Electrolyte additive for high performance iodine or bromine based electrochemical device

Abstract

The disclosure relates to an ionic liquid additive that is useful in an electrolyte solution for an electrochemical device. The ionic liquid additive is capable of interacting with polyiodide and / or polybromide species to mitigate shuttle effect for the electrochemical device and stabilise the positive electrode and the negative electrode of the electrochemical device.

Description

ELECTROLYTE ADDITIVE FOR HIGH PERFORMANCE IODINE OR BROMINE BASED ELECTROCHEMICAL DEVICE PRIORITY DOCUMENT The present application claims priority from Australian Provisional Patent Application No.2024900214 titled “ELECTROLYTE ADDITIVE FOR HIGH PERFORMANCE IODINE OR BROMINE BASED ELECTROCHEMICAL DEVICE” and filed on 30 January 2024, the content of which is hereby incorporated by reference in its entirety. TECHNICAL FIELD

[0001] The present disclosure relates generally to aqueous electrochemical devices, such as iodine or bromine based batteries. In particular, the present disclosure relates to electrolyte additives for aqueous electrochemical devices and to electrochemical devices comprising the additive. BACKGROUND

[0002] Energy crises and looming environmental concerns have driven the rapid development of reliable, low-cost, and eco-friendly electrochemical energy storage devices. Among these developments, organic electrolyte-based batteries show high energy densities which are, in principle, suitable for large scale energy storage. However, they suffer from inherent instability and safety issues caused by the use of expensive yet highly volatile and flammable organic solvents (for example dimethyl carbonate and diethyl carbonate) and of chemically unstable and toxic salts (for example lithium hexafluorophosphate (LiPF6)). The leakage hazard of organic solvents is also a concern for large scale energy storage. Aqueous batteries show promise in resolving these issues and have shown enormous potential for large scale energy storage due to their cost effectiveness, high ionic conductivity and much improved safety.

[0003] The abundance of iodine (about 50 µg L−1) and bromine (about 65 mg L-1) in seawater as well as the highly reversible redox chemistry of iodine or bromine make iodine-based or bromine-based batteries a promising cheap energy storage system. Accordingly, increasing attention has been drawn to iodine-based batteries and bromine-based batteries, for example, metal-iodine or metal-bromine batteries, such as aqueous zinc-iodine (Zn-I2) batteries, aqueous zinc-bromine (Zn-Br2) batteries, aqueous lithium- iodine (Li-I2) batteries, non-aqueous Li-I2 batteries, aqueous sodium-iodine (Na-I2) batteries, non-aqueous magnesium-bromine (Mg-Br2) batteries, aqueous aluminum-iodine (Al-I2) batteries, and non-aqueous Al- I2 batteries.

[0004] Aqueous zinc-based batteries with high safety and low cost provide a candidate for large scale energy storage. [1] Among zinc-based batteries, rechargeable Zn-I2 batteries are appealing because of theabundant reserves of iodine in seawater [2], stability of metallic zinc in water, high specific capacity (211 mAh giodine‒1)[3] and high discharge potential plateau (1.38 V vs. Zn / Zn2+)[4]. Besides, the liquid phase conversion mechanism of I‒ / I2 at the cathode endows a Zn-I2 system with excellent rate capability.[5] In addition, rechargeable aqueous Zn-Br2 batteries can present a higher theoretical specific capacity (355 mAh g-1) and higher theoretical voltage plateau (1.85 V vs. Zn / Zn2+) compared with rechargeable aqueous Zn-I2 batteries.

[0005] However, the existing rechargeable aqueous and non-aqueous metal-iodine or metal-bromine batteries are still far from satisfactory due to the challenges of intermediate dissolution, severe self- discharge, and metal anode corrosion.[2] Particularly, weak interaction between the conductive support of the cathode and iodine / bromine species leads to the dissolution of intermediate polyiodide or polybromide anions or molecules in the aqueous or non-aqueous electrolyte and the subsequent migration of polyiodide or polybromide through the separator towards the metal anode, i.e. the shuttle effect. The polyiodide or polybromide shuttle effect typically occurs in five steps: (i) formation of long-chain polyhalogen anions, (ii) detachment of polyhalogen anions from the halogen source, (iii) dissolution of polyhalogen anions into the electrolyte solution, (iv) migration of polyhalogen anions toward the anode, and (v) reaction between polyhalogen anions and the anode metal. The shuttle effect can result in irreversible loss of iodine or bromine, corrosion of the anode metal, rapid capacity fading and poor Coulombic efficiency of the batteries, severe self-discharge, and poor cycling stability.[6-8] In aqueous electrolytes, Zn-I2batteries present a reversible I‒ / I2redox reaction, in which polyiodide species form as highly-soluble intermediates such as I5- and I3- that cause the shuttle effect, lead to irreversible loss of active mass (such as Zn), and hinder improvement of depth-of-discharge.[5, 11, 16] Therefore, it is important to develop an economical electrolyte capable of stabilising both cathode and anode so as to increase utilisation of iodine or bromine based batteries.

[0006] To address these challenges, several approaches have been proposed, which include designing porous carbon hosts [9-12] to confine polyiodides or polybromide and suppress shuttle effects, altering the reaction pathways of iodine through confinement effects in pores

[0013] , or implementing protective measures for the Zn anode [8, 14, 15] to prevent its reaction with I3−. Although some good results have been reported, they were primarily achieved with coin cells at low electrode loadings (<2 mg cm−2) and high testing rates (>5 C). However, the low mass loading and high testing currents could mask the shuttle effects of iodine cathode and side reactions of Zn anode, and thus it is difficult for the small-size battery configuration to be effectively applied in large scale energy storage.

[0007] Accordingly, there remains a need for a cost-effective electrolyte solution and an electrochemical device comprising the same which may solve or alleviate one or more of the above problems and may be of practical use in large scale energy storage.SUMMARY

[0008] In a first aspect, provided herein is an ionic liquid additive, which is used in an electrolyte solution for an electrochemical device and is capable of interacting with polyiodide and / or polybromide species to mitigate shuttle effects for the electrochemical device.

[0009] In some embodiments of the first aspect, the ionic liquid useful as the additive has at least one heteroatom within a conjugated ring. In some embodiments, the heteroatom is selected from nitrogen (N), sulfur (S) and oxygen (O). In certain of these embodiments, the heteroatom is nitrogen (N). In some further embodiments, the ionic liquid useful as the additive has two heteroatoms within a conjugated ring. In even further embodiments, the two heteroatoms are nitrogen (N) and sulfur (S), nitrogen (N) and oxygen (O) or both of them are nitrogen (N).

[0010] In some embodiments of the first aspect, the ionic liquid useful as the additive is selected from the group consisting of an imidazolium-based ionic liquid, a pyrazolium-based ionic liquid, an oxazolium-based ionic liquid, a thiazolium-based ionic liquid, a pyridinium-based ionic liquid, and a picolinium-based ionic liquid. In some embodiments, the ionic liquid additive is a mixture of ionic liquids.

[0011] In some embodiments of the first aspect, the ionic liquid useful as the additive has an anion selected from halides, hydroxide, carboxylates, sulfites, sulfates (such as hydrogen sulfate and methyl sulfate), sulfonates, sulfamates, carbonates, nitrate, nitrites, perchlorate, dicyanamide, thiocyanate, sulfonylimides, antimonates, arsenates, phosphates, phosphonates, phosphinates, phosphites, phosphonites, phosphinites, borates, and imides. In some embodiments, the ionic liquid useful as the additive has an anion selected from Cl−, Br−, nitrate (NO3−), sulfate (SO42−), hydrogen sulfate (HSO4−), perchlorate (ClO4−), acetate (CH3COO−), trifluoroacetate (CF3COO−), trichloroacetate (CCl3COO−), tetrafluoroborate (BF4−), hexafluorophosphate (PF6−), hexafluoroantimonate (SbF6−), hexafluoroarsenate (AsF6−), methyl sulfate (CH3SO4−), ethyl sulfate (C2H5SO4−), methanesulfonate (CH3SO3−), trifluoromethanesulfonate (CF3SO3−), perfluorobutanesulfonate (C4F9SO3−), bis(fluorosulfonyl)imide (FSO2)2N–), bis(trifluoromethanesulfonyl)imide (CF3SO2)2N−, heptafluorobutanoate (C3F7COO−), and bis(pentafluoroethanesulfonyl)imide (C2F5SO2)2N−.

[0012] In some embodiments of the first aspect, the ionic liquid useful as the additive has an organic cation, for example, one cation (i.e. a monocationic ionic liquid) and two cations (i.e. a dicationic ionic liquid). In some embodiments, the organic cation is selected from the group consisting of imidazolium cations, pyrazolium cations, oxazolium cations, thiazolium cations, pyridinium cations, and picolinium cations. In some embodiments, the ionic liquid useful as the additive has an organic cation selected from imidazolium cations, pyrazolium cations, pyridinium cations, and picoliunium cations. In certain of theseembodiments, the organic cation is a pyrazolium cation. In certain other of these embodiments, the organic cation is a pyridinium cation. In some further embodiments, the organic cation has a substituent selected from the group consisting of alkyl, alkoxy, hydroxyalkyl, cycloalkyl, heterocycalkyl, alkenyl, alkynyl, aryl, and heteroaryl. In even further embodiments, the substituent on the organic cation is selected from C1-C8 alkyl, C1-C8 alkoxy, C1-C8 hydroxyalkyl, C3-C8 cycloalkyl, C3-C8 heterocycalkyl, C2- C8 alkenyl, C2-C8 alkynyl, C6-C12 aryl, and C3-C12 heteroaryl.

