U.S. patent application number 12/323983 was filed with the patent office on 2009-08-13 for batteries and electrodes for use thereof.
This patent application is currently assigned to MASSACHUSETTS INSTITUTE OF TECHNOLOGY. Invention is credited to Yet-Ming Chiang, Timothy E. Chin, Can K. Erdonmez, Wei Lai, Ryan C. Wartena.
Application Number | 20090202903 12/323983 |
Document ID | / |
Family ID | 41650177 |
Filed Date | 2009-08-13 |
United States Patent
Application |
20090202903 |
Kind Code |
A1 |
Chiang; Yet-Ming ; et
al. |
August 13, 2009 |
BATTERIES AND ELECTRODES FOR USE THEREOF
Abstract
The present invention generally relates to batteries or other
electrochemical devices, and systems and materials for use in
these, including novel electrode materials and designs. In some
embodiments, the present invention relates to small-scale batteries
or microbatteries. For example, in one aspect of the invention, a
battery may have a volume of no more than about 5 mm.sup.3, while
having an energy density of at least about 400 W h/l. In some
cases, the battery may include a electrode comprising a porous
electroactive compound. In some embodiments, the pores of the
porous electrode may be at least partially filled with a liquid
such as a liquid electrolyte. The electrode may be able to
withstand repeated charging and discharging. In some cases, the
electrode may have a plurality of protrusions and/or a wall (which
may surround the protrusions, if present); however, in other cases,
there may be no protrusions or walls. The electrode may be formed
from a unitary material. In certain embodiments, a nonporous
electrolyte may be disposed onto the electrode. Such an electrolyte
may allow ionic transport (e.g., of lithium ions) while preventing
dendritic formation due to the lack of pores. In certain
embodiments the porous electrode has a surface that is denser than
its interior. Other aspects of the invention are directed to
techniques of making such electrodes or batteries, techniques of
forming electrical connections to and packaging such batteries,
techniques of using such electrodes or batteries, or the like.
Inventors: |
Chiang; Yet-Ming;
(Framingham, MA) ; Wartena; Ryan C.; (Los Angeles,
CA) ; Chin; Timothy E.; (Cambridge, MA) ;
Erdonmez; Can K.; (Cambridge, MA) ; Lai; Wei;
(Cambridge, MA) |
Correspondence
Address: |
WOLF GREENFIELD & SACKS, P.C.
600 ATLANTIC AVENUE
BOSTON
MA
02210-2206
US
|
Assignee: |
MASSACHUSETTS INSTITUTE OF
TECHNOLOGY
Cambridge
MA
|
Family ID: |
41650177 |
Appl. No.: |
12/323983 |
Filed: |
November 26, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12126841 |
May 23, 2008 |
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12323983 |
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60931819 |
May 25, 2007 |
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Current U.S.
Class: |
429/203 ;
252/182.1; 429/209; 429/231.3; 429/231.95 |
Current CPC
Class: |
H01M 4/485 20130101;
H01M 4/1391 20130101; H01M 10/0436 20130101; H01M 4/525 20130101;
H01M 10/0565 20130101; H01M 4/664 20130101; H01M 4/0426 20130101;
H01M 4/1393 20130101; H01M 10/0562 20130101; H01M 4/1397 20130101;
H01M 4/661 20130101; H01M 4/505 20130101; H01M 10/0525 20130101;
H01M 4/131 20130101; H01M 2004/021 20130101; H01M 4/667 20130101;
Y02E 60/10 20130101; H01M 10/0472 20130101; H01M 10/052 20130101;
Y02P 70/50 20151101; H01M 4/0471 20130101 |
Class at
Publication: |
429/203 ;
429/209; 429/231.95; 429/231.3; 252/182.1 |
International
Class: |
H01M 6/04 20060101
H01M006/04; H01M 4/02 20060101 H01M004/02; H01M 4/58 20060101
H01M004/58; H01M 4/48 20060101 H01M004/48; H01M 4/88 20060101
H01M004/88 |
Goverment Interests
GOVERNMENT FUNDING
[0002] Research leading to various aspects of the present invention
were sponsored, at least in part, by the U.S. Department of
Defense, Grant No. NMA501-03-1-2004. The U.S. Government has
certain rights in this invention.
Claims
1. An article, comprising: an electrode formed from a sintered
ceramic, the electrode having a porosity of no more than about 50%,
at least some of the pores of the electrode being filled with an
electrolyte that is a liquid or a polymer.
2-4. (canceled)
5. The article of claim 1, wherein the electrode has a linear
strain differential of less than about 20% when the electrode is
infiltrated with Li ions.
6. The article of claim 1, wherein the electrode has a linear
strain differential of less than about 2% when the electrode is
infiltrated with Li ions.
7. (canceled)
8. The article of claim 1, further comprising a nonporous
electrolyte disposed on the electrode.
9. The article of claim 1, wherein the porosity is no more than
about 30%.
10. The article of claim 9, wherein the porosity is no more than
about 20%.
11. The article of claim 10, wherein the porosity is no more than
15%.
12. The article of claim 1, wherein the ceramic comprises
LiCoO.sub.2.
13. (canceled)
14. The article of claim 1, wherein the ceramic comprises
Li.sub.x(Ni.sub.a, Co.sub.b, Al.sub.c)O.sub.2, wherein x is between
about 0 and about 1.5, and the sum of a, b, and c is about 1.
15. The article of claim 1, wherein the ceramic comprises
Li.sub.x(Mn.sub.a, Ni.sub.b, Co.sub.c)O.sub.2, wherein x is between
about 0 and about 1.5, and the sum of a, b, and c is about 1.
16-17. (canceled)
18. The article of claim 8, wherein the nonporous electrolyte
comprises LiPON.
19. The article of claim 1, wherein the article is a battery.
20. (canceled)
21. The article of claim 19, wherein the battery has a storage
density of at least about 400 W h/l.
22-27. (canceled)
28. The article of claim 1, wherein the ceramic has a smallest
dimension that is at least about 0.2 mm.
29. The article of claim 28, wherein the ceramic has a smallest
dimension that is at least about 0.4 mm.
30. The article of claim 29, wherein the ceramic has a smallest
dimension that is at least about 0.6 mm.
31-68. (canceled)
69. An article, comprising: an electrode formed from a sintered
ceramic that is able to retain at least about 50% of its initial
storage capacity after at least 6 charge-discharge cycles at a C/20
rate.
70-181. (canceled)
182. An article, comprising: an electrode formed from a sintered
ceramic, the electrode having a density of at least about 50%, a
thickness of at least about 0.25 mm.
183. (canceled)
184. The article of claim 182, wherein the electrode has a density
of at least about 70%.
185. The article of claim 182, wherein the electrode has a density
of at least about 80%.
186-197. (canceled)
198. An article, comprising: an electrode formed from a sintered
ceramic, the electrode having a product of density, wherein the
density is measured as a percent of the theoretical density of the
ceramic, and thickness, wherein the thickness is measured in
millimeters, of between about 10%-mm and about 150%-mm.
199-217. (canceled)
218. An article, comprising: an electrode formed from a sintered
ceramic, the electrode having a charge capacity per unit area of
electrode of between about 10 mAh/cm.sup.2 and about 100
mAh/cm.sup.2.
219-239. (canceled)
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 12/126,841, filed May 23, 2008, entitled
"Batteries and Electrodes For Use Thereof," by Chiang, et al.,
which application claims the benefit of U.S. Provisional Patent
Application Ser. No. 60/931,819, filed May 25, 2007, by Chiang, et
al., each incorporated herein by reference.
FIELD OF INVENTION
[0003] The present invention generally relates to batteries or
other electrochemical devices, and systems and materials for use in
these, including novel electrode materials and designs. In some
embodiments, the present invention relates to small-scale batteries
or microbatteries.
BACKGROUND
[0004] Since the time of Volta, batteries and other electrochemical
devices have been fabricated by the manual assembly of critical
components. The advent of distributed and autonomous electronics
requiring very small and high energy density power sources, as well
as continuing demand in larger batteries for low cost energy and
power, has created a need for entirely new designs and fabrication
approaches for batteries and the like. Current devices range in
length from micrometer-thick thin film batteries, to lithium
rechargeable batteries based on wound laminate films, to the
macroassemblies used in common alkaline and lead-acid batteries.
However, the laminated construction techniques of current high
energy density batteries (e.g., lithium ion batteries), now
approaching their engineering limits, have inefficient mass and
volume utilization, with only 30% to 40% of the available device
volume being used for ion storage. Attempts to increase power
density, for instance by using thinner electrodes, typically has
come at the expense of energy density. Furthermore, as the size
scale of powered devices continues to shrink, there is a growing
need for distributed high energy density power sources of
comparable size scale.
SUMMARY OF THE INVENTION
[0005] The present invention generally relates to batteries or
other electrochemical devices, and systems and materials for use in
these, including novel electrode materials and designs. In some
embodiments, the present invention relates to small-scale batteries
or microbatteries. The subject matter of the present invention
involves, in some cases, interrelated products, alternative
solutions to a particular problem, and/or a plurality of different
uses of one or more systems and/or articles.
[0006] In one aspect, the invention is directed to an article. In
one set of embodiments, the article includes a battery comprising
an entire anode, an electrolyte, and an entire cathode, where the
battery has a volume of no more than about 5 mm.sup.3 or about 10
mm.sup.3 and an energy density of at least about 200 W h/l or at
least about 400 W h/l. In another set of embodiments, the article
includes a rechargeable battery having an energy density of at
least about 1000 W h/l.
[0007] The article, in yet another set of embodiments, includes an
electrode formed from a sintered ceramic and/or a ceramic
composite, where the electrode has a porosity of no more than about
50%. In some cases, at least some of the pores of the electrode are
filled with an electrolyte that is a liquid, a gel, a solid
polymer, and/or a solid inorganic compound. In still another set of
embodiments, the article includes an electrode formed from a
sintered ceramic and/or a ceramic composite that is able to retain
at least about 50% of its initial storage capacity after at least 6
charge-discharge cycles at a C/20 rate.
[0008] In one set of embodiments, the sintered electrode has a
thickness of between 100 microns and 2000 microns and a porosity
between 10% and 70% by volume, and more preferably a thickness
between 300 microns and 1000 microns and porosity between 15% and
50% by volume.
[0009] In yet another set of embodiments, the article includes a
electrode formed from a sintered ceramic or ceramic composite. The
compound or compounds of the electrode may have, in some cases, a
molar volume difference between the charged and discharged state of
the cell of less than about 30%, less than about 15%, less than
about 10%, or less than about 5%. In some embodiments the compound
or compounds of the electrode has a linear or a volumetric strain
between the charged and discharged state of the cell of less than
about 20%, less than about 15%, less than about 10%, less than
about 5%, less than about 3%, less than about 2%, or less than
about 1%. In some embodiments, the compounds of the electrode
include at least one compound that increases in molar volume at
least some compositions during use and at least one compound that
decreases in molar volume at least some compositions during use. In
some embodiments, the net volume change of the electrode between
the charged and discharged state of the battery is decreased by
combining at least one compound that has a net positive volume
change between the charged and discharged state, with at least one
compound that has a net negative volume change between the charged
and discharged state of the battery.
[0010] In one set of embodiments, the article includes an electrode
formed from a sintered ceramic and/or a ceramic composite. The
electrode may be micromachined in some cases. In some embodiments,
the ceramic comprises a lithium metal oxide LiMO.sub.2 where M is
at least one transition metal, or a lithium transition metal
phosphate olivine. In some embodiments, the sintered ceramic is
LiCoO.sub.2 and/or LiFePO.sub.4. In certain instances, the ceramic
comprises Li.sub.x(Ni.sub.a, Co.sub.b, Al.sub.c)O.sub.2, wherein x
is between about 0 and about 1.5, and the sum of a, b, and c is
about 1. In another set of embodiments, the article includes a
micromachined electrode formed from a porous sintered ceramic
and/or a ceramic composite. In yet another set of embodiments, the
article includes a micromachined electrode formed from a sintered
ceramic and/or a ceramic composite, where the ceramic has a linear
or a volumetric strain differential of less than about 20%, less
than about 10%, less than about 3%, or less than about 2%.
[0011] The article, according to another set of embodiments,
includes an electrode having a base and a plurality of protrusions
extending at least about 50 micrometers away from the base of the
electrode, where at least some of the protrusions comprises
LiCoO.sub.2, and where substantially all of the protrusions have a
surface and a bulk and being sized such that substantially all of
the bulk is no more than about 25 micrometers away from the
surface. The electrode may be nonporous (dense) or porous. In some
cases, the article may also include a nonporous electrolyte
disposed on the surfaces of the protrusions. In another embodiment,
at least some of the protrusions comprises Li.sub.x(Ni.sub.a,
Co.sub.b, Al.sub.c)O.sub.2 or Li.sub.x(Mn.sub.a, Ni.sub.b,
Co.sub.c)O.sub.2, wherein x is between about 0 and about 1.5, and
the sum of a, b, and c is about 1.
[0012] According to yet another set of embodiments, the article
includes an electrode comprising a base and a plurality of
protrusions extending from the base, and a wall extending from the
base and surrounding the plurality of protrusions. In some cases,
the protrusions and the wall are formed from a unitary material. In
another set of embodiments, the article includes an electrode
comprising, on one surface, a plurality of protrusions and a wall
surrounding the plurality of protrusions. In some cases, the
electrode can be formed using laser micromachining.
[0013] According to yet another set of embodiments, the article
includes a battery that comprises only solid phases. In another set
of embodiments, the article includes a battery that comprises a
liquid electrolyte. In another set of embodiments, the article
includes a battery that comprises both a solid electrolyte and a
liquid electrolyte.
[0014] In one set of embodiments, the article includes an electrode
having a plurality of protrusions. In some cases, the protrusions
have an aspect ratio of at least about 3:1 and a pitch of at least
about 2:1. In one embodiment, the electrode is formed using laser
micromachining. In another embodiment, the electrode is formed from
a unitary material.
[0015] According to another set of embodiments, the article
includes a lithium metal electrode, a nonporous electrolyte
contacting the lithium metal electrode, and a porous sintered
electrode contacting the lithium metal electrode.
[0016] In one set of embodiments, the article includes an electrode
formed from a sintered ceramic or a ceramic composite. In certain
cases, the electrode has a density of at least about 50%, a
thickness of at least about 0.25 mm. In another set of embodiments,
the electrode has a density that is at least about 50% of the
theoretical density of the ceramic, and a thickness of at least
about 0.25 mm. In yet another set of embodiments, the electrode has
an open porosity of no more than about 50%, and a thickness of at
least about 0.25 mm.
[0017] In one embodiment, the invention is directed to an electrode
formed from a sintered ceramic and/or a ceramic composite. The
electrode can have a product of density, where density is measured
in percent of the theoretical density of the ceramic, and
thickness, where thickness is measured in millimeters, of between
about 10%-mm and about 150%-mm. In another embodiment, the
electrode can have a product of porosity and thickness, e.g.,
having a product that is between 150%-mm and 2.5%-mm.
