U.S. patent application number 10/040651 was filed with the patent office on 2002-07-18 for separators for electrochemical cells.
Invention is credited to Carlson, Steven A., Deng, Zhongyi, Skotheim, Terje A., Ying, Qicong.
Application Number | 20020092155 10/040651 |
Document ID | / |
Family ID | 25541380 |
Filed Date | 2002-07-18 |
United States Patent
Application |
20020092155 |
Kind Code |
A1 |
Carlson, Steven A. ; et
al. |
July 18, 2002 |
Separators for electrochemical cells
Abstract
This invention pertains to separators for electrochemical cells
which comprise a microporous pseudo-boehmite layer; electrolyte
elements comprising such separators; electrical current producing
cells comprising such separators; and methods of making such
separators, electrolyte elements and cells.
Inventors: |
Carlson, Steven A.;
(Cambridge, MA) ; Ying, Qicong; (Tucson, AZ)
; Deng, Zhongyi; (Tucson, AZ) ; Skotheim, Terje
A.; (Tucson, AZ) |
Correspondence
Address: |
Jacqueline M. Nicol
Moltech Corporation
9062 South Rita Road
Tucson
AZ
85747-9108
US
|
Family ID: |
25541380 |
Appl. No.: |
10/040651 |
Filed: |
October 22, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10040651 |
Oct 22, 2001 |
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09215112 |
Dec 17, 1998 |
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09215112 |
Dec 17, 1998 |
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08995089 |
Dec 19, 1997 |
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Current U.S.
Class: |
29/623.5 ;
427/126.4; 429/247; 429/251 |
Current CPC
Class: |
H01M 10/052 20130101;
H01M 50/443 20210101; H01M 2300/0017 20130101; Y10T 29/49115
20150115; H01M 50/46 20210101; Y02P 70/50 20151101; H01M 50/431
20210101; H01M 10/0525 20130101; B01D 71/025 20130101; H01M
2300/0068 20130101; H01M 50/409 20210101; H01M 50/434 20210101;
H01M 4/136 20130101; H01M 10/0563 20130101; Y02E 60/10 20130101;
H01M 6/14 20130101; H01M 10/05 20130101; H01M 10/0565 20130101 |
Class at
Publication: |
29/623.5 ;
429/247; 429/251; 427/126.4 |
International
Class: |
H01M 010/04; H01M
002/16; B05D 005/12 |
Claims
1. A separator for an electric current producing cell, wherein said
separator comprises a microporous layer comprising pseudo-boehmite
and a binder.
2. The separator of claim 1, wherein said binder is present in an
amount of 3 to 200% of the weight of pseudo-boehmite in said
microporous layer.
3. The separator of claim 1, wherein said binder is present in an
amount of 5 to 70% of the weight of pseudo-boehmite in said
microporous layer.
4. The separator of claim 1, wherein said binder is an organic
binder.
5. The separator of claim 1, wherein said binder is selected from
the consisting of: polyvinyl alcohols, polyethylene oxides,
alkylated polyethylene oxides, polyvinyl pyrrolidones, polyvinyl
butyrals, cellulosics, polymers comprising quaternary ammonium
groups, polyacrylamides and non-hydrolyzed derivatives and
partially hydrolyzed derivatives, polyvinyl ethers,
polyethyleneimines, polyurethanes, epoxies, melamine formaldehydes,
urea formaldehydes; copolymers thereof, copolymers of maleic
anhydride and derivatives and esters thereof; gelatin; starch; and
mixtures of the foregoing binders.
6. The separator of claim 1, wherein said binder is an inorganic
binder.
7. The separator of claim 1, wherein said binder is selected from
the group consisting of: colloidal silicas, colloidal non-hydrated
aluminum oxides, colloidal tin oxides, colloidal titanium oxides,
colloidal zirconium oxides, and colloidal zinc oxides.
8. The separator of claim 1, wherein said binder further comprises
one or more additives selected from the group consisting of:
pigments, crosslinking agents, catalysts for non-radiation curing,
sensitizers for radiation curing, plasticizers, surfactants, and
dispersants.
9. The separator of claim 1, wherein pores of said microporous
layer are impregnated with a composition comprising a polymer.
10. The separator of claim 9, wherein said polymer is ionically
conductive.
11. The separator of claim 9, wherein said composition further
comprises a crosslinking agent.
12. The separator of claim 1, wherein said microporous layer has a
thickness of from 1 micron to 50 microns.
13. The separator of claim 1, wherein said microporous layer has a
thickness of from 1 micron to 25 microns.
14. The separator of claim 1, wherein said microporous layer has a
thickness of from 2 microns to 15 microns.
15. An electrolyte element for an electric current producing cell,
wherein said electrolyte element comprises: (a) a separator
comprising a microporous layer comprising pseudo-boehmite and a
binder; and, (b) an organic electrolyte contained within pores of
said microporous layer.
16. The electrolyte element of claim 15, wherein said binder is
present in an amount of 3 to 200% of the weight of pseudo-boehmite
in said microporous layer.
17. The electrolyte element of claim 15, wherein said binder is
present in an amount of 5 to 70% of the weight of pseudo-boehmite
in said microporous layer.
18. The electrolyte element of claim 15, wherein said binder is an
organic binder.
19. The electrolyte element of claim 15, wherein said binder is
selected from the group consisting of: polyvinyl alcohols,
polyethylene oxides, alkylated polyethylene oxides, polyvinyl
pyrrolidones, polyvinyl butyrals, cellulosics, polymers comprising
quaternary ammonium groups, polyacrylamides and non-hydrolyzed
derivatives and partially hydrolyzed derivatives, polyvinyl ethers,
polyethyleneimines, polyurethanes, epoxies, melamine formaldehydes,
urea formaldehydes; copolymers thereof; copolymers of maleic
anhydride and derivatives and esters thereof; gelatin; starch; and
mixtures of the foregoing binders.
20. The electrolyte element of claim 15, wherein said binder is an
inorganic binder.
21. The electrolyte element of claim 15, wherein said binder is
selected from the group consisting of: colloidal silicas, colloidal
non-hydrated aluminum oxides, colloidal tin oxides, colloidal
titanium oxides, colloidal zirconium oxides, and colloidal zinc
oxides.
22. The electrolyte element of claim 15, wherein said binder
further comprises one or more additives selected from the group
consisting of: pigments, crosslinking agents, catalysts for
non-radiation curing, sensitizers for radiation curing,
plasticizers, surfactants, and dispersants.
23. The electrolyte element of claim 15, wherein said organic
electrolyte comprises one or more materials selected from the group
consisting of: liquid electrolytes, gel polymer electrolytes, and
solid polymer electrolytes.
24. The electrolyte element of claim 15, wherein said microporous
layer has a thickness of from 2 microns to 15 microns.
25. A method of making a separator for an electric current
producing cell, said separator comprising a microporous layer
comprising pseudo-boehmite and a binder, wherein said method
comprises the steps of: (a) coating onto a substrate a liquid
mixture comprising a boehmite sol, a binder, and a liquid medium;
and, (b) drying the coating formed in step (a) to yield said
microporous layer.
26. The method of claim 25, wherein said liquid medium comprises
water.
27. The method of claim 25, wherein said liquid medium comprises
one or more organic solvents.
28. The method of claim 25, wherein said liquid medium comprises
one or more protic organic solvents selected from the group
consisting of: alcohols and glycols.
29. The method of claim 25, wherein said liquid medium comprises
one or more protic organic solvents selected from the group
consisting of: methanol, ethanol, isopropanol, 1-propanol,
1-butanol, 2-butanol, 2-methoxyethanol, 2-ethoxyethanol,
2-butoxyethanol, ethylene glycol, and propylene glycol.
30. The method of claim 25, wherein said liquid medium comprises
ethanol.
31. The method of claim 25, wherein said liquid medium comprises
water and one or more organic solvents.
32. The method of claim 25, wherein said binder is present in an
amount of3 to 200% of the weight of pseudo-boehmite in said
microporous layer.
33. The method of claim 25, wherein said binder is present in an
amount of 5 to 70% of the weight of pseudo-boehmite in said
microporous layer.
34. The method of claim 25, wherein said binder is an organic
binder.
35. The method of claim 25, wherein said binder is selected from
the group consisting of: polyvinyl alcohols, polyethylene oxides,
alkylated polyethylene oxides, polyvinyl pyrrolidones, polyvinyl
butyrals, cellulosics, polymers comprising quaternary ammonium
groups, polyacrylamides and non-hydrolyzed derivatives and
partially hydrolyzed derivatives, polyvinyl ethers,
polyethyleneimines, polyurethanes, epoxies, melamine formaldehydes,
urea formaldehydes; copolymers thereof; copolymers of maleic
anhydride and derivatives and esters thereof; gelatin; starch; and
mixtures of the foregoing binders.
36. The method of claim 25, wherein said binder is an inorganic
binder.
37. The method of claim 25, wherein said binder is selected from
the group consisting of: colloidal silicas, colloidal non-hydrated
aluminum oxides, colloidal tin oxides, colloidal titanium oxides,
colloidal zirconium oxides, and colloidal zinc oxides.
38. The method of claim 25, wherein said binder further comprises
one or more additives selected from the group consisting of:
pigments, crosslinking agents, catalysts for non-radiation curing,
sensitizers for radiation curing, plasticizers, surfactants, and
dispersants.
39. The method of claim 25, wherein pores of said microporous layer
are impregnated with a composition comprising a polymer.
40. The method of claim 39, wherein said polymer is ionically
conductive.
41. The method of claim 39, wherein said composition further
comprises a crosslinking agent.
42. The method of claim 25, wherein said microporous layer has a
thickness of from 1 micron to 50 microns.
43. The method of claim 25, wherein said microporous layer has a
thickness of from 1 micron to 25 microns.
44. The method of claim 25, wherein said microporous layer has a
thickness of from 2 microns to 15 microns.
45. The method of claim 25, wherein at least one outermost surface
of said substrate comprises a cathode layer and said liquid mixture
is coated onto said cathode layer.
46. The method of claim 45, further comprising, subsequent to step
(b), the steps of: (c) contacting a surface of said microporous
layer with a solution comprising heat- or radiation-curable
monomers or oligomers, thereby causing infusion of said monomers or
said oligomers into pores of said microporous layer; and, (d)
curing said monomers or said oligomers with an energy source
selected from the group consisting of: heat, ultraviolet light,
visible light, infrared radiation, and electron beam radiation;
thereby forming a polymer.
47. The method of claim 46, wherein said polymer formed in step (d)
is an ionic conductive polymer.
48. The method of claim 25, wherein at least one outermost surface
of said substrate comprises a release layer and said liquid mixture
is coated onto said release layer.
49. The method of claim 48, further comprising, subsequent to step
(b), the step of: (c) delaminating said microporous layer from said
substrate.
50. The method of claim 48, further comprising, subsequent to step
(b), the steps of: (c) contacting a surface of said microporous
layer with a solution comprising heat- or radiation-curable
monomers or oligomers, thereby causing infusion of said monomers or
said oligomers into pores of said microporous layer; and, (d)
curing said monomers or said oligomers with an energy source
selected from the group consisting of: heat, ultraviolet light,
visible light, infrared radiation, and electron beam radiation;
thereby forming a polymer.
51. The method of claim 50, wherein said polymer formed in step (d)
is an ionic conductive polymer.
52. The method of claim 50, further comprising, subsequent to step
(d), the step of: (e) delaminating said microporous layer from said
substrate.
53. The method of claim 48, further comprising, subsequent to step
(b), the steps of: (c) contacting a surface of said microporous
layer with a coating solution comprising a solid material and a
liquid medium; and, (d) drying the coating formed in step (c) to
yield a coating layer comprising said solid material.
54. The method of claim 53, further comprising, subsequent to step
(d), the step of: (e) delaminating said microporous layer from said
substrate.
55. The method of claim 53, wherein said coating layer formed in
step (d) is a cathode layer.
56. The method of claim 55, further comprising, subsequent to step
(d), the step of: (e) delaminating said microporous layer from said
substrate.
57. A method of making an electrolyte element for an electric
current producing cell, said electrolyte element comprising a
microporous layer comprising pseudo-boehmite and a binder, wherein
said method comprises the steps of: (a) coating onto a substrate a
liquid mixture comprising a boehmite sol, a binder, and a liquid
medium; (b) drying the coating formed in step (a) to yield said
microporous layer; and, (c) contacting a surface of said
microporous layer with an organic electrolyte, thereby causing
infusion of said electrolyte into pores of said microporous
layer.
58. The method of claim 57, wherein said liquid medium comprises
water.
59. The method of claim 57, wherein said liquid medium comprises
one or more organic solvents.