[0013] In some embodiments of the first aspect, the ionic liquid useful as the additive has the general formula (I):wherein each of R1, R2, R3, R4and R5is independently selected from the group consisting of a H atom and an organic substituent having 1 to 20 carbon atoms, for example 1 to 12 carbon atoms or 1 to 8 carbon atoms, and L−is a compatible anion. In some embodiments, R2is H, each of R1, R3, R4and R5is independently selected from the group consisting of a H atom and an organic substituent having 1 to 20 carbon atoms. In some further embodiments, the organic substituent having 1 to 20 carbon atoms is selected from the group consisting of alkyl, alkoxy, hydroxyalkyl, cycloalkyl, heterocycalkyl, alkenyl, alkynyl, aryl, and heteroaryl. In even further embodiments, the organic substituent having 1 to 20 carbon atoms is selected from the group consisting of C1-C8 alkyl, C1-C8 alkoxy, C1-C8 hydroxyalkyl, C3-C8 cycloalkyl, C3-C8 heterocycalkyl, C2-C8 alkenyl, C2-C8 alkynyl, C6-C12 aryl, and C3-C12 heteroaryl. In some embodiments, L−is selected from those described herein above for the anion of the ionic liquid.

[0014] In some embodiments of the first aspect, the ionic liquid useful as the additive has the general formula (I’):wherein each of R1and R3is independently selected from the group consisting of a H atom and an organic group having 1 to 20 carbon atoms, for example 1 to 12 carbon atoms or 1 to 8 carbon atoms, L−is a compatible anion. In some embodiments, each of R1and R3is independently selected from the group consisting of H, C1-C8 alkyl, C1-C8 alkoxy, C1-C8 hydroxyalkyl, C3-C8 cycloalkyl, C3-C8 heterocycalkyl, C2-C8 alkenyl, C2-C8 alkynyl, C6-C12 aryl, and C3-C12 heteroaryl. In some further embodiments, each of R1and R3is independently selected from the group consisting of C1-C8 alkyl, C1-C8 hydroxyalkyl, and C2- C8 alkenyl. In some embodiments, L−is selected from those described herein above for the anion of the ionic liquid.

[0015] In some embodiments of the first aspect, the ionic liquid useful as the additive has an organic cation selected from the group consisting of 1-ethyl-3-methylimidazolium, 1-propyl-3- methylimidazolium, 1-butyl-3-methylimidazolium, 1-pentyl-3-methylimidazolium, 1-hexyl-3- methylimidazolium, 1-ethyl-3-vinylimidazolium, 1-octyl-3-methylimidazolium, 1-allyl-3- methylimidazolium, 1-butyl-2-methylpyrazolium, 1-butylpyridinium, 1-pentylpyridinium, 1-hexyl-3- picolinium, 1-hexyl-4-picolinium, and 1-(3-hydroxypropyl)-3-methylimidazolium.

[0016] In some embodiments, the ionic liquid useful as the additive is selected from the group consisting of 1-propyl-3-methylimidazolium methanesulfonate, methyl sulfate, trifluoromethanesulfonate, chloride, or nitrate; 1-butyl-3-methylimidazolium methanesulfonate, methyl sulfate, trifluoromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-pentyl-3-methylimidazolium methanesulfonate, methyl sulfate, trifluoromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate;1-hexyl-3-methylimidazolium methanesulfonate, methyl sulfate, trifluoromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-octyl-3-methylimidazolium methanesulfonate, methyl sulfate, trifluoromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-ethyl-3-vinylimidazolium methanesulfonate, methyl sulfate, trifluoromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-ethyl-3-methylimidazolium trifluoroacetate, methyl sulfate, trifluoromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-butyl-3-methylimidazolium trifluoroacetate, methyl sulfate, trifluoromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-butyl-2-methylpyrazolium bis(trifluoromethanesulfonyl)imide, 1-butylpyridinium methanesulfonate, methyl sulfate, trifluoromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-pentylpyridinium trifluoroacetate; 1-hexyl-3-picolinium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; and 1-hexyl-4-picolinium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate.

[0017] In a second aspect, provided herein is an electrolyte solution for an electrochemical device, wherein the electrolyte solution comprises the ionic liquid additive of the first aspect.

[0018] In a third aspect, provided herein is an electrochemical device comprising an electrolyte solution, wherein the electrolyte solution comprises the ionic liquid additive of the first aspect.

[0019] In a fourth aspect, provided herein is use of an ionic liquid as an additive for the electrolyte solution to mitigate shuttle effects for an electrochemical device, wherein the ionic liquid additive is the one of the first aspect.

[0020] In a fifth aspect, provided herein is a method of mitigating shuttle effects caused by polyiodide and / or polybromide species for an electrochemical device, wherein the method includes introducing the ionic liquid additive of the first aspect into the electrolyte solution of an electrochemical device.

[0021] In some embodiments of the first, second, third, fourth or fifth aspect, the electrolyte solution comprises the ionic liquid additive in a concentration of 0.01 M to 0.5 M, or 0.06 M to 0.15 M, or 0.07 M to 0.12 M, or 0.08 M to 0.10 M. In some embodiments, the ionic liquid additive and the electrolyte are in a molar ratio of 0.025: 1 to 0.1: 1, or 0.03:1 to 0.075:1, or 0.035:1 to 0.06:1, or 0.04:1 to 0.05:1.

[0022] In some embodiments of the first, second, third, fourth or fifth aspect, the electrolyte solution is water-based. In some embodiments, the electrolyte in the electrolyte solution is selected from the group consisting of ZnSO4, ZnCl2, ZnNO3, zinc acetate, zinc bis(trifluoromethylsulfonyl)imide (Zn(TFSI)2), zinc trifluoromethanesulfonate (Zn(OTf)2), Zn(ClO4)2, and a combination thereof.

[0023] In some embodiments of the first, second, third, fourth or fifth aspect, the electrochemical device comprises a positive electrode and a negative electrode.

[0024] In some embodiments of the first, second, third, fourth or fifth aspect, a source of iodine and / or bromine as a positive electrode active material is contained within the positive electrode and / or within the electrolyte solution. In some embodiments, the source of iodine and / or bromine as a positive electrode active material is selected from elemental iodine (I2), LiI, NaI, KI, ZnI2, NH4I, quaternary ammonium iodides such as trimethylammonium iodide, quaternary ammonium bromides such as trimethylammonium bromide, triiodomethane (CH3I), poly(vinylpyrrolidone)-Iodine (PVP-I), elemental bromine (Br2), LiBr, KBr, ZnBr2and combinations thereof.

[0025] In some embodiments of the first, second, third, fourth or fifth aspect, the negative electrode comprises a source of zinc, lithium, sodium, aluminium or magnesium as a negative electrode active material.

[0026] In some embodiments of the first, second, third, fourth or fifth aspect, the electrochemical device is an aqueous electrochemical device. In some embodiments, the electrochemical device is an iodine based electrochemical device or a bromine based electrochemical device. In some further embodiments, the aqueous electrochemical device is selected from an aqueous metal-iodine electrochemical device and an aqueous metal-bromine electrochemical device. In some further embodiments, the aqueous electrochemical device is an aqueous zinc-iodine electrochemical device or an aqueous zinc-bromine electrochemical device. In even further embodiments, the aqueous electrochemical device is an aqueous zinc-iodine electrochemical device wherein metallic zinc (Zn) is used as the negative electrode active material and elemental iodine (I2) is used as the positive electrode active material.

[0027] In some embodiments of the first, second, third, fourth or fifth aspect, the electrochemical device comprises a separator which is selected from glass fibre separators, ceramic separators, polyolefin separators, nonwoven separators, and porous polymer separators.

[0028] In some embodiments of the first, second, third, fourth or fifth aspect, the electrochemical device further comprises a conductive agent in the positive electrode and / or a negative electrode. In some embodiments, the conductive agent is selected from carbon black, Ketjen black, graphene, active carbon, conductive nano carbon fiber (VGCF), carbon nanotubes (CNTs), multi-walled carbon nanotubes (MWCNTs) and combinations thereof.

[0029] In some embodiments of the first, second, third, fourth or fifth aspect, the electrochemical device delivers a low self-discharge rate of about 11.7 % capacity loss per month at a depth of discharge of 44.0 % and room temperature. In some embodiments, the electrochemical device exhibits a long lifespan with about 94.0 % capacity retention after 750 cycles. In some embodiments, the electrochemical device maintains about 83.5 % of its capacity at a high rate of 20 mA cm−2(~6.7 C) at an areal loading over 15 mg cm−2. In some embodiments, the electrochemical device exhibits a depth-of-discharge (DOD) of about 68.2 % and a capacity retention of about 90.0 % after 2,500 cycles. BRIEF DESCRIPTION OF THE FIGURES

[0030] Non-limiting embodiments of the present disclosure will be discussed with reference to the accompanying figures wherein:

[0031] Figure 1 illustrates the suppression of iodine shuttle effects by 1-butyl-3-methylimidzolium methanesulfonate (BMIS): (a) Chemical structure of BMIS; (b) Charge density differences of I3−and BMI+; (c) The Coulombic efficiency (CE) comparison of batteries using electrolyte with various concentrations of BMIS; (d) The self-discharge performance of batteries using 2 M ZnSO4electrolyte and 2 M ZnSO4+0.1 M BMIS electrolyte respectively (1 C= 211 mA

[0032] Figure 2 shows the reaction between I3−and BMIS: (a) 0.02 M KI3and 0.02 M KI3as prepared; (b) 0.02 M KI3with added water and 0.02 M KI3with added 0.1 M BMIS; (c) 0.02 M KI3with added water and 0.02 M KI3with added 0.1 M BMIS after 12 hours.

[0033] Figure 3 depicts Operando analysis of I3−shuttle effect during battery cycling and resting: Operando UV vis spectrum of Zn / iodine batteries during first discharging process using (a) 2 M ZnSO4and (b) 2 M ZnSO4+0.1 M BMIS electrolytes (inserted figures are discharge voltage profiles of batteries); Operando Raman map of Zn / iodine batteries after the first discharging process using (c) 2 M ZnSO4and (d) 2 M ZnSO4+0.1 M BMIS electrolytes (left figures are charge / discharge voltage profiles of batteries);Operando Raman map of Zn / iodine batteries resting process after charge to 100 % state of charge (SOC) using (e) 2 M ZnSO4 and (f) 2 M ZnSO4+0.1 M BMIS electrolytes.

[0034] Figure 4 shows IR spectra of BMIS aqueous solution before and after KI3 solution being added.

[0035] Figure 5 depicts self-discharge performance of coin cells using a 2 M ZnSO4 electrolyte.