[0018] The article, in another set of embodiments, is directed to
an electrode formed from a sintered ceramic and/or a ceramic
composite. The article may have, in one embodiment, an electrode
having a charge capacity per unit area of electrode of between
about 10 mAh/cm.sup.2 and about 100 mAh/cm.sup.2.
[0019] Another aspect of the invention is drawn to a method. In one
set of embodiments, the method includes an act of fabricating an
electrode from a unitary material. In some cases, the electrode
comprises, on one surface, a plurality of protrusions and a wall
surrounding the plurality of protrusions.
[0020] In another set of embodiments, the method includes acts of
providing a Li-containing substrate that Li metal will not wet,
depositing a metal layer on the substrate, and adding Li metal to
the metal layer. In some cases, the Li reacts with the metal layer
to wet the surface.
[0021] In another aspect, the present invention is directed to a
method of making one or more of the embodiments described herein,
for example, a small-scale battery or a or microbattery. In another
aspect, the present invention is directed to a method of using one
or more of the embodiments described herein, for example, a
small-scale battery or a microbattery.
[0022] Other advantages and novel features of the present invention
will become apparent from the following detailed description of
various non-limiting embodiments of the invention when considered
in conjunction with the accompanying figures. In cases where the
present specification and a document incorporated by reference
include conflicting and/or inconsistent disclosure, the present
specification shall control. If two or more documents incorporated
by reference include conflicting and/or inconsistent disclosure
with respect to each other, then the document having the later
effective date shall control.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] Non-limiting embodiments of the present invention will be
described by way of example with reference to the accompanying
figures, which are schematic and are not intended to be drawn to
scale. In the figures, each identical or nearly identical component
illustrated is typically represented by a single numeral. For
purposes of clarity, not every component is labeled in every
figure, nor is every component of each embodiment of the invention
shown where illustration is not necessary to allow those of
ordinary skill in the art to understand the invention. In the
figures:
[0024] FIGS. 1A-1D illustrate electrodes having protrusions,
according to one embodiment of the invention;
[0025] FIGS. 2A-2C are photomicrographs of an embodiment of the
invention, illustrating an electrode having ribs;
[0026] FIG. 3 illustrates a sloped protrusion, in accordance with
one embodiment of the invention;
[0027] FIGS. 4A-4C are photomicrographs of various embodiments of
the invention having sloped protrusions;
[0028] FIGS. 5A-5B illustrate electrodes having walls, according to
another embodiment of the invention;
[0029] FIGS. 6A-6E are photomicrographs of various embodiments of
the invention, illustrating electrodes having walls;
[0030] FIGS. 7A-7D are photomicrographs of another embodiment of
the invention, illustrating an electrode having walls;
[0031] FIGS. 8A-8B are photomicrographs of yet another embodiment
of the invention, illustrating electrodes having substantially
planar surfaces;
[0032] FIGS. 9A-9C are photomicrographs of still another embodiment
of the invention, illustrating an electrode that does not show any
obvious degradation or cracking;
[0033] FIG. 10 is a schematic diagram of one embodiment of the
invention;
[0034] FIG. 11 is a schematic diagram of a method of fabricating a
battery, in accordance with another embodiment of the
invention;
[0035] FIGS. 12A-12D illustrate an embodiment of the invention
using colloidal-scale self-organization to produce an
electrode;
[0036] FIGS. 13A-13B illustrate the energy densities of batteries
using various materials, in accordance with certain embodiments of
the invention;
[0037] FIG. 14 illustrates energy density as a function of volume
for various batteries, in yet another embodiment of the
invention;
[0038] FIGS. 15A-15B illustrate the deposition of liquid lithium on
a wet oxide surface, in accordance with still another embodiment of
the invention;
[0039] FIGS. 16A-16B show electrochemical test results of porous
LiCoO.sub.2 electrodes prepared in accordance with certain
embodiments of the invention;
[0040] FIGS. 17A-17B show the specific capacity measured by
galvanostatic cycling over 40 cycles of a sintered doped olivine
phosphate cathode produced in accordance with one embodiment of the
invention;
[0041] FIGS. 18A-18B show a conformal lithium phosphorus oxynitride
layer sputtered onto a porous sintered LiCoO.sub.2 cathode, in
accordance with another embodiment of the invention;
[0042] FIG. 19 shows a galvanostatic test of a porous sintered
LiCoO.sub.2 cathode conformally coated with an approximately
.about.0.5 micrometer thick film of lithium phosphorus oxynitride,
in yet another embodiment of the invention;
[0043] FIGS. 20A-20B illustrates microbattery packaging comprising
an electroformed gold can and copper foil lid, in still another
embodiment of the invention;
[0044] FIG. 21 shows the first charge curve for two microbatteries
made using sintered electrodes, in one embodiment of the
invention;
[0045] FIG. 22 shows the first discharge curves for two
microbatteries produced in accordance with another embodiment of
the invention;
[0046] FIG. 23 shows the first four discharge curves for a
microbattery produced in yet another embodiment of the invention,
showing voltage vs. the specific capacity of the sintered
LiCoO.sub.2 cathode;
[0047] FIGS. 24A-24C illustrate a bicell fabricated using sintered
LiCoO.sub.2 cathodes, and test results using the bicell, in still
another embodiment of the invention;
[0048] FIGS. 25A-25B illustrate discharge characteristics for
certain electrodes of the invention;
[0049] FIG. 26 illustrates the effect of increasing electronic
conductivity upon delithiation of electrodes of another embodiment
of the invention; and
[0050] FIGS. 27A-27B illustrate a model of an electrode having a
stacked prismatic cell configuration, in yet another embodiment of
the invention.
DETAILED DESCRIPTION
[0051] The present invention generally relates to batteries or
other electrochemical devices, and systems and materials for use in
these, including novel electrode materials and designs. In some
embodiments, the present invention relates to small-scale batteries
or microbatteries. For example, in one aspect of the invention, a
battery may have a volume of no more than about 5 mm.sup.3 or about
10 mm.sup.3, while having an energy density of at least about 200 W
h/l or at least about 400 W h/l. In some cases, the battery may
include a electrode comprising a porous electroactive compound, for
example, LiCoO.sub.2, which may be formed, in some cases, by a
process including but not limited to sintering of a particle
compact. In some embodiments, the pores of the porous electrode may
be at least partially filled with a liquid such as a liquid
electrolyte comprising alkyl carbonates and/or a lithium salt such
as LiPF.sub.6, a polymer such as a polymer electrolyte comprising
polyethylene oxide and/or a lithium salt, a block copolymer
lithium-conducting electrolyte, and/or an inorganic electrolyte
such as a lithium phosphorus oxynitride compound, lithium iodide,
and the like. The electrode may be able to withstand repeated
charging and discharging. In some cases, the electrode may have a
plurality of protrusions and/or a wall (which may surround the
protrusions, if present); however, in other cases, there may be no
protrusions or walls present. The electrode may be formed from a
unitary material, e.g., formed using laser micromachining, dry
etching processes such as plasma or reactive ion etching, wet
chemical etching, or similar techniques. In some instances, the
electrode may be formed in a desired shape from a powder or powder
suspension using methods such as tape-casting, interrupted
tape-casting, slip-casting, pressing, and embossing, and may be
fired to obtain a sintered material after forming. In certain
embodiments, a nonporous electrolyte, such as lithium phosphorus
oxynitride, a polymer electrolyte such as one based on polyethylene
oxide and/or a lithium salt, a block-copolymer lithium conducting
electrolyte, and/or a polyelectrolyte multilayer film (which may be
formed by a layer-by-layer deposition process) may be disposed onto
the electrode. Such an electrolyte may allow ionic transport (e.g.,
of lithium ions) while preventing dendritic formation due to the
lack of pores. In certain embodiments the porous electrode has a
surface that is denser than its interior. The denser surface may be
formed by laser processing, rapid thermal annealing, formation of a
surface layer with a higher powder particle packing density prior
to sintering, filling of the surface with finer particles,
application of a surface coating by a vapor phase deposition or a
sol-gel coating process, or other such methods. Other aspects of
the invention are directed to techniques of making such electrodes
or batteries, techniques of forming electrical connections to and
packaging such batteries, techniques of using such electrodes or
batteries, or the like.
[0052] Various aspects of the invention are directed to batteries
or other electrochemical devices. Generally, a battery includes an
anode, a cathode, and an electrolyte separating the anode and the
cathode. Current collectors may be electrically connected to the
anode and the cathode, and current drawn from the battery using the
current collectors. Typically, current is produced by the battery
when the current collectors are put into electrical communication
with each other, e.g., through a load, such as a light, a motor, an
electrical circuit, a sensor, a transmitter, an electrical device,
etc. Within the battery, ions flow through the electrolyte between
the anode and the cathode during discharge. The electrolyte may be
a solid, a liquid, a gel, or the like, and the electrolyte may be
organic, inorganic, or a combination. In one aspect of the
invention, the battery is a Li ion (Li.sup.+) battery, i.e., the
battery uses Li.sup.+ as a charge carrier (alone, or in conjunction
with other charge carriers) within the electrolyte.
[0053] In some embodiments, the battery is "dry," meaning that it
is substantially free of liquid or gel components. In other
embodiments, however, the battery includes one or more liquid or
gel electrolytes, which may fill or partially fill the interior of
the battery cell. In some embodiments, the battery includes both
solid and liquid electrolytes. For instance, in some cases, the
solid electrolyte can be used as a conformal film coating the
surface of an electrode, and/or as a separator between the
electrodes.
[0054] In some cases, the battery is disposable after being
discharged once. In other cases, however, the battery is
rechargeable, i.e., the battery can be charged and discharged more
than once. For example, the battery may be able to withstand at
least 3 cycles, at least 6 cycles, or at least 10 cycles of
charging and discharging (for example at a C/20 rate, where 1 C=280
mA/g) with a retention of its initial storage capacity (e.g., as
measured in W h) of at least about 50%, at least about 60%, at
least about 70%, at least about 75%, at least about 80%, at least
about 85%, at least about 90%, or at least about 95% relative to
the initial charge of the battery after its first full charging. In
some cases, the battery can withstand even greater numbers of
cycles, e.g., at least 20 cycles, at least 30 cycles, at least 40
cycles, at least 50 cycles, at least 60 cycles, etc. Examples of
batteries exhibiting such cycles are discussed in the examples,
below. In addition, in some cases, the battery may exhibit such
charging cycles, even under more rapid charging or discharging
conditions. For instance, the battery may be charged or discharged
at C/50, C/25, C/15, C/10, C/5, C/3, C/2, C, 2C, or 3C rates for at
least the numbers of cycles discussed above ("C-rates" of charging
or discharging). A rechargeable lithium battery typically has
electrodes that exchange lithium during charge and discharge. For a
cathode or positive electrode material, Li.sup.+ and electrons are
adsorbed during the discharge of the battery, and this process is
reversed during the charge. Though the present invention is not
limited to cathodes, as used herein, "charging" indicates lithium
removal from the positive electrode and "discharging" refers to
lithium insertion into the positive electrode.
[0055] In some embodiments of the present invention, the battery is
a "microbattery," i.e., a battery having a volume of less than
about 10 mm.sup.3, including the entire anode, cathode,
electrolyte, current collectors, and exterior packaging that form
the battery. In some cases, the volume of the battery may be less
than about 5 mm.sup.3, less than about 3 mm.sup.3, or less than
about 1 mm.sup.3. For example, the battery may be generally
cube-shaped, having dimensions of less than about 3 mm, less than
about 2.5 mm, less than about 2 mm, less than about 1.5 mm, or less
than about 1 mm on each side. Of course, other shapes are also
possible, for example, rectangular parallelepiped, disc, rod,
plate, or spherical shapes, in other embodiments of the invention.
In some embodiments of the invention, the battery may contain an
electrode having a smallest dimension of at least about 0.2 mm, and
in some cases, at least about 0.4 mm, at least about 0.6 mm, at
least about 0.8 mm, at least about 1.0 mm, at least about 1.5 mm,
or at least about 2.0 mm.
[0056] In some embodiments, the battery may have a volume, mass,
energy, and/or power suitable for use in portable electronic
devices such as wireless headsets (e.g., Bluetooth), cellular
telephones, laptop computers, cordless power tools or other
appliances, vehicles, backup power systems, or in large scale
energy storage systems.
[0057] In one set of embodiments, the battery has an energy density
of at least about 200 W h/l, i.e., the battery is able to produce
200 W h of energy for each liter of volume of the battery
(including the entire anode, cathode, and electrolyte forming the
battery). In some embodiments, even higher energy densities can be
obtained, for instance, at least about 300 W h/l, at least about
400 W h/l, at least about 800 W h/l, at least about 1000 W h/l, at
least about 1200 W h/l, at least about 1400 W h/l, or at least
about 1600 W h/l. In other such embodiments, such energy densities
can be obtained even when the current collector and packaging of
the cell are included in the battery volume.
[0058] In one aspect of the present invention, such energy
densities may be achieved by using a cathode having a shape such
that substantially all of the cathode may be able to participate in
lithium ion exchange, e.g., with the electrolyte during charge or
discharge. For instance, in some embodiments, the electrode has a
shape that allows a relatively high degree of exposure between the
electrode and the electrolyte contacting the electrode, and/or a
relatively thin cross-sectional dimension, which may facilitate
transport of ions into and out of the electrode. In one set of
embodiments, the electrode may have the form of a base and a
plurality of protrusions, for instance, as is shown in FIG. 1A in
side view. In this figure, an electrode 10 includes a base 15, and
a plurality of protrusions 18 that extend away from the surface of
the base. As used herein, the base of the electrode is defined as a
generally flat, contiguous, featureless surface, and the
protrusions are defined a series of extensions that each extend
away from the base, although the base and the protrusions, in some
embodiments are made from a unitary material, as discussed
below.
[0059] As shown in FIG. 1, the protrusions are each shown as being
generally rectangular; however, in other embodiments, the
protrusions may be cylindrical, cone shaped, irregular,
rectangular, pyramidal, etc., and may be distributed on the surface
of the base in any manner, e.g., regularly or randomly arranged,
etc. The protrusions on the base may each be substantially the same
shape and/or size, as is shown in FIG. 1A, or the protrusions may
have different sizes.
[0060] FIG. 1B shows an example of one electrode having a
two-dimensional array of protrusions. In this figure, the
cross-sections of the protrusions are generally square, although in
other embodiments, other shapes are possible, e.g., rectangles or
circles. FIGS. 1C and 1D shows a battery that includes such
two-dimensional arrays of protrusions, used as a cathode and an
anode, in exploded view (FIG. 1C), and when assembled (FIG. 1D),
including top and bottom current collectors, in electrical
communication with the anode and cathode, respectively. In FIG. 1C,
battery 20 includes an anode 12, a cathode 14, and an electrolyte
13. In FIG. 1D, the battery is shown assembled, with a top current
collector 17 in electrical communication with anode 12, and a
bottom current collector 19 in electrical communication with
cathode 14. Additionally, in FIG. 1D, as a non-limiting example,
dimensions of a microbattery that could be formed using such
electrodes are illustrated.
