Multi-Functional Hermetic Seal for Non-Aqueous Electrochemical Cells

1. Field of the Invention

The present invention relates to the field of non-aqueous electrochemical cells which contain highly corrosive chemical components and so require hermetic sealing. More particularly, the invention concerns improvement in the hermetic seals of such cells which reduces corrosion and allows the electrical connector bridging the seal or pin terminal to be either positive or negative.

2. Description of the Prior Art

Non-aqueous electrochemical cells and particularly lithium batteries contain highly reactive components including, for example, such species as lithium, lithium salts, SO.sub.2, SOCl.sub.2, and SO.sub.2 Cl.sub.2. Packaging these materials properly is critical, since corrosion can readily destroy the most efficient battery through by-products and leakage. For that reason, stainless steel has become the preferred packing materials for lithium batteries. Stainless steel, particularly 300 series stainless steels are not attached by the corrosive cell materials.

In addition, one other requirement in these high energy batteries is that the cells be hermetically sealed to prevent outside contamination of the cells and prevent leakage which can result in loss of performance and/or damage to the environment. Accordingly, efforts have been made to hermetically seal the battery about the pin terminal, using stainless steel, as mentioned above, and glass which has been found to be an impervious insulator suitable for sealing the battery and preventing shorting of the cell between the anode and cathode.

One such successful device is found in U.S. Pat. No. 4,358,514. That invention involves an hermetic seal (which also is described in detail with regard to FIG. 1 of this application) includes a stainless steel header sized to hermetically fit the body and a glass annular ring bonded to the header. A terminal pin is bonded to the ring with the pin consisting of a solid central core of a material selected from nickel and nickel alloys and a stainless steel sleeve on the core.

That header device permits a hermetic seal between the header, the terminal pin and the body of the cell which does succeed in achieving a good glass-to-metal seal because the glass has a coefficient of expansion close enough to that of the nickel core over the contemplated temperature range. It has certain drawbacks, however. This is true, particularly with respect to corrosion problems which limit the use of such a terminal to the negative terminal only.

The prior art device is based on the premise that nickel must be used for the pin as it must take up the bulk of the expansion/contraction and the thermal coefficient of expansion of nickel closely follows that of the glass. However, the nickel, if made positive, is susceptible to anodic corrosion in the cell environment particularly with respect to the presence of SO.sub.2, SOCl.sub.2, SO.sub.2 Cl.sub.2 and other highly reactive solvent depolarizers commonly used in such cells. Therefore, it has heretofore been necessary to make the pin terminal of the cell negative so that it is then protected by the more reactive lithium or other alkali metal in the cell which preferentially decomposes.

The present invention solves the dual problem of glass-to-metal hermetic sealing of non-aqueous electrochemical cells and prevention of corrosion or decomposition of the pin terminal. More importantly, this also allows the pin terminal to be the positive terminal.

It has been discovered that a pin terminal of the combination of a center core pin of a material having a thermal coefficient of expansion substantially matching that of the glass annular ring and an outer case of a metal chemically resistant to the species of the electrochemical system arranged so that the case material alone is exposed to the cell contents provides the long-sought breakthrough.

In the preferred embodiment, the assembled device comprises a stainless steel annualar ring header sized to hermetically fit the cell body and a glass central annular ring bonded inside of the annular ring of the header. A composite central pin terminal is bonded inside the glass annular ring. The composite terminal pin has a solid central core of a stainless steel material electrically compatible and readily weldable normally by brazing to a stainless steel cup-shaped case having walls and a bottom which is exposed to the contents of the cell. The pin has a thermal coefficient of expansion close to the glass of the annular ring. The case has a nominal wall thickness of 0.005" and a bottom thickness of 0.020". The pin core is normally 0.230" in diameter. The pin core and the case are normally brazed together to form a good mechanical and electrical connection. The bonding of the pin to the glass ring and the stainless ring to the header are accomplished at a nominal elevated temperature of 1600.degree. F.

In one embodiment the central pin core was made of a high chrome stainless steel such as 446 stainless, the case of a 300 series stainless (316L) and the glass annulus was made of aluminoborate glass. The case and pin core were joined by high temperature braze.

