Note: Descriptions are shown in the official language in which they were submitted.
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...
Alkali Polysulfide Flow Battery
BACKGROUND OF THE INVENTION
In balancing intermittent supplies of power, such as available from renewable
energy systems, with variable demand, means of energy storage that provide
availability for storage and recovery of energy are desirable. In particular,
power
storage options that are capable of rapid charge and discharge on demand and
multiple cycling are advantageous. Whilst pumped hydro offers a large scale
energy storage option and hydrogen is a useful energy storage and transport
medium, for rapid cycling of power and charge and discharge on demand,
supercapacitors and batteries are possible options. Flow batteries are
recognised
as high capacity stationary power storage and cycling system.
A flow battery typically comprises an electrochemical cell and has at least
one
liquid electrolyte and more usually two liquid electrolytes. Typically, the
electrolyte flows through an electrochemical cell from an electrolyte
reservoir and
is charged or discharged at an electrode. A charge carrier species typically
passes
through a charge-carrier porous membrane separating the catholyte and anolyte.
Flow batteries, whilst offering the potential for large scale energy storage
have
typically suffered from certain disadvantages including energy density,
cycling
issues and corrosive material issues.
Various types of flow battery are available. A particularly interesting form
of
flow battery that has the potential to address some of the shortcomings of
flow
batteries are lithium-polysulfide systems.
Lithium polysulfide systems are known for use in flow batteries
and solid state batteries alike, in which lithium ions are the charge
carriers. In a
lithium polysulfide system, in the course of discharge, the lithium
polysulfide
converts from a polysulfide species (Li2S8) to, potentially, a highly
lithiated
lithium sulfide species (Li2S), via a number of lithium sulfide intermediates,
Li2S6,
Li2S4, Li2S3 and Li2S2. The more highly lithiated species offer the greatest
charge
carrying capacity (and their use thereby substantially improves power
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density of the battery), but also suffer from being highly insoluble species
(compared with the less highly lithiated species). A second issue with such
lithium polysulfide systems is the issue of polysulfide shuttle whereby
polysulfide
species manage to pass the separator or membrane to contaminate the lithium
anode or anolyte
US 8889300 (Bugga et al) is concerned with a high energy density
flow battery comprising an anode of a lithium solvated electron solution (Li-
SES)
anolyte or a solid lithium anode and a cathode of a Li-SES catholyte or a
lithium
polysulfide solution catholyte, the cathode and anode separated by a lithium
ion
conductive membrane. According to US 8889300, on charging of the flow
battery, the lithium polysulfide is de-lithiated through to a S8 species and
on
discharge, the S8 associates with lithium ions to form successive lithiated
species
Li2S8, Li2S6, Li2S4 and Li253. It states that the catholyte conductive
solution
includes lithium polysulfides in the form of LiõSn where n is from 3 to 8,
because
Li252 and Li2S species are insoluble. Thus the US 8889300 system does not
utilise the highly lithiated lithium sulfide species.
The present inventor has invented a new and improved battery
system and flow battery.
PROBLEM TO BE SOLVED BY THE INVENTION
There is a need for improvements in flow batteries and lithium
sulfide batteries.
It is an object of this invention to provide a flow battery and/or an
alkali (especially lithium) sulfide battery with improved performance
characteristics, especially improved energy density.
SUMMARY OF THE INVENTION
In accordance with a first aspect of the invention, there is provided
an ion-selective separator composition for a battery having an anode and an
alkali
metal sulfide or polysulfide cathode, the separator composition comprising an
alkali metal ion conducting separator film for separating the anode and the
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cathode, a carbon layer disposed to a cathode side of the film and an alkali
metal
ion conductor layer disposed to an anode side of the carbon layer.
In a second aspect of the invention, there is provided an alkali
metal ion flow battery comprising:
a flow battery electrochemical cell comprising an anode half-cell and a
cathode half-cell separated by an ion-selective separator, the electrochemical
cell
having at least one liquid electrode or electrolyte;
at least one electrolyte reservoir and a flow circulation system for
facilitating flow of electrolyte to and from the electrochemical cell and
electrolyte
reservoir; and optionally
a power convertor for two way conversion of current to and from the
electrochemical cell and a load or supply,
wherein the flow battery further comprises one or more of:
a) an ion-selective separator comprising an ion-selective separator
composition as defined above;
b) an alkali-metal polysulfide catholyte
c) an anode selected from a solid alkali metal, alkali metal alloy,
alkali metal composition and an alkali-metal based anolyte;
d) an electrolyte flushing system for flushing an electrolyte through
the electrochemical cell in a short burst or pulse;
e) a flow circulation system for facilitating flow of electrolyte which
is configured to enable the circulating electrolyte to pass through the power
convertor to act as a coolant for the power convertor; and
f) a dosing and/or filtering system for use with the at least one
electrolyte.
In a third aspect of the invention, there is provided an alkali-metal
polysulfide catholyte for use in a flow battery as defined above.
In a fourth aspect of the invention, there is provided an anode for
use in a flow battery as defined above, the anode being selected from a solid
alkali
metal, alkali metal alloy, alkali metal composition and an alkali-metal based
anolyte.
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In a fifth aspect of the invention, there is provided an electrolyte
flushing system for use in a flow battery as defined above, the electrolyte
flushing
system being for flushing an electrolyte through an electrochemical cell in a
short
burst or pulse.
In a sixth aspect of the invention, there is provided a flow
circulation system for facilitating flow of electrolyte which is configured to
enable
the circulating electrolyte to pass through a power convertor to act as a
coolant for
the power convertor.
In a seventh aspect of the invention, there is provided a dosing
and/or filtering system for use with at least one electrolyte in a flow
battery as
defined above.
