ELECTRONICS COMPRISING ELECTROCHEMICAL DOUBLE LAYER CAPACITOR

Field of the invention

The present invention relates to electronics comprising an electrochemical double-layer capacitor, which comprises two electrodes in contact with an electrolyte, the electrodes comprising conductive particles and binder holding the conductive partices together.

The invention also relates to an electrode structure for a electrochemical double-layer capacitor, said electrode structure comprising conductive particles and binder holding the conductive partices together. Background of the invention

An electrochemical double layer capacitor comprises polarizable electrodes, a positive electrode and a negative electrode. Between the electrodes there is an electrolyte which is in contact with the conductive material of both electrodes. Charge and discharge under load is possible due to the formation of electrochemical double layers at the electrode surfaces during the charge and discharge processes. For connecting the electrodes to the electric circuitry, current collectors in contact over a large area with the electrodes on their outer sides are used. Electrochemical double layer capacitors are energy storage devices where the energy is stored in an electric field like in traditional capacitors. In pseudocapacitors energy is stored by applying redox-reactions e.g. in ruthenium oxide. Typically the power output of the double layer capacitors is higher than in batteries but lower than in traditional capacitors. On the other hand, the capacity to store energy is higher than in traditional capacitors but lower than in batteries. Compared with a battery, the charging time of a double layer capacitor is shorter, of the order af a few seconds. An electrochemical double layer capacitor is also known by the name supercapacitor.

A double layer capacitor provides large amounts of power and therefore it can complement battery technique in targets where more power production is needed during peak load. On the other hand, batteries can be made smaller when power peaks can be provided by the capacitors. Practical examples

include electric vehicles, UPS systems and other devices requiring reserve power. The largest number of double layer capacitors are used in electronics, such as in clocks and memories as reserve power sources.

Electrochemical double layers are formed at the surfaces of the electrolyte and electrode, and the capacitor of this type consist in a way of two capacitors connected in series. A large surface area for the electrodes is provided by the porosity of the electrode (pores between the particles) and porosity of the particles forming the electrode. The thickness of the double layers is of the order of nanometres. Thus the capacitance values are several decades higher than in the case of conventional capacitors. Because of the operational principle the charging and discharging rates are very fast compared with batteries. Typically electrochemical double layer capacitors can be charged or discharged in a few seconds. The lifetime can be even more than million duty cycles.

Manufacturing electronic'components by printing methods and integration to functional circuits facilitates various novel applications. These include, inter alia, smart packages, single-use sensors for rapid diagnostics, ubiquitous applications, and active displays. In many applications momentary large power is required. This kind of power peaks are difficult to produce with conventional printed power sources.

The electrodes of double layer capacitors are typically made of activated carbon powder or fabric having large specific surface area. In case of powder it is necessary to use a binder which holds the particles together so that they form a coherent electrode layer. European patent 449145 shows the use of fluoropolymers as binder of electrode particles in the double-layer capacitors.

The use of fluoropolymers as binder is problematic in the sense that their use decreases the recycling possibilities of the capacitor. Further, fluoropolymers require mechanical processing (in the case of PTFE) or the use of organic solvents (PVDF) so that they could be used as binders. Fluoropolymers are known to be hydrophobic which restricts the choice of the electrolyte to non- aqueous organic solutions. Aluminium foil is used as current collector in the double-layer capacitor of European patent 449145.

Summary of the invention

A purpose of the invention is to overcome the above-described drawbacks and provide a double layer capacitor, which provides a component producing instantaneous power for functional circuits, especially electronic circuits made by printing methods.'

The double layer capacitor for electronics according to claim 1 of the present application enables the use of relatively large amounts of power in printed electronics. It can replace a separate power source or a battery or a conventional capacitor manufactured by printing methods. Specifically, it can be used next to a power source providing only a small amount of power to provide more power required instantaneously. The power and energy provided are the right size for various applications. The possibility for quick charging increases the number of applications. In the invention it is substantial that the entire double layer capacitor is manufactured by printing methods and it is used as a part of printed electronics.