[0036] Figure 6 depicts self-discharge performance of coin cells using a 2 M ZnSO4+0.1 M BMIS electrolyte.

[0037] Figure 7 shows a quartz cell construction for operando UV-vis test.

[0038] Figure 8 depicts a calibration curve for determination of total I3−content in the 2M ZnSO4 electrolyte.

[0039] Figure 9 shows Raman spectra of batteries using the 2M ZnSO4 and the 2 M ZnSO4 + 0.1 M BMIS electrolyte after being charged to 1.6 V.

[0040] Figure 10 depicts electrochemical performance of Zn–I2 batteries: (a) Cycling performance of batteries using 2 M ZnSO4 and 2 M ZnSO4+0.1 M BMIS electrolytes at 0.5 C; (b) Charge / discharge voltage profiles of batteries using 2 M ZnSO4+0.1 M BMIS electrolyte at 0.5 C: (c) Rate performance of batteries using 2 M ZnSO4 and 2 M ZnSO4+0.1 M BMIS electrolytes; (d) Cycling performance of battery using 2 M ZnSO4+0.1 M BMIS electrolyte at 10 C.

[0041] Figure 11shows charge / discharge voltage profile of batteries using (a) 2M ZnSO4and (b) 2 M ZnSO4+ 0.1 M BMIS electrolyte at various current densities.

[0042] Figure 12 depicts the formation mechanism of the micro-solvation layer: (a) Molecular dynamics (MD) simulations of 2 M ZnSO4+0.1 M BMIS electrolyte during discharge process; (b) Operando Raman test of Zn / I2full batteries during the first discharge process; (c) Operando ATR−IR test of Zn / I2full batteries during the first discharge process; (d) photo of iodine electrode in various concentration BMIS / water solution.

[0043] Figure 13 shows the configuration of Operando Raman cell.

[0044] Figure 14 shows the configuration of Operando ATR-FTIR cell.

[0045] Figure 15 depicts Operando Raman test of Zn / I2 full batteries after first discharge.

[0046] Figure 16 depicts operando EIS spectra of Zn / I batteries at various charge transfer resistances (Rct) using 2M ZnSO4 and 2 M ZnSO4+0.1 M BMIS electrolyte at 1 C at (a) pristine state, (b) after discharge to 1.2 V, (c) after discharge to 1.6 V and (d) after charge to 1.3 V.

[0047] Figure 17 depicts CV curves of Zn / I batteries using (a) 2M ZnSO4 and (b) 2 M ZnSO4 + 0.1 M BMIS electrolyte at various current density at various sweep rates and (c) the relationship between peak currents and sweep rates.

[0048] Figure 18 shows (a) GITT plot collected at a current density of 1 C after first discharge process; diffusion coefficient versus specific capacity, calculated from GITT measurements during (b) charging process and (c) discharging process.

[0049] Figure 19 depicts rate performance of pouch cell using 2 M ZnSO4+0.1 M BMIS electrolyte.

[0050] Figure 20 depicts (a) Charge / discharge voltage profiles of 1 Ah pouch cell at 8.3 mA cm−2; and (b) Cycling performance of 1 Ah pouch cell at 8.3 mA cm−2.

[0051] Figure 21 shows Zn metal morphology after cycling in (a) 2M ZnSO4 and (b) 2 M ZnSO4+0.1 M BMIS electrolytes.

[0052] Figure 22 shows Cu morphology after Zn deposit in (a) to (b) 2M ZnSO4 and (c) to (d) 2 M ZnSO4+0.1 M BMIS electrolytes.

[0053] Figure 23 shows small angle X-ray scattering (SAXS) images of Zn metal after cycling in (a) 2M ZnSO4and (b) 2 M ZnSO4+0.1 M BMIS electrolytes.

[0054] Figure 24 depicts SAXS patterns of Zn metal after cycling in (a) 2M ZnSO4and (b) 2 M ZnSO4+0.1 M BMIS electrolyte.

[0055] Figure 25 shows electrochemical performance of high Zn DOD pouch cell using 2 M ZnSO4+0.1 M BMIS electrolytes: (a) Electrodeposition-dissolution CE profiles of Zn metal on Cu current collector in the current of 2 mA cm−2; (b) Self-discharge performance of pouch cell at Zn DOD of 44 %; (c) Cycling performance of pouch cell at Zn DOD of 55.1 % in the current of 1.25 mA cm−2; (d) Cycling performance of pouch cell at Zn DOD of 82.0 % in the current of 7.94 mA cm−2; (e) Cycling performance of pouch cell at Zn DOD of 68.3 % in the current of 15.9 mA cm−2.

[0056] Figure 26 depicts charge / discharge voltage profiles of Zn / Cu pouch cell.

[0057] Figure 27 depicts electrochemical performance of Zn–I2 batteries using 2 M ZnSO4+0.1 M 1- ethyl-3-methylimidazolium chloride electrolytes at 1 C.

[0058] Figure 28 depicts electrochemical performance of Zn–I2 batteries using 2 M ZnSO4+0.1 M 1- ethyl-3-methylimidazolium acetate electrolytes at 1 C. DESCRIPTION OF EMBODIMENTS

[0059] 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, but are not limited to, a battery, a cell, a capacitor, a supercapacitor and a photovoltaic device. The electrochemical device may be an electrochemical device without use of a fuel (or rather than a fuel cell) or may be an electrochemical device rather than a dye sensitized solar cell (DSSC). Examples of a battery include, but are not limited to, a static (non-flow) battery or a flow battery.

[0060] The term “ionic liquid” used herein refers to compounds completely composed of ions with a melting point below 100 °C. An ionic liquid consists of a large organic cation and typically a smaller inorganic or organic anion. In some circumstances, an ionic liquid may have two cations, which is called dicationic ionic liquid, for example diethylene glycol-bis(3-methylimidazolium) dihydroxide.

[0061] The term “additive” used in relation to an electrolyte solution refers to a substance that is present at a relatively small amount compared to the base electrolyte and is used to modify or improve the performance of an electrochemical device. For example, sodium sulfate (Na2SO4) can be added to the H2SO4electrolyte to improve its electroconductivity on deep discharge and at low temperatures.

[0062] The term “aqueous electrolyte solution” used herein generally refers to a water-based electrolyte solution or an electrolyte solution comprising water as a solvent. However, this does not exclude the possibility of presence of other constituents, for example an electrolyte additive and an organic solvent (e.g. acetone).

[0063] The term “depth of discharge (DOD)” used herein is defined as the capacity that is discharged from a fully charged electrochemical device, divided by the nominal capacity of the device. It indicates the percentage of an electrochemical device that has been discharged relative to the overall capacity thereof. Depth of discharge is normally expressed as a percentage.

[0064] The term “rate capability” used herein refers to a certain amount of specific charge (e. g., in units of mAh / g) is transferred while maintaining a certain voltage limit. High rate capability enables anelectrochemical device like mobile electronic devices to be recharged in a very short time, enhancing their user-friendliness.

[0065] The term “Coulombic efficiency” used herein describes the charge efficiency by which electrons are transferred in an electrochemical device and is also called faradaic efficiency or current efficiency. CE is defined as the quotient of the discharge capacity and its antecedent charge capacity. It is a measure of how reversible the electrochemical energy storing reactions are.

[0066] The term “areal capacity” used herein is the battery capacity exerted by the unit area of electrode.

[0067] 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 a composite comprising the active material in order to prepare the desirable electrodes.

[0068] 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−.

[0069] The term “polybromide” used herein refers to a class of polyhalogen anions composed entirely of bromine atoms. Examples of polybromides commonly involved in the shuttle effect of an electrochemical device include Br3−, Br5−and Br7−.

[0070] The disclosure arises from the inventors’ research into the shuttle effect that occurs with iodine or bromine based electrochemical device. It has been surprisingly found that an ionic liquid can be introduced as an additive in a small amount into the electrolyte solution to interact with species like polyiodide to suppress the shuttle effect. It has also been surprisingly found that the cations of an ionic liquid accumulated at the cathode surface during discharging process. Without being bound by any theory, it is believed by the inventors that the cations (such as 1-butyl-3-methylimidazolium cations) of the ionic liquid that accumulate at the cathode form a micro-solvation layer at the cathode surface, and that the solubility of solid I2in the micro-solvation layer is increased due to the higher solubility of iodine in the ionic liquid, which promotes the I2conversion kinetics and contributes to enhanced rate performance. Consequently, the shuttle effect was effectively mitigated or suppressed, the anti-self- discharge performance and cycling stability were enhanced for iodine-based batteries.

[0071] Accordingly, disclosed herein is an ionic liquid additive for use in an electrolyte solution of an electrochemical device. The ionic liquid additive is capable of interacting with polyiodide and / or polybromide species to mitigate shuttle effect for the electrochemical device.

[0072] Ionic liquid generally consists of a large organic cation and typically a smaller inorganic or organic anion. Most ionic liquids are soluble in a variety of organic and inorganic reagents and can be highly water soluble. The number of organic cation within an ionic liquid may vary. For example, there can be one cation (i.e. a monocationic ionic liquid) or two cations (i.e. a dicationic ionic liquid).

[0073] Any suitable ionic liquid can be used for the present disclosure. If needed, a mixture of ionic liquids can be considered. It has been found by the inventors that the ionic liquid which has at least one heteroatom embedded within a conjugated ring could interact with polyiodide and / or polybromide species, for example through formation of N-I bonds. A simple test can be done to preliminarily determine if an ionic liquid is capable of interacting with polyiodide and / or polybromide species, wherein a solution of the ionic liquid is introduced into a KI3 solution to see whether a precipitate is formed and / or the yellow I3 solution changes colour, for example turns colourless. The charge density difference between the cation of the ionic liquid and polyiodide and / or polybromide species (such as I3−) may assist in determining potential presence of the interaction. Other methods such as the Attenuated total reflection Fourier transform infrared (ATR−FTIR) may also be used to determine if an ionic liquid is capable of interacting with polyiodide and / or polybromide species. In some circumstances, the cation (C+) of the ionic liquid additive may form a C+- I3−interaction or a C+- Br3−interaction. For the present disclosure, the heteroatom embedded within a conjugated ring may include, but is not limited to, nitrogen (N), sulfur (S) and / or oxygen (O). In some embodiments, it might be preferable for the conjugated ring to include nitrogen (N) as the heteroatom or heteroatoms, which could lead to the presence of an amine group and / or an imine group in the ionic liquid. In some further embodiments, the conjugated ring may comprise both nitrogen (N) and sulfur (S) as the heteroatoms.

