U.S. patent application number 15/675798 was filed with the patent office on 2017-12-14 for polymer composition with electrophilic groups for stabilization of lithium sulfur batteries.
The applicant listed for this patent is Seeo, Inc.. Invention is credited to Hany Basam Eitouni, Russell Clayton Pratt, Kulandaivelu Sivanandan.
Application Number | 20170358822 15/675798 |
Document ID | / |
Family ID | 54322746 |
Filed Date | 2017-12-14 |
United States Patent
Application |
20170358822 |
Kind Code |
A1 |
Pratt; Russell Clayton ; et
al. |
December 14, 2017 |
POLYMER COMPOSITION WITH ELECTROPHILIC GROUPS FOR STABILIZATION OF
LITHIUM SULFUR BATTERIES
Abstract
A polymer to be used as a binder for sulfur-based cathodes in
lithium batteries that includes in its composition electrophilic
groups capable of reaction with and entrapment of polysulfide
species. Beneficial effects include reductions in capacity loss and
ionic resistance gain.
Inventors: |
Pratt; Russell Clayton; (San
Mateo, CA) ; Eitouni; Hany Basam; (Oakland, CA)
; Sivanandan; Kulandaivelu; (Fremont, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Seeo, Inc. |
Hayward |
CA |
US |
|
|
Family ID: |
54322746 |
Appl. No.: |
15/675798 |
Filed: |
August 14, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14718084 |
May 21, 2015 |
9774058 |
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15675798 |
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PCT/US14/62415 |
Oct 27, 2014 |
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14718084 |
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61981732 |
Apr 18, 2014 |
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61981735 |
Apr 18, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/622 20130101;
H01M 10/0565 20130101; H01M 4/5815 20130101; H01M 4/625 20130101;
B01J 39/19 20170101; H01M 4/136 20130101; H01M 10/052 20130101;
Y02E 60/10 20130101; H01M 2300/0082 20130101; H01M 2004/028
20130101 |
International
Class: |
H01M 10/0565 20100101
H01M010/0565; B01J 39/19 20060101 B01J039/19; H01M 10/052 20100101
H01M010/052; H01M 4/136 20100101 H01M004/136; H01M 4/62 20060101
H01M004/62; H01M 4/58 20100101 H01M004/58 |
Claims
1. An electrolyte comprising: a polymer that comprises: a first
monomer that is ionically conductive; and a second monomer that
includes an electrophilic group capable of nucleophilic
substitution; and an electrolyte salt; wherein the polymer
comprises: ##STR00018## wherein 1-x is the mole fraction of the
acrylonitrile monomer component.
2. The polymer of claim 1 wherein the electrolyte salt is a lithium
salt.
3. A cathode comprising: elemental sulfur; carbon; and the
electrolyte of claim 1.
4. The cathode of claim 3 wherein the elemental sulfur is mixed
with one or more additives selected from the group consisting of
carbon, silica, aluminum oxide, and titanium dioxide to form a
sulfur composite
5. The cathode of claim 3 further comprising a current collector in
electrical communication with the cathode.
6. An electrochemical cell comprising: a cathode comprising;
elemental sulfur; carbon; and the electrolyte of claim 1; a lithium
metal anode; and a separator between the cathode and the anode, the
separator providing a path for ionic conduction between the cathode
and the anode.
7. The electrochemical cell of claim 6 further comprising a layer
of the electrolyte between the cathode and the separator.
8. A block copolymer electrolyte comprising: a first lamellar
domain comprising a plurality of first polymer blocks comprising
the polymer of claim 1; and an electrolyte salt; wherein the first
lamellar domain forms a conductive portion of the electrolyte
material; and a second lamellar domain comprising a plurality of
second polymer blocks, the second lamellar domain adjacent to the
first lamellar domain; wherein the second lamellar domain forms a
structural portion of the electrolyte material.
9. The block copolymer electrolyte of claim 8 wherein the first
lamellar domain and the second lamellar domain comprise a plurality
of linear diblock copolymers.
10. The block copolymer electrolyte of claim 9 wherein the linear
diblock copolymer has a molecular weight of at least 150,000
Daltons.
11. The block copolymer electrolyte of claim 9 wherein the linear
diblock copolymer has a molecular weight of at least 350,000
Daltons.
12. The block copolymer electrolyte material of claim 8 wherein the
first lamellar domain and the second lamellar domain comprise a
plurality of linear triblock copolymers.
13. The block copolymer electrolyte of claim 8 wherein the second
polymer blocks comprise a non-ionic-conducting polymer with a bulk
modulus greater than 10.sup.7 Pa at 90 degrees C.
14. The block copolymer electrolyte of claim 8 wherein the second
polymer blocks comprise a component selected from a group
comprising styrene, methacrylate, vinylpyridine, vinylcyclohexane,
imide, amide, propylene, alphamethylstyrene and combinations
thereof.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Divisional Application of U.S. patent
application Ser. No. 14/718,084, filed May 21, 2015 which, in turn,
is a Continuation-in-Part Application of International Application
Number PCT/US14/62415, filed Oct. 27, 2014, which is a
Non-Provisional Application of U.S. Provisional Application No.
61/981,732, filed Apr. 18, 2014 and U.S. Provisional Application
No. 61/981,735, filed Apr. 18, 2014, all of which are included by
reference herein.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] This invention relates generally to lithium metal
electrochemical cells, and, more specifically, to sulfur-based
cathodes in lithium metal batteries with polymeric
electrolytes.
[0003] Sulfur-based materials are attractive cathode active
materials for lithium batteries due to their high lithium
capacities. For example, the theoretical lithium capacity for
elemental sulfur is 1675 mAh/g, and capacities for sulfur compounds
can be as high as 800 mAh/g or so, as compared with capacities
around 170 mAh/g for conventionally used cathode materials such as
lithium iron phosphate. However, lithium batteries that have
sulfur-based cathodes tend to have poor cycling stability due to
the formation and migration of lithium polysulfide salts (e.g.,
LiSx, 3<x<8) as well as the formation and/or diffusion of
elemental sulfur out of the cathode layer. These unbound
sulfur-containing species separate from the cathode layer, causing
irreversible capacity loss, and can migrate to the anode and
decompose, causing an increase in internal ionic resistance of the
cell or outright decomposition of the anode.
[0004] Lithium metal-based materials are attractive anode active
materials for lithium batteries due to their high specific
capacities of 3860 mAh/g. Coupling lithium metal anodes to
sulfur-containing cathodes would provide a very high specific
capacity cell, and would result in a high specific energy cell.
However, stable cycling and safe operation of batteries containing
lithium metal have proved elusive, no matter what cathode material
is used, due to either a reaction of the lithium metal with the
electrolyte or formation of lithium dendrites upon cycling.
[0005] Improvements in stability, cyclability and lifetime of
lithium-sulfur batteries are usually sought through the use of
sulfur composites in which inactive materials are combined with
sulfur to prevent diffusion of polysulfide and sulfur species.
Examples include using carbon structures or other molecular
encaging species that can physically and/or chemically sequester
sulfur and/or lithium polysulfides, or can react with sulfur to
form immobile species such as graphite or cyclized PAN that
chemically sequester the sulfur. Another example is using
single-ion conductors that allow transport of Li cations, but not
anions or elemental sulfur species. Examples of such single-ion
conductors include Li.sub.3N, LISICON, LIPON, Thio-LISICON,
Li.sub.2S--P.sub.2S.sub.5, and the like. Suppression of lithium
dendrites has been attempted by use of high modulus electrolytes
such as cross-linked PEO, block copolymer electrolytes, and
inorganic conductors.
[0006] What is really needed is a way to take full advantage of the
high lithium capacity of sulfur-containing cathode materials
coupled with lithium metal electrodes to make stable, long life
cycle electrochemical cells.
[0007] Lithium-sulfur couples have been studied as they have the
potential to produce batteries with higher capacity and higher
energy than conventional Li-ion batteries. However, there are many
problems with these systems. One problem is that sulfur is very
soluble in typical liquid electrolytes. In a conventional
sulfur-based electrochemical cell system, the sulfur in the cathode
(in the form of polysulfides, for example) dissolves in the
electrolyte and diffuses to the anode where it reacts with the
lithium to form lithium sulfides. Trapped at the anode in the
reduced state, the sulfur cannot be reoxidized to the original form
and be returned to the cathode. This leads to rapid capacity fade
and high impedance, resulting ultimately in cell death.
[0008] Another problem associated with lithium-sulfur systems
arises from loss of surface area in the electrodes. During cycling,
sulfur in the electrode region aggregates into larger particles,
permanently changing the morphology of the cathode. The change in
morphology results in reduced ionic and electronic conductivity.
Thus it has not been possible to produce viable battery systems
from lithium-sulfur couples.
[0009] It would be useful to construct a battery in which sulfur
could be used as the active cathode material in order to exploit
the high capacity and high energy that sulfur can provide.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The foregoing aspects and others will be readily appreciated
by the skilled artisan from the following description of
illustrative embodiments when read in conjunction with the
accompanying drawings.
