Patent Publication Number: US-10770698-B1

Title: Miniature electrochemical cell having a casing comprising opposed ceramic substrates secured together using a precious metal braze

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. provisional patent application Ser. No. 62/517,408, filed on Jun. 9, 2017. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the art of electrochemical cells. More particularly, the present invention relates to an improved miniature electrochemical cell, which is defined as a cell having a total volume that is less than 0.5 cc. 
     2. Prior Art 
     Conventional miniature electrochemical cells are primarily designed as coin cells, which is a convenient configuration for the assembly of small cells. A typical coin cell has a metal base and a metal lid that serve as the opposite polarity terminals for the cell and are electrically isolated from each other by a plastic gasket. A seal is formed by crimping the metal base around the gasket. The plastic gasket then electrically isolates the base and lid from each other. However, seals formed by crimping a metallic member onto a plastic gasket are difficult to form consistently and are known to be unacceptably porous for hermetic applications. A compromised hermetic seal may present an escape path for electrolyte or allow for atmospheric contaminants to get inside the cell and disrupt its operation. Ingress of water is particularly problematic for lithium electrochemical cells. In that respect, crimped plastic seals may not provide the type of consistent longevity from one cell to the next that is required for some applications, particularly when the cell is intended to power a device implanted in a human or animal body. 
     In addition, the crimped metal/plastic seal has a certain peripheral volume requirement that reduces space for the electrodes, thereby reducing cell capacity and energy density. A conventional coin cell with a crimped seal may not have sufficient energy per unit volume to power the associated electronic device for an acceptable period of time. Further, there is a limit to how small a crimped seal can be made; below a certain size, it may not be possible to construct the coin cell at all. 
     U.S. Pat. No. 8,697,278 to Wyser et al. describes a coin cell construction having a metal lid and a metal base. The lid is anodized to form an insulation layer on selected portions thereof. This insulating material is then coated with a formable material, such as gold, and the lid is press-fitted to the metal base so that the anodized layer acts as an insulator between the opposite polarity base and lid while the formable material seals the anodized layer to the base. In this prior art cell construction, however, the sealing strength between the formable layer and the metal base may not be strong enough to withstand the pressure of evolved gas or electrode expansion. This may cause the base and lid to gradually separate from each other, resulting in leakage of electrolyte from the cell and loss of function. 
     Therefore, what is needed is an improved miniature electrochemical cell having a total volume that is less than 0.5 cc. The cell enclosure must be both mechanically robust and resistive to corrosion of the type that can occur when the cell is implanted in an animal or human body. The miniature electrochemical cells of the present invention meet these needs. 
     SUMMARY OF THE INVENTION 
     Miniature electrochemical cells according to the present invention employ mechanically robust sealing approaches that are capable of maintaining hermeticity for 10 years, or more. A ceramic material serves as the insulating layer disposed between the anode and cathode terminals. The insulating ceramic is joined to the opposite polarity terminals using a metal-containing feedthrough formed by co-firing a metallic paste in a via hole extending through a green ceramic. 
     In that respect, as with every cell, miniature electrochemical cells according to the present invention have two terminals that are electrically isolated from each other. However, in order for the cell to operate for 10 years or more, novel robust sealing methodologies are required to prevent ingress of moisture or egress of electrolyte while still providing adequate electrical isolation between the terminals. The problem is that conventional sealing methods are often not practical when cell sizes drop below 0.5 cc. That is because the seals themselves become a major portion of the overall cell volume. On the other hand, the present invention describes multiple encapsulation methodologies that are designed to provide a miniature electrochemical cell having a total volume of less than 0.5 cc with a hermetic seal exhibiting high volumetric efficiency. 
     In addition, as cells become smaller and smaller, it becomes more difficult to find space for the electrolyte fill port, and to find a practical means of plugging and sealing the fill port. The present invention describes a novel fill port design and hermetic fill port sealing process that is readily adapted to miniature electrochemical cells having a total volume of less than 0.5 cc. 
     Moreover, with cell sizes below 0.5 cc, it may become advantageous to use a solid electrolyte so that filling with a liquid is not required. However, conventional solid electrolyte cells are known to undergo expansion and contraction during cycling and consequently require hermetic encapsulation structures that are designed to accommodate the expected dimensional changes. Enclosure designs for miniature cells activated with solid electrolyte systems do not currently exist. Therefore, one aspect of the present invention is to provide encapsulation structures that can be applied to miniature solid-state electrochemical cells to provide a hermetic seal while accommodating the required dimensional changes. 
     In the case of solid-state cells, ceramic substrates are well known in the art, but are often formed with rough surfaces. Rough substrate surfaces can lead to defective coatings such as interrupted interconnection traces, protrusions which may cause electric shorting between layers, uneven reaction leading to poor cycling or dendrite formation, and the like. Miniature solid-state electrochemical cells according to the present invention use physical vapor deposition (PVD) for laying down the non-active layers, for example the anode and cathode current collectors, onto ceramic substrates that have been lapped and polished with slurry or ceramic coated grinding paper followed by diamond polishing to a smooth surface finish. 
     Another problem with conventional miniature electrochemical cell designs is the need for the materials from which the cell is constructed to be both chemically compatible with each other and not susceptible to undesirable corrosion reactions. In the case of a ceramic housing, a miniature electrochemical cell according to the present invention uses various electrically conductive materials in the form of a conductive paste that is filled into a via hole extending through a ceramic plate-shaped substrate to thereby provide a hermetic feedthrough. A current collector is then deposited on an inner surface of the lapped and polished ceramic substrate in contact with the via fill material. In addition to providing electrical conduction from the electrode active layer to the via fill material, the current collector protects the fill material from corrosive reactions with other battery components while exhibiting good adhesion to the ceramic substrate or to an adhesion layer on the substrate should there be one. Exemplary current collectors according to the present invention are from about 0.1 microns to about 50 microns thick and comprised of a metallic layer that is deposited on the ceramic substrate using a PVD process so that the deposited metal covers the via. Exemplary current collector materials include copper and titanium. 
