U.S. patent application number 13/987923 was filed with the patent office on 2017-05-04 for lithium ion battery capable of being discharged to zero volts.
The applicant listed for this patent is Clay Kishiyama, Mikito Nagata, Hiroshi Nakahara, Tiehua Piao, Hisashi Tsukamoto. Invention is credited to Clay Kishiyama, Mikito Nagata, Hiroshi Nakahara, Tiehua Piao, Hisashi Tsukamoto.
Application Number | 20170125838 13/987923 |
Document ID | / |
Family ID | 52666626 |
Filed Date | 2017-05-04 |
United States Patent
Application |
20170125838 |
Kind Code |
A9 |
Tsukamoto; Hisashi ; et
al. |
May 4, 2017 |
LITHIUM ION BATTERY CAPABLE OF BEING DISCHARGED TO ZERO VOLTS
Abstract
A lithium ion battery particularly configured to be able to
discharge to a very low voltage, e.g. zero volts, without causing
permanent damage to the battery. More particularly, the battery is
configured to define a Zero Volt Crossing Potential (ZCP) which is
lower than a Damage Potential Threshold (DPT).
Inventors: |
Tsukamoto; Hisashi; (Santa
Clarita, CA) ; Kishiyama; Clay; (San Francisco,
CA) ; Nagata; Mikito; (Saugus, CA) ; Nakahara;
Hiroshi; (Santa Clarita, CA) ; Piao; Tiehua;
(Valencia, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Tsukamoto; Hisashi
Kishiyama; Clay
Nagata; Mikito
Nakahara; Hiroshi
Piao; Tiehua |
Santa Clarita
San Francisco
Saugus
Santa Clarita
Valencia |
CA
CA
CA
CA
CA |
US
US
US
US
US |
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|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20150074987 A1 |
March 19, 2015 |
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|
Family ID: |
52666626 |
Appl. No.: |
13/987923 |
Filed: |
September 16, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11978081 |
Oct 25, 2007 |
8535831 |
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13987923 |
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10217967 |
Aug 13, 2002 |
7993781 |
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11978081 |
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09675287 |
Sep 29, 2000 |
6596439 |
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10217967 |
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60199895 |
Apr 26, 2000 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/661 20130101;
H01M 10/0525 20130101; H01M 4/133 20130101; H01M 10/058 20130101;
Y02E 60/10 20130101; H01M 4/131 20130101; H01M 4/662 20130101; Y02P
70/50 20151101; Y10T 29/49108 20150115; Y10T 29/4911 20150115; A61N
1/378 20130101 |
International
Class: |
H01M 10/058 20060101
H01M010/058; H01M 4/66 20060101 H01M004/66; H01M 4/131 20060101
H01M004/131; A61N 1/378 20060101 A61N001/378 |
Claims
1. A method of manufacturing a battery, comprising: providing a
positive electrode and a negative electrode, said positive
electrode comprising a metal substrate having a first active
material comprising lithium formed thereon and said negative
electrode comprising a metal substrate having a second active
material comprising carbon formed thereon, wherein said positive
and negative electrodes define a Zero Volt Crossing Potential (ZCP)
relative to a reference level when the voltage between said
electrodes is zero; providing an electrolyte wherein said negative
electrode can react with said electrolyte to form a solid
electrolyte interface (SEI) layer, said SEI layer being susceptible
of damage when a voltage potential exceeding a Film Dissolution
Potential (FDP) is applied thereto, and wherein said FDP is lower
than the maximum positive operating potential of the battery;
wherein said positive and negative electrodes are selected to
establish ZCP at a lower level than FDP to prevent dissolution of
said SEI layer during storage at a predetermined temperature.
2. The method of claims 1 wherein said negative electrode substrate
is susceptible of permanent damage when a potential exceeding a
Substrate Dissolution Potential (SDP) is applied thereto; and
wherein said positive and negative electrodes are selected and
configured to establish ZCP at a lower level than SDP in order to
prevent dissolution of the negative substrate during storage at
said predetermined temperature.
3. The method of claim 1 wherein said negative electrode substrate
is formed of a material selected from the group consisting of
titanium and titanium alloy.
4. The method of claim 1 wherein said negative electrode substrate
is formed of stainless steel.
