Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1998-12-03
2001-08-28
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C029S623300, C029S623500, C427S123000
Reexamination Certificate
active
06280879
ABSTRACT:
The present invention relates to a method for the production of electrode/current collector laminates for an electrochemical device, an electrode paste, a gravure roll and a primer-coated current collector foil for use in the method, a primer paste for the production of primer-coated current collector foils, an electrode paste for the production of electrode/current collector laminates and a gravure roll. The invention furthermore relates to the electrode/current collector laminates produced.
Traditionally, components for electrochemical cells have been bulky and have been produced by mould casting, pressing, punching and rolling techniques.
The increasing demand for high capacity and high power density batteries has resulted in the development of laminar batteries. Such batteries are generally produced by laminating together thin layers of current collectors, electrodes and optionally a mechanical separator. Electrode/current collector components for laminar cells are conventionally produced by pressing, extrusion, pasting or doctor blade coating techniques (see e.g. “Handbook of Batteries”, McGraw-Hill, 1995).
U.S. Pat. No. 4,935,317 describes a process for making a solid state laminar cell involving doctor blade coating of a cathode composition layer on a current collector, followed by rolling of the external surface of said cathode composition to provide an external surface having minimal surface discontinuities.
EP 411 949 describes a method for the formation of a thin layer of lithium metal onto a substrate by transferring molten metal, which is projected above the surface of a vessel, to a substrate by directly or indirectly contacting the molten metal with the surface of the metal.
For the production of the thin cell components used in laminar electrochemical systems like lithium polymer batteries and supercapacitors, however, none of the above-mentioned methods are appropriate, since they do not allow for continuous high speed production of components of uniform thickness.
The thickness of the components of laminar electrochemical devices is in the range of 5-200 &mgr;m. Such laminar components should be of uniform thickness, since non-uniformity may lead to a non-uniform resistance distribution over the laminate area, non-uniform current distribution and eventually hot spots and cell failure. Accordingly, these devices call for a manufacturing method which allows the application of layers of uniform thickness. In particular, a need exists for a method for the production of electrode/current collector laminates which allows for application of an electrode layer of uniform thickness on a current collector despite the presence of substrate irregularities.
Rechargeable electrochemical cells, such as lithium polymer batteries, may be manufactured in their charged as well as their discharged state. However, due to the fact that the electrodes of such cells are generally highly reactive in their charged state, charged-state production calls for production conditions involving inert atmosphere, adding significantly to the production costs. Thus, production of electrochemical cells in their discharged state, which do not call for such precautions, is generally desirable.
The electrode/electrolyte interface of laminar electrochemical cells typically consists of a structure including electrode components as well as electrolyte components, intimately mixed to high percolation. This interface structure may be obtained by an in-situ one-step operation, in which a paste containing electrode as well as electrolyte components is applied to a current collector. Due to the hygroscopicity of non-aqueous electrolytes, this operation must be performed in an inert atmosphere.
Following an alternative route of manufacture, however, electrolyte-free electrode/current collector laminates may be produced, which are subsequently impregnated with electrolyte. In this case the part of the manufacturing process, which does not involve the electrolyte, can take place in ambient atmosphere. Electrodes manufactured in this way should display controlled porosity as to allow the subsequent absorption of the electrolyte.
Therefore, although a number of techniques are known for the manufacture of laminar components for electrochemical systems, there still exist a need for a technique, which fulfils the requirements for fast, reproducible manufacture of electrode/current collector laminates of uniform thickness, despite the presence of irregularity of the surfaces of the current collector foil.
It is thus an object of the invention to provide an efficient and economically advantageous method for continuous high speed production of electrode/current collector laminates having an electrode layer of uniform thickness despite the presence of current collector foil irregularities, the thickness of the electrode layer being at least 20 &mgr;m.
This object is accomplished by a method for the production of an electrode/current collector laminate for an electrochemical device, said electrode layer of the laminate having a thickness of at least 20 &mgr;m, wherein said current collector foil of the laminate, optionally coated with a conductive primer, is provided with an electrode paste coating on at least one of its sides, said electrode paste comprises particles of electrode material, a solvent and a binder, said method comprises the step of:
a) filling said paste into the cells or grooves of a patterned matrice roll;
b) transferring paste from the matrice roll onto the current collector foil by a printing operation involving contacting the matrice roll and the current collector foil or alternatively contacting the matrice roll with an offset roll which in turn is in contact with the current collector foil;
c) drying the layer of paste printed onto the current collector; and
d) optionally repeating steps a)-c) until the desired thickness of the electrode layer is obtained.
In the present context the expression “electrochemical devices” i.a. encompass batteries, including rechargeable batteries, preferably lithium-polymer batteries, fuel cells, capacitors, including so-called supercapacitors, electromechanical reactors and electrochromic devices here under electrochromic displays, including “smart windows”.
Electrochromic displays, which are also referred to as variable transmission windows or smart windows, are transparent electrochromic systems, the colour of which can be controlled upon variation of the applied potential. Those electrochromic displays comprise a working glass electrode, traditionally comprising WO
3
, which is bleached upon charge and coloured upon discharge and a counter glass electrode. Both of these electrodes are coated onto a transparent current collector layer, traditionally of ITO glass, and sandwiched between said electrodes, is the electrolyte.
It is preferred that the steps a)-c) of the method according to the invention is performed using gravure printing technique.
The gravure printing technique involves transfer of a paste of pigment and/or printing ink from the engraved cells of a gravure roll onto a web, alternatively onto a second roll and subsequently onto a web (offset gravure printing). Traditionally, the conditions of a gravure printing operation involves coating of pastes having a viscosity of 15-1500 mPas at a printing speed of up to 300 m/min. The virtually non-porous coatings obtained typically have a thickness in the range of 1-10 &mgr;m.
The gravure printing technique is basically a loading procedure, in which a fixed amount of material is applied on a substrate. The general conditions and measures to be taken to perform gravure printing are e.g. described in Cohen, Edward and Gutoff, Edgar: Modern Coating and Drying Technology, VCH, pp. 103-108 and Leach and Pierce: The printing ink manual, Chapmann & Hall, 1993, pp. 474-546.
Application of gravure printing in other types of battery and capacitor manufacturing processes has been described in the literature:
U.S. Pat. No. 5,227,223 describes the gravure printing of metallic catalytic inks having a very low viscosity (20
Andersen Torben Paarup
Lindbjerg Jens
Yde-Andersen Steen
Danionics A/S
Darby & Darby
Weiner Laura
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