Patent Publication Number: US-2003228520-A1

Title: Process for manufacturing thermal battery with thin fiber separator

Description:
RELATED APPLICATIONS  
     [0001] This application, pursuant to 37 C.F.R. 1.78(c), claims priority based on provisional application serial No. 60/386,859 filed on Jun. 6, 2002. 
    
    
     [0002] This invention was developed with Navy SBIR funding under Contract no. Contract N00167-99-C-0071. The U.S. government may have certain rights in the invention. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0003] The present invention relates to a unique material and method of making same which is useful as, among other things, a thin separator in thermal batteries. This thin separator material facilitates a continuous process to produce dense (50 vol % loading) electrode layers onto the separator layer a without high-tonnage hydraulic press.  
       [0004] The origin of articles of thin layers of ceramic fiber for separations at high temperature (greater than 300° C.) and extreme chemical activity (eg. Lithium and sulfur) are the production of BN fiber (Economy, U.S. Pat. No. 3,668,059) and formation of thin BN fiber mats (Hamilton, U.S. Pat. No. 4,284,610 and Maczuga, U.S. Pat. No. 4,354,986).  
       [0005] This material exhibited structural stability (compressive strength), small intertestices as particle barrier and could hold a large volume fraction (65-85%) of a working fluid, such as molten halide electrolyte. Because of its unacceptable high production cost (difficult high temperature fabrication), and difficulty in initiating wetting with molten halide, it was later abandoned for high-surface-area MgO powder. The MgO is a relatively cheap material, had chemical stability and could immobilize 65-85 Vol % of electrolyte. The drawback of MgO is limited structural stability and therefore limits on thinness. The MgO powder separator is a paste at the thermal cell operating temperature.  
       [0006] Attempts to make MgO fiber have resulted in a highly frangible product which is reduced to particles under compressive load application. Frankle U.S. Pat. No. 4,104,395 taught the impregnation of organic fibers with percursors to form mineral fibers after high temperature (1400° C.) processing. Smith, etal. U.S. Pat. No. 4,992,341 teaches production of fiber-like sheet of MgO. Threads of MgO powder in a combustible binder are layered into a sheet. Then, the “green” sheet is sintered to form a structure.  
       [0007] Due its frangible, formation of an MgO structure capable of compressive strength is limited to about 50% open volume fraction for a working fluid such as a molten halide electrolyte. This is taught in Briscoe etal. U.S. Pat. No. 5,714,283 in which an MgO structure is developed in conjunction with microporous sintered metal screen supports and the MgO sinter has in the range of 20-50% volume for electrolyte for a thickness of 3-25 mil. The low free volume imposes performance limitations, especially if there is structural disintegration.  
       [0008] Thermal batteries are the reserve power “of choice” on board many weapon and defense systems. Their outstanding quality is very long shelf life, about 25 years. They are essentially in a frozen state. Thermal batteries are activated using heat sources and within milliseconds they can produce very high pulse power. The power serves for guidance, communication, and arming of these systems. Accordingly, as a part of these systems, thermal batteries play a critical role in our national defense.  
       [0009] In thermal battery technology, the trend is toward higher power density. The design approach is for thinner cells. The battery itself is produced from pressed powder wafers: heat pellet, Li-alloy negative, MgO separator, and FeS 2  positive. Each wafer is about 1 mm thick. Battery performance would benefit from thinner MgO separators which physically and electrically separate anode from cathode. The separator is the ionic coupling between anode and cathode and should have high electrolyte content with connected porosity for high performance. Added features of dimensional stability and flexibility are desired separator characteristics. In one application, these wafer thin components limit battery pulse power to about 5.5 kW. A thinner MgO wafer would boost the proportion of active materials in the battery. The MgO powder wafer has limited handling strength. Thinner MgO may crack or break to destroy the entire batteries integrity. Larger diameter pieces exacerbate the handling problems. Additionally, the durability of the MgO powder wafer in operation requires a substantial thickness. Volumetric changes of the active material tend to distort the electrolyte/separator interface, which leads to cell shorting.  