[0011] FIG. 1A is a schematic drawing of a diblock copolymer
molecule, according to an embodiment of the invention.
[0012] FIG. 1B is a schematic drawing that shows how multiple
diblock copolymer molecules, as shown in FIG. 1A, arrange
themselves into a domain structure, according to an embodiment of
the invention.
[0013] FIG. 1C is a schematic drawing that shows how multiple
repeat domains, as shown in FIG. 1B, form a continuous
nanostructured block copolymer material, according to an embodiment
of the invention.
[0014] FIG. 2A is a schematic drawing of a triblock copolymer
molecule that includes two different polymer blocks, according to
an embodiment of the invention.
[0015] FIG. 2B is a schematic drawing that shows how multiple
triblock copolymer molecules, as shown in FIG. 2A, arrange
themselves into a domain structure, according to an embodiment of
the invention.
[0016] FIG. 2C is a schematic drawing that shows how multiple
repeat domains, as shown in FIG. 2B, form a continuous
nanostructured block copolymer material, according to an embodiment
of the invention.
[0017] FIG. 3A is a schematic drawing of a triblock copolymer
molecule that includes three different polymer blocks, according to
an embodiment of the invention.
[0018] FIG. 3B is a schematic drawing that shows how multiple
triblock copolymer molecules, as shown in FIG. 3A, arrange
themselves into a domain structure, according to an embodiment of
the invention.
[0019] FIG. 3C is a schematic drawing that shows how multiple
repeat domains, as shown in FIG. 3B, form a continuous
nanostructured block copolymer material, according to an embodiment
of the invention.
[0020] FIG. 4 is a schematic illustration of a lithium metal cell
with a sulfur-based cathode that uses a sulfur-sequestering
catholyte, according to an embodiment of the invention.
[0021] FIG. 5 is a schematic illustration of a lithium metal cell
with a sulfur-based cathode that has a layer of sulfur-sequestering
electrolyte between the cathode and the separator, according to an
embodiment of the invention.
[0022] FIG. 6 is a schematic illustration of a lithium metal cell
with a sulfur-based cathode in which individual cathode active
material particles are coated with a sulfur-sequestering
electrolyte, according to an embodiment of the invention.
[0023] FIG. 7 is a cross-sectional schematic drawing of an
electrochemical cell, according to an embodiment of the
invention.
[0024] FIG. 8 is a cross-sectional schematic drawing of an
electrochemical cell, according to another embodiment of the
invention.
SUMMARY
[0025] A new polymer composition is disclosed in the embodiments of
the invention. The new composition is an ionically conductive
polymer that includes an electrophilic group capable of or
configured to undergo nucleophilic substitution. The polymer may
have at least two different monomers, wherein a first monomer is
ionically conductive and a second monomer comprises the
electrophilic group capable of nucleophilic substitution. The
ionically conductive monomer may be any of ethylene oxides,
acrylonitriles, phosphoesters, ethers, amines, imides, amides,
alkyl carbonates, nitriles, siloxanes, phosphazines, olefins,
dienes, and combinations thereof. The electrophilic group may be
any of alkyl halides, alkyl sulfonates, alkyl phosphates, alkyl
carbonates, oxiranes, aryl halides, and/or aryl sulfonates. The
polymer may be used as an electrolyte with the addition of an
electrolyte salt, such as a lithium salt.
[0026] The polymer composition may combine with elemental sulfur,
carbon, and a metal salt to form a cathode. The elemental sulfur
may also have one or more additives, such as carbon, silica,
aluminum oxide, and titanium dioxide, to form a sulfur composite.
There may also be a current collector in electrical communication
with the cathode.
[0027] In one embodiment of the invention, an electrochemical cell
has a cathode as described above that includes a Li salt, a lithium
metal anode, and a separator between the cathode and the anode. The
separator provides a path for ionic conduction between the cathode
and the anode. In one arrangement, there is also a layer of the
polymer composition described above between the cathode and the
separator.
[0028] In another embodiment of the invention, a block copolymer
electrolyte has a first lamellar domain comprising a plurality of
first polymer blocks made from the polymer composition described
above and a salt and a second lamellar adjacent to the first
lamellar domain and comprising a plurality of second polymer blocks
the second domain. The first domain forms a conductive portion of
the electrolyte material. The second domain forms a structural
portion of the electrolyte material.
[0029] The first lamellar domain and the second lamellar domain may
comprise a plurality of linear diblock copolymers. The linear
diblock copolymer may have a molecular weight of at least 150,000
Daltons or at least 350,000 Daltons.
[0030] The first lamellar domain and the second lamellar domain may
comprise a plurality of linear triblock copolymers.
[0031] The second polymer blocks may comprise a
non-ionic-conducting polymer with a bulk modulus greater than
10.sup.7 Pa at 90 degrees C. The second polymer blocks may comprise
a component selected from a group comprising styrene, methacrylate,
vinylpyridine, vinylcyclohexane, imide, amide, propylene,
alphamethylstyrene and combinations thereof.
[0032] An electrochemical cell is disclosed. The cell has a cathode
that contains at least a SPAN cathode active material, an
electronically conducting agent, and a first polymer electrolyte
that contains a lithium salt, all mixed together. The cell has a
lithium anode and a separator positioned between the cell and the
anode. The anode may be a lithium metal film.
[0033] In one arrangement, the first polymer electrolyte is a
liquid and the cell also contains a binder. In another arrangement,
the first polymer electrolyte is a solid polymer electrolyte. The
separator may contain a second solid polymer electrolyte.
[0034] At least one of the first polymer electrolyte and the
separator may be configured to react chemically with elemental
sulfur. At least one of the first polymer electrolyte and the
separator may contain a radical-generating species, such as
bromine, TEMPO groups or pendant methacrylate groups.
[0035] At least one of the first polymer electrolyte and the
separator may be configured to react chemically with lithium
polysulfide. At least one of the first polymer electrolyte and the
separator may include an electrolyte salt and an ionically
conductive polymer that includes an electrophilic group capable of
nucleophilic substitution or an ionically conductive polymer that
includes an olefinic group capable of polysulfide addition.
[0036] At least one of the first polymer electrolyte and the
separator may be configured to sequester sulfur by physical
interaction. At least one of the first polymer electrolyte and the
separator may contain a linear copolymer of carbonates, ethylene
oxide (P(LC-EO)), and P(LC-EO)s which incorporate thioethers
linkages in addition to ether linkages (P(LC-TEO)). At least one of
the first polymer electrolyte and the separator may contain a
molecule that has a polymer backbone to which polar groups are
attached. At least one of the first polymer electrolyte and the
separator may contain a molecule that has a polyether backbone with
cyclic carbonates grafted as side groups (P(GC-EO)). The polymer
backbone may be any of (P(GN-EO) or P(GP-EO)), polyalkanes,
polyphosphazenes, or polysiloxanes. The polar groups may be any of
nitrile groups (GN), phosphonate groups (GP), prises poly
phosphorus esters.
[0037] At least one of the first polymer electrolyte and the
separator may be configured to sequester lithium polysulfide by
physical interaction. In one arrangement, at least one of the first
polymer electrolyte and the separator contains a polyelectrolyte,
such as a cationic polymer with counterions such as any of
Cl.sup.-, TFSI.sup.-, BETI.sup.-, ClO.sub.4.sup.-, BF.sub.4.sup.-,
PF.sub.6.sup.-, Triflate.sup.-, and BOB.sup.-. In another
arrangement, at least one of the first polymer electrolyte and the
separator contains an anionic polymer, such as any of Nafion.RTM.,
poly(styrene sulfonate), polyvinyl and sulfonate.
[0038] In one embodiment of the invention, at least one of the
first polymer electrolyte and the separator contains a block
copolymer that has a first lamellar domain made of first polymer
blocks that contain an ionically conductive polymer that includes
an electrophilic group capable of nucleophilic substitution and an
electrolyte salt and a second lamellar domain made of second
polymer blocks, the second domain adjacent the first lamellar
domain and forming a structural portion of the electrolyte
material. The second polymer blocks may have a component such as
styrene, methacrylate, vinylpyridine, vinylcyclohexane, imide,
amide, propylene, alphamethylstyrene and combinations thereof. The
first lamellar domain and the second lamellar domain may contain a
plurality of linear block copolymers, which may be diblock or
triblock copolymers.
[0039] In one embodiment of the invention, a cathode contains at
least SPAN cathode active material, an electronically conducting
agent, and a first polymer electrolyte with a lithium salt, all
mixed together. There may be a layer of a second polymer
electrolyte on the surface of the cathode film that faces the
separator layer in the battery. In one arrangement, the first
polymer electrolyte and the second polymer electrolyte are the
same. There may also be a current collector on the surface of the
cathode film that faces away from the separator.
