Abstract:
A battery comprises a substrate having a cathode with a lower surface contacting the substrate and an opposing upper surface. A cathode current collector comprises conducting lines that contact the upper surface of the cathode. An electrolyte at least partially extends through the cathode current collector and contacts the cathode. An anode contacts the electrolyte, and optionally, an anode current collector contacts the anode. Also, because the cathode is formed on the substrate before the cathode current collector, the cathode current collector advantageously does not have to be fabricated out of a metal that is capable of withstanding further processing of the cathode, such as annealing of the cathode.

Description:
CROSS-REFERENCE  
       [0001]    The present application is a continuation-in-part of U.S. patent application Ser. No. 09/656,012, by Krasnov, et al, filed on Sep. 7, 2000, entitled “Thin Film Battery and Method of Manufacture”, and which is incorporated herein by reference in its entirety. 
     
    
     
       BACKGROUND  
         [0002]    Embodiments of the present invention relate to thin film batteries and their methods of manufacture.  
           [0003]    A thin film battery  20  typically comprises a substrate  22  having one or more thin films  24 ,  26 ,  28  thereon, as for example, shown in FIG. 1. In a conventional thin film battery  10 , typically, a cathode current collector  24  is deposited on the substrate  22 , and thereafter, a cathode  26  is deposited on the cathode current collector  24 . An electrolyte  28  is formed in contact with the cathode  26 , and an anode (not shown) and optional anode current collector (also not shown) are on the other side of the electrolyte  28 . The thin films are typically formed by thin film fabrication processes, such as for example, physical or chemical vapor deposition methods (PVD or CVD), oxidation, nitridation or electro-plating, on a substrate that is has good mechanical strength. The thin film battery is typically formed by thin film processes such as physical or chemical vapor deposition methods (PVD or CVD), oxidation, nitridation, plating, or other such processes.  
           [0004]    It is desirable for the cathode  26  to have a crystalline microstructure. When the cathode  26  comprises a thin film having an amorphous or microcrystalline structure, the energy that can be stored in such films is usually less than that stored in a microcrystalline film. Furthermore, the charge and discharge rate of the amorphous or microcrystalline film is also smaller than that of a crystalline material film with the same chemical composition. To crystallize an amorphous or microcrystalline thin film to form the cathode  26 , the as-deposited thin film is annealed in a separate process step. The crystallization or annealing temperature that is required to crystallize the amorphous oxide film may be a relatively high temperature. For example, the crystalline microstructure of a thin film cathode comprising LiCoO 2  is dependent upon an annealing step that is conducted subsequent to deposition of an amorphous or microcrystalline thin film of LiCoO 2 . The typical annealing temperature is about 700° C. The high temperature annealed crystalline LiCoO 2  provides good cathode performance, such as high energy density (0.07 mAh/cm 2 /mm) and high charge to discharge current (more than 5 mA/cm 2 ).  
           [0005]    Low temperature processes that produce high quality crystalline LiCoO 2  cathode materials have also been developed, for example, to deposit LiCoO 2  in at least a partially crystalline form. A 200 to 600° C. low temperature anneal process step in oxygen improves the performance such the as-deposited LiCoO 2  to that of a high temperature annealed cathode material.  
           [0006]    However, in both the high and low temperature processes for making the cathode  26 , oxidation of underlying cathode current collector  24  is a problem. The annealing process, which is often carried out in a flow of oxygen, limits the materials that may be used to form the underlying current collector  24  because of melting, oxidation, or inter-diffusion problems. This problem may be reduced by making the cathode current collector  24  out of a noble metal, such as Pt or Au. However, such metals increase the cost of battery  20 . Also, the annealing process can generate thermal stresses due to the thermal expansion coefficient difference between the substrate  22 , cathode  26 , and cathode current collector  24 . These stresses can result in peeling or de-lamination of these layers from the battery  20 .  
           [0007]    Thus it is desirable to have a battery having a cathode and cathode current collector capable of providing good properties, such as for example, desirable energy storage and conductor properties, respectively. It is further desirable to be able to reduce the cost of fabrication of the battery. It is also desirable to be able to minimize any thermal stresses which may be caused by annealing of thermally mismatched materials in the fabrication of the batteries.  
         SUMMARY  
         [0008]    A battery comprises a substrate having a cathode thereon, the cathode having a surface. A cathode current collector comprising one or more conducting lines that contact the surface of the cathode. An electrolyte at least partially extends through the conducting lines of the cathode current collector to contact the cathode. An anode contacts the electrolyte.  
