Shuttle-free zinc-iodine batteries

Abstract

A solid positive electrode for an electrochemical device is disclosed. The solid positive electrode comprises a functionalised porous carbon substrate comprising a redox active metallocene, or a derivative thereof, and iodine.

Description

SHUTTLE-FREE ZINC-IODINE BATTERIES PRIORITY DOCUMENT

[0001] The present application claims priority from Australian Provisional Patent Application No.2024900451 titled “SHUTTLE-FREE ZINC-IODINE BATTERIES” and filed on 23 February 2024, the content of which is hereby incorporated by reference in its entirety. TECHNICAL FIELD

[0002] The present disclosure relates generally to electrochemical devices and methods for their preparation. In particular, the present disclosure relates to shuttling-free electrochemical devices wherein the shuttle effect caused by polyiodide is suppressed, and methods for their preparation. BACKGROUND

[0003] Zinc (Zn)-iodine batteries show tremendous promise in the field of large-scale energy storage. However, the generation of polyiodide anions such as I3−and I5−during cycling of zinc-iodine batteries leads to low Coulombic efficiency, fast battery degradation, and short lifespan.

[0004] Aqueous batteries show great promise as an alternative to organic electrolyte-based batteries for use in large scale energy storage systems. This is because organic electrolyte-based batteries suffer from inherent instability and safety issues caused by the use of expensive yet highly volatile and flammable organic solvents and chemically unstable toxic salts. For this reason, there is increasing attention on aqueous batteries. Furthermore, the abundance of iodine in seawater as well as highly reversible redox chemistry of iodine make iodine-based aqueous batteries a promising alternative to organic electrolyte- based batteries for energy storage. Iodine-based aqueous electrolyte batteries include zinc-iodine (Zn-I2) batteries, lithium-iodine (Li-I2) batteries, sodium-iodine (Na-I2) batteries, and aluminum-iodine (Al-I2) batteries. Rechargeable Zn-I2batteries are particularly appealing because of the abundant reserves of iodine in seawater, their high specific capacity (211 mAh giodine‒1) and high discharge potential plateau (1.38 V vs. Zn / Zn2+). In addition, the liquid phase conversion mechanism of I‒ / I2at the cathode endows a Zn-I2system with excellent rate capability.

[0005] Unfortunately, existing rechargeable aqueous and non-aqueous metal-iodine batteries are still unsatisfactory due to the challenges of intermediate dissolution as well as metal anode corrosion. Particularly, weak interaction between the conductive support of the cathode and iodine species leads to the dissolution of intermediate polyiodide anions or molecules in the aqueous or non-aqueous electrolyte and the subsequent migration of polyiodide through the separator towards the metal anode. Thisphenomenon is known as the “shuttle effect”. The polyiodide shuttle effect typically occurs in five steps: (i) formation of long-chain polyiodide anions, (ii) detachment of polyiodide anions from the iodine source, (iii) dissolution of polyiodide anions into the electrolyte solution, (iv) migration of polyiodide anions toward the anode, and (v) reaction between polyiodide anions and the anode metal. The shuttle effect can result in irreversible loss of iodine, corrosion of the anode metal, rapid capacity fading and poor Coulombic efficiency of the batteries. In aqueous electrolytes, Zn-I2 batteries present a reversible I‒ / I2 redox reaction, in which polyiodide species form as highly-soluble intermediates such as I3- that cause the shuttle effect and lead to irreversible loss of active mass. Therefore, it is important to suppress the shuttle effect and stabilise the iodine cathode and alleviate the corrosion of anode metal, which in turn expands the life span of iodine based batteries.

[0006] There remains a need for electrochemical devices which solve or alleviate one or more of the above problems and may be of practical use in large scale energy storage. SUMMARY

[0007] The present disclosure arises from the inventors’ research into the shuttle effect that occurs with iodine based batteries. It has been surprisingly found that a ferrocene-carbon-iodine (Fc / C-I2) complex can be used in a solid positive electrode to substantially prevent or minimise dissolution of polyiodide in an electrolyte solution and thereby suppress the shuttle effect and alleviate the corrosion of anode metal. Ferrocene as well as other redox active metallocenes and their derivates can be oxidised to form, for example, ferrocenium (the chemical valence of iron changes from +2 to +3). This ferrocenium, or related oxidised metallocene, can strongly bond with polyiodide to form insoluble ionic compounds which suppress the shuttle effect of zinc-iodine batteries. It has also been found that derivatives of ferrocene, such as ferrocenecarboxylic acid and 1,1′-dimethylferrocene, can also be used. It is also suggested that other redox active metallocenes can also be used.

[0008] In a first aspect, provided herein is a solid positive electrode for a shuttling-free electrochemical device, the solid positive electrode comprising a functionalised porous carbon substrate comprising a redox active metallocene, or a derivative thereof, and iodine.

[0009] In a second aspect, provided herein is a shuttling-free electrochemical device comprising a negative electrode, a solid positive electrode, and an electrolyte solution, the solid positive electrode comprising a functionalised porous carbon substrate comprising a redox active metallocene, or a derivative thereof, and iodine.

[0010] In a third aspect, provided herein is a method of fabricating a shuttling-free electrochemical device comprising a negative electrode, a solid positive electrode, and an electrolyte solution, the methodcomprising introducing into the solid positive electrode a functionalised porous carbon substrate comprising a redox active metallocene, or a derivative thereof, and iodine.

[0011] In a fourth aspect, provided herein is use of a functionalised porous carbon substrate comprising a redox active metallocene, or a derivative thereof, and iodine in a shuttling-free electrochemical device comprising a negative electrode, a solid positive electrode, and an electrolyte solution, wherein the functionalised porous carbon substrate is introduced into the solid positive electrode.

[0012] In certain embodiments of the first, second, third and fourth aspects, the redox active metallocene is selected from the group consisting of ferrocene, cobaltocene, nickelocene, chromocene, iridocene, vanadocene, manganocene, titanocene salts, ruthenocene, zirconocene salts, osmocene, and derivatives of any of the aforementioned. In certain specific embodiments of the first, second, third and fourth aspects, the redox active metallocene is ferrocene or a derivative thereof.

[0013] In certain embodiments of the second, third or fourth aspect, the functionalised porous carbon substrate comprising a redox active metallocene, or a derivative thereof, and iodine is capable of confining polyiodide species to the positive electrode during operation of the device.

[0014] In certain embodiments of the second, third or fourth aspect, the negative electrode has a source of zinc, lithium, sodium, aluminium or magnesium as a negative electrode active material.

[0015] In certain embodiments of the first, second, third or fourth aspect, the shuttling-free electrochemical device is an aqueous battery. In some embodiments, the aqueous battery is an aqueous metal-iodine battery. In some further embodiments, the aqueous battery is an aqueous zinc-iodine battery. In even further embodiments, the aqueous battery is an aqueous zinc-iodine battery wherein metallic zinc (Zn) is used as the negative electrode active material and elemental iodine (I2) is used as the positive electrode active material.

[0016] In certain embodiments of the first, second, third or fourth aspect, the carbon of the porous carbon substrate is selected from carbon black, Ketjen Black (KB), graphene, graphene oxide, carbon nanotubes (CNT), carbon nanofiber (CNF), carbon cloth, carbon felt, carbon paper, carbon fibre, carbon pellet, carbon powder, hollow carbon spheres, metal-organic frameworks (MOF), carbonised MOF, active carbon cloth / polyvinylpyrrolidone (ACC / PVPI) composite, and combinations thereof.

[0017] In certain embodiments of the first, second, third or fourth aspect, when the shuttling-free electrochemical device is an aqueous battery, the electrolyte is selected from ZnSO4, ZnI2, ZnBr2, ZnCl2, ZnNO3, zinc acetate, zinc bis(trifluoromethylsulfonyl)imide (Zn(TFSI)2), zinc trifluoromethanesulfonate (Zn(OTf)2), Zn(ClO4)2, a mixture of KI and ZnBr2, a mixture of ZnSO4and ZnI2, a mixture of ZnSO4andLi2SO4, a mixture of ZnCl2 and KCl, a mixture of ZnCl2, LiCl and acetonitrile. In some embodiments, when the shuttling-free electrochemical device is an aqueous battery, the electrolyte is selected from ZnSO4, ZnCl2, ZnNO3, and a mixture of ZnSO4 and Li2SO4. In other embodiments, when the shuttling- free electrochemical device is an aqueous zinc-iodine battery, an electrolyte for the solid positive electrode (i.e. catholyte) is a mixture of LiI and I2, an electrolyte for the negative electrode (i.e. anolyte) is a mixture of ZnSO4 and Li2SO4.

[0018] In certain embodiments of the first, second, third or fourth aspect, the shuttling-free electrochemical device comprises a separator which is selected from glass fibre separators, ceramic separators, polyolefin separators, nonwoven separators, and porous polymer separators.

[0019] In certain embodiments of the first, second, third or fourth aspect, the electrochemical device delivers a specific capacity of about 250 mAh g‒1and a Coulombic efficiency of about 99.5% at 0.2 A g−1. BRIEF DESCRIPTION OF THE FIGURES

[0020] Embodiments of the present disclosure will be discussed with reference to the accompanying figures wherein:

[0021] Figure 1 shows coupling between Fc+and polyiodides. a-c, Schematics for shuttle effects in Zn-I2 battery, Zn-Fc battery, and Fc coupled Zn-I2 battery. d, The crystal structure of Fc solid. e, XRD patterns of Fc and FcI3. Inset is the photograph of I2 / ethanol, Fc / ethanol solutions, and their mixture. f, Raman spectra of Fc and FcI3. The inset curves are the magnified Raman peaks;

[0022] Figure 2 shows material characterization of cathodes. a, HAADF-TEM image (top) and EDX mapping analysis (bottom) of Fc / KB. b, HR-TEM image of Fc / KB. c, XRD patterns of KB, Fc / KB, and Fc / KB-I2. d, HAADF-TEM image (top) and EDX mapping analysis (bottom) of Fc / KB-I2. e, HR-TEM image of Fc / KB. Inset is the magnified HR-TEM showing FcIxcrystals, labeled with circles. f, The TGA- MS profiles of KB-I2and Fc / KB-I2;

[0023] Figure 3 shows electrochemical performance of Zn-I2coin cells. a, CV profiles of Fc / KB, KB- I2, and Fc / KB-I2. b, Cycling performance and c, GCD curves of KB-I2(top) and Fc / KB-I2(bottom) at 0.2 A g−1. d, Rate capability and e, cycling stability of KB-I2and Fc / KB-I2batteries at 2.0 A g−1;

[0024] Figure 4 shows the conversion mechanism of Fc coupled Zn−I2batteries. a, Operando synchrotron XRD of Fc / KB-I2. b, Potential-dependent intensity evolution of Fc and FcIx. In-situ Raman spectra of KB-I2(c) and Fc / KB-I2(d). e, Comparison of peak positions of I3−(top) and I5−(bottom) duringcharge and discharge. f, Comparison of intensity evolution of I3−and I5−during charge and discharge in KB-I2 (top) and Fc / KB-I2 (bottom). The GCD curves (left) and dQ / dV plots (right) of KB-I2 (g) and Fc / KB-I2 (h);

