WO2023023749A1

WO2023023749A1 - Electrochemical cell conditioning cycle

Info

Publication number: WO2023023749A1
Application number: PCT/AU2022/050976
Authority: WO
Inventors: Nathan Coad; Lance Maul; Lu Chen
Original assignee: Gelion Technologies Pty Ltd
Priority date: 2021-08-25
Filing date: 2022-08-24
Publication date: 2023-03-02

Abstract

The present invention provides a method of equalising the state of charge of a stack of cells, or battery, by electrically resetting each electrochemical cell (if required) within that stack, wherein the electrochemical cell comprises a metal-halogen redox couple. The electrochemical cell comprises: an anode, a cathode, a semipermeable barrier disposed between the anode and the cathode, a catholyte composition comprising polyhalide ions disposed between the cathode and the semipermeable barrier, and an anolyte composition disposed between the anode and the semipermeable barrier. The method comprising the step of applying a first, reverse voltage of between 0 and about −1.8 V between the anode and the cathode, such that polyhalide ions comprised in the catholyte migrate to the anolyte. A further preferred embodiment of the invention includes applying a second voltage of between 0 and about +1.8 V between the anode and the cathode, such that halide and/or polyhalide ions located in the anolyte migrate to the catholyte.

Description

ELECTROCHEMICAL CELL CONDITIONING CYCLE

Field

[0001] The present invention relates to a method of conditioning metal halide electrochemical cells. However, it will be appreciated that the invention is not limited to this particular field of use.

Background

[0002] The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. It should be appreciated, however, that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0003] For practical deployment of metal halide electrochemical cells in real world energy storage systems, multiple cells must be connected together (in series, parallel, or both) to form a battery pack. However, in such a battery pack, each individual cell may be slightly different to the others in terms of, for example, its capacity, resistance, etc., in spite of great efforts to minimise any discrepancy. Such discrepancies may ultimately lead to poor performance of the battery pack. For example, if one cell in the middle of the pack originally has lower capacity than others, then during charging, this cell will be fully charged before the others and continued charging can give rise to irreversible damage of this cell. With continued cycling, the performance of this cell will gradually decrease, further enhancing the discrepancy with the other cells. As a result, the performance of the battery pack drops.

[0004] For zinc-bromine cells, one reason for imbalances in cell capacity is due to the diffusion of polybromide ions towards the anode side of the cell. In this case, during charging the applied potential will be used to preferentially reduce polybromide ions rather than zinc ions, as the reduction of polybromides is more thermodynamically favourable than the reduction of zinc ions. In this process, the zinc-bromine redox couple is not established, and therefore no electricity is stored. Only once all the polybromide is fully depleted will the reduction of zinc ions commence, the desired redox reaction of zinc and bromine start to take place, and electrical energy be stored. [0005] Metal halide batteries possess the ability to discharge to zero volts, which chemically resets the cell chemistry, eliminates dendrites (uneven metal deposits on the anode that impact cell performance) and creates a zero state of charge condition. This can be used to reduce imbalances between individual cells in a battery pack. However, ensuring that a cell is discharged to 0 V requires an adequate stripping process, which is further complicated in a battery pack comprising multiple cells.

[0006] Existing metal halide battery technologies typically employ a method of short circuiting or open circuiting the cell and waiting a certain period of time so that the cell reaches equilibrium at a zero-energy state. However, the present inventors have determined that these methods are often ineffective in both single cells and battery packs with cells connected in series, in particular in static (i.e. non-flow) metal halide cells. Specifically, at open circuit not all dendrites are removed, and at short circuit, in strings, there is significant cell reversal that creates an unstable system. For example, during shorting, a cell with low capacity will be reverse charged by neighbouring cells which have higher capacity, further increasing the imbalance in the string.

[0007] Existing technologies have also relied on reverse charging cells to reach a zero state of charge condition, for example as described in US Patent No. 9,570,753. However, such methods based on reverse charging are not suitable for static (i.e. non-flow) metal halide cells. In a flow cell, the flowing electrolyte provides lower potential gradients, or put alternatively, high electrolyte uniformity. This means that the half-cell reactions are less likely to be limited (e.g. by electrode surface area) or poorly distributed over the electrode area (leading to dendrites and stratification). The flowing electrolyte also removes gassing from the electrode surfaces, which further improves homogeneity. This results in cell designs where the anode and cathode can be relatively similar, allowing for a degree of function in the reverse polarity state. However, in a non-flow battery the electrodes need to more customised to the half-cell reactions, hence the anode and cathode are relatively different to one another and do not function well in a reverse polarity state. Non-flow cells also do not tolerate rapid gas generation, which can result from operation in a reverse polarity state. This leads non-flow cells to be unsuitable for prolonged or high current reverse polarity cycling (i.e. in the voltage ranges required for, for example, the zinc-bromine redox reaction). [0008] As the key reason of the capacity imbalance between cells in a battery pack is the diffusion of polyhalide ions, the present inventors have invented a method to shift any polyhalide ions found in the anolyte back to the catholyte. Further, the inventors have found that this method may also be used to shift polyhalide ions from the catholyte to the anolyte, where they may be used to react with and dissolve any dendrites. The method devised by the inventors is advantageously suitable for use in static, i.e. non-flow, metal halide cells.

[0009] It is an object of the present invention to overcome or ameliorate one or more the disadvantages of the prior art, or at least to provide a useful alternative.

