U.S. patent application number 12/862581 was filed with the patent office on 2011-03-31 for electrochemical cells comprising porous structures comprising sulfur.
This patent application is currently assigned to Sion Power Corporation. Invention is credited to John D. Affinito, Christopher T.S. Campbell, Igor Kovalev, Yuriy V. Mikhaylik, Vladimir Oleshko, Chariclea Scordilis-Kelley.
Application Number | 20110076560 12/862581 |
Document ID | / |
Family ID | 43648036 |
Filed Date | 2011-03-31 |
United States Patent
Application |
20110076560 |
Kind Code |
A1 |
Scordilis-Kelley; Chariclea ;
et al. |
March 31, 2011 |
ELECTROCHEMICAL CELLS COMPRISING POROUS STRUCTURES COMPRISING
SULFUR
Abstract
The present invention relates to the use of porous structures
comprising sulfur in electrochemical cells. Such materials may be
useful, for example, in forming one or more electrodes in an
electrochemical cell. For example, the systems and methods
described herein may comprise the use of an electrode comprising a
conductive porous support structure and a plurality of particles
comprising sulfur (e.g., as an active species) substantially
contained within the pores of the support structure. The inventors
have unexpectedly discovered that, in some embodiments, the sizes
of the pores within the porous support structure and/or the sizes
of the particles within the pores can be tailored such that the
contact between the electrolyte and the sulfur is enhanced, while
the electrical conductivity and structural integrity of the
electrode are maintained at sufficiently high levels to allow for
effective operation of the cell. Also, the sizes of the pores
within the porous support structures and/or the sizes of the
particles within the pores can be selected such that any suitable
ratio of sulfur to support material can be achieved while
maintaining mechanical stability in the electrode. The inventors
have also unexpectedly discovered that the use of porous support
structures comprising certain materials (e.g., metals such as
nickel) can lead to relatively large increases in cell performance.
In some embodiments, methods for forming sulfur particles within
pores of a porous support structure allow for a desired
relationship between the particle size and pore size. The sizes of
the pores within the porous support structure and/or the sizes of
the particles within the pores can also be tailored such that the
resulting electrode is able to withstand the application of an
anisotropic force, while maintaining the structural integrity of
the electrode.
Inventors: |
Scordilis-Kelley; Chariclea;
(Tucson, AZ) ; Mikhaylik; Yuriy V.; (Tucson,
AZ) ; Kovalev; Igor; (Vail, AZ) ; Oleshko;
Vladimir; (Tucson, AZ) ; Campbell; Christopher
T.S.; (Tucson, AZ) ; Affinito; John D.;
(Tucson, AZ) |
Assignee: |
Sion Power Corporation
Tucson
AZ
|
Family ID: |
43648036 |
Appl. No.: |
12/862581 |
Filed: |
August 24, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61237903 |
Aug 28, 2009 |
|
|
|
Current U.S.
Class: |
429/218.1 |
Current CPC
Class: |
H01M 4/602 20130101;
H01M 4/38 20130101; H01M 4/668 20130101; H01M 4/364 20130101; H01M
4/60 20130101; Y10T 29/49108 20150115; H01M 4/583 20130101; Y02T
10/70 20130101; Y10T 29/49115 20150115; H01M 4/5815 20130101; Y02E
60/10 20130101; H01M 2010/4292 20130101; H01M 2004/021 20130101;
Y02E 60/13 20130101; H01G 11/06 20130101; H01M 4/136 20130101; Y02P
70/50 20151101; H01M 4/80 20130101; H01M 4/64 20130101; H01M 4/0402
20130101 |
Class at
Publication: |
429/218.1 |
International
Class: |
H01M 4/58 20100101
H01M004/58 |
Claims
1. An electrode for use in an electrochemical cell, comprising: a
porous support structure comprising a plurality of pores; and a
plurality of particles comprising an electrode active material
comprising sulfur substantially contained within the pores of the
porous support structure, wherein each particle of the plurality of
particles has a maximum cross-sectional dimension; each particle of
the plurality of particles has a particle volume, and the plurality
of particles has a total particle volume defined by the sum of each
of the individual particle volumes; and at least about 50% of the
total particle volume is occupied by particles having maximum
cross-sectional dimensions of between about 0.1 microns and about
10 microns.
2. An electrode for use in an electrochemical cell, comprising: a
porous support structure comprising a plurality of pores; and a
plurality of particles comprising an electrode active material
comprising sulfur substantially contained within the pores of the
porous support structure, wherein the plurality of particles
together defines a total quantity of particulate material, and
wherein at least about 50% of the total quantity of particulate
material is made up of particles having maximum cross-sectional
dimensions of between about 0.1 microns and about 10 microns.
3. An electrode for use in an electrochemical cell, comprising: a
porous support structure comprising a plurality of pores; and an
electrode active material comprising sulfur substantially contained
within the pores of the porous support structure, wherein each pore
of the plurality of pores has a pore volume, and the plurality of
pores has a total pore volume defined by the sum of each of the
individual pore volumes; and at least about 50% of the total pore
volume is occupied by pores having cross-sectional diameters of
between about 0.1 microns and about 10 microns.
4. An electrode for use in an electrochemical cell, comprising: a
porous support structure comprising a plurality of pores; and an
electrode active material comprising sulfur substantially contained
within the pores of the porous support structure, wherein the
plurality of pores of the porous support structure together defines
a total pore volume, and at least about 50% of the total pore
volume is defined by pores having cross-sectional diameters of
between about 0.1 microns and about 10 microns.
5. An electrode for use in an electrochemical cell, comprising: a
porous support structure comprising a plurality of pores; and an
electrode active material comprising sulfur substantially contained
within the pores of the porous support structure, wherein an
electrochemical cell comprising the electrode is capable of
utilizing at least about 65% of the total sulfur in the cell
through at least 1 charge and discharge cycles subsequent to a
first charge and discharge cycle, wherein 100% utilization
corresponds to 1672 mAh per gram of sulfur in the electrode.
6. An electrode as in claim 1, wherein the standard deviation of
the maximum cross-sectional dimensions of the particles is less
than about 50%.
7. An electrode as in claim 3, wherein the standard deviation of
the cross-sectional diameters of the pores is less than about
50%.
8. An electrode as in claim 1, wherein the porous support structure
comprises an electrically conductive material.
9. An electrode as in claim 8, wherein an electrically conductive
material is deposited on the porous support structure.
10. An electrode as in claim 1, wherein the porous support
structure comprises at least one of carbon, a metal, a polymer, a
ceramic, and a semiconductor.
11. An electrode as in claim 1, wherein the porous support
structure comprises carbon.
12. An electrode as in claim 1, wherein the porous support
structure comprises a metal.
13. An electrode as in claim 1, wherein the porous support
structure comprises silicon dioxide.
14. An electrode as in claim 1, wherein the sulfur comprises at
least one of elemental sulfur, polymeric sulfur, inorganic
sulfides, inorganic polysulfides, organic sulfides, organic
polysulfides, and sulfur organic compounds.
15. An electrode as in claim 1, wherein the sulfur comprises
elemental sulfur.
16. An electrode as in claim 1, wherein the electrode comprises at
least about 20 wt % sulfur.
17. An electrode as in claim 1, wherein the electrode active
material occupies at least about 10% of the accessible pore volume
of the porous support structure.
18. An electrode as in claim 1, wherein the electrode has a void
volume of at least about 1 cm.sup.3 per gram of sulfur.
19. An electrode as in claim 1, wherein an electrochemical cell
comprising the electrode is capable of utilizing at least about 65%
of the total sulfur in the cell through at least 1 charge and
discharge cycles subsequent to a first charge and discharge cycle,
wherein 100% utilization corresponds to 1672 mAh per gram of sulfur
in the electrode.
20. An electrode as in claim 1, wherein an electrochemical cell
comprising the electrode is capable of utilizing at least about 65%
of the total sulfur in the cell through at least 10 charge and
discharge cycles subsequent to a first charge and discharge cycle,
wherein 100% utilization corresponds to 1672 mAh per gram of sulfur
in the electrode.
21. An electrode as in claim 1, wherein an electrochemical cell
comprising the electrode is capable of achieving a current density
of at least about 100 mA per gram of sulfur in the electrode during
at least one charge and discharge cycle subsequent to a first
charge and discharge cycle.
22. An electrode as in claim 1, wherein an electrochemical cell
comprising the electrode is capable of achieving a current density
of at least about 100 mA per gram of sulfur in the electrode during
at least 10 charge and discharge cycles subsequent to a first
charge and discharge cycle.
23. An electrode as in claim 1, wherein the porous support
structure comprises a porous continuous structure.
24. An electrode as in claim 23, wherein the maximum
cross-sectional dimension of the porous continuous structure within
the electrode is at least about 50% of the maximum cross sectional
dimension of the electrode.
25. An electrode as in claim 1, wherein: the electrode has a
surface, at least about 50% of the area of the surface of the
electrode defines a uniform area with a first average concentration
of sulfur, and any continuous area that covers about 10% of the
uniform area of the surface of the electrode includes a second
average concentration of sulfur that varies by less than about 25%
relative to the first average concentration of sulfur across the
uniform area.
26. An electrode as in claim 1, wherein: the electrode has a
surface, at least about 50% of the area of the surface of the
electrode defines a first, continuous area of essentially uniform
sulfur distribution, and the first area has a first average
concentration of sulfur, and any continuous area that covers about
10% of the first, continuous area of the surface of the electrode
includes a second average concentration of sulfur that varies by
less than about 25% relative to the first average concentration of
sulfur across the first, continuous area.
27. An electrode as in claim 1, wherein: the electrode has a
thickness and a cross-section substantially perpendicular to the
thickness, at least about 50% of the cross-section defines a
uniform area with a first average concentration of sulfur, and any
continuous area that covers about 10% of the uniform area of the
cross-section includes a second average concentration of sulfur
that varies by less than about 25% relative to the first average
concentration of sulfur across the uniform area.
28. An electrode as in claim 1, wherein: the electrode has a
thickness and a cross-section substantially perpendicular to the
thickness, at least about 50% of the area of the cross-section
defines a first, continuous area of essentially uniform sulfur
distribution, and the first area has a first average concentration
of sulfur, and any continuous area that covers about 10% of the
first, continuous area of the cross-section includes a second
average concentration of sulfur that varies by less than about 25%
relative to the first average concentration of sulfur across the
first, continuous area.
29. An electrode as in claim 1, wherein at least about 70% of the
total particle volume is occupied by particles having maximum
cross-sectional dimensions of between about 0.1 microns and about
10 microns.
30. An electrode as in claim 1, wherein at least about 80% of the
total particle volume is occupied by particles having maximum
cross-sectional dimensions of between about 0.1 microns and about
10 microns.
31. An electrode as in claim 3, wherein at least about 70% of the
total pore volume is occupied by pores having cross-sectional
diameters of between about 0.1 microns and about 10 microns.
32. An electrode as in claim 3, wherein at least about 80% of the
total pore volume is occupied by pores having cross-sectional
diameters of between about 0.1 microns and about 10 microns.
33. An electrode as in claim 1, wherein the electrode contains less
than about 20 wt % binder.
34. An electrode as in claim 1, wherein the ratio of the average
maximum cross-sectional dimension of the particles of material
within the porous support structure to the average cross-sectional
diameter of the pores within the porous support structure is
between about 0.001:1 and about 1:1.
35. An electrochemical cell comprising an electrode as in claim 1
and an electrolyte, wherein the ratio of electrolyte to sulfur, by
mass, within the electrochemical cell is less than about 6:1.
36. An electrochemical cell as in claim 35, wherein the ratio of
electrolyte to sulfur, by mass, within the electrochemical cell is
less than about 5:1.
37. An electrochemical cell as in claim 35, wherein the ratio of
electrolyte to sulfur, by mass, within the electrochemical cell is
less than about 4:1.
38. An electrochemical cell as in claim 35, wherein the ratio of
electrolyte to sulfur, by mass, within the electrochemical cell is
less than about 3:1.
39. An electrochemical cell as in claim 35, wherein the ratio of
electrolyte to sulfur, by mass, within the electrochemical cell is
less than about 2:1.
Description
RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C.
.sctn.119(e) to U.S. Provisional Patent Application No. 61/237,903,
filed Aug. 28, 2009, and entitled "Electrochemical Cells Comprising
Porous Structures Comprising Sulfur," which is incorporated herein
by reference in its entirety for all purposes.
FIELD OF INVENTION
[0002] The present invention relates to electrochemical cells, and
more specifically, to systems and methods involving electrochemical
cells comprising porous structures comprising sulfur.
BACKGROUND
[0003] A typical electrochemical cell includes a cathode and an
anode which participate in an electrochemical reaction. Generally,
electrochemical reactions are facilitated by an electrolyte, which
can contain free ions and can behave as an electrically conductive
medium. The performance of an electrochemical cell can be enhanced
by increasing the amount of contact between an electrode active
material and the electrolyte (e.g., by employing porous
electrodes), which can lead to an increase in the rate of the
electrochemical reaction within the cell. In addition, the
performance of an electrochemical cell can be enhanced by
maintaining a high degree of electrical conductivity within the
bulk of the electrodes (e.g., between an electrode active material
and a support on which it is deposited). Accordingly, systems and
methods that increase the amount of contact between electrode
active materials and electrolytes as well as increase the
electrical conductivity within the electrodes would be
beneficial.
SUMMARY OF THE INVENTION
[0004] The present invention relates to electrochemical cells, and
more specifically, to systems and methods involving electrochemical
cells comprising porous structures comprising sulfur. 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.
[0005] In one aspect, an electrode is described. In some
embodiments, the electrode can comprise a porous support structure
comprising a plurality of pores, and a plurality of particles
comprising an electrode active material comprising sulfur
substantially contained within the pores of the porous support
structure. In some embodiments, each particle of the plurality of
particles has a maximum cross-sectional dimension, each particle of
the plurality of particles has a particle volume, and the plurality
of particles has a total particle volume defined by the sum of each
of the individual particle volumes. In some cases, at least about
50% of the total particle volume is occupied by particles having
maximum cross-sectional dimensions of between about 0.1 microns and
about 10 microns.
[0006] In some cases, the electrode can comprise a porous support
structure comprising a plurality of pores; and a plurality of
particles comprising an electrode active material comprising sulfur
substantially contained within the pores of the porous support
structure, wherein the plurality of particles together defines a
total quantity of particulate material, and wherein at least about
50% of the total quantity of particulate material is made up of
particles having maximum cross-sectional dimensions of between
about 0.1 microns and about 10 microns.
[0007] The electrode can comprise, in some embodiments, a porous
support structure comprising a plurality of pores, and an electrode
active material comprising sulfur substantially contained within
the pores of the porous support structure, wherein each pore of the
plurality of pores has a pore volume, and the plurality of pores
has a total pore volume defined by the sum of each of the
individual pore volumes. In some instances, at least about 50% of
the total pore volume is occupied by pores having cross-sectional
diameters of between about 0.1 microns and about 10 microns.
[0008] In some instances, the electrode can comprise a porous
support structure comprising a plurality of pores; and an electrode
active material comprising sulfur substantially contained within
the pores of the porous support structure, wherein the plurality of
pores of the porous support structure together defines a total pore
volume, and at least about 50% of the total pore volume is defined
by pores having cross-sectional diameters of between about 0.1
microns and about 10 microns.
[0009] The electrode can comprise, in some cases, a porous support
structure comprising a plurality of pores, and an electrode active
material comprising sulfur substantially contained within the pores
of the porous support structure, wherein an electrochemical cell
comprising the electrode is capable of utilizing at least about 65%
of the total sulfur in the cell through at least 1 charge and
discharge cycles subsequent to a first charge and discharge cycle,
wherein 100% utilization corresponds to 1672 mAh per gram of sulfur
in the electrode.
[0010] 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. All patents and patent applications
disclosed herein are incorporated by reference in their entirety
for all purposes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] 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:
[0012] FIG. 1 is a schematic illustration of an exemplary
electrochemical cell;
[0013] FIG. 2 is a schematic illustration of an electrochemical
cell, according to another set of embodiments;
[0014] FIG. 3 is a schematic illustration of an exemplary
electrochemical cell;
[0015] FIGS. 4A-4B include scanning electron micrographs (SEMs) of
exemplary electrodes;
[0016] FIGS. 5A-5B include (A) a plot of specific discharge
capacity as a function of the number of charge-discharge cycles and
(B) a plot of capacity as a function of C-rate, according to one
set of embodiments;
[0017] FIGS. 6A-6F include (A) a secondary electron image of a
sulfur-carbon composite, (B-C) X-ray spectral images of the
composite in FIG. 6A, (D) a secondary electron image of a
cross-section of a sulfur-carbon composite, and (E-F) X-ray
spectral images of the composite in FIG. 6D, according to one set
of embodiments;
[0018] FIG. 7 includes a plot of specific discharge capacity as a
function of the number of charge-discharge cycles, according to one
set of embodiments;
[0019] FIGS. 8A-8B include secondary electron images of electrodes,
according to one set of embodiments;
[0020] FIGS. 9A-9F include X-ray spectral images outlining the
distributions of (A) sulfur in a composite cathode, (B) carbon in a
composite cathode, (C) aluminum in a composite cathode, (D) sulfur
in a mechanically mixed cathode, (E) carbon in a mechanically mixed
cathode, and (F) aluminum in a mechanically mixed cathode,
according to one set of embodiments;
[0021] FIG. 10 includes a plot of specific discharge capacity as a
function of the number of charge-discharge cycles for exemplary
electrochemical cells;
[0022] FIG. 11 includes an exemplary plot of percentage capacity as
a function of C-rate, according to one set of embodiments;
[0023] FIG. 12 includes, according to one set of embodiments, a
plot of cathode thickness as a function of applied pressure;
and
[0024] FIG. 13 includes an exemplary plot of specific discharge
capacity as a function of cycle number, according to some
embodiments.
