U.S. patent application number 13/368387 was filed with the patent office on 2012-05-31 for control of silver vanadium oxide surface areas as a means of controlling voltage delay and rdc growth in an electrochemical cell.
This patent application is currently assigned to Greatbatch Ltd.. Invention is credited to Hong Gan, Joseph Lehnes, Robert S. Rubino, Esther S. Takeuchi.
Application Number | 20120133341 13/368387 |
Document ID | / |
Family ID | 45787882 |
Filed Date | 2012-05-31 |
United States Patent
Application |
20120133341 |
Kind Code |
A1 |
Gan; Hong ; et al. |
May 31, 2012 |
Control of Silver Vanadium Oxide Surface Areas as a Means of
Controlling Voltage Delay and RDC Growth in an Electrochemical
Cell
Abstract
An electrochemical cell comprising a lithium anode, a cathode
comprising a blank cut from a free-standing sheet of a silver
vanadium oxide mixture contacted to a current collector. The active
material has having a relatively lower surface area and an
electrolyte activating the anode and the cathode is described. By
optimizing the cathode active material surface area in a
SVO-containing cell, the magnitude of the passivating film growth
at the solid-electrolyte interphase (SEI) and its relative
impermeability to lithium ion diffusion is reduced. Therefore, by
using a cathode of an active material, in a range of from about 0.2
m.sup.2/gram to about 2.6 m.sup.2/gram, and preferably from about
1.6 m.sup.2/gram to about 2.4 m.sup.2/gram, it is possible to
eliminate or significantly reduce undesirable irreversible Rdc
growth and voltage delay in the cell and to extend its useful life
in an implantable medical device.
Inventors: |
Gan; Hong; (Williamsville,
NY) ; Lehnes; Joseph; (Williamsville, NY) ;
Rubino; Robert S.; (Williamsville, NY) ; Takeuchi;
Esther S.; (East Amherst, NY) |
Assignee: |
Greatbatch Ltd.
Clarence
NY
|
Family ID: |
45787882 |
Appl. No.: |
13/368387 |
Filed: |
February 8, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11696867 |
Apr 5, 2007 |
8133614 |
|
|
13368387 |
|
|
|
|
60791258 |
Apr 12, 2006 |
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Current U.S.
Class: |
320/166 ;
156/50 |
Current CPC
Class: |
Y10T 29/49108 20150115;
H01M 4/382 20130101; H01M 6/16 20130101; H01M 4/623 20130101; H01M
4/54 20130101; H01M 6/164 20130101; H01M 4/06 20130101; H01M 4/661
20130101; H01M 4/5835 20130101; H01M 6/166 20130101; H01M 2300/0037
20130101 |
Class at
Publication: |
320/166 ;
156/50 |
International
Class: |
H02J 7/00 20060101
H02J007/00; H01B 13/00 20060101 H01B013/00 |
Claims
1. A method for preparing a cathode for use in an electrochemical
cell having the cathode and an anode which is electrochemically
oxidized to form metal ions in the cell upon discharge to generate
electron flow in an external electrical circuit connected to the
cell, wherein the electron flow is generated by intercalation of
the metal ions into a cathode active material comprising the
cathode and the cell is characterized by an ionically conductive
electrolytic solution associated with the anode and the cathode,
which comprises: a) providing a cathode active material comprising
a metallic material, wherein the metallic material comprises from
between about 80 weight percent to about 98 weight percent of the
cathode active material; b) mixing the cathode active material with
a solvent material to form a paste comprising the cathode active
material; c) subjecting the paste to a first pressing step forming
the paste into a cathode sheet which is intercalatable by the metal
ions formed by the anode; d) removing any residual solvent material
from the cathode sheet; e) subjecting the cathode sheet to a
forming means that serves to provide at least one cathode plate
having a variety of geometric shapes; and f) laminating at least
one of the thusly formed cathode plates on at least one side of a
current collector means by subjecting the cathode plate to a second
pressing step to form a laminated cathode component as the cathode
for use in the electrochemical cell, wherein during the laminating
step the cathode active material does not lose its ability to
intercalate metal ions formed by the anode.
2. The method of claim 1 including providing the paste comprising
the cathode material further comprising the addition of binder and
conductor materials.
3. The method of claim 1 including selecting the solvent material
from the group consisting of water and an inert organic
material.
4. The method of claim 1 including providing the cathode active
material in a range from about 0.2 m.sup.2/gram to about 2.6
m.sup.2/gram prior to mixing with the solvent material forming the
paste.
5. The method of claim 1 including forming the paste into the
cathode sheet by feeding the paste through a roll mill as the first
pressing step.
6. The method of claim 5 further including the step of first
feeding the paste into a compaction means that serves to provide
the cathode active material in a pellet form prior to introduction
to the roll mill as the first pressing step.
7. The method of claim 1 including selecting the cathode active
material from the group consisting of silver vanadium oxide, copper
silver vanadium oxide, manganese dioxide, copper vanadium oxide,
titanium disulfide, copper oxide, copper sulfide, iron sulfide,
iron disulfide, and mixtures thereof.
8. The method of claim 1 including providing the cathode sheet
having a thickness in the range of from between about 0.0015 inches
to about 0.020 inches.
9. The method of claim 1 including removing the residual solvent
material from the cathode active material by drying.
10. The method of claim 1 including providing the cathode having a
configuration selected from the group consisting of: SVO/current
collector/CF.sub.x/current collector/SVO, SVO/current
collector/SVO/CF.sub.x/SVO/current collector/SVO, and SVO/current
collector/CF.sub.x, with the SVO facing the lithium anode.
