GB2249425A - Reserve electrochemical cells having high strength anode substrate - Google Patents

Abstract

In a high energy density, non-aqueous active metal electrochemical cell of the reserve type for extremely high stress applications, anode substrate 23 fabricated from powdered stainless steel utilizing sintered powder metal techniques to form a high strength porous body of the required porosity is provided for supporting the active metal e.g. lithium of the anode. <IMAGE>

Description

2 24 94 715

IMPROVED HIGH STRENGTH ANODE SUBSTRATE BACKGROUND OF THE INVENTION I. Field of the Invention

The present invention is directed generally to the field of high energy density, non-aqueous active metal electrochemical cells and, more particularly, to a high strength anode substrate to support the active metal of the anode for such a cell for use in extremely high stress environment applications.

II. Description of the Related Art

Active metal cells of the class described, typically consist of a light, strongly reducing active anode material, normally an alkali metal such as lithium, an aprotic, nonaqueous solvent depolarizer such as thionyl chloride into which an appropriate quantity of a salt of the anode metal has been dissolved to form a conductive solution, and an oxidizing agent as the cathode active material. In many application,,- the cells are assembled as deferred action or reserve cells in which the cells remain in an inactive state, in which the electrochemical couple is disabled or otherwise remains inoperable until such time as the cell is activated. In these cells, the electrolyte system is typically stored separated from one or both electrodes contained in a readily rupturable container such as a glass ampule. In this state, the cell may be stored over a period of many years and later activated. Activation is normally accomplished by applying an external force __Z_ in a predetermined manner to rupture the glass ampule thereby allowing the electrolyte system to complete the electrochemica continuity between anode and cathode. For many applications, once activation starts, the rapid dispersal of the electrolyte system to complete the circuit in an extremely short time is critical. In a typical configuration, the cells have a casing constituted by a thin-walled, generally cylindrical metal container in the form of a stainless steel can closed by a cover, the rim of the can being hermetically sealed around the periphery of the cover, normally by welding, to seal off the contents within the can. A cylindrically shaped glass ampule containing the electrolyte system is centrally disposed within the can surrounded in close proximity by the active metal anode material which is carried by an anode support member which may be in the form of a stainless steel or nickel wire mesh, or a thin stainless steel member with a number of small holes in it typically formed by an acid etching procedure and formed in a hollow cylindrical shape. A layer of separator material, which may be paper or ceramic, is located outside of the anode support member. It, in turn, is surrounded by a cathode collector member, I normally nickel, which carries the cathode active material, normally carbon or acetylene black. Teflon or other binder may be used to help adee the carbon blzick to the cathode collector member.

1 1 For certain very high stress applications of such cel'L,-. such as their use in fuses or arming devices for high velocity ar-_illery shells, which achieve speeds;>- Mach V, or the like, the use of mesh or acid etched plates as the anode substrate has been found to be unsatisfactory as they lack the necessary strength to make the cell. reliably operable upon application of very high "g" forces. Recent applications will require such cells to be operable after surviving acceleration forces in excess of 30,000 g.

SUMMARY OF THE INVENTION

In accordance with the present invention, problems associated with the lack of physical strength with regard to very high stress applications of non-aqueous reserve cells are solved by the provision of a very high strength, porous anode support which not only accommodates the desired amount of lithium or other active metal, but also contains the proper amount of porosity to accommodate the desired rate of dispersal of the electrolyte system through the anode support upon activation of the cell. The present invention, then, provides a high rate, non-aqueous active metal cell able to withstand extreme acceleration. The cell is particularly characterized by the improved anode substrate which is able to withstand very high ("g") forces.

The improved high strength porous anode support substrate is one preferably formed using powdered metallurgical techniques. The process includes mixing the metal powder, which may be nickel or stainless steel (316 L stainless steel has been employed successfully in the preferred embodiment) with a filler material, which may be a compatible waxy material such as a paraffin wax or lithium stearate. A green article, generally in the f orm. of an elongated, thin"tubular member is f ormed or molded f rom the mixture by well-knbwn hydrostatic pressing techniques. The article is then fired using known methods to sinter the powdered metal for a time and at a temperature suf f icient to achieve the desired strength and average porosity. The sintered article may then be further heat treated if such is necessary to further modify the physical properties to achieve those desired. The sintered article is then cut to length and the desired lithium or other active metal is pressed onto the sintered article.

