U.S. patent application number 11/076367 was filed with the patent office on 2005-08-11 for cathodes comprising electroactive sulfur materials and secondary batteries using same.
Invention is credited to Boguslavsky, Leonid I., Deng, Zhongyi, Gorkovenko, Alexander, Mukherjee, Shyama P., Skotheim, Terje A., Xu, Zhe-Sheng.
Application Number | 20050175895 11/076367 |
Document ID | / |
Family ID | 25540964 |
Filed Date | 2005-08-11 |
United States Patent
Application |
20050175895 |
Kind Code |
A1 |
Gorkovenko, Alexander ; et
al. |
August 11, 2005 |
Cathodes comprising electroactive sulfur materials and secondary
batteries using same
Abstract
The present invention pertains to solid composite cathodes which
comprise (a) an electroactive sulfur-containing cathode material
which, in its oxidized state, comprises a polysulfide moiety of the
formula, --S.sub.m--, wherein m is an integer from 3 to 10; and (b)
a non-electroactive particulate material having a strong adsorption
of soluble polysulfides. The present invention also pertains to
electric current producing cells comprising such solid composite
cathodes, and methods of making such solid composite cathodes and
electric current producing cells.
Inventors: |
Gorkovenko, Alexander;
(Tucson, AZ) ; Skotheim, Terje A.; (Tucson,
AZ) ; Xu, Zhe-Sheng; (Tucson, AZ) ;
Boguslavsky, Leonid I.; (Tucson, AZ) ; Deng,
Zhongyi; (Tucson, AZ) ; Mukherjee, Shyama P.;
(Tucson, AZ) |
Correspondence
Address: |
Squire, Sanders & Dempsey L.L.P.
Two Renaissance Squire,
Suite 2700
40 North Central Avenue
Phoenix
AZ
85004-4498
US
|
Family ID: |
25540964 |
Appl. No.: |
11/076367 |
Filed: |
March 8, 2005 |
Related U.S. Patent Documents
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Application
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Filing Date |
Patent Number |
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11076367 |
Mar 8, 2005 |
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10172598 |
Jun 14, 2002 |
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6878488 |
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10172598 |
Jun 14, 2002 |
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09703348 |
Oct 31, 2000 |
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6406814 |
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09703348 |
Oct 31, 2000 |
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08994708 |
Dec 19, 1997 |
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6210831 |
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Current U.S.
Class: |
429/218.1 ;
252/182.1; 429/213; 429/232 |
Current CPC
Class: |
H01M 4/0485 20130101;
Y02E 60/10 20130101; H01M 10/3909 20130101; Y10S 977/753 20130101;
Y10S 977/948 20130101; H01M 2004/028 20130101; H01M 4/623 20130101;
H01M 4/60 20130101; H01M 4/602 20130101; Y10T 29/49108 20150115;
H01M 4/40 20130101; H01M 4/581 20130101; H01M 4/583 20130101; H01M
4/136 20130101; H01M 4/62 20130101; H01M 4/624 20130101; H01M 4/587
20130101; H01M 4/137 20130101; H01M 4/5815 20130101; H01M 2004/027
20130101; H01M 4/36 20130101; H01M 4/622 20130101; H01M 10/052
20130101 |
Class at
Publication: |
429/218.1 ;
429/232; 429/213; 252/182.1 |
International
Class: |
H01M 004/58; H01M
004/62; H01M 004/60 |
Claims
1-31. (canceled)
32. A solid cathode for use in an electric current producing cell,
the cathode comprising: (a) an electroactive sulfur-containing
material; (b) a non-electroactive particulate non-fibrous material
having an adsorption of at least 40% of lithium octasulfide in a
0.03 M solution of lithium octasulfide in tetraglyme with the
particulate non-fibrous material present at a weight ratio of the
particulate non-fibrous material to lithium octasulfide of 6.2 to
1, and wherein the particulate non-fibrous material comprises a
carbon.
33. The cathode of claim 32, wherein the electroactive
sulfur-containing material comprises elemental sulfur.
34. The cathode of claim 32, wherein the electroactive
sulfur-containing material comprises a carbon-sulfur polymer.
35. The cathode of claim 32, wherein the adsorption by the
particulate non-fibrous material of the lithium octasulfide in the
solution is at least 60%.
36. The cathode of claim 32, wherein the adsorption by the
particulate non-fibrous material of the lithium octasulfide in the
solution is at least 87%.
37. The cathode of claim 32, wherein the cathode further comprises
a conductive filler that includes one or more materials selected
from the group consisting of conductive carbons, graphites, active
carbon fibers, metal flakes, metal powders, electrically conductive
polymers, and electrically conductive metal chalcogenides; wherein
the conductive filler adsorbs less than 40% of lithium octasulfide
in a 0.03 M solution of lithium octasulfide in tetraglyme with the
conductive filler present at the weight ratio of the conductive
filler to lithium octasulfide of 6.2 to 1.
38. The cathode of claim 32, wherein the cathode further comprises
a binder.
39. The cathode of claim 32, wherein the cathode comprises from 50
to 96% by weight of the electroactive sulfur-containing material,
and from 5 to 50% by weight of the non-electroactive particulate
material.
40. The cathode of claim 38, wherein the cathode comprises less
than about 15% by weight of the binder.
41. An electric current producing cell comprising: (a) an anode
comprising lithium; (b) a solid cathode comprising: (i) an
electroactive sulfur-containing material; and (ii) a
non-electroactive particulate non-fibrous material having an
adsorption of at least 40% of lithium octasulfide in a 0.03 M
solution of lithium octasulfide in tetraglyme with the particulate
non-fibrous material present at a weight ratio of the particulate
non-fibrous material to lithium octasulfide of 6.2 to 1, and
wherein the particulate non-fibrous material comprises a carbon;
and (c) an electrolyte interposed between the anode and the
cathode.
42. The cell of claim 41, wherein the electroactive
sulfur-containing material comprises elemental sulfur.
43. The cell of claim 41, wherein the electroactive
sulfur-containing material comprises a carbon-sulfur polymer.
44. The cell of claim 41, wherein the adsorption by the particulate
non-fibrous material of the lithium octasulfide in the solution is
at least 60%.
45. The cell of claim 41, wherein the adsorption by the particulate
non-fibrous material of the lithium octasulfide in the solution is
at least 87%.
46. The cell of claim 41, wherein the solid cathode further
comprises a conductive filler that includes one or more materials
selected from the group consisting of conductive carbons,
graphites, active carbon fibers, metal flakes, metal powders,
electrically conductive polymers, and electrically conductive metal
chalcogenides; wherein the conductive filler adsorbs less than 40%
of lithium octasulfide in a 0.03 M solution of lithium octasulfide
in tetraglyme with the conductive filler present at the weight
ratio of the conductive filler to lithium octasulfide of 6.2 to
1.
47. The cell of claim 41, wherein the cathode further comprises a
binder.
48. The cell of claim 41, wherein the anode comprises one or more
anode active materials selected from the group consisting of
lithium metal, lithium-aluminum alloys, lithium-tin alloys,
lithium-intercalated carbons, and lithium-intercalated
graphites.
49. The cell of claim 41, wherein the electrolyte comprises one or
more materials selected from the group consisting of liquid
electrolytes, gel polymer electrolytes, and solid polymer
electrolytes.
Description
TECHNICAL FIELD
[0001] The present invention relates generally to the field of
cathodes and rechargeable electric current producing cells. More
particularly, the present invention pertains to solid composite
cathodes which comprise (a) an electroactive sulfur-containing
material which, in its oxidized state, comprises a polysulfide
moiety of the formula, --S.sub.m--, wherein m is an integer from 3
to 10; and, (b) a non-electroactive particulate material having a
strong adsorption of soluble polysulfides. This strongly adsorbing
particulate material significantly reduces or retards the diffusion
of sulfur-containing electroactive materials from the cathode into
the electrolyte and other cell components when incorporated into
the cathode of an electric current producing cell. The present
invention also pertains to electric current producing cells
comprising such composite cathodes, and methods of making such
solid composite cathodes and electric current producing cells.
BACKGROUND
[0002] Throughout this application, various publications, patents,
and published patent applications are referred to by an identifying
citation. The disclosures of the publications, patents, and
published patent applications referenced in this application are
hereby incorporated by reference into the present disclosure to
more fully describe the state of the art to which this invention
pertains.
[0003] As the rapid evolution of portable electronic devices
continues, the need for safe, long-lasting, high capacity
rechargeable batteries becomes increasingly evident. Under such
circumstances, high energy density lithium secondary batteries are
rapidly being developed that will eventually replace the
conventional lead acid, nickel-cadmium, and nickel metal hydride
batteries in many applications. In recent years, there has been
considerable interest in developing high energy density cathode
active materials and alkali metals as anode active materials for
high energy density lithium secondary batteries to meet these
needs.
[0004] Lithium and sulfur are highly desirable as the
electrochemically active materials for the anode and cathode,
respectively, of rechargeable or secondary battery cells because
they provide nearly the highest energy density on a weight (2500
Wh/kg) or volume (2800 Wh/l) basis possible of any of the known
combinations of materials. To obtain high energy densities, the
lithium can be present as the pure metal, in an alloy or in an
intercalated form, and the sulfur can be present as elemental
sulfur or as a component in an organic or inorganic material with a
high sulfur content, preferably greater than 50 weight percent
sulfur.
[0005] Hereinafter, anodes containing the element lithium in any
form are referred to as lithium-containing anodes. Cathodes
containing the element sulfur in any form are hereinafter referred
to as sulfur-containing cathodes.
[0006] Many battery systems comprising alkali metal containing
anodes and sulfur-containing cathodes have been described.
