Patent Application
20110159342
The invention is concerned with an anode for a thin battery and a method for preparing such an anode.
The basic components of a battery are the electrodes with terminals (electric connections) to connect to an external circuit, a separator to keep the electrodes apart and prevent them from shorting, the electrolyte which carries the charged ions resulting from the chemical reactions taking place at the electrodes and a cover to contain the active chemicals and hold the electrodes in place.
“Wet” cells refer to galvanic cells where the electrolyte is in liquid form and is allowed to flow freely within the cell casing. “Dry” cells are cells that use a solid or powdery electrolyte. Cells with liquid electrolyte can be classified as “dry” if the electrolyte is immobilized by some mechanism, such as by gelling it or by holding it in place with an absorbent substance such as paper.
The most common type of battery used today is the “dry cell” battery used in e.g. relatively large batteries such as “flashlight” batteries and in miniaturized versions used for wristwatches or calculators.
Batteries are often classified by the type of electrolyte used in their construction. There are three common classifications; acid, mildly acid, and alkaline.
All batteries utilize similar procedures to create electricity; however, variations in materials and construction have produced different types of batteries.
One battery type consists of a layered structure, i.e. those called thin film batteries. Thin film batteries, which term in this text is to be understood as “layered-structured batteries” regardless of size, can be applied directly onto film applications in any shape or size, and flexible batteries can be made by printing on to paper, plastics, or other kind of thin foil.
Thin film batteries have e.g. a wide range of uses as power sources for consumer products and for micro-sized applications. Thin film batteries are also suitable for powering smart cards and Radio Frequency IDentification (RFID) tags.
Methods for manufacturing such a thin battery can e.g. in accordance with the Finnish patent application of the applicant 20070584 includes wetting a separator paper with an electrolyte solution and applying an anode material and a cathode material as pastes on the separator paper(s), the anode paste on one side and the cathode paste on the opposite side. The anode and cathode materials can be applied on the separator papers with different methods such as by coating or printing. The coating and printing processes generally involve the application of a thin film of functional material to a substrate, such as roll of paper, fabric, film or other textile.
With the term paste, it is in this text just meant, a viscous water-based dispersion of materials.
However, also the outmost separator layers will always contain electrolyte, since they absorb electrolyte from the wetted layer and furthermore, the electrolyte solution, that contains additives, might also be mixed with the anode active material and cathode active materials to form so called anode and cathode pastes.
The electrodes of a thin battery are formed of the anode and the cathode. The anode material is a paste or ink containing an anode active material and usually electrolyte solution with additives and the cathode material is a paste or ink containing a cathode active material and usually electrolyte solution with additives. The paste is quite viscous so that the electrolyte is not capable of flowing out. The anode active material is often zinc (Zn) and the cathode active material is manganese dioxide (MnO2). The electrolyte solution can e.g. contain ZnCl2 as a main ingredient as well as additives, such as binder(s). The additive(s) in the electrolyte comprises binder(s) in order to provide other properties to the electrolyte solution for example binding the electrode material particles together to form a paste. The binder is e.g. polyvinylalcohol (PVA).
Conductive material is added to the anode and cathode pastes. The conductive material can be carbon powder, such as graphite powder, soot, carbon black, carbon nanotube, conductive ink or combinations thereof. The electrodes (consisting of the anode and cathode pastes inclusive the conductive material) are connected to a collector material and the whole product is covered by films of sealing material. The sealing material can be of e.g. polypropylene, polyethylene, polyester or other known film materials. The collector material is formed to have terminals to be connected to an external circuit. The collector material can be conductive carbon ink, carbon film or other material, which is chemically inert but conductive enough for the purpose.
The combined layers are cut in desired sizes to form products.
Different formats of anode materials have been tested to facilitate the manufacturing of a thin battery.
US patent application 2006/0216586 is presented as prior art. It discloses a thin electrochemical cell. The anode of the cell can be a zinc strip. The problem with this solution is, however, that the connection of the zinc strip to the collector is problematic and that the zinc self discharge is high because of lacking coverage.
U.S. Pat. No. 6,379,835 is concerned with a thin film battery. The anode is hydrophilic being a water based zinc ink or a zinc ink in an organic solvent+water swellable polymer mixture and therefore the battery has high self discharge rate. Furthermore because of the low conductivity of the anode, an extra collector at the anode is needed.
US patent application 2006/0115717 presents a flexible thin printed battery. A conductive aqueous zinc material to constitute the anode is printed directly onto a non-conductive substrate and is made sufficiently conductive to eliminate the need for a distinct anode current collector. The necessary conductivity is achieved by excess zinc+2 cations (from zinc acetate) and polymeric binder (polyvinylpyrrolidone) to improve particle contact. However, also here, the hydrophilic zinc anode causes high zinc self discharge and unstable conductivity during the discharging process.