[0040] The SPAN material may be mixed with one or more additives
such as any of carbon, silica, aluminum oxide, and titanium
dioxide, to form a sulfur composite.
[0041] The electronically conductive agent may be any of carbon
black, graphite, conductive carbons, and conductive polymers.
Exemplary conductive polymers include polythiophene, polyphenylene
vinylene, polypyrrole, polyphenylene sulfide, and cyclized
polyacrylonitrile.
[0042] In one arrangement, the cathode contains no fluorinated
polymers.
[0043] The first polymer electrolyte may be a solid block copolymer
that is either a diblock copolymer or a triblock copolymer. The
first block of the diblock or triblock copolymer may be ionically
conductive, such polyethers, polyamines, polyimides, polyamides,
poly(alkyl carbonates), polynitriles, polysiloxanes,
polyphosphazenes, polyolefins, polydienes, and combinations
thereof. The first block of the diblock or triblock copolymer may
be an ionically-conductive comb polymer that has a backbone and
pendant groups. The backbone may be any of polysiloxanes,
polyphosphazines, polyethers, polydienes, polyolefins,
polyacrylates, polymethacrylates, and combinations thereof. The
pendants may be any of oligoethers, substituted oligoethers,
nitrile groups, sulfones, thiols, polyethers, polyamines,
polyimides, polyamides, poly(alkyl carbonates), polynitriles, other
polar groups, and combinations thereof.
[0044] The second block of the diblock or triblock copolymer may be
any of polystyrene, hydrogenated polystyrene, polymethacrylate,
poly(methyl methacrylate), polyvinylpyridine, polyvinylcyclohexane,
polyimide, polyamide, polypropylene, polyolefins, poly(t-butyl
vinyl ether), poly(cyclohexyl methacrylate), poly(cyclohexyl vinyl
ether), poly(t-butyl vinyl ether), polyethylene, fluorocarbons,
polyvinylidene fluoride, and copolymers that contain styrene,
methacrylate, and/or vinylpyridine.
[0045] At least one of the first polymer electrolyte and the second
polymer electrolyte may sequester sulfur or lithium polysulfide by
chemical bonding. At least one of the first polymer electrolyte and
the second polymer electrolyte may contain an ionically conductive
polymer that includes an olefinic group capable of polysulfide
addition and an electrolyte salt. In one arrangement, the ionically
conductive polymer includes a first monomer and a second monomer:
the first monomer is ionically conductive and the second monomer
contains an olefinic group capable of polysulfide addition. The
ionically conductive monomer may be any of ethylene oxides,
acrylonitriles, phosphoesters, ethers, amines, imides, amides,
alkyl carbonates, nitriles, siloxanes, phosphazines, olefins,
dienes, and combinations thereof. The olefinic group may be an
allyl group, such as an allyloxymethyl and a vinyl group. In one
arrangement, at least one of the first polymer electrolyte and the
second polymer electrolyte contains a radical-generating species,
such as any of bromine, TEMPO
((2,2,6,6-Tetramethylpiperidin-1-yl)oxy or
(2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl) groups, and pendant
methacrylate groups.
[0046] At least one of the first polymer electrolyte and the second
polymer electrolyte may be configured to sequester sulfur or
lithium polysulfide physically. At least one of the first polymer
electrolyte and the second polymer electrolyte may contain a
molecule that has a polymer backbone to which polar groups are
attached. The backbone may be a polyether molecule that has cyclic
carbonates grafted as side groups (P(GC-EO)). The polymer backbone
may be any of (P(GN-EO), P(GP-EO)), polyalkanes, polyphosphazenes,
and polysiloxanes and the polar groups are can be either nitrile
groups (GN) or phosphonate groups (GP). The polar groups may be
poly phosphorus esters. At least one of the first polymer
electrolyte and the second polymer electrolyte may contain a linear
copolymer of carbonates, ethylene oxide (P(LC-EO)), or analogs of
P(LC-EO) that incorporate thioethers linkages in addition to ether
linkages (P(LC-TEO)). At least one of the first polymer electrolyte
and the second polymer electrolyte may contain a polyelectrolyte,
such as Cl.sup.-, TFSI.sup.-, BETI.sup.-, ClO.sub.4.sup.-,
BF.sub.4.sup.-, PF.sub.6.sup.-, Triflate.sup.-, or BOB.sup.-. At
least one of the first polymer electrolyte and the second polymer
electrolyte may contain an anionic polymer, such as any of
Nafion.RTM., poly(styrene sulfonate), polyvinyl and sulfonate.
DETAILED DESCRIPTION
[0047] The preferred embodiments are illustrated in the context of
lithium metal-sulfur electrochemical cells. The skilled artisan
will readily appreciate, however, that the materials and methods
disclosed herein will have application in a number of other
contexts where diffusion of sulfur or polysulfides is
undesirable.
[0048] All publications referred to herein are incorporated by
reference in their entirety for all purposes as if fully set forth
herein.
[0049] In the embodiments of the invention, a lithium metal battery
that has a sulfur-based cathode and a sulfur-sequestering
electrolyte is disclosed. The electrolyte may be internal to the
cathode or it may form a coating between the cathode and the
separator. The sulfur-sequestering electrolyte may act in any of a
few different ways. It may prevent formation and diffusion of
unbound lithium polysulfide and/or elemental sulfur. The
sequestration may be physical or it may be chemical. Beneficial
aspects of such a battery include high specific capacity and long
cycle life.
[0050] In this disclosure, the terms "negative electrode" and
"anode" are both used to mean "negative electrode." Likewise, the
terms "positive electrode" and "cathode" are both used to mean
"positive electrode."
[0051] The term "separator" is used herein to mean either: [0052] a
permeable membrane placed between a battery's anode and cathode,
which is filled with a liquid or gel electrolyte, or [0053] the
region between a battery's anode and cathode, which is filled with
a solid electrolyte membrane.
[0054] The term "solid polymer" is used herein to include solid
homopolymers, solid random copolymers and solid block
copolymers.
High Energy Cell with Lithium Metal and SPAN Electrodes
[0055] In one embodiment of the invention, a cell design combines a
lithium metal anode, a block copolymer separator electrolyte, a
composite cathode containing sulfur-bound cyclized
polyacrylonitrile (SPAN), and a cathode electrolyte (catholyte)
that acts as a sulfur and/or polysulfide barrier to prevent loss of
elemental sulfur and/or lithium polysulfides from the cathode
layer.
[0056] Generally, a cathode has at least the following components:
[0057] an electrochemically active cathode material; [0058]
electronically conductive additives; [0059] a polymeric binder to
hold the active material and conductive additives in place; and
[0060] a current collector backing the electrochemically active
material.
[0061] Examples of electronically conductive additives include
carbon black, graphite, vapor-grown carbon fiber (VGCF), graphene,
SuperP, Printex, Ketjenblack, and carbon nanotubes (CNTs). Examples
of binders include polyvinylidene fluoride (PVDF), poly(vinylidene
fluoride-hexafluoropropylene) (PVDF-HFP), polyacrylonitrile (PAN),
polyacrylic acid (PAA), alginate, polyethylene oxide (PEO),
carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR),
polypropylene oxide, and copolymers thereof. In a dry polymer
battery, the polymeric binder may or may not be ionically
conductive and may be accompanied by an additional polymer
electrolyte (catholyte) that contains a dissolved metal salt and
functions as a metal ion transporter. For lithium batteries,
lithium salts are used. Typical lithium salts include lithium
bis(trifluoromethanesulfonyl) imide (LiTFSI), lithium
bis(perfluoroethanesulfonyl) imide (LiBETI), LiClO.sub.4,
LiBF.sub.4, LiPF.sub.6, lithium trifluoromethanesulfonate (Li
triflate), and lithium bis(oxalato) borate (LiBOB). Salts of other
metals can be used if other metals form the basis of the cell.
Examples of such metals include Na, K, Mg, Ca, and Al.
[0062] In a sulfur-based cell, the cathode is generally fabricated
in the charged state (oxidized) and requires a source of lithium.
Anode choices may include lithium metal foil, lithiated graphite,
lithiated silicon, or the like. Use of lithium metal anodes in a
rechargeable cell requires a specialized separator that is
chemically stable to lithium as well as capable of preventing
dendritic growth during charging. Block copolymer electrolytes
provide a means of achieving the required mechanical strength and
ionic conductivity.
[0063] The separator layer between the anode and cathode is an
ionically conductive, but electronically insulative layer. Such a
layer may be a liquid-electrolyte-soaked porous plastic membrane in
conventional lithium-ion cells or a solid polymer electrolyte
coating in dry polymer cells. Combinations of these are also
possible. For a polymer electrolyte such as PEO that is a viscous
liquid or gel with poor mechanical properties, greater mechanical
strength can be achieved by forming block copolymers that have a
first PEO polymer block that is ionically conductive and a second
polymer block that is mechanically-stabilizing. In order for the
second block to provide mechanical stability, the cell is operated
at a temperature below the melting temperature (Tm) for crystalline
polymers or the glass transition temperature (T.sub.g) for
amorphous polymers.