[0061] However, in some cases, the protrusions extend along one
dimension of the electrode, thereby giving the appearance of
"ribs," that, when viewed in cross-section, has an appearance
similar to that shown in FIG. 1A. An example of an electrode having
such a series of extended protrusions is shown in FIG. 2A-2C at
different magnifications. The electrode in this example was
laser-machined from a porous sintered LiCoO.sub.2 material,
although other materials and other forming processes can also be
used.
[0062] In some embodiments, the protrusions may extend a distance
of at least about 25 micrometers away from the base of the
electrode, i.e., the maximum separation of the end of the
protrusion away from the surface of the base of the electrode is
about 25 micrometers. In other cases, the protrusions may extend a
distance of at least about 50 micrometers, at least about 75
micrometers, at least about 100 micrometers, etc., away from the
base of the electrode. As mentioned above, not all of the
protrusions may extend the same difference away from the surface of
the base. In some cases, the protrusion may have an aspect ratio
(i.e., the ratio of the distance the protrusion extends away from
the base to the maximum thickness of the protrusion) of at least
about 3:1, and in some cases, at least about 5:1, at least about
10:1, at least about 15:1, at least about 20:1, etc.
[0063] In some cases, the protrusions have sloped sides, i.e.,
sides that are not orthogonal to the surface of the base. For
example, a protrusion may have a pitch of at least about 2:1, and
in some embodiments, the pitch may be at least about 3:1, at least
about 5:1, or at least about 10:1. The "pitch" of a protrusion, as
used herein, is the slope of the protrusion, or the ratio of its
"rise" to "run." The sides of the protrusion need not all have the
same pitch. As shown in FIG. 3, a protrusion may have sloped sides,
and the pitch is the ratio of the rise of the slope 22 of the
protrusion to its run 24. Photomicrographs of such sloped
protrusions are shown in FIGS. 4A-4C. FIG. 4A shows sloped
protrusions formed from polycrystalline graphite; FIG. 4B shows
sloped protrusions formed from polygraphite on alumina, and FIG. 4C
shows sloped protrusions formed from HOPG (highly ordered pyrolytic
graphite) on alumina. Materials that can be used to form the
electrode and/or the protrusions are discussed in detail below.
[0064] In some cases, the protrusions may have a shape and/or size
such that the protrusion, or at least a substantial fraction of the
protrusion, is not more than a certain distance away from the
surface of the protrusion. Such a protrusion, for example, may
offer a limited distance for Li ions to be transported within the
electrode before reaching the surface or the electrolyte, and thus,
in some cases, substantially all of the protrusion may participate
in Li ion exchange during charging or discharging of the electrode,
thereby increasing the efficiency and/or the power density of the
electrode. For instance, a protrusion may have a surface and a
bulk, where the protrusion has a shape and/or size such that
substantially all of the bulk is no more than about 5 micrometers,
about 10 micrometers, about 15 micrometers, about 20 micrometers,
about 25 micrometers, about 50 micrometers, about 75 micrometers,
or about 100 micrometers away from the surface of the
protrusion.
[0065] In certain embodiments, the protrusions on the base of the
electrode may be at least partially surrounded by a wall or a
"can." For example, as is shown in FIG. 5A in cross section,
electrode 10 includes a base 15, a plurality of protrusions 18 that
extend away from the surface of the base, and a wall 11 surrounding
the protrusions. A three-dimensional view can be seen in FIG. 5B,
and photomicrographs of such electrodes are shown in FIGS. 6A-6E.
In FIGS. 6A and 6B, the height of the walls and the protrusion is
about 0.5 mm, and the width of the protrusions is about 100
micrometers. In FIGS. 6C-6E, the protrusions have a 100 micrometer
pitch, and a feature width of 80 micrometers. The walls, as shown
in this example, have a square or rectangular arrangement, but in
other embodiments, other shapes are possible, for example,
circular, hexagonal, triangular, etc.
[0066] The wall may be same thickness as the protrusions, or of a
different thickness. For instance, the wall may have a thickness of
less than about 200 micrometers, less than about 175 micrometers,
less than about 150 micrometers, less than about 125 micrometers,
less than about 100 micrometers, less than about 75 micrometers,
less than about 50 micrometers, or less than about 25 micrometers,
and the wall thickness may be uniform or non-uniform. The wall may
also be orthogonal to the base, or in some cases, the wall may have
sloped or tapered sides. A non-limiting example of an electrode
having a tapered wall is shown in FIGS. 7A-7D. In addition, as can
be seen in FIGS. 7A-7D, an electrode may have a wall on the base
without necessarily having any protrusions, in certain embodiments
of the invention.
[0067] The wall may, in certain embodiments of the invention, be
useful to contain an electrolyte and/or other materials within the
electrode, i.e., such that it remains in contact with the
protrusions of the electrode. The wall may also protect the
protrusion from external factors, for example, from forces that
might cause the protrusions to deform or break. In some cases, the
wall may facilitate the construction of integrated electrode
arrays, for example, for microbattery applications. In some cases,
as discussed below, the wall is formed, along with the base and
optionally the protrusions, from a unitary material. By forming the
wall and the base from a unitary material, an airtight or hermetic
seal between the wall and the base is naturally formed, which
prevents leakage to or from the battery, e.g., leaking of the
electrolyte contained within the electrode. In one set of
embodiments, the walls and the protrusions are micromachined from a
unitary ceramic material, as is discussed in detail below.
[0068] It should be noted here that not all embodiments of the
present invention necessarily must include protrusions and/or
walls. For example, in some embodiments, the electrode has a
substantially planar surface, e.g., as is shown in FIGS. 8A and 8B
for an example of an electrode formed in a monolithic shape from
sintered LiCoO.sub.2, and having a density of about 85%. Thus,
according to another aspect of the invention, relatively high
energy densities may be achieved, regardless of the shape of the
electrode (i.e., whether or not the electrode is planar or has
protrusions, walls, or the like), due to the porosity of the
electrode. In some cases, as discussed below, due to the
electrolyte-filled porosity of the electrode, substantially all of
the electrode may be able to participate in Li ion exchange, e.g.,
with the electrolyte during charge or discharge.
[0069] In some cases, the electrode may have a smallest dimension
(which may be the base or a protrusion) that is at least about 0.2
mm, and in some cases, at least about 0.25 mm, at least about 0.3
mm, at least about 0.4 mm, at least about 0.6 mm, at least about
0.8 mm, at least about 1.0 mm, at least about 1.5 mm, or at least
about 2.0 mm.
[0070] As used herein, "porous" means containing a plurality of
openings; this definition includes both regular and irregular
openings, as well as openings that generally extend all the way
through a structure as well as those that do not (e.g.,
interconnected, or "open" pores, as opposed to at least partially
non-connected, or "closed" pores). The porous electrode may have
any suitable porosity. For example, the porous electrode may have a
porosity of up to about 15%, up to about 20%, up to about 25%, up
to about 30%, up to about 40%, or up to about 50% (where the
percentages indicate void volume within the electrode).
Equivalently, the porous electrode may have a density of at least
about 50%, and up to about 70%, up to about 75%, up to about 80%,
up to about 85%, up to about 90%, or up to about 95%, where the
density is the amount of non-void volume present within the
electrode material. In some cases, the porous electrode may have an
average pore size of less than about 300 micrometers, for example,
less than about 100 micrometers, between about 1 micrometer and
about 300 micrometers, between about 50 micrometers and about 200
micrometers, or between about 100 micrometers and about 200
micrometers. The average pore size may be determined, for example,
from density measurements, from optical and/or electron microscopy
images, or from porosimetry, e.g., by the intrusion of a
non-wetting liquid (such as mercury) at high pressure into the
material, and is usually taken as the number average size of the
pores present in the material. Such techniques for determining
porosity of a sample are known to those of ordinary skill in the
art. For example, porosimetry measurements can be used to determine
the average pore size of the porosity that is open to the exterior
of the material based on the pressure needed to force a liquid,
such as mercury, into the pores of the sample. In some embodiments,
some or all of the porosity is open porosity, for example to
facilitate filling of the pores by electrolyte. Techniques for
forming a porous electrode are discussed in detail below.
[0071] Without wishing to be bound by any theory, it is believed
that the pores facilitate transport of Li.sup.+ or other ions from
the electrode to the electrolyte. In a material having a porous
structure, some of which pores may be filled with an electrolyte
(such as described below), Li.sup.+ or other ions have a shorter
distance to travel from the electrode to the electrolyte and vice
versa, thereby increasing the ability of the electrode to
participate in energy storage, and/or increasing the energy density
of the electrode. In addition, as discussed below, in some
embodiments, porous electrodes may be fabricated that have a
relatively low dimensional strain upon charge and discharge, and
such materials can withstand a surprising number of charging or
discharging cycles.
[0072] In some cases, the volume fraction porosity of the electrode
is not constant throughout the electrode, but can vary. For
example, the porosity of the surface of the electrode may be lower
than the bulk of the electrode, one end of the electrode may have a
higher or lower porosity than another end of the electrode, etc. In
one embodiment, the surface is nonporous, although the bulk of the
electrode is porous. In some cases, porosity differences in an
electrode may be created during the process of creating the porous
electrode, e.g., during the firing of a powder compact to form a
ceramic. However, in other cases, the porosity differences may be
intentionally controlled or altered, for example, by laser
treatment of the surface, rapid thermal annealing of the ceramic,
physical vapor or chemical vapor deposition, by adding particles or
other materials to the electrode surface, by coating the electrode
with a material, such as a sol-gel material, or the like. The
porosity at the surface and variation in porosity with distance
from the surface are readily observed and quantified using
techniques such as electron microscopy and image analysis of the
plan and cross-sectional views of the sample.
[0073] Electrodes such as those described above (e.g., porous,
having protrusions and/or walls, etc.) may be formed, according to
another aspect of the present invention, from a ceramic or ceramic
composite. A ceramic is typically an inorganic non-metal material,
although the ceramic can include metal ions within its structure,
e.g., transition metals or alkali ions such as Li.sup.+ or Na.sup.+
or K.sup.+, as discussed below. A ceramic composite is typically a
mixture including one or more ceramic materials, e.g. a mixture of
different ceramic phases, or a mixture of a ceramic and a metal or
a ceramic and a polymer, and may have improved properties compared
to the ceramic alone. For example, a ceramic-ceramic composite may
have an ion storage ceramic combined with a fast-ion conducting
ceramic to impart higher ionic conductivity to the composite while
still retaining ion storage functions. A ceramic-metal composite
may have improved electronic conductivity and improved mechanical
strength or fracture toughness compare to a pure ceramic. A
ceramic-polymer composite may have improved ionic conductivity if
the polymer is an electrolyte having higher ionic conductivity than
the ceramic, as well as having improved fracture toughness or
strength. Combinations of these and/or other composites are also
contemplated. In some embodiments, the electrode consists
essentially of a ceramic, and in some cases, the electrode is
formed from a unitary ceramic material. In some embodiments, the
electrode material having the lower electronic conductivity is
formed from a unitary ceramic or ceramic composite, which may
improve electron transport to and from the electrode during use of
the battery. Non-limiting examples of suitable ceramic materials
include those which are able to transport Li ions during
charging/discharging. The ceramic may be one in which Li ions can
be removed during charging (a "Li-extraction" ceramic), i.e., the
ceramic is one that contains Li ions that can be removed to form a
limiting composition material (e.g., Li ions can be extracted from
LiCoO.sub.2 to produce Li.sub.0.5CoO.sub.2, from LiNiO.sub.2 to
produce Li.sub.0.3NiO.sub.2, etc.). Examples of potentially
suitable ceramic materials comprising Li include, but are not
limited to, LiCoO.sub.2, LiNiO.sub.2, LiMn.sub.2O.sub.4, or
Li.sub.2Mn.sub.2O.sub.4 spinel, LiMnO.sub.2 of the orthorhombic or
monoclinic polymorphs, LiMPO.sub.4, olivines where M may be one or
more of Ni, Co, Mn, and Fe, Li.sub.4Ti.sub.5O.sub.12, derivatives
or modified compositions of these compounds, and/or physical
mixtures of one or more of these compounds, or the like. In some
cases, as discussed below, the ceramic has a relatively small
volumetric or linear strain differential during the insertion and
removal of an ion. Examples of such ceramics include LiCoO.sub.2,
LiNiO.sub.2, LiFePO.sub.4, and Li.sub.4Ti.sub.5O.sub.12, and their
derivative compositions and structures as well as mixtures of such
oxides. Still other examples include compounds such as Li.sub.x(Ni,
Co, Al)O.sub.2 (often referred to as "NCA"), Li.sub.x(Mn, Ni,
Co)O.sub.2, ("MNC" or "1/3 1/3 1/3"), and compounds that are
intergrowths or nanoscale mixtures between any of the structures of
"layered" ordered rock salt type or spinel type, including those in
the Li--Mn--Ni--O family. In these formulae, x may by any number
between 0 and 1.5, depending on the Li content of the synthesized
material and the charge/discharge excursions during use, e.g.,
between about 0.80 and about 1, between about 0.90 and about 1,
between about 0.95 and about 1, etc., in the lithiated (i.e.,
charged) state, and the elements within the parenthesis may be
present in any amount or any relative amount as long as standard
chemical rules of charge balance are obeyed. Typically, the sum of
the elements within the parenthesis is about 1, i.e., for
Li.sub.x(Ni.sub.a, Co.sub.b, Al.sub.c)O.sub.2 and
Li.sub.x(Mn.sub.a, Ni.sub.b, Co.sub.c)O.sub.2, the sum of a, b, and
c is about 1, although each of a, b, and c may be any number
between 0 (including 0) and about 1.
[0074] Generally, the electrode may be formed out of a single,
unitary "block" of ceramic, e.g., by "carving" the ceramic in some
fashion, for instance, through micromachining or etching techniques
or the like, to produce the final shape of the electrode. The
electrode may also be formed in a desired shape from a powder or
powder suspension, in some embodiments, using any suitable
technique, for instance, techniques such as tape-casting,
interrupted tape-casting, slip-casting, pressing, and embossing,
and the powder or powder suspension may be fired to obtain a
sintered material after its formation.
