U.S. patent application number 09/750701 was filed with the patent office on 2001-05-10 for autoclavable electrochemical cell.
Invention is credited to Takeuchi, Esther S..
Application Number | 20010001055 09/750701 |
Document ID | / |
Family ID | 25080791 |
Filed Date | 2001-05-10 |
United States Patent
Application |
20010001055 |
Kind Code |
A1 |
Takeuchi, Esther S. |
May 10, 2001 |
Autoclavable electrochemical cell
Abstract
An autoclavable elctrochemical cell which may be used in an
implantable medical device. The anode active material is lithium or
other material from groups IA and IIA of the Periodic Table and
having a melting point greater than about 150 degrees C. The
cathode active material is silver vanadium oxide or other metal
oxide or carbon monofluoride. The solvent for the electrolyte has a
boiling point greater than about 100 degrees C. and a dielectric
constant greater than about 5 so that the cell may be dimensionally
and chemically stable during repeated exposures of about one hour
each to the autoclaving temperatures.
Inventors: |
Takeuchi, Esther S.;
(Williamsville, NY) |
Correspondence
Address: |
BANNER & WITCOFF
1001 G STREET N W
SUITE 1100
WASHINGTON
DC
20001
US
|
Family ID: |
25080791 |
Appl. No.: |
09/750701 |
Filed: |
January 2, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09750701 |
Jan 2, 2001 |
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09551830 |
Apr 18, 2000 |
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09551830 |
Apr 18, 2000 |
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08403570 |
Mar 14, 1995 |
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6150057 |
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08403570 |
Mar 14, 1995 |
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08273604 |
Jul 12, 1994 |
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08273604 |
Jul 12, 1994 |
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07987584 |
Dec 8, 1992 |
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07987584 |
Dec 8, 1992 |
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07767855 |
Sep 30, 1991 |
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Current U.S.
Class: |
429/330 ;
429/219; 429/335 |
Current CPC
Class: |
H01M 4/5835 20130101;
H01M 4/663 20130101; H01M 4/661 20130101; H01M 4/66 20130101; H01M
4/58 20130101; H01M 4/667 20130101; H01M 2300/0037 20130101; H01M
50/449 20210101; H01M 50/489 20210101; H01M 50/417 20210101; H01M
6/16 20130101; H01M 4/669 20130101; H01M 6/164 20130101; H01M 4/48
20130101; H01M 50/411 20210101 |
Class at
Publication: |
429/330 ;
429/335; 429/219 |
International
Class: |
H01M 006/16; H01M
010/40 |
Claims
What is claimed is:
1. An electrochemical cell comprising: a casing; an anode having as
active material a material which has a melting point greater than
about 150 degrees C. and which is selected from groups IA and IIA
of the Periodic Table; a cathode having as active material a
material which is selected from the group of materials consisting
of metal oxides, metal oxide bronzes, and carbon monofluoride; and
an electrolyte comprising a lithium salt and an organic solvent,
said solvent characterized by having a boiling point greater than
about 100 degrees C. and a dielectric constant greater than about
5.
2. A cell according to claim 1 wherein said anode active material
is lithium.
3. A cell according to claim 1 wherein said cathode active material
is silver vanadium oxide.
4. A cell according to claim 1 wherein said anode active material
is lithium and wherein said cathode active material is silver
vanadium oxide.
5. A cell according to claim 4 wherein said solvent is a mixture of
propylene carbonate and diglyme and wherein said salt is lithium
trifluoromethane sulfonate.
6. A cell according to claim 4 wherein said solvent is a mixture of
propylene carbonate and diglyme.
7. A cell according to claim 6 further comprising a separator means
between said anode and said cathode, said separator means composed
of a material which is porous for passage of said electrolyte
therethrough and which is characterized by being wettable to said
electrolyte and by having a melting point which is greater than
about 130 degrees C.
8. A cell according to claim 7 wherein said separator material is
composed of a laminate of a polypropylene membrane and a
polypropylene mesh.
9. A cell according to claim 7 wherein said salt is lithium
trifluoromethane sulfonate.