       [0010] In one aspect, this invention relates to the design and manufacture of thermal batteries using a thin Ceramic Fiber Separator (CFS) as a substitute to the conventional pressed MgO powder separator can. The thinner CFS results in higher power (higher voltage and current) thermal batteries. A greater portion of the battery height is utilized to increase the number of cells (or voltage) in a battery. Less battery volume and thermal mass is inactive (as in the separator) such that significantly higher battery energy is developed. Pulse power may increase by 50%. Higher power density is a dominant theme in thermal battery development. In one application, the CFS may increase battery pulse power from 5.5 kW to about 8 kW.  
       [0011] The prevailing construction and chemistry of a state-of-the-art thermal battery has been around for about 25 years. It uses Li-alloy/metal sulfide electrodes and lithium halide salt. The salt becomes molten electrolyte upon thermal activation. The battery is composed of a stack of wafers of pelletized powders. Wafer fabrication and battery assembly involve substantial hand labor. The pressing operation has received some automation, but the battery assembly relies on hand stacking of components.  
       [0012] Current thermal battery manufacturing employs uniaxial powder pressing technology to form active cell components. Uniaxial powder pressing is limited in thickness, diameter, and overall geometry (parts are typically cylindrical). Thickness of uniaxially pressed parts range from approximately 1-10 mm. Production of thinner or thicker parts is notably difficult, commonly resulting in low yields and therefore, higher labor costs. Thinner parts require precise, even die loading while thicker parts require the use of organic binders to distribute the applied pressure evenly. Similarly, large diameter parts are difficult to uniaxially press due to increasingly larger processing equipment required to provide the necessary mechanical loads effectively to form the powders (typically &gt;10,000 psi). These limitations preclude many advanced battery designs.  
       [0013] For electrode pellet manufacture, the high tonnage press is typically required to achieve 50 volume % active material loading having a portion of electrolyte salt present to form cold-pressed pellets. The metal sulfide electrode material, FeS 2  (also used as brake lining material), is a very hard material and does not compact on its own. For example, the pressed electrode uses FeS 2  coated with electrolyte salt to facilitate the powder compaction. The resultant cold-pressed pellet would have 50 volume % FeS 2 , 30 volume % electrolyte salt and 20 volume % void. An unpressed powder layer would typically have 50 volume % void. To achieve the desired 50 volume % active material loading, the high tonnage press must displace 30 volume % void that is needed for the electrolyte salt. This is crucial, in that unpressed electrodes with 20-30 volume % loading have poor performance—low energy density and low power.  
       [0014] Separator material in previous development of molten salt battery was MgO high-surface area powder, such a Maglite S or Maglite D (Calgon), and more recently Marinco OL (Marine Magnesium Company). These materials mixed with electrolyte salt have been the materials of choice for pressed powder separators. Alternative materials, Boron long-life rechargeable molten salt battery. Only the pressed-powder MgO/salt separator has found commercial application. Both alternatives have proven too costly by comparison and both have physical properties that have hindered design, production, or operation of thermal batteries.  
       [0015] This invention when used as a CFS for thermal battery separators has unique chemical stability to Li activity and wetability to molten halides. In addition, the CFS has excellent handling characteristics (flexure strength) and durability in molten salt. As a ceramic film compared to an MgO particle bed, the thin CFS withstands distortion from the volume changes in cell capacity discharge. Additionally, CFS provides the foundation for a new, more-economic manufacture of thermal batteries. The structural stability of CFS in molten salt permits electrode to be applied to itself in a continuous process. Further, the ability to manipulate large diameter, thin CFS enables new designs for a high power thermal battery. Experiments have shown the ability of CFS to increase battery power and energy density.  
       SUMMARY OF THE INVENTION  
       [0016] The principal object of the present invention is to provide a thin and flexible fibrous ceramic article useful as a filter or separator in a molten salt thermal battery.  
       [0017] Another object of the present invention is to provide a separator for a thermal battery, comprising a porous film of electrically non-conductive ceramic fibers and electrically non-conductive ceramic binder, the ceramic fibers being present in the range of from about 70% to about 90% by weight, the ceramic binder being present in the range of from about 10% to about 30% by weight, the film having a porosity of not less than about 50% by volume.  
