U.S. patent application number 09/772680 was filed with the patent office on 2001-07-05 for electrochemical cell activated with a nonaqueous electrolyte having a sulfate additive.
Invention is credited to Gan, Hong, Takeuchi, Esther S..
Application Number | 20010006751 09/772680 |
Document ID | / |
Family ID | 26679609 |
Filed Date | 2001-07-05 |
United States Patent
Application |
20010006751 |
Kind Code |
A1 |
Gan, Hong ; et al. |
July 5, 2001 |
Electrochemical cell activated with a nonaqueous electrolyte having
a sulfate additive
Abstract
An alkali metal, solid cathode, non-aqueous electrochemical cell
capable of delivering high current pulses, rapidly recovering its
open circuit voltage and having high current capacity, is
described. The stated benefits are realized by the addition of at
least one organic sulfate additive to an electrolyte comprising an
alkali metal salt dissolved in a mixture of a low viscosity solvent
and a high permittivity solvent. A preferred solvent mixture
includes propylene carbonate, 1,2-dimethoxyethane and a sulfate
additive having at least one unsaturated hydrocarbon containing a
C(sp or sp.sup.2)-C(sp.sup.3) bond unit having the C(sp.sup.3)
carbon directly connected to the --OSO.sub.3-- functional
group.
Inventors: |
Gan, Hong; (E. Amherst,
NY) ; Takeuchi, Esther S.; (E. Amherst, NY) |
Correspondence
Address: |
Michael F. Scalise
Hodgson Russ LLP
Suite 2000
One M&T Plaza
Buffalo
NY
14203-2391
US
|
Family ID: |
26679609 |
Appl. No.: |
09/772680 |
Filed: |
January 30, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09772680 |
Jan 30, 2001 |
|
|
|
09460035 |
Dec 13, 1999 |
|
|
|
6180283 |
|
|
|
|
09460035 |
Dec 13, 1999 |
|
|
|
09009537 |
Jan 20, 1998 |
|
|
|
5797144 |
|
|
|
|
Current U.S.
Class: |
429/340 ;
429/231.2; 429/231.95; 429/332; 429/333; 429/334; 429/335 |
Current CPC
Class: |
H01M 6/168 20130101;
H01M 6/166 20130101; H01M 2300/0037 20130101 |
Class at
Publication: |
429/340 ;
429/332; 429/333; 429/334; 429/335; 429/231.95; 429/231.2 |
International
Class: |
H01M 010/40 |
Claims
What is claimed is:
1. An electrochemical cell, which comprises: a) an anode comprising
an alkali metal; b) a cathode comprising a cathode active material;
c) a non-aqueous electrolyte operatively associated with the anode
and the cathode, the non-aqueous electrolyte comprising a sulfate
additive having the general formula R.sup.1OS (.dbd.O.sub.2)
(OR.sup.2), wherein R.sup.1 and R.sup.2 are selected from the group
consisting of: i) at least one of R.sup.1 and R.sup.2 is an organic
group containing at least 3 carbon atoms and having either a first
bond structure of the type C(sp)-C(sp.sup.3) directly connected to
the --OSO.sub.3-- functional group or having a second bond
structure of the type C(sp.sup.2)-C(sp.sup.3) directly connected to
the --OSO.sub.3-- functional group; and ii) at least one of R.sup.1
and R.sup.2 is an unsaturated inorganic group.
2. The electrochemical cell of claim 1 wherein when the sulfate
additive has at least one of R.sup.1 and R.sup.2 having the first
bond structure, the second bond structure, or being the unsaturated
inorganic group, then the at least one of R.sup.1 and R.sup.2 is
selected from the group consisting of benzyl, allyl, propargyl and
cyanomethyl.
3. The electrochemical cell of claim 2 wherein the other of R.sup.1
and R.sup.2 which is not the first bond structure, the second bond
structure, or the unsaturated inorganic group is either a linear or
a cyclic organic group having 1 to 13 carbon atoms or an inorganic
group.
4. The electrochemical cell of claim 1 wherein the sulfate additive
is present in the electrolyte in a range of about 0.001M to about
0.40M.
5. The electrochemical cell of claim 1 including discharging the
cell to deliver at least one current pulse of an electrical current
of a greater amplitude than that of a pre-pulse current immediately
prior to the pulse.
6. The electrochemical cell of claim 5 wherein there are at least
two pulses delivered in succession with or without an open circuit
period between the pulses.
7. The electrochemical cell of claim 1 wherein the nonaqueous
electrolyte comprises a first solvent selected from the group
consisting of an ester, an ether, a dialkyl carbonate, and mixtures
thereof, and a second solvent selected from the group consisting of
a cyclic carbonate, a cyclic ester, a cyclic amide, and mixtures
thereof.
