higher intercalation and sorptive capacity. To achieve this goal, precursors were used based on fluorinated graphite intercalation com- pounds (FGIC) with organic and inorganic volatile substances.
  • FGIC fluorinated graphite intercalation com- pounds
  • intercalation compound based on dicarbon fluoride with CIF3 having the empirical composition C2FCly «zClF3.
  • y is an integer such as 1, 2 or 3
  • z is also an integer, for example 1, 2 n, where n could be up to 10 or larger.
  • This intercalation compound can be synthesized by fluorination of graphite with liquid CIF3 or its solutions in anhydrous HF:
  • x is an arbitrary integer, i.e. 1, 2, 3...n where n can be a large number.
  • composition of the matrix in all similar intercalates synthesized by the reaction of graphite with different halogen fluorides is uniform; the composition is changed very little (from C2Fo.94 # yClF3 to C2Fo.98*yClF3) and does not depend on the synthesis temperature and used solvent.
  • y is again 1, 2 or 3.
  • FGIC FGIC
  • a bulk density of 1.3-1.8 kg/m 3 a particle size of 6-8 nm
  • Figs. 1 to 5 show, in a manner well understood by a person skilled in the art, the especially good properties of a superfine expanded graphite prepared in accordance with the method of the present invention.
  • the open pore nature of the expanded graphite with interconnecting pores and its ability to absorb gases such as nitrogen, which is also indicative of its ability to absorb other intercalating species can be seen from Fig. 1
  • Fig. 2 shows the pore size distribution in the expanded graphite sample and it can be seen from this diagram that a large percentage of the pores, at least 20% of them and especially 40% of them and actually more than 50% of them lie in the range from 0.5nm to IOnm.
  • Figs. 3 to 5 show in detail the structure of the expanded graphite material with it being apparent from Fig. 5 in particular that the graphene layers are closely spaced from each other and that multilayer graphene structures are randomly orientated with larger interconnected pores between them which also cinterconnect to the spaces between the individual graphene layers.
  • the SEM micrographof Fig. 3 shows that the texture of the sample resem- ples corrugated crumpled paper.

Abstract

This application relates to the use of an expanded graphite material prepared by adding C1F3 in liquid form to solid carbon, by heating the mixture at a reaction temperature between 22 to 100°C to form intercalated compounds of the formula C2F.xC1F3 and C1Fgas with the C2F being a solid material present in layers and the C1F3 being a gaseous material present between adjacent layers of C2F, according to the reaction C (solid) + (x+1/2) C1F3 (liquid) → C2F.xC1F3 (solid) +1/2C1F(gas). The method further comprises the step of subsequently heating the intercalated compound to expel the C1F3 gas and to simultaneously form CF4 gas, with the gas formation and expulsion serving to expand the structure formed by the C2F layers, with the C2F layers changing composition to carbon layers with a percentage of fluorine in the range up to 5 %, said heating temperature lying in the range from 400 to 500°C. Such an expanded graphite material is particularly useful in electrochemical devices, e.g as an anode in a lithium ion battery or as a catalyst support.

Description

Use of a superfine expanded graphite and preparation thereof
The present invention relates to uses of superfine expanded graphite and to methods for the preparation thereof. The use of the superfine expanded graphite material preferably takes place in an electrochemical device, for example in secondary batteries, such as in a lithium battery, in a super- capacitor, in an electrochromic device or in a solar energy cell.
By way of example, lithium batteries are known in rechargeable form. Such batteries comprise positive and negative electrodes with a non- aqueous electrolyte disposed between them.