Summary of Invention

[00010] In a first aspect of the invention, there is provided a method of equalising an electrochemical cell comprising a metal-halogen redox couple, wherein the electrochemical cell comprises: a. an anode, b. a cathode, and c. a semipermeable barrier disposed between the anode and the cathode, d. a catholyte composition comprising polyhalide ions, disposed between the cathode and the semipermeable barrier, and e. an anolyte composition disposed between the anode and the semipermeable barrier, the method comprising the step of:

(i) applying a first, reverse voltage of between 0 and about -1.8 V between the anode and the cathode, such that the polyhalide ions comprised in the catholyte migrate to the anolyte.

[00011] The present invention provides a method of equalising the state of charge of a stack of cells, or battery, by electrically resetting each electrochemical cell (if required) within that stack. The skilled person will appreciate that the catholyte composition is in electrical communication with the cathode and the semipermeable barrier and the anolyte composition is in electrical communication with the anode and the semipermeable barrier.

[00012] The following options may be used in conjunction with the first aspect of the invention, either individually or in any combination. [00013] The first, reverse voltage may be between about -0.1 and about -1.5 V, optionally between about -0.3 and about -1.2 V. The first, reverse voltage may be about -0.5 V. The first, reverse voltage may be between about 0 to -0.1, -0.1 to -0.2, -0.2 to -0.3, -0.3 to -0.4, -0.4 to - 0.5, -0.5 to -0.6, -0.6 to -0.7, -0.7 to -0.8, -0.8 to -0.9, -0.9 to -1.0, -1.0 to -1.1, -1.1 to -1.2, -1.2 to -1.3, -1.3 to -1.4, -1.4 to -1.5, -1.5 to -1.6, -1.6 to -1.7, or -1.7 to -1.8 V. The first, reverse voltage may be about -0.1, -0.2, -0.3, -0.4, -0.5, -0.6, -0.7, -0.8, -0.9, -1.0, -1.1, -1.2, -1.3, -1.4, - 1.5, -1.6, -1.7, or -1.8 V.

[00014] The first, reverse voltage may be applied for a duration of between about 1 minute to about 10 hours. The first, reverse voltage may be applied for a duration of about 30 minutes. The first, reverse voltage may be applied for a duration of about 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26 , 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52 , 54, 56, 58, or 60 minutes, or 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12 hours.

[00015] In step (i) of the method of the first aspect of the invention, the polyhalide in the anolyte may react with metal plated on the anode.

[00016] The method of the first aspect of the invention may further comprise the step of:

(ii) applying a second voltage of between 0 and about +1.8 V between the anode and the cathode, such that halide and/or the polyhalide ions located in the anolyte migrate to the catholyte.

[00017] The catholyte and anolyte may comprise a plating additive. The plating additive may be selected from the group consisting of Li⁺, Mg²⁺, Al³⁺, In³⁺, Bi³⁺, Sn²⁺, Pb²⁺, Ag⁺ and Cu²⁺, or in any combination thereof. The plating additive may be In³⁺ orBi³⁺. The plating additive may be In³⁺. The plating additive may be Bi³⁺. The plating additive may be In³⁺ and Bi³⁺. In step (ii) of the method of the first aspect of the invention, the plating additive may be reduced at the anode, and may function as the strike layer for Zn plating. It will be appreciated by the skilled person that the sequence of metallic additive plating is dependent on the applied voltage.

[00018] In step (ii) of the method of the first aspect of the invention, the second voltage may be between about +0.1 V and +1.5 V, optionally the second voltage may be between about +0.3 V and +1.2 V. The second voltage may be about +0.5 V. The second voltage may be between about 0 to 0.1, 0.1 to 0.2, 0.2 to 0.3, 0.3 to 0.4, 0.4 to 0.5, 0.5 to 0.6, 0.6 to 0.7, 0.7 to 0.8, 0.8 to 0.9, 0.9 to 1.0, 1.0 to 1.1, 1.1 to 1.2, 1.2 to 1.3, 1.3 to 1.4, 1.4 to 1.5, 1.5 to 1.6, 1.6 to 1.7, or 1.7 to 1.8 V. The second voltage may be about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, or 1.8 V.

[00019] In step (ii) of the method of the first aspect of the invention, the second voltage may be applied for a duration of between about 1 minute and 1 hour. The second voltage may be applied for a duration of about 30 minutes. The second voltage may be applied for a duration of about 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, or 60 minutes, or 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12 hours.

[00020] The metal may be zinc. The halogen may be bromine.

[00021] The electrochemical cell may be in electrical connection with one or more electrochemical cells as defined in the first aspect of the invention. The electrical connection may be a series connection or a parallel connection.

[00022] The electrochemical cell may not be a flow cell.

Brief Description of Drawings

[00023] Figure 1. Schematic of cell cases, assembled cell, and assembled battery pack.

[00024] Figure 2. Photograph of assembled battery packs.

[00025] Figure 3. Voltage profile during cycling of the cell pack with cycling regime of: CC_Chg 220 mA (current density of 5 mA cm^-2) for 1 hour with cut off voltage of 60 V, rest for 5 minutes, CC_Dchg 220 mA, (current density of 5 mA cm^-2) for 1 hour, with cut off voltage of 0.1 V, rest for 5 minutes, then repeat the above cycle (CC_Chg, Rest, CC_Dchg). Each cycling curve represents a single cell within the stack.

[00026] Figure 4. Voltage profile during cycling of the same cell pack as Figure 3, after cell equilisation according to the method of the invention. The cell pack was cycled with CC_Chg 220 mA (current density of 5 mA cm^-2) for 4 hour with cut off voltage of 60 V, rest for 5 minutes, CC_Dchg 220 mA, (current density of 5 mA cm^-2) for 4 hour, with cut off voltage of 0.1 V, rest for 5 minutes, then repeat the above cycle (CC_Chg, Rest, CC_Dchg). Each cycling curve represents a single cell within the stack.