8. The electrochemical cell of claim 7 wherein the first solvent is
selected from the group consisting of tetrahydrofuran, methyl
acetate, diglyme, triglyme, tetraglyme, 1,2-dimethoxyethane,
diisopropylether, 1,2-diethoxyeltane, 1-ethoxy,2-methoxyethane,
dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl
methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate,
and mixtures thereof.
9. The electrochemical cell of claim 7 wherein the second solvent
is selected from the group consisting of propylene carbonate,
ethylene carbonate, butylene carbonate, acetonitrile, dimethyl
sulfoxide, dimethyl formamide, dimethyl acetamide,
.gamma.-valerolactone, .gamma.-butyrolactone,
N-methyl-pyrrolidinone, and mixtures thereof.
10. The electrochemical cell of claim 1 including providing an
alkali metal salt in the electrolyte, the alkali metal salt being
selected from the group consisting of LiPF.sub.6, LiBF.sub.4,
LiAsF.sub.6, LiSbF.sub.6, LiClO.sub.4, LiAlCl.sub.4, LiGaCl.sub.4,
LiC(SO.sub.2CF.sub.3).sub.3, LiO.sub.2,
LiN(SO.sub.2CF.sub.3).sub.2, LiSCN, LiO.sub.3SCF.sub.2CF.sub.3- ,
LiC.sub.6F.sub.5SO.sub.3, LiO.sub.2CCF.sub.3, LiSO.sub.3F,
LiB(C.sub.6H.sub.5).sub.4, LiF.sub.3SO.sub.3, and mixtures
thereof.
11. The electrochemical cell of claim 1 wherein the cathode active
material is selected from the group consisting of silver vanadium
oxide, copper silver vanadium oxide, vanadium oxide, copper
vanadium oxide, and mixtures thereof.
12. The electrochemical cell of claim 1 wherein the anode is
comprised of lithium or a lithium-aluminum alloy.
13. The electrochemical cell of claim 1 wherein the cathode
comprises from about 80 to about 99 weight percent of the cathode
active material.
14. The electrochemical cell of claim 1 wherein the cathode further
comprises a binder material and a conductive additive.
15. The electrochemical cell of claim 14 wherein the binder
material is a fluoro-resin powder.
16. The electrochemical cell of claim 14 wherein the conductive
additive is selected from the group consisting of carbon, graphite
powder, acetylene black, titanium powder, aluminum powder, nickel
powder, stainless steel powder, and mixtures thereof.
17. The electrochemical cell of claim 1 wherein the cathode
comprises from about 0 to 3 weight percent carbon, about 1 to 5
weight percent of a powder fluoro-resin and about 94 weight percent
of the cathode active material.
18. The electrochemical cell of claim 1 including powering an
implantable medical device with the electrochemical cell.
19. A method for reducing voltage delay in a pulse discharging
electrochemical cell activated with a nonaqueous electrolyte,
comprising the steps of: a) providing an anode comprising an alkali
metal; b) providing a cathode including a mixed metal oxide
comprised of vanadium oxide and a second metal "SM" selected from
the group consisting of Groups IB, IIB, IIIB, IVB, VIB, VIIB and
VIII of the Periodic Table of the Elements, the mixed metal oxide
having the general formula SM.sub.xV.sub.2O.sub.y, wherein
0.30.ltoreq.x.ltoreq.2.0 and 4.5.ltoreq.y.ltoreq.6.0; c) activating
the electrochemical cell with the non-aqueous electrolyte
operatively associated with the anode and the cathode, the
non-aqueous electrolyte comprising: i) a first solvent selected
from the group consisting of an ester, an ether, a dialkyl
carbonate, and mixtures thereof; ii) a second solvent selected from
the group consisting of a cyclic carbonate, a cyclic ester, a
cyclic amide, and mixtures thereof; iii) a sulfate additive having
the general formula R.sup.1OS(.dbd.O.sub.2) (OR.sup.2), wherein at
least one of R.sup.1 and R is an organic group containing at least
3 carbon atoms and being selected from the group consisting of a
first bond structure of the type C(sp)-C(sp.sup.3) directly
connected to the --OSO.sub.3-- functional group, a second bond
structure of the type C(sp.sup.2)-C(sp.sup.3)- directly connected
to the --OSO.sub.3-- functional group, and an unsaturated inorganic
group; and iv) an alkali metal salt dissolved therein; and d)
discharging the cell to deliver at least one current pulse of an
electrical current of a greater amplitude than that of a pre-pulse
current immediately prior to the pulse.
20. The method of claim 19 including providing the sulfate additive
having at least one of R.sup.1 and R.sup.2 having the first bond
structure, the second bond structure, or being the unsaturated
inorganic group, then the at least one of R.sup.1 and R.sup.2 is
selected from the group consisting of benzyl, allyl, propargyl and
cyanomethyl.
