WO2019081907A1

WO2019081907A1 - Ionic liquid

Info

Publication number: WO2019081907A1
Application number: PCT/GB2018/053053
Authority: WO
Inventors: Christopher Hardacre; Alex NEALE; Peter Goodrich; Johan JACQUEMIN; Peter Nockemann; Jorge ALVAREZ VICENTE
Original assignee: Johnson Matthey Public Limited Company
Priority date: 2017-10-26
Filing date: 2018-10-23
Publication date: 2019-05-02
Also published as: GB201717576D0

Abstract

Ionic liquids are described which have a very broad electrochemical stability window and which are therefore of high utility for a wide range of electrochemical energy storage device applications.

Description

Ionic Liquid

Related applications

This application claims the priority of UK patent application number 1717576.1 filed on 26 October 2017, the contents of which are incorporated herein by reference in their entirety.

Technical field

The invention relates generally to ionic liquids, and the use of those ionic liquids as an electrolyte or component of an electrolyte in an electrochemical energy storage device, such as a metal-ion battery (e.g. lithium-ion battery), metal-air battery, super capacitor or hybrid capacitor.

Background art

Ionic liquids (IL) are salts in the liquid state. In some cases, ionic liquids are defined as salts which have a melting point below a certain (relatively low) temperature, e.g. 100 °C. In some cases, ionic liquids are "room-temperature ionic liquids" (RTIL), being in the liquid state at room temperature and pressure, i.e. at around 298 K and 100 kPa. They have found use as electrolytes in battery applications, for example in lithium or metal ion batteries. They can be used alone as the electrolyte or can be mixed with other solvents as a component of an electrolyte.

When selecting an electrolyte for use in electrochemical energy storage devices, the electrochemical stability window (ESW) of the electrolyte is an important parameter. The ESW of a substance is the voltage range within which the substance is stable, i.e. neither oxidised nor reduced. In other words, the ESW is the difference between the cathodic limit (the potential at which reduction of the electrolyte takes place) and the anodic limit (the potential at which oxidation of the electrolyte takes place).

In the case of lithium ion battery applications, it is the cathodic and anodic limits of the electrolyte which determine its usefulness. In a lithium-ion battery the anode potential is set by Li/Li⁺, so the cathodic limit of the electrolyte relative to Li/Li⁺ determines whether the electrolyte would be reduced by lithium metal. The anodic limit of the electrolyte then determines the allowable voltage of the cathode.

In capacitor applications, the important factor is not the cathodic or anodic limits themselves, but the breadth of the ESW. So, an electrolyte with a wider ESW allows a wider range of counter-electrodes to successfully operate in a lithium-ion battery, opening up new design possibilities, and provides higher performance capacitors.

JP 2011/037769 (the disclosure of which is incorporated herein by reference) describes the use of phosphonate moieties within the anion of an I L or within a zwitterion.

There is a need for new ILs with wide ESW as well as cathodic limits which make them suitable for use in a lithium-ion battery. This would mean that electrochemical energy storage devices including an electrolyte comprising such an IL could operate at higher voltages than existing devices without degradation of the electrolyte.

Summary of the invention

Any sub-titles herein are included for convenience only, and are not to be construed as limiting the disclosure in any way.

A first aspect of the invention is an ionic liquid comprising an anion and a cation, wherein the cation is a cation according to Formula (I):

(I) wherein Y¹ and Y² are each independently selected from a single bond, O, N, NH and S; Y³ is selected from a single bond and O;

R¹ and R² are each independently selected from linear or branched C1-12 saturated alkyl optionally substituted with one or more groups R^1A;

L is C1-20 linear saturated alkylene optionally substituted with one or more groups R^L; G is selected from N(R³)(R⁴)(R⁵) and P(R⁶)(R⁷)(R⁸); wherein R³ is linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^3A; R⁴ and R⁵ are each independently selected from linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^4A, or R⁴ and R⁵ together with the N to which they are attached form a 4- to 12-membered heterocyclic group, optionally substituted with one or more groups R^4B; R⁶ is linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^6A; and

R⁷ and R⁸ are each independently selected from linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^7A, or R⁷ and R⁸ together with the P to which they are attached form a 4- to 12-membered heterocyclic group, optionally substituted with one or more groups R^7B; wherein R^1A, R^L, R^3A, R^4A, R^4B, R^6A, R^7A and R^7B are each independently selected from saturated C1-3 linear unsubstituted alkyl;

-OH, -0(R^A),

-F, -CI,

-CN, and

wherein R^A is saturated C1-6 linear unsubstituted alkyl and each R^N is independently selected from H and C1-6 linear unsubstituted alkyl.

Such an ionic liquid has a very broad electrochemical stability window and therefore provides a very useful I L for a wide range of electrochemical energy storage device applications. The IL also has a cathodic limit less than 0 V against Li/Li⁺, ensuring that reduction of the I L would not take place during operation of a lithium-ion battery.