       [0018] Yet another object of the present invention is to provide a separator and electrolyte combination for a thermal battery, comprising a porous film of electrically non-conductive ceramic fibers and electrically non-conductive ceramic binder, the ceramic fibers being present in the range of from about 70% to about 90% by weight, the ceramic binder being present in the range of from about 10% to about 30% by weight, and an alkali metal halide electrolyte present in the porous film up to about 95% by volume of the combination.  
       [0019] Yet another object of the present invention is to provide a cell comprising a lithium-containing anode and a powder cathode separated by a thin film less than about 12 mils in thickness of electrically non-conductive ceramic fibers and electrically non-conductive ceramic binder, the ceramic fibers being present in the range of from about 70% to about 90% by weight, the ceramic binder being present in the range of from about 10% to about 30% by weight, and an alkali metal halide electrolyte present in the thin film up to about 95% by volume.  
       [0020] Yet another object of the present invention is to provide a method of making a ceramic combination, comprising providing a suspension of ceramic fibers, filtering the suspension of ceramic fibers leaving a mat of ceramic fibers, introducing a ceramic binder or precursor thereof into the mat of ceramic fibers, drying the ceramic fibers and ceramic binder or precursor thereof, and heating at a temperature and for a time sufficient to convert the precursor of the ceramic binder if present to the binder to provide a ceramic of fibers and binder having a porosity not less than about 50% by volume.  
       [0021] Yet another object of the present invention is to provide a more economical manufacture of thermal batteries. Integration of ceramic fiber separator with a electrode particle bed is accomplished by molten salt infiltration. The resulting component containing a dense electrode (50 vol % active material) has improved handling for thin-cell battery production.  
       [0022] A final object of the present invention is to provide a continuous manufacturing method for thermal battery cells in which handleable electrode of 40-60% active material is produced without the conventional high tonnage hydraulic press. A thermal process integrates a ceramic fiber separator with a bed of electrode (active material) particles. The ceramic fiber separator, CFS, enables the electrolyte to infiltrate metal-sulfide, particle-bed and retain the initial particle-bed density of 50 volume %. The CFS has structural integrity; so as the electrolyte salt melts, the CFS regulates the flow of the salt over to the electrode powder bed to keep from fluidizing.  
       [0023] Additional advantages, objects and novel feature of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0024]FIG. 1 depicts in cross-section, the process for making a laminate of Ceramic Fiber Separator (CFS) along with a pressed-powder FeS 2 /electrolyte electrode;  
     [0025]FIG. 2 depicts the conveyor belt (a) process for making a laminate cross-section b-f of Ceramic Fiber Separator (CFS) along with a metal-sulfide, particle-bed, which after electrolyte addition becomes an electrode;  
     [0026]FIG. 3 is a graph of the Modulus of Rupture in psi for a Ceramic Fiber Paper/electrolyte and the standard MgO/electrolyte Pressed Pellet;  
     [0027]FIG. 4 is a cell voltage/time plot of LiSi/COS 2  cell having CFS with 10 sec pulses at 0.9A/cm 2 ;  
     [0028]FIG. 5 is a graph illustration comparing the final voltage of 10 second pulses at 0.9A/cm 2  current density for the direct substitution of 10 mil CFS for 10 mil pressed MgO powder separator; and  