21. The method of claim 20 including providing the other of R.sup.1
and R.sup.2 which is not of the first bond structure, the second
bond structure, or the unsaturated inorganic group is either a
linear or a cyclic organic group having 1 to 13 carbon atoms or an
inorganic group.
22. The method of claim 19 wherein the sulfate additive is present
in the electrolyte in a range of about 0.001M to about 0.40M.
23. A method for providing an electrochemical cell, comprising the
steps of: a) providing an anode comprising an alkali metal; b)
providing a cathode of a cathode active material; c) activating the
anode and the cathode with a non-aqueous electrolyte comprising a
sulfate additive having the general formula R.sup.1OS
(.dbd.O.sup.2) (OR.sup.2), wherein at least one of the R.sup.1 and
R is an organic group containing at least 3 carbon atoms and being
selected from the group consisting of a first bond structure of the
type C(sp)-C(sp.sup.3) directly connected to the --OSO.sub.3--
functional group, a second bond structure of the type
C(sp.sup.2)-C(sp.sup.3) directly connected to the --OSO.sub.3--
functional group, and an unsaturated inorganic group.
24. The method of claim 23 including providing the sulfate additive
having at least one of R.sup.1 and R.sup.2 having the first bond
structure, the second bond structure, or being the unsaturated
inorganic group, then the at least one of R.sup.1 and R.sup.2 is
selected from the group consisting of benzyl, allyl, propargyl and
cyanomethyl.
25. The method of claim 24 wherein the other of R.sup.1 and R.sup.2
which is not the first bond structure, the second bond structure,
or the unsaturated inorganic group is either a linear or a cyclic
organic group having 1 to 13 carbon atoms or an inorganic
group.
26. The method of claim 23 including providing the electrolyte
comprising a first solvent selected from the group consisting of an
ester, an ether, a dialkyl carbonate, and mixtures thereof, and a
second solvent selected from the group consisting of a cyclic
carbonate, a cyclic ester, a cyclic amide, and mixtures
thereof.
27. The method of claim 23 including providing the electrolyte
having an alkali metal salt dissolved therein.
28. The method of claim 23 wherein the sulfate additive is present
in the electrolyte in a range of about 0.001M to about 0.40M.
29. The method of claim 24 wherein in the cathode active material
is selected from the group consisting of silver vanadium oxide,
copper silver vanadium oxide, manganese dioxide, lithium cobalt
oxide, lithium nickel oxide, copper vanadium oxide, vanadium oxide,
titanium disulfide, copper oxide, copper sulfide, iron sulfide,
iron disulfide, carbon, fluorinated carbon, and mixtures thereof.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of application
Ser. No. 09/460,035, now U.S. Pat. No. 6,180,283 to Gan et al,
which is a continuation-in-part of application Ser. No. 09/009,537,
now U.S. Pat. No. 6,013,394 to Gan et al.
BACKGROUND OF INVENTION
[0002] 1. Field of the Invention
[0003] The present invention generally relates to an alkali metal
electrochemical cell, and more particularly, to an alkali metal
cell suitable for current pulse discharge applications with reduced
or no appreciable voltage delay. These benefits are realized by the
provision of a sulfate additive in the electrolyte.
[0004] 2. Prior Art
[0005] The initial drop in cell potential during the application of
a short duration pulse is termed voltage delay, and reflects the
resistance of the cell, i.e., the resistance due to the cathode,
the cathode-electrolyte interphase, the anode, and the
anode-electrolyte interphase. In the absence of voltage delay, the
resistance due to passivated films on the anode and/or cathode is
negligible. However, the formation of a surface film is unavoidable
for alkali metal, and in particular, lithium metal anodes, and for
lithium intercalated carbon anodes, due to their relatively low
potential and high reactivity towards organic electrolytes. Thus,
the ideal anode surface film should be electrically insulating and
ionically conducting. While most alkali metal, and in particular,
lithium electrochemical systems meet the first requirement, the
second requirement is difficult to achieve. In the event of voltage
delay, the resistance of these films is not negligible, and as a
result, impedance builds up inside the cell due to this surface
layer formation which often results in reduced discharge voltage
and reduced cell capacity. In other words, the drop in potential
between the background voltage and the lowest voltage under pulse
discharge conditions, excluding voltage delay, is an indication of
the conductivity of the cell, i.e., the conductivity of the
cathode, anode, electrolyte, and surface films, while the gradual
decrease in cell potential during the application of the pulse
train is due to the polarization of the electrodes and the
electrolyte.