Definitions

[00027] In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments of the invention only and is not intended to be limiting.

[00028] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one having ordinary skill in the art to which the invention pertains.

[00029] As used in this application, the singular form “a”, “an” and “the” include plural references unless the context clearly dictates otherwise.

[00030] Unless the context clearly requires otherwise, throughout the description and the claims, the terms “comprise”, “comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”. For example, a composition, mixture, process or method that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, mixture, process or method.

[00031] The transitional phrase "consisting of’ excludes any element, step, or ingredient not specified. If in the claim, such would close the claim to the inclusion of materials other than those recited except for impurities ordinarily associated therewith. When the phrase "consisting of" appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole.

[00032] The transitional phrase "consisting essentially of" is used to define a composition, process or method that includes materials, steps, features, components, or elements, in addition to those literally disclosed, provided that these additional materials, steps, features, components, or elements do not materially affect the basic and novel characteristic(s) of the claimed invention. The term "consisting essentially of" occupies a middle ground between "comprising" and "consisting of".

[00033] Where the applicant has defined an invention or a portion thereof with an open-ended term such as "comprising", it should be readily understood that (unless otherwise stated) the description should be interpreted to also describe such an invention using the terms "consisting essentially of' or "consisting of." In other words, with respect to the terms “comprising”, “consisting of’, and “consisting essentially of’, where one of these three terms is used herein, the presently disclosed and claimed subject matter may include the use of either of the other two terms. Thus, in some embodiments not otherwise explicitly recited, any instance of “comprising” may be replaced by “consisting of’ or, alternatively, by “consisting essentially of’

[00034] Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used herein are to be understood as modified in all instances by the term “about”. The examples are not intended to limit the scope of the invention. In what follows, or where otherwise indicated, “%” will mean “weight %”, “ratio” will mean “weight ratio” and “parts” will mean “weight parts”.

[00035] The terms “predominantly” and “substantially” as used herein shall mean comprising more than 50% by weight, unless otherwise indicated.

[00036] The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention.

[00037] It will be understood that use of the term “between” herein when referring to a range of numerical values encompasses the numerical values at each endpoint of the range. For example, a temperature of between 80 °C and 150 °C is inclusive of a temperature of 80 °C and a temperature 150 °C.

[00038] The “oxidant” refers to the element which is reduced during the discharge of the battery. The “reductant” refers to the element which is oxidised during the discharge of the battery. This terminology may be applied to each element regardless of whether the battery is charging or discharging. Accordingly, during charging the “oxidant” is oxidised and the “reductant” is reduced. For example, in a metal-halogen battery, the halogen species may be referred to as the oxidant and the metal species may be referred to as the reductant.

[00039] The “anode” refers to the electrode at which the reductant is oxidised during discharge of the battery. The “cathode” refers to the electrode at which the oxidant is reduced during discharge of the battery. This terminology may be applied to each electrode regardless of whether the battery is charging or discharging. Accordingly, during charging, the oxidant is oxidised at the cathode and the reductant is reduced at the anode. For example, the halogen is reduced and oxidised at the cathode, and the metal is oxidised and reduced at the anode.

[00040] For the purposes of description, all documents referred to herein are hereby incorporated by reference in their entirety unless otherwise stated.

Description of Embodiments

[00041] The skilled addressee will understand that the invention comprises the embodiments and features disclosed herein as well as all combinations and/or permutations of the disclosed embodiments and features.

[00042] The present inventors have surprisingly discovered that a step of applying a first, low reverse (relative to the normal polarity for charging) voltage to a metal-halide electrochemical cell causes polyhalide ions to diffuse to the anolyte under the influence of the applied voltage and come into contact with the anode. The polyhalide ions will then progressively react with any metal that has been plated on the anode, preferably during the resting and discharging period. This dissolves any dendrites from the anode, and also causes the cell to reach a zero state of charge as the metal and halogen are consumed, or nearly consumed. The inventors have surprisingly found that application of such a low reverse voltage is effective in bringing a static metal halide cell to a zero state of charge, without the disadvantages associated with high voltage reverse charging. The inventors have also found that this step can be followed by a second step of applying a second, low voltage of normal charging polarity, which causes any remaining polyhalide to migrate to the catholyte. The second low voltage application step also has the additional advantage of establishing a strike layer on the anode, where the electrolyte contains metallic plating additives that have a lower reduction potential for plating than zinc. [00043] The method of the invention is applicable to an electrochemical cell comprising a metal-halogen redox couple, i.e. a metal-halogen electrochemical cell. Such a cell generates electrical energy by the oxidation of a metal and the reduction of a halogen. During discharge, the reductant, which often comprises elemental metal, is oxidised at the anode to produce metal cations. At the cathode, the halogen species, typically a molecular halogen, is reduced to halide ions.

[00044] For example, the oxidation reaction of a divalent metal at the anode during battery discharge may be represented by the forward direction of Equation 1 :

. . .Equation 1

The reduction reaction at the cathode during battery discharge may be represented by the forward direction of Equation 2:

. . .Equation 2

[00045] During charging of the battery, an electrical current is applied such that the reverse of Equations 1 and 2 take place.