In an eighth aspect of the invention, there is provided an
electrochemical cell for a flow battery as defmed above, the electrochemical
cell
comprising an anode half-cell and a cathode half-cell separated by an ion-
selective
separator and having at least one liquid electrode or electrolyte.
In a ninth aspect of the invention, there is provided a method of
operating an alkali metal ion flow battery as defmed above, the method
comprising, during the discharge cycle, periodically drawing a high current
for a
short period (e.g. of up to 10 ms) and detecting the voltage variation during
that
period and, in dependence of no or minimal drop in voltage, causing the
positive
electrolyte to be circulated at a higher flow rate for a predetermined
duration (e.g.
5s).
In a tenth aspect of the invention, there is provided a method of
enhancing performance of an electrolyte in a flow battery, the method
comprising
dosing the electrolyte with a functional additive.
ADVANTAGES OF THE INVENTION
The invention provides an ion-selective separator composition for
a battery and an alkali metal ion flow battery which has improved performance
over existing systems in terms of energy density whilst inhibiting the problem
of
polysulfide shuttle suffered by conventional lithium polysulfide batteries,
thereby
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improving cycle efficiency and charge/discharge efficiency and doing so in a
cost-
effective manner.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a diagrammatic representation of a flow battery
according to one embodiment of an aspect of the invention having an anolyte
and
a catholyte.
Figure 2 is a diagrammatic representation of a flow battery
according to another embodiment of an aspect of the invention having a
catholyte
and solid anode.
Figure 3 is a diagrammatic representation of an electrochemical
cell of an aspect of the invention having an ion selective separator of the
invention.
Figure 4 is a graph of discharge capacity against cycle number for
an electrochemical cell according to one embodiment of the invention.
Figure 5 is a graph of voltage against discharge capacity for an
electrochemical cell according to one embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The invention provides for an improved battery and improved flow
battery and components thereof.
Preferably, the battery is a flow battery.
The term 'flow battery' as used herein may refer to a conventional
dual liquid flow battery or a hybrid single liquid flow battery. A
conventional
dual liquid flow battery is one where the cathode (catholyte) and anode
(anolyte)
are both liquids. A hybrid single liquid flow battery is a battery where there
is a
liquid electrode on one side of the cell (typically the cathode, forming a
catholyte)
and a solid electrode on the other side of the cell.
As used herein, the term anode is the negative electrode and the
term anolyte is an electrolyte in or circulating through the anode or negative
half
cell of an electrochemical cell. The term cathode is the positive electrode
and the
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term catholyte is an electrolyte in or circulating through the cathode half
cell of an
electrochemical cell.
The flow battery according to the present invention is an alkali
metal ion flow battery having a flow battery electrochemical cell. The
electrochemical cell has an anode half-cell and a cathode half-cell separated
by an
ion-selective separator and has at least one liquid electrode or electrolyte.
The
flow battery will have at least one electrolyte reservoir and a flow
circulation
system (typically comprising conduits connecting the electrochemical cell or a
plurality of electrochemical cells in a cell stack with the electrolyte
storage
reservoir and a pump or other means for causing the electrolyte to circulate
therethrough as required). The flow battery will typically further comprise or
be
associated with a power convertor for two-way conversion of current to and
from
the electrochemical cell with a load or supply (e.g. from a/c to d/c). The
flow
battery further comprises one or more and preferably all of the following
features,
which are preferred features of the flow battery of the invention and
independently
further aspects of invention:
a) an ion-selective separator comprising an ion-selective separator
composition;
b) an alkali-metal polysulfide catholyte
c) an anode selected from a solid alkali metal, alkali metal alloy,
alkali metal composition, an intercalated alkalki metal host composition
and an alkali-metal based anolyte;
d) an electrolyte flushing system for flushing an electrolyte through
the electrochemical cell in a short burst or pulse;
e) a flow circulation system for facilitating flow of electrolyte which
is configured to enable the circulating electrolyte to pass through the
power convertor to act as a coolant for the power convertor; and
0 a dosing and/or filtering system for use with the at least
one
electrolyte.
In a preferred embodiment, the flow battery is a lithium ion flow
battery and preferably comprises a lithium polysulfide catholyte as the
positive
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electrode or cathode and as the negative electrode or anode there is
optionally a
solid lithium-based anode, an intercalated lithium and solid composite host
material or a solvated lithium-based anolyte. Therefore, in a preferred flow
battery of the invention lithium or lithium ions are the charge carrying
species
which are conducted across a separator membrane between the anode half-cell
and
cathode half-cell during charge/discharge cycles. Typically, current
collectors are
provided in relation to each of the cathode or anode, especially where a
liquid
cathode or anode is provided. Whilst a lithium-based system is preferred,
there
will be described hereinafter systems related to an alkali metal ion flow
battery
and to a lithium flow battery, where the alkali metal and lithium may be read
interchangeably where the context allow, but the preferred system is where the
alkali metal is lithium.
In a preferred embodiment of the flow battery of the invention and
in a further aspect, there is an ion-selective separator composition for a
battery
having an anode and an alkali metal sulfide or polysulfide cathode or
catholyte.
The separator composition comprises an alkali metal ion conducting separator
film for separating the anode and the cathode, a carbon layer disposed to a
cathode
side of the film and an alkali metal ion conductor layer disposed to an anode
side
of the carbon layer.
Preferably, the catholyte comprises a carrier medium in which
alkali metal polysulfide species are soluble.
Preferably, the alkali metal is lithium and the cathode is a lithium
polysulfide catholyte. Lithium polysulfide batteries and flow batteries are
known,
but suffer from polysulfide shuttle where polysulfide species cross over from
the
cathode side of an electrochemical cell to the anode side which has the effect
of
degrading the anode and decreasing the capacity of the catholyte. The
structure
and composition of the ion-selective separator (preferably a lithium-ion
selective
separator) is such as to reduce and inhibit polysulfide shuttle.