5. The method of claim 1 wherein said negative electrode substrate
is formed of a material selected from the group consisting of
nickel and nickel alloy.
6. The method of claim 1 wherein said positive electrode active
material comprises cobalt.
7. The method of claim 6 wherein said positive electrode active
material further comprises nickel.
8. The method of claim 1 wherein said positive electrode active
material consists of an oxide.
9. The method of claim 8 wherein said oxide comprises lithium,
nickel, and cobalt.
10. The method of claim 1 wherein said negative electrode active
material consists of carbon.
11. The method of claim 1 wherein said electrolyte consists of a
liquid electrolyte.
12. The method of claim 11 wherein said liquid electrolyte
comprises a lithium salt dissolved in EC:DEC.
13. The method of claim 12 wherein said lithium salt is
LiPF.sub.6.
14. The method of claim 1 wherein said predetermined temperature is
body temperature.
15. The method of claim 1 wherein said predetermined temperature is
37.degree. C.
16. The method of claim 1 wherein said predetermined temperature is
25.degree. C.
17. The method of claim 1 further comprising providing a battery
management circuit that attempts to stop battery discharge when the
battery voltage reaches 2.5 V.
18. The method of claim 1 wherein said ZCP is greater than about 3
V vs. Li/Li.sup.+.
19. The method of claim 1 wherein said positive electrode has a
positive discharge curve having a negative slope over most of said
positive discharge curve, wherein said negative slope is more
negative than the negative slope over most of the discharge curve
of LiCoO.sub.2.
20. The method of claim 1 further comprising the step of: housing
said positive and negative electrodes in a case, wherein said case
is configured for implanting in a patient's body.
21. The method of claim 20 further comprising the step of
hermetically sealing the case.
22. The method of claim 20 wherein said case has a volume of less
than 30 cc.
23. A method for making a rechargeable lithium ion battery
comprising: providing a positive electrode comprising a metal
substrate having a first active material comprising lithium formed
thereon; providing a negative electrode comprising a substrate
selected from the group consisting of titanium and titanium alloy
having a second active material comprising carbon formed thereon;
said negative electrode being susceptible of damage when a voltage
exceeding a Damage Potential Threshold (DPT) is applied thereto,
and wherein said DPT is lower than the maximum positive operating
potential of the battery; said positive and negative electrodes
defining a Zero Volt Crossing Potential (ZCP) relative to a
reference level when the voltage between said electrodes is zero;
and wherein said positive and negative electrodes are selected and
configured to define a value of ZCP which is less than the value of
DPT at a predetermined temperature.
24. The method of claim 23, wherein the first active material
comprises cobalt.
25. The method of claim 24, wherein the first active material
further comprises nickel.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/978,081, filed on Oct. 25, 2007, entitled
Lithium-Ion Battery Capable of Being Discharged to Zero Volts; U.S.
patent application Ser. No. 11/978,081 is a continuation of U.S.
patent application Ser. No. 10/217,967, filed on Aug. 13, 2002,
entitled Method for Making a Lithium-Ion Battery Capable of Being
Discharged to Zero Volt, now issued as U.S. Pat. No. 7,993,781;
which is a divisional of U.S. patent application Ser. No.
09/675,287, filed Sep. 29, 2000, entitled Lithium-Ion Battery
Capable of being discharged to Zero Volts, now issued as U.S. Pat.
No. 6,596,439 B1; which claims the benefit of U.S. Provisional
Application 60/199,895 filed Apr. 26, 2000; each of which is
incorporated herein in its entirety.
FIELD
[0002] This invention relates generally to rechargeable electric
batteries particularly suited for applications, e.g., implanted
medical devices, where a battery cannot be easily replaced. More
particularly, the invention relates to rechargeable lithium
batteries configured to tolerate deep discharging to zero volts
without permanently damaging the battery's energy storing
capacity.