[0074] Examples of the organic cation that may be comprised by the ionic liquid for the present disclosure include, but are not limited to, imidazolium cations, pyrazolium cations, non-substituted or substituted oxazolium cations, thiazolium cations, pyridinium cations, and picolinium cations. In some embodiments, the ionic liquid useful as the additive has an organic cation selected from imidazolium cations, pyrazolium cations, pyridinium cations, and picoliunium cations.

[0075] For the purpose of illustration, the organic cation mentioned above may have a substituent, and the substituent may be selected from alkyl, alkoxy, hydroxyalkyl, cycloalkyl, heterocycalkyl, alkenyl, alkynyl, aryl, and heteroaryl. Preferably, the substituent may have 1 to 20 carbon atoms, for example, 1 to 12 carbon atoms, or 1 to 10 carbon atoms, or 1 to 8 carbon atoms. Examples of the substituent on the organic cations include, but are not limited to, C1-C8alkyl, C1-C8alkoxy, C1-C8hydroxyalkyl, C3-C8cycloalkyl, C3-C8 heterocycalkyl, C2-C8 alkenyl, C2-C8 alkynyl, C6-C12 aryl, and C3-C12 heteroaryl. In some circumstances, the length of the substituent on the organic cations may affect solubility of the ionic liquid in a solvent. It might be the case that when water is used as a solvent or a co-solvent for an electrolyte solution, the longer the substituent is, the less soluble the ionic liquid is in the electrolyte solution. High solubility of the ionic liquid is good for the electrolyte preparation process.

[0076] Illustrative examples of the organic cations are 1-ethyl-3-methylimidazolium, 1-propyl-3- methylimidazolium, 1-butyl-3-methylimidazolium, 1-pentyl-3-methylimidazolium, 1-hexyl-3- methylimidazolium, 1-ethyl-3-vinylimidazolium, 1-octyl-3-methylimidazolium, 1-allyl-3- methylimidazolium, 1-butyl-2-methylpyrazolium, 1-butylpyridinium, 1-pentylpyridinium, 1-hexyl-3- picolinium, 1-hexyl-4-picolinium, and 1-(3-hydroxypropyl)-3-methylimidazolium.

[0077] Examples of the anion that may be comprised by the ionic liquid for the present disclosure include, but are not limited to, halides (such as Cl−, Br−, I−), hydroxide (OH−), carboxylates, sulfites, sulfates (such as hydrogen sulfate and methyl sulfate), sulfonates, sulfamates, carbonates (such as CO32−and CH3CO3−), nitrate, nitrites, perchlorate, dicyanamide, thiocyanate, sulfonylimides, antimonates, arsenates, phosphates, phosphonates, phosphinates, phosphites, phosphonites, phosphinites, borates, and imides. In particular, an anion suitable for the present disclosure can be Cl−, Br−, nitrate (NO3−), sulfate (SO42−), hydrogen sulfate (HSO4−), perchlorate (ClO4−), acetate (CH3COO−), trifluoroacetate (CF3COO−), trichloroacetate (CCl3COO−), tetrafluoroborate (BF4−), hexafluorophosphate (PF6−), hexafluoroantimonate (SbF6−), hexafluoroarsenate (AsF6−), methyl sulfate (CH3SO4−), ethyl sulfate (C2H5SO4−), methanesulfonate (CH3SO3−), trifluoromethanesulfonate (CF3SO3−), perfluorobutanesulfonate (C4F9SO3−), bis(fluorosulfonyl)imide (FSO2)2N–), bis(trifluoromethanesulfonyl)imide (CF3SO2)2N−, heptafluorobutanoate (C3F7COO−), and bis(pentafluoroethanesulfonyl)imide (C2F5SO2)2N−. The effect of the anion on the stability of the anode may be taken into account when selecting an anion.

[0078] In addition to solubility, consideration may be given to ionic conductivity in choosing an ionic liquid, which represents the ability of charged ions to move through the electrolyte solution. For industrial applications, the price of the ionic liquid may be considered when choosing a suitable ionic liquid useful as the additive.

[0079] For the purpose of illustration, the ionic liquid useful as the additive may have the general formula (I):wherein each of R1, R2, R3, R4and R5is independently selected from the group consisting of a H atom and an organic substituent having 1 to 20 carbon atoms, for example 1 to 12 carbon atoms or 1 to 8 carbon atoms, L−is a compatible anion, which may correspond to the anion generally described herein above for the ionic liquid. In some embodiments, R2is H, each of R1, R3, R4and R5is independently selected from the group consisting of a H atom and an organic substituent having 1 to 20 carbon atoms. In some further embodiments, the organic substituent having 1 to 20 carbon atoms is selected from the group consisting of alkyl, alkoxy, hydroxyalkyl, cycloalkyl, heterocycalkyl, alkenyl, alkynyl, aryl, and heteroaryl. In even further embodiments, the organic substituent having 1 to 20 carbon atoms is selected from the group consisting of C1-C8 alkyl, C1-C8 alkoxy, C1-C8 hydroxyalkyl, C3-C8 cycloalkyl, C3-C8 heterocycalkyl, C2-C8 alkenyl, C2-C8 alkynyl, C6-C12 aryl, and C3-C12 heteroaryl.

[0080] Further, the ionic liquid that may be suitable for the present disclosure has the general formula (I’):(I’)wherein each of R1and R3is independently selected from the group consisting of a H atom and an organic substituent having 1 to 20 carbon atoms, for example 1 to 12 carbon atoms or 1 to 8 carbon atoms, L−is a compatible anion, which may correspond to the anion generally described herein above for the ionic liquid. Examples of each of R1and R3are H, C1-C8 alkyl, C1-C8 alkoxy, C1-C8 hydroxyalkyl, C3- C8 cycloalkyl, C3-C8 heterocycalkyl, C2-C8 alkenyl, C2-C8 alkynyl, C6-C12 aryl, and C3-C12 heteroaryl. In some circumstances, each of R1and R3may be selected from C1-C8 alkyl, C1-C8 hydroxyalkyl, and C2-C8 alkenyl.

[0081] In accordance with the above, ionic liquids that may be suitable for the present disclose include, but are not limited to, the following: 1-propyl-3-methylimidazolium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, or nitrate; 1-butyl-3-methylimidazolium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-pentyl-3-methylimidazolium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-hexyl-3-methylimidazolium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-octyl-3-methylimidazolium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-ethyl-3-vinylimidazolium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-ethyl-3-methylimidazolium trifluoroacetate, methyl sulfate, trifluoromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-butyl-3-methylimidazolium trifluoroacetate, methyl sulfate, trifluoromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate, 1-butyl-2-methylpyrazolium bis(trifluoromethanesulfonyl)imide, 1-butylpyridinium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate;1-pentylpyridinium trifluoroacetate, 1-hexyl-3-picolinium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate, and 1-hexyl-4-picolinium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate.

[0082] The ionic liquids may be prepared through a method known in the art. Imidazolium-based ionic liquids are available by alkylating imidazoles, which may be formed from the reaction of an amino acid, ammonia, formaldehyde, and glyoxal in alkaline solution. It is also possible to directly alkylate imidazole and pyridine derivatives using Brønsted acids or ammonium salts in the presence of an orthoester to give high yields of the corresponding ionic liquids with low levels of residual chloride.

[0048] Alternatively, an ionic liquid useful for the present disclosure may also be commercially available. For example, 1-butyl-3- methylimidazolium methyl sulfate, 1-butyl-3-methylimidazolium methanesulfonate, 1-butyl-3- methylimidazolium trifluoromethanesulfonate, 1-ethyl-3-methylimidazolium acetate and 1-ethyl-3- methylimidazolium chloride can be purchased from Sigma-Aldrich.

[0083] Also disclosed herein is an electrolyte solution of an electrochemical device comprising the ionic liquid additive. If the electrochemical device is effectively divided into two compartments and there are catholyte and anolyte, it is preferable for the ionic liquid to be comprised at least in the catholyte solution that is in contact with the positive electrode.

[0084] Introducing the ionic liquid additive into the electrolyte solution is intended to mitigate or suppress shuttle effect caused by polyiodide and / or polybromide species for an electrochemical device. Apart from the ionic liquid additive, the electrolyte solution may further comprise a solvent or a mixture of solvents, an electrolyte or a mixture of electrolytes, and / or another additive such as a SEI (solid electrolyte interphases)-forming additive or a pH buffer etc. The electrolyte solution may be water-based (i.e. an aqueous electrolyte solution). For an aqueous electrolyte solution, potassium iodide (KI) or other iodide aqueous solutions may be added to increase the solubility of iodine. Presence of a solvent (such as acetone) other than water is not excluded. If desirable, the electrolyte solution may be formulated by using an organic solvent, such as tetraethylene glycol dimethyl ether (tetraglyme).

[0085] A concentration of the ionic liquid additive may be selected to effectively interact with polyiodide and / or polybromide species and mitigate or suppress the shuttle effect caused thereby. However, a higher concentration of the ionic liquid additive might not be cost effective. In some circumstances, an electrolyte solution comprises the ionic liquid additive in a concentration of about 0.01 M to 0.5 M, or about 0.06 M to 0.15 M, or about 0.07 M to 0.12 M, or about 0.08 M to 0.10 M. Taking anaqueous ZnSO4 electrolyte solution as an example, the ionic liquid may be present at a concentration of about 0.10 M. In some embodiments, the ionic liquid additive and the electrolyte are in a molar ratio of about 0.025:1 to 0.1:1, or about 0.03:1 to 0.075:1, or about 0.035:1 to 0.06:1, or about 0.04:1 to 0.05:1.

