WO2022003157A1

WO2022003157A1 - Electrochemical device

Info

Publication number: WO2022003157A1
Application number: PCT/EP2021/068329
Authority: WO
Inventors: Robert A. W. DRYFE; Andinet EJIGU; Lewis W. LE FEVRE
Original assignee: The University Of Manchester
Priority date: 2020-07-03
Filing date: 2021-07-02
Publication date: 2022-01-06
Also published as: GB202010210D0

Abstract

The present invention relates to electrochemical devices based on zinc-ion chemistries. It uses a carefully formulated water-in-salt gel electrolyte. The present invention also relates to a cathode material ideally suited to use with such an electrolyte.

Description

ELECTROCHEMICAL DEVICE

Field of the Invention

The present invention relates to electrochemical devices, and particularly, although not exclusively, to batteries and supercapacitors utilising zinc chemistries.

Background

Aqueous rechargeable batteries are a promising class of batteries for grid-scale electrochemical energy storage owing to their low cost, easy fabrication, high ionic conductivity and high operational safety^1-3. Research on aqueous batteries in recent years has been gaining momentum from application in low voltage divalent zinc-ion battery (ZIB) to high voltage monovalent lithium ion batteries (LIBs)^4-6. In particular, ZIBs have attracted substantial interest as one of the most promising next-generation technologies because: (i) it uses abundant metal in earth’s crust as a negative electrode and is air stable unlike Li; (ii) it is attractive for grid-scale energy storage, due to its low cost, good safety and environmental friendliness; and (iii) its energy storage per volume is approximately three times higher than that of Li²· ^7-9.

The development of ZIBs is, however, hindered by a number of factors such as issues with the aqueous electrolytes, formation of Zn dendrites at the anode and lack of efficient cathode materials⁷ due to the strong binding of divalent zinc ions with the host lattice. Furthermore, the co-decomposition of water molecules during deposition of Zn²⁺ is known to affect the reversibility of the Zn striping/deposition and depletes the electrolyte due to the sustained water consumption.

Therefore there is a need for improved electrolytes for zinc-based electrochemical devices.

Furthermore, improved cathode materials are also needed. The lack of an efficient cathode material for ZIBs remains a severe challenge¹³·¹⁴. A few of the cathode materials developed to date are Prussian blue analogues^15-17, manganese oxides^18-22 and vanadium oxides¹³·¹⁴. Each of these cathode materials suffer from limited specific capacity far below the theoretical capacity of the Zn anode (820 mA h g^_1), low voltage plateau (<1 .4 V) and low rate capability due to their poor electronic conductivity.

WO 2019/183224 A1 describes an aqueous solution electrolyte for a rechargeable zinc-metal battery.

The electrolyte contains a zinc ion and a first fluorine containing anion, and a secondary salt (generally lithium) and a second fluorine containing anion. This solution based electrolyte suffers from safety concerns as well as processing complexities.

US 2019/312303 A1 describes a lithium/zinc ion hybrid battery; it utilises an electrolyte solution containing a zinc salt of a fluoroalkylsulfonyl anion.

The present invention has been devised in light of the above considerations. Summary of the Invention

The present inventors have found that improvements can be achieved by careful composition of a gel electrolyte in a zinc-ion cell. In particular, by inclusion of certain ions within the gel the electrolyte can operate with improved electrochemical and cycling stability while providing higher safety levels than previously proposed electrolytes.

The inventors have also found that inclusion of zinc halides in a carbon-based electrode can yield improvements in zinc-ion cells.

In a first aspect, the present invention provides an electrochemical device comprising a positive electrode; a negative electrode comprising metallic zinc; and an electrolyte; wherein the electrolyte is a water-in-salt gel electrolyte comprising: a source of Zn²⁺ ions; a source of [trifluoromethanesulfonate]- ions; a source of one or more compounds of the following formula (1 ); and optionally, ZnSC ;

where, in formula (1 ), R¹ and R² are each independently selected from C1-10 fluorinated alkyl (that is, C1-10 alkyl wherein one or more hydrogens is replace with fluorine).

Suitably, the electrolyte comprises Zn[trifluoromethanesulfonate]2, Li[bis(trifluoromethylsulfonyl)imide] or Zn[bis(trifluoromethylsulfonyl)imide]2 and ZnSC . This combination of salts in the gel electrolyte has been found to be remarkably effective.

The electrolyte described above may advantageously be used in devices where the positive electrode comprises a carbonaceous material and at least one compound of formula ZnX2, wherein each X is independently selected from F, Cl, Br and I. Each X may be the same or different. Each X may independently be selected from Br and I; alternatively, the positive electrode comprises ZnCl2, ZnBr2 and Znl₂.

In view of this, in a second aspect the present invention provides an electrode material, suitably to be used as the positive electrode in a zinc-ion based device, comprising a carbonaceous material and at least one compound of formula ZnX2, wherein X is selected from F, Cl, Br and I. It will be recognised that each feature of such a material can be applied to this second aspect of the invention as well as to preferred embodiments of the first aspect.

The carbonaceous material of the positive electrode may suitably selected from graphite and activated carbon. These materials have been found to have particular advantages in certain uses. For example, in some embodiments the carbonaceous material of the positive electrode is activated carbon. Such embodiments are found to be particularly suitable as supercapacitors. In some embodiments, the carbonaceous material of the positive electrode is graphite. Such embodiments are found to be particularly suitable as batteries.

The positive electrode may further comprise a polymeric binder. The polymeric binder may be selected from polytetrafluoroethylene. polyvinylidene fluoride and sodium alginate; polytetrafluoroethylene is particularly suitable.

In the present invention, the electrolyte may suitably comprise ZnSC , Zn[trifluoromethanesulfonate]2 and either Li[bis(trifluoromethanesulfonyl)imide] or Zn[bis(trifluoromethanesulfonyl)imide]2, in a mass ratio of about 1 :1 :2. These particular combinations have been found to provide high performance.