Furthermore, the ILs have fire-retardant properties due to the presence of the

phosphonate moiety, and devices which incorporate them are therefore safer to use. In some embodiments, the ESW of the I L is greater than 5 V. In some embodiments, the ESW of the I L is greater than 5.5 V. In some embodiments, the ESW of the I L is greater than 6 V. In some embodiments, the ESW of the I L is greater than 6.5 V. In some embodiments of the first aspect, the cation according to Formula (I) in the ionic liquid is not selected from any of the cations (X1 )-(X7):

A second aspect of the invention is an electrolyte comprising an ionic liquid according to the first aspect.

A third aspect of the invention is an electrochemical energy storage device comprising an electrolyte according to the second aspect.

In some embodiments of the third aspect, the cation according to Formula (I) in the ionic liquid is not selected from (X1). In some embodiments of the third aspect, the cation according to Formula (I) in the ionic liquid is not selected from any of the cations (X1)- (X7).

In some embodiments, the electrochemical energy storage device is selected from a metal-ion battery, a metal-air battery, a super capacitor and a hybrid capacitor. In some embodiments, the electrochemical energy storage device is selected from a lithium-ion battery and a lithium-air battery

In some embodiments, the electrochemical energy storage device is a lithium-ion battery.

A fourth aspect of the invention is a method of preparing an ionic liquid precursor halide salt, comprising the steps of reacting an intermediate according to formula (IV):

with a compound according to formula (V) or formula (VI):

(V) (VI)

wherein

Y\ Y², Y³, R¹ , R², L, R³, R⁴, R⁵, R⁶, R⁷ and R⁸ are as defined in the first aspect; and Hal¹ is selected from F, CI, Br and I.

A fifth aspect is a method of preparing an ionic liquid comprising the step of reacting an ionic liquid halide precursor salt according to formula (VII):

(VII) with a lithium salt comprising an anion and a lithium cation, wherein R¹ , R², Y¹ , Y², Y³, L and G are as defined in the first aspect; and Hal¹ is selected from F, CI, Br and I.

A sixth aspect is a salt comprising an anion and a cation according to formula (I):

(I) wherein R¹ , R², Y¹ , Y², Y³, L and G are as defined in the first aspect.

In some embodiments of the sixth aspect, the cation according to Formula (I) in the salt is not selected from any of the cations (X1)-(X7).

A seventh aspect of the invention is the use of the ionic liquid according to the first aspect in an electrochemical energy storage device.

In some embodiments of the seventh aspect, the cation according to Formula (I) in the ionic liquid is not selected from the cation (X1). In some embodiments of the seventh aspect, the cation according to Formula (I) in the ionic liquid is not selected from any of the cations (X1)-(X7).

An eighth aspect of the invention is the use of the ionic liquid according to the first aspect to provide improved electrochemical stability of an electrolyte in an electrochemical energy storage device or increased anodic limit of an electrolyte in an electrochemical energy storage device.

In some embodiments of the eighth aspect, the cation according to Formula (I) in the ionic liquid is not selected from the cation (X1). In some embodiments of the eighth aspect, the cation according to Formula (I) in the ionic liquid is not selected from any of the cations

(X1)-(X7).

A ninth aspect is the use of a salt according to the sixth aspect in the manufacture of an ionic liquid.

A tenth aspect is an ionic liquid made by a method according to the fifth aspect. Definitions

Substituted: The phrase "optionally substituted" as used herein, pertains to a parent group which may be unsubstituted or which may be substituted. Unless otherwise specified, the term "substituted" as used herein, pertains to a parent group which bears one or more substituents, for example one, two or three substituents. The term

"substituent" is used herein in the conventional sense and refers to a chemical moiety which is covalently attached to, or if appropriate, fused to, a parent group. A wide variety of substituents are well known, and methods for their formation and introduction into a variety of parent groups are also well known.

Alkyl: The term "C1-12 alkyl" as used herein, pertains to a monovalent moiety obtained by removing a hydrogen atom from a carbon atom of a hydrocarbon compound having from 1 to 12 carbon atoms, which may be aliphatic or alicyclic. Examples of saturated alkyl groups include, but are not limited to, methyl (Ci), ethyl (C2), propyl (C3), butyl (C₄), pentyl (C5), hexyl (Ce) and heptyl (C₇).

Examples of saturated linear alkyl groups include, but are not limited to, methyl (Ci), ethyl (C2), n-propyl (C3), n-butyl (C₄), n-pentyl (amyl) (C5), n-hexyl (Ce) and n-heptyl (C₇).

Examples of saturated branched alkyl groups include iso-propyl (C3), iso-butyl (C₄), sec- butyl (C₄), tert-butyl (C₄), iso-pentyl (C5), and neo-pentyl (C5).

Alkylene: The term "C1-20 linear alkylene" as used herein, pertains to a bivalent moiety obtained by removing two hydrogen atoms, one from each terminal carbon atom of a hydrocarbon compound having from 1 to 20 carbon atoms, which may be aliphatic or alicyclic. In the case of methylene, the two hydrogen atoms are removed from the same carbon atom of methane. Examples of saturated linear alkylene groups include, but are not limited to, methylene (Ci), ethylene (C2), n-propylene (C3), n-butylene (C₄), n-pentylene (C5) and n-hexylene (Ce).