     Thus, the present invention describes various concepts for miniature electrochemical cells. The first cell concept relates to a hermetically-sealed electrochemical cell activated with either a liquid or a solid electrolyte. One or both of the anode and cathode terminals consist of a metal or metal/ceramic mixture surrounded by a ceramic substrate with the metal or metal/ceramic mixture being hermetically bonded or sealed to the substrate. The bond is preferably formed by co-firing a metallic material filled into a via hole extending through a green ceramic body. The metal is preferably platinum or a platinum/ceramic composite and the ceramic is 3% YSZ or alumina. 
     A miniature electrochemical cell activated with a liquid electrolyte comprises a ceramic enclosure containing a secondary via serving as a fill port. A metallic material is filled into the secondary via with an open hole in the center of the fill material. After the cell is filled with electrolyte, the metallic material in the electrolyte fill via is melted, preferably using a laser, to form a hermetic seal. The metal is preferentially platinum or platinum ceramic composite and the ceramic is 3% YSZ or alumina. 
     A hermetically-sealed electrochemical cell comprises a ceramic lid and a ceramic base. The lid and base are joined by welding a peripheral band that is attached to the ceramic by co-firing the band with the ceramic so that they are chemically-bonded together. Preferably, the metal periphery band consists of platinum co-fired with a ceramic or a composite of platinum/ceramic co-fired with additional filler metals selected from gold, platinum, and palladium at the weld joint. 
     In the case of a miniature electrochemical cell according to the present invention that is activated with a solid electrolyte, the hermetic ceramic enclosure is provided with a welded periphery and metallic feedthroughs in which the bottom surface of the lid is between 1 and 100 um above the upper surface of the cell stack. The gap between the cell stack and the lid provides sufficient space for dimensional changes to the cell stack without damaging the enclosure or disrupting the seal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of one embodiment of a miniature electrochemical cell  10  according to the present invention. 
         FIG. 2  is a cross-sectional view of another embodiment of a miniature electrochemical cell  10 A according to the present invention. 
         FIG. 3  is a cross-sectional view of another embodiment of a miniature electrochemical cell  100  according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A miniature electrochemical cell according to the present invention is not limited to any particular type. The miniature electrochemical cell can be an alkaline cell, a primary lithium cell, a rechargeable lithium cell, a Ni/cadmium cell, a Ni/metal hydride cell, a supercapacitor, a thin film solid-state cell, and the like. An exemplary miniature electrochemical cell is of a lithium-ion chemistry having a carbon-based anode and a metal oxide-based cathode, such as a cathode of LiCoO 2  or LiNi a Mn b Co 1-a-b O 2 , or a solid-state thin film cell with a lithium anode, a metal-oxide based cathode and a solid electrolyte, for example a LiPON (Li x PO y N z , with x ranging from 3 to 4, y ranging from 3 to 4 and z ranging from 0.1 to 1)) electrolyte. If the cell is not of a solid-state chemistry, it is activated with a liquid electrolyte. The liquid electrolyte is added to the cell stack prior to joining the case and lid, or alternatively, a fill port is provided in the metallic or ceramic casing so that the fill port and consequentially the casing can be hermetically sealed after filling with electrolyte. 
     Referring now to the drawings,  FIG. 1  illustrates a first embodiment of a miniature electrochemical cell  10  according to the present invention. The cell  10  comprises a first monolithic substrate or plate  12  of a ceramic material, for example, of alumina that is spaced from a second monolithic substrate or plate  14  of a ceramic material, for example, of alumina. According to one embodiment of the present invention, the first and second substrates  12 ,  14  may be fabricated from a pressed ceramic slurry in tape form, which is widely available commercially. Also according to the present invention, a substrate of 92%-99% alumina (Al 2 O 3 ) may be used for the ceramic substrates  12 ,  14 . The substrate material may be bought in sheet form, which may be flexible or rigid. 
     The first ceramic substrate  12  has a major sidewall  12 A extending to an upper surface  12 B and a lower surface  12 C. The upper and lower surfaces  12 B,  12 C are substantially co-planar. The major sidewall  12 A is not limited to any particular shape; it can have a curved annular shape or comprise front and back sidewall portions meeting right and left sidewall portions. As those skilled in the art will readily understand, a myriad of other shapes are contemplated for the major sidewall  12 A of the first ceramic substrate  12 , the shape being limited only by the form factor for the particular application or device that the cell  10  is intended to power. 
     A depending annular portion  12 D extends downwardly from the lower surface  12 C of the ceramic substrate  12 . The annular portion  12 D comprises an inner annular surface  12 E spaced from an outer annular surface  12 F. Both annular surfaces  12 E and  12 F meet an annular face  12 G. As shown, the annular portion  12 D is spaced inwardly from the major sidewall  12 A. 
     The first ceramic substrate  12  has a first via hole  18  extending from the upper surface  12 B to the lower surface  12 C thereof. The first via hole  18  is provided with an electrically conductive pathway  20  that extends to the spaced apart upper and lower ceramic substrate surfaces  12 B,  12 C. The electrically conductive pathway  20  is formed by filling the via hole  18  with a ceramic reinforced metal composite or CRMC paste  20 A and then, in the green state, the inner diameter of the CRMC paste is drilled out so that the via hole can be re-filled with a substantially pure platinum paste, or a platinum wire  20 B. In the case of a substantially pure platinum paste  20 B, the paste formulation is substantially devoid of a ceramic material. 