In a rechargeable lithium ion battery (secondary battery) the positive electrode of the battery can, for example, be LiCoθ2 (referred to as the "cathode" in the Li-battery community) and the negative electrode can for ex- ample be carbon (referred to as the "anode" in Li-battery community). In a non-rechargeable battery (primary battery) the positive electrode can for example be Mnθ2 and the negative electrode can be lithium metal. Various different types of electrolyte are known. For example there is the class of liquid electrolytes comprising at least one ionically conducting salt, such as Li(TFSI), i.e. lithium bis(trifluorosulphonyl)imide, LiPFό, i.e. lithium hexafluorophosphate, LiBOB (lithium bis(oxaltoborate) or LiClO4, i.e. lithium perchlorate, which are present, with a low degree of dissociation within a non-aqueous solvent, such as a mixture of DME (dimethylethane) and EC (ethylene carbonate), a mixture of DEC (diethylene carbonate) and EC, or a mixture of DMC (dimethyl carbonate) and EC or PC (propylene carbonate) or combinations thereof. A useful range for the degree of disso- ciation lies in the range from 1 x 1O1 to 108 HmoH. In addition there are so-called dry polymer electrolytes. In these electrolytes the salt is selected as before (i.e. for example from Li(TFSI), LiPFe, LiBOB or LiClO4) and is dispersed in a polymer or mixture of polymers. Suitable polymers comprise PEO (polyethylene oxide), PVDF (polyvinylene di-fluoride), PAN (poly- acrylonitrile) , and PMMA (polymethyl methyl acrylate).
As far as lithium batteries are concerned, various materials have been investigated as anodes, of which graphite is one of the most relevent candidates. One of the major advantages of graphite is the high chemical poten- tial of lithium in its intercalate resulting in a low voltage, e.g. 0.0 IV for LiC6 versus lithium. However the most important drawback of graphite is that its specific capacity is limited to 372 mAh g-1.
The object of the present invention is to provide a novel material in the form of an expanded graphite having a smaller particle size and a higher specific surface than previous expanded graphites which is well- suited for use in such electrochemical devices, for example as an electrode material, and also to propose a method for the preparation of such "superfine" expanded graphite which is beneficial for a large number of applications.
More specifically, the present invention proposes the use of superfine expanded graphite having a bulk density in the range 1.3 to 1.8 kg/πr3 and comprising ribbon-like particles having a thickness of 2 - 3 nm and a length of 13 nm to 100 nm and comprising a plurality of spaced apart graphene layers with a spacing of from 0.2 to 0.4 nm, the ribbon-like par- tides being randomly orientated relative to one another with pores therebetween, and a specific surface in the range from 150 m2 g"1 to 500 m2 g 1, particularly in the range from 250 m2g"1 to 280 m2g"1. Each ribbon-like particle typically comprises a plurality of spaced apart graphene layers.
It has been found that the use of the use of the porous superfine expanded graphite as a negative electrode in lithium- based electrochemical storage devices leads to high reversible capacities greater than 700 mAh/g and sufficient cycling performance over a wide voltage range (0 to 3 V). It is important for the present invention that the material is porous with interconnected pores and with the pore sizes ranging from typically about 0.5 nm to 50 nm and with a majority of pores preferably being below 10 nm in size. Another important feature of the material of the present invention is that there is some fluorine present in the carbon material and this fluorine increases the capacity of the structure which is beneficial for lithium intercalation, i.e. lithium storage properties. The fluorine content is normally from 0.5 to 5 % and more preferably in the range from 2 to 3 %.
Thus, in order to increase the specific capacity, carbonaceuos materials with disordered structures and morphologies have been investigated. Furthermore, apart from the increase in the specific capacity, in particular the excellent electronic conductivity and good Li mobility, such carbon e- lectrodes exhibit good reversibility and rate performance.
The invention particularly relates to the use of a material prepared by adding CIF3 in liquid form to solid carbon, heating the mixture at a reaction temperature between 22 to 1000C to form intercalated compounds of the formula C2F»xClF3 and ClFgas with the C2F being a solid material present in layers and the CIF3 being a gaseous material present between adjacent layers of C2F, according to the reaction
C (solid) + (X+ 1 / 2) ClF3 (liquid) → C2F»xClF3 (solid) + l/2ClF(gas)
the method further comprising the step of subsequently heating the intercalated compound to expel the CIF3 gas and to simultaneously form CF4 gas, with the gas formation and expulsion serving to expand the structure formed by the C2F layers, with the C2F layers changing composition to carbon layers with a percentage of fluorine in the range up to 5 %, said heating temperature lying in the range from 400 to 5000C. In the above equation x is an arbitrary integer, i.e. 1, 2, 3...n where n can be a large number.