11. A method of powering an implantable medical device with an
electrochemical cell, the cell comprising a lithium anode coupled
to a cathode of a cathode active material activated with an
electrolyte, comprising the steps of: a) providing the anode; b)
providing the cathode of a cathode active material having a surface
area from about 0.2 m.sup.2/gram to about 2.6 m.sup.2/gram; c)
positioning the anode and the cathode inside a casing with an
intermediate separator preventing direct physical contact between
them; activating the anode and the cathode with an electrolyte; e)
connecting a negative terminal and a positive terminal of the cell
to the implantable medical device; f) powering the implantable
medical device with the cell; and g) periodically discharging the
cell to deliver at least one pulse of electrical current of
significantly greater amplitude than that of a pre-pulse current or
open circuit voltage immediately prior to the pulse discharge.
12. The method of claim 11 including selecting the cathode active
material from the group consisting of silver vanadium oxide, copper
silver vanadium oxide, manganese dioxide, copper vanadium oxide,
titanium disulfide, copper oxide, copper sulfide, iron sulfide,
iron disulfide, fluorinated carbon, and mixtures thereof.
13. The method of claim 11 wherein the cathode active material has
a surface area from about 1.6 m.sup.2/gram to about 2.4
m.sup.2/gram.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 11/696,867, filed on Apr. 5, 2007, now U.S. Pat. No. ______ to
Gan et al., which claims priority from U.S. provisional application
Ser. No. 60/791,258, filed on Apr. 12, 2006.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention generally relates to the conversion of
chemical energy to electrical energy. More particularly, this
invention relates to an alkali metal/solid cathode electrochemical
cell having reduced voltage delay and irreversible Rdc growth. A
preferred couple is a lithium/silver vanadium oxide (Li/SVO) cell.
In such cells, voltage delay and permanent or irreversible Rdc
growth typically occur from about 25% to about 70%
depth-of-discharges (DoD). According to the present inventions,
these phenomena are limited by the use of relatively low surface
area cathode active materials. The low surface area active material
is preferably provided in a free-standing sheet form.
[0004] 2. Description of Related Art
[0005] Voltage delay is a phenomenon typically exhibited in an
alkali metal/solid cathode cell, such as of the lithium/silver
vanadium oxide couple (Li/SVO) that has been depleted of about 25%
to 70% of its capacity and that is subjected to high current pulse
discharge applications. It is theorized that in a Li/SVO cell,
vanadium compounds become soluble in the cell electrolyte from the
cathode and are subsequently deposited onto the lithium anode
surface. The resulting anode surface passivation film provides
additional resistance, which leads to cell polarization.
[0006] The voltage response of a cell that does not exhibit voltage
delay during the application of a short duration pulse or pulse
train has distinct features. First, the cell potential decreases
throughout the application of the pulse until it reaches a minimum
at the end of the pulse, and second, the minimum potential of the
first pulse in a series of pulses is higher than the minimum
potential of the last pulse. FIG. 1 is a graph showing an
illustrative discharge curve 10 as a typical or "ideal" waveform of
a cell during the application of a series of pulses as a pulse
train that does not exhibit voltage delay.
[0007] On the other hand, the voltage response of a cell that
exhibits voltage delay during the application of a short duration
pulse or during a pulse train can take one or both of two forms.
One form is that the leading edge potential of the first pulse is
lower than the end edge potential of the first pulse. In other
words, the voltage of the cell at the instant the first pulse is
applied is lower than the voltage of the cell immediately before
the first pulse is removed. The second form of voltage delay is
that the minimum potential of the first pulse is lower than the
minimum potential of the last pulse when a series of pulses have
been applied. FIG. 2 is a graph showing an illustrative discharge
curve 12 as the voltage waveform of a cell that exhibits both forms
of voltage delay.
[0008] Decreased discharge voltages and the existence of voltage
delay are undesirable characteristics of a pulse dischargeable
lithium/solid cathode cell, such as a Li/SVO cell, in terms of
their influence on devices such as implantable medical devices
including pacemakers and automatic implantable cardiac
defibrillators. Depressed discharge voltages and voltage delay are
undesirable because they limit the effectiveness and even the
proper functioning of both the cell and the associated electrically
powered device under current pulse discharge conditions.
[0009] Heretofore, a number of patents have disclosed Li/SVO cells
and various reforming methods and algorithms to minimize
irreversible Rdc growth and voltage delay. For example, U.S. Pat.
No. 6,982,543 to Syracuse et al., which is assigned to the assignee
of the present invention and incorporated herein by reference,
describes methodologies for accurately determining the precise
boundaries of irreversible Rdc growth and voltage delay in the
about 25% to about 70% DoD region of a Li/SVO cell. This is so that
more frequent pulse discharging for the purpose of cell reform is
confined to the limits of the region.
[0010] Additionally, U.S. Pat. No. 6,930,468 to Syracuse et al.,
which is assigned to the assignee of the present invention and
incorporated herein by reference, describes methodologies for
minimizing the occurrence of irreversible Rdc growth and voltage
delay in the about 25% to about 70% DoD region by subjecting Li/SVO
cells to novel discharge regimes. An optimum discharge regime for a
particular cell configuration and electrode material set is
determined by subjecting groups of exemplary cells of a particular
configuration and material set to a range of different discharge
regimes to determine their affects on cell performance.
[0011] Additionally, U.S. Pat. No. 7,026,791 to Palazzo et al.,
which is assigned to the assignee of the present invention and
incorporated herein by reference, describes conditioning
methodologies for minimizing the occurrence of irreversible Rdc
growth and voltage delay in the about 35% to about 70% DoD region
by subjecting Li/SVO cells to alternative novel discharge regimes
consisting of relatively short high current pulses separated by a
relatively short rest period between pulses.