The ion-selective separator, which is preferably a lithium ion
selective separator, comprises an alkali metal ion conducting separator film
which
functions to conduct the alkali metal ion which will typically be the charge
carrier
for a battery or system between the anode and cathode sides of a cell during
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charge and discharge cycles. The alkali metal ion conducting separator film,
or
membrane, may be suitable ceramics, glasses, polymers gels or combinations
thereof, such as an organic polymer, an oxide glass, an oxynitride glass, a
sulfide
glass, an oxysulfide glass, a crystalline ceramic electrolyte, a perovskite, a
nasicon
type phosphate, a lisicon type oxide, a metal halide, a metal nitride, a metal
phosphide, a metal sulfide, a metal sulfate, a silicate, an aluminosilicate or
a boron
phosphate. Preferably, the ion conductive film is a polymer film, which is
preferably a polyethylene oxide based polymer, a polystyrene, polyethylene or
polysulfone or polypropylene optionally having pendant moieties such as crown
ethers (e.g. 12-crown-4-ether) and more preferably the ion conductive film is
a
polypropylene film. This may be of any suitable thickness but preferably has a
thickness of up to 250 gm, more preferably up to 100 gm, still more preferably
up
to 751.1m and most preferably up to 50 gm. It should ideally have a thickness
sufficient to maintain integrity, optionally with the support of one or more
coatings, but in any case preferably a thickness of at least 5 gm, more
preferably
at least 10 gm. Optionally, a polypropylene lithium ion selective film has a
thickness in the range of 15 to 40 gm, more preferably 25 to 30 gm. The
polypropylene film for use in the separator is typically referred to as a
porous or
lithium ion selective conducting film.
Disposed to the cathode side of the film is a carbon layer,
optionally a composite carbon and polymer (e.g. polyvinylidene fluoride)
layer.
Preferably, the carbon-containing layer is obtainable by coating a composition
of
carbon powder, polyvinylidene fluoride and N-methyl-2-pyrrolidinone to what
will be the cathode side of the ion-selective film and preferably directly on
to the
ion conducting separator film. The carbon layer preferably has a thickness of
up
to 50 gm, more preferably up to 25 gm, still more preferably up to 20 gm,
still
more preferably up to 15 gm and most preferably up to 10 gm. Preferably, it
has
a thickness of at least 0.1 gm, more preferably at least 0.5 gm, still more
preferably 0.75 gm and most preferably at least 1 gm. Optionally the carbon
layer
may have a thickness of from 2 to 10 gm, e.g. 5 to 7.5 gm. Preferably, the
carbon
layer is loaded in an amount of 0.01 mg/cm2 to 1 g/cm2 of the alkali metal ion
conducting separator film.
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The function of the carbon layer is preferably in weakening the
polysulfide shuttle effect. It is intended to inhibit sulfur species from the
cathode/catholyte (e.g. S8, S6, etc) crossing over to anode or the anolyte
side. It is
believed that the carbon layer is effective by activating the polysulfide
species, so
they will react with alkali metal ions (typically lithium ions) in the
catholyte
solution (on the polysulfide side of the separator) and become lithiated
rather than
shuttling towards the anode.
A carbon powder-containing layer is preferably obtainable by
coating a composition of carbon powder, polyvinylidene fluoride and N-methy1-2-
pyrrolidinone onto a substrate (e.g. a polypropylene metal ion conducting
separator film).
Optionally, the carbon layer is coated directly on to the cathode
side of the alkali metal ion conducting separator film or is separated from
the film
by one or more further layers.
Preferably, the ion-selective separator composition comprises an
alkali metal ion conductor layer disposed to an anode side of the carbon
layer,
which may optionally be disposed to the anode side of the ion-conducting
separator film or may be disposed to the cathode side of the ion-conducting
separator film (i.e. between the separator film and the carbon layer) or both.
Preferably, the alkali metal ion conductor layer comprises aluminium oxide.
The
aluminium oxide forming the ion conductor layer is believed to be effective in
conducting alkali ions (preferably lithium ions) across the membrane or
separator,
thus improving the charge/discharge efficiency, whilst not facilitating
transfer of
other species. The aluminium oxide-containing layer is preferably provided in
a
layer thickness of from 0.5 gm to 100 gm, more preferably 1 gm to 50 gm, e.g.
10
gm to 35 gm.
Preferably, the alkali metal ion conductor layer further comprises
titanium oxide, which may form a layer on and/or potentially melded with or
within the aluminium oxide-containing layer. Preferably, the titanium oxide-
containing layer is provided in a layer thickness of from 0.5 gm to 100 gm,
more
preferably 1 gm to 50 gm, e.g. 10 gm to 35 gm. The titanium oxide is, it is
believed, effective in inhibiting alkali metal ion (e.g. lithium) polysulfide
species
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from passing through the alkali metal ion conductor layer (and on through the
ion
selective separator to the anode side). The titanium oxide is particularly
effective
when used with an aluminium oxide layer for a lithium polysulfide catholyte,
in
which case it is believed that the titanium oxide is a lithium polysulfide gap
filler.
In a preferred embodiment of the flow battery of the invention and
in a further aspect, there is an alkali-metal polysulfide catholyte. The
catholyte is
preferably lithium polysulfide. The catholyte used in the flow battery of a
preferred embodiment utilizes lithium sulfide species of Li2Sõ (where n = 1 to
8)
which provides a significantly increased charge/discharge capacity for the
catholyte since the discharge capacity in moving from Li2S3 to Li2S through
Li2S2
is a significant increase over the S8 to S3 species. More highly lithiated
species,
such as Li2S2 and Li2S are understood to be considerably less soluble in
carrier
media and so the choice of media, choice of additives and system or flow
battery
structure or components may be selected to enhance or maximize solubility or
stability of highly lithiated polysulfide species in the carrier medium and/or
to
inhibit precipitation and/or mitigate the effects of precipitation of highly
lithiated
polysulfide species in the catholyte.
