WO2019004974A2

WO2019004974A2 - A carbon nanofiber and production method thereof

Info

Publication number: WO2019004974A2
Application number: PCT/TR2018/050183
Authority: WO
Inventors: Mustafa Edhem KAHRAMAN; Aras MUTLU; Hasan Alper ONDUR; Bülent Oğuz BALLI; Orhan Çalişkan
Original assignee: Aksa Akri̇li̇k Ki̇mya Sanayi̇i̇ Anoni̇m Şi̇rketi̇
Priority date: 2017-04-28
Filing date: 2018-04-21
Publication date: 2019-01-03
Also published as: WO2019004974A3

Abstract

The invention is related to a carbon nanofiber production method that comprises the steps of obtaining nanofiber bundles by means of the electrospinning method from a precursor fiber solution, subjecting the obtained nanofiber bundles to a stabilization process, and then subjecting the nanofiber bundles obtained from the stabilization process to a carbonization process.

Description

A CAR B ON NANOFIB E R AND P RODUCTION ME T HOD TH E R E OF TE C H NICAL FIE L D

The invention is related to a carbon nanofiber production method and a carbon nanofiber produced by means of this method.

The invention is particularly related to a nanofiber production method and a carbon nanofiber produced by means of this method having mechanical and chemical features that may be used as anode material in lithium ion batteries.

P RIOR ART

Carbon based fibers are being used in several technical fields nowadays. The main reason for this is that carbon fibers are lighter in weight in comparison to other alternatives and that it provides higher resistance. One of the known features of carbon fibers is that it provides an amount of electrical conductivity. E lectrical conductivity enables said fiber type to be a suitable material for usage in batteries. The batteries mentioned herein are lithium-ion batteries that are used in various electronic appliances. The fiber is usually produced as nano finer and is used as anode material.

One of the carbon nanofiber production methods (CVD) that have been mentioned is the chemical vapor deposition method. In the United S tates Patent and Trademark Office (US PTO) with the publication number US 2009053608, a carbon nanofiber anode active agent is mentioned for a lithium back-up battery. According to the production method of the present invention, after an amorphous silicon or metal material is processed, a support that is made of an amorphous silicon alloy is prepared, a catalyst selected from F e, C o, Ni, C u, Mg, Mn, Ti, S n, S i, Zr, Zn, G e, P b or In is distributed over said support made of amorphous silicon alloy, and a carbon source selected from carbon monoxide, methane, acetylene or ethylene is used to grow the carbon nanofiber on said support by means of the chemical vapor deposition method. As the chemical vapor deposition method is very expensive and highly dangerous, the manufacturers in the sector have slowly tended to use different production methods. One of these methods is the production of fiber using the electrospinning method. F ibers that are produced using this method usually have low mechanical and chemical characteristics. For this reason, following the electrospinning process, in order to reinforce fibers, thermal processes such as stabilization and/or carbonization are applied. Despite all of these processes, carbon fibers still do not provide the desired and/or sufficient features that are required in order for them to be able to be used in lithium ion batteries. The surface area of the carbon nanofibers that have been established with the known methods cannot be increased to the desired level and the low surface area directly proportionally reduces the battery capacity. Another disadvantage is that the thickness of the material cannot be reduced. As the thickness of the material increases, the diffusion distance also increases.

One of the studies carried out on said electrospinning method is described in the C N1044661 68 numbered C hinese Intellectual P roperty R ights Office (S IP O) patent document titled ^'P reparation method of cobaltosic oxide-carbon porous nanofiber and application of cobaltosic oxide-carbon porous nanofiber to preparation of lithium ion battery_, The method subject to the above mentioned invention, comprises the steps of adding dropwise, a soluble cobalt salt solution into a dimethylformamide solution of polyacrylonitrile and polymethyl methacrylate and continuous stirring and storing at a temperature between 80-100 eC for 24 hours. F ollowing this, electrospinning is used to prepare a polymer/cobalt salt mixture composite fiber supported on aluminum foil. The polymer/cobalt salt mixture composite fiber, is heated up to 280 eC such that the temperature is increased by 2 to 4eC per minute and it is kept for 2 hours at this temperature. Afterwards it is continued to be heated for 4 hours at inert atmosphere and is cooled to obtain a cobaltosic oxide-carbon porous nanofiber. Another known method is the patent document titled ^'Batteries prepared by spinning^" described in the US PTO document numbered US 201 6036037. A method of forming a lithium- ion battery by spinning and a battery formed thereby are disclosed. The anode and/or cathode layers of the battery includes polyacrylonitrile (PAN) fibers. To render the anode and cathode layers conductive, they may be carbonized using a heat source (e.g., a laser). Another method is the US PTO patent document with the publication number US 2013126794 titled ^'Carbon nanofiber containing metal oxide or intermetallic compound, preparation method thereof, and lithium secondary battery using same _. In this document a method for producing carbon nanofiber comprising metal oxide is described, wherein the method comprises the steps of: adding a tin precursor or copper precursor to the carbon fiber precursor material in order to produce a fiber precursor composite; spinning the fiber precursor composite to manufacture a fiber; and heat treating the fiber.

