TECHNICAL FIELD

Present invention addresses the flexible ultracapacitor composed of ultra-thin carbon composite electrodes. Present invention also concerns with producing flexible 20-300μιη thick electrodes by electro-spinning technology.

BACKGROUND ART

Present invention is related to the energy storage device also called as supercapacitor or ultracapacitor. Supercapacitor is an energy storage device of high efficiency, which can be either electric double-layer capacitor (EDLC), which stores the energy electrostatically on the surface of chemically inert electrodes in the so-called electric double layer, or redox capacitor (ROC), in which the electrical charge is stored in the reversible redox-processes. In reality, the EDLC always is accompanied by few redox processes and so the ROC includes the EDL (electric double-layer) capacitance. Since the amount of energy stored in supercapacitor is proportional to the area of electric double layer, the electrodes of supercapacitors are commonly manufactured from the highly porous carbon with large surface area. Supercapacitors are characterised by a very rapid charge-discharge cycle, from few minutes to few seconds. In order to achieve the good energy and power output parameters of the supercapacitor the composition of its components need to be well optimized. A significant factor is the compliance of electrode materials with each other, i.e. microporous carbon of large surface area and the electrolyte. The electrolyte can be aqueous or non-aqueous, which determines the range of working voltage of the supercapacitor.

In order to achieve the maximum energy density and specific capacity of a super capacitor it is important to balance the positive and negative electrode capacities in the electrode pair. Balancing of electrodes by masses is described for example in US 2006/0148112 Al (, MAXWELL TECHNOLOGIES INC., 06. 07.2006. The super capacitor described in current invention achieves superior energy and power output properties by balancing the thicknesses of carbon composite electrodes of varying specific capacity. The pores on a negative electrode are bigger and the electrode has inasmuch smaller density as the positive electrode, which is necessary for achieving the good mobility of ions and low internal resistance of the electrochemical system. While the specific capacity of the negative electrode is somewhat smaller than that of the positive electrode, it leads to employing negative electrodes, which are up to 10% thicker than positive electrodes in order to equalise the electrode capacities.

An important requirement for achieving the low internal resistance of the super capacitor is the low charge transport resistance between the carbon electrode and current collector. Abrading of current collectors and improving the bonding by a carbon layer are known in prior art as the mechanical treatments of the aluminium foil layer, used as the mechanical current collector, described for example by PCT/US2009/050324 (WO2010/011509, 28.01.2010), MAXWELL TECHNOLOGIES INC and PCT/US2009/050122 (WO2010/006179, 14.01.2010), MAXWELL TECHNOLOGIES INC. For improving the electric conductivity between the carbon layer and the current collector, an electrically conductive and adhesion-improving intermediate layer is used, which is normally a polymer (e.g. polyvinylidene fluoride) including carbon nano particles (e.g. lampblack, nanographite, etc.).

DISCLOSURE OF INVENTION

High surface area, appropriate porosity and good conductivity are required in the materials for supercapacitor electrodes. Besides traditional capacitor electrodes such as in the form of carbon fiber fabric, films or activated carbon paste, the electrospun nanofibers can be used for capacitor electrodes due to their high surface area and porosity. To produce such nanofibers with proper characteristics by electrospinning method, the polymer solution must be prepared. Solution properties and conditions of electrospinning process are the main factors that influence the nanofiber producing. For achieving the above named properties of the nanofibers for electrodes the electrospinning solution must contain polymer matrix to provide mechanical properties and stiffness of the electrospun nanofibrous material, conductive carbon allotropes of high specific surface area, specific additives to improve conductivity and porosity and solvent to make solution.

The homogenity of the solution used for electrospinning is the main characteristic to get fibrous material with suitable morphology and improved electrical and mechanical properties when additives are used. That is why in solution preparation it is significant to establish the method to achieve the homogenity of the components added to the solution.

BRIEF DESCRIPTION OF DRAWINGS

The flexible ultracapacitor and a method of making such ultracapacitor according to the present invention will be described below in detail with reference to the drawings where in Fig 1 is illustrated the cycling voltammogram of the samples S4, S 13 and S26 with the voltage scan rate of 5mV/s;

Fig 2 is illustrated the constant current charge-discharge curve for the cell;

Fig 3 is illustrated general scheme of the flexible electrospun supercapacitor according to present invention (general production example of flexible supercapacitor);

Fig 4 is illustrated the carbon composite electrode half-cell of the flexible ultracapacitor composed of ultra-thin carbon composite electrodes according to the method of the present invention;

Fig 5 is illustrated the flexible ultracapacitor cell composed of ultra-thin carbon composite electrode half-cells according to the method of the present invention;

Fig 6 is illustrated a layered structure of the carbon composite electrode half-cell in the non- conductive substrate of the flexible ultracapacitor composed according to the method;

Fig 7 is illustrated the composite electrode half-cell layered structure in fig 6 with cut-off before folding said composite electrode half-cell to flexible ultracapacitor;

Fig 8 is illustrated the flexible ultracapacitor with current terminals composed of ultra-thin carbon composite electrode half-cells with layered structure in fig 7.