The alkali metal polysulfide or lithium polysulfide catholyte
preferably comprises a fluid (preferably liquid) carrier medium which is
preferably any suitable medium capable of solvating or dissolving one or more
lithium polysulfide species. Suitable such carrier media include, for example
one
or a mixture of two or more of tetrahydrofuran, dimethyl sulfoxide, dimethyl
formamide, 1,3-dioxolane, dimethyl acetamide and tetra(ethylene glycol)
dimethyl
ether and tetraethylene glycol dimethyl ether¨lithium
trifluoromethanesulfonate.
Preferably, the liquid carrier medium comprises tetra(ethylene glycol)
dimethyl
ether and dimethyl sulfoxide. Preferably, the tetra(ethylene glycol) dimethyl
ether
is present in an amount of 50% to 95% by volume of the liquid carrier medium
and dimethyl sulfoxide is present in an amount of 5% to 50% by volume of the
liquid carrier medium. Optionally, the liquid carrier medium further comprises
1,3-dioxolane, which is preferably present in an amount of up to 15% by volume
of the liquid carrier medium, preferably from 2% to 10% by volume (e.g. about
5%). In a particularly preferred embodiment, a liquid carrier medium comprises
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tetra(ethylene glycol) dimethyl ether, dimethyl sulfoxide and 1,3-dioxolane in
the
by volume proportions of 55% to 80% tetra(ethylene glycol) dimethyl ether, 15%
to 35% dimethyl sulfoxide and about 5% (or e.g. from 3%) to about 10% (or e.g.
to 12%) 1,3-dioxolane. Two particularly preferred solvent formulations are: 5%
1,3-dioxolane, 15-25% dimethyl sulfoxide and 70-80% tetra(ethylene glycol)
dimethyl ether; and 10% 1,3-dioxolane, 25-35% dimethyl sulfoxide and 55-65%
tetra(ethylene glycol) dimethyl ether.
A liquid carrier medium or solvent for the catholyte should
preferably have a flash point of at least 125 C, more preferably 150 C (to
enable a
reasonable working temperature for the cell) and still more preferably at
lease
180 C and should preferably have a melting point not higher than 0 C, more
preferably not higher than -5 C.
Preferably, the alkali metal ion polysulfide catholyte (e.g. a lithium
polysulfide catholyte) in a carrier medium further comprises an alkali metal
ion
polysulfide species solvating additive, which is preferably capable of
solvating the
more highly lithiated species such as Li2S2 and Li2S. Preferably, the
catholyte
comprises a phosphorus pentasulfide, preferably in an amount of up to 20% by
weight of polysulfide in the catholyte, more preferably up to 15%, still more
preferably up to 10% and yet more preferably up to 5% and preferably at least
1%,
more preferably at least 2% and still more preferably at least 4%.
Optionally, the catholyte further comprises an alkali metal nitrate,
such as lithium nitrate. The effect of the lithium nitrate, it is believed, is
at the ion
selective separator or membrane where it inhibits the formation of lithium
polysulfide in or close to the anode. The alkali metal nitrate may be provided
in
an amount of up to 5% by weight of polysulfide in the catholyte.
The concentration of the catholyte is preferably as high as operably
practicable since a higher concentration corresponds with a higher charge
density.
The concentration achievable may depend upon the solvent system chosen and the
solubility of the polysulfide species in that solvent system. In one
embodiment in
which the solvent is an organic polar solvent, such as tetrahydrofuran, the
concentration of polysulfide in the catholyte, based upon moles of sulfur, may
be
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up to 12 M, e.g. from 8 to 10 M. In another embodiment in which the solvent is
glycol based (e.g. a mixture of tetra(ethylene glycol) dimethyl ether,
dimethyl
sulfoxide and optionally 1,3-dioxolane), the concentration of polysulfide in
the
catholyte, based upon moles of sulfur, may be up to 8M, e.g. from 4 to 6 M,
preferably about 5 M.
A cathode half-cell should further be provided with a current
collector. Any suitable current collector may be used, e.g. aluminium, steel
or
carbon. Preferably, the cathode current collector is an aluminium foil and
more
preferably is an aluminium foil coated with a carbon coating (e.g. carbon
black or
graphite).
The flow battery of the present invention may comprise an anode
which is a solid alkali metal, a solid alkali metal alloy, a solid alkali
metal
composition or an alkali metal-based anolyte. Preferably, in each case, the
alkali
metal is lithium.
In one embodiment, in which the anode is a solid electrode and in
which the battery is thereby a single electrolyte or hybrid flow battery, the
anode
may be a pure metal anode, an alloy anode or an alkali metal composition or an
alkali metal intercalation host material. The intercalation host material may
be
selected from carbon, silicon, tin and cobalt tin titanium. Preferably, the
intercalation material is graphite and more preferably, the anode is lithium
intercalated in graphite.
A preferred lithium intercalation host material for use in the anode
half-cell (e.g. as the anode) is also provided as another aspect of the
invention.