     [0029]FIG. 6 is a graph illustrating the low impedance of LiSi/CoS2 cell with 10 mil CFS, based on 10-sec pulses of 0.9A/cm 2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0030] This invention relates to ceramic articles and methods of making same which are electric insulators useful for a variety of purposes. However, for illustration purposes, the invention will be described in connection with a thermal battery. A CFS separator that consists of 10/90 to about 30/70 weight ratio of ceramic fiber/ceramic binder is a preferred embodiment for a thermal battery. Fiber diameter is about 10 microns and fiber length is about 1000 microns. Objective to form a separator for thermal battery consisting of resilient ceramic fiber formed into a film of 3-12 mil thickness that allows at least 85% electrolyte volume to impart high ionic conductivity. A most preferred separator composition is 56% A1 2 O 3  fiber, 19% AlSiO 2  fiber, 25% MgO binder, by weight. An example of the fiber blend is 75/25 weight % A1 2 O 3 /AlSiO 2 . Fiber sources are Altra Al 2 O 3  from Rath (Wilmington, Del.) and Fiberfrax AlSiO 2  from Carborundum. The MgO binder constituent after “firing” has a 30% weight contribution. SiO 2  or ceramic/glasses with higher levels of SiO 2  have been fund to be of insufficient chemical stability for the high Li-activity Li-alloy electrodes that are generally used in thermal batteries. Other fibers may be sulfide ceramics such as CaAl 2 S 4 , YALS 3 , LiAlS 3  or AlN. Our use of a binder that decomposes into MgO further enhances the chemical stability of the separator. Other candidate binders are aluminum nitrate, aluminum acetate and other like organometallic compounds available through from Sigma-Aldrich Co. As a replacement for MgO powder, the film separator with MgO binder possesses durability and flexibility. After the fiber is “dropped” onto a fine polyester mesh for a 3 mg/cm 2  to a given 130 micron thick layer, a series dry/infiltrate steps coats and connects the interstices of the fiber mat with Mg Acetate from Sigma-Aldrich (Milwaukee, Wis.). (Dropping refers to a papermaking process in which the fiber is collected on a fine screen as the aqueous suspension of fiber rushes through the fine screen). The Mg Acetate is applied as a 0.6 g/cc aqueous solution about two and a half times (as explained in the subsequent procedure). Mg carbonate or Mg hydroxide are other soluble magnesium compounds which could be substituted for Mg acetate. Addition of isopropyl alcohol or the like to the aqueous Mg acetate solution helps wet the ceramic fiber and enhance the drying. Drying is done in flowing air at about 75-100° C. After drying about 2 hours, the ceramic fiber paper CFS can be peeled from the fine polyester mesh. (Use of a combustible carrier is a good option to the peeling step.) The CFS is sufficiently rigid for good cutting and use of die punch; yet it can be picked up having sufficient flexure strength that it doesn&#39;t easily crack and break apart. CFS as thin as 100 micron can be handled in sheets as large as 250 mm diameter. The ceramic film has been made using the following procedure:  
     [0031] 1. Fiber blending with water in impeller.  
     [0032] 2. Fiber dispersion into water.  
     [0033] 3. Release trap door on bottom of tank.  
     [0034] 4. Collect fiber mat onto fine mesh (polyester vale).  
     [0035] 5. Drip dry.  
     [0036] 6. Damp mat is infiltrated with aqueous solution containing binder.  
     [0037] 7. Completely dry in flowing hot-air drier.  
     [0038] 8. Repeat binder infiltration.  
     [0039] 9. Repeat drying in hot-air drier.  
     [0040] 10. Peel fiber paper from fine mesh fabric.  
     [0041] 11. Cut to desired size.  
     [0042] 12. Process at 600-650° C. in air for 3-6 hours.  
     [0043] Other methods useful for making the ceramic composition of this invention are as follows:  
     [0044] 1. A vacuum roller pulls fiber from a fluidized bath containing ceramic fiber onto a belt (e.g. fine mesh screen or combustible carrier).  
     [0045] 2. Ceramic fiber in an aqueous suspension is sprayed onto a belt (e.g. fine mesh screen or combustible carrier).  
     [0046] 3. Ceramic fiber in gelatinous medium is slip cast onto a belt (e.g. fine mesh screen or combustible carrier).  
     [0047] 4. Ceramic fiber in an air dispersion is blow onto a belt (e.g. fine mesh screen or combustible screen).  