In general, the electrolyte suitably comprises water and a polymeric binder. The binder may be, for example, polytetrafluoroethylene. This facilitates gel formation.

The electrolyte may contain water in an amount of about 10-30 mass% relative to the salt content of the electrolyte. This amount is suitable to produce a water-in-salt gel electrolyte; of course, it will be recognised that such gel electrolytes are intrinsically and structurally different from aqueous or other solution electrolytes.

Accordingly a third aspect of the present invention relates to an electrolyte for use in a zinc-ion cell or electrochemical device, the electrolyte being a water-in-salt gel electrolyte comprising: a source of Zn²⁺ ions; a source of [trifluoromethanesulfonate]^· ions; a source of one or more compounds of the following formula (1 ); and optionally, ZnSC ;

where, in formula (1 ), R¹ and R² are each independently selected from C1-10 fluorinated alkyl (that is, C1-10 alkyl wherein one or more hydrogens is replace with fluorine). It will be recognised that each feature of such an electrolyte can be applied to this third aspect of the invention as well as to preferred embodiments of the first aspect.

The present invention may permit the development of devices which do not require a separator between the electrodes. Accordingly, the device preferably does not comprise a separator between the positive electrode and the negative electrode. The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

Summary of the Figures

Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

Figure 1 shows (A) TGA traces recorded for a WiTS (water-in-trisalt) gel electrolyte obtained by ramping the temperature from 30 °C to 800 °C at a rate of 10 °C min^-1 in N2. The inset shows photographic images of the WiTS gel electrolyte. (B) CV (cyclic voltammetry) recorded at a 3-mm-diameter GC (glassy carbon) electrode at 10 mV s^_1 using the present WiTS gel electrolyte between -0.2 V to 4.2 V. (C) CVs recorded at symmetrical Znl Zn cell at 1 mV s^_1 in the present WiTS gel electrolyte before and after cycling for 100 h using galvanostatic charge-discharge. (D) galvanostatic charge-discharge curve obtained using symmetrical Zn | Zn cell in the present WiTS gel electrolyte at 0.2 mA cm^-2. (E) SEM image of fully discharged Zn substrate, with inset; the scale bars represent 20 pm for the main image and 5 pm for the inset, and (F) fully charged Zn substrate, with the scale bar representing 20 pm. The inset shows the XRD pattern of a Zn anode and cathode after 50 stripping/plating cycles in WiTS gel electrolyte in Znl Zn cell.

Figure 2 shows (A) Cyclic voltammograms recorded at 1 mV s^_1 in the present WiTS gel electrolyte using coin cells constructed from the present G-ZnX2 positive electrodes and a Zn foil negative electrode. (B) Ex-situ XPS (x-ray photoelectron spectroscopy data) of G-ZnCl2-ZnBr2 obtained for fully charged cell and (C) fully discharged cell. (D) Galvanostatic discharge curves vs capacity obtained at a current density of 50 mA g^_1 using the present positive electrodes (G-ZnX2) and Zn negative electrode in the present WiTS gel electrolyte.

Figure 3 shows in situ Raman spectral series of Znl G-ZnX2 cell in the present WiTS gel electrolyte during full (A) charge and (B) discharge. The insets in (A) and (B) show the graphite G-band region. In each figure and inset, from the top the spectra represent G-ZnC , G-ZnCl2-ZnBr2, G-ZnBr2, G-ZnE and all samples are at OCP (open circuit potential).

Figure 4 shows (A) charge-discharge curve obtained using coin cells constructed from AC-ZnCI/Br/l positive electrode and Zn foil negative electrode at indicated gravemetric current. In each case, the voltage range was between 0.1 V to 2.0 V vs Zn/Zn²⁺and the gel electrolyte was the present WiTS gel electrolyte. (B) Specific capacity vs gravimetric current using shown positive electrodes, (C) CVs recorded using various Zn/AC-ZnX2 coin cells at 1 .0 mV s^_1 in the present WiTS gel electrolyte and (D) Capacity retention and coulombic efficiency of Zn|AC-ZnCI/Br/l cell cycled at 1 A g^_1 using the present WiTS gel electrolyte.

Figure 5 shows the initial and final parts of a galvanostatic charge-discharge curve obtained using symmetrical Zn | Zn cell in the present WiTS gel electrolyte at 0.1 mA cm^-2. The inset shows the magnification of the first ten cycles. It can be seen that no change occurs between the settled initial part and the final part of the test. It is noted that the central part has not been reproduced because of data file processing difficulties.

Figure 6 shows galvanostatic discharge curves vs capacity obtained at a current density of 0.05 A g^_1 using (G-ZnCl2-ZnBr2) positive electrodes and Zn negative electrode in the present WiTS gel electrolyte.

Figure 7 shows charge-discharge curve obtained using coin cells constructed from the present positive electrode and Zn foil negative electrode at 0.05 A g^_1 using the present WiTS gel electrolyte

Figure 8 shows (A)CVs recorded at Zn | AC-ZnCI/Br/l coin cells at 0.5, 2, and 3.0 mV s^_1 in the present WiTS gel electrolyte and (B) graphs of peak current versus scan rate for the oxidation and reduction process shown in (A).

Figure 9 shows Zn | AC-ZnCI/Br/l coin cells charged at 1 .0 A g^_1 (left side Y-axis ) and discharged at 0.05 A g^_1 (right side Y-axis) using the present WiTS gel electrolyte.

Figure 10 shows (A) Galvanostatic discharge curves vs capacity obtained at a gravimetric current density of 0.02A g^_1 using a-Mhq2 positive electrode and Zn negative electrode in the present WiTS gel electrolyte and (B) Capacity as a function of cell (Zh|a-Mhq2) cycling at 0.02 A g^_1 using the present WiTS gel electrolyte.