According to this aspect and preferred embodiment, a lithium intercalation
host
material of graphite is provided, which preferably comprises layer coated onto
a
substrate (typically a current collector of stainless steel or other suitable
material)
a layer of graphite typically up to 200 gm, more preferably up to 100 gm and
more preferably from 25 to 75 gm (e.g. about 50 gm) thick. The graphite layer
preferably comprises graphite particles (of less than 20 gm, more preferably
less
than 10 gm) and a binder which is preferably an alkaline binder. Preferably,
the
binder comprises a cellulose and/or an acrylic acid and more preferably a
carboxytnethyl cellulose preferably with a polyacrylic acid, preferably in a
ratio of
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from 1:4 to 1:1 more preferably from 1:3 to 1:1.5 and most preferably in a
ratio of
1:2. The graphite layer preferably further comprises graphene nano platelets
(e.g.
in an amount up to 10% by weight) and/or a conductive additive super C65 (e.g.
available from TIMCAL as Super C65 and e.g. in an amount of up to 5% by
weight). Preferably, the graphite layer comprises up to 20 wt% of the binder,
more preferably up to 15 wt%, still more preferably at least 2 wt% and most
preferably from 4 to 8 wt%, e.g. 5 to 7 wt%. Preferably, the graphite coating
material is obtainable by ball milling a coarse graphite powder (with particle
size
from 10 to 20 gm) in an amount of at least 50% by weight, more preferably at
least 70% optionally with a fine graphite powder (with particle size from 5 to
10
gm), with optional graphene nano platelet and conductive additive carbon black
along with the above binder composition in aqueous solution of at least 20 wt%
water (e.g. sodium carboxymethyl cellulose, 20% by weight, and sodium
polyacrylic acid, 40% by weight, and water, 40% by weight). The slurry is
preferably ball milled for up to 10 hours, preferably from 4 to 6 hours. After
coating onto a substrate (e.g. stainless steel foil, as a current collector),
such as by
blade coating or blade casting, the graphite layer-coated foil is dried (e.g.
under
vacuum at up to 95 C, preferably about 90 C for at least 6 hours, e.g. up to
24
hours and preferably from 10 to 14 hours). The resulting anode is effective in
intercalating lithium and is capable of withstanding contact with glycol-based
catholyte solvents and, in particular, a solvent comprising a mixture of
tetra(ethylene glycol) dimethyl ether, dimethyl sulfoxide and optionally 1,3-
dioxolane.
In another embodiment, in which the anode comprises a liquid
anolyte, typically an alkali metal-based anolyte such as a lithium anolyte,
the
battery is a double electrolyte flow battery. The anolyte preferably comprises
an
alkali-metal polyaromatic hydrocarbon complex in a liquid carrier medium. The
polyaromatic hydrocarbon is preferably selected from one or a mixture of
byphenyl and naphthalene, which form a solvated electron solution with lithium
in
a solvent or carrier medium which is preferably tetrahydrofuran. Optionally,
the
anolyte comprises an alkali metal nitrate.
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An anode half-cell should further be provided with a current
collector. Any suitable current collector may be used. Preferably, the current
collector is steel and more preferably stainless steel, such as a stainless
steel foil
of up to 200 gm thickness, more preferably from 50 to 150 gm, still more
preferably from 80 to 120 gm such as about 100 gm thickness.
In another preferred embodiment of the flow battery of the
invention, there further comprises an electrolyte flushing system for flushing
an
electrolyte through the electrochemical cell in a short burst or pulse. Such a
system finds particular application when configured for use with a lithium
polysulfide catholyte to inhibit precipitation of highly lithiated lithium
polysulfide
species (such as Li2S2 or Li2S) in the electrochemical cell during a discharge
cycle. Preferably, the electrolyte flushing system is configured to cause
electrolyte to be circulated at an increased flow rate for a pre-determined or
determined duration in dependence of a pre-defmed trigger. The pre-defined
trigger may comprise a determination of a change in voltage in response to an
active change in current. Preferably, the electrolyte flushing system is
configured
to cause electrolyte to be circulated at an increased flow rate for a pre-
determined
or determined duration in dependence of a pre-defined trigger which trigger is
the
detection of no or minimal reduction in voltage in response to a high current
draw
test, which may comprise drawing a high current (e.g. at least 25% more than
actually being drawn during the discharge to meet a demand and no more than
say
100% more, e.g. up to 80% more and more preferably up to about 60% more,
typically 50% more) for a period of up to 500 ms (milliseconds), more
preferably
up to 100 ms, still more preferably up to 50 ms, still more preferably up to
25 ms
and most preferably up to 20 ms. The current draw is ideally for at least 1 ms
and
still more preferably at least 5 ms.
Optionally, the pre-defined trigger may identify the point during a
discharge cycle when a flushing procedure (comprising periodic or occasional
flushing) should begin or may trigger an instance of flushing.
Preferably, the electrolyte flushing system is configured to cause
electrolyte to be circulated at an increased flow rate for up to 5 minutes,
more
preferably up to 1 minutes, still more preferably up to 30 seconds, more
preferably
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up to 20 seconds and most preferably up to 10 seconds. Preferably, the
electrolyte
is circulated at an increased flow rate of at least 1 second, more preferably
at least
seconds.
The flushing system may be configured to circulate fluid at a rate
that is increased upon the necessary circulation for the particular point of
discharge (e.g. up to 200% increase, more preferably up to 100% increase and
preferably at least 10% increase and still more preferably at least 20%
increase
and optimally from 30 to 75% increase, e.g. from 40 to 60% increase).
Optionally, the flushing (or the pre-defined trigger test) can be carried out
at
predefmed intervals of power discharge (rather than intervals of time) and
optionally decreasing power discharge intervals, for example every 250 Wh or
up
to every 200 Wh, preferably up to every 150 Wh, still more preferably up to
every
120 Wh, preferably at least every 50 Wh and most preferably up to every 100
Wh's discharged per cell in the flow battery system. Optionally, the flushing
may
take place at an increased rate at greater depth of discharge. The flushing is
ideally only activated when likelihood of highly lithiated species is great,
such as
during discharge and toward the tail end of the depth of discharge (DOD), e.g.
from at least 70% DOD, more preferably at least 75% DOD, still more preferably
at least 80% and yet more preferably from 80-95% DOD.