Exemplary of high temperature cells incorporating molten alkali
metal anodes and molten sulfur cathodes separated by a solid
electrolyte are those described in U.S. Pat. Nos. 3,993,503,
4,237,200 and 4,683,179. For operation, these storage cells must be
heated to temperatures greater than about 320.degree. C. Of
considerable recent interest are cells comprising alkali metal
anodes and cathodes containing elemental sulfur that operate at
considerably lower temperatures, particularly those with solid
cathodes operating at ambient temperatures. Rechargeable lithium
sulfur battery cells operating at room temperature have been
described by Peled et al. in J. Power Sources, 1989, 26, 269-271,
wherein the solid sulfur-containing cathodes are comprised of a
porous carbon loaded with elemental sulfur. The nature of the
porous carbon was not described, but cells constructed with these
cathodes provided only a maximum of 50 cycles. The decline in
capacity with cycling was attributed to loss of cathode active
material.
[0007] U.S. Pat. No. 3,639,174 to Kegelman describes solid
composite cathodes comprising elemental sulfur and a particulate
electrical conductor. U.S. Pat. No. 4,303,748 to Armand et al.
discloses solid composite cathodes containing an ionically
conductive material together with elemental sulfur, transition
metal salts, or other cathode active materials for use with lithium
or other anodes in which, for example, the active sulfur or other
cathode active material and the inert compounds with electrical
conduction, such as graphite powder, are both particles of between
1 and 500 microns in diameter. Further examples of cathodes
comprising elemental sulfur, an electrically conductive material
and an ionically conductive material that operate in the
temperature range from -40.degree. C. to 145.degree. C. are
described in U.S. Pat. Nos. 5,523,179, 5,532,077, 5,582,623 and
5,686,201 to Chu. U.S. Pat. Nos. 5,552,244 and 5,506,072 to Griffin
et al. describe metal-sulfur batteries using a cathode comprising a
mixture of finely divided sulfur and graphite packed around a
conductive electrode and covered with a porous separator. A minimum
of 10 weight percent of graphite is needed to achieve sufficient
conductivity in the cathode structure. No function other than
providing conductivity is described for the graphite.
[0008] In spite of the many known systems, as for example described
above, employing a solid cathode comprising elemental sulfur in
rechargeable alkali metal sulfur battery systems has been
problematic in obtaining good electrochemical efficiency and
capacity, cycle life, and safety of the cells owing to the
diffusion of sulfur active materials from the sulfur-containing
cathode into the electrolyte and anode components. This has been
particularly true in battery cells comprising a sulfur-containing
cathode in combination with a lithium-containing anode. U.S. Pat.
No. 3,907,591 to Lauck and an article by Yamin et al. in J. Power
Sources, 1983, 9, 281-287 describe the reduction of elemental
sulfur during the discharging of a lithium/sulfur cell to soluble
lithium polysulfides in high concentrations in the electrolyte.
Even partial reduction of the solid sulfur in the cathode forms
polysulfides, such as lithium octasulfide, that are soluble in the
organic electrolytes. In battery cells, these soluble polysulfides
diffuse from the cathode into the surrounding electrolyte and may
react with the lithium anode leading to its fast depletion. This
leads to reduced capacity of the battery cell.
[0009] In attempts to reduce the problems associated with the
generation of soluble polysulfides in alkali metal battery cells
comprising elemental sulfur, battery cells have been developed
utilizing cathodes comprised of sulfur-containing materials in
which sulfur is chemically bound to an organic or carbon polymer
backbone or to a low molecular weight organic compound. One such
class of electroactive sulfur-containing materials has been
referred to as organo-sulfur materials. Herein, the term
"organo-sulfur materials" means a material containing organic
sulfur compounds with only single or double carbon-sulfur bonds or
sulfur-sulfur bonds forming disulfide (--S--S--) linkages.
[0010] U.S. Pat. Nos. 4,833,048 and 4,917,974 to Dejonghe et al.
disclose liquid sulfur-containing cathodes comprising organo-sulfur
materials of the formula, (R(S).sub.y).sub.n, where y=1 to 6; R is
one or more different aliphatic or aromatic organic moieties having
1 to 20 carbon atoms; and n is greater than 1. U.S. Pat. No.
5,162,175 to Visco et al. describes the use of 1 to 20 weight
percent of conductor particles, such as carbon black, in solid
composite cathodes containing organo-sulfur materials having
disulfide electroactive groups. These organo-sulfur materials
undergo polymerization (dimerization) and de-polymerization
(disulfide cleavage) upon the formation and breaking of the
disulfide bonds. The de-polymerization which occurs during the
discharging of the cell results in lower molecular weight polymeric
and monomeric species, namely soluble anionic organic sulfides,
which may dissolve into the electrolyte and cause self discharge,
reduced capacity, and eventually complete cell failure, thereby
severely reducing the utility of organo-sulfur materials as a
cathode-active material in secondary batteries. Although the
soluble discharge products are typically soluble organic sulfides
rather than the inorganic polysulfides of the type formed with
elemental sulfur, the detrimental effects on electrochemical
efficiency and cycle life are similar. In addition, the
organo-sulfur materials typically contain less than 50 weight
percent of sulfur so they have a much lower energy density or
theoretical specific capacity than elemental sulfur.
[0011] U.S. Pat. No. 5,324,599 to Oyama et al. discloses a solid
composite cathode comprising a combination of a compound having a
disulfide group and a conductive polymer, or an organo-disulfide
derivative of a conductive polymer. In one variation, a complex is
formed from a disulfide compound and a conductive polymer in a
composite cathode layer so that the disulfide compound is not
likely to leak out of the composite cathode into the electrolyte of
the rechargeable battery.
[0012] In a similar approach to overcome the dissolution problem
with organo-sulfur materials, U.S. Pat. No. 5,516,598 to Visco et
al. discloses solid composite cathodes comprising
metal/organo-sulfur charge transfer materials with one or more
metal-sulfur bonds, wherein the oxidation sate of the metal is
changed in charging and discharging the positive electrode or
cathode. The metal ion provides high electrical conductivity to the
cathode, although it significantly lowers the cathode energy
density and capacity per unit weight of the polymeric organo-sulfur
material. There is no mention of retarding the transport of soluble
reduced sulfide or thiolate anion species formed during charging or
discharging of the cell.
[0013] Another class of electroactive sulfur-containing materials
comprises carbon-sulfur polymer materials, for example, as
described in U.S. Pat. Nos. 5,529,860, 5,601,947 and 5,609,702, and
in copending U.S. patent application Ser. No. 08/602,323 to
Skotheim et al. These references also describe the use of
conductive carbons and graphites, conductive polymers, and metal
fibers, powders, and flakes as conductive fillers with
carbon-sulfur polymer materials. Herein, the term "carbon-sulfur
polymer materials" means materials comprising carbon-sulfur
polymers with carbon-sulfur single bonds and with sulfur-sulfur
bonds comprising trisulfide (--S--S--S--), tetrasulfide
(--S--S--S--S--), or higher polysulfide linkages. The carbon-sulfur
polymer materials comprise, in their oxidized state, a polysulfide
moiety of the formula, --S.sub.m--, wherein m is an integer equal
to 3 or greater.
[0014] Several approaches have been described to inhibit or retard
the transport or diffusion of soluble polysulfides from the cathode
to the electrolyte. U.S. Pat. No. 3,806,369 to Dey et al. describes
an ion exchange membrane between the cathode and the
electrolyte/separator layer to inhibit the passage of polysulfides
or other anions from the cathode into the electrolyte. Without this
barrier layer, the soluble polysulfides or other anions form
insoluble films on the cathode and shorten the cycle life of the
cell. U.S. Pat. No. 3,532,543 to Nole et al. describes the attempt
to use copper halide salts to limit the formation of polysulfides
in a solid cathode containing elemental sulfur. U.S. patent
application Ser. No. 08/859,996, titled "Novel Composite Cathodes,
Electrochemical Cells Comprising Novel Composite Cathodes, and
Processes for Fabricating Same" to the common assignee, discloses
the addition of a class of electroactive transition metal
chalcogenide materials to sulfur-containing cathodes to encapsulate
or entrap the sulfur-containing materials to retard the transport
of soluble polysulfides and sulfides from the cathode into the
electrolyte.
[0015] Barrier layers, as for example those described heretofore,
can be effective in preventing excessive diffusion of soluble
cathode reduction products, such as inorganic polysulfides, into
the electrolyte, thereby improving cycle life and safety from the
levels obtained when excessive inorganic polysulfides and other
soluble cathode reduction products are present in the electrolyte.
However, these barrier layers may have disadvantages. Besides the
cost and the non-cathode active volume occupied by the materials,
they may effectively block the transport of desirable soluble or
insoluble anionic species into the electrolyte. Also, the barrier
layer may be only partially effective so that there is a slow
buildup of soluble cathode reduction products in the electrolyte.
While low concentrations of polysulfides initially may be
acceptable in the early cycles of the cell, in the later
charge-discharge cycles of the cell, the concentrations of the
soluble polysulfides and other anions may become too high or
excessive, thereby shortening the cycle life and decreasing cell
safety.
[0016] Japanese Patent Publication No. 09-147868, published Jun. 6,
1997, describes the use of active carbon fibers to absorb
electroactive sulfur-containing materials in cathodes of secondary
batteries and to provide increased cycle life at high discharge
currents. These active carbon fibers are characterized by highly
microporous structures with specific surface areas above 1000
m.sup.2/g, which absorb large amounts of sulfur-containing
materials such as 30 to 50 weight percent, into the pores. These
active carbon fibers also have diameters greater than 1 micron,
typically in the range of 2 to 6 microns.
[0017] Despite the various approaches proposed for the fabrication
of high energy density rechargeable cells comprising elemental
sulfur, organo-sulfur or carbon-sulfur polymer materials in a solid
cathode, there remains a need for materials and cell designs that
prevent the excessive out-diffusion of sulfides and polysulfides
from the cathode layers in these cells, improve the electrochemical
utilization of cathode active materials and cell efficiencies, and
provide safe rechargeable cells with high rates and capacities over
many cycles.