Heterocyclic: The term "4- to 12-membered heterocyclic" as used herein, pertains to a monovalent moiety obtained by removing a hydrogen atom from a ring atom of a heterocyclic compound, which moiety has from 4 to 12 ring atoms, of which from 1 to 6 are ring heteroatoms. Preferably, each ring has from 3 to 7 ring atoms, of which from 1 to 4 are ring heteroatoms. More preferably, each ring has from 4 to 7 ring atoms, of which from 1 to 4 are ring heteroatoms. The term "heterocyclic" as used herein does not encompass aromatic groups.

Ionic liquid: The term "ionic liquid" as used herein, refers to a salt (that is, an ionic compound made up of cations and anions) which is in the liquid state. In some cases, the term "ionic liquid" as used herein, refers to a salt which has a melting point below 100°C under standard pressure (100 kPa). In some cases, an ionic liquid is a salt which is in the liquid state under standard conditions of temperature and pressure, i.e. 298 K and 100 kPa (also known as room temperature ionic liquids, "RTILs"). When an ionic liquid is specified herein simply by reference to the cation, it will be understood by the skilled person that a suitable counter-anion is also present in the compound. Suitable counter-anions which may be combined with the specified cations to form an ionic liquid will be apparent to the skilled person.

Further options and preferences

Ionic liquid

The groups Y¹ and Y²

In some embodiments, Y¹ is O. In some embodiments, Y² is O. In some embodiments, Y¹ and Y² are each O. In some embodiments, one of Y¹ and Y² is a single bond and the other is O.

In some embodiments, Y¹ is N. In some embodiments, Y² is N. In some embodiments, Y¹ and Y² are each N. In some embodiments, one of Y¹ and Y² is a single bond and the other is N.

In some embodiments, Y¹ is NH. In some embodiments, Y² is NH. In some

embodiments, Y¹ and Y² are each NH. In some embodiments, one of Y¹ and Y² is a single bond and the other is NH.

In some embodiments, Y¹ is S. In some embodiments, Y² is S. In some embodiments, Y¹ and Y² are each S. In some embodiments, one of Y¹ and Y² is a single bond and the other is S. The group Y³

In some embodiments, Y³ is a single bond. In other embodiments, Y³ is O. The groups R¹ and R²

In some embodiments, R¹ and R² are each independently selected from linear saturated C1-12 alkyl optionally substituted with one or more groups R^1A. In some embodiments, R¹ and R² are each independently selected from linear unsubstituted saturated C1-12 alkyl. In some embodiments, R¹ and R² are each independently selected from linear unsubstituted saturated C1-6 alkyl. In some embodiments, R¹ and R² are each independently selected from linear unsubstituted saturated C2-4 alkyl.

In some embodiments, R¹ and R² are each independently selected from linear saturated C1-10 alkyl optionally substituted with one or more groups R^1A, such as linear saturated Ci- 8 alkyl optionally substituted with one or more groups R^1A, such as linear saturated C2-6 alkyl optionally substituted with one or more groups R^1A, such as linear saturated C2-4 alkyl optionally substituted with one or more groups R^1A.

In some embodiments, one of R¹ and R² is methyl or ethyl, and the other is selected from linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^1A.

In some embodiments, one of R¹ and R² is methyl or ethyl, and the other is selected from linear saturated C1-12 alkyl optionally substituted with one or more groups R^1A.

In some embodiments, one of R¹ and R² is methyl or ethyl, and the other is selected from linear saturated C1-12 unsubstituted alkyl.

In some embodiments, one of R¹ and R² is methyl or ethyl, and the other is selected from linear saturated CMO unsubstituted alkyl. In some embodiments, one of R¹ and R² is methyl or ethyl, and the other is selected from linear saturated C2-6 unsubstituted alkyl.

In some embodiments, R¹ and R² are each independently selected from methyl, ethyl and n-propyl. In some embodiments, R¹ and R² are each independently selected from ethyl and n-propyl. In some embodiments, R¹ and R² are each ethyl. The group R

R^1A is selected from

saturated C1-3 linear unsubstituted alkyl;

-OH, -0(R^A),

-F, -CI,

-CN, and wherein R^A is saturated C1-3 linear unsubstituted alkyl.

In some embodiments, R^1A is selected from

-OH, -0(R^A),

-F, -CI,

-CN, and wherein R^A is saturated C1-3 linear unsubstituted alkyl.

In some embodiments, R^1A is selected from

-OH, -0(R^A),

-F and -CI,

wherein R^A is saturated C1-3 linear unsubstituted alkyl.

In some embodiments, when R^1A is -0(R^A), R^A is methyl.

The group L

In some embodiments, L is saturated C3-20 linear alkylene optionally substituted with one or more groups R^L. In some embodiments, L is saturated C3-10 linear alkylene optionally substituted with one or more groups R^L. In some embodiments, L is saturated C3-6 linear alkylene optionally substituted with one or more groups R^L. In some embodiments, L is saturated C3-4 linear alkylene optionally substituted with one or more groups R^L.