Nanostructured Block Copolymer Electrolytes
[0064] FIG. 1A is a simplified illustration of an exemplary diblock
polymer molecule 100 that has a first polymer block 110 and a
second polymer block 120 covalently bonded together. In one
arrangement both the first polymer block 110 and the second polymer
block 120 are linear polymer blocks. In another arrangement, either
one or both polymer blocks 110, 120 has a comb (or branched)
structure. In one arrangement, neither polymer block is
cross-linked. In another arrangement, one polymer block is
cross-linked. In yet another arrangement, both polymer blocks are
cross-linked.
[0065] Multiple diblock polymer molecules 100 can arrange
themselves to form a first domain 115 of a first phase made of the
first polymer blocks 110 and a second domain 125 of a second phase
made of the second polymer blocks 120, as shown in FIG. 1B. Diblock
polymer molecules 100 can arrange themselves to form multiple
repeat domains, thereby forming a continuous nanostructured block
copolymer material 140, as shown in FIG. 1C. The sizes or widths of
the domains can be adjusted by adjusting the molecular weights of
each of the polymer blocks.
[0066] In one arrangement the first polymer domain 115 is ionically
conductive, and the second polymer domain 125 provides mechanical
strength to the nanostructured block copolymer.
[0067] FIG. 2A is a simplified illustration of an exemplary
triblock polymer molecule 200 that has a first polymer block 210a,
a second polymer block 220, and a third polymer block 210b that is
the same as the first polymer block 210a, all covalently bonded
together. In one arrangement the first polymer block 210a, the
second polymer block 220, and the third copolymer block 210b are
linear polymer blocks. In another arrangement, either some or all
polymer blocks 210a, 220, 210b have a comb (or branched) structure.
In one arrangement, no polymer block is cross-linked. In another
arrangement, one polymer block is cross-linked. In yet another
arrangement, two polymer blocks are cross-linked. In yet another
arrangement, all polymer blocks are cross-linked.
[0068] Multiple triblock polymer molecules 200 can arrange
themselves to form a first domain 215 of a first phase made of the
first polymer blocks 210a, a second domain 225 of a second phase
made of the second polymer blocks 220, and a third domain 215b of a
first phase made of the third polymer blocks 210b as shown in FIG.
2B. Triblock polymer molecules 200 can arrange themselves to form
multiple repeat domains 225, 215 (containing both 215a and 215b),
thereby forming a continuous nanostructured block copolymer 230, as
shown in FIG. 2C. The sizes of the domains can be adjusted by
adjusting the molecular weights of each of the polymer blocks.
[0069] In one arrangement the first and third polymer domains 215a,
215b are ionically conductive, and the second polymer domain 225
provides mechanical strength to the nanostructured block copolymer.
In another arrangement, the second polymer domain 225 is ionically
conductive, and the first and third polymer domains 215 provide a
structural framework.
[0070] FIG. 3A is a simplified illustration of another exemplary
triblock polymer molecule 300 that has a first polymer block 310, a
second polymer block 320, and a third polymer block 330, different
from either of the other two polymer blocks, all covalently bonded
together. In one arrangement the first polymer block 310, the
second polymer block 320, and the third copolymer block 330 are
linear polymer blocks. In another arrangement, either some or all
polymer blocks 310, 320, 330 have a comb (or branched) structure.
In one arrangement, no polymer block is cross-linked. In another
arrangement, one polymer block is cross-linked. In yet another
arrangement, two polymer blocks are cross-linked. In yet another
arrangement, all polymer blocks are cross-linked.
[0071] Multiple triblock polymer molecules 300 can arrange
themselves to form a first domain 315 of a first phase made of the
first polymer blocks 310a, a second domain 325 of a second phase
made of the second polymer blocks 320, and a third domain 335 of a
third phase made of the third polymer blocks 330 as shown in FIG.
3B. Triblock polymer molecules 300 can arrange themselves to form
multiple repeat domains, thereby forming a continuous
nanostructured block copolymer 340, as shown in FIG. 3C. The sizes
of the domains can be adjusted by adjusting the molecular weights
of each of the polymer blocks.
[0072] In one arrangement the first polymer domains 315 are
ionically conductive, and the second polymer domains 325 provide
mechanical strength to the nanostructured block copolymer. The
third polymer domains 335 provides an additional functionality that
may improve mechanical strength, ionic conductivity, chemical or
electrochemical stability, may make the material easier to process,
or may provide some other desirable property to the block
copolymer. In other arrangements, the individual domains can
exchange roles.
[0073] Choosing appropriate polymers for the block copolymers
described above is important in order to achieve desired
electrolyte properties. In one embodiment, the conductive polymer
(1) exhibits ionic conductivity of at least 10.sup.-5 Scm.sup.-1 at
electrochemical cell operating temperatures when combined with an
appropriate salt(s), such as lithium salt(s); (2) is chemically
stable against such salt(s); and (3) is thermally stable at
electrochemical cell operating temperatures. In one embodiment, the
structural material has a modulus in excess of 1.times.10.sup.5 Pa
at electrochemical cell operating temperatures. In one embodiment,
the third polymer (1) is rubbery; and (2) has a glass transition
temperature lower than operating and processing temperatures. It is
useful if all materials are mutually immiscible.
[0074] In one embodiment of the invention, the conductive phase can
be made of a linear or branched polymer. Conductive linear or
branched polymers that can be used in the conductive phase include,
but are not limited to, polyethers, polyamines, polyimides,
polyamides, alkyl carbonates, polynitriles, and combinations
thereof. The conductive linear or branched polymers can also be
used in combination with polysiloxanes, polyphosphazines,
polyolefins, and/or polydienes to form the conductive phase.
[0075] In another exemplary embodiment, the conductive phase is
made of comb (or branched) polymers that have a backbone and
pendant groups. Backbones that can be used in these polymers
include, but are not limited to, polysiloxanes, polyphosphazines,
polyethers, polydienes, polyolefins, polyacrylates,
polymethacrylates, and combinations thereof. Pendants that can be
used include, but are not limited to, oligoethers, substituted
oligoethers, nitrile groups, sulfones, thiols, polyethers,
polyamines, polyimides, polyamides, alkyl carbonates, polynitriles,
other polar groups, and combinations thereof.
[0076] Further details about polymers that can be used in the
conductive phase can be found in International Patent Application
Number PCT/US09/45356, filed May 27, 2009 (PCT Publication
WO2009146340 published Dec. 3, 2009), International Patent
Application Number PCT/US09/54709, filed Aug. 22, 2009 (U.S. Pat.
No. 8,691,928 issued Apr. 8, 2014), International Patent
Application Number PCT/US10/21065, filed Jan. 14, 2010 (PCT
Publication WO2010083325 published Jul. 22, 2010), International
Patent Application Number PCT/US10/21070, filed Jan. 14, 2010 (PCT
Publication WO2010083330 published Jul. 22, 2010), U.S.
International Patent Application Number PCT/US10/25680, filed Feb.
26, 2009 (PCT Publication WO2010101791 published Sep. 10, 2010),
and International Patent Application Number PCT/US10/25690, filed
Feb. 26, 2009 (U.S. Pat. No. 8,598,273 issued Dec. 3, 2013, all of
which are included by reference herein.
[0077] There are no particular restrictions on the electrolyte salt
that can be used in the block copolymer electrolytes. Any
electrolyte salt that includes the ion identified as the most
desirable charge carrier for the application can be used. It is
especially useful to use electrolyte salts that have a large
dissociation constant within the polymer electrolyte.
[0078] Suitable examples include alkali metal salts, such as Li
salts. Examples of useful Li salts include, but are not limited to,
LiPF.sub.6, LiN(CF.sub.3SO.sub.2).sub.2,
Li(CF.sub.3SO.sub.2).sub.3C, LiN(SO.sub.2CF.sub.2CF.sub.3).sub.2,
LiB(C.sub.2O.sub.4).sub.2, Li.sub.2B.sub.12F.sub.xH.sub.12-x,
Li.sub.2B.sub.12F.sub.12, and mixtures thereof.
[0079] In one embodiment of the invention, single ion conductors
can be used with electrolyte salts or instead of electrolyte salts.
Examples of single ion conductors include, but are not limited to
sulfonamide salts, boron based salts, and sulfates.
[0080] In one embodiment of the invention, the structural phase can
be made of glassy or crystalline polymers such as polystyrene,
hydrogenated polystyrene, polymethacrylate, poly(methyl
methacrylate), polyvinylpyridine, polyvinylcyclohexane, polyimide,
polyamide, polypropylene, poly (2,6-dimethyl-1,4-phenylene oxide)
(PXE), polyolefins, poly(t-butyl vinyl ether), poly(cyclohexyl
methacrylate), poly(cyclohexyl vinyl ether), polyethylene,
fluorocarbons, such as polyvinylidene fluoride, or copolymers that
contain styrene, methacrylate, or vinylpyridine.