           [0009]    A method of fabricating a battery comprises forming a substrate, forming a cathode on the substrate, the cathode having a surface, forming a cathode current collector comprising one or more conducting lines that contact the surface of the cathode, forming an electrolyte at least partially extending through the conducting line of the cathode current collector to contact the cathode, and forming an anode contacting the electrolyte. 
       
    
    
     DRAWINGS  
       [0010]    These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description, appended claims, and accompanying drawings, which illustrate embodiments of the present invention that may be used separately or in combination with one another, where:  
         [0011]    [0011]FIG. 1 (prior art) is a schematic sectional view of a conventional thin film battery;  
         [0012]    [0012]FIG. 2 is a schematic sectional side view of an embodiment of a battery according to the present invention;  
         [0013]    [0013]FIG. 3 is a schematic top view of the battery along section  2 - 2  of FIG. 1;  
         [0014]    [0014]FIG. 4 is a schematic top view of another embodiment of a battery according to the present invention; and  
         [0015]    [0015]FIG. 5 is a graph showing discharge curves of an embodiment of a battery according to the present invention. 
     
    
     DESCRIPTION  
       [0016]    An embodiment of a battery  100  having exemplary features according to the present invention is illustrated in FIG. 2. The battery  100  is formed on a substrate  104  which may be a dielectric, insulator, semiconductor, or conductor material. The substrate  104  should also have sufficient mechanical strength to support layers formed thereon during temperatures reached during processing or operation of the battery  100 . Typically, the substrate  104  is a dielectric material, such as silicon dioxide, aluminum oxide, titanium, or a polymer. A preferred substrate  100  comprises mica which has good tensile strength and temperature resistance, as described in aforementioned commonly owned U.S. patent application Ser. No. 09/656,012 which is incorporated herein by reference in its entirety. In one version, the mica layer comprises a thickness of less than about 100 microns, and more preferably less than 25 microns, to reduce the weight and volume of the battery  100 .  
         [0017]    The materials deposited on the substrate  104  may have a number of different configurations, arrangements, and shapes, and should not be limited to the exemplary configurations, arrangements, and shapes, which are described herein to illustrate exemplary embodiments of the invention. Typically, the materials are deposited or otherwise formed as one or more thin films on the substrate  104 . These thin films are typically thin layers that have a thickness of from about 1 to about 1000 microns. The layers may be continuous, segmented or patterned. Optionally, certain layers, such as an adhesion layer (not shown), may be deposited on the substrate  104  or on other already deposited layers, to improve the adhesion of any overlying layers. Suitable adhesion layers may be made from metal containing materials, such as, for example, titanium, cobalt, aluminum, other metals, or ceramic containing materials, such as for example, LiCoO x , which may comprise a mixed stoichiometry that includes LiCoO 2 .  
         [0018]    In one configuration, the cathode  108  that serves as the positive electrode of the battery  100  is initially formed on the substrate  104 . In this embodiment, the cathode  108  is deposited directly on the substrate  100 , without an underlying current collector. The cathode  108  may comprise, for example, an electrochemically active material, such as for example, amorphous vanadium pentoxide, V 2 O 5 , or one of several intercalation compounds that may be deposited in thin-film form, such as crystalline TiS 2 , LiMn 2 O 2  or LiCoO 2 . In one exemplary embodiment, the cathode  108  comprises a crystalline LiCoO 2  film that is formed on the substrate  104 . The LiCoO 2  film can be deposited on the substrate at relatively low temperatures, such as below 600° C. by a PVD process, such as RF or DC magnetron sputtering of a target with a relatively high plasma density, as for example, described in aforementioned U.S. patent application Ser. No. 09/656,012, which is incorporated herein by reference in its entirety. The deposition chamber may be a vacuum chamber comprising one or more sputtering targets and a process gas distribution manifold for distributing process gases into the chamber. A mixture of argon and oxygen gases is introduced into the chamber with a total pressure of 5 to 25 mTorr and a volumetric flow rate ratio of Ar/O 2  of from about 1 to about 45 sccm. The target comprises a disc of LiCoO x . Radio frequency (RF) sputtering of the target was performed at a power density level of 1 to 20 W/cm 2 . Thereafter, the deposited cathode material is thermally annealed to a temperature of from about 150 to 600° C. in an annealing gas comprising ambient oxygen to crystallize the cathode material.  