In some embodiments, L is unsubstituted saturated C3-20 linear alkylene. In some embodiments, L is unsubstituted saturated C3-10 linear alkylene. In some embodiments, L is unsubstituted saturated C3-6 linear alkylene. In some embodiments, L is selected from n-propylene, n-butylene and n-pentylene. In some embodiments, L is selected from n-propylene and n-butylene. In some

embodiments, L is n-propylene. The group R^L

R^L is selected from

saturated C1-3 linear unsubstituted alkyl;

-OH, -0(R^A),

-F, -CI,

-CN, and wherein R^A is saturated C1-3 linear unsubstituted alkyl.

In some embodiments, R^L is selected from saturated C1-3 linear unsubstituted alkyl. In some embodiments, R^L is selected from methyl and ethyl. In some embodiments, R^L is methyl.

In some embodiments, when R^L is -0(R^A), R^A is methyl. The group G

It will be understood that the group G includes a net positive charge, for example provided by an atom which carries a positive charge, or a delocalised positive charge, and the group G is therefore a cationic moiety. In some embodiments, G is N(R³)(R⁴)(R⁵). In other words, in some embodiments G is a group containing a quaternary ammonium ion. In other embodiments, G is P(R⁶)(R⁷)(R⁸). In other words, in some embodiments G is a group containing a quaternary phosphonium ion. When G is N(R³)(R⁴)(R⁵), R³ is linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^3A; and R⁴ and R⁵ are each independently selected from linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^4A, or R⁴ and R⁵ together with the N to which they are attached form a 4- to 12-membered heterocyclic group, optionally substituted with one or more groups R^4B. In some embodiments R³ is selected from linear saturated C1-12 alkyl, for example C1-8 alkyl, C1-6 alkyl or C1-3 alkyl, optionally substituted with one or more groups R^3A.

In some embodiments R³ is selected from linear unsubstituted saturated C1-12 alkyl, for example C1-8 alkyl, C1-6 alkyl or C1-3 alkyl.

In some embodiments, R³ is selected from methyl and ethyl. In some embodiments, R³ is methyl. In some embodiments, R⁴ and R⁵ are each independently selected from linear saturated C1-12 alkyl, for example C1-8 alkyl, C1-6 alkyl or C1-3 alkyl, optionally substituted with one or more groups R^4A.

In some embodiments R⁴ and R⁵ are each independently selected from linear

unsubstituted saturated C1-12 alkyl, for example C1-8 alkyl, C1-6 alkyl or C1-3 alkyl.

In some embodiments R⁴ and R⁵ are each independently selected from methyl and ethyl.

The 4- to 12-membered heterocyclic group contains at least one heteroatom (i.e. at least the nitrogen atom of N(R³)(R⁴)(R⁵)). In some embodiments, the 4- to 7-membered heterocyclic group contains up to three heteroatoms selected from N, O and S. In some embodiments, the 4- to 7-membered heterocyclic group contains up to two heteroatoms selected from N, O and S. In some embodiments, the 4- to 7-membered heterocyclic group contains only one heteroatom (i.e. only the nitrogen atom of N(R³)(R⁴)(R⁵)).

In some embodiments, R⁴ and R⁵ together with the N to which they are attached form a 5- to 10-membered heterocyclic group, optionally substituted with one or more groups R^4B. In some embodiments, R⁴ and R⁵ together with the N to which they are attached form a 5- to 7-membered heterocyclic group, optionally substituted with one or more groups R^4B. In some embodiments, R⁴ and R⁵ together with the N to which they are attached form a 5- or 6-membered heterocyclic group, optionally substituted with one or more groups R^4B.

In some embodiments, the heterocyclic group is substituted with one or two groups R^4B. In some embodiments, the heterocyclic group is substituted with one group R^4B. In some embodiments, the heterocyclic group is unsubstituted. In some embodiments, R⁴ and R⁵ together with the N to which they are attached form a 4- to 12-membered unsubstituted heterocyclic group. In some embodiments, R⁴ and R⁵ together with the N to which they are attached form a 5- to 10-membered unsubstituted heterocyclic group. In some embodiments, R⁴ and R⁵ together with the N to which they are attached form a 5- to 7-membered unsubstituted heterocyclic group. In some embodiments, R⁴ and R⁵ together with the N to which they are attached form a 5- or

6- membered unsubstituted heterocyclic group.

In some embodiments, R⁴ and R⁵ together with the N to which they are attached form a 4- to 12-membered unsubstituted heterocyclic group containing a single heteroatom. In some embodiments, R⁴ and R⁵ together with the N to which they are attached form a 5- to 10-membered unsubstituted heterocyclic group containing a single heteroatom. In some embodiments, R⁴ and R⁵ together with the N to which they are attached form a 5- to

7- membered unsubstituted heterocyclic group containing a single heteroatom. In some embodiments, R⁴ and R⁵ together with the N to which they are attached form a 5- or

6-membered unsubstituted heterocyclic group containing a single heteroatom. In some embodiments, R⁴ and R⁵ together with the N to which they are attached form a heterocyclic group selected from azetidinyl, pyrrolidinyl, piperidinyl, azepanyl and azocanyl. In some embodiments, R⁴ and R⁵ together with the N to which they are attached form a heterocyclic group selected from pyrrolidinyl, piperidinyl and azepanyl.