60. The method of claim 57, wherein said liquid medium comprises
one or more protic solvents selected from the group consisting of:
alcohols and glycols.
61. The method of claim 57, wherein said liquid medium comprises
water and one or more organic solvents.
62. The method of claim 57, wherein said binder is present in an
amount of 3 to 200% of the weight of pseudo-boehmite in said
microporous layer.
63. The method of claim 57, wherein said binder is present in an
amount of 5 to 70% of the weight of pseudo-boehmite in said
microporous layer.
64. The method of claim 57, wherein said binder is an organic
binder.
65. The method of claim 57, wherein said binder is selected from
the group consisting of: polyvinyl alcohols, polyethylene oxides,
alkylated polyethylene oxides, polyvinyl pyrrolidones, polyvinyl
butyrals, cellulosics, polymers comprising quaternary ammonium
groups, polyacrylamides and non-hydrolyzed derivatives and
partially hydrolyzed derivatives, polyvinyl ethers,
polyethyleneimines, polyurethanes, epoxies, melamine formaldehydes,
urea formaldehydes; copolymers thereof; copolymers of maleic
anhydride and derivatives and esters thereof; gelatin; starch; and
mixtures of the foregoing binders.
66. The method of claim 57, wherein said binder is an inorganic
binder.
67. The method of claim 57, wherein said binder is selected from
the group consisting of: colloidal silicas, colloidal non-hydrated
aluminum oxides, colloidal tin oxides, colloidal titanium oxides,
colloidal zirconium oxides, and colloidal zinc oxides.
68. The method of claim 57, wherein said binder further comprises
one or more additives selected from the group consisting of:
pigments, crosslinking agents, catalysts for non-radiation curing,
sensitizers for radiation curing, plasticizers, surfactants, and
dispersants.
69. The method of claim 57, wherein said organic electrolyte
comprises one or more materials selected from the group consisting
of: liquid electrolytes, gel polymer electrolytes, and solid
polymer electrolytes.
70. The method of claim 57, wherein said organic electrolyte
comprises heat- or radiation-curable monomers or oligomers.
71. The method of claim 70, further comprising, subsequent to step
(c), the step of: (d) curing said monomers or said oligomers with
an energy source selected from the group consisting of: heat,
ultraviolet light, visible light, infrared radiation, and electron
beam radiation.
72. The method of claim 57, wherein at least one outermost surface
of said substrate comprises a cathode layer and said liquid mixture
is coated onto said cathode layer.
73. The method of claim 72, further comprising, subsequent to step
(b) and prior to step (c), the steps of: (i) contacting a surface
of said microporous layer with a solution comprising heat- or
radiation-curable monomers or oligomers, thereby causing infusion
of said monomers or said oligomers into pores of said microporous
layer; and, (ii) curing said monomers or said oligomers with an
energy source selected from the group consisting of: heat,
ultraviolet light, visible light, infrared radiation, and electron
beam radiation; thereby forming a polymer.
74. The method of claim 72, further comprising, subsequent to step
(b) and prior to step (c), the steps of: (i) contacting a surface
of said microporous layer with a coating solution comprising a
solid material and a liquid medium; and, (ii) drying the coating
formed in step (i) to yield a coating layer comprising said solid
material.
75. An electric current producing cell comprising a cathode, an
anode, and an electrolyte element interposed between said cathode
and said anode, wherein said electrolyte element comprises: (a) a
separator comprising a microporous layer, which microporous layer
comprises pseudo-boehmite and a binder; and, (b) an organic
electrolyte contained within pores of said microporous layer.
76. The cell of claim 75, wherein said binder is present in an
amount of 3 to 200% of the weight of pseudo-boehmite in said
microporous layer.
77. The cell of claim 75, wherein said binder is an organic
binder.
78. The cell of claim 75, wherein said binder is an inorganic
binder.
79. The cell of claim 75, wherein said cell is a secondary electric
current producing cell.
80. The cell of claim 75, wherein said cell is a primary electric
current producing cell.
81. The cell of claim 75, wherein said 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.
82. The cell of claim 75, wherein said cathode comprises one or
more cathode active materials selected from the group consisting
of: transition metal chalcogenides, conductive polymers, and
sulfur-containing materials.
83. The cell of claim 75, wherein said cathode comprises a
sulfur-containing materials.
84. The cell of claim 83, wherein said sulfur-containing material
comprises elemental sulfur.
85. The cell of claim 83, wherein said sulfur-containing material
comprises a sulfur-containing polymer comprising a covalent
polysulfide moiety of the formula, --S.sub.m--, wherein m is an
integer equal to or greater than 3.
86. The cell of claim 83, wherein said sulfur-containing material
comprises a sulfur-containing polymer comprising a covalent
polysulfide moiety of the formula, --S.sub.m--, wherein m is an
integer equal to or greater than 8.
87. The cell of claim 85, wherein said sulfur-containing polymer
has a polymer backbone chain comprising conjugated segments.
88. The cell of claim 85, wherein said sulfur-containing polymer
has a polymer backbone chain and said polysulfide moiety,
--S.sub.m--, is covalently bonded by one or both of its terminal
sulfur atoms on a side group to said polymer backbone chain.
89. The cell of claim 85, wherein said sulfur-containing polymer
has a polymer backbone chain and said polysulfide moiety,
--S.sub.m--, is incorporated into said polymer backbone chain by
covalent bonding of terminal sulfur atoms of said polysulfide
moiety.
90. The cell of claim 85, wherein said sulfur-containing polymer
comprises greater than 75 weight percent of sulfur.
91. The cell of claim 83, wherein said sulfur-containing material
comprises a sulfur-containing polymer comprising an ionic
polysulfide moiety selected from the group consisting of; ionic
--S.sub.m.sup.- moieties and ionic S.sub.m.sup.2- moieties; wherein
m is an integer equal to or greater than 3.
92. The cell of claim 83, wherein said sulfur-containing material
comprises a sulfur-containing polymer comprising an ionic
polysulfide moiety selected from the group consisting of: ionic
--S.sub.m.sup.- moieties and ionic S.sub.m.sup.2- moieties; wherein
m is an integer equal to or greater than 8.
93. The cell of claim 91, wherein [the polymer backbone chain of]
said sulfur-containing polymer has a polymer backbone chain
comprising conjugated segments.
94. The cell of claim 91, wherein said sulfur-containing polymer
has a polymer backbone chain and said polysulfide moiety,
--S.sub.m--, is covalently bonded by one or both of its terminal
sulfur atoms on a side group to said [the] polymer backbone chain
[of said sulfur-containing polymer].
95. The cell of claim 91, wherein said sulfur-containing polymer
comprises greater than 75 weight percent of sulfur.
96. The cell of claim 75, wherein said organic electrolyte
comprises one or more materials selected from the group consisting
of: liquid electrolytes, gel polymer electrolytes, and solid
polymer electrolytes.
97. The cell of claim 75, wherein said organic electrolyte
comprises a liquid electrolyte.
98. The cell of claim 75, wherein said organic electrolyte
comprises a gel polymer electrolyte.
99. The cell of claim 75, wherein said organic electrolyte
comprises a solid polymer electrolyte.
100. A method of forming an electric current producing cell, said
method comprising the steps of: (a) providing an anode; (b)
providing a cathode; and, (c) interposing an electrolyte element
according to claim 15 between said anode and said cathode.
101. The method of claim 100, wherein the organic electrolyte of
said electrolyte element comprises one or more materials selected
from the group consisting of: liquid electrolytes, gel polymer
electrolytes, and solid polymer electrolytes.
102. The method of claim 100, further comprising, subsequent to
step (c), the step of: (d) imbibing a solution comprising one or
more ionic electrolyte salts and one or more electrolyte solvents
into said electrolyte element.
103. The method of claim 101, wherein said organic electrolyte
after step (c) and prior to step (d) does not comprise an ionic
electrolyte salt.
Description
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 08/995,089, filed Dec. 19, 1997, the contents
of which are incorporated herein by reference in their
entirety.
TECHNICAL FIELD
[0002] This invention is in the field of separators for
electrochemical cells. More particularly, this invention pertains
to separators for electrochemical cells which comprise a
microporous pseudo-boehmite layer; electrolyte elements comprising
such separators; electrical current producing cells comprising such
separators; and methods of making such separators, electrolyte
elements, and cells.
BACKGROUND
[0003] 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 specifications 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.
[0004] An electroactive material that has been fabricated into a
structure for use in a battery is referred to as an electrode. Of a
pair of electrodes used in a battery, herein referred to as an
electric current producing cell, the electrode on the
electrochemically higher potential side is referred to as the
positive electrode, or the cathode, while the electrode on the
electrochemically lower potential side is referred to as the
negative electrode, or the anode.
[0005] An electrochemically active material used in the cathode or
positive electrode is referred to hereinafter as a cathode active
material. An electrochemically active material used in the anode or
negative electrode is hereinafter referred to as an anode active
material. An electric current producing cell or battery comprising
a cathode with the cathode active material in an oxidized state and
an anode with the anode active material in a reduced state is
referred to as being in a charged state. Accordingly, an electric
current producing cell comprising a cathode with the cathode active
material in a reduced state, and an anode with the anode active
material in an oxidized state, is referred to as being in a
discharged state.
[0006] Discharging an electric current producing cell in its
charged state by allowing electrons to flow from the anode to the
cathode through an external circuit results in the electrochemical
reduction of the cathode active material at the cathode and the
electrochemical oxidation of the anode active material at the
anode. To prevent the undesirable flow of the electrons in a short
circuit internally from the anode to the cathode, an electrolyte
element is interposed between the cathode and the anode. This
electrolyte element must be electronically non-conductive to
prevent the short circuiting, but must permit the transport of
positive ions between the anode and the cathode. The electrolyte
element should also be stable electrochemically and chemically
toward both the anode and the cathode.
[0007] Typically, the electrolyte element contains a porous
material, referred to as a separator (since it separates or
insulates the anode and the cathode from each other), and an
aqueous or non-aqueous electrolyte, which typically comprises an
ionic electrolyte salt and ionically conductive material, in the
pores of the separator. A variety of materials have been used for
the porous layer or separator of the electrolyte element in
electric current producing cells. These porous separator materials
include polyolefins such as polyethylenes and polypropylenes, glass
fiber filter papers, and ceramic materials. Usually these separator
materials are supplied as porous free standing films which are
interleaved with the anodes and the cathodes in the fabrication of
electric current producing cells. Alternatively, the porous
separator layer can be applied directly to one of the electrodes,
for example, as described in U.S. Pat. No. 5,194,341 to Bagley et
al.
[0008] Porous separator materials have been fabricated by a variety
of processes including, for example, stretching combined with
special heating and cooling of plastic films, extraction of a
soluble plasticizer or filler from plastic films, and plasma
oxidation. The methods for making conventional free standing
separators typically involve extrusion of melted polymeric
materials either followed by a post-heating and stretching or
drawing process or followed by a solvent extraction process to
provide the porosity throughout the separator layer. U.S. Pat. No.
5,326,391 to Anderson et al. and references therein, describe the
fabrication of free standing porous materials based on extraction
of a soluble plasticizer from pigmented plastic films. U.S. Pat.
No. 5,418,091 to Gozdz et al. and references therein, describe
forming electrolyte layers by extracting a soluble plasticizer from
a fluorinated polymer matrix either as a coated component of a
multilayer battery structure or as an individual separator film.
U.S. Pat. No. 5,194,341 to Bagley et al. describes an electrolyte
element with a microporous silica layer and an organic electrolyte.
The silica layer was the product of the plasma oxidation of a
siloxane polymer. These manufacturing methods for free standing
separators are complex and expensive and are not effective either
in providing ultrafine pores of less than 1 micron in diameter or
in providing separator thicknesses of less than 15 microns.
[0009] The methods for making a separator coated directly on
another layer of the cell typically involve a solvent extraction
process after coating to provide the porosity throughout the
separator layer. As with the free standing separators, this solvent
extraction process is complex, expensive, and not effective in
providing ultrafine pores of less than 1 micron in diameter.
[0010] As the electrolyte in the pores of the separator in the
electrolyte element, a liquid organic electrolyte containing
organic solvents and ionic salts is typically used. Alternatively,
a gel or solid polymer electrolyte containing an ionically
conductive polymer and ionic salts, and optionally organic
solvents, might be utilized instead of the liquid organic
electrolyte. For example, U.S. Pat. Nos. 5,597,659 and 5,691,005 to
Morigaki et al. describe a separator matrix formed of a microporous
polyolefin membrane impregnated in its pores with an ionic
conductive gel electrolyte.
