BACKGROUND OF THE INVENTION

This invention relates to an electrochemical cell, a current

collector for the cell and a method for cell construction. In particular,

the invention relates to a hermetically sealed cell having a reactive

cathode and a lithium anode. The electrochemical cell may be

used to power body implantable medical devices such as heart

pacemakers.

In constructing an electrochemical cell for use in implantable

medical devices, a known method of making a cathode pellet is to

compress a mixture of powdered metal oxide, a conductive matrix

such as graphite or carbon black and a binding material such as

polytetrafluroethylene (PTFE). In such a cell, it is essential for uniform

discharge of the cell to maintain good contact between the

cathode current collector and the cathode material. One known

way of providing such contact is to imbed the current collector inside

the cathode powder mixture and then compress the mixture into a

pellet.

A drawback of imbedding the current collector in the cathode

material is that the volume of the cathode typically expands as the

cathode is discharged. This expansion of the cathode can cause

degradation of the contact between the cathode and the current

collector, causing changes in overall cell impedance as the cell is

discharged. During latter stages of discharge, contact degradation

may cause such an increase in impedance that it causes a

significant decrease in cell capacity. Since high reliability in a

surgically implanted medical device is essential, an unexpected

reduction in cell capacity means that the device must be surgically

removed and replaced much earlier than usual.

It should be understood that not all cells with imbedded

current collectors will undergo a dramatic impedance increase that

requires replacement of the cell. However, even without the

problem of early cell replacement, the variability of cell impedance

complicates the use of the cell. Powering critical implantable

medical devices such as pacemakers, neurostimulators and drug

infusing pumps, requires compactness and efficiency of circuit

design. If the cells used are too variable in their output, the device

would need to be larger and less efficient since the circuits must

either increase in complexity to compensate for the output variability

or must include capacitors which can provide additional energy

storage.

Of course, it is known to provide cathodes without imbedded

current collectors. For example, U.S. Patent 3,440,1 10 issued to Arbter

discloses a cathode assembly which includes a support ring into

which a cathode material is pressed. This cathode assembly is then

pressed into intimate contact with the bottom of the cell housing so

that the housing itself can contact the cathode material and act as

a current collector. However, in a high reliability electrochemical cell

for use in critical medical device applications, reliance on this

contact between the cathode and the case can still raise a concern

about undesirable impedance variations.

It is, therefore, an object of the present invention to provide an

electrochemical cell in which a cathode material which is subject to

swelling during discharge has a current collector which will obviate

cell impedance variability.

SUMMARY OF THE INVENTION

We have discovered an electrochemical cell having a metal

housing and an anode and cathode within the metal housing in

which the cathode assembly includes a ring-shaped current

collector into which the cathode material is pressed. The cathode

is formed into a pellet shape with a flat top surface, a flat bottom

surface and a peripheral edge extending between the top and

bottom surfaces. The cathode current collector circumferentially

surrounds the cathode pellet and is in contact with the peripheral

edge of the cathode pellet. Since the cathode is comprised of a

material which expands as the cell is discharged, the expansion of

the cathode material against the confining ring-shaped current

collector will serve as a stable connection between the current

collector and the cathode material. The current collector is then

electrically connected to the metal housing to allow current flow

between the current collector and housing. The housing may

therefore be used as one terminal of the cell.

In one aspect of the invention, the cathode current collector

can be a ring having an open top portion exposing the top surface

of the cathode pellet and an open bottom portion exposing the

bottom surface of the cathode pellet. Since neither the top nor

bottom portions of the cathode pellet are confined by the current

collector, the expansion of the cathode material during discharge

may be distributed more evenly on both sides of the ring. Also,

another advantage for the open ring shape is that it allows the

entire thickness of the cathode assembly to be filled with reactive

cathode material to maximize cathode capacity in the cell.

In yet another aspect of the invention, the cathode current

collector can have a non-circular shape such as a D-shape in

which the ring is reinforced in order to maintain its shape during

cathode discharge. The reinforcement can be, for example, a

flange extending around the ring or placed selectively at portions

of the ring which are susceptible to deformation. Typically, the

flange would be provided at the top portion or the bottom portion

of the current collector ring. Preferably, the flange extends inwardly

around the ring to assist in the retention of the cathode pellet but

still exposing the center portion of the cathode pellet.

In yet another aspect of the invention, the metal housing of the

cell is provided with an insulator material which electrically

separates the cathode material from the metal housing. The ring-

shaped current collector then provides the connection to the

housing. A connector tab can extend outwardly from the cathode

current collector to a location remote from the cathode material

where it can be attached to the metal housing by welding or some

other means for providing electrical contact.