[0006] Thus, the existence of voltage delay is an undesirable
characteristic of alkali metal/mixed metal oxide cells subjected to
current pulse discharge conditions in terms of its influence on
devices such as medical devices including implantable pacemakers
and cardiac defibrillators. Voltage delay is undesirable because it
limits the effectiveness and even the proper functioning of both
the cell and the associated electrically powered device under
current pulse discharge conditions.
[0007] The present invention is directed to the provision of
sulfate additives in the electrolyte of an alkali metal
electrochemical cell to beneficially modify the anode surface film.
The sulfate additives preferably include at least one organic group
containing a C(sp or sp.sup.2)-C(sp.sup.3) bond unit having the
C(sp.sup.3) carbon directly connected to the --OSO.sub.3--
functional group, and are provided as a co-solvent with commonly
used organic aprotic solvents. The organic sulfate additives are in
a condensed phase which makes them easy to handle in electrolyte
preparation. When used as a co-solvent in an activating
electrolyte, the sulfate additives interact with the alkali metal
anode to form an ionically conductive surface protective layer
thereon. The conductive surface layer improves the discharge
performance of the alkali metal electrochemical cell and minimizes
or even eliminates voltage delay in the high current pulse
discharge of such cells.
SUMMARY OF THE INVENTION
[0008] The object of the present invention is to improve the pulse
discharge performance of an alkali metal electrochemical cell, and
more particularly a primary lithium electrochemical cell, by the
provision of at least one of a family of sulfate additives as a
co-solvent in the cell's activating nonaqueous electrolyte
solution. Due to the high reduction potentials of the sulfate group
vs. lithium, the sulfate additives can compete effectively with the
other electrolyte co-solvents or the solute to react with the
lithium anode. Lithium sulfate or the lithium salt of sulfate
reduction products are believed to be the major reaction products.
These lithium salts are believed to deposit on the anode surface to
form an ionically conductive protective film thereon. As a
consequence, the chemical composition and perhaps the morphology of
the anode surface protective layer is changed, and this proves
beneficial to the discharge characteristics of the cell. The thusly
fabricated cell exhibits reduced or no appreciable voltage delay
under current pulse discharge usage, which is an unexpected
result.
[0009] These and other objects of the present invention will become
increasingly more apparent to those skilled in the art by reference
to the following description.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0010] As used herein, the term "pulse" means a burst of electrical
current of a greater amplitude than that of a pre-pulse current
immediately prior to the pulse. A pulse train consists of at least
two pulses of electrical current delivered in relatively short
succession with or without open circuit rest between the pulses.
Voltage delay is calculated as the pulse end potential minus the
pulse minimum potential.
[0011] The electrochemical cell of the present invention includes
an anode selected from Group IA, IIA or IIIB of the Periodic Table
of Elements, including lithium, sodium, potassium, etc., and their
alloys and intermetallic compounds including, for example Li--Si,
Li--B and Li--Si--B alloys and intermetallic compounds. The
preferred anode comprises lithium, and the more preferred anode
comprises a lithium alloy, the preferred lithium alloy being a
lithium-aluminum alloy. The greater the amount of aluminum present
by weight in the alloy, however, the lower the energy density of
the cell.
[0012] The form of the anode may vary, but preferably the anode is
a thin metal sheet or foil of the anode metal, pressed or rolled on
a metallic anode current collector, i.e., preferably comprising
nickel, to form an anode component. In the exemplary cell of the
present invention, the anode component has an extended tab or lead
of the same material as the anode current collector, i.e.,
preferably nickel, integrally formed therewith such as by welding
and contacted by a weld to a cell case of conductive metal in a
case-negative electrical configuration. Alternatively, the anode
may be formed in some other geometry, such as a bobbin shape,
cylinder or pellet to allow an alternate low surface cell
design.
[0013] The cathode is preferably of a solid material and the
electrochemical reaction at the cathode involves conversion of ions
which migrate from the anode to the cathode in atomic or molecular
forms. The solid cathode material may comprise a metal, a metal
oxide, a mixed metal oxide, a metal sulfide or a carbonaceous
compound, and combinations thereof. The metal oxide, the mixed
metal oxide and the metal sulfide can be formed by the chemical
addition, reaction, or otherwise intimate contact of various metal
oxides, metal sulfides and/or metal elements, preferably during
thermal treatment, sol-gel formation, chemical vapor deposition or
hydrothermal synthesis in mixed states. The active materials
thereby produced contain metals, oxides and sulfides of Groups IB,
IIB, IIIB, IVB, VB, VIB, VIIB and VIII, which includes the noble
metals and/or other oxide and sulfide compounds.
[0014] One preferred mixed metal oxide has the general formula
SM.sub.xV.sub.2O.sub.y wherein SM is a metal selected from Groups
IB to VIIB and VIII of the Periodic Table of Elements, wherein x is
about 0.30 to 2.0 and y is about 4.5 to 6.0 in the general formula.