That is to say the invention relates to an expanded graphite structure prepared by adding CIF3 in liquid form to solid carbon, heating the mixture at a reaction temperature between 22 to 1000C to form intercalated compounds of the formula C2F»xClF3 and ClFgas with the C2F being a solid material present in layers and the CIF3 being a gaseous material present be- tween adjacent layers of C2F, according to the reaction
C (solid) + (X+ 1 / 2) ClF3 (liquid) → C2F-XClF3 (solid) + l/2ClF(gas)
the method further comprising the step of subsequently heating the inter- calated compound to expel the CIF3 gas and to simultaneously form CF4 gas, with the gas formation and expulsion serving to expand the structure formed by the C2F layers, with the C2F layers changing composition to carbon layers with a percentage of fluorine in the range up to 5 %, said heating temperature lying in the range from 400 to 5000C. In the above equation x is an arbitrary integer, i.e. 1, 2, 3...n where n can be a large number.
The solid carbon is initially present in the form of graphite, either as chunks of graphite or graphite powder or graphite flakes. Preferred uses of the material are set forth in the dependent claim.
The expanded graphite is specifically useful as an electrode material of the present invention especially as a negative electrode material (anode material in the lithium battery community) due to its high storage capacity and excellent electronic conductivity.
Thus, the porous expanded graphite when used as anode in Li-based electrochemical storage device show high reversible capacities (> 70OmAh/ g) and sufficient cycling performance over wide voltage range (0-3V).
The invention will now be described in more detail with reference to the accompanying drawings in which are shown:
Fig. 1 the nitrogen adsorption/ desorption isotherms of the as pre- pared expanded graphite sample,
Fig. 2 the pore size distribution of the prepared expanded graphite sample,
Fig. 3 a scanning electron micrograph of the expanded graphite sample,
Fig. 4 a low resolution transmission electron micrograph of the expanded graphite sample, Fig. 5 a high resolution transmission electron micrograph of the expanded graphite sample,
Fig. 6 a graph showing the specific capacity of the expanded graph- ite material of the present invention when used in a lithium battery as the negative electrode material at a charge rate of C/5,
Fig. 7 a graph showing that the specific capacity of the expanded graphite material of the present invention when used in a lithium battery as the negative electrode remains substantially constant over a large number of charge/ discharge cycles at a charge rate of C, and
Fig. 8 the specific capacity of the expanded graphite material of the present invention when used in a lithium battery as the negative electrode showing that the specific capacity remains substantially constant over a large number of charge/ discharge cycles at charge rates from 10 C to 60 C.
Turning first to Fig. 6, the graph shows a typical galvostatic charge/ discharge curve for material produced in accordance with the above method. As is usual with such materials, the specific capacity of the electrode on the first discharge is considerably higher than after repeated charge /discharge cycles, which is why the limb of the curve which extends to about 2400 mAh/g and which is labeled "first discharge" differs from the limb which finishes at about 700 to 1000 mAh/g which shows the specific capacity for repeated charge/ discharge cycles. The curve which extends upwardly to the right from the origin shows the charging property of the electrode material. The inset shows the specific capacity of the electrode material as a result of the cycle number and it can be seen that the drop-off in specific capacity for up to 10 charge cycles is only minimal and indeed the curve flattens off and thus further cyclic charging and discharging of the electrode does not result in a significant change of the specific capacity of the electrode material.