     The second ceramic substrate  14  has a major sidewall  14 A extending to an upper surface  14 B that is substantially coplanar with a lower surface  14 C. In a similar manner as with the major sidewall  12 A of the first ceramic substrate  12 , the shape of the major sidewall  14 A is not limited to any particular form as it may have a myriad of shapes dictated by the form factor for the particular application or device that the cell  10  is intended to power. Preferably, however, the major sidewalls  12 A,  14 A of the respective first and second ceramic substrates  12 ,  14  have similar peripheral shapes. 
     An annular portion  14 D extends upwardly from the upper surface  14 B of the ceramic substrate  14 . The upstanding annular portion  14 D comprises an inner annular surface  14 E spaced from an outer annular surface  14 F. Both annular surfaces  14 E and  14 F meet an annular face  14 G. As shown, the annular portion  14 D is spaced inwardly from the major sidewall  14 A. 
     The second substrate  14  preferably comprises a monolithic alumina substrate that has a second via hole  22  extending from the upper surface  14 B to the lower surface  14 C thereof. The second via hole  22  is provided with a conductive pathway  24  that extends to the spaced apart upper and lower substrate surfaces  14 B,  14 C. Conductive pathway  24  is similar to the conductive pathway  20  disposed through the first via hole  18  of the first ceramic substrate  12  and is comprised of a CRMC paste  24 A surrounding a cylinder of substantially pure platinum paste  24 B. The CRMC paste  24 A is similar to the CRMC paste  20 A and the platinum cylinder  24 B is similar to the substantially pure platinum cylinder  20 B of the first conductive pathway  20 . 
     The first ceramic substrate  12  also has a third via hole  26  extending to the upper and lower surfaces  12 B,  12 C. However, instead of filling or occupying the entire volume of the third via hole in a similar manner as the first and second conductive pathways  20 ,  24  occupy the first and second via holes  18 ,  22  in the respective first and second ceramic substrates  12 ,  14 , the third platinum fill  28  has the shape of a sleeve provided with an opening  30  extending to the upper and lower surfaces  12 B,  12 C. In that respect, the third via hole  26  is provided with an outer cylinder of CRMC paste  28 A surrounding an inner cylinder of substantially pure platinum paste  28 B. As will be described in detail hereinafter, opening  30  serves as a passageway for filling an electrolyte into the interior of the electrochemical cell  10 . 
     Examples of CRMC ceramic/metal pairings include, but are not limited to: 
     a) Alumina (Al 2 O 3 ) or zirconia (ZrO 2 ) including various stabilized or partially stabilized zirconia like zirconia toughened alumina (ZTA) and alumina toughened zirconia (ATZ) with platinum (Pt) or palladium (Pd). 
     b. Alumina (Al 2 O 3 ) or zirconia (ZrO 2 ) with iridium, rhenium, rhodium, various Pt alloys (e.g., Pt—Ir, Pt—Pd, Pt—Rh, Pt—Re, Pt—Au, Pt—Ag etc.), Pd alloys (e.g., Pd—Ir, Pd—Re, Pd—Rh, Pd—Ag, Pd—Au, Pd—Pt, Pd—Nb, etc.), Au alloys (e.g., Au—Nb, Au—Ti, etc.), Au alloys (e.g., Au—Nb, Au—Ti, etc.), and Ti alloys (e.g., Ti—Al—V, Ti—Pt, Ti—Nb, etc.) 
     Other non-limiting biocompatible metals and alloys that may be used in place of platinum include niobium, platinum/palladium, stainless steels, and titanium. 
     Furthermore any of the following materials may be used alone or in combination with any of the materials already discussed or within this list: gold (Au), silver (Ag), iridium (Ir), rhenium (Re), rhodium (Rh), titanium (Ti), tantalum (Ta), tungsten (W), zirconium (Zr), and vanadium (V); cobalt chromium molybdenum alloy, cobalt chromium nickel iron molybdenum manganese alloy, cobalt chromium tungsten nickel iron manganese alloy, cobalt nickel chromium iron molybdenum titanium alloy, cobalt nickel chromium iron molybdenum tungsten titanium alloy, cobalt nickel chromium molybdenum alloy, copper aluminum nickel alloy, copper zinc alloy, copper zinc aluminum nickel alloy, copper zinc silver alloy, gold platinum palladium silver indium alloy, iron chromium alloy, iron chromium nickel alloy, iron chromium nickel aluminum alloy, iron chromium nickel copper alloy, iron chromium nickel copper molybdenum niobium alloy, iron chromium nickel copper niobium alloy, iron chromium nickel copper titanium niobium alloy, iron chromium nickel manganese molybdenum alloy, iron chromium nickel molybdenum alloy, iron chromium nickel molybdenum aluminum alloy, iron chromium nickel titanium molybdenum alloy, iron manganese chromium molybdenum nitrogen alloy, nickel platinum alloy, nitinol, nickel titanium alloy, nickel titanium aluminum alloy, niobium-titanium alloy, platinum iridium alloy, platinum palladium gold alloy, titanium aluminum vanadium alloy, titanium based aluminum iron alloy, titanium based aluminum molybdenum zirconium alloy, titanium based molybdenum niobium alloy, titanium based molybdenum zirconium iron alloy, titanium based niobium zirconium alloy, titanium based niobium zirconium tantalum alloy, titanium molybdenum alloy, titanium niobium alloy, titanium platinum alloy, and titanium-based molybdenum zirconium tin alloy. 