BACKGROUND
[0003] Rechargeable electric batteries are employed in a wide range
of applications, e.g., consumer products, medical devices, and
aerospace/military systems, which respectively impose different
performance requirements. In some applications, e.g., implanted
medical devices, it is important that the battery be able to
reliably maintain its performance characteristics over a long
useful life despite extended periods of inactivity. Implanted
medical device applications impose special requirements on a
battery because the medical device needs to be highly reliable to
perform critical tasks, the battery may remain inactive and
uncharged for extended periods, e.g., several months, and it is
difficult and/or expensive to replace a battery. Analogous
conditions exist in various aerospace/military applications. For
example, a rechargeable battery may be deployed to power a
satellite in deep space where it cannot be replaced and must be
able to operate over a long life under varying conditions,
including long periods of inactivity. Military applications often
demand similar performance specifications since military hardware
can be unused for several months but must remain ready to be
activated. Current battery technology requires stored batteries to
be charged every few months to avoid a permanent reduction in
energy storing capacity.
[0004] In order to avoid unnecessary surgery to replace a damaged
battery in an implanted medical device, it is desirable that a
battery perform reliably over a very long life, i.e., several
years, under a variety of conditions. Such conditions can include
extended periods of non-use which may allow the battery to deeply
self discharge to zero volts. It is typical for prior art
rechargeable lithium batteries to suffer a permanent capacity loss
after discharging below 2.5 volts. To avoid such capacity loss, it
is important to regularly charge prior art lithium batteries.
[0005] Existing rechargeable lithium batteries typically consist of
a case containing a positive electrode and a negative electrode
spaced by a separator, an electrolyte, and feedthrough pins
respectively connected to the electrodes and extending externally
of the case. Each electrode is typically formed of a metal
substrate that is coated with a mixture of an active material, a
binder, and a solvent. In a typical battery design, the electrodes
comprise sheets which are rolled together, separated by separator
sheets, and then placed in a prismatic or cylindrical case.
Positive and/or negative feed through pins (i.e., terminals) are
then connected to the respective electrodes and the case is filled
with electrolyte and then sealed. The negative electrode is
typically formed of a copper substrate carrying graphite as the
active material. The positive electrode is typically formed of an
aluminum substrate carrying lithium cobalt dioxide as the active
material. The electrolyte is most commonly a 1:1 mixture of EC:DEC
in a 1.0 M salt of LiPF.sub.6. The separator is frequently a
microporous membrane made of a polyolephine, such as a combination
of polyethylene and/or polypropylene which can, for example, be
approximately 25 microns thick.
[0006] Batteries used in implanted medical devices can be charged
from an external power source utilizing a primary coil to transfer
power through a patient's skin to a secondary coil associated with
the implanted medical device. The secondary coil and an associated
charging circuit provide a charging current to the battery.
Protection circuitry is typically used in conjunction with prior
art lithium batteries to avoid the potential deleterious effects of
over or under charging the battery. Such protection circuitry can
terminate charging if the voltage or temperature of the battery
exceeds a certain level. Moreover, it is common to also incorporate
low voltage protection to disconnect the battery from its load if
the voltage of the battery falls below a certain lower level. This
latter precaution is taken to prevent permanent damage to the
battery that will likely occur if the voltage on an electrode
exceeds a Damage Potential Threshold (DPT). For example, it is well
known in the industry that discharging a lithium battery to below
2.5 volts and storing it for an extended period of time will likely
result in a permanent loss of battery capacity. Despite
incorporating low voltage cutoff protection to disconnect the
battery from its load if the voltage falls below a certain
threshold, typical prior art batteries will slowly self-discharge
further causing the voltage of an electrode to exceed the Damage
Potential Threshold.
SUMMARY
[0007] The present invention is directed to a rechargeable lithium
battery particularly configured to permit it to discharge to a very
low voltage, e.g. zero volts, without causing permanent damage to
the battery. More particularly, a battery in accordance with the
invention is configured to define a Zero Volt Crossing Potential
(ZCP) which is lower than the battery's Damage Potential Threshold
(DPT).
[0008] ZCP refers to the voltage on the positive and negative
electrodes relative to a lithium reference (Li/Li+) when the
battery potential, i.e., the voltage between the electrodes, is
zero. The Damage Potential Threshold (DPT) is attributable to at
least two factors, i.e., a Substrate Dissolution Potential (SDP)
and a Film Dissolution Potential (FDP). SDP refers to the voltage
of the negative electrode, relative to the lithium reference, above
which the electrode substrate starts to corrode or decompose to
permanently damage the substrate. FDP refers to the voltage of the
negative electrode, relative to the lithium reference, above which
a solid electrolyte interface (SEI) layer begins to dissolve. The
SEI, or film, comprises a passivation layer which, in normal
operation, forms on the negative electrode and functions to inhibit
a continuing reaction between the negative electrode active
material and the electrolyte. When the voltage of the negative
electrode relative to the lithium reference, exceeds either SDP or
FDP, physical damage to the electrode is likely to occur thereby
permanently impairing the battery's capacity.