[0011] In addition to being porous and chemically stable to the
other materials of the electric current producing cell, the
separator should be flexible, thin, economical in cost, and have
good mechanical strength. These properties are particularly
important when the cell is spirally wound or is folded to increase
the surface area of the electrodes and thereby improve the capacity
and high rate capability of the cell. Typically, free standing
separators have been 25 microns (.mu.m) or greater in thickness. As
batteries have continued to evolve to higher volumetric capacities
and smaller lightweight structures, there is a need for separators
that are 15 microns or less in thickness with a substantial
increase in the area of the separator in each battery. Reducing the
thickness from 25 microns to 15 microns or less greatly increases
the challenge of providing porosity and good mechanical strength
while not sacrificing the protection against short circuits or not
significantly increasing the total cost of the separator in each
battery.
[0012] This protection against short circuits is particularly
critical in the case of secondary or rechargeable batteries with
lithium as the anode active material. During the charging process
of the battery, dendrites can form on the surface of the lithium
anode and can grow with continued charging. A key feature of the
separator in the electrolyte element of lithium rechargeable
batteries is that it have a small pore structure, such as 10
microns or less in pore diameter, and sufficient mechanical
strength to prevent the lithium dendrites from contacting the
cathode and causing a short circuit with perhaps a large increase
in the temperature of the battery leading to an unsafe explosive
condition.
[0013] Another highly desirable feature of the separator in the
electrolyte element is that it is readily wetted by the electrolyte
materials which provide the ionic conductivity. When the separator
material is a polyolefinic material which has nonpolar surface
properties, the electrolyte materials (which typically have highly
polar properties) often poorly wet the separator material. This
results in low capacities in the battery due to the nonuniform
distribution of electrolyte materials in the electrolyte
element.
[0014] Further it would be highly advantageous to be able to
prepare free standing separators by a relatively simple process of
coating which directly provides ultrafine pores as small as 1 nm in
diameter and can readily provide a range of thicknesses from 50
microns or greater down to 1 micron. Also, it would be advantageous
to be able to prepare separators with ultafine pores and a wide
range of thicknesses coated directly on another layer of the
electric current producing cell by a process of coating without
requiring any subsequent solvent extraction or other complex
process which is costly, difficult to control, and not effective in
providing ultrafine pores.
[0015] A separator, particularly one with a thickness less than 25
microns, which is applicable for electric current producing cells,
and which can avoid the foregoing problems often encountered with
the use of polyolefinic and other conventional porous materials
made using extrusion, extraction, or other processes would be of
great value to the battery industry.
SUMMARY OF THE INVENTION
[0016] The separator of the present invention for use in an
electric current producing cell comprises a microporous
pseudo-boehmite layer. In one embodiment, the pseudo-boehmite layer
has a pore volume from 0.02 to 2.0 cm.sup.3/g. In a preferred
embodiment, the pseudo-boehmite layer has a pore volume from 0.3 to
1.0 cm.sup.3/g. In a more preferred embodiment, the pseudo-boehmite
layer has a pore volume from 0.4 to 0.7 cm.sup.3/g.
[0017] In one embodiment, the pseudo-boehmite layer of the
separator has an average pore diameter from 1 to 300 nm. In a
preferred embodiment, the pseudo-boehmite layer has an average pore
diameter from 2 to 30 nm. In a more preferred embodiment, the
pseudo-boehmite layer has an average pore diameter from 3 to 10
nm.
[0018] In another embodiment of the invention, the pseudo-boehmite
layer further comprises a binder. In one embodiment, the binder is
present in the amount of 5 to 70% by weight of the pseudo-boehmite
in the separator. In a preferred embodiment, the binder comprises
polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone,
copolymers of the foregoing, or a combination thereof. In another
preferred embodiment, the pseudo-boehmite layer is impregnated with
an ionic conductive polymer. In a most preferred embodiment, the
ionic conductive polymer comprises a polymer resin cured with heat,
ultraviolet light, visible light, infrared radiation, or electron
beam radiation.
[0019] In one embodiment, the pseudo-boehmite layer of the
separator has a thickness of from 1 micron to 50 microns. In a
preferred embodiment, the pseudo-boehmite layer has a thickness of
from 1 micron to 25 microns. In a more preferred embodiment, the
pseudo-boehmite layer has a thickness of from 5 microns to 15
microns.
[0020] In another embodiment of the invention, a method for forming
a separator for use in electric current producing cells is
provided. The method comprises coating onto a substrate a liquid
mixture comprising a boehmite sol and then drying the coating to
form the microporous pseudo-boehmite layer, as described herein. In
one embodiment, the liquid mixture comprising a boehmite sol
further comprises a binder and then is dried to form the
microporous pseudo-boehmite layer with binder present, as described
herein. In a preferred embodiment, there is a subsequent step of
delaminating the pseudo-boehmite layer from the substrate, thereby
providing a free standing separator comprising the pseudo-boehmite
layer with binder present. In a most preferred embodiment, the
substrate comprises a cathode coating layer on at least one
outermost surface and the liquid mixture comprising the boehmite
sol is coated onto the cathode coating layer.
[0021] Another aspect of the separators and of the methods of
forming the separator of this invention pertains to a method of
making the separator comprising a microporous layer, which
microporous layer comprises pseudo-boehmite and a binder, and to a
separator formed by such method, wherein the method comprises the
steps of: (a) coating onto a substrate a liquid mixture comprising
a boehmite sol, a binder, and a liquid medium; and, (b) drying the
coating formed in step (a) to yield the microporous layer. In one
embodiment, the liquid medium comprises water. In one embodiment,
the liquid medium comprises one or more organic solvents. In one
embodiment, the liquid medium comprises one or more protic organic
solvents selected from the group consisting of: alcohols and
glycols. Suitable organic solvents for the liquid medium include,
but are not limited to, methanol, ethanol, isopropanol, 1-propanol,
1-butanol, 2-butanol, ethylene glycol, propylene glycol,
2-methoxyethanol, 2-ethoxyethanol, and 2-butoxyethanol. In a
preferred embodiment, the liquid medium comprises ethanol. In one
embodiment, the liquid medium comprises water and one or more
organic solvents.
[0022] A wide variety of binders that are compatible with the
liquid mixture of the methods of this invention may be utilized in
the present invention. In one embodiment, the binder is an organic
binder. Suitable organic binders include, but are not limited to,
polyvinyl alcohols, polyethylene oxides, alkylated polyethylene
oxides, polyvinyl pyrrolidones, polyvinyl butyrals, cellulosics,
polymers comprising quaternary ammonium groups, polyacrylamides and
non-hydrolyzed derivatives and partially hydrolyzed derivatives,
polyvinyl ethers, polyethyleneimines, polyurethanes, epoxies,
melamine formaldehydes, urea formaldehydes; copolymers thereof;
copolymers of maleic anhydride and derivatives and esters thereof;
gelatin; starch; and mixtures of the foregoing binders. In one
embodiment, the binder is an inorganic binder. Suitable inorganic
binders include, but are not limited to, colloidal silicas,
colloidal non-hydrated aluminum oxides, colloidal tin oxides,
colloidal titanium oxides, colloidal zirconium oxides, and
colloidal zinc oxides. In one embodiment, the binder further
comprises one or more additives. Suitable additives include, but
are not limited to, pigments, crosslinking agents, catalysts for
non-radiation curing, sensitizers for radiation curing,
plasticizers, surfactants, and dispersants. Since the densities of
some binders, especially those comprising inorganic binders and
pigments with high specific gravities, may be high, in one
embodiment, the binder is present in an amount of 3 to 200% of the
weight of the pseudo-boehmite in the microporous layer. In a
preferred embodiment, the binder is present in an amount of 5 to
70% by weight of the pseudo-boehmite in the microporous layer. In
one embodiment, the microporous layer has a thickness of from 1
micron to 50 microns, preferably from 1 micron to 25 microns, and,
more preferably, from 2 to 15 microns.
[0023] In one embodiment of the methods of forming the separator of
the present invention, at least one outermost surface of said
substrate comprises a cathode layer and said liquid mixture is
coated onto said cathode layer.
[0024] In one embodiment of the methods of forming the separator of
the present invention, the substrate comprises a release layer on
at least one outermost surface, and the liquid mixture is coated
onto the release layer. In one embodiment, after step (b) of drying
the coating to yield the microporous layer, there are subsequent
steps of: (c) contacting a surface of the microporous layer with a
coating solution comprising a solid material and a liquid medium;
and, (d) drying the coating formed in step (c) to yield a coating
layer comprising the solid material. In one embodiment, the coating
layer in step (d) is a cathode layer. In one embodiment, there is a
subsequent step of delaminating the microporous layer, as described
herein, from the substrate comprising the release layer.
[0025] In one embodiment of the methods of forming the separator of
the present invention, pores of said microporous layer are
impregnated with a composition comprising a polymer, preferably the
polymer is ionically conductive. In one embodiment of the methods
of forming the separator of the present invention, after step (b)
of drying the coating to yield the microporous layer, there are
subsequent steps of: (c) contacting a surface of said microporous
layer with a solution comprising heat- or radiation-curable
monomers or oligomers, thereby causing infusion of said monomers or
said oligomers into pores of said microporous layer; and, (d)
curing said monomers or said oligomers with an energy source
selected from the group consisting of: heat, ultraviolet light,
visible light, infrared radiation, and electron beam radiation;
thereby forming a polymer.
[0026] In another embodiment of the methods of forming the
separator of this invention, there is a subsequent step of
contacting the surface of the pseudo-boehmite layer with a solution
comprising heat- or radiation-curable monomers or oligomers to
thereby cause the infusion of the monomers or oligomers into the
pores of the pseudo-boehmite layer and then curing the monomers or
oligomers with heat, ultraviolet light, visible light, infrared
radiation, or electron beam radiation to form a composition
comprising a polymer, preferably an ionically conductive polymer.
In one embodiment, the solution further comprises a crosslinking
agent, thereby forming a composition further comprising the
crosslinking agent. In one embodiment, after the infusion and
curing of the heat- or radiation-curable monomers or oligomers,
there is a subsequent step of delaminating the pseudo-boehmite
layer from the substrate, thereby providing a free standing
separator comprising the pseudo-boehmite layer with a polymer
present, preferably an ionically conductive polymer, in the pores
of the pseudo-boehmite layer.
[0027] In another embodiment of the invention, an electrolyte
element for use in an electric current producing cell comprises the
separator comprising a microporous pseudo-boehmite layer, as
described herein, and an organic electrolyte contained within the
pores of the microporous pseudo-boehmite layer. Suitable materials
for use as the organic electrolyte include liquid electrolytes, gel
polymer electrolytes, and solid polymer electrolytes.
[0028] In another embodiment of the invention, a method of making
an electrolyte element for an electric current producing cell is
provided. The method comprises the methods of forming the separator
comprising a microporous pseudo-boehmite layer, as described
herein, and then contacting the surface of the microporous
pseudo-boehmite layer with an organic electrolyte thereby causing
the infusion of the electrolyte into the pores of the microporous
pseudo-boehmite layer. In one embodiment, the organic electrolyte
comprises heat- or radiation-curable monomers or oligomers. In a
preferred embodiment, there is a subsequent step of curing the
heat- or radiation-curable monomers or oligomers of the organic
electrolyte with heat, ultraviolet light, visible light, infrared
radiation, or electron beam radiation to form a polymer, preferably
an ionically conductive polymer. In a most preferred embodiment, at
least an outermost surface of the substrate is a cathode coating
layer, and the pseudo-boehmite layer is coated on the cathode
layer.
[0029] In another aspect of the invention, pertains to a method of
making an electrolyte element for an electric current producing
cell, wherein the electrolyte element comprises a microporous layer
comprising pseudo-boehmite and a binder, wherein the method
comprises the steps of: (a) coating onto a substrate a liquid
mixture comprising a boehmite sol, a binder, and a liquid medium;
(b) drying the coating formed in step (a) to yield said microporous
layer; and, (c) contacting a surface of said microporous layer with
an organic electrolyte, thereby causing infusion of said
electrolyte into pores of said microporous layer.
[0030] In another embodiment of the invention, an electric current
producing cell is provided. The cell is comprised of a cathode and
an anode, and an electrolyte element interposed between the cathode
and the anode, wherein the electrolyte element comprises the
separator comprising a microporous pseudo-boehmite layer, as
described herein, and an organic electrolyte contained within the
pores of the microporous pseudo-boehmite layer. In one embodiment,
the cell is a secondary battery. In one embodiment, the cell is a
primary electric current producing cell. In a preferred embodiment,
the anode active material of the cell is selected from the group
consisting of: a lithium metal, lithium-aluminum alloys,
lithium-tin alloys, lithium-intercalated carbons, and
lithium-intercalated graphites.