[0086] An electrolyte serves to ship electrons between the positive electrode (cathode) and the negative electrode (anode). Any suitable electrolyte may be considered for the present disclosure. 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. Factors such as balanced viscosity, ionic conductivity, and control of byproducts during the redox process may play a role in selecting the electrolyte. In some circumstances, it is desirable for the electrolyte to be dissolved in water. For example, the electrolyte may comprise an anion selected from SO42−, NO3−, ClO4−, PO43−, CO32−, CH3COO−, Cl−, Br−, I−, OH−, bis(trifluoromethylsulfonyl)imide, and trifluoromethanesulfonate. For example, the electrolyte may comprise a cation selected from Na+, K+, Li+, Mg2+, Zn2+, Ca2+, and Al3+. Accordingly, illustrative examples of the electrolyte include LiOH, LiNO3, LiI, KCl, KI, K2CO3, KClO4, ZnSO4, MgNO3, Ca (NO3)2, KNO3, Li2SO4, AlCl3, ZnI2, ZnBr2, ZnCl2, ZnNO3, zinc acetate, zinc bis(trifluoromethylsulfonyl)imide (Zn(TFSI)2), zinc trifluoromethanesulfonate (Zn(OTf)2), Zn(ClO4)2, and a combination thereof. A blend of different electrolytes is likely to result in an optimized system. For the purpose of illustration, the electrolyte solution may comprise an electrolyte at a concentration of about 0.1 M to 10.0 M, for example about 0.5 M to 3.0 M, or about 1.0 M to 2.0 M.

[0087] The pH of the electrolyte solution may be neutral (e.g. close to 7 at room temperature, such as 6.5 to 7.5). Optionally, the pH of the electrolyte may be altered by adding some additional H+ ionic species to make the electrolyte solution more acidic, for example by adding an acid such as H2SO4. A neutral, mild acidic or alkalescent aqueous electrolyte solution may be recommended as strong alkalinity could result in direct reaction with I2or Br2.

[0088] For the present purpose, the electrochemical device comprising the electrolyte solution can be an iodine based electrochemical device or a bromine based electrochemical device. An iodine based electrochemical device is typically based on an iodine redox reaction, which mainly involves the triiodide / iodide (I3− / I−) redox couple. A bromine based electrochemical device is typically based on a bromine redox reaction, which mainly involves the bromine / bromide (Br2 / Br−) redox couple. It would be appreciated that the ionic liquid electrolyte additive and the electrolyte solution can find use in any suitable electrochemical device, for example an iodine based or bromine based electrochemical device. The present disclosure thus also provides a method of mitigating shuttle effect caused by polyiodide and / or polybromide species for an electrochemical device, which includes introducing an ionic liquid capable of interacting with polyiodide and / or polybromide species as an additive into the electrolyte solution of an electrochemical device.

[0089] An electrochemical device known in the art can also be adapted to employ the electrolyte solution disclosed herein. The electrochemical device may be water based and thus an aqueous electrochemical device. In some circumstances, the electrochemical device can be a metal-iodine electrochemical device and a metal-bromine electrochemical device. In some embodiments, the electrochemical device is an aqueous zinc-iodine electrochemical device or an aqueous zinc-bromine electrochemical device.

[0090] A source of iodine and / or bromine may be present as a positive electrode active material in a positive electrode of the electrochemical device. In addition or alternatively, the source of iodine and / or bromine as a positive electrode active material may be contained within the electrolyte solution in contact with a positive electrode. For example, a certain amount of I2 and / or LiI may be comprised in the positive electrode. In some circumstances, the positive electrode is loaded with the source of iodine (for example, I2 and LiI) in the range of about 0.01 to about 20 mg·cm-2or the positive electrode is loaded with the source of bromine (for example, Br2 and LiBr) in the range of about 0.01 to about 20 mg·cm-2.

[0091] In some circumstances, the source of iodine and / or bromine as a positive electrode active material is selected from elemental iodine (I2), LiI, NaI, KI, NH4I, ZnI2, quaternary ammonium iodides such as trimethylammonium iodide, quaternary ammonium bromides such as trimethylammonium bromide, triiodomethane (CH3I), poly(vinylpyrrolidone)-Iodine (PVP-I), elemental bromine (Br2), LiBr and combinations thereof. The negative electrode of the 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, an electrochemical device known in the art may be adapted to the present disclosure through incorporating the ionic liquid additive disclosed herein. In some embodiments, the electrochemical device is an aqueous metal-iodine electrochemical device or an aqueous metal-bromine electrochemical device.

[0092] 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, most importantly, 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-I2batteries wherein metallic magnesium is used as the negative electrode active material and a composite of active carbon cloth (ACC) and I2is 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.

[0093] 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 thepositive 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.

[0094] Consideration may also be given to aqueous zinc-iodine (Zn-I2) batteries and aqueous zinc- bromine (Zn-Br2) batteries. Aqueous Zn- I2 batteries 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. Aqueous Zn-Br2 batteries may have advantages in their higher theoretical specific capacity of 355 mAh g-1and higher theoretical voltage plateau (1.85 V vs. Zn / Zn2+) compared with Zn-I2 batteries. When the negative electrode active material is metallic zinc, the latter may be in the form of a zinc foil or a zinc deposited copper.

[0095] Factors such as wettability and electronic conductivity, and fixation and promotion of the redox reaction (for example, in relation to elemental iodine, bromine and sulfur) might be considered in selecting components of the positive electrode. A conductive substrate may be used in fabricating the positive electrode to support the positive electrode active material and enhance the conductivity of the positive electrode. In some circumstances, a substrate with a large surface area and pore volume may be desirable. The conductive substrate used herein may be a conductive carbonaceous substrate and examples thereof include active carbon, carbon 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. The conductive substrate may be commercially available or prepared through a method known in the art. For instance, carbon nanotubes (CNT) are commercially available, for example, from Sigma-Aldrich, or grown by chemical vapor deposition (CVD) techniques. For the purpose of the present disclosure, the positive electrode active material such as I2may be dissolved in solutions and then continuously adsorbed on the conductive substrate such as carbon cloth, so as to allow uniform distribution of the positive electrode active material within the conductive substrate.

[0096] It is acknowledged that carbon, being non-polar in nature, is not able to adsorb polar species such as I- and Br-. A functional carbohydrate may also be introduced into the positive electrode as a host to effectively trap the guest species such as polyiodide or polybromide and limit the exposure of the guest species to the electrolyte solution. Functional carbohydrates that may be used for this purpose include, but are not limited to, polysaccharides, oligosaccharides, and combinations thereof. In this regard, PCT / AU2023 / 050315 is incorporated herein by reference in its entirety.

[0097] 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 withno 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 glass fibre separators, ceramic separators, polyolefin separators (e.g. polyolefin porous membrane), nonwoven separators, and porous polymer separators.

[0098] 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), and carboxymethyl cellulose (CMC).

[0099] 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.

[0100] The electrochemical device using the ionic liquid electrolyte additive disclosed herein may be advantageous in many aspects and it may especially achieve significant improvement in electrochemical performance and stability. In some embodiments, the electrochemical device delivers a low self-discharge rate of about 11.7 % capacity loss per month at a depth of discharge of 44.0 % and room temperature. In some embodiments, the electrochemical device exhibits a long lifespan with about 94.0 % capacity retention after 750 cycles. In some embodiments, the electrochemical device maintains about 83.5 % of its capacity at a high rate of 20 mA cm−2(~6.7 C) at an areal loading over 15 mg cm−2. In some embodiments, the electrochemical device exhibits a depth-of-discharge (DOD) of about 68.2 % and a capacity retention of about 90.0 % after 2,500 cycles. In some embodiments, the electrochemical device exhibits a capacity retention of 98.8 % with a high CE of 99.7 % in 0.5 C after 1000 cycles and an ultralong cycling performance of over 15,000 cycles with a capacity retention of 91.57 % at 10 C. The electrochemical device using the ionic liquid electrolyte additive may maintain a remarkable discharge capacity of 182.1 mAh g−1at a high current density of 10 C.

[0101] EXAMPLES

[0102] Chemical reagents

[0103] Zn foil, three-electrode cell, Cu foil, aluminium-plastic film, Ag / AgCl electrode, and hydrophobic polytetrafluoroethylene (PTFE) were purchased from Shenzhen Kejing Star Technology. ZnSO4·7H2O (≥ 99.0 %), iodine (≥ 99.8 %), and 1-butyl-3-methylimidazolium methanesulfonate (≥ 98.0 %) were purchased from Sigma-Aldrich Chemical Co. Ketjen Black (KB) was purchased from Guangdong Canrd New Energy Technology Co. All other reagents were analytical grade and used directly without purification. Deionized water was used to prepare all aqueous electrolytes.

[0104] Preparation of electrodes

[0105] The iodine (70 %) and Ketjen Black (30 %) were mixed and ground in a mortar for half an hour. The resulting mixture was then transferred to a small glass vial and subjected to vacuum drying at 80 degrees Celsius for 3 hours, followed by natural cooling, yielding I2@KB. For coin cell, the cathode was prepared via mechanically mixing 70 wt.% I2@KB, 20 wt.% KB and 10 wt.% polytetrafluoroethylene (PTFE) binder and dispersing them in a tiny amount of ethanol solvent (AR). The mixture was pressed on a Ti-mesh and dried at 50 °C for 1 hour in a vacuum oven. For pouch cell, the cathode was prepared via mechanically mixing 80 wt.% I2@KB, 10 wt.% KB and 10 wt.% polytetrafluoroethylene (PTFE) binder dispersed in tiny amount of ethanol solvent (AR). The mixture was pressed on a Ti-mesh and dried at 50 °C for 1 hour in a vacuum oven.

[0106] Preparation of electrolytes

[0107] 28.7 g ZnSO4·7H2O was dissolved in 50 mL of water to obtain 2 M ZnSO4electrolyte. Desired amounts of 1-butyl-3-methylimidazolium methanesulfonate (BMIS) were added into the 2 M ZnSO4electrolyte to obtain 2 M ZnSO4+ 0.1(0.02, 0.05,0.2) M BMIS electrolyte.