[0075] During processes such as those described above, portions of
the unitary starting material are removed in some fashion, to
produce the final shape of the electrode. Thus, the unitary
starting material is of a size larger than the final electrode that
is "carved" from the starting material. As discussed below, such
unitary ceramic materials may have several advantages, including
smaller strain differentials, lack of stress-concentrating
features, or the lack of joints or seams by which ions, fluids, or
gases could pass through. As used herein, the term "unitary" is not
meant to include structures, such as conjoined individual
particles, that are formed as separate, individual units which are
then agglomerated together in some fashion to form the final
structure; instead, a unitary material is one that is processed
(e.g., by sintering) such that any individual particles used to
form the material cease to be readily separable as individual
particles.
[0076] For example, a unitary material may be formed from a ceramic
precursor, e.g., a powder, through a sintering process. For
example, the ceramic precursor may be pressed and/or heated such
that the powder particles are bonded together, forming a unitary
whole. Porosity may be created within the sintered ceramic
material, for example, by controlling the initial powder particle
size distribution, powder packing density, the firing temperature
and time, rate of heating during various stages of the firing
process, and/or the firing atmosphere. Methods to control the
shrinkage (densification) and evolution of porosity in powder-based
materials to create a desired density or porosity are known to
those of ordinary skill in the art.
[0077] In some instances the electrode comprising a unitary
material may be formed in its desired shape from a powder mixture
or powder suspension using such processes as tape casting,
interrupted tape casting, slip casting, pressing, rolling,
extruding, embossing, or other such processes.
[0078] The compound or compounds of the electrode may have, in some
cases, a molar volume difference between the charged and discharged
state of the cell of less than about 30%, less than about 15%, less
than about 10%, or less than about 5%. In some embodiments the
compound or compounds of the electrode has a linear or a volumetric
strain between the charged and discharged state of the cell of less
than about 20%, less than about 15%, less than about 10%, less than
about 5%, less than about 3%, less than about 2%, or less than
about 1%. In some embodiments, the compounds of the electrode
include at least one compound that increases in molar volume at
least some compositions during use and at least one compound that
decreases in molar volume at at least some compositions during use.
In some embodiments, the net volume change of the electrode between
the charged and discharged state of the battery is decreased by
combining at least one compound that has a net positive volume
change between the charged and discharged state, with at least one
compound that has a net negative volume change between the charged
and discharged state of the battery. In one set of embodiments, the
electrode is fabricated from a ceramic material having a relatively
small linear or a volumetric strain differential when the electrode
is infiltrated with Li ions.
[0079] Non-limiting examples of such materials include LiCoO.sub.2
(having a linear strain differential averaged along all
crystallographic orientations of about +0.6% upon delithiating to a
composition of about Li.sub.0.5CoO.sub.2) and LiNiO.sub.2 (having a
linear strain differential of about -0.9% upon delithiating to a
composition of about Li.sub.0.3NiO.sub.2). Such a material may be
able to withstand a relatively large number of charging or
discharging cycles while remaining free of cracks or otherwise
degrading, as the material does not expand or contract
significantly during charging or discharging. Linear strain is
generally defined as the change in length of a material with
respect to the initial length (.DELTA.L/L.sub.0), and volumetric
strain is similarly defined, but with respect to the initial
volume. For example, a material of the instant invention may be
able to withstand at least 6 cycles, at least 10 cycles, at least
15 cycles, or at least 20 cycles of complete charging and
discharging (e.g., at a C/20 rate), while remaining free of
identifiable cracks or other degradations (e.g., chips, peeling,
etc.) that can be observed under scanning electron microscopy. As
an example, in FIGS. 9A-9C, a ceramic material used as an electrode
was fully charged and discharged (i.e., "cycled") at a C/20 rate 6
times, and then studied using scanning electron microscopy (SEM).
Thus, in another set of embodiments, the electrode is able to
retain at least 50% of its initial storage capacity after at least
6 charge-discharge cycles at a C/20 rate. As can be seen in these
figures (at different magnifications, as shown by the scale bars),
no obvious degradation or cracking of the ceramic material was
observed. In contrast, many prior art materials are unable to
withstand such conditions.
[0080] It is unexpected that a sintered ceramic electrode as
described herein could be electrochemically cycled repeatedly
without substantial evidence of mechanical failure. Firstly,
intercalation compounds such as the lithium transition metal oxides
typically have a rock salt or ordered rock salt structure, spinel
structure, olivine structure, or rutile structure, amongst others.
These typically have high elastic moduli and low fracture toughness
and are brittle. For such compounds, the linear strain to failure
is typically less than about 1%, an amount that is exceeded by the
typical linear strain induced upon charging and discharging. Also,
several studies have shown that particles of intercalation
compounds used in rechargeable lithium batteries sustain fracture
and disorder and defect formation in their crystalline structure
upon being charged and discharged. In addition, the strains induced
upon charging and discharging may, in some cases, be larger than
the thermal strains typically induced in ceramic parts during
thermal shock that leads to fracture, such as the thermal shock of
a glass body. Thus, it is unexpected that the electrodes could
sustain the differential strains during charging and discharging,
which necessarily induce strain and stress gradients since
different portions of an electrode undergo different degrees of
expansion or contraction as ions are added from the opposing
electrode. As an example, Table 1 shows the linear strain to
failure of several example compounds induced upon charging and
discharging. Table 1 also shows a listing of several well-known
lithium storage compounds and their volumetric and average linear
strains during charging and discharging.
TABLE-US-00001 TABLE 1 Volume Linear Lithium Storage Limiting
Strain Strain* Potential Compound Composition .DELTA.V/V.sub.0
.DELTA.L/L.sub.0 vs. Li/Li.sup.+ Li-extraction LiCoO.sub.2
Li.sub.0.5CoO.sub.2 +1.9% +0.6% 4.0 V LiFePO.sub.4 FePO.sub.4 -6.5%
-2.2% 3.4 V LiMn.sub.2O.sub.4 Mn.sub.2O.sub.4 -7.3% -2.5% 4.0 V
LiNiO.sub.2 Li.sub.0.3NiO.sub.2 -2.8% -0.9% 3.8 V Li-insertion C
(graphite) 1/6 LiC.sub.6 +13.1% +4.2% 0.1 V
Li.sub.4Ti.sub.5O.sub.12 Li.sub.7Ti.sub.5O.sub.12 0.0% 0.0% 1.5 V
Si Li.sub.4.4Si +311% +60% 0.3 V .beta.-Sn Li.sub.4.4Sn +260% +53%
0.4 V *for a randomly oriented polycrystal
[0081] As shown in the examples below, dense sintered electrodes of
intercalation oxides with substantial strain during charge and
discharge can be electrochemically cycled without experiencing
detrimental mechanical failure, in contrast to the prior art.
Detrimental mechanical failure would include fracture or multiple
fractures that propagate across the electrode, crumbling or
comminuting of the sintered particles causing a loss of
connectivity between the particles, or a significant loss of
electrochemical storage capability due to such events. This is
observed in compounds such as LiCoO.sub.2, in which the
differential strain during charge and discharge is near the strain
to failure of a brittle ceramic, as well as in compounds such as
nanoscale doped olivines in which the differential strain is above
that which would be expected to cause failure.
[0082] Without being bound by any particular scientific
interpretation, it is believed that during electrochemical cycling
of various electrodes of the invention, microcracking of particles
and at grain boundaries between particles may occur, but that such
damage remains localized and does not propagate across the
electrode causing failure as it would in a typical sintered ceramic
of similar physical properties and sintered density subjected to
the same strain. Instead, the strains induced during
electrochemical cycling may be anisotropic at the crystal level,
and/or may be able to accommodate microcracks distributed widely
throughout the material, which may dissipate stored elastic energy
without causing failure on a length scale much larger than the
particle size. Such ceramics do not exhibit high strength in
comparison to other ceramics of comparable density and particle and
pore size, but can be damage tolerant in some cases. Considered in
such manner, various electrodes of the invention can be made
damage-tolerant by taking into account the differential strain
during charging and discharging, the crystalline anisotropy in
strain, the crystallite size, agglomerate size, sintered density,
and other microstructural and processing considerations well-known
to those skilled in the art of ceramic materials processing. For
example, the larger crystalline strain of LiFePO.sub.4 compared to
LiCoO.sub.2 necessitates a smaller particle size to avoid damaging
fracture events, all other factors such as density, particle size
distribution and pore size distribution being constant.
[0083] Thus in some embodiments, a porous electrode of the present
invention that does not comprise additional ductile phases
providing mechanical toughness may have a differential volume
change of less than about 20%, less than about 15%, or less than
about 10% between the charged and discharged state. In some
embodiments, the microstructure of the sintered electrode, as
characterized by well-known measures such as grain size, grain
shape, grain size distribution, pore volume, the relative fractions
of open and closed porosity, pore size distribution, or pore
topology, is adjusted to permit reversible cycling with relatively
low capacity loss. In some embodiments, the particle size may be
reduced to improve damage tolerance, for example using particles
having a primary (single crystallite) size of less than about 500
nm, less than 200 nm, or less than about 100 nm. In some
embodiments the particles have an anisometric shape, including
being in the shape of a rod or plate in which the aspect ratio
(ratio of the longest dimension to the shortest) is at least a
factor of 2, at least a factor of 5, or at least a factor of 10,
which may improve damage tolerance in some cases.
[0084] In yet another set of embodiments, the electrodes may
comprise a mixture of compounds, such compounds being selected to
achieve a desired volumetric or linear differential strain upon
charging and discharging the battery. By selecting compounds in
this manner, the electrode may attain improved tolerance to
electrochemical cycling induced mechanical damage, and/or the total
volume change of the cell constituents, including both anode and
cathode, during cycling may be reduced. As a non-limiting example,
referring to Table 1, LiCoO.sub.2 can be seen to experience a net
volume contraction of about 1.9% upon being charged to the
composition Li.sub.0.5CoO.sub.2, whereas LiFePO.sub.4,
LiMn.sub.2O.sub.4, and LiNiO.sub.2 all exhibit volumetric expansion
upon being charged. For a mixture of LiCoO.sub.2 with one or more
of the latter three compounds, under particular charging conditions
such as voltage and current rate and time, each of the constituent
materials reaches a particular lithium concentration, and therefore
a particular change in volume compared to the starting discharged
state. Accordingly, in one embodiment, the electrode is selected to
comprise a mixture of compounds, such compounds being selected to
achieve a volumetric or linear differential strain of less than
about 20%, less than about 15%, less than about 10%, less than
about 5%, less than about 3%, less than about 2%, or less than
about 1%, upon charging and discharging the battery
[0085] These volumetric changes are readily determined by methods,
such as X-ray diffraction of the charged electrode, that are
well-known to those of ordinary skill in the art. For instance, the
net volume change of the electrode at any particular
state-of-charge may be selected by mixing the constituents in
certain ratios easily determined by calculation or experimentation.
As an example, a mixture of LiCoO.sub.2 and LiNiO.sub.2 may be
selected to provide net zero expansion between the charged and
discharged states.
[0086] In some embodiments, the porous sintered electrode is
selected to comprise the less electronically conductive of the
cathode and anode materials. The porous sintered electrode
construction may provide a continuous interconnected material
and/or improve the electronic conductivity of the ion storage
material network compared to, for example, a compacted powder that
has not been sintered. Thus the electronic conductivity of the
sintered porous electrode can be as good as, or better than, that
of a conventional lithium ion battery electrode that typically
comprises an active material powder, conductive additive such as
carbon black, and polymer binder, while having lesser or no
additive phases, and having a higher volume fraction of additive
material. As shown in the examples, a sintered LiCoO.sub.2 or
sintered lithium metal phosphate olivine cathode can have much
higher volume packing density, e.g., as high as 70-85% density, and
can be electrochemically cycled without incorporating any
conductive additive or binder in the electrode.
[0087] In some embodiments, the sintered electrode comprises a
lithium storage compound that increases in electronic or ionic
conductivity when alkali ions are removed or inserted into said
compound. As a non-limiting example, Li.sub.1-xCoO.sub.2 may
exhibit increased electronic conductivity with increasing x, and
may undergo a semiconductor to metal transition at x.about.0.03.
Thus, a benefit may be provided under certain conditions by
utilizing LiCoO.sub.2 or other compounds exhibiting such behavior
in a battery, in one embodiment of the invention. As the battery is
charged and lithium is extracted from the LiCoO.sub.2, the
impedance of the electrode decreases, which may facilitate
electrochemical use of the electrode. A further benefit may be
realized in some cases based on the typical behavior of lithium
rechargeable cells where there is a first cycle irreversible loss
of lithium due to the formation of side-reaction products. The
irreversible consumption of lithium may cause the LiCoO.sub.2 to
remain lithium-deficient thereafter, in certain cases, even in the
discharged state of the cell, and thereby may cause the sintered
cathode to retain a high electronic conductivity in some
embodiments of the invention.
[0088] In some embodiments, a porous electrode of the present
invention may contain an electrolyte within the pores of the porous
electrode. The electrolyte, in some cases, may be a liquid
electrolyte, such as a mixture of alkyl carbonates and a lithium
salt such as LiPF.sub.6, or a polymer electrolyte, such as
polyethylene oxide or a block copolymer. The electrolyte may also
be, for instance, a gel or an inorganic compound. Non-limiting
examples of inorganic electrolytes include a lithium phosphorus
oxynitride compound, lithium iodide, or the like. In some cases,
the electrolyte can comprises any combination of these and/or other
materials.
[0089] In some cases, the electrolyte and/or the electrode may
contain a lithium salt to impart lithium ion conductivity.
Formulations for such electrolytes, including additives to improve
safety, cycle life, and/or calendar life amongst other attributes,
are known to those skilled in the art, and it should be understood
that any such formulation may be used, based on the desired
attributes of the battery for a particular application. The
electrolyte contained within the electrode may or may not have the
same concentration or composition as the electrolyte that separates
the electrode from an opposite electrode (i.e., separating the
cathode and the anode within a battery). A liquid electrolyte may
be useful, for example, to facilitate flow of Li ions into and out
of the porous electrode. In some cases, the liquid electrolyte may
comprise Li ions. An example of such an electrolyte is one using
LiPF.sub.6 as the lithium salt. Depending on the porosity of the
electrode, the liquid electrolyte may be introduced into the pores
of the electrode by exposing the pores to the liquid electrolyte,
for instance, as discussed below. The electrolyte, in some cases,
may also surround the protrusions of the electrode (if protrusions
are present). For example, the electrolyte may be contained within
the electrode (e.g., within walls of an electrode, if a wall is
present), bathing the protrusions in electrolyte.
[0090] In some aspects of the invention, various combinations of
density, porosity, thickness, and C-rates of operation may impart
novel, unexpected performance characteristics on the electrodes
described herein. Examples of densities, porosities, thicknesses,
C-rates (i.e., charging or discharging rates) have been discussed
above, and these parameters may be combined in any suitable
combination. In one set of embodiments, the product of the density
(expressed in %, relative to the theoretical density of the
ceramic, i.e., a 100% dense ceramic) and the thickness (or minimum
dimension, expressed in mm) of the electrode can be used to define
a novel combination of density or porosity and thickness (expressed
in %-mm) that produces such unexpected performance characteristics.
