The present invention relates to a polyacrylonitrile-based graphite fiber, a textile fabric containing the polyacrylonitrile-based graphite fiber, a method for the manufacture of the graphite fiber and the textile fabric and its use as 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 graphitization 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 sp ² -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.

Graphite fibers feature high electrical and thermal conductivity rather than, for example, high tensile strength as carbon fibers do. Thus, the focus of graphite fibers 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 2,200°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 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 2,200°C or above.

The d002 distance and the Lc of a polyacrylonitrile-based carbon fiber which is carbonized at a temperature of 2,500°C are about 0.343 nm and 5 nm respectively (Sources: M. Inagaki, F. Kang, Materials Science and Engineering of Carbon:

Fundamentals, Elsevier, 2 ^nd Edition, 2014 and M.R. Buchmeister et al.,

Carbonfasern: Prakursor-Systeme, Verarbeitung, Struktur und Eigenschaften, Angewandte Chemie 2014, vol. 126, pages 5364-5403). In JP2014167039 (A) a catalytic graphitization method is described, wherein the activation energy of the carbonization reaction can be lowered so that the respective carbon fiber carbonized at lower temperatures has the same crystallinity as a typical polyacrylonitrile-based carbon fiber carbonized at higher temperatures. However, carbon fibers having a higher grade of crystallinity are not described. The method includes a copolymeri- zation of polyacrylonitrile with an organometallic compound of a group VIII metal. The so obtained copolymer is then used as a precursor polymer in the known carbon fiber production method. While the performance of a copolymerization method with an organometallic compound is a rather expensive method and the reached crystallinity of the resulting carbon fiber still not exceeds that of typical

polyacrylonitrile-based carbon fibers, better solution should become available.

It is, therefore, the object of the present invention to provide a graphite fiber with a high grade of graphitization which can be manufactured by a simple and cost efficient method.

The solution of the present invention solving this object is based on the finding that solid fine milled particles can be employed in the polyacrylonitrile precursor composition as graphitization catalysts. From such a composition precursor fibers can be spun which in turn can be further processed to stable graphite fibers having a higher grade of graphitization than typical polyacrylonitrile-based carbon fibers without graphitization catalysts processed under the same conditions. As a first aspect, the present invention provides a polyacrylonitrile-based graphite fiber, wherein the material of the graphite fiber exhibits an interlayer spacing (d002 distance) of below 0.344 nm, preferably below 0.341 nm and a mean crystallite size (Lc) of at least 12 nm, preferably at least 20 nm. One of the advantages of the graphite fiber of the present invention is that it has a high thermal conductivity of about 1 100 W/m-K. The electrical conductivity is also very high compared to standard carbon fibers. Furthermore, the graphite fiber of the present invention is less cost-intensive than mesophase-pitch based graphite fibers, since it is based on polyacrylonitrile.

The term "polyacrylonitrile-based" means that the graphite fiber is obtained by starting with polyacrylonitrile as the base material for the respective precursor fiber 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.

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 graphite fiber of the present invention is not limited with regard to any coatings or the like being present, for example, on the surface of the fiber. Any such additional material is not encompassed by the expression "the material of the graphite fiber". The "material of the graphite fiber" rather refers to the graphitic material based on polyacrylonitrile. According to a preferred embodiment of the present invention the material of the polyacrylonitrile-based graphite fiber exhibits an interlayer spacing (d002 distance) of below 0.341 nm, preferably below 0.339 nm after being heated to a temperature of at least 2,500°C for 30 minutes. Even more preferably, the material of the graphite fiber exhibits an interlayer spacing (d002 distance) of below 0.339 nm, preferably below 0.337 nm after being heated to a temperature of at least 3,000°C for 30 minutes. The material of the fiber 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.

According to a preferred embodiment of the present invention the material of the polyacrylonitrile-based graphite fiber exhibits a mean crystallite size (Lc) of at least 12 nm, preferably at least 20 nm after being heated to a temperature of at least 2,500°C for 30 minutes. Even more preferably, the material of the graphite fiber exhibits a mean crystallite size (Lc) of at least 20 nm, preferably at least 30 nm after being heated to a temperature of at least 3,000°C for 30 minutes. This high grade of crystallinity also leads to high thermal and electrical conductivity and advantageous frictional properties.

