EP3558867A1

EP3558867A1 - Graphite material

Info

Publication number: EP3558867A1
Application number: EP17818147.5A
Authority: EP
Inventors: Manfred Jung; Daniel Steppich; Oswin ÖTTINGER; Rainer Schmitt
Original assignee: SGL Carbon SE
Current assignee: SGL Carbon SE
Priority date: 2016-12-23
Filing date: 2017-12-20
Publication date: 2019-10-30
Also published as: WO2018115177A1

Abstract

The present invention relates to a graphite material based on a precursor material comprising at least one of the group consisting of polyacrylonitrile, rayon, resin, pitch and natural carbon precursors, wherein the graphite material has an interlayer spacing (d002 distance) of below 0.344 nm, preferably below 0.341 nm, more preferably of below 0,340 nm and a mean crystallite size (Lc) of at least 12 nm, preferably at least 20 nm, wherein the graphite material is porous having voids with a mean diameter of not larger than 50 nm, preferably not larger than 10 nm, more preferably not larger than 5 nm, and wherein the graphite material is in the form of either graphite fibers or graphite powder. The graphite material is produced by catalytic graphitization via coating.

Description

Graphite material

The present invention relates to graphite fibers and graphite powder, a method for the manu- facturing the same and its uses for battery felts used in redox-flow batteries or NaS batteries, for carbon fiber papers used as gas diffusion layers in fuel cells and electrolysis cells, in satellite structure parts, in heat spreaders in the area of thermal management, as electrode material or for current collectors in desalination units, batteries, preferably redox-flow batteries, LiS batteries and Li-ion batteries, preferably as anode material in lithium-ion battery applications, as conductive filler for polymers, as a catalyst carrier, as additive for friction materials, preferably in brake pads or brake discs typically for aircraft and racing cars, wet friction, preferably for clutches, and for deicing applications, as filter material, for electromagnetic shielding, for flash protection and as material for electrical contacts. Graphite fibers compared to carbon fibers are characterized by their high grade of graphitiza- tion resulting in the molecular structure and microstructure of graphite. The microstructure of graphite is defined by the crystallographic parameters (i) interlayer spacing (d002 distance) which is the distance between adjacent molecular layers of covalently bonded sp²-hybridized carbon and (ii) mean crystallite size (Lc) which is a measure of the extension of the perfect graphite structure within a material. In a perfect graphite single crystal, the d002 distance is 0.3354 nm and Lc corresponds to the size of the macroscopic crystal. While the d002 distance of a carbon fiber which is based on polyacrylonitrile is about 0.345 nm and Lc is about 3 nm after carbonization at 1400 °C, one can talk about a graphite fiber, if the d002 distance is 0.344 or lower accompanied with an increase of Lc to 12 nm or higher.

A graphitic material, such as a graphite fiber, features high electrical and thermal conductivity rather than, for example, high tensile strength as carbon fibers do. Thus, the focus of graphite materials lies on applications requiring suitable thermo-electrical properties rather than good mechanical properties. Graphite fibers can, for example, be obtained by graphitization of mesophase pitch-based carbon fibers. That means, carbon fibers based on mesophase pitch are heated to a sufficiently high temperature, typically above 2200 °C, so that the apriori amorphous carbon converts into a graphitic structure. Mesophase pitch-based carbon fibers, however, are very expensive. Compared to typical polyacrylonitrile-based carbon fibers mesophase pitch-based carbon fibers can cost about 100 times more. Polyacrylonitrile-based carbon fibers, however, do not graphitize as good as mesophase pitch-based carbon fibers even if they are heated up to 2200 °C or above.

The d002 distance and the Lc of a polyacrylonitrile-based carbon fiber which is carbonized at a temperature of 2500 °C are about 0.343 nm and 5 nm respectively (Sources: M. Inagaki, F. Kang, Materials Science and Engineering of Carbon: Fundamentals, Elsevier, 2nd Edition, 2014 and M.R. Buchmeister et al., Carbonfasern: Prakursor-Systeme, Verarbeitung, Struktur und Eigenschaften, Angewandte Chemie 2014, vol. 126, pages 5364-5403). According to WO2017/178492 a polyacrylnitrile based graphite fiber is produced by adding nano-additives to the polyacrylnitrile spinning process before conducting the steps of stabilization, carbonization and graphitization. However, this process has the disadvantage that an increased porosity or higher defect numbers or defect sizes of the graphite fibers is generated due to the nano- additives after their reaction in the catalytic process. Typically, a pore (defect) is introduced at the place where the additive was incorporated in the fiber. Consequently, the mechanical prop- erties of these graphite fibers are deteriorated and the surface of these graphite fibers is more prone to oxidation.