[0031] Suitable cathode active materials for the electric current
producing cells of this invention include, but are not limited to,
transition metal chalcogenides, conductive polymers, and
sulfur-containing materials. In a preferred embodiment, the cathode
of the cell comprises a sulfur-containing material.
[0032] In one embodiment, the sulfur-containing material comprises
elemental sulfur. In one embodiment, the sulfur-containing material
comprises a sulfur-containing polymer, wherein the electroactive
sulfur-containing polymer, in its oxidized state, comprises a
covalent polysulfide moiety of the formula, --S.sub.m--, wherein m
is an integer equal to or greater than 3, preferably m is an
integer from 3 to 10, most preferably m is an integer equal to or
greater than 6, and particularly most preferably m is an integer
equal to or greater than 8. In one embodiment, the
sulfur-containing polymer has a polymer backbone chain comprising
conjugated segments. In one embodiment, the sulfur-containing
polymer has a polymer backbone chain 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. In one
embodiment, the sulfur-containing polymer materials polymer has a
polymer backbone chain and the polysulfide moiety, --S.sub.m--, is
incorporated into the polymer backbone chain by covalent bonding of
terminal sulfur atoms of the polysulfide moiety. In one embodiment
the sulfur-containing polymer comprises greater than 75 weight
percent of sulfur.
[0033] In one embodiment, the electroactive sulfur-containing
material comprises a sulfur-containing polymer comprising an ionic
polysulfide moiety selected from the group of consisting of: ionic
--S.sub.m.sup.- moieties and ionic S.sub.m.sup.2- moieties, wherein
m is an integer equal to or greater than 3, and preferably m is an
integer equal to or greater than 8. In one embodiment, the
sulfur-containing polymer comprising an ionic polysulfide moiety
has a polymer backbone comprising conjugated segments. In one
embodiment, the polysulfide moiety, --S.sub.m.sup.-, is covalently
bonded by one of its terminal sulfur atoms on a side group to the
polymer backbone chain. In one embodiment, the sulfur-containing
polymer comprising an ionic polysulfide moiety comprises greater
than 75 weight percent of sulfur.
[0034] In a preferred embodiment, the cathode of the secondary
battery comprises an electroactive sulfur-containing cathode
material, wherein the electroactive sulfur-containing cathode
material, in its oxidized state, comprises a polysulfide moiety of
the formula, --S.sub.m--, wherein m is an integer equal to or
greater than 3, preferably m is an integer from 3 to 10, and most
preferably m is an integer equal to or greater than 6, and
particularly most preferably m is an integer equal to or greater
than 8. In a most preferred embodiment, the electroactive
sulfur-containing cathode material is selected from the group
consisting of: elemental sulfur, carbon-sulfur polymer materials
with their --S.sub.m-- groups covalently bonded by one or more of
their terminal sulfur atoms on a side group on the polymer backbone
chain; carbon-sulfur polymer materials with their --S.sub.m--
groups incorporated into the polymer backbone chain by covalent
bonding of their terminal sulfur atoms; and carbon-sulfur polymer
materials with greater than 75 weight percent of sulfur in the
carbon-sulfur polymer material. In one embodiment, the
carbon-sulfur polymer materials comprise conjugated segments in the
polymer backbone chain.
[0035] In another embodiment of the cell of this invention, the
organic electrolyte in the electrolyte element comprises one or
more materials selected from the group consisting of: liquid
electrolytes, gel polymer electrolytes, or solid polymer
electrolytes.
[0036] In another embodiment of the invention, a method for forming
an electric current producing cell is provided. The method
comprises providing an anode and a cathode, and enclosing an
electrolyte element, as described herein, interposed between the
anode and the cathode. In one embodiment of the method, the organic
electrolyte of the electrolyte element comprises one or more
materials selected from the group consisting of: liquid
electrolytes, gel polymer electrolytes, and solid polymer
electrolytes. In a preferred embodiment of the method, after the
step of enclosing the electrolyte element between the anode and the
cathode, there is a subsequent step comprising the imbibition of a
solution comprising one or more ionic electrolyte salts and one or
more electrolyte solvents into the electrolyte element. In a most
preferred embodiment, after the step of enclosing the electrolyte
element between the anode and the cathode, there is substantially
no ionic electrolyte salt present in the electrolyte element, and
there is a subsequent step comprising the imbibition of a solution
comprising one or more ionic electrolyte salts and one or more
electrolyte solvents into the electrolyte element.
[0037] As will be appreciated by one of skill in the art, features
of one aspect or embodiment of the invention are also applicable to
other aspects or embodiments of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0038] The separators of the present invention provide superior
electric current producing cell properties, particularly in cells
utilizing separators with thicknesses below about 25 microns.
Conventional separators, such as porous polyolefins, porous
fluoropolymers where the porosity is provided by a solvent
extraction process, and glass fiber papers, and the like, are
difficult and costly to manufacture, especially at thicknesses
below about 25 microns. Due to the nature of the processes used to
manufacture these separators and the relatively large pore sizes
intrinsic to these separators, electrical shorting may be a
significant challenge at separator thicknesses of below about 25
microns, especially at thicknesses below about 15 microns. To
overcome these limitations, the separators of the present invention
for use in electric current producing cells comprise a microporous
pseudo-boehmite layer.
[0039] Separators Comprising Microporous Pseudo-Boehmite
[0040] Upon heating, Al(OH).sub.3 (which occurs naturally as
gibbsite) transforms to a mixed oxide hydroxide, AlO.OH (which
occurs naturally as diaspore and boehmite). Upon further heating
above about 450.degree. C., it further transforms to gamma-alumina,
Al.sub.2O.sub.3. The term "pseudo-boehmite," as used herein,
pertains to hydrated aluminum oxides having the chemical formula
Al.sub.2O.sub.3.xH.sub.2O wherein x is in the range of from 1.0 to
1.5. Terms used herein, which are synonymous with
"pseudo-boehmite," include "boehmite," "AlOOH," and "hydrated
alumina." The materials referred to herein as "pseudo-boehmite" are
distinct from 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.xH.sub.2O wherein x is less than 1.0 or
greater than 1.5.
[0041] The term "microporous," is used herein to describe the
material of a layer, which material possesses pores of diameter of
about 10 microns or less which are connected in a substantially
continuous fashion from one outermost surface of the layer through
to the other outermost surface of the layer. Porous separators
which are made from fibers, such as glass, TEFLON (a trademark for
polytetrafluoroethylene available from DuPont Corporation,
Wilmington, Del.), and polypropylene, are generally characterized
as non-woven separator materials and typically have pore diameters
too large to be called microporous, thereby making them
unacceptable for rechargeable cells where dendrite formation is a
potential concern.
[0042] The amount of these pores in the layer may be characterized
by the pore volume, which is the volume of pores per unit weight of
the layer. The pore volume may be measured by filling the pores
with a liquid having a known density and then calculated by the
increase in weight of the layer with the liquid present divided by
the known density of the liquid and then dividing this quotient by
the weight of the layer with no liquid present, according to the
equation: 1 Pore Volume = [ W 1 - W 2 ] / d W 2 I
[0043] where W.sub.1 is the weight of the layer when the pores are
completely filled with the liquid of known density, W.sub.2 is the
weight of the layer with no liquid present in the pores, and d is
the density of the liquid used to fill the pores. Also, the pore
volume may be estimated from the apparent density of the layer by
subtracing the reciprocal of the theoretical density of the
materials (assuming no pores) comprising the microporous layer from
the reciprocal of the apparent density or measured density of the
actual microporous layer, according to the equation: 2 Pore Volume
= ( 1 d 1 - 1 d 2 ) II
[0044] where d.sub.1 is the density of the layer which is
determined from the quotient of the weight of the layer and the
layer volume as determined from the measurements of the dimensions
of the layer, and d.sub.2 is the calculated density of the
materials in the layer assuming no pores are present or, in other
words, d.sub.2 is the density of the solid part of the layer as
calculated from the densities and the relative amounts of the
different materials in the layer. The porosity or void volume of
the layer, expressed as percent by volume, can be determined
according to the equation: 3 Porosity = 100 ( Pore Volume ) [ Pore
Volume + 1 / d 2 ] III
[0045] where pore volume is as determined above, and d.sub.2 is the
calculated density of the solid part of the layer, as described
above.
[0046] In one embodiment, the pseudo-boehmite layer of the present
invention has a pore volume from 0.02 to 2.0 cm.sup.3/g. In a
preferred embodiment, the pseudo-boehmite layer has a pore volume
from 0.3 to 1.0 cm.sup.3/g. In a more preferred embodiment, the
pseudo-boehmite layer has a pore volume from 0.4 to 0.7 cm.sup.3/g.
Below a pore volume of 0.02 cm.sup.3/g, the transport of ionic
species is inhibited by the reduced pore volume. Above a pore
volume of 2.0 cm.sup.3/g, the amount of voids are greater which
reduces the mechanical strength of the microporous pseudo-boehmite
layer.
[0047] In contrast to conventional microporous separators which
typically have pore diameters on the order of 1 to 10 microns, the
separators of the present invention have pore diameters which range
from about 1 micron down to less than 0.002 microns. In one
embodiment, the separator of the present invention has an average
pore diameter from 0.001 microns or 1 nm to 0.3 microns or 300 nm.
In a preferred embodiment, the separator of the present invention
has an average pore diameter from 2 nm to 30 nm. In a more
preferred embodiment, the separator of the present invention has an
average pore diameter from 3 nm to 10 nm.
[0048] The pore diameters of the separators of the present
invention can be measured by preparing a sample of a cross-section
of the layer by conventional techniques and viewing this
cross-section through a microscope having the necessary resolution.
For the ultrafine pore diameters of 2 to 30 nm, a transmission
electron microscope (TEM) can provide the necessary resolution. The
average pore diameter of the separators of the present invention
may be determined by mercury porosimetry using conventional
techniques.
[0049] One distinct advantage of separators with much smaller pore
diameters on the order of 0.001 to 0.03 microns is that insoluble
particles, even colloidal particles with diameters on the order of
0.05 to 1.0 microns, can not pass through the separator because of
the ultrafine pores. In contrast, colloidal particles, such as the
conductive carbon powders often incorporated into cathode
compositions, can readily pass through conventional separators,
such as microporous polyolefins, and thereby can migrate to
undesired areas of the cell.
[0050] Another significant advantage of the pseudo-boehmite
separators of the present invention over conventional separators is
that the nanoporous structure of the layer may function as an
ultrafiltration membrane and, in addition to blocking all particles
and insoluble materials, may block the diffusion of soluble
materials of relatively low molecular weights, such as 2,000 or
higher, while permitting the diffusion of soluble materials with
molecular weights below this cutoff level. This property may be
utilized to advantage in selectively impregnating or imbibing
materials into the separator during manufacture of the electric
current producing cell or in selectively permitting diffusion of
very low molecular weight materials through the separator during
all phases of the operation of the cell while blocking or
significantly inhibiting the diffusion of insoluble materials or of
soluble materials of medium and higher molecular weights.
[0051] Another important advantage of the extremely small pore
diameters of the separators of the present invention is the strong
capillary action of these tiny pores which enhances the capability
of the separators to readily take up or imbibe electrolyte liquids
and to retain these materials in the pores.
[0052] The microporous pseudo-boehmite layer may optionally further
comprise a binder, preferably an organic polymer binder. The binder
is usually selected on the basis of improving the mechanical
strength of the layer without significantly impacting the
properties of the microporous structure, which includes transport
of low molecular weight materials through the layer while blocking
the transport of colloidal or larger particles and high molecular
weight materials. The preferred amount of binder is from 5% to 70%
of the weight of the pseudo-boehmite in the layer. Below 5 weight
percent, the amount of binder is usually too low to provide a
significant increase in mechanical strength. Above 70 weight
percent, the amount of binder is usually too high and fills the
pores to an excessive extent which may interfere with the transport
of ionic species and other low molecular weight materials through
the microporous layer.
[0053] Any binder that is compatible with the boehmite sol during
mixing and processing into the microporous layer and provides the
desired mechanical strength and uniformity of the layer without
significantly interfering with the desired microporosity is
suitable for use in this invention. Examples of suitable binders
include, but are not limited to, polyvinyl alcohols, polyethylene
oxides, polyvinyl pyrrolidones, copolymers thereof, and mixtures
thereof. Preferred binders are water soluble polymers and have
ionically conductive properties.