The polyacrylonitrile-based graphite fiber according to the present invention is not limited with regard to any other ingredients within the fiber besides the graphitic polyacrylonitrile-based material. In a preferred embodiment, however, the graphite fiber contains one or more elements selected from the group consisting of Ca, Ti, Si, B, V, Cr, Ni, Mn, Fe, W and Cu in the form of its carbides, oxides, borides and/or nitrides, wherein the carbides, oxides, borides or nitrides are embedded in the fiber in the form of particles having a mean particle size in the range of 5 nm to 1 μιτι, preferably 10 nm to 0.35 μιτι and more preferably 50 nm to 0.1 μιτι. In the graphite fiber, such particles may have some advantageous functions depending on the final application of the fiber. In the context of the present invention, the mean particle size corresponds to the d50-value. This particle size can be measured by using the laser diffraction method according to ISO 13320. The main function of these particles lies in the catalysis of the graphitization reaction which leads to the high grade of crystallinity according to the present invention of the material of the graphite fiber. More preferably, the graphite fiber comprises one or more elements selected from the group consisting of Ti, Si and Ni in the form of its carbides and/or oxides, wherein the carbides or oxides are embedded in the fiber in the form of particles having a mean particle size in the range of 5 nm to 1 μιτι, preferably 10 nm to 0.35 μιτι and more preferably 50 nm to 0.1 μιτι.

According to a still further preferred embodiment of the present invention the particles are present in an amount of 0.01 to 3 weight-percent (wt.-%), preferably of 0.01 to 1 .5 wt-%, based on the total weight of the graphite fiber. Within these ranges the highest grades of graphitization can be observed. The amount can be measured simply by burning the graphite fiber in an oxygen-containing atmosphere, weighing the residual ashes and analyzing the ashes. The total ash content of the graphite fiber of the present invention is not particularly limited. Preferably, however, the graphite fiber has an ash content in the range 0.01 to 3 wt-%, preferably of 0.01 to 1 .5 wt-%, based on the total weight of the graphite fiber, wherein at least 60% the ash consists of NiC, NiO, TiC, T1O2, SiC, S1O2 or mixtures thereof.

In another embodiment of the present invention the graphite fiber has an ash content of below 0.1 wt-%, preferably below 0.05 wt-% and more preferably below 0.01 wt-% based on the total weight of the graphite fiber and that the fiber is porous and contains voids having a mean diameter in the range of 5 nm to 1 μιτι. Such a fiber can be obtained according to a preferred method described below, wherein the graphite fiber is subjected to a purification method, for example, with chlorine gas converting the particles mentioned above into volatile substances and, thus, removing them from the fiber. As a result, the preferred graphite fiber of the present invention is of high purity and high crystallinity. The residual pores resulting from the particles previously present in the graphite fiber preferably have mean diameter in the range of 5 nm to 1 μιτι, preferably 10 nm to 0.35 μιτι and more preferably 50 nm to 0.1 μιτι. It is assumed that the particles mentioned above are completely removed during the purification process leaving pores having the corresponding size of the particles. The pore volume of such a purified fiber is preferably in the range of 0.005 Vol.-% to 1 .5 Vol.-%. The polyacrylonitrile-based graphite fiber of the present invention can be further processed into any textile. In another aspect, the present invention relates to a textile fabric comprising the polyacrylonitrile-based graphite fiber of the present invention being in the form of a woven fabric, a nonwoven or a paper. These textiles can be advantageously used as 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.

In another aspect, the present invention relates to a method for the manufacture of a graphite fiber comprising the following steps:

a) providing a liquid polymer comprising polyacrylonitrile,

b) adding carbide, oxide, boride or nitride particles of one or more elements selected from the group consisting of Ca, Ti, Si, B, V, Cr, Ni, Mn, Fe, W and Cu to the liquid polymer, mixing the same and obtaining a precursor composition for the graphite fiber,

c) spinning the precursor composition in order to obtain a precursor fiber,

d) subjecting the precursor fiber to stabilization in order to obtain a stabilized precursor fiber,

e) subjecting the stabilized precursor fiber to carbonization in order to obtain a carbon fiber and

f) subjecting the carbon fiber to graphitization in order to obtain the graphite fiber.

Preferably, the graphite fiber of the present invention is manufactured by the method of the present invention. The term "liquid polymer comprising polyacrylonitrile" in the context of the present invention encompasses any liquid, including but not limited to solutions or melts, containing any polyacrylonitrile-containing polymer, including but not limited to a polyacrylonitrile polymer, a copolymer with polyacrylonitrile. Any known

polyacrylonitrile-based precursor compositions for carbon fiber production can be used as the liquid polymer.

The particles added in step b) work as graphitization catalysts in the later

graphitization step f), resulting in a graphite fiber of a high grade of graphitization. Preferably, the particles added in step b) comprise carbide or oxide particles of Ti, Si or Ni and the particles have a mean particle size in the range of 5 nm to 1 μιτι, preferably in the range of 10 nm to 0.35 μιτι and more preferably in the range of 50 nm to 0.1 μιτι. Most preferred are T1O2 and SiC particles having the respective particle size. The preferred particles, in particular, show a high catalytic effect.

Foreign particles in a precursor composition for carbon fibers or graphite fibers always lead to a weakening of the mechanical properties, like tensile strength, of the final fibers. The preferred particle sizes, however, lead to a low weakening of the mechanical properties. As a preferred embodiment of the method of the present invention the particles are added in an amount of 0.05 to 1 .5 wt.-%, more preferably 0.1 to 1 wt.-% based on the total weight of the liquid polymer provided in step a). If the liquid polymer is a solution, then the amount of added particles refers to the proportion of polymer present in the solution. It has been surprisingly found that the grade of graphitization is even higher, if less catalyst is used, for example, using 0.6 wt.-% of T1O2 leads to a higher grade of graphitization than using 1 .5 wt.-% or more. This will be described further below in the examples.