In some embodiments, R³ is selected from linear saturated C1-3 unsubstituted alkyl, and R⁴ and R⁵ together with the N to which they are attached form a 5 to 7-membered heterocyclic group optionally substituted with one or more groups R^4B.

When G is P(R⁶)(R⁷)(R⁸), R⁶ is linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^6A; and R⁷ and R⁸ are each independently selected from linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^7A, or R⁷ and R⁸ together with the P to which they are attached form a 4- to 12-membered heterocyclic group, optionally substituted with one or more groups R^7B.

In some embodiments R⁶ is selected from linear saturated C1-12 alkyl, for example C1-8 alkyl, C1-6 alkyl or C1-3 alkyl, optionally substituted with one or more groups R^6A. In some embodiments R⁶ is selected from linear unsubstituted saturated C1-12 alkyl, for example C1-8 alkyl, C1-6 alkyl or C1-3 alkyl. In some embodiments, R⁶ is selected from methyl and ethyl. In some embodiments, R⁶ is methyl. In some embodiments, R⁷ and R⁸ are each independently selected from linear saturated C1-12 alkyl, for example C1-8 alkyl, C1-6 alkyl or C1-3 alkyl, optionally substituted with one or more groups R^7A.

In some embodiments R⁷ and R⁸ are each independently selected from linear

In some embodiments each of R⁶, R⁷ and R⁸ are independently selected from linear unsubstituted saturated C1-12 alkyl, for example C2-8 alkyl. In some embodiments R⁷ and R⁸ are each independently selected from methyl and ethyl. In some embodiments, R⁶ and R⁷ are each methyl, and R⁸ is selected from methyl and ethyl.

The 4- to 12-membered heterocyclic group contains at least one heteroatom (i.e. at least the phosphorus atom of P(R⁶)(R⁷)(R⁸)). In some embodiments, the 4- to 7-membered heterocyclic group contains up to three heteroatoms selected from P, N, O and S. In some embodiments, the 4- to 7-membered heterocyclic group contains up to two heteroatoms selected from P, N, O and S. In some embodiments, the 4- to 7-membered heterocyclic group contains only one heteroatom (i.e. only the phosphorus atom of P(R⁶)(R⁷)(R⁸)).

In some embodiments, R⁷ and R⁸ together with the P to which they are attached form a 5- to 10-membered heterocyclic group, optionally substituted with one or more groups R^7B. In some embodiments, R⁷ and R⁸ together with the P to which they are attached form a 5- to 7-membered heterocyclic group, optionally substituted with one or more groups R^7B. In some embodiments, R⁷ and R⁸ together with the P to which they are attached form a 5- or 6-membered heterocyclic group, optionally substituted with one or more groups R^7B.

In some embodiments, the heterocyclic group is substituted with one or two groups R^7B. In some embodiments, the heterocyclic group is substituted with one group R^7B. In some embodiments, the heterocyclic group is unsubstituted. In some embodiments, R⁷ and R⁸ together with the P to which they are attached form a 4- to 12-membered unsubstituted heterocyclic group. In some embodiments, R⁷ and R⁸ together with the P to which they are attached form a 5- to 10-membered unsubstituted heterocyclic group. In some embodiments, R⁷ and R⁸ together with the P to which they are attached form a 5- to 7-membered unsubstituted heterocyclic group. In some embodiments, R⁷ and R⁸ together with the P to which they are attached form a 5- or 6-membered unsubstituted heterocyclic group. In some embodiments, R⁷ and R⁸ together with the P to which they are attached form a 4- to 12-membered unsubstituted heterocyclic group containing a single heteroatom. In some embodiments, R⁷ and R⁸ together with the P to which they are attached form a 5- to 10-membered unsubstituted heterocyclic group containing a single heteroatom. In some embodiments, R⁷ and R⁸ together with the P to which they are attached form a 5- to 7-membered unsubstituted heterocyclic group containing a single heteroatom. In some embodiments, R⁷ and R⁸ together with the P to which they are attached form a 5- or 6-membered unsubstituted heterocyclic group containing a single heteroatom.

In some embodiments, R⁷ and R⁸ together with the P to which they are attached form a heterocyclic group selected from phospholanyl and phosphinanyl.

The group R^3A

R^3A is selected from

saturated C1-3 linear unsubstituted alkyl;

-OH, -0(R^A),

-F, -CI,

-CN, and wherein R^A is saturated C1-6 linear unsubstituted alkyl and each R^N is independently selected from H and C1-6 linear unsubstituted alkyl.

In some embodiments, R^3A is selected from saturated C1-3 linear unsubstituted alkyl. In some embodiments, R^3A is selected from methyl and ethyl. In some embodiments, R^3A is methyl. In some embodiments, when R^3A is -0(R^A), R^A is saturated C1-3 linear unsubstituted alkyl, preferably methyl.