The anode and cathode

[00046] The electrochemical cell of the invention comprises an anode and a cathode. The anode is the electrode at which the oxidation reaction takes place during discharge. The cathode is the electrode at which the reduction reaction takes place during discharge. An electrode is an electrical conductor used to make contact with a non-metallic part of a circuit. The anode and cathode may comprise any suitable material, typically an inert conductor. Suitable anode materials include carbon-filled polymers, carbon fibre felts, metals (including zinc), alloys, conductive organic polymers, conductive metallo-organic polymers, and binder-held carbon powders. Suitable cathode materials include carbon fibre felts, halogen resistant metals, carbon- filled polymers, and binder-held carbon powders. The anode and the cathode may each comprise a collector which is typically a conductive plate or mesh which is connected to the wires of an external circuit. The semipermeable barrier

[00047] The electrochemical cell of the invention may comprise a separator, which is a semipermeable barrier disposed between the cathode and the anode. A “semipermeable barrier” refers to a material which is typically electrically non-conductive and allows electrolyte ions to move between the anode and the cathode sides of the battery to balance charge, but reduces the diffusion of the oxidant and reductant between the two sides of the battery. Other roles of the separator may also include providing a controlled space between the anode and cathode to ensure an evenly distributed electro-chemical potential and current, and to provide a physical barrier to dendrites and other uneven deposits of the oxidant and/or reductant species. Examples of suitable materials for the semipermeable barrier include micro-porous lead-acid battery membranes, polyolefin membranes, fluorinated polymer membranes, porous ceramic layers such as absorptive glass matt (AGM), or ion-exchange membranes.

The electrolyte

[00048] The electrochemical cell comprises an electrolyte disposed between the anode and the cathode. The electrolyte is split into two regions by the semipermeable barrier. The region of the electrolyte disposed between the cathode and the semipermeable barrier is referred to as the catholyte, and the region of the electrolyte disposed between the anode and the semipermeable barrier is referred to as the anolyte. The catholyte and the anolyte may have substantially the same composition or may have different compositions.

[00049] The electrolyte (both the catholyte and the anolyte) contains a dissolved electrolyte salt which provides ionic conductivity as well as ionic neutrality for the charges formed at the cathode and anode during charging and discharging. For example, the dissolved electrolyte salt may be KC1, KBr, LiCl, LiBr, NaCl, NaBr, NH₄C1, NH₄Br, or LiC10₄,. The salt may be KC1.

[00050] The electrolyte (both the catholyte and the anolyte) may contain water. The electrolyte may contain no water. The electrolyte may contain less than about 5% w/w water (a water in salt electrolyte). The electrolyte may contain more than about 50% w/w water.

[00051] The electrolyte may be a solution. Alternatively, the electrolyte may be a gel.

Alternatively, the electrolyte may be a highly viscous water in salt electrolyte. For example, the electrolyte may be a gel as described in International PCT Publication No. W02020/186307 Al. [00052] The electrolyte also comprises the redox species, i.e. the metal-halogen redox couple. The electrolyte comprises a salt of the metal redox species. For example, an acetate, sulfate, triflate permanganate, oxide, hydroxide, dichromate, perchlorate, or halide salt of Zn²⁺, Co²⁺, Cu²⁺, Pb²⁺’ Sn²⁺, Fe²⁺, Ni²⁺, or Ni³⁺. The electrolyte may comprise one, two, three, four or more salts of the metal redox species. For example, the electrolyte may comprise zinc bromide, zinc chloride, or a mixture of zinc bromide and zinc chloride.

[00053] The electrolyte may further comprise a halogen sequestering agent (HSA). The role of the HSA is to form a complex with the halogen redox species, which not only reduces vapour pressure of the halogen, for safety purposes, but also reduces diffusion of the halogen (i.e. crossover to the anolyte). Crossover is believed to be one of the biggest contributors towards self-discharging. The HSA is typically an organic compound comprising a moiety capable of sequestering the halogen redox species. The moiety capable of sequestering the halogen may be a tertiary or quaternary ammonium group, a phosphonium group, or a sulfonium group. Preferably the moiety capable of sequestering the halogen is a quaternary ammonium group. The HSA may be an ionic liquid comprising a quaternary ammonium group. For example, the HSA may comprise (a) one or more anions selected from the group consisting of bromide, chloride, iodide, bis(trifluoromethylsulfonyl)imide, bis(fluorosulfonyl)imide, acetate, propionate, pentanoate, hexanoate, hexafluorophosphate, and tris(pentafluoro)trifluorophosphate; and (b) one or more cations selected from the group consisting of 1 -butylpyridinium, 1 -octylpyridinium, l-(2-hydroxyethyl)pyridinium, l-ethyl-3- methylimidazolium, l-butyl-3-methylimidazolium, l-pentyl-3-methylimidazolium,l-hexyl-3- methylimidazolium, 1 -(2-methoxyethyl)-3-methylimidazolium, 1 -( 1 -methoxymethyl)-3- methylimidazolium, l-methyl-3-octylimidazolium, 1 -methyl- 1-ethylpyrolidinium, 1-methyl-l- butylpyrrolidinium, 1 -methyl- 1 -hexylpyrolidinium, 1 -(2-methoxy ethyl)- 1 -methylpyrrolidinium, 1 -( 1 -methoxymethyl)- 1 -methylpyrrolidinium, tetrabutylphosphonium, tributy locty Ipho sphonium, tributy 1(2 -methoxy ethy l)pho sphonium, tributy 1-tert- butylphosphonium, tributyl( 1 -methoxymethyl)phosphonium, tetraethylammonium, tetrabutylammonium, tributyloctylammonium, tributyl(2-methoxyethyl)ammonium, tributyl(l- methoxymethyl)ammonium, and tributyl-tert-butylammonium. For example, the HSA may be 1- methylethypyrrolidinium bromide. Any HSA is suitable to be used with any halogen.