It is, therefore, the object of the present invention to provide a graphite fiber with a high grade of graphitization having improved mechanical properties and not being prone to oxidation.

The solution of the present invention is based on the finding that precursor materials having small diameter, like fibers or powder, can be catalytically graphitized by applying a graphitization catalyst on the surface of the respective precursor material, like polyacrylonitrile fibers or powder, rayon fibers or powder, resin fibers or powder, pitch fibers or powder, or natural (or „bio"-) precursor fibers or powder, like lignin fibers or powder. The thus applied coating catalyzes the transformation of the precursor material via carbon to graphite at elevated temperatures somehow from the surface region of the material to the center, i.e., the inner region of the material. The prescribed catalytic graphitization via coating also works well when the respective stabilized (or chemically activated) precursor material or carbon material is coated with the graphitization catalyst.

The present invention, thus, provides as a first aspect a graphite material based on a precursor material comprising at least one of the group consisting of polyacrylonitrile, rayon, resin, pitch and natural carbon precursors, wherein the graphite material has an interlayer spacing (d002 distance) of below 0.344 nm, preferably below 0.341 nm, more preferably of below 0.340 nm and a mean crystallite size (Lc) of at least 12 nm, preferably at least 20 nm, wherein the graphite material is porous having voids with a mean diameter of not larger than 50 nm, preferably not larger than 30 nm, more preferably not larger than 20 nm, even more preferably not larger than 10 nm, most preferably not larger than 5 nm, and wherein the graphite material is in the form of either graphite fibers or graphite powder. The mean diameter of the voids can be determined using electron microscopy like scanning electron microscopy (SEM) or transmission electron microscopy (TEM).

In the framework of the present invention the terms„graphitic material" and„graphite material" are synonyms and refer to the substance itself, regardless of the form in which the substance is present. The graphite material of the present invention is in the form of fibers and powders. The fibers and powders of the present invention can, of course, be further processed to, for example, textiles and composites as described further below which are also encompassed by the present invention. Physical properties with respect to the graphite material of the present invention refer to the substance as such. The voids of the present invention, for example, are present in the fibers or in the powder. Further voids may be present in a textile or composite between the fibers or between the powder particles, but such voids are not meant by the voids of the present invention. In case that some features of the aspects and embodiments of the present invention including the corresponding advantages are described in connection with either fibers only or a powder only, this is not to be understood as limiting. Unless explicitly described otherwise or the occurrence of a logical contradiction, all features described herein can be combined with both forms, fibers and powder, of the graphitic material of the present invention, its respective production methods and its uses. The interlayer spacing (d002 distance) and mean crystallite size (Lc) are crystallographic standard parameters which can be easily measured by known methods, like X-ray diffraction, for example, according to the method described by J. Maire and J. Mehring in "Graphitization of soft carbons" in Chemistry and Physics of Carbon, Vol. 6, Marcel Dekker, P.L. Walker Jr. (Publisher), New York, 1970, pages 125-190.

The material of the present invention is, thus, getting close to the ideal graphite structure featuring high thermal and electrical conductivity. Furthermore, also the frictional properties are advantageous, in particular for applications like as friction materials in clutches, wet-friction, brake discs and brake pads. One of the advantages of the graphite material of the present invention is that it has a high thermal conductivity of about 1 100 W/m-K (determined according to DIN 51908). The electrical conductivity is also very high compared to standard carbon materials, like carbon fibers. Furthermore, the graphite material of the present invention is less cost-intensive than mesophase- pitch based graphite materials, since it is based on the precursor materials polyacrylonitrile, rayon, resin, like phenolic resin, pitch or natural carbon precursors, like lignin. A preferred precursor material of the present invention is polyacrylonitrile. Polyacrylonitrile-based carbon fibers, for example, are well known in the art and exhibit a lot of advantageous properties while being relatively cost-effective at the same time.