In the US PTO document with the publication number US 2007048521 describes activated carbon fibers, methods of their preparation, and devices comprising activated carbon fibers. According to an embodiment of said invention, it is defined that the fiber must be able to be electrospun and carbonized.

As a result, all of the problems mentioned above have brought about the obligation for a novel method to be provided in the related field.

B RIE F DE S C RIPTION OF TH E INVE NTION

The invention is related to a carbon nanofiber production having mechanical and chemical properties that can be used as anode material in lithium ion batteries and carbon nanofibers that can be produced by means of said method. Parallel nanofiber bundles are established besides a nanofiber web shaped like a spider s web by means of a precursor fiber solution electrospinning method in order to produce said nanofiber. This nanofiber is then subjected to two different thermal processes one of which is the stabilization and the other, a carbonization process. Tensile stress is applied to said nanofiber bundle particularly during stabilization.

AIM OF T H E INV E NTION

The present invention has been provided as a solution to the obligation to provide a novelty to the related prior art in order to eliminate the problems mentioned above.

The primary aim of the invention is to produce a bicarbon nanofiber having mechanical and chemical properties that can be used as an anode material in lithium ion batteries and to provide the carbon nanofiber structure that has been provided with said method. T he present invention is related to carbon nanofiber production in order to reach all of the aims mentioned above and which sha ll be better understood from the deta iled description below. Accordingly,

- Obta ining nanofiber bundles from precursor fiber solution by means of the electrospinning method,

- S ubjecting the obtained nanofiber bundles to a stabilization process,

- S ubjecting the nanofiber bundles obtained from the stabilization process to a carbonization process, characterized in that; during the stabilization process, the nanofiber bundles are subjected to tensile stress.

T herefore, by increasing the chemical and physical properties of the carbon nanofiber that has been produced it is ena bled for the carbon nanofiber to be more suitable to anode applications. In a preferred embodiment of the invention, the polymer, pitch to be used in the precursor solution is selected from oxidized PAN, PAN, saran, rayon and ramie. As a result it has been made possible to manufacture with different starter components.

In another preferred embodiment of the invention the solvent in the precursor solution is selected as D MF (Dimethylformamide). As a result it is possible to dissolve said components in the most suitable way.

In another preferred embodiment of the invention, the PAN is obtained from AN (Acrylonitrile) and a comonomer added between 5%-1 0% by weight. T herefore the crystal formation of the PAN is reduced and tensile stress strength thereof can be increased as it will act as a plasticizer. In another preferred embodiment of the invention tensile stress is applied during said carbonization process. As a result if rayon is used, the desired mechanical and chemical features are provided.

In another preferred embodiment of the invention, graphitization process is applied following the mentioned carbonization process. T herefore the crystallite size is increased, the preferred collimation is improved and the fiber is enabled to be more graphite.

In a preferred embodiment of the invention, the mentioned inert atmos phere is provided by nitrogen. As a result healthy graphitization up to 2000 LC can be provided. In another preferred embodiment of the invention the mentioned inert atmos phere is provided with a rgon. As a result healthy graphitization up to 2000 LC can be provided. In yet another preferred embodiment of the invention, the tension established by means of tensile strength is provided by tightly placing the nanofiber bundle between 2 metal cylinders. As a result the desired tension is provided in the most cost effective way possible.

D E TAIL E D D E S C RIPTIO N O F T H E INV E NTIO N

In this detailed description the carbon nanofiber and production method thereof subject to the invention, has been described together with examples that should not be cons idered to be limiting the scope of the invention but which have been provided for clarity purposes.

T he invention is related to the carbon nanofiber production method and carbon nanofibers produced using said method.