10. A cell according to claim 1 wherein said solvent is a mixture
of propylene carbonate and diglyme and wherein said salt is lithium
trifluoromethane sulfonate.
11. A cell according to claim 1 further comprising a separator
means between said anode and said cathode, said separator means
composed of a material which is porous for passage of said
electrolyte therethrough and which is characterized by being
wettable to said electrolyte and by having a melting point which is
greater than about 130 degrees C.
12. A cell according to claim 1 comprising means for maintaining
the cell dimensionally and chemically stable during repeated
exposures each of about one hour to a temperature of about 130 to
135 degrees C.
13. A cell according to claim 1 wherein said casing is hermetically
sealed and is composed of corrosion-resistant material.
14. A cell according to claim 1 wherein said cathode active
material is carbon monoflouride, said cathode includes a current
collector composed of a material selected from the group consisting
of a superferrite material and carbon coated titanium, said salt is
selected from the group consisting of lithium tetrafluoroborate and
lithium trifluoromethane sulfonate, and said solvent is
gammabutyrolactone.
15. An autoclavable electrochemical cell comprising a casing; an
anode having as active material a material which has a melting
point greater than about 150 degrees C. and which is selected from
groups IA and IIA of the Periodic Table; a cathode having as active
material a material which is selected from the group of materials
consisting of metal oxides, metal oxide bronzes, and carbon
monoflouride; and an electrolyte comprising a lithium salt and an
organic solvent, said solvent characterized by having a boiling
point greater than about 100 degrees C. and a dielectric constant
greater than about 5, the cell further characterized by being
dimensionally and chemically stable during repeated exposures each
of about one hour to a temperature of about 130 to 135 degrees
C.
16. A cell according to claim 15 wherein said anode active material
is lithium and wherein said cathode active material is silver
vanadium oxide.
17. A cell according to claim 15 further comprising a separator
means between said anode and said cathode, said separator means
composed of a material which is porous for passage of said
electrolyte therethrough and which is characterized by being
wettable to said electrolyte and by having a melting point which is
greater than about 130 degrees C.
18. An electrochemical cell comprising: a casing; a lithium anode;
a silver vanadium oxide cathode; and an electrolyte comprising
lithium trifluoromethane sulfonate and an organic solvent, said
solvent being a mixture of propylene carbonate and diglyme.
19. A cell according to claim 18 further comprising a separator
means between said anode and said cathode, said separator means
composed of a material which is porous for passage of said
electrolyte therethrough and which is characterized by being
wettable to said electrolyte and by having a melting point which is
greater than about 130 degrees C.
20. A cell according to claim 18 comprising means for maintaining
the cell dimensionally and chemically stable during repeated
exposures each of about one hour to a temperature of about 130 to
135 degrees C.
Description
1. The present invention relates generally to the art of
electrochemical cells and more particularly autoclavable
electrochemical cells or batteries such as may be used, for
example, in implantable medical devices.
2. Numerous power sources have been developed for use in
implantable devices such as implantable drug pumps and pacemakers.
It is important that the medical devices be sterilized prior to
implantation in the body. Medical devices have been sterilized by
treatment with an oxide gas such as ethylene oxide (filling oxide
gas treatment). However, in addition to being considered
environmentally unsafe, ethylene oxide gas is necrotic to tissue.
During sterilization the ethylene oxide gas may become trapped in
spaces within a medical device with the result that its eventual
release, after implantation of the device, may lead to potentially
severe tissue damage in the patient.
3. An alternative to ethylene oxide gas treatment is sterilization
of the medical device in an autoclave. For such sterilization the
implantable medical device, and the electrochemical cell which
serves as its power source, must be capable of withstanding the
repeated prolonged exposures to heat soak and other autoclaving
conditions at the high temperatures on the order of 130 to 135
degrees C. encountered.