[0025] Figure 5 shows determination of self-discharging in Zn-I2 batteries. a, Comparison of self- discharge rates at fully charged state after different aging times. b, The GCD curves after 12-hour aging for KB-I2 (top) and Fc / KB-I2 (bottom). c, In-situ Raman spectra of Zn-I2 battery using KB-I2 during 12- hour aging. d, Time-dependent intensity evolution of I3−(top) and I5−(bottom). e, In-situ Raman spectra of Zn-I2 battery using Fc / KB-I2 cathode during 12-hour aging. f, Time-dependent intensity evolution of I3−(top) and I5−(bottom);

[0026] Figure 6 shows practical demonstration of Ah-level Zn-I2 pouch cells. a, Schematic for pouch cell configuration using Zn foils. b, GCD curves and c, cycling performance of 1.2 Ah Zn-I2 pouch cell with excess Zn under 15 mA cm−2(equal to approximately 0.4 A g−1). d, Schematic for pouch cell configuration with controlled Zn DoD. e, GCD curves and f, cycling performance of a 0.45 Ah Zn-I2 pouch cell with a high Zn DoD (40%) at the current density of 20 mA cm−2(equal to approximately 0.6 A g−1). g, The comparison of energy density and cycling life with reported AZIB systems. h, The projected gravimetric energy density of Zn-I2 batteries. Inset is the relationship between I2 proportion and energy density at 50% Zn DoD. i, The comparison of Zn-I2 battery system with alternative AZB systems;

[0027] Figure 7 shows representations of Fc / ethanol solution (left), I2 / ethanol solution (middle), and their mixture solution (right);

[0028] Figure 8 shows the XRD pattern of Fc, I2, and Fc+I3-;

[0029] Figure 9 shows the XRD and simulated XRD of Fc+I3-;

[0030] Figure 10 shows Raman spectra of Fc and Fc+I3-;

[0031] Figure 11 shows Fe K-edge X-ray absorption spectroscopy (XAS) of Fc and Fc+ I3-;

[0032] Figure 12 shows (a) TEM images and corresponding elemental mapping result of Fc / KB complex, and (b) the high-resolution TEM image of Fc / C complex;

[0033] Figure 13 shows (a) TEM images and corresponding elemental mapping result of Fc / KB-I2complex, and (b) the high-resolution TEM image of Fc / KB-I2complex;

[0034] Figure 14 shows the XRD pattern of Fc+ I3-, Fc / KB, and Fc / KB-I2complex;

[0035] Figure 15 shows thermogravimetric analysis together with mass spectroscopy for the materials;

[0036] Figure 16 shows in-situ Raman spectra of Zn-I2 batteries using (a) KB-I2 and (b) Fc / KB-I2. (c) The calculated evolution of Raman shift for I3- and I5- species;

[0037] Figure 17 shows in-situ synchrotron powder diffraction reveals the coupling of Fc / Fc+ redox and I- / I2 redox;

[0038] Figure 18 shows the cycling performance of Zn-I2 battery at a current density of 0.2 A / g;

[0039] Figure 19 shows the charge-discharge curves of Zn-I2 battery using KB-I2 and Fc / KB-I2 at 0.2A / g;

[0040] Figure 20 shows the self-discharge experiments of Zn-I2 battery;

[0041] Figure 21 shows the rate performance of Zn-I2 battery;

[0042] Figure 22 shows the long-term cyclability of Zn-I2 battery; and

[0043] Figure 23 shows the cycling performance of 1.2 Ah Zn-I2 pouch cell. DESCRIPTION OF EMBODIMENTS

[0044] The term “electrochemical device” used herein refers to a device that can convert chemical energy into electrical energy through an electrochemical reaction. Examples of the electrochemical device include a battery and a cell such as a coin cell.

[0045] The term “shuttling-free” used herein means that the shuttling of polyiodide species to the negative electrode of the electrochemical device is effectively suppressed without adversely affecting the performance of the electrochemical device.

[0046] The term “aqueous electrolyte” used herein generally refers to a water-based electrolyte solution. However, this does not exclude the possibility of presence of other constituents like ionic liquids.

[0047] The term “negative electrode active material” used herein refers to an active material for the negative electrode of the electrochemical device. The term “positive electrode active material” used herein refers to an active material for the positive electrode of the electrochemical device. The active materials may be combined with other materials such as a conductive agent and a component to form acomposite comprising the active material in order to prepare the desirable electrodes.

[0048] The term “polyiodide” used herein refers to a class of polyhalogen anions composed entirely of iodine atoms. Examples of polyiodides commonly involved in the shuttle effect of an electrochemical device include I3−and I5−.

[0049] The term “metallocene” used herein refers to organometallic compounds that contain a metal atom sandwiched between two aromatic cyclopentadienyl (Cp) rings. A metallocene may have the general formula Me(C5H5)2 where Me is a metal atom. Depending on the valency of the metal, the metallocene may also contain one or more counterion to account for the valency of the metal. Counterions can be inorganic, such as halides, or organic, such as carbonyls. One exemplary metallocene disclosed herein is ferrocene. The terms “ferrocene” and “Fc” are used interchangeably herein and refer to an organometallic compound with the formula Fe(C5H5)2. Ferrocene is redox active and undergoes a one- electron oxidation at a low potential. Throughout the following description reference will be made to ferrocene for ease of explanation however it will be appreciated by the skilled person that other redox active metallocenes could be used in place of ferrocene or in addition to ferrocene.

[0050] Disclosed herein are a solid positive electrode for a shuttling-free electrochemical device and a shuttling-free electrochemical device. The shuttling-free electrochemical device comprises a negative electrode, a solid positive electrode, and an electrolyte solution. A functionalised porous carbon substrate comprising a redox active metallocene, or a derivative thereof, and iodine is contained within the solid positive electrode. The functionalised porous carbon substrate is capable of confining polyiodide species to the positive electrode during operation of the device.

[0051] The shuttling-free electrochemical device disclosed herein may be in the form of a battery or a cell. The functionalised porous carbon substrate contained in the solid positive electrode provides a source of iodine as a positive electrode active material.

[0052] The negative electrode of the shuttle-free electrochemical device may have a source of zinc, lithium, sodium, aluminium or magnesium as a negative electrode active material, for example zinc foil. If needed, the batteries known in the art may be adapted to the present disclosure through incorporating the ferrocene-carbon-iodine complex as detailed below.

[0053] The negative electrode and the positive electrode are electrically connected to each other by an electrolyte solution. In some circumstances, the electrolyte (i.e. anolyte) used for the negative electrode is different from the electrolyte (i.e. catholyte) used for the positive electrode. At least the following factors may be considered when choosing an electrolyte solution for the aqueous electrolyte: (i) chemical inertness; (ii) wide liquid range and thermal stability; (iii) balanced viscosity; (iv) high ionic and noelectronic conductivity; (v) interphase properties and control of by-products during the redox process; and (vi) availability. It may be desirable for the electrolyte solution to be modified by introducing corrosion inhibitors or complexing agents to render the electrolyte solution less corrosive. It is also possible to introduce an additive into the aqueous electrolyte to optimise electrochemical performance in the electrochemical device. Factors such as balanced viscosity, ionic conductivity, and control of by-products during the redox process may play a role in selecting the electrolyte. It is possible for the shuttling-free electrochemical device disclosed herein to have an aqueous electrolyte or a non-aqueous electrolyte such as an organic electrolyte. Iodine is soluble in organic solvents because of its small dielectric constant but exhibits low solubility in aqueous solutions. Organic solvents particularly with low dielectric properties, e.g. carbon disulphide, diethyl ether and hexane, may be suitable non-aqueous electrolytes. For an aqueous electrolyte, potassium iodide (KI) or other iodide aqueous solutions, may be added to increase the solubility of iodine.

[0054] High abundance (1.5 wt% in earth crust and 0.13 wt% in sea water), high volumetric energy density (3,833 mAh cm−3), very negative reduction potential (-2.37 V versus Standard Hydrogen Electrode) and highly reversible dendrite-free deposition, have made magnesium (Mg) metal a competent candidate for the negative electrode active material. In this regard, reference may be made to Mg-I2 batteries wherein metallic magnesium is used as the negative electrode active material and a composite of active carbon cloth (ACC) and I2 is used to prepare the positive electrode, and the electrolyte solution can be synthesised by reacting magnesium bis(trimethylsilyl)amide ((HMDS)2Mg) with aluminum chloride (AlCl3) and magnesium chloride (MgCl2) in tetraglyme (TEGDME) in situ.

[0055] Aluminum-iodine batteries that suffer from the polyiodide shuttle effect due to dissolution of polyiodide in an ionic liquid electrolyte may also be considered. For the purpose of illustration, the aluminum-iodine batteries may comprise Al foil as the negative electrode active material and I2as the positive electrode active material, and the ionic liquid electrolyte may be a mixture of 1-ethyl-3- methylimidazolium chloride (EMIC) and AlCl3, for example in a ratio of 1:1.3.

[0056] In some embodiments, the shuttling-free electrochemical device is an aqueous battery. The aqueous battery may be an aqueous metal-iodine battery. A particular example of the metal mentioned here is zinc. Accordingly, consideration may be given to aqueous zinc-iodine (Zn-I2) batteries. Aqueous Zn- I2batteries may be advantageous owing to the abundance of iodine in seawater, the high theoretical specific capacity of 211 mAh g-1and high discharge plateau (1.38 V vs. Zn / Zn2+). Furthermore, iodine has high chemical stability in the majority of commonly available solvents, even water.

[0057] When the negative electrode active material is made from metallic zinc, the latter may be in the form of a zinc foil.

[0058] Iodine present in the functionalised porous carbon substrate is an electrode active material within the solid positive electrode. The active mass loading of iodine for the positive electrode may be in the range of from about 0.01 mg / cm2to about 100 mg / cm2.

[0059] Non-limiting examples of the electrolyte that can be used for the aqueous Zn-I2 batteries include ZnSO4, ZnI2, ZnBr2, KCl, ZnCl2, ZnNO3, zinc acetate, zinc bis(trifluoromethylsulfonyl)imide (Zn(TFSI)2), zinc trifluoromethanesulfonate (Zn(OTf)2), Zn(ClO4)2, a mixture of KI and ZnBr2, a mixture of ZnSO4 and ZnI2, a mixture of ZnSO4 and Li2SO4, a mixture of ZnCl2 and KCl, a mixture of ZnCl2, LiCl and acetonitrile. ZnSO4, ZnCl2, ZnNO3, and a mixture of ZnSO4 and Li2SO4 may be preferred in some circumstances. A neutral, acidic or alkalescent aqueous electrolyte of the aqueous Zn-I2 batteries may be recommended as strong alkalinity could result in direct reaction with I2.

[0060] To form the positive electrode, a current collector may be used to support the functionalised porous carbon substrate and enhance the conductivity of the positive electrode. The current collector may be a conductive metal electrode material, such as iron or titanium. The current collector may be in the form of a sheet, a mesh, etc.

[0061] Factors such as wettability and electronic conductivity, and fixation and promotion of the redox reaction might be considered in selecting components of the positive electrode.