[0108] A similar procedure was followed to produce electrolytes comprising 2 M ZnSO4+0.1 M 1- ethyl-3-methylimidazolium chloride and 2 M ZnSO4+0.1 M 1-ethyl-3-methylimidazolium acetetate electrolytes.

[0109] Electrochemical measurements & materials characterisation

[0110] Cyclic voltammetry curves (CV) and electrochemical impedance spectroscopy are examined using a VMP3 (CHI 760E, Chenhua). The galvanostatic intermittent titration technique (GITT) was applied to analyze the reaction and diffusion kinetics at a current density of 0.5 C and a charge / discharge time is 5 minutes, interval of 20 minutes for each step. The galvanostatic cycling studies are performedusing a LAND battery testing system at room temperature. The working voltage of the cells was set from 0.6 V to 1.6 V versus Zn / Zn2+.

[0111] Characterisation

[0112] Operando Attenuated Total Reflectance Infrared (ATR-IR) spectroscopy was performed with a Thermo-Fisher Nicolet iS20 equipped with a liquid, nitrogen-cooled HgCdTe (MCT) detector using a VeeMax III ATR accessory (Pike Technologies). DEMS was used to monitor volatile gases of H2 produced during battery operation at RT (Hidden HPR40). A UV-Vis spectrophotometer (UV2600, Shimadzu, Japan) was utilized to obtain the operando UV-Vis diffuse reflectance spectra and UV-Vis absorption spectra. Data collected by SEM and EDS with a field emission scanning electron microscope (FEI Quanta 450). Operando Raman spectroscopy for the Ni / C coating layer was determined using a confocal Raman microscope (Horiba LabRAM HR Evolution) with a 50X (1.0 N.A) objective (Olympus). Laser wavelength was 532 nm.

[0113] Molecular Dynamics (MD) simulations

[0114] MD simulations were performed on aqueous electrolytes added with ZnSO4 salt with different anti-solvent volume ratios. Simulations were carried out using an NAMD package. [27-31] The forced field parameters for all particles were obtained from CHARMM36 force fields.

[0032] The TIP3P water model was employed for H2O.

[0033] The time step was set to be 2 fs. The cutoff radius for vdW were 12 Å and the electrostatic interactions was 10 Å. The standard periodic boundary condition was used in all simulations. After minimization of initial structure for 5,000 steps (10 ps), each system was heated from 200K to 300K by performing Langevin dynamics temperature control for 400 ps. The systems were further relaxed for another 100 ps under NPT by Nosé-Hoover Langevin piston pressure control method at 1.01325 bar. After relaxation, the system was simulated for 2 ns under canonical ensemble (NVT) for data collection and statistical analysis. The periodic boundary cell length (Å) is: 31.92(x), 31.92(y), 35.21(z). And the solution model contained a one layer armchair graphene sheet with 540 carbon atoms, 40 ZnSO4molecules, 1110 water molecules, 2 CH3SO3molecules and 2 C8H15N2molecules. The data was presented and analyzed using Visual Molecular Dynamics (VMD)

[0034] .

[0115] Density Function Theory (DFT) calculations

[0116] First-principles calculation was performed with The Vienna Ab Initio Simulation Package (VASP) code.

[0035] The Perdew-Burke-Ernzerhof functional was employed to compute electron exchange-correlation energy.

[0036] Projector Augmented Wave (PAW) potentials were used to describe the ionic cores.

[0037] Atomic relaxations were carried out with the quasi-Newton minimization scheme, until the maximum force on any atom was < 0.02 eV / Å.

[0038] the Becke-Johnson (BJ) damping was usedin the DFT-D3 method

[0039] . Geometry optimizations were performed with a plane-wave cutoff of 520 eV. An irreducible 2×2×2 Monkhorst Pack k-point grid was used,

[0040] with the centre shifted to the gamma point.

[0117] Results and Discussion

[0118] Figure 1a depicts the chemical structure of BMIS which has N heteroatoms embedded within a π-conjugated ring. To confirm the strong capability of BMIS in confining the shuttle effect of I3−, the BMIS solution was added into in KI3 solution to check whether there would be precipitate formation. As demonstrated in Figure 2, upon the addition of BMIS solution, the previously yellow KI3 solution rapidly formed a precipitate, and turned nearly colorless after a resting period of 12 hours. This observation provides persuasive evidence for the effective interaction between BMIS and I3−within the electrolyte. To further support this finding, the charge density difference between BMI+and I3−was investigated, as illustrated in Figure 3b. The charge accumulation is visualised as the yellow region proximate to the I3−ion, while the charge depletion is depicted as the blue region near the N atoms. These results strongly suggest formation of N−I bonds, substantiating the conclusion that an interaction occurs between BMI+and I3−. Moreover, the Attenuated total reflection Fourier transform infrared (ATR−FTIR) was also conducted to investigate the interaction between BMI+and I3−(Figure 4). The stretching mode peaks in the range of 2750 cm−1to 3000which are attributed to the butyl chain [17, 18], showed a redshift. Additionally, the C-H normal mode vibrations in the imidazolium ring exhibited increased intensity at 3083 3106and 3149 cm−1 23. These findings also provide strong evidence of the existence of BMI+−I3−interaction.

[0119] To assess BMIS addictive’s influence on real battery cycling process, the BMIS was incorporated into 2 M aqueous ZnSO4electrolyte with various concentrations. Since the Coulombic efficiency (CE) value at a low current density is one of the most important parameters to evaluate the shuttle effect of I2cathode, thus the CEs of Zn−I2batteries in 2 M ZnSO4with / without BMIS were collected at a low current density of 1 C, as depicted in Figure 1c. In 2 M ZnSO4, the battery only displays an average CE of approximately 97.1 %. In comparison, the CE is improved notably from 97.1 % to 99.9 % when the concentration of BMIS concentration increase from 0.01 M to 0.2 M. As the average CEs for batteries with 0.1 M BMIS and 0.2 M BMIS are about the same, but higher concentration of BMIS will lead to higher electrolyte expenses. Consequently, 0.1 M BMIS electrolyte was selected as an optimal concentration for the present disclosure.

[0120] In addition to the CE, the self−discharge performance can be regarded as another important indicator of the shuttle effect, which was further studied. The results show that the battery using 2 M ZnSO4delivers capacity retentions of 44.35 %, 33.94 %, 26.14 % and 17.28 % after 12 hours, 24 hours, 48 hours and 168 hours duration (Figure 1d, Figure 5 and Figure 6). Remarkably, the batteryincorporating 2 M ZnSO4+0.1 M BMIS maintained a capacity retention of 89.27 % even after 168 hours, demonstrating suppression of the shuttle effects after the addition of BMIS.

[0121] In light of the notable enhancements in CE and a significant reduction in self-discharge exhibited by batteries incorporating the BMIS additive, it can be concluded that the BMIS effectively suppresses the shuttle effect induced by polyiodides during battery cycling. To in-depth study the working mechanism of BMIS, a comprehensive series of operando experiments were meticulously conducted. For the operando ultraviolet-visible spectroscopy (UV−vis) test, a specially designed homemade quartz cell was utilized to facilitate operando UV−vis measurements, as depicted in Figure 7. The results obtained from Figure 3a demonstrate that when employing a 2 M ZnSO4 electrolyte, the cell displayed an evident increase in the absorbance of the I3−signal during the initial discharging process, indicative of substantial I3−dissolution. The concentration of I3−reached approximately 0.008 mM throughout the discharging process after calculated based on the calibration curve in Figure 8. However, upon the introduction of the BMIS additive to the electrolyte, the absorbance of I3−consistently maintained at a significantly low level (~0.0002 mM) throughout the entire discharging process, as illustrated in Figure 3b. This finding demonstrates the strong interaction between I3−with the electrolyte additive of BMIS, leading to the low concentration of free I3−in electrolyte.

[0122] Figures 3c and 3d display the operando Raman spectra recorded during the charge / discharge process of batteries cycled with 2 M ZnSO4and 2 M ZnSO4+0.1 M BMIS electrolytes, respectively. Raman peaks at approximately 110 cm−1and 160 cm−1correspond to I3−and I5−, respectively21

[0019] . With 2 M ZnSO4, the appearance and disappearance of signals for I3−and I5−occur simultaneously during charging and discharging (Figure 3c), indicating the iodine transformation reaction is not following the linear reaction pathway for I−→I3−→I5−→I2. This may be caused by the high solubility of I3−, which results in a rapid conversion of I3−to I5−. Conversely, the interaction between I3−and BMIS make the reaction route strictly follow the linear reaction I−→I3−→I5−→I2, as proved by the delayed appearance of the I5−(after charging to 1.5 V) signal compared to that of I3−(after charge to 1.25 V, Figure 3d). Moreover, the blue shift of I3−peak in the battery system using 2 M ZnSO4+0.1 M BMIS electrolyte also confirms the interaction between I3−and BMIS (Figure 9). Furthermore, Raman spectra were also collected during battery resting to reveal the effects of BMIS additive on self-discharging. During the resting process, I5−will gradually convert back to I3−in the battery employing 2 M ZnSO4. Subsequently, the highly soluble I3−dissolves in the electrolyte and migrates to the Zn anode, leading to significant self−discharge of the battery. Finally, the peak intensities of I3−and I5−decrease in the Raman spectra (Figure 3e). However, with the addition of BMIS, while the peak intensity of I5−continues to decrease, remarkably, the intensity of I3−gradually increases, indicating the migration of I3−has been suppressed by BMIS.

[0123] The impact of BMIS on the cycling stability and the rate capability of Zn−I2 coin cells has also been investigated, in which all specific capacities and current densities were reported based on the mass of I2 in the cells. Benefiting from the suppression of shuttle effect, when testing the battery cycling stability at a ultra-low current density of 0.5 C, the battery with BMIS additive exhibits an excellent long lifespan for over 1000 times with a high average CE of 99.7 % (the CE of the battery using a 2 M ZnSO4 electrolyte is only 92.1 %), as evidenced by Figure 10a and 10b. Compared to the battery employing a 2 M ZnSO4 electrolyte, the battery with BMIS additive exhibits significant improvements in the rate performance, as depicted in Figure 10c and Figure 11. At a high current density of 10 C, the battery with BMIS additive maintains a remarkable discharge capacity of 182.1 mAh g−1, outperforming the ZnSO4−based battery (129.4 mAh g−1). The cycling stability of the BMIS−based battery under higher current densities has also been studied, as illustrated in Figure 10d. Under 10 C, the battery achieves a remarkable capacity retention of 91.57 % after an ultra−long cycling life of over 15,000 cycles, indicating the effectiveness of BMIS in stabilizing the Zn−I2 performance in small-size coin cell.

