GB2197117A - Hermetically sealed nonaqueous cell with positive terminal pin and perchlorate electrolyte - Google Patents

Abstract

An electrochemical cell has a conductive housing that contains an anode and a cathode. The cathode is electrically isolated from the housing and the active anode material is electrically connected to the housing, thus making the housing the negative electrode terminal of the cell. The cell includes an electrically conductive pin that protrudes from the cell through an orifice in the cell housing and is electrically connected to the cathode to make the pin the positive electrode terminal of the cell. An electrically insulating member is disposed between the pin member and the wall of the orifice. The cell electrolyte comprises a metal perchlorate salt dissolved in a nonaqueous liquid. substantially preventing the build up of a conductive, corrosive deposit on the insulating member.

Description

SPECIFICATION Hermetically sealed nonaqueous cell with positive terminal pin and perchlorate electrolyte This invention relates to nonaqueous galvanic cells, and more particularly to a construction for such cells that substantially reduces or eliminates self-discharge of the cell with time.

Galvanic cells are often constructed so that the cell container is used as one of the electrode terminals of the cell. In consequence, it is necessary to dispose an electrical insulator between the cell container and the other cell electrode terminal to prevent the cell from shorting.

In addition, galvanic cells typically are sealed to prevent leakage and consequent loss of electrolyte, and thus, in the cell construction described above, the electrical insulator should be securely bonded in a leakproof manner to both the cell container and the other cell electrode terminal. However, a consequence of such sealing is that certain operating conditions can cause the internal pressure of the cells to increase markedly. In cells utilizing a highly reactive anode material, such as lithium, external sources, such as fire, or internal sources, such as heat generated during charging, can cause the anode to melt and vigorously react with the cathode and/or electrolyte, thereby resulting in a sharp increase in internal cell pressure.In the case of other galvanic cells, such as alkaline-zinc cells, carbon-zinc cells, etc., large quantities of gas are generated under certain conditions of use. Thus, if any of the foregoing cells were permanently sealed, the build-up of internal pressure within the cell could cause the cell container to leak, bulge or even rupture, with the attendant possibility of property and/or bodily damage.

It is therefore necessary to provide a vent for galvanic cells which remains sealed during normal operating conditions, but which opens when the pressure within the cell substantially increases. To meet these objectives, cells have been made with a vent release mechanism.

Referring to the situation where an electrical insulator is disposed between the cell container and a cell electrode terminal, the insulator can act both to isolate the cell container from a cell electrode terminal and as a vent release mechanism. Specifically, the insulator can be made of glass or ceramic material that is sufficiently thin so as to be frangible. The insulator is then disposed within and secured to a vent orifice that is usually located in the cell cover, so as to hermetically seal the vent orifice, and the cell electrode terminal passes through the central region of the insulator, When the pressure within the cell exceeds a predetermined limit, the frangible member fractures to release the excess pressure.

In one type of cell, referred to as a flat cell, a short, cylindrical container holds a wafer-like anode comprising an active anode material, such as lithium, disposed over and separated from a wafer-like cathode comprising an active cathode material, such as manganese dioxide. A ferrous metal, such as stainless steel, is commonly used for the container. This is because stainless steel is generally corrosion resistant, is easily formed or machined into an appropriate container shape and is electrically conductive so that the container itself can form one terminal of the cell.

A container cover disposed over and separated from the anode is hermetically sealed to the cell container. The cathode is disposed to rest on the bottom of the cell container, thereby making the container the positive electrode terminal of the cell. In contrast, the anode is electrically isolated from the container.

So that electrical contact can be established with the anode, a disc-shaped current collector plate is disposed over and placed in physical (and thus electrical) contact with the anode, and a collector insulator is placed between the current collector plate and the cover to maintain the electrical isolation of the anode from the container. A cylindrical pin, typically made of a ferrous material, such as stainless steel, is placed in electrical contact with the current collector plate and is disposed to protrude through an orifice in the cell cover to form the negative electrode terminal of the cell. An annular seal, typically made of glass, is disposed within the orifice between the pin and the cell cover to hermetically seal the cell, This seal will fracture when pressure within the cell substantially increases, thereby relieving the pressure.