Detailed Description of the Invention

Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

The present invention relates to electrochemical devices/cells, which include for example batteries and supercapacitors. As explained below, the behaviour of the claimed devices can be tuned by selection of their components.

Herein, certain parts are sometimes referred to as the ‘cathode’ and ‘anode’. It will be understood that these terms refer to the identity of the electrodes during discharge of the claimed device. At that time the anode releases electrons (it is the negative electrode); the cathode absorbs electrons (it is the positive electrode). That is, where a cathode is mentioned it is equivalent to the positive electrode; where an anode is mentioned it is equivalent to the negative electrode.

Negative Electrode / Anode

In the present invention the negative electrode is generally one comprising zinc (Zn), that is, one which can provide Zn²⁺ ions to the electrolyte during electrochemical reaction. Generally it contains metallic zinc due to the reactions which occur at the electrode. Most commonly the negative electrode is in the form of zinc metal or a zinc alloy. The negative electrode may be greater than about 50 wt% zinc metal, for example greater than about 70%, or even greater than about 95% zinc metal.

The negative electrode may be, for example, a zinc foil. It may be a (optionally porous) zinc metal powder electrode, for example made by slurry casting. In such an electrode a binder is present, and optionally a conductive additive. Such components are known in the art; binders and additives discussed herein may be used. The negative electrode might be a high surface area zinc electrode, such as zinc nanoparticles or electrodeposited zinc structures on a current collector.

Inclusion of a binder, for example, may aid manufacture of the electrode, particularly if the zinc content is in the form of or includes, for example, powdered zinc or nanoparticulate zinc (average particle diameter 1 to 100 nm, measured by SEM per ISO/DIS 19749(en)). The binder may be selected from polytetrafluoroethylene. polyvinylidene fluoride and sodium alginate; polytetrafluoroethylene is particularly suitable.

Positive Electrode / Cathode

The present invention is applicable in devices using a variety of different types of positive electrode; however, a useful choice is one (for example a composite) comprising carbon; that is, a carbonaceous material. Carbon electrodes are well known in the art, and are suitable or use in the present invention.

In preferred embodiments of the present invention, the positive electrode comprises not only carbon (or a carbonaceous material) but also a zinc halide compound. The carbonaceous material may be carbon, for example graphite, activated carbon, hetero-atom (for example nitrogen or boron) doped carbon, carbon nanotubes, hard carbon, graphene or carbon black. The zinc halide may be one or more of ZnCb, ZnBr2 and Znl2.

Where the positive electrode comprises graphite as the main carbon material, the present devices operate better as batteries. On the other hand, where the positive electrode comprises activated carbon as the main carbon material, the present devices operate better as supercapacitors. Explanation of this is provided in the Examples below.

Here, the ‘main carbon material’ means greater than about 50 wt%, preferably greater than about 70 wt%, and more preferably greater than about 90 wt% of the carbon material. The main carbon material may even be the only carbon material included in the positive electrode (excluding impurities).

Activated carbon is widely available, and may usefully be, for example, a powdered carbon. It may have a surface area (M²/g, BET method) of greater than or equal to about 500, for example greater than or equal to about 1000 or greater than or equal to about 2000. It may have an average particle size (Dso, pm) of about 5-50, for example about 5-20.

The graphite material suitable for use in the invention is not particularly limited either. The positive electrode may be a composite comprising the carbon material mentioned above, the zinc halide, and a binder material. It may be made by, for example, mixing the ingredients then drying. For example, the desired amount of zinc halide may be dissolved in a solvent such as water; the carbon material is then added while mixing. A binder, for example a resin or polymeric binder in suspension, can then be mixed in too. Excess solvent can then be removed by, for example, drying or otherwise heating.

The binder may be, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF) or sodium alginate (NaAIg). Of these PTFE may be preferred.

The positive electrode may also contain a conductive additive to assist in performance. For example, it may include carbon black. Embodiments where the carbon material is activated carbon may particularly benefit from the addition of such a conductive additive.

For example, 5 to 10 wt% carbon black or graphene may be added to the electrode.

The contents of the ingredients in the positive electrode material mix may be controlled to be in certain ratios. For example, the content of binder in the positive electrode material mix may be from about 0.5 to about 10 wt% with respect to the mass of carbon material, for example about 5 wt%.

In the non-binder content, the mass (or weight) ratio between the zinc halide and the carbon material may be about (0.5-2) : (3.5-2). The ratio may be from about 0.5:3.5 (= 1 :7) to about 2:2 (= 1 :1 ). For example, it may be about 1 :3.

As for the zinc halide, as mentioned above suitably one or more of ZnF_å, ZnCk, ZnBr2 and Znl2 is used. One of these alone, two of these, three of these or all four may be used. Alternatively, suitably one or more of ZnC , ZnBr2 and Znl2 is used. One of these alone, two of these, all three may be used. In other embodiments one or more mixed halide can be used, alone or in conjunction with the halides already discussed. A mixed halide may be ZnX¹X² wherein X¹ and X² are different and are each independently selected from F, Cl, Br and I. In some embodiments X¹ and X² are each independently selected from Cl, Br and I. For example, ZnBrCI can be used.

The halide ions can act both as charge carriers and redox centres. For example, the halide ions can undergo a “conversion-intercalation/adsorption” chemistry inside the carbon structure when the cell is charged for example using a Zn²⁺ conducting water-in-salt and suitably water-in-trisalt (WiTS) gel electrolyte.

In some embodiments, particularly where the carbon material is graphite, the zinc halide is not ZnCl2. In other embodiments, ZnCl2 is only included in combination with ZnBr2 or Znl2. This is because, as explained below, some less preferred performance characteristics are observed when ZnCl2 is used alone with a graphite carbon material.

That is, in some embodiments the zinc halide comprises ZnCl2 and at least one of ZnBr2 and ZnE.