DETAILED DESCRIPTION
[0025] The present invention relates to the use of porous
structures comprising sulfur in electrochemical cells. Such
materials may be useful, for example, in forming one or more
electrodes in an electrochemical cell. For example, the systems and
methods described herein may comprise the use of an electrode
comprising a conductive porous support structure and a plurality of
particles comprising sulfur (e.g., as an active species)
substantially contained within the pores of the support structure.
The inventors have unexpectedly discovered that, in some
embodiments, the sizes of the pores within the porous support
structure and/or the sizes of the particles within the pores can be
tailored such that the contact between the electrolyte and the
sulfur is enhanced, while the electrical conductivity and
structural integrity of the electrode are maintained at
sufficiently high levels to allow for effective operation of the
cell. Also, the sizes of the pores within the porous support
structures and/or the sizes of the particles within the pores can
be selected such that any suitable ratio of sulfur to support
material can be achieved while maintaining mechanical stability in
the electrode. The inventors have also unexpectedly discovered that
the use of porous support structures comprising certain materials
(e.g., metals such as nickel) can lead to relatively large
increases in cell performance. In some embodiments, methods for
forming particles comprising electrode active material (e.g.,
comprising sulfur) within pores of a porous support structure allow
for a desired relationship between the particle size and pore size.
The sizes of the pores within the porous support structure and/or
the sizes of the particles within the pores can also be tailored
such that the resulting electrode is able to withstand the
application of an anisotropic force, while maintaining the
structural integrity of the electrode.
[0026] In developing the systems and methods described herein, the
inventors have identified several challenges associated with
producing electrodes comprising sulfur. First, sulfur possesses a
relatively low electrical conductivity (e.g., about 5.0)(10.sup.-14
S cm.sup.-1 for elemental sulfur), which can inhibit the electrical
conductivity of the electrode and hence, cell performance. In
addition, small particle sulfur, which can be useful in producing
uniform thickness and high surface-area electrodes, can be
difficult to produce using traditional mechanical milling, as the
particles that are produced can quickly re-agglomerate. Moreover,
high surface area carbon, which can yield relatively high specific
capacity and cycle life, can be difficult to process as a
traditional slurry because it possesses a high absorption stiffness
resulting in a slurry with a relatively low amount of solids.
Finally, traditional slurry processing of sulfur-containing
electrode materials can lead to re-distribution of the slurry
components, which can produce uneven porosity within the cathode
and decreased anode utilization. The inventors have unexpectedly
discovered that these traditional disadvantages can be overcome by
disposing particles comprising sulfur within the pores of a support
material to produce an electrode that includes relatively uniform
porosity, particle size, and component distribution.
[0027] The porous structures described herein can be used in
electrochemical cells for a wide variety of devices, such as, for
example, electric vehicles, load-leveling devices (e.g., for solar-
or wind-based energy platforms), portable electronic devices, and
the like. In some cases, the porous structures described herein may
be particularly useful as electrodes in secondary batteries (i.e.,
rechargeable batteries) such as lithium-sulfur (L-S) batteries.
[0028] In one aspect, an electrode for use in an electrochemical
cell is described. The electrode may comprise a porous support
structure comprising a plurality of pores. As used herein, a "pore"
refers to a pore as measured using ASTM Standard Test D4284-07, and
generally refers to a conduit, void, or passageway, at least a
portion of which is surrounded by the medium in which the pore is
formed such that a continuous loop may be drawn around the pore
while remaining within the medium. Generally, voids within a
material that are completely surrounded by the material (and thus,
not accessible from outside the material, e.g. closed cells) are
not considered pores within the context of the invention. It should
be understood that, in cases where the article comprises an
agglomeration of particles, pores include both the interparticle
pores (i.e., those pores defined between particles when they are
packed together, e.g. interstices) and intraparticle pores (i.e.,
those pores lying within the envelopes of the individual
particles). Pores may comprise any suitable cross-sectional shape
such as, for example, circular, elliptical, polygonal (e.g.,
rectangular, triangular, etc.), irregular, and the like.
[0029] The porous support structure can comprise any suitable form.
In some instances, the porous support structure can comprise a
porous agglomeration of discreet particles, within which the
particles can be porous or non-porous. For example, the porous
support structure might be formed by mixing porous or non-porous
particles with a binder to form a porous agglomeration. Electrode
active material might be positioned within the interstices between
the particles and/or the pores within the particles (in cases where
porous particles are employed) to form the inventive electrodes
described herein.
[0030] In some embodiments, the porous support structure can be a
"porous continuous" structure. A porous continuous structure, as
used herein, refers to a continuous solid structure that contains
pores within it, with relatively continuous surfaces between
regions of the solid that define the pores. Examples of porous
continuous structures include, for example, a piece of material
that includes pores within its volume (e.g., a porous carbon
particle, a metal foam, etc.). One of ordinary skill in the art
will be capable of differentiating between a porous continuous
structure and, for example, a structure which is not a porous
continuous structure but which is a porous agglomeration of
discreet particles (where the interstices and/or other voids
between the discrete particles would be considered pores) by, for
example, comparing SEM images of the two structures.
[0031] The porous support structure may be of any suitable shape or
size. For example, the support structure can be a porous continuous
particle with any suitable maximum cross-sectional dimension (e.g.,
less than about 10 mm, less than about 1 mm, less than about 500
microns, etc.). In some cases, the porous support structure (porous
continuous or otherwise) can have a relatively large maximum
cross-sectional dimension (e.g., at least about 500 microns, at
least about 1 mm, at least about 10 mm, at least about 10 cm,
between about 1 mm and about 50 cm, between about 10 mm and about
50 cm, or between about 10 mm and about 10 cm). In some
embodiments, the maximum cross-sectional dimension of a porous
support structure within an electrode can be at least about 50%, at
least about 75%, at least about 90%, at least about 95%, at least
about 98%, or at least about 99% of the maximum cross sectional
dimension of the electrode formed using the porous continuous
structure.
[0032] In some embodiments, the support structure can be an article
with one relatively thin dimension relative to the other two, such
as, for example, a film. For example, the support structure can be
an article with a thickness of less than about 1 mm, less than
about 500 microns, less than about 100 microns, between about 1
micron and about 5 mm, between about 1 micron and about 1 mm,
between about 10 microns and about 5 mm, or between about 10
microns and about 1 mm, and a width and/or length at least about
100, at least about 1000, or at least about 10,000 times greater.
As used herein, the "maximum cross-sectional dimension" of an
article (e.g., a porous support structure) refers to the largest
distance between two opposed boundaries of an article that may be
measured. Porous support structures described herein may also be of
any suitable shape. For example, the support structure can be
spherical, cylindrical, or prismatic (e.g., a triangular prism,
rectangular prism, etc.). In some cases, the morphology of the
support structure may be selected such that the support structure
can be relatively easily integrated into an electrode for use in,
for example, an electrochemical cell. For example, the support
structure may comprise a thin film upon which additional components
of an electrochemical cell (e.g., an electrolyte, another
electrode, etc.) can be formed.
[0033] In some cases, porous particles can be used as a porous
continuous structure. In some such embodiments, material (e.g.,
electrode active material) can be deposited within the pores of the
particles, and the particles can be used to form an electrode. For
example, porous particles containing electrode active material
within their pores might be bound together (e.g., using binder or
other additives) to form a composite electrode. Exemplary processes
for forming such composite electrodes are described, for example,
in U.S. Pub. No.: 2006/0115579, filed Jan. 13, 2006, entitled
"Novel composite cathodes, electrochemical cells comprising novel
composite cathodes, and processes for fabricating same", which is
incorporated herein by reference in its entirety.
[0034] In some embodiments, the porous support structure might
comprise a relatively large-scale porous continuous structure that,
unlike the porous particles described above, is sized and shaped to
serve as an electrode. Such structures can be formed of a variety
of materials such as, for example, metals (e.g., a metal foam),
ceramics, and polymers. Examples of such materials are described in
more detail below. In some embodiments, the maximum cross-sectional
dimension of a porous continuous structure within an electrode can
be at least about 50%, at least about 75%, at least about 90%, at
least about 95%, at least about 98%, or at least about 99% of the
maximum cross sectional dimension of the electrode formed using the
porous continuous structure.
[0035] The use of such relatively large porous continuous
structures can, in some embodiments, ensure that little or no
binder is located within the electrode because binder would not be
required to hold together small particles to form the porous
support structure. In some embodiments, the electrode can include
less than about 20 wt %, less than about 10 wt %, less than about 5
wt %, less than about 2 wt %, less than about 1 wt %, or less than
about 0.1 wt % binder. In this context, "binder" refers to material
that is not an electrode active material and is not included to
provide an electrically conductive pathway for the electrode. For
example, an electrode might contain binder to facilitate internal
cohesion within the cathode.
[0036] The porous support structure may comprise any suitable
material. In some embodiments, the porous support structure can be
used as an electrical conductor within the electrode (e.g., as an
electrolyte-accessible conductive material). Accordingly, the
porous support structure may comprise an electrically conductive
material. Examples of electrically conductive materials that may be
suitable for use include, but are not limited to, metals (e.g.,
nickel, copper, aluminum, iron, or any other suitable metal or
combination in pure or alloyed form), carbon (e.g., graphite,
carbon black, acetylene black, carbon fibers, carbon nanofibers,
hallow carbon tubes, graphene, carbon filaments, etc.),
electrically conductive polymers, or any other suitable
electrically conductive material. In some embodiments, the bulk of
the porous support structure may be formed from an electrically
conductive material. In some cases, the porous support structure
may comprise an electrically non-conductive material that is at
least partially coated (e.g., via solution-based deposition,
evaporative deposition, or any other suitable technique) with a
conductive material. In some embodiments, the porous support
structure may comprise a glass (e.g., silicon dioxide, amorphous
silica, etc.), a ceramic (e.g., aluminum oxide, tin oxide, vanadium
oxide, and others described below), a semiconductor (e.g., silicon,
germanium, gallium arsenide, etc.), non-conductive polymers, and
the like.
[0037] The porous support structure may comprise pores with a size
distribution chosen to enhance the performance of the
electrochemical cell. In some cases, the porous support structure
may comprise pores than are larger than sub-nanometer scale and
single-nanometer scale pores, which can be too small to allow for
the passage of electrolyte (e.g., liquid electrolyte) into the
pores of the electrode due to, for example, capillary forces. In
addition, in some cases, the pores may be smaller than
millimeter-scale pores, which may be so large that they render the
electrode mechanically unstable. In some embodiments, the porous
support structure can comprise a plurality of pores, wherein each
pore of the plurality of pores has a pore volume, and the plurality
of pores has a total pore volume defined by the sum of each of the
individual pore volumes. In some embodiments, at least about 50%,
at least about 70%, at least about 80%, at least about 90%, at
least about 95%, at least about 99%, or substantially all of the
total pore volume is occupied by pores having cross-sectional
diameters of between about 0.1 microns and about 10 microns. In
some embodiments, at least about 50%, at least about 70%, at least
about 80%, at least about 90%, at least about 95%, at least about
99%, or substantially all of the total pore volume is occupied by
pores having cross-sectional diameters of between about 0.1 microns
and about 20 microns, between about 1 micron and about 10 microns,
or between about 1 micron and about 3 microns. Stated another way,
in some embodiments, the plurality of pores of the porous support
structure together defines a total pore volume, and at least about
50% (or at least about 70%, at least about 80%, at least about 90%,
at least about 95%, at least about 99%, or substantially all) of
the total pore volume is defined by pores having cross-sectional
diameters of between about 0.1 microns and about 10 microns (or
between about 0.1 microns and about 20 microns, between about 1
micron and about 10 microns, or between about 1 micron and about 3
microns).
[0038] In some embodiments, it may be advantageous to use porous
materials wherein the plurality of pores has an average
cross-sectional diameter within a designated range. For example, in
some cases, the porous support material may comprise a plurality of
pores wherein the average cross-sectional diameter of the plurality
of pores is between about 0.1 microns and about 10 microns, between
about 1 micron and about 10 microns, or between about 1 micron and
about 3 microns.
[0039] As described below, the pore distributions described herein
can be achieved, in some cases, while an anisotropic force (e.g.,
defining a pressure of between about 4.9 Newtons/cm.sup.2 and about
198 Newtons/cm.sup.2, or any of the ranges outlined below) is
applied to the electrochemical cell. This can be accomplished by
fabricating the porous support structure from materials (e.g.,
metals, ceramics, polymers, etc.) capable of maintaining their
porosities under applied loads. Fabricating an electrode from a
material which resists deformation under an applied load can allow
the electrode to maintains its permeability under pressure, and
allows the cathode to maintain the enhanced rate capabilities
described herein. In some embodiments, the yield strength of the
porous support structure (and the resulting electrode produced from
the porous support structure) can be at least about 200
Newtons/cm.sup.2, at least about 350 Newtons/cm.sup.2, or at least
about 500 Newtons/cm.sup.2. Methods of fabricating such structures
are described in more detail below.
[0040] As used herein, the "cross-sectional diameter" of a pore
refers to a cross-sectional diameter as measured using ASTM
Standard Test D4284-07. The cross-sectional diameter can refer to
the minimum diameter of the cross-section of the pore. The "average
cross-sectional diameter" of a plurality of pores refers to the
number average of the cross-sectional diameters of each of the
plurality of the pores.
[0041] One of ordinary skill in the art would be capable of
calculating the distribution of cross-sectional diameters and the
average cross-sectional diameter of the pores within a porous
structure using mercury intrusion porosimetry as described in ASTM
standard D4284-92, which is incorporated herein by reference in its
entirety. For example, the methods described in ASTM standard
D4284-92 can be used to produce a distribution of pore sizes
plotted as the cumulative intruded pore volume as a function of
pore diameter. To calculate the percentage of the total pore volume
within the sample that is occupied by pores within a given range of
pore diameters, one would: (1) calculate the area under the curve
that spans the given range over the x-axis, (2) divide the area
calculated in step (1) by the total area under the curve, and (3)
multiply by 100%. Optionally, in cases where the article includes
pore sizes that lie outside the range of pore sizes that can be
accurately measured using ASTM standard D4284-92, porosimetry
measurements may be supplemented using BET surface analysis, as
described, for example, in S. Brunauer, P. H. Emmett, and E.
Teller, J. Am. Chem. Soc., 1938, 60, 309, which is incorporated
herein by reference in its entirety.
[0042] In some embodiments, the porous material may comprise pores
with relatively uniform cross-sectional diameters. Not wishing to
be bound by any theory, such uniformity may be useful in
maintaining relatively consistent structural stability throughout
the bulk of the porous material. In addition, the ability to
control the pore size to within a relatively narrow range can allow
one to incorporate a large number of pores that are large enough to
allow for fluid penetration (e.g., electrolyte penetration) while
maintaining sufficiently small pores to preserve structural
stability of the porous material. In some embodiments, the
distribution of the cross-sectional diameters of the pores within
the porous material can have a standard deviation of less than
about 50%, less than about 25%, less than about 10%, less than
about 5%, less than about 2%, or less than about 1% of the average
cross-sectional diameter of the plurality of pores. Standard
deviation (lower-case sigma) is given its normal meaning in the
art, and can be calculated as:
.sigma. = i = 1 n ( D i - D avg ) 2 n - 1 ##EQU00001##
wherein D.sub.i is the cross-sectional diameter of pore i,
D.sub.avg is the average of the cross-sectional diameters of the
plurality of pores, and n is the number of pores. The percentage
comparisons between the standard deviation and the average
cross-sectional diameters of the pores outlined above can be
obtained by dividing the standard deviation by the average and
multiplying by 100%.
[0043] The electrodes described herein can also comprise a material
substantially contained within the pores of the porous support
structure. A material that is said to be "substantially contained"
within a pore is one that at least partially lies within the
imaginary volume defined by the outer boundaries of the pore. For
example, a material substantially contained within a pore can be
fully contained within the pore, or may only have a fraction of its
volume contained within the pore, but a substantial portion of the
material, overall, is contained within pores. In one set of
embodiments, material (e.g., material comprising sulfur) is
provided, at least 30% of which by mass is contained within pores
of a porous support structure. In other embodiments, at least 50%,
70%, 80%, 85%, 90%, or 95% by mass of the material is contained
within the pores of the support structure.