The anode support of the invention is a thin-walled, e.g., 0.009 inch thick, sintered powdered metal tube having a porosity selected to be commensurate with passing liquid electrolyte systems upon activation within a desired time limit. Relative porosity can be controlled by controlling particle size, time and temperature of firing, and relative amount of filler material. The material of the tube is extremely strong and able to withstand very high "g" forces (e.g., 30,000 g) required of certain applications. The porous anode support of the invention also has sufficient porosity surf ace roughness to readily retain the pressed lithium or the anode active material with relatively little pressing. The pressed lithium active material, which may be in foil form, also is dispersed into the surface of the porous anode support in a manner which does not clog the pores or allow the lithium to penetrate through the support member and contact the glass ampule of the cell.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, wherein like numerals are utilized to designate like parts throughout the same:

Figure I is a cross-sectional view of a cell of the class in which the anode support of the present invention may be used; Figures 2A and 2B depict a prior art anode support device;

Figure 3 is a perspective view of an anode support in accordance with the present invention; and Figure 4 is a schematic block diagram of a process for making the powdered metal porous anode support of the invention.

DETAILED DESCRIPTION

Most of the improvements in the art related to active metal, non-aqueous electrochemical cells of the class described have generally dealt with improvements to the electrolyte systems or in the cathode materials. In these electrochemical couples or known cell embodiments, lithium or other typical low density, strongly reducing metals or alloys of lithium are utilized. However, the lithium or other active metal of the anode together with the substrate or other member supporting the lithium metal, if such is 6 used, have generally not been Ehe sun3ect of such improvements. The present invention deals with an improvement in the anode supporting structure for such a cell which renders it suitable for certain very high stress environment applications, but also provides a viable alternative active metal support.system suitable for many applications.

Figure I illustrates a cross-section of a typical cell of a class for which the present anode support system has been designed. The cell is shown generally at 10 and includes a generally cylindrical stainless steel, cold formed outer shell or can member 11 which itself may be positively charged by virtue of being in intimate contact with one or more cathode leads 12. The cell contains a glass ampule 13 for containing the electrolyte system which is contained between a bottom separator member 14 (with a bottom outside insulator ring 15) and a pair of top ampule support pads 16 and 17. With respect to cell construction, there is further provided an ampule support shim 18 which cooperates with an insulator disk 19, normally polytetrafluoroethylene (PTFE). The upper end of the cell is sealed utilizing a metal terminal plate 20 separated from a central anode terminal 21 by a glass sealing ring 22. The coefficient of expansion of the glass sealing ring is coordinated to be compatible with that of the terminal plate and the anode terminal 21 such that an hermetic seal is maintained over k -- -7 a fairly wide temperature range. The connection of the anode terminal is discussed further below.

The anode support 23 in cylindrical form is located next to and surrounding the glass ampule. The anode support carries a thin layer of lithium metal anode material. Cell output is increased by adjusting the anode area/volume ratio so that a relatively thin layer of large area is available to produce the necessary high level instantaneous energy output. The anode material is separated from the cathode by a cylindrical vertical separator shown at 25.

The vertical separator 25 may be of paper or matted glass fiber or a porous ceramic material but must be capable of allowing permeation by the electrolyte but prevent direct anode/ cathode contact. The cathode system includes a layer of cathode composition 26 and a metal cathode current collector substrate 27.

The material of the cathode mix composition may be, for example, approximately 85% acetylene black (100% compressed) and approximately 15% finely divided PTFE binder which produces a good mix capable of readily adhering to the substrate and providing sufficient path to the cathode current collector substrate which is normally of stainless steel or nickel. A lead 28 is provided from the anode substrate 23 to connect it with the anode pin terminal 21.

Figures 2A and 2B depict a prior method of constructing an anode substrate in which a conductive electrical lead 30 is attached, as by welding, to a stainless steel plate member 31 which q has been subjected to an acid etching procedure which imparts a certain pinhole porosity to the member 31. The strip 31 is then rolled and welded to complete the piece part for assembly into the cell as shown in Figure 2B. In some assemblies processed, the number 31 is shaped and welded in situ. The assembled rolled strip has a nominal wall thickness at 32 of approximately 0.001 inch. The lithium anode active material may be pressed onto the member 31 in a well-known manner; however, the assembled anode, while quite functional electrochemically for some applications, lacks the necessary strength to hold up under applications which impart excessive stress to the system.