     It is understood that throughout this disclosure when CRMC paste and pure platinum paste are referred to, those pastes include solvents and binders that will be baked out during sintering. Suitable binders are selected from the group consisting of ethyl cellulose, acrylic resin, polyvinyl alcohol, polyvinyl butyral and a poly(alkylene carbonate) having the general formula R—O—C(═O)—O with R═C1 to C5. Poly(ethylene carbonate) or poly(propylene carbonate) are preferred poly(alkylene carbonates). Suitable solvents are selected from the group consisting of terpineol, butyl carbitol, cyclohexanone, n-octyl alcohol, ethylene glycol, glycerol, water, and mixtures thereof. 
     In that respect, an important aspect of a miniature electrochemical cell according to the present invention centers around three enabling areas: (1) packing the ceramic substrate via holes  18 ,  22  and  26  with a high platinum solids loading in the CRMC paste and in the platinum paste, (2) compression of the ceramic substrate against the CRMC paste and in turn the CRMC paste against the platinum paste during binder bake-out and sintering, and (3) a controlled cool down rate in combination with interfacial bonding sufficient to tolerate coefficient of thermal expansion (CTE) mismatch. That is because metal/ceramic compatibility is an important factor in manufacturing hermetic structures. The difference in CTEs of the metal and ceramic is recognized as a major parameter in predicting compatibility. 
     The thermal expansion of a metal is generally considerably greater than that of a ceramic material. For example, at a bake-out temperature of 500° C., the CTE of alumina is 7.8×10 −6 /K and of platinum is 9.6×10 −6 /K. Historically, a CTE difference within the range of 0.5×10 −6 /K to 1.0×10 −6 /K between the mating metal and ceramic material is adequate to sustain hermetic bonding between these materials. However, it is believed that a difference beyond that range at the bake-out temperature for the alumina/platinum pair may produce sufficient tensile stresses at the interface during cooling to cause spontaneous bonding failure. Hence, given the significant difference in CTEs, even at a relatively low temperature of 500° C., achieving a hermetic seal between the platinum metal and alumina ceramic material would not be expected if the difference in CTE between the sintered alumina and the platinum metal exceeds the range of 0.5×10 −6 /K to 1.0×10 −6 /K. 
     In addition, a highly irregular surface at the material interface between the alumina substrate and the platinum metal particles within the via hole provides a mechanical contribution to adherence and robustness of the hermetic seal. A surface roughness produced by drill bits, sandblasting, grit blasting or chemical etching of the ceramic substrate can increase the surface area and, in so doing, provide for a stronger mechanical attachment along the mutually conformal interface. Examples of sandblasting and grit blasting media include sand, sodium bicarbonate, walnut shells, alumina particles, and other equivalent media. 
     According to the present invention, to achieve sustainable hermeticity, the following is required. Because the CTE of platinum is sufficiently higher than that of alumina, it is not theoretically possible for alumina to provide compressive forces on a platinum body in a via hole. Hence, to overcome the CTE differences between these two materials, the CRMC paste preferably has a ratio of alumina to platinum that ranges, by weight %, from about 10:90 to about 90:10, more preferably from about 50:50, still further preferably 70:30 to 30:70. 
     The via holes  18 ,  22  and  26  through the alumina substrates  12 ,  14  must be packed with the CRMC paste to occupy at least about 90% of the available space. In a preferred embodiment, the CRMC paste is packed to occupy about 95% of the via hole space. In a more preferred embodiment, the CRMC paste is packed to occupy about 99% of the via hole. The CRMC paste is a smooth, soft mass having a pliable consistency and comprising alumina particles, pure platinum particles, a binder material and a solvent. 
     Further, the platinum contacting the CRMC mixture in the via that was previously drilled through the CRMC material is a platinum paste occupying at least about 90% of the available space in the drilled CRMC via hole. In a preferred embodiment, the platinum paste is packed within the CRMC via to occupy about 95% of the space. In a more preferred embodiment, the platinum paste is packed to occupy about 99% of the CRMC via. In the case of the substantially pure platinum, a “paste” is defined as a smooth, soft mass having a pliable consistency and comprising pure platinum particles, a binder material and a solvent. 
     Moreover, shrinkage of the alumina substrate against the CRMC mixture and of the alumina in the CRMC mixture against the platinum paste must be no greater than about 20% of that of the platinum in the via hole. In a preferred embodiment, shrinkage is about 14%. In a more preferred embodiment, shrinkage is about 16%. 
     Importantly, the CRMC mixture  20 A closely matches the thermal coefficient of expansion of the alumina substrate  12 . This results in a very good hermetic seal between the CRMC mixture  20 A and the alumina substrate  12  and between the CRMC mixture  20 A and the cylinder of substantially pure platinum  20 B. Further, the CRMC mixture  20 A which surrounds the pure platinum cylinder  20 B forms a buffer during sintering and subsequent thermal shock so that there is a gradation of the differential and thermal coefficient of expansion. The thickness of the CRMC paste  20 A can be adjusted along with the diameter of the pure platinum cylinder  20 B so that during co-firing no stress cracks are induced in the ceramic substrate  12 . 
     The electrode assembly of the cell  10  comprises an anode current collector  32  seated against the lower surface  12 C of the upper substrate  12 . An anode active material  34  contacts the opposite side of the current collector  32 . Similarly, a cathode current collector  36  is seated against the upper surface  14 B of the lower substrate  14 . A cathode active material  38  is contacted to the opposite side of the cathode current collector  36 . An alumina spacer  40  serving as a separator resides between the anode and the cathode active materials  34 ,  38 . Preferably, the separator  40  is in direct contact with the anode active material  34  and the cathode active material  38  opposite their respective current collectors  32 ,  26 . That way, the separator  40  prevents direct physical contact between the anode active material  34  and the cathode active material  38  while allowing for ionic transport and conductivity therebetween. 