[00054] The electrolyte, specifically the catholyte, comprises the halogen redox species in the form of a molecular halogen. For example, the catholyte may comprise bromine (Bn), chlorine (Ch), or iodine (I2). The molecular halogen typically forms charged species in the electrolyte which are referred to herein as ‘polyhalides’. Polyhalides may be polyanions of the halogen redox species itself, or they may be negatively charged complexes of the halogen redox species with a halogen sequestering agent (HSA). For example, if the halogen is bromine, polyhalide species present in the electrolyte may include [Bra]-, [HSA^Bra]-, [HSA^Brs]-, and [HSA^Br?]-.

[00055] The electrolyte may also comprise additives. Some additives may serve, for example to modify the electrode/electrolyte interface and thus reduce gassing and reduce corrosion of the electrode. These are typically polymer additives. A wide range of polymers maybe used. Examples of polymer additives include polyethylene glycol (PEG), neopentyl Glycol, 2- [4- (2,4,4-trimethylpentan-2-yl)phenoxy]ethanol (Triton X 100), polyethylenimine (PEI), polydimethylsiloxane (PDMS), polyethyleneglycol alkyl (3 -sulfopropyl) diether, potassium salt. As an illustrative example, PEG may be used in an amount of between 1 to 5 % w/v, or 2 to 4, or 4% w/v. The PEG may have a molecular weight of 1000 to 10,000, or 2000-8000, or 6000 gmol ¹. Other additives are used in a similar manner.

[00056] Another type of additive is a plating additive. The metal of the redox couple may adhere poorly to the anode material, leading to a lack of uniformity, increasing the chance of detachment in the following cycling, therefore compromising battery performance. Plating additives are metallic ions which are added to the electrolyte and are reduced at a lower potential than the metal of the redox couple, thus forming a (typically very thin) layer on the anode before reduction of the metal of the redox couple (i.e. plating of the metal of the redox couple) commences. This thin layer of the metallic plating additive is called the strike layer in the electroplating industry. The strike layer makes the cathode material substrate ready for electroplating. The metal of the redox couple adheres better to the strike layer than it does to the electrode material, resulting in more uniform plating. During charging, the strike layer is immediately covered by the metal of the redox couple, therefore it does not affect the charging step. Suitable plating additives include Li⁺, Mg²⁺, Al³⁺, In³⁺, Bi³⁺, Sn²⁺, Pb²⁺, Ag⁺ and Cu²⁺, or in any combination thereof, and salts thereof. Preferred plating additives include In³⁺ and/or Bi³⁺.

[00057] Alternatively, the electrolyte may contain no additives. Static cell

[00058] The method of the invention is applicable to both flow cells and non-flow cells, but is particularly advantageous in non-flow cells. Thus the electrochemical cell of the invention is preferably not a flow cell (i.e. it is preferably a static cell). That is, the electrochemical cell of the invention or a battery comprising the electrochemical of the invention does not comprise a pump and/or external electrolyte reservoirs, by means of which electrolyte is caused to flow past the anode and cathode.

A battery pack comprising the electrochemical cell of the invention

[00059] Multiple electrochemical cells according to the invention may be placed in electrical connection with one another to form a battery pack. The cells may be connected in series, or in parallel, or both. Preferably, the cells are connected in series to facilitate higher voltage systems.

[00060] A battery pack comprising multiple cells according to the invention will also contain a means for carrying out the method of the invention, i.e. for applying the voltages of step (i) and step (ii) as described below. These steps can be facilitated solely or jointly, within the following system components: the cell Battery Management System (BMS), the String Management System (SMS) or the System Power Electronics (SPE).

[00061] The BMS is a cell or cell-pack level electronics device that typically provides protection to the cells by preventing cell operation in the case of current, voltage and temperature being outside of the recommended operating ranges. In the case of a static cell capable of discharge to 0V, the BMS may contain a bypass switch (FET, diode, solenoid, contact, etc. in parallel with the cell terminals) that short-circuits a cell in a string, before it is fully reversed. The surrounding string will still try to drive the cell into a negative polarity but the voltage experienced by the cell is tunable by the switch impedance, or a resistor in series with said switch (as they are in parallel with the cell), e.g. a high impedance bypass will force more current through the cell and drive it to a lower voltage.

[00062] A SMS may achieve something similar by regulating current through the string using the cell voltage feedback for regulation, e.g. a DCDC converter may be used to limit current as a cell enters a reversal state, and this current reduction will lower the reversal voltage to the desired level. The SPE may also regulate current (e.g. using an inverter) to limit the reversal voltage in the same manner.

[00063] For recharge at a partial voltage, the current regulation equipment can be used to limit the current so as to prevent full cell current. At the BMS level the switch can be used in a Pulse Width Modulation (PWM) state to also limit the charge voltage with the cell capacitance providing smoothing of the voltage in this operation state.

Step (!) of the method of the invention

[00064] In normal operation of a metal-halide electrochemical cell, the metal of the redox couple may adhere unevenly to the surface of the anode leading to non-uniform plating. Repeated cycling of the battery exacerbates this lack of uniformity and dendrites of the metal begin to develop on the anode. It is advantageous to dissolve all or nearly all of the metal plated on the anode and achieve a zero state of charge to reset the battery chemistry and refresh the anode surface.