[0012] With these methodologies, energy consumption for cell
reforming may be a significant portion of the overall discharge
capacity. For example, in the embodiments disclosed in the '791
patent of Palazzo et al., up to about 10% DoD may be consumed in
cell reforming.
[0013] Therefore, there remains a need for a lithium/silver
vanadium oxide cell that is dischargeable to deliver the high
capacity needed for powering implantable medical devices and the
like, but that experiences little, if any, irreversible Rdc growth
and voltage delay during pulse discharging, especially at about 25%
to about 70% DoD. It is preferable that such a cell does not
require the use of a complex discharge regime for cell reforming,
nor the process control capability to detect the onset of Rdc
growth and then initiate such a discharge regime. In other words,
there is a need for a cell with minimal irreversible Rdc growth and
voltage delay that is attained solely by the choice of electrode
active materials and structures, rather than by the use of complex
and power consuming discharge regimes.
SUMMARY OF THE INVENTION
[0014] It is therefore an object of this invention to provide an
electrochemical cell comprising a cathode of a relatively low
surface area active material that results in minimal irreversible
Rdc growth and voltage delay.
[0015] According to the present invention, there is provided an
electrochemical cell comprising a lithium anode, a cathode
comprising a cathode active material having a relatively low
surface area, and an electrolyte activating the anode and the
cathode. The cathode active material may be selected from the group
consisting of silver vanadium oxide, copper silver vanadium oxide,
manganese dioxide, copper vanadium oxide, titanium disulfide,
copper oxide, copper sulfide, iron sulfide, iron disulfide,
fluorinated carbon, and mixtures thereof. In one preferred
embodiment, the cathode active material is comprised of silver
vanadium oxide having a surface area from about 0.2 m.sup.2/gram to
about 2.6 m.sup.2/gram, and preferably from about 1.6 m.sup.2/gram
to about 2.4 m.sup.2/gram, in the form of a blank cut from a
free-standing sheet of an active material mixture and contacted to
a current collector.
[0016] Also according to the present invention, there is further
provided a method of controlling irreversible Rdc growth and
voltage delay in an electrochemical cell comprising a lithium anode
coupled to a cathode of a cathode active material activated with an
electrolyte, the method comprising the steps of: providing the
anode, providing the cathode comprised of a free-standing sheet of
cathode active material having a relatively low surface area from
about 0.2 m.sup.2/gram to about 2.6 m.sup.2/gram, and preferably
from about 1.6 m.sup.2/gram to about 2.4 m.sup.2/gram, and
positioning the anode and the cathode in a casing connected to
opposite polarity terminals, and activating them with an
electrolyte.
[0017] The foregoing and additional, objects, advantages, and
characterizing features of the present invention will become
increasingly more apparent upon a reading of the following detailed
description together with the included drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a graph showing an illustrative pulse discharge
waveform or curve 10 of an exemplary electrochemical cell that does
not exhibit voltage delay.
[0019] FIG. 2 is a graph showing an illustrative pulse discharge
waveform or curve 12 of an exemplary electrochemical cell that
exhibits voltage delay.
[0020] FIG. 3 is a graph illustrating the discharge profile of a
typical Li/SVO cell.
[0021] FIG. 4 is a graph constructed from the average background
voltages and minimum pulse voltages of five groups of Li/SVO cells
discharged at 72.degree. C. under a constant load but subjected to
a 10-second current pulse every 15 days at 37.degree. C. and having
a range of cathode active material surface areas.
[0022] FIG. 5 is a graph illustrating the average waveforms of the
10-second current pulse train 1 taken at beginning-of-life for the
cell groups shown in FIG. 4.
[0023] FIG. 6 is a graph illustrating the average waveforms of the
10-second current pulse train 2 taken at about 13% DoD for the cell
groups shown in FIG. 4.
[0024] FIG. 7 is a graph illustrating the average waveforms of the
10-second current pulse train 4 taken at about 33% DoD for the cell
groups shown in FIG. 4.
[0025] FIG. 8 is a graph derived from the data used to construct
the graph of FIG. 7 and relating SVO surface area to voltage delay
during pulse train 4.
[0026] FIG. 9 is a graph illustrating the average waveforms of the
10-second current pulse train 5 taken at about 42% DoD for the cell
groups shown in FIG. 4.
[0027] FIG. 10 is a graph displaying the relationship between SVO
surface area and the P.sub.min voltages data at pulse train 5 for
the cell groups used to construct the graph of FIG. 9.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] In describing the present invention, the following terms are
used.
[0029] The term "surface area" is defined as the BET surface using
nitrogen as the adsorption gas.
[0030] The term percent of depth-of-discharge (DoD) is defined as
the ratio of delivered capacity to theoretical capacity times
100.
[0031] The term "pulse" means a short burst of electrical current
of significantly greater amplitude than that of a pre-pulse current
or open-circuit voltage immediately prior to the pulse. A pulse
train consists of at least one pulse of electrical current. The
pulse is designed to deliver energy, power or current. If the pulse
train consists of more than one pulse, they are delivered in
relatively short succession with or without open circuit rest
between the pulses.
[0032] In performing accelerated discharge testing of a cell, an
exemplary pulse train may consist of one to four 5- to 20-second
pulses (23.2 mA/cm.sup.2) with about a 10 to 30 second rest,
preferably about 15 second rest, between each pulse. A typically
used range of current densities for cells powering implantable
medical devices is from about 15 mA/cm.sup.2 to about 50
mA/cm.sup.2, and more preferably from about 18 mA/cm.sup.2 to about
35 mA/cm.sup.2. Typically, a 10-second pulse is suitable for
medical implantable applications. However, it could be
significantly shorter or longer depending on the specific cell
design and chemistry and the associated device energy requirements.