Printed electronics, a part of which the double layer capacitor is, also comprises a power-consuming circuit, to which the double layer capacitor is connected. The double layer capacitor and the circuit are manufactured by printing methods on the same substrate. The electronics may also comprise a power source with which the double layer capacitor is connected in parallel.

In addition, the power source (battery or a component providing energy from light) may be manufactured by printing methods on the same substrate.

Printing method here refers to arranging the conductive material in a pattern on the surface of the substrate and specifically, when forming a double layer capacitor, the lamination of different layers on the substrate as well.

It is the purpose of the present invention to also provide a double layer capacitor of the general structure described above under the heading "Background of the invention", but which is simpler and less expensive to manufacture and better recyclable or more safely disposable than the double layer capacitors of the prior art. For achieving this, the double layer capacitor according to the invention is mainly characterized in that the binder of the conductive particles is cellulose carbamate (CCA).

Cellulose carbamate is a cellulose derivative which is manufactured starting from natural substance, cellulose, through chemical modification. Cellulose carbamate, its manufacturing method and prior patent documents are discussed for example in International Publication WO 03/064476. Compared with cellulose, CCA contains omly nitrogen as additional chemical element.

It has been found that GCA works well as binder for conductive particles. The electrode structure comprising electrically conductive particles and cellulose carbamate binder holding them together is recyclable. The electrode, due to the hydrophilic nature of the CCA binder, is also suitable for use in double layer capacitors where the electrolyte is water-based, allowing the electrolyte to penetrate to the porous structure of the electrode. Water-based electrolytes are a safe alternative compared with organic solvents. The cellulose carbamate is also stable in contact with water-based elctrolytes, that is, it retains its adhesive power under the operation conditions of the capacitor.

The conductive particles of the electrode are preferably activated carbon. The cellulose carbamate is able to bind the activated carbon particles to a coherent electrode layer so that the carbon particles a re in suficient contact with each other but it does not cover substantialy the pores of carbon particles, both characteristics being essential for the operation of the capacitor. , ' '

According to one embodiment, the mechanical properties of the CCA are modified by a plastifier, preferably glyserol. This reduces the brittleness of the CCA and avoids the formation of cracks. Other well-known plastifiers can also be used. This alternative is useful if the electrode is to be bent or wound when the capacitor is manufactured.

Still according to one embodiment, the current collectors in contact with the electrodes are made of graphite. Graphite is suitable as material in contact with electrodes, because the CCA binder has alkalinity, which corrodes aluminium collectors. A double layer capacitor having activated carbon particles and CCA binder as constituents of the electrodes, water-based electrolyte between the ¹ electrodes, and current collectors made of graphite,

is made in its entirety of environmentally friendly materials, which can be recycled or safely disposed, for example by burning.

In the following description, the invention will be described in more detail with reference to the appended drawings, in which

fig. 1 shows the manufacture of a printed double layer capacitor by printing technique in different steps,

fig. 2 shows an example of the circuit where the double layer capacitor can be used, and

fig. 3 shows a structure of the electrolytic double-layer capacitor in cross-section.

In the following an example of how a double layer capacitor can be manufactured for the needs of printed electronics is described. The substrate on which the double layer capacitor is formed by printing methods may comprise the other components of the printed electronics or they may be formed on the substrate after the formation of the double layer capacitor.

In the next example the invention is described in more detail by referring to Figure 1 which shows how electrochemical double-layer capacitor can be manufactured by applying printing methods.

Manufacturing example: ¹

1. Conductive ink including e.g. silver or carbon. As substrate, paper or polymer foil can be used. Alternatively these current collectors can be made using a lamination method or a substrate that contains electrically conducting layers.

2. Ink containig activated carbon, including binder.

3. Printing or lamination of electrolyte. This layer can include gel or liquid electrolyte and can be porous.

4. Ink containig activated carbon, including binder. This layer can be similar to the layer of step 2.

5. Conductive layer, can be similar to the layer of step 1.

6. Encapsulation of the electrochemical double-layer capacitor.

The activated carbon ink in steps 2 and/or 4 can be replaced entirely or partly with a material containing conductive polymer.