In yet another aspect of the invention, the ring-shaped current

collector allows for construction of the electrochemical cell by a

convenient method. The current collector is formed and placed into a circumferentially fitting die where the cathode material can

be pressed into a self-supporting pellet which is retained within the

cathode current collector. The combined cathode current

collector and cathode material can then be placed as a unit into

the metal housing of the cell and electrically connected to a

portion of the metal housing. Also, a portion of the flange of the

current collector ring can be provided with an outward bend to

provide a tab to secure the current collector to the metal housing

at a portion of the housing that is not insulated. This would also

have the effect of making a stable electrical connection between

the current collector and the housing. This method is particularly

useful when assembling a cell with a pocket-like, D-shaped housing

since the current collector assembly can be readily inserted into the

pocket of the housing and secured to the housing by welding at the

pocket opening.

In yet another aspect of the invention, the cathode current

collector allows for the use of an alternative connection to a

feedthrough pin which would allow the metal housing of the cell to

be used as a negative terminal for the cell (i.e. connecting the

anode to the metal housing and the cathode to a feedthrough pin)

or for the housing to be neutral with respect to the terminals of the

cell (i.e. the anode and cathode each connected to a feedthrough

pin) .

BRIEF DESCRIPTION OF DRAWINGS

The invention will be further described with reference to the

accompanying drawings, in which:

FIG. 1 shows a cross-sectional, elevational view of the cell.

FIG. 2 shows a cross-sectional, side view of the cell along line 2-

2 of FIG. 1.

FIG. 3 shows an elevational view of the non-circular ring current

collector of FIG. 1.

FIG. 4 shows a cross-sectional view, along line 4-4, of the

current collector of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 , a cell 1 according to the invention is shown,

including a metal housing in two parts, a D-shaped cell housing body

10 and a housing cover 15, both comprised of a metal such as

stainless steel or titanium. The housing cover 15 is placed in the open

end of housing body 10 and is hermetically sealed to the housing

body 10 with a laser weld around the entire edge of the housing

cover 15. The housing cover 15 has an opening 20 to allow a

conducting pin 25 to be externalized through a feedthrough with a

glass seal 28 which insulates the pin 25 from the housing cover 15 and

hermetically seals the opening 20. The housing cover 15 also has a fill

port 30 which allows filling of the cell 1 with an electrolyte after the

housing cover 15 is welded to the housing body 10. After the cell 1 is

charged with a liquid electrolyte, a disc 35 is welded into the fill port

30 to provide α hermetic seal. The liquid electrolyte charged into

the cell 1 can include an organic solvent in combination with an

ionizing solute. The organic solvent can be, for example, diethyl

carbonate, dimethylcarbonate, butylene carbonate,

3-methyl-2-oxazolidone, sulfolane, tetrahydrofuran, methyl-substituted

tetrahydrofuran, 1 ,3-dioxolane, propylene carbonate (PC), ethylene

carbonate, gamma -butyrolactone, ethylene glycol sulfite,

dimethylsulfite, dimethyl sulfoxide or mixtures thereof and also, for

example, low viscosity cosolvents such as tetrahydrofuran (THF),

methyl-substituted tetrahydrofuran (Met-THF), dioxolane (DIOX),

dimethoxyethane (DME), dimethyl isoxazole (DMI), diethyl

carbonate(DEC), ethylene glycol sulfite (EGS), dioxane, dimethyl

sulfite (DMS) or the like. The ionizing solute can be a simple or double

salt or mixtures thereof, for example, LiBF ₄ , LiAsFό, LiPFό, LiCI0 ₄ ,

LiN(SOCl2)3, or UC(Sθ2CF3)2, which will produce an ionically

conductive solution when dissolved in one or more solvents.

FIG. 2 shows various elements contained within the housing

body 10. An anode layer 40 of an active metal is pressed onto a thin

anode current collector 45 comprised of a conducting metal such as

stainless steel, nickel or titanium. One end of the pin 25 is attached

by welding to the anode current collector 45. Active metal anode

materials can include, for example, aluminum, the alkali metals,

alkaline earth metals and alloys of alkali metals or alkaline earth

metals with each other and other metals. The preferred anode

materials are lithium, sodium, potassium, calcium and alloys thereof.

A cathode assembly, including a cathode pellet 55 and

cathode current collector 60 is spaced apart from the anode layer

40 by separator 50 which is comprised of a porous or a microporous

material, preferably polypropylene or polyethylene. The

separator 50 completely surrounds and seals the anode 40 and

anode current collector 45. The reactive cathode material is a which

will swell upon discharge. Manganese dioxide is one such material.