The flushing system finds particular application in combination
with an alkali metal ion polysulfide (e.g. lithium polysulfide) catholyte
having a
polysulfide solvating agent (e.g. P2S5).
In another preferred embodiment of the flow battery of the
invention, there is a flow circulation system for facilitating flow of
electrolyte
which is configured to enable the circulating electrolyte to pass through the
power
convertor to act as a coolant for the power convertor.
In another preferred embodiment of the flow battery of the
invention, there is a dosing and/or filtering system for use with the at least
one
electrolyte. This is preferably configured to dose the at least one
electrolyte with
a functional additive, ideally after pre-determined periods of time, after a
pre-
determined number of charge-discharge cycles or a pre-determined number of
charge-discharge cycles over a pre-determined depth of discharge, in response
to a
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performance measurement outside a pre-determined range and/or to maintain the
concentration of the functional additive in the electrolyte within a pre-
determined
range. In a preferred embodiment, the electrolyte is an alkali metal
polysulfide
catholyte and the functional additive is selected from a highly alkaliated
alkali
metal polysulfide species solvating agent and an alkali metal nitrate. The
solvating agent may be phosphorus pentasulfide which is dosed into the
catholyte
to maintain the concentration thereof within the range 0.5% to 5%, preferably
0.75% to 1%, by weight of polysulfide in the catholyte.
In another embodiment of the dosing system, the electrolyte is an
alkali metal solvated electronic solution and the functional additive is an
alkali
metal nitrate, preferably when the alkali metal solvated electronic solution
has a
lithium ion concentration of greater than 10 molar.
Optionally, there is provided an electrolyte filtering system for
periodically removing precipitated material from the at least one electrolyte.
This
may be configured to remove precipitated material from electrolyte in the
electrolyte storage reservoir, such as the base or sump thereof, by for
example
circulating electrolyte from the storage reservoir through an electrolyte
filtration
circuit.
In a further aspect, a flow battery having a circulating electrolyte
and a power convertor is configured to enable the circulating electrolyte to
pass
through the power convertor to act as a coolant for the power convertor.
In a further aspect, there is provided an electrochemical cell for use
in a flow battery as defined above, which cell comprises an anode half-cell
and a
cathode half-cell. It is preferably separated by an ion-selective separator
and
having at least one liquid electrode or electrolyte. The electrochemical cell
and/or
ion-selective separator may be as further defined above. Preferably in an
electrochemical cell, each cell half configured for operating with an
electrolyte
(e.g. a hybrid or single electrolyte battery or conventional or dual
electrolyte
battery) is provided with a current collector. The current collector may be of
any
suitable material as is known in the art or as defined above.
In a preferred embodiment of the invention, a flow battery or
electrochemical cell of a flow battery comprises an anode half-cell comprising
a
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lithium intercalated graphite host anode (preferably as defined above) and a
cathode half-cell comprising a lithium polysulfide catholyte which is
configured
to circulate via a flow circulation system to and from a catholyte storage
reservoir.
The cell halves are preferably separated by an ion selective separator
composition
(preferably as defmed above), preferably comprising a lithium conducting
separator film (such as a microporous polypropylene), a graphene layer on the
cathode side of the film and an aluminium nitrate coating on the anode side of
the
film. The catholyte carrier medium is preferably a glycol-based medium
comprising TEGDME and optionally one or both of DMSO and 1,3-dioxolane
(preferably as defmed above) and preferably with phosphorus pentasulfide
additive. The system is preferably configured with a pulse-flushing system for
use at high depth of discharge to circulate the catholyte. The charge-
discharge
cycle is preferably controlled by a control system to allow formation of
highly
lithiated sulfide species including Li2S2 and Li2S in order to increase the
discharge
capacity of the cell. As such, according to a preferred embodiment the flow
battery and electrochemical cell are capable of providing a discharge capacity
of
at least 250 mAh/g of sulfur and more preferably at least 275 mAh/g of sulfur,
still
more preferably at least 300 mAh/g of sulfur, still more preferably at least
315
mAh/g of sulfur. Preferably this is achieved even when allowing up to 95%
depth
of discharge of the cell. Preferably, the cell and flow battery according to
this
preferred embodiment can provide stable multiple cycling of charge-discharge
cycles and preferably at least 1000 cycles, more preferably at least 2000,
still
more preferably at least 5000 and still more preferably at least 6000 (and in
laboratory verification a cell can demonstrate in excess of 7000 cycles) with
the
afore mentioned preferred discharge capacity, before significant (e.g. greater
than
30%) deterioration.
The invention will now be described in more detail, without
limitation, with reference to the accompanying Figures.
In Figure 1, a dual electrolyte flow battery system 1 is illustrated,
which comprises a cell stack 3 comprising a stack or multiple electrochemical
cells 5 (shown in Figure 3), a catholyte reservoir 7 and flow circulation
system 9
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including pump 11 for circulating catholyte through the flow circulation
system 9
and reservoir 7 and through the cathode half cell 19 (see Figure 3) of each
cell 5
and an anolyte reservoir 13 and flow circulation system 15 including pump 17
for
circulating anolyte through the flow circulation system 15 and reservoir 13
and
through the anode half cell 21 (see Figure 3) of a cell 5. Current is captured
(during discharge) and delivered (during charge) to the electrochemical cells
5 via
current collectors 23,25 (see Figure 3) and imported or exported via a two-way
power convertor 27. Thus, when there is surplus energy on the electricity grid
or
from a renewable energy device, for example, the flow battery may be charged
whereby current via the power convertor is delivered to the system via current
collectors 23,25 causing charge carriers (in this case lithium ions) in the
catholyte
in the cathode half cell 19 to migrate to the anode half cell 21. Charge is
stored in
the electrolyte solutions. During discharge, a load is applied to the power
convertor 27 which draws current from current collectors 23,25 and charge
carrier
species (lithium ions) migrate from the anode half cell 21 to the cathode half
cell
19. The catholyte 29 in the system 1 is preferably lithium polysulfide system
in a
carrier medium such as THF. The anolyte 31 in the system is preferably a
lithium
solvated electronic solution (Li-SES) of lithium complexed with polycyclic
hydrocarbons such as naphthalene in a solvent such as THF.