     An optional weld band  42  surrounds the electrode assembly comprising the stacked anode current collector  32 , anode active material  34 , separator  40 , cathode active material  38  and cathode current collector  36 . An upper edge  42 A of the weld band  42  contacts the lower surface  12 C of the first substrate  12  while a lower edge  42 B thereof contacts the upper surface  14 B of the second substrate  14 . 
     An annular air space  44  resides between the weld band  42  and the peripheral edges of the anode and cathode current collectors  32 ,  36 , anode active material  34 , cathode active material  38  and the intermediate separator  40 . Opposite the annular air space, the weld band  42  contacts the depending and upstanding portions  12 D and  14 D of the respective first and second substrate  12 ,  14 . 
     An upper annular ring  46  of CRMC paste surrounds the depending portion  12 D of the first substrate  12  while a lower annular ring  48  of CRMC paste surrounds the upstanding portion  14 D of the second ceramic substrate  14 . The upper and lower annular rings  46  and  48  are spaced inwardly from the major sidewalls  12 A and  14 A of the respective ceramic substrates  12 ,  14 . This provides a space for an upper annular ring  50  of a substantially pure platinum paste that surrounds the upper annular ring  46  of CRMC paste. Similarly, a lower annular ring  52  of a substantially pure platinum paste surrounds the lower annular ring  48  of CRMC paste. The pairs of the depending and upstanding ceramic portions  12 D and  14 D, the upper and lower CRMC rings  46  and  48 , and the upper and lower substantially pure platinum rings  50  and  52  meet each other at respective interfaces that are aligned with each other. 
     In that manner, the miniature electrochemical cell  10  is constructed by first positioning the CRMC rings  46  and  48  followed by the substantially pure platinum rings  50  and  52  into position with the respective ceramic substrates  12  and  14 , all in a green state. The first and third via holes  18  and  26  are provided with their respective CRMC and platinum structures in a green state. Similarly, the second via hole  22  of the second ceramic substrate  14  is provided with its CRMC and platinum structure in a green state. 
     After the ceramic substrate subassemblies  12 ,  14  including their filled via holes  18 ,  22  and  26 , respective CRMC rings  46 ,  48  and respective substantially pure platinum rings  50  and  52  are assembled, the substrate subassemblies are subjected to a controlled binder bake-out, sintering and cool down processing profile. The binder bake-out portion is performed at a temperature of from about 400° C. to about 700° C. for a minimum of about 4 hours. A preferred binder bake-out is at a temperature of from about 550° C. to about 650° C. A more preferred binder bake-out is at a temperature of from about 500° C. to about 600° C. 
     The sintering profile is preferably performed at a temperature ranging from about 1,400° C. to about 1,900° C. for up to about 6 hours. A preferred sintering profile is at a temperature ranging from about 1,500° C. to about 1,800° C. A more preferred sintering temperature ranges from about 1,600° C. to about 1,700° C. 
     The cool down profile occurs either by turning off the heating chamber and allowing the chamber to equalize to room temperature or, preferably by setting the cool down portion at a rate of up to about 5° C./min from the hold temperature cooled down to about 1,000° C. At about 1,000° C., the chamber is allowed to naturally equalize to room temperature. A more preferred cool down profile is at a rate of about 1° C./min from the hold temperature to about 1,000° C. followed by allowing the heating chamber to naturally equalize to room temperature. In so doing, the desired outcome of achieving a robust hermetic seal is achieved between the mating materials of the alumina and platinum. It is noted that these materials have a CTE mismatch beyond the limits heretofore recognized as adequate for sustained bonding. 
     During processing, compression is imparted by the alumina substrate around the CRMC paste and in turn the CRMC paste against the platinum paste within the via due to the shrinkage of the alumina being greater than that of the CRMC mixture and the shrinkage of the CRMC mixture being greater than that of the substantially pure platinum. Furthermore, the platinum is sufficiently malleable at this phase to be favorably deformed by the compressive forces being applied by the alumina in the substrates  12 ,  14  and in the CRMC paste. The combination of the platinum solids loading, the platinum packing in the via hole and the shrinkage of the alumina being greater than that of platinum results in the platinum taking the shape of the mating CRMC surface and in turn the CRMC surface taking the shape of the alumina substrate surface. 
     In other words, the compressive forces that result from the greater shrinkage of the alumina substrate than that of the CRMC mixture and in turn the shrinkage of the CRMC mixture being greater than that of the platinum within the via holes  18 ,  22  and  26  limit expansion of the platinum and force the platinum to deform such that it forms a hermetic seal. Thus, an interface between the alumina and CRMC mixture and between the CRMC mixture and the platinum material that conform to the respective interface surfaces and result in a nearly exact mirror image of the interfacing surfaces is formed, thereby creating a hermetic bond therebetween. This mutually conformal interface is critical. 
     Under certain processing conditions CRMC bodies may form a thin glass layer or even an alumina layer over the via ends. It may be necessary therefore to remove this formed layer with an additional manufacturing step, such as acid etch, lapping or mechanical abrasion. 
