Abstract:
A method of providing a packaging system for a thin film battery ( 20 ) is now provided wherein the battery cell is treated with an epoxy ( 41 ) prior to the placement of an overlying glass layer ( 42 ). The positioning of the glass layer causes the migration or spreading of the epoxy layer across the battery cell.

Description:
TECHNICAL FIELD 
     This invention relates to thin film battery construction, and more particularly to a method of producing thin film batteries having a protective packaging. 
     BACKGROUND OF THE INVENTION 
     The metal lithium of thin film batteries reacts rapidly upon exposure to atmospheric elements such as oxygen, nitrogen, carbon dioxide and water vapor. Thus, the lithium anode of a thin film battery will react in an undesirable manner upon exposure to such elements if the anode is not suitably protected. Other components of a thin film battery, such as a lithium electrolyte and cathode films, also require protection from exposure to air although these components are commonly not as reactive as thin metal anode films. It should therefore be desirable to incorporate within a lithium battery, which includes an anode of lithium and other air-reactive components, a packaging system that satisfactorily protects the battery components from exposure to air. 
     In the past packaging systems for batteries have been devised which included a shield which overlays the active components of the battery. These shields have been made of a ceramic material, a metallic material, and a combination of ceramic and metallic material. The construction of thin film batteries however has proven to be quite difficult to produce and in providing an appropriate barrier as gas pockets may be captured between the anode and the protective layer during construction. 
     Another thin film battery packaging system has been devised wherein alternating layers of parylene and titanium are laid over the active components. The alternating layers are provided to restrict the continuation of pin holes formed in the layers during construction. This method of producing a protective layer has been difficult to achieve and has provided a protective layer which remains effective for only a few months. 
     Accordingly, it is seen that a need remains for a method of producing a protective packaging of a thin film battery which protects the active components from atmospheric elements. It is to the provision of such therefore that the present invention is primarily directed. 
     SUMMARY OF THE INVENTION 
     In a preferred form of the invention a method of sealing a battery cell comprising the steps of positioning a quantity of epoxy upon the battery cell, and positioning a layer of air impermeably material upon the quantity of epoxy so as to cause the epoxy layer to spread across the battery cell and thereby force gases to be expelled from between the battery cell and the overlying glass layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross sectional view of a thin film battery made in accordance with the method of the present invention. 
     FIGS. 2-4 are a sequence of views showing the construction of a thin film battery packaging, which show in sequence, the thin film battery components being protected with a packaging. 
    
    
     DETAILED DESCRIPTION 
     With reference next to the drawings, there is shown a thin film battery  20  which incorporates a packaging system in accordance with the present invention. The battery  20  includes components which have been built up onto a substrate  22 . The battery  20  includes a cathode  24 , an electrolyte  26  and an anode  28 , wherein each component is provided by a film deposited in a predetermined fashion upon the substrate  22 . It is a feature of the battery  20  that its packaging system is in the form of a protective sheath  30  which overlies and coats the anode of the battery so that the sheath provides a barrier against the penetration of air and water vapor. 
     The substrate underlying the battery  20  may be comprised of glass, alumina, sapphire or various semiconductor or polymer materials. To enable electrical power to be withdrawn from the battery  20 , two current collector films  32  and  34  are deposited upon the substrate  22 , and then the cathode film  24  is deposited upon the collector  32 . The current collector film  32  and  34  are separated from each other as shown in FIG.  1 . 
     The electrolyte film  26  is deposited in place so as to cover the cathode film  24 . Preferably, the electrolyte  26  is an amorphous lithium phosphorus oxynitride having the composition Li x PO y N z . The anode  28  is comprised of lithium and is deposited upon the previously formed films  24 ,  26  and  28  so as to directly overlie a substantial portion of the electrolyte  26 . 
     The protective shield  30  overlies and covers the entire top surface of the anode  28  which would otherwise be left exposed to the atmosphere. In addition, the shield  30  is advantageous in that it acts as an impermeable barrier through which air and water vapor cannot penetrate. Such an advantage can be readily appreciated in connection with the component films which include an amount of lithium, which is highly reactive to atmospheric elements. The protective shield itself includes two layers, a top, glass layer  40  and an epoxy layer  41 . The glass layer  40  preferably allows the transmission of ultraviolet light so that an ultraviolet light curable epoxy may comprise the epoxy layer  41 . The glass layer  40  is sized and shaped to conform with the underlying components of the battery cell. 
     To produce the packaging system in accordance with the present invention the manufacturing process commences with a battery cell wherein the cathode  24 , electrolyte  26  and anode  28  are produced in any conventional manner. Once the cell is produced a small amount of liquid epoxy  41 , such as Micro-Lite 3010 made by MLT/Micro-Lite Technology Corporation of Mesa, Ariz., is placed proximate the center of the anode  28 , as shown in FIG.  2 . The epoxy may be applied in any conventional manner such as with the use of a nozzle or syringe. A glass layer, made of material such as Pyrex, model nos. #7950 or #9741 by Corning, Inc of Corning, N.Y. and measuring approximately 0.2 mm, is then positioned upon the liquid epoxy  41  wherein a sufficient force is placed upon the underlying epoxy  41 , either by the is weight of the glass layer alone or with an additional outside force, to cause the epoxy to spread outwardly between the glass layer  40  and the anode  28 , as shown in FIG.  3 . The epoxy  41  spreads until it completely fills the void between the glass layer  40  and the anode  28 , as shown in FIG.  4 . The amount of epoxy required is determined by the surface area the epoxy must cover once spread. Once the epoxy coating is complete an ultraviolet light is passed through the glass layer  40  to cure the epoxy in a rapid manner. 
     It should be understood that by placing a quantity of epoxy upon the anode all gases are expelled from between the glass layer  40  and the anode  28  as the epoxy spreads outwardly, thereby preventing the capturing of gas pockets between the glass layer and the anode which would cause the degradation of the anode. 
     It should be understood that the present invention also includes the use of non-ultraviolet curable epoxies but is not intended to be limited to such as other types of epoxies may be utilized in practicing the present invention. Should non-ultraviolet curable epoxies be utilized and the glass may be substituted with any appropriate ceramic material. 
     It should also be understood that the present invention is not limited to a method wherein the epoxy is centrally positioned. For example, a peripheral layer of epoxy may be applied to the anode while constructed in an argon atmosphere, thereby encapsulating the argon within the peripheral seal of epoxy. This method may be preferred where it may be necessary to allow flexing or volume changes of the anode, i.e. it allows for expansion. 
     While this invention has been described in detail with particular reference to the preferred embodiments thereof, it should be understood that many modifications, additions and deletions, in addition to those expressly recited, may be made thereto without departure from the spirit and scope of invention as set forth in the following claims.