[00065] In step (i) of the method of the invention a first, low reverse voltage is applied between the anode and the cathode, such that the anode becomes positively charged. The voltage applied is below the redox potential of the electrochemical cell (1.75 V for a zinc -bromine cell), such that no redox reactions of the metal-halogen redox couple take place. However, under the influence of the applied potential, polyhalides in catholyte undergo reduction, and halide anions in the anolyte undergo oxidation. Therefore, it appears as if negatively charged polyhalide ions in the catholyte move towards the positively charged anode and migrate to the anolyte. Once in the anolyte, the polyhalide ions will react with any metal that is plated on the anode, resulting in the formation of metal cations and halide ions.

[00066] The first reverse voltage applied in step (i) of the method of the invention may be between 0 V and the redox potential of the cell. For the purposes of this specification, a voltage of the opposite polarity to the voltage used to charge the cell is designated a ‘reverse’ voltage and indicated with a minus sign. The reverse voltage may be between about 0 and about -1.8 V, or between about 0 and about -1.75 V, or between about 0.1 and -1.5 V, about 0.1 and -1.2 V, or between about -0.3 and about -1.2 V. The reverse voltage may be about -0.1 V, or about -0.3, or about -0.5 V. [00067] The first reverse voltage may be applied continuously or may be applied in pulses.

When the first reverse voltage is applied in pulses the desired electrochemical reaction proceeds further to completion because the rest time in between pulses allows the mass transport of active species to the electrode surface. The pulse frequency may be between about 1 Hz to 1 kHz. The pulse frequency may be between about 1 to 50, 1 to 100, 1 to 200, 1 to 300, 1 to 400, 1 to 500,

1 to 600, 1 to 700, 1 to 800, 1 to 900, 50 to 100, 50 to 200, 50 to 300, 50 to 400, 50 to 500, 50 to

600, 50 to 700, 50 to 800, 50 to 900, 50 to 1000, 100 to 200, 100 to 300, 100 to 400, 100 to 500,

100 to 600, 100 to 700, 100 to 800, 100 to 900, 100 to 1000, 200 to 300, 200 to 400, 200 to 500,

200 to 600, 200 to 700, 200 to 800, 200 to 900, 200 to 1000, 300 to 400, 300 to 500, 300 to 600,

300 to 700, 300 to 800, 300 to 900, 300 to 1000, 400 to 500, 400 to 600, 400 to 700, 400 to 800,

400 to 900, 400 to 1000, 500 to 600, 500 to 700, 500 to 800, 500 to 900, 500 to 1000, 600 to 700, 600 to 800, 600 to 900, 600 to 1000, 700 to 800, 700 to 900, 700 to 1000, 800 to 900, 800 to 1000, or 900 to 1000 Hz. The pulse frequency may be 1 Hz, or 50, 100, 200, 300, 400, 500, 600, 700, 800, or 900 Hz, or 1 kHz. The amplitude may be the same as when applying a continuous reverse voltage.

[00068] The first reverse voltage that is applied in step (i) of the method of the invention may be applied for a duration of between about 1 minute and 10 hours. The reverse voltage may be applied for a duration of between about 1 to 2 minutes, or about 1 to 5, 1 to 10, 1 to 15, 1 to 30,

2 to 5, 2 to 10, 5 to 10, 5 to 15, 5 to 20, 5 to 30, 30 to 40, 40 to 50, or between about 50 to 60 minutes. The first reverse voltage may be applied for a duration of between about 1 to 2.5 hours, or between about 1 to 5, 1 to 7.5, 1 to 10, 2.5 to 5, 2.5 to 7.5, 2.5 to 10, 5 to 7.5, 5 to 10, or 7.5 to 10 hours. The reverse voltage may be applied for a duration of about 1 minute, or about 2, 5, 10, 15, 20, 30, 40, 50, or 60 minutes, or for a duration of 2 hours, or about 3, 4, 5, 6, 7, 8, 8, or 10 hours. The reverse voltage may be applied for a duration of about 30 minutes. The first reverse voltage of step (i) of the method of the invention can be applied for longer than the minimum required duration in order to ensure that all metal plated on the anode is removed.

Step (ii) of the method of the invention

[00069] At the conclusion of step (i) of the invention, all or nearly all metal redox species plated on the anode will be dissolved and all or nearly all of the halogen redox species will be in the form of halide. However, as a result of polyhalides migrating to the anolyte in step (i), the anolyte may contain an excess of polyhalide ions which need to be shuttled to the catholyte. In step (ii) of the method of the invention, a second low voltage is applied between the anode and the cathode, such that the cathode becomes positively charged. The voltage applied is below the redox potential of the electrochemical cell (1.75 V for a zinc-bromine cell), such that no redox reactions of the metal-halogen redox couple take place. However, under the influence of the applied potential, the polyhalides in anolyte will undergo reduction while the halide ions in the catholyte will undergo oxidation. Therefore it appears as if negatively charged polyhalide ions in the anolyte will move towards the positively charged cathode and migrate to the catholyte, ready to engage in the normal redox reaction upon charging of the cell.

[00070] While the second low voltage of step (ii) is below the redox potential of the cell, it may be sufficiently high to cause plating additives in the catholyte and anolyte to be reduced and form a strike layer on the anode. This has the added advantage of improving plating quality of the metal redox species in subsequent charging steps.

[00071] The second voltage that is applied in step (ii) of the method of the invention may be between 0 V and the redox potential of the cell. For the purposes of this specification, a voltage of the polarity that is used to charge the cell is designated a ‘positive’ voltage and indicated with a plus sign. The second voltage may be between about 0 and about +1.8 V, or between about 0 and +1.75 V, or between about +0.1 and +1.5 V, about +0.1 and +1.2 V, or between about +0.3 and about +1.2 V. The second voltage may be about +0.1 V, or about +0.3, or about +0.5 V.