After steps 2 and 4 the active layer can be impregnated with an electrolyte, if necessary.

If the substrate material is porous, it can be necessary to print the encapsulation layer similar to the one in step 6 before printing the first conductive ink.

The described electrochemical double layer capacitor can be printed together with other components in the printed electronic circuit. E.g. the conductors of the whole system can be printed simultaneously.

It is also possible to manufacture the electrochemical double-layer capacitor by making the electrodes separately and then complete the component by installing them face-to-face. Between the electrolytes there is needed an ion- conducting layer that does not cause short-circuit between the electrodes.

In connection with manufacturing the double layer capacitor the different layers can be laminated together and use, for example, stamping methods. The possible printing methods include gravure, flexo-print, offset, screen printing, tampo printing, intaglio and ink jet techniques.

The electrochemical double-layer capacior can be connected in parallel with, for example, a component providing energy from light or a battery. In fig. 2 the capacitor C is formed on the common substrate with a power source V and a power-consuming Circuit R.

In the following, a preferred embodiment of the capacitor itself is described in more detailed with reference to fig. 3. This capacitor structure is particularly suitable as component in the printed electronics as described above, but it can be used in other applications as well.

The capacitor comprises two porous electrodes 2, anode and cathode, in contact with an electrolyte 4. The electrolyte 4 can be e.g. water- or organic

solvent based salt solution. With organic solvents higher voltage windows can be used than with water based electrolytes. However, with water based electrolytes the resistance of the double layer capacitor can be decreased compared with organic electrolytes and environmantally safer water can be used. In contact with the outer sides of the electrodes there are electrically conducting current collectors 1 to connect the anode and cathode, respectively, of the capacitor to the rest of the circuitry to which the capacitor belongs. Although aluminium is typically used as current collector 1 , instead of aluminium carbon-based materials such as graphite foil or material made of graphite ink are used. Electrodes are separated by a porous separator 3 that be made e.g. of cellulose or polymer material.

Now it has been proved that alkali-soluble cellulose derivative, cellulose carbamate (CCA), works well as binder for electrically conductive particles which from the conductive material of both electrodes, anode and cathode. The advantages of CCA are easy applicability, that is mixability with activated carbon particles. CCA is widely available at low costs. The CCA has been obtained by reacting cellulose with urea, which involves completing the reaction above 130 degrees C. The reaction is started advantageously in a solvent-free dry method according to WO 03/064476 where cellulose is reacted with urea at a high dry matter content (low water content) in presence of auxiliary agent, such as alkali metal hydroxide or hydrogen peroxide, the mixture is subjected to mechanical working at the low water content, whereafter the reaction is completed at elevated temperature above 130 degrees. The result is alkali-soluble cellulose carbamate (CCA), which can be dissolved in alkaline solution.

When used as binder, cellulose carbamate is first dissolved in aqueous alkaline solution, for example alkali metal hydroxide, especially sodium hydroxide, whereafter it is mixed with the electrically conductive particles and optional plastifier (for example glycerol) so that an electrode layer where the particles are held together by the binder can be formed on a substrate, whereafter it is dried. In' practice; the electrode is formed by applying the mixture on a foil which will form the current collector 1 of the same capacitor.

The current collectors 1 are referably carbon based, such as graphite, because they are alkali-resistant and they can be used with electrodes where

the binder contains high alkalinity, such as CCA which contains alkali metal hydroxide of the original solution.

The cellulose carbamate can also be used as material for the separator 3 separating the electrodes 2 from each other by preventing the electronic conduction between the anode and cathode. The constituents of the mixture from which the separator film or membrane is formed are the same, except that conductive particles are omitted.

The electrolyte 4 can be based on water as solvent, but an organic solvent, such as propylene carbonate can also be used.