A non-limiting example of such materials is discussed below with
reference to Example 8.
[0091] In one set of embodiments, the product of density and
thickness may be between about 10%-mm and about 150%-mm, between
about 20%-mm and about 150%-mm, between about 30%-mm and about
150%-mm, between about 40%-mm and about 150%-mm, between about
50%-mm and about 150%-mm, between about 60%-mm and about 150%-mm,
between about 75%-mm and about 150%-mm, or between about 100%-mm
and about 150%-mm, etc, In another set of embodiments, the product
of porosity and thickness may be between about 150%-mm and about
2.5%-mm, between about 150%-mm and about 5%-mm, between about
150%-mm and about 10%-mm, between about 150%-mm and about 20%-mm,
between about 150%-mm and about 30%-mm, between about 150%-mm and
about 40%-mm, between about 150%-mm and about 50%-mm, between about
150%-mm and about 75%-mm, or between about 150%-mm and about
100%-mm, etc.
[0092] Another aspect of the present invention is directed to a
separator. The anode and the cathode in a battery or other
electrochemical device are generally electronically insulated from
each other while having an electrolyte to permit ion exchange. A
porous "separator" material that is infused with an ion-conducting
electrolyte can serve this function. According to one set of
embodiments, a separator is used in a battery that comprises a
porous polymer film, and/or a porous ceramic layer. In some cases,
the film or layer may have a pore fraction of between 10% and 70%
by volume, or between 25% and 75% by volume, and a thickness
between about 5 micrometers and about 500 micrometers, between
about 100 micrometers and about 2000 micrometers, between about 300
micrometers and about 1000 micrometers, etc. The film or layer may
also have a porosity of at least about 30%, at least about 40%, or
at least about 50%, and/or the porosity may be no more than to
about 60%, about 65%, about 70%, or about 75%. The thickness may
also be less than about 300 micrometers, or less than about 100
micrometers, and/or greater than 10 micrometers, greater than 30
micrometers, or greater than about 50 micrometers. In some
embodiments, a relatively thick porous ceramic separator may be
useful in decreasing the frequency of occurrence of internal short
circuits due to lithium dendrite formation.
[0093] According to another set of embodiments, the electrolyte is
nonporous (i.e., solid), i.e., the electrolyte does not contain
"pinholes" or defects (such as pores or cracks) through which Li
dendrite formation leading to short circuits can occur, even after
tens, hundreds, or thousands of cycles of charging or discharging.
In some cases, the electrolyte comprises Li ions, which may be
useful, to facilitate flow of Li ions into and out of the adjacent
electrodes. Amongst numerous possible choices, one example of such
an electrolyte is LiPON (lithium phosphorus oxynitride), an
inorganic material typically made in thin-film form by sputtering.
Another example of an electrolyte is lithium iodide (LiI). In one
set of embodiments, the electrolyte is present as a film, which can
be deposited by sputtering or other physical vapor or chemical
vapor methods. In some cases, the electrolyte is a conformal film
formed upon the electrode surface using layer-by-layer deposition,
i.e., where discrete molecular layers of electrolyte material are
added to the electrode until a suitably thick layer of electrolyte
has been built up. Those of ordinary skill in the art will be aware
of suitable layer-by-layer deposition techniques, which typically
involve the application of molecular layers of alternating positive
and negative charge from wet chemical solution.
[0094] The nonporous electrolyte may be used, in some embodiments,
to seal the electrode surface, and in some cases, to create a
hermetically sealed compartment containing the electrode and an
electrolyte, such as a liquid or a polymer electrolyte, within the
sealed compartment. Thus, the hermetically sealed compartment may
be defined by the walls of the cell, the base of the electrode, and
the lid formed by the nonporous electrode. A non-limiting example
of a battery having such a nonporous electrolyte is shown in FIG.
10, in which the nonporous electrolyte layer 16 seals the
compartment beneath it formed by the walls of the electrode 15,
within which an electrolyte resides. The volume of the cell outside
of this compartment may or may not be also filled with electrolyte.
The nonporous electrolyte may have any suitable size and/or shape.
For example, portions of the electrolyte may extend into the
interior space of the electrode, or the electrolyte may essentially
define a substantially planar layer or "lid" above the walls of the
electrode, e.g., as in FIG. 10. For instance, the nonporous
electrolyte may have a thickness of at least about 1 micrometer, at
least about 3 micrometers, at least about 5 micrometers, at least
about 10 micrometers, at least about 20 micrometers, at least about
30 micrometers, at least about 50 micrometers, etc.
[0095] Yet another aspect of the invention is directed to
techniques for making such electrodes and batteries or
microbatteries. In one set of embodiments, a unitary ceramic
material is used, and in some, but not all embodiments, the
material may be etched in some fashion, for example, using
micromachining techniques such as laser micromachining, or dry
etching or wet chemical etching methods well known to those skilled
in the art of fabricating microelectromechanical systems (MEMS).
Such machining processes may be used to form the walls and/or
protrusions on the surface of the base of the electrode. In another
set of embodiments, the protrusions or walls of the electrode are
produced directly by forming a starting powder or composite mixture
under pressure using a die having the inverse of the desired final
geometry. The electrode thus formed may be used directly or may be
sintered after forming.
[0096] In the non-limiting example of a completed battery shown in
FIG. 10, the cathode 14 has a plurality of protrusions 18 that
extend away from the surface of a base 15 of the cathode,
surrounded by a wall 11. In addition, the battery may be contained
within a packaging material 27, as is shown in FIG. 10. Packaging
materials for batteries are known to those skilled in the art. For
lithium batteries, non-limiting examples include polymers,
polymer-metal laminates, thin-walled metal containers, metal
containers sealed with polymers, and laser-welded metal containers.
For the batteries of the invention, one embodiment uses inorganic
compounds such as insulating oxides as the packaging material. Such
compounds may be applied to the exterior of the battery by physical
vapor deposition or coating from wet chemical solutions or particle
suspensions, or the package may be pre-formed and the battery
inserted within.
[0097] The cathode may be laser-micromachined, and has a height of
about 500 micrometers in the particular example in FIG. 10. The
cathode is in electrical communication with a current collector 19,
such as a gold current collector, which in turn is positioned on a
substrate 23, for instance, an alumina substrate. The collector may
have any suitable thickness, for example, about 25 micrometers,
about 50 micrometers, about 75 micrometers, about 100 micrometers,
etc. In some cases, the electrode may have a thickness of between
about 100 micrometers and about 2000 micrometers, or between about
between 300 micrometers and about 1000 micrometers. Similarly, the
substrate may have any suitable shape and/or dimensions, depending
on the cathode. For instance, the base may have a thickness of at
least about 0.25 mm, at least about 0.3 mm, at least about 0.4 mm,
at least about 0.5 mm, at least about 0.75 mm, at least about 0.8
mm, at least about 1 mm, at least about 2 mm, etc.
[0098] In some embodiments, within the walls of cathode 15, which
may be porous, is a liquid electrolyte 13, for example about 1.0 M
to about 1.5 M, e.g., about 1.33 M, of LiPF.sub.6 dissolved in a
mixture of organic and/or alkyl carbonates. Such liquid
electrolytes are well-known to those skilled in the art of
nonaqueous batteries, and may, in some cases, contain additive
compounds that stabilize the solid-electrolyte interface (SEI)
between the electrode and the electrolyte, improve the temperature
range over which the battery may be used, provide flame retardance,
suppress gas formation, and/or retard the growth of lithium
dendrites. The liquid electrolyte is contained within the electrode
via a nonporous electrolyte 16, for example, a solid inorganic or a
polymeric electrolyte. The nonporous electrolyte may also
conformally cover the surfaces of cathode 15. The nonporous
electrolyte may be able to conduct electrons and/or ions back and
forth between the cathode and the anode, and may have any suitable
thickness or shape, for example, a thickness of at least about 1
micrometer, at least about 3 micrometers, at least about 5
micrometers, at least about 10 micrometers, at least about 20
micrometers, at least about 30 micrometers, at least about 50
micrometers, etc.
[0099] In the example of FIG. 10, the anode 12, positioned adjacent
to the nonporous electrode, is in electrical communication with an
anode current collector 17, such as a metal current collector
(e.g., Cu). The anode current collector may have any suitable
thickness, for example, at least about 1 micrometer, at least about
3 micrometers, at least about 5 micrometers, at least about 10
micrometers, at least about 25 micrometers, at least about 50
micrometers, at least about 75 micrometers, at least about 100
micrometers, etc., and may or may not be the same thickness and/or
comprise the same materials as the cathode current collector,
depending on the embodiment and the application. In instances where
nonporous electrolyte 16 conformally coats the surface of electrode
15, anode 12 may also conformally coat the film of electrolyte 16
in some cases, or may fill the space between the protrusions of
electrode 15 while remaining everywhere separated from the
electrode 15 by the conformal electrolyte film in certain
embodiments. In some embodiments the electrode 15 is the initial
source of the alkali ions that are stored in the electrodes during
charge and discharge, and no anode is used, but simply a negative
current collector.
[0100] In some cases, alkali ions, such as lithium, are deposited
at the negative current collector as alkali metal upon charging of
the battery, and/or are removed and deposited in the positive
electrode upon discharge. In some embodiments, disposed on the
negative current collector is a material to facilitate the further
deposition of alkali metal during charging of the battery. This
material may be an alkali metal, such as lithium metal, or may be
an anode-active compound for lithium ion batteries that
intercalates or alloys with lithium metal without enabling the
precipitation of metallic lithium. Such compounds include carbon
materials such as graphite or hard carbons, intercalation oxides
such as Li.sub.4Ti.sub.5O.sub.12, metals and metalloids such as B,
Al, Ag, Au, Bi, Ge, Sn, Si, Zn, alloys comprising one or more of
such metals and metalloids, and mixtures of such metals or
metalloids or their alloys. In some embodiments, the amount of such
anode-active material is at least sufficient to completely absorb
the lithium supplied by the cathode-active material during charge,
as is the case in conventional lithium-ion batteries. In other
embodiments, however, the amount of such material is lower, and the
material may both saturate with the alkaline metal and provide a
location for the further deposition of the alkali metal as the
battery is charged.
[0101] As mentioned above, the ceramic electrode may be formed, for
example, by sintering particles together, e.g., forming a unitary
material. However, the invention is not limited to sintered
ceramics; for instance, other ceramic materials or composites may
be used. Techniques for sintering particles to form a ceramic are
known to those of ordinary skill in the art, e.g., forming a
sintered ceramic by pressing and/or heating a precursor to form the
ceramic. In one set of embodiments, such sintering may be used to
form a porous unitary structure. As discussed, porosity may be
created within the sintered ceramic material, for example, by
controlling the sintering temperature and pressure, and such
process conditions can be optimized to create a desired density or
porosity using routine optimization techniques known to those of
ordinary skill in the art. In some embodiments, porosity is
introduced into the sintered electrode by incorporating with the
starting powder a constituent that can be later removed, which may
thus leave behind pores under some conditions. Such constituent may
be referred to as "fugitive material." For example, a fugitive
material that is incorporated into the compacted powder that
becomes the sintered electrode may be removed by any suitable
technique, for example, chemical dissolution, melting and draining
of the melted liquid, sublimation, oxidation, and/or pyrolysis,
while leaving the material of the sintered electrode behind.
Examples of such fugitive materials include, but are not limited
to, ice, which may be moved by melting or sublimation, naphthalene,
which may be sublimed, polymer constituents such as latex spheres
or polymer fibers, which may be chemically dissolved, melted,
and/or pyrolysed, and carbonaceous particles or platelets or
fibers, which may be removed by oxidation at elevated temperatures.
Such carbonaceous particles may be, for instance, carbon or
graphitic spherical particles, graphite platelets, graphite or
carbon fibers, vapor-grown carbon fibers (VGCF), and carbon
nanofibers or carbon nanotubes. As a specific example, LiCoO.sub.2
is typically fired in oxidizing gaseous atmosphere such as air or
oxygen. By including carbon fibers in a compact made from
LiCoO.sub.2 powder, and pyrolyzing the carbon fibers upon firing in
oxidizing atmosphere, elongated pore channels may be left behind in
the sintered LiCoO.sub.2 compact which, when filled with
electrolyte, may be useful for ion transport and thus to the
battery's power and energy utilization.
[0102] The desired shape of the electrode may be fashioned using
micromachining techniques such as laser micromachining, deep
reactive-ion etching, ion-milling, or the like. Those of ordinary
skill in the art will be familiar with such techniques. For
instance, in laser micromachining, a laser is directed at the
unitary ceramic material. The laser light, when interacting with
the ceramic material, may melt, ablate, or vaporize the material,
which may be used to control the shape of the final electrode.
Thus, laser micromachining can produce an object having a desired
shape by removing, in some fashion using a laser, everything that
does not belong to the final shape. The laser may have any suitable
frequency (wavelength) and/or power able to destroy or otherwise
remove such ceramic materials in order to produce the final
structure for use in a battery or other electrochemical device.
[0103] The following is a non-limiting example of a method of
manufacturing an embodiment of the invention. Referring now to FIG.
11, in pathway A, the creation of a battery, which may be a
microbattery, having a plurality of protrusions and a wall
surrounding the plurality of protrusions, is shown. A unitary
ceramic material is formed into an electrode having a plurality of
protrusions and a wall surrounding the plurality of protrusions
using techniques such as laser micromachining. The electrode may
also contain a current collector, for instance comprising gold or
another metal, such as silver.
[0104] In one technique, a separator or electrolyte layer
comprising LiPON and/or a polymer or organic electrolyte is first
added to the electrode. As shown in FIG. 10, LiPON may be sputtered
onto the electrode, or a polymer or organic separator may be
deposited onto the electrode in some fashion, for instance, using
coating from sol-gel solution, electrodeposition techniques, or
layer-by-layer assembly.
[0105] Next, the counterelectrode is added to substantially fill
the remaining space. In one technique, the interior space defined
by the walls of the electrode is filled with a colloidal
suspension, the colloidal particles being the negative electrode
material and optionally additive particles such as conductive
additives or binders. However, in another technique, a "flux and
solder" approach is used, which Au is first sputtered onto the
separator, then Li (e.g., Li solder) is melted onto the Au. Such a
technique may be useful in cases where the electrode and/or the
electrolyte contains a material that Li metal, when in a liquid
state, will not "wet" or substantially adhere to. In such cases,
gold or another compatible metal that Li will "wet" when Li is in a
liquid state, is used to facilitate bonding. Without wishing to be
bound by any theory, it is believed that Li is able to react with
the metal to wet the surface. The top current collector (e.g., a
metal, such as Cu, is then added, and optionally, the battery is
sealed. The battery can then be packaged, e.g., by depositing
parylene and/or a metal hermetic oxide or thick film onto the
battery.