BEST MODE FOR CARRYING OUT THE INVENTION

In the manufacture of a flexible ultracapacitor according to the method of the present are used the following components.

Polymer matrix

A soluble thermoplastic polymer as Polyacrylonitrile (PAN), polystyrene (PS), styrene acrylonitrile copolymer (SAN), cellulose derivates or other have been chosen as a polymer matrix to prepare electrospinnable solution to manufacture an electrode layer and/or a separator layer.

The concentration of polymer in solution depends on molecular weight of polymer used and is directly related to the viscosity of solution [Nandana Bhardwaj, Subhas C. Kundu. Electrospinning: A fascinating fiber fabrication technique. Biotechnology Advances 28, p. 325- 347, 2010 II Quynh P. Pham, Upma Sharma, Ph.D., and Antonios G. Mikos, Ph.D. Electrospinning of Polymeric Nano fibers for Tissue Engineering Applications: A Review. Tissue Engineering, Vol. 12 (5), 2006 II Seeram Ramakrishna, Kazutoshi Fujihara, Wee-Eong Teo.Teik-Cheng Lim & Zuwei Ma. An Introduction to Electrospinning and Nanofibers. World Scientific Publishing Co., 2005, p. 381].

The concentration of the polymer matrix is adjusted to be in the lowest limit, firstly, to be able to produce fibrous membrane by electrospinning and, secondly, to add carbon allotropes of higher concentration. The possible lowest ratio of polymer matrix and the possible highest ratio of carbon allotropes in electrospinnable solution can help to avoid the blocking of pores of the filler by polymer matrix, which is isolator. The proper amount of polymer in electrospinnable system also influences the mechanical properties of produced material that limits the using of very small amount of polymer matrix. The concentration of polymer matrix is calculated on the solution amount.

The concentration of PAN can change from 4 wt% up to 10 wt%. This is the concentration range where PAN solution is spinnable according to the literature data results [D. S. Gomes, A. N. R. d Silva, N. I. Morimoto, L. T. F. Mendes, R. Furlan, I. Ramos. Characterization of an electrospinnable process using different PAN/DMF concentrations. Polimeros: Ciencia e Tecnologia, vol. 17, n 3, p. 206-211, 2007. II ϋ. Ozkoc, H.I. Icoglu and A. Kirecci. Effect of concentration on polyacrylonitrile (PAN) nanofibers. Journal of Materials Science and Engineering 5, p. 277-280, 2011] and the results got by our experiments in the laboratory.

The concentration of cellulose derivates will depend on the molecular weight and the solvent environment to dissolve cellulose. The amount of cellulose in solution can vary from 1.5 wt% up to 3 wt% (wt% - percentage by weight).

The concentration of PS polymer as a matrix will be used in the range 10 wt% -20 wt% [Ladawan Wannatong, Anuvat Sirivat and Pitt Supaphol. Effect of solvents on electrospun polymeric fibers: preliminary study on polystyrene. Polymer International, 53, p. 1851-1859, 2004 II Tamer Uyar, Flemming Besenbacher. Electrospinning of uniform polystyrene fibers: The effect of solvent conductivity. Polymer, 49, p. 5336-5343, 2008 II Chia-Ling Pai, Mary C. Boyce, and Gregory C. Rutledge. Morphology of Porous and Wrinkled Fibers of Polystyrene Electrospun from Dimethylformamide. Macromolecules, 42, p. 2102-2114, 2012 II Todsapon Nitanan, Praneet Opanasopit, et. Al. Effects of processing parameters on morphology of electrospun polystyrene nanofibers. Korean J. Chem. Eng., 29 (2), p. 173-181, 2012].