[0081] Additional species can be added to nanostructured block
copolymer electrolytes to enhance the ionic conductivity, to
enhance the mechanical properties, or to enhance any other
properties that may be desirable.
[0082] The ionic conductivity of nanostructured block copolymer
electrolyte materials can be improved by including one or more
additives in the ionically conductive phase. An additive can
improve ionic conductivity by lowering the degree of crystallinity,
lowering the melting temperature, lowering the glass transition
temperature, increasing chain mobility, or any combination of
these. A high dielectric additive can aid dissociation of the salt,
increasing the number of Li+ ions available for ion transport, and
reducing the bulky Li+[salt] complexes. Additives that weaken the
interaction between Li+ and PEO chains/anions, thereby making it
easier for Li+ ions to diffuse, may be included in the conductive
phase. The additives that enhance ionic conductivity can be broadly
classified in the following categories: low molecular weight
conductive polymers, ceramic particles, room temp ionic liquids
(RTILs), high dielectric organic plasticizers, and Lewis acids.
[0083] Other additives can be used in the polymer electrolytes
described herein. For example, additives that help with overcharge
protection, provide stable SEI (solid electrolyte interface)
layers, and/or improve electrochemical stability can be used. Such
additives are well known to people with ordinary skill in the art.
Additives that make the polymers easier to process, such as
plasticizers, can also be used.
[0084] Further details about block copolymer electrolytes are
described in U.S. patent application Ser. No. 12/225,934, filed
Oct. 1, 2008 (U.S. Pat. No. 8,563,168 issued Oct. 22, 2013), U.S.
patent application Ser. No. 12/271,829, filed Nov. 14, 2008 (U.S.
Pat. No. 8,268,197 issued Sep. 18, 2012, and International Patent
Application Number PCT/US09/31356, filed Jan. 16, 2009 (U.S. Pat.
No. 8,889,301 issued Nov. 18, 2014), all of which are included by
reference herein.
SPAN Cathode Active Material
[0085] If the cathode of a lithium-sulfur cell consisted completely
of elemental sulfur, in theory, an energy density of more than
1,000 Wh/kg could be achieved. However, sulfur is neither ionically
nor electrically conductive, so a sulfur cathode includes additives
that supply these properties and which lower the energy density. In
addition, during the discharge of a lithium-sulfur cell, elemental
sulfur is usually reduced to soluble polysulfides. The polysulfides
can diffuse into other regions of the cell (and can even reach the
anode), where they are no longer able to participate in the
electrochemical reactions of subsequent charge/discharge cycles. In
addition, polysulfides may be dissolved in the electrolyte, where
they cannot be reduced further. Therefore, at present, the energy
density of lithium-sulfur cells is much lower than the theoretical
value, and is estimated to be between 400 Wh/kg and 600 Wh/kg. Even
worse, the service life of lithium-sulfur cells may be limited to
as few as 100 complete cycles or even less. Cycle life may be
affected by diffusion of polysulfides from the cathode to the anode
where they can react with the lithium metal anode and shorten its
life. Sulfur utilization in lithium-sulfur battery cells can be
significantly increased when the sulfur is bound to cyclized
polyacrylonitrile. Thus it is advantageous to use
polyacrylonitrile-sulfur (SPAN) composite as a cathode active
material.
[0086] In one embodiment, the cathode active material contains a
polyacrylonitrile-sulfur (SPAN) composite. SPAN is a composite
material that is produced by reacting polyacrylonitrile (PAN) with
sulfur (S). SPAN material has sulfur-carbon bonds which can bond
polysulfides to the SPAN polymer matrix. In such a SPAN composite,
the sulfur is fixedly bonded to a polymer structure on a
sub-nanometer/nanometer scale. In addition the sulfur is finely or
homogeneously distributed within the SPAN structure. SPAN has been
shown to offer good cycling stability with a high sulfur
utilization rate. In addition, SPAN has shown such good performance
even at high discharge rates (C rates).
[0087] In one embodiment of the invention, SPAN is produced by
reacting polyacrylonitrile with an excess of sulfur at a
temperature greater than or equal to 300.degree. C. In some
arrangements, temperatures greater than or 550.degree. C. are used.
The ratio of excess sulfur to polyacrylonitrile that is used
depends on the reaction temperature. The sulfur atoms may be in the
polyacrylonitrile-sulfur composite material both directly by
covalent sulfur-carbon bonds, as well as indirectly through one or
more covalent bonds, sulfur-sulfur and one or more sulfur-carbon
bonds may be connected to a particular cyclized polyacrylonitrile.
In this case, at least a portion of the sulfur atoms of the SPAN
composite material, for example in the form of polysulfides, is
covalently linked to a cyclized polyacrylonitrile. In such
composite materials are indications of sulfur-carbon bond, which
tie the polysulfides to the polymer matrix. Consequently, there is
a sulfur polyacrylonitrile composite having various functional
groups and chemical bonds, which can all have different properties
with respect to electrochemical performance, and aging
behavior.
[0088] In another embodiment of the invention, SPAN composite
material is produced by (a) converting polyacrylonitrile to
cyclized polyacrylonitrile, and (b) converting the cyclized
polyacrylonitrile with sulfur to form a polyacrylonitrile-sulfur
composite material. In step (a), an electrically conductive
cyclized polyacrylonitrile (cPAN) base is formed. In step (b), the
cPAN is reacted with electrochemically active sulfur takes place,
bonding the sulfur covalently to the electrically conductive
skeleton of the cPAN, thus forming polyacrylonitrile-sulfur
composite material (SPAN). By using a two-step method, reaction
conditions can be optimized for each partial reaction. It may be
interesting to note that step (a) is similar to a dehydrogenation
reaction known from the preparation of carbon fiber, and step (b)
is similar to a reaction from a different technical field, namely
the vulcanization reaction of rubber.
[0089] This cell design has numerous embodiments related to
different architectural configurations of a barrier catholyte as
well as numerous mechanistic processes for preventing loss of
sulfur and/or polysulfides. Different catholyte materials or
additives can be used to capture sulfur species and prevent
capacity fade.
[0090] The embodiments of the invention include various solid
polymer electrolyte materials that can be used as active or passive
barriers to prevent sulfur/polysulfide loss. Such polymers may or
may not be cross-linked. In general the disclosed materials and
related mechanism for preventing sulfur loss can be classified into
two groups: active (or chemical) and passive (or physical) barriers
to prevent sulfur/polysulfide loss. It should be understood that
for any such polymer material disclosed below, various structural
configurations are possible. For example, the monomers that make up
any such polymer may be organized as random copolymers or in blocks
to make block copolymer structures. In addition, the polymers (and
the monomers therein) disclosed below may be combined with yet
other polymers to form random copolymer or block copolymer
structures.
[0091] In a solid-state lithium polymer battery, electrophilic
groups are included in the polymer used as the ionically conductive
binder in a sulfur-based cathode. During operation of the battery,
the electrophilic groups are positioned to rapidly react with any
free lithium polysulfide species that are formed, forming an
electrochemically stable carbon-sulfur bond and a lithium salt and
preventing further migration of the polysulfide species. Depending
on the identity of the polysulfide species, it may be able to
continue lithium redox cycles at moderately reduced capacity
relative to the original cathode; regardless, the polysulfide
species is prevented from diffusing to the anode and causing higher
internal resistance to ion flow.
[0092] In an operating lithium-sulfur cell, lithium polysulfides
are formed as intermediates in the reduction of elemental sulfur in
the cathode to dilithium sulfide:
S.sub.8.fwdarw.Li.sub.2S.sub.8.fwdarw.Li.sub.2S.sub.6.fwdarw.Li.sub.2S.s-
ub.4.fwdarw.Li.sub.2S.sub.3.fwdarw.Li.sub.2S.sub.2.fwdarw.Li.sub.2S
[0093] The leftmost and rightmost species in this sequence
(S.sub.8, Li.sub.2S.sub.2, and Li.sub.2S) have low solubilities in
most ionically conductive media and therefore form immobilized
solid precipitates. The polysulfide species shown in bold tend to
dissolve in ionically conductive media, and then diffuse away in
electrolyte from their point of origin in the cathode. When they
diffuse far enough from the cathode into the separator between the
cathode and anode, they lose electrical contact with the cathode
and cannot be reduced further, resulting in an irreversible loss of
cell capacity. If these species diffuse across the entire separator
and reach the anode, they are likely to be spontaneously, fully
reduced to dilithium sulfide. This reaction both degrades the anode
material itself and forms an inhibiting layer of precipitated
dilithium sulfide at the anode surface, further degrading cell
performance.
[0094] Prevention of the diffusion of polysulfide species is a
major hurdle in the development of lithium-sulfur batteries with
long cycle life. Most reported research involves the creation of
sulfur composites that form the active material of the cathode.