Where the carbon material is activated carbon, it may be preferred that ZnCl2, ZnBr2 and Znk are all used in combination. The variety of the halogen species means that the characteristics such as battery voltage of the cell can be fine-tuned and increased. For example, the standard redox potential of IVI2, Br/Br2, CIVCI2, and FYF2 ions are 0.54 V, 1 .09 V, 1 .36 V and 2.8 V vs standard hydrogen electrode respectively²³. The combination of the zinc halide-carbon cathode with a Zn anode can generate an open-circuit voltage that ranges between 1 .3 V to 3.5 V depending on the halide type.

The advantages of using halide ion conversion-adsorption within carbon cathodes are (i) they are lighter than most organic anions so that the migration/diffusion of ions is faster, (ii) they are present within the carbon structure so they don’t have to diffuse to the surface from bulk electrolyte and (iii) they undergo reversible fast redox reactions which substantially provide an extra charge unlike inert anions.⁶ The combination of these factors is responsible for the high performance. The cells can also truly combine the characteristic high energy density of a battery with the high power density of a supercapacitor device

In some aspects, the present invention may provide an electrode material, for example a cathode material for a zinc-ion battery or supercapacitor, as described above as the positive electrode.

As mentioned above, as detailed in the Examples section below, the use of graphite in the composite electrode produced a battery like behaviour where the voltage plateau was related to the standard potential of the halogen species. However, when activated carbon was used in the composite, the cell acted as a hybrid Zn-ion capacitor due to the fast reversible halogen ion electrosorption/desorption in the carbon pores. The ZnX2-activated carbon composite delivers an exceptionally high capacity and cell energy density while retaining capacity after a large number of cycles.

Electrolyte

The electrolyte of the present invention is a water-in-salt gel electrolyte.

Water-in-salt gel electrolytes (WiSGE) are generally well known, as are water-in-salt electrolytes (WiSE). They contain a low amount of water (that is, a high concentration of the contained salt), such that the hydrated ions outnumber free water⁵· ^10_12. Generally the water content needs to be minimal relative to the salt content.

In other words, there is no free water to react at the electrode surface so the overall cell voltage can be increased. The combination of small cations and large fluorinated anions in water alters the hydration behaviour of the ions where the cation is strongly solvated, but the anion is not. The less solvated fluorinated anions can easily reduce to form a passivating solid electrolyte interface (SEI) at electrode surface⁵. This SEI formation significantly suppresses the hydrogen evolution reaction and is largely responsible for the overall electrochemical stability window of WiSEs⁵.

Typically the water content of the WiSGE is from about 10 to 30 mass% by mass of the salt(s). The highest voltage window (4.9 V) recorded for WiSEs contains small metal cations (Li+) and large fluorinated anions such as bis(trifluoromethanesulfonyl)imide ([TFSI]-) and trifluoromethanesulfonate ([TFO] )⁶.

In a WiSE, the salt-water ratio is generally larger than 1 .

A water-in-salt electrolyte or gel electrolyte may be further defined by the number of salts in them. Where there are two different salts, for example, it is a bisalt electrolyte (which may be denoted as WiBS); where three are present, for example, it is a trisalt electrolyte (which may be denoted as WiTS).

In the present invention, the electrolyte also includes salt materials. It contains a source of Zn²⁺ ions; a source of [trifluoromethanesulfonate]- ions (TFO ); a source of one or more of compounds of the following formula (1 ); and, optionally, ZnS04.

[That is, R¹-S0₂-N -S0₂-R²] where, in formula (1 ), R¹ and R² are each independently selected from C1-10 fluorinated alkyl (that is, C1-10 alkyl wherein one or more hydrogens is replace with fluorine); in particular perfluoro C1-10 alkyl (that is, Ci- io alkyl wherein all hydrogens are replaced with fluorines). R¹ and R² may preferably be each independently selected from trifluoromethyl, pentafluoroethyl, and fluoromethyl. R¹ and R² may preferably be the same.

The compound of formula (1 ) may be, for example, [bis(trifluoromethylsulfonyl)imide] (TFSI ), [bis(pentafluoroethlylsulfonyl)imide] (BETI ), or [bis(fluoromethylsulfonyl)imide]^· (FSI ). It may suitably be [bis(trifluoromethanesulfonyl)imide] (TFSh).

The electrolyte may suitably contain at least one salt of Zn²⁺, at least one [TFO]- salt, at least one [TFSI]- salt, and at least one SC ^2- salt. These ions may be combined to reduce the total number of salts included.

LiTFSI or Zn[TFSI]₂ may preferably be used as a source of [TFSI]- anion since it is believed that the reduction of [TFSI]- is responsible for the formation of the SEI which passivates the surface to extend the overall electrochemical window.

The inventors have also found that the absence of ZnSC in the formulation produces a gel type material which is very sticky to handle while the absence of ZnTFO simply produces a rigid dry material as excess ZnSC pulls water from the mixture. Accordingly inclusion of both of these is also preferred. The present WiSGE contains a binder to assist with gelling. The binder may be, for example, a resin or polymeric (polymer) binder, such as polytetrafluoroethylene (PTFE). PTFE is preferred as it enables facile gel electrolyte formation.

The binder content is important for the provision of a gel electrolyte. Typically, in the present invention the binder content of the electrolyte is 5-10 mass% by mass of the salt(s).

The binder may be dissolved in the water before addition to the salt(s) to form the water-in-salt gel electrolyte described herein.

The content of the salts in the electrolyte can be selected appropriately. In particular their mass ratio can be selected appropriately.

For example, where ZnSC and ZnTFO are included, they may be included in the same mass amount (1 :1 mass ratio), or within a ratio range of about 1 :2 to about 2:1 .

Where ZnSC and LiTFSI are included, they may be within a ratio range of about 1 :1 to about 1 :4, for example about 1 :2. Where ZnTFO and LiTFSI are included, they may be within a ratio range of about 1 :1 to 1 :4, for example about 1 :2.