Current densities are based on square centimeters of the cathode
electrode.
[0033] An electrochemical cell that possesses sufficient energy
density and discharge capacity required to meet the vigorous
requirements of implantable medical devices comprises an anode of
lithium. An alternate anode comprises a lithium alloy such as a
lithium-aluminum alloy. The greater the amounts of aluminum present
by weight in the alloy, however, the lower the energy density of
the cell.
[0034] The form of the anode may vary, but preferably it is a thin
metal sheet or foil of lithium metal, pressed or rolled on a
metallic anode current collector, i.e., preferably comprising
titanium, titanium alloy or nickel. Copper, tungsten and tantalum
are also suitable materials for the anode current collector. The
anode current collector has an extended tab or lead 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 for a low surface cell design.
[0035] The electrochemical cell of the present invention further
comprises a cathode of electrically conductive material that serves
as the counter electrode. The cathode is preferably of solid
materials having the general formula SM.sub.xV.sub.2O.sub.y where
SM is a metal selected from Groups IB to VIIB and VIIIB of the
Periodic Table of Elements, and 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 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.
[0036] Another preferred composite transition metal oxide cathode
material includes V.sub.2O.sub.z wherein z.ltoreq.5 combined with
Ag.sub.2O with silver in either the silver(II), silver(I) or
silver(0) oxidation state and CuO with 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, the 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.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. No. 5,472,810 to Takeuchi
et al. and U.S. Pat. No. 5,516,340 to Takeuchi et al., both of
which are assigned to the assignee of the present invention and
incorporated herein by reference.
[0037] Other useful cathode active materials include manganese
dioxide, copper vanadium oxide, titanium disulfide, copper oxide,
copper sulfide, iron sulfide, iron disulfide, fluorinated carbon,
and mixtures thereof. Preferred fluorinated carbon compounds are
represented by the formula (CF.sub.x).sub.n wherein x varies
between about 0.1 to 1.9 and preferably between about 0.5 and 1.2,
and (C.sub.2F).sub.n wherein the n refers to the number of monomer
units which can vary widely.
[0038] Before fabrication into an electrode for incorporation into
an electrochemical cell, the cathode active material is 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,
stainless steel, and mixtures thereof. The preferred cathode active
mixture thus includes a powdered fluoro-polymer binder present at a
quantity of at least about 3 weight percent, a conductive diluent
present at a quantity of at least about 3 weight percent and from
about 80 to about 98 weight percent of the cathode active
material.
[0039] Cathode components for incorporation into the cell may be
prepared by rolling, spreading or pressing the cathode active
mixture onto a suitable current collector selected from the group
consisting of stainless steel, titanium, tantalum, platinum, gold,
aluminum, cobalt nickel alloys, highly alloyed ferritic stainless
steel containing molybdenum and chromium, and nickel-, chromium-,
and molybdenum-containing alloys. For a silver vanadium oxide or
copper silver vanadium oxide cathode, the current collector is
preferably of aluminum or titanium with the latter being
preferred.
[0040] A preferred method of cathode preparation is by contacting a
blank cut from a free-standing sheet of cathode active material to
a current collector. Blank preparation starts by taking granular
silver vanadium oxide and adjusting its particle size to a useful
range in an attrition or grinding step. In one preferred
embodiment, the SVO is "high temperature" silver vanadium oxide
(ht-SVO), prepared according to the methods described in U.S. Pat.
No. 6,566,007 to Takeuchi et al. This patent is assigned to the
assignee of the present invention and incorporated herein by
reference. The preparation protocols of the '007 patent to Takeuchi
et al. result in SVO having a surface area of from about 0.2
m.sup.2/gram to about 0.8 m.sup.2/gram. This material can be used
as is or subsequently subjected to an attriting step to arrive at a
desired surface area for cathode sheet preparation. If an attrited
active material is desired, a ball mill or vertical ball mill is
preferred and typical grinding time ranges from between about 10 to
15 minutes. Preferably, attriting results in an active material
having a surface area up to about 2.6 m.sup.2/gram.
[0041] In any event, the finely divided active material is then
preferably mixed with carbon black and/or graphite as conductive
diluents and a powder fluoro-resin such as polytetrafuoroethylene
powder as a binder material to form a depolarizer admixture. This
is typically done in a solvent of either water or an inert organic
medium such as mineral spirits. The mixing process provides for
fibrillation of the fluoro-resin to ensure material integrity.
After mixing sufficiently to ensure homogeneity in the admixture,
the active admixture is removed from the mixer as a paste.
[0042] Following the mixing step, the solvent is vacuum filtered
from the paste to adjust the solvent content to about 0.25 cc to
about 0.35 cc per gram of solids, i.e., the solids comprising the
electrode active material (SVO), the conductive diluent and the
binder. The resulting filter cake is fed into a series of roll
mills that compact the active admixture into a thin sheet having a
tape form, or the active filter cake is first run through a
briquette mill. In the latter case, the active admixture is formed
into small pellets which are then fed into the roll mills.
[0043] Typically, the compacting step is performed by roll mills
comprising two to four calender mills that serve to press the
admixture between rotating rollers to provide a free-standing sheet
of the active material as a continuous tape. In a preferred method,
cathodes are made from blanks prepared as described in U.S. Pat.