[0106] In another set of embodiments, a battery, such as a
microbattery, having a plurality of protrusions and a wall
surrounding the plurality of protrusions can be created as follows.
Referring again to FIG. 11, in pathway B, the creation of a battery
may proceed by allowing self-organization of the counter electrode
and the separator to occur. In this approach, repulsive forces
between the electrode and the counterelectrode are used to create a
separation that is spontaneously filled by separator or electrolyte
material. The repulsive forces used to self-organize the two
electrodes with respect to each other include but are not limited
to van der Waals forces, steric forces, acid-base interactions, and
electrostatic forces. Subsequently, as before, a top current
collector (e.g., a metal, such as Cu, is then added, and
optionally, the battery is sealed). The battery can then be
packaged, e.g., by depositing parylene and/or a metal hermetic
oxide or thick film onto the battery.
[0107] U.S. patent application Ser. No. 10/021,740, filed Oct. 22,
2001, entitled "Reticulated and Controlled Porosity Battery
Structures," by Chiang, et al., published as U.S. Patent
Application Publication No. 2003/0082446 on May 1, 2003, and U.S.
patent application Ser. No. 10/206,662, filed Jul. 26, 2002,
entitled "Battery Structures, Self-Organizing Structures, and
Related Methods," by Chiang, et al., published as U.S. Patent
Application Publication No. 2003/0099884 on May 29, 2003, are
incorporated herein by reference. Also incorporated herein by
reference are U.S. Provisional Patent Application Ser. No.
60/931,819, filed May 25, 2007, by Chiang, et al.; U.S. Provisional
Patent Application Ser. No. 61/027,842, filed Feb. 12, 2008, by
Marinis, et al.; and U.S. patent application Ser. No. 10/329,046,
filed Dec. 23, 2002, entitled "Conductive Lithium Storage
Electrode," by Chiang, et al., published as U.S. Patent Application
Publication No. 2004/00055265 on Jan. 8, 2004. Also incorporated
herein by reference are U.S. patent application Ser. No.
12/126,841, filed May 23, 2008, entitled "Batteries and Electrodes
For Use Thereof," by Chiang, et al.; International Patent
Application No. PCT/US2008/006604, filed May 23, 2008, entitled
"Batteries and Electrodes For Use Thereof," by Chiang, et al.; and
a U.S. provisional application filed on even date herewith,
entitled "Small Scale Batteries and Electrodes For Use Thereof," by
Chiang, et al.
[0108] The following examples are intended to illustrate certain
embodiments of the present invention, but do not exemplify the full
scope of the invention.
Example 1
[0109] This example illustrates an integrally packaged, solid-state
lithium rechargeable microbattery with a 3-dimensional
interpenetrating-electrode internal architecture, in accordance
with one embodiment of the invention. Such microbatteries may have
the capability for outer package aspect ratios of (for example)
less than 5:1 for maximum to minimum dimensions (i.e., not
restricted to thin planar configurations), active materials
packaging fraction of >75% in a 1 mm.sup.3 volume, under which
conditions they will exceed an initial energy density target of 200
W h/l by a factor of 3 to 7. The approach in this example will use
currently available and proven cathode and anode materials, but
does not exclude higher energy or higher rate active materials in
the future.
[0110] The microbatteries in this example will allow energy
densities of about 200 W h/l to about 1500 W h/L to be achieved,
depending on the electrochemical couple used, and specific design
parameters, as discussed below. Microbatteries of this form could
be used to power a wide variety of small systems from simple
sensors to systems with integrated ultrahigh density packaging.
[0111] A microfabricated structure of 3D electrode arrays is
co-fabricated with an integral hermetic package, e.g., as is
illustrated in FIGS. 6A-6E. This particular demonstration uses
graphite and laser micromachining as the fabrication method. Using
highly-oriented pyrolytic graphite (HOPG) that was laser-machined
to about 200 micrometer half-thickness, cycling rates of about C/20
were demonstrated in lithium half-cells. In graphite, a ten-fold
increase in rate to 2C would require a factor of 10.sup.1/2=3.2
reduction in cross-sectional dimension (e.g., diffusion time
t=x.sup.2/D, where x is the diffusion length and D the diffusion
coefficient). These dimensions are achievable with laser
micromachining technology. In order to maximize energy density, the
electrode cross-sectional dimensions should be as large as possible
while still supplying the desired rate capability (since the
inactive materials fraction increases as the feature size
decreases). In some microbattery applications, electrodes having
micrometer to tens of micrometer dimensionality may be
sufficient.
[0112] FIG. 6A shows that laser micromachining can produce
individual electrode features in graphite having about 50
micrometer half-thickness and 0.5 mm height with a slight
(controllable) taper, forming a 3 mm.times.3 mm array (4.5 mm.sup.3
volume). Furthermore, the lateral resolution and taper of the kerf
in laser machining is strongly impacted by the thermal conductivity
of the material being machined, with high thermal conductivity
decreasing resolution and increasing taper. In lithium
intercalation oxides of low thermal conductivity compared to
graphite, it is expected that closely-spaced features of about 10
to about 20 micrometer total width to be possible at feature
heights of about 0.5 mm to about 1 mm. In this example,
3-dimensional (3D) electrodes of similar morphology but having
smaller cross-sections can be fabricated from lithium storage
compounds, by laser-micromachining or other microfabrication
processes, for example, amenable to simultaneous fabrication of
many devices. These continuous and dense 3D electrode arrays can be
fabricated from the active material of lower electronic
conductivity, usually the cathode, in order to decrease electronic
polarization and increase the rate capability of the final
device.
[0113] Using the microfabricated electrode/package structures as
the starting template, three example paths to fabrication of the
completed battery are demonstrated, with reference to FIG. 11, as
discussed below.
[0114] In one path, conformal deposition of a solid inorganic
electrolyte film (e.g., LiPON) is performed by sputtering, which
can create an electronically insulating layer of 1 micrometer to 3
micrometer thickness, which may cover the upward-facing surfaces.
The taper of these electrode features can be "tuned" through
instrumental parameters to allow conformal coating. At such
thickness, the impedance of the electrolyte film during subsequent
use as a battery may be low enough that the rate capability can be
primarily determined by the electrodes. After electrolyte
deposition, the remaining free volume within the cell can be filled
by the counterelectrode. The counterelectrode will, in one
instance, be Li or a Li alloy, melt-infiltrated (about 180.degree.
C.) into the coated electrode array using a "flux and solder"
process to enable high surface tension liquid lithium to wet oxide
surfaces, as discussed above. An advantage of using lithium metal
is that its high volumetric capacity allows the negative electrode
to be of small volume, for example only about one-fourth that of
the positive electrode, if LiCoO.sub.2 is used. Thus, a negative
electrode film of only several micrometer dimensions filling the
pore space of the electrode array may be needed for cell balancing.
Alternatively, the counterelectrode can be applied in the form of a
powder suspension where a solid polymer electrolyte (e.g.,
PEO-based) is included in the formulation to provide a fully
solid-state device. Subsequently, a top current collector can be
applied by physical vapor deposition or thick film paste
technology, following which a hermetic sealing layer including a
sputtered oxide or CVD-applied polymer layer (parylene) is used to
complete the packaging.
[0115] In another path, similar to the path outlined above, the
electrolyte film is an electrodeposited layer of a solid polymer
electrolyte. Methods for the electrodeposition of electronically
insulating polymer films can be applied in this project to form
electrolytic layers. Alternatively, a layer-by-layer deposition
approach may be used. The counterelectrode may be powder suspension
based, since even the modest melting temperature of Li alloys could
damage polymeric electrolytes. The subsequent packaging steps are
similar as described above.
[0116] In yet another path, a colloidal-scale self-organization
approach may be applied. LiCoO.sub.2 and graphite immersed in a
suitable solvent may be mutually repulsive due to short-range
dispersion and electrostatic forces. FIGS. 12A-12D shows key
results in which the mutual repulsion between sintered dense
LiCoO.sub.2 and an MCMB (mesocarbon microbead) suspension formed a
rechargeable lithium battery under the influence of the surface
forces. The constituents of solid polymer electrolytes were
dissolved in the solvent without negatively affecting the
interparticle forces. FIG. 12A shows a cell schematic. FIG. 12B
shows the open circuit potential (OCP) between LiCoO.sub.2 and MCMB
upon forced contact, showing an electrical short-circuit upon
contact for acetonitrile, but an open circuit for MEK (methyl ethyl
ketone) due to repulsive surface forces. FIG. 12C shows reversible
galvanostatic cycling of a self-organized battery using MEK and 0.1
M LiClO.sub.4 as the electrolyte. FIG. 12D shows measurements of
the potential difference between a Li titanate reference electrode
and the LiCoO.sub.2 working (W) and MCMB counter (C) electrodes,
conducted in MEK and 0.1 M LiClO.sub.4 and 1 wt % PEG 1500
(poly(ethylene glycol)). All potentials referenced to Li/Li.sup.+.
Potentials observed during each stage of the test demonstrate
Faradic activity, with the LiCoO.sub.2 being delithiated and MCMB
being lithiated. In the present configuration, an MCMB suspension
can be used to fill the integral container formed from the
LiCoO.sub.2 and a self-forming separator obtained upon drying.
Subsequent application of a top current collector and outer
packaging will be carried out in the same manner as the above.
[0117] The energy densities are determined in these devices by the
volume fraction of active materials present in the cell, and the
degree of electrochemical utilization of those materials. In FIGS.
13A and 13B, plots of the expected energy density for
microbatteries made from 5 different electrochemical couples using
the present fabrication approach is plotted against the volume
fraction of inactive material in the packaged cell due to the
electrolyte layer, integral package wall, current collectors, and
outer packaging for 5 mm.sup.3 (FIG. 13A) and 1 mm.sup.3 (FIG. 13B)
volumes. In each case, the relative volumes of the positive and
negative electrode are as needed for a charge-balanced cell. The
theoretical energy density (at zero percent inactive material) of
these systems exceeds 350 W h/L by a factor of 2.3 to 5. The
results for 5 mm.sup.3 microbatteries of the configuration in this
example are calculated assuming realistic component dimensions: 50
micrometer electrode diameter with 100 micrometer or 60 micrometer
integral package wall thickness, 2 micrometer electrolyte layer
thickness, and 10 micrometer thick current collectors. The
thickness of the outer packaging is treated as a variable, ranging
from 25 micrometer to 150 micrometer thickness. Also shown in FIG.
13A is an experimental data point (identified as 21), which
illustrates that substantially all of the LiCoO.sub.2 has been
utilized.
[0118] FIG. 14 compares the results in FIGS. 13A and 13B against
recent data for commercially-available small batteries, as well as
data for various embodiments of the invention at various discharge
rates. Based on this figure, the performance envelope represented
by this approach appears to represent a major improvement in the
performance of small batteries.
Example 2
[0119] In this example, 3D batteries having periodic or aperiodic
interpenetrating electrodes are used since their electronic
conductivity is typically higher than ionic conductivity in battery
materials. Interpenetrating electrodes of high aspect ratio can
have shorter ion diffusion length between electrodes while still
taking advantage of the higher electronic conductivity along the
electrodes to extract current. In the solid-state diffusion limit,
the dimension that may determine the utilization of the battery
capacity is the half-width x of the electrode features, for which
the discharge time is t=x.sup.2/D.sub.Li.
[0120] Using tabulated room-temperature lithium chemical
diffusivities (D.sub.Li) for spinel and layered structure
intercalation oxides, which fall in the range 1.times.10.sup.-9
cm.sup.2/sec to 5.times.10.sup.-9 cm.sup.2/sec, for a maximum 2C
discharge rate (t=1800 sec), a half-thicknesses of about 6 to about
30 micrometers is useful. These kinetics and their limitations on
particle dimensions are well-known to the battery field;
LiCoO.sub.2 is typically used as particles of 5 to 10 micrometers
dimension, while LiMn.sub.2O.sub.4 has a higher and also isotropic
lithium diffusion coefficient allowing roughly 25 micrometer
particles to be used. LiFePO.sub.4, on the other hand, has a much
lower lithium diffusion coefficient requiring particle dimensions
of <100 nm for high energy and power. Li.sub.4Ti.sub.5O.sub.12
is similar to LiFePO.sub.4 in this respect. Such materials may be
used as fine-scale porous materials filled with suitable
electrolytes. For LiCoO.sub.2 and LiMn.sub.2O.sub.4, as well as
related layered oxide and spinel compounds, a total electrode
dimension of 10 micrometers to 30 micrometers may be desired. Also,
for any reticulated structure, the smaller the feature size, the
greater the inactive volume occupied by electrolyte/separators,
binders and/or conductive additives. The results plotted in FIGS.
13 and 14 show that these materials, combined with a low lithium
potential anode such as Li metal, Li alloys, or carbon-based
electrodes, have desirable energy densities at the proposed
electrode dimensions.
[0121] For non-planar form factors, a second issue in the
fabrication of microbatteries is the electrode aspect ratio or
feature height. While various lithography-based processes have been
used recently to fabricate 3D electrodes, these experiments focus
on laser micro-machining due to its suitability for fabricating
highly aspected features with controlled taper. FIG. 4 illustrates
these two geometric parameters, as well as the ability to design in
controlled pore fraction for the counterelectrode. FIG. 4A shows
1.2 mm height at 200 micrometer to 250 micrometer feature width;
FIGS. 4B and 4C illustrate the ability to control taper. As
mentioned earlier, the spatial resolution of laser-micromachining
can be determined by the thermal conductivity of the material.
Preliminary laser-machining results on densified LiMn.sub.2O.sub.4
as one example indicates that it is possible to fabricate 3D
electrodes having 5:1 to 20:1 aspect ratios at the cross-sectional
dimensions desired.
[0122] Too high of an aspect ratio may be undesirable in some cases
from the viewpoint of electronic polarization (voltage drop along
the electrode), for example, in highly reticulated electrodes of
thin cross-section. For LiCoO.sub.2 and LiMn.sub.2O.sub.4 and
related compositions, which have electronic conductivities
>10.sup.-3 S/cm at room temperature, the voltage drop at these
aspect ratios is negligible (<0.1 V).
[0123] While laser-machining with a single focused beam is one
approach, resulting in individually fabricated devices, scale-up to
fabrication methods capable of producing many simultaneous devices
from an oxide "wafer" (e.g., produced by hot-pressing) is also
possible. Laser-machining remains an option for scaleup, using
diffuse beams and physical masks, for example. However, other
methods used in MEMS fabrication such as deep reactive ion etching
are also possible.
[0124] The electrolyte layer may be LiPON. LiPON is a thin film
electrolyte, which at 1 micrometer to 2 micrometers thickness
provides a low impedance, high rate, low self-discharge
electrolyte. The fabricated 3D electrode structures can be
sputtered with LiPON. The uniformity of LiPON coverage can be
evaluated by electron microscopy and electrical tests after
deposition of the counterelectrode.