The group R^4A

R^4A is selected from

saturated C1-3 linear unsubstituted alkyl;

-OH, -0(R^A),

-F, -CI,

-CN, and wherein R^A is saturated C1-3 linear unsubstituted alkyl and each R^N is independently selected from H and C1-6 linear unsubstituted alkyl.

In some embodiments, R^4A is selected from

-OH, -0(R^A),

-F, -CI,

In some embodiments, when R^4A is -0(R^A), R^A is saturated C1-3 linear unsubstituted alkyl, preferably methyl. The group R^4B

R^4B is selected from

saturated C1-3 linear unsubstituted alkyl;

-OH, -0(R^A),

-F, -CI,

-CN, and wherein R^A is saturated C1-6 linear unsubstituted alkyl and each R^N is independently selected from H and C1-6 linear unsubstituted alkyl. In some embodiments, R^4B is selected from saturated C1-3 linear unsubstituted alkyl. In some embodiments, R^4B is selected from methyl and ethyl. In some embodiments, R^4B is methyl. In some embodiments, when R^4B is -0(R^A), R^A is saturated C1-3 linear unsubstituted alkyl, preferably methyl.

The group R^6A

R^6A is selected from

saturated C1-3 linear unsubstituted alkyl;

-OH, -0(R^A),

-F, -CI,

In some embodiments, R^6A is selected from saturated C1-3 linear unsubstituted alkyl. In some embodiments, R^6A is selected from methyl and ethyl. In some embodiments, R^6A is methyl.

In some embodiments, when R^6A is -0(R^A), R^A is saturated C1-3 linear unsubstituted alkyl, preferably methyl. The group R^7A

R^7A is selected from

saturated C1-3 linear unsubstituted alkyl;

-OH, -0(R^A),

-F, -CI,

-CN, and wherein R^A is saturated C1-6 linear unsubstituted alkyl and each R^N is independently selected from H and C1-6 linear unsubstituted alkyl. In some embodiments, R^7A is selected from

-OH, -0(R^A), -F, -CI,

In some embodiments, when R^7A is -0(R^A), R^A is saturated C1-3 linear unsubstituted alkyl, preferably methyl. The group R^7B

R^7B is selected from

saturated C1-3 linear unsubstituted alkyl;

-OH, -0(R^A),

-F, -CI,

-CN, and wherein R^A is saturated C1-6 linear unsubstituted alkyl and each R^N is independently selected from H and C1-6 linear unsubstituted alkyl. In some embodiments, R^7B is selected from saturated C1-3 linear unsubstituted alkyl. In some embodiments, R^7B is selected from methyl and ethyl. In some embodiments, R^7B is methyl.

In some embodiments, when R^7B is -0(R^A), R^A is saturated C1-3 linear unsubstituted alkyl, preferably methyl.

Further preferences

In some embodiments,

Y¹ and Y² are each independently selected from a single bond and O;

Y³ is selected from a single bond and O;

G is N(R³)(R⁴)(R⁵), wherein R³ is linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^3A; and R⁴ and R⁵ together with the N to which they are attached form a 4- to 12-membered heterocyclic group, optionally substituted with one or more groups R^4B;

R¹ and R² are each independently selected from linear or branched saturated

unsubstituted C1-3 alkyl; and L is saturated C1-20 linear alkylene optionally substituted with one or more groups R^L.

In some embodiments,

Y¹ and Y² are each O;

Y³ is a single bond;

R¹ and R² are each independently selected from linear or branched saturated

unsubstituted C1-3 alkyl; and

L is saturated C1-20 linear alkylene optionally substituted with one or more groups R^L.

In some embodiments,

Y¹ and Y² are each independently selected from a single bond and O;

Y³ is selected from a single bond and O;

G is selected from N(R³)(R⁴)(R⁵) and P(R⁶)(R⁷)(R⁸);

wherein R³ is linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^3A;

R⁴ and R⁵ are each independently selected from linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^4A;

R⁶ is linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^6A;

R⁷ and R⁸ are each independently selected from linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^7A;

R¹ and R² are each independently selected from linear or branched saturated C3-12 alkyl optionally substituted with one or more groups R^1A, and methyl;

L is saturated C1-20 linear alkylene optionally substituted with one or more groups R^L. In some embodiments,

Y¹ and Y² are each independently selected from a single bond and O;

Y³ is selected from a single bond and O;

G is selected from N(R³)(R⁴)(R⁵) and P(R⁶)(R⁷)(R⁸);

wherein R³ is linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^3A; R⁴ and R⁵ are each independently selected from linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^4A;

R¹ and R² are each independently selected from linear or branched saturated C3-12 alkyl optionally substituted with one or more groups R^1A;

In some embodiments,

Y¹ and Y² are each independently selected from a single bond and O;

Y³ is selected from a single bond and O;

G is selected from N(R³)(R⁴)(R⁵) and P(R⁶)(R⁷)(R⁸);

R¹ and R² are each independently methyl;

In some embodiments,

Y¹ and Y² are each independently selected from a single bond and O;

Y³ is selected from a single bond and O;