No. 6,582,545 to Thiebolt III et al. This patent is assigned to the
assignee of the present invention and incorporated herein by
reference. It teaches that the basis weight of an electrode active
admixture such as one including silver vanadium oxide is formed
into an electrode structure from an admixture paste subjected to a
calendering process using a secondary calendering step performed in
a direction reverse or orthogonal to that used to form the initial
sheet tape. Orthogonal or reverse feed of the electrode active
admixture provides for fibrillation of the fluoro-polymeric binder
in other than the initial direction. This lets the binder spread in
directions transverse to the initial direction. In a broader sense,
however, the secondary step is in any direction other than the
first direction to provide the electrode active sheet tape having a
second thickness less than the first thickness. It is believed that
when the electrode active admixture is calendered in a single
direction the binder is fibrillated to an extent near its maximum
tensile strength. If the electrode active sheet tape is calendered
in a secondary direction, the active admixture spreads in
directions other than, and preferably transverse to, the initial
direction. Accordingly, the secondary calendering step forms a
thinner sheet tape with a broader footprint having a lower basis
weight, defined as grams/in.sup.2 of the cathode active admixture,
than the sheet material formed from the primary calendering.
Preferably, the electrode active sheet tape comprises the active
material having a basis weight of less than about 340
mg/in.sup.2.
[0044] The tape preferably has a thickness in the range of from
about 0.0015 inches to about 0.020 inches. The outer edges of the
tape leaving the rollers are trimmed and the resulting tape is
subsequently subjected to a drying step under vacuum conditions.
The drying step serves to remove any residual solvent and/or water
from the active material. Alternatively, the process can include
the drop wise addition of a liquid electrolyte into the active
mixture prior to the initial calendering step to enhance the
performance and rate capacity of an assembled electrochemical cell.
The active sheet tape can be stored for later use, or fed on a
conveyor belt to a punching machine. The punching operation forms
the sheet tape into active blanks of any dimension needed for
preparation of an electrode component for use in a high energy
density electrochemical cell.
[0045] U.S. Pat. Nos. 5,435,874 and 5,571,640, both to Takeuchi et
al., describe the preparation of a cathode component by an SVO
sheeting process. These Takeuchi et al. patents are also assigned
to the assignee of the present invention and incorporated herein by
reference. Prior to the present invention, the Takeuchi et al.
sheeting process used an SVO material having a surface area range
of from about 2.65 m.sup.2/gram to about 3.5 m.sup.2/gram. However,
it has now been discovered that a reduced surface area as described
herein results in improved Rdc and voltage delay performance from
this active material.
[0046] Alternate preparation techniques are shown in U.S. Pat. Nos.
4,830,940 and 4,964,877, both to Keister et al., which describe
manufacturing a cathode by pressing a powdered admixture of SVO,
conductive diluent and binder material onto a current collector.
The above Keister et al. patents are assigned to the assignee of
the present invention and incorporated herein by reference.
Nonetheless, cathode blanks prepared as described above 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".
[0047] In one embodiment, the cathode has one of the above active
materials, for example SVO, as a blank cut from a free-standing
sheet and contacted to both sides of the cathode current collector.
In another embodiment, the cathode has a sandwich design as
described in U.S. Pat. No. 6,551,747 to Gan. The sandwich cathode
design comprises a first active material of a relatively high
energy density but a relatively low rate capability in comparison
to a second cathode active material. Fluorinated carbon is a
preferred first cathode active material. One preferred second
active material is silver vanadium oxide. Another is the previously
described copper silver vanadium oxide. Preferably, both, but at
least the high rate active material is in the form of a blank cut
from a free-standing sheet contacted to a current collector.
[0048] One exemplary sandwich cathode electrode has the following
configuration: SVO/current collector/CF/current collector/SVO.
[0049] Another exemplary sandwich cathode electrode configuration
is: SVO/current collector/SVO/CF.sub.x/SVO/current
collector/SVO.
[0050] Still another configuration for an electrochemical cell with
a sandwich electrode has a lithium anode and a cathode
configuration of: SVO/current collector/CF.sub.x, with the SVO
facing the lithium anode.
[0051] In a broader sense, it is contemplated by the scope of the
present invention that the second active material of the sandwich
cathode design is any material, which has a relatively lower energy
density but a relatively higher rate capability than the first
active material. In that respect, other than silver vanadium oxide
and copper silver vanadium oxide, V.sub.2O.sub.5, MnO.sub.2,
LiCoO.sub.2, LiNiO.sub.2, LiMn.sub.2O.sub.4, TiS.sub.2, CuS, FeS,
FeS.sub.2, CuO, copper vanadium oxide (CVO), and mixtures thereof
are useful as the second active material. And, in addition to
fluorinated carbon, Ag.sub.2O, Ag.sub.2O.sub.2, CuF,
Ag.sub.2CrO.sub.4, MnO.sub.2, and even SVO itself, are useful as
the second active material. The theoretical volumetric capacity
(Ah/ml) of CF.sub.x is 2.42, Ag.sub.2O.sub.2 is 3.24, Ag.sub.2O is
1.65 and AgV.sub.2O.sub.5.5 is 1.37. Thus, CF.sub.x,
Ag.sub.2O.sub.2, Ag.sub.2O, all have higher theoretical volumetric
capacities than that of SVO.
[0052] In order to prevent internal short circuit conditions, the
cathode is physically segregated from the lithium anode by a
separator. The separator is of electrically insulative material
that 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 there through of the electrolyte
during the electrochemical reactions of the cell. Illustrative
separator materials include fabrics woven from fluoropolymeric
fibers including polyvinylidine fluoride,
polyethylenetetrafluoroethylene, and
polyethylenechlorotrifluoroethylene used either alone or laminated
with a fluoropolymeric microporous film, non-woven glass,
polypropylene, polyethylene, glass fiber materials, ceramics, a
polytetrafluoroethylene membrane commercially available under the
designation ZITEX.RTM. (Chemplast Inc.), a polypropylene membrane
commercially available under the designation CELGARD.RTM. (Celanese
Plastic Company, Inc.), a membrane commercially available under the
designation DEXIGLAS.RTM. (C. H. Dexter, Div., Dexter Corp.), and a
membrane commercially available under the designation
TONEN.RTM..
