Laminate structures are used in many applications including but not limited to electronic devices, packaging material, and solar reflectors. Laminate structures in electronic devices are found in devices including but not limited to circuit elements and electrochromic devices wherein conductive polymer layers are combined and may include a metal layer. Electrochromic devices include but are not limited to switchable mirrors and switchable windows. Circuit elements include active elements, for example fuel cells and batteries, and passive elements, for example capacitors.
Presently, many laminate structures are made with solid polymer laminate layers. In packaging material and solar reflectors, a metal layer may be added to enhance optical reflectance. In electronic devices, a metal layer may be added to enhance electrical conductivity. In packaging material and solar reflectors, it is not necessary that the polymer layer or layers be conductive, whereas in electronic devices, especially batteries, the polymer layers must be conductive to act as electrolytes, anodes, and cathodes. Certain polymers when doped with selected salts are known to make suitable solid polymer ion conductive layers. Polymers known to be useful include but are not limited to polyethyleneoxide, polypropyleneoxide, polyorgansulfides, and polyanaline. Suitable salts include but are not limited to lithium salts, for example lithium perchlorate, and lithium hexafluoroarsenate. Although the anode, cathode, and electrolyte layers may all be of solid polymer material, when making a lithium polymer battery, it is preferred to have a layer of lithium metal as an anode.
Other polymers having added compounds, including but not limited to conductive powders and dyes, may be made by the present invention.
Presently, mass production of polymer and metal laminate structures used for electronic devices, and especially batteries, relies upon assembling preformed layers of polymer with a thin metal foil. Polymer layers are formed in production quantities by depositing a thin layer of a monomer onto a moving substrate that carries the monomer layer while and until it is cured. Many means for forming polymer layers are available, including but not limited to physical or mechanical liquid-monomer spreading apparati; for example, roll coaters, gravure roll coaters, wire wound rods, doctor blades, and slotted dies, as well as means for evaporation and deposition of a monomer vapor, for example polymer multilayer deposition. In any means having a moving substrate, the substrate has a velocity different from a nozzle or bath that deposits the liquid monomer onto the substrate. Hence, the term "moving substrate" as used herein excludes a situation wherein there is no relative motion or velocity differential between substrate and liquid-monomer dispensing means.
The polymer multilayer deposition technique is distinct from liquid-monomer spreading techniques because polymer multilayer deposition requires flash evaporation of the monomer. First, a monomer is atomized into a heated chamber that is under vacuum. Within the heated chamber the monomer droplets are evaporated, then exit the heated chamber, and monomer vapor condenses upon a substrate and is subsequently cured.
Curing may be done by any means including but not limited to heat, infrared light, ultraviolet light, electron beam, and other radiation.
When fabricating a battery, several techniques are used to combine a thin metal layer with a conductive polymer layer. One technique of battery fabrication is to combine a metal foil with a conductive polymer layer by press bonding a metal foil layer to a solid conductive polymer layer. Another technique is to spread uncured conductive monomer onto a metal foil and subsequently cure the conductive monomer to form a solid conductive polymer layer. Use of metal foil, especially lithium metal foil, results in minimum metal thicknesses of from about 1.5 mils (40 micrometers) to about 2 mils (50 micrometers).
Other battery fabrication techniques include making a thin metal layer by sputtering, plating, or vacuum depositing metal onto a metal substrate. A conductive polymer is then placed in contact with the metal. Either solid conductive polymer or uncured conductive monomer may be brought into contact with the metal to form the battery. Polymer laminate structures, including but not limited to batteries, are made by a procedure wherein individual layers are sequentially and separately formed then combined.
The performance and lifetime of polymer/polymer and polymer/metal laminate structures depend upon the quality of bonding between laminate layers. Bonding quality is affected by the presence of small, even microscale, areas of non-bonding at an interface between laminate layers. The bonding is especially critical between dissimilar layers; for example, polymer and metal layers. In batteries, reduced bond quality between polymer and lithium metal layers results in greater internal resistance of a battery produced with the laminate material and potential for "hot spots" upon recharging. In any structure, another problem with bonding dissimilar materials is chemical interaction between the materials. Areas of non-bonding can enhance chemical interaction because they may contain non-inert species or provide different surface characteristics at a boundary between bonded and unbonded areas.
Bonding between layers is therefore of great importance and is enhanced by several means, including but not limited to mechanical presses, and application of a second layer as a liquid with subsequent solidification upon a first solid layer at atmospheric pressure. The difficulty with these methods is that the low cost assembly of pressing or liquid application leads to low quality bonding as identified in U.S. Pat. No. 4,098,965, issued Jul. 4, 1976, to Kinsman, column 1, lines 47-50, wherein he states "[g]ases usually air, [that] are included in the void regions of the battery during assembly . . . ".
It is of great interest to those skilled in the art, then, to make batteries having high bond quality while making them in a cost effective manner.