Further prior art solutions are presented in e.g. WO publication 03/100893 and US006045942.
Intensive work is currently done for making constantly improved batteries by solving some of the acknowledged problems in connection with said self discharge.
Another problem in traditional manufacturing methods to overcome is that the paper sheets become quite wet when they are impregnated with the electrolyte and also the paste remains wet after it is applied. This situation influences on the surface properties of the layers so that further manufacturing steps will be difficult to carry out and makes the total manufacturing process of battery difficult.
There is electrolyte in the anode and cathode pastes, too. As is well known, the zinc is quite easily oxidized under wet environment containing electrolyte salt and self-discharge is a problem. Furthermore, the printing or coating of especially the anode paste is also difficult due to the fast changes of the properties of the paste.
The object of the invention is to solve the above discussed problems.
The anode material of the invention for use in a printed battery comprises a substrate, an anode active material, a conductive material, a binder. The conductive material, the anode active material, the solvent and the binder have been selected to form a film on the substrate at drying after mixing.
The method of the invention for manufacturing an anode material for use in a printed battery comprises providing a substrate, an anode active material, a conductive material, a solvent, and a binder. The method further comprises mixing said solvent, said binder, said anode active material and said conductive material to form anode ink, applying said anode ink on said substrate, drying said anode ink on said substrate. In response to said drying, said solvent evaporates and said anode ink forms a film on said substrate.
The invention is characterized by the enclosed claims and the advantageous features are presented in the subclaims.
The present invention may be suitable for alternate anode active components, such as magnesium, cadmium, and copper, in combination with conductive material such as carbon ink. Preferably, the anode active material is zinc (Zn).
Water or electrolyte is needed for any electrode reaction and ionic transfer between the electrodes of a battery.
Zinc is more easily oxidized in water or in an electrolyte environment than in a dry environment. In many thin and flexible batteries of prior art, the zinc anode is therefore made of an aqueous paste of zinc or of a hydrophilic material. The disadvantage of such a prior art battery is, however, that the self discharge is fast. As a consequence of the self-discharge, the lifetime of the battery becomes short and its capacity smaller.
In the invention, the anode is hydrophobic and dry, which makes it difficult for water or electrolyte to directly penetrate into the deep layer of the zinc anode. The hydrophobic layer in the dry anode of the invention works like a wall to keep the moisture level of the battery stable.
A suitable moisture level in the battery is of course important for good performance, and when using an anode according to the invention in a battery, the water can transfer from the cathode to the surface of the anode after combination of the different layers. The oxidation reaction takes place at the surface of the anode between the anode active metal, electrolyte and water.
The core of the invention is that dry anode ink is used instead of wet/humid anode paste and a separate anode collector as used in prior art. In the invention, the dry anode ink can (after drying) also provide for the transfer of electrons from the anode material to the (−) terminal on the surface of the battery.
In prior art batteries an anode collector was needed to function as a (−) terminal. At that time a separate anode collector was used together with a wet anode paste because otherwise 1) the wet anode paste would flow out from the battery through the (−) terminal hole in the sealing material and because 2) a change in the ambient humidity level would affect the wet anode paste and alter its characteristics by making the performance worse. Both these problems are avoided with the anode material of the invention which eliminates the use of an anode collector.
When the anode collector is eliminated it is now possible to place the terminals on the same side of the sheet like battery. This will enable wide range of applications using the power of the battery to be connected more easily to the battery. Namely it is easier to design and produce the applications if the power inlet terminals for them are on a same side of a sheet like application e.g. a printed RFID tag or a chip.
The anode of the invention is flexible, stable and mass producible and useful in batteries used as a power source for many applications, where a thin flexible disposal power source is needed.
Commercially available conductive carbon inks, and commercially available zinc powders can be used to produce the anode without extra collector in the battery. The anode electrode consists of an anode material, which is a homogenous blend of zinc powder and conductive carbon ink.
The anode material, for the battery of the invention is prepared by adding zinc powder to conductive carbon ink and by stirring until a homogeneous mixture is obtained.
The zinc powder used for this purpose can be a commercial battery grade product with certain properties with respect to the particle size and purity. A particle size less than 50 μm and purity over 99% have been found suitable. Examples of usable commercial zinc powders are e.g. Grillo-Werke Aktiengesellschaft GZN 3-0 and Xstrata EC-100.
The conductive carbon ink used can be a commercial product with certain properties. The ink to be used should possess high conductivity, high flexibility, high binding strength and high hydrophobicity. Examples of usable commercial conductive carbon ink are e.g. XZ302-1 HV and XZ302-1 MV Conductive Carbon, 26-8203 Conductive Graphite, Creative Material 116-19 Low Temperature Curing Conductive Ink, DuPont 7105 Carbon, Asahi FTU-20D3, and Acheson EB-412.