Corrosion problems in the foregoing cell construction have arisen in connection with the glass seal. Specifically, in the case of alkali metal anodes, especially lithium, it has been found that during storage a conductive corrosive deposit grows from the negative electrode terminal pin across and into the seal undersurface toward the cell container, which is the positive electrode terminal of the cell. This deposit grows until the glass seal is bridged and the cell is shorted, thereby causing the cell to self-discharge. Moreover, during the course of its growth, the deposit corrodes the glass seal, which gives rise to the possibility of cell leakage.

While the exact nature and cause of the conductive corrosive deposit are not known, it is believed to be a lithium-modified ferrous compound caused by a complex reaction that is at least a function of the cell potential and the material compositions of the anode, those portions of the cell structure that are in electrical contact with the cathode material, and the glass seal.

Efforts to prevent premature failure of the cell and thereby prolong the shelf life of lithium cells have for the most part concentrated on the seal composition and/or effective coatings for the seal.

For example, Sandia Report #83-2314 of September, 1984, "Glass Corrosion in Liquid Lith ium", suggests that certain glass compositions are better able to withstand corrosion by liquid lithium than others. In U.S. Patent No. 4,168,351, corrosion of a seal is retarded by coating the entire glass surface exposed to the interior of the cell with a protective material such as a metal oxide, polyolefin or fluorocarbon polymer. In U.S. Patent No. 4,233,372, an inert polymeric coating is applied over the glass surface exposed to the cell environment to reduce chemical attack on the glass, and in European Patent No. 35,074, the exposed glass surface is protected by a silicone layer. A still further solution to the problem of glass corrosion is proposed by U.S.

Patent No. 4,308,323, wherein the resistance of the glass to chemical attack is improved by a graded seal composed of one glass composition bonded to the terminal pin and another glass composition bonded to the wall of the container.

An alternative approach to solving the corrosion problem is to be found in U.S. Patent No.

4,609,598. In that patent, all metal components of the cell electronically connected to the cathode are made of a non-ferrous metal, such as molybdenum. This construction decreases the deposition of conductive corrosive material on the glass seal and, thus, the resulting seal corrosion.

It has now been surprisingly found that, when the electrolyte is a nonaqueous solution of a metal perchlorate salt, build up of the conductive, corrosive material is substantially prevented, greatly reducing or eliminating cell self-discharge and seal corrosion.

Thus, in a first aspect of the present invention, there is provided an electrochemical cell comprising an electrically conductive cell housing containing an anode, a cathode and an electrolyte, the anode being electrically connected to the housing to make the housing the negative terminal of the cell, the cathode being electrically isolated from the housing and connected to an electrically conductive pin to make the pin the positive terminal, the pin protruding from the cell through an orifice in the housing from which it is electrically insulated, and the electrolyte comprising a metal perchlorate salt dissolved in a nonaqueous liquid.

The creation and growth of the conductive corrosive deposit is dramatically reduced as, at the onset of the corrosion reaction, the perchlorate salt causes the formation of a thin passivation layer on the ferrous metal components in contact with the cathode material, preventing their further corrosion and thus arresting the growth of the lithium-modified conductive ferrous deposit on the undersurface of the insulative seal.

Examples of perchlorate salts which may be used in this invention include alkali and alkaline earth metal perchlorates, such as lithium perchlorate. The nonaqueous liquid solvent can be an organic solvent, such as a mixture of propylene carbonate and dimethoxyethane, preferably a mixture of equal parts by volume of propylene carbonate and dimethoxyethane. Provided that the perchlorate salt is the predominant solute, small amounts of other solutes may be included in the electrolyte.

Any suitable solvent may be used to dissolve the perchlorate salt. Suitable examples include; propylene carbonate, dimethoxyethane, dtoxolane, 3-methyl-2-oxazolidone, 3,5-dimethylisoxazole, as well as mixtures thereof.

In a specific embodiment, the electrolyte preferably comprises lithium perchlorate dissolved in equal parts of propylene carbonate and dimethoxyethane.

Since the passivation layer has been found to be chromium-rich, it is preferred that the ferrous metal components in electrical contact with the cathode material should have a high chromium content.

While this invention can be utilized to prevent the growth of corrosive conductive deposits in any cell having an insulating seal, it is believed to be especially useful in connection with the higher voltage lithium cell systems.

The invention is further illustrated with reference to the accompanying drawings, in which: Figure 1 is an elevation, partly broken away and in section, of a flat electrochemical cell embodying the present invention; and Figures 2 and 3 are graphs showing the variation in open circuit voltage of each of a number of cells with time.