[0053] The electrochemical cell of the present invention further
includes a nonaqueous, ionically conductive electrolyte serving as
a medium for migration of ions between the anode and the cathode
electrodes during electrochemical reactions of the cell. The
electrochemical reaction at the electrodes involves conversion of
ions in atomic or molecular forms that migrate from the anode to
the cathode. Thus, suitable nonaqueous electrolytes are
substantially inert to the anode and cathode materials, and they
exhibit those physical properties necessary for ionic transport,
namely, low viscosity, low surface tension and wettability.
[0054] A suitable electrolyte has an inorganic, ionically
conductive lithium salt dissolved in a mixture of aprotic organic
solvents comprising a low viscosity solvent and a high permittivity
solvent. Preferred lithium salts include LiPF.sub.6, LiBF.sub.4,
LiAsF.sub.6, LiSbF.sub.6, LiClO.sub.4, LiO.sub.2, LiAlCl.sub.4,
LiGaCl.sub.4, LiC(SO.sub.2CF.sub.3).sub.3,
LiN(SO.sub.2CF.sub.3).sub.2, LiSCN, LiO.sub.3SCF.sub.3,
LiC.sub.6FSO.sub.3, LiO.sub.2CCF.sub.3, LiSO.sub.6F,
LiB(C.sub.6H.sub.5).sub.4, LiCF.sub.3SO.sub.3, and mixtures
thereof.
[0055] Low viscosity solvents useful with the present invention
include esters, linear and cyclic ethers and dialkyl carbonates
such as tetrahydrofuran (THF), methyl acetate (MA), diglyme,
trigylme, tetragylme, dimethyl carbonate (DMC), 1,2-dimethoxyethane
(DME), 1,2-diethoxyethane (DEE), 1-ethoxy,2-methoxyethane (EME),
ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl
carbonate, diethyl carbonate, dipropyl carbonate, and mixtures
thereof. High permittivity solvents include cyclic carbonates,
cyclic esters and cyclic amides such as propylene carbonate (PC),
ethylene carbonate (EC), butylene carbonate, acetonitrile, dimethyl
sulfoxide, dimethyl formamide, dimethyl acetamide,
.gamma.-valerolactone, .gamma.-butyrolactone (GBL),
N-methyl-2-pyrrolidone (NMP), and mixtures thereof. In the present
invention, the preferred electrolyte for a Li/SVO cell is 0.8M to
1.5M LiAsF.sub.6 or LiPF.sub.6 dissolved in a 50:50 mixture, by
volume, of propylene carbonate and 1,2-dimethoxyethane.
[0056] The preferred form of the electrochemical cell is a
case-negative design wherein the anode/cathode couple is inserted
into a conductive metal casing connected to the anode current
collector, as is well known to those skilled in the art. A
preferred material for the casing is stainless steel, although
titanium, mild steel, nickel, nickel-plated mild steel and aluminum
are also suitable. The casing header comprises a metallic lid
having a sufficient number of openings to accommodate the
glass-to-metal seal/terminal pin feedthrough for the cathode. The
anode is preferably connected to the case or the lid. An additional
opening is provided for electrolyte filling. The casing header
comprises elements having compatibility with the other components
of the electrochemical cell and is resistant to corrosion. The cell
is thereafter filled with the electrolyte solution described
hereinabove and hermetically sealed, such as by close-welding a
stainless steel plug over the fill hole, but not limited thereto.
The cell of the present invention can also be constructed in a
case-positive design.
[0057] As shown in FIG. 3, the background discharge profile of a
typical Li/SVO cell consists of four regions: regions 1 and 3 are
referred to as the plateau regions while regions 2 and 4 are
transition regions. Lithium/silver vanadium oxide cells generally
have stable internal resistance (Rdc) in regions 1 and 2. When
cathodes are prepared from an SVO powder process as described in
the previously referenced Keister et al. patents, the initiation
point of irreversible Rdc growth and voltage delay is typically
found at the beginning of region 3, or around a background voltage
of about 2.6V. However, when SVO cathodes are prepared using the
sheet process of the previously referenced Takeuchi et al. patents,
initiation of irreversible Rdc growth and voltage delay are
typically found in the middle of region 2, or at a background
voltage in the range of about 2.8V to 2.9V. This means that even
though irreversible Rdc growth and voltage delay are a function of
cathode processing, they are not typically observed until the
latter parts of region 2 to the beginning of region 3. This
correlates to about 25% to 40% DoD.
[0058] Voltage delay and Rdc growth are impacted by cell design
parameters including electrode geometry and selection of active
materials. Without wishing to be bound by any particular theory, it
is believed that voltage delay in lithium/solid cathode cells, for
example Li/SVO cells, is caused when a film forms on the anode,
causing the initial pulse voltage to drop substantially. It is
thought that this film forms as a result of vanadium dissolution
from the silver vanadium cathode active material, which is then
reductively deposited on the anode surface. The vanadium forms a
passivating film which increases Rdc.
[0059] From extensive accelerated discharge studies with Li/SVO
cells, it has been discovered that by adjusting the surface area of
the cathode active material facing the anode, the magnitude of
growth of the solid-electrolyte interphase (SEI) passivating film
can be limited. Alternately, to the extent that a passivating film
does grow, it has a structure that allows for sufficient lithium
ion migration there through so that Rdc and voltage delay are
limited to acceptable levels and overall discharge performance is
not significantly degraded. For SVO, the surface area is from about
0.2 m.sup.2/gram to about 2.6 m.sup.2/gram, and preferably from
about 1.6 m.sup.2/gram to about 2.4 m.sup.2/gram.