T he present invention relates to a method for producing a carbon nanofiber, comprising the steps of obtaining nanofiber bundles by a precursor solution electro-spinning method; subjecting the obtained na nofiber bundles to a stabilization process; and carbonizing the nanofiber bundles resulting from the stabilization process. S aid precursor solution is a fiber solution, preferably a polymer base precursor fiber solution.

During the method of carbon nanofiber production subject to the invention, firstly a precursor solution is prepared in the step of obtaining nanofiber bundles by the predissolved electro- spinning method. While the precursor solution is being prepared, the polymer is being dissolved in a suitable solvent or is melted with heat. P referably polyacrylonitrile (PAN) polymer is used as it has high carbon efficiency and high tension (stretch-draw) features in the precursor solution that is to be used in carbon nanofiber production subject to the invention. T he precursor solution, is obta ined by dissolving polyacrylonitrile polymer (PAN) in dimethylformamide (D MF ) solvent. However due to its hydrogen binding structure the processability of polyacrylonitrile (PAN) homopolymer is limited. Due to this reason, in order to increase (PAN) processability, polyacrylonitrile (PAN) copolymer with added comonomer between 5% to 10% is used. C omonomer, reduces the crystallity of the polyacrylonitrile (PAN) structure and as a result, it acts like an inner plasticizer and increases the tens ile strength of polyacrylonitrile (PAN). As a comonomer, at least one of acrylic acid, methacrylic acid, itaconic acid, methacrylate or acrylamide is used. In a preferred embodiment of the invention the solvent used to dissolve polyacrylonitrile (PAN) when preparing a precursor solution is DMAC . In a preferred embodiment of the invention the solvent used to dissolve polyacrylonitrile (PAN) when preparing a precursor solution is DMS O.

In a preferred embodiment of the invention the polymer used in the precursor solution is selected from previously oxidized PAN, rayon, pitch, saran or ramie.

Nanofiber bundles are obtained by means of the electros pinning method from the prepared precursor solution.

E lectrospinning, is a method which provides nano sized fibers from precursor solution by means of electric receiving forces. Basically electrospinning is formed of a high voltage power source, small dimensioned pipette/or metallic needle (spinneret) and a collector. High voltage is applied via an electrode to the pipette system that has been filled with precursor solution that has been prepared for the fiber to be obtained and the fibers are produced in this high voltage electrical field. The precursor solution inducted in an electrical field formed by high voltage, is moved towards a grounded collector or a rotating collector as an opposite pole and the fibers become fine and nanometer sized fibers are formed. The mentioned voltage is between 1 5kV-25kV, where the distance between the pipette/needle collector is between 10cm to 30cm.

The precursor solution is placed into a small dimensioned tube which has a small hole at the end. The tube that is mentioned can be a pipette, capillary tube or syringe. The small hole at the end of the tube comprises a need or capillary structure. The pump placed at the other end of the tube, provides continuous pressure that can push the precursor solution towards the tip of the tube. A drop of the precursor solution is discharged out of the tip of the needle and the liquid stays suspended before it drops by means of surface tension. When the solution free surface is exposed to an electric field, a charge is created between the air and the solution surface. The load-driving force causes a force to resist surface tension. When the voltage value exceeds the threshold value, the electrostatic forces exceed the surface tension and cause the dropletto drift away from the precursor solution. When the force of the electrical field is increased, the semi-spherical shaped surface of the drop, sags to form a conical shape. This is called a Taylor cone. The drop that is about to be sputtered is called a jet. The jet inside the solvent evaporates as it moves towards the collector, and is collected as a web formed of random nanofibers on the collector. In a preferred embodiment of the invention the solution is fed into the system from a plurality of tubes.

The collector plate of the electrospinning method, can be a flat plate, a grill, a carrier conveyor, etc. or it can be used in other different shapes. In the carbon nanofiber production method of the invention, a rotating type collector is used. When the collector rotates at a rotation speed, the nanofiber accelerates on the rotating collector, and it is oriented and becomes parallel.

In a preferred embodiment of the invention, the collector that is mentioned is a drum like collector which can rotate like a cylinder.

In a preferred embodiment of the invention the collector is a disc type collector. The drum like collectors are grounded collectors having cylindrical shape and which have a certain diameter and length. The drum like collector used in the production of carbon nanofibers subject to the invention is a collector that weighs approximately 500 grams, has a 28cm width and 6cm diameter. Due to its relatively heavy weight it causes oscillation during the electrospinning process and as the electrospinning process is carried out in high speed, the collection of the nanofiberweb is performed in a spider web like shape as it is not oriented to a certain direction.