[0062] A functionalised porous carbon substrate with large surface area and / or pore volume may be desirable. The carbon component of the functionalised porous carbon substrate used herein may be a conductive carbonaceous substrate and examples include carbon black, acetylene black, Ketjen Black (KB), graphene, graphene oxide, carbon nanotubes (CNT), carbon nanofiber (CNF), carbon cloth, carbon felt, carbon paper, carbon fibre, carbon pellets, carbon powder, hollow carbon spheres, metal-organic frameworks (MOF), carbonised MOF, active carbon cloth / polyvinylpyrrolidone (ACC / PVPI) composite, and combinations thereof. In certain embodiments, the carbon component of the functionalised porous carbon substrate is Ketjen black. The carbon component of the functionalised porous carbon substrate may be commercially available or prepared through a method known in the art. For example, Ketjen black is commercially available, for example, from Canrd New Energy Technology Co., Ltd. Other carbon components may be prepared by chemical vapor deposition (CVD) techniques.

[0063] The positive electrode active material is I2that is adsorbed on the porous carbon substrate which also incorporates the ferrocene or derivative thereof. The I2is substantially uniformly distributed within the porous carbon substrate.

[0064] The functionalised porous carbon substrate may be formed by combining the porous carbon material, the redox active metallocene or derivative and the iodine in a suitable manner. The componentsmay be combined all at once, or sequentially. For example, the redox active metallocene or derivative may be combined with the porous carbon substrate in a first stage and the resultant metallocene / carbon complex (i.e. “Mc / C complex or “Fc / C complex”) may then be combined with iodine. The components may be combined my milling (e.g. ball milling), heating, shearing or any combination of these. For example, ferrocene or a derivative thereof may be mixed with porous carbon by milling, and then heated to a temperature of from about 30 ºC to about 300 ºC to provide the Fc / C complex. The amount of Fc in the Fc / C complex may be in the range of from about 0.01 wt.% to about 99.99 wt.%. The prepared Fc / C complex may then be mixed with iodine (I2) by milling and heated to a temperature of from about 30 ºC to about 300 ºC to provide an Fc / C-I2 complex. The amount of I2 in the Fc / C-I2 complex may be in the range of from about 0.01 wt.% to about 99.99 wt.%.

[0065] The Fc / C-I2 complex may be mixed with a conductive agent and a binder, and the mixture may then be coated onto a current collector.

[0066] During the charge / discharge process in a typical Zn-I2 battery, polyiodide (e.g. I3-) will dissolve into the electrolyte, leading to the shuttle effect. In the same process, ferrocene also will turn to soluble Fc+during the charge process, which will also lead to Fc+shuttling. However, in the shuttling-free electrochemical device disclosed herein an insoluble Fc+I3- complex will form during the electrochemical process, strongly avoiding the shuttling of both Fc+and I3-.

[0067] The redox active metallocene may be any redox active complex of the general formula Me(C5H5)2.Xn where Me is an oxidisable metal, X is a counterion (if present) and n is 0, 1, 2, or 3. Examples of redox active metallocenes include ferrocene (i.e. Me is Fe), cobaltocene (i.e. Me is Co), nickelocene (i.e. Me is Ni), manganocene (i.e. Me is Mn), chromocene (i.e. Me is Cr), iridocene (i.e. Me is Ir), molybdocene / molybdenocene (i.e. Me is Mo), tungstocene (i.e. Me is W), vanadocene (i.e. Me is V), titanocene (i.e. Me is Ti), ruthenocene (i.e. Me is Ru), zirconocene (i.e. Me is Zr), and osmocene (i.e. Me is Os). The redox active metallocene may be a derivative of any of the aforementioned. The redox active metallocene may be a mixture of any two or more of the aforementioned.

[0068] For the purposes of further description, reference will be made to ferrocene as the metallocene. The ferrocene (“Fc”) may be ferrocene or a derivative thereof. Ferrocene and derivatives comprise two cyclopentadienyl rings bound to a central iron atom and they are redox active compounds that can be oxidised to ferrocenium (the chemical valence of iron changes from +2 to +3). The resultant ferrocenium can strongly bond with polyiodide to form insoluble ionic compounds, which dramatically suppress the shuttle effect of zinc-iodine batteries. It will be evident from the foregoing that ferrocene as well as any ferrocene that is redox active can be used.

[0069] Suitable ferrocene derivatives include ferrocene aldehydes or ketones, such as 1,2-diformylferrocene, 3-ferrocenyl acrylaldehyde, (4-formylphenyl) ferrocene, octamethylformylferrocene, chloroacetyl ferrocene, 1-acetyl-1'-cyano ferrocene, α-οxο-1,1'-trimethylene ferrocene, β-οxο-1,1'- tetramethylene ferrocene, 1,1'-diacetyl ferrocene, (1,3-dioxobutyl) ferrocene, 1-acetyl-1'-acetylamino ferrocene, (2-chlorobenzoyl) ferrocene, benzoyl ferrocene, 1,1'-di(3-cyano-propionyl) ferrocene, phenylacetyl ferrocene, (2-methoxybenzoyl) ferrocene, 1,1'-di(acetoacetyl) ferrocene, 1-acetyl-1'-p- chlorobenzoyl ferrocene, 1-ferrocenyl-3-phenyl-2-propen-1-one, 3-ferrocenyl-1-phenyl-2-propen-1-one, (2,4-dimethoxy benzoyl) ferrocene, 1,1'-di(propionoacetyl) ferrocene, bisferrocenyl methyl ketone, 2- acetyl-biferrocene, 1,1'-di(pentafluorobenzoyl) ferrocene, 1,2-bisferrocenyl acyl ethane, 1,3- bis(ferrocenyl methylidene) acetone, 1'-acetyl-2,2-bisferrocenyl propane, 1,1'-di(benzoylacetyl) ferrocene.

[0070] Other suitable ferrocene derivatives include compounds of ferrocene carboxylic acid and its derivatives, such as ferrocene carboxylic acid, 2-hydroxy ferrocene carboxylic acid, ferrocene acetic acid, ferrocene thioacetic acid, 3-ferrocenyl acrylic acid, ferrocene propionic acid, ferrocene methylthio acetic acid, 1,1'-ferrocene diacetic acid, ferrocene butyric acid, ferrocene pentanoic acid, 2,2-dimethyl-3- ferrocenyl propionic acid, 1,1'-ferrocene dipropionic acid, ferrocene hexanoic acid, 1,1'-ferrocene dibutyric acid, 4,4'-bisferrocenyl pentanoic acid, 1,1'-ferrocene diformyl chloride, 1,2-ferrocene dicarboxylic anhydride, 1,1'-ferrocene diacetic anhydride, 2-(1'-carboxymethyl ferrocene) benzoic anhydride, ferrocene formic anhydride, dimethyl ferrocene-1,1'-dicarboxylate, 3-ferrocenyl ethyl acrylate, 1,1'"-di(methoxycarbonyl)-biferrocene, 4,4'-bisferrocenyl methyl pentanoate, ferrocene formamide, ferrocene formyl hydroxylamine, ferrocene formyl hydrazide, acetamido ferrocene, ferrocene formyl azirdine, 1'-vinyl ferrocene formamide, N-(2-cyanoethyl) ferrocene formamide, N-acetyl-2-ferrocenyl ethylamine, N-butyl ferrocene formamide, 1,1'-ferrocene diformyl azirdine, N,N,N',N'-tetramethyl-1,1'- ferrocene diformamide, N-phenyl ferrocene formyl hydroxylamine, N-ferrocenyl phthalimide, N-benzoyl- 2-ferrocenyl ethylamine, 4,4-bisferrocenyl valeramide, cyano ferrocene, 1,1'-dicyano ferrocene.

[0071] Still other suitable ferrocene derivatives include compounds of ferrocene alcohols, phenols or ethers, such as α-hydroxy ferrocene acetonitrile, ferrocene dimethanol, 1,2-ferrocene dimethanol, 1,1'- di(1-ethoxyl) ferrocene, octamethyl ferrocene methanol, ferrocenyl-(2,4,6-trimethoxyphenyl) methanol, bisferrocenyl methanol, α,α-diphenyl ferrocene methanol, 4-(2-ferrocenyl-2-ethoxyl)-4'-methyl-2,2'- bipyridine, 2-methyl-α,α-diphenyl ferrocene methanol, 1,4-bisferrocenyl-1,4-butanediol, 4,4- bisferrocenyl-1-pentanol, 4,4'-di(2-ferrocenyl-2-ethoxyl)-2,2'-bipyridine, 1,1'-di(diphenylhydroxymethyl) ferrocene, (4-hydroxyphenyl) ferrocene, 2-oxa-1,1'-trimethylene ferrocene, 1,3-dimethyl-2-oxa-1,1'- trimethylene ferrocene, bis(ferrocenyl methyl) ether, 1,1-bisferrocenyl methyl tert-butyl ether.

[0072] Still other suitable ferrocene derivatives include hydrocarbon compounds, such as 1,1'-dimethyl ferrocene, 1,1'-trimethylene ferrocene, 1,1'-diethyl ferrocene, 1 -vinyl-1' -chloroferrocene, 1,1'-di(α- cyclopentadienyl ethylidene) ferrocene, phenylethynyl ferrocene, bisferrocenyl acetylene, 1,1'-di(phenylethynyl) ferrocene, 1,1'-bis(ferrocenyl ethynyl) ferrocene, 1,1',2,2'-tetrachloro ferrocene, fluoroferrocene, biferrocene, 2,2-bisferrocenyl propane, 1,1-bisferrocenyl pentane, 1',1'"-di(triphenyl methyl) biferrocene.

[0073] Still other suitable ferrocene derivatives include nitrogen-containing ferrocene compounds such as (2-nitrovinyl) ferrocene, (4-nitrophenyl) ferrocene, 2-hydroxy-2-ferrocenyl ethylamine, N,N'- bisferrocenyl ethylenediamine, N,N'-bisferrocenyl methyl ethylenediamine, N,N'-di(bisferrocenyl methyl) ethylenediamine, 2-hydroxy-5-nitrobenzylimino ferrocene, benzoyl ferrocene oxime, ferrocene methyl diazomethyl ketone, 1,1'-diphenyl azoferrocene, ferrocenyl phenyl methylimino benzene, 1,6- diferrocenyl-2,5-diaza-1,5-hexadiene.

[0074] Still other suitable ferrocene derivatives include sulfur-containing or phosphorus-containing ferrocene compounds, such as 1,1'-ferrocene disulfonyl chloride, 1,1'-ferrocene disulfonyl azide, ferrocene sulfonyl chloride, ferrocene sulfinic acid, ferrocene sulfonic acid, (diethyl-dithiocarbamate)- ferrocene, 1,1'-di(dimethyl-dithiocarbamate)-ferrocene, ferrocene methyl phenyl sulfone, thiolferrocenyl- ferrocene sulphonate, bisferrocenyl disulfide, N,N'-dicyclohexyl-1,1'-disulfonamide ferrocene, (diphenylphosphino)-ferrocene; and silicon-containing ferrocene compounds such as 1,1'-dichloro-2- trichlorosilanyl-ferrocene, bis(1,1'-dichloro-2,2'-ferrocenylene)-silane, (1,1'-octamethyl-ferrocenylene)- dimethylsilane, (1,1'-dichloro-2,2' -ferrocenylene)-diphenylsilane, 1,1' -di[α-hydroxy-α- (trisilylpropyl)ethyl]ferrocene, 1,1'-di(phthalimide methyldisilyl)ferrocene.