[0009] A battery's ZCP level relative to the lithium reference is
dependent in part on the materials used for the positive and/or
negative electrodes. In accordance with a preferred embodiment of
the invention, a positive electrode active material
LiNi.sub.xCo.sub.1-xO.sub.2 is selected which exhibits a discharge
voltage curve appropriate to achieve a relatively low Zero Crossing
Potential (ZCP) level. This feature of the preferred embodiment
facilitates the implementation of a battery characterized by a ZCP
less than its Damage Potential Threshold (DPT). It has been
recognized that as more Ni is substituted for Co (i.e., increasing
x), the magnitude of the discharge voltage profile decreases. It
has been determined that values of x between 0.5 and 1.0 optimally
achieve the desired ZCP/DPT relationship in accordance with the
present invention, i.e., LiNi.Co.sub.1-xO.sub.2 (where
0.5.ltoreq.x.ltoreq.1.0). For x<0.5, there is a noticeable
degradation in capacity retention after storage at zero volts
indicating that some corrosion or internal battery degradation has
occurred at low voltage similar to the results seen from a
conventional lithium battery design.
[0010] In accordance with a preferred embodiment of the invention,
LiNi.sub.0.8Co.sub.0.2O.sub.2 is used for the positive active
material on a thin metal substrate, e.g., aluminum. The negative
electrode is preferably formed of a titanium or titanium alloy
substrate carrying a layer of negative active material, e.g.,
graphite coated on both faces of the substrate.
[0011] Batteries in accordance with the present invention are
particularly suited for use in critical applications where physical
access to the battery is difficult and/or expensive, such as in
medical devices configured to be implanted in a patient's body.
Such a medical device is typically comprised of a hermetically
sealed housing formed of biocompatible material and dimensioned to
be implanted without interfering with normal body function. A
battery in accordance with the invention includes a case configured
for mounting in the device housing. The battery case can be of a
variety of shapes, e.g., prismatic or cylindrical, and typically
defines a volume of between 0.05 cc and 30 cc. Batteries within
this range exhibit capacities between 1.0 milliamp hours and 10 amp
hours. An exemplary battery for use in a neurostimulation device
includes a prismatic hermetically sealed battery casing having
dimensions of 35 mm.times.17 mm.times.5.5 mm. A wide variety of
medical device applications are discussed in the medical and patent
literature; see, for example, U.S. Pat. No. 6,185,452.
BRIEF DESCRIPTION OF THE FIGURES
[0012] The above and other features and uniqueness of the invention
will be better visualized from the following drawings and
schematics.
[0013] FIG. 1A schematically depicts positive and negative battery
electrodes rolled around a mandrel for placement in a battery case
and FIG. 1B depicts in cross-section a complete battery;
[0014] FIG. 2 shows a typical deep discharge curve for a
conventional lithium ion battery using copper as the negative
electrode substrate and lithium cobalt dioxide LiCoO2 as the
positive electrode active material;
[0015] FIG. 3 shows a typical deep discharge curve for a lithium
ion battery in accordance with the present invention using titanium
as the negative electrode substrate;
[0016] FIG. 4 shows a typical deep discharge curve for a lithium
ion battery in accordance with the present invention using
LiNi.sub.xCo.sub.1-xO.sub.2 (0.5.ltoreq.x.ltoreq.1.0) as the
positive electrode active material;
[0017] FIG. 5 is a table showing test results of various battery
configurations including a preferred embodiment in accordance with
the present invention; and
[0018] FIG. 6 schematically depicts a battery in accordance with
the invention contained within an implantable medical device
housing.