The disc type collectors have a wider diameter in comparison to a drum like collector and are shorter in length and are preferably produced from polyethylene. The disc type collector used in the production of carbon nanofiber subject to the invention has a membrane thickness of approximately 2,5cm and has a 1 5 cm diameter. As it is lighter in weight, it can operate at speeds that are 8-10 times faster than drum type collectors. As they can operate at higher speeds and they are lighter in weight, the collection of nanofibers is enabled such that they are oriented. This orientation allows the nanofibers to be parallel as they can be aligned on the collector and oriented fibers are obtained. After nanofiber bundles are obtained from a precursor solution by means of an electrospinning method, the nanofibers obtained are subjected to nanofiber stabilization (oxidation) process at lower temperatures in comparison to carbonization in order to ensure that the fibers are resistant to higher temperatures. S everal reactions during oxidative stabilization carried out such as cyclization, dehydrogenation, aromatization, and oxidation and cross linking reactions. Stabilization converts the triple bonds located between carbon and nitrogen that are found in polyacrylonitrile (PAN) besides minimum vaporization from the carbon section into double bonds, and forms a cross bond between polyacrylonitrile (PAN) molecules and therefore enables polyacrylonitrile (PAN), and in turn the nanofibers to be processed at high temperatures. The cross bonds enable the polymer chains to become parallel to the fiber and to be linear without any kind of tensioning application. The dehydrogenation and cyclization reactions form a stair polymer structure that is thermally stable and can withstand high temperatures during the pyrolysis process. During the carbon nanofiber production method subject to the invention, tensile stress by drawing is applied during stabilization, in order to improve the mechanical, morphological, crystallity degrees and molecular orientation of the nanofibers obtained by means of electrospinning. As a result, the parallelization of the nanofibers is increased during stabilization.

The stabilization process can be carried out in different ways such as non oriented nanofibers that are in a spider web form or as oriented nanofibers.

The non oriented nanofibers have a spider web structure and as a result are wound tightly around a glass plate in order to provide the required tension and are subjected to a thermal process. The oriented nanofibers are fixed from both ends between two metal cylinders. The oriented nanofibers, can be attached around the metal cylinders due to the electrostatic forces that occurs between the nanofibers and the metal cylinder. During stabilization, the tension formed by tensile stress, is provided by tightly placing the nanofiber bundle between two metal cylinders.

While stabilization is being carried out, in order to prevent the nanofibers from contacting any surface inside the heater, metal wheels have been mounted at both ends of the metal cylinders. The nanofibers that have been fixed between two metal cylinders are subjected to thermal processing. Due to the fact that nitrile conjugation cross bonds are formed between polyacrylonitrile (PAN) polymer chains following heating, approximately 25% shrinkage is observed. In order to prevent this shrinkage and to ens ure the elongation of the fiber, tensile stress is applied to nanofibers. The required tensile stress is applied in a direction that is opposite to the shrinkage direction of the nanofiber bundles. In order to obtain high mechanical resistant nanofibers, tension is applied to nanofibers by drawing, during the stabilization process. As a result during the tensile stress process applied against size shrinkage of the nanofibers, mechanical and morphological features are improved.

As the stabilization process creates the establishment of activation centers for cyclization, in order to form some groups that comprise oxygen in the backbone of the fibers, the process is carried out in the air or in an oxidation atmosphere between 1 80-400 LC . T he mentioned atmosphere preferably comprises 10% oxygen. T he groups that comprise oxygen help the stair cha ins to fuse during carbonization. T he nanofiber bundles that are obtained from the stabilization process are subjected to a carbonization process. In order to prevent the unwanted reactions from occurring during carbonization, to reduce the waste toxic gas created during carbonization, and to prevent the entrance of ambient air, an inert atmosphere is needed. As a result preferably argon gas is used to create an inert atmos phere.

In a preferred embodiment of the invention, nitrogen is used for inert atmosphere.

T he stabilized nanofibers, are placed into an alumina vessel in orderto prevent the nanofibers from roving.