[0075] Still other suitable ferrocene derivatives include ferrocene derivatives used in the present invention can also be heterocyclic ferrocene compounds such as 2-ferrocenyl-1,3-dithiane, 5-ferrocenyl- methylidene-1-aza-3-oxa-4-oxo-2-phenyl-1-cyclopentene, 1,3-bisferrocenyl imidazoline, 2,5- bisferrocenyl tetrahydrofuran.

[0076] Still other suitable ferrocene derivatives include for example, 1,1'-dicopper ferrocene, chloromercury ferrocene, ferrocene boric acid, ferrocenyl cuprous acetylide, bisferrocenyl titanocene.

[0077] Particularly suitable ferrocene derivatives include, but are not limited to, ferrocenecarboxylic acid and 1,1′-dimethylferrocene.

[0078] In fabricating the electrochemical device, other components such as a separator, a binder, a conductive agent, and a current collector may be employed. A separator serves to provide a barrier with no electrical conductivity between the negative electrode (anode) and the positive electrode (cathode) while allowing ion transport from one electrode to the other electrode. The separator is expected to retain chemical stability in the electrolyte while also having a high affinity for the electrolyte. It is also desirable for the separator to have good mechanical stability. Non-limiting examples of the separator include glassfibre separators, ceramic separators, polyolefin separators (e.g. polyolefin porous membrane), nonwoven separators, and porous polymer separators.

[0079] When powdered materials are used for the electrodes, a binder may be added to the electrodes to bring various components together and provide consistent mixing of electrode components so as to allow the electrodes to conduct the requisite amount of electrons and guarantee electronic contact during cycling of the electrochemical device. Non-limiting examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), sodium alginate, and carboxymethyl cellulose (CMC).

[0080] The primary role of a conductive agent is to enhance conductivity of the electrodes. In some circumstances, the conductive agent used can be identical to the porous carbon substrate used functionalised porous carbon substrate. Non-limiting examples of the conductive agent include carbon black, Ketjen black, graphene, conductive nano carbon fiber (VGCF), carbon nanotubes (CNTs), and multi-walled carbon nanotubes (MWCNTs). In some embodiments, a conductive agent may be introduced into the solid positive electrode in addition to the conductive substrate combined with the functional carbohydrate.

[0081] A current collector is a bridging component that collects electrical current generated at the electrodes and connects with external circuits. It can have influence on the capacity, rate capability and long-term stability of the electrochemical device. Non-limiting examples of the current collector include aluminium (Al) foil, copper (Cu) foil, carbon-coated aluminium, carbon-coated titanium (Ti) foil, and carbonaceous materials.

[0082] The shuttling-free electrochemical device disclosed herein may be advantageous in many aspects and it may especially achieve significant improvement in electrochemical performance and stability. It has been surprisingly found by the present inventors that the electrochemical device disclosed herein may deliver a specific capacity of about 250 mAh / g. This can be contrasted with a Zn-I2battery using KB-I2which has an initial capacity of about 195 mAh / g. Whereas a Zn-I2battery using KB-I2the initial Coulombic efficiency was only ~90%, proving a serious shuttle effect. In contrast, the electrochemical device disclosed herein maintained a higher Coulombic efficiency of ~99.5%, indicating successful suppression of the shuttle effect.

[0083] On this basis, a method of fabricating a shuttling-free electrochemical device comprising a negative electrode, a solid positive electrode, and an electrolyte solution has been developed. The method comprises introducing into the solid positive electrode, a functionalised porous carbon substrate comprising a redox active metallocene, or a derivative thereof, and iodine. Methods of fabricating an electrochemical device such as a battery are known in the art and can be adapted to the present disclosure.

[0084] Also disclosed herein is use of a functionalised porous carbon substrate comprising a redox active metallocene, or a derivative thereof, and iodine in a shuttling-free electrochemical device comprising a negative electrode, a solid positive electrode, and an electrolyte solution, wherein the functionalised porous carbon substrate is introduced into the solid positive electrode. EXAMPLES

[0085] Chemicals

[0086] Ferrocene (>98%), iodine (≥99%), zinc sulfate heptahydrate (≥99%), and polytetrafluoroethylene aqueous dispersion (PTFE, 60 wt.% solid content) were purchased from Sigma- Aldrich without further purification. Ketjen black, zinc foils (100 ^m in thickness), glass fiber (Whatman, GF / A), and titanium mesh (100 mesh) were provided by Canrd New Energy Technology Co., Ltd.

[0087] Example 1 - Preparation of FcIx ionic crystals

[0088] The FcIx crystals were prepared by a coprecipitation method. I2 and Fc were dissolved into ethanol, respectively, with a concentration of 50 mM. Typically, the I2 solution was added drop-wise into Fc solution at room temperature with vigorous stirring. Black FcIx immediately began to precipitate from the solution. The reaction was taken to completion by warming the mixture for 30 minutes at 50 °C and leaving it to stand for 24 hours at room temperature in the dark without disruption. The precipitate was then filtered off, washed with ethanol, and dried under vacuum at 40 °C for 2 hours. The obtained FcIx crystals were purified by recrystallizing using acetone. The crude FcIx crystals were dissolved in warm acetone (50 °C) with concentration of 15 mL g−1. About 60% acetone was then removed from the solutions by rotatory evaporation. The pure FcIx crystals were obtained by recrystallizing at −20 °C, filtered off, and dried under vacuum at 40 °C for 2 hours. Different types of FcIx were obtained by controlling the stoichiometric ratios of Fc and I2, as shown in Table 1.

[0089] Table 1 - Synthesis of FcIx and their ICP results

[0090] Example 2 - Preparation of cathode materials

[0091] Fc / KB-I2 composites were prepared by a two-step thermal-diffusion method. In the first step, the Fc solid and KB were uniformly mixed after hand-grinding for 10 minutes. The mixture then was sealed in hermetic glass bottles and heated at 130 ºC for 4 hours to provide Fc / KB composites. In the second step, Fc / KB and I2 in a weight ratio of 3: 7 were homogeneously mixed by hand-grinding for 10 minutes and following heating at 80 ºC for 4 hours to prepare the Fc / KB-I2 composites. As a comparison, the KB-I2 composites were obtained by mixing KB and I2 in a weight ratio of 3:7 and following thermal treatment at 80 ºC for 4 hours.

[0092] Material characterization

[0093] XRD patterns were collected on a Rigaku MiniFlex 600 X-Ray diffractometer (Cu K^, λ=1.5418 Å). HADDF-TEM imaging and EDX mapping analysis were determined on a FEI Titan Themis 80-200, operating at 200 kV. UV-Vis spectroscopy was collected on a UV-2600i spectrophotometer (Shimadzu, Tokyo, Japan). TGA-MS characterization was performed on a Netzsch STA 449 C-QMS 403 C instrument. TGA was performed in high purity nitrogen at a flow rate of 80 ml min−1. Mass analysis was carried out using a spectrometer with an electron-impact ion source (70 eV). The connection between TGA and MS was done by means of a quartz capillary at 280 °C as well. The Fe and I quantification was analyzed via ICP-MS, Agilent 7500cx instrument. The solutions were diluted prior to measurement.2% nitric acid solution and 1% ammonium hydroxide aqueous solution were selected to dilute the solutions for Fe and I analysis, respectively. Synchrotron X-ray absorption spectroscopy (XAS) measurements were conducted at the Australian Synchrotron (ANSTO), Melbourne. The data obtained were analysed and processed with Athena and Artemis software.

[0094] Example 3 - Assembly of coin cells

[0095] Cathode electrodes were prepared by mixing the KB-I2 or Fc / KB-I2 with KB and PTFE at 8: 1: 1 in weight, and then pressed onto Ti mesh. The cathodes were then dried at 40 ºC for 6 hours to evaporate all residual solvent. The mass loading of I2 in cathodes was controlled at approximately 15 mg cm−2. The Zn-I2 batteries were assembled by stacking the cathodes, glass fiber (GF / A, Whatman) separator, and Zn foils (100 ^m) in coin cells (CR-2032).2M ZnSO4aqueous solution was used as electrolyte. The Zn-Fc batteries were assembled by a similar method, and the mass loading of Fc was about 5 mg cm−2.

[0096] Example 4 - Assembly of pouch cells

[0097] The cathode electrodes were prepared by mixing the Fc / KB-I2 with KB and PTFE at 9: 0.5: 0.5in weight, and then pressed onto two sides of Ti mesh (80 mm × 90 mm), following by drying at 40 ºC. The 1.2 Ah pouch cell was assembled by stacking two cathode electrodes with three Zn foils (100 ^m, 82 mm × 92 mm), where glass fiber (GF / A, Whatman) and 2M ZnSO4aqueous solution were used as separators and electrolytes. The loading of I2 was ~50 mg cm−2. As for the pouch cell with high Zn DoD, the cathode electrode (80 mm × 90 mm) was fabricated by the same method, and the loading of I2 was controlled at ~35 mg cm−2. The Zn anodes with designed Zn capacity were obtained by using a Cu-Zn pouch cell, where two Cu foils (82 mm × 92 mm) and a Zn foils (80 mm × 90 mm) were stacked, and glass fiber (GF / A, Whatman) and 2 M ZnSO4 aqueous solution were used as separators and electrolytes. The Cu-Zn pouch cell was pre-cycled at 500 mA and 10 minutes for 10 times for activation, and then Zn with fixed capacity (1000 mAh, total capacity on two Cu foils) was plated on the Cu foils. The Cu-Zn pouch cell was then disassembled, the Zn@Cu foils were washed with water to remove residual electrolyte, and then dried under vacuum at 50 °C for 2 hours. The Fc / KB-I2 electrodes were sandwiched by Zn@Cu anodes to assemble the pouch cell, and glass fiber (GF / A, Whatman) was used as separator. The electrolyte was prepared by dissolving 2M ZnCl2in H2O / ^-butyrolactone (6 / 4 v / v). The injection of electrolyte was controlled at approximately 8 mL Ah−1.

[0098] Electrochemical measurement

[0099] Galvanostatic discharge / charge measurements were conducted at a potential range of 0.6-1.6 V using the battery test system (CT-4008T, Neware, Shenzhen, China) under the ambient environment. All current densities were calculated based on the mass of iodine. For the self-discharge tests, the Zn-I2 batteries were firstly charged to upper cut-off voltage (1.6 V). After aging at given times without any interruption, the batteries were then discharged to 0.6 V. The self-discharge rates of Zn-I2 batteries, which reflects the capacity loss after aging, were calculated according to following equation: Self-discharge Rate = (1 −Discharge capacity after aging Charge capacity) × 100%

[0100] The CV profiles of batteries were performed on the electrochemical workstation (CHI660E, Chenhua, Shanghai) at the voltage range of 0.6-1.6 V with a scan rate of 0.2 mV s‒1.

[0101] Operando synchrotron XRD characterizations

[0102] The Operando synchrotron XRD pattens were collected continuously each 2 minutes from the powder diffraction beamline of ANSTO with a wavelength (λ) of 0.59 Å. Home-made CR-2032 coin- cells were used for operando synchrotron XRD data collection. Both sides of the cell cases were punched with holes (0.3 cm in diameter) and sealed with Kapton films to allow X-ray transmission. The coin-cells were connected to a Neware battery tester to conduct the galvanostatic charge-discharge tests. As-assembled battery was firstly discharge to 0.6 V before test. The current density was set as 0.3 A g−1.