Other suitable cathode materials could be used instead of

manganese dioxide, including vanadium oxide (V2O5), silver

vanadium oxide (Ag2V ₄ On) carbon monoflouride, C0O2, Niθ2, and

TiS2. The cathode pellet 55 may include binders and conductivity

enhancers in addition to the reactive cathode material. Binders

which may be typically employed in the cathode of the present

invention are polytetrafluoroethylene, ethylene/propylene

copolymers and the like. Representative of the conductive materials

which may be employed as a conductivity enhancer are graphite,

carbon and the like. In a cathode pellet 55 having manganese

dioxide as the reactive component, binders may comprise between

about 1 and about 10 weight percent, preferably between about 1

and about 5 weight percent, of the cathode mix used to make the

cathode pellet 55 while the conductive material may comprise

between about 1 and about 12 weight percent, preferably between

about 3 and about 10 weight percent, of the cathode mix. The solid

cathode materials used to make the cathode pellet 55 are in finely

divided form so they can be intimately mixed. The cathode mixture

may then be pressed into the cathode current collector 60 such that

a self supporting cathode pellet 55 is formed within the current

collector with peripheral edges of the cathode pellet 55 pressed into

intimate contact with the current collector 60. The intimate contact

of the current collector 60 around the cathode pellet 55 has the

effect of confining the cathode pellet 55 from expanding in diameter

as the cathode pellet 55 expands in volume during cell discharge.

A lining material 65, such as polyethylene, electrically insulates

the anode 40 and cathode pellet 55 from the interior of the housing

body 10. The lining material 65 can be provided in a thin, molded

pocket-shaped item or a porous or microporous material which fits

closely within the housing body 10. The cathode current collector 60

is electrically connected to the housing body 10 at connector tab 75

which is welded to the housing cover 15.

Referring now to FIG. 3, the cathode current collector 60 has a

generally D-shaped outline that is partly semicircular and partly

polygonal. The cathode current collector 60 is comprised of a wall 80

with a rim portion 70 and an flange portion 65. The purpose of the

flange 65 is to provide a reinforcing means which will retain the

shape of the cathode current collector 60 as the cathode pellet

expands. Accordingly, the reinforcing means may also include

internal or external flanges or a thickened portion of the cathode

current collector 60 or other known means for retaining a shape. As

in FIG 3, the current collector 60 preferably has a uniform width, as

measured from the rim portion 70 to the inward flange 65.

The cathode current collector 60, comprising an electrically

conductive material such as stainless steel or titanium, can be

formed by stamping a metal sheet into a cup, cutting excess steel or

titanium sheet around the cup but leaving a rectangular tab on a

straight edge of the cup. An aperture can then be punched out at

the bottom of the cup forming the inward flange 65 portion of the

curent collector 60. The rectangular tab can then be bent perpendicularly to create the electrical connector tab 75 of the

current collector 60.

After the cathode current collector 60 is formed, a collector-

cathode assembly is made. The cathode current collector 60 is

placed in a closely fitting die fixture such that the die maintains the

shape of the cathode current collector 60 as the collector-cathode

assembly is made. A measured amount of cathode mixture

comprising powdered manganese dioxide, an inert binding material

such as PTFE and conductivity enhancer such as graphite or carbon

black is placed into the die inside the current collector. The cathode

mixture is compressed in a press within the cathode current collector

60 to form a self-supporting cathode pellet 55 having opposite, flat

surfaces exposed. It will be appreciated by those skilled in the art

that other active cathode materials which cause the cathode pellet

55 to expand as the cell is discharged could be used in in place of

the manganese dioxide. For example, Mnθ2, CF _X , V2O5, and a silver

vanadium oxide (e.g. Ag2V ₄ On) could be used alone or in

combination as active cathode materials in the present invention.

FIG. 4 provides α cross-sectionαl view of the cathode current

collector 60, including the wall 80 with rim portion 70 and inward

flange portion 65. The connector tab 75 has a flat surface 85 which is

electrically connected to the housing cover 15.

As an alternative embodiment of the invention, the connector

tab 75 on the cathode current collector 60 can be electrically

connected to the housing 10 or the cover 15 via a separate,

intervening conductor. One end of the conductor may be

electrically connected to the housing cover 15. The other end of the

conductor is electrically connected to the connector tab 75 of the

cathode current collector. The electrical connections can be made

by welding.

In yet another embodiment of the invention, a case negative

version of the cell (i.e. with the case at the anode potential) can be

easily made by connecting the anode current collector 45 to the

metal housing cover 15 by a tab similar to the connector tab 75 on

the cathode current collector 60 and the anode curent collector to

the conducting pin 25. Also, a case neutral version of the cell (i.e.

with the case at neither the anode or cathode potential) can be

made by adding a second conductive pin to the feedthrough or

adding a second feedthrough so that each of the current collectors

45, 60 can be connected to a separate conductor pins passing

through the metal housing.

It will be appreciated by those skilled in the art that while the

invention has been described above in connection with particular

embodiments and examples, the invention is not necessarily so

limited and that numerous other embodiments, examples, uses,

modifications and departures from the embodiments, examples and

uses may be made without departing from the inventive concepts.