Referring to Figure 1, there is shown a cross-sectional view of a cylindrical cell 10 that employs the present invention, although other geometric shapes are equally suitable. The housing of the cell is defined principally by an open-ended cell container 12 made of a conductive material, such as stainless steel, and a cell cover 14 also made of a conductive material, such as stainless steel. The cover 14 is secured to cell container 12, as, for example, by laser welding the two components together. The cell 10 contains anode 26 and cathode 30.

Anode 26 is generally a consumable metal and can be an alkali metal, an alkaline earth metal, or an alloy of one or more alkali metals and/or one or more alkaline earth metals with each other and/or with other metals ("alloy" as used herein includes mixtures, solid solutions such as lithium-magnesium, and intermetallic compounds such as lithium monoaluminide), The preferred materials for anode 26 are the alkali metals, particularly lithium, sodium or potassium, and the alkaline earth metals, particularly calcium or magnesium, Lithium is especially preferred.

In the embodiment shown in Fig. 1, the anode 26 is in the form of a relatively thin wafer of a suitable material such as lithium having one of its flat surfaces disposed against the inner surface of the container bottom wall 16. This electrically connects the anode 26 to the container 12, thereby rendering container 12 the negative electrode terminal of the cell.

Also disposed within the container is a cathode assembly 28 that includes a cathode 30 and a current collector plate 32. The active cathode material of cathode 30 is a solid, such as manganese dioxide, iron disulphide, titanium disulphide, antimony trisulphide, molybdenum disulphide, molybdenum trisulphide, niobium triselenide, bismuth oxide, vanadium pentoxide, or a polycarbon fluoride such as (C2F)n or (CFX,n (where x ranges from greater than 0.0 to about 1.2 and n is the degree of polymerisation), or a mixture of anY two or more thereof. The active cathode material is mixed with a binder and a conductor to form cathode 30.

Cathode 30 is separated from anode 26 by a separator 40. Separator 40 should be electrically non-conductive, but ionically permeable so as to allow ion transport between anode 26 and cathode 30. Accordingly, separator 40 can be a felted glass fibre fabric that is impregnated with a liquid having a composition that is described in greater detail below.

Disc-shaped current collector plate 32 is made of a ferrous or non-ferrous metal and is positioned against, and in electrical contact with, cathode 30. In the present invention, ferrous metals are preferred for collector plate 32 over non-ferrous metals for cost-efficiency. Current collector plate 32 is placed in intimate contact with cathode face 34 of cathode 30, Preferably, the surface of current collector plate 32 that is in contact with cathode face 34 is pre-coated with a conductive carbon coating.

Both cathode 30 and current collector plate 32 are electrically insulated from cell container 12 and cell cover 14 by insulator 42, which comprises a disc-shaped insulating section 43 disposed between collector plate 32 and cell cover 14, and an insulative skirt 44 depending from the edge of insulating section 43 so as to circumscribe the periphery of each of collector plate 32, cathode 30, and separator 40, as well as a portion of the periphery of anode 26. Insulator 42 should be made of a material that is compatible with the cell components, such as polypropylene or Tefzel (trade mark of E.I, du Pont de Nemours & Co., Wilmington, Delaware).

Cell cover 14 contains an orifice 50, which may be defined by an upturned circular flange 20.

An electrically conductive generally cylindrical pin member 22, which has a first portion 51, protrudes through the orifice 50. The current collector plate 32, in turn, is connected to a second portion 36 of pin member 22 by any suitable mechanical and electrical connection which electronically connects the collector plate 32 and the pin member 22, thereby rendering the pin member 22 the positive electrode terminal of the cell, Pin member 22 can be releasably secured to collector plate 32 by press-fitting pin member 22 into orifice 33 of collector 32, thus improving the venting of pressure generated in the cell during abnormal operating conditions (more fully described in British Patent Application No. 8716859). As with collector plate 32, for cost-efficiency reasons, it is preferred to make pin member 22 of a ferrous metal, as opposed to a non-ferrous metal.