The term "polyacrylonitrile-based" means that the graphite material is obtained by starting with polyacrylonitrile as the base material for the respective precursor fiber or powder as it is analogously the case with polyacrylonitrile-based carbon fibers which are well known in the art. The term "polyacrylonitrile-based" also includes co-polymers of polyacrylonitrile with other monomers.

Natural carbon precursors mean bio precursors for carbon materials like, for example, lignin.

According to a preferred embodiment of the present invention, the graphite material contains at least one element selected from the group consisting of Ca, Si, B, Al, Ti, V, Cr, Ni, Mn, Fe, Mo, W and Cu. As will become apparent further below with regard to the second aspect of the present invention, these elements originate from a coating which has been applied on the respective fibers or powder before graphitization. The elements may diffuse somehow into the material during graphitization and, thus, traces of the elements may still be present in the graphite material of the present invention and may have some advantageous functions depending on the final application of the graphite material. Besides the above pure elements, the graphite material can also contain nano-forms, oxides, carbides, nitrides, borides, inorganic salts and organic salts thereof. Graphite fibers are preferred forms of the graphite material of the present invention. The present invention, thus, provides a graphite fiber comprising the graphite material of the present invention. The graphite fibers of the present invention can be present as single fibers, fiber bundles, stretch broken yarn and/or any kind of textile and in all of these forms, the fibers can be either staple fibers or continuous fibers. Textiles include woven fabrics, braidings, non- wovens, non-crimp fabrics and paper. In an embodiment, the graphite fibers of the present invention are preferably present in the form of a woven fabric. In another embodiment, the graphite fibers of the present invention are preferably present in the form of a braided fabric. In another embodiment, the graphite fibers of the present invention are preferably present in the form of a non-woven. In another embodiment, the graphite fibers of the present invention are preferably present in the form of a non-crimp fabric. In another embodiment, the graphite fibers of the present invention are preferably present in the form of a graphite fiber paper. Furthermore, the graphite fibers of the present invention can also be present in the form of milled fibers. The milling grade of the milled fibers is not limited and, thus, the milled fibers according to an embodiment are preferably present in the form milled down to a powder. From all the preceding forms of the graphite fibers of the present invention, further products can be formed, like fiber composite materials, which are also encompassed by the present invention.

According to a second aspect of the present invention, a method for manufacturing a graphite material is provided comprising the following steps:

a) providing a liquid precursor composition comprising polyacrylonitrile, rayon, resin, pitch or natural carbon precursors,

b) forming a precursor material selected from the group consisting of precursor fibers and precursor powder by either spinning the precursor composition in order to obtain precursor fibers or forming a precursor powder from the precursor composition, respectively, c) optionally subjecting the precursor material to stabilization in order to obtain a stabilized precursor material,

d) subjecting the precursor material or the stabilized precursor material to carbonization in order to obtain a carbon material and

e) subjecting the carbon material to graphitization in order to obtain the graphite material, wherein either after step b), c) or d) the material obtained in the respective step is subjected to a coating step comprising applying a compound on the surface of the material, wherein the compound contains at least one of the group consisting of Ca, Si, B, Al, Ti, V, Cr, Ni, Mn, Fe, Mo, W and Cu as well as oxides, carbides, nitrides, borides, inorganic salts and organic salts thereof.

By using this method, a graphite fiber based on PAN precursor can be produced with a inter- layer distance of 0.336 nm; cf. the examples further below: The XRD (X-Ray diffraction) structure of the former Ni-coated fiber is more or less identical with a structure which can only be done for C-fibers with mesophase as a precursor or even better. If active material with a low vaporization temperature can be used (e.g. Ni), then the temperature for graphitization can be lowered to e.g. 2000 °C and even lower. This increases efficiency of the process and reduces energy costs. The respective Lc-value of the fiber is also very high close to maximum of the measurable level (> 250 nm). As a resin, any carbonizable resin can be used, i.e., any resin which, under inert atmosphere and elevated temperatures, essentially transforms into carbon. Exemplary resins are phenolic resins, epoxy resins or polyimide based resins, wherein phenolic resins are preferred due to the higher carbon yield after carbonization. Under„elevated temperature" in this regard, temperatures are to be understood which are sufficient for carbonizing organic matter, typical tem- peratures for carbonization (step d of the method) are above 900 °C, preferably 1 100 °C or above.