     [0048] The present invention provides Ceramic Fiber Separator (CFS) at 4-12 mil thick to overcome the present thinnest limits of 25 mil for cell pressed wafers at 10 mm diameter or larger. Because it is also supplied in the form of a flexible sheet, CFS offers cost saving options for thermal battery manufacture. The cost of conventional thin-cell thermal batteries is inflated by the poor handling of the wafer-thin components. A thirty percent parts loss rate is presently typical. Even at the conventional thicknesses, the battery would benefit from applying CFS. The fragility of the wafer pellets has required expensive hand assembly. The durability of CFS (it is bendable and passes a “drop” test) permits automated, faster assembly. Withdrawal of human error from the assembly process, improves quality control, thereby further increasing the profitability for thin-cell thermals. Wetting with molten halide, bending strength, flexibility, small pore-size, low density, tortuosity are properties required for an improved separator of a thin-cell, high-power thermal battery. This combustion enables significant cost reduction and improved quality control for the production of thin-cell thermal batteries.  
     [0049] CFS provides superior thinness and strength to increase battery power and energy density. For larger sizes (&gt;3 in. diameter), handling difficulty prohibited thin-cell battery construction and/or caused significantly increased cost.  
     [0050] Cell Fabrication with CFS.  
     [0051] The structural stability of CFS in the presence of molten salt becomes the basis for an improved method for thermal cell manufacture. In this thermal battery manufacture, CFS serves as a buffer to limit molten electrolyte to metal-sulfide, particle-bed electrode. Too much electrolyte would fluidize the particle bed; thus destroying the packing-density and the physical dimensioning of the wafer-thin electrode. The CFS is used in three variations of cell manufacture which results in the fabrication of a CFS/metal-sulfide electrode laminate. The laminate significantly enhances the handling strength of a separator/electrode component and also gives insurance of components mating and being flat for stacking. The form of the metal-sulfide electrode dictates the fabrication procedure. The conventional metal-sulfide electrode is a pressed powder bed of metal-sulfides and electrolyte salt. The other two approaches do not contain electrolyte salt; they are a simple particle bed contained within a cup, or a typecast layer that is particle bed contained by an expendable binder matrix. The process enables the electrolyte to infiltrate these metal-sulfide, particle-bed matrices and retain the initial particle-bed density of 50 volume %. In prior art, production of metal-sulfide electrode pellet with particle-bed density that approached 50 vol % would require high-tonnage hydraulic presses. The salt component of the pellet was compacted between the metal-sulfide particles by the elimination of void volume. The present invention infiltrates molten salt in a controlled fashion into a particle-bed can achieve the same particle-bed density, 50 vol %, without the high-tonnage hydraulic presses.  
     [0052] The cross-sectional views FIGS.  1 ( a )-( d ) depicts the process for making a laminate of Ceramic Fiber Separator (CFS) along with a pressed-powder FeS 2 /electrolyte electrode. The laminated component 30 enables thin, large-diameter separator and cathode to be assembled into a thermal battery. In FIG. 1 a,  the first step on conveyor through a tunnel furnace is an amount of electrolyte powder  10  is placed onto the conveyor. This is the amount necessary to infiltrate the CFS. It can be dispensed by a shoe (a powder filled hopper) traveling over a cavity (not shown) or a die punched, piece of tapecasted electrolyte powder. The CFS piece  12  is placed onto electrolyte powder  10 . In turn, the pressed powder FeS 2 /electrolyte electrode  16  is placed onto the CFS piece  12 . The stacked components then travel through a tunnel furnace at 550° C. for 2 minutes. As in FIG. 1 b,  the electrolyte powder  10  is melted and infiltrated into CFS piece  12  to form CFS/electrolyte salt  20 . The pressed-powder FeS 2 /electrolyte electrode  16  remains on top. The stacked components then travel out the tunnel furnace, where as in FIG. 1 c,  a copper chill block  42  is put in place. After cooling to room temperature in 2 minutes, FIG. 1 d,  the laminated CFS/cathode  30  emerges. The frozen electrolyte salt unitizes the layers for superior handling. The laminated CFS/cathode  30  (along with anode, heat pellet and current-collector sheet, (now shown) is immediately available for assembly of a thermal battery.  