[0125] An alternative to LiPON is the electrodeposition of solid
polymer electrolytes (SPEs) such as PEO-based compositions, or a
polyelectrolyte multilayer approach. Recent work on
electrophoretically formed batteries shows that electrodeposition
is an effective conformal deposition technique for PEO-based
electrolytes. For typical room temperature conductivities of
10.sup.-5 S/cm to 10.sup.-4 S/cm, the electrolyte is not limiting,
at a few micrometers thickness.
[0126] Selection and deposition of the counterelectrode may be
performed as follows. 3D micromachined structures may be formed out
of the positive electrode for electronic conductivity reasons
discussed earlier. For the negative electrode that will fill the
pore space after deposition of the electrolyte film, lithium metal,
a lithium metal allow such as LiAl, or a graphite-based suspension
can be used, with a cell structure designed to achieve cell
balance. Graphite based anodes such as MCMB can be formulated
similarly to conventional lithium ion anodes, except that in the
absence of liquid electrolytes, SPE can be used as a binder phase.
These suspensions can be used to infiltrate the pore space in the
electrolyte-coated 3D structure.
[0127] For the deposition of 0.5 mm to 1 mm thick lithium metal,
given the low melting point (181.degree. C.) of lithium metal, it
would be attractive to use liquid metal infiltration to fill the 3D
structure. A difficulty is that, like other liquid metals, lithium
has a high surface tension and does not as easily wet oxides or
polymers. Thus, a "flux and solder" method is used in this example,
by which liquid lithium can be made to wet oxide surfaces. By first
sputtering a thin layer of a metal that alloys with Li, such as Au,
reactive wetting of the sputtered surface occur readily. This was
demonstrated on glass surfaces, as shown in FIG. 15, with various
configurations and various discharge rates. Thus, a sputtered metal
layer applied to the electrolyte surface can be used to enable
subsequent infiltration by lithium metal, filling the 3D electrode
structure (FIG. 11). In order to control the amount of lithium
metal that is deposited, the liquid lithium may be dispensed
through a syringe or to dispense and then melt the solid lithium
metal powder (SLMP) available from FMC corporation, which is
passivated with a surface phosphate layer to allow handling in air
and certain organic solvents.
[0128] Self-organization as an assembly method may also be used for
selection and deposition of the counterelectrode. A colloidal-scale
self-assembly method for bipolar-devices may be used in which
repulsive forces between dissimilar materials are used to form
electrochemical junctions at the same time that attractive forces
between like material are used to form percolating conductive
networks of a single electrode material. A demonstration of this
approach is shown in FIG. 12, in which the percolating network is
MCMB. The present 3D forms a dense and continuous 3D electrode from
the less conductive material.
[0129] One of the challenges in microbattery technology, including
thin-film batteries, has been the development of effective hermetic
packaging with minimal contributed volume. The 3D design in this
example uses densified oxide for hermetic sealing on all except the
top surface (FIG. 11). Thus final sealing of the battery can be
accomplished by deposition from the top of a suitable packaging
material. A parylene-based packaging material, on top of which is
typically sputtered a metal film for hermeticity may be used, or a
dense insulating oxide coating by physical vapor methods may also
be used.
Example 3
[0130] In this example, it is shown that a porous sintered
electrode of LiCoO.sub.2 of greater than 0.5 mm minimum
cross-sectional dimension that is infused with a liquid electrolyte
can, surprisingly and unexpectedly, be electrochemically cycled
while obtaining nearly all of the available ion storage capacity
over at least 20 cycles at C/20 rate with minimal capacity fade and
no apparent detrimental mechanical damage to the electrode. This
shows that such electrodes can effectively be used in certain
batteries of the invention.
[0131] A battery grade LiCoO.sub.2 powder from Seimi Corporation
(Japan) having 10.7 micrometers d.sub.50 particle size was pressed
and fired at 1100.degree. C. in air to form a porous sintered
ceramic having about 85% of the theoretical density of LiCoO.sub.2.
In one instance, a plate of this electrode having 0.66 mm thickness
was prepared, as shown in FIGS. 8A and 8B. This electrode plate was
attached to a gold foil current collector and assembled for testing
in a sealed polymer pouch-cell, using lithium metal foil as the
counterelectrode, a copper current collector at the negative
electrode, a porous polymer separator of 20 micrometer thickness,
and a liquid electrolyte having a 1.33 M concentration of
LiPF.sub.6 in a mixture of alkyl carbonates.
[0132] FIG. 16A shows the 6.sup.th and 7.sup.th charge-discharge
cycles of this cell. The charge protocol used a constant current at
C/20 rate to an upper voltage of 4.3 V, followed by a constant
voltage hold until the current decayed to C/100 rate, followed by
an open-circuit rest, followed by a constant current discharge to
2.5 V. FIG. 16B shows the charge and discharge capacities observed
over 20 cycles at C/20 discharge rate, followed by discharges at
C/5 and 1C rate. The C/20 discharge capacity was about 130 mAh/g,
essentially the same as the value observed for this LiCoO.sub.2
over this voltage range in standardized tests. This shows that this
porous electrode was able to accept and discharge nearly all of the
lithium storage capacity at C/20 rate. Even at C/5 rate, the
capacity was above 90 mAh/g. Furthermore, there was very little
capacity fade over 20 cycles at C/20 rate. When this electrode is
packaged as a complete microbattery according to the earlier
described construction and methods, the volume is 6.4 mm.sup.3 and
the projected energy density based on the measured cathode
performance is 954 W h/L.
[0133] Remarkably, this sample was found to exhibit no apparent
signs of mechanical failure after this electrochemical test, as
shown in FIG. 9.
[0134] In other instances, the electrodes shown in FIGS. 2 and 7
were produced from the same starting sintered ceramic using laser
micromachining, and were assembled into a test cell and
electrochemically tested in the same manner. These test electrodes
exhibited similar electrochemical performance to the electrode of
FIG. 16. Based on the electrochemical tests of each of these
electrodes, in fully packaged form, the electrode of FIG. 2
produces a battery of 5.72 mm.sup.3 volume and 1022 W h/L energy
density, while the electrode of FIG. 7 produces a battery of 5.74
mm.sup.3 volume and 1300 W h/L.
Example 4
[0135] In this example, it is shown that a porous sintered
electrode of a lithium transition metal phosphate olivine that is
infused with a liquid electrolyte can, surprisingly and
unexpectedly, be electrochemically cycled while obtaining nearly
all of the available ion storage capacity over at least 30 cycles
at C/10 rate with minimal capacity fade. This shows that such
electrodes can effectively be used in certain batteries of the
invention.
[0136] A powder of a Nb-doped, nanoscale lithium iron phosphate
material such as is described in U.S. patent application Ser. No.
10/329,046, filed Dec. 23, 2002, entitled "Conductive Lithium
Storage Electrode," by Chiang, et al., published as U.S. Patent
Application Publication No. 2004/00055265 on Jan. 8, 2004
(incorporated herein by reference), was uniaxially pressed into a
1/2 inch disk at a pressure of 20,000 psi (1 psi=6.89475
kilopascals) and sintered in a tube furnace at 775.degree. C. for 2
hours in Ar atmosphere.
[0137] After sintering, the material was observed using scanning
electron microscopy to have a primary particle size of 100-200 nm.
The density of the disk was measured to be 72% by the Archimedes
method. The disk was polished to 0.305 mm thickness using 5 micron
grit size silicon carbide polishing paper and cut using a diamond
wire saw to a rectangular dimension of 3.48 mm by 2.93 mm by 0.305
mm. The sample weight was 7.3 mg. The sample was assembled as the
positive electrode in an electrochemical test cell made using
Swagelok fittings using 150 micrometer Li foil ( 7/16'' inch in
diameter) as both the counter and reference electrode. Celgard 2320
(1/2'' inch in diameter) was used as the separator. A liquid
electrolyte having a 1.33 M concentration of LiPF.sub.6 in a
mixture of alkyl carbonates was used. The cell was
galvanostatically charged at C/20 for the first cycle and at C/10
for all the subsequent cycles. All the discharge rates are C/10
unless otherwise indicated. The voltage window was between 2 and
4.2 V.
[0138] FIG. 17A shows the specific capacity as a function of cycle
number for the cathode, which comprised sintered doped olivine
phosphate, and shows that almost no capacity fade occurred over 40
cycles. FIG. 17B shows voltage vs. time of the 30.sup.th
galvanostatic charge/discharge cycle of the cathode. The cathode
had a density of 72% and was 0.305 mm thick. These results
demonstrate that the sintered cathodes of the invention can be
usefully employed in the batteries of the invention.
Example 5
[0139] This example demonstrates a sintered porous electrode onto
which is conformally deposited a dense solid electrolyte film and
that it can be used as an electrode in the batteries of certain
embodiments of the invention. LiCoO.sub.2 powder with a mean
particle size of 10-11 micrometers was purchased from a commercial
vendor. 35 g of the powder was milled for 5 days in a zirconia jar
mill using zirconia milling balls. After milling, the mean particle
diameter fell to 4-5 micrometers. 3.5 g of the milled powder was
pressed into a 1/2-inch diameter pellet (about 1.27 cm) under a
pressure of 100 MPa in a uniaxial press. The pellet was placed onto
an alumina plate, covered with loose LiCoO.sub.2 powder, covered by
an inverted alumina jar and sintered under air for 1.5 hrs at
950.degree. C. The densified cylindrical pellet was recovered and
sliced into 0.8 mm thick disks.
[0140] One of the LiCoO.sub.2 disks was simultaneously thinned down
to 0.4-0.5 mm thickness and polished to a mirror-like finish using
silicon carbide abrasive pads of increasingly finer grit size down
to 1.0 micrometer. The disk was affixed onto an alumina plate and
diced into 2.2 mm.times.2.2 mm squares. The squares were mounted
into a metallic fixture and placed into a custom-built vacuum
deposition chamber. In several hours, the exposed top surface of
each square was coated with an .about.0.5 micrometer thick lithium
phosphorous oxynitride (LiPON) coating that was also visible to the
eye by its iridescence. The coated electrode was assembled and
tested in an electrochemical cell as described in Example 4.
[0141] FIG. 18 shows a scanning electron microscope image showing
the continuous, conformal LiPON coating. FIG. 19 shows that in
galvanostatic cycling, such a film presented very little additional
resistance compared to an uncoated electrode.
Example 6
[0142] This example demonstrates high energy density packaged
microbatteries made using certain sintered porous electrodes of the
invention. Two microbatteries are described in this particular
example, made using the following procedure. A sintered porous
LiCoO.sub.2 electrode (2.20 mm by 2.20 mm by 0.37 mm), made as
described in Example 3, was put into a electroformed gold can (2.5
mm by 2.5 mm by 0.7 mm), shown in FIG. 20, using a conductive paste
made of polyvinylidene fluoride (PVDF), vapor grown carbon fibers
(VGCF), and high surface area carbon black. A Celgard 2320
separator was glued onto the flange of the can on three sides using
a visible light curable glue, Loctite 3972. A small piece of Li was
put on a 10 micrometer thick copper foil lid cut to fit on top of
the can, and heated at 100.degree. C. for 20 minutes. Four holes
were punched around the Li using a small needle to allow for
subsequent infiltration by liquid electrolyte. The copper foil lid,
with the lithium metal negative electrode facing the open top of
the can, was glued onto the separator using Loctite 3972 on the
same three. The whole cell was immersed in a liquid electrolyte, of
the kind described in Example 3, for 24 hours and then was
galvanostatically charged to 4.6 V at a C/12 rate and discharged at
a C/2.7 rate to 3V.
[0143] FIG. 21 shows that both cells can be charged smoothly to
4.6V. FIG. 22 shows that in the first discharge, both cells
exhibited high energy densities of 676 W h/L and 658 W h/L
respectively, at about 200 W/L power. After the first cycle, excess
electrolyte was cleaned from the surface of the cell and the
electrolyte infiltration holes were sealed using Loctite 3972. The
cell was then sealed on all its surfaces with Hardman fast-setting
3 minute epoxy and tested further. FIG. 23 shows the specific
capacity of the cathode during the first 4 cycles of one of the
cells. In the second and third discharges under the same current as
the first cycle, the capacity and energy had decreased, but
remained still very high. The 4.sup.th cycle was conducted at a
C/12 rate, and shows that the cell had diminished in its capacity
to about 100 mAh/g. This behavior corresponds to the behavior
reported in the literature for LiCoO.sub.2 charged to 4.6 V, and
shows that the sintered cathode in the microbatteries of the
invention can be used to prepare high energy density
microbatteries.
Example 7
[0144] This example demonstrates a high energy density bicell
battery made according to certain embodiments of the invention.
Sintered LiCoO.sub.2 electrodes were made according to the method
of Example 5 and sliced into two 0.8 mm thick disks that were then
thinned down to 0.4 mm thickness and polished to a mirror-like
finish using silicon carbide abrasive pads of increasingly finer
grit size down to 3 micrometers.
[0145] Aluminum current collector strips with a wide end size
matched to the circular LiCoO.sub.2 was cut out of 35 micrometer
thick aluminum foil. The wide ends were coated with a thin layer of
a conductive paste made of polyvinylidene fluoride (PVDF), vapor
grown carbon fibers (VGCF), and high surface area carbon black. The
LiCoO.sub.2 disks were attached to the current collector strips
using the conductive paste. The strips were air dried for an hour
first and then vacuum-dried for 12 hours at 90.degree. C. After
drying, the LiCoO.sub.2 disks were found to be bonded well to the
aluminum strip. The end of the strip with the attached LiCoO.sub.2
disks was soaked in a liquid electrolyte mixture for 12 hours to
ensure infiltration.
[0146] Lithium negative electrodes were cut from a 150
micrometer-thick lithium sheet to match the size of the disk
cathodes. These lithium pieces were pressed onto two sides of a 10
.quadrature.m thick copper foil, serving as the negative current
collector.
[0147] An electrochemical bicell as illustrated in FIGS. 24A-24C
was constructed from the positive and negative electrodes, with a
layer of Celgard 2320 separator separating the two, and polymer
packaging heat-sealed around the electrode assemblies. Some
additional liquid electrolyte was added to the cell before vacuum
sealing. FIG. 24 shows that the bicell could be charged and
discharged between 4.3 V and 2.5 V, but exhibited a high energy
density and specific energy compared to other lithium ion cells of
comparable size (e.g., about 0.5 cm.sup.3 volume), of 275 W h/L and
213 W h/kg respectively.
Example 8
[0148] In this example, it is shown that porous sintered electrodes
produced according to one embodiment of the invention exhibit high
levels of capacity utilization and high energy density that exhibit
surprising combinations of low electrode porosity, high electrode
thickness, high capacity per unit area of electrode, while being
utilized at relatively high C-rates. For instance, this example
shows that sintered LiCoO.sub.2 electrodes with 74% to 87% of the
theoretical density, and 0.26 mm to 0.80 mm thickness, yield
specific capacities greater than 85% of that expected for the same
powder when prepared as a conventional battery electrode.