The kind of spinning method in step b) is not particularly limited. Possible spinning methods are air gap spinning, dry spinning, melt spinning and wet spinning. Preferably, however, the spinning step b) is a wet spinning method, in particular solvent spinning, or air gap spinning. The wet spinning and air gap spinning method are known in the art of polyacrylonitrile-based carbon fiber production. From the spinning method, a precursor fiber can be obtained. The fiber can be further processed according to the process of the invention as a continuous fiber. It is, however, also possible making any kind of textile fabric from the fiber before further pro- cessing. That is, a textile can be formed from the fiber either in the state of a precursor fiber or a stabilized precursor fiber or a carbon fiber. As a preferred embodiment, thus, a textile is formed from the stabilized precursor fibers, then the textile is subjected to the coating step, to carbonization and finally to graphitization. As another preferred embodiment, thus, a textile is formed from the coated stabilized precursorfibers, then the textile is subjected to carbonization and finally to graphitization. As still another preferred embodiment, thus, a textile is formed from the carbon fibers, then the textile is subjected to the coating step and finally to graphitization. As still another preferred embodiment, thus, a textile is formed from the coated carbon fibers, then the textile is subjected to graphitization. As still another preferred embodiment, thus, a textile is formed from the coated precursor fibers, then the textile is subjected to stabi- lization, to carbonization and finally to graphitization. As still another preferred embodiment, thus, a textile is formed from the precursor fibers, then the textile is subjected to the coating step, to stabilization, to carbonization and finally to graphitization. The present method can also be performed using polyacrylonitrile powder materials, leading to highly graphitized round-shaped powders which can be used e.g. as anode material with Lithium-ion Battery (LiB)-applications. The term ..stabilization" according to the optional method step c) is to be understood generally as a kind of chemical activation step which can be carried out under a variety of conditions depending on the kind of precursor material chosen. Whether or not the step of stabilization is performed depends on the choice of the precursor material. Stabilization is highly preferred when the precursor material is of polyacrylonitrile. Stabilization of polyacrylonitrile typically is conducted under elevated temperatures in the presence of oxygen in the atmosphere which is well known in the art of carbon fiber production. Other materials like cellulose or rayon may be pyrolized at about 400 °C and then carbonized right away in one step. Polyethylene, for example, may be chemically activated before carbonization by a change of the C-H groups into C- hetero atom groups. Pitch fibers are typically oxidized between 200 °C and 400 °C before carbonization. These and other methods for stabilization/chemical activation are known in the art and any of them can be used in the process of the present invention, if suitable.

Typical graphitization temperatures (step e of the method) are within the range of 2400 °C to 3000 °C. Depending on the choice of the graphitization catalyst, the graphitization temperature can be lowered to about 800 °C. Thus, the preferred temperature range for the graphitization is from 800 °C to 3000 °C, more preferably from 800 °C to 2500 °C. According to a preferred embodiment of the present invention, step e) is conducted in an inert atmosphere.

The compound applied in the method of the present invention is or contains a graphitization catalyst or its respective precursor. According to a preferred embodiment of the present invention, the compound applied in the coating step contains at least one of the group consisting of Ni, Fe, oxides of Ni and Fe, carbides of Ni and Fe, nitrides of Ni and Fe, borides of Ni and Fe, inorganic and organic salts of Ni and Fe, nano-silicon, S1O2, SiC, AI2O3, T1B2, T1O2 and TiC. Ni-acetate is more preferred. Due to variation of the additive content, i.e., the catalyst material, and high temperature treatment conditions interlayer spacing can be controlled and properties like elastic modules and conductivities can be designed.

The coating step can be performed after step b). That is, the coating, i.e., the compound, is applied on the surface of the precursor fiber or precursor powder. Preferably, the coating is applied onto the carbon material, like a carbon fiber. If the optional stabilization step or, as explained above, the chemical activation step c) is carried out, it is preferred that the coating step is performed after the stabilization step.