4. Batteries for implantable medical devices may include anodes
having as active material lithium or other alkali metal, cathodes
having as active material silver vanadium oxide or other metal
oxide or carbon monoflouride, electrolytes composed of a lithium
salt and an organic solvent, and a separator material between the
electrodes and which is porous for passage of the electrolyte
therethrough for ionic transfer between the electrodes for
generating a current. Examples of such batteries are disclosed in
U.S. Pat. Nos. 4,057,679; 4,618,548; and 4,830,940. While batteries
have been provided which have operating temperatures within the
range of minus 55 to plus 225 degrees C., as discussed in related
U.S. Pat. Nos. 4,310,609 and 4,391,729, which are assigned to the
assignee of the present invention, the ability of a battery to
operate in such a temperature range does not determine whether it
has the ability to withstand the heat soak and other conditions of
autoclaving at temperatures of about 130 to 135 degrees C.
5. Other patents which may be of interest include U.S. Pat. Nos.
4,751,157; 4,751,158; 4,146,685; 4,574,113; 4,615,959; 4,668,594;
4,668,595; and 4,735,875.
6. As discussed by the inventors of the present invention in an
article entitled "Autoclavable Li/Silver Vanadium Oxide Cell",
Progress in Batteries & Solar Cells, Volume 8 (1989), at pages
122-125, a desirable characteristic of some medical cells is the
ability of the cells to withstand repeated high temperature
excursions that occur during sterilization in an autoclave without
loss of deliverable capacity.
7. Such batteries as disclosed in the aforesaid patents are
deficient for purposes of autoclaving since their compositions are
such that one or more of their components may render the cell
dimensionally and/or chemically unstable during repeated exposures
at autoclavable temperatures or cause a significant reduction in
the cell's capacity as a consequence of such exposures.
8. It is accordingly an object of the present invention to provide
an electrochemical cell which can withstand repeated exposure to
autoclave environments without significant loss of capacity.
9. In order to provide such an autoclavable electrochemical cell,
in accordance with the present invention the anode is provided to
have as active material a material which has a melting point
greater than about 150 degrees C. and the solvent for the
electrolyte is characterized by having a boiling point greater than
about 100 degrees C., and a dielectric constant greater than about
5.
10. The above and other objects, features, and advantages of the
present invention will be apparent in the following detailed
description of the preferred embodiments taken in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
11. FIG. 1 is a graph showing discharge under a five kilohm load of
a cell of the present invention which has not undergone
autoclaving.
12. FIG. 2 is a graph showing discharge of the cell of FIG. 1 after
it has been subjected to five exposures each of about one hour to
an autoclaving environment at a temperature of about 130 degrees
C.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
13. A battery suitable as a power source for an implantable medical
device is comprised of a casing, cathode plates having as active
material silver vanadium oxide (a metal oxide bronze), an anode
having as active material lithium, a non-aqueous electrolyte
solution which includes a lithium salt and an organic solvent, and
a separator material encapsulating either or both of the
electrodes, as discussed in greater detail in the aforesaid U.S.
Pat. No. 4,830,940, which is assigned to the assignee of the
present invention and the disclosure of which is hereby
incorporated herein by reference. Other suitable active materials
for the anode may be selected from groups IA and IIA of the
Periodic Table. The cathode active material may suitably comprise
other metal oxides, which are meant to include the metal oxide
bronzes, and carbon monoflouride. Examples of metal oxides for the
cathode material include, but are not limited to, manganese
dioxide, vanadium oxide, and cobalt oxide. The reference herein and
in the claims to lithium is meant to include alloys thereof.
14. The exposure of such a cell as described above to elevated
temperatures during repeated periods of autoclaving requires that
the cell be constructed so that it remains dimensionally and
chemically stable. Thus, the cell suitably should retain
dimensional and chemical stability during repeated exposures each
of about one hour to a temperature of about 130 to 135 degrees C.
For-the purposes of this specification and the claims, the term
"repeated" is meant to refer to at least five such exposures. The
term "dimensionally stable" refers to the ability of the cell to
resist swelling. As the temperature increases the pressure inside
the cell increases. This may result in cell swelling if a cell
includes an unsuitable electrolyte The case walls undesirably bulge
during such cell swelling due to high vapor pressure. The term
"chemically stable" is meant to refer to maintenance of the
chemical composition of the cell components so that performance of
the cell is not significantly compromised by the autoclaving heat
whereby it does not have reduced capacity or increased cell
resistance or decreased cell life.