In Figure 2, a hybrid flow battery system 33 has a single electrolyte
(catholyte 29) and a solid anode (see Figure 3). The battery 33 comprises a
cell
stack 3 comprising a stack or multiple electrochemical cells 5 (see Figure 3),
a
catholyte reservoir 7 and flow circulation system 9 including pump 11 for
circulating catholyte through the flow circulation system 9 and reservoir 7
and
through the cathode half cell 19 (see Figure 3) of each cell 5. The anode half-
cell
21 (see Figure 3) comprises a solid anode 35 of intercalated graphite hosting
lithium as a charge carrier (see Figure 3) and so no liquid anolyte or anolyte
tank
etc. Current is captured (during discharge) and delivered (during charge) to
the
electrochemical cells 5 via current collectors 23,25 (see Figure 3) and
imported or
exported via a two-way power convertor 27. Thus, when there is surplus energy
on the electricity grid or from a renewable energy device, for example, the
flow
battery may be charged whereby current via the power convertor is delivered to
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the system via current collectors 23,25 causing charge carriers (in this case
lithium
ions) in the catholyte 29 in the cathode half cell 19 to migrate to the anode
half
cell 21 where it is accepted into the intercalation host graphite anode 35.
During
discharge, a load is applied to the power convertor 27 which draws current
from
current collectors 23,25 and charge carrier species (lithium ions) migrate
from the
anode half cell 21 to the cathode half cell 19. The catholyte 29 in the system
1 is
preferably lithium polysulfide system in a carrier medium, which is preferably
glycol-based, e.g. tetra(ethylene glycol) dimethyl ether, DMSO and 1,3-
dioxolane.
The anode 35 is preferably lithium intercalated graphite, a solid anode for
hosting
lithium ions.
Figure 3 shows an electrochemical cell 5 used in a preferred
embodiment of the invention and, in particular, in a hybrid flow battery of
Figure
2. The electrochemical cell 5 has an anode half-cell 21 and a cathode half-
cell 19
separated by an ion selective separator 37. The anode half-cell 21 comprises a
solid anode 35 lithium intercalated graphite in a 50 gm coating on a 100 gm
stainless steel foil as an anode current collector 25. The cathode half-cell
19
comprises a catholyte 29 of lithium polysulfide in a carrier medium of
tetra(ethylene glycol) dimethyl ether, DMSO and 1,3-dioxolane, dosed with
phosphorus pentasulfide and lithium nitrate. A cathode current collector 23 of
aluminium foil 100 gm has a carbon black film coating 39 of 25 gm thickness on
the cathode side of the current collector 23, which is disposed to provide a
catholyte space 41 through which catholyte 29 may flow to and from a reservoir
7
via a flow circulation system 9. The ion selective separator 37 comprises a 10
gm
microporous polypropylene separator film 43 which allows conduction or
migration of lithium ions, an aluminium oxide lithium ion conducting layer 45
coated on the anode side of the film 43 and a 5 gm thick graphene layer 47
formed
on the cathode side of the film 43.
During discharge, lithium ions hosted in the intercalated graphite
anode 35 migrate or are conducted across the ion selective separator 37 and
react
with S8 or lithium sulfide species such as Li2S8, Li2S6 or Li2S4 in the
catholyte 29
to form more highly lithiated species. The graphene layer 47 helps activate or
catalyse reaction of polysulfide species with lithium ions in solution and
reduce
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the risk of polysulfide shuttle (across the separator 37) which would degrade
the
battery or its performance.
Examples
Example 1
An electrochemical cell for use in a hybrid flow battery in which
the electrochemical cell has a solid anode of lithium intercalated in graphite
and a
lithium polysulfide based catholyte was prepared.
An anode half-cell electrode slurry was made up using 75% wt
coarse graphite powder (CGP) (20um > CGP particle size >10um), 8% wt fine
graphite powder (FGP) (10um> FGP particle size >5um), 2% wt conductive
additive super C65, 3% wt graphene nano platelets, 2 wt% LiNO3 and 10% wt
binder composition. The binder composition was made up of 20% wt sodium
carboxymethyl cellulose (CMC-Na) and 40% wt sodium polyacrylic acid (PAA-
Na) dissolved in 40% wt water. The slurry was ball milled for 5 hours and then
blade casted on a 100 gm stainless steel foil which is used as the anode
current
collector. The layer coated foil was dried at vacuum for 12 hours at 90 C. The
resulting dried layer coated foil provides the stainless steel current
collector and
the intercalation graphite anode for the anode half-cell.
A cathode half-cell is prepared using a 100 gm aluminium foil as
the current collector which is coated with a carbon black film coating on the
cathode side of the in a thickness of 25 gm, the carbon black film coating
formed
by coating a slurry comprising KetjenblackTM, an electroconductive carbon
black
available from Alczo Nobel in an amount of 60% by weight, graphene nano
platelets in an amount of 30% by weight and 10% polyvinylidene fluoride
dissolved in N-methyl-2-pyrrolidone.