[00072] The second voltage may be applied continuously or may be applied in pulses. When the second voltage is applied in pulses the desired electrochemical reaction proceeds further to completion because the rest time in between pulses allows the mass transport of active species to the electrode surface. The pulse frequency may be between about 1 Hz to 1 kHz. The pulse frequency may be between about 1 to 50, 1 to 100, 1 to 200, 1 to 300, 1 to 400, 1 to 500, 1 to 600, 1 to 700, 1 to 800, 1 to 900, 50 to 100, 50 to 200, 50 to 300, 50 to 400, 50 to 500, 50 to 600, 50 to 700, 50 to 800, 50 to 900, 50 to 1000, 100 to 200, 100 to 300, 100 to 400, 100 to 500,

400 to 900, 400 to 1000, 500 to 600, 500 to 700, 500 to 800, 500 to 900, 500 to 1000, 600 to 700, 600 to 800, 600 to 900, 600 to 1000, 700 to 800, 700 to 900, 700 to 1000, 800 to 900, 800 to 1000, or 900 to 1000 Hz. The pulse frequency may be 1 Hz, or 50, 100, 200, 300, 400, 500, 600, 700, 800, or 900 Hz, or 1 kHz. The amplitude may be the same as when applying a continuous positive voltage.

[00073] The second voltage applied in step (i) of the method of the invention may be applied for a duration of between about 1 minute and 10 hours. The second voltage may be applied for a duration of between about 1 to 2 minutes, or about 1 to 5, 1 to 10, 1 to 15, 1 to 30, 2 to 5, 2 to 10, 5 to 10, 5 to 15, 5 to 20, 5 to 30, 30 to 40, 40 to 50, or between about 50 to 60 minutes. The second voltage may be applied for a duration of between about 1 to 2.5 hours, or between about 1 to 5, 1 to 7.5, 1 to 10, 2.5 to 5, 2.5 to 7.5, 2.5 to 10, 5 to 7.5, 5 to 10, or 7.5 to 10 hours. The second voltage may be applied for a duration of about 1 minute, or about 2, 5, 10, 15, 20, 30, 40, 50, or 60 minutes, or for a duration of 2 hours, or about 3, 4, 5, 6, 7, 8, 8, or 10 hours. The second voltage may be applied for a duration of about 30 minutes.

[00074] The method of the invention can comprise applying solely step (i) described above. Alternatively, the method of the invention can comprise applying solely step (ii) described above. Carrying out only step (ii) can result in an improved strike layer and reduced dendrite formation, and can be of assistance in case the current collector/anode is not compatible with bromine, Alternatively, the method of the invention can comprise applying both step (i) and step (ii).

Examples

Cell components

[00075] The anolyte, catholyte, and cathode filler material were prepared as follows:

[00076] Anolyte composition: 2.5 M ZnBr₂, 0.5 M ZnCl₂, 1.5 M KC1, 1 M MEPBr, 9.4 g/L (6000 mg/kg) PEG6000.

[00077] Catholyte composition: 2.5 M ZnBr₂, 0.5 M ZnCl₂, 1.5 M KC1, 1 M MEPBr, 9.4 g/L (6000 mg/kg) PEG6000, blended with 3 g/100 ml fumed silica (0.2 - 0.3 micron, Sigma Aldrich) + 3 ml/L Br₂ for spiking.

[00078] Cathode filler (carbon gel): the cathode filler is comprised of EG30 graphite power (SGL, 4.8g/100 ml standard electrolyte), ketjenblack EC300J carbon black (4.8 g/100 ml standard electrolyte), blended with standard electrolyte. The mixing process was performed with a kitchen mixer at a very slow rate.

Cell construction

[00079] The following components are arranged in order to form the Zn side of the cell.

1. Zn side casing (carbon plastic, mainly HDPE)

2. 1 x carbon felt (SGL GFD, 2.5 mm thickness, 75 mm diameter), which was thermally treated (500 °C, 2 hours, in air) prior to use, “fuzzy side” up

3. 1 x absorbent glass mat (AGM), with thickness of 0.9 mm, diameter of 76.6 mm, sourced from HBL, India (pre-soaked in electrolyte)

4. 1 x Carbon paper (AvCarb® P50) with diameter of 76.6 mm

5. 1 x Amersil FF50 separator with diameter of 83.5 mm (pre-soaked by the anolyte)

[00080] Then, 28 g of cathode filler was dispersed against the bromine side casing.

[00081] The cell was then sealed by using a hydraulic press, and silicone sealant was applied around the edge of the seal. Each cell requires around 20 mL anolyte and 16 mL of catholyte (catholyte contain 0.048 ml of Bn. as the concentration of Bn is 0.3% v/v). Such a cell has current density of 5 mA/cm², thus current will be 220 mA, and C/8 can reach 2.65 Wh.

Pack construction

[00082] Individual cells as described above may be combined together to form a pack. Thirty cells as described above were connected in series, with carbon felt (AGL) placed between each two cells to decrease the contact resistance. In the pack, the compression of the carbon felt is around 50%. Three metal strings were used to secure the whole pack. This is depicted in Figure 2. For monitoring purposes, the potential of each individual cell and the whole pack were recorded. [00083] Testing of the pack was performed with a Neware ® Battery cycler, 5A 60 V (Shenzhen, China) on a constant current charging/discharging regime.

