CA3071502A1

CA3071502A1 - Alkali polysulfide flow battery

Info

Publication number: CA3071502A1
Application number: CA3071502A
Authority: CA
Inventors: Pasidu Mihikara Pallawela
Original assignee: Individual
Current assignee: Individual
Priority date: 2020-02-05
Filing date: 2020-02-05
Publication date: 2021-08-05

Abstract

This invention pertains generally to the field of energy storage and batteries such as for smoothing supply, demand power profiles and for electrical vehicles. More particularly, the invention relates to an alkali polysulfide flow battery, components, systems and compositions for use with an alkali polysulfide flow battery and a method of manufacturing and operating a flow battery system. Figure 1 is a basic representative model of the related invention.

Description

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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

- 2 -_ 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 _

- 3 --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.

- 4 -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

- 5 -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

- 6 -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

- 7 -_ 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

- 8 -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.

..

- 9 -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 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

- 11 -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

- 12 -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

- 13 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 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 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.

- 14 -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

- 15 -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 =

- 16 -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

- 17 -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 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

- 18 -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 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 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 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

- 20 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 ..

- 21 -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 ...

- 22 -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.

=
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 -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.

Claims

CLAIMS:

1. 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 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.

2. An ion-selective separator composition as claimed in claim 1, wherein the alkali metal is lithium.

3. An ion-selective separator composition as claimed in claim 2, wherein the cathode is a lithium polysulfide catholyte.

4. An ion-selective separator composition as claimed in claim 3, wherein the catholyte comprises a carrier medium in which lithium polysulfide species are soluble.

5. An ion-selective separator composition as claimed in any one of the preceding claims, wherein the anode is lithium based.

6. An ion-selective separator composition as claimed in claim 5, wherein the anode is selected from a solid lithium anode, a solid lithium alloy and/or composite structure and a solvated lithium anolyte.

7. An ion-selective separator composition as claimed in any one of the preceding claims, wherein the alkali metal ion conducting separator film is a polypropylene film.

8. An ion selective separator composition as claimed in any one of the preceding claims, wherein alkali metal ion conducting separator film has a thickness of from 10 to 50 gm.

9. An ion selective separator composition as claimed in any one of the preceding claims, wherein the carbon layer is a graphene-containing layer.

10. An ion selective separator composition as claimed in claim 9, wherein the carbon-containing layer is obtainable by coating a composition of carbon powder, polyvinylidene fluoride and N-methy1-2-pyrrolidinone.

11. An ion-selective separator composition as claimed in claim 9 or claim 10, wherein the carbon powder -containing layer has a thickness of from 1 to 10 gm.

12. An ion-selective separator composition as claimed in any one of claims 9 to 11, wherein the carbon powder -containing layer is loaded in an amount of 0.01 mg/cm2 to 1 g/cm2 of the alkali metal ion conducting separator film.

13. An ion-selective separator composition as claimed in any one of claims 8 to 12, wherein the carbon layer is coated directly onto the cathode side of the alkali metal ion conducting separator fihn.

14. An ion-selective separator composition as claimed in any one of claims 1 to 12, wherein the alkali metal ion conductor layer is disposed on the cathode side of the alkali metal ion conducting separator film, between the carbon layer and the alkali metal ion conducting separator film.

15. An ion-selective separator composition as claimed in any one of claims 1 to 13, wherein the alkali metal ion conductor layer is disposed on the anode side of the alkali metal ion conducting separator film.

16. An ion-selective separator composition as claimed in any one of the preceding claims, wherein the alkali metal ion conductor layer comprises aluminium oxide.

17. An ion-selective separator composition as claimed in claim 16, wherein the aluminium oxide is provided in a layer thickness of from 1 p.m to 100 p.m.

18. An ion-selective separator composition as claimed in claim 16 or claim 17, wherein the alkali metal ion conductor layer further comprises titanium oxide, which his preferably provided in a layer thickness of from 1 p.m to 50 gm.

19. 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 in any one of claims 1 to 18;
b) an alkali-metal polysulfide catholyte c) an anode selected from a solid alkali metal, alkali metal alloy, an alkali metal and/or graphite 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.

20. A flow battery as claimed in claim 19, which comprises an alkali-metal polysulfide catholyte.

21. A flow battery as claimed in claim 20, wherein the catholyte further comprises a solvating additive for polysulfide species.

22. A flow battery as claimed in claim 20 or claim 21, wherein the catholyte further comprises a phosphorus pentasulfide in an amount of up to 5% by weight of polysulfide in the catholyte.

23. A flow battery as claimed in any one of claims 20 to 22, wherein the catholyte further comprises an alkali metal nitrate.

24. A flow battery as claimed in any one of claims 20 to 23, wherein the catholyte comprises a liquid carrier medium selected from 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 tetra ethylene glycol dimethyl ether¨lithium trifluoromethanesulfonate.

25. A flow battery as claimed in claim 24, wherein the liquid carrier medium comprises tetra(ethylene glycol) dimethyl ether and dimethyl sulfoxide.

26. A flow battery as claimed in claim 25, wherein 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.

27. A flow battery as claimed in claim 25 or claim 26, wherein the liquid carrier medium further comprises 1,3-dioxolane.

28. A flow battery as claimed in claim 27, wherein the 1,3-dioxolane is present in an amount of up to 15% by volume of the liquid carrier medium, preferably from 5% to 10%.

29. A flow battery as claimed in any one of claims 19 to 27, which comprises an anode selected from a solid alkali metal, a solid alkali metal alloy, a solid alkali metal composition and an alkali-metal based anolyte.