By way of illustration, and in no way intended to be limiting, one
exemplary cathode active material comprises silver vanadium oxide
(SVO) having the general formula Ag.sub.xV.sub.2O.sub.y in any one
of its many phases, i.e., .beta.-phase silver vanadium oxide having
in the general formula x=0.35 and y=5.8, .gamma.-phase silver
vanadium oxide having in the general formula x=0.80 and y=5.40 and
.epsilon.-phase silver vanadium oxide having in the general formula
x=1.0 and y=5.5, and combination and mixtures of phases thereof.
For a more detailed description of such a cathode active material
reference is made to U.S. Pat. No. 4,310,609 to Liang et al., which
is assigned to the assignee of the present invention and
incorporated herein by reference.
[0015] Another preferred composite cathode active material includes
V.sub.2O.sub.z wherein z.ltoreq.5 combined with Ag.sub.2O with the
silver in either the silver(II), silver(I) or silver(0) oxidation
state and CuO with the copper in either the copper(II), copper(I)
or copper(0) oxidation state to provide the mixed metal oxide
having the general formula Cu.sub.xAg.sub.yV.sub.2O.sub.z, (CSVO).
Thus, this composite cathode active material may be described as a
metal oxide-metal oxide-metal oxide, a metal-metal oxide-metal
oxide, or a metal-metal-metal oxide and the range of material
compositions found for Cu.sub.xAg.sub.yV.sub.2O.sub.z is preferably
about 0.01.ltoreq.x.ltoreq.1.0, about 0.01.ltoreq.y.ltoreq.1.0 and
about 5.01.ltoreq.z.ltoreq. 6.5. Typical forms of CSVO are
Cu.sub.0.16Ag.sub.0.67V.sub.2O.sub.z with z being about 5.5 and
Cu.sub.0.5Ag.sub.0.5V.sub.2O.sub.z with z being about 5.75. The
oxygen content is designated by z since the exact stoichiometric
proportion of oxygen in CSVO can vary depending on whether the
cathode material is prepared in an oxidizing atmosphere such as air
or oxygen, or in an inert atmosphere such as argon, nitrogen and
helium. For a more detailed description of this cathode active
material reference is made to U.S. Pat. Nos. 5,472,810 to Takeuchi
et al. and 5,516,340 to Takeuchi et al., both of which are assigned
to the assignee of the present invention and incorporated herein by
reference.
[0016] Additional cathode active materials include manganese
dioxide, lithium cobalt oxide, lithium nickel oxide, copper
vanadium oxide, vanadium oxide, titanium disulfide, copper oxide,
copper sulfide, iron sulfide, iron disulfide, carbon, fluorinated
carbon, and mixtures thereof. Preferably, the cathode comprises
from about 80 to about 99 weight percent of the cathode active
material.
[0017] Cathode active materials prepared as described above are
preferably mixed with a binder material such as a powdered
fluoro-polymer, more preferably powdered polytetrafluoroethylene or
powdered polyvinylidene fluoride present at about 1 to about 5
weight percent of the cathode mixture. Further, up to about 10
weight percent of a conductive diluent is preferably added to the
cathode mixture to improve conductivity. Suitable materials for
this purpose include acetylene black, carbon black and/or graphite
or a metallic powder such as powdered nickel, aluminum, titanium
and stainless steel. The preferred cathode active mixture thus
includes a powdered fluoro-polymer binder present at about 3 weight
percent, a conductive diluent present at about 3 weight percent and
about 94 weight percent of the cathode active material. The cathode
active mixture may be in the form of one or more plates operatively
associated with at least one or more plates of anode material, or
in the form of a strip wound with a corresponding strip of anode
material in a structure similar to a "jellyroll".
[0018] In order to prevent internal short circuit conditions, the
cathode is separated from the Group IA, IIA or IIIB anode material
by a suitable separator material. The separator is of electrically
insulative material, and the separator material also is chemically
unreactive with the anode and cathode active materials and both
chemically unreactive with and insoluble in the electrolyte. In
addition, the separator material has a degree of porosity
sufficient to allow flow therethrough of the electrolyte during the
electrochemical reaction of the cell. Illustrative separator
materials include woven and non-woven fabrics of polyolefinic
fibers or fluoropolymeric fibers including polyvinylidene fluoride,
polyethylenetetrafluoroethylene, and
polyethylenechlorotrifluoroethylene laminated or superposed with a
polyolefinic or a fluoropolymeric microporous film. Suitable
microporous films include a polytetrafluoroethylene membrane
commercially available under the designation ZITEX (Chemplast
Inc.), polypropylene membrane commercially available under the
designation CELGARD (Celanese Plastic Company, Inc.) and a membrane
commercially available under the designation DEXIGLAS (C. H.