         [0019]    In one embodiment, a cathode current collector  112  is then formed on the cathode  108 . The current collector  112  is typically a conductive layer, comprising, for example, a metal containing material, such as a metal, metal alloy, or metal silicide. Because such a current collector  112  may be formed after annealing of the cathode  108 , many conducting metal containing materials may be used and it is no longer necessary to use only a non-reactive material. Thus, the current collector  112  may be absent a non-reactive metal containing material, such as for example, silver, gold or platinum, because it is no longer subject to an oxidizing or high temperature treatment that may be used to crystallize the cathode  108 . Instead, the current collector  112  may be made from conducting reactive materials, including for example, oxidizing materials or relatively low melting point metals, such as for example, aluminum, cobalt, copper, nickel, titanium, tantalum, vanadium, zirconium, and alloys and compounds mixtures thereof. Preferred conductor materials may comprise aluminum, copper or indium-tin oxide. These metals or metal compounds are typically relatively inexpensive and thus also be advantageously used to reduce the cost of the battery  100 . The residual stress is also lowered since thermal stresses that may arise from the use of metals which have high thermal expansion coefficients is also avoided. In a preferred embodiment, the metal comprises  
         [0020]    The current collector  112  provides a conducting surface from which electrons may be dissipated or collected from the cathode  108 . Thus, the current collector  112  is shaped to increase electron conductivity to or from the cathode  108 . However, because the current collector  112  is on the side of the cathode  108  that faces an electrolyte  118  of the battery  100 , it is also shaped to reduce blockage of the positive ions that move between the electrolyte and the cathode  108 . Thus, the current collector  112  has the conflicting requirements of trying to have a large area in contact with the cathode  108  to increase electron transport efficiency while also trying to reduce the area that may block transport of ions between the electrolyte  118  and the cathode  108 .  
         [0021]    A suitable current collector  112  comprises one or more conducting lines  128  covering the surface of the cathode  108 . In one embodiment, the conducting lines  128  are formed by placing a substrate in a sputtering process chamber (not shown), and placing on the substrate, a mask (not shown) having patterned lines etched therethrough. Conducting material is then deposited on the cathode  110  using a sputtering system similar to the one used for deposition of the cathode  110 . However, the sputtering gas may be pure argon and DC instead of RF magnetron sputtering may also be used to sputter a target. The mask may be a stainless steel plate having the desired pattern of the conducting lines etched therethrough. To deposit a conducting pattern comprising copper material, the target material comprises copper and a gas comprising Ar is introduced into the chamber at a pressure of about 1 to 10 mTorr. The gas may be energized with DC energy at a power level of from about 0.5 to about 5 kw, and more preferably about 1 kw. The temperature of the substrate may be maintained at less than 100° C. This is performed for 240 seconds to deposit patterned conducting lines of copper having a thickness of about 0.3 microns on the substrate.  
         [0022]    In one example, the conducting lines  128  are arranged to form a grid defined by a plurality of elongated prongs  116  that extend outwardly from a base prong  117 , as for example, illustrated in the embodiment shown in FIG. 3. The effective resistance of a cathode  108  having such a structure for the current collector  112  is given by: 
           R   t =⅙× Ri×L/W/N 2, 
         [0023]    where the length of the base member  117  is ‘L’, the length of each elongated prong  116  is ‘W’, the total number of elongated prongs  116  is ‘N’, the thickness of the cathode  108  is ‘T’, and the resistivity of the cathode material is ‘Ri’. For a cathode  108  comprising crystalline LiCoO 2  having a top surface area of 1 cm×1 cm and that is 10 micron thick, and a current collector  112  comprising  10  elongated prongs  116 , the effective resistance R t  is about 4 ohm.  
         [0024]    In an exemplary embodiment, the cathode current collector  112  comprises ten elongated prongs  116  which are equally spaced apart across a rectangular shaped cathode  108  and connected to a base prong  117  that forms an edge of the cathode. In one embodiment, the effective resistance of the elongated prongs  116  is about 1.5 ohm, and each member  116  is sized about 0.1 microns thick, 0.05 mm wide, and 1 cm long. Such a current collector  112  may be made from copper. The reduction of effective area of the cathode/electrolyte interface, in this current collector structure, is only about 5%. Considering that the resistance of an electrolyte  118  comprising lithium phosphorous oxynitride having an area of 1 cm 2  and that is 1 micron thick, is about 50 ohm, the resistance of this current collector  112  is acceptable for many applications. For a battery  100  having a small area and that is operated at a low discharge current, the current collector  112  may comprise only the base prong  117  without the elongated prongs  116 . While the internal resistance of such a battery is higher, the higher resistance does not significantly affect the battery performance because it is discharged at a relatively low current level.  