     After the substrate subassemblies  12 ,  14  have been subjected to the binder bake-out, sintering and cool down processing, the optional weld band  42  is positioned into one of the upper and lower ceramic substrates  12 ,  14 . The electrode assembly comprising the previously described anode current collector,  32 , anode active material  34 , separator  40 , cathode active material  38  and cathode current collector  36  is nested inside the weld band  42 . The other of the upper and lower ceramic substrate  12 ,  14  is then moved into position so that their respective annular faces  12 G and  14 G contact each other. A laser (not shown) is then used to direct a laser beam at the annular interface between the pure platinum rings  50  and  52 . This serves to melt the platinum material in the rings and also possibly melt the platinum in the CRMC rings  46  and  48  to thereby hermetically seal the ceramic subassemblies  12  and  14  together. If desired, the laser beam can also be directed at the annular interface between the upper platinum ring  50  and the upper ceramic substrate  12  and at the annular interface between the lower platinum ring  52  and the lower ceramic substrate  14 . 
     The electrode assembly housed inside the hermetically seal ceramic subassemblies  12  and  14  is then activated with an electrolyte filled through the fill opening  30  in the third via hole  26  in the upper substrate  12 . Finally, a hermetic electrochemical cell  10  is provided by sealing the fill opening  30  with a laser beam in a similar manner as previously described with respect to sealing the ceramic subassemblies  12  and  14  together with a laser beam directed at the annular interface between the platinum rings  50 ,  52 . 
     The electrochemical cell  10 A illustrated in  FIG. 2  is similar to the cell  10  shown in  FIG. 1  except that the depending portion  12 D of the ceramic substrate  12  and the upstanding portion  14 D of the lower ceramic substrate do not abut or contact each other. Instead, they are spaced apart from each other. Similarly, the respective CRMC rings  46 A and  48 A, and the respective platinum rings  50 A and  52 A are spaced apart from each other. An annular disc  54  of a precious metal, for example a disc selected from gold, platinum and palladium resides in this gap. A laser beam is directed at the precious metal disc  54  to fuse the disc to the platinum rings  50 A,  52 A and to the CRMC rings  46 A,  48 A to thereby seal the upper and lower ceramic subassemblies  12  and  14  together. 
     An activating electrolyte is filled through the fill hole  30  into contact with the electrode assembly housed therein and the fill hole is sealed with a laser beam as previously described. 
       FIG. 3  illustrates another embodiment of a miniature electrochemical cell  100  according to the present invention. The cell  100  comprises a first monolithic substrate or plate  102  of a ceramic material, for example, of alumina that is spaced from a second monolithic substrate or plate  104  of a ceramic material, for example, of alumina. In a similar manner as the various substrates described for the electrochemical cells  10  and  10 A illustrated in  FIGS. 1 and 2 , the first and second substrates.  102 ,  104  may be fabricated from a pressed ceramic slurry in tape form, for example 92%-99% alumina (Al 2 O 3 ). The substrate material may be flexible or rigid. 
     The first ceramic substrate  102  has a major sidewall  102 A extending to an upper surface  102 B and a lower surface  102 C. The upper and lower surfaces  102 B,  102 C are substantially co-planar. The major sidewall  102 A is not limited to any particular shape; it can have a curved annular shape or comprise front and back sidewall portions meeting right and left sidewall portions. In fact, a myriad of other shapes are contemplated for the major sidewall  102 A of the first ceramic substrate  102 , the shape being limited only by the form factor for the particular application or device that the cell  100  is intended to power. 
     A depending annular portion  102 D extends downwardly from the lower surface  102 C of the ceramic substrate  102 . The annular portion  102 D comprises an inner annular surface  102 E spaced from an outer annular surface  102 F. Both annular surfaces  102 E and  102 F meet an annular face  102 G. As shown, the annular portion  102 D is spaced inwardly from the major sidewall  102 A. 
     The second ceramic substrate  104  has a first sidewall  104 A extending to an upper surface  104 B and a lower surface  104 C. The upper and lower surfaces  104 B,  104 C are substantially co-planar. The second ceramic substrate  104  further has a second sidewall  104 D extending to an upper surface  104 E and a lower surface  104 F. The upper and lower surfaces  104 E,  104 F are substantially co-planar. Moreover, the upper surface  104 E meets the first sidewall  104 A as a monolithic structure. Importantly, the second sidewall  104 D is spaced outwardly from the first sidewall  104 A to thereby form an annular ledge where the second upper surface  104 E meets the first sidewall  104 A. The second annular sidewall  104 D is substantially aligned with the first sidewall  102 A of the first substrate  102 . 
     The second substrate  104  preferably comprises a monolithic alumina substrate that has a first via hole  106  and a second via hole  108 , both extending from the upper surface  104 B to the lower surface  104 C thereof. The via holes  106  and  108  are provided with respective conductive pathways  110  and  112  that each extend to the spaced apart upper and lower second substrate surfaces  104 B and  104 C. The conductive pathway  108  residing in the via hole  106  comprises a CRMC paste  110 A contacting the alumina substrate  104  and a substantially pure platinum paste  110 B disposed inside a via hole drilled through the CRMC paste. Similarly, the conductive pathway  112  residing in the second via hole  108  comprises a CRMC paste  112 A contacting the alumina substrate  104  and a substantially pure platinum paste  112 B disposed inside a via hole drilled through the CRMC paste. This is in a similar manner as the previously described conductive pathways  20  and  24  illustrated in  FIGS. 1 and 2  for electrochemical cells  10  and  10 A. 
     The electrode assembly of the cell  100  comprises a cathode current collector  114  seated against the upper surface  104 B of the lower substrate  104  aligned with the first via hole  106 . An anode current collector  116  is seated against the upper surface  104 B of the lower substrate aligned with the second via hole  108 . In their aligned orientations, the cathode current collector  114  contacts the platinum  110 B in via hole  106  and the anode current collector  116  contacts the platinum  112 B in via hole  108 . 