30. A flow battery as claimed in claim 29, wherein the anode comprises an alkali-metal based anolyte.

31. A flow battery as claimed in claim 30, wherein the anolyte comprises an alkali-metal polyaromatic hydrocarbon complex in a liquid carrier medium.

32. A flow battery as claimed in claim 31, wherein the polyaromatic hydrocarbon is selected from one or a mixture of biphenyl and naphthalene.

33. A flow battery as claimed in claim 31 or claim 32, wherein the liquid carrier medium is tetra ethylene glycol dimethyl ether

34. A flow battery as claimed in any one of claims 31 to 33, wherein the anolyte further comprises an alkali metal nitrate.

35. A flow battery as claimed in claim 29, wherein the anode is a solid alkali metal composition.

36. A flow battery as claimed in claim 29 or claim 35, wherein the anode comprises an alkali metal in an intercalation host material.

37. A flow battery as claimed in claim 36, wherein the intercalation host material is selected from carbon, silicon, tin and cobalt tin titanium.

38. A flow battery as claimed in claim 36 or claim 37, wherein the anode comprises an alkali metal intercalated in graphite.

39. A flow battery as claimed in any one of claims 19 to 38, which comprises an electrolyte flushing system for flushing an electrolyte through the electrochemical cell in a short burst or pulse.

40. A flow battery as claimed in claim 39, wherein the electrolyte flushing system is configured cause electrolyte to be circulated at an increased flow rate for a pre-determined or determined duration in dependence of a pre-defined trigger.

41. A flow battery as claimed in claim 40, wherein the pre-defmed trigger comprises a determination of a change in voltage in response to an active change in current.

42. A flow battery as claimed in claim 40 or claim 41, wherein the electrolyte flushing system is configured for use with a lithium polysulfide catholyte to inhibit precipitation of highly lithiated lithium polysulfide species in the electrochemical cell during a discharge cycle.

43. A flow battery as claimed in claim 42, wherein 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 which trigger is the detection of no or minimal reduction in voltage in response to a high current draw test.

44. A flow battery as claimed in claim 43, wherein the high current draw test comprises drawing a high current for a period of up to 20 ms.

45. A flow battery as claimed in any one of claims 39 to 44, wherein the electrolyte flushing system is configured to cause electrolyte to be circulated at an increased flow rate for up to 10 seconds.

46. A flow battery as claimed in any one of claims 39 to 45, wherein the electrolyte flushing system is configured to flush a catholyte during discharge at greater than 75% discharge.

47. A flow battery as claimed in any one of claims 19 to 46, which further comprises 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.

48. A flow battery as claimed in any one of claims 19 to 47, which further comprises a dosing and/or filtering system for use with the at least one electrolyte.

49. A flow battery as claimed in claim 48, which comprises a dosing system configured to dose the at least one electrolyte with a functional additive.

50. A flow battery as claimed in claim 49, wherein the dosing system is configured to dose the at least one electrolyte with a functional additive 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 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.

51. A flow battery as claimed in claim 49 or claim 50, wherein 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.

52. A flow battery as claimed in claim 51, wherein the functional additive is 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.

53. A flow battery as claimed in claim 51 or claim 52, wherein the functional additive is an alkali metal nitrate.

54. A flow battery as claimed in claim 49, wherein 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.

55. A flow battery as claimed in any one of claims 48 to 54, which further comprises an electrolyte filtering system for periodically removing precipitated material from the at least one electrolyte.

56. A flow battery as claimed in claim 55, wherein electrolyte filtering system is 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.

57. A flow battery as claimed in any one of claims 19 to 56, wherein the alkali metal is lithium.

58. An alkali-metal polysulfide catholyte for use in a flow battery as defmed in claim 19, the catholyte as further defmed in any one of claims 20 to 28.

59. An anode for use in a flow battery as defmed in claim 19, the anode being selected from a solid alkali metal, alkali metal alloy, alkali metal composition and an alkali-metal based anolyte and being as further defmed in any one of claims to 38.

60. An electrolyte flushing system for use in a flow battery as defmed in claim 19, the electrolyte flushing system being for flushing an electrolyte through an electrochemical cell in a short burst or pulse and as further defmed in any one of claims 39 to 46.

61. 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.

62. A dosing and/or filtering system for use with at least one electrolyte in a flow battery as defmed in claim 19, the dosing and/or filtering system being as further defmed in claim 47.

63. An electrochemical cell for a flow battery as defmed in claim 19, 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, wherein the electrochemical cell and/or ion-selective separator is as further defmed in any one or combination of claims 1 to 18 and 20 to 57.

64. A method of operating an alkali metal ion flow battery as defined in any one of claims 19 to 63, 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).

65. A method of enhancing performance of an electrolyte in a flow battery, the method comprising dosing the electrolyte with a functional additive.

66. The method of claim 65, wherein the dosing of the electrolyte with the functional additive is conducted periodically, to maintain a defined concentration, responsive to a performance demand or according to a further criterion measurement.

,

67. A flow battery as hereinbefore described with reference to the drawings and/or examples.

68. An alkali-metal polysulfide catholyte as hereinbefore described with reference to the drawings and/or examples

69. An anode for use in a flow battery, the anode as hereinbefore described with reference to the drawings and/or examples.

70. An electrolyte flushing system as hereinbefore described with reference to the drawings and/or examples.

71. A flow circulation system as hereinbefore described with reference to the drawings and/or examples.

72. A dosing and/or filtering system as hereinbefore described with reference to the drawings and/or examples.

73. An electrochemical cell as hereinbefore described with reference to the drawings and/or examples.

74. An ion selective separator composition as hereinbefore described with reference to the drawings and/or examples.