[0054] The microporous layer comprising pseudo-boehmite and binder
may further comprise an ionic conductive polymer which is
impregnated into the microporous layer. The term "ionic conductive
polymer," as used herein, denotes a polymer which, when combined
with an appropriate ionic electrolyte salt, may provide ionically
conductive properties to a gel polymer electrolyte or a solid
polymer electrolyte. The ionic conductive polymer may further
increase mechanical strength, in addition to providing ionically
conductive properties to the microporous layer. Since the
microporous layer of the present invention typically blocks
insoluble materials and soluble materials above a low molecular
weight such as 2,000, the ionic conductive polymer is preferably
formed by impregnating the microporous layer with suitable heat- or
radiation-curable monomers or oligomers with molecular weights
below 2,000 and then crosslinking these monomers or oligomers in
situ to form an ionic conductive polymer resin by exposure to an
energy source. Suitable energy sources include heat, ultraviolet
light, visible light, infrared radiation, and electron beam
radiation. Where the monomers or oligomers are not sensitive to the
radiation (such as ultraviolet light), sensitizers may be added to
the monomers or oligomers to activate the crosslinking or curing to
form a polymer resin.
[0055] The thickness of the microporous pseudo-boehmite layer, with
or without additional binder, for use as a separator may vary over
a wide range since the basic properties of microporosity and
mechanical integrity are present in layers of a few microns in
thickness as well as in layers with thicknesses of hundreds of
microns. For various reasons including cost, overall performance
properties as a separator, and ease of manufacturing, the desirable
thicknesses are in the range of 1 micron to 50 microns. Preferred
are thicknesses in the range of 1 micron to 25 microns. More
preferred are thicknesses in the range 2 to 15 Microns. The most
preferred thicknesses are in the range of 5 microns to 15 microns.
Conventional separators, such as the porous polyolefin materials,
are typically 25 to 50 microns in thickness so it is particularly
advantageous that the microporous separators of this invention can
be effective and inexpensive at thicknesses well below 25
microns.
[0056] The present invention provides a method of forming a
separator for use in electric current producing cells which
overcomes the disadvantages of the aforementioned conventional
methods for forming separators. One aspect of the method of the
present invention comprises the steps of coating a liquid mixture
comprising a boehmite sol onto a substrate and subsequently drying
the coating to form a microporous pseudo-boehmite layer. This
pseudo-boehmite layer has a pore volume from 0.02 to 2.0 cm.sup.3/g
and has an average pore diameter from 1 nm to 300 nm, as described
previously for the microporous pseudo-boehmite separator.
[0057] If increased mechanical strength or some other improvement
in the properties of the separator are desired, the liquid mixture
comprising the boehmite sol may further comprise a binder, and the
resulting liquid mixture is then dried to form the microporous
pseudo-boehmite layer with binder present.
[0058] One aspect of the separators and of the methods of forming
the separator of this invention pertains to a method of making the
separator comprising a microporous layer, which microporous layer
comprises pseudo-boehmite and a binder, and to a separator formed
by such method, wherein the method comprises the steps of: (a)
coating onto a substrate a liquid mixture comprising a boehmite
sol, a binder, and a liquid medium; and, (b) drying the coating
formed in step (a) to yield the microporous layer. In one
embodiment, the liquid medium comprises water. In one embodiment,
the liquid medium comprises one or more organic solvents. In one
embodiment, the liquid medium comprises one or more protic organic
solvents selected from the group consisting of: alcohols and
glycols. Suitable organic solvents for the liquid medium include,
but are not limited to, methanol, ethanol, isopropanol, 1-propanol,
1-butanol, 2-butanol, ethylene glycol, propylene glycol,
2-methoxyethanol, 2-ethoxyethanol, and 2-butoxyethanol. In a
preferred embodiment, the liquid medium comprises ethanol. In one
embodiment, the liquid medium comprises water and one or more
organic solvents. The use of organic solvents in the liquid medium
may be advantageous in allowing a wider choice and amount of
binders to be utilized with the boehmite sol, in providing a faster
rate of drying, and in reducing the presence of water in the
microporous layer, where the water may lead to undesirable
reactions in the electric current producing cell.
[0059] A wide variety of binders that are compatible with the
liquid mixture of the methods of this invention may be utilized in
the present invention. In one embodiment, the binder is an organic
binder. Suitable organic binders include, but are not limited to,
polyvinyl alcohols, polyethylene oxides, alkylated polyethylene
oxides such as the dimethyl ethers of polyethylene oxides,
polyvinyl pyrrolidones, polyvinyl butyrals, cellulosics such as
hydroxyethyl cellulose and alcohol-soluble cellulose acetate
propionate, polymers comprising quaternary ammonium groups,
polyacrylamides and non-hydrolyzed derivatives and partially
hydrolyzed derivatives, polyvinyl ethers, polyethyleneimines,
polyurethanes, epoxies, melamine formaldehydes, urea formaldehydes;
copolymers thereof; copolymers of maleic anhydride and derivatives
and esters thereof; gelatin; starch; and mixtures of the foregoing
binders. In one embodiment, the binder is an inorganic binder.
Suitable inorganic binders include, but are not limited to,
colloidal silicas, colloidal non-hydrated aluminum oxides,
colloidal tin oxides, colloidal titanium oxides, colloidal
zirconium oxides, and colloidal zinc oxides. These colloidal oxide
materials have the typical colloidal particle sizes of less than 1
micron and typically are present in the liquid mixture as colloidal
sols which form inorganic gel networks with binder properties upon
drying. In one embodiment, the binder further comprises one or more
additives, as are known in the art of coatings with organic or
inorganic binders. Suitable additives include, but are not limited
to, pigments, crosslinking agents such as isocyanates, catalysts
for non-radiation curing such as organic acids, sensitizers for
radiation curing, plasticizers, surfactants, and dispersants.
[0060] Since the densities of some binders, such as, for example,
those comprising inorganic binders and pigments with high specific
gravities, may be high, in one embodiment, the binder is present in
an amount of 3 to 200% of the weight of the pseudo-boehmite in the
microporous layer. In a preferred embodiment, the binder is present
in an amount of 5 to 70% by weight of the pseudo-boehmite in the
microporous layer. In one embodiment, the microporous layer has a
thickness of from 1 micron to 50 microns, preferably from 1 micron
to 25 microns, and, more preferably, from 2 to 15 microns.
[0061] These methods of forming a microporous pseudo-boehmite layer
with or without a binder present may be used to produce either a
free standing separator or a separator coated directly onto a layer
of an electric current producing cell. In a most preferred
embodiment of the methods of forming the separators of this
invention, the separator is coated directly onto the cathode layer
of the electric current producing cell by application of a liquid
mixture comprising a boehmite sol and a binder onto the outermost
surface of a cathode coating layer on a suitable current collector
substrate and then drying this liquid mixture to form the
microporous pseudo-boehmite separator layer. The term "cathode
coating layer" or "cathode layer," as used herein, refers to an
outer most layer or surface of a cathode of an electric current
producing cell. Typically, the outer most layer of the cathode is
the cathode active layer. However, the cathode active layer may
have another layer over it such that the cathode active layer is an
intermediate layer in the cathode and not an outer most layer. The
term "cathode active layer," as used herein, pertains to any layer
of the cathode that comprises the cathode active material.
[0062] A further distinct advantage of this coating method to
produce a separator is that it is flexible in the pattern in which
the separator layer may be applied to the substrate. For example,
the separator layer may be applied over the entire outermost
surface of the cathode including the top surface and sides of the
cathode coating on the current collector and cathode substrate. The
cathode may thus be completely encapsulated on all outermost
surfaces, including the edges or sides of the cathode coating which
are not contacted or covered by conventional free standing
polyolefin or other porous separators, by coating the separator
layer in a pattern over all the outermost, exposed surfaces of the
cathode. This complete encapsulation by the separator layer of the
present invention is very advantageous to safety and battery
performance in providing an insulating surface to prevent any short
circuits by the cathode during fabrication and during use of the
electric current producing cell. This encapsulation is also very
advantageous to high cell capacity and long cycle life in acting as
a total barrier in inhibiting the migration of any insoluble or
high molecular weight species in the cathode to outside the cathode
area and similarly in retarding the diffusion of any low molecular
weight species, such as soluble polysulfides, in the cathode to
outside the cathode area.
[0063] In a preferred embodiment, a free standing separator is
formed by application of a liquid mixture comprising a boehmite sol
and a binder onto a substrate, subsequently drying the liquid
mixture to form the microporous pseudo-boehmite layer, and then
delaminating this pseudo-boehmite layer from the substrate to
provide a free standing microporous pseudo-boehmite separator with
binder present. The substrate is selected to have weak adhesion to
the pseudo-boehmite layer so that the coating can be readily
delaminated from the substrate without damaging the separator.
Suitable substrates include papers with release coatings on the
surface that receives the pseudo-boehmite liquid mixture and
flexible plastic films, such as polyester and polystyrene films,
which have weak adhesion to the coating of boehmite sol and binder.
The width of the separator layer when it is delaminated from the
substrate may be the full width as coated on the substrate or the
coated separator may be cut to a narrower width, such as the width
desired for use in the specific electric current producing cell. In
a most preferred embodiment, before delaminating the microporous
pseudo-boehmite layer with binder present from the substrate, the
surface of the pseudo-boehmite layer is contacted with heat- or
radiation-curable monomers or oligomers, as described herein, to
impregnate the pores of the pseudo-boehmite layer. After curing the
monomers or oligomers with heat or radiation to form a polymer, the
pseudo-boehmite layer is delaminated from the substrate to provide
a free standing separator with polymer present, preferably an
ionically conductive polymer.
[0064] In one embodiment of the methods of forming the separator of
the present invention, the substrate comprises a release layer on
at least one outermost surface, and the liquid mixture is coated
onto the release layer. In one embodiment, after step (b) of drying
the coating to yield the microporous layer, there are subsequent
steps of: (c) contacting a surface of the microporous layer with a
coating solution comprising a solid material and a liquid medium;
and, (d) drying the coating formed in step (c) to yield a coating
layer comprising the solid material. In one embodiment, the coating
layer in step (d) is a cathode layer. In one embodiment, there is a
subsequent step of delaminating the microporous layer, as described
herein, from the substrate comprising the release layer.
[0065] In one embodiment of the methods of this invention for
producing a separator coated directly on the cathode active layer,
the surface of the pseudo-boehmite layer is contacted with a
solution comprising heat- or radiation-curable monomers or
oligomers, as described herein, to impregnate the pores of the
pseudo-boehmite layer. Curing the monomers or oligomers with heat
or radiation to form a composition comprising a polymer provides a
microporous pseudo-boehmite separator with polymer, preferably an
ionically conductive polymer, present coated directly on the
cathode active layer. In one embodiment, the solution further
comprises a crosslinking agent, thereby forming a composition
further comprising the crosslinking agent. Suitable crosslinking
agents include, but are not limited to, isocyanates and
polyaziridines.
[0066] Electrolyte Elements
[0067] The present invention provides an electrolyte element for
use in an electric current producing cell by combining the
separator of the present invention, as described herein, with an
electrolyte contained within the pores of the pseudo-boehmite layer
of the separator. The electrolyte may be any of the types of
non-aqueous and aqueous electrolytes known in the art.
[0068] The methods of making electrolyte elements for an electric
current producing cell of the present invention comprise the steps
of first coating a liquid mixture comprising a boehmite sol and a
liquid medium, and optionally a binder, onto a substrate and then
drying the coating to form a microporous pseudo-boehmite layer, as
described herein in the methods of making a separator, and
subsequently contacting a surface of this pseudo-boehmite layer
with an electrolyte, preferably an organic electrolyte, to cause
the electrolyte to infuse into the pores of the pseudo-boehmite
layer. Prior to the infusion of the electrolyte, the
pseudo-boehmite layer has a pore volume from 0.02 to 2.0 cm.sup.3/g
and an average pore diameter from 1 nm to 300 nm, as described
herein for the microporous pseudo-boehmite separator.
[0069] If increased mechanical strength or some other improvements
such as improved adhesion to the substrate or coating uniformity
are desired, the liquid mixture comprising the boehmite sol may
further comprise a binder and then is dried to form a microporous
pseudo-boehmite layer with binder present. The types of the binders
such as polyvinyl alcohols, the amounts of the binder materials in
the range of 3 to 200% of the weight of the pseudo-boehmite in the
layer, and the thicknesses of the pseudo-boehmite layer with binder
in the range of 1 to 50 microns, preferably 1 to 25 microns, more
preferably 2 to 15 microns, and most preferably 5 to 15 microns,
are as described herein for the microporous pseudo-boehmite
separator and the methods of making the microporous pseudo-boehmite
separator.
[0070] Examples of suitable electrolytes for use in the electrolyte
elements of 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.
[0071] Examples of useful liquid electrolyte solvents include, but
are not limited to, those comprising electrolyte solvents selected
from the group consisting of: N-methyl acetamide, acetonitrile,
carbonates, sulfones, sulfolanes, aliphatic ethers, cyclic ethers,
glymes, siloxanes, dioxolanes, N-alkyl pyrrolidones, substituted
forms of the foregoing, and blends thereof, to which is added an
appropriate ionic electrolyte salt.