     A cathode active material  118  contacts the cathode current collector  114  opposite the platinum  110 B in via hole  106 . A separator  120  has an annular shape in plain view with a recess  1120 A that is sized and shaped to receive the cathode active material  118  supported on the cathode current collector  114 . An outer annular edge  120 B of the separator contacts or is otherwise supported on the upper surface  104 B of the second substrate  104 . 
     A shaped anode  122  contacts the anode current collector  116  opposite the platinum material  112 B in the via hole  108 . The anode active material  122  also contacts the separator  120 . In that respect, the anode  122  has a curved sidewall  122 A extending to an upper surface  122 B. A shaped lower surface  122 C opposite the upper surface  112 B contacts the anode current collector  116  and separator  120  to thereby provide the anode with a backwards and sideways L-shape in cross-section. 
     An exemplary chemistry for the miniature electrochemical cell  100  shown in  FIG. 3  has lithium as an exemplary anode active material, LiCoO 2  as a cathode active material and the separator is of LiPON (Li x PO y N z , with x ranging from 3 to 4, y ranging from 3 to 4 and z ranging from 0.1 to 1). LiPON is a solid material that both serves as both the separator and the electrolyte for the Li/LiCoO 2  couple. 
     An optional weld band  124  surrounds the side-by-side anode  122  and cathode  118  including their respective current collectors  116 ,  114  and the intermediate separator  118 . In that respect, an upper edge  124 A of weld band  124  contacts a lower surface  102 C of the upper substrate  102  while a lower edge  124 B of the band rests on the upper surface  104 B of the lower substrate  104 . An outer surface  124 C of the weld band is spaced inwardly from the peripheral sidewalls  102 D of the first substrate  102 . An air gap  126  resides between the weld band  124  and the electrode assembly. 
     An upper annular ring  126  of CRMC paste surrounds the depending portion  102 D of the first substrate  102  while a lower annular ring  128  of CRMC paste surrounds the sidewall  104 A of the second ceramic substrate  104 . The upper and lower annular rings  126  and  128  are spaced inwardly from the sidewalls  102 A and  104 D of the respective ceramic substrates  102 ,  104 . This provides a space for an upper annular ring  130  of a substantially pure platinum paste that surrounds the upper annular ring  126  of CRMC paste. Similarly, a lower annular ring  132  of a substantially pure platinum paste surrounds the lower annular ring  128  of CRMC paste. The pairs of the depending and upstanding ceramic portions  102 D and  104 A, the upper and lower CRMC rings  126  and  128 , and the upper and lower substantially pure platinum rings  130  and  132  meet each other at respective interfaces that are aligned with each other. 
     In that manner, the miniature electrochemical cell  100  is constructed by first positioning the CRMC rings  126  and  128  followed by the substantially pure platinum rings  130  and  132  into position with the respective ceramic substrates  102  and  104 , all in a green state. The first and second via holes  108  and  106  are provided with their respective CRMC and platinum structures in a green state. 
     After the ceramic substrate subassemblies  102 ,  104  including their filled via holes  106  and  108 , respective CRMC rings  126 ,  128  and respective substantially pure platinum rings  130  and  132  are assembled, the substrate subassemblies are subjected to a controlled binder bake-out, sintering and cool down processing profile as previously described with respect to electrochemical cells  10  and  10 A. 
     After binder bake-out, sintering and cool down processing, the optional weld band  124  is positioned in one of the upper and lower ceramic substrates  102 ,  104 , preferably the lower substrate. The electrode assembly comprising the previously described anode current collector  116 , anode active material  122 , separator  120 , cathode active material  118  and cathode current collector  114  is nested inside the weld band  124 . The other of the upper and lower ceramic substrate  102 ,  104 , preferably the upper substrate, is then moved into position so that their respective annular surfaces  102 G and  104 B contact each other. A laser (not shown) is then used to direct a laser beam at the annular interface between the pure platinum rings  130  and  132 . This serves to melt the platinum material in the rings and also possibly melt the platinum in the CRMC rings  126  and  128  to thereby hermetically seal the ceramic subassemblies  102  and  104  together. If desired, the laser beam can also be directed at the annular interface between the upper platinum ring  130  and the upper ceramic substrate  102  and at the annular interface between the lower platinum ring  132  and the lower ceramic substrate  104 . 
     In a broad sense, the miniature electrochemical cells  10 ,  10 A and  100  can each be of a primary or a secondary chemistry. If a primary chemistry, the anode can also comprise metals capable of alloying with lithium at potentials below 1.0 V vs. lithium such as Sn, Si, Al, B, Si—B, and composites of those metals with inactive metals to reduce volume expansion. The form of the anode may vary, but preferably it is of a thin sheet or foil that is pressed, evaporation, or rolled on the metallic anode current collector. 
     The cathode of a primary cell is of electrically conductive material, preferably a solid material. The solid cathode may comprise a metal element, a metal oxide, a mixed metal oxide, and a metal sulfide, and combinations thereof. A preferred cathode active material is selected from the group consisting of silver vanadium oxide, copper silver vanadium oxide, manganese dioxide, cobalt nickel, nickel oxide, copper oxide, copper sulfide, iron sulfide, iron disulfide, titanium disulfide, copper vanadium oxide, carbon monofluoride, and mixtures thereof. 
     Before fabrication into an electrode for incorporation into an electrochemical cell, the cathode active material is mixed with a binder material such as a powdered fluoro-polymer, more preferably powdered polytetrafluoroethylene or powdered polyvinylidene fluoride (PVDF) present at about 1 to about 5 weight percent of the cathode mixture. Further, up to about 10 weight percent of a conductive diluent is preferably added to the cathode mixture to improve conductivity. Suitable materials for this purpose include acetylene black, carbon black and/or graphite or a metallic powder such as powdered nickel, aluminum, titanium and stainless steel. The preferred cathode active mixture for the electrochemical cells  10  and  10 A, but not for cell  100  thus includes a powdered fluoro-polymer binder present at about 3 weight percent, a conductive diluent present at about 3 weight percent, and about 94 weight percent of the cathode active material. 