SUMMARY OF THE INVENTION
[0018] One aspect of the present invention pertains to a solid
composite cathode for use in an electric current producing cell
comprising (a) an electroactive sulfur-containing cathode material,
which material, in its oxidized state, comprises a polysulfide
moiety of the formula, --S.sub.m--, wherein m is an integer from 3
to 10, and (b) a non-electroactive particulate material having a
strong adsorption of soluble polysulfides, wherein the adsorption
by said particulate material is characterized by adsorption of at
least 40% of the lithium octasulfide in a 0.03 M solution of
lithium octasulfide in tetraglyme with said particulate material
present at the weight ratio of said particulate material to lithium
octasulfide of 6.2 to 1.
[0019] In one embodiment, the adsorption by said particulate
material of the lithium octasulfide in said solution is at least
60%. In one embodiment, the adsorption by said particulate material
of the lithium octasulfide in said solution is at least 87%. In one
embodiment, the adsorption by said particulate material of the
lithium octasulfide in said solution is at least 93%. In one
embodiment, the adsorption by said particulate material of the
lithium octasulfide in said solution is at least 97%.
[0020] In one embodiment, the non-electroactive particulate
material having strong adsorption of soluble polysulfides is
selected from the group consisting of: carbons, silicas, aluminum
oxides, transition metal chalcogenides, and metals. In one
embodiment, the non-electroactive particulate material having
strong adsorption of soluble polysulfides comprises a carbon. In
one embodiment, the non-electroactive particulate material having
strong adsorption of soluble polysulfides comprises a silica. In
one embodiment, the non-electroactive particulate material having
strong adsorption of soluble polysulfides comprises an aluminum
oxide. In a particularly preferred embodiment, said aluminum oxide
comprises pseudo-boehmite. In one embodiment, the non-electroactive
particulate material having strong adsorption of soluble
polysulfides comprises a transition metal chalcogenide. In a
preferred embodiment, said chalcogenide comprises a
non-electroactive vanadium oxide. In a most particularly preferred
embodiment, said chalcogenide comprises an aerogel of a crystalline
vanadium oxide. In one embodiment, the non-electroactive
particulate material having strong adsorption of soluble
polysulfides comprises a metal.
[0021] The solid composite cathodes of the present invention
comprise an electroactive sulfur-containing material, which
material, in its oxidized state, comprises a polysulfide moiety of
the formula, --S.sub.m--, wherein m is an integer from 3 to 10. In
one embodiment, m is an integer from 3 to 8. In one embodiment, m
is an integer from 3 to 6. In one embodiment, m is an integer from
6 to 10. In one embodiment, the polysulfide linkage comprises
--S--S--S-- (i.e., trisulfide). In one embodiment, the polysulfide
linkage comprises --S--S--S--S-- (i.e., tetrasulfide). In one
embodiment, the polysulfide linkage comprises --S--S--S--S--S--
(i.e., pentasulfide). In one embodiment, the polysulfide linkage
comprises --S--S--S--S--S--S-- (i.e., hexasulfide). In one
embodiment, the polysulfide linkage comprises
--S--S--S--S--S--S--S-- (i.e., heptasulfide). In one embodiment,
the polysulfide linkage comprises --S--S--S--S--S--S--S--S-- (i.e.,
octasulfide).
[0022] In one embodiment, the electroactive sulfur-containing
material of the solid composite cathodes of the present invention
comprises elemental sulfur. In one embodiment, the electroactive
sulfur-containing material comprises a carbon-sulfur polymer.
[0023] In one embodiment, the solid composite cathodes of the
present invention further comprise a conductive filler not having
strong adsorption of soluble polysulfides. Examples of suitable
conductive fillers include, but are not limited to, carbons,
graphites, active carbon fibers, metal flakes, metal powders, metal
fibers, electrically conductive polymers, and electrically
conductive metal chalcogenides.
[0024] In one embodiment, the solid composite cathodes of the
present invention further comprise a binder.
[0025] In one embodiment, the solid composite cathodes of this
invention further comprise an electrolyte.
[0026] In one embodiment, the solid composite cathodes of this
invention further comprise a non-electroactive metal oxide not
having a strong adsorption of soluble polysulfides, wherein the
metal oxide is selected from the group consisting of: silicas,
aluminum oxides, silicates, and titanium oxides.
[0027] Another aspect of the present invention pertains to electric
current producing cells which comprise an anode; a solid composite
cathode of the present invention, as described herein; and an
electrolyte interposed between the anode and the composite
cathode.
[0028] Examples of suitable anode active materials for use in the
anodes of the cells of the present invention include, but are not
limited to, lithium metal, lithium-aluminum alloys, lithium-tin
alloys, lithium-intercalated carbons, and lithium-intercalated
graphites.
[0029] Examples of suitable electrolytes for use in cells of the
present invention include, but are not limited to, liquid
electrolytes, gel polymer electrolytes, and solid polymer
electrolytes.
[0030] In a preferred embodiment, the electrolyte comprises one or
more ionic electrolyte salts and 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; and blends of the foregoing.
[0031] In a preferred embodiment, the electrolyte for the cell of
this invention comprises one or more ionic electrolyte salts and
one or more electrolyte solvents selected from the group consisting
of: N-methyl acetamide, acetonitrile, sulfolanes, sulfones,
carbonates, N-alkyl pyrrolidones, dioxolanes, glymes, and
siloxanes.
[0032] Yet another aspect of the present invention pertains to
methods of manufacturing a solid composite cathode, as described
herein.
[0033] Still another aspect of the present invention pertains to
methods of manufacturing an electric current producing cell which
employs a solid composite cathode, as described herein.
[0034] As one of skill in the art will appreciate, features of one
embodiment and aspect of the invention are applicable to other
embodiments and aspects of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 shows a composite cathode on a current collector
incorporating a cathode configuration wherein the electroactive
sulfur-containing cathode material is encapsulated with a thin
coating comprising the non-electroactive particulate material
having a strong adsorption of soluble polysulfides. The thin
coating acts as a polysulfide retarding barrier layer material.
These "core-shell" cathode materials are bound together in a
composite cathode optionally using a binder containing a conductive
filler.
[0036] FIG. 2 shows a composite cathode configuration on a current
collector wherein the non-electroactive particulate material having
a strong adsorption of soluble polysulfides remains as an interface
layer at the boundaries of the electroactive sulfur-containing
cathode materials. The composite cathode can be represented as a
composite having the electroactive sulfur-containing cathode
materials dispersed in a matrix comprising the non-electroactive
particulate material having a strong adsorption of soluble
polysulfides in a matrix composition which optionally contains a
binder and a conductive filler.
[0037] FIG. 3 shows a solid composite cathode design on a current
collector wherein the electroactive sulfur-containing cathode
material is coated or impregnated with a layer comprising the
non-electroactive particulate material having a strong adsorption
of soluble polysulfides.
[0038] FIG. 4 shows volumetric capacity (mAh/cm.sup.3 of cathode
coating) versus cycle number for the AA cell of example 2
incorporating an elemental sulfur cathode of the present invention
comprising a particulate carbon of the present invention (PRINTEX
XE-2, a tradename for carbon available from Degussa Corporation,
Arkon, Ohio) with polyethylene oxide (PEO) as a binder.
[0039] FIG. 5 shows a plot of cell specific capacity versus cycle
number for up to 100 cycles for the AA lithium battery cell of
Example 3 with a solid composite cathode comprising a carbon-sulfur
polymer with a particulate carbon and silica pigment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0040] One aspect of the present invention pertains to a solid
composite cathode for use in an electric current producing cell
comprising (a) an electroactive sulfur-containing cathode material,
which material, in its oxidized state, comprises a polysulfide
moiety of the formula, --S.sub.m--, wherein m is an integer from 3
to 10, and (b) a non-electroactive particulate material having a
strong adsorption of soluble polysulfides, wherein the adsorption
by said particulate material is characterized by adsorption of at
least 40% of the lithium octasulfide in a 0.03 M solution of
lithium octasulfide in tetraglyme with said particulate material
present at the weight ratio of said particulate material to lithium
octasulfide of 6.2 to 1. The solid composite cathodes of the
present invention are particularly preferred for use in
electrolytic cells, rechargeable batteries, fuel cells, and the
like, which comprise organic electroactive sulfur-containing
cathodes and which require high energy density.
[0041] Non-Electroactive Particulate Materials
[0042] The term "particulate," as used herein, pertains to the
non-fibrous shape and structure of the materials. This particulate
shape and structure may have a regular shape, as for example,
spherical, or may have an irregular shape, but does not have a
needle-like, fiber, or filament shape or structure.
[0043] The term "electroactive," as used herein, pertains to the
electrochemical property of a material which takes part in the
electrochemical reaction of charge or discharge in an electric
current producing cell. The term "non-electroactive," as used
herein, pertains to the electrochemical property of a material
which does not take part in the electrochemical reaction of charge
or discharge in an electric current producing cell.
[0044] The term, "adsorption," as used herein, pertains to the
tendency of molecules or adsorbate from a liquid phase to adhere to
the surface of a solid. This tendency is an equilibrium-based
selectivity among the different molecules in the liquid phase to
compete, depending on their relative affinity for the surface of
the solid, for physical adsorption, but not chemisorption, on this
surface, as described in the Kirk-Othmer Encyclopedia of Chemical
Technology, 4th Edition, Vol. 1, pages 493-528, John Wiley &
Sons, New York, 1991, Howe-Grant editor. When the solid is
microporous, this physical adsorption includes the amount of
adsorbate on the surfaces of the solid and in the micropore volume
of the solid. The term, "adsorbs," as used herein, pertains to
undergoing the process of adsorption.
[0045] The relative ability of a non-electroactive material to
retard the diffusion of electroactive materials from a solid
cathode in an electric current producing cell may be evaluated by
determining the efficiency with which the non-electroactive
material physically adsorbs a soluble species of the electroactive
material, when that soluble species is dissolved in a suitable
solvent. For example, when the electroactive material in the solid
cathode is a sulfur-containing material, a typical soluble species
is an inorganic polysulfide, such as lithium octasulfide, and a
typical suitable solvent is an ether, such as tetraethylene glycol
dimethyl ether or tetraglyme. One method for characterizing the
efficiency of the non-electroactive particulate material in the
practice of this invention is by adding a known amount of the
non-electroactive particulate material to a solution of the
dissolved species of the electroactive material in a solvent at the
temperature desired, typically room temperature; allowing the
non-electroactive material to adsorb the dissolved species until
equilibrium is reached; separating the non-electroactive material
from the solution; and measuring the amount of the dissolved
species adsorbed by the non-electroactive material and the amount
remaining dissolved in the solvent.