Nanostructured layers of carbon, titanium dioxide, or other
materials can form barriers around sulfur particles that limit the
diffusion of polysulfide species, increasing the stability and
lifetime of cells, though there is some appreciable loss of
capacity due to the inclusion of inactive components in the
cathode. Depending on the complexity of the nanostructured
composites, scaling up to commercial quantities may be more or less
practical and successful in creating "contained sulfur."
[0095] A cathode has of multiple components, the active material
being only one. Generally, a cathode also has a polymeric binder to
hold active material particles in place on a current collector
backing, and an electrically conductive additive such as carbon
black, graphite, or vapor-grown carbon fiber (VGCF) to ensure
electrical connectivity to all of the active material particles. In
a dry polymer battery, the polymeric binder must also function as a
lithium ion transporter, and will contain some amount of a
dissolved lithium salt. The separator layer between the anode and
cathode is also essential to the operation of a cell as an
ionically conductive but electrically insulative layer. The
separator may be a liquid-electrolyte-soaked porous plastic layer
in conventional lithium-ion cells or a solid plastic coating in dry
polymer cells, or some combination thereof. All components of a
lithium-sulfur cell may provide additional opportunities to
stabilize the operation of lithium-sulfur batteries.
Active Barriers for Lithium Polysulfides
[0096] In one embodiment, a catholyte polymer is an active
polysulfide barrier that captures unbound lithium polysulfides by
chemical reaction (actively). During operation of a battery cell,
reactive groups of the catholyte are available to react rapidly
with any free lithium polysulfide species that are generated,
forming stable carbon-sulfur bonds, thus sequestering the
polysulfides and preventing migration of the polysulfide species
away from the cathode. Depending on the particular polysulfide
species that are sequestered, there may still be enough capacity
left in the cathode that the battery can continue to cycle at an
acceptable, though reduced capacity relative to the original
cathode.
[0097] In one embodiment of the invention the catholyte is an
epichlorohydrin-ethylene oxide copolymer (P(EC-EO)). The catholyte
may also be used as a cathode binder. The structure of P(EC-EO)
is:
##STR00001##
wherein n is the total number of repeat units, and x is the mole
fraction of EO units, leaving 1-x as the mole fraction of EC units.
Values of x can range from 0 to 0.99, and values of n can range
from about 10 to 200,000 or greater. P(EC-EO) can be dissolved in
common organic solvents, and the solution can be useful for forming
coatings. The structure of P(EC-EO) represents a perturbation of
the poly(ethylene oxide) homopolymer, which is a known lithium ion
conductor when mixed with a lithium salt. The pendant chloromethyl
groups derived from the epichlorohydrin monomers only modestly
decrease the lithium ion conductivity of the polymer relative to
PEO homopolymer, as long as the mole ratio of EC (1-x) is kept low
(e.g., 0<1-x<0.5).
[0098] The pendant chloromethyl group of the EC portion of P(EC-EO)
is electrophilic, meaning that a nucleophilic unit ("Nu-") can
displace a chloride ion while forming a new bond to the adjacent
carbon atom:
##STR00002##
[0099] Unbound lithium polysulfide species generated during
operation of lithium-sulfur batteries (general formula
"Li.sub.mS.sub.y", m=1 or 2, y=3 to 8) are known to be good
nucleophiles. If they are formed in the cathode in the presence of
a P(EC-EO) binder, it is highly likely that reactions similar to
the following will take place (as a spectator ion, Li.sup.+ is not
included in the reaction):
##STR00003##
[0100] The substitution reaction is expected to be permanent: the
C--S bond does not break under normal cell operating conditions.
The polysulfide species has thus been trapped in the cathode, is
still in electrical contact with the cathode, and cannot migrate or
diffuse to the anode. The formation of the C--S bond causes an
irreversible loss in cathode capacity as the sulfur can no longer
be fully oxidized, but this loss is smaller than what would result
from diffusion of the entire polysulfide species away from the
cathode. Therefore, a sulfur or sulfur composite cathode formulated
with some portion of a P(EC-EO) polymer has higher stability than
one made without, due to the reduced diffusion of lithium
polysulfide species away from the cathode.
[0101] While the above describes one example, there are other
embodiments with structures that satisfy the general criteria of an
electrophilic group susceptible to nucleophilic substitution
incorporated into an ionically conductive polymer. The P(EC-EO)
structure can be generalized to
##STR00004##
in which the electrophilic methyl group is substituted with, for
example, Z=chloride, bromide, iodide, methanesulfonate,
p-methyltoluenesulfonate, or p-nitrobenzenesulfonate. These can be
synthesized using precursors such as epichlorohydrin,
epibromohydrin, and glycidol. Other halide-bearing structures
exhibit a similar capability to undergo nucleophilic substitution,
such as vinyl-chloride monomer groups.
[0102] In other embodiments, PEG-brush polyacrylates and
polymethacrylates of the following structure satisfy similar
criteria, wherein the PEG side chains provide ionic conductivity
and alternative side chains provide the electrophilic
component:
##STR00005##
in which R.dbd.H (acrylate) or Me (methacrylate), x is the mole
ratio of the PEG monomer component ranging from values of 0 to
0.99, n has the same values as above, 2<k<50 is the number of
repeat units in the PEG side chain, and R.sup.Z is an electrophilic
group susceptible to substitution by polysulfide such as:
2-chloroethyl, 2-bromoethyl, .omega.-chloropoly(ethylene glycol),
.omega.-bromopoly(ethylene glycol),
.omega.-methanesulfonatopoly(ethylene glycol),
.omega.-(p-toluenesulfonato)poly(ethylene glycol), glycidyl, or
.omega.-glycidylpoly(ethylene glycol).
[0103] PEG-brush poly(vinyl ethers) of the following structure
satisfy similar criteria, wherein the PEG side chains provide ionic
conductivity and alternative side chains provide the electrophilic
component:
##STR00006##
in which x is the mole ratio of the PEG monomer component having
the same values as above, n has the same values as above,
2<k<50 is the number of repeat units in the PEG side chain,
and R.sup.Z is an electrophilic group susceptible to substitution
by polysulfide such as: 2-chloroethyl, 2-bromoethyl,
.omega.-chloropoly(ethylene glycol), .omega.-bromopoly(ethylene
glycol), .omega.-methanesulfonatopoly(ethylene glycol),
.omega.-(p-toluenesulfonato)poly(ethylene glycol), glycidyl, or
.omega.-glycidylpoly(ethylene glycol).
[0104] PEG-brush polyphosphazenes and PEG-brush polycarbonates can
follow similar substitution patterns:
##STR00007##
in which x, k, n, and R.sup.Z are as defined previously, and
R.sup.c is a short alkyl chain, a short alkyl chain bearing an acyl
side group, or a short PEG chain.
[0105] Polyphosphoesters can also be included of the type:
##STR00008##
in which R=alkyl C.sub.1-C4, R.sup.Z and n are as defined
previously, and x and 1-x are the mole ratios of the monomers
defined previously. These polymers are differentiated by the
reactivity of the phosphoester backbone, as a polysulfide
nucleophile could, by substitution, cleave C--O bonds in the
backbone or the --OR group instead of reacting only with the
electrophilic R.sup.Z side chain.
[0106] In another embodiment of the invention, polyacrylonitrile
can be copolymerized with chlorinated vinyl monomers to make
polymers similar to:
##STR00009##
in which x and 1-x are the mole fractions of the monomers, in which
x can range from 0.01 to 1. The value n is defined previously. The
chloride group in the backbone can be displaced by a polysulfide
nucleophile, while polyacrylonitrile is an ionically conductive
polymer.
[0107] Several of the polymers given as examples above exist as
viscous liquids or gels with poor mechanical properties, especially
if they are mixed with a plasticizing lithium salt such as LiTFSI.
If greater mechanical strength is desired, it is possible to form
block copolymers wherein a 1.sup.st polymer sequence serves as the
ionic conductor and polysulfide trap as described above and a
2.sup.nd polymer sequence serves as a mechanical block. Examples of
suitable mechanical blocks include polystyrene, poly(methyl
methacrylate), poly(cyclohexyl methacrylate). The polymers
typically are chosen such that they have microphase separation
behavior.
[0108] In another embodiment of the invention, any of the polymers
discussed above is made into an ionic conductor by formulating it
with an appropriate salt. Examples include, but are not limited to,
lithium salts such as LiTFSI, LiBETI, LiClO.sub.4, LiBF.sub.4,
LiPF.sub.6, Li triflate, LiBOB, LiPF.sub.6,
LiN(CF.sub.3SO.sub.2).sub.2, Li(CF.sub.3SO.sub.2).sub.3C,
LiN(SO.sub.2CF.sub.2CF.sub.3).sub.2, LiB(C.sub.2O.sub.4).sub.2,
Li.sub.2B.sub.12F.sub.xH.sub.12-x, Li.sub.2B.sub.12F.sub.12, and
mixtures thereof. If a metal other than Li is the basis of the
cell, such as Na, K, Mg, Ca, or Al, other appropriate salts can be
used.