[0044] The material within the support structure can comprise, in
some cases, particles, which may be substantially solid or porous.
In some embodiments, the material substantially contained within
the pores may comprise isolated particles or agglomerated
particles. In some embodiments, the material may comprise a film
(which may be substantially solid or porous) on at least a portion
of the pores within the support structure. In some embodiments, the
material may substantially fill at least a portion of the pores
within the support structure, such that the material assumes the
shape and/or size of the portion of the pores.
[0045] The material within the support structure may comprise, in
some cases, an electrode active material. As used herein, the term
"electrode active material" refers to any electrochemically active
species associated with an electrode. For example, a cathode active
material refers to any electrochemically active species associated
with the cathode, while an anode active material refers to any
electrochemically active species associated with an anode.
[0046] In some embodiments, the electrodes of the present invention
may comprise a relatively large amount of material comprising
electrode active material within the pores of the porous support.
For example, in some embodiments, the electrode (e.g., cathode) may
comprise at least about 20 wt %, at least about 35 wt %, at least
about 50 wt %, at least about 65 wt %, or at least about 75 wt %
material comprising electrode active material, such as the
electroactive sulfur-containing materials described herein.
[0047] The material within the pores of the porous support
structure may comprise a variety of compositions. In some
embodiments, the material within the pores can comprise sulfur. For
example, the material within the pores can comprise electroactive
sulfur-containing materials. "Electroactive sulfur-containing
materials," as used herein, refers to electrode active materials
which comprise the element sulfur in any form, wherein the
electrochemical activity involves the oxidation or reduction of
sulfur atoms or moieties. As an example, the electroactive
sulfur-containing material may comprise elemental sulfur (e.g.,
S.sub.8). In another embodiment, the electroactive
sulfur-containing material comprises a mixture of elemental sulfur
and a sulfur-containing polymer. Thus, suitable electroactive
sulfur-containing materials may include, but are not limited to,
elemental sulfur, sulfides or polysulfides (e.g., of alkali metals)
which may be organic or inorganic, and organic materials comprising
sulfur atoms and carbon atoms, which may or may not be polymeric.
Suitable organic materials include, but are not limited to, those
further comprising heteroatoms, conductive polymer segments,
composites, and conductive polymers.
[0048] In some embodiments, an electroactive sulfur-containing
material of a cathode active layer comprises at least about 40 wt %
sulfur. In some cases, the electroactive sulfur-containing material
comprises at least about 50 wt %, at least about 75 wt %, or at
least about 90 wt % sulfur.
[0049] Examples of sulfur-containing polymers include those
described in: U.S. Pat. Nos. 5,601,947 and 5,690,702 to Skotheim et
al.; U.S. Pat. Nos. 5,529,860 and 6,117,590 to Skotheim et al.;
U.S. Pat. No. 6,201,100 issued Mar. 13, 2001, to Gorkovenko et al.
of the common assignee, and PCT Publication No. WO 99/33130. Other
suitable electroactive sulfur-containing materials comprising
polysulfide linkages are described in U.S. Pat. No. 5,441,831 to
Skotheim et al.; U.S. Pat. No. 4,664,991 to Perichaud et al., and
in U.S. Pat. Nos. 5,723,230, 5,783,330, 5,792,575 and 5,882,819 to
Naoi et al. Still further examples of electroactive
sulfur-containing materials include those comprising disulfide
groups as described, for example in, U.S. Pat. No. 4,739,018 to
Armand et al.; U.S. Pat. Nos. 4,833,048 and 4,917,974, both to De
Jonghe et al.; U.S. Pat. Nos. 5,162,175 and 5,516,598, both to
Visco et al.; and U.S. Pat. No. 5,324,599 to Oyama et al.
[0050] While sulfur, as the active electrode species, is described
predominately, it is to be understood that wherever sulfur is
described as the active electrode species herein, any suitable
electrode active species may be used. Those of ordinary skill in
the art will appreciate this and will be able to select species
(e.g., from the list described below) for such use.
[0051] In embodiments in which the material within the pores
comprises particles (e.g., particles of electrode active material),
the particles can be of any suitable shape. For example, in some
embodiments, the particles may be substantially spherical. In some
cases, a particle can be similar in shape to the pore it occupies
(e.g., cylindrical, prismatic, etc.).
[0052] The size of the particles (e.g., particles of electrode
active material) within the pores of the porous support structure
can be selected to enhance the performance of the electrochemical
cell. In some embodiments, each particle of the plurality of
particles within the pores of the porous support structure has a
particle volume, and the plurality of particles has a total
particle volume defined by the sum of each of the individual
particle volumes. In addition, in some embodiments, each particle
of the plurality of particles within the pores of the porous
support structure has a maximum cross-sectional dimension. In some
instances, at least about 50%, at least about 70%, at least about
80%, at least about 90%, at least about 95%, at least about 99%, or
substantially all of the total particle volume within the pores of
the porous support structure is occupied by particles having
maximum cross-sectional dimensions of between about 0.1 microns and
about 10 microns. In some embodiments, at least about 50%, at least
about 70%, at least about 80%, at least about 90%, at least about
95%, at least about 99%, or substantially all of the total particle
volume within the pores of the porous support structure is occupied
by particles having maximum cross-sectional dimensions of between
about 1 micron and about 10 microns, or between about 1 micron and
about 3 microns. Stated another way, in some embodiments, the
plurality of particles together defines a total quantity of
particulate material, and at least about 50% (or at least about
70%, at least about 80%, at least about 90%, at least about 95%, at
least about 99%, or substantially all) of the total quantity of
particulate material is made up of particles having maximum
cross-sectional dimensions of between about 0.1 microns and about
10 microns (or between about 1 micron and about 10 microns, or
between about 1 micron and about 3 microns).
[0053] As used herein, the "maximum cross-sectional dimension" of a
particle refers to the largest distance between two opposed
boundaries of an individual particle that may be measured. The
"average maximum cross-sectional dimension" of a plurality of
particles refers to the number average of the maximum
cross-sectional dimensions of the plurality of particles.
[0054] One of ordinary skill in the art would be capable of
measuring the maximum cross-sectional dimension of a particle by,
for example, analyzing a scanning electron micrograph (SEM) of a
particle. In embodiments comprising agglomerated particles, the
particles should be considered separately when determining the
maximum cross-sectional dimensions. The measurement could be
performed by establishing imaginary boundaries between each of the
agglomerated particles, and measuring the maximum cross-sectional
dimension of the hypothetical, individuated particles that result
from establishing such boundaries. The distribution of maximum
cross-sectional dimensions and particle volumes could also be
determined by one of ordinary skill in the art using SEM analysis.
The total particle volume of the particles within the pores could
be determined by one of ordinary skill in the art by employing
mercury intrusion porosimetry according to ASTM Standard Test
D4284-07 (optionally with BET surface analysis) to measure the
volume within the pores before and after the particles are disposed
within the pores. When the material inside the pores of the support
structure is itself porous, mercury intrusion porosimetry
measurements (with optional BET surface analysis) may be
supplemented with visual analysis of SEM micrographs to determine
the volume occupied by the material (e.g., particles) within the
pores.
[0055] In some embodiments, the particles of material (e.g.,
electrode active material) within the porous support structure may
have an average maximum cross-sectional dimension within a
designated range. For example, in some cases, the particles of
material (e.g., electrode active material) within the porous
support structure can have an average maximum cross-sectional
dimension of between about 0.1 microns and about 10 microns,
between about 1 micron and about 10 microns, or between about 1
micron and about 3 microns. In some embodiments, the ratio of the
average maximum cross-sectional dimension of the particles of
material within the porous support structure to the average
cross-sectional diameter of the pores within the porous support
structure can be between about 0.001:1 and about 1:1, between about
0.01:1 and about 1:1, or between about 0.1:1.
[0056] In some embodiments, particles within the pores of the
porous support structure can have relatively uniform maximum
cross-sectional dimensions. Not wishing to be bound by any theory,
such uniformity may be useful in producing relatively consistent
performance along a surface of an electrode comprising electrode
active material particles. In some embodiments, the distribution of
the cross-sectional dimensions of the pores within the porous
material can have a standard deviation of less than about 50%, less
than about 25%, less than about 10%, less than about 5%, less than
about 2%, or less than about 1% of the average cross-sectional
diameter of the plurality of pores. Standard deviation (lower-case
sigma) is given its normal meaning in the art, and can be
calculated, and expressed as a percentage relative to an average,
as described above.
[0057] In some embodiments, the material (e.g., particles) within
the pores of the porous support structure may occupy a relatively
large percentage of the pore volume. For example, in some
embodiments, the material within the porous support structure
(e.g., particles comprising an electrode active material) can
occupy at least about 10%, at least about 20%, at least about 35%,
at least about 50%, at least about 70%, or more of the accessible
pore volume of the porous support structure. As used herein, the
"accessible pore volume" is consistent with the above definition of
a pore and refers to the percentage of the pore volume that is
exposed to the external environment surrounding a porous article,
as opposed to pore volume that is completely enclosed by the
material forming the porous article. The volume occupied by
material within the pores should be understood to include an
imaginary volume that surrounds the outer boundaries of the
material (e.g., particles) within the pores, which may include
material (e.g. particle) void volume in cases where the material
within the pores is itself porous. One of ordinary skill in the art
is capable of calculating the percentage of accessible pore volume,
for example, using mercury intrusion porosimetry measurements
according to ASTM Standard Test D4284-07, optionally supplemented
by BET surface analysis. The percentage of accessible pore volume
within a porous article that is occupied by particles can be
calculated, for example, by performing mercury intrusion
porosimetry measurements (optionally with BET surface analysis) of
the porous article before and after the particles are positioned
within the pores. When the material inside the pores of the support
structure is itself porous, mercury intrusion porosimetry
measurements (with optional BET surface analysis) may be
supplemented with visual analysis of SEM micrographs to determine
the volume occupied by the material (e.g., particles) within the
pores.
[0058] The electrodes comprising the porous support structure may
comprise a relatively high percentage of electrode active material
(e.g., sulfur), in some cases. In some embodiments, the electrodes
comprising the porous support structure can comprise, for example,
at least about 20 wt %, at least about 30 wt %, at least about 40
wt %, or more electrode active material. It should be understood
that, for the purposes of calculating the amount of electrode
active material within an electrode, only the weight of the
electrode active species is counted. For example, in cases where
electroactive sulfur-containing materials such as polysilfides or
organic materials comprising sulfur, only the sulfur content of the
electroactive sulfur-containing materials is counted in determining
the percentage of electrode active material within the electrode.
In some embodiments, the electrodes comprising the porous support
structure can comprise at least about 20 wt %, at least about 30 wt
%, at least about 40 wt %, or more sulfur.
[0059] The electrodes described herein can comprise any suitable
weight ratio of electrode active material and support material
(e.g., any suitable ratio of sulfur to carbon). For example, in
some embodiments, the electrode can comprise a weight ratio of
sulfur to carbon of at least about 1:1, at least about 2:1, at
least about 3:1, at least about 4:1, at least about 5:1, or at
least about 6:1. In some embodiments, the electrode can comprise a
weight ratio of sulfur to carbon of less than about 6:1, less than
about 5:1, less than about 4:1, less than about 3:1, less than
about 2:1, or less than about 1:1.
[0060] In some cases, the concentration of the electrode active
material (e.g., sulfur within a cathode) can be relatively
consistent across one or more surfaces of the electrode, or across
any cross-section of the electrode. In some embodiments, at least
about 50%, at least about 75%, at least about 85%, at least about
90%, at least about 95%, or at least about 98% of the area of the
surface of an electrode (e.g., cathode) defines a uniform area that
includes a uniform distribution of electrode active material (e.g.,
sulfur). In some embodiments, at least about 50%, at least about
75%, at least about 85%, at least about 90%, at least about 95%, or
at least about 98% of the area of a surface of a cross-section
substantially perpendicular to the thickness of an electrode (e.g.,
a cathode) defines a uniform area that includes a uniform
distribution of electrode active material (e.g., sulfur).
[0061] In this context, a "surface of an electrode" refers to the
geometric surface of the electrode, which will be understood by
those of ordinary skill in the art to refer to the surface defining
the outer boundaries of the electrode, for example, the area that
may be measured by a macroscopic measuring tool (e.g., a ruler) and
does not include the internal surface area (e.g., area within pores
of a porous material such as a foam, or surface area of those
fibers of a mesh that are contained within the mesh and do not
define the outer boundary, etc.). In addition, a "cross-section of
an electrode" defines an approximate plane viewed by cutting
(actually or theoretically) the electrode to expose the portion one
wishes to analyze. After the electrode has been cut to observe the
cross-section, the "surface of the cross-section of the electrode"
corresponds to the exposed geometric surface. Stated another way,
"surface of an electrode" and "surface of the cross-section of the
electrode" refer, respectively, to the geometric surface of the
electrode and the geometric surface of a cross-section of the
electrode.
[0062] In some embodiments, an electrode active material (e.g.,
sulfur) is uniformly distributed when any continuous area that
covers about 10%, about 5%, about 2%, or about 1% of the uniform
area (described in the preceding paragraphs) includes an average
concentration of the electrode active material (e.g., sulfur) that
varies by less than about 25%, less than about 10%, less than about
5%, less than about 2%, or less than about 1% relative to the
average concentration of the electrode active material (e.g.,
sulfur) across the entirety of the uniform area. In this context,
the "average concentration" of an electrode active material refers
to the percentage of the surface area of the electrode (e.g.,
exposed surface area, surface area of a cross section of the
electrode) that is occupied by electrode active material when the
electrode is viewed from an angle substantially perpendicularly to
the electrode.
[0063] One of ordinary skill in the art would be capable of
calculating average electrode active material concentrations within
a surface or a cross-section of an electrode, and the variance in
concentrations, by analyzing, for example, X-ray spectral images of
an electrode surface or cross-section. For example, one could
obtain an x-ray spectral image of an electrode surface or
cross-section (e.g., by physically slicing the electrode to produce
the cross-section), such as the images shown in FIG. E6A-E6C. To
calculate the average concentration of sulfur over a given area in
such an image, one would determine the percentage of the image that
is occupied by the color corresponding to sulfur over that area. To
determine whether the average concentration within a sub-area
varies by more than X % relative to the average concentration
within a larger area, one would use the following formula:
Variance ( % ) = C L - C sub C L 100 % ##EQU00002##
wherein C.sub.L is the average concentration within the larger area
(expressed as a percentage), C.sub.sub is the average concentration
within the sub-area (expressed as a percentage). As a specific
example, if the average concentration of the electrode active
material within a sub-area is 12%, and the average concentration of
the electrode active material within a larger area is 20%, the
variance would be 40%.
[0064] Stated another way, in some embodiments, at least about 50%
(or at least about 75%, at least about 85%, at least about 90%, at
least about 95%, or at least about 98%) of the area of the surface
of the electrode (or of a cross-section of the electrode) defines a
first, continuous area of essentially uniform sulfur distribution,
the first area having a first average concentration of sulfur. In
some cases, any continuous area that covers about 10% (or about 5%,
about 2%, or about 1%) of the first, continuous area of the surface
of the electrode (or of the cross section of the electrode)
includes a second average concentration of sulfur that varies by
less than about 25% (or less than about 10%, less than about 5%,
less than about 2%, or less than about 1%) relative to the first
average concentration of sulfur across the first, continuous
area.
[0065] In another aspect, a method of making an electrode for use
in an electrochemical cell is described. The method may comprise,
in some embodiments, depositing a material (e.g., particles)
substantially within the pores of a porous support structure. The
material deposited in the pores of the porous support structure can
comprise an electrode active material such as, for example, sulfur.
The porous support structure and the material can possess any of
the characteristics (e.g., materials, sizes, porosities, etc.)
described herein.
[0066] Porous support structures (and resulting electrodes) can be
fabricated using a variety of methods. For example, in some
embodiments, particles can be suspended in a fluid, and the fluid
can be subsequently removed (e.g., via heat drying, vacuum drying,
filtration, etc) to produce the porous support structure in which
the particles are adhered to each other. As mentioned above, in
some cases, a binder can be used to adhere particles to form a
composite porous support structure.
[0067] In some embodiments, porous support structures can be
fabricated by heating individual particles of a material until the
particles are altered to form a porous support structure (e.g., a
porous continuous structure). In some embodiments, particles (e.g.,
metallic particles, ceramic particles, glass particles, etc.) can
be arranged such that they are in contact with each other, with
interstices located between the particles. The particles can then
be sintered to form a fused structure in which the interstices
between the particles constitute the pores in the sintered
structure. As used herein, "sintering" is given its normal meaning
in the art, and is used to refer to a method for making objects
from particles, by heating the particles below their melting point
until the particles adhere to each other. The total porosity, size
of the pores, and other properties of the final structure could be
controlled by selecting appropriate particles sizes and shapes,
arranging them to form a desired packing density prior to
sintering, and selecting appropriate sintering conditions (e.g.,
heating time, temperature, etc.).