Additives

Carbon allotropes are chosen as good candidates for capacitor electrode due to their good electrical conductivity, high surface area with appropriate pore size and mechanical stability. Carbon black (CB) is used as filler in polymer solution to produce electrically conductive fibrous membranes. For achieving higher conductivity, the concentration of CB filler will vary in the range from 5 wt% up to 50 wt% calculated on dry matter in polymer solution. The highest limit of CB concentration is defined by solution viscosity needful to perform electrospinning. CB sample with particle size no larger than 0.3 μιη (0,3 micrometre) will be used to prepare the homogenous dispersion of CB filler in polymer solution.

Carbide derived carbon (CDC) or other activated carbons are used as another filler in the electrode to increase specific surface area and conductivity. The beneficial property of CDC or other activated carbons is large specific surface area, highly micro- or mesoporous structure with good mechanical properties. The particle size of CDC or other activated carbons has not to exceed the particle size of CB used to get the homogeneous solution with two fillers. Conductive polymers as a polyaniline (PANi) are used in polymer - carbon allotropes composite to increase the electrical conductivity of electropsun fibrous membrane. The ratio of a conductive polymer to the carbon allotropes can influence solution viscosity and will vary from 5/95 up to 40/60. The idea of using conductive polymers is also related to producing highly conductive membranes by co-axial electrospinning. In co-axial electrospinning process two solutions, core one and the shear one are prepared. Using conductive polymer in core layer solution and CB filler in polymer solution of the outer layer the conductivity of coaxially electrospun membranes can be increased.

Ionic liquids (ILs) are used as an additive due to their unique properties such as negligible vapor pressure, excellent chemical stability, high thermal stability and high ionic conductivity. ILs show good compatibility and solubility in organic polymer solutions. ILs are also known as "green solvents". These properties make ILs ideal for using as additive in polymer solutions for electrospinning [H. Olivier-Bourbigou, L. Magna, D. Morvan. Ionic liquids and catalysis: recent progress from knowledge to applications. Applied Catalysis A: General, 373, p. 1-56, 2010 II P. Kubisa. Ionic liquids as solvents for polymerization processes- progress and challenges. Progress in Polymer Science, 34, p. 1333-1347, 2009 II T. Tsuda and C. L. Hussey. Electrochemical applications of room-temperature ionic liquids. The electrochemical Society Interface, p. 42-49, 2007]. The idea of using ILs on carbon solutions is to fill the pores with ILs to avoid the blocking them by polymer matrix (isolator) thus improving the specific surface and capacitance of electrospun fibrous membrane. ILs can improve the dispersion of carbon allotropes in polymer solution. They increase also ionic conductivity of the electrode. Solvent

The choice of the solvents for solution preparation is based on several defined parameters. Firstly, the solvent must be appropriate for electrospinning process. Secondly, the solvent must be suitable to prepare homogeneous solution by mixing polymer matrix with added fillers. Solvent evaporation rate influences fiber morphology. By using volatile solvent, porous fibers can be obtained [R. Khajavi, M. Abbasiour. Electrospinning as a versatile method for fabrication coreshell, hollow and porous nanofibers. Scientia Iranica F, 19 (6), p. 2029- 2034, 2012. II S. J. Eichhorn and W.W. Sampson. Relationships between specific surface area and pore size in electrospun polymer fibre networks. Journal of the Royal Society, 7, p. 641- 649, 2010].

The following solvents can be used for solution preparation:

1. Dimethyl sulphoxide (DMSO) can be chosen as a good solvent to dissolve PAN for electrospinning. The previous experimental results have shown that DMSO is the best solvent to disperse CB as filler in PAN solution.

2. Dimethyl formamide (DMF) is the solvent that dissolves both PAN and PS. The volatility of DMF is higher than of DMSO which can help to increase the porosity of electrospun fibrous material.

3. Mixture of DMSO - Acetone can be used to improve the evaporation of DMSO in the solution during electrospinning process.

4. Mixture of Nitromethane - Water can be used to dissolve PAN matrix polymer. By using this mixture of solvents in PAN solution for electrospinning the porosity of post- dried membranes increases.

Electrolyte

The electrolytes used for filling the electrochemical system of the ultra-thin capacitors may comprise any anhydrous organic salt or saline solution, existing in ion pairs in its liquid state, which has been selected from the quaternary ammonium salts or quaternary phosphonium salts or mixtures thereof, from ionic fluids based on imidazolium derivates, whereas the electrolyte salt cation can include RiR ₂ R3R ₄ N ⁺ or RiR ₂ R3R ₄ P ⁺ , in which Ri, R ₂ , R3 and R ₄ are alkyl groups from -CH3 to -C5H11 or cyclic phenyl radical -C6H5 and anion can include BF ₄ ^_ , PFe ^" , AsFe ^" , BPh ^~ , CF3SO3 ^"

The electrolyte solvents have been selected from nitriles, cyclic carbonates and propylene carbonates, lactones, sulfolanes, esters, ethers, tetrahydrofurans, N,N-dimethylformamides, dimethyl sulfoxides and pyridine derivatives. In alternative embodiment the electrical system of the ultra-thin supercapacitor is filled with an aqueous electrolyte, which can include alkaline solutions, acidic solutions or ionic salts dissolved in the water.