[0046] The greater the percentage of dissolved species that are
adsorbed by the non-electroactive particulate material, the
stronger the adsorption of the dissolved or soluble species by the
non-electroactive particulate material, and thus the more effective
is the non-electroactive particulate material in retarding the
out-diffusion and transport of these soluble electroactive
materials from the cathode into the electrolyte and other parts of
the electric current producing cell.
[0047] For the solid composite cathodes of the present invention
comprising electroactive sulfur-containing materials, suitable
non-electroactive particulate materials are those non-electroactive
particulate materials having a strong adsorption of soluble
polysulfides. To differentiate these non-electroactive particulate
materials of the present invention from those non-electroactive
particulate materials not having a strong adsorption of soluble
polysulfides and thus not a part of the present invention, an
adsorption test procedure was used to characterize the relative
adsorptive strength or affinity for soluble polysulfides. To
characterize the capability of non-electroactive particulate
materials to effectively retard the diffusion of polysulfides and
related anionic reduction products of sulfur-containing cathodes
from the cathode to other parts of the electric current producing
cell, the adsorption affinity of the non-electroactive particulate
material for these anionic reduction products was evaluated by a
physical adsorption experiment using lithium octasulfide as a
representative adsorbate for a polysulfide. The more strongly the
lithium octasulfide is adsorbed by the non-electroactive
particulate material, the more effective the non-electroactive
particulate material will be in retarding the out-diffusion and
transport of lithium octasulfide and related anionic reduction
products to outside of the solid composite cathode.
[0048] For example, a conductive carbon particulate material widely
reported in experiments with sulfur-containing cathodes is SAB-50
(a tradename for Shawingan Acetylene Black, a conductive carbon
pigment available from Chevron Corporation, Baytown, Tex.). The
physical adsorption experiment of mixing 0.5 g of SAB-50 carbon
with a 10 ml solution of 0.03 M Li.sub.2S.sub.8 (lithium
octasulfide) in tetraglyme, showed only a 14% adsorption of the
lithium octasulfide. 86% of the lithium octasulfide remained in the
tetraglyme solution. Similarly, another conductive carbon
particulate material used in sulfur-containing cathodes is VULCAN
XE72R (a tradename for a carbon available from Cabot Corporation,
Billerica, Mass.). By the same physical adsorption test, the XE72R
carbon only adsorbed 31% of the lithium octasulfide. By contrast,
three conductive carbon particulate materials of the present
invention (PRINTEX XE; BLACK PEARL 2000, a tradename for carbon
available from Cabot Corporation, Billerica, Mass.; and FW200, a
tradename for carbon available from Degussa Corporation, Arkon,
Ohio) showed adsorption of 50%, 65%, and 95%, respectively, of the
lithium octasulfide using this same test procedure for physical
adsorption.
[0049] The non-electroactive particulate materials of the solid
composite cathodes of the present invention comprise those
non-electroactive particulate materials having a strong adsorption
of soluble polysulfides, wherein this strong adsorption by the
particulate material is characterized by the adsorption of at least
40% of the lithium octasulfide in a 0.03 M solution of lithium
octasulfide in tetraglyme with the particulate material present at
the weight ratio of the particulate material to lithium octasulfide
of 6.2 to 1. In one embodiment, the adsorption by said particulate
particle of the lithium octasulfide in said solution is at least
60%. In one embodiment, the adsorption by said particulate material
of the lithium octasulfide in said solution is at least 87%. In one
embodiment, the adsorption by said particulate material in said
solution is at least 93%. In one embodiment, the adsorption by said
particulate material of the lithium octasulfide in said solution is
at least 97%.
[0050] In one embodiment, the non-electroactive particulate
material having strong adsorption of soluble polysulfides of the
solid composite cathodes of the present invention is selected from
the group consisting of: carbons, silicas, aluminum oxides,
transition metal chalcogenides, and metals; wherein this strong
adsorption by the particulate material is characterized by the
adsorption of at least 40% of the lithium octasulfide in a 0.03 M
solution of lithium octasulfide in tetraglyme with the particulate
material present at the weight ratio of the particulate material to
lithium octasulfide of 6.2 to 1.
[0051] In one embodiment, the non-electroactive particulate
material having strong adsorption of soluble polysulfides comprises
a carbon. Suitable particulate carbons in the present invention are
those which adsorb at least 40% of the lithium octasulfide in a
0.03 M solution of lithium octasulfide in tetraglyme with the
carbon present at the weight ratio of the carbon to lithium
octasulfide of 6.2 to 1, and include, but are not limited to,
carbon aerogel (available from, for example, GenCorp Aerojet,
Sacramento, Calif., or Ocellus, Inc., San Carlos, Calif.), PRINTEX
XE-2, BLACK PEARL 2000, and FW200. Preferred particulate carbons of
the present invention are those which adsorb at least 60% of the
lithium octasulfide in a 0.03 M solution of lithium octasulfide in
tetraglyme with the carbon present at the weight ratio of the
carbon to lithium octasulfide of 6.2 to 1, and include, but are not
limited to, BLACK PEARL 2000 and FW200. Particularly preferred
particulate carbons of the present invention are those which adsorb
at least 87% of the lithium octasulfide in a 0.03 M solution of the
lithium octasulfide in tetraglyme with the carbon present at the
weight ratio of the carbon to lithium octasulfide of 6.2 to 1, and
include, but are not limited to, FW200. More particularly preferred
particulate carbons of the present invention are those which adsorb
at least 93% of the lithium octasulfide in a 0.03 M solution of the
lithium octasulfide in tetraglyme with the carbon present at the
weight ratio of the carbon to lithium octasulfide of 6.2 to 1, and
include, but are not limited to, FW200. Most particularly preferred
particulate carbons of the present invention are those which adsorb
at least 97% of the lithium octasulfide in a 0.03 M solution of the
lithium octasulfide in tetraglyme with the carbon present at the
weight ratio of the carbon to lithium octasulfide of 6.2 to 1.
[0052] Particulate carbons that are not suitable for strong
adsorption of soluble polysulfides in the solid composite cathodes
of the present invention are those which adsorb less than 40% of
the lithium octasulfide in a 0.03 M solution of the lithium
octasulfide in tetraglyme with the carbon present at the weight
ratio of the carbon to lithium octasulfide of 6.2 to 1, and
include, but are not limited to, SAB-50, PRINTEX L (a tradename for
carbon available from Degussa Corporation, Arkon, Ohio), PRINTEX L6
(a tradename for carbon available from Degussa Corporation, Arkon,
Ohio), Monarch 700 (a tradename for carbon available from Cabot,
Billerica, Mass.), and XE72R.
[0053] In one embodiment, the non-electroactive particulate
material having said strong adsorption of soluble polysulfides
comprises a silica. Suitable particulate silicas in the present
invention are those which adsorb at least 40% of the lithium
octasulfide in a 0.03 M solution of lithium octasulfide in
tetraglyme with the silica present at the weight ratio of the
silica to lithium octasulfide of 6.2 to 1, and include, but are not
limited to, silica aerogel (available from, for example, GenCorp
Aerojet, Sacramento, Calif.), CABOSIL M5, AEROSIL 380, and CABOSIL
530 (a tradename for silica available from Cabot, Tuscola, Ill.).
Preferred particulate silicas of the present invention are those
which adsorb at least 60% of the lithium octasulfide in a 0.03 M
solution of lithium octasulfide in tetraglyme with the silica
present at the weight ratio of the silica to lithium octasulfide of
6.2 to 1, and include, but are not limited to, AEROSIL 380 and
CABOSIL 530. Particularly preferred particulate silicas of the
present invention are those which adsorb at least 87% of the
lithium octasulfide in a 0.03 M solution of the lithium octasulfide
in tetraglyme with the silica present at the weight ratio of the
silica to lithium octasulfide of 6.2 to 1, and include, but are not
limited to, CABOSIL 530. More particularly preferred particulate
silicas of the present invention are those which adsorb at least
93% of the lithium octasulfide in a 0.03 M solution of the lithium
octasulfide in tetraglyme with the silica present at the weight
ratio of the silica to lithium octasulfide of 6.2 to 1, and
include, but are not limited to, CABOSIL 530. Most particularly
preferred particulate silicas of the present invention are those
which adsorb at least 97% of the lithium octasulfide in a 0.03 M
solution of the lithium octasulfide in tetraglyme with the silica
present at the weight ratio of the silica to lithium octasulfide of
6.2 to 1.
[0054] Particulate silicas that are not suitable for strong
adsorption of soluble polysulfide in the solid composite cathodes
of the present invention are those which adsorb less than 40% of
the lithium octasulfide in a 0.03 M solution of the lithium
octasulfide in tetraglyme with the silica present at the weight
ratio of the silica to lithium octasulfide of 6.2 to 1, and
include, but are not limited to, CABOSIL TS720 (a tradename for
silica available from Cabot Corporation, Tuscola, Ill.) and CABOSIL
L90 (a tradename for silica available from Cabot Corporation,
Tuscola, Ill.).
[0055] In one embodiment, the non-electroactive particulate
material having strong adsorption of soluble polysulfides comprises
an aluminum oxide. Suitable aluminum oxides in the present
invention are those which adsorb at least 40% of the lithium
octasulfide in a 0.03 M solution of lithium octasulfide in
tetraglyme with the aluminum oxide present at the weight ratio of
the aluminum oxide to lithium octasulfide of 6.2 to 1, and
includes, but is not limited to, pseudo-boehmite.