[0068] In some cases, particles (e.g., polymeric particles,
metallic particles, glass particles, ceramic particles, etc.)
particles arranged such that they are in contact with each other
can be heated such that the particles melt to form a porous
continuous structure. The interstices of the original structure can
form the pores of the porous continuous structure in some such
embodiments. The total porosity, size of the pores, and other
properties of the final structure could be controlled by selecting
appropriate particles sizes and shapes, arranging them to form a
desired packing density prior to heating, and selecting appropriate
heating conditions (e.g., heating time, temperature, etc.).
[0069] In some embodiments, the particles can be controllably
arranged prior to melting or sintering. For example, in some cases
in which the particles are used to form a porous layer, it can be
advantageous to arrange the particles such that they are
distributed relatively evenly and relatively flatly against a
substrate. This can be achieved, for example, by suspending the
particles in a solvent that is volatile (e.g., at room
temperature), and pouring the solvent onto the substrate on which
the porous structure is to be formed. After the particle solvent is
deposited, the volatile solvent can be allowed to evaporate,
leaving behind a relatively well-ordered array of particles.
[0070] The sintering and/or melting processes described herein can
be carried out in a controlled atmosphere, in some cases. For
example, the volume in which melting or sintering is performed can
be filled with a relatively inter gas (e.g., nitrogen, argon,
helium, and the like), in some cases. In some instances, the
melting and/or sintering can be carried out in the substantial
absence of oxygen, which can reduce or eliminate oxidation and/or
combustion of the material used to form the porous support
structure. In some embodiments, a reducing atmosphere (e.g.,
forming gas with the balance nitrogen and/or argon, hydrogen, or
the like) can be used to reduce the final oxygen content of the
sintered and/or melted article.
[0071] The sintering and/or melting temperature can be selected
based upon the material being used to form the porous support
structure. For example, when melting particles to form the porous
support structure, the heating temperature can be selected such
that it is above the melting temperature of the material from which
the particles are made. One of ordinary skill in the art would be
capable of selecting an appropriate sintering temperature, based
upon the type of material being sintered. For example, suitable
sintering temperatures for nickel might be between about
700.degree. C. and about 950.degree. C.
[0072] As mentioned above, the sizes and shapes of the particles
used to form the porous support structure can be selected to
achieve a desired porosity. In some embodiments, the particles can
be substantially spherical, although particles with other
cross-sectional shapes (e.g., ellipses, polygons (e.g., rectangles,
triangles, squares, etc.), irregular, etc.) can also be used. The
particles can be relatively small (e.g., in the form of a powder),
in some embodiments. For example, in some cases, at least about
50%, at least about 70%, at least about 80%, at least about 90%, at
least about 95%, at least about 99%, or substantially all of the
particles have maximum cross-sectional dimensions of between about
0.5 microns and about 20 microns or between about 3 microns and
about 5 microns. Such particle sizes can be useful in producing
porous support structures with the advantageous porosity properties
described elsewhere in this application.
[0073] In some embodiments, the porous support structure can be
formed by combining a first material with a second material, and
forming the pores of the support structure by removing one of the
materials from the mixture. Removing one of the materials from the
mixture can leave behind voids which ultimately form the pores of
the porous support structure. In some cases, the structure of the
non-removed material can be substantially maintained while one or
more of the materials within the mixture is removed. For example,
in some cases, the support structure material (e.g., a metal,
ceramic, glass, polymer, etc. which might be melted) or a precursor
to the support structure material (e.g., which might be converted
to form the material of the porous support structure via, for
example, a reaction (e.g., polymerization, precipitation, etc.)),
can be mixed with a plurality of templating entities. The
templating entities can be arranged such that they form an
interconnected network within the support structure material or
precursor. After the templating entities have been arranged within
the support structure material, they can be removed from the
support structure material to leave behind pores. The support
structure material can be hardened before the templating entities
are removed and/or during the removal of the templating entities.
As used herein, the term "hardened" is used to refer to the process
of substantially increasing the viscosity of a material, and is not
necessarily limited to solidifying a material (although in one set
of embodiments, a porous support structure material is hardened by
converting it into a solid). A material can be hardened, for
example, by gelling a liquid phase. In some instances, a material
can be hardened using polymerization (e.g., IR- or UV-induced
polymerization). In some cases, a material can being hardened can
go through a phase change (e.g., reducing the temperature of a
material below its freezing point or below its glass transition
temperature). A material may also be hardened by removing a solvent
from a solution, for example, by evaporation of a solvent phase,
thereby leaving behind a solid phase material.
[0074] The templating entities can be of any suitable phase. In
some cases, the templating entities can be solid particles. For
example, the templating entities might comprise silica particles,
which can be dissolved out of a porous structure using, for
example, hydrofluoric acid. As another example, the templating
entities might comprise ammonium bicarbonate, which can be removed
by dissolving it in water. In some embodiments, the templating
entities can comprise fluid (e.g., liquid and/or gas) bubbles.
[0075] The templating entities can also have any suitable shape,
regular or irregular, including, but not limited to, spheres,
cubes, pyramids, or a mixture of these and/or other shapes. The
templating entities may also each be formed of any suitable size.
In some embodiments, the templating entities may have an average
maximum cross-sectional dimension roughly equivalent to the size of
the desired pores within the porous support structure.
[0076] As a specific example, a metallic porous support structure
can be fabricated using metal injection molding. In an exemplary
process, a "green" of metal particles, binder, and templating
entities can be formed into a suitable structure (e.g., a
relatively thin sheet) via injection molding. As the green is
heated, the metal particles can be melted or sintered together
while the binder and templating entities can be burned off, leaving
behind a series of pores.
[0077] Porous ceramic structures can also be produced using a
templating methods. For example, in some cases, a ceramic foam can
be produced by including ceramic particles and templating entities
within a polyaphron solution (i.e., a bi-liquid foam). The
resulting mixture can be used in a sol gel solution, which can form
a stable emulsion with the use of, for example, appropriate
surfactants. Once the gel has been hardened, the templating
entities can be removed by heat treatment. The size of the
polyaphrons can be controlled by varying the type and amount of the
surfactants in the bi-liquid foam.
[0078] Templating methods can also be used to produce porous
polymeric structures. For example, a plurality of solid particles
might be dispersed within a monomer solution. After the monomer is
polymerized to form a polymer, the solid particles can be
selectively dissolved out of the mixture to leave behind a series
of pores within the rest of the polymeric structure.
[0079] Another method that might be used to produce the porous
support structures described herein includes 3D printing. 3D
printing is known to those of ordinary skill in the art, and refers
to a process by which a three dimensional object is created by
shaping successive layers, which are adhered on top of each other
to form the final object. 3D printing can be used with a variety of
materials, including metals, polymers, ceramics, and others.
[0080] A variety of materials (e.g., in particle form, in melt
form, or other forms mentioned herein) can be used to form the
porous support structure. The material used to form all or part of
the porous support structure can include a metal or a metal alloy,
in some embodiments. Suitable metals include, but are not limited
to, nickel, copper, magnesium, aluminum, titanium, scandium, and
alloys and/or combinations of these. In some embodiments, the metal
or metal alloy from which the particles are formed can have a
density of less than about 9 g/cm.sup.3 or less than about 4.5
g/cm.sup.3.
[0081] In some embodiments, a polymeric material can be used to
form all or part of the porous support structure. Suitable polymers
for use in forming porous support structures include, but are not
limited to, polyvinyl alcohol (PVA), phenolic resins
(novolac/resorcinol), lithium polystyrenesulfonate (LiPSS),
epoxies, UHMWPE, PTFE, PVDF, PTFE/vinyl copolymers,
co-polymers/block co-polymers of the above and others. In some
embodiments, two polymers can be used for their unique
funcionalities (e.g. PVA for adhesion, and LiPSS for rigidity, or
resorcinol for rigidity and an elastomer for
flexibility/toughness). The material used to form the porous
support structure might include one or more conductive polymers
such as, for example, poly(3,4-ethylenedioxythiphene) (PEDOT),
poly(methylenedioxythiophene) (PMDOT), other thiophenes,
polyaniline (PANI), polypyrrole (PPy). Those of ordinary skill in
the art would be capable of selecting a counter ion for a
conductive polymer system, which can be selected from a variety of
chemical species such as PSS for PEDOT, other well known conductive
polymers, and co and block co-polymers as above.
[0082] A ceramic material might be used to form all or part of a
porous support structure, in some instances. Suitable ceramics
include, but are not limited to, oxides, nitrides, and/or
oxynitrides of aluminum, silicon, zinc, tin, vanadium, zirconium,
magnesium, indium, and alloys thereof. In some cases, the porous
support structure can include any of the oxides, nitrides, and/or
oxynitrides above doped to impart desirable properties, such as
electrical conductivity; specific examples of such doped materials
include tin oxide doped with indium and zinc oxide doped with
aluminum. The material used to form the porous support structure
can comprise glass (e.g., quartz, amorphous silica, chalcogenides,
and/or other conductive glasses) in some embodiments. The porous
support structure can include, in some cases, aerogels and/or xero
gels of any of the above materials. In some cases, the porous
support structure can include a vitreous ceramic.
[0083] In some embodiments in which the bulk of the porous support
structure is made of a material that is substantially electrically
non-conductive, electrically conductive material can be deposited
within the pores of the support structure to impart electrical
conductivity. For example, the bulk of the porous support structure
might comprise a ceramic (e.g., glass) or an electrically
non-conductive polymer, and a metal might be deposited within the
pores of the support structure. The electrically conductive
material can be deposited, for example, via electrochemical
deposition, chemical vapor deposition, or physical vapor
deposition. In some cases, after the deposition of the electrically
conductive material, an electrode active material can be deposited
within the pores of the porous support structure. Suitable
materials for placement within the pores of the porous support
structure to impart electrical conductivity include, but are not
limited to carbon and metals such as nickel and copper, and
combinations of these.
[0084] The bulk of the porous support structure can be made
electrically conductive, in some embodiments, by incorporating one
or more electrically conductive materials into the bulk of the
porous support structure material. For example, carbon (e.g.,
carbon black, graphite or graphene, carbon fibers, etc.), metal
particles, or other electrically conductive materials might be
incorporated into a melt (e.g., a non-conductive polymeric melt, a
glass melt, etc.) which is used to form a polymeric porous support
structure to impart electrical conductivity to the porous support
structure. After the melt is hardened, the electrically conductive
material can be included within the bulk of the porous support
structure.
[0085] The mechanical properties of the porous support structure
can also be enhanced by incorporating materials that structurally
reinforce the porous support structure into the bulk of the porous
support structure. For example, carbon fibers and/or particulate
fillers can be incorporated into a melt (e.g., a metallic melt, a
glass melt, a polymeric melt, etc.) which is hardened to form a
porous support structure. In some cases, carbon fibers and/or
particulate fillers can be incorporated into a solution in which
the porous support structure is formed (e.g., in some cases in
which the porous support structure comprises a polymer).
[0086] In some embodiments, the surfaces on or within of the porous
support structure may be activated or modified prior to depositing
the material, for example, to provide for enhanced attachment of
material to the surfaces of the porous support structure. Porous
support structures can be activated or modified by exposing the
porous material to reactive or unreactive gasses or vapors. In some
embodiments, the activation or modification steps can be performed
at elevated temperatures (e.g., at least about 50.degree. C., at
least about 100.degree. C., at least about 250.degree. C., at least
about 500.degree. C., at least about 750.degree. C., or higher)
and/or aub-atmospheric pressures (e.g., less than about 760 torr,
less than about 250 torr, less than about 100 torr, less than about
10 torr, less than about 1 torr, less than about 0.1 torr, less
than about 0.01 torr, or lower).
[0087] Electrode active material (e.g., particles, films, or other
forms comprising electrode active material) may be deposited within
the pores of the porous support structure via a variety of methods.
In some embodiments, electrode active material is deposited by
suspending or dissolving a particle precursor (e.g., a precursor
salt, elemental precursor material such as elemental sulfur, and
the like) in a solvent and exposing the porous support structure to
the suspension or solution (e.g., via dipping the porous support
structure into the solvent, by spraying the solvent into the pores
of the porous support structure, and the like). The particle
precursor may subsequently form particles within the pores of the
support structure. For example, in some cases, the precursor may
form crystals within the pores of the support structure. Any
suitable solvent or suspension medium may be used in conjunction
with such a technique including aqueous liquids, non-aqueous
liquids, and mixtures thereof. Examples of suitable solvents or
suspension media include, but are not limited to, water, methanol,
ethanol, isopropanol, propanol, butanol, tetrahydrofuran,
dimethoxyethane, acetone, toluene, xylene, acetonitrile,
cyclohexane, and mixtures thereof. Of course, other suitable
solvents or suspension media can also be used as needed.
[0088] Electrode active material can also be deposited within the
pores of the support structure, in some cases, by heating a
material above its melting point or boiling point (optionally
adjusting the surrounding pressure to, for example, aid in
evaporation). The heated material may then be flowed or vaporized
into the pores of the support material such that particulate
deposits or other solids are formed. As a specific example,
elemental sulfur powder can be positioned next to a porous support
material and heated above the melting point of sulfur, such that
the sulfur flows into the pores of the material (e.g., via
sublimation, via liquid flow). The composite can then be cooled
such that the sulfur deposits within the pores.
[0089] In some embodiments, electrode active material can be
deposited within the pores of the support structure via
electrochemical deposition, chemical vapor deposition, or physical
vapor deposition. For example, metals such as aluminum, nickel,
iron, titanium, and the like, can be electrochemically deposited
within the pores of a porous support structure. Alternatively, such
materials may be deposited, for example, using a physical vapor
deposition technique such as, for example, electron beam
deposition.
[0090] In some embodiments, catalyst may be deposited within the
pores of the support structure in addition to the electrode active
material (e.g., before or during the deposition of the electrode
active material). In some cases, the catalyst may catalyze the
electrochemical conversion of the electrode active material (e.g.,
the conversion of sulfur to Li.sub.2S and/or the conversion of
Li.sub.2S to sulfur). Suitable catalyst can include, for example,
cobalt phthalocyanine and transition metal salts, complexes, and
oxides (e.g., Mg.sub.0.6Ni.sub.0.4O).
[0091] The electrodes described herein may comprise one or more
advantageous properties. In some embodiments, the electrode may
exhibit a relatively high porosity. In some cases, the porosity of
the electrode may be at least about 30%, at least about 40%, at
least about 50%, at least about 60%, at least about 70%, at least
about 80%, or at least about 90%. Such porosities (and any of the
pore distributions described herein) may be achieved, in some
cases, while an anisotropic force (e.g., defining a pressure of
between about 4.9 Newtons/cm.sup.2 and about 198 Newtons/cm.sup.2,
or any of the ranges outlined below) is applied to the
electrochemical cell. As used herein, the percentage porosity of an
electrode (e.g., the cathode) is defined as the void volume of the
electrode divided by the volume within the outer boundary of the
electrode, expressed as a percentage. "Void volume" is used to
refer to portions of an electrode that are not occupied by
electrode active material (e.g., sulfur), conductive material
(e.g., carbon, metal, etc.), binder, or other materials that
provide structural support. The void volume within the electrode
may comprise pores in the electrode as well as interstices between
aggregates of the electrode material. Void volume may be occupied
by electrolyte, gases, or other non-electrode materials. In some
embodiments, the void volume of the electrode may be at least about
1, at least about 2, at least about 4, or at least about 8 cm.sup.3
per gram of electrode active material (e.g., sulfur) in the
electrode.
[0092] In some embodiments, the electrode can comprise a relatively
large electrolyte accessible conductive material area. As used
herein, "electrolyte accessible conductive material area" is used
to refer to the total surface area of the conductive material
(e.g., carbon) within the electrode that can be contacted by
electrolyte. For example, electrolyte accessible conductive
material area may comprise conductive material surface area within
the pores of the electrode, conductive material surface area on the
external surface of the electrode, etc. In some instances,
electrolyte accessible conductive material area is not obstructed
by binder or other materials. In addition, in some embodiments,
electrolyte accessible conductive material area does not include
portions of the conductive material that reside within pores that
restrict electrolyte flow due to surface tension effects. In some
cases, the electrode comprises an electrolyte accessible conductive
material area (e.g., an electrolyte accessible carbon area, an
electrolyte accessible metal area) of at least about 1 m.sup.2, at
least about 5 m.sup.2, at least about 10 m.sup.2, at least about 20
m.sup.2, at least about 50 m.sup.2, or at least about 100 m.sup.2
per gram of electrode active material (e.g., sulfur) in the
electrode. The relatively large electrolyte accessible conductive
material areas described above can be achieved, in some cases,
while an anisotropic force (e.g., defining a pressure of between
about 4.9 Newtons/cm.sup.2 and about 198 Newtons/cm.sup.2, or any
of the ranges outlined below) is applied to the electrochemical
cell.