Figure 3 depicts generally at 40 an anode substrate shaped in accordance with that of the present invention. The anode substrate of the present invention is rigid and able to withstand high stress and is made utilizing powdered metallurgy technology. The member 40 is formed in its final tubular shape as shown. It may have a nominal thickness, as at 41, of perhaps 0.009 inch. The lithium metal is then pressed on the member 40 and adheres readily to it forming the anode of applicable to a cell as shown in Figure 1.

Figure 4 depicts a preferred method of making the anode support of the present invention. The process begins at 50 where powdered metal, normally nickel or, preferably, 316 L stainless steel, is obtained or ground in powder form to the desired average particle size to produce the strength and porosity required for the - q particular application. Tile powdered metal is mixed %,!itll all amount of filler material 51,,Ijich also aids in determini"19 the Felat-ive density and porosity of the final sintered material. The pore formant may be, for example, an acro wax compound or a paraffin wax. The powdered metal and filler material are blended together to form a mixture as at 52 which, in the desired proportion, is then pressed using well-known hydrostatic pressing and molding techniques into a tubular form of desired thickness at 53. This forns an elongated tubular structure in the green state of the desired thickness but normally of a length which will accommodate many anode substrates. - The- green article is then fired at 54 to a temperature and for a time such that the powdered metal actually sinters to the degree desired. For 316 L stainless steel, for example, this may be in the range of 2200 to 2400F for approximately 30 minutes. The organic wax compound material, of course, is vaporized, leaving void spaces'. After cooling, if the process requires additional heat treating steps, which may include additional hardening or annealing steps, these may be accomplished at 55.

The sintered article at that point is of the proper porosity, hardness and strength for its application and may be cut to size to fit the particulaV electrochemical cell as at 56. The layer of lithium metal may then be pressed on using standard dry room techniques in a well-known manner as illustrated by the block 57.

io In accordance with the present invention, the anode is then ready for assembly into the electrochemical cell as illustrated in Figure 1. The anodes built in accordance with the present invention have shown excellent properties of survival and use under extremely high stress applications.

The invention has been described herein in considerable detail in order to comply with the Patent Statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct. and use such specialized components as are required. However, it to be understood that the invention can be carried out using specifically different materials, equipment and devices and that various modifications can be accomplished without departing from the scope of the invention itself.

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Claims (9)

1. CLAIMS 1. A non-aqueous active metal electrochemical cell of the reserve

   type for high stress applications comprising: a substantially cylindrical, rupturable glass ampule for containing the non-aqueous electrolyte system; a high strength, stress resistant thin-walled, substantially cylindrical porous sintered stainless steel anode support member substantially surrounding the side wall of the ampule and carrying active metal lithium anode material pressed into the porous structure thereof; a substantially cylindrical porous separator member in the shape of juxtaposed and surrounding the anode support member; and a cathode system surrounding the separator member comprising a cathode active material and a cathode current collector member.
2. 2. An electrochemical cell according to claim 1 wherein the active metal is selected from the group consisting of lithium or lithium alloys.
3. 3. An electrochemical cell according to claim 1 wherei'.r. the sintered metal is 316 L stainless steel and the actLve metal anode material is lithium or lithium alloy.
4. 4. An electrochemical cell according to any one of the preceding claims wherein the anode support member is sufficiently pervious to said electrolyte to allow rapid dissemination of the electrolyte and, thus, rapid activation of the cell upon rupture of the ampule.

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5. 5. A non-aqueous active metal electrochemical cell constructed, arranged and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.
6. 6. A high strength anode structure suitable for use in an electrochemical cell according to claim 1 having a relatively large surface area for use in a high rate, nonaqueous active metal electrochemical cell comprising a shaped, sintered porous metal member and an amount of the active anode metal material pressed into the surface of the shaped, sintered porous metal member.
7. 7. The high strength anode structure of claim 6 wherein the active metal is lithium or a lithium alloy.
8. 8. The high strength anode structure of claim 6 wherein the sintered metal is stainless steel and the active metal anode material is lithium.
9. 9. The high strength anode structure of claim 8 wherein the sintered material is type 316 L stainless steel.