This is shown in more detail for up to 50 charge cycles in Fig. 7, and in- deed at a higher rate C meaning the theoretical charge capacity for charging in a period of one hour: Whereas Fig. 6 relates to C/ 5, i.e. theoretical charge capacity for charging in a period of five hours, it can be seen that even at the higher charge rate of C, the specific capacity evens out at around 100 mAh/g.
Fig. 8 shows the specific capacity of the electrode material for significantly higher charge rates of 10, 20, 30, 40, 50 and 60 C, with 10 C meaning that the charge cycle lasts 1/ 10 of an hour, and 60 C meaning that it lasts one 1/60 of an hour, i.e. one minute (60 C). It can be seen that even at very high charge rates, such as one minute, the electrode material still has a specific capacity of around 75 mAh/g and this is a very good value when compared with the prior art.
Expanded graphite (EG) is known per se. The known EG is prepared by thermal decomposition of some graphite intercalation compounds (GIC) in the regime of "thermal shock". This method allows to prepare EG with a bulk density of ~3 kg/m3, a particle size of -40 nm, and specific surface up to 150 m2/g. The patent applicants have developed a method for preparation of more superfine EG with smaller particle size and higher specific surface, and also with higher intercalation and sorptive capacity. To achieve this goal, precursors were used based on fluorinated graphite intercalation com- pounds (FGIC) with organic and inorganic volatile substances. One effective precursor has been found to be the intercalation compound based on dicarbon fluoride with CIF3 having the empirical composition C2FCly«zClF3. In this expression y is an integer such as 1, 2 or 3 and z is also an integer, for example 1, 2 n, where n could be up to 10 or larger. This intercalation compound can be synthesized by fluorination of graphite with liquid CIF3 or its solutions in anhydrous HF:
C (solid) + (x+l/2)ClF3 (iiquid) → C2F»xClF3 (solid) + l/2ClF(gaS) (reaction temperature 22— 1000C)
In the above equation x is an arbitrary integer, i.e. 1, 2, 3...n where n can be a large number.
The composition of the matrix in all similar intercalates synthesized by the reaction of graphite with different halogen fluorides is uniform; the composition is changed very little (from C2Fo.94#yClF3 to C2Fo.98*yClF3) and does not depend on the synthesis temperature and used solvent. Here y is again 1, 2 or 3.
The principal difference between two approaches in preparation of traditional EG and superfine EG lies in the scheme of thermal decomposition of starting precursors - GIC and FGIC. In the case of GIC, the expansion of layered graphite matrix takes place only due to a fast increase of the vapor pressure of volatile intercalated substances (the regime of "thermal shock"). In the case of FGIC, the expansion of graphite particles takes place not only due to a fast increase of the vapor pressure of volatile intercalated substances but additionally thanks to the formation of gaseous fluorocarbons (mainly CF4) in the interlaminar spaces.
The schematic model of the intercalation compound (with C 1 F3) decomposition is following:
C2F^xClF3 → EG + CF4 + CF2Cl2
It has been shown that the use of FGIC as precursors allows the preparation of EG with the following parameters: a bulk density of 1.3-1.8 kg/m3, a particle size of 6-8 nm, a specific surface in the range from 150 m2g-1 to 500 m2g-1 and especially of 250- 280 m2/g and sorptive capacity up to
250 — 300 g/g (organic liquids). Such EG is a true nanomaterial. This new material surpasses known traditional EG appreciably on all characteristics. The effectiveness of its application in lithium chemical power engineering is much higher than that of EG obtained by traditional methods.
The Figs. 1 to 5 show, in a manner well understood by a person skilled in the art, the especially good properties of a superfine expanded graphite prepared in accordance with the method of the present invention. The open pore nature of the expanded graphite with interconnecting pores and its ability to absorb gases such as nitrogen, which is also indicative of its ability to absorb other intercalating species can be seen from Fig. 1 Fig. 2 shows the pore size distribution in the expanded graphite sample and it can be seen from this diagram that a large percentage of the pores, at least 20% of them and especially 40% of them and actually more than 50% of them lie in the range from 0.5nm to IOnm.