[0093] Although the electrodes described herein can find use in a
wide variety of electrochemical devices, an example of one such
device is provided in FIG. 1 for illustrative purposes only. A
general embodiment of an electrochemical cell can include a
cathode, an anode, and an electrolyte layer in electrochemical
communication with the cathode and the anode. In some cases, the
cell also may comprise a containment structure. The components may
be assembled, in some cases, such that the electrolyte is placed
between the cathode and anode in a stacked configuration. FIG. 1
illustrates an electrochemical cell of the invention. In the
embodiment shown, cell 10 includes a cathode 30 that can be formed
on a substantially planar surface of substrate 20. While the
cathode and substrate in FIG. 1 are shown as having a planar
configuration, other embodiments may include non-planar
configurations, as will be discussed in more detail later. As noted
above, the cathode and/or anode can include a porous support
structure within which electrode active material is contained. For
example, in a lithium-sulfur battery, the cathode can comprise a
porous support structure within which a electroactive
sulfur-containing material is contained.
[0094] The cathode may comprise a variety of cathode active
materials. For example, the cathode may comprise a
sulfur-containing material, wherein sulfur is the cathode active
material. Other examples of cathode active materials are described
more fully below. In some embodiments, cathode 30 comprises at
least one active surface (e.g., surface 32). As used herein, the
term "active surface" is used to describe a surface of an electrode
that is in physical contact with the electrolyte and at which
electrochemical reactions may take place.
[0095] An electrolyte 40 (e.g., comprising a porous separator
material) can be formed adjacent the cathode 30. In some
embodiments, electrolyte 40 may comprise a non-solid electrolyte,
which may or may not be incorporated with a porous separator. As
used herein, the term "non-solid" is used to refer to materials
that are unable to withstand a static shear stress, and when a
shear stress is applied, the non-solid experiences a continuing and
permanent distortion. Examples of non-solids include, for example,
liquids, deformable gels, and the like.
[0096] Electrochemical cells described herein may make use of a
relatively low mass of electrolyte relative to the mass of the
cathode active material or the anode active material, in some
embodiments. For example, in some instances, the ratio of
electrolyte to cathode active material (e.g., sulfur) or anode
active material, by mass, within the electrochemical cell is less
than about 6:1, less than about 5:1, less than about 4:1, less than
about 3:1, or less than about 2:1.
[0097] An anode layer 50 can be formed adjacent electrolyte 40 and
may be in electrical communication with the cathode 30. The anode
may comprise a variety of anode active materials. For example, the
anode may comprise a lithium-containing material, wherein lithium
is the anode active material. Other examples of anode active
materials are described more fully below. In some embodiments,
anode 50 comprises at least one active surface (e.g., surface 52).
The anode 50 may also be formed on an electrolyte layer positioned
on cathode 30, for example on electrolyte 40. Of course, the
orientation of the components can be varied, and it should be
understood that there are other embodiments in which the
orientation of the layers is varied such that, for example, the
anode layer or the electrolyte layer is first formed on the
substrate.
[0098] Optionally, the cell may also include, in some embodiments,
containment structure 56. In addition, the cell may also,
optionally, include additional layers (not shown), such as a
multi-layer structure that protects an electroactive material
(e.g., an electrode) from the electrolyte, may be present, as
described in more detail in U.S. patent application Ser. No.
11/400,781, filed Apr. 6, 2006, entitled, "Rechargeable
Lithium/Water, Lithium/Air Batteries" to Affinito et al., which is
incorporated herein by reference in its entirety. Additionally,
non-planar arrangements, arrangements with proportions of materials
different than those shown, and other alternative arrangements are
useful in connection with the present invention. A typical
electrochemical cell also would include, of course, current
collectors, external circuitry, housing structure, and the like.
Those of ordinary skill in the art are well aware of the many
arrangements that can be utilized with the general schematic
arrangement as shown in the figures and described herein.
[0099] While FIG. 1 illustrates an electrochemical cell arranged in
a stacked configuration, it is to be understood that any
electrochemical cell arrangement can be constructed, employing the
principles of the present invention, in any configuration. For
example, FIG. 2 illustrates a cross-sectional view of an
electrochemical cell arranged as a cylinder. In the embodiment
shown in FIG. 2, cell 100 includes an electrode 130, an electrolyte
140, and electrode 150. In some embodiments, electrode 130 may
comprise an anode while electrode 150 may comprise a cathode, while
in other embodiments, their order may be reversed. Optionally, the
cell may contain a core 170, which may be solid, hollow, or contain
a channel or channels. Cell 100 also includes active surfaces 132
and 152. Optionally, the cell may also include, in some
embodiments, containment structure 156. As shown in FIG. 2,
electrode 130 is formed on core 170, electrolyte 140 is formed on
electrode 130, and electrode 150 is formed on electrolyte 140.
However, in some embodiments, electrode 130 may be proximate core
170, electrolyte 140 may be proximate electrode 130, and/or
electrode 150 may be proximate electrolyte 140, optionally
including one or more intervening sections of material between
components. In one set of embodiments, electrode 130 may at least
partially surround core 170, electrolyte 140 may at least partially
surround electrode 130, and/or electrode 150 may at least partially
surround electrolyte 140. As used herein, a first entity is "at
least partially surrounded" by a second entity if a closed loop can
be drawn around the first entity through only the second entity,
and does not imply that the first entity is necessarily completely
encapsulated by the second entity.
[0100] In another set of embodiments, illustrated in FIG. 3, the
electrochemical cell is in the shape of a folded stack. The cell
200 illustrated in FIG. 3 comprises electrolyte 240 separating
anode 230 and cathode 250. The electrochemical cell in FIG. 3
comprises an electrolyte including three folded planes parallel to
arrow 260. In other embodiments, electrochemical cells may comprise
electrolytes including any number of folded planes parallel to
arrow 260. Optionally, the cell may also include, in some
embodiments, containment structure 256. In addition to the shapes
illustrated in FIGS. 1-3, the electrochemical cells described
herein may be of any other shape including, but not limited to,
prisms (e.g., triangular prisms, rectangular prisms, etc.),
"Swiss-rolls," non-planar stacks, etc. Additional configurations
are described in U.S. patent application Ser. No. 11/400,025, filed
Apr. 6, 2006, entitled, "Electrode Protection in both Aqueous and
Non-Aqueous Electrochemical Cells, including Rechargeable Lithium
Batteries," to Affinito et al., which is incorporated herein by
reference in its entirety.
[0101] Electrochemical cells using the electrodes described herein
may be capable of achieving enhanced performance. In some
embodiments, the electrochemical cell may exhibit high electrode
active species utilization. As used herein, "utilization" refers to
the extent to which the electrode active material (e.g., sulfur as
the cathode active material) within a cell reacts to form desirable
reaction products, such that the electrochemical performance (as
measured by the discharge capacity) is enhanced. For example, an
electrochemical cell is said to utilize 100% of the total sulfur in
the cell when all of the sulfur in the cell is completely converted
to the desired reaction product (e.g., S.sup.2- in the case of
sulfur as the active cathode material), thus providing the
theoretical discharge capacity of 1672 mAh/g of total sulfur in the
cell. It is to be understood that wherever "sulfur" is used as an
exemplary electrode active material (e.g., cathode active
material), any other electrode active material suitable for use
with the invention can be substituted. The theoretical capacity of
any electrode active material can be calculated by the following
formula:
Q = n F 3600 M ##EQU00003##
wherein,
[0102] Q=Theoretical capacity, Ah/g (ampere hour per gram)
[0103] n=number of electrons involved in the desired
electrochemical reaction,
[0104] F=Faraday constant, 96485 C/equi,
[0105] M=Molecular mass of electrode active material, gram
[0106] 3600=Number of seconds in one hour.
Those of ordinary skill in the art would be able to calculate the
active material theoretical capacity and compare it to the
experimental active material capacity for a particular material to
determine whether or not the experimental capacity is at least some
percent (e.g., 60%), or greater, of the theoretical capacity. For
example, when elemental sulfur (S) is used as the cathode active
material and S.sup.2- is the desired reaction product, the
theoretical capacity is 1672 mAh/g. That is, a cell is said to
utilize 100% of the total sulfur in the cell when it produces 1672
mAh/g of total sulfur in the cell, 90% of the total sulfur in the
cell when it produces 1504.8 mAh/g of total sulfur in the cell, 60%
of the total sulfur in the cell when it produces 1003.2 mAh/g of
total sulfur in the cell, and 50% of the total sulfur in the cell
when it produces 836 mAh/g of total sulfur in the cell.
[0107] In some embodiments, it is possible for the amount of sulfur
(or other active material) in the region of the cell that is
enclosed by the cathode and anode ("available" sulfur) to be less
than that of the total sulfur in the cell. In some cases the
electrolyte may be located both in the region enclosed by the anode
and cathode and the region not enclosed by the cathode and anode.
For example, during charge/discharge cycles under pressure, it is
possible for the un-reacted species in the region enclosed by anode
and cathode to move out either by diffusion or by the movement of
the electrolyte. The utilization expressed based on this
"available" electrode active material is the measure of the ability
of the cathode structure to facilitate the conversion of the
electrode active material in the region enclosed between the
cathode and anode to desirable reaction product (e.g., S.sup.2- in
the case of sulfur as the active cathode material). For example, if
all the sulfur available in the region enclosed between the cathode
and anode is completely converted to desired reaction product, then
the cell will be said to utilize 100% of the available sulfur, and
will produce 1672 mAh/g of available sulfur.
[0108] In some embodiments, the electrochemical cell can be
designed in such a way that either all of the electrolyte is
located in between the region enclosed by the anode and cathode or
the transport of un-reacted species from the enclosed region to the
outside is completely eliminated. For such embodiments, the
utilization expressed as mAh/g of available sulfur will be equal to
that expressed as mAh/g of total sulfur in the cell.
[0109] Electrode active material (e.g., sulfur) utilization may
vary with the discharge current applied to the cell, among other
things. In some embodiments, electrode active material utilization
at low discharge rates may be higher than electrode active material
utilization at high discharge rates. In some embodiments, the cell
is capable of utilizing at least about 60%, at least about 65%, at
least about 70%, at least about 75%, at least about 80%, at least
about 85%, at least about 90%, or at least about 92% of the total
electrode active material (e.g., sulfur) in the cell over at least
one charge and discharge cycle. In some embodiments, the cell is
capable of utilizing at least about 60%, at least about 65%, at
least about 70%, at least about 75%, at least about 80%, at least
about 85%, at least about 90%, or at least about 92% of the
available electrode active material (e.g., sulfur) over at least
one charge and discharge cycle.
[0110] In some cases, the utilization rates of electrochemical
cells described herein may remain relatively high through a
relatively large number of charge and discharge cycles. As used
herein, a "charge and discharge cycle" refers to the process by
which a cell is charged from 0% to 100% state of charge (SOC) and
discharged from 100% back to 0% SOC. In some embodiments, the
electrochemical cell may be capable of utilizing at least about
60%, at least about 65%, at least about 70%, at least about 75%, at
least about 80%, at least about 85%, or at least about 90% of the
sulfur (e.g., total sulfur in the cell, available sulfur) or other
electrode active material through at least a first charge and
discharge cycle and at least about 1, 2, 10, 20, 30, 50, 75, 100,
125, or 135 charge and discharge cycles subsequent to the first
charge and discharge cycle. In certain embodiments, electrochemical
cells of the present invention will cycle at least 1 time, at least
2 times, at least 10 times, at least 20 times, at least 30 times,
at least 50 times, at least 75 times, at least 100 times, at least
125 times, or at least 135 times subsequent to a first charge and
discharge cycle with each cycle having a sulfur utilization
(measured as a fraction of 1672 mAh/g sulfur (e.g., total sulfur in
the cell, available sulfur) output during the discharge phase of
the cycle) or other electrode active material utilization of at
least about 40-50%, at least about 50-60%, at least about 40-60%,
at least about 40-80%, at least about 60-70%, at least about 70%,
at least about 70-80%, at least about 80%, at least about 80-90%,
or at least about 90% when discharged at a moderately high
discharge current of at least about 100 mA/g of sulfur (e.g., a
discharge current between 100-200 mA/g, between 200-300 mA/g,
between 300-400 mA/g, between 400-500 mA/g, or between 100-500
mA/g).
[0111] Some of the electrochemical cells described herein may
maintain capacity over a relatively large number of charge and
discharge cycles. For example, in some cases, the electrochemical
cell capacity decreases by less than about 0.2% per charge and
discharge cycle over at least about 2, at least about 10, at least
about 20, at least about 30, at least about 50, at least about 75,
at least about 100, at least about 125, or at least about 135
cycles subsequent to a first charge and discharge cycle.
[0112] In some embodiments, the electrochemical cells described
herein may exhibit relatively high capacities after repeated
cycling of the cell. For example, in some embodiments, after
alternatively discharging and charging the cell three times, the
cell exhibits at least about 50%, at least about 80%, at least
about 90%, or at least about 95% of the cell's initial capacity at
the end of the third cycle. In some cases, after alternatively
discharging and charging the cell ten times, the cell exhibits at
least about 50%, at least about 80%, at least about 90%, or at
least about 95% of the cell's initial capacity at the end of the
tenth cycle. In still further cases, after alternatively
discharging and charging the cell twenty-five times, the cell
exhibits at least about 50%, at least about 80%, at least about
90%, or at least about 95% of the cell's initial capacity at the
end of the twenty-fifth cycle.
[0113] In some embodiments, the electrochemical cells described
herein may achieve relatively high charge efficiencies over a large
number of cycles. As used herein, the "charge efficiency" of the
Nth cycle is calculated as the discharge capacity of the (N+1)th
cycle divided by the charge capacity of the Nth cycle (where N is
an integer), and is expressed as a percentage. In some cases,
electrochemical cells may achieve charge efficiencies of at least
about 60%, at least about 70%, at least about 80%, at least about
90%, at least about 95%, at least about 97%, at least about 98%, at
least about 99%, at least about 99.5%, or at least about 99.9% for
the first cycle. In some embodiments, charge efficiencies of at
least about 60%, at least about 70%, at least about 80%, at least
about 90%, at least about 97%, at least about 98%, at least about
99%, at least about 99.5%, or at least about 99.9% may be achieved
for the 10th, 20th, 30th, 50th, 75th, 100.sup.th, 125th, or 135th
cycles subsequent to a first charge and discharge cycle.
[0114] The electrochemical cells described herein may be operated
using relatively high discharge current densities, in some cases.
As used herein, the "discharge current density" refers to the
discharge current between the electrodes, divided by the area of
the electrode over which the discharge occurs, as measured
perpendicular to the direction of the current. For the purposes of
discharge current density, the area of the electrode does not
include the total exposed surface area of the electrode, but
rather, refers to an imaginary plane drawn along the electrode
surface perpendicular to the direction of the current. In some
embodiments, the electrochemical cells may be operated at a
discharge current density of at least about 0.1 mA/cm.sup.2, at
least about 0.2 mA/cm.sup.2, at least about 0.4 mA/cm.sup.2 of the
cathode surface, or higher. The cells described herein may also be
operated, in some cases, at a high discharge current per unit mass
of active material. For example, the discharge current may be at
least about 100, at least about 200, at least about 300, at least
about 400, or at least about 500 mA per gram of active material in
an electrode (e.g., sulfur in the cathode), or higher.
[0115] Some embodiments may include electrochemical devices in
which the application of force is used to enhance the performance
of the device. For example, the force may provide improved
electrical conductivity between conductive material in an electrode
(e.g., carbon in a cathode). In some instances, the application of
force to the electrochemical cell may reduce the amount of
roughening of one or more surfaces of one or more electrodes which
may improve the cycling lifetime and performance of the cell. Any
of the electrode properties (e.g., porosities, pore size
distributions, etc.) and/or performance metrics outlined above may
be achieved, alone or in combination with each other, while an
anisotropic force is applied to the electrochemical cell (e.g.,
during charge and/or discharge of the cell). The magnitude of the
anisotropic force may lie within any of the ranges mentioned
below.
[0116] In some embodiments, the application of force can be used to
enhance the performance of the device. In some embodiments, the
force comprises an anisotropic force with a component normal to the
active surface of the anode. In the case of a planar surface, the
force may comprise an anisotropic force with a component normal to
the surface at the point at which the force is applied. For
example, referring to FIG. 1, a force may be applied in the
direction of arrow 60. Arrow 62 illustrates the component of the
force that is normal to active surface 52 of anode 50. In the case
of a curved surface, for example, a concave surface or a convex
surface, the force may comprise an anisotropic force with a
component normal to a plane that is tangent to the curved surface
at the point at which the force is applied. Referring to the
cylindrical cell illustrated in FIG. 2, a force may be applied to
an external surface of the cell in the direction of, for example,
arrow 180. In some embodiments, the force may be applied from the
interior of the cylindrical cell, for example, in the direction of
arrow 182. In some embodiments, an anisotropic force with a
component normal to the active surface of the anode is applied
during at least one period of time during charge and/or discharge
of the electrochemical cell. In some embodiments, the force may be
applied continuously, over one period of time, or over multiple
periods of time that may vary in duration and/or frequency. The
anisotropic force may be applied, in some cases, at one or more
pre-determined locations, optionally distributed over the active
surface of the anode. In some embodiments, the anisotropic force is
applied uniformly over the active surface of the anode.