[0056] The term "pseudo-boelunite," as used herein, pertains to
hydrated aluminum oxides having the chemical formula
Al.sub.2O.sub.3.H.sub.2O wherein x is in the range of from 1.0 to
1.5. Terms which are synonymous with "pseudo-boehmite," include
"boehmite," "AlOOH," and "hydrated alumina." The materials referred
to herein as "pseudo-boehmite" are distinct from other aluminum
oxides, as for example, anhydrous aluminas (Al.sub.2O.sub.3, such
as alpha-alumina and gamma-alumina) and hydrated aluminum oxides of
the formula Al.sub.2O.sub.3.H.sub.2O wherein x is less than 1.0 or
greater than 1.5.
[0057] Preferred particulate aluminum oxides of the present
invention are those which adsorb at least 60% of the lithium
octasulfide in a 0.03 M solution of lithium octasulfide in
tetraglyme with the aluminum oxide present at the weight ratio of
the aluminum oxide to lithium octasulfide of 6.2 to 1, and include,
but are not limited to, psuedo-boehmite. Particularly preferred
particulate aluminum oxides of the present invention are those
which adsorb at least 87% of the lithium octasulfide in a 0.03 M
solution of lithium octasulfide in tetraglyme with the aluminum
oxide present at a weight ratio of the aluminum oxide to lithium
octasulfide of 6.2 to 1, and include, but are not limited to,
pseudo-boehmite. More particularly preferred particulate aluminum
oxides of the present invention are those which adsorb at least
93%, and most particularly preferably at least 97%, of the lithium
octasulfide in a 0.03 M solution of lithium octasulfide in
tetraglyme with the aluminum oxide present at the weight ratio of
the aluminum oxide to lithium octasulfide of 6.2 to 1.
[0058] Particulate aluminum oxides that are not suitable for strong
adsorption of soluble polysulfides in the solid composite cathodes
of the present invention are those which adsorb less than 40% of
the lithium octasulfide in a 0.03 M solution of lithium octasulfide
in tetraglyme with the aluminum oxide present at the weight ratio
of the aluminum oxide to lithium octasulfide of 6.2 to 1.
[0059] In one embodiment, the non-electroactive particulate
material having strong adsorption of soluble polysulfides comprises
a non-electroactive transition metal chalcogenide. As used herein,
the term "non-electroactive transition metal chalcogenide" means a
non-electroactive material, wherein the transition metal is at
least one selected from the group consisting of: Ti, V, Cr, Mn, Fe,
Nb, Mo, Ta, W, Co, Ni, Cu, Y, Zr, Ru, Rh, Pd, Hf, Re, Os, and Ir;
and the chalogenide is at least one selected from the group
consisting of: O, S, and Se. Suitable particulate transition metal
chalcogenides in the present invention are those which adsorb at
least 40% of the lithium octasulfide in a 0.03 M solution of
lithium octasulfide in tetraglyme with the transition metal
chalcogenide present at the weight ratio of the chalcogenide to
lithium octasulfide of 6.2 to 1, and include, but are not limited
to, crystalline vanadium oxide aerogel. Preferred particulate
transition metal chalcogenides of the present invention are those
which adsorb at least 60% of the lithium octasulfide in a 0.03 M
solution of lithium octasulfide in tetraglyme with the transition
metal chalcogenide present at the weight ratio of the chalcogenide
to lithium octasulfide of 6.2 to 1, and include, but are not
limited to, crystalline vanadium oxide aerogel. Particularly
preferred particulate transition metal chalcogenides of the present
invention are those which adsorb at least 87% of the lithium
octasulfide in a 0.03 M solution of the lithium octasulfide in
tetraglyme with the transition metal chalcogenide present at the
weight ratio of the chalcogenide to lithium octasulfide of 6.2 to
1, and include, but are not limited to, crystalline vanadium oxide
aerogel. More particularly preferred particulate transition metal
chalcogenides of the present invention are those which adsorb at
least 93% of the lithium octasulfide in a 0.03 M solution of
lithium octasulfide in tetraglyme with the transition metal
chalcogenide present at the weight ratio of the chalcogenide to
lithium octasulfide of 6.2 to 1, and include, but are not limited
to, crystalline vanadium oxide aerogel. Most particularly preferred
particulate transition metal chalcogenides of the present invention
are those which adsorb at least 97% of the lithium octasulfide in a
0.03 M solution of lithium octasulfide in tetraglyme with the
transition metal chalcogenide present at the weight ratio of the
chalcogenide to lithium octasulfide of 6.2 to 1, and include, but
are not limited to, crystalline vanadium oxide aerogel.
[0060] Particulate transition metal chalcogenides that are not
suitable for the strong adsorption of soluble polysulfides in the
solid composite cathodes of the present invention are those which
adsorb less than 40% of the lithium octasulfide in a 0.03 M
solution of the lithium octasulfide in tetraglyme with the
transition metal chalcogenide present at the weight ratio of the
chalcogenide to lithium octasulfide of 6.2 to 1.
[0061] In one embodiment, the non-electroactive particulate
material having strong adsorption of soluble polysulfides comprises
a metal. The particulate material comprising a metal could be
essentially a pure metal or alloy of metals or, optionally, a metal
deposited on the surface of another material, such as, for example,
palladium on carbon. Suitable particulate metals in the present
invention are those which adsorb at least 40% of the lithium
octasulfide in a 0.03 M solution of lithium octasulfide in
tetraglyme with the metal present at the weight ratio of metal to
lithium octasulfide of 6.2 to 1, and include, but are not limited
to, palladium, copper, nickel, silver, iron, cobalt, manganese,
chromium, platinum, and gold. Preferred particulate metals of the
present invention are those which adsorb at least 60%, and
particularly preferably at least 87%, of the lithium octasulfide in
a 0.03 M solution of lithium octasulfide in tetraglyme with the
metal present at the weight ratio of the metal to lithium
octasulfide of 6.2 to 1. More particularly preferred particulate
metals of the present invention are those which adsorb at least
93%, and most particularly preferably at least 97%, of the lithium
octasulfide in a 0.03 M solution of lithium octasulfide in
tetraglyme with the metal present at the weight ratio of the metal
to lithium octasulfide of 6.2 to 1.
[0062] Particulate metals that are not suitable for the strong
adsorption of soluble polysulfides in the solid composite cathodes
of the present invention are those which adsorb less than 40% of
the lithium octasulfide in a 0.03 M solution of lithium octasulfide
in tetraglyme with the metal present at the weight ratio of the
metal to lithium octasulfide of 6.2 to 1.
[0063] Electroactive Sulfur-Containing Cathode Materials
[0064] One aspect of the present invention pertains to a solid
composite cathode for use in an electric current producing cell
comprising (a) an electroactive sulfur-containing cathode material,
which material, in its oxidized state, comprises a polysulfide
moiety of the formula, --S.sub.m--, wherein m is an integer from 3
to 10, and (b) a non-electroactive particulate material having a
strong adsorption of soluble polysulfides, as described herein.
[0065] The term "sulfur-containing cathode material," as used
herein, relates to cathode active materials which comprise the
elemental sulfur in any form, wherein the electrochemical activity
involves the breaking or forming of sulfur-sulfur covalent
bonds.
[0066] The nature of the electroactive sulfur-containing cathode
materials useful in the practice of this invention may vary widely.
The electroactive properties of elemental sulfur and of
sulfur-containing materials are well known in the art, and include
the reversible formation of lithiated or lithium ion sulfides
during the discharge or cathode reduction cycle of the battery
cell.
[0067] In one embodiment, the electroactive sulfur-containing
cathode material comprises elemental sulfur.
[0068] In one embodiment, the electroactive sulfur-containing
cathode material is organic, that is, it comprises both sulfur
atoms and carbon atoms.
[0069] In one embodiment, the electroactive sulfur-containing
cathode material is polymeric. In one embodiment, the polymeric
electroactive sulfur-containing cathode material comprises a
carbon-sulfur polymer, and the polysulfide moiety, --S.sub.m--, is
covalently bonded by one or both of its terminal sulfur atoms on a
side group to the polymer backbone chain of said carbon-sulfur
polymer material. In one embodiment, the polymeric electroactive
sulfur-containing cathode material comprises a carbon-sulfur
polymer, and the polysulfide moiety, --S.sub.m--, is incorporated
into the polymer backbone chain of said carbon-sulfur polymer by
covalent bonding of said polysulfide moiety's terminal sulfur
atoms.
[0070] Examples of polymeric electroactive sulfur-containing
materials include, but are not limited to, those comprising one or
more carbon-sulfur compounds of formulae (CS.sub.x).sub.n and
(C.sub.2S.sub.z).sub.n. Compositions of the general formula
--(CS.sub.x).sub.n-- (formula I), wherein x ranges from 1.2 to 2.3,
and n is an integer equal to or greater than 2, are described in
U.S. Pat. No. 5,441,831 to Okamoto et al. Additional examples
include those wherein x ranges from greater than 2.3 to about 50,
and n is equal to or greater than 2, as described in U.S. Pat. Nos.
5,601,947 and 5,690,702 to Skotheim et al.
[0071] Further examples of polymeric electroactive
sulfur-containing materials are those of the general formula
--(C.sub.2S.sub.z).sub.n-- (formula II) wherein z ranges from
greater than 1 to about 100, and n is equal to or greater than 2,
as described in U.S. Pat. No. 5,529,860 and copending U.S. patent
application Ser. No. 08/602,323 to Skotheim et al.
[0072] The preferred materials of general formulae I and II, in
their oxidized state, comprise a polysulfide moiety of the formula,
--S.sub.m--, wherein m is an integer equal to or greater than 3, or
more preferably, wherein m is an integer from 3 to 10. In one
embodiment, m is an integer from 3 to 8. In one embodiment, m is an
integer from 3 to 6. In one embodiment, m is an integer from 6 to
10. In one embodiment, the polysulfide linkage comprises
--S--S--S-- (i.e., trisulfide). In one embodiment, the polysulfide
linkage comprises --S--S--S--S-- (i.e., tetrasulfide). In one
embodiment, the polysulfide linkage comprises --S--S--S--S--S--
(i.e., pentasulfide). In one embodiment, the polysulfide linkage
comprises --S--S--S--S--S--S-- (i.e., hexasulfide). In one
embodiment, the polysulfide linkage comprises
--S--S--S--S--S--S--S-- (i.e., heptasulfide). In one embodiment,
the polysulfide linkage comprises --S--S--S--S--S--S--S--S-- (i.e.,
octasulfide).