Furthermore, this example shows that more than 85% of the intrinsic
specific capacity of the material can be obtained at a C/3 rate in
electrodes simultaneously having greater than 0.25 mm thickness and
greater than 70% sintered density. At this lower limit of thickness
and density, in the case of LiCoO.sub.2 cycled over a potential
range yielding 140 mAh/g specific capacity and at a C/3 rate, the
area capacity was 12.3 mAh/cm.sup.2, which is more than about 3.5
times greater than that of conventional powder-based LiCoO.sub.2
electrodes. For a sintered electrode of 0.40 mm thickness and 85%
of the theoretical density, tested at C/3 rate, the specific
capacity reached about 155 mAh/g, providing an area capacity of
about 26.4 mAh/cm.sup.2. Electrodes that are able to deliver such
high values of area capacity at these C-rates are novel in the
field of lithium rechargeable batteries and in battery technology
in general.
[0149] To design such electrodes, active materials were first
selected that had a particle or crystallite size in the final
electrode that was small enough to permit high utilization at the
current rates used for the electrode. That is, solid state
diffusion in the sintered particulates should not present the
rate-limiting step in electrochemical cycling. Methods to determine
the particle or crystallite or grain size of a sintered material
are well-known to those skilled in the art. The ability of a
particular particle size or size distribution to provide high
utilization at the desired C-rates is readily tested, for instance
by preparing a very thin sintered electrode or by crushing the
sintered electrode to a powder that is then tested as a
conventional powder electrode. In the present example, LiCoO.sub.2
had sufficiently high capacity at high C-rates, for instance
greater than 130 mAh/g capacity at 10 C rate in a powder based
electrode, that the particles themselves were not rate-limiting.
Sintered electrodes were made according to the procedure discussed
in Example 3, except that the starting battery grade LiCoO.sub.2
powder from Seimi Corporation (Japan) having 10.7 micrometers
d.sub.50 particle size was first milled in a zirconia jar mill
using zirconia milling media in order to reduce the d.sub.50
particle size to about 4 micrometers. The powder was pressed into
disc shaped samples of about 12.7 mm (0.5 inch) diameter and about
1 mm thickness, then fired in air at various times and temperatures
to form porous sintered ceramics having a range of sintered
densities. For example, to obtain 74% of the theoretical sintered
density, firing was conducted using a 1:50 ramp in temperature
(i.e., a 1 hour and 50 minute ramp) to 950.degree. C., a 1:30 hold
at 950.degree. C., and a 1:50 cooldown to room temperature; and to
obtain 87% of the theoretical density, firing was conducted using a
1:50 ramp to 1050.degree. C., a 1:30 hold at 1050.degree. C., and a
1:50 cooldown to room temperature. Afterwards, both sides of the
pellet were polished to remove dense surface material, and to
obtain uniform thickness. The sintered samples were then cut into
plates of uniform thickness and tested in pouch-type or
Swagelok-type cells using lithium metal foil as the
counterelectrode, a copper current collector at the negative
electrode, a porous polymer separator of 20 micrometer thickness,
and a liquid electrolyte having a 1.33 M concentration of
LiPF.sub.6 in a mixture of alkyl carbonates.
[0150] FIG. 25A shows the specific discharge capacity vs. C-rate
obtained from the LiCoO.sub.2 electrodes of varying thickness (260
micrometers to 800 micrometers) and active materials volume
fraction (74% to 87%), charged to 4.25 V and 4.6 V. The discharge
capacity is shown for several electrodes in which the thickness and
density were varied. The lower two curves show cases in which the
thickness and/or density of the electrodes were relatively high and
the intrinsic capacity is available at C/20 rate or lower. The
upper two curves show instances where >85% or more of the
theoretical capacity at the respective charge voltages (4.25 V and
4.6 V) were obtained at .about.C/3 rate.
[0151] From these results, combinations of the sintered density of
the electrode and the thickness of the electrode that allow high
utilization at useful C-rates can be determined. For instance, by
taking the density as the percentage of the theoretical density of
the compound, and the thickness in units of millimeters, ranges for
the product of the two can be identified, in units of %-mm, that
comprise useful electrodes. For example, a 3 mm thick electrode of
fairly low sintered density of 50% may be effective in some cases.
This produces a product of 150%-mm. Also, a 90% dense electrode of
0.25 mm thickness can be effective, yielding the product 22.5%-mm.
Or, for higher rate capability, a 50% dense electrode of 0.25 mm
thickness may be effective. This electrode has a product 12.5%-mm.
The same dimensional units may be applied in the case where the
product of the open porosity of the electrode and the thickness are
considered. Total porosity is (100-% density); however, open
porosity is what is accessed by infiltrated electrolyte. The open
porosity in the electrodes of the invention may range from 10% to
50%. The product of open porosity and thickness of useful
electrodes may span a range from about 150%-mm to about
2.5%-mm.
[0152] Yet another metric is the total capacity per unit area of
electrode. The electrodes of the invention are able, in some
embodiments, to provide a higher capacity per unit area compared to
conventional electrodes, e.g., between about 10 mAh/cm.sup.2 and
about 100 mAh/cm.sup.2. The latter number represents, for example,
a 3 mm thick LiCoO.sub.2 electrode of 50% density, having a density
of 5.01 g/cm.sup.3 and a specific capacity of 140 mAh/g, which may
provide useful utilization at low rates based on the data of this
example.
[0153] FIG. 25B shows the discharge capacity obtained upon cycling
a sintered nanoscale olivine Li.sub.0.99Nb.sub.0.01FePO.sub.4
electrode of 418 micrometer thickness and 82% by volume of active
material at various rates out to cumulatively 40 cycles. The C-rate
profile was designed to show the capacity upon cycling to a higher
rate and then returning to lower rates. Notice that the discharge
capacity was not noticeably degraded upon returning to each C-rate.
Examination after cycling showed no detectable mechanical
damage.
[0154] The results of this and other experiments demonstrates
another optional feature of the invention, which is the selection
of active materials that have a higher level of electronic
conductivity over the majority of the composition range over which
they are cycled, compared to either the fully lithiated or
delithiated states. For instance, the electronic conductivity of
typical positive electrode compounds is, generally speaking, orders
of magnitude lower that that of commonly used negative electrodes
such as graphite or the metal alloys. This has necessitated the
addition of conductive additives to positive electrode
formulations. However, by providing a dense interconnected network
of this low-conductivity component, lower cathode impedance with
less or no conductive additive can be achieved in some cases.
[0155] Also, some intercalation oxides increase in their electronic
conductivity as they are delithiated; Li.sub.1-xCoO.sub.2 as one
example undergoes an insulator-to-metal transition at x.about.0.03.
However, the impact of increasing electronic conductivity upon
delithiation is illustrated in FIG. 26. This figure shows the
effect of increasing electronic conductivity upon delithiation of
Li.sub.1-xCoO.sub.2 in the polarization occurring at very low
state-of-charge. The polarization decreased rapidly as the
composition reached values of x corresponding to the
insulator-to-metal transition. This may be attributed to the fact
that the Li.sub.1-xCoO.sub.2 network in the electrode becomes
increasingly conductive, or even metallic, as the x value increases
beyond about 0.03. Impedance spectra (not shown) taken on these
cells were consistent with this interpretation; these results
suggested that proper electrode design and compound selection can
provide low impedance, very high capacity electrodes that have
essentially no conductive additive.
[0156] High specific and volumetric energies were also achievable
when such electrodes were deployed in real cells. The mass
utilization of active material and the resulting specific energy
and energy density of full-scale cells based on the invention were
readily computed: FIGS. 27A-27B shows results (energy density and
cell utilization, respectively) for a stacked prismatic cell having
90 cm.sup.3 volume and 21 Ah capacity, using 10 cathode layers of 1
mm thickness (the half-cell thickness is equivalent to an electrode
thickness of 0.50 mm in FIG. 25A, assuming graphite negative
electrodes. For LiCoO.sub.2 electrodes of the densities shown in
the above examples, the specific energy exceeded 300 Wh/kg and the
energy density exceeded 900 Wh/L.
[0157] It should be understood that other cathode materials that
are isostructural or structural derivatives of LiCoO.sub.2,
generally known as "layered" oxides within the ordered rock salt
structure family, may be substituted in whole or in part for
LiCoO.sub.2. These include, but are not limited to, compounds such
as LiNiO.sub.2, Li.sub.x(Ni, Co, Al)O.sub.2 (often referred to as
"NCA"), Li.sub.x(Mn, Ni, Co)O.sub.2, ("MNC" or "1/3 1/3 1/3").
[0158] Also included are compounds that are intergrowths or
nanoscale mixtures between any of structures of "layered" ordered
rock salt type or spinel type, including those in the Li--Mn--Ni--O
family. In these formulae, x may by any number between 0 and about
1.5 (or other ranges discussed herein), depending on the Li content
of the synthesized material and the charge/discharge excursions
during use, and the elements within the parenthesis may be present
in any amount and any relative amount as long as standard chemical
rules of charge balance are obeyed. Typically, the sum of the
elements within the parenthesis is about 1, i.e., for
Li.sub.x(Ni.sub.a, Co.sub.b, Al.sub.c)O.sub.2 and
Li.sub.x(Mn.sub.a, Ni.sub.b, Co.sub.c)O.sub.2, the sum of a, b, and
c is about 1, although each of a, b, and c may be any number
between 0 (including 0) and about 1.
[0159] Most of these compounds also exhibited the characteristic
useful in this invention of having an electronic conductivity that
increases upon partial delithiation of a fully lithiated starting
material, e.g., due to the formation of multivalent transition
metals. The expansion characteristics of these compounds may be
selected or optimized so as to provide improved stability and life
to the sintered electrode. For example, LiCoO.sub.2 exhibits a
decrease in volume when delithiated, while LiNiO.sub.2 exhibits an
expansion. Thus, simple physical mixtures of the two compounds may
be used to obtain a range of expansion characteristics including
zero expansion. These compounds also differ in their thermal
stability and safety characteristics. Thus, mixtures of such
compounds may be selected to improve the safety of the sintered
electrode and batteries based on such electrodes, using no more
than routine optimization.
[0160] While several embodiments of the present invention have been
described and illustrated herein, those of ordinary skill in the
art will readily envision a variety of other means and/or
structures for performing the functions and/or obtaining the
results and/or one or more of the advantages described herein, and
each of such variations and/or modifications is deemed to be within
the scope of the present invention. More generally, those skilled
in the art will readily appreciate that all parameters, dimensions,
materials, and configurations described herein are meant to be
exemplary and that the actual parameters, dimensions, materials,
and/or configurations will depend upon the specific application or
applications for which the teachings of the present invention
is/are used. Those skilled in the art will recognize, or be able to
ascertain using no more than routine experimentation, many
equivalents to the specific embodiments of the invention described
herein. It is, therefore, to be understood that the foregoing
embodiments are presented by way of example only and that, within
the scope of the appended claims and equivalents thereto, the
invention may be practiced otherwise than as specifically described
and claimed. The present invention is directed to each individual
feature, system, article, material, kit, and/or method described
herein. In addition, any combination of two or more such features,
systems, articles, materials, kits, and/or methods, if such
features, systems, articles, materials, kits, and/or methods are
not mutually inconsistent, is included within the scope of the
present invention.
[0161] All definitions, as defined and used herein, should be
understood to control over dictionary definitions, definitions in
documents incorporated by reference, and/or ordinary meanings of
the defined terms.
[0162] The indefinite articles "a" and "an," as used herein in the
specification and in the claims, unless clearly indicated to the
contrary, should be understood to mean "at least one."
[0163] The phrase "and/or," as used herein in the specification and
in the claims, should be understood to mean "either or both" of the
elements so conjoined, i.e., elements that are conjunctively
present in some cases and disjunctively present in other cases.
Multiple elements listed with "and/or" should be construed in the
same fashion, i.e., "one or more" of the elements so conjoined.
Other elements may optionally be present other than the elements
specifically identified by the "and/or" clause, whether related or
unrelated to those elements specifically identified. Thus, as a
non-limiting example, a reference to "A and/or B", when used in
conjunction with open-ended language such as "comprising" can
refer, in one embodiment, to A only (optionally including elements
other than B); in another embodiment, to B only (optionally
including elements other than A); in yet another embodiment, to
both A and B (optionally including other elements); etc.
[0164] As used herein in the specification and in the claims, "or"
should be understood to have the same meaning as "and/or" as
defined above. For example, when separating items in a list, "or"
or "and/or" shall be interpreted as being inclusive, i.e., the
inclusion of at least one, but also including more than one, of a
number or list of elements, and, optionally, additional unlisted
items. Only terms clearly indicated to the contrary, such as "only
one of" or "exactly one of," or, when used in the claims,
"consisting of," will refer to the inclusion of exactly one element
of a number or list of elements. In general, the term "or" as used
herein shall only be interpreted as indicating exclusive
alternatives (i.e. "one or the other but not both") when preceded
by terms of exclusivity, such as "either," "one of," "only one of,"
or "exactly one of." "Consisting essentially of," when used in the
claims, shall have its ordinary meaning as used in the field of
patent law.
[0165] As used herein in the specification and in the claims, the
phrase "at least one," in reference to a list of one or more
elements, should be understood to mean at least one element
selected from any one or more of the elements in the list of
elements, but not necessarily including at least one of each and
every element specifically listed within the list of elements and
not excluding any combinations of elements in the list of elements.
This definition also allows that elements may optionally be present
other than the elements specifically identified within the list of
elements to which the phrase "at least one" refers, whether related
or unrelated to those elements specifically identified. Thus, as a
non-limiting example, "at least one of A and B" (or, equivalently,
"at least one of A or B," or, equivalently "at least one of A
and/or B") can refer, in one embodiment, to at least one,
optionally including more than one, A, with no B present (and
optionally including elements other than B); in another embodiment,
to at least one, optionally including more than one, B, with no A
present (and optionally including elements other than A); in yet
another embodiment, to at least one, optionally including more than
one, A, and at least one, optionally including more than one, B
(and optionally including other elements); etc.
[0166] It should also be understood that, unless clearly indicated
to the contrary, in any methods claimed herein that include more
than one step or act, the order of the steps or acts of the method
is not necessarily limited to the order in which the steps or acts
of the method are recited.
[0167] In the claims, as well as in the specification above, all
transitional phrases such as "comprising," "including," "carrying,"
"having," "containing," "involving," "holding," "composed of," and
the like are to be understood to be open-ended, i.e., to mean
including but not limited to. Only the transitional phrases
"consisting of" and "consisting essentially of" shall be closed or
semi-closed transitional phrases, respectively, as set forth in the
United States Patent Office Manual of Patent Examining Procedures,
Section 2111.03.
* * * * *