The cell was assembled with a separator comprising a 10 gm
microporous polypropylene separator film having a 5 gm graphene nanoplatelet
layer coated on the cathode side and an aluminium oxide layer coated on the
anode side. The cell was sealed and filled with a lithium polysulfide solution
Li2S4 in a carrier medium and sufficient additional lithium added to convert
all the
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polysulfide to its highly lithiated species Li2S which is effective to cause
the
excess lithium to occupy the interalated graphite anode in a charge balanced
state.
The cell was then sealed for use in testing.
Example 2
A solvent system for use as a carrier medium for a lithium
polysulfide catholyte in an electrochemical cell of a hybrid flow battery
(such as
that produced according to Example 1) was explored.
The preferred solvent system was identified as having a flash point
of 180 C and a melting point of no higher than -5 C.
Mixtures of three solvents, TEGDME, DMSO and 1,3-dioxolane,
were investigated. Table 1 illustrates the findings for a range of % v/v
solvent
mixtures.
Table 1: solvent system test findings
Percentage % WV
Solution Flash Melting point
Number TEGDME DMSO DOL point >180C not higher
than -5C
1 5 90 5 Yes No
2 10 85 5 Yes No
3 15 80 5 Yes no
4 20 75 5 Yes No
25 70 5 Yes No
6 30 65 5 Yes No
7 35 60 5 Yes no
8 40 55 5 Yes No
9 45 50 5 Yes No
50 45 5 Yes No
11 55 40 5 Yes no
12 60 35 5 Yes No
13 65 30 5 Yes No
14 70 25 5 Yes yes
75 20 5 Yes Yes
16 80 15 5 Yes yes
17 85 10 5 Yes Yes
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18 90 5 5 Yes yes
19 5 85 10 Yes No
20 10 80 10 Yes No
21 15 75 10 Yes No
22 20 70 10 Yes No
23 25 65 10 Yes No
24 30 60 10 Yes No
25 35 55 10 Yes No
26 40 50 10 No No
27 45 45 10 No No
28 50 40 10 No Yes
29 55 35 10 Yes Yes
30 60 30 10 Yes Yes
31 65 25 10 Yes Yes
32 70 20 10 Yes Yes
33 75 15 10 Yes Yes
34 80 10 10 Yes Yes
35 85 5 10 Yes Yes
As can be seen from Table 1, solvent formulations 14 to 18 and 29
to 35 meet both the defined requrements. Thus, a suitable solvent system
includes
solvent mixtures of 5% v/v 1,3-dioxolane with from 70-90 % v/v TEGDME and
5-25 % v/v DMSO and solvent mixtures of 10% v/v 1,3-dioxolane with from 55-
85 % v/v TEGDME and 5-35 % v/v DMSO.
Example 3
An electrochemical cell for a hybrid flow battery was constructed
using the method and materials of Example 1 in which a polysulfide catholyte
was
used. The lithium polysulfide catholyte was prepared using stoichiometric
quantities of lithium and sulfur mixed together with a nominal formula of
Li2S4
and stirred in a TEGDME/DMSO/DOL solvent system (70% /25% / 5% v/v
respectively ¨ an acceptable solvent system according to Example 2) at 80 C
for
two days to formulate a 5M sulfur solution. (5 moles of sulfur in a litre). 1
wt%
LiNO3, 1 wt% lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 0.2 wt%
Lithium bis(oxalato) borate (LiBOB) and 0.2 wt% P2S5 were added to the
solution
and stirred for 3 hours.
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=
The catholyte solution was added to the cell in the manner
described in Example 1 and the cell sealed.
The anode and cathode current collectors were connected to an
electronic test bed. The cell was charged in a constant current at 30mA/cm2,
which varied from 2.4V ¨ 2.8V. Lithium polysulfide species becomes a mixture
of dissolved S8 and Li2S8 and additional lithium gets intercalated in the
anode.
Cell testing was started after performing about 20 charge-discharge
cycles which intercalate and de-intercalate lithium ions in the anode. The
cell was
tested for performance in terms of discharge capacity, cycle number without
degradation and voltage efficiency.
Figures 4 and 5 show graphs of the test results. Figure 4 shows a
graph of discharge capacity in mAh/g of sulfur versus cycle number. Figure 5
shows a graph of voltage versus discharge capacity (mAh/g of sulfur).
Figure 4 shows the number of cycles the battery can achieve
against the cell discharge capacity. The cell was discharged to a depth of
discharge close to 95%. LiNO3 and P2S5 were dosed every 250 cycles in very
small quantities. (0.1% wt of Sulfur). Pulse flushing was used (as described
in the
general description) if highly lithiated species were detected (from about 1.9
V).
The cell showed that it has a discharge capacity of about 325
mAh/g of sulfur which when operated at 95% depth of discharge manifested as a
discharge capacity of approximately 315 mAh/g of sulfur, which was stable for
more than 7000 cycles after which degradation of discharge capacity began and
became rapid after 8000 cycles.
The capacity degradation shown is mainly due to the degradation
of the lithium intercalation graphite anode.
The use of alkali salts of carboxymethyl cellulose (CMC-Alkali)
and polyacrylic acid (PAA-alkali) were used in the production of the anode,
which
greatly enhanced the performance compared to standard PVDF binder used in
graphite intercalation anodes.
In Figure 4, a graph of discharge capacity vs cell voltage is shown
for the cell. Higher voltages correspond to sulfur being converted to higher
order
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-
lithium polysulfide species such as Li2S8. Voltage starts to drop during the
formation of highly lithiated species (e.g. Li2S2and Li2S). The battery
control
system stopped drawing power after 1.9V. This demonstrates an effective
discharge capacity close to 325mAh/g of sulfur.
The invention has been described with reference to a preferred
embodiment. However, it will be appreciated that variations and modifications
can be effected by a person of ordinary skill in the art without departing
from the
scope of the invention.
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