Where ZnS04, ZnTFO and LiTFSI are included, the content of ZnS04 and ZnTFO together may be approximately equal to the content of LiTFSI. The [ZnS04 + ZnTFO] : LiTFSI ratio may be from about 2:1 to about 1 :2.

In preferred embodiments, the ratio of ZnS04, ZnTFO and LiTFSI is about 1 :1 :2 mass ratio.

These ratios may also be used in mixing. For example, the electrolyte may be formed by mixing about 70 mass% of the salts with about 20 mass% water along with about 10 mass% binder. The content of the salts may vary from about 50 mass% to about 90 mass%; the content of water may vary from about 5 mass% to about 30 mass%; the content of binder may vary from about 5 mass% to about 10 mass%.

The content of the ingredients can be selected so as to form the desired gel material.

The electrolytes of the present invention are flexible and non-flammable semi-solids. They therefore have improved processing and safety possibilities. It can exhibit fast electrode kinetics for Zn oxidation and reduction without the formation of Zn dendrites.

By using the present WiSGE, it is possible for the separator normally needed in electrochemical devices to be omitted. Accordingly, in some embodiments the device of the present invention does not include a separator. This will significantly reduce the contact resistance of the interface. The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example +/- 10%.

Examples

1. Experimental Methods 1.1 Materials and Apparatus

All chemicals used were analytical grade and obtained from Sigma-Aldrich, Fluorochem or Alfa Aesar. X- ray photoelectron spectroscopy (XPS) was performed using a Kratos Axis Ultra DLD spectrometer with a monochromated Al Ka X-ray source (E = 1486.6 eV, 10 mA emission). Scanning electron microscope (SEM) analysis was carried out using a FEI Quanta 650 FEG environmental SEM. Powder X-ray diffraction analysis was performed using a Philips X'pert PRO diffractometer with Cu Ka radiation (l =

0.154 nm) and operating at 40 kV and 30 mA. 1.2 WiTS gel electrolyte preparation

2 g of ZnS04.7H20 (99.99 %, Sigma-Aldrich), 2 g of zinc trifluoromethanesulfonate (98 % , Fluorochem) and 4 g of lithium bis(trifluoromethanesulfonyl)imide, (99 %, Fluorochem) were mixed together with 1 .5 g of water in a mortar and pestle until a uniform white paste formed. Subsequently, 1 .3 g (10 % of total salt) of a 60 % PTFE suspension was added and mixed with a mortar and pestle. The mixture was then heated at 80 °C on a hot plate for 30 min to remove any excess water. A semi-solid elastic gel was formed.

1.3 Electrode preparation

For the preparation of a carbon-ZnX2 cathode, first the desired amount of ZnX2 (anhydrous ZnCL, ZnBr2, Znl2 or mixture of two or more of these) was dissolved in 0.5g of water followed by slow addition of natural graphite (-325 mesh, 99.8 %, Sigma-Aldrich) or activated carbon (YEC-8B, Fuzhou Yihuan Carbon Co., Ltd) while mixing homogenously with a mortar and pestle. The mass ratio between ZnX2 and carbon was 1 :3. PTFE suspension (5 % with respect to the mass of carbon) was added to the thick slurry and mixed to uniformly coat the mixture with the polymer binder. The excess water was removed by heating the mixture on a hot plate. The resulting carbon-ZnX2 clay was quite flexible and could be made as a free standing film or rolled on to a pre-punched (15 mm diameter) titanium (99.99 %, Alfa Aeser) foil current collector. The composite electrode was dried in a vacuum oven at 80 °C overnight.

The typical mass loading of the carbon-ZnX2 composite electrode ranged from 2 to 5 mg crrr².

1.4 Battery assembly and electrochemical measurements

The full cells were assembled in CR2032-type coin cells using carbon-ZnX2 as the cathode and Zn foil as the anode. The flexible gel electrolyte was spread on to the Zn foil with an approximate thickness of 0.5- 1 .0 mm and acted as both the electrolyte and separator. The coin cell was sealed using a hydraulic crimping machine (MSK-160D) in ambient atmosphere. Three-electrode cell electrochemical measurements were conducted using a WiTS gel electrolyte that was rolled onto a microscopy glass slide for electrode connection. A glassy carbon working electrode, Pt wire counter electrode and Zn metal reference electrode were used. Electrochemical measurements were performed using an Autolab potentiostat (model PGSTAT302N, Metrohm Autolab, The Netherlands). The Charge-discharge battery tests were carried out using a Basytec Cell Test System (GmbH, Germany) with 32 independent test channels.

1.5 In situ Raman spectroscopy measurement

Raman spectra were obtained using a Renishaw inVia microscope with a 532 nm excitation laser operated at a power of 0.274 mW with a grating of 1800 lines/mm and 50c objective. The In situ Raman cell was obtained from ECC-Opto-Std/EI-Cell and the cell was comprised of a free standing carbon-ZnX2 positive electrode and a Zn foil negative electrode with a WiTS gel electrolyte. A titanium current collector which contained a small hole in its middle (diameter ca. 1 mm) was used as a current collector for the positive electrode. The exciting laser beam was shone through a thin glass window onto the rear of the free standing carbon-ZnX2 film through the small hole in the centre of the Ti foil. Spectral scans were collected in a backscattering configuration. The Raman measurements were collected at various voltages as the cell charged and discharged at 1 mV s^-1.