Dexter, Div., Dexter Corp.). The separator may also be composed of
non-woven glass, glass fiber materials and ceramic materials.
[0019] The form of the separator typically is a sheet which is
placed between the anode and cathode electrodes and in a manner
preventing physical contact therebetween. Such is the case when the
anode is folded in a serpentine-like structure with a plurality of
cathode plates disposed intermediate the anode folds and received
in a cell casing or when the electrode combination is rolled or
otherwise formed into a cylindrical "jellyroll" configuration.
[0020] The electrochemical cell of the present invention further
includes a nonaqueous, ionically conductive electrolyte operatively
associated with the anode and the cathode electrodes. The
electrolyte serves as a medium for migration of ions between the
anode and the cathode during the electrochemical reactions of the
cell and nonaqueous solvents suitable for the present invention are
chosen so as to exhibit those physical properties necessary for
ionic transport (low viscosity, low surface tension and
wettability). Suitable nonaqueous solvents are comprised of an
inorganic salt dissolved in a nonaqueous solvent and more
preferably an alkali metal salt dissolved in a mixture of aprotic
organic solvents comprising a low viscosity solvent including
organic esters, ethers, dialkyl carbonates, and mixtures thereof,
and a high permittivity solvent including cyclic carbonates, cyclic
esters, cyclic amides, and mixtures thereof. Low viscosity solvents
include tetrahydrofuran (THF), methyl acetate (MA), diglyme,
triglyme, tetraglyme, 1,2-dimethoxyethane (DME), dimethyl carbonate
(DMC), diisopropylether, 1,2-diethoxyethane,
1-ethoxy,2-methoxyethane, diethyl carbonate (DEC), dipropyl
carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl
carbonate (MPC), ethyl propyl carbonate (EPC), and mixtures
thereof. High permittivity solvents include propylene carbonate
(PC), ethylene carbonate (EC), butylene carbonate (BC),
acetonitrile, dimethyl sulfoxide, dimethyl formamide, dimethyl
acetamide, .gamma.-valerolactone, .gamma.-butyrolactone (GBL),
N-methyl-pyrrolidinone (NMP), and mixtures thereof.
[0021] The preferred electrolyte comprises an inorganic alkali
metal salt, and in the case of an anode comprising lithium, the
alkali metal salt of the electrolyte is a lithium based salt. Known
lithium salts that are useful as a vehicle for transport of alkali
metal ions from the anode to the cathode include LiPF.sub.6,
LiBF.sub.4, LiAsF.sub.6, LiSbF.sub.6, LiClO.sub.4, LiAlCl.sub.4,
LiGaCl.sub.4, LiC(SO.sub.2CF.sub.3).sub.3, LiO.sub.2,
LiN(SO.sub.2CF.sub.3).sub.2, LiSCN, LiO.sub.3SCF.sub.2CF.sub.3- ,
LiC.sub.6F.sub.5SO.sub.3, LiO.sub.2CCF.sub.3, LiSO.sub.3F,
LiB(C.sub.6H.sub.5).sub.4, LiCF.sub.3SO.sub.3, and mixtures
thereof. Suitable salt concentrations typically range between about
0.8 to 1.5 molar, and a preferred electrolyte for a
lithium/transition metal oxide electrochemical cell includes
LiAsF.sub.6 or LiPF6 dissolved in a 50:50 mixture, by volume, of PC
and DME.
[0022] A distinguishing characteristic of a primary lithium metal
cell, in comparison to that of a secondary lithium ion cell, is
that the lithium anode material is consumed during discharge. In
other words, the surface of the anode is shrinking or fading away
from the electrode/electrolyte interphase during discharge. This is
especially the case during high current pulse discharge when the
anode surface passivation film breaks up to expose a fresh lithium
surface. At this stage, a new passivation film forms and its
chemical composition and surface morphology depends on the
competition reactions of the existing components in the
electrolyte, which, in turn, depends on the cell depth of
discharge.
[0023] In a lithium/silver vanadium oxide cell, there are two
characteristic voltage plateaus in the discharge profile. The first
one occurs at about 3.2V and the second plateau occurs at about
2.6V. At the 3.2V discharge plateau, Li/SVO cells are known to be
relatively stable and exhibit consistent long term pulse discharge
behavior. This indicates that the lithium anode is effectively
passivated by the electrolyte. However, when the cell is discharged
to the 2.6V plateau, voltage delay is present in long term pulse
discharge tests.
[0024] It has been determined that voltage delay in an alkali
metal/transition metal oxide cell is caused by the growth of anode
surface impedance. The chemical composition and the passivation
ability of the anode surface layer change in the time that the
Li/SVO cell discharges from the 3.2V plateau to the 2.6V plateau.
Consequently, the anode surface impedance increases over time and
results in voltage delay during high current pulse discharge in
long term tests.