[0117] An "anisotropic force" is given its ordinary meaning in the
art and means a force that is not equal in all directions. A force
equal in all directions is, for example, internal pressure of a
fluid or material within the fluid or material, such as internal
gas pressure of an object. Examples of forces not equal in all
directions include forces directed in a particular direction, such
as the force on a table applied by an object on the table via
gravity. Another example of an anisotropic force includes a force
applied by a band arranged around a perimeter of an object. For
example, a rubber band or turnbuckle can apply forces around a
perimeter of an object around which it is wrapped. However, the
band may not apply any direct force on any part of the exterior
surface of the object not in contact with the band. In addition,
when the band is expanded along a first axis to a greater extent
than a second axis, the band can apply a larger force in the
direction parallel to the first axis than the force applied
parallel to the second axis.
[0118] A force with a "component normal" to a surface, for example
an active surface of an anode, is given its ordinary meaning as
would be understood by those of ordinary skill in the art and
includes, for example, a force which at least in part exerts itself
in a direction substantially perpendicular to the surface. For
example, in the case of a horizontal table with an object resting
on the table and affected only by gravity, the object exerts a
force essentially completely normal to the surface of the table. If
the object is also urged laterally across the horizontal table
surface, then it exerts a force on the table which, while not
completely perpendicular to the horizontal surface, includes a
component normal to the table surface. Those of ordinary skill can
understand other examples of these terms, especially as applied
within the description of this document.
[0119] In some embodiments, the anisotropic force can be applied
such that the magnitude of the force is substantially equal in all
directions within a plane defining a cross-section of the
electrochemical cell, but the magnitude of the forces in
out-of-plane directions is substantially unequal to the magnitudes
of the in-plane forces. For example, referring to FIG. 2, a
cylindrical band may be positioned around the exterior of cell 100
such that forces (e.g., force 180) are applied to the cell oriented
toward the cell's central axis (indicated by point 190 and
extending into and out of the surface of the cross-sectional
schematic diagram). In some embodiments, the magnitudes of the
forces oriented toward the central axis of the cell are different
(e.g., greater than) the magnitudes of the forces applied in out of
plane directions (e.g., parallel to central axis 190).
[0120] In one set of embodiments, cells of the invention are
constructed and arranged to apply, during at least one period of
time during charge and/or discharge of the cell, an anisotropic
force with a component normal to the active surface of the anode.
Those of ordinary skill in the art will understand the meaning of
this. In such an arrangement, the cell may be formed as part of a
container which applies such a force by virtue of a "load" applied
during or after assembly of the cell, or applied during use of the
cell as a result of expansion and/or contraction of one or more
portions of the cell itself.
[0121] The magnitude of the applied force is, in some embodiments,
large enough to enhance the performance of the electrochemical
cell. The anode active surface and the anisotropic force may be, in
some instances, together selected such that the anisotropic force
affects surface morphology of the anode active surface to inhibit
increase in anode active surface area through charge and discharge
and wherein, in the absence of the anisotropic force but under
otherwise essentially identical conditions, the anode active
surface area is increased to a greater extent through charge and
discharge cycles. "Essentially identical conditions," in this
context, means conditions that are similar or identical other than
the application and/or magnitude of the force. For example,
otherwise identical conditions may mean a cell that is identical,
but where it is not constructed (e.g., by brackets or other
connections) to apply the anisotropic force on the subject
cell.
[0122] Electrode materials or structures and anisotropic forces can
be selected together, to achieve results described herein, by those
of ordinary skill in the art. For example, where the electrode(s)
is relatively soft, the component of the force normal to the active
anode surface may be selected to be lower. Where the electrode(s)
is harder, the component of the force normal to the active surface
may be greater. Those of ordinary skill in the art can easily
select electrode materials, alloys, mixtures, etc. with known or
predictable properties, or readily test the hardness or softness of
such surfaces, and readily select cell construction techniques and
arrangements to provide appropriate forces to achieve what is
described herein. Simple testing can be done, for example by
arranging a series of active materials, each with a series of
forces applied normal (or with a component normal) to the active
surface, to determine the morphological effect of the force on the
surface without cell cycling (for prediction of the selected
combination during cell cycling) or with cell cycling with
observation of a result relevant to the selection.
[0123] In some embodiments, an anisotropic force with a component
normal to the active surface of the anode is applied, during at
least one period of time during charge and/or discharge of the
cell, to an extent effective to inhibit an increase in surface area
of the anode active surface relative to an increase in surface area
absent the anisotropic force. The component of the anisotropic
force normal to the anode active surface may, for example, define a
pressure of at least about 4.9, at least about 9.8, at least about
24.5, at least about 49, at least about 78, at least about 98, at
least about 117.6, at least about 147, at least about 175, at least
about 200, at least about 225, or at least about 250 Newtons per
square centimeter. In some embodiments, the component of the
anisotropic force normal to the anode active surface may, for
example, define a pressure of less than about 250, less than about
225, less than about 196, less than about 147, less than about
117.6, less than about 98, less than about 49, less than about
24.5, or less than about 9.8 Newtons per square centimeter. In some
cases, the component of the anisotropic force normal to the anode
active surface is may define a pressure of between about 4.9 and
about 147 Newtons per square centimeter, between about 49 and about
117.6 Newtons per square centimeter, between about 68.6 and about
98 Newtons per square centimeter, between about 78 and about 108
Newtons per square centimeter, between about 4.9 and about 250
Newtons per square centimeter, between about 49 and about 250
Newtons per square centimeter, between about 80 and about 250
Newtons per square centimeter, between about 90 and about 250
Newtons per square centimeter, or between about 100 and about 250
Newtons per square centimeter. While forces and pressures are
generally described herein in units of Newtons and Newtons per unit
area, respectively, forces and pressures can also be expressed in
units of kilograms-force and kilograms-force per unit area,
respectively. One of ordinary skill in the art will be familiar
with kilogram-force-based units, and will understand that 1
kilogram-force (kg.sub.f) is equivalent to about 9.8 Newtons.
[0124] In some embodiments, the surface of an electrode layer can
be enhanced during cycling (e.g., for lithium, the development of
mossy or a rough surface of lithium may be reduced or eliminated)
by application of an externally-applied (in some embodiments,
uniaxial) pressure. The externally-applied pressure may, in some
embodiments, be chosen to be greater than the yield stress of a
material forming the electrode material layer. For example, for an
electrode material comprising lithium, the cell may be under an
externally-applied anisotropic force with a component defining a
pressure of at least about 8 kg.sub.f/cm.sup.2, at least about 9
kg.sub.f/cm.sup.2, or at least about 10 kg.sub.f/cm.sup.2. This is
because the yield stress of lithium is around 7-8
kg.sub.f/cm.sup.2. Thus, at pressures (e.g., uniaxial pressures)
greater than this value, mossy Li, or any surface roughness at all,
may be reduced or suppressed. The lithium surface roughness may
mimic the surface that is pressing against it. Accordingly, when
cycling under at least about 8 kg.sub.f/cm.sup.2, at least about 9
kg.sub.f/cm.sup.2, or at least about 10 kg.sub.f/cm.sup.2 of
externally-applied pressure, the lithium surface may become
smoother with cycling when the pressing surface is smooth. As
described herein, the pressing surface may be modified by choosing
the appropriate material(s) positioned between the anode and the
cathode. For instance, in some cases the smoothness of the lithium
surface (or surface of other active electrode materials) can be
increased, during application of pressure, by the use of an
electrically non-conductive material layer as described herein.
[0125] In some cases, one or more forces applied to the cell have a
component that is not normal to an active surface of an anode. For
example, in FIG. 1, force 60 is not normal to anode active surface
52, and force 60 includes component 64, which is substantially
parallel to anode active surface 52. In addition, a force 66, which
is substantially parallel to anode active surface 52, could be
applied to the cell in some cases. In one set of embodiments, the
sum of the components of all applied anisotropic forces in a
direction normal to the anode active surface is larger than any sum
of components in a direction that is non-normal to the anode active
surface. In some embodiments, the sum of the components of all
applied anisotropic forces in a direction normal to the anode
active surface is at least about 5%, at least about 10%, at least
about 20%, at least about 35%, at least about 50%, at least about
75%, at least about 90%, at least about 95%, at least about 99%, or
at least about 99.9% larger than any sum of components in a
direction that is parallel to the anode active surface.
[0126] In some embodiments, the cathode and anode have yield
stresses, wherein the effective yield stress of one of the cathode
and anode is greater than the yield stress of the other, such that
an anisotropic force applied normal to the surface of one of the
active surface of the anode and the active surface of the cathode
causes the surface morphology of one of the cathode and the anode
to be affected. In some embodiments, the component of the
anisotropic force normal to the active anode surface is between
about 20% and about 200% of the yield stress of the anode material,
between about 50% and about 120% of the yield stress of the anode
material, between about 80% and about 120% of the yield stress of
the anode material, between about 80% and about 100% of the yield
stress of the anode material, between about 100% and about 300% of
the yield stress of the anode material, between about 100% and
about 200% of the yield stress of the anode material, or between
about 100% and about 120% of the yield stress of the anode
material.
[0127] The anisotropic force described herein may be applied using
any method known in the art. In some embodiments, the force may be
applied using compression springs. Forces may be applied using
other elements (either inside or outside a containment structure)
including, but not limited to Belleville washers, machine screws,
pneumatic devices, and/or weights, among others. In some cases,
cells may be pre-compressed before they are inserted into
containment structures, and, upon being inserted to the containment
structure, they may expand to produce a net force on the cell.
Suitable methods for applying such forces are described in detail,
for example, in U.S. Provisional Application No. 61/086,329, filed
Aug. 5, 2008, entitled "Application of Force in Electrochemical
Cells" to Scordilis-Kelley et al., and U.S. patent application Ser.
No. 12/535,328, filed Aug. 4, 2009, entitled "Application of Force
in Electrochemical Cells" to Scordilis-Kelley et al. which are
incorporated herein by reference in their entirety.
[0128] In some embodiments, the application of a force as described
herein may allow for the use of smaller amounts of anode active
material (e.g., lithium) and/or electrolyte within an
electrochemical cell, relative to the amounts used in essentially
identical cells in which the force is not applied. In cells lacking
the applied force described herein, active anode material (e.g.,
lithium metal) may be, in some cases, redeposited unevenly on an
anode during charge-discharge cycles of the cell, forming a rough
surface. In some cases, this may lead to an increase in the rates
of one or more undesired reactions involving the anode metal. These
undesired reactions may, after a number of charge-discharge cycles,
stabilize and/or begin to self-inhibit such that substantially no
additional active anode material becomes depleted and the cell may
function with the remaining active materials. For cells lacking the
applied force as described herein, this "stabilization" is often
reached only after a substantial amount of anode active material
has been consumed and cell performance has deteriorated. Therefore,
in some cases where forces as described herein have not been
applied, a relatively large amount of anode active material and/or
electrolyte has often been incorporated within cells to accommodate
for loss of material during consumption of active materials, in
order to preserve cell performance.
[0129] Accordingly, the application of force as described herein
may reduce and/or prevent depletion of active materials such that
the inclusion of large amounts of anode active material and/or
electrolyte within the electrochemical cell may not be necessary.
For example, the force may be applied to a cell prior to use of the
cell, or in an early stage in the lifetime of the cell (e.g., less
than five charge-discharge cycles), such that little or
substantially no depletion of active material may occur upon
charging or discharging of the cell. By reducing and/or eliminating
the need to accommodate for active material loss during
charge-discharge of the cell, relatively small amounts of anode
active material may be used to fabricate cells and devices as
described herein. In some embodiments, the invention relates to
devices comprising an electrochemical cell having been charged and
discharged less than five times in its lifetime, wherein the cell
comprises an anode, a cathode, and an electrolyte, wherein the
anode comprises no more than five times the amount of anode active
material which can be ionized during one full discharge cycle of
the cell. In some cases, the anode comprises no more than four,
three, two, or 1.5 times the amount of lithium which can be ionized
during one full discharge cycle of the cell.
[0130] In some cases, the devices described herein can comprise an
electrochemical cell, wherein the cell comprises an anode active
material, a cathode active material, and an electrolyte, wherein
the ratio of the amount of anode active material in the anode to
the amount of cathode active material in the cathode is less than
about 5:1, less than about 3:1, less than about 2:1, or less than
about 1.5:1 on a molar basis. For example, a cell may comprise
lithium as an active anode material and sulfur as an active cathode
material, wherein the molar ratio Li:S is less than about 5:1. In
some cases, the molar ratio of lithium to sulfur, Li:S, is less
than about 3:1, less than about 2:1, or less than about 1.5:1. In
some embodiments, the ratio of anode active material (e.g.,
lithium) to cathode active material by weight may be less than
about 2:1, less than about 1.5:1, less than about 1.25:1, or less
than about 1.1:1. For example, a cell may comprise lithium as the
active anode material and sulfur as the active cathode material,
wherein the ratio Li:S by weight is less than about 2:1, less than
about 1.5:1, less than about 1.25:1, or less than about 1.1:1.
[0131] The use of smaller amounts of active anode material and/or
electrolyte material may advantageously allow for electrochemical
cells, or portions thereof, having decreased thickness. In some
embodiments, the anode layer and the electrolyte layer together
have a maximum thickness of about 500 microns. In some cases, the
anode layer and the electrolyte layer together have a maximum
thickness of about 400 microns, about 300 microns, about 200
microns, about 100 microns, about 50 microns, or, in some cases,
about 20 microns.
[0132] The anodes described herein may include a variety of
electroactive materials. Suitable electroactive materials for use
as anode active materials in the anode of the electrochemical cells
described herein include, but are not limited to, lithium metal
such as lithium foil and lithium deposited onto a conductive
substrate, and lithium alloys (e.g., lithium-aluminum alloys and
lithium-tin alloys). While these are preferred negative electrode
materials, the current collectors may also be used with other cell
chemistries. In some embodiments, the anode may comprise one or
more binder materials (e.g., polymers, etc.).
[0133] In some embodiments, an electroactive lithium-containing
material of an anode active layer comprises greater than 50 wt %
lithium. In some cases, the electroactive lithium-containing
material of an anode active layer comprises greater than 75 wt %
lithium. In still other embodiments, the electroactive
lithium-containing material of an anode active layer comprises
greater than 90 wt % lithium.
[0134] Positive and/or negative electrodes may optionally include
one or more layers that interact favorably with a suitable
electrolyte, such as those described in U.S. patent application
Ser. No. 12/312,764, filed May 26, 2009 and entitled "Separation of
Electrolytes," by Mikhaylik et al., which is incorporated herein by
reference in its entirety.
[0135] The electrolytes used in electrochemical or battery cells
can function as a medium for the storage and transport of ions, and
in the special case of solid electrolytes and gel electrolytes,
these materials may additionally function as a separator between
the anode and the cathode. Any liquid, solid, or gel material
capable of storing and transporting ions may be used, so long as
the material facilitates the transport of ions (e.g., lithium ions)
between the anode and the cathode. The electrolyte is
electronically non-conductive to prevent short circuiting between
the anode and the cathode. In some embodiments, the electrolyte may
comprise a non-solid electrolyte.
[0136] In some embodiments, the electrolyte comprises a fluid that
can be added at any point in the fabrication process. In some
cases, the electrochemical cell may be fabricated by providing a
cathode and an anode, applying an anisotropic force component
normal to the active surface of the anode, and subsequently adding
the fluid electrolyte such that the electrolyte is in
electrochemical communication with the cathode and the anode. In
other cases, the fluid electrolyte may be added to the
electrochemical cell prior to or simultaneously with the
application of the anisotropic force component, after which the
electrolyte is in electrochemical communication with the cathode
and the anode.
[0137] The electrolyte can comprise one or more ionic electrolyte
salts to provide ionic conductivity and one or more liquid
electrolyte solvents, gel polymer materials, or polymer materials.
Suitable non-aqueous electrolytes may include organic electrolytes
comprising one or more materials selected from the group consisting
of liquid electrolytes, gel polymer electrolytes, and solid polymer
electrolytes. Examples of non-aqueous electrolytes for lithium
batteries are described by Dorniney in Lithium Batteries, New
Materials, Developments and Perspectives, Chapter 4, pp. 137-165,
Elsevier, Amsterdam (1994). Examples of gel polymer electrolytes
and solid polymer electrolytes are described by Alamgir et al. in
Lithium Batteries, New Materials, Developments and Perspectives,
Chapter 3, pp. 93-136, Elsevier, Amsterdam (1994). Heterogeneous
electrolyte compositions that can be used in batteries described
herein are described in U.S. patent application Ser. No.
12/312,764, filed May 26, 2009 and entitled "Separation of
Electrolytes," by Mikhaylik et al., which is incorporated herein by
reference in its entirety.