THE METHOD FOR PREPARING A FLEXIBLE ULTRACAPACITOR

Several methods can be used for preparing homogeneous polymer solutions with proper dispersion of carbon additives:

1. Mechanical mixing (by magnetic stirrer for example) at different temperatures during 24 hr or 48 hr. The temperature choice will depend on the solvent used in polymer system.

2. Treatment of carbon derivates solution by ultrasound (US) for 2-6 hour at room temperature can improve the dispersion of carbon particles in the solution.

3. The combination of mechanical mixing of solution with the treatment by US.

Electrospinning

The parameters of electrospinning process such as distance between needle tip and collector, nozzle diameter, applied voltage and flow rate affect the properties of the fibrous material. Environmental factors such as humidity and temperature also play an important role in fiber uniformity, diameter and porosity.

Porous nanofibers can be produced under certain controlled environmental conditions [R. Khajavi, M. Abbasipour. Electrospinning as a versatile method for fabrication core shell, hollow and porous nanofibers. Scientia Iranica F, 19 (6), p. 2029-2034, 2012 II M. Bognizki, W. Czado, T. Frese, A. Shaper, M. Hellwig, M. Steinhart, A. Greiner, and J.H. Wendorff. Nanostructured fibers via electrospinning. Adv. Mater., 13 (1), 2001]. The described methods are incorporated herein by reference.

Examples of the electrochemical characteristics of the electrospun electrodes

The fibrous electrode material containing 14,6% to 21,2% CDC in the dry electrospun electrode were used in variable different thicknesses: 33μιη (micrometre) to 115μιη (micrometre) in the electrochemical cells.

From the fibrous electrode material the EDL capacitors S4, SI 3, S24 and S26 were assembled, whereas the visible surface area of electrode was about 12 cm ² . The cells were vacuum dried at 105 deg C° and filled with the 1.8M Triethylmethylammonium/Acetonitrile (TEMA/ACN) supercapacitor electrolyte. Thereafter the cells were electrochemically tested as typical EDL capacitors, by using cycling voltammetry, galvanostatic and impedance measurement techniques. The cycling voltammograms, expressed on on Fig. 1 indicates that all samples behave as typical EDL capacitor materials (the cycling voltammogram of the samples S4, S13 and S26, the voltage scan rate of 5mV/s).

To analyse the EDL capacitance and resistance of fibrous electrode material the constant-current charge/discharge was applied to cells in the voltage range of 0-2.0V, shown on Fig. 2. The EDL capacitance was calculated from the fifth discharge cycle at the current of

The specific capacitance in Table 1, is calculated per dry CDC material of the cell. The inner resistance was calculated in accordance with Ohmic law by applying the current pulse of 10 msec to cell provided by the Arbin hardware.

Table 1. The electrochemical testing results of the fibrous electrode materials in EDL capacitors.

Production example of flexible supercapacitor (Fig. 3-8)