[0103] In−situ Raman spectroscopy

[0104] The Raman spectroscopy was conducted on a Renishaw inViaTM Raman microscope with a 532 nm solid laser as an excitation source. The in-situ Raman spectroscopy was collected by using a two- electrode battery testing the Raman cell (Gaoss Union, B002-RM). During the test, galvanostatic charge- discharge or prolonged aging procedure of Zn-I2 batteries were performed on the electrochemical workstation (CHI660E, Chenhua, Shanghai), and Raman spectra were in-situ collected by the Raman microscope from the quartz window of Raman cell.

[0105] Results

[0106] Polyiodides coupled by Fc+. As illustrated in Fig.1a, the I− / I2 electrochemical conversion in aqueous Zn-I2 batteries will generate soluble I3−. The soluble I3−will shuttle to the anode upon cycling, leading to serious self-discharging and Zn corrosion. The I3−dissolution is observed by using a three- electrode system, evidenced by the yellow electrolyte and strong I3−absorbance in Ultraviolet-visible (UV-Vis) spectra after one cycle. When the Zn metal was directly immersed into the electrolyte containing I3−, the yellow electrolyte turns to colorless after two-hour resting. This result indicates that I3−can be reduced by the Zn metal, which consumes the active Zn. The active Zn consumption would further shorten the battery cycling life and limit the battery application1. As an organometallic compound, Fc features a unique structure consisting of two cyclopentadienyl rings (Cp−) bound to a central Fe atom, which is possible to occur the reversible Fc / Fc+conversion by altering the valence state of its Fe center2. The Fc / Fc+redox is also demonstrated as the reversible conversion in aqueous media with a theoretical specific capacity of 144 mAh g−1(based on Fc)3, 4. However, since Fc+cation is highly soluble in electrolyte, which results in the shuttle effect of Fc+upon battery cycling as well (Fig.1b). The Fc / Fc+conversion revealed in the three-electrode system demonstrates the severe dissolution of Fc+, as proved by the blue color and strong UV-vis absorbance of Fc+in the electrolyte after one cycle. Similarly, the shuttled Fc+can be reduced by metallic Zn and deposited on the Zn surface, leading to serious active mass loss and battery degradation. As depicted in Fig 1c, when coupling I2and Fc conversion together, the Fc+and I3−can form solid-state FcI3ionic crystals, thus suppressing the shuttle of Fc+and I3−simultaneously. The electrolyte after one cycle remains colorless with only slight absorbance in UV-Vis spectra, strongly supporting the shuttle-free conversion when coupling these two redoxes.

[0107] To better understand the interaction between Fc+and polyiodides, various studies were performed. As illustrated in Fig.1d, Fc solid is a molecular crystal and constructed by arranging Fc molecules through van der Waals forces in a monoclinic crystal structure with a space group of P21 / a. Benefiting from its abundant interstitial sites, the inner structure of Fc allows for the oxidativeincorporation of bulky anions during the oxidation from Fc to Fc+. Specifically, the elemental I2 can oxidize Fc by forming FcIx, as evidenced by the visible precipitation when mixing Fc and I2 ethanol solutions (inset of Fig.1e). The X-ray diffraction (XRD) pattern of the precipitate exhibits distinct diffraction peaks compared to the pristine Fc solid. It is consistent with the simulated XRD pattern of FcI3 trigonal crystal with a space group of R-3m (Fig.1e). The presence of a strong Raman band at 106 cm−1confirms the incorporation of bulky I3−anions in the FcI3 crystals (Fig.1f). Fc presents characteristic Raman bands at 303 cm−1and 1103 cm−1, associating to the ring-metal stretch mode and ring breathing mode5, 6. In FcI3 crystals, two characteristic Raman bands of Fc are also detected, indicating the oxidation by I2 would not destruct the molecular structure of Fc. The Fe K-edge X-ray absorption near-edge spectroscopy (XANES) further validates the unchangeable Fc molecular structure after oxidation. The positive shift of edge energy verifies the valence change of Fe in Fc7, 8.

[0108] Since the long Fe‒I distance (~4.8 Å), FcI3 crystal allows the incorporation of excess I2 inside its interstitial sites. Thus, by controlling the stoichiometric ratios of Fc and I2 according to the previous report9, a series of FcIx crystals with higher-order polyiodides were synthesized. Raman spectra of obtained FcIx were collected to investigate their anion constitution. When the stoichiometric ratio of Fc and I2 is controlled at 1:1.5, only Raman band for I3−is observed, indicating the formation of FcI3. After increasing the proportion of I2 during synthesis, the Raman band for I5−located at around 165 cm−1is gradually intensified, indicating the extra I2 involved and coupled with I3−. The similar tendency was further convinced by XRD results. Under relatively low I2concentration, the XRD pattern of FcIxwell matches with that of FcI3crystal (JCPDS No.00-036-1962). After increasing the I2concentration, the generated FcIxis more inclined to the FcI4.3crystal (JCPDS No.00-036-1965), indicating more I2incorporation. The molecular formulas of FcIxwith different stoichiometric ratios of Fc and I2are identified as FcI3, FcI4, FcI4.3and FcI5, respectively, as confirmed by inductively coupled plasma mass spectrometry (ICP-MS). Furthermore, the thermogravimetric analysis (TGA) of FcIxwas conducted. FcI3and FcI4showcase high thermal stability, sharply losing their weight when the temperature is higher than 160 ºC. Differently, FcI4.3and FcI5present obvious additional weight loss before 160 ºC compared to the cases of FcI3and FcI4. Polyiodides higher than I3−can be described as a combination of I3−and I2, which showcases relatively poor stability compared to I3−. Thus, this weight loss occurred in the low- temperature region, would indicate the decomposition of high-order polyiodide to generate lower-order one, implying that the excessive I2embedded inside the crystals is weakly bonded with the I3−. The results clearly demonstrate that Fc has the remarkable capability of coupling multiple stoichiometric fractions of I2within itself, underscoring its strong potential for confining polyiodides.

[0109] Coupling Fc in I2cathodes. As strong ability of polyiodide confinement of Fc, the introduction of Fc in the I2cathodes should be effective to suppress the shuttle effect. Owing to both Fc and I2solids are sublimable at elevated temperature, thereby the Fc coupled I2cathodes were prepared step-wisely through thermal diffusion of Fc and I2 into the pores of a commercial porous carbon (Ketjen black, KB).In the first step, Fc and KB were mixed with different ratios and followed by the thermal heating at 130 ºC to prepare the Fc / KB composites. In Fc / KB composites, the maximum weight ratio of Fc is approximately 60 wt.%, as higher Fc ratios surpass the affordability limits of KB. And in the next step, I2 was further diffused into Fc / KB at a temperature of 80 ºC by a weight ratio of 70 wt.%, which is denoted as Fc / KB-I2. The precise constitution of Fc, KB and I2 is calculated in Table 2.

[0110] Table 2 - Constitution of Fc / KB-I2 with different Fc ratiosaI / H ratio stands for the weight ratio of I2and hosts (Fc / KB).

[0111] It is demonstrated that higher Fc ratio in Fc / KB-I2presents better suppression of shuttle effect, as gradually improved battery CEs. Therefore, the optimized Fc ratio in Fc / KB composite should be 60 wt.%, which is served as the host material for I2. After further loading 70 wt.% I2in the Fc / KB composite, the optimized Fc / KB-I2cathodes is constituted by 12 wt.% KB, 18 wt.% Fc, and 70 wt.% I2. No special emphasis, the Fc / KB and Fc / KB-I2discussed below stand for the corresponding composites with optimized constitutions.

[0112] In Fc / KB, the Fc solid is dispersed uniformly within KB without observable Fc crystals, as evidenced by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-TEM) image (top of Fig.2a). Overlapped energy dispersive X-ray (EDX) mapping of Fe and C further confirms its uniform distribution (bottom of Fig.2a). High-resolution TEM (HR- TEM) image demonstrates the Fc molecular crystals are homogeneously dispersed in the pores of KB (Fig.2b). In the XRD pattern of Fc / KB, the diffraction peaks of Fc crystal are reduced to a broad peak located at approximately 16º (Fig.2c), confirming the well dispersion of Fc inside the pores of KB4. Without Fc, the KB-I2 presents only diffraction of KB, indicating that I2 solid is decentralized by KB (Supplementary Fig.12). With Fc, however, the infiltrated I2 would oxidate the Fc to yield FcIx crystals,as supported by the appearance of new intense diffraction peaks belonged to FcIx crystals in Fc / KB-I2 (Fig.2c). Solid I2 presents a strong Raman shift at 180 cm−1, which is assigned to the I−I stretching vibration10. When I2 is loaded into Fc / KB, two Raman bands located at 106 cm−1and 163 cm−1are observed, which is attributed to the I−I stretching vibration for I3−and I5−, respectively11. This is because that the I2 accept the electron from the Fc to form Fc+and polyiodides, evidencing the coupling of Fc+and polyiodides by strong ionic bond. The generated FcIx crystals are uniformly decentralized on the KB substrate, as evidenced by the absence of large FcIx crystals in HAADF-TEM image and coincident Fe and I distribution in EDX mapping analysis (Fig.2d). In the HR-TEM images, observable FcIx nanocrystals with a diameter of approximately 2 nm are discretely anchored on the KB substrates (Fig. 2e). These results validate the uniform impregnation of both Fc and I2 inside the KB substrates, which would be beneficial to facilitate the conversion.

[0113] The TGA combined with online mass spectrometer (TGA-MS) was applied to investigate the thermal stability of KB-I2 and Fc / KB-I2, and online analyze the evolved gases (Fig.2f). The elemental I2 starts to lose weight before 50 ºC, and totally sublimates at 120 °C. For the I2 / KB, the adsorption of KB only slightly improves the thermal stability of I2, demonstrating the poor confinement ability of KB owing to its weak physical adsorption. Notably, the Fc / KB-I2 presents two-stage weight loss. In the stage I, a gentle weight loss at the temperature range from 80 °C to 157 °C with a total weight loss of ~21 wt.% is observed. Subsequently, a sharp weight loss occurs within 157 °C to 175 °C, corresponding to ~24 wt.% of the initial weight. The MS profiles demonstrate that no Fc signal (m / z = 187) is detected. This result evidences the Fc is oxidated by I2to form Fc+, which features higher thermal stability compared to the Fc solid. The I2signal (m / z=127) showcases two distinguished weight loss procedures, which accords to the two-stage weight loss in the TGA. This evidence supports that Fc is oxidatively incorporated with polyiodides to form FcIxwith high-order polyiodides, which step-wisely decreases the orders of iodine chains by releasing the I2. These results further emphasize that Fc+displays a strong capacity for anchoring the polyiodides.

[0114] Electrochemical performance of Fc coupled Zn-I2batteries. The cyclic voltammetry (CV) profiles of Fc / KB, KB-I2, and Fc / KB-I2electrodes were collected, as shown in Fig.3a. The Fc / KB electrode delivers the reversible conversion between Fc and Fc+, locating at 1.46 V and 1.32 V (versus Zn / Zn2+). For the KB-I2cathode, redox peaks for the conversion between I−and I2are observed at 1.26 V and 1.18 V, which is lower than the Fc / Fc+conversion. When incorporating these two conversions, the oxidation and reduction potentials can be found at 1.22 V and 1.16 V, respectively, which is lower than the two conversions. This is because the formation of solid-state FcIxwould decrease the concentration of Fc+cations and polyiodide anions simultaneously, which negatively shifts the equilibrium potentials, as interpreted by Nernst equation4.