[0138] Examples of useful non-aqueous liquid electrolyte solvents
include, but are not limited to, non-aqueous organic solvents, such
as, for example, N-methyl acetamide, acetonitrile, acetals, ketals,
esters, carbonates, sulfones, sulfites, sulfolanes, aliphatic
ethers, cyclic ethers, glymes, polyethers, phosphate esters,
siloxanes, dioxolanes, N-alkylpyrrolidones, substituted forms of
the foregoing, and blends thereof. Fluorinated derivatives of the
foregoing are also useful as liquid electrolyte solvents.
[0139] In some cases, aqueous solvents can be used as electrolytes,
for example, in lithium cells. Aqueous solvents can include water,
which can contain other components such as ionic salts. As noted
above, in some embodiments, the electrolyte can include species
such as lithium hydroxide, or other species rendering the
electrolyte basic, so as to reduce the concentration of hydrogen
ions in the electrolyte.
[0140] Liquid electrolyte solvents can also be useful as
plasticizers for gel polymer electrolytes, i.e., electrolytes
comprising one or more polymers forming a semi-solid network.
Examples of useful gel polymer electrolytes include, but are not
limited to, those comprising one or more polymers selected from the
group consisting of polyethylene oxides, polypropylene oxides,
polyacrylonitriles, polysiloxanes, polyimides, polyphosphazenes,
polyethers, sulfonated polyimides, perfluorinated membranes (NAFION
resins), polydivinyl polyethylene glycols, polyethylene glycol
diacrylates, polyethylene glycol dimethacrylates, polysulfones,
polyethersulfones, derivatives of the foregoing, copolymers of the
foregoing, crosslinked and network structures of the foregoing, and
blends of the foregoing, and optionally, one or more plasticizers.
In some embodiments, a gel polymer electrolyte comprises between
10-20%, 20-40%, between 60-70%, between 70-80%, between 80-90%, or
between 90-95% of a heterogeneous electrolyte by volume.
[0141] In some embodiments, one or more solid polymers can be used
to form an electrolyte. Examples of useful solid polymer
electrolytes include, but are not limited to, those comprising one
or more polymers selected from the group consisting of polyethers,
polyethylene oxides, polypropylene oxides, polyimides,
polyphosphazenes, polyacrylonitriles, polysiloxanes, derivatives of
the foregoing, copolymers of the foregoing, crosslinked and network
structures of the foregoing, and blends of the foregoing.
[0142] In addition to electrolyte solvents, gelling agents, and
polymers as known in the art for forming electrolytes, the
electrolyte may further comprise one or more ionic electrolyte
salts, also as known in the art, to increase the ionic
conductivity.
[0143] Examples of ionic electrolyte salts for use in the
electrolytes of the present invention include, but are not limited
to, LiSCN, LiBr, LiI, LiClO.sub.4, LiAsF.sub.6, LiSO.sub.3CF.sub.3,
LiSO.sub.3CH.sub.3, LiBF.sub.4, LiB(Ph).sub.4, LiPF.sub.6,
LiC(SO.sub.2CF.sub.3).sub.3, and LiN(SO.sub.2CF.sub.3).sub.2. Other
electrolyte salts that may be useful include lithium polysulfides
(Li.sub.2S.sub.x), and lithium salts of organic polysulfides
(LiS.sub.xR).sub.n, where x is an integer from 1 to 20, n is an
integer from 1 to 3, and R is an organic group, and those disclosed
in U.S. Pat. No. 5,538,812 to Lee et al.
[0144] In some embodiments, electrochemical cells may further
comprise a separator interposed between the cathode and anode. The
separator may be a solid non-conductive or insulative material
which separates or insulates the anode and the cathode from each
other preventing short circuiting, and which permits the transport
of ions between the anode and the cathode. In some embodiments, the
porous separator may be permeable to the electrolyte.
[0145] The pores of the separator may be partially or substantially
filled with electrolyte. Separators may be supplied as porous free
standing films which are interleaved with the anodes and the
cathodes during the fabrication of cells. Alternatively, the porous
separator layer may be applied directly to the surface of one of
the electrodes, for example, as described in PCT Publication No. WO
99/33125 to Carlson et al. and in U.S. Pat. No. 5,194,341 to Bagley
et al.
[0146] A variety of separator materials are known in the art.
Examples of suitable solid porous separator materials include, but
are not limited to, polyolefins, such as, for example,
polyethylenes (e.g., SETELA.TM. made by Tonen Chemical Corp) and
polypropylenes, glass fiber filter papers, and ceramic materials.
For example, in some embodiments, the separator comprises a
microporous polyethylene film. Further examples of separators and
separator materials suitable for use in this invention are those
comprising a microporous xerogel layer, for example, a microporous
pseudo-boehmite layer, which may be provided either as a free
standing film or by a direct coating application on one of the
electrodes, as described in U.S. Pat. Nos. 6,153,337 and 6,306,545
by Carlson et al. of the common assignee. Solid electrolytes and
gel electrolytes may also function as a separator in addition to
their electrolyte function.
[0147] The following documents are incorporated herein by reference
in their entireties for all purposes: U.S. Provisional Patent
Application No. 61/237,903, filed Aug. 28, 2009, and entitled
"Electrochemical Cells Comprising Porous Structures Comprising
Sulfur;" U.S. Pat. No. 7,247,408, filed May 23, 2001, entitled
"Lithium Anodes for Electrochemical Cells"; U.S. Pat. No.
5,648,187, filed Mar. 19, 1996, entitled "Stabilized Anode for
Lithium-Polymer Batteries"; U.S. Pat. No. 5,961,672, filed Jul. 7,
1997, entitled "Stabilized Anode for Lithium-Polymer Batteries";
U.S. Pat. No. 5,919,587, filed May 21, 1997, entitled "Novel
Composite Cathodes, Electrochemical Cells Comprising Novel
Composite Cathodes, and Processes for Fabricating Same"; U.S.
patent application Ser. No. 11/400,781, filed Apr. 6, 2006,
entitled "Rechargeable Lithium/Water, Lithium/Air Batteries";
International Patent Apl. Serial No.: PCT/US2008/009158, filed Jul.
29, 2008, entitled "Swelling Inhibition in Lithium Batteries"; U.S.
patent application Ser. No. 12/312,764, filed May 26, 2009,
entitled "Separation of Electrolytes"; International Patent Apl.
Serial No.: PCT/US2008/012042, filed Oct. 23, 2008, entitled
"Primer for Battery Electrode"; U.S. patent application Ser. No.
12/069,335, filed Feb. 8, 2008, entitled "Protective Circuit for
Energy-Storage Device"; U.S. patent application Ser. No.
11/400,025, filed Apr. 6, 2006, entitled "Electrode Protection in
both Aqueous and Non-Aqueous Electrochemical Cells, including
Rechargeable Lithium Batteries"; U.S. patent application Ser. No.
11/821,576, filed Jun. 22, 2007, entitled "Lithium Alloy/Sulfur
Batteries"; patent application Ser. No. 11/111,262, filed Apr. 20,
2005, entitled "Lithium Sulfur Rechargeable Battery Fuel Gauge
Systems and Methods"; U.S. patent application Ser. No. 11/728,197,
filed Mar. 23, 2007, entitled "Co-Flash Evaporation of
Polymerizable Monomers and Non-Polymerizable Carrier Solvent/Salt
Mixtures/Solutions"; International Patent Apl. Serial No.:
PCT/US2008/010894, filed Sep. 19, 2008, entitled "Electrolyte
Additives for Lithium Batteries and Related Methods"; International
Patent Apl. Serial No.: PCT/US2009/000090, filed Jan. 8, 2009,
entitled "Porous Electrodes and Associated Methods"; U.S. patent
application Ser. No. 12/535,328, filed Aug. 4, 2009, entitled
"Application of Force In Electrochemical Cells"; U.S. patent
application Ser. No. 12/727,862, filed Mar. 19, 2010, entitled
"Cathode for Lithium Battery"; U.S. patent application Ser. No.
12/471,095, filed May 22, 2009, entitled "Hermetic Sample Holder
and Method for Performing Microanalysis Under Controlled Atmosphere
Environment"; a U.S. patent application, filed on even date
herewith, entitled "Release System for Electrochemical cells (which
claims priority to Provisional Patent Apl. Ser. No. 61/236,322,
filed Aug. 24, 2009, entitled "Release System for Electrochemical
Cells"); a U.S. Provisional application, filed on even date
herewith, entitled "Separator for Electrochemical Cell;" a U.S.
patent application, filed on even date herewith, entitled
"Electrochemical Cell;" and 3 U.S. patent applications, filed on
even date herewith, entitled "Electrochemical Cells Comprising
Porous Structures Comprising Sulfur."
[0148] 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
[0149] This example describes the fabrication and testing of a
cathode comprising a porous support structure in which particles
comprising sulfur were deposited. 100 grams of elemental sulfur
(available from Aldrich Chemical Company, Milwaukee, Wis.), were
dissolved in 1200 mL of toluene (Aldrich) at 103.degree. C. in a
round bottom flask fitted with a condenser. 100 grams of
Printex.RTM. XE-2 (a carbon pigment available from Degussa
Corporation, Akron, Ohio) conductive carbon (surface area 800-1000
m.sup.2/g, absorption stiffness 350-410 mL dibutyl phthalate
(DBP)/100 g XE-2), was added to the solution. The solution was
quickly absorbed by the carbon. After a couple of hours of
stiffing, the mixture was cooled to room temperature, where sulfur
has lower solubility in toluene (84 mM). After cooling, the sulfur
crystallized in the carbon pores, and the excess toluene was
filtered off.
[0150] The sulfur-filled carbon material was dried and mixed with
appropriate amounts of polyvinyl alcohol binder (Celvol 425 from
Celanese Corporation) dissolved in a 1:1 weight ratio mixture of
isopropanol and water. The cathode slurry was coated onto a
conductive carbon coated Aluminum foil substrate (7-.mu.m-thick
from All Foils). After drying, the coated cathode active layer
thickness was about 110 microns. The resultant cathode was easy to
coat, included a homogeneous distribution of small particle size
sulfur, and had uniform porosity, as shown in FIG. 4A.
[0151] An electrochemical cell including the cathode was assembled
for testing. Lithium metal (>99.9% Li, 2 mil thick foil from
Chemetall-Foote Corp., Kings Mountain, N.C.) was used for the
anode. The electrolyte included 8 parts of lithium bis
(trifluoromethane sulfonyl) imide, (lithium imide available from 3M
Corporation, St. Paul, Minn.), 3.8 parts lithium nitrate (available
from Aldrich Chemical Company, Milwaukee, Wis.), 1 part guanidine
nitrate (also available from Aldrich Chemical Company, Milwaukee,
Wis.) and 0.4 parts pyridine nitrate (synthesized in-house from
pyridine and nitric acid) in a 1:1 weight ratio mixture of
1,3-dioxolane and dimethoxyethane. The electrolyte included a water
content of less than 50 ppm. A porous separator comprising 9-.mu.m
SETELA (a polyolefin separator available from Tonen Chemical
Corporation, Tokyo, Japan, and from Mobil Chemical Company, Films
Division, Pittsford, N.Y.) was included between the anode and the
cathode. The anode, cathode, separator, and electrolyte were
stacked into a layered structure of
6.times.(cathode/separator/anode), which was compressed between two
parallel plates at a pressure of 196 Newtons/cm.sup.2 (about 20
kg-force/cm.sup.2). The liquid electrolyte filled the void areas of
the separator and cathode to form prismatic cells with an electrode
area of about 100 cm.sup.2. After sealing, the cells were stored
for 24 hours. Charge-discharge cycling was performed at 40 mA and
25 mA, respectively. The discharge cutoff voltage was 1.7V and the
charge cutoff voltage was 2.5V. FIG. 5A includes a plot of specific
discharge capacity for the composite electrodes (indicated with
asterisks) as a function of the number of cycles. FIG. 5B includes
a plot of the capacity of the cells (indicated with asterisks),
expressed as a percentage relative to the initial maximum, as a
function of C-rate. The specific discharge capacities remained
relatively high over multiple cycles. In addition, the cells
maintained relatively high capacities at relatively high
C-rates.
Comparative Example 1
[0152] This example describes the fabrication and testing of a
cathode comprising a mechanical mixture of sulfur and carbon. The
first attempt included a 1:1 carbon-to-sulfur ratio in the mixture.
However, the 1:1 mixture could not be effectively deposited on the
coater. In subsequent experiments, the cathode was prepared by
preparing a mixture comprising 55 parts of elemental sulfur
(Aldrich Chemical Company, Milwaukee, Wis.), 40 parts of a
conductive carbon pigment Printex.RTM. XE-2, and 5 parts of
polyvinyl alcohol binder. The mixture was dissolved in a 1:1 weight
ratio mixture of isopropanol and water. The solution was coated
onto a 7-micron-thick conductive carbon coated Aluminum foil
substrate. After drying, the coated cathode active layer thickness
was about 90 microns. The resultant cathode was very heterogeneous
with large Sulfur particles and carbon agglomerates, as shown in
FIG. 4B.
[0153] An electrochemical cell including the cathode was assembled
for testing according to the process described in Example 1. FIG.
5A includes a plot of specific discharge capacity of cells
comprising mechanically mixed cathodes (indicated with diamonds) as
a function of the number of cycles. FIG. 5B includes a plot of the
capacity of cells comprising mechanically mixed cathodes (indicated
with diamonds), expressed as a percentage relative to the initial
maximum, as a function of C-rate. The specific discharge capacities
for cells comprising the mechanically mixed cathodes were
relatively low over the number of tested cycles. In addition, the
cells comprising mechanically mixed cathodes exhibited much larger
decreases in capacity at higher C-rates.
Example 2
[0154] This example describes the use of a thermal processing
scheme to deposit sulfur in porous support materials comprising
conductive carbons. The following conductive carbons were tested:
Printex.RTM. XE-2, Vulcan XC72R (Cabot Corporation, Tampa, Tex.)
and SAB (Shawinigan Acetylene Black, formerly available from
Chevron Phillips, The Woodlands, Tex.). The conductive carbons were
heated in an evacuated round bottom flask at 300-450.degree. C. for
5 to 6 hours prior to mixing with sulfur. Sulfur powder (Alfa Aesar
Corporation, Ward Hill, Mass.) and conductive carbon were mixed
together under an inert Ar atmosphere.
[0155] The mixture was heated past the meting point of sulfur to
160.degree. C. under vacuum for 5 to 6 hours to produce a yellow,
low viscosity fluid comprising Sg. Heating the mixture beyond the
melting point of sulfur resulted in depositing liquid sulfur in the
pores of the carbon particles without the use of solvents. The
temperature was then raised to 250 to 300.degree. C. under reduced
pressure for 4 to 6 hours to perform polymerization of adsorbed
sulfur. In addition, the heating helped in producing a uniform
surface distribution. After the polymerization step, the composites
were rapidly cooled to form composites comprising polymeric sulfur
and carbon.
[0156] Sulfur-filled carbon composites were fabricated with S:C
ratios varying from 6:1 to 1:1, depending on the available pore
volume and surface area of the carbon were fabricated using various
types of carbon. FIG. 6A includes a secondary electron image of the
external surface of a sulfur-carbon composite particle. FIGS. 6B-6C
include X-ray spectral images outlining the distributions of (B)
sulfur and (C) carbon on the external surface of the composite
particle shown in FIG. 6A. The images in FIGS. 6B-6C indicate that
the sulfur and carbon were evenly distributed across the outside
surface of the composite particle. FIG. 6D includes a secondary
electron image of an internal surface of a sulfur-carbon composite
particle. FIGS. 6E-6F include X-ray spectral images outlining the
distributions of (E) sulfur and (F) carbon on the internal surface
of the composite particle shown in FIG. 6D. The images in FIGS.
6E-6F indicate that the sulfur and carbon were evenly distributed
across the internal surface of the composite particle.
[0157] The sulfur-carbon composites were mixed with appropriate
amounts of polyvinyl alcohol binder or Gelatin B (Sigma-Aldrich
Chemical Company, Milwaukee, Wis.) and dissolved in a 1:1 weight
ratio mixture of iso-propanol and water. The cathode slurry was
coated onto a conductive, carbon-coated Aluminum foil substrate (7
microns thick) in a similar fashion as described in Example 1.
[0158] The resultant composite cathode materials had a relatively
high density, uniform porosity, and homogeneous sulfur
distribution. The sulfur-carbon composites contained extended,
clean sulfur-carbon interfaces that could be useful in promoting
structure and conductivity. In addition, this process produced
little to no waste, and mechanical milling was not required. In
addition, the carbons and composites could be easily modified by
various gas or vapor treatments during thermo-vacuum activation at
any processing stage, providing for a flexible means of
functionalization (e.g., via metals, oxide powders, sulfide
powders, nanotubes, polymer macromolecules, etc.). The composite
cathodes were also relatively easy to coat, and were relatively
stable and compositionally uniform during cycling.