[0124] An intriguing question arose, that is, considering that the high−rate capability of I2−based batteries is primarily attributed to the soluble I3−, why the interaction between BMIS and I3−that effectively reduces the mobility and solubility of I3−could lead to enhancement of rate performance after adding BMIS to the electrolyte. It would have been expected that the rate performance of batteries should be little compromised by the addition of BMIS. However, compared with the unmodified system, the BMIS-based system shows an enhanced rate performance. This abnormal phenomenon is worth further study.

[0125] There are several factors that could contribute to the enhanced rate performance of the batteries. One potential factor is catalytic effects. Certain carbon-based catalysts can reduce the dissociation energy barrier of polyiodides / iodide conversion, thereby improving the rate performance of batteries

[0020] . However, in this system, BMIS, being an electrochemically inert IL, has not previously been reported to possess any catalytic activity. Therefore, the most likely reason for the improved rate performance could be the alteration of the electrolyte structure resulting from the addition of BMIS.

[0126] As show in Figure 12a, upon dissociation in water, BMIS yields BMI+cations and methyl sulfate anions. After applying a negative electric field to simulate the battery discharge process, BMI+will migrate to the cathode under the electric field, which may lead to accumulation of BMI+at the cathode surface. Operando ATR−FTIR and Raman spectroscopy also proved the accumulation of BMI+at cathode surface during the discharge process. The operando Raman tests were conducted using the cell depicted in Figure 13. The peaks around 650 cm−1and 750attributed to the anti−anti (AA)

[0021] conformation of the imidazolium ring, increase in intensity as the depth of battery discharge increases (Figure 12b). This observation further confirmed the accumulation of BMI+at the I2cathode surface. Additionally, operando ATR−FTIR signals were detected from the surface of the I2 cathode, as shown inFigure 14. Following the discharge process, the peak at around 1085 cm−1, attributed to the in−plane and out−of−plane bending vibrations of C−H of the imidazolium ring, gradually blue shifts (Figure 12c). This also indicates an improvement in BMI+concentration near the cathode's surface during discharge [18, 22].

[0127] The accumulation of BMI+at the cathode surface can create a micro-solvation layer (MSL) with a higher concentration of ionic liquid (IL). As a result, the solubility of iodine in this layer is increased due to the higher solubility of iodine in IL. According to Figure 12d, the fabricated I2 electrode was immersed in an electrolyte with various concentrations of BMIS to investigate the influence of incorporating BMIS on the solubility of I2 in the electrolyte. The relatively low amount of added BMIS does not seem to effectively improve the solubility of I2. However, as discussed above, the BMI+will accumulate on the cathode surface to form a MSL which has a higher portion of IL. This change could improve the solubility of iodine in this layer, therefore, making the transformation of iodine easier. Additionally, operando Raman testing after the first discharge indicated that the formed MSL could be maintained effectively. Figure 15 demonstrates that the peak intensity of the imidazolium ring remained stable during the subsequent charge and discharge cycle, suggesting that the concentration of BMI+at the I2 cathode surface remained relatively stable during cycling. This stability is attributed to the interaction between BMI+and I3−, limiting the mobility of BMI+near the surface of cathode even under a reversed electric field. Furthermore, considering that the quantity of IL in the electrolyte remains constant, the proportion of IL in the MSL increases, while the amount in the bulk electrolyte decreases. As a result, iodine is primarily confined within the MSL rather than dissolving into the bulk electrolyte solution, thereby preventing a loss of active materials.

[0128] The boosted I2conversion speed in Zn−I2batteries by use of the ionic liquid additive was further confirmed through operando Electrochemical Impedance Spectroscopy (EIS). In the EIS spectra, a semicircle in the high−frequency region corresponds to the charge transfer resistance (Rct)

[0023] . As illustrated in Figure 16, the addition of BMIS leads to a reduction in Rctfrom 125 to 60 Ω, indicating a higher conversion speed of I−to I2. Notably, during the discharge process, the difference in Rctfurther increases (75 to 23 Ω), indicating a further acceleration in the iodine conversion rate after the accumulation of BMIS at the cathode surface. Cyclic voltammetry (CV), operando Electrochemical Impedance Spectroscopy (EIS) and the galvanostatic intermittent titration technique (GITT) were used to investigate the reaction kinetic of I2conversion after the formation of MSL (after first discharge). The CV curves of batteries at different sweep rates (ranging from 0.1 to 0.8 mV s−1) support the improved I2conversion rate due to the formation of MSL (Figure 17). The kinetic analysis, based on log i—log ν plots, reveals the slope of the peaks in Figure 17b and Figure 17c. When using a 2 M ZnSO4electrolyte, the battery displays a lower slope (both for reduction and oxidation peaks) compared to that using a 2 M ZnSO4+0.1 M BMIS electrolyte. This indicates that the conversion of I−to I2is in pure ZnSO4electrolyte is lower than that after adding BMIS. To further confirm this assumption, the GITT was employed. TheGITT plots (Figure 18a) were collected at a current density of 1 C, where the capacities were similar. The diffusion coefficient (D) obtained from GITT exhibited a sudden drop between the voltage range of 1.3 V to 1.4 V, which was due to the conversion from liquid I3−to solid I2. However, after adding BMIS, the diffusion coefficient (D) remained stable during the whole charging process, indicating that the BMIS−induced MSL can improve the solubility of the solid state I2 and facilitate a conversion reaction of I2 (Figure 18b and 18c). In summary, the combination of operando Raman spectroscopy, operando ATR−FTIR, operando EIS, CV and GITT provided compelling evidence for the accumulation of BMI+at the I2 cathode surface, the formation of a stable MSL, and its beneficial effects on the rate performance of the batteries.

[0129] In order to prove the effectiveness of BMIS in large scale application, the battery was upscaled to a pouch cell. As shown in Figure 19, the formation of the MSL contributed to the battery's outstanding rate performance, even at a high cathode areal capacity exceeding 3 mAh cm−2. At a high rate of 20 mA cm−2(~8 C), the 0.2 Ah pouch cell still retained 83.5 % of its capacity compared to the capacity at 2 mA cm−2. Furthermore, the pouch cell was further upscaled to a multilayer pouch cell with a capacity of nearly 1 Ah and an areal capacity of 7.5 mAh cm−2. As depicted in Figure 20, the battery polarization of the Ah−level pouch cell remained stable even after 400 cycles at 8.3 mA cm−2(~1.1 C). Additionally, the capacity retention of the Ah−level pouch cell reached an impressive 94.0 % after 750 cycles.

[0130] In the context of practical applications, the Zn depth-of-discharge (DOD) has long been a subject of concern within pure aqueous systems. Thus, it is plausible that BMIS might also confer advantageous effects on the Zn anode. To investigate the impact of BMIS on utilization of zinc anode in a scaled-up system, we conducted experiments using pouch-type Zn−Cu batteries. Scanning electron microscope (SEM) images of cycled Zn metal reveal that Zn metal cycled with 2 M ZnSO4experiences corrosion, resulting in the formation of cracks and holes (Figure 21a). In contrast, the image of Zn metal cycled with 2 M ZnSO4+0.1 M BMIS demonstrates a notably smoother surface (Figure 21b). The morphologies of deposited Zn on the Cu foil in the 2 M ZnSO4 electrolyte and the formulated electrolyte are presented in Figure 22. In Figure 22a and Figure 22b, dendritic and agglomerated Zn protrusions are observed, while in Figure 22c and Figure 22d, a more uniform Zn deposition is evident. The smoother Zn morphologies was attributed to BMIS's preference for adsorption onto the (100) and (101) crystal planes, which impedes the deposition of Zn2+ions on these surfaces [24-26]. This encourages selective Zn2+ion deposition on the (002) plane, suppressing Zn dendrite growth. Supporting this, small-angle X-ray scattering (SAXS) images and patterns (Figure 23 and Figure 24) indicate a significant increase in (002) plane-associated peaks upon BMIS inclusion. Therefore, after the addition of BMIS in the electrolyte, an initial CE of 98.9 % and an average CE of 99.91 % at a current density of 4.0 mA cm−2and areal capacity of 2.0 mA h cm−2were achieved in Zn / Cu pouch cell (Figure 25a and Figure 26).