         [0025]    Other patterns for the conducting lines  128  can also be used, such as an arrangement of one or more of meandering lines, circular lines, random lines, radial lines, horizontal lines, vertical lines and diagonal lines. For example, FIG. 4 shows an embodiment of the current collector  112  comprising concentric arcuate lines  132  that are connected to radially extending lines  134 . The concentric arcuate lines  132  extend from a number of alternating radial lines  134 , and are interleaved with one another to cover a surface of the cathode  108 . In one embodiment, the width of the arcuate and radial lines  132 ,  134 , is about 0.05 mm, and the spacing between the arcuate lines  132  is about 1 mm. The electrical resistance and the surface coverage are both similar to the patterned line embodiment shown in FIG. 3.  
         [0026]    Thereafter, an electrolyte  118  maybe formed over the cathode current collector  112 , as illustrated in FIG. 2. The electrolyte  118  may comprise, for example, amorphous lithium phosphorus oxynitride material. The lithium phosphorous oxynitride is deposited over the conducting lines  128  of the current collector  112  and the exposed portions of the cathode  110 . Deposition of lithium phosphorous oxynitride may be carried out in a vacuum chamber similar to that used for deposition of the cathode  110  and cathode current collector  112 . For example, the lithium phosphorous oxynitride may be deposited by RF sputtering of a lithium phosphate (Li 3 PO 4 ) target in pure nitrogen at a power density level of from about 1 to about 20 W/cm 2 . The flow rate of nitrogen gas is from about 100 to about 300 sccm, and the gas is maintained at a pressure of less than about 15 mTorr, and more preferably at least about 1 mTorr. The resultant material has an ionic conductivity of 2×10 −6  S. The sample is then annealed in nitrogen or in air at 200° C. for 10 minutes to increase the ionic conductivity of electrolyte and to reduce the resistance of any interfaces.  
         [0027]    An anode  120  that serves as the negative terminal of the battery  100  is then deposited over the electrolyte  118 . The anode  120  comprises a conductor film, that may be for example, a metal film, such as a copper film, that is deposited directly on the electrolyte  118 . In one version, an optional anode current collector  124  is deposited on the anode  120  (as shown). The anode  120  may also be deposited to overlap a portion of the anode current collector  124 , for example, by forming the anode current collector  124  below an edge or boundary of the anode  120 . The anode current collector  124  is especially useful when the anode  120  is made from a material having a relatively low conductivity. The materials used to fabricate the anode  120  and the optional anode current collector  124  may be the same as the materials used to fabricate the cathode  108  and the cathode current collector  112 , respectively, or they may be materials having different conductivities. In another version, the anode  120  is made from an in-situ deposited lithium film which is sufficiently conductive to also serve as an anode current collector  124 , and the two films  120 ,  124  are the same film. Further layers may be formed over or below the substrate  104 , for example, to provide damage, environmental, or corrosion protection, the protective layers including for example, polymer, parylene, lithium phosphorous oxynitride, or copper layers.  
         [0028]    [0028]FIG. 5 is a typical discharge curve of a battery  100  having a top surface area of about 1 cm 2 . The battery  100  comprises a substrate  104  that is a  10  m-thick layer of mica. A cathode  108  comprising crystalline LiCoO 2  is formed on the substrate  104 , as for example, illustrated in aforementioned U.S. patent application Ser. No. 09/656,012., which is incorporated herein by reference in its entirety. The energy capacity of the battery is about 0.05 mAh. A cathode current collector  112  comprising one or more conducting lines made of 0.3 μm thick copper is formed on the cathode  108 . Thereafter, an electrolyte  118  and anode  120 , and the optional anode current collector  124  is formed on the substrate  104 . The graph of FIG. 5 shows that the cut off voltage of the battery  100  is well defined at 3.6 Volts. After 10 charge/discharge cycles, the performance of the battery  100  is unchanged from the first charge/discharge cycle, indicating the good charging and recharging quality of the battery  100 .  
         [0029]    Although the present invention has been described in considerable detail with regard to certain preferred versions thereof, other versions would be apparent to those of ordinary skill in the art. For example, a current collector according to the present invention may be used with other types electronic devices or structures, and for other methods or purposes. Also, the structure or operation of the battery may be modified as would be apparent to one of ordinary skill in the art. Thus, the appended claims should not be limited to the description of the preferred versions contained herein.