DESCRIPTION
[0019] A rechargeable battery in accordance with the present
invention is particularly suited for use in medical devices
intended to be implanted in a patient's body. Such medical devices
are extensively discussed in the medical and patent literature. For
example, U.S. Pat. No. 6,1895,452 describes a Battery-Powered
Patient Implantable Device utilizing a rechargeable battery
depicted in alternative constructions in FIGS. 8A through 8G. The
present invention is directed to an improved rechargeable lithium
battery, useful in devices of the type described in U.S. Pat. No.
6,185,452, configured to tolerate deep discharging without
significantly impairing the battery's ability to recover its
original storage capacity.
[0020] FIGS. 1A and 1B schematically depict a typical lithium
battery construction 10 comprising a prismatic case 12 containing a
positive electrode 14 and a negative electrode 16, rolled around a
mandrel 18. Separator sheets 20, 22 are incorporated in the rolling
to electrically separate the electrodes. The case 12 also typically
includes electrolyte material (not shown) and positive and negative
feed through pins (i.e., terminals) 26, 28 which are respectively
connected to the electrodes 14, 16 and extend externally of the
case 12.
[0021] Typical prior art lithium ion batteries include a positive
electrode 14 comprised of a thin metal substrate, e.g., aluminum,
carrying a layer of positive active material, e.g., lithium cobalt
dioxide LiCoO.sub.2 mixed with a binder, and coated on both faces
of the substrate. The negative electrode 16 is typically comprised
of a thin metal substrate, e.g., copper, carrying a layer of
negative active material, e.g., graphite coated on both faces of
the substrate.
[0022] Two layers of separator 20, 22 electrically separate the
electrodes 14, 16 from each other, enabling the electrodes to be
rolled around mandrel 18. Each separator layer can comprise a micro
porous membrane made of a combination of polypropylene and is
approximately 25 .mu.m thick. The electrolyte is most commonly a
1:1 mixture of EC:DEC in a 1.0 M salt of LiPF.sub.6.
[0023] FIG. 2 shows typical deep discharge performance curves for a
conventional lithium ion battery. The y-axis represents voltage
relative to a lithium reference (Li/Li+) or counter electrode and
the x-axis represents time. Curves 50 and 52 respectively depict
the discharge curves for the positive and negative electrodes. The
battery output voltage is the difference between the positive
electrode voltage and the negative electrode voltage. During
discharge, the positive electrode voltage decreases relative to the
lithium reference and the negative voltage increases, primarily
near the end of discharge. A protection or management circuit (not
shown) is typically provided to disconnect the load to stop the
discharge when the battery voltage reaches 2.5 volts. If, however,
the discharge continues (attributable, for example, to
self-discharge over a long period of time), the negative electrode
potential will rise until it reaches the potential of the positive
electrode. This constitutes the Zero Volt Crossing Potential (ZCP)
and is typically about 3.6 volts in conventional lithium ion
battery constructions. The negative electrode potential at ZCP can
exceed the Substrate Dissolution Potential (SDP) of the negative
electrode substrate, e.g., 3.3 volts for copper, and cause
decomposition and permanent damage to the substrate. The present
invention is directed to battery improvements to assure that the
value of SDP is greater than the value of ZCP, as represented in
FIG. 3.
[0024] FIG. 3 depicts deep discharge performance curves for a
lithium battery in accordance with the present invention in which
the negative electrode substrate is formed of titanium instead of
copper. The use of titanium increases the knee of the negative
electrode curve 54 to position the SDP level above the ZCP level.
This relationship considerably reduces potential damage to the
negative electrode substrate. In addition to commercially pure
titanium, i.e., titanium CP, other materials can be used to raise
the SDP sufficiently, e.g. titanium alloys, nickel, nickel alloys,
and stainless steel.
[0025] FIG. 3 demonstrates how the SDP level can be increased
relative to the ZCP by proper choice of the negative electrode
substrate material. Alternatively, or additionally, the ZCP level
can be decreased relative to the SDP by proper choice of the
positive electrode active material, as depicted in FIG. 4.