2. Electrolyte

2.1 Thermal Decomposition

The thermal decomposition of the gel electrolyte was studied by thermogravimetric analyser and it shows four mass losses due to the decomposition of H2O (<140 °C), [SC ]^2- (182-284 °C), [TFO] (323-450 °C) and [TFSI]- (>450 °C).^24-26

2.2 Ionic Conductivity

The ionic conductivity of the gel electrolyte was determined using AC impedance and was 6 mS cm⁴, which is comparable to that of non-aqueous electrolytes (9.0 mS cm ¹) used in commercial LIBs.⁵

2.3 Electrochemical Window

The electrochemical window of the gel was investigated using cyclic voltammetry (CV) at a glassy carbon disk electrode. The gel electrolyte exhibited fast kinetics for Zn oxidation and reduction and achieved a potential window of 3.0 V (Figure 1B). This potential window is comparable to WiSE-based in 21 m LiTFSI.⁵

2.4 Electrolyte Assessment

The long term electrochemical reversibility of Zn plating and stripping processes in WiTS gel electrolyte was investigated using a Zn|Zn symmetric cell under galvanostatic and CV methods. Figure 1C shows the CV obtained before and after several charge discharge cycles; and in each case the gel electrolyte exhibited the reversibility of Zn chemistry with the ratio between the anodic (/p,_a) and cathodic peak current (/p,_c) being one. The kinetics of Zn oxidation/reduction, however, significantly improved after cycling, as exemplified by the peak-to-peak (DEr) separation which decreased from -0.4 V to 0.2 V. The decrease in AEp with cycling is most likely due to the removal of surface oxides from Zn which impede electron transfer. In addition, the current due to Zn oxidation/reduction is increased by a factor of five after cycling which could be due to the increase in surface area. The charge-discharge curve also showed similar behaviour where the overpotential (7) for Zn stripping/plating decreased from about 0.6V to 0.2V after ten cycles, and stabilized at 0.2V even after the cell was cycled for 100 h at 0.2 mA cm^-2. The cell can operate continuously without short circuiting (no formation of dendrites) for over 400h at 0.1 mA cm^-2 with much lower 7 for Zn | Zn²⁺ redox reactions (see Figure 5). These observations demonstrate that the gel electrolyte is an excellent Zn²⁺ conductor with very facile Zn stripping/plating kinetics. The stability and the fast kinetics of Zn chemistry in WiTS gel electrolyte are superior to that of solution based aqueous and non-aqueous ionic liquid-based electrolytes.^27-30

2.5 Dendrite Formation

The presence or absence of Zn dendrite formation in the gel electrolyte was also examined using SEM after galvanostatic cycling of a Zn|Zn symmetric cell for 400 h. The Zn plated substrate exhibited a dense and uniform layered-like structure with the absence of any substantial dendrites (Figure 1 E). After Zn stripping, this dense structure is completely removed and the original surface is retained without the formation of ZnO (Figure 1 F inset) indicating the reversibility of Zn chemistry in the gel electrolyte.

3. Aqueous Zn|(Graphite-ZnX₂) Cell

The electrochemistry of confined Zn halides within a graphite electrode was examined using the WiTS gel electrolyte in full-cell Zn batteries. The freestanding graphite-ZnX2 cathodes were prepared by mixing the desired halide and natural graphite at a mass ratio of 1 :3 with 5 % PTFE binder. Figure 2A show the CVs recorded at a graphite-Znl2 (G-Znl2), graphite-ZnBr2 (G-ZnBr2) and graphite-ZnC (G-ZnCb) cathode and Zn anode cells using the WiTS gel electrolyte. Significantly, both G-ZnE and G-ZnBr2 showed reversible redox reaction at characteristic formal potentials of IVI2 (1 .17V vs Zn/Zn²⁺) and Br/Br2 (1 .67V vs Zn/Zn²⁺). The oxidative redox reactions are due to the conversion of the halogen (halide) ion (l^~, Br) to elemental halogen (l° or Br°) which are stabilized by sequential intercalation/adsorption into graphite galleries to form a solid graphite intercalation compound (Equation 1 ).⁶ _’ ³¹ _’ ³² This oxidation process releases Zn²⁺ which can conduct through the gel electrolyte to reversibly plate on the Zn anode (Equation 2). The reduction process at the cathode is due to the de-intercalation and reduction of l°/Br° to recombine with Zn²⁺ (n is the molar ratio of carbon atoms to the intercalated/adsorbed halogens in the GIC). The ratio between /p,_a and /p,b being one demonstrates the high reversibility of Equation 1

C_n + ZnX₂ C_n[X] + Zn²⁺ + 2e^~ (Equation 1 )

Zn²⁺ + 2e^~ <-> Zn (Equation 2)

In contrast, the CV obtained using the G-ZnCl2 cathode showed lower current and a lower degree of reversibility with a sharp undefined oxidation peak and a small reduction peak. The poor reversibility of G-ZnCl2 suggests the formation of an irreversible product during battery charging. It is thought that the chemical intercalation of ZnCl2 into graphite forms a strong complex with the carbon species where the Zn atom reacts to form a tetrahedral coordination with carbon atom.³³ Indeed, characterisation of a fully charged G-ZnCl2 containing electrode using XPS showed the formation of C-Zn-C bonds at a low binding energy (283.2eV) when analysing the high resolution C1s (Figure 2B).³⁴ Furthermore, the signal due to Zn carbide is still present and increased for the fully discharged cathode which confirms the irreversibility of the process, exhibited in the CV response (Figure 2C). These data demonstrate that the use of the ZnCl2-based redox active cathode material with sp²-carbon for ZIB is not preferred.

Figure 2D presents the discharge curves at 50 mAg^_1of the ZIB full-cells with the WiTS gel electrolyte for different cathode electrodes. As expected, G-ZnC showed very low performance with the absence of any useable voltage plateau due to the formation of an irreversible product. The G-Znl2 and G-ZnBr2 cathodes showed voltage plateaus that are equivalent to their respective redox reactions, G-Znl2 at 1 .17V and G-ZnBr2 at 1 .67V vs Zn/Zn²⁺, in agreement with the CV data. The cathode made from the equimolar mixture of ZnCl2 and ZnBrå showed two discharges voltage plateaus: one small at 1 .90V due to the CIVCI2 redox reaction and the other at 1 .67V due to the Br/Br2 redox reaction. Nonetheless, the specific capacity of each cathode is much lower than the theoretical capacity of a halogen GIC (309 mAh g ¹ for MBr_n, and 632 mAh g^_1 for MCln).⁶ Among the cathodes tested, the best specific capacity (55 mAh g ¹) was obtained using G-ZnCl2-ZnBr2 with the others being lower than 30 mAh g^_1. However, the capacity decayed by more than 50 % after 200 cycles due to the continual formation of zinc carbide species (Figure 6). The specific capacity quoted is based on the total mass of cathode (mass of graphite plus mass of halide).