[0060] A cardiac defibrillator essentially consists of an
electrochemical cell as a power source for charging at least one
electrolytic capacitor to deliver an electrical shock therapy to
the patients heart. Microprocessors powered by the cell perform
heart sensing and pacing. These functions require electrical
current of about 1 microampere to about 100 milliamperes. 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 therapy to the heart to treat a
tachyarrhythmia, the irregular, rapid heartbeats that can be fatal
if left uncorrected. This requires electrical current of about 1
ampere to about 4 amperes.
[0061] In automatic implantable cardiac defibrillator applications,
one very important parameter is the charge time to achieve a
pre-determined energy for therapy delivery. In other words, the
time to charge a capacitor to a required voltage is affected by Rdc
growth and voltage delay. A typical automatic implantable cardiac
defibrillator requires energy in the range of from about 20 Joules
to about 90 Joules per Li/SVO cell for electrical shock therapy.
The relationship is shown below:
Energy(J)=I(amp).times.V(volt).times.t(sec.)
Therefore, t(s)=energy(J)/I.times.V.
[0062] If the required delivered energy (J) and pulsing current
(amp) are both defined, then the charge time in seconds is
inversely proportional to the average voltage under pulsing. To
maintain a relatively low charge time, the cell must deliver higher
voltage under pulsing. This requirement is, however, compromised by
the previously discussed irreversible Rdc growth and voltage delay
phenomena in the Li/SVO system possibly beginning at the middle of
discharge life region 2 (starting at about 25% DoD). Under severe
conditions, cell voltage under pulsing becomes so low that the
charge time is considered too long for the required therapy. Since
irreversible Rdc growth and voltage delay start at about 25% to 40%
DoD, it is possible that only about 25% of the theoretical capacity
of a particular Li/SVO cell (region 1 and a portion of region 2)
may actually be delivered. The remaining capacity (regions 3 and 4)
is wasted, which translates into a shorter device service life.
[0063] Not only do Li/SVO cells experience irreversible Rdc growth
and voltage delay problems beginning at about 25% DoD, but
electrolytic capacitors can experience degradation in their
charging efficiency after long periods of inactivity. It is
believed that the anodes of electrolytic capacitors, which are
typically of aluminum or tantalum, develop micro-fractures in their
dielectric oxides after extended periods of non-use. These
micro-fractures consequently result in extended charge times and
reduced breakdown voltages. Degraded charging efficiency ultimately
requires that a Li/SVO cell progressively expend more and more
energy to charge the capacitors for providing therapy.
[0064] To repair this degradation, microprocessors controlling the
automatic implantable cardiac defibrillator are programmed to
regularly charge the electrolytic capacitors to or near a
maximum-energy breakdown voltage (the voltage corresponding to
maximum energy) before discharging them internally through a
non-therapeutic load. The capacitors can be immediately discharged
once the maximum-energy voltage is reached or they can be held at
maximum-energy voltage for a period of time, which can be rather
short, before being discharged. These periodic charge-discharge or
charge-hold-discharge cycles for capacitor maintenance are called
"reforms." Reforming automatic cardiac implantable defibrillator
capacitors at least partially restores and preserves their charging
efficiency.
[0065] An industry-recognized standard is to reform implantable
capacitors by pulse discharging the connected electrochemical cell
about once every 90 to 180 days throughout the useful life of the
medical device, which is typically dictated by the life of the
cell. However, during the irreversible Rdc growth and voltage delay
region at about 25% to 70% DoD, it is desirable to pulse discharge
the Li/SVO cell at a more frequent rate, as taught by the
previously discussed U.S. Pat. No. 6,930,468 to Syracuse et al.,
U.S. Pat. No. 6,982,543 to Syracuse et al. and U.S. Pat. No.
7,026,791 to Palazzo et al. The reason for this more frequent pulse
discharging is to break up the passivation layer forming at the
solid-electrolyte-interphase (SEI) at the anode so that when the
medical device is required to charge the capacitors during device
activation mode, the charge time is relatively low. The consequence
is that more useful energy is expended for non-therapeutic
applications by these more frequent pulse discharges, even more
than is needed for capacitor reform.
[0066] Thus, the basis for the present invention is driven by the
desire to substantially reduce, if not completely eliminate,
irreversible Rdc growth and voltage delay in a Li/SVO cell while at
the same time maintaining sufficient discharge capacity to
periodically reform the connected capacitors to maintain them at
their rated breakdown voltages. That is without using one of the
previously described energy-consuming cell reforming methodologies
described in Syracuse et al. '468 and '543 patents and the Palazzo
et al. '791 patent. Instead, in the present invention, the
reduction in irreversible Rdc growth and voltage delay is
accomplished by providing the cathode comprising a blank cut from a
free-standing sheet of the active material mixture contacted to a
current collector. The active material has a relatively low surface
area active material from about 0.2 m.sup.2/gram to about 2.6
m.sup.2/gram, and preferably from about 1.6 m.sup.2/gram to about
2.4 m.sup.2/gram. This means that the cell is usable well into its
DoD range without the need for subjecting the cell to comparatively
more energy-consuming cell reforming pulse sequences in lieu of the
more conservative capacitor reforming pulse sequences. In spite of
there being less total cathode active material surface area
available for lithium ion intercalation, the useful life of a low
surface area cathode active material cell is extended beyond that
of a higher surface area cathode active material cell.
[0067] Accordingly, the following examples describe the manner and
process of an electrochemical cell according to the present
invention, and they set forth the best mode contemplated by the
inventors of carrying out the invention, but they are not to be
construed as limiting.