Figs. 3 to 5 show in detail the structure of the expanded graphite material with it being apparent from Fig. 5 in particular that the graphene layers are closely spaced from each other and that multilayer graphene structures are randomly orientated with larger interconnected pores between them which also cinterconnect to the spaces between the individual graphene layers.
The SEM micrographof Fig. 3 shows that the texture of the sample resem- ples corrugated crumpled paper.

Claims

Patent Claims
1. An expanded graphite having a bulk density in the range 1.3 to 1.8 kg/πr3 and comprising ribbon-like particles having a thickness of 2 - 3 nm and a length of 13 nm to 100 nm and comprising a plurality of spaced apart graphene layers with a spacing of from 0.2 to 0.4 nm, the ribbon-like particles being randomly orientated relative to one another with pores therebetween, and a specific surface in the range from 150 m2 g"1 to 500 m2 g-1, particularly in the range from 250 to 280 In2R &-1.
2. Use of the material in accordance with claim 1 in an electrode for an electrochemical device.
3. Use of the material in accordance with claim 1 or claim 2 as a negative electrode in a rechargeable lithium battery.
4. Use of the material in accordance with any one of the preceding claims to accommodate lithium ions as intercalating species be- tween adjacent graphene layers.
5. Use of the material in accordance with claim 1 or claim 2 in a fuel cell as a catalyst support, with the catalyst being present on the external surfaces of the ribbon-like particles.
6. Use of the material in accordance with claim 5 as a conductive catalyst support without additional conductivity promoting materials.
7. Use of the material in accordance with claim 1 or claim 2 as an elec- trode material in a supercapacitor.
8. A method of preparing an expanded graphite material as claimed in claim 1 or as used in any one of the preceding claims, the material being prepared by adding CIF3 in liquid form to solid carbon, heat- ing the mixture at a reaction temperature between 22 to 1000C to form intercalated compounds of the formula C2F»xClF3 and ClFgas with the C2F being a solid material present in layers and the CIF3 being a gaseous material present between adjacent layers of C2F, according to the reaction
C (solid) + (X+ 1 / 2) ClF3 (liquid) → C2F-XClF3 (SoUd) + l/2ClF(gas)
the method further comprising the step of subsequently heating the intercalated compound to expel the CIF3 gas and to simultaneously form CF4 gas, with the gas formation and expulsion serving to expand the structure formed by the C2F layers, with the C2F layers changing composition to carbon layers with a percentage of fluorine in the range up to 5 %, said heating temperature lying in the range from 400 to 5000C.
9. An expanded graphite structure prepared by adding CIF3 in liquid form to solid carbon, heating the mixture at a reaction temperature between 22 to 100°C to form intercalated compounds of the formula C2F»xClF3 and ClFgas with the C2F being a solid material present in layers and the CIF3 being a gaseous material present between adjacent layers of C2F, according to the reaction
C (solid) + (X+ 1 / 2)C1F3 (liquid) → C2F-XClF3 (SoUd) + l/2ClF(gas) the method further comprising the step of subsequently heating the intercalated compound to expel the CIF3 gas and to simultaneously form CF4 gas, with the gas formation and expulsion serving to expand the structure formed by the C2F layers, with the C2F layers changing composition to carbon layers with a percentage of fluorine in the range up to 5 %, said heating temperature lying in the range from 400 to 5000C.
10. An expanded graphite structure prepared as in claim 9, wherein the solid carbon is initially present in the form of graphite, either as chunks of graphite or graphite powder or graphite flakes.
PCT/EP2009/004439 2008-06-20 2009-06-19 Use of a superfine expanded graphite and preparation thereof WO2009153051A1 (en)

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RU2554933C1 (en) * 2014-01-09 2015-07-10 Общество с ограниченной ответственностью "АкКо Лаб" Composite carboniferous material for chemical current source and method of its obtaining

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