[0115] The shuttle effect and cycling performance of batteries were studied by the galvanostaticcharge / discharge (GCD). The Zn-Fc battery using Fc / KB electrode delivers an initial capacity of around 100 mAh gFc−1at a current density of 0.1 A g−1(based on Fc), but it loses ~70% of initial capacity within 200 cycles. The strong Fe signal was detected in EDX mapping of Zn surface after 200 cycles, indicating the severe active mass loss owing to the shuttle effect of Fc+. As depicted in Fig.3b, the Zn-I2 battery using KB-I2 electrode deliver an initial capacity of 193.9 mAh g−1at 0.2 A g−1and retains 82.8% of initial capacity after 200 cycles, manifesting the poor cycling stability. CE at small current density is a critical parameter to evaluate the shuttle effect in Zn-I2 batteries12. The shuttled I3−will be reduced to I−by Zn anodes, which moves back to cathodes for further oxidation, resulting in low CE. The CE for KB-I2 is only 92.1% at initial cycle and maintains at a low level (<98%) throughout the cycling, validating the serious shuttle effect. These results reveal that both Fc+and I3−would dissolve into electrolyte and trigger the shuttle effect.

[0116] The coupling of Fc in Zn-I2 batteries dramatically suppresses the shuttle effect of Fc+and I3−and enhances the cycling stability. As shown in Fig.3b and c, Fc / KB-I2 delivers a high specific capacity of 225 mAh g−1and areal capacity of > 3.0 mAh cm−2after several cycles’ activation at a current density of 0.2 A g−1under a high I2 loading of ~15 mg cm−2. The energy density can reach over 260 Wh kg−1(based on I2). Only 5% capacity decay and ignorable increase in battery polarization after 200 cycles proves the desirable reversibility despite under the condition of such high I2 proportion and I2 loading. More importantly, the CE can remain at >99.5% throughout the cycling, strongly validating the elimination of shuttle effect by Fc.

[0117] Fig.3d compares the rate capability, showing that the Fc / KB-I2electrode displays higher capacities at all current densities compared to the counterpart using KB-I2electrode. Even at 10 A g−1, Fc / KB-I2electrode remains a high capacity of 166.7 mAh g−1, which is over double the KB-I2electrode (73.5 mAh g−1). Moreover, the cycling stability of different batteries at a current density of 2.0 A g−1was compared (Fig.3e). Fc / KB-I2electrode delivers a high capacity of 190.4 mAh g−1and only decays <5% of initial capacity after 5,000 cycles. In sharp comparison, a lower initial specific capacity of 160.5 mAh g−1and faster capacity fading with 76.1% capacity retention are observed in KB-I2electrode.

[0118] Conversion mechanism study on the Zn-I2battery with Fc. Benefiting from the strong ionic bond, the coupling of Fc in Zn-I2batteries would generate a variety of crystalline FcIx. The detection of FcIxformation and their conversion process upon battery operation are critical to understand the role of Fc on enhancing performance of Zn-I2batteries. Fig.4a presents the operando synchrotron XRD of Fc / KB-I2electrodes during two GCD cycles. The potential-relevant intensity evolution of Fc and FcIxduring electrochemical procedure is shown in Fig.4b. At the beginning of charging, only diffraction peaks of Fc can be found. With the potential increases, these peaks gradually decrease and disappear at approximately 1.28 V. While the diffraction peaks belonging to FcI3start to emerge at around 1.19 V and gradually increase the intensity with the charging processing. Specifically, with the potential elevating tohigher than 1.30 V, diffraction peaks associated to FcIx with higher-order polyiodides (i.e., FcI4.3 or FcI5) are also detected, indicating that I2 produced in the following charging is further captured and embedded into FcI3 crystals. In the discharging, the reverse tendency for Fc and FcIx is observed, where FcIx gradually decreases and completely converts to Fc at the end of discharging. The same tendency is observed in the second GCD cycle, indicating this conversion in reversible. This periodical and reversible conversion between Fc and FcIx is concomitant throughout the whole iodine conversion. Fc+functions to form solid-state FcIx with a variety of polyiodides by strong ionic bond, which transforms the soluble polyiodide conversion to solid state, fundamentally eliminating the shuttle effect.

[0119] Although operando synchrotron XRD provides general understanding for the roles of Fc / Fc+on the I− / I2 conversion, it fails to unfold the internal polyiodide conversion. In-situ Raman spectra were collected from Zn-I2 batteries employing KB-I2 and Fc / KB-I2 electrodes to investigate the electrochemistry of polyiodides, since Raman spectroscopy is highly sensitive to polyiodides, making it favorable for polyiodide identification (Fig.4c, d). Raman spectra exhibits distinct peaks at around 110 cm−1and 165 cm−1, corresponding to the I−I stretching vibration of I3−and I5−, respectively. Both battery configurations share the same polyiodide intermediates but showcase differences in Raman shift and intensity evolution. Fig.4e illustrates that the Fc / KB-I2 exhibits a lower Raman shift for the I3−(~106.5 cm−1) compares to the KB−I2 (~109 cm−1) throughout the charging and discharging. This is because that the strong ionic bond between Fc+and polyiodides lowers the I−I force constant related to an increase of bond distance13. In the battery utilizing the KB-I2electrode, the Raman band of the I5−signal gradually moves to higher Raman shift, indicating the oxidation of I5−to higher-order polyiodides. Furthermore, the attendance of Fc / Fc+redox also caused a down-shift in the Raman band of I5−compared to the case of KB-I2, further confirming the strong coupling by Fc+cations with high-order polyiodides.

[0120] To better understand the polyiodide electrochemistry, the potential-dependent intensity evolution of polyiodides was calculated and displayed in Fig.4f. It is observed that the I3−and I5−signals in the battery using the KB-I2cathode intensified simultaneously during charging, while synchronously diminished during discharging (top of Fig.4f). This result declaims the indistinguishable I− / I3−and I3− / I2redoxes in the KB-I2, which is in accords to the previous reports in aqueous media1. The GCD curve of KB-I2shows the typical features for I− / I2conversion with unseparated I− / I3−and I3− / I2conversion, as reflected by only one pair of board redox peaks in dQ / dV plot (Fig.4g). In contrast, in the Fc / KB-I2, the intensity of I3−Raman band intensifies sharply in the early charging, reaching its highest level at a potential of 1.22 V, while the I5−signal remains at a low intensity in this period (bottom of Fig.4f). This result manifests the oxidation of Fc and I−will convert to FcI3preferentially, as FcI3is the most stable one in various FcIx. In the following charging, the intensity of I3−Raman band starts to decline while intensity of I5−Raman band gradually enhances, which further reveals that the I2generated in following oxidation is embedded inside the FcIxcrystals. The reverse tendency is observed in discharging process, manifesting the reversibility of this conversion procedure. The GCD curves of Fc / KB-I2 are variedsegmentally with two distinguished voltage plateaus (left in Fig.4h). A pair of sharp redox peaks (1.17 V and 1.13 V) and a couple of shoulder peaks (1.30 V and 1.23 V) are observed in derived dQ / dV plot (right in Fig.4h). The former should be attributed to the I− / I3−conversion, the latter is assigned to the I3− / I2 conversion. It should be noted that the Fc coupled Zn-I2 also enables a reversible capacity of 240 mAh giodine−1. The additional capacity surpassed the I− / I2 conversion is contributed by the Fc / Fc+electrochemistry, indicating the Fc+coupling could improve the energy density of batteries. Among various polyiodides, I3−serves as most dangerous species causing shuttle effect owing to its high solubility in electrolyte. The Fc+has a strong capability to bond polyiodides, especially I3−, and allows unobstructed conversion between various polyiodides, which avoids the I3−dissolution and shuttle on the premise of ensuring battery performance.

[0121] Suppression of self-discharging. Self-discharge rate represents the capacity loss after aging at high state of charge or full charge states, which is a descriptor to indicate the battery’s ability for energy storage under the state of rest or transportation. In Zn-I2 battery, the self-discharge phenomenon is related to the hydrolysis of I2 (Eq.1) and parasitic reaction between shuttled I3−and metallic Zn (Eq.2)12, 14. I2 + H2O ↔ HIO + I−+ H+(1) Zn + I3−↔ Zn2++ 3I−(2)

[0122] In which the hydrolysis of I2 will generate I−(Eq.1) that bonds I2 to boost its dissolution by forming I3−and I5−. The soluble I3−can migrate to the Zn anode and chemically reacts with metallic Zn, which consumes the stored energy by releasing heat (Eq.2). Thus, the self-discharging is critical to evaluate the practical application of I2-based batteries.

[0123] Fig.5a compares the self-discharging of Zn-I2batteries, which reflects the capacity loss during aging. The KB-I2present a self-discharge rate of <10% after 2-hour aging, but sharply increase when extended the aging time. It loses 81.2 % of initial capacity when the aging time prolongs to one week (168 hours), showcasing serious self-discharging. Remarkably, the counterparts using Fc / KB-I2cathodes deliver a much lower self-discharge rate of <5% within 24-hour aging, and only suffers from a capacity loss of 27.4% even after one-week aging. The GCD curves further demonstrate that the integration of Fc in Zn-I2batteries can significantly retard the voltage decay during prolonged aging (Fig.5b), supporting the outstanding self-discharging suppression by introducing Fc.

[0124] The understanding of the root of capacity loss during aging is critical to effectively retard the self-discharging of Zn-I2batteries. In-situ Raman spectra enable real-time monitor the evolution of polyiodides during aging, which is feasible to unravel the mystery of self-discharging of Zn-I2batteries. As depicted in Fig.5c, the KB-I2at full charge state shows a strong Raman band at 170 cm−1and a weakRaman band at 110 cm−1. Typically, elemental I2 is Raman-active, showing a Raman band at 180 cm−1. In the KB-I2, relatively lower Raman shift at 170 cm−1may indicate the existence of long-chain polyiodides at full charge state, such as I7−or I9−.13The stability of polyiodides decreases with the iodine chain extends, which is easily hydrolysed in aqueous media to yield short-chain polyiodides (i.e., I3−and I5−). As evidence, in the KB-I2, the Raman band at 170 cm−1gradually shifts to 166 cm−1in the first 5.5-hour aging, implying the hydrolysis of long-chain polyiodides to I5−. To in-depth understand the evolution of polyiodides, the relative intensity of polyiodides during aging is summarized, as revealed in Fig.5d. In the first-hour aging, the Raman band of I3−and I5−intensified significantly, indicating the generation of I3−and I5−in the electrode owing to the hydrolysis of long-chain polyiodides. In the following two-hour aging, the intensity of I5−remains static and the intensity of I3−continues to increase. This result indicates that I5−is saturated in the electrode, thus the hydrolysis of long-chain polyiodides further contributes to the more I3−in the cathodes. Further extending the aging time, the I5−starts to decompose, showing downtrend in intensity. Insufficient confinement ability of KB cannot prevent the dissolution of polyiodides, leading to identical declination of both I3−and I5−. Spontaneous reaction between Zn anodes and I3−aggravates the concentration gradient of I3−between cathode and anode, resulting in severe shuttle effect. These findings highlight that the self-discharging of Zn-I2batteries is stemming from the water- induced hydrolysis reaction and polyiodide shuttling. The effective suppression of self-discharging requires not only polyiodide confinement but also hydrolysis retardation.