An annular seal member 24 is disposed in orifice 50 between circular flange 20 and pin member 22. Seal member 24 is bonded (in the case of a glass seal, fused, and in the case of a ceramic seal, brazed) to both circular flange 20 and pin member 22 to hermetically seal the cell and secure pin member 22 in its proper location. Seal member 24 is made of an insulative material that will electrically isolate pin member 22 from cell cover 14, and preferably is made sufficiently thin to be frangible. Thus, when the pressure within the cell reaches a pre-defined level, the material will fracture to form a path, or contribute to forming a path, from inside the cell to the atmosphere for the release of excess pressure. Seal member 24 is preferably made of a glass, such as borosilicate glass, or a ceramic material, such as alumina.

Cathode face 34 of cathode 30 has a portion of its surface defining a recess 38. The second portion 36 of pin member 22 extends through an opening 33 in the cathode collector plate 32 and is received in the recess. The recess 38 is large enough so that the bottom 39 of the recess is spaced from the second portion 36 of pin member 22 to provide clearance around the end portion of the pin member 22. This clearance ensures that seal member 24 is not subjected to stress when the cell is assembled. In this respect, during the course of fabrication, cover 14 together with pin member 22 and seal member 24 are preassembled and then attached as a unit to container 12. Should the second portion 36 of the pin member 22 press against the cathode 30 during such assembly, the resulting axial force on the pin member 22 could cause damage to, or even failure of, seal member 24.The clearance about pin member 22, as provided by the recess 38, ensures that such contact does not occur.

The internal arrangement of cell 10 shown in Figure 1 results in pin member 22 being the positive electrode terminal of cell 10, with anode 26 being positioned away from seal member 24 and in electrical contact with cell container 12. Any corrosive deposit that does occur starts at the outer periphery of the undersurface of seal member 24, rather than at the junction of pin member 22 and seal member 24. In addition, the deposit grows inwardly at a slower rate and in a physical form less conducive to shorting across seal member 24, than is the case when pin member 22 is the negative electrode terminal of cell 10.

Separator 40 is impregnated with a nonaqueous solution of a metal perchlorate salt.

In the embodiment shown in Figure 1, it is preferred that pin member 22 should be made of a 400 series stainless steel, specifically and preferably 446 stainless steel.

To fabricate the cell shown in Figure 1, seal member 24 is positioned within orifice 50 of cell cover 14, and pin member 22 is positioned within seal member 24. This assembly is then heated to seal pin member 22 and the periphery of orifice 50 to seal member 24. Next, the assembly is inverted, a preformed insulator 42 is placed over pin member 22, and current collector plate 32 having precoated surface 34 is placed over pin member 22 and on top of preformed insulator 42, such that the edges of aperture 33 of the current collector plate 32 are in contact with pin member 22. Next, cathode 30 is disposed over and contacts surface 34 of current collector plate 32, in the manner shown in Figure 1. A preformed separator 40 is then placed onto cathode 30 and electrolyte is dispensed onto the separator 40.Anode 26 is secured, as by ultrasonic or cold welding, to the inner surface of the container bottom wall 16, to form an anode assembly. This anode assembly is inverted and placed over the abovedescribed cathode assembly to form cell 10. The cell 10 is then turned right-side up, cover 14 is properly seated within container 12, and the cell 10 is welded shut.

Figure 2 compares the average open circuit voltages during elevated temperature storage (85 C) of four cell lots (five cells per cell lot) fabricated as shown in Figure 1 but using different electrolytes. The curves shown in Figure 2, labeled A, B, C and D, reflect the performance of cell lots having the following Cell Lot Electrolyte A 1.OM LiC104 dissolved in: 4 50% propylene carbonate; and 50% dimethoxyethane.

B l.OM LCF SO dissolved in: 40% dioxolane: 30% dimethoxyethane; 30% 3-methyl-2-oxazolidone; and 0.2% 3,5-diniethylisoxazole.

C 1.OM LiCF3S03 dissolved in: 50% propylene carbonate; and 50% dimethoxyethane.

D 50% electrolyte of Cell C and 50% electrolyte of Cell A.

The above percentages are by volume.

Otherwise, each of the cells was identically constructed with a lithium anode, a manganese dioxide cathode, a pin member 22 made of 446 stainless steel, and a seal member 24 consisting of a type 364U uncoloured alkali silicate glass member (available from Glass Beads Company, 580 Monastery Drive, Latrobe, Pennsylvania 15650).