[0026] More particularly, FIG. 4 shows the discharge curve 60 for a
positive electrode using lithium nickel cobalt dioxide
LiNi.sub.xCo.sub.1-xO.sub.2 (where 0<x.ltoreq.1) as the active
material, i.e., as the intercalation compound. Note that the curve
of FIG. 4 exhibits a greater negative slope than the analogous
curve 50 of FIG. 2 representing the standard intercalation compound
LiCoO.sub.2. The effect of the increased negative slope is to lower
the ZCP level relative to the lithium reference and the SDP. As was
the case in connection with FIG. 3, this reduces the potential
damage to the negative electrode substrate. Additionally, however,
the ZCP level also falls below a Film Dissolution Potential (FDP)
which is the voltage above which a solid electrolyte interface
(SEI) layer begins to dissolve. The SEI, or film, comprises a
passivation layer which forms on the negative electrode and
functions to inhibit a continuing reaction between the negative
electrode active material and the electrolyte. Dissolution of the
SEI can noticeably damage the negative electrode active
material.
[0027] Experiments have been performed at two different
temperatures employing the aforedescribed techniques depicted in
FIGS. 3 and 4. The preliminary results are summarized in the table
of FIG. 5. Four different battery configurations were constructed
as shown. Configuration (1) corresponds to the conventional
arrangement represented in FIG. 2 comprising a copper substrate for
the negative electrode and LiCoO.sub.2 for the positive active
material. The battery was built and then recycled once to get an
initial capacity measurement. The battery was then shorted between
the positive and negative leads to achieve a zero volt state. This
zero volt condition was held for one week and then recharged and
discharged to get a capacity measurement after zero-volt storage.
The capacity retention is calculated by dividing the discharge
capacity after zero volt storage by the initial capacity and
multiplying by 100%. In this manner, this percentage reflects any
damage that had occurred to the battery while in the zero volt
state.
[0028] As represented in FIG. 5, the capacity retention for battery
configuration (1) is below 80%, thus suggesting that the zero volt
condition had significantly damaged the battery. After opening the
battery and examining the electrodes, it was seen that dissolution
of the negative electrode copper substrate had occurred. This
battery (1) configuration performed poorly at both temperature
settings.
[0029] The battery configuration (2) used LiCoO.sub.2 as the
positive active material and a titanium substrate as the negative
substrate corresponding to the arrangement represented in FIG. 3.
The results show that at 25.degree. C. the capacity retention was
at about 98% after the zero volt condition. However, at a higher
temperature (37.degree. C.) exemplary of medical implant
conditions, performance deteriorates to below 80%. This suggests
that perhaps the zero volt crossing potential was sufficiently
below SDP to avoid substrate dissolution but still high enough to
exceed FDP and cause damage to the negative electrode active
material. Accordingly, attempts were made to lower ZCP further to
avoid damage both to the negative active material and the negative
electrode substrate.
[0030] The battery configuration (3) utilizes
LiNi.sub.xCo.sub.1-xO.sub.2 (where x=0.8) as the positive electrode
active material and a conventional copper negative electrode
substrate. The results show that at 37.degree. C., the capacity
retention is quite high at 90%. However, examination after the
test, revealed that some dissolution of the copper substrate had
occurred. Battery configuration (4) uses both
LiNi.sub.0.8Co.sub.0.2O.sub.2 as the positive active material and
titanium as the negative electrode substrate material. Results show
that this configuration gives the best capacity retention after
zero volt storage.
[0031] From the curves of FIG. 4 and the table of FIG. 5, it
appears that some performance gain is achieved by configuration (2)
using a titanium negative electrode substrate and by configuration
(3) using LiNi.sub.xCo.sub.1-xO.sub.2 (where x=0.8) as the positive
active material. However, maximum performance gain appears in
configuration (4) which combines both of these features.
[0032] FIG. 6 schematically depicts a battery 60 in accordance with
the invention mounted in a housing 64 (shown partially open for the
purposes of illustration) of a medical device 66 configured for
implanting in a patient's body. The housing 64 is preferably formed
of biocompatible material and hermetically sealed. The device 66 is
typically used for monitoring and/or affecting body parameters. For
example, the device can be used to electrically stimulate nerves.
The casing 68 of battery 64 can, for example, have dimensions of 35
mm.times.17 mm.times.5.5 mm. Other configurations and sizes are
suggested in the literature, e.g., U.S. Pat. No. 6,185,452.
[0033] While the invention has been described with reference to
specific exemplary embodiments and applications, it should be
recognized that numerous modifications and variations will occur to
those skilled in the art without departing from the spirit and
scope of the invention set forth in the appended claims.
* * * * *