Testing

[00084] A baseline capacity test on a 48 V/32 series cell pack was carried out according to the cycling regime below. Discharging of the pack to 0.1 V typically induces reversal in about half of the cells. The BMS facilitates the fractional reverse voltage (step (i) of the method of the invention, if required) and the cycler can facilitate the fractional positive voltage (step (ii) of the method of the invention).

[00085] The following cycling regime was used: CC_Chg (constant current charging) at 220 mA (current density of 5 mA cm^-2) for 1 - 4 hours, rest for 5 minutes, CC_Dchg (constant current discharging) at 220 mA (cut off voltage of discharging is 0.1 V), rest for 5 minutes, CR_Dchg (constant resistance discharging) with resistance of 0.5 to 5 Ohms, rest for 5 minutes. The above steps are considered one complete cycle. The total cut off of the charging voltage is 60 V.

[00086] For newly completed cells, the voltage quickly increases once charging start and the Zn and Br2 redox couple is formed. The voltage is the combination of the theoretical redox potential and other overpotentials, under a current density of 5 mA/cm². However, several cells shown in Figure 3 do not respond to the constant current charging as the voltage does not increase as fresh cells typically do. The inventors have found this behavior is due to the presence of polybromide on the Zn side of the battery (i.e. the anolyte), mainly in the AGM, which prevents the creation of the Zn and Bn redox couple. As the electrochemical reduction of polybromide on the Zn side (i.e. in the anolyte) is much quicker than reduction of Zn²⁺ (plating), most of the charge is used for reduction (consumption) of polybromide first. Plating of Zn²⁺ only takes place once the majority of polybromide on the Zn side is depleted, and is then followed by establishing the Zn/Br2 redox couple, which results in the charging voltage increasing to a normal value (around 1.8 V). As can be seen in Figure 3, several cells do not show the normal charging voltage as late as over halfway through the charging step. In other words, more than half of the electricity is wasted to consume polybromide first.

[00087] Figure 4 shows the same cell pack as Figure 3, but after cell equilisation according to the method of the invention. A low positive voltage charging step (+0.2 V for 1 hour) is applied, prior to constant current charging. It is evident that the charging voltages become more “normal”, with most of the cells charging immediately once charging starts. This is the evidence of the successful cell equilisation according to the method of the invention.

[00088] Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms in particular features of any one of the various described examples may be provided in any combination in any of the other described examples. Various modifications and alterations to this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention. It should be understood that this invention is not intended to be unduly limited by the illustrative embodiments and examples set forth herein and that such examples and embodiments are presented by way of example only with the scope of the invention intended to be limited only by the claims set forth herein as follows.

Claims

1. A method of equalising an electrochemical cell comprising a metal -halogen redox couple, wherein the electrochemical cell comprises: a. an anode, b. a cathode, and c. a semipermeable barrier disposed between the anode and the cathode, d. a catholyte composition comprising polyhalide ions, disposed between the cathode and the semipermeable barrier, and e. an anolyte composition disposed between the anode and the semipermeable barrier, the method comprising the step of:

(i) applying a first, reverse voltage of between 0 and about -1.8 V between the anode and the cathode, such that polyhalide ions comprised in the catholyte migrate to the anolyte.

2. The method of claim 1, wherein the first, reverse voltage is between about -0.1 and about -1.5 V.

3. The method of claim 1 or claim 2, wherein the first, reverse voltage is between about -0.3 and about -1.2 V.

4. The method of any one of claims 1 to 3, wherein the first, reverse voltage is about -0.5 V.

5. The method of any one of claims 1 to 4, wherein the first, reverse voltage is applied for a duration of between about 1 minute to about 10 hours.

6. The method of any one of claims 1 to 5, wherein the first, reverse voltage is applied for a duration of about 30 minutes.

7. The method of any one of claims 1 to 6, wherein in step (i) the polyhalide in the anolyte reacts with metal plated on the anode.

8. The method of any one of claims 1 to 7, further comprising the step of:

(ii) applying a second voltage of between 0 and about +1.8 V between the anode and the cathode, such that halide and/or polyhalide ions located in the anolyte migrate to the catholyte.

9. The method of claim 8, wherein the catholyte and anolyte comprise a plating additive.

10. The method of claim 9, wherein the plating additive is selected from the group consisting of Li⁺, Mg²⁺, Al³⁺, In³⁺, Bi³⁺, Sn²⁺, Pb²⁺, Ag⁺ and Cu²⁺, or in any combination thereof.

11. The method of claim 9 or claim 10, wherein the plating additive is In³⁺ or Bi³⁺.

12. The method of any one of claims 7 to 11, wherein in step (ii) the plating additive is reduced at the anode.

13. The method of any one of claims 8 to 12, wherein the second voltage is between about +0.1 V and +1.5 V.

14. The method of any one of claims 8 to 13, wherein the second voltage is between about +0.3 V and +1.2 V.

15. The method of any one of claims 8 to 14, wherein the second voltage is about +0.5 V.

16. The method of any one of claims 8 to 15, wherein the second voltage is applied for a duration of between about 1 minute and 1 hour.

17. The method of any one of claims 8 to 16, wherein the second voltage is applied for a duration of about 30 minutes.

18. The method of any one of claims 1 to 17, wherein the metal is zinc.

19. The method of any one of claims 1 to 18, wherein the halogen is bromine.

20. The method of any one of claims 1 to 19, wherein the electrochemical cell is in electrical connection with one or more electrochemical cells as defined in claim 1.

21. The method of claim 20, wherein the electrical connection is a series connection or a parallel connection.

22. The method of any one of claim 1 to 21, wherein the electrochemical cell is not a flow cell.