[0109] In one arrangement, the polymers described above are used as
admixtures other known ionically conducting formulations of
polymers and metal salts.
[0110] In another arrangement, the polymers described above, when
formulated with a metal salt, can be used as a separator in an
electrochemical cell with a sulfur-based cathode. Such a separator
can improve cell performance by limiting polysulfide migration to
the anode, but may allow polysulfides to diffuse away from
electrical contact with the cathode which would be observed as an
irreversible loss in cell capacity.
[0111] Olefin containing polymers can also be included. In one
example of the invention the catholyte is an allyl glycidyl
ether-ethylene oxide copolymer (P(AGE-EO)). The structure of
P(AGE-EO) is:
##STR00010##
wherein n is the total number of repeat units having values defined
above, and x is the mole fraction of EO units leaving 1-x as the
mole fraction of AGE units, in which x can range from 0 to 0.99.
The structure of P(AGE-EO) represents a perturbation of the
poly(ethylene oxide) homopolymer, which is a known lithium ion
conductor when mixed with a lithium salt. The pendant
allyloxymethyl groups derived from the AGE monomers only modestly
decrease the lithium ion conductivity of the polymer relative to
PEO homopolymer, as long as the mole ratio of AGE (1-x) is kept low
(e.g. 0<1-x<0.5). It is an example of an olefin, the common
name for the functional group consisting of 2 carbon atoms forming
a double bond. Olefins are susceptible to a number of reactions,
include radical-induced polymerization, addition reactions, and
cycloaddition reactions.
[0112] Lithium polysulfide species are capable of addition
reactions to olefins as has been described in de Graaf, "Laboratory
simulation of natural sulphurization: I. Formation of monomeric and
oligomeric isoprenoid polysulphides by low-temperature reactions of
inorganic polysulphides with phytol and phytadienes," Geochim.
Cosmochim. Acta 1992, 56, 4321-4328 and in de Graaf,
"Low-temperature addition of hydrogen polysulfides to olefins:
formation of 2,2'-dialkyl polysulfides from alk-1-enes and cyclic
(poly)sulfides and polymeric organic sulfur compounds from
.alpha.,.omega.-dienes," J. Chem. Soc. Perkin. Trans. 1 1995,
635-640. This is similar to the industrial process of rubber
vulcanization, in which sulfur is used to form crosslinks in
natural latex (polyisoprene) as described in Carroll,
"Polysulfides--Nature's organic soluble sulfur," Phosphorus, Sulfur
and Silicon and Related Elements 1994, 95, 517-518. If polysulfides
are formed in the cathode in the presence of a P(AGE-EO) binder, it
is possible for reactions similar to the following to take
place:
##STR00011##
[0113] In this reaction, a portion of the polysulfide
Li.sub.mS.sub.y (m=1 or 2, y=3 to 8) reacts to form a sulfur link
between carbon atoms, with a depreciated lithium polysulfide being
the .omega.-product. The AGE-sulfur addition reactions are expected
to be permanent: a C--S bond does not break under normal cell
operating conditions. A portion of the polysulfide species has thus
been trapped in the cathode, is still in electrical contact with
the cathode, and cannot migrate or diffuse to the anode. The
formation of the C--S bond causes an irreversible loss in cathode
capacity as the sulfur can no longer be fully oxidized, but this
loss is smaller than what would result from diffusion of the entire
polysulfide species away from the cathode. The lithium polysulfide
.omega.-product is more fully reduced than the original lithium
polysulfide and is likely to be Li.sub.2S.sub.2 or Li.sub.2S
species, which have poor solubility and mobility, and will
therefore tend to stop diffusing out of the cathode and may be
trapped by other mechanisms proposed herein. Therefore, a sulfur or
sulfur composite cathode formulated with some portion of a
P(AGE-EO) polymer may be expected to show higher stability than one
made without, due to the reduced diffusion of lithium polysulfide
species away from the cathode.
[0114] In other embodiments of the invention, various structures
that have an olefinic group capable of polysulfide addition
incorporated into an ionically conductive polymer can be used.
[0115] A person of ordinary skill in the art will understand that
there are many other structures that satisfy general criteria of an
electrophilic group or an olefinic group susceptible to polysulfide
addition incorporated into an ionically conductive polymer.
[0116] Several of the polymers given as examples above exist as
viscous liquids or gels with poor mechanical properties, especially
if they are mixed with a plasticizing lithium salt such as LiTFSI.
If greater mechanical strength is desired, it is possible to form
block copolymers wherein a 1.sup.st polymer sequence serves as the
ionic conductor and polysulfide or sulfur trap as described above
and a 2.sup.nd polymer sequence serves as a mechanical block.
Examples of suitable mechanical blocks include polystyrene,
poly(methyl methacrylate), poly(cyclohexyl methacrylate). The
polymers typically are chosen such that they have microphase
separation behavior.
[0117] In another embodiment of the invention, an of the polymers
discussed above is made into a ionic conductor by formulating it
with an appropriate salt. Examples include, but are not limited to,
lithium salts such as LiTFSI, LiBETI, LiClO.sub.4, LiBF.sub.4,
LiPF.sub.6, Li triflate, LiBOB, LiPF.sub.6,
LiN(CF.sub.3S.sub.02).sub.2, Li(CF.sub.3SO.sub.2).sub.3C,
LiN(SO.sub.2CF.sub.2CF.sub.3).sub.2, LiB(C.sub.2O.sub.4).sub.2,
Li.sub.2B.sub.12F.sub.xH.sub.12-x, Li.sub.2B.sub.12F.sub.12, and
mixtures thereof. In a metal other than Li is the basis of the
cell, such as Na, K, Mg, Ca, Al, etc, other appropriate salts can
be used, in which case the polymer is formulated with a salt of the
metal that is the basis of the cell.
[0118] In one arrangement, the polymers described above are used as
admixtures other known ionically conducting formulations of
polymers and metal salts.
[0119] In another arrangement, the polymers described above, when
formulated with a metal salt, can be used as a separator in an
electrochemical cell with a sulfur-based cathode. Such a separator
can improve cell performance by limiting polysulfide migration to
the anode, but may allow polysulfides to diffuse away from
electrical contact with the cathode which would be observed as an
irreversible loss in cell capacity.
Active Barriers for Elemental Sulfur
[0120] In another embodiment, an additional active mechanism serves
to capture elemental sulfur through chemical reaction (actively).
If elemental sulfur is formed during charging of the battery, it
can diffuse out of the cathode causing capacity loss and eventually
react at the surface of the anode causing resistance increase and
anode decomposition. During operation of the battery cell, reactive
groups of the catholyte are positioned to react rapidly with any
elemental sulfur species that are generated, forming stable
carbon-sulfur bonds and preventing further migration of the sulfur
species. Sulfur bound in the catholyte in this way may still be
available for further electrochemical activity during cycling of
the battery cell.
[0121] In one embodiment of the invention, the catholyte contains a
radical-generating species that is active at operating temperatures
of the cell. In one example, the catholyte contains bromine, TEMPO
groups or pendant methacrylate groups. Representative examples
include polymers with the following structures:
##STR00012##
[0122] If elemental sulfur species form during operation of the
cell, radical generating species such as those described herein may
react with the elemental sulfur creating C--S bonds thus preventing
loss of the sulfur from the cathode. The formation of the C--S bond
causes an irreversible loss in cathode capacity as the sulfur can
no longer be fully oxidized, but this loss is smaller than what
would result from diffusion of the entire sulfur species away from
the cathode. Therefore, a sulfur or sulfur composite cathode
formulated with some portion of a radical forming polymer may be
expected to show higher stability than one made without such a
polymer, due to the reduced diffusion of sulfur species away from
the cathode.
Passive Barriers for Lithium Polysulfides
[0123] In another embodiment, the cell uses a catholyte that acts
as a passive (or physical) barrier to diffusion of lithium
polysulfides. During discharge of the battery, free lithium
polysulfide species may be generated. Due to polarity and/or
specific interactions of the catholyte and the unbound lithium
polysulfides, the catholyte limits the dissolution of the lithium
polysulfide species forming it into a precipitate and preventing
migration of the polysulfide species out of the cathode. The choice
of catholyte can also affect the regioselectivity of the reduction
of sulfur species. For example, proper choice of catholyte may
result in formation of low index lithium polysulfides that have
poorer solubility, resulting in entrapment in the cathode
layer.
[0124] In one embodiment of the invention the catholyte is a
polyelectrolyte or a polymerized ionic liquid. Representative
examples include cationic polymers with the following structures,
in which X is an anion such as Cl.sup.-, TFSI.sup.-, BETI.sup.-,
ClO.sub.4.sup.-, BF.sub.4.sup.-, PF.sub.6.sup.-, Triflate.sup.-,
BOB.sup.-, and the like.