15. In order to minimize the generation of gas as the temperature
is increased during autoclaving for dimensional stability, the
electrolyte solvent is selected to have a high boiling point, i.e.,
at least about 100 degrees C. The solvent is also selected to have
a high dielectric constant, i.e., at least about 5 so that the cell
capacity may be maintained during the repeated exposures to an
autoclaving environment. The electrolyte solvents are also selected
to be thermally stable in the presence of the electrode-active
materials at the autoclaving temperature. Examples of solvents
which are suitable for use with cells having lithium anodes and
silver vanadium oxide cathodes include, but are not limited to,
diglyme, sulfolane, propylene carbonate, ethylene carbonate, and
mixtures thereof.
16. The salt and solvent combination should be such that high
conductivity is provided, high thermal stablity is present, and the
cell can discharge effectively at room temperature and at 37
degrees C. By "thermal stability" is meant the ability of a
material not to weaken or melt or degrade at the autoclaving
temperature. This includes the ability of the salt not to
precipitate out of the electrolyte solution upon exposure to the
autoclaving temperatures. Suitable lithium salts include, but are
not limited to, lithium tetrafluoroborate, lithium trifluoromethane
sulfonate, lithium hexafluoroarsenate, lithium hexafluorophosphate
and lithium perchlorate.
17. The separator material is composed of an electrically
insulative material to prevent an internal electrical short circuit
between the electrodes, is chemically unreactive with the electrode
materials, is both chemically unreactive with and insoluble in the
electrolyte solution, and has a sufficient porosity to allow
flow-through of the electrolyte solution during the electrochemical
reaction in the cell. If the separator material were insufficiently
wettable by the electrolyte solvent, there would be too high of
resistance to flow of the electrolyte through the pores thereof due
to the increased surface tension. In addition, melting or a
tendency to melt of the separator material, if it has insufficient
thermal stability, may tend to clog the openings therein to thereby
undesirably prevent or reduce electrolyte flow. In order to have
adequate thermal stability, the separator material is chosen to
have a melting point of preferably at least about 130 degrees.
Examples of suitable separator materials for the aforementioned
electrolytes include, but are not limited to, polypropylene
non-woven material, polypropylene membrane material, polypropylene
laminate of non-woven and membrane material, a Teflon membrane
material in conjunction with a polypropylene non-woven layer, and
halogenated polymeric membranes such as Tefzel membranes provided
by Scimat, Inc.
18. The active material of the anode, as well as that of the
cathode, must also be thermally stable, i.e., have a melting point
which is greater than about 150 degrees C., so that it can
withstand the autoclaving temperatures without undesirably melting
or degrading.
19. Repeated exposure to the elevated temperatures during
autoclaving can aggravate corrosion problems. In order to prevent
such corrosion, the various metallic cell components are suitably
made of corrosion-resistant materials such as, for example,
stainless steel or titanium. Glass seals providing feed-throughs
for the electrodes are suitably composed of corrosion resistant
glass. The cathode may suitably be enclosed in the separator
material and then placed into the case containing the anode plates,
which may also be enclosed in separator material, and the cell then
vacuum filled with electrolyte after which a final close welding
provides an hermetic seal of the case.
20. A preferred electrolyte for the cells of the present invention
comprises lithium trifluoromethane sulfonate salt, having a good
thermal stability, in a high conductivity solvent comprising a
mixture of propylene carbonate and diglyme (2-methoxy ethyl ether),
both having high boiling points and good thermal stability as well
as the combination of solvents providing a higher conductivity than
any of the aforementioned suitable solvents alone. While lithium
trifluoromethane sulfonate is preferred, it should be understood
that other suitable salts may be used with the mixture of propylene
carbonate and diglyme. A suitable ratio, by volume, for the mixture
of propylene carbonate and diglyme is 50:50.
21. A separator material composed of a laminate of a polypropylene
membrane and a polypropylene mesh is not suitably wettable by
propylene carbonate alone due to the low viscosity of propylene
carbonate. However, the polypropylene laminate is sufficiently
wettable by and may be used with the combination of propylene
carbonate and diglyme, wherein the diglyme comprises at least about
10 percent by volume of the mixture, since the diglyme tends to
thin the mixture.