EXAMPLE I
[0068] As shown in Table 1, a set of Li/SVO cells were built
comprising a cathode of a blank cut from a free-standing sheet of
the active material mixture contacted to a current collector. The
cathode active material had different ht-SVO active material
surface areas. Each group included five identical cells.
TABLE-US-00001 TABLE 1 Surface Area Related Reference Numerals In:
Group (m.sup.2/gram) FIG. 4 FIG. 6 FIG. 7 FIG. 9 A 2.0 22 32 42 54
B 2.4 24 34 44 56 C 2.8 26 36 46 58 D 3.3 28 38 48 60 E 3.8 30 40
50 62
[0069] Each cell was discharged at 72.degree. C. under a constant
load of 11.4 k.OMEGA. for 6 months to 100% DoD. The cells were also
subjected to a single 10-second, 3.67 amp pulse every 15 days
administered at 37.degree. C., resulting in a current density of 40
mA/cm.sup.2. The average discharge data from selected pulse train
waveforms are listed in Table 2 and displayed in FIGS. 5 to 7 and
9.
TABLE-US-00002 TABLE 2 Background Voltage SVO Surface Pulse Voltage
P1 min Delay Group Area (m.sup.2/g) Train % DOD (mV) (mV) (mV) A
2.0 1 2 3198 2129 0 B 2.4 1 2 3199 2181 0 C 2.8 1 2 3197 2156 0 D
3.3 1 2 3204 2140 0 E 3.8 1 1 3206 2135 0 A 2.0 2 12 3165 1971 7 B
2.4 2 13 3157 1992 47 C 2.8 2 13 3152 1976 62 D 3.3 2 13 3160 1900
113 E 3.8 2 12 3167 1867 146 A 2.0 4 32 2823 1808 0 B 2.4 4 34 2685
1321 444 C 2.8 4 35 2640 1332 335 D 3.3 4 33 2677 1185 484 E 3.8 4
33 2687 994 658 A 2.0 5 40 2603 1483 62 B 2.4 5 42 2574 1266 140 C
2.8 5 44 2566 1096 69 D 3.3 5 42 2574 845 0 E 3.8 5 41 2575 621
0
[0070] FIG. 4 shows the average background voltages 20 and the
respective average minimum pulse (P.sub.min) voltages 22, 24, 26,
28 and 30 for each of the five surface area groups of Table 1. The
average background voltages 20 exhibited the typical discharge
regions previously described and shown in FIG. 3. The average
P.sub.min voltages for the five groups exhibited Rdc growth in the
range of about 30% DoD to about 40% DoD, with a minimum value of
P.sub.min occurring in region 2 at about 40% to 50% DoD. What is
noteworthy is that for the lowest surface area cells represented by
data sets 22, 24, the average Rdc growth is reversible, while for
the highest surface area cells represented by data set 30, the
average Rdc growth is irreversible. The average P.sub.min voltages
22, 24 for the two lowest surface area cell groups recovered from
their troughs at about 35% to 45% DoD, and then followed a
trajectory that was comparable to the background voltage curve 20.
The P.sub.min voltage 30 for the highest surface area cell showed
virtually no recovery, in spite of the continued pulsing that was
administered for the purpose of capacitor reforming. The P.sub.min
voltages 26 and 28 of the intermediate surface area cells showed
varying degrees of reversal of Rdc growth, with the trend clearly
being that cells of lower surface areas showed greater degrees of
recovery.
[0071] FIG. 5 is graph illustrating the waveform of the 10-second
current pulses of a selected cell from each of the groups A to E
shown in FIG. 4 taken at beginning-of-life (BOL) (i.e., pulse train
1) for the groups of cells shown in FIG. 4. This graph shows that
there is little difference in the waveforms of the respective
groups at this early stage of discharge.
[0072] FIG. 6 then shows that as early as the second pulse train at
about 13% DoD, voltage delay is observed and its magnitude tracks
directly with SVO surface area. The trend relating SVO surface area
to voltage delay at the second pulse train indicates that a surface
area of about 1.9 m.sup.2/gram would result in zero voltage
delay.
[0073] FIG. 7 then displays that significant voltage delay is
observed at the 4.sup.th pulse train at about 33% DoD. The
distinction between groups is magnified with the highest surface
area groups 48, 50 clearly performing the worst and the lowest
surface area group 42 performing the best.
[0074] As shown in FIG. 8, the trend curve 52 relating SVO surface
area to voltage delay at the fourth pulse train indicates that at
an SVO surface area of about 1.6 m.sup.2/g would result in zero
voltage delay.
[0075] In FIG. 9, by the fifth pulse train at about 42% DoD the
waveforms demonstrate that all cell groups are experiencing some
degree of Rdc growth, with the higher surface area groups 58, 60
and 62 displaying lower pulse voltages.
[0076] The graph in FIG. 10 displays the relationship between SVO
surface area and the P.sub.min voltages for groups A to E at the
fifth pulse train with a very good resulting correlation
coefficient of 0.9989. The implication derived from this graph is
that as the attrited surface area trends towards 0.2 m.sup.2/gram,
the pulse minimum voltage increases. While there is some limit to
this, a surface area range of from about 0.2 m.sup.2/gram to about
2.6 m.sup.2/gram, and preferably from about 1.6 m.sup.2/gram to
about 2.4 m.sup.2/gram, is useful in a sheeting process to provide
a cathode electrode. When combined with a lithium anode, the
resulting cell has decreased irreversible Rdc and reduced voltage
delay.
[0077] While this invention has been described in conjunction with
preferred embodiments thereof, it is evident that many
alternatives, modifications, and variations will be apparent to
those skilled in the art. Accordingly, it is intended to embrace
all such alternatives, modifications and variations that fall
within the spirit and broad scope of the appended claims.
* * * * *