[0073] The backbone of polymeric electroactive sulfur-containing
materials may comprise --S.sub.m-- main chain linkages as well as
covalently bound --S.sub.m-- side groups. Owing to the presence of
multiple linked sulfur atoms, --S.sub.m--, where m is an integer
equal to or greater than 3, in these materials, they possess
significantly higher energy densities or specific capacities than
corresponding materials containing the disulfide linkage, --S--S--,
alone.
[0074] Other preferred polymeric electroactive sulfur-containing
materials are those comprising carbocyclic repeat groups, as
described in copending U.S. Pat. Application titled,
"Electroactive, Energy-Storing, Highly-Crosslinked,
Polysulfide-Containing Organic Polymers for Use in Electrochemical
Cells," filed on even date herewith.
[0075] Polymeric electroactive sulfur-containing materials of the
present invention typically have elemental compositions containing
between about 50 weight percent and 98 weight percent sulfur.
Preferred polymeric electroactive sulfur-containing materials have
greater than 75 weight percent sulfur. Particularly preferred
polymeric electroactive sulfur-containing materials have greater
than 86 weight percent sulfur, and most particularly preferred are
polymeric electroactive sulfur-containing materials with greater
than 90 weight percent sulfur.
[0076] With carbon-sulfur polymer materials in solid composite
cathodes, polysulfides are formed during discharge. The term
"polysulfides," means as used herein, S-containing materials with
two or more S.sup.- groups present. The disulfides of organo-sulfur
materials form polysulfides (RS.sup.-, where R is the organo-sulfur
moiety to which two or more S.sup.- groups are attached) during
reduction or discharge. Since the carbon-sulfur polymer materials
contain large amounts of (--S.sub.m--) groups where m is an integer
from 3 to 10, they form organic polysulfides of the general formula
(R'S.sub.x.sup.-) during reduction or discharge, where x is 2 or
greater and R' is the carbon-sulfur moiety to which the polysulfide
group is attached. These polysulfides are typically insoluble
because of their attachment to the polymer backbone, but upon
continued discharge, they are further reduced to form some soluble
organic polysulfides and inorganic polysulfides of the formula
(S.sub.x.sup.2-), where x is 2 or greater.
[0077] Even though the carbon-sulfur polymer materials show
improvements over organo-sulfur materials as cathode active
materials because of lower amounts of soluble sulfides and because
of much higher specific capacities from the multiple polysulfide
linkages and the typically higher weight percent of sulfur (of over
50 percent, and often above 85 percent of sulfur by weight), there
is typically still some formation of soluble organic polysulfides
and inorganic polysulfides during the electrochemical cycling of
the carbon-sulfur polymer materials. The non-electroactive
particulate materials of the present invention have a strong
adsorption of soluble polysulfides and retard their out-diffusion
from the solid composite cathode and thereby enhance their
availability during charging to regenerate a carbon-sulfur polymer
material and to improve the reversible capacity and self-discharge
properties.
[0078] Solid Composite Cathodes
[0079] One aspect of the present invention pertains to solid
composite cathodes for use in an electric current producing cell,
which composite cathodes comprise (a) an electroactive
sulfur-containing cathode material, which material, in its oxidized
state, comprises a polysulfide moiety of the formula, --S.sub.m--,
wherein m is an integer from 3 to 10, and (b) a non-electroactive
particulate material having a strong adsorption of soluble
polysulfides, as described herein.
[0080] In one embodiment, the solid composite cathode is fabricated
from a mixture comprising the electroactive sulfur-containing
cathode material and the non-electroactive particulate material
having a strong adsorption of soluble polysulfides, which mixture
is deposited onto a substrate. Optionally, the mixture may further
comprise conductive additives, polymeric binders, electrolytes, and
other additives to further improve the electrochemical
recycleability and capacity of the cells.
[0081] In one embodiment, the solid composite cathodes of this
invention further comprise a non-electroactive metal oxide, which
is added to the cathode coating layer to further improve the access
to the electrolyte during the filling process and during the
cycling of the cell. This is especially beneficial in increasing
the energy density and capacity above that achieved with only the
electroactive sulfur-containing material (e.g., elemental sulfur
and carbon-sulfur polymer materials) and the non-electroactive
particulate material having a strong adsorption of soluble
polysulfides of the present invention. Examples of these
non-electroactive metal oxides include silicas, aluminum oxides,
silicates, and titanium oxides which adsorb less than 40% of the
lithium octasulfide in a 0.03 M solution of lithium octasulfide in
tetraglyme with the metal oxide present at the weight ratio of the
metal oxide to lithium octasulfide of 6.2 to 1, and include, but
are not limited to, CABOSIL TS720 and CABOSIL L90.
[0082] The relative amounts of electroactive sulfur-containing
material, non-electroactive material having a strong adsorption of
soluble polysulfides, and optional components such as conductive
additives, polymeric binders, electrolytes, non-electroactive metal
oxides, and other additives in the solid composite cathode may vary
widely. Generally these relative amounts are determined by
experimentation and chosen so as to optimize the amount of
electroactive cathode material present, the energy storage capacity
of the cathode, and the electrochemical performance of the solid
composite cathode in an electric current producing cell. Typically,
the amount of electroactive sulfur-containing material used in the
solid composite cathode of the present invention will vary from
about 50 weight percent to 96 weight percent. Preferred are solid
composite cathodes comprising between 60 weight percent and 96
weight percent sulfur-containing material. Especially preferred are
those containing greater than 80 weight percent of
sulfur-containing material.
[0083] The relative amounts of sulfur-containing cathode active
material and non-electroactive particulate material having a strong
adsorption of soluble polysulfides in the solid composite cathode
may vary widely so long as sufficient strongly adsorbing
particulate material is present to effectively provide the
adsorption of soluble polysulfides for efficient utilization and
cycling of the sulfur-containing cathode active material consistent
with the volumetric density requirements for loading of cathode
active material in the cell. Typically, the amount of strongly
adsorbing non-electroactive particulate materials used in the solid
composite cathodes will vary from about 5 weight percent to about
100 weight percent of the weight of sulfur-containing cathode
active material in the cathode coating layer. Preferred solid
composite cathodes are those which comprise between 5 weight
percent and 50 weight percent of strongly adsorbing particulate
materials based on the weight of sulfur-containing cathode active
material. Most preferred solid composite cathodes comprise between
10 weight percent and 25 weight percent of strongly adsorbing
particulate materials based on the weight of the sulfur-containing
cathode active material.
[0084] The solid composite cathodes of the present invention may
further comprise one or more materials selected from the group of
conductive additives, polymeric binders, electrolytes, and other
additives, usually to improve or simplify their fabrication as well
as improve their electrical and electrochemical
characteristics.
[0085] Useful conductive additives are those conductive materials
that provide electrical connectivity to the majority of the
electroactive materials in the solid composite cathode. Examples of
useful conductive additives include, but are not limited to,
conductive carbons (e.g., carbon blacks), graphites, metal flakes,
metal powders, and electrically conductive polymers. Where these
useful conductive additives are particulate materials, the useful
particulate conductive additives in the present invention are those
which adsorb less than 40% of the lithium octasulfide in a 0.03 M
solution of lithium octasulfide in tetraglyme with the conductive
additive present at the weight ratio of the conductive additive to
lithium octasulfide of 6.2 to 1, and include, but are not limited
to, SAB-50, PRINTEX L, PRINTEX L6, M700, and XE72R.
[0086] Further useful conductive additives in the composite cathode
of the present invention are non-activated carbon nanofibers, as
described in present applicant's copending U.S. Pat. Application
titled "Electrochemical Cells with Carbon Nanofibers and
Electroactive Sulfur Compounds", filed on even date herewith.
[0087] The choice of polymeric binder material may vary greatly so
long as it is inert with respect to the solid composite cathode
active materials. Useful binders are those materials that allow for
ease of processing of battery electrode composites and are
generally known to those skilled in the art of electrode
fabrication. Examples of useful binders include, but are not
limited to, organic polymers such as polytetrafluoroethylenes,
polyvinylidene fluorides, ethylene-propylene-diene (EPDM) rubbers,
polyethylene oxides (PEO), UV curable acrylates, UV curable
methacrylates, and heat curable divinyl ethers. Examples of other
useful binders are cationic polymers with quaternary ammonium salt
groups, as described in applicant's copending U.S. Pat. Application
titled "Electrochemical Cells with Cationic Polymers and
Electroactive Sulfur Compounds", filed on even date herewith.
[0088] Examples of useful electrolytes include, but are not limited
to, liquid, solid, or solid-like materials capable of storing and
transporting ions, so long as the electrolyte material is stable
electrochemically and chemically with respect to the anode and
composite cathode materials, facilitates the transport of ions
between the anode and the cathode, and is electronically
non-conductive to prevent short circuiting between the anode and
the cathode.
[0089] In those cases where polymeric binder and conductive
additive are desired, the amounts of binder and conductive additive
can vary widely and the amounts present will depend on the desired
performance. Typically, when binders and conductive additives are
used, the amount of binder will vary greatly, but will generally be
less than about 15 weight percent of the solid composite cathode.
Preferred amounts are less than 10 weight percent. The amount of
conductive additive used will also vary greatly and will typically
be less than 15 weight percent of the solid composite cathode.
Preferred amounts of conductive additives are less than 12 weight
percent. Where the strongly adsorbing non-electroactive particulate
material of the present invention is electrically conductive, as
for example, PRINTEX XE-2 carbon particles, and is present in the
solid composite cathode, the amounts of conductive additives may be
zero or considerably reduced from their typical levels.