The kind of spinning method in step c) is not particularly limited. Possible spinning methods are air gap spinning, dry spinning, melt spinning and wet spinning.

Preferably, however, the spinning step c) 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. The graphite fiber of the present invention can be further treated by various methods. Preferably, however, after step f) the graphite fiber is thermally treated at above 1000°C in the presence of chlorine gas or chlorofluorocarbon gas. This treatment leads to a leach out of the particles added in step b) and, thus, to a purification of the graphite fiber. Preferred chlorofluorocarbon gases are dichlorodifluoromethane and 1 ,1 ,1 ,2-tetrafluorethane.

In another aspect, the present invention relates to a method for the manufacture of a textile fabric incorporating the method for the manufacture of a graphite fiber according to the present invention wherein additionally a textile fabric forming process step is performed either with the precursor fiber before step d), with the stabilized precursor fiber before step e), with the carbon fiber before step f) or with the graphite fiber after step f) and that the textile fabric forming process step is one of a woven fabric forming process step, a nonwoven forming process step or a paper forming process step.

If the textile forming process step is performed with the graphite fiber after step f), then the above described preferred purification step can be performed before or after the textile forming process step.

In another aspect, the present invention relates to the use of the graphite fiber according to the present invention as conductive filler for polymers or as additive for friction materials. For this use, the fibers are preferably cut into a suitable length before use.

In another aspect, the present invention relates to the use of the textile fabric according to the present invention as an electrode material or a current collector in desalination units, batteries, preferably redox-flow batteries, LiS batteries and Li-ion batteries, as a gas diffusion layer in electrolysis or in fuel cells, as a catalyst carrier, as a friction material, preferably in the form of a composite material, preferably in brake pads or brake discs typically for aircraft and racing cars, wet friction, preferably for clutches or as a heat spreader in the area of thermal management, in particular for the distribution of heat.

Brief description of the drawings:

Figure 1 : A diagram showing the differences in d002 distance between mesophase pitch based carbon fibers and PAN-based carbon fibers in dependence on the temperature. (Source: M. Inagaki, F. Kang, Materials Science and Engineering of Carbon: Fundamentals, Elsevier, 2nd Edition, 2014)

Figure 2: A diagram showing the differences in crystallite size (Lc) between mesophase pitch and PAN in dependence on the temperature. (Source: M.R.

Buchmeister et al., Carbonfasern: Prakursor-Systeme, Verarbeitung, Struktur und Eigenschaften, Angewandte Chemie 2014, vol. 126, pages 5364-5403)

Figure 3: A diagram showing measurement results. Examples

Four batches of a standard polyacrylonitrile solution for carbon fiber production have been prepared. ΤΊΟ2 powder has been milled until the mean particle size reached the value of d50 = 70 nm. Then the T1O2 powder was mixed in the batches according to the following recipes (the following relative amounts are based on the weight of polyacrylonitrile polymer present in the solution of each of the batches):

Batch no. 1 : 0.35 weight-% of TiO ₂

Batch no. 2: 0.6 weight-% of TiO ₂

Batch no. 3: 1 .5 weight-% of TiO ₂

Batch no. 4: no ΤΊΟ2 was added.

With each of the batches polyacrylonitrile fibers have been spun to precursor fibers by a solvent-spinning method. The respective precursor fibers were further processed by a stabilization process which is heating the precursor fiber under tension in the presence of oxygen, followed by a carbonization step in which the fiber is heated up to 1400°C in the absence of oxygen. The method steps of solvent- spinning, stabilization and carbonization are well known methods in the field of carbon fiber production.

All four carbon fibers have been finally heated up to 2800°C and the temperature was held at 2800°C for 30 minutes. After cooling, crystallographic measurements were carried out on the resulting carbon or graphite fibers, respectively, which are given in figure 3.

For comparative purposes, mesophase pitch fibers have been heated as well up to 2800°C for 30 minutes. The crystallographic results are as well given in figure 3.

It can be clearly derived form figure 3 that catalytic graphitization takes place in all batches 1 -3. Surprisingly, while the graphite fiber obtained from batch no. 3 (1 .5 weight-% ΤΊΟ2) exhibits nearly the same values of d002 distance and crystallite size Lc as mesophase pitch fiber after heating at 2800°C does, the grade of graphitization is even higher when fewer catalyst particles are employed. The graphite fiber of batch no.1 has a d002 distance of about 0.337 nm and crystallite size Lc of about 70 nm. When looking at figure 1 it is well obvious that such good graphitization values cannot be reached only by heating typical PAN-based carbon fibers without catalyst.