     [0053] The cross-sectional view of FIGS.  2 ( a )-( f ) depict the process for making a laminate of ceramic fiber separator (CFS) along with a FeS 2  powder bed (in essence eliminating the hydraulic pressing step). The laminated component  30  enables thin, large diameter separator and cathode to be assembled into a thermal battery. As in FIG. 2 a,  the process uses a conveyor belt  50  that consists of plates with shallow cups. In FIG. 2 b,  the first step on conveyor through a tunnel furnace is an amount of metal sulfide powder  8  is placed onto the conveyor. It can be dispensed by a shoe (a powder filled hopper) traveling over a cavity (not sown) or a die punched, piece of tapecasted electrode powder. In FIG. 2 c,  the CFS piece  12  is placed onto electrode powder  8 . In turn, FIG. 2 d,  the electrolyte powder  10  is placed onto the CFS piece  12 . This is the amount of electrolyte powder  10  necessary to infiltrate both the CFS  12  and the FeS 2  powder bed  8 . Again, the electrolyte powder can be dispensed by a shoe (a powder filled hopper) traveling over a cavity (not shown) or a die punched, piece of tapecasted electrolyte powder. The stacked components then travel through the tunnel furnace at 550° C., 2 minutes. As in FIG. 2 e,  the electrolyte powder  10  is melted and infiltrated into CFS to form piece  20  and the FeS 2  cathode  16  with molten electrolyte salt. The stacked components then travel through the tunnel furnace at 550° C., 2 minutes. The stacked components then travel out the tunnel furnace, where after cooling to room temperature in 2 minutes as in FIG. 2 f,  the laminated CFS/cathode  30  emerges. Laminated part  30  is ejected from the cavity by part-ejector  40  (a push plate) that is at the bottom of each cup on the conveyor. The frozen electrolyte salt along with the high Modulus of Rupture (MOR) of the CFS unitizes the layers for superior handling. The laminated CFS cathode  30  (along with anode, heat pellet and current-collector sheet, not shown) is immediately available for assembly of a thermal battery.  
     [0054] The preferred fiber for the CFS is about 10μ diameter and about 1 mm long. Ceramic fibers are generally manufactured to a nominal fiber diameter of between 3-4μ (micrometers), although a typical range of actual diameters is 0.2-8.0μ.  
     [0055] Below are manufacturers/supplier trade name/form of material.  
     [0056] Carborundum/Fiberfrax/Bulk loose, Felt, and paper, Rope and braid.  
     [0057] Morganite/Triton Kaowool/Bulk loose, Blanket, Felt and paper.  
     [0058] Bells Thermalag/Kaowool/Pyrotek M6 felt.  
     [0059] ICI/Saffil/Saffil bulk A1 2 O 3  fiber.  
     [0060] Manville/Cerafiber/Cerawool.  
     [0061] Rath/Altra/bulk A1 2 O 3  fiber.  
     [0062] Rath/HTZ/bulk AlSiO 2  fiber.  
     EXAMPLES  
     Example 1  
     [0063] Production of a CFS Part.  
     [0064] A CFS composition of 75/25 weight % A1 2 O 3 /AlSiO 2  fiber (e.g. Saffil by ICI/Fiberfrax by Carborundum) is prepared by using a blender to suspend 3.0 g fiber in 0.5 liter water. In a papermaking machine, the fiber is dispersed in 8 liters of water. After the fiber is “dropped” onto a fine polyester mesh for a 6 mg/cm 2  loading to give a 250 micron thick layer of 250 mm diameter, a series dry/infiltrate steps coats and connects the interstices of the fiber mat with Mg acetate. The Mg acetate is applied as a 0.6 g/cc aqueous solution about two and a half times (as explained earlier, the first application is done with the fiber mat not totally dry). Addition of 5 vol % isopropyl alcohol to the aqueous Mg acetate solution helps wet the ceramic fiber and enhance the drying. Drying is done in flowing air at about 75-100° C. After drying about 2 hours, the ceramic fiber paper CFS can be peeled from the fine polyester mesh. The pieces of CFS are cut to desired size using Exacto knife and precision form, such as 2.05″ diameter. The CFS piece is processed at 600-650° C. in air for 3-6 hours to convert the Mg acetate is MgO. Electrolyte is infiltrated into the CFS by placing a weighed amount of electrolyte powder onto the CFS, placing it onto 500° C. hot plate, just long enough to melt the electrolyte. The electrolyte-infiltrated piece is placed onto chill-block (e.g. a Mo plate) and cooled under a weight. This separator piece is then stacked between pressed-pellets of LiSi/electrolyte and CoS 2 /electrolyte to form a test cell.  