[0131] Then, pouch cells configuration with high DOD (calculated based on the total capacity of Zn and the real cell capacity) is used for testing. The pouch cells were assembled by stacking a Zn-deposited Cu foil anode, glass fiber separator, and I2 cathode with a BMIS-based electrolyte. Primarily, the self- discharge rate, a critical metric for evaluating large-scale energy storage battery systems, was tested in a practical situation. The lab-made ~0.2 Ah pouch cell with BMIS-based electrolyte demonstrated a self- discharge rate of 11.74 % capacity decay per month at a high DOD of 44.0 %, outperforming mature commercial nickel-metal hydride (NiMH) batteries (20–30 % per month, Figure 25b). Regarding cycling performance, the 0.27 Ah pouch cell, operating at a high DOD of 55.1 %, maintained a capacity retention of 93.5 % after 200 cycles at a lower rate of 1.25 mA cm−2(~0.36 C, Figure 25c). Even with an extremely high Zn DOD of 82.0 %, the pouch cell still maintained a high capacity retention of 94.1 % after 150 cycles. Also, a practical-level pouch cell with a high DOD of 68.3 % provided an areal capacity of 6.5 mAh cm−2with 90.0 % capacity retention after 2,500 cycles at higher rate of 15.9 mA cm−2(~2.4 C) with an average CE over 99.996 %, representing a pinnacle in stable high DOD pouch cell performance (Figure 25e and Table 1). Even in comparison with other commercial or nearly commercial aqueous battery systems, this battery system showcases a substantial advantage including energy density lifespan, cost efficacy, rate capability, and acceptable self−discharge rate (Table 2). Table 1: Comparison of this work with other high Zn utilization rate Zn pouch batteries capacity retention; Cathode areal capacity Battery system Zn DOD h cm- cycle numbers; rate (mA2) (%, cycles, C) 82 5.1 94.1, 150, ~1.2 This work 54.4 3.4 93.5, 200, ~0.36 68.3 6.5 90.0, 2500, ~2.4 3DGs

[0041] 57.5 3.8 ~53, 150, ~10 VVLP

[0042] 74.1 15.7 79.1, 80, 0.10 RME

[0043] 31.3 5.5 ~80, 390, ~0.20 12-C-4

[0044] 45.5 13.0 ~78, 160, 0.16 70SL

[0045] 46.5 2.8 88, 300, 0.20 ZLT-DMC

[0046] 43.5 2.5 ~77, 500, ~0.36 BPE

[0047] 33.3 2.0 83.8, 50, ~0.50Table 2: Comparison of this work with other aqueous batteries Energy Self-discharge Lifespan Rate Cost- Battery system density performance (Cycles) capability efficacy11.74 % per This work 2,500 ~100 9 C High month Li aqueous batteries 18.7 % per 1,000 ~100 3 C Medium

[0016] month 3-20 % per Lead-acid [6] 300-1,000 25-40 0.2 C High month 20–30 % per Ni-MH [6] 200-1,200 50-85 0.5 C Medium month 15–30 % per Ni-Cd

[0016] 1500 45-80 1 C Medium month

[0132] Attaining stable performance in Zn-based pouch cells under practical conditions, encompassing low electrolyte cost, high DOD, and absence of intricate Zn interface modifications, are of great significance. Non-limiting examples above introduces an ionic liquid electrolyte for example BMIS into the economical 2 M ZnSO4 electrolyte, offering a feasible strategy to counter iodine shuttle effects and enhance self-discharge performance in Zn-I2 batteries. This intervention yields an exceptional self- discharge rate of 11.74 % capacity decay per month in pouch cell. Additionally, the creation of MSL during discharging augments iodine solubility at the cathode surface, thereby enhancing battery rate capability even under a commercial level areal capacity loading. Notably, even at a demanding rate of 20 mA cm−2(~8 C), the pouch cell with an areal capacity of 3 mAh cm–2sustains 83.5 % of its capacity compared to its capacity at 2 mA cm−2(~0.8 C). BMIS also promotes selective Zn2+ion deposition on the (002) plane, effectively suppressing Zn dendrite formation. Consequently, an areal capacity of 6.5 mAh cm−2, along with 90.0 % capacity retention after 2,500 cycles at a high DOD of 68.2 %, achieved an average CE of 99.996 %.

[0133] 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.

[0134] 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.

[0135] 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” is intended to cover: a, b, c, a-b, a-c, b-c, and a-b-c.

[0136] 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. An ionic liquid additive, which is used in an electrolyte solution for an electrochemical device and is capable of interacting with polyiodide and / or polybromide species to mitigate shuttle effect for the electrochemical device.

2. The ionic liquid additive according to claim 1, wherein the ionic liquid has at least one heteroatom embedded within a conjugated ring.

3. The ionic liquid additive according to any one of claims 1 to 2, wherein the heteroatom is selected from nitrogen (N), sulfur (S) and oxygen (O).

4. The ionic liquid additive according to any one of claims 1 to 3, wherein the ionic liquid is selected from the group consisting of an imidazolium-based ionic liquid, a pyrazolium-based ionic liquid, an oxazolium-based ionic liquid, a thiazolium-based ionic liquid, a pyridinium-based ionic liquid, and a picolinium-based ionic liquid.

5. The ionic liquid additive according to any one of claims 1 to 4, wherein the ionic liquid comprises an anion selected from the group consisting of halides, hydroxide, carboxylates, sulfites, sulfates, sulfonates, sulfamates, carbonates, nitrate, nitrites, perchlorate, dicyanamide, thiocyanate, sulfonylimides, antimonates, arsenates, phosphates, phosphonates, phosphinates, phosphites, phosphonites, phosphinites, borates, and imides.

6. The ionic liquid additive according to any one of claims 1 to 5, wherein the ionic liquid comprises an anion selected from the group consisting of Cl−, Br−, nitrate (NO3−), sulfate (SO42−), hydrogen sulfate (HSO4−), perchlorate (ClO4−), acetate (CH3COO−), trifluoroacetate (CF3COO−), trichloroacetate (CCl3COO−), tetrafluoroborate (BF4−), hexafluorophosphate (PF6−), hexafluoroantimonate (SbF6−), hexafluoroarsenate (AsF6−), methyl sulfate (CH3SO4−), ethyl sulfate (C2H5SO4−), methanesulfonate (CH3SO3−), trifluoromethanesulfonate (CF3SO3−), perfluorobutanesulfonate (C4F9SO3−), bis(fluorosulfonyl)imide (FSO2)2N–), bis(trifluoromethanesulfonyl)imide (CF3SO2)2N−, heptafluorobutanoate (C3F7COO−), bis(pentafluoroethanesulfonyl)imide (C2F5SO2)2N−.

7. The ionic liquid additive according to any one of claims 1 to 6, wherein the ionic liquid has the general formula (I):wherein each of R1, R2, R3, R4and R5is independently selected from the group consisting of a H atom and an organic substituent having 1 to 20 carbon atoms, for example 1 to 12 carbon atoms or 1 to 8 carbon atoms, L−is a compatible anion.

8. The ionic liquid additive according to claim 7, wherein R2is H, each of R1, R3, R4and R5is independently selected from the group consisting of a H atom and an organic substituent having 1 to 20 carbon atoms.

9. The ionic liquid additive according to claim 8, wherein the organic substituent having 1 to 20 carbon atoms is selected from the group consisting of alkyl, alkoxy, hydroxyalkyl, cycloalkyl, heterocycalkyl, alkenyl, alkynyl, aryl, and heteroaryl.

10. The ionic liquid additive according to any one of claims 1 to 9, wherein the ionic liquid useful as the additive has an organic cation selected from the group consisting of 1-ethyl-3- methylimidazolium, 1-propyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-pentyl-3- methylimidazolium, 1-hexyl-3-methylimidazolium, 1-ethyl-3-vinylimidazolium, 1-octyl-3- methylimidazolium, 1-allyl-3-methylimidazolium, 1-butyl-2-methylpyrazolium, 1-butylpyridinium, 1-pentylpyridinium, 1-hexyl-3-picolinium, 1-hexyl-4-picolinium, and 1-(3-hydroxypropyl)-3- methylimidazolium.

11. The ionic liquid additive according to any one of claims 1 to 10, wherein the ionic liquid useful as the additive is selected from the group consisting of: 1-propyl-3-methylimidazolium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, or nitrate;1-butyl-3-methylimidazolium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-pentyl-3-methylimidazolium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-hexyl-3-methylimidazolium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-octyl-3-methylimidazolium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-ethyl-3-vinylimidazolium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-ethyl-3-methylimidazolium trifluoroacetate, methyl sulfate, trifluoromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-butyl-3-methylimidazolium trifluoroacetate, methyl sulfate, trifluoromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-butyl-2-methylpyrazolium bis(trifluoromethanesulfonyl)imide; 1-butylpyridinium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; 1-pentylpyridinium trifluoroacetate; 1-hexyl-3-picolinium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate; and 1-hexyl-4-picolinium methanesulfonate, methyl sulfate, trifloromethanesulfonate, chloride, tetrafluoroborate, acetate, or nitrate.

12. An electrolyte solution for an electrochemical device, wherein the electrolyte solution comprises the ionic liquid additive according to any one of claims 1 to 11.

13. An electrochemical device comprising an electrolyte solution, wherein the electrolyte solution comprises the ionic liquid additive according to any one of claims 1 to 11.

14. Use of an ionic liquid as an additive for the electrolyte solution to mitigate shuttle effect for an electrochemical device, wherein the ionic liquid additive is the one according to any one of claims 1 to 11.

15. A method of mitigating shuttle effect caused by polyiodide and / or polybromide species for an electrochemical device, wherein the method includes introducing the ionic liquid additive according to any one of claims 1 to 11 into the electrolyte solution of an electrochemical device.

16. The ionic liquid additive, the electrolyte solution, the electrochemical device, the use and the method according to any one of claims 1 to 15, wherein the electrolyte solution comprises the ionic liquid additive in a concentration of 0.01 M to 0.5 M, or 0.06 M to 0.15 M, or 0.07 M to 0.12 M, or 0.08 M to 0.10 M.

17. The ionic liquid additive, the electrolyte solution, the electrochemical device, the use and the method according to any one of claims 1 to 16, wherein the electrolyte solution is water-based.

18. The ionic liquid additive, the electrolyte solution, the electrochemical device, the use and the method according to any one of claims 1 to 17, wherein the electrochemical device comprises a source of iodine and / or bromine as a positive electrode active material within the positive electrode and / or within the electrolyte solution.

19. The ionic liquid additive, the electrolyte solution, the electrochemical device, the use and the method according to any one of claims 1 to 18, wherein the electrochemical device is selected from an aqueous metal-iodine electrochemical device and an aqueous metal-bromine electrochemical device.

20. The ionic liquid additive, the electrolyte solution, the electrochemical device, the use and the method according to any one of claims 1 to 19, wherein the electrochemical device delivers a low self-discharge rate of about 11.7 % capacity loss per month at a depth of discharge of 44.0 % and room temperature; the electrochemical device exhibits a long lifespan with about 94.0 % capacity retention after 750 cycles; the electrochemical device maintains about 83.5 % of its capacity at a high rate of 20 mA cm−2(~6.7 C) at an areal loading over 15 mg cm−2; and / or the electrochemical device exhibits a depth-of-discharge (DOD) of about 68.2 % and a capacity retention of about 90.0 % after 2,500 cycles.

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