The conductive ink and the proportion of zinc and the conductive ink chosen depend on the properties of the materials, e.g. the conductive carbon ink sheet resistance, the conductive carbon ink flexibility, binding strength, viscosity and conductive carbon ink hydrophobic properties.
From the production point of view, the anode of the invention to be used in a battery has the following advantages for many applications of interest
1) It is easier to produce the anode separately form the battery manufacture, since it can be done in advance and e.g. be stored on rolls after printing on separator. As each step for making the battery does not have to be made simultaneously, the manufacturing process for making the battery is less vulnerable when e.g problems occur in a certain process step. This also means that a bad quality in a product of some step does not render the whole production obsolete, only the product step in question.
2) It is easy to print the anode material of the invention on separator in the battery manufacture;
3) The anode used in the invention reduces self discharge as it is made of hydrophobic ink and does not absorb electrolyte.
5) The second consequence of the fact that no separate collector for the anode is needed in the first embodiment of the invention is that a battery comprising an anode of the invention becomes thinner
6) High stability due to the hydrophobic property of the ink;
In the following the invention is further described by means of some illustrative figures and examples. The invention is, however, not restricted to the details of the following description.
Commercially available conductive carbon inks and commercially available zinc powders can be used to produce the anode material as described earlier more in detail. The anode material is a homogenous blend of zinc powder and conductive carbon ink.
An anode material printed on separator produced in advance e.g. in the way presented in PREPARATION EXAMPLE 1 by means of some printing method known in itself on a web of paper supported by a compression roll. The web on which the anode material is printed goes through a nip formed of a compression roll and another printing roll on which the printed anode material is. Thereafter the web is dried.
Different known methods can be used in the process, e.g. screen printing, Gravue printing, offset printing, and flexo printing can be used as printing method. Also drying can be performed by heat curing, IR curing and UV curing, and by the force of hot air. The anode material might be cut, and then the cutting method may be e.g. die cutting, laser cutting, perforating, slitting, punching or other known method.
The anode material (i.e. anode ink) of the invention is prepared by adding zinc powder to conductive carbon ink and keeping stirring until homogeneous as mentioned early. The ink and zinc powder used in the test are Acheson EB 412 and EC-100.
The conductive carbon ink is stirred with a speed of e.g. 300 rpm and the zinc powder is added gradually. When the zinc powder has been added, the speed of the mixer is gradually increased up to 2000 rpm and the blend is further stirred in about 30 minutes.
The mixing in the way it is done in the invention gives a good result of anode performance, since peeling problems, conductivity problems an uneven conductivity is avoided. A proper viscosity of the anode material is needed. If the viscosity is too low, then zinc particles will deposit to the bottom. A suitable viscosity of the ink is: Brookfield 20° C. 20 RPM 20000 mPaS to 28000 mPa·S. The mixing quality can be controlled by taking samples and checking their density or can be measured by electronic microscope.
Therefore, as the anode material (i.e. anode ink) is produced in this way, it can be printed and dried in advance and e.g. be stored on rolls after printing on separator.
This is a real advantage for the later manufacturing of a battery. As each step for making the battery does not have to be made simultaneously, the manufacturing process for making the battery is less vulnerable when e.g problems occur in a certain process step.
Some properties of the anode material (i.e. anode ink) can be seen as indications of the quality for a battery to be made:
Some principles for measuring these properties are described in the following:
The hydrophobicity was evaluated by applying water on the anode, and the hydrophobicity was considered good if the water did not dissolve, the anode did not break and/or the sheet resistance did not change
If VO is the open loop voltage of the battery; VL is the voltage of load, which is measured 1 minute late after discharge starts; RL is the resistance value of the load, the internal resistance is
Sheet resistance is measured after conductive ink or anode ink is printed on a substrate and dried. The sheet resistance value is the average resistance value at 6 different positions on the sheet. Each resistance is measured by using two probes of multimeter through a distance of 1 cm.
Anode inks made by different proportions of zinc powder and conductive carbon ink were made by the method of preparation example 1.
The zinc powders used in the tests were
A: Grillo-Werke Aktiengesellschaft GZN 3-0 (Zinc content 99.8%, particle size >25 μm 14%, <25 μm 86%) and
B: Xstrata EC-100 (Zinc content 99.9%, particle size >75 μm 0.1%, <45 μm 96%).
The conductive carbon ink used were
Some properties of these conductive carbon inks are presented in the following table:
The proportions of the conductive carbon ink and the zinc powder in the anodes tested were 1:1
Regarding the hydrophobicity of the anode ink it was tested from two aspects mentioned after having applied water on the anode.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20085838 | Sep 2008 | FI | national |
| 20085840 | Sep 2008 | FI | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FI09/00081 | 9/4/2009 | WO | 00 | 3/7/2011 |