     [0065] An important specification related to the handling strength of separators is the Modulus of Rupture (MOR) or bending strength before breaking. As the separator is thinned, it becomes easier to break. A three-point break apparatus is used to evaluate modulus of rupture (MOR). The MOR is determined by incrementally-loading the three-point fixture (usually three rods in parallel) until the specimen snaps. Separator MOR values are determined from a group of repeated tests. The MOR is normalized for varying cross-section.  
     [0066] The CFS has an MOR of 2,000 up to 4,500 compared to only about 100 for the conventional pressed powder MgO/salt pellet, as illustrated in FIG. 3 and this is what is meant by use of the term “flexible” regarding the ceramic articles of the present invention. Since the bending moment increases for a larger diameter separator, the MOR becomes more critical. The MOR for CFS is 20 times greater than that of the MgO powder separator, or it has the same handling strength at 5% the thickness of the MgO separator. It is therefore understandable that the CFS at 50% thickness of the MgO separator thickness has superior handling strength. It is therefore understandable that the CFS at 50% thickness has superior handling strength, and also fulfills the targeted power density of the emerging thermal battery market. Additionally, the ceramic material of this invention when used as separators in a cell have the chemical and physical properties necessary to meet the high current density at high power. These are 85-95% open volume (high electrolyte content) and the chemical stability to provide resistance to Li corrosion. Unlike pressed MgO powder separator, full-size 3.66″ diameter CFS separators pass the “drop test”, and display physical flexibility even after electrolyte filling. The CFS can reduce cell thickness and weight, due to a lowered piece-loss rate. Approximately 15% more cells may be added to a battery by substituting CFS for pressed, MgO powder separator.  
     Example 2  
     [0067] CFS Laminated with Pressed Electrode Pellet.  
     [0068] A CFS composition of 75/25 weight % A1 2 O 3 /AlSiO 2  fiber (e.g. Altra Al 2 O 3  from Rath (Wilmington, Del.)/Z-90 SAZ P-15 AlSiO 2  fiber from K Industrial (Livonia Mich.) is prepared by using a blender to suspend 1.5 g fiber in 0.5 liter water. In a papermaking machine, the fiber is dispersed in 8 liters of water. After the fiber is “dropped” onto a fine polyester mesh for a 6 mg/cm 2  loading to give 250 micron thick layer of 250 mm diameter, a series dry/infiltrate steps coats and connects the interstices of the fiber mat with Mg acetate. The Mg acetate is applied as a 0.6 g/cc aqueous solution about two and a half times (as explained earlier, the first application is done with the fiber mat not totally dry). Addition of 5 vol. % isopropyl alcohol to the aqueous Mg acetate solution helps wet the ceramic fiber and enhance the drying. Drying is done in flowing air at about 75-100° C. After drying about 2 hours, the ceramic fiber paper CFS can be peeled from the fine polyester mesh. The pieces of CFS are cut to desired size using an Exacto knife and precision form, such as 2.05″ diameter. The CFS piece is processed at 600-650° C. in air for 3-6 hours to convert the Mg acetate is MgO for a 70/30 weight ratio of ceramic fiber/MgO binder. This separator piece is then stacked with COS 2 /electrolyte pressed-pellet as in FIG. 1. Electrolyte is infiltrated into the CFS by placing a weighed amount of LiCl—LiBr—KBr electrolyte powder, 1.28 g, onto a 500° C. hot late, just long enough to melt the electrolyte into the CFS. The electrolyte-infiltrated CFS piece becomes laminated to the CoS 2 /electrolyte pressed-pellet and is placed onto chill block (e.g. a Mo plate) and cooled under a weight. The electrolyte/separator weight ratio is 87/13. A LiSi/electrolyte pressed-pellet is stacked with the CFS/CoS 2  electrode pellet laminate to form a test cell.  