G is selected from N(R³)(R⁴)(R⁵) and P(R⁶)(R⁷)(R⁸);

provided that at least two of R³, R⁴ and R⁵ are different from one another;

R⁶ is linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^6A; R⁷ and R⁸ are each independently selected from linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^7A;

provided that at least two of R⁶, R⁷ and R⁸ are different from one another;

R¹ and R² are each independently selected from linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^1A;

In some embodiments,

Y¹ and Y² are each independently selected from a single bond and O;

Y³ is selected from a single bond and O;

G is N(R³)(R⁴)(R⁵);

In some embodiments,

Y¹ and Y² are each independently selected from a single bond and O;

Y³ is selected from a single bond and O;

G is N(R³)(R⁴)(R⁵);

wherein R³ is methyl;

The anion

One aspect of the invention is any salt comprising a cation according to formula (I) and an anion. Such a salt may be in any physical state (for example, solid, liquid or solution) and may have any physical structure. Such salts may be useful as precursors in the manufacture of an ionic liquid according to the invention, or may themselves be an ionic liquid. Another aspect of the invention is an ionic liquid comprising a cation according to formula (I) and an anion.

It will be understood that the anion is a moiety carrying a net negative charge. The anion may carry a single negative charge or may be more highly charged. For example, the anion may carry a net charge of (-1), (-2), (-3) or (-4). In some embodiments, the anion carries a net charge of (-1) or (-2). In some embodiments, the anion carries a net charge of (-1). In some embodiments, the anion represents a single type of anion, i.e. the ionic liquid contains only one type of anion. In other embodiments, the anion represents more than one type of anion, i.e. the ionic liquid contains more than one type of anion.

The anion in the ionic liquid of the first aspect may be selected from any suitable anionic species which, when combined with the cation according to formula (I) to produce a salt, provides an ionic liquid as defined herein. The skilled person will be able to select a suitable anion for this purpose.

For example, the anion may be selected from one or more of acetate (CH3CO2^"), F6^", BF4-, triflate (CF3SO3 ), nonaflate (CF₃(CF₂)₃S0₃ ^"), bis(triflyl)amide ((CF₃S0₂)₂N^"), trifluoroacetate (CF3CO2^"), heptafluorobutanoate (CF₃(CF₂)2C02^"), AICU^", bistriflimide (TFSI; [(CF₃S0₂)₂N]-), 2,2,2-trifluoro-/V-(trifluoromethylsulfonyl)acetamide (TSAC;

CF₃S0₂NCOCF₃-), bis(perfluoroethylsulfonyl)imide (BETI; [(C₂F₅S0₂)₂N]-),

bis(fluorosulfonyl)imide (FSI; [(FS0₂)₂N]^"), fluoroalkyl phosphate (FAP; PF₃(C₂F₅)₃), 4,5- dicyano-2-(trifluoromethyl) imidazolide (TDI; CF₃-CN₂C₂(CN)₂) and asymmetric imides such as fluorosulfonyltriflimide (FTFSI; [(CF₃S0₂)(FS0₂)N]^") and [(C₂F₅S0₂)(FS0₂)N]-).

In some embodiments, the anion is selected from TFSI^", FSI^", PF6^" and BF₄ ^". The anion in the salt according to the fifth aspect may be selected from any anionic species which forms a salt with the cation according to formula (I). In some embodiments the anion is selected from one or more of halide (F^", CI^", Br, I^"), hydroxide (OH^"), peroxide (0₂ ^2"), sulphide (S^2"), hydrogen sulphide (HS^"), selenide (Se^2"), nitride (N^3"), azide (N₃ ^"), phosphide (P^3"), arsenide (As^3"), carbide (C^4"), cyanide (CIST), sulfate (S0₄ ^"), sulfite (S0₃ ^2" ), hydrogen sulfate (HS0₄ ^"), thiosulfate (S₂0₃ ^2"), carbonate (C0₃ ^2") , bicarbonate (HCO3 ), phosphate (P0₄ ^3"), phosphite (P0₃ ^3"), hydrogen phosphate (HP0₄ ^2"), dihydrogen phosphate (H2PO4^"), oxalate (C2O4^2"), cyanate (NCO^"), isocyanate (OCN^"), thiocyanate (SCN^"), chromate (Cr0₄ ^2_), dichromate (Cr20₇ ^2"), permanganate (Mn04^"), nitrate (NO3^"), nitrite (NO2 ), acetate (CH3CO2 ), PFe^", BF₄ ^", triflate (CF3SO3 ), nonaflate (CF₃(CF₂)3S0₃-), bis(triflyl)amide ((CF3S02)2N^"), trifluoroacetate (CF3CO2^"), heptafluorobutanoate

(CF₃(CF₂)2C0₂-), AICI4-, bistriflimide (TFSI; [(CFsSC^N]^"), 2,2,2-trifluoro-/V- (trifluoromethylsulfonyl)acetamide (TSAC; CF3SO2NCOCF3^"),

bis(perfluoroethylsulfonyl)amide (BETI; [(C2FsS02)2N]^"), bis(fluorosulfonyl)imide (FSI; [(FS0₂)₂N]-), fluoroalkyl phosphate (FAP; PF₃(C₂F₅)3), 4,5-dicyano-2-(trifluoromethyl) imidazolide (TDI; CF3-CN2C2(CN)2) and asymmetric imides such as fluorosulfonyltriflimide (FTFSI; [(CF₃S0₂)(FS0₂)N]-) and [(C₂F₅S02)(FS02)N]-). Other anions will be apparent to the skilled person.