[0159] Electrochemical cells including the cathodes were assembled
for testing. Lithium metal (>99.9% Li, 2 mil thick foil) was
used as the anode. The electrolyte included 8 parts of lithium
bis(trifluoromethane sulfonyl) imide, 3.8 parts lithium nitrate, 1
part guanidine nitrate, and 0.4 parts pyridine nitrate (synthesized
in-house from pyridine and nitric acid) in a 1:1 weight ratio
mixture of 1,3-dioxolane and dimethoxyethane. The electrolyte
included a water content of less than 50 ppm. A 9-micron SETELA
porous separator was also used.
[0160] The above components were assembled into a layered structure
of cathode/separator/anode, which was folded in half to make a
bicell. The bicell was placed in a foil pouch with approximately
0.4 grams of a liquid electrolyte. After storing for 24 hours, the
cells were tested without compression. FIG. 7 includes a plot of
specific discharge capacity of cells comprising the composite
cathodes (indicated with asterisks) as a function of the number of
cycles. The cells comprising the composite cathodes exhibited
relatively high discharge capacities relative to cells comprising
mechanically mixed cathodes.
Comparative Example 2
[0161] This example describes the fabrication and testing of a
cathode comprising a mechanical mixture of sulfur and carbon in a
1:1 ratio. As was observed in Comparative Example 1, a 1:1
carbon-to-sulfur formulation was extremely difficult to produce on
the coater. In this example, hand drawn-down coatings were
fabricated. The electrodes fabricated by this process exhibited
dispersities more than two orders of magnitude lower compared to
those observed in the composite structures described in Example 2.
In addition, the hand drawn-down coatings were more difficult to
coat and exhibited relatively high compositional instabilities
during cycling, relative to the composites described in Example
2.
[0162] FIGS. 8A and 8B include secondary electron images of a
pristine composite cathode (as described in Example 2) and a
mechanically mixed cathode, respectively, each comprising a S:XE2
ratio of 84:16. FIGS. 9A-9C include X-ray spectral images outlining
the distributions of (A) sulfur, (B) carbon, and (C) aluminum in
the composite cathode (Example 2). In addition, FIGS. 9D-9F include
X-ray spectral images outlining the distributions of (D) sulfur,
(E) carbon, and (F) aluminum in the mechanically mixed cathode. The
composite cathode includes a relatively uniform distribution of all
three elements, compared to the mechanically mixed cathode. In
addition, the domain structure in the composite cathode described
in Example 2 was not developed. The thicknesses of the cracks in
the composite cathode were two times smaller than those in the
mechanically mixed cathode, and the crack density of the composite
cathode was significantly lower than that observed in the
mechanically mixed cathode.
[0163] An electrochemical cell including the mechanically mixed
cathode was assembled for testing according to the process
described in Example 2. FIG. 7 includes a plot of specific
discharge capacity of cells comprising the mechanically mixed
cathodes (indicated with squares) as a function of the number of
cycles. The cells comprising the mechanically mixed cathodes
exhibited relatively low discharge capacities relative to cells
comprising composite cathodes.
Example 3
[0164] This example describes the fabrication and testing of a
cathode fabricated using a nickel foam. The cathode was prepared by
filling the pores of a nickel foam (Incofoam supplied by Novamet,
450 micron pores, density of 320 g/cm.sup.2) with a mixture of 75
parts of elemental sulfur, 20 parts of Printex.RTM. XE-2, 4 parts
of graphite powder (Aldrich Chemical Company, Milwaukee, Wis.), and
1 part of polyvinyl alcohol (Celvol 425 from Celanese Corporation)
dissolved in a 1:1 weight ratio mixture of isopropanol and water.
Upon adding the mixture, pores of less than 10 microns in diameter
were formed within the pores of the nickel foam, in which the
sulfur was deposited.
[0165] An electrochemical cell including the cathode was assembled
for testing. Lithium metal (>99.9% Li, 2 mil thick foil) was
used as the anode. The electrolyte included 14 parts of lithium
bis(trifluoromethane sulfonyl) imide and 4 parts lithium nitrate in
a 1:1 weight ratio mixture of 1,3-dioxolane and dimethoxyethane.
The electrolyte included a water content of less than 50 ppm. A
9-micron SETELA porous separator was also used.
[0166] The above components were assembled into a layered structure
of cathode/separator/anode, which was folded in half to make a
bicell. The bicell was placed in a foil pouch with approximately
0.4 grams of a liquid electrolyte. After storing for 24 hours, half
of the cells were tested without compression, and the other half
were compressed between two parallel plates at a pressure of 98
Newtons/cm.sup.2 (about 10 kg-force/cm.sup.2). The liquid
electrolyte filled the void areas of the separator and cathode to
form prismatic cells with an electrode area of about 33 cm.sup.2.
Discharge-charge cycling was performed at 13.7 mA and 7.8 mA,
respectively. The discharge cutoff voltage was 1.7V and the charge
cutoff voltage was 2.5V. FIG. 10 includes a plot of specific
discharge capacity as a function of the number of charge-discharge
cycles for electrochemical cells comprising the cathodes fabricated
in this example. The nickel foam electrodes continued to exhibit
relatively high discharge capacities, even after 40
charge-discharge cycles. The application of 98 Newtons/cm.sup.2
(about 10 kg-force/cm.sup.3) of pressure to the cells led to more
consistent cycling performance after about 30 cycles. The use of
nickel foam and the application of pressure led to reduced fade
rates and extended cycle life. In addition, the rate of
polarization increase was much slower for the nickel foam cells,
especially when pressure was applied.
Comparative Example 3
[0167] In this example, a cathode was prepared by coating a mixture
of 75 parts of elemental sulfur, 20 parts of Printex.RTM. XE-2, 4
parts graphite powder, and 1 part of polyvinyl alcohol dissolved in
a 1:1 weight ratio mixture of isopropanol and water onto a 7-micron
thick, conductive carbon coated aluminum foil substrate. After
drying, the coated cathode active layer thickness was about 90
microns. Cells were assembled and tested as outlined in Example 3.
Cycling results are summarized in FIG. 10. The cell comprising the
cathode deposited on the aluminum foil substrate exhibited
relatively lower specific discharge capacity relative to the cell
comprising the nickel foam cathode.
Example 4
[0168] This example describes the fabrication and testing of
electrochemical cells comprising sintered nickel cathodes. Cells
with sintered nickel cathodes were prepared using Inco filamentary
nickel powder type 255 (Inco Special Products). The nickel
particles had an apparent density of 0.5 to 0.65 g/cm.sup.3. In
addition, the particles had diameters of bewteen about 1 micron and
about 100 microns, with a median diameter of around 20 microns. The
nickel powder was suspended in acetone, a relatively volatile
liquid, by vigorously mixing the powder and the suspension fluid to
produce a slurry. The slurry was poured into a crucible, and the
slurry was smoothened to distribute the nickel powder evenly and
flatly across the bottom surface of the crucible. The volatile
suspension fluid was then allowed to evaporate at room temperature,
leaving behind a relatively ordered array of nickel particles.
[0169] At this point the nickel powder was ready for sintering. The
sintering process was conducted in a reducing atmosphere comprising
95% nitrogen and 5% hydrogen. The nickel powder was sintered by
ramping the temperature to 800.degree. C. at a rate of 5.degree.
C./min, holding for 10 minutes, and allowing the samples to furnace
cool with the heating elements turned off. The final sintered
structure had a thickness of about 250 microns, with a pore size
distribution centered at about 15 microns.
[0170] Sulfur was then added to the sintered nickel porous support
structure. To incorporate the sulfur, an oil bath was prepared and
heated to 85.degree. C. A beaker containing toluene saturated with
sulfur was placed in the bath and allowed to come to equilibrium.
To ensure a saturated solution a small amount of sulfur was
maintained in solid form as a second phase in the beaker, by adding
sulfur to the toluene as necessary. Toluene was added to the beaker
as necessary to maintain nearly the same volume of toluene in the
beaker at all times. Each time a significant addition of reagent
(sulfur or toluene) was added, the system was allowed to come to
equilibrium. The nickel electrodes were dipped into the beaker with
sulfur saturated toluene, and dried with an argon stream. Once all
the electrodes in a batch were dipped, they were baked in a vacuum
oven at 80.degree. for several hours (anywhere from 1 to overnight
.about.14, with 3-4 being the most common, appeared to have no
effect as long as toluene smell was not present when the oven was
opened). The electrodes were weighed and compared with pre-dipping
weight to determine the amount of sulfur present. If the amount of
sulfur was below the desired amount the dipping was repeated. If
the amount of sulfur was above the desired amount the electrodes
were quickly dipped in pure toluene, then baked according to the
previous procedure. All the electrodes were loaded with between 1.5
and 2 mg S.sub.8/cm.sup.2. The porosity of the final structure was
substantially the same as the nickel once the sulfur was
dissolved.
[0171] An electrochemical cell including the cathode was assembled
for testing. The anode included a vapor deposited lithium film 26
microns in thickness. The electrolyte included 8 parts of lithium
bis(trifluoromethane sulfonyl) imide and 4 parts lithium nitrate in
a 1:1 weight ratio mixture of 1,3-dioxolane and dimethoxyethane.
The electrolyte included a water content of less than 50 ppm. A
16-micron SETELA porous separator was also used. The above
components were assembled into a layered structure of single sided
anode/separator/2.times.(2 sided cathode/separator/2.times. anode
back to back/separator)/2 sided cathode/separator/single sided
anode. This flat cell was placed in a foil pouch with approximately
0.62 grams of a liquid electrolyte. After storing for 24 hours the
cells were compressed between two parallel plates at a pressure of
98 Newtons/cm.sup.2 (about 10 kg-force/cm.sup.2). The liquid
electrolyte filled the void areas of the separator and cathode to
form flat cells with an electrode area of about 99.441 cm.sup.2.
Discharge-charge cycling was performed at 40 mA and 25 mA,
respectively. The discharge cutoff voltage was 1.7V and the charge
cutoff voltage was 2.5V
[0172] FIG. 11 includes a plot of the percentage capacity of the
tested cells as a function of C-rate. The cells cycled normally
until the 15.sup.th discharge when standard rate testing was
completed.
[0173] Electrochemical cells were fabricated using milestone
cathodes for comparison purposes. The milestone cathodes were made
essentially as described in Comparative Example 1, except in this
case, the cathodes included a 55/20/20/4 mixture of
sulfur/XE-2/Vulcan XC72R/PVOH. As can be seen in FIG. 11, the cells
including the sintered nickel cathodes exhibited a higher
percentage capacity at higher rates relative to the cells with
milestone cathodes.
Example 5
[0174] This example describes the fabrication and testing of
electrochemical cells comprising cathodes fabricated using a
polymeric porous support structure. The polymeric porous support
structure was produced by combining a solution of polyvinyl alcohol
(PVA) with ammonium bicarbonate as a blowing agent, Vulcan carbon,
TIMCAL Ks6 graphite, and carbon fibers were added to the solution
of PVA to increase the electrical conductivity and act as
reinforcing agents to improve the mechanical properties of the PVA
matrix.
[0175] The ammonium bicarbonate was pre-milled in an attritor mill
to reduce the particle size to the 1 to 2 micron range. Isopropyl
alcohol (IPA) was used as a carrier solvent during this milling and
was vacuum filtered from the milled ammonium bicarbonate. To remove
the final amounts of IPA, the ammonium bicarbonate was allowed to
dry in an open pan over night. During this overnight drying about
20% of the ammonium bicarbonate was lost to sublimation.
[0176] To prepare the final slurry, a solution containing Vulcan
XC72R Carbon and Timcal KS6 graphite (6 micron diameter platelets)
was milled with a soluble binder solution of PVOH in water, IPA and
2-(2-ethoxyethoxy)ethanol (i.e., dowanol; Dow Chemical) solvents.
This milled solution is designated as VKC2.
[0177] In a second milling step, the VKC2 solution was milled for
20 minutes in a attritor mill with Polygraff PR-24 carbon fibers
(Pyrograf Products, Inc., 8 micron diameter, 100-150 micron length)
and a small amount of additional water. Water was added to improve
the adhesion of the foamed primer to the aluminum substrate (the
same type of aluminum foil substrate described in Example 1). The
pre-milled ammonium bicarbonate was then added at a ratio of 8
parts ammonium bicarbonate to one part, by weight, of the original
primer solids. This final mixture was then milled for 10 minutes
before being discharged from the attritor mill.
[0178] To further reduce the final pore sizes formed by the
ammonium bicarbonate, the VKC2/fiber/ammonium bicarbonate mixture
was passed thru a Micro-fluidizer. A single 400 micron chamber was
used, and the discharge pressure was set to 5 kpsi.
[0179] One the same day that it was prepared, the slurry was coated
onto an aluminum substrate using a slot die. The coating was dried
in a four zone, air convection oven (MEGTEC Systems, DePere, Wis.).
The temperatures of each zone were controlled between 25.degree. C.
and about 85.degree. C. to allow of the formation of the pores in
the final cathode structure and control the brittleness/adhesion of
the cast slurry.
[0180] The thickness of the dried, porous polymer matrix prior to
compression was about 216.7 microns. Upon applying a force of about
98 Newtons/cm.sup.2 (about 10 kg.sub.f/cm.sup.2), the polymer
matrix thickness was about 112.1 microns. The weight of the polymer
matrix was about 1.064 mg/cm.sup.2. The dibutyl phthalate (DBP)
available void space was measured by placing a weighed sample in a
bag with an excess amount of DBP to saturate the sample, and
re-weighing the resulting saturated sample. The DFP available void
space was about 0.0038 cm.sup.3/cm.sup.2 per side, which was
significantly larger than the 0.0015 cm.sup.3/cm.sup.2 required for
1.85 mg S.sub.8/cm.sup.2.
[0181] BET surface area measurements of the polymer matrix
indicated that the available surface area was about 39
m.sup.2/g.
[0182] FIG. 12 includes a plot of the polymer matrix thickness as a
function of the applied pressure (in kg.sub.f/cm.sup.2, which can
be converted to Newtons/cm.sup.2 by multiplying by about 9.8).
Three samples were tested. The applied force was increased from 0
to 20 kg.sub.f/cm.sup.2, for a total of four cycles per sample
(labeled #1, #2, #3, and #4 in FIG. 12). As the force was applied,
the thickness of the sample was measured. After the initial cycle,
the thickness of each sample returned to only about 45% of its
original thickness. As the samples were exposed to the forces, the
DBP uptake of the foam did not vary significantly before and after
initial compression. The data reported in this example was taken
after initial compression.
[0183] After formation of the conductive polymer matrix, sulfur was
added by dipping the polymer matrix into a hot bath of toluene
saturated with sulfur. An electrochemical cell including the
cathode was assembled for testing. Lithium metal (>99.9% Li, 2
mil thick foil from Chemetall-Foote Corp., Kings Mountain, N.C.)
was used for the anode. The electrolyte included 8 parts of lithium
bis(trifluoromethane sulfonyl) imide, (lithium imide available from
3M Corporation, St. Paul, Minn.), 3.8 parts lithium nitrate
(available from Aldrich Chemical Company, Milwaukee, Wis.), 1 part
guanidine nitrate (also available from Aldrich Chemical Company,
Milwaukee, Wis.) and 0.4 parts pyridine nitrate (synthesized
in-house from pyridine and nitric acid) in a 1:1 weight ratio
mixture of 1,3-dioxolane and dimethoxyethane. The electrolyte
included a water content of less than 50 ppm. A porous separator
comprising 9-.mu.m SETELA (a polyolefin separator available from
Tonen Chemical Corporation, Tokyo, Japan, and from Mobil Chemical
Company, Films Division, Pittsford, N.Y.) was included between the
anode and the cathode. The dual sided cathode was wrapped with the
separator and anode foil and then placed in a foil pouch. 0.42
grams of the liquid electrolyte are then added to the foil pouch.
The liquid electrolyte fills the void areas of the separator and
cathode to form prismatic cells with an electrode area of about
31.8 cm.sup.2. After sealing, the cells were stored for 24 hours.
Prior to being placed on test the cells are compressed between two
parallel plates at a pressure of 98 Newtons/cm.sup.2 (about 10
kg.sub.f/cm.sup.2). Charge-discharge cycling was performed at 13.7
mA and 7.8 mA, respectively. The discharge cutoff voltage was 1.7V
and the charge cutoff voltage was 2.5V. The electrochemical cells
were exposed to a compressive force of 10 kg.sub.f/cm.sup.2 (about
98 Newtons/cm.sup.2). FIG. 13 includes a plot of the specific
discharge capacity of the cells as a function of the
charge/discharge cycle. The cells exhibited comparable performance
through 20 cycles.
[0184] 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.
[0185] 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."
[0186] 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.
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 unless clearly
indicated to the contrary. 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
without B (optionally including elements other than B); in another
embodiment, to B without A (optionally including elements other
than A); in yet another embodiment, to both A and B (optionally
including other elements); etc.
[0187] 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.
[0188] 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.
[0189] In the claims, as well as in the specification above, all
transitional phrases such as "comprising," "including," "carrying,"
"having," "containing," "involving," "holding," 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.
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