     Test Results for Example 2  
     [0069] This film ceramic fiber separator (CFS) is produced at less than about 12 mils and preferably at 4-6 mil thickness. Electrolyte infiltration and handling are improved.  
     [0070] The CFS was tested under state thermal conditions using the LiSi/CoS 2  couple with molten halide electrolyte. LiSi/COS 2  cells having CFS were tested with 10 second pulses using 0.9 Acm 2  current density at 500° C., see FIG. 4. Outstanding performance of greater than 1.6 volts for the pulse voltage for the first 75% of cell capacity showed that CFS could meet or exceed the performance of the pressed MgO powder separator.  
     Example 3  
     [0071] CFS Laminated with Electrode Particle Bed  
     [0072] CFS with an electrode from a powder bed are introduced into a cup (no hydraulic pressing of a pellet) to form a laminate. A CFS composition of 70/30 weight % Al 2 O 3 /AlSiO 2  fiber is formed as in Example 1 with a 75/25 weight ratio of ceramic fiber/MgO binder. As in FIG. 2, the 250 micron thick CFS is positioned onto the cavity that has been filled with FeS 2  particles. The cavity is coated with BN to eliminate sticking to the cup. In this procedure, the LiCl—LiBr—KBr electrolyte addition, 2,3 g, also includes the portion for the electrode infiltration. The arrangement of materials is then passed through a 550° C. tunnel furnace. After the electrolyte melts and infiltrates the two component layers, the electrolyte/separator weight ratio is 87/13 and the electrolyte/FeS 2  cathode weight ratio is 25/75. The electrolyte-infiltrated CFS piece becomes laminated to the FeS 2 /electrolyte pellet. The laminated component possessing superior handling strength is available for battery stack assembly.  
     Example 4  
     [0073] CFS Laminated with Tapecast Electrode Particle Bed  
     [0074] CFS with electrode from tapecast, cathode material powder bed to form a laminate. The tapecast electrode is comprised of particles held together in 2-5 vol % polymer matrix. The polymer matrix is a handling method for the electrode particle bed; it is removed in the thermal processing.  
     [0075] A CFS composition of 80/20 weight % Al 2 O 3 /AlSiO 2  fiber is formed as in Example 2 with a 80/20 weight ratio of ceramic fiber/MgO binder. As in FIG. 2, the 125 micron thick CFS is positioned onto the typecast sheet of cathode, a powder bed of FeS 2 —CuFeS 2  particles (50 vol. % FeS 2 —CuFeS 2  particles). The pieces of CFS laminated with tapecast cathode are cut to desired size using Exacto knife and precision form, such as 2.05″ diameter. The conveyor belt is coated with BN to eliminate sticking. In this procedure, the LiCl—LiBr—KBr electrolyte addition, 2,3 g, also includes the portion for the electrode infiltration. The arrangement of materials is then passed through a 550° C. tunnel furnace. After the electrolyte melts and infiltrates the two component layers, the electrolyte/separator weight ratio is 89/11 and the electrolyte/cathode weight ratio is 22/78. The electrolyte-infiltrated CFS piece becomes laminated to the cathode/electrolyte pellet. The laminated CFS/cathode component (possessing superior handling strength) stacked with LiSi/electrolyte pellet is available for battery stack operation.  
                               Ceramic materials of the present invention       have the following proprties:                                                MOR of Salt-Loaded Parts   &gt;2000 psi           Average bulk density (without electrolytes)   0.3 g/cm 3             Open Volume   90-95%           Thickness   0.003-0.25″           Tensile Strength   &gt;350 g/in           Maximum Use Temperature   1200° C.                      
 
     [0076]               TABLE 1                          12/21 Separator Comparison                         Characteristics   MgO Powder Pellet   Ceramic Fiber Film Separator               Thickness   10-25 mil   5-10 mil       Cost, % of Total   3%   10% of Thin Cell Materials       Size Limitation   3.5 in Diameter   Up to 10 in Diameter               achieved       Handling   Brittle   Flexture       Electrolyte Content   70 vol. %   85 vol. %                    
     [0077] While there has been disclosed what is considered to be the preferred embodiment of the present intention, it is understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.