Reference is hereby made to the drawings in which:

FIG. 1 is a schematic view, in section, of a prior art seal of the class described,

FIG. 2 is a schematic top view showing the preferred embodiment of the present invention; and

FIG. 3 is a schematic view, in section, of the device shown in FIG. 2.

The header device of the prior art device of U.S. Pat. No. 4,358,514 is shown generally in FIG. 1. That invention accomplished the forming of an hermetic seal between the header, the terminal pin and the body of an electrical chemical cell. The stainless steel header 11 is sized to hermetically fit the body of a cell, whereby welding or other hermetically bonding can be accomplished in a conventional manner. An annular glass ring 12 is fitted inside and bonded to the stainless steel header 11. A pin terminal which consists of a central nickel core 13, and a stainless steel sleeve 14 is fitted within the annular glass ring 12. High temperature nickel brazing 15 is applied to keep the thin outer shell 14 solidly bonded to the nickel core 13.

The temperatures of fabrication, which may range from at least 1800.degree. F. to in excess of 2000.degree. F. The brazing 15 further prevents circumferential growth of the sleeve 14. The nickel braze 15 also provides a low resistance electrical contact through the seal to the terminal pin. By making the sleeve 14 slightly longer than the nickel core 13, subsequent trimming of the sleeve and welding of the enclosures into the inside diameter of the sleeve can be accomplished. The glass 12 and the nickel core 13 have relatively closely matched coefficients of expansion, particularly when compared to the coefficient of expansion of the stainless steel.

Now, the cell sealing structure contemplated by the present invention, will be described with respect to FIGS. 2 and 3. The conventional stainless steel torus-shaped header is depicted, in part, at 20 (FIG. 3). Of course, this header is sized to hermetically fit the body of the cell, normally also stainless steel, for which it is designed and can be bonded thereto in any compatible conventional manner such as by welding. Concentrically, inside the header is a glass ring 21 which is typically made of an aluminoborate glass such as Fusite #425 glass manufactured by Fusite Corporation of Cincinnati, Ohio. The terminal pin of the invention is located inside the glass ring 21 and is made up of a composite structure including an outer case surrounding and increasing the lower portion of a solid central pin member 23 and fixed thereto as by high temperature brazing at 24 to present a solid unit with regard to the integrity of the opening in the glass ring 21.

The combination including the glass ring 21, case structure 22, and pin 23 must nbt only present a compatible unit with respect to maintaining the physical sealing integrity of the header unit, but it must also stand up to the contents of the cell chemically so that the pin may be made the positive terminal in the electrochemical couple without fear of deterioration. In addition, the pin assembly must provide good electrical conductivity through the member 23 while remaining insulated from the cell case and header by means of the glass ring.

Accordingly, it has been discovered that a case member made from a 300 series or high nickel type stainless steel such as type 316L in combination with a solid internal pin of a high chrome type stainless steel, such as type 446, functions very well when used as the terminal pin assembly within a ring of aluminoborate glass. The nominal pin diameter is about 0.248" and the nominal wall thickness of the case member is 0.005". The case may have a bottom thickness of about 0.020".

Control of the chemical reactivity of pin terminals, in addition to the coefficient of expansion, had not been considered in the prior art. Because with the type 316L or similar stainless steel case, unlike the nickel within the sleeve of the prior art, is impervious to the corrosive species in the internal cell environment, the polarity of the pin terminal is no longer a critical factor as it was in the prior art. This chemical compatibility allows the cell to be used much more efficiently with regard to its surroundings than was heretofore possible.

Many applications for cells of this type require placement of one or more of the cells in a metal container. Until the present invention it had been necessary to insulate each cell body from its surroundings when placed in such a container to prevent shorting because the cell body had to be made the positive terminal. This made construction of items employing these cells much more difficult than it would have been had the cell bodies been negative and the pin terminals positive. Now negative cell bodies can be grounded directly to the container into which they are mounted.

The invention, then, allows all the external cell connections to be made with stainless steel and allows flexibility in choosing pin and case polarities.