[0125] The in-situ Raman spectra were also conducted to unravel the self-discharging in Fc / KB-I2electrode (Fig.5e). At full charge state, the strong Raman band at 163 cm−1and a weak Raman band at 106 cm−1are observed. The Raman bands of polyiodides remain invariable in the period of 12-hour aging, indicating the retardation of hydrolysis. Besides, the band intensity of I3−and I5−with ignorable declination indicates the inhabitation of shuttle effect as well (Fig.5f). These results comprehensively reveal that the strong ionic bond between Fc+and polyiodides dramatically enhances the stability of polyiodides against the hydrolysis, while simultaneously reducing polyiodide solubility to mitigate the shuttle effect. These improvements contribute to the effective suppression of self-discharge of Zn-I2batteries.

[0126] Demonstration of large-scale application by Ah-level Zn-I2pouch cells. To assess suitability of Zn-I2batteries for large-scale applications, an Ah-level Zn-I2pouch cell with a total capacity of 1.2 Ah was manufactured (Fig.6a). The pouch cell was assembled by stacking two Fc / KB-I2electrodes and three Zn foils (100 ^m) with the size of 80 × 90 mm. It is noteworthy that the I2 loading of the Fc / KB-I2electrode can reach a recorded-high level of approximately 50 mg cm−2. The tap density of Fc / KB-I2is 0.561 g cm−3, two-fold higher than the KB-I2(0.268 g cm−3), making it more beneficial for achieving high I2loading. As demonstrated in Figs.6b and c, the Zn-I2pouch cell delivers an initial capacity of 1.18 Ah at a current density of ~15 mA cm−2, corresponding to a high energy density of 195Wh kg−1(based on Fc / KB-I2). After 350 cycles, the pouch cell still retains 91.5% of initial capacity (1.08 Ah) with only slight increase in battery polarization. The CE of pouch cells is 96.9% for the first cycle and reaches >99.9% after initial 10 cycles, verifying the ignorable shuttle effect and excellent reversibility.

[0127] One should keep in mind that the energy density of AZBs is over-estimated when using thick Zn foils15. The massive use of Zn anodes not only results in serious material waste but also severely compromises the energy density at the cell level. Therefore, the practical application of AZBs requires the matching of high-areal capacity cathodes with limited Zn supplement to fully hunter the advantage of its high energy density. In conventional 2 M ZnSO4 aqueous electrolyte, insufficient Zn plating / stripping CEs and dendrites formation would not meet the requirements of high cyclability of AZBs, especially under condition of limited Zn supplement. In our previous effort, we demonstrated a highly reversible Zn anode in the ZnCl2aqueous electrolyte containing ^-butyrolactone as co-solvent, which achieved stable cycling under 50% Zn DoD16. This modified electrolyte (2 M ZnCl2 dissolved in H2O / ^-butyrolactone (6 / 4 v / v)) was applied to estimate the Fc coupled Zn-I2 pouch cells under limited Zn supplement to ensure the desirable Zn reversibility.

[0128] Herein, a Zn-I2 pouch cells with a total capacity of ~0.45 Ah and limited Zn supplement was assembled. The loading of active mass is 35 mgiodine cm−2, and the electrolyte injection is approximately 8 mL Ah−1. To control the Zn supplement, the Zn was electrochemically plated on the Cu foil (Zn@Cu) under a given Zn capacity of 1.0 Ah by using Zn-Cu pouch cells. As-prepared Zn@Cu anodes were then matched with Fc / KB-I2 cathodes to assemble the Zn-I2 pouch cells (Fig.6d). As shown in Fig.6e, Zn-I2 pouch cell presents a reversible capacity of 0.45 Ah at a current density of 10 mA cm−2(equal to ~0.2 A g−1). Under a current density of ~20 mA cm−2(equal to ~0.4 A g−1), the pouch cell still delivers a capacity of 0.40 Ah, corresponding to the Zn DoD of approximately 40%. Under such high Zn DoD, the Zn-I2 pouch cell can remain stable cycling for 800 cycles with high capacity retention of 80.0% (Fig.6f). The gravimetric energy density (Em) and volumetric energy density (EV) of this Zn-I2 pouch cell are 105 Wh kg−1(based on Fc / KB-I2 cathode and Zn anode) and 94.5 Wh L−1(based on whole pouch cell), respectively. This successful demonstration of Zn-I2 pouch cells under high Zn DoD presents outstanding energy density and desirable cycling life compared to reported AZBs pouch cells (Fig.6g).

[0129] The practical energy density of pouch cell often much smaller than the theoretical value, which is root from the necessary inactive components of pouch cells during fabrication, such as binders, conductive agents, electrolytes, separators, and packing materials. Thus, a fictional Zn-I2 batteries was proposed to project the practical energy density of Zn-I2 batteries, as shown in Table 3.

[0130] Table 3 - Technical parameters of a projected Zn-I2 pouch cell.aI / H ratio stands for the weight ratio of I2and host materials.bPress density is obtained by measuring the electrodes.

[0131] At a Zn DoD of 50%, the Emof Zn-I2batterie using Fc / KB-I2can reach 95.0 Wh kg−1, which is much higher than counterpart using KB-I2(83.1 Wh kg−1) (Fig.6h). It worth to note that the energy density of Zn-I2battery is highly related to the I2proportion, which sharply increases with the increment of I2proportion (inset of Fig.6h). Thus, high I2proportion of ≥70 wt.% should be achieved to meet the requirement of high energy density. Moreover, the high tap density of Fc / KB-I2 dramatically reduces the thickness of cathode electrodes, therefore the projected Ev of pouch cell based on Fc / KB-I2 can achieve 300 Wh L−1under 50% Zn DoD, which is almost triply higher than the counterparts using KB-I2 (111 Wh L−1) (Table 3). This value far exceeds those of o ther aqueous systems, including aqueous Li-ion batteries (50-90 Wh kg−1)17, 18, 19, aqueous Na-ion batteries (30-40 Wh kg−1)20, 21, 22commercial lead-acid (~40 Wh kg−1) and Ni-Cd technologies (~50 Wh kg−1)23. Besides, the energy density of alternative AZB systems based on MnO2, V2O5, and Prussian blue analog (PBA) cathodes were also calculated (Fig.6i). It can be known that the energy density of Zn-I2batteries (90.2 Wh kg−1) is over the Zn-V2O5(~85.6 Wh kg−1) and Zn-PBA systems (~81.6 Wh kg−1) when Zn DoD is 50%. Among alternatives, Zn-MnO2batteries ispredicted to have highest energy density of (~128 Wh kg−1at 50% Zn DoD). However, the specific capacity and cycling life of MnO2 cathodes in the practical application is still far from satisfactory, thus the energy density of Zn-MnO2 batteries would be much lower than this calculated value. According to the above discussion, Zn-I2 batteries are demonstrated as a promising candidate for the large-scale applications profiting from their competent energy density, excellent lifespan, and outstanding rate performance.

[0132] Discussion

[0133] In summary, we report a new-type ionic bond between Fc+and polyiodides to settle the shuttle effect in Zn-I2 batteries, which presents much stronger polyiodides confinement ability compared to physical adsorption. Fc+functions to integrate with polyiodides, which converts soluble polyiodides to solid state, enabling a transformation of polyiodide conversion from liquid-phase to solid-phase pathway, thus fundamentally suppressing the shuttle effect. Operando synchrotron XRD and in-situ Raman techniques unfold this solid-phase conversion between Fc and FcIx. The effectiveness of shuttle effect suppression is demonstrated by the high CEs (>99.5%) and degradation-free cycling at 0.2 Aalso supported by the extraordinary self-discharging elimination with 27.2% capacity loss after one-week aging. Besides, the coupling between Fc+and polyiodides also promotes the conversions of both I− / I2 and Fc / Fc+redoxes, proving by excellent rate performance (166.7 mAh g−1at 10 A g−1), and stable cycling at 2 A g−1for 5000 cycles with < 5% capacity fading. An Ah-level pouch cell (1.2 Ah) with record-high I2loading (50 mg cm−2) presents 350 deep cycles with 91.5% retentive capacity. Moreover, a 0.45 Ah Zn−I2pouch cell with thin Zn anodes (40% DoD) sustains 800 cycles with 80% capacity retention, exhibiting a remarkable energy density of 105 Wh kg−1(based on cathodes and anodes) and 94.5 Wh L−1(based on whole pouch cell) under practical conditions. We also estimated the projected energy density of Zn−I2batteries, which is superior to conventional AZBs systems. Benefiting from the high specific capacity, excellent kinetics, and stable conversion stability, the Zn-I2batteries showcase competitive energy density and far exceeded cyclability, which acts as promising candidate among various aqueous battery systems.

[0134] Figure 7 demonstrates the reaction between Fc and I2. Fc and I2can be dissolved in ethanol to form a uniform solution. After mixing the Fc and I2solution, a precipitate formed and it was separated from the solution, demonstrating that the Fc can react with I2to form insoluble products. The chemical reaction between Fc and I2is Fc + I2→ Fc+I3−(Fc+is ferrocerium). During the charge / discharge process in a Zn-I2battery, polyiodide, especially I3- will dissolve into the electrolyte, leading to the shuttle effect. The Fc also will turn to soluble Fc+during the charge process, which also will lead to Fc+shuttling. By coupling Fc and I2, an insoluble Fc+I3- complex will form during the electrochemical process, strongly avoiding the shuttling of both Fc+and I3-.

[0135] As shown in Figure 8, both Fc and I2show well-defined crystal structures. The as formed Fc+I3-precipitate showed a completely different XRD pattern compared to Fc and I2, thereby demonstrating new Fc+I3- crystal formation. The as-generated XRD pattern of Fc+I3- also matched a simulated one, further demonstrating the Fc+I3- crystal formation.

[0136] The Raman spectra of Fc shows a series of strong Raman peaks. The peak located at 303 cm-1is the ring-metal stretch mode of Fc, and the peak at 1103 cm-1is associated to the ring-breathing of C5H5−in Fc. Other features of Fc including sym. right title, C-H bending, and C-C stretch were also observed. In the Fc+I3-, the features of the Fc molecular structure including the ring-metal stretch and the ring- breathing mode of Fc were well preserved, indicating the formation of Fc+I3- does not destroy the molecular structure of Fc. A new strong Raman peak located at 107 cm-1was observed, and this is associated the triiodide (I3-), proving the main anion in the Fc+I3- complex is I3-.

[0137] The Fc presents typical features in K-edge XAS spectra. The small peak at ~7190 eV is associated with the Fe 1s→3d transition, while another peak at ~7123 eV corresponds to the transition of Fe 1s to the conjugated ^ ring (C5H5−). The strongest peak located at ~7132 eV is indexed as the Fe 1s→4p transition. After reacting with I2, Fc+I3- preserves all typical features of Fc, indicating the Fc structure remains stable. However, the edge energy shifts to higher energy region, manifesting the oxidation state of Fe increase.