[0025] In the absence of a sulfate additive to the electrolyte
according to the present invention, the anode surface passivation
film, which reforms on the newly exposed, fresh lithium surface
during discharge, becomes more resistive ionically, or does not
effectively passivate the anode electrically, and therefore,
additional amounts of electrolyte decomposition occur on the anode
surface. In turn, the impedance of the anode passivation layer
increases over long term discharge tests, which results in
unacceptable voltage delay and decreased pulse minimum potentials
over those experienced without the undesirable anode surface
passivation film. This is the reason for voltage delay during high
current pulse discharge, especially in long term tests when the
cell has been depleted of 40% to 70% of its capacity.
[0026] According to the present invention, the undesirable or "bad"
passivation film formed on the lithium anode at the 2.6V depth of
discharge in a Li/SVO cell is prevented by the presence of an
electrically insulating and ionically conductive or "good"
passivation layer formed on the anode surface. This passivation
layer does not break up, or does not break up easily, during high
current pulse discharge. If the passivation film does break up, it
is readily reformed before an undesirable passivation layer
replaces it. Thus, it is believed that exposure of fresh lithium is
prevented or minimized during discharge.
[0027] An electrically insulating and ionically conducting or
"good" passivation layer is formed on the lithium anode in
accordance with the present invention by the provision of at least
one sulfate additive as a co-solvent in the electrolyte solution of
the previously described alkali metal electrochemical cell. The
sulfate additive preferably has the general formula
R.sup.1OS(.dbd.O).sub.2(OR.sup.2), wherein at least one of R.sup.1
and R.sup.2 is an organic group containing at least 3 carbon atoms
and having an sp or sp.sup.2 hydridized carbon atom bonded to an
sp.sup.3 hydridized carbon atom bonded to the oxygen atom bonded to
the sulfur atom, or those were at least one of R.sup.1 and R.sup.2
is an unsaturated inorganic group. Sulfate additive coming under
the purview of the present invention include those where at least
one of R.sup.1 and R.sup.2 has a first bond structure of the type
C(sp)-C(sp.sup.3) directly connected to the --OSO.sub.3--
functional group, those where at least one of R.sup.1 and R.sup.2
has a second bond structure of the type C(sp.sup.2)-C(sp.sup.3)
directly connected to the --OSO.sub.3-- functional group, or those
where at least one of R.sup.1 and R.sup.2is an unsaturated
inorganic group. Sulfate compounds having at least one of the first
or the second bond structures or which have an unsaturated
inorganic group comprise, for example, R.sup.1 and R.sup.2 selected
from the group of benzyl, allyl, propargyl and cyanomethyl. In that
case, the other of R.sup.1 and R.sup.2 not being the first bond
structure or the second bond structure is either a linear or a
cyclic alkyl group having 1 to 13 carbon atoms or an unsaturated
inorganic group. Exemplary alkyl groups include: methyl, ethyl,
propyl, iso-propyl, butyl, iso-butyl, tert-butyl, pentyl, hexly,
cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl,
cyclooctyl, cyclononanyl, cyclodecanyl, cycloundecyl, cyclododecyl,
phenyl, tolyl and cyanomethyl.
[0028] Examples of sulfate compounds having at least one of R.sup.1
and R.sup.2 of the first bond structure of the type
C(sp)-C(sp.sup.3) directly connected to the --OSO.sub.3--
functional group include: 1
[0029] dipropargyl sulfate (R=propargyl) or
[0030] "alkyl" propargyl sulfate (R.noteq.propargyl)
[0031] Examples of sulfate compounds having at least one of R.sup.1
and R.sup.2 being an unsaturated inorganic group include: 2
[0032] dicyanomethyl sulfate (R=cyanomethyl) or
[0033] cyanomethyl "alkyl" sulfate (R.noteq.cyanomethyl)
[0034] Examples of sulfate compounds having at least one of R.sup.1
and R.sup.2 of the second bond structure of the type
C(sp.sup.2)-C(sp.sup.3) directly connected to the --OSO.sub.3--
functional group include: 3
[0035] dibenzyl sulfate (R=benzyl) or
[0036] "alkyl" benzyl sulfate (R.noteq.benzyl) 4
[0037] diallyl sulfate (R=allyl) or
[0038] "alkyl" allyl sulfate (R.noteq.allyl)
[0039] The above described compounds are only intended to be
exemplary of those that are useful with the present invention, and
are not to be construed as limiting. Those skilled in the art will
readily recognize sulfate compounds which come under the purview of
the general formula set forth above and which will be useful as
additives for the electrolyte to reduce voltage delay according to
the present invention.