In situ Raman spectroscopy was used to understand the intercalation mechanism of halogen and to understand the performance of the G-ZnX2 using the WiTS gel electrolyte. Figure 3 shows the fully charged/discharged Raman spectra for each cathode. The free standing sample for each cathode showed a similar response at open circuit potential (OCP); with the characteristic G-band at 1580 cm^-1 and small D-band at -1350 cm^-1 being shown and no other bands associated with Zn halides. The fully charged G-Znl2 electrode displayed an intense Raman signal at 172 cm^-1 due to surface bound iodine species.³⁵ _’ ³⁶ However, the absence of G-band splitting indicates no intercalation of the iodide species into graphite galleries (see inset of Figure 3A). While other halogens including chlorine and bromine intercalate into graphite, iodine has strong affinity to adsorb than intercalating into graphite.³⁷ _’ ³⁸ G-ZnBr2 also exhibited similar behaviour when fully charged where it showed a signal at 240 cm^-1 due to the stretching mode of Br2.⁶ _’ ³⁹ The G-band of the material, however, split into two Raman modes: the E2g2i mode at -1580 cm^-1 due to the interior unintercalated original layers and the E2g2b at 1604 cm^-1 due to the bounding layers next to the intercalants. The fact that the intensity of the E2g2i mode is twice that of the E2g2b togther with the wavenumber postion for the E2g2b indicates the dilute intercalation of bromine species into graphite galleries.⁴⁰ The reverse trends were observed during discharge which involves the desorption/de-intercalation of the halogen at each cathode (Figure 3D) characterised by the absence of halogen bands further confirming the reversibility of the process. This in-situ Raman spectroscopy data shows that reversible surface adsorption/desorption is the dominant reaction mechanism in G-Znl2 and G- ZnBr2 electrodes. This observation is similar to Na chemistry at a graphite electrode where Na plates on the graphite surface rather than intercalating due to its weaker chemical interaction with the graphite plane.⁴¹

The electrode that contained ZnCl2 (in neat or mixed with ZnBr2) on the other hand showed relatively high intercalation density reaching a stage-3 GIC. However, the G-band splitting remained after the battery was fully discharged which demonstrates that the intercalation process is non-reversible, in agreement with the XPS analysis (inset of Figure 3D). Overall, the analysis of Raman spectra indicate that the size of halogens significantly impacts the reaction mechanism at the graphite cathode. The conversion- adsorption process occurs when the size of the halogen is larger, for example in G-ZnE, and the conversion-intercalation process occurs for smaller halogens such as G-ZnC . Even though ZnC can reach a reasonably high intercalation staging, the irreversible formation of zinc carbide is the hindering factor for use in certain ZIBs. The low capacity of the full-cell battery at each electrode is explained by the conversion-adsorption reaction mechanism which needs a high surface area carbon rather than the low surface area graphite.

4. Aqueous Zn|(Activated carbon-ZnX₂) cell Various Zn halides were mixed with high surface area activated carbon (AC) and their performance was tested in full-cell coin cells. Figure 4A and Figure 7 show the representative charge-discharge curves of Zn|AC-ZnX2 batteries using the present WiTS gel electrolyte at various current densities between 0.1 to 2.0 V. Significantly, the charge-discharge curves show a near-triangular shape with little deviation from an ideal capacitor response. This implies that the kinetics of the halogen conversion-adsorption reaction at the activated carbon electrode is extremely facile in the present WiTS gel electrolyte. The specific capacities-based for Zn|AC-ZnCl2, Zn|AC-ZnBr2 and Zn|AC-Znl2 cells are approximately 281 , 232, and 196 mAh g^-1, respectively, at a current density of 0.05A g ¹. These capacities decreased to 102, 90 and 65 rnAhg ¹ when the current density was increased to 1 .0 A g^-1, and the corresponding capacity fade is over 60 % for each cell (Figure 4B). The capacities obtained at these electrodes are higher than Zn-ion capacitors using an activated carbon cathode and Zn anode.⁴² _’ ⁴³

The AC composite cathode made from the combination between ZnCl2, ZnBr2 and Znl2 with equimolar ratio of the halides achieved specific capacities twice that of individual ZnX2 at all current densities studied (Figure 4B). The improved capacity in the Zn|AC-Zn/CI/Br/l cell relative to Zn|ZnX2 could be due to the dense adsorption of halide ions on the carbon microstructure. For example, the columbic repulsion within the carbon microstructure host is expected to be minimal when the three halide species (Cl-, Br and I ) are adsorbed next to each other. The low columbic repulsion in the AC-ZnCI/Br/l electrode over individual AC-ZnX2 can enhance the packing density of halogens on carbon surface.

It is noted that herein where ‘ZnCI/Br/G is mentioned it is meant that the electrode comprises zinc ions along with chloride, bromide and iodide ions. It is not intended to be indicative of the relative content or ratio between those ions.