[0138] By heating the mixture of Fc and porous carbon (Ketjen black) (Fc: KB = 6: 4 by weight), the Fc can uniformly diffuse into the pores of the KB. From the TEM image (Figure 12a), most of the Fc was dispersed by the KB with no obvious Fc crystal observed. Elemental mapping further confirmed the uniform distribution of Fc in KB. In the high-resolution TEM image, some single atom like bright dots can be observed, which may be Fe atoms. Single atom like distribution of Fe atoms also demonstrates the uniform distribution of Fc in KB.

[0139] After further loading the I2 inside the Fc / KB complex, the TEM image also demonstrates the iodine is well dispersed in the Fc / KB complex. Elemental mapping shows the homogenous distribution of Fe and I elements. In the high-resolution TEM image, some tiny Fc+I3- crystals can be found in the KB substrate, which supports the strong interaction between Fc and I2.

[0140] No obvious strong diffraction was observed in Fc / C complex which further supports the uniform dispersion of Fc in KB. While in Fc / KB-I2, some obvious diffraction peaks can be observed, which are well matched with the XRD pattern of Fc+I3-, indicating the formation of Fc+I3- in Fc / KB-I2.

[0141] I2 easily sublimates, which can be reflected by rapid mass loss at relatively low temperature. The confinement of I2 by the KB substrate can extend the thermal stability of I2. Mass spectroscopy further emphasises the I2 loss in KB-I2 starts from about 60 ºC and suffers from rapid mass loss attemperatures higher than 80 ºC. In the Fc / KB-I2, I2 sublimation occurs in two stages. When the temperature reached 80 ºC, obvious mass loss was seen. When the temperature was further elevated to ~147 ºC, a sharp mass loss was observed, which is associated with the decomposition of Fc+I3-. These results strongly highlight the interaction between Fc and I2 can extend the thermal stability of I2, suggesting higher shuttling suppression.

[0142] The Fc / KB-I2 and KB-I2 was matched with Zn foils to assemble Zn-I2 batteries. Both Zn-I2 batteries using Fc / KB-I2 and KB-I2 demonstrated similar evolution of polyiodide conversion during the charge-discharge process. To observe the function of Fc / Fc+redox on enhancing polyiodide confinement, the corresponding Raman shift of I3- and I5- during charge-discharge process was calculated and compared. In KB-I2, the I3- shows a Raman shift of ~108 cm-1. The introduction of Fc / Fc+downshifts the Raman peak to ~106 cm-1. This Raman shift means the strong interaction of Fc+with I3- suppresses vibration of I-I bonding. Besides, the Raman shift of I5- in KB-I2 is ~162 cm-1, and gradually increases during the charge process, indicating extension of the iodine chain, while gradually decreases during the discharge process, further illustrating the decomposition of the iodine chain. When the Fc / Fc+redox is introduced, the Raman shift of the I5- species is significantly downshifted, also proving the strong interaction between Fc+and I5-. These results demonstrate the strong interaction of Fc with polyiodide during whole charge-discharge process, which can suppress the shuttle effect efficiently.

[0143] In the beginning of the charge, the diffraction of Fc is found. During the charge process, the diffraction of Fc gradually decreases and then disappears. Instead, the diffraction peaks related to the Fc+I3- are found. During the discharge process, the Fc+I3- gradually decreases and disappears at the end of discharging, and the diffraction of Fc reappears. A similar trend of Fc and Fc+I3- was also found in the second cycle, validating the good reversibility. This result strongly underlines the effect of the Fc / Fc+redox on anchoring the polyiodide, which leads to suppression of the shuttle effect.

[0144] The Zn-I2batteries using KB-I2presented an initial capacity of ~195 mAh / g, and they suffered from fast capacity loss in the following cycles. Besides, the initial Coulombic efficiency of Zn-I2batteries was only ~90%, proving a serious shuttle effect. In contrast, the Fc / KB-I2delivered a much higher specific capacity of ~250 mAh / g. Additional capacity (only 211 mAh / g for I- / I2conversion) originated from the Fc / Fc+redox. More importantly, the Coulombic efficiency of batteries was maintained at a high level of ~99.5%, indicating successful suppression of the shuttle effect.

[0145] The Zn-I2battery using KB-I2(upper figure) showed high charge capacity and low discharge capacity, indicating a serious shuttle effect. After 200 cycles, an obvious capacity loss was observed. In contrast, the Zn-I2battery using Fc / KB-I2presented a high initial area capacity of ~3.2 mAh / cm2. Stable capacity delivery and negligible polarization increase was observed even after 200 cycles.

[0146] The self-discharge rate can indicate the capacity maintenance during storage. When tested, the Zn-I2 battery was charged to cut-off potential (1.6 V vs. Zn / Zn2+), and discharged after several hours resting. The Coulombic efficiency declination reflects the capacity loss during storage. The self-discharge rate is defined according to the equation: self-discharge rate = 100% - Coulombic efficiency. When using KB-I2, the battery suffers from ~5% capacity loss after 0.5 h resting. When the resting time increase to 24 h, a capacity loss of ~55% was observed, indicating serious polyiodide shuttling. For the Fc / KB-I2, negligible self-discharge rate was observed when resting less than 2 h, and 90% capacity was preserved after 24 h resting, proving excellent polyiodide confinement.

[0147] Compared to the battery using KB-I2, the counterpart using Fc / KB-I2 presents excellent capacity delivery at ultra-high current density (10.0 A / g) of ~170 mAh / g, validating good rate capability.

[0148] The Zn-I2 battery with KB-I2 suffers from constant capacity fading. For the battery using Fc / KB-I2, stable cycling with negligible capacity fading for over 5000 cycles was achieved.

[0149] With Fc / KB-I2, industrial-level pouch cells with a total capacity of 1.2 Ah can present a stable cycling performance for over 350 cycles. A 1.2 Ah Zn-I2 pouch cell with record-high I2 loading (50 mg cm−2) demonstrated extraordinary cycling. By controlling the depth of discharge of Zn (40%), a 400 mAh Zn-I2 pouch cell exhibited high energy density of 105 Wh kg−1and 94.5 Wh L−1under practical conditions, and sustained 800 deep cycles with 80% retentive capacity.

[0150] The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that such prior art forms part of the common general knowledge.

[0151] It will be understood that the terms “comprise” and “include” and any of their derivatives (e.g. comprises, comprising, includes, including) as used in this specification, and the claims that follow, is to be taken to be inclusive of features to which the term refers, and is not meant to exclude the presence of any additional features unless otherwise stated or implied.

[0152] In some cases, a single embodiment may, for succinctness and / or to assist in understanding the scope of the disclosure, combine multiple features. It is to be understood that in such a case, these multiple features may be provided separately (in separate embodiments), or in any other suitable combination. Alternatively, where separate features are described in separate embodiments, these separate features may be combined into a single embodiment unless otherwise stated or implied. This also applies to the claims which can be recombined in any combination. That is a claim may be amended to include a feature defined in any other claim. Further a phrase referring to “at least one of” a list of items refers to any combination of those items, including single members. As an example, “at least one of: a, b, or c” isintended to cover: a, b, c, a-b, a-c, b-c, and a-b-c.

[0153] It will be appreciated by those skilled in the art that the disclosure is not restricted in its use to the particular application or applications described. Neither is the present disclosure restricted in its preferred embodiment with regard to the particular elements and / or features described or depicted herein. It will be appreciated that the disclosure is not limited to the embodiment or embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope as set forth and defined by the following claims.REFERENCES

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Claims

CLAIMS 1. A solid positive electrode for an electrochemical device, the solid positive electrode comprising a functionalised porous carbon substrate comprising a redox active metallocene, or a derivative thereof, and iodine.

2. The solid positive electrode according to claim 1, wherein the functionalised porous carbon substrate comprising a redox active metallocene, or a derivative thereof, and iodine is capable of confining polyiodide species to the positive electrode during operation of the device.

3. The solid positive electrode according to any one of claims 1 to 2, wherein the redox active metallocene is selected from the group consisting of ferrocene, cobaltocene, nickelocene, chromocene, iridocene, vanadocene, manganocene, titanocene, ruthenocene, zirconocene, and osmocene.

4. The solid positive electrode according to claim 3, wherein the redox active metallocene is ferrocene.

5. The solid positive electrode according to any one of claims 1 to 4, wherein the carbon of the porous carbon substrate is selected from carbon black, Ketjen Black (KB), graphene, graphene oxide, carbon nanotubes (CNT), carbon nanofiber (CNF), carbon cloth, carbon felt, carbon paper, carbon fibre, carbon pellet, carbon powder, hollow carbon spheres, metal-organic frameworks (MOF), carbonised MOF, active carbon cloth / polyvinylpyrrolidone (ACC / PVPI) composite, and combinations thereof.

6. The solid positive electrode according to any one of claims 1 to 5, wherein the electrochemical device is an aqueous battery.

7. The solid positive electrode according to claim 6, wherein the aqueous battery is an aqueous zinc- iodine battery.

8. An electrochemical device comprising a negative electrode, a solid positive electrode according to any one of claims 1 to 5, and an electrolyte solution.

9. The electrochemical device according to claim 8, wherein the electrochemical device is an aqueous battery.

10. The electrochemical device according to claim 9, wherein the aqueous battery is an aqueous zinc- iodine battery.

11. The electrochemical device according to claim 10, wherein the aqueous battery is an aqueouszinc-iodine battery wherein metallic zinc (Zn) is used as the negative electrode active material and elemental iodine (I2) is used as the positive electrode active material.

12. The electrochemical device according to any one of claims 8 to 11, wherein the electrolyte is ZnSO4.

13. The electrochemical device according to any one of claims 8 to 12, wherein the negative electrode has a source of zinc, lithium, sodium, aluminium or magnesium as a negative electrode active material.

14. Use of a functionalised porous carbon substrate comprising a redox active metallocene, or a derivative thereof, and iodine in an electrochemical device comprising a negative electrode, a solid positive electrode, and an electrolyte solution, wherein the functionalised porous carbon substrate is introduced into the solid positive electrode.

15. The use according to claim 14, wherein the functionalised porous carbon substrate comprising a redox active metallocene, or a derivative thereof, and iodine is capable of confining polyiodide species to the positive electrode during operation of the device.

16. The use according to any one of claims 14 to 15, wherein the redox active metallocene is selected from the group consisting of ferrocene, cobaltocene, nickelocene, chromocene, iridocene, vanadocene, manganocene, titanocene, ruthenocene, zirconocene, and osmocene.

17. The use according to claim 16, wherein the redox active metallocene is ferrocene.

18. The use according to any one of claims 14 to 17, wherein the carbon of the porous carbon substrate is selected from carbon black, Ketjen Black (KB), graphene, graphene oxide, carbon nanotubes (CNT), carbon nanofiber (CNF), carbon cloth, carbon felt, carbon paper, carbon fibre, carbon pellet, carbon powder, hollow carbon spheres, metal-organic frameworks (MOF), carbonised MOF, active carbon cloth / polyvinylpyrrolidone (ACC / PVPI) composite, and combinations thereof.

19. The use according to any one of claims 14 to 18, wherein the electrochemical device is an aqueous battery.

20. A method of fabricating an electrochemical device comprising a negative electrode, a solid positive electrode, and an electrolyte solution, the method comprising introducing into the solid positive electrode a functionalised porous carbon substrate comprising a redox active metallocene, or a derivative thereof, and iodine.

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