[0090] The solid composite cathodes of the present invention may
also further comprise a current collector. Suitable current
collectors for use in the present invention are those known in the
art for solid electroactive sulfur-containing cathodes. Examples of
suitable current collectors include, but are not limited to, metal
films, foils, nets, and expanded metal grids made from metals such
as nickel, titanium, aluminum, tin, and stainless steel, and
plastic films with conductive layers comprising metals such as
aluminum, stainless steel, nickel, titanium, and tin. Such metallic
current collectors may optionally have a layer comprising
conductive carbon or graphite coated on the metallic layer.
[0091] Methods of Making Composite Cathodes
[0092] One aspect of the present invention pertains to methods for
fabricating solid composite cathodes, as described herein.
[0093] One method employs a physical mixture of an electroactive
sulfur-containing cathode material, a non-electroactive particulate
material having a strong adsorption of soluble polysulfides, and
optionally polymeric binders, conductive additives, electrolytes,
non-electroactive metal oxides, and other additives, either as dry
solids, or as a slurry in a solvent or mixture of solvents. The
mixture is fabricated into a solid cathode structure of desired
dimensions, for example, by casting, doctor blade coating, roll
coating, dip coating, extrusion coating, calendering, and other
means known in the art.
[0094] Mixing of the various components can be accomplished using
any of a variety of methods so long as the desired dissolution or
dispersion of the components is obtained. Suitable methods of
mixing include, but are not limited to, mechanical agitation,
grinding, ultrasonication, ball milling, sand milling, and
impingement milling.
[0095] The formulated dispersions can be applied to supports by any
of a variety of well-known coating methods and dried using
conventional techniques. Suitable hand coating techniques include,
but are not limited to, the use of a coating rod or gap coating
bar. Suitable machine coating methods include, but are not limited
to, the use of roller coating, gravure coating, slot extrusion
coating, curtain coating, and bead coating. Removal of some or all
of the liquid from the mixture can be accomplished by any of a
variety of conventional means. Examples of suitable methods for the
removal of liquid from the mixture include, but are not limited to,
hot air convection, heat, infrared radiation, flowing gases,
vacuum, reduced pressure, extraction, and by simply air drying if
convenient.
[0096] Once formed, the solid composite cathode may optionally be
calendered to provide a solid composite cathode with a desired
thickness, porosity, and volumetric density of electroactive
material.
[0097] Thus, in one embodiment, the present invention pertains to a
method for the preparation of a solid composite cathode, said
method comprising the steps of:
[0098] (a) dispersing or suspending, in a liquid medium, an
electroactive sulfur-containing cathode material, as described
herein, and a non-electroactive particulate material having a
strong adsorption of soluble polysulfides, as described herein;
[0099] (b) casting the mixture formed in step (a) onto a substrate
or placing the mixture formed in step (a) into a mold; and,
[0100] (c) removing some or all of the liquid medium from the
mixture of step (b) to form a solid or gel-like composite cathode
in the shape or form desired.
[0101] Examples of liquid media suitable for use in the methods of
the present invention include, but are not limited to, aqueous
liquids, non-aqueous liquids, and mixtures thereof. Preferred
liquids are non-aqueous liquids such as methanol, ethanol,
isopropanol, 1-propanol, butanol, tetrahydrofuran, dimethoxyethane,
acetone, toluene, xylene, acetonitrile, heptane, and
cyclohexane.
[0102] Optionally, polymeric binders, conductive additives,
electrolytes, non-electroactive metal oxides, and other additives
may be added to the mixture at one or more of the various steps in
the methods, usually at steps which involve dissolving, dispersing,
or mixing. Such additives often facilitate adhesion, cohesion,
current collection, and ion transport.
[0103] Another method of making the solid composite cathodes of
this invention incorporates a solid composite cathode comprised of
particulate sulfur-containing materials, generally less than 25
microns in diameter, individually coated with an encapsulating
layer comprising the non-electroactive particulate material of the
present invention. A solid composite cathode fabricated from such a
"core-shell" configuration of materials is shown in FIG. 1. Here,
the solid composite cathode layer 1 in contact with a current
collector 2 comprises particles of the composite cathode. Each
composite cathode particle is comprised of a core 3 of the
sulfur-containing cathode active material with an outer shell 4 of
a retarding barrier layer comprising the non-electroactive
particulate material of this invention. Optionally, such a solid
composite cathode may contain fillers 5 comprising conductive
materials, binders, and other additives, as described herein.
[0104] FIG. 2 illustrates a solid composite cathode structure 1 in
contact with a current collector 2 wherein the solid composite
cathode was made by the method of dispersing the sulfur-containing
cathode materials 6 in a liquid medium also comprising the
non-electroactive particulate materials of the present invention,
and optionally binders, conductive materials, and other additives
as described herein, and coating and drying the medium, as
described herein, to form a matrix of the sulfur-containing cathode
materials 6 dispersed in a phase 7 comprising the non-electroactive
particulate materials, and optionally other additives. The phase 7
retards the transport of soluble polysulfides from the solid
composite cathode to the electrolyte or other layers or parts of
the electric current producing cell.
[0105] Another method of making the solid composite cathode of this
invention is one where a coating comprising the sulfur-containing
cathode material is encapsulated or impregnated by a thin coherent
film coating comprising the non-electroactive particulate material
of the present invention, as shown in FIG. 3. Here, the
sulfur-containing cathode structure 8 in contact with the current
collector 2 is effectively encapsulated with a layer 9 comprising
the non-electroactive particulate material. Both structure 8 and
layer 9 may optionally comprise the binders, conductive materials,
and other additives of the present invention, as described
herein.
[0106] Another method useful in this invention relates to the
fabrication of a solid composite cathode by a sol-gel method
wherein the sulfur-containing cathode active material, and
optionally conductive fillers and binders, are suspended or
dispersed in a liquid medium containing a colloidal sol of the
non-electroactive particulate material of the present invention,
for example, a boehmite sol or a crystalline vanadium oxide sol.
From the sol, during the drying process of the coating, a sol-gel
or gel is formed from an inorganic polymerization reaction that
results in an interconnected, rigid network, typically having
sub-micron pores.
[0107] These sol-gel methods may be used to provide solid composite
cathodes in at least two different configurations. One relates to a
configuration in which particulate sulfur-containing cathode active
material is encapsulated with a layer comprising the
non-electroactive particulate sol-gel material of the present
invention. The other relates to a composite structure in which the
sulfur-containing cathode active material is embedded in a
continuos network comprising the non-electroactive particulate
sol-gel material of the present invention.
[0108] Rechargeable Battery Cells and Methods of Making Same
[0109] One aspect of the present invention pertains to an electric
current producing cell which comprises:
[0110] (a) an anode;
[0111] (b) a solid composite cathode, as described herein; and,
[0112] (c) an electrolyte interposed between said anode and said
cathode.
[0113] Another aspect of the present invention pertains to a method
of forming an electric current producing cell, which method
comprises the steps of:
[0114] (a) providing an anode;
[0115] (b) providing a solid composite cathode, as described
herein; and,
[0116] (c) enclosing an electrolyte between said anode and said
cathode.
[0117] In one embodiment, the electric current producing cell is a
secondary (rechargeable) electric current producing cell.
[0118] Suitable anode active materials for the electrochemical
cells of the present invention include, but are not limited to, one
or more metals or metal alloys or a mixture of one or more metals
and one or more alloys, wherein said metals are selected from the
Group IA and IIA metals in the Periodic Table. Examples of suitable
anode active materials include, but are not limited to,
alkali-metal intercalated conductive polymers, such as lithium
doped polyacetylenes, polyphenylenes, polypyrroles, and the like,
and alkali-metal intercalated graphites and carbons. Anode active
materials comprising lithium are particularly useful for the anode
of the cells of the present invention. Preferred anode materials
are lithium metal, lithium-aluminum alloys, lithium-tin alloys,
lithium-intercalated carbons, and lithium-intercalated
graphites.
[0119] The electrolytes used in battery cells function as a medium
for storage and transport of ions and, in the special case of solid
electrolytes, these materials may additionally function as
separator materials between the anode and the cathode. Any liquid,
solid, or solid-like material capable of storing and transporting
ions may be used, so long as the material is electrochemically and
chemically inert with respect to the anode and the cathode, the
material facilitates the transport of ions between the anode and
the cathode, and the material is electronically non-conducting to
prevent short circuiting between the anode and the cathode.
[0120] Examples of suitable electrolytes for use in the present
invention include, but are not limited to, organic electrolytes
comprising one or more materials selected from the group consisting
of: liquid electrolytes, gel polymer electrolytes, and solid
polymer electrolytes.
[0121] Examples of useful liquid electrolyte solvents include, but
are not limited to, N-methyl acetamide, acetonitrile, carbonates,
sulfones, sulfolanes, glymes, siloxanes, dioxolanes, N-alkyl
pyrrolidones, substituted forms of the foregoing, and blends
thereof.
[0122] These liquid electrolyte solvents are themselves useful as
gel forming (plasticizing) agents for gel-polymer electrolytes.
Further examples of useful gel-polymer electrolytes include, but
are not limited to, those comprising polymers selected from the
group consisting of: polyethylene oxides (PEO), polypropylene
oxides, polyacrylonitriles, polysiloxanes, polyimides, polyethers,
sulfonated polyimides, perfluorinated membranes (Nafion.TM.
resins), divinyl polyethylene glycols, polyethylene
glycol-bis-(methyl acrylates), polyethylene glycol-bis-(methyl
methacrylates), derivatives of the foregoing, copolymers of the
foregoing, crosslinked and network structures of the foregoing, and
blends of the foregoing; to which is added an appropriate
electrolyte salt.
[0123] Examples of useful solid polymer electrolytes include, but
are not limited to, those comprising polymers selected from the
group consisting of: polyethers, polyethylene oxides (PEO),
polypropylene oxides, polyimides, polyphosphazenes,
polyacrylonitriles (PAN), polysiloxanes, derivatives of the
foregoing, copolymers of the foregoing, crosslinked and network
structures of the foregoing, and blends of the foregoing; to which
is added an appropriate electrolyte salt. Ionically conductive
solid polymer electrolytes may additionally function as separator
materials between the anode and the cathode.