As can be seen in Figure 2, Cell Lot A showed an essentially constant open-circuit voltage for the 18 week test period and exhibited remarkably better performance than Cell Lot C, which differed only in the composition of the electrolyte solute. Cell Lot A's performance was also significantly better than the performance of Cell Lot B (note that Cell Lot B contained the same solute as, but a different solvent than, Cell Lot C). Indeed, the present invention reduces the growth rate of conductive corrosive deposits on the undersurface of seal member 24 to such an extent that a thirty year shelf life at 200C is predicted.

For purposes of comparison, Figure 3 shows the average open circuit voltages during elevated temperature storage (85 C) of four cell lots of the same construction used in the tests shown in Figure 2, except that each was fabricated so that its cell polarity was the reverse of the Figure 2 cells. Thus, the pin member 22 of each cell was electrically connected to the anode, and the cell container 12 of each was electrically connected to the cathode.

As shown in Figure 3, the open circuit voltages of all of the cells decreased substantially with time, relative to their Figure 2 counterparts. Indeed, the open circuit voltage of Cell Lot A, which uses a perchlorate electrolyte in accordance with this invention, for the most part fell more rapidly than did that of Cell Lot D, which had the mixed electrolyte. Thus Figure 3 demonstrates the need for the polarity of the Figure 1 cell along with the disclosed perchlorate electrolyte.

Claims (19)

1. An electrochemical cell comprising an electrically conductive cell housing containing an anode, a cathode and an electrolyte, the anode being electrically connected to the housing to make the housing the negative terminal of the cell, the cathode being electrically isolated from the housing and connected to an electrically conductive pin to make the pin the positive terminal, the pin protruding from the cell through an orifice in the housing from which it is electrically insulated, and the electrolyte comprising a metal perchlorate salt dissolved in a nonaqueous liquid.

2. An electrochemical cell according to Claim 1, in which the metal perchlorate salt is an alkali or alkaline earth metal perchlorate salt.

3. An electrochemical cell according to Claim 1 or 2, in which the perchlorate salt is lithium perchlorate.

4. An electrochemical cell according to any one of Claims 1 to 3, in which the pin is made from a ferrous metal.

5. An electrochemical cell according to Claim 4, in which the pin is made from stainless steel.

6. An electrochemical cell according to Claim 5, in which the pin is made from 446 stainless steel.

7. An electrochemical cell according to any one of the preceding Claims, in which the active anode material is lithium, sodium, potassium, calcium, magnesium, or an alloy of any two or more thereof with each other, or an an alloy of any one or more thereof with another metal or metals.

8. An electrochemical cell according to Claim 7, in which the active anode material is lithium.

9. An electrochemical cell according to any one of the preceding Claims, in which the cathode consists of manganese dioxide, iron disulphide, antimony trisulphide, titanium disulphide, molybdenum disulphide, molybdenum trisulphide, niobium triselenide, bismuth oxide, vanadium pentoxide, polycarbon fluorides, or a mixture of any two or more thereof.

10. An electrochemical cell according to Claim 9, in which the cathode consists of manganese dioxide.

11. An electrochemical cell according to any one of the preceding Claims, in which the nonaqueous liquid is an organic solvent.

12. An electrochemical cell according to Claim 11, in which the organic solvent is a mixture of dimethoxyethane and propylene carbonate.

13. An electrochemical cell according to Claim 12, in which the organic solvent is a mixture of about equal volumes of dimethoxyethane and propylene carbonate.

14. An electrochemical cell according to any one of the preceding Claims, in which the pin is electrically insulated from the housing by a frangible member.

15. An electrochemical cell according to Claim 14, in which the frangible insulating member is made of a glass and/or a ceramic.

16. An electrochemical cell according to any preceding Claim, wherein any ferrous components in contact with the cathode contain chromium.

17. An electrochemical cell according to any preceding Claim, wherein the housing is made from stainless steel.

18. An electrochemical cell comprising an electrically conductive cell housing containing a lithium anode, a manganese dioxide cathode and an electrolyte, the anode being electrically connected to the housing to make the housing the negative terminal of the cell, the cathode being electrically isolated from the housing and connected to a stainless steel pin to make the pin the positive terminal, the pin protruding from the cell through an orifice in the housing from which it is electrically insulated by a frangible glass seal disposed between the pin and the wall of the orifice, and the electrolyte comprising lithium perchlorate dissolved in a mixture of propylene carbonate and dimethoxyethane.

19. An electrochemical cell according to Claim 1, substantially as described herein.