[0072] The electrolyte solvents of these liquid electrolytes are
themselves useful as plasticizers for semi-solid or gel polymer
electrolytes. Examples of useful gel polymer electrolytes include,
but are not limited to, those comprising, in addition to the
electrolyte solvents sufficient to provide the desired semi-solid
or gel state, ionic conductive polymers selected from the group
consisting of: polyethylene oxides (PEO), polypropylene oxides,
polyacrylonitriles, polysiloxanes, polyimides, polyphosphazenes,
polyethers, sulfonated polyimides, perfluorinated membranes
(NAFION.TM. resins), polydivinyl polyethylene glycols, polyethylene
glycol diacrylates, polyethylene glycol dimethacrylates,
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 ionic electrolyte
salt.
[0073] Examples of useful solid polymer electrolytes include, but
are not limited to, those comprising ionic conductive polymers
selected from the group consisting of: polyethers, polyethylene
oxides (PEO), polypropylene oxides, polyimides, polyphosphazenes,
polyacrylonitriles (PAN), polysiloxanes, polyether grafted
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 ionic
electrolyte salt. The solid polymer electrolytes suitable for use
in this invention may optionally further comprise one or more
electrolyte solvents, typically less than 20% by weight.
[0074] To improve the ionic conductivity and other electrochemical
properties of the electrolyte element, the organic electrolyte
typically comprises one or more ionic electrolyte salts. As used
herein, liquid electrolytes, gel polymer electrolytes, and solid
polymer electrolytes comprise an ionic electrolyte salt.
[0075] 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, MSCN, MI, MBr, 1
[0076] and the like, where M is Li or Na. Other electrolyte salts
useful in the practice of this invention are lithium polysulfides,
lithium salts of organic ionic polysulfides and those disclosed in
U.S. Pat. No. 5,538,812 to Lee et al. Preferred ionic electrolyte
salts are LiI, LiSCN, LiSO.sub.3CF.sub.3 (lithium triflate) and
LiN(SO.sub.2CF.sub.3).sub.2 (lithium imide).
[0077] Since the microporous pseudo-boehmite layer of this
invention is usually impermeable to high molecular weight materials
such as the ionic conductive polymers typically used in gel polymer
electrolytes and solid polymer electrolytes, it is preferable to
introduce the ionic conductive polymer component of the electrolyte
in a low molecular weight monomer or oligomer form into the pores
of the pseudo-boehmite layer. Subsequently, the low molecular
weight monomer or oligomer may be cured into an ionic conductive
polymer to provide the desired type of polymer electrolyte.
Suitable monomers or oligomers include, but are not limited to,
heat- or radiation-curable monomers or oligomers. Examples include
divinyl ethers such as tetraethyleneglycol divinyl ether. To
provide sensitivity to ultraviolet (UV) or visible radiation where
the monomers or oligomers do not absorb significantly, a
conventional photosensitizer may be added to cause curing of the
monomers or oligomers into a polymeric material. For example, a
small amount, such as 0.2% by weight of the monomers or oligomers,
of a UV sensitizer, such as ESCURE KTO (a trademark for a
photosensitizer available from Sartomer Inc., Exton, Pa.), may be
added to the monomers or oligomers.
[0078] The fraction of the pores of the pseudo-boehmite layer that
are filled with ionic conductive polymer may vary from 2 to 100%
depending on the type of polymer electrolyte desired. For gel
polymer electrolytes, it is preferred to fill 15 to 80% of the
pores with the ionic conductive polymer and then to fill the
remainder of the pores with electrolyte solvents and ionic
electrolyte salts. Typically, the ionic conductive polymer swells
in the presence of the electrolyte solvents and ionic electrolyte
salts to form a semi-solid or gel polymer electrolyte. A particular
advantage of the ultrafine pores and strong capillary action of the
pseudo-boehmite separator of the present invention is its excellent
wetting by a broad variety of electrolytes and retention of these
electrolytes in the pores. Accordingly, it is possible to
incorporate liquid or tacky electrolyte materials into the
nanoporous matrix of the pseudo-boehmite separator without having a
significant excess of liquid or tacky material on the surface.
Preferably, the electrolyte material is held below the outermost
surface of the pseudo-boehmite separator during the cell
fabrication process. For example, this is useful in preventing the
tacky surfaces of solid or gel polymer electrolytes from
interfering with the fabrication processes of winding or layering a
multiple layer construction of an electric current producing cell.
For liquid organic electrolytes, no polymer is required, and the
electrolyte composition may consist of only electrolyte solvents
and ionic electrolyte salts.
[0079] In a most preferred embodiment, the method of producing the
electrolyte element comprises a substrate with a cathode coating
layer on at least one of its outermost surfaces, and the liquid
mixture comprising the boehmite sol is coated onto this cathode
coating layer and, after drying, the surface of the pseudo-boehmite
layer is contacted with an organic electrolyte to cause the
infusion of the electrolyte into the pores of the pseudo-boehmite
layer.
[0080] Electric Current Producing Cells
[0081] The present invention provides an electric current producing
cell comprising a cathode and an anode and an electrolyte element
interposed between the cathode and the anode, wherein the
electrolyte element comprises a microporous pseudo-boehmite layer
and an electrolyte, preferably an organic electrolyte, contained
within the pores of the pseudo-boehmite layer, as described herein
for the microporous pseudo-boehmite layer or separator and the
electrolyte element of the present invention. This pseudo-boehmite
layer has a pore volume from 0.02 to 2.0 cm.sup.3/g, before the
introduction of the electrolyte, and has an average pore diameter
from 1 nm to 300 nm, as described herein for the microporous
pseudo-boehmite separator.
[0082] Although the electric current producing cell of the present
invention may be utilized for a wide variety of primary and
secondary batteries known in the art, it is preferred to utilize
these cells in secondary or rechargeable batteries where the many
features of a free standing or directly coated microporous
pseudo-boehmite separator may be employed to help control the
chemistry of the active materials through many repeated discharge
and charge cycles.
[0083] Suitable anode active materials for the electric current
producing 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 of the present
invention 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 especially useful. Preferred anode
active materials are lithium metal, lithium-aluminum alloys,
lithium-tin alloys, lithium-intercalated carbons, and
lithium-intercalated graphites.
[0084] Suitable cathode active materials for use in the cathode
active layers of the cathodes for the electric current producing
cells of this invention include, but are not limited to,
electroactive transition metal chalcogenides, electroactive
conductive polymers, and electroactive sulfur-containing materials.
The term "sulfur-containing material," as used herein, relates to
cathode active materials which comprise the element sulfur in any
form, wherein the electrochemical activity involves the braking or
forming of sulfur-sulfur covalent bonds. In one embodiment, the
electroactive sulfur-containing material comprises elemental
sulfur. In one embodiment, the electroactive sulfur-containing
material is organic, that is, it comprises both sulfur atoms and
carbon atoms. In one embodiment, the electroactive
sulfur-containing material comprises a sulfur-containing polymer,
wherein the electroactive sulfur-containing polymer, in its
oxidized state, comprises a covalent polysulfide moiety of the
formula, --S.sub.m--, wherein m is an integer equal to or greater
than 3, preferably m is an integer from 3 to 10, most preferably m
is an integer equal to or greater than 6, and particularly most
preferably m is an integer equal to or greater than 8.
[0085] Cathode active materials for use in the cathodes for the
electric current producing cells of the present invention include,
but are not limited to, electroactive sulfur-containing cathode
materials which, 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, preferably m is an integer from 3 to 10, most
preferably m is an integer equal to or greater than 6, and
particularly most preferably m is an integer equal to or greater
than 8. Examples of these preferred cathode materials include
elemental sulfur and carbon-sulfur polymer materials, as described
in U.S. Pat. Nos. 5,529,860; 5,601,947; and 5,690,702; in U.S.
patent application Ser. No. 08/602,323, all by Skotheim et al.; and
in U.S. patent application Ser. No. 08/995,112 to Gorkovenko et
al., all to the common assignee.
[0086] In a most preferred embodiment, the polysulfide moiety,
--S.sub.m--, of the carbon-sulfur polymer material is covalently
bonded by one or both of its terminal sulfur atoms on a side group
to the polymer backbone chain of the polymer material. In another
most preferred embodiment, the polysulfide moiety, --S.sub.m--, of
the carbon-sulfur polymer material is incorporated into the polymer
backbone chain of the polymer material by covalent bonding of the
polysulfide moiety's terminal sulfur atoms. In another most
preferred embodiment, the carbon-sulfur polymer material with
polysulfide, --S.sub.m--, groups, wherein m is an integer equal to
or greater than 3, comprises greater than 75 weight percent of
sulfur.
[0087] In one embodiment, the electroactive sulfur-containing
material comprises a sulfur-containing polymer comprising an ionic
polysulfide moiety selected from the group consisting of: ionic
--S.sub.m.sup.- moieties and ionic S.sub.m.sup.2- moieties, wherein
m is an integer equal to or greater than 3, and preferably m is an
integer equal to or greater than 8. Examples of these
sulfur-containing materials include sulfur-containing polymers
comprising ionic --S.sub.m.sup.- moieties, as described in U.S.
Pat. No. 4,664,991 to Perichaud et al, and sulfur-containing
polymers comprising ionic S.sub.m.sup.2- moieties, as described in
the aforementioned U.S. Pat. No. 4,664,991 and in European Pat. No.
250,518 B1 to Genies. In one embodiment, the polymer backbone chain
of the sulfur-containing polymer having an ionic polysulfide moiety
comprises conjugated segments. In one embodiment, the polysulfide
moiety, --S.sub.m.sup.-, is covalently bonded by one of its
terminal sulfur atoms on a side group to the polymer backbone chain
of the sulfur-containing polymer. In one embodiment, the
sulfur-containing polymer having an ionic polysulfide moiety
comprises greater than 75 weight percent of sulfur.
[0088] In another embodiment of the electric current producing cell
of the present invention, the electrolyte of the electrolyte
element is an organic electrolyte comprising one or more materials
selected from the group consisting of: liquid electrolytes, gel
polymer electrolytes and solid polymer electrolytes.
[0089] A method for forming the electric current producing cell of
the present invention comprises providing an anode and a cathode
and interposing an electrolyte element of the present invention, as
described herein, between the anode and the cathode.
[0090] In one embodiment of the methods of forming the electric
current producing cell, the electrolyte of the electrolyte element
is an organic electrolyte comprising one or more materials selected
from the group consisting of: liquid electrolytes, gel polymer
electrolytes, and solid polymer electrolytes.
[0091] The flexibility of the product designs and methods of
forming the separators and electrolyte elements of the present
invention provide the option of effectively incorporating the ionic
electrolyte salt into the electric current producing cell at a
later or final stage of fabricating materials into the
electrochemically active cell. This may be advantageous when the
ionic electrolyte salt is hygroscopic and difficult to coat as part
of a gel polymer or solid polymer electrolyte composition and then
difficult to keep from absorbing water before fabrication and
hermetic sealing of the cell in a dry room facility. This may also
be advantageous when the ionic electrolyte salt increases the
viscosity and otherwise interferes with the wetting and penetration
of a liquid or polymer electrolyte into the separator and cathode
layers during the filling of the cell. In a preferred embodiment,
after the electrolyte element is enclosed between the anode and
cathode, there is a subsequent step comprising the imbibition of a
solution comprising one or more ionic electrolyte salts and one or
more electrolyte solvents into the electrolyte element. In a most
preferred embodiment, the electrolyte element that is enclosed
between the anode and the cathode initially comprises the
microporous layer and electrolyte solvents, but contains no ionic
electrolyte salt; and the ionic electrolyte salts as may be
required for the electrolyte element are provided by a subsequent
step of imbibing a solution comprising one or more ionic
electrolyte salts and one or more electrolyte solvents into the
electrolyte element. To achieve the desired final concentration of
ionic electrolyte salts in the organic electrolyte, the
concentration of ionic electrolyte salts in the imbibed solution
will be correspondingly much greater than this desired final
concentration.
EXAMPLES
[0092] 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
[0093] A microporous layer of pseudo-boehmite with polyvinyl
alcohol binder present was prepared according to the following
procedure. A coating mixture with a solids content of about 8%
comprising 7 weight percent (solid content) of CATALOID AS-3 (a
tradename for boehmite sol available from Catalysts & Chemicals
Ind. Co., Ltd., Tokyo, Japan) and 0.7 weight percent (solid
content) of AIRVOL 125 (a trademark for polyvinyl alcohol polymer
available from Air Products, Inc., Allentown, Pa.) in water was
prepared. This coating mixture was coated on 100 micron thick
MELINEX 516 film (a trademark for polyethylene terephthalate (PET)
film available from ICI Polyester, Wilmington, Del.) using a gap
coater so that the dry pseudo-boehmite coating thickness would be
25 microns and followed by drying at 130.degree. C. Alternatively,
the dry coating thickness of 25 microns was obtained by multiple
passes of coating application and drying such as two consecutive
coating applications with thicknesses of about 12.5 microns of dry
coating. After drying, the coated film was soaked in a solution of
67:33 by volume of water:isopropanol. This lowered the adhesion of
the coating to the PET film so that the 25 micron pseudo-boehmite
coating could be delaminated from the PET film substrate.