The improved capacity may also be caused by the electrocatalytic effect since the halide ion conversion- adsorption reaction is an electrocatalytic process where the reaction is greatly affected by the surface composition of the electrode.⁴⁴ _’ ⁴⁵ CVs were used to see if this electrocatalytic effect is in play and Figure 4C shows the CVs recorded for each electrode. At the AC-ZnCI/Br/l, a sharp oxidation peak at 1 .18V due to the conversion and adsorption of iodide species was seen along with a corresponding reduction peak at 1 .06V due to the reduction and recombination of iodide with Zn. Electrochemical data also suggest that the \^~lk redox reaction is a surface-controlled process at the AC electrode as both /_p,a and /_P,c are proportional to the scan rate (Figure 8). The DEr for the IVI2 redox couple at AC-ZnCI/Br/l was 0.12 V in contrast to over 1 .0 V at AC-Znl2. This demonstrates that the IVI2 redox reaction is kinetically much faster at AC-ZnCI/Br/l than at AC-Znl2. Similarly, the DEr for the Br/Br2 redox couple decreased from 0.65V at AC-ZnBr2 to 0.1V at AC-ZnCI/Br/l which confirms the electrocatalytic effect of the ternary halide mixture over the individual halides.

The specific capacity of Zn|AC-ZnCI/Br/l is 479 mAhg^-1 at 0.05 A g ¹ which decreased to 230 mAhg^-1 at 1 .0 A g^-1. Based on the mass of the cathode (mass of activated carbon plus mass of ZnXå) an energy densities of 422 and 160 Wh kg^-1 at power densities of 122.8 and 1071 .7 W kg^-1 were obtained. These values are higher than the energy density of other cathode materials reported for ZIBs including Mn0₂,⁴⁶ _’ ⁴⁷ V2O5,¹⁴’⁴⁸ Zh₃n₂q7.2H2q,⁴⁹ Zn_0.25V₂O5-nH₂O,⁵⁰ CuHCF,⁵¹ and VS₂ nanosheets.⁵² The cell energy density ranged between 55 Wh kg ¹ (at 1 .0 A g^-1) and 140 Wh kg ¹ (at 0.05 A g^-1) assuming the weight of the cathode material within a pouch cell configuration is a third of the total mass of the cell.⁴⁶ These values are much higher than that of the typical commercial supercapacitors (5-10 Wh kg ¹), lead- acid batteries (30-40 Wh kg^-1) and Zn-ion capacitors.^53-55 The energy density of the Zn|AC-ZnCI/Br/l cell is even higher than a Li-ion capacitor where the energy density is varies between 30 to 90 Wh kg^-1.⁵⁶ Li- ion capacitors often use intercalation-type anodes and an adsorption-type cathodes (adsorption of large complex anions ([PFe] , [TFSI] , [BF4] etc) where their overall energy density is limited by the capacitor- type electrode.⁵⁶

This cell can be fully charged within a few minutes (6 min) at high power and can be discharged for over 5 hr at lower rates (see Figure 9).

The Zn|AC-ZnCI/Br/l cell also exhibited excellent cyclic stability when the cell was cycled at 1 .0 A g^-1. The cell capacity retention is 95% after 500 cycles with 99 % columbic efficiency throughout the cycles (Figure 4D). Although the capacity initially decreased by 10 %, an increase in capacity was observed after 250 cycles. The increase in capacity could be due to the gradual activation of the electrode which increases the number of active electrochemical sites for ion adsorption.

Finally, the WiTS gel electrolyte performance was tested using traditional a-Mn0₂ cathode and Figure 10 shows the charge-discharge curve obtained at Zn|a-Mn0₂ cell. The gel electrolyte exhibited a coulombic efficiency of over 99 % with specific capacities that were increased from 162 rnAhg^-1 to 210 rnAhg^-1 after 50 cycles. This indicates that the WiTS gel electrolyte is as efficient as traditional solution electrolytes (such as 1 M ZnSC ), but with additional advantages like absence of parasitic water reduction reaction, elimination of the separator and other advantages that semi-solid state devices provide.

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Claims

Claims:

1 . An electrochemical device comprising a positive electrode; a negative electrode comprising metallic zinc; and an electrolyte; wherein the electrolyte is a water-in-salt gel electrolyte comprising: a source of Zn²⁺ ions; a source of [trifluoromethanesulfonate]- ions; a source of one or more compounds of the following formula (1 ); and optionally, ZnSC ;

2. An electrochemical device according to claim 1 , wherein the electrolyte comprises Zn[trifluoromethanesulfonate]2, Li[bis(trifluoromethylsulfonyl)imide] or Zn[bis(trifluoromethylsulfonyl)imide]2 and ZnSC .

3. An electrochemical device according to claim 1 or claim 2, wherein the positive electrode comprises a carbonaceous material and at least one compound of formula ZnX2, wherein each X is independently selected from F, Cl, Br and I.

4. An electrochemical device according to claim 3, wherein each X is independently selected from Br and I.

5. An electrochemical device according to claim 3, wherein the positive electrode comprises ZnCl2, ZnBr2 and Znl2.

6. An electrochemical device according to any one of claims 3 to 5, wherein the carbonaceous material of the positive electrode is selected from graphite and activated carbon.

7. An electrochemical device according to claim 6, wherein the carbonaceous material of the positive electrode is activated carbon.

8. An electrochemical device according to any one of claims 3 to 7, wherein the positive electrode further comprises a polymeric binder.

9. An electrochemical device according to claim 8, wherein the polymeric binder is selected from polytetrafluoroethylene. polyvinylidene fluoride and sodium alginate.

10. An electrochemical device according to any one of the preceding claims, wherein the electrolyte comprises ZnSC , Zn[trifluoromethanesulfonate]2 and either Li[bis(trifluoromethanesulfonyl)imide] or Zn[bis(trifluoromethanesulfonyl)imide]2, in a mass ratio of about 1 :1 :2.

11. An electrochemical device according to any one of the preceding claims, wherein the electrolyte comprises water and a polymeric binder.

12. An electrochemical device according to claim 11 , wherein the polymer binder is polytetrafluoroethylene.

13. An electrochemical device according to any one of the preceding claims, wherein the electrolyte contains water in an amount of about 10-30 mass% relative to the salt content of the electrolyte.

14. An electrochemical device according to any one of the previous claims, wherein the device does not comprise a separator between the positive electrode and the negative electrode.