##STR00013##
[0125] Alternative choices for catholytes that affect polysulfide
formation and diffusion are anionic polymers such as Nafion.RTM.,
poly(styrene sulfonate), polyvinyl sulfonate, and the like.
Representative examples include anionic polymers with the following
structures:
##STR00014##
[0126] The advantage of passive barriers for lithium polysulfides
as compared to active barriers is that they do not reduce the
capacity of the cathode, in that no reduction of the polysulfide
occurs.
Passive Barriers for Elemental Sulfur
[0127] In another embodiment, the cathode uses a catholyte that
acts as a passive barrier to diffusion of elemental sulfur. During
charging of the battery, free sulfur species S.sub.x (e.g.,
elemental sulfur) may be generated. Such a catholyte has low
solubility for elemental sulfur, so there is little dissolution of
the elemental sulfur species. Instead sulfur precipitates are
formed, preventing migration of the sulfur species out of the
cathode. Low solubility can be achieved if the catholyte is highly
polar. Polarity of materials may be quantified by their dielectric
constant. For example a low polarity material may have a dielectric
constant of around 8 or less, while a highly polar material may
have a dielectric constant greater than about 30. In one
arrangement, such polarity is effected when a high concentration of
salt species is dissolved in the catholyte. In another arrangement,
a polymer that is intrinsically polar is used as the catholyte. For
the embodiments of the invention, as disclosed herein, polar
materials are considered to have low enough solubility of sulfur to
be useful as physical barriers to elemental sulfur diffusion when
they have a dielectric constant greater than 20, or greater than
25, or greater than 30, or any range therein.
[0128] In one embodiment of the invention the catholyte is a linear
copolymer of carbonates and ethylene oxide (P(LC-EO)). In another
example different analogs of P(LC-EO) can be used which incorporate
thioethers linkages in addition to ether linkages (P(LC-TEO)).
Examples of some structure of these types are:
##STR00015##
[0129] In another embodiment, polar groups are attached to a
polymer backbone, resulting in increased polarity, which reduces
elemental sulfur solubility. In one arrangement, the catholyte is a
polyether backbone with cyclic carbonates grafted as side groups
(P(GC-EO)). In another arrangement, different polar groups such as
nitrile groups (GN) or phosphonate groups (GP) are grafted off the
backbone polymer (P(GN-EO) or P(GP-EO)). Alternative backbones such
as polyalkanes, polyphosphazenes, or polysiloxanes may be used.
Representative examples of structure of these types include:
##STR00016##
in which n and x have the values as described above.
[0130] In another embodiment, poly phosphorus esters are used as
catholytes to limit elemental sulfur dissolution and diffusion.
Representative examples of different structure of these types
are:
##STR00017##
in which R=methyl, ethyl, isopropyl, 2,2,2-trifluoroethyl, etc.
Cell Architecture and Barrier Configurations
[0131] A barrier layer preventing sulfur loss from a cathode can be
configured in a variety of ways. A catholyte material which
prohibits diffusion of lithium polysulfides or elemental sulfur may
be located in different regions of the cathode depending on its
properties like conductivity, binding ability, and surface
compatibility with other cell components.
[0132] FIG. 4 is a schematic illustration of a lithium metal cell
400 with a sulfur-based cathode 420 that uses a sulfur-sequestering
catholyte 430, according to an embodiment of the invention. The
cell 400 also has a lithium metal anode 440 and a separator 450
between the anode 440 and the cathode 420. The sulfur-sequestering
catholyte 430 is included in the bulk of the cathode 420 and can be
seen as light grey stripes. The cathode 420 also has
sulfur-containing active material particles 423 and electronically
conductive particles 426. In one arrangement, the cathode active
material particles 423 are made of SPAN. The cathode 420 may also
contain additional electrolytes or binders (not shown). There may
also be a current collector 460 adjacent to the cathode 420.
[0133] FIG. 5 is a schematic illustration of a lithium metal cell
500 with a sulfur-based cathode 520 that uses a sulfur-sequestering
electrolyte, according to another embodiment of the invention. The
cell 500 also has a lithium metal anode 540 and a separator 550
between the anode 540 and the cathode 520. There is a layer of
sulfur-sequestering electrolyte 530 between the separator 550 and
the cathode 520 and can be seen as light grey stripes. The cathode
520 has sulfur-containing active material particles 523 and
electronically conductive particles 526. In one arrangement, the
cathode active material particles 523 are made of SPAN. The cathode
520 also contains a second electrolyte 529 and may contain binders
(not shown). There may also be a current collector 560 adjacent to
the cathode 520.
[0134] FIG. 6 is a schematic illustration of a lithium metal cell
600 with a sulfur-based cathode 620 that uses a sulfur-sequestering
catholyte, according to another embodiment of the invention. The
cell 600 also has a lithium metal anode 640 and a separator 650
between the anode 640 and the cathode 620. The cathode 620 has
sulfur-containing active material particles 623 and electronically
conductive particles 626. In one arrangement, the cathode active
material particles 623 are made of SPAN. The sulfur-containing
active material particles 623 are coated with a layer of
sulfur-sequestering catholyte 630, which can be seen as dark grey
edges 630 around the particles 623. In some arrangements, a charge
transfer tie-layer (not shown) may be used to improve charge
transfer between the catholyte coating 630 and the cathode
particles 623. The cathode 620 also contains a second electrolyte
629 and may contain binders (not shown). There may also be a
current collector 660 adjacent to the cathode 620.
[0135] In another embodiment, two or more of the barrier
configurations, as shown in FIGS. 4, 5, and 6 are used in the same
cell. For example, a catholyte particle coating may be used to
minimize the amount of elemental sulfur that is formed. In addition
a catholyte binder may be used to act as a barrier to elemental
sulfur diffusion. Finally, an overcoat layer may be used to prevent
lithium polysulfide and elemental sulfur diffusion.
[0136] FIG. 7 is a cross-sectional schematic drawing of an
electrochemical cell 702 with a positive electrode assembly 700,
according to an embodiment of the invention. The positive electrode
assembly 700 has a positive electrode (cathode) film 710 and an
optional current collector 740. The positive electrode film 710 has
positive electrode active material particles 720, such as elemental
sulfur or a sulfur composite material, embedded in a matrix of
electrolyte 730 that also contains small, electronically-conductive
particles (as indicated by small grey dots) such as carbon black.
The polymer electrolyte 730 can be a polymer or a block copolymer,
as described above. Combinations of the polymer and block copolymer
electrolyte are also possible. There is an optional positive
electrode current collector 740 that may be a continuous or
reticulated metal film. There is a negative electrode (anode) 760
that is a metal layer, such as a lithium layer, that acts as both
negative electrode active material and negative electrode current
collector.
[0137] There is a separator region 750 filled with an electrolyte
that provides ionic communication between the positive electrode
film 710 and the negative electrode 760. In one arrangement, the
separator region 750 contains a solid electrolyte and can be the
same electrolyte (without the carbon particles) 730 as is used in
the positive electrode film 710.
[0138] FIG. 8 is a cross-sectional schematic drawing of an
electrochemical cell 802 with a positive electrode assembly 800,
according to another embodiment of the invention. The positive
electrode assembly 800 has a positive electrode film 810 and an
optional current collector 840. The positive electrode film 810 has
positive electrode active material particles 820, such as elemental
sulfur or a sulfur composite material, embedded in a matrix of
electrolyte 830 that also contains small, electronically-conductive
particles (as indicated by small grey dots) such as carbon black.
The polymer electrolyte 830 can be a polymer or a block copolymer,
as described above. Combinations of the polymer and block copolymer
electrolyte are also possible. There is an optional positive
electrode current collector 840 that may be a continuous or
reticulated metal film. There is a negative electrode 860 that is a
metal layer, such as a lithium layer, that acts as both negative
electrode active material and negative electrode current
collector.
[0139] There is a separator region 850 that has two regions, 850a
and 850b. Region 850a is a layer of a polymer or a block copolymer,
as described above, and provides extra protection against diffusion
of polysulfide species away from the cathode 810. When such a
protective layer 850a is used, it is possible to use a different
kind of electrolyte within the cathode 810 itself, as the
protective layer 850a may be able to prevent all polysulfide
species from leaving the cathode 810 by itself. Region 850b is
filled with an electrolyte that provides ionic communication
between the positive electrode film 810 (through the protective
layer 850a) and the negative electrode 860. In one arrangement, the
separator region 850 contains a solid electrolyte and can be the
same electrolyte (without the carbon particles) 830 as is used in
the positive electrode film 810 and or as used in the protective
layer 850a.
[0140] This invention has been described herein in considerable
detail to provide those skilled in the art with information
relevant to apply the novel principles and to construct and use
such specialized components as are required. However, it is to be
understood that the invention can be carried out by different
equipment, materials and devices, and that various modifications,
both as to the equipment and operating procedures, can be
accomplished without departing from the scope of the invention
itself.
* * * * *