[0094] The pore volume of this 25 micron free standing
pseudo-boehmite film was determined by the method of soaking a
piece of film with a known area in liquid polyethylene glycol
dimethyl ether, wiping the excess liquid from the film surface, and
weighing the film before and after imbibing the liquid. By this
method, using Equation I: 4 Pore Volume = [ W 1 - W 2 ] / d W 2
I
[0095] wherein W.sub.1 is the weight of the film when the pores are
completely filled with the liquid, W.sub.2 is the weight of the
film with no liquid present in the pores, and d is the density of
the liquid, the pore volume of the 25 micron pseudo-boehmite film
was measured to be 0.67 cm.sup.3/g. The porosity was calculated as
65% using Equation III. Using the same method, the porosity of
CELGARD 2500 polyethylene separator film (trademark for porous
polyolefin films, such as this 25 micron thick polyethylene film,
available from Hoechst Celanese Corporation, Charlotte, N.C.) was
calculated as 48%. As an alternative method, the actual density of
the 25 micron pseudo-boehmite film was measured as 1.0 g/cm.sup.3,
and the theoretical density (assuming no pores) was calculated as
2.8 g/cm.sup.3. From this, by subtracting the reciprocal of the
theoretical density from the reciprocal of the measured density as
in Equation II, the pore volume was estimated as 0.64 cm.sup.3/g.
The porosity was calculated as 64%.
Example 2
[0096] A 5% by weight solution of a 1:1 ratio by weight of PHOTOMER
4028 (a trademark for radiation curable oligomers, such as this
ethoxylated bisphenol A diacrylate, available from Henkel Corp.,
Ambler, Pa.) and PHOTOMER 6210 (an ethoxylated aliphatic urethane
diacrylate also available from Henkel Corp., Ambler, Pa.) was
prepared by dissolving these oligomers in methyl acetate. To this
solution, 0.2% by weight (based on the total weight of the
oligomers) of ESCURE KTO (a trademark for photosensitizers
available from Sartomer Inc., Exton, Pa.) was added and dissolved
by stirring to give the final solution for treatment of the
pseudo-boehmite film.
[0097] A 25 micron thick free-standing pseudo-boehmite film,
prepared according to Example 1, was dipped in the above solution
of oligomers and photosensitizer so that the pores of the
pseudo-boehmite film were completely filled with the solution.
Excess liquid was removed from the surface of the pseudo-boehmite
film by using a smooth coating blade. The film was then dried at
25.degree. C. in a laboratory hood with moderate exhaust air flow.
The dried film was then cured by placing it on the conveyor belt of
a FUSION Model P300 UV exposure unit (a tradename for radiation
curing equipment available from Fusion Systems Company, Torrance,
Calif.) and exposing it to the UV lamps for 30 seconds. The
resulting 25 micron thick pseudo-boehmite separator film with
polymer treatment was insoluble in water and in 1,3-dioxolane,
methyl acetate, and dimethoxyethane (DME). By weighing the
pseudo-boehmite separator film before and after the polymer
treatment, the increase in weight from a single dipping process and
subsequent cure, as described above, corresponded to filling about
18% of the available void volume of the pseudo-boehmite layer with
cured polymer materials. Since the porosity or void volume of the
pseudo-boehmite layer of Example 1 is about 65%, the void volume of
the pseudo-boehmite layer remaining after this polymer treatment
was calculated to be about 53%.
Comparative Example 1
[0098] A cathode was prepared by coating a mixture of 55 parts of
elemental sulfur, 15 parts of CAB-O-SIL TS-530 silica (a trademark
for silica pigment available from Cabot Corporation, Tuscola,
Ill.), 15 parts of conductive carbon pigment (Shawingan Acetylene
Black or SAB-50, a trademark for carbon pigments available from
Chevron Corporation, Baytown, Tex.), and 15 parts of polyethylene
oxide (PEO) binder (5,000,000 molecular weight available from
Polysciences Inc., Warrington, Pa.) in acetonitrile as the solvent
onto an 18 micron thick nickel foil substrate to give a cathode
coating thickness of about 25 microns. The anode was lithium foil
of about 175 microns in thickness. The electrolyte was a 1.0 M
solution of lithium triflate in tetraethylene glycol dimethyl ether
(tetraglyme). The porous separator used was CELGARD 2500.
[0099] The above components were combined into a layered structure
of cathode/separator/anode with the liquid electrolyte filling the
void areas of the separator and cathode to form disc-shaped CR2032
coin cells of 2 cm.sup.2 in area.
[0100] Discharge-charge cycling on these cells was done at 0.2 mA
or 0.1 mA/cm.sup.2 for the first two cycles and then at 1 mA or 0.5
mA/cm.sup.2 for additional cycles. The average capacity for 4 coin
cells was 1.9 mAh on the first cycle and 1.3 mAh on the fifth
cycle.
[0101] Two different coin cells were cycled at 0.4 mA (0.2
mA/cm.sup.2) discharge and charge from the first cycle. The average
capacity for these 2 coin cells with CELGARD 2500 polyethylene
separator was 0.8 mAh (specific capacity of 466 mAh/g of elemental
sulfur in the cell) on the fifth cycle and was 0.7 mAh (specific
capacity of 408 mAh/g) on the 15th cycle.
Example 3
[0102] Coin cells were constructed as in Comparative Example 1
except that the 25 micron pseudo-boehmite separator film from
Example 1 was substituted for the 25 micron CELGARD 2500
polyethylene film. Discharge-charge cycling was done under the same
first set of conditions (0.2 mA for first two cycles and then 1 mA)
as in Comparative Example 1. The average capacity for 2 coin cells
with the 25 micron pseudo-boehmite separator of Example 1 was 1.9
mAh on the first cycle and 1.4 mAh on the fifth cycle. This shows
cell performance and cycling similar to Comparative Example 1 with
the conventional porous polyethylene separator (CELGARD 2500).
[0103] Another coin cell was cycled at a second set of conditions,
namely 1.0 mA (0.5 mA/cm.sup.2) discharge and charge from the first
cycle. The capacity of this coin cell with pseudo-boehmite
separator was 1.0 mAh (specific capacity of 633 mAh/g of elemental
sulfur in the cell) on the fifth cycle and was 0.7 mAh (specific
capacity of 466 mAh/g) on the 33rd cycle. Although the
significantly higher discharge and charge rates for this cell would
be expected to lower its capacity and cycling stability compared to
the two coin cells in Comparative Example 1 with lower
discharge-charge rates, this cell with pseudo-boehmite separator
had better capacity and cycle stability than the two cells with the
porous polyethylene separator tested at 0.4 mA charge and discharge
in Comparative Example 1.
Comparative Example 2
[0104] A cathode was prepared by coating a mixture of 80 parts of a
carbon-sulfur polymer with an average polysulfide chain length of
about 5 and with about 90% sulfur content (made by the process
described in Example 2 in a copending U.S. patent application Ser.
No. 08/995,112 to Gorkovenko et al. to the common assignee); 10
parts of a conductive carbon pigment, PRINTEX XE-2 (a trademark for
carbon pigments available from Degussa Corporation, Akron, Ohio); 5
parts of PYROGRAF-III carbon nanofiber (a trademark for carbon
filaments available from Applied Sciences, Inc., Cedarville, Ohio);
and 5 parts of a 4:1 by weight ratio of
poly(acrylamide-co-diallyldimethylammonium chloride) (available
from Aldrich Chemical Company, Milwaukee, Wis.) and of polyethylene
oxide or PEO (as described in Comparative Example 1) at about 12%
solids in a blend of 13:1 ethanol:water onto a 17 micron thick
conductive carbon and resin coated aluminum foil substrate (Product
No. 60303 available from Rexam Graphics, South Hadley, Mass.). The
cathode coating thickness was about 25 microns. The anode was
lithium foil of about 75 microns in thickness. The electrolyte was
a 0.75 M solution of lithium imide (available from 3M Corporation,
St. Paul, Minn.) in a 50:50 by volume mixture of 1,3-dioxolane and
dimethoxyethane (DME). The porous separator used was E25 SETELA (a
trademark for polyolefin separators available from Tonen Chemical
Corporation, Tokyo, Japan, and also available from Mobil Chemical
Company, Films Division, Pittsford, N.Y.).
[0105] The above components were combined into a layered structure
of cathode/separator/anode with the liquid electrolyte filling the
void areas of the separator and cathode to form vial cells of about
40 cm.sup.2 in area. Discharge-charge cycling on these cells was
done at 8 mA or 0.2 mA/cm.sup.2 with discharge cutoff at a voltage
of 1.3 V and charge cutoff at 150% overcharge of the previous
discharge capacity or 2.8 V, whichever came first. Typical capacity
of these cells was 32 mAh (specific capacity of 575 mnAh/g of
carbon-sulfur polymer in the cell) at the fifth cycle with a total
capacity fade of about 25% over the next 25 cycles. The charging at
each cycle was to the 150% overcharge limit. The 2.8 V cutoff was
not reached on the charging half of the cycle. Thus, charging was
relatively inefficient and required at least a 50% extra charge on
each cycle to recharge after the discharge.
Comparative Example 3
[0106] Vial cells were constructed as in Comparative Example 2
except that 80 parts of elemental sulfur were substituted for the
80 parts of carbon-sulfur polymer in the mixture used to prepare
the cathode. Discharging of the cells was done at 0.33 mA/cm.sup.2,
and charging of the cells was done at 0.22 mA/cm.sup.2. The
discharge and charge cutoff conditions were the same as in
Comparative Example 2. Typical specific capacity of these cells was
460 mAh/g of elemental sulfur in the cell at the 5th cycle with no
significant loss in specific capacity out to 70 cycles. As in
Comparative Example 2, the charging at each cycle on these cells
was to the 150% overcharge limit. The 2.8 V cutoff was not reached
on the charging half of the cycle.
Example 4
[0107] Vial cells were constructed as in Comparative Example 2
except that the 25 micron pseudo-boehmite separator film with
polymer treatment from Example 2 was substituted for the 25 micron
E25 SETELA separator film. Discharge-charge cycling was done under
the same set of conditions as in Comparative Example 2. Typical
capacity of these cells was 25 mA (specific capacity of 613 mAh/g
of carbon-sulfur polymer in the cell) at the fifth cycle with a
total capacity fade of about 14% over the next 25 cycles and of
about 28% over the next 65 cycles. The charging at each cycle was
to the 2.8 V cutoff. The 150% overcharge limit was not reached on
the charging half of the cycle. The amount of overcharge before
reaching the 2.8 V cutoff remained in the range of 105 to 120%
during the cycling. This data shows the specific capacities,
capacity fading, and charging conditions and efficiencies of the
cells with the pseudo-boehmite separator to be significantly better
than that of the cells with the conventional polyolefin separator
of Comparative Example 2.
[0108] A self-discharge test was performed by resting the cells for
24 hours after the 5th, 10th, and 30th cycles and then measuring
the loss in capacity. The self-discharge was 5.8%, 2.6%, and 2.7%
after the 5th, 10th, and 30th cycles, respectively. By contrast the
self-discharge of cells from Comparative Example 2 was in the range
of 10 to 20%.
Example 5
[0109] Vial cells were constructed as in Comparative Example 3
except that the 25 micron pseudo-boehmite separator film with
polymer treatment from Example 2 was substituted for the 25 micron
E25 SETELA separator film. Discharge-charge cycling was done under
the same set of conditions as in Comparable Example 3. Typical
specific capacity of these cells was 600 mAh/g of elemental sulfur
in the cell at the 5th cycle with a total capacity fade of about
15% out to 70 cycles. The charging at each cycle was to the 2.8 V
cutoff. The 150% overcharge limit was not reached on the charging
half of the cycle. The amount of overcharge before reaching the 2.8
V cutoff remained in the range of 110 to 140% during the cycling.
This data shows the specific capacities and charging conditions and
efficiencies of the cells with the pseudo-boehmite separator to be
significantly better than that of the cells with the conventional
polyolefin separator of Comparative Example 3. The capacity fade of
the cells with the pseudo-boehmite separator is greater than in
Comparative Example 3, but the specific capacity at the 70th cycle
and the accumulated capacity out to 70 cycles are significantly
better than Comparative Example 3.