A method for manufacture of the electrochemical system of the supercapacitor of flexible ultra-thin structure (fig 3) comprising the steps where: a) a flexible layer of a metal (for example thin gold or aluminium film) as current collector 1 , is mounted/feed into an electrospinning device/line. In alternative embodiment (see fig 6), a flexible layer of a metal as current collector on a non-conductive substrate 4 (for example metal layer is pre-laminated to the non-conductive substrate such as plastic substrate) is mounted/feed into an electrospinning device/line. In further embodiment said non-conductive substrate may form a flexible case 8 (see fig 8) of the supercapacitor; b) a thin layer (approximately 20 - 300 micrometre) of fibrous electrode material prepared as described above is electrospinned on top of the metal layer (for example on the top of thin gold or aluminium film) or alternatively on top of the metal layer (gold or aluminium film) pre- laminated to the plastic substrate, wherein the electrode layer 2, 2' is formed. In alternative embodiment the fibrous electrode layer is impregnated with electrolyte. The fibrous electrode material electrospinned on top of the current collector is a carbon-polymer composite as described above and comprises and ionic liquid or liquids as additives, further the carbon polymer composite may comprise further solvents and/or other additives for improving electrical conductivity and mobility of the electrolyte in the electrode layer; c) thereafter a thin separator layer 3, 3' (approximately 10 - 50 micrometres) for example of carboxymethyl cellulose is electrospinned on top of the electrode layer 2, 2' . In one embodiment the electrode layer and separator layer is impregnated after step b) with electrolyte 9 (ionic liquid or some other conductive liquid) by electrospraying or other suitable method. In another embodiment the carbon-polymer slurry electrospinned to the current collector comprises a ionic liquid or ionic liquids (ILs) as an additives, solvents or other additives to improve the dispersion of carbon allotropes in polymer solution due to their good solubility, to increase ionic conductivity of the electrode etc; d) the product comprising the following layers in the following order: the flexible layer of current collector 1, 1 ', carbon-polymer layer (as electrode layer 2, 2') and separator layer3, 3' (see fig 4) is then removed from the electrospinning device/line; e) a rectangular of suitable size as half-cell of the electrode of the electrochemical system of the supercapacitor is cut from said multilayered material; f) thereafter the current terminals 7, 7' are attached/connected to the current collector layer 1, , and g) two rectangular half-cells of the electrode are placed to each other so that the separator layers of both half-cells of the electrode are forming inner layer (middle layer) of the supercapacitor as product (see fig 5).

In other embodiment the rechargeable flexible electrodes and the separator layer, both, are directly coated by electrospinning in the right sequence onto the current collector, which is an electrically conductive foil, which after electrospinning of electrodes and separator layer is inserted in the nonconductive hermetically sealed casing such as a plastic bag. In alternative embodiment after step d) the multilayered material is fold up along shorter edge to form electrode of the supercapacitor. Before folding one separator layer to other separator layer the electrode layer and separator layer is cut-off. In another embodiment where current collector is metal layer laminated to the plastic substrate the Separator layer, electrode layer and metal layer is cut-off too to break conductivity (see fig 6, 7). The plastic substrate is left uncut, between two half-cell is cut-off 5 which width corresponds approximately to double high of the current layer, electrode layer and separator layer. After folding two half-cells of electrodes the separator layers of half-cells will be the inner layer of the supercapacitor as product. h) further the edges of case 8 of the product will be cleaned, welded, glued or sealed with a method, which avoids short circuiting of the electrode and collector layers. Before final sealing the electrode layer and separators layers are impregnated after with electrolyte 9 (ionic liquid or some other) and thereafter the electrochemical system of the supercapacitor is sealed and the supercapacitor (see fig 8) is directed to the further processing (training, conditioning to the working voltage etc).

In another embodiment the method according to the present invention for manufacture of the electrochemical system of the supercapacitor of flexible ultra-thin structure comprises the steps where:

a) a flexible layer of a metal as current collector is feed into an electrospinning device; b) a layer approximately 20-300 micrometre of fibrous electrode material such as carbon- polymer composite is electrospinned on top of the metal layer wherein the electrode layer is formed; thereafter

c) a separator layer approximately 10-50 micrometre for example of carboxymethyl cellulose is electrospinned on top of the electrode layer;

d) a product comprising the following layers in the following order: the flexible layer of current collector, carbon-polymer layer (as electrode layer) and separator layer is then removed from the electrospinning device/line; and

e) a rectangular of suitable size as half-cell of the electrochemical system of the supercapacitor is cut from said multilayered material, thereafter

f) a current terminals are attached to the current collector layer, and

g) two rectangular half-cells are placed to each other so that the separator layers of both half- cell are forming inner layer (middle layer) of the supercapacitor as product, thereafter h) edges of case of the product will be cleaned, welded, glued or sealed with a suitable method to avoid short circuiting of the electrode and collector layers; and i) before final sealing the electrode layer and separators layers are impregnated after with electrolyte (ionic liquid or some other) and thereafter the electrochemical system of the supercapacitor is sealed and the supercapacitor is directed to the further processing.

It is clear to the person skilled in the art that the steps after electrospinning the fibrous electrode layer and separator layer of the electrochemical system of a supercapacitor of flexible ultra-thin structure can be modified within the scope of the present description and claims.

List of elements:

1, 1 ' - Current collector

2, 2' - Electrode layer

3, 3' - Separator layer

4 - Non-conductive substrate

5 - Cut-off

6 - Non-conductive material

7, 7' - Current terminal

8 - Case

9 - Electrolyte