[0124] In addition to solvents, gelling agents and ionically
conductive polymers as known in the art for organic electrolytes,
the organic electrolyte further comprises one or more ionic
electrolyte salts, also as known in the art, to increase the ionic
conductivity.
[0125] Examples of ionic electrolyte salts for use in the present
invention include, but are not limited to, MClO.sub.4, MAsF.sub.6,
MSO.sub.3CF.sub.3, MSO.sub.3CH.sub.3, MBF.sub.4, MB(Ph).sub.4,
MPF.sub.6, 1
[0126] and the like, where M is Li or Na. Other electrolyte salts
useful in the practice of this invention are disclosed in U.S. Pat.
No. 5,538,812 to Lee et al. Preferred ionic electrolyte salts are
LiSO.sub.3CF.sub.3 (lithium triflate) and
LiN(SO.sub.2CF.sub.3).sub.2 (lithium imide).
EXAMPLES
[0127] Several embodiments of the present invention are described
in the following examples, which are offered by way of illustration
and not by way of limitation.
Example 1
[0128] The following general procedure was used to determine the
relative adsorptive strength of various non-electroactive
particulate materials for soluble lithium polysulfide solvates such
as Li.sub.2S.sub.8 (lithium octasulfide). The non-electroactive
particulate material to be evaluated was dried under vacuum at
80.degree. C. for 18 hours. To 10 ml of a 30 mM (0.03 M) solution
of lithium octasulfide in dry tetraglyme at 25.degree. C. in an
argon filled glove box was added 0.5 g of the non-electroactive
particulate material. This dispersion was stirred for 18 hours. It
was then transferred to a sealed centrifuge tube and centrifuged
for 30 minutes at 4000 rpm under argon. The supernatant liquid was
filtered through a 0.45 micron syringe filter and analyzed by
absorption spectroscopy using the absorbance band at 450 nm for
lithium octasulfide. The concentration of lithium octasulfide
remaining in the solvent was then calculated using a calibration
curve generated by using standardized solutions of lithium
octasulfide in tetraglyme. The amount of lithium octasulfide
adsorbed by the non-electroactive particulate material was then
calculated from the loss of lithium octasulfide from the solution.
Table I summarizes the adsorption strength of various carbons,
silicas, and other particulate materials for lithium
octasulfide.
1TABLE I The adsorption strengths of various carbons, silicas and
other particulate materials for lithium octasulfide. Final
Li.sub.2S.sub.8 Wt of Surface Concen- Li.sub.2S.sub.8 Area.sup.f
tration Adsorbed % Wt % Li.sub.2S.sub.8 Sample m.sup.2/g mM mg/g
Uptake Adsorbed FW 200.sup.a 460 1.73 152.8 15.3 94.2 Cabosil
M5.sup.b 100 12.88 92.7 9.3 57.1 Cabosil 530.sup.b 200 2.06 151
15.1 93.1 Carbon 1000 15.1 80.6 8.1 49.7 Printex XE-2.sup.a Carbon
250 15.2 79.8 8 49.3 Aerogel.sup.c Carbon BP 1500 10.64 105 10.5
64.5 2000.sup.b Aerosil 380.sup.a 380 8.6 115.8 11.6 71.3 Pseudo-
15.sup.g 3.17 145.1 14.5 89.4 Boehmite.sup.d Cabosil L90.sup.b 90
19.8 55.1 5.5 34.0 Carbon 150 24.2 31.1 3 19.3 Printex L.sup.a
Carbon 84 25.72 23.2 2.3 14.3 SAB-50.sup.e Crystalline 25 0.33
160.5 16.1 98.9 Vanadium Oxide Aerogel.sup.h .sup.aDegussa
.sup.bCabot Corporation .sup.cGenCorp Aerojet .sup.dCatalysts &
Chemicals Ind. Co., Ltd, Tokyo, Japan; Cataloid AS-3 supplied as a
7 wt % colloidal boehmite sol in water and dried to a
pseudo-boehmite powder in-house. .sup.eChevron .sup.fAs supplied by
Manufacturer .sup.gMeasured .sup.hPrepared from vanadium
acetoacetonate and not electroactive after heat treatment at
140.degree. C
Example 2
[0129] A solid composite cathode comprising elemental sulfur and a
particulate carbon material was fabricated and evaluated in AA
cells in the following way. A cathode slurry formulation of 85 wt.
% elemental sulfur, 10 wt. % PRINTEX XE2, and 5 wt. % polyethylene
oxide (PEO) binder (5,000,000 molecular weight available from
Polysciences Inc., Warrington, Pa.), using acetonitrile as the
solvent, was prepared by conventional techniques. The slurry was
cast by hand coating using a gap coater bar onto a two side coated
18 micron thick conductive carbon coated aluminum foil substrate
(Product No. 60303 available from Rexam Graphics, South Hadley,
Mass.) as a current collector and dried in a laboratory hood with
exhaust of the ambient air to provide flowing air over the coating.
The coating and drying process was repeated for the second side of
the substrate. The total composite cathode thickness was 12 microns
with an electroactive sulfur loading of 1.05 mg/cm.sup.2. The
volumetric density of elemental sulfur in the cathode was about
1050 mg/cm.sup.3. The solid composite cathode was then wound into a
AA cell with a 50 micron lithium foil anode and a 25 micron E25
SETELA (a tradename for a polyolefin separator available from Tonen
Chemical corporation, Tokyo, Japan, and also available from Mobil
Chemical Company, Films Division, Pittsford, N.Y.) separator and
filled with a liquid electrolyte (50% 1,3-dioxolane, 35% diglyme,
10% dimethoxyethane, and 5% o-xylene by volume with 1.0 M lithium
triflate salt (available from 3M corporation, St. Paul, Minn.).
[0130] The first discharge-charge cycle was done at a current of
100 mA. Subsequent cycling was done with a discharge current of 275
mA and a charge current of 200 mA. FIG. 4 shows the volumetric
capacity in mA/cm.sup.3 of the cathode coating for this AA cell.
After the first cycle, this volumetric capacity was still very high
(above 500 mAh/cm.sup.3) and stable for more than 60 cycles.
Similar AA cells, except that either 10% SAB-50 carbon or 10%
VULCAN XE72R carbon was substituted for the 10% PRINTEX XE-2,
showed in both cases a volumetric capacity at the second
discharge-charge cycle that was more than 10% lower than that of
the AA cell with 10% PRINTEX XE-2 carbon and lost more than 15% of
this volumetric capacity when cycled to 60 cycles.
Example 3
[0131] Composite cathodes were fabricated from carbon-sulfur
polymer (made by the process described in Example 2 in copending
U.S. Pat. Application titled "Electroactive, Energy-Storing, Highly
Crosslinked, polysulfide-Containing Organic Polymers for use in
Electrochemical Cells," filed on even day herewith by common
assigneee). The polymer was first pre-ground to disperse any
clumping of polymer particles (typical mean size <10 microns). A
cathode slurry was prepared with a formulation of 70% carbon-sulfur
polymer, 10% conductive carbon pigment (PRINTEX XE-2), 5%
non-activated PYROGRAF-III carbon nanofibers (a tradename for
carbon nanofibers available from Applied Sciences, Inc.,
Cedarville, Ohio), 5% silica pigment (AEROSIL 380), and 10%
polyethylene oxide (PEO with a molecular weight of 5,000,000
available from Polysciences Inc., Warrington, Pa.) by weight, in a
mixed solvent of water and n-propanol (80:20 volume ratio) in a
ball mill jar containing ceramic cylinders. The solids content of
the slurry was 12 wt %. The mixture was ball milled for 20 hours.
The slurry was cast (hand drawn with a gap coater bar) onto both
sides of a 17.5 micron thick conductive carbon coated aluminum foil
(Product No. 60303, Rexam Graphics) as a current collector. The
coating was dried under ambient conditions overnight, and then
under vacuum at 60.degree. C. for one hour. The resulting dry
cathode coating had a thickness as about 20 to 25 microns on each
side of the current collector with a density or loading of
carbon-sulfur polymer in the range of 0.63 to 0.97 mg/cm.sup.2. The
volumetric density of the carbon-sulfur polymer in the solid
composite cathode layer was in the range of 319 to 385
mg/cm.sup.3.
[0132] Wound AA size cells were fabricated from these cathodes with
a 75 micron lithium foil anode and a 25 micron E25 SETELA
separator. The cell were filled with a liquid electrolyte (50%
1,3-dioxolane, 20% diglyme, 10% sulfolane, and 20% dimethoxyethane
by volume with 1.0 M lithium triflate salt). The cells were cycled
at a rate of charge and discharge of C/3 (0.2 mA/cm.sup.2) and C/2
(0.33 mA/cm.sup.2) respectively. Cell performance data at
25.degree. C. FIG. 5) showed that the carbon-sulfur polymer cathode
had excellent capacity and good stability, with specific capacities
of about 1000 mAh/g for the first 10 cycles and 700 mAh/g at the
100th cycle. The cells showed a low rate of capacity loss with
cycling with a value of about 0.29% per cycle.
[0133] Similar AA cells, except that 10% SAB-50 was substituted for
the 10% PRINTEX XE-2, showed specific capacities for the
carbon-sulfur polymer that were more than 30% lower at the 100th
cycle compared to the AA cells with 10% PRINTEX XE-2 present.
[0134] Similar AA cells with 10% PRINTEX XE-2, except that 5%
CABOSIL L90 was substituted for the 5% AEROSIL 380, showed specific
capacities for the carbon-sulfur polymer that were more than 15%
lower at the 100th cycle compared to the AA cells with 10% PRINTEX
XE-2 and 5% AEROSIL 380.
[0135] While the invention has been described in detail and with
reference to specific embodiments thereof, it will be apparent to
one skilled in the art that various changes and modifications can
be made without departing from the spirit and scope thereof.
* * * * *