Any kind of tensile stress by drawing is not applied during the carbonization process. As the polyacrylonitrile (PAN) nanofiber backbone is formed of carbon atoms following stabilization, it is not mandatory to perform drawing during carbonization. However, in cases where rayon is used in the precursor solution, an oxygen atom is present per the monomer unit of a backbone, and therefore restructuring arrangement occurs during the carbonization process. C arbonization is carried out in fina l temperatures between 900 - 1 500 LC . T he nanofibers that are obtained following the stabilization process, are heated from room temperature to 200 LC , at a ratio of 1 0 LC /minute heat. F ollowing this, 5 LC /minute heat up to 600 LC is applied and then it is heated up to final temperature at 3 LC /minute. After the final temperature is reached, it is cooled to room temperature at 1 0 LC /minute. During carbonization, approximately 50% of the weight of the fibers, are eliminated by the effect of the gasses such as H20, N H3, H C N, C O, C 02, N2 and H2. After subjecting the nanofiber bundles obtained from the stabilization process to a carbonization process, carbon nanofibers are formed.

In a preferred embodiment of the invention, graphitization process is applied after the carbonization process. The carbonization process above 1 500 LC is called high temperature carbonization or graphitization.

If nitrogen is used at high temperatures, it may damage the chemical and physical structure of the carbon nanofibers. F or this reason nitrogen can be used up to 2000 LC as inert atmosphere, above this temperature, the reaction between nitrogen and carbon, forms cyanogens and creates a toxic effect. For this reason argon or helium is preferred as inert atmosphere if the temperature is higher than 2000 LC . Following the graphitization process, the crystallite size of the carbon nanofiber increases and as the preferred orientation is developed, carbon nanofiber becomes graphite. The protection scope of the invention has been mentioned in the attached claims, and it cannot be limited to the embodiments described above which have been given for illustration purposes. It is obvious to those specialized in the art that similar embodiments can be achieved without departing from the main scope of the invention by following the descriptions above.

Claims

C LAIMS

1 . The invention is related to a carbon nanofiber production method comprising the steps of; a) obtaining nanofiber bundles from precursor fiber solution by means of the electrospinning method,

b) subjecting the obtained nanofiber bundles to a stabilization process, c) subjecting the nanofiber bundles obtained from the stabilization process to a carbonization process,

c haracterized in that; during the stabilization process, the nanofiber bundles are subjected to tensile stress.

2. A carbon nanofiber production method according to claim 1 c haracterized in that the polymer pitch to be used in the precursor solution is selected from previously oxidized PAN, PAN, saran, rayon and ramie.

3. A carbon nanofiber production method according to claim 1 c haracterized in that the polymer to be used in the precursor solution is selected as PAN.

4. A carbon nanofiber production method according to claim 1 c haracterized in that the polymer to be used in the precursor solution is selected as rayon.

5. A carbon nanofiber production method according to any of the claims 1 to 4 c haracterized in that the solvent in the precursor solution is selected as DMF .

6. A carbon nanofiber production method according to claim 3 characterized in that the PAN, AN is obtained from a comonomer added between 5%-10% by weight.

7. A carbon nanofiber production method according to claim 6 characterized in that said comonomer is selected from acrylic acid, methacrylic acid, itaconic acid, methacrylate or acrylamide.

8. A carbon nanofiber production method according to claim 1 c haracterized in that said stabilization is carried out at temperatures between 1 80-400 LC .

9. A carbon nanofiber production method according to claim 8 c haracterized in that said stabilization is carried out at oxidation atmosphere.

1 0. A carbon nanofiber production method according to claim 1 c haracterized in that said carbonization process is carried out at temperatures between 400- 1 500LC .

1 1 . A carbon nanofiber production method according to claim 4 c haracterized in that tensile stress is applied during said carbonization process.

1 2. A carbon nanofiber production method according to claim 1 c haracterized in that a graphitization process is applied following said carbonization process.

1 3. A carbon na nofiber production method according to claim 1 1 c haracterized in that said graphitization is carried out at temperatures between 1 500- 3000 LC .

14. A carbon nanofiber production method according to claim 1 1 or 1 2 c haracterized in that the graphitization is carried out in inert atmosphere.

1 5. A carbon na nofiber production method according to claim 13 c haracterized in that the inert atmos phere is provided by nitrogen.

1 6. A carbon nanofiber production method according to claim 1 3 c haracterized in that the inert atmos phere is provided by argon.

1 7. A carbon nanofiber production method according to claim 1 , characterized in that the tensile stress is provided by tightly placing the nanofiber bundle between 2 metal cylinders.

1 8. C arbon nanofiber produced with the carbon nanofiber production method according to any of the preceding claims.