Activated carbon is commonly used as conductive particles in electrodes of double layer capacitors. The particles of the electrodes bound by CCA can be any other substance than activated carbon, for example ruthenium oxide.

The electrochemical double-layer capacitor forms a typical layered structure whose structural layers can be assembled in a variety of ways. It should also be noted that the capacitor according to the invention can be straight (stack of the various structural layers forming a prismatic structure) or cylindrical (the structural layers wound to a roll). The electrically conductive particles and binder can be applied by any suitable method for forming the capacitor electrode. One possible way is to use printing techniques to apply the mixture of particles and the binder on a substrate. It is also possible to form the separator by printing technique by using ink comprsing CCA dissolved in aqueous alkaline solution, for example alkali metal hydroxide.

The capacitor according to the invention can be used as an electronic component in various circuits where temporary storage of electricity is needed. The capacitor' can be manufactured in various sizes depending on the capacity and power required. The available electrode surface area can vary for example between 1 mm ² and 100 cm ² .

Cellulose carbamate can be used as binder of particles in other electric storage devices as well, for example in batteries.

In the following examples, the percentage and ratios are based on weights of various constituents unless indicated otherwise.

Example 1 PT001G3

Ink of the following materials was mixed:

25 g activated carbon Norit Super 30

2.25 g CCA (8.3 % of total activated acrbon + CCA, dry substance) 6 g NaOH - 66.75 g H ₂ O

The ink was applied on Sigraflex graphite foil on 2 cm ² . The ink was dried and after this the electrode was rinsed in order to remove remaining NaOH. After water had evaporated the mass of the electrode (carbon + CCA) was 11.4 mg. Since the capacitor consists of two electrodes, the total electrode mass in the component was 22.8 mg. Cellulose paper (NKK TF40-50) was used as separator. NaCI:water (1 :5) was used as electrolyte.

The capacitor was characterized according to IEC 62391 standard with 10 mA and 50 mA discharge currents between 0.96 V and 0.48 V. The efficiencies were defined as the ratio of discharge and charge energy between 0.2 V and 1.2 V. The capacitance obtained for 10 mA current was

0.68 F corresponding about 30 F/g electrode mass. For 50 mA the capacitance was 0.40 F. The series resistance of the capacitor was 1 ohm. The efficiency values measured for 10 mA and 50 mA charge and discharge were 76 and 57 %, respectively.

Example 2 PT001 G1

A similar component to the one described in example 1 was constructed. Instead of water based electrolyte 0.5 M tetraethyl ammonium tetrafluoroborate in propylene carbonate was used as electrolyte. The capacitance measured with 10 mA discharge current was 0.27 F.

Example 3 PT002A

A new ink with the following composition was made: 20 g activated carbon Norit Super 30

1.2 g CCA (5.7 % of total activated carbon + CCA, dry substance) 6.4 g NaOH 72.4 g H2O

The ink was applied on current collectors in the same way as in example 1. The electrode mass for the whole capacitor (2 electrodes) was 15.1 mg.

The capacitance obtained for 10 mA current was 0.49 F corresponding about 33 F/g electrode mass. For 50 mA the capacitance was 0.42 F. The series resistance of the capacitor was 0.6 ohm. For 10 mA and 50 mA the efficiency values were 89 and 80 %.

Example 4 PT005D

A similar component to the one in example 3 was prepared. The cellulose paper used as separator was replaced by a layer of CCA ink. This ink did not contain activated carbon. Replacing separator with ink makes roll to roll printing process easier. The capacitance obtained for 10 mA current was 0.29 F corresponding about 20 F/g electrode mass. For 50 mA the capacitance was 0.14 F. The series resistance of the capacitor was 0.9 ohm.

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Example 5 (PT006AA and AB)

The ink and manufacturing method of example 3 were used. Because of the brittle nature of the CCA bound activated carbon, the CCA binder was plasticized with 20 % glycerol in water. The result was a flexible electrode structure. The measured specific capacitance values were again about 30 F/g.