According to a preferred embodiment of the present invention, the compound is applied on the surface of the material either chemically, electrochemically or physically by contacting the material with the compound in the coating step.

According to a preferred embodiment of the present invention, the compound to be applied is in the form of dry particles, in the form of suspended particles in a liquid medium, in the form of a solution in a liquid solvent or in gaseous form. A preferred coating step is performed by contacting the material to be coated with a liquid medium containing the compound, i.e., the graphitization catalyst, preferably a ΤΊΟ2 dispersion, a Si-based sizing or a Ni-salt solution for chemical deposition of Ni. According to a preferred embodiment of the present invention, the precursor material is a precursor fiber and the coating step comprises a fiber sizing step. The fiber sizing step can be similar to any sizing method known in carbon fiber production.

The liquid precursor composition encompasses any liquid, including but not limited to solutions or melts, containing a polyacrylonitrile polymer, a rayon polymer, a resin polymer, pitch or natural carbon precursors. Preferred, however, is a polyacrylonitrile solution. Most preferably, the liquid precursor composition comprises a polyacrylonitrile precursor for carbon fiber production. Also preferred is pitch melt, being then subjected to melt spinning. According to a preferred embodiment of the present invention, after step e) the graphite material is thermally treated at above 1000 °C in the presence of chlorine gas or chlorofluorocarbon gas, preferably dichlorodifluoromethane and/or 1 ,1 ,1 ,2-tetrafluorethane. This treatment is suitable to remove remaining graphitization catalyst and, thus, to purify the graphite product. In another aspect, the present invention provides a use of the graphite material of the present invention or of the graphite material obtainable from the method of the present invention for battery felts used in redox-flow batteries or NaS batteries; for carbon fiber papers used as gas diffusion layers in fuel cells and electrolysis cells; in satellite structure parts; in heat spreaders in the area of thermal management; as electrode material or for current collectors in desalina- tion units, batteries, preferably redox-flow batteries, LiS batteries and Li-ion batteries, preferably as anode material in lithium-ion battery applications; as conductive filler for polymers; as a catalyst carrier; and as additive for friction materials, preferably in brake pads or brake discs typically for aircraft and racing cars, wet friction, preferably for clutches; for deicing applica- tions; as filter material; for electromagnetic shielding; for flash protection and as material for electrical contacts.

EXAMPLES: Example 1 : Ni Coating

For Ni-coating of a polyacrylonitrile (PAN)-based carbon fiber the following method can be exemplarily used (among other known Ni-Coating procedures): For cleaning the surface of the carbon fiber, the fiber is rinsed with water and sonicated afterwards in distilled water, acetone, NaOH, HCI and a second time with distilled water each for 15 minutes.

Then the dried fiber is heated to 50 °C in a solution of SnC /HCI for 4 hours and afterwards in a solution of PdC /HCI for 15 minutes.

Then a coating solution is prepared by mixing 30 g/l NiC , 10 g/l sodium hypophosphate, 12 g/l sodium citrate, 10 ml/l acetic acid in an aqueous solution. The ph was adjusted to 4-6 by adding NH4OH. Temperature of the solution was then heated to 90 °C and the fiber dipped in for coating.

The coated fiber was then heated up to 1800°C in an inert atmosphere and XRD data was measured afterwards in comparison to the same PAN-based carbon fiber without Ni coating. Fiber without coating: d002 [nm] = 0,346; Lc [nm] = 4.

Fiber with Ni-coating: d002 [nm] = 0,336; Lc [nm] = 131 . Example 2: T1O2 Coating

For coating of an oxidized PAN-fiber an aqueous dispersion of titanium dioxide is prepared using titanium dioxide with a primary particle size of d50 = 14 nm and d50 = 70 nm in aggregate state (available under the trade name AEROXIDE® ΤΊΟ2 P90 from Evonik).

A dispersion with 2 g/l is prepared and treated with ultrasonic for 10 minutes for particle dispersion. Every 2 minutes the dispersion was allowed to cool down for 5 minutes at room tem- perature.