[0040] While not intending to be bound by any particular mechanism,
it is believed that due to the presence of the sulfate additive
comprising at least one unsaturated hydrocarbon containing a C(sp
or sp.sup.2)-C(sp.sup.3) bond unit having the C(sp.sup.3) carbon
directly connected to the --OSO.sub.3-- functional group or being
an unsaturated inorganic group, the bond between oxygen and at
least one of the R.sup.1 and R.sup.2 groups is readily severed and
the sulfate intermediate is able to compete effectively with the
other electrolyte solvents or solutes to react with lithium and
form a sulfate salt of the formula S(.dbd.O.sub.2) (O--Li).sub.
n(OR).sub.m where n=1 or 2 and m=0 or 1, i.e., lithium sulfate, or
the lithium salt of a sulfate reduction product on the surface of
the anode. The resulting salt is more conductive than lithium oxide
which may form on the anode in the absence of the organic sulfate
additive. In fact, it is believed that the lithium sulfate or the
lithium salt of a sulfate reduction product on the surface of the
anode provides for the existence of charge delocalization due to
resonance equilibration at the anode surface. This equilibration
allows lithium ions to travel easily from one molecule to the other
via a lithium ion exchange mechanism. As a result, beneficial ionic
conductance is realized.
[0041] In the present invention, the anode is lithium metal and the
cathode is preferably the transition mixed metal oxide
AgV.sub.2O.sub.5.5 (SVO). The preferred electrolyte is 1.0M to 1.2M
LiAsF.sub.6 dissolved in an aprotic solvent mixture comprising at
least one of the above listed low viscosity solvents and at least
one of the above listed high permittivity solvents. The preferred
aprotic solvent mixture comprises a 50/50, by volume, mixture of
propylene carbonate and 1,2-dimethoxyethane. The concentration of
the above discussed sulfate additives according to the present
invention should preferably be in the range of between about 0.001M
to about 0.40M. The positive effects of these additives in reducing
voltage delay in a pulse discharging alkali metal cell have been
achieved both at room temperature as well as at temperatures up to
about 37.degree. C. This makes the novel electrolyte solution of
the present invention particularly useful for activating an alkali
metal/transition metal oxide cell incorporated into an implantable
medical device such as a cardiac defibrillator to minimize or even
eliminate voltage delay under high current pulse discharge
conditions.
[0042] As is well known by those skilled in the art, an implantable
cardiac defibrillator is a device that requires a power source for
a generally medium rate, constant resistance load component
provided by circuits performing such functions as, for example, the
heart sensing and pacing functions. From time to time, the cardiac
defibrillator may require a generally high rate, pulse discharge
load component that occurs, for example, during charging of a
capacitor in the defibrillator for the purpose of delivering an
electrical shock to the heart to treat tachyarrhythmias, the
irregular, rapid heartbeats that can be fatal if left uncorrected.
Reduction and even elimination of voltage delay during a current
pulse application is important for proper device operation and
extended device life.
[0043] The assembly of the cell described herein is preferably in
the form of a wound element cell. That is, the fabricated cathode,
anode and separator are wound together in a "jellyroll" type
configuration or "wound element cell stack" such that the anode is
on the outside of the roll to make electrical contact with the cell
case in a case-negative configuration. Using suitable top and
bottom insulators, the wound cell stack is inserted into a metallic
case of a suitable size dimension. The metallic case may comprise
materials such as stainless steel, mild steel, nickel-plated mild
steel, titanium, tantalum or aluminum, but not limited thereto, so
long as the metallic material is compatible for use with components
of the cell.
[0044] The cell header comprises a metallic disc-shaped body with a
first hole to accommodate a glass-to-metal seal/terminal pin
feedthrough and a second hole for electrolyte filling. The glass
used is of a corrosion resistant type having up to about 50% by
weight silicon such as CABAL 12, TA 23 or FUSITE 425 or FUSITE 435.
The positive terminal pin feedthrough preferably comprises titanium
although molybdenum, aluminum, nickel alloy, or stainless steel can
also be used. The cell header comprises elements having
compatibility with the other components of the electrochemical cell
and is resistant to corrosion. The cathode lead is welded to the
positive terminal pin in the glass-to-metal seal and the header is
welded to the case containing the electrode stack. The cell is
thereafter filled with the electrolyte solution comprising at least
one of the sulfate additives described hereinabove and hermetically
sealed such as by close-welding a stainless steel ball over the
fill hole, but not limited thereto.
[0045] The above assembly describes a case-negative cell, which is
the preferred construction of the exemplary cell of the present
invention. As is well known to those skilled in the art, the
exemplary electrochemical system of the present invention can also
be constructed in a case-positive configuration.
[0046] It is appreciated that various modifications to the
inventive concepts described herein may be apparent to those of
ordinary skill in the art without departing from the spirit and
scope of the present invention as defined by the appended
claims.
* * * * *