     The primary electrochemical cells  10  and  10 A include a nonaqueous, ionically conductive electrolyte having an inorganic, ionically conductive salt dissolved in a nonaqueous solvent and, more preferably, a lithium salt dissolved in a mixture of a low viscosity solvent and a high permittivity solvent. The salt serves as the vehicle for migration of the anode ions to intercalate or react with the cathode active material and suitable salts include LiPF 6 , LiBF 4 , LiAsF 6 , LiSbF 6 , LiClO 4 , LiO 2 , LiAlCl 4 , LiGaCl 4 , LiC(SO 2 CF 3 ) 3 , LiN(SO 2 CF 3 ) 2 , LiSCN, LiO 3 SCF 3 , LiC 6  F 5 SO 3 , LiO 2 CCF 3 , LiSO 6 F, LiB(C 6 H 5 ) 4 , LiCF 3 SO 3 , and mixtures thereof. 
     Suitable low viscosity solvents include esters, linear and cyclic ethers and dialkyl carbonates such as tetrahydrofuran (THF), methyl acetate (MA), diglyme, trigylme, tetragylme, dimethyl carbonate (DMC), 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), 1-ethoxy,2-methoxyethane (EME), ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, diethyl carbonate, dipropyl carbonate, and mixtures thereof. High permittivity solvents include cyclic carbonates, cyclic esters and cyclic amides such as propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate, acetonitrile, dimethyl sulfoxide, dimethyl, formamide, dimethyl acetamide, γ-valerolactone, γ-butyrolactone (GBL), N-methyl-pyrrolidinone (NMP), and mixtures thereof. The preferred electrolyte for a lithium primary cell is 0.8M to 1.5M LiAsF 6  or LiPF 6  dissolved in a 50:50 mixture, by volume, of PC as the preferred high permittivity solvent and DME as the preferred low viscosity solvent. 
     By way of example, in an illustrative electrochemical cell, the active material of the cathode is silver vanadium oxide as described in U.S. Pat. Nos. 4,310,609 and 4,391,729 to Liang et al., or copper silver vanadium oxide as described in U.S. Pat. Nos. 5,472,810 and 5,516,340 to Takeuchi et al., all assigned to the assignee of the present invention, the disclosures of which are hereby incorporated by reference. 
     In the exemplary secondary electrochemical cell  100 , in addition to lithium, the anode  122  can comprise a material capable of intercalating and de-intercalating an alkali metal, and preferably lithium. A carbonaceous anode comprising any of the various forms of carbon (e.g., coke, graphite, acetylene black, carbon black, glassy carbon, etc.), which are capable of reversibly retaining the lithium species, is preferred. Graphite is particularly preferred due to its relatively high lithium-retention capacity. Regardless of the form of the carbon, fibers of the carbonaceous material are particularly advantageous because they have excellent mechanical properties that permit them to be fabricated into rigid electrodes capable of withstanding degradation during repeated charge/discharge cycling. 
     The cathode  118  of the exemplary secondary electrochemical cell  100  preferably comprises a lithiated material that is stable in air and readily handled. Examples of such air-stable lithiated cathode materials include oxides, sulfides, selenides, and tellurides of such metals as vanadium, titanium, chromium, copper, molybdenum, niobium, iron, nickel, cobalt and manganese. The more preferred oxides include LiNiO 2 , LiMn 2 O 4 , LiCoO 2 , LiCo 0.92 Sn 0.08 O 2 , LiCo 1-x  Ni x O 2 , LiFePO 4 , LiNi x Mn y Co 1-x-y O 2 , and LiNi x Co y Al 1-x-y O 2 . 
     For electrochemical cells  10  and  10 A, the lithiated active material is preferably mixed with a conductive additive selected from acetylene black, carbon black, graphite, and powdered metals of nickel, aluminum, titanium and stainless steel. The cathode further comprises a fluoro-resin binder, preferably in a powder form, such as PTFE, PVDF, ETFE, polyamides and polyimides, and mixtures thereof. 
     The respective current collectors are selected from stainless steel, titanium, tantalum, platinum, gold, aluminum, cobalt nickel alloys, highly alloyed ferritic stainless steel containing molybdenum and chromium, and nickel-, chromium- and molybdenum-containing alloys. 
     Suitable secondary electrochemical systems are comprised of nonaqueous electrolytes of an inorganic salt dissolved in a nonaqueous solvent and more preferably an alkali metal salt dissolved in a quaternary mixture of organic carbonate solvents comprising dialkyl (non-cyclic) carbonates selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), and ethyl propyl carbonate (EPC), and mixtures thereof, and at least one cyclic carbonate selected from propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), and vinylene carbonate (VC), and mixtures thereof. Organic carbonates are generally used in the electrolyte solvent system for such battery chemistries because they exhibit high oxidative stability toward cathode materials and good kinetic stability toward anode materials. 
     Whether of a primary or a secondary chemistry, the cells  10  and  10 A are filled with the appropriate electrolyte described hereinabove and hermetically sealed such as by close-welding the fill openings  30  using a laser beam. 
     Now, it is therefore apparent that the present invention relates to various embodiments for miniature electrochemical cells having a total volume of less than 0.5 cc. Moreover, while embodiments of the present invention have been described in detail, such is for the purpose of illustration, not limitation.