22. Titanium reacts with carbon monofluoride at high temperatures
to increase cell impedance. If the cathode active material is
carbon monofluoroide, the current collector therefor is suitably
composed of superferrite or titanium coated with a carbon paint or
other suitable conductor. In order to provide an electrolyte which
has good stability when used with the carbon monofluoroide cathode
material, the electrolyte therefor is preferably selected to be
lithium tetrafluoroborate or lithium trifluoromethane sulfonate in
gammabutyrolactone solvent, which has a suitably high boiling
point.
23. The following is an example of a cell made in accordance with
the present invention and a comparison of its operating
characteristics before and after repeated exposures thereof to an
autoclaving environment, it being understood that the following
example is being provided for illustrative purposes only and not
for purposes of limitation.
24. Cells, that have a half-round shape of dimensions 7 mm.times.
28 mm.times.43 mm, were constructed in accordance with the present
invention. The cathode material comprised, by total weight-percent,
98% silver vanadium oxide (SVO), 1% Teflon 7A material, and 1%
graphite. The anode material was composed of lithium, and the
electrodes of the cells were prohibited from coming into contact
with each other by using separators composed of Goretex
polypropylene laminate material of a membrane and a non-woven mesh,
a product of W. L. Gore & Assoc. The electrolyte used was
comprised of 1M lithium trifluoromethane sulfonate as the salt
component, and a 1:1 mixture of propylene carbonate: diglyme as the
organic solvent component. The-cell components were composed of
corrosion-resistant materials, and the casing was hermetically
sealed. The cells have a theoretical capacity of about 2.3 Ah with
a volumetric energy density of about 870 Wh/L and a gravimetric
density of about 260 Wh/kg.
25. The performance of the cells under 1 or 5 kilohm loads was
observed. Cells that were subjected to five autoclave cycles, each
cycle attaining a temperature to about 130 degrees C. for about one
hour, were compared to identical cells which were not autoclaved.
The comparison, as shown in Table I, indicates that there is no
significant difference in delivered capacity between cells that
were autoclaved and those that were not. Cells discharged under 1
kilohm delivered an average of 1.93Ah or 84% of theoretical
capacity to a 2 volt cutoff when they were autoclaved compared to
1.93 Ah or 84% when they were not. The 5 kilohm group delivered
2.06 Ah or 90% of theoretical capacity to a 2 volt cutoff when they
were autoclaved and 2.07 Ah or 90% theoretical capacity when they
were not autoclaved. Typical discharge curves of the autoclaved and
non-autoclaved cells under 5 kilohm load are shown in FIGS. 1 and 2
respectively.
1 TABLE I Experimental to 2V % Theoretical 2V (Ah) (Ah) 1 kilohm
Discharge Autoclaved 1.93 84 Non-autoclaved 1.93 84 5 kilohm
Discharge Autoclaved 2.06 90 Non-autoclaved 2.07 90
26. The discharge behavior of the cells was also observed. The
self-discharge of the cells was estimated from heat dissipated as
measured by calorimetry. Microcalorimetry testing at 37 degrees C.
was performed on cells stored at open circuit after 2, 5 and 8
months. At 2 months, the autoclaved cells showed annual
self-discharge rates of 0.6% to 0.8% while the non-autoclaved cells
showed rates of 1.2% to 1.3%. At 5 months there was still slightly
less heat dissipation from the autoclaved cells than from
non-autoclaved cells, but the difference was narrowed (avg. 0.55%
vs. 0.67%) and after 8 months the averages were 0.28% vs. 0.43% for
the average annual self-discharge. Thus, as presented in Table II,
the microcalorimetry testing indicated less than 1% self-discharge
per year for cells of this autoclavable design.
2TABLE II Microcalorimetry Test Results % Self-Discharge 2 Months 5
Months 8 Months Autoclaved 0.72 0.55 0.28 Non-autoclaved 1.28 0.67
0.43
27. The above detailed description and examples are intended for
purposes of illustrating the invention and are not to be construed
as limiting. The invention can be embodied otherwise without
departing from the principles thereof, and such other embodiments
are meant to come within the scope of the present invention as
defined by the appended claims.
* * * * *