The oxidized PAN-fiber was then dip coated in the prepared dispersion, that is, dipped in, taken out and shortly dripped of. Afterwards the fiber was dried for 3 hours at 120 °C. Based on this dip coating about 0,5 weight-% T1O2 based on the total weight of the oxidized PAN-fiber was applied on the oxidized PAN-fiber.

The coated fiber was then heated up to 2800 °C in an inert atmosphere and XRD data was measured afterwards in comparison to the same oxidized PAN fiber without T1O2 coating.

Fiber without coating: d002 [nm] = 0,341 ; Lc [nm] = 10.

Fiber with 0,5% Ti02 coating: d002 [nm] = 0,337; Lc [nm] = 24.

Example 3:

A nickel coated carbon fiber from Toho Tenax (PAN based), Tenax-J HTS40 A23 12K 1420 tex MC (diameter: 7,5 μηι; incl. 0.25 μηι Ni) was graphitized in N2-atmosphere at 2000°C and in Lengthwise Graphitization at 2600°C. XRD analysis yields interlayer spacing of 0,336 nm and Lc of 320 nm.

For comparison a pitch fiber from Mitsubishi Rayon (DIALEAD K63712) yields interlayer spacing of 0,337 nm (Lc of 58 nm) only and is therefore less graphitized.

Claims

A graphite material based on a precursor material comprising at least one of the group consisting of polyacrylonitrile, rayon, resin, pitch and natural carbon precursors, wherein the graphite material has an interlayer spacing (d002 distance) of below 0.344 nm, and a mean crystallite size (Lc) of at least 12 nm, wherein the graphite material is porous having voids with a mean diameter of not larger than 50 nm, wherein the graphite material is in the form of either graphite fibers or graphite powder.

The graphite material according to claim 1 , characterized in that the material is based on polyacrylonitrile.

The graphite material according to claim 1 or 2, characterized in that it contains at least one element selected from the group consisting of Ca, Si, B, Al, Ti, V, Cr, Ni, Mn, Fe, Mo, W and Cu.

A graphite fiber comprising the graphite material according to any of claims 1 to 3.

A method for manufacturing a graphite material according to any of claims 1 to 4 comprising the following steps:

b) forming a precursor material selected from the group consisting of precursor fibers and precursor powder by either spinning the precursor composition in order to obtain precursor fibers orforming a precursor powderfrom the precursor composition, respectively,

c) optionally subjecting the precursor material to stabilization in order to obtain a stabilized precursor material,

e) subjecting the carbon material to graphitization in order to obtain the graphite material,

wherein either after step b), c) or d) the material obtained in the respective step is subjected to a coating step comprising applying a compound on the surface of the material, wherein the compound contains at least one of the group consisting of Ca, Si, B, Al, Ti, V, Cr, Ni, Mn, Fe, Mo, W and Cu as well as oxides, carbides, nitrides, borides, inorganic salts and organic salts thereof.

The method according to claim 5, characterized in that the compound applied in the coating step contains at least one of the group consisting of Ni, Fe, oxides of Ni and Fe, carbides of Ni and Fe, nitrides of Ni and Fe, borides of Ni and Fe, inorganic and organic salts of Ni and Fe, nano-silicon, S1O2, SiC, AI2O3, T1B2, T1O2 and TiC.

The method according to claim 5, characterized in that the compound is applied on the surface of the material either chemically, electrochemically or physically by contacting the material with the compound in the coating step.

The method according to claim 5, characterized in that the compound to be applied is in the form of dry particles, in the form of suspended particles in a liquid medium, in the form of a solution in a liquid solvent or in gaseous form.

The method according to claim 5, characterized in that the precursor material is a precursor fiber and that the coating step comprises a fiber sizing step.

Use of the graphite material according to any of claims 1 to 4 for battery felts used in redox-flow batteries or NaS batteries; for carbon fiber papers used as gas diffusion layers in fuel cells and electrolysis cells; in satellite structure parts; in heat spreaders in the area of thermal management; as electrode material or for current collectors in desalination units, batteries, preferably redox-flow batteries, LiS batteries and Li-ion batteries; as conductive filler for polymers; as a catalyst carrier; as additive for friction materials; for de- icing applications; as filter material; for electromagnetic shielding; for flash protection and as material for electrical contacts.