Example 6
[0110] Vial cells were constructed as in Example 5 except that an
11 micron thick pseudo-boehmite separator film with polymer
treatment was used instead of the 25 micron thick pseudo-boehmite
separator film with polymer treatment. The 11 micron
pseudo-boehmite separator film with polymer treatment was prepared
as in Examples 1 and 2, except that DISPAL 11N7-12 (a trademark for
boehmite sol available from CONDEA Vista Company, Houston, Tex.)
was used instead of CATALOID AS-3, and the gap of the gap coater
was reduced so that the dry pseudo-boehmite coating thickness was
11 microns. Discharge-charge cycling performed under the same set
of conditions as in Example 5 gave similar specific capacities,
capacity fading, and charging properties as in Example 5. This
shows that the large reduction in thickness in the pseudo-boehmite
separator from 25 to 11 microns still provides satisfactory
performance and retains significant advantages over the thicker 25
micron polyolefin separators. No formation of short circuits was
observed during cycling of these cells to over 100 cycles. This is
further evidence that the pseudo-boehmite separator is stable
during cycling and is not degrading such that the formation of
short circuits occurs.
Example 7
[0111] Vial cells were constructed as in Example 6, except that a
12 micron thick pseudo-boehmite separator layer was coated directly
on the cathode instead of using the free standing separator of
Example 6; the oligomer treatment with curing was done after the
direct coating of the separator; and changes were made in the
cathode and the electrolyte. As in Example 6, DISPAL 11N7-12
instead of CATALOID AS-3 was used for the boehmite sol. The
approximately 8% by weight solids mixture of 10:1 DISPAL 12
N7-12:AIRVOL 125 in water was applied in two coating passes of
approximately equal coating weight application to give a total
pseudo-boehmite layer thickness of 12 microns. The cathode used was
the same as in Comparative Example 3, except that 85 parts of
elemental sulfur was used instead of 80 parts; the amount of
conductive carbon pigment, PRINTEX XE-2, was increased to 12 parts;
the amount of PYROGRAF-III carbon nanofiber was reduced to 2 parts;
and no polymeric binder was present. The cathode coating thickness
was about 16 microns. The UV curable oligomer and photosensitizer
solution was the same as in Example 2. This solution was applied by
a blade coating to the outermost surface of the pseudo-boehmite
separator layer coated on the cathode so that the pores of the
pseudo-boehmite layer were completely filled with the solution. The
excess liquid was removed from the surface and the pseudo-boehmite
layer then dried and cured by UV radiation, as described in Example
2. The electrolyte was a 1.0 M solution of lithium trifate
(available from 3M Corporation, St. Paul, Minn.) in a 50:35:10:5 by
volume mixture of 1,3-dioxolane:diethylene glycol dimethyl ether
(diglyme):DME:o-xylene.
[0112] Discharge-charge cycling was done under the same set of
conditions as in Example 5. Typical specific capacities of these
cells were 550 mAh/g of elemental sulfur in the cell at the 5th
cycle with a total capacity fade of about 20% out to 100 cycles.
The charging at each cycle was to a 3.0 V cutoff. The 150%
overcharge limit was not reached on the charging half of the cycle.
The amount of overcharge before reaching the 3.0 V cutoff remained
in the range of 125 to 145% during the cycling. This data shows
that cells with a pseudo-boehmite separator layer coated directly
on the cathode provided comparable specific capacities, capacity
fading, and charging properties to cells with a free standing
pseudo-boehmite separator of similar composition and had
satisfactory performance at a dry thickness of only 12 microns.
Example 8
[0113] Vial cells were constructed as in Example 7, except that a
50:50 by weight mixture of PEO (600,000 molecular weight):PEO
(5,000,000 molecular weight), both available from Polysciences
Inc., Warrington, Pa., was used instead of the AIRVOL 125 polyvinyl
alcohol polymer; and the electrolyte as described in Comparative
Example 2 was used. Discharge, charge, and cutoff conditions were
the same as in Example 7. The specific capacities, capacity fading,
and charging properties were similar to those reported in Example
7. This illustrates that other polymeric binders, such as
polyethylene oxides, can be used instead of polyvinyl alcohol to
form the microporous separator of the present invention.
Comparative Example 4
[0114] The carbon electrode was prepared using 85 parts of graphite
powder (MCMB 10-28 available from Osaka Gas, Osaka, Japan), 5 parts
of conductive carbon (SAB-50), and 10 parts of KYNAR 500LVD binder
(a trademark for vinylidene fluoride-tetrafluoroethylene copolymer
available from Elf Atochem North America, Inc., Philadelphia, Pa.)
in N-methyl pyrrolidone as the solvent to give a coating thickness
of about 75 microns on a 13 micron thick copper foil substrate. The
other electrode was lithium foil of about 175 microns in thickness.
The electrolyte was a 1.0 M solution of LiPF.sub.6 in an 1:1 by
volume mixture of ethylene carbonate (EC): dimethyl carbonate
(DMC). The porous separator used was CELGARD 3401, a special grade
of polyolefin separator with a wetting agent treatment present.
[0115] The above components were combined into a layered structure
of carbon electrode/separator/lithium foil with the liquid
electrolyte filling the void areas of the separator and cathode to
form disc-shaped CR2032 coin cells of 2 cm.sup.2 in area.
[0116] Discharge-charge cycling on these cells was done at 0.2 mA
or 0.1 mA/cm.sup.2 for the first two cycles and then at 1 mA or 0.5
mA/cm.sup.2 for additional cycles. The average capacity for 8 of
these coin cells was 8.9 mAh on the second cycle.
Example 9
[0117] A coin cell was constructed as in Comparative Example 4,
except that the 25 micron pseudo-boehmite separator film from
Example 1 was substituted for the 25 micron CELGARD 3401 polyolefin
film. Discharge-charge cycling was done under the same conditions
as in Comparative Example 4. The carbon-lithium coin cell with the
25 micron pseudo-boehmite separator of Example 1 had a capacity of
6.0 mAh on the second cycle and a capacity of 6.2 mAh on the sixth
cycle. Although the capacity with the pseudo-boehmite separator is
lower on the second charge-discharge cycle than with the CELGARD
3401 polyolefin separator, the capacity is still good. The lack of
capacity fade upon charge-discharge cycling with the
pseudo-boehmite separator is also positive.
[0118] The pseudo-boehmite separator of this invention with its
excellent wetting properties exhibits flexibility in its broad
compatibility with various cathode and electrolyte solvent/lithium
salt combinations, such as sulfur-based cathodes and electrolyte
compositions for both sulfur-based and for lithium ion type
batteries, without requiring a special treatment with a wetting
agent. In particular, the wettability of the separator to the
different electrolytes commonly used in batteries is often
problematical. For example, the combination of electrolyte solvent
and lithium salt of Comparative Example 4 does not wet conventional
polyolefin separators, such as CELGARD 2500, adequately so a
surfactant or wetting agent treatment must be applied to the
separator surface. However, there is a concern about possible
negative effects of a surfactant treatment on battery
performance.
Comparative Example 5
[0119] A cathode was prepared by coating a mixture of 75 parts of
elemental sulfur (available from Aldrich Chemical Company,
Milwaukee, Wis.), 20 parts of PRINTEX XE-2 conductive carbon
pigment, and 5 parts of PYROGRAF-III carbon nanofibers dispersed at
10% solids in isopropanol onto a 17 micron thick conductive carbon
and resin coated aluminum foil substrate (Product No. 60303). After
heating at 130.degree. C. for 1 minute and calendering, the
thickness of the cathode active layer was about 10 microns. The
anode was lithium foil of about 50 microns in thickness. The
electrolyte was a 1.424 M solution of lithium imide in a 41.5:58.5
volume ratio of 1,3-dioxolane and dimethoxyethane. The porous
separator used was 16 micron thick E25 SETELA polyolefin
separator.
[0120] The above components were combined into a layered structure
of cathode/separator/anode, which was wound and compressed, with
the liquid electrolyte filling the void areas of the separator and
cathode active layer to form prismatic cells of 500 cm.sup.2 in
area. Discharge/charge cycling on these cells was done at 0.5/0.3
mA/cm.sup.2, respectively, with discharge cutoff at 1.3 V and
charge cutoff at 3 V or 120% of the discharge capacity of the last
cycle, whichever came first. Typical capacity of these cells was
about 500 mAh (specific capacity of 1030 mAh/g of the elemental
sulfur in the cell) at the 10th cycle with a total capacity fade of
about 14% over the next 50 cycles.
Example 10
[0121] A microporous layer comprising pseudo-boehmite with binder
present was prepared according to the following procedure. A
water-based coating mixture with a solids content of about 8%
comprising 86 parts by weight (solid content) of DISPAL 11N7-12
boehmite sol, 6 parts by weight (solid content) of AIRVOL 125
polyvinyl alcohol polymer, 3 parts by weight of polyethylene glycol
(molecular weight of 900,000 available from Aldrich Chemical
Company, Milwaukee, Wis.), and 5 parts by weight of polyethylene
glycol dimethylether, M-250 (available from Fluka Chemical Company,
Ronkonkoma, N.Y.), in water was prepared. This coating mixture was
coated directly on a cathode with a cathode active layer prepared
as described in Comparative Example 5, using a slot die extrusion
coating application with two successive coating applications of
similar thickness to provide a total dry pseudo-boehmite coating
thickness of about 12 microns.
[0122] A 5% by weight solution of a 3:2 ratio by weight of CD 9038
(a tradename for ethoxylated bisphenol A diacrylate available from
Sartomer Inc., Exton, Pa.) and CN 984 (a tradename for a urethane
acrylate available from Sartomer Inc., Exton, Pa.) was prepared by
dissolving these oligomers in ethyl acetate. To this solution, 0.2%
by weight (based on the total weight of acrylate oligomers) of
ESCURE KTO was added. This coating solution also contained 5% by
weight of CAB-O-SIL TS-530 silica pigment, which was dispersed in
the oligomer solution by sonication. The oligomer solution was
coated on the 12 micron thick microporous layer comprising
pseudo-boehmite and binder and then dried to remove the solvent
present. The coating thickness of the dried coating on top of the
pseudo-boehmite layer was about 4 microns. The dried film was then
cured by UV curing using a Model F450T UV exposure unit (available
from Fusion Systems Corporation, Rockville, Md.). After heating at
110.degree. C. for 4 minutes, these cathodes were ready for
assembly into electric current producing cells.
[0123] Prismatic cells of 500 cm.sup.2 in area were constructed as
described in Comparative Example 5. The electrolyte was the same as
that used in Comparative Example 5. Discharge/charge cycling was
performed on these cells at the same current densities as described
in Comparative Example 5. The specific capacity of the cells was
about 850 mAh/g for the elemental sulfur in the cell at the 5th
cycle with a total capacity fade of about 17% over the next 80
cycles. After 100 cycles, the average capacity of these cells was
77% of the specific capacity at the 5th cycle.
Example 11
[0124] 18% by weight of DISPAL 11N7-80 boehmite powder (available
from CONDEA Vista Company, Houston, Tex.) was added slowly into
ethanol solvent under stirring by a 1 horsepower laboratory mixer
and emulsifier (Ross Model ME-100LX) and stirred at about 2500 rpm
for 5 minutes followed by additional stirring for 15 minutes at
4750 rpm.
[0125] A microporous layer comprising pseudo-boehmite was prepared
according to the following procedure. A coating mixture with a
solid content of about 14% comprising 85 parts by weight (solid
content) of DISPAL 11N7-80 boehmite powder (18% solids dispersion
in ethanol), 12 parts by weight (solid content) of MOWITAL B60H (a
trademark for polyvinyl butyral binders available from Hoechst
Celanese Corporation, Charlotte, N.C.), and 3 parts by weight of
M-250 polyethylene glycol dimethyl ether in ethanol was prepared
and then coated as described in Example 10.
[0126] Prismatic cells were prepared as described in Example 10,
except that the microporous layer comprising pseudo-boehmite was
obtained by using the above ethanol-based boehmite coating mixture.
The electrolyte used was the same as that used in Example 10.
Discharge/charge cycling was performed on these cells at the same
current densities as in Example 10. The specific capacity of the
cells was about 1030 mAh/g of the elemental sulfur in the cell at
the 10th cycle with a total capacity fade of only about 13% over
the next 130 cycles. After 150 cycles, the average capacity of
these cells was above 85% of the specific capacity at the 10th
cycle.
[0127] While the invention has been described in detail and with
reference to specific and general embodiments thereof, it will be
apparent to one skilled in the art that various changes and
modifications can be made therein without departing from the spirit
and scope thereof.
* * * * *