In some embodiments, the anion in the salt according to the fifth aspect may be selected from halide (F^", CI^", Br or I^"). In some embodiments, the anion in the salt according to the fifth aspect is Br.

The cation according to formula (I)

In some embodiments, IA):

(IA) wherein n, m and p are each an integer independently selected from 1 to 12, L^A is a saturated C3-10 linear unsubstituted alkylene group and s and t are each an integer independently selected from 1 to 3.

In some embodiments, n is an integer independently selected from 1 to 4 and m and p are each an integer independently selected from 1 to 12. In some embodiments, n and m are each an integer independently selected from 1 to 4 and p is an integer independently selected from 1 to 12. In some embodiments, n, m and p are each an integer independently selected from 1 to 8. In some embodiments, n, m and p are each an integer independently selected from 2 to 4.

In some embodiments, L^A is a saturated C3-6 linear unsubstituted alkylene group. In some embodiments, L^A is n-propylene [-(CH2)3-]. In some embodiments, L^A is n-butylene KCH₂)₄-].

In some embodiments, s and t are each an integer independently selected from 1 and 2. In some embodiments, s and t are each 1.

In some embodiments, the cation is a cation according to formula (IB):

(IB)

wherein n, m and p are each an integer independently selected from 1 to 12, L^B is a saturated C3-10 linear unsubstituted alkylene group and s and t are each an integer independently selected from 1 to 3. In some embodiments, n is an integer independently selected from 1 to 4 and m and p are each an integer independently selected from 1 to 12. In some embodiments, n and m are each an integer independently selected from 1 to 4 and p is an integer independently selected from 1 to 12. In some embodiments, n, m and p are each 8.

In some embodiments, n, m and p are each an integer independently selected from 1 to 8. In some embodiments, n, m and p are each an integer independently selected from 2 to 4. In some embodiments, L^B is a saturated C3-6 linear unsubstituted alkylene group. In some embodiments, L^B is n-propylene [-(CH2)3-]. In some embodiments, L^B is n-butylene

In some embodiments, the cation is a cation according to formula (IC):

(IC) wherein n is an integer independently selected from 1 to 12, L^c is a saturated C3-10 linear unsubstituted alkylene group, s and t are each an integer independently selected from 1 to 3, and R¹³ and R¹⁴, together with the N atom to which they are attached, form a 4-7 membered unsubstituted heterocyclic group.

In some embodiments, n is an integer independently selected from 1 to 8, for example 1 to 4, for example 2 to 4.

In some embodiments, L^c is a saturated C3-6 linear unsubstituted alkylene group. In some embodiments, L^c is n-propylene [-(CH2)3-]. In some embodiments, L^c is n-butylene KCH₂)₄-].

The heterocyclic group formed by R¹³ and R¹⁴ includes at least one heteroatom (i.e., at least the N atom of the group NR¹³R¹⁴). In some embodiments, the heterocyclic group formed by R¹³ and R¹⁴ includes only one heteroatom (i.e., only the N atom of the group NR¹³R¹⁴). In some embodiments, the heterocyclic group formed by R¹³ and R¹⁴ includes 1 , 2 or 3 additional heteroatoms (for example 1 or 2 additional heteroatoms) each independently selected from N, O and S.

In some embodiments, R¹³ and R¹⁴, together with the N atom to which they are attached, form a 4-6 membered unsubstituted heterocyclic group.

In some embodiments, the cation according to formula (I) is selected from:

Ionic liquids

The ionic liquid according to the invention is made up of cations and anions, in stoichiometric proportions which balance the overall charge of the ionic liquid. The ionic liquid comprises at least one type of cation according to formula (I). The ionic liquid may comprise only one type of cation according to formula (I), or may comprise two or more types of cation according to formula (I). In addition to the one or more types of cation according to formula (I), in some embodiments the ionic liquid comprises one or more other types of cation not encompassed by formula (I). These other cations may be selected from any cation suitable for use in an ionic liquid. Non-limiting examples include imidazolium cations (such as 1-ethyl-3-methylimidazolium (EMIM⁺), 1-butyl-3- methylimidazolium (BMIM⁺), 1 ,2-dimethyl-3-propylimidazolium (Me₂Prlm⁺), 1 ,2-diethyl- 3,4-dimethylimidazolium (Et2Me2lm⁺), 1-methyl-3-alkylimidazolium (MeRlm⁺), 1 ,2- dimethyl-3-butylimidazolium (BuMe2lm⁺), 1-cyanomethyl-3-methylimidazolium (CMMelm⁺) and 1-cyanopropyl-3-methylimidazolium (CPMelm⁺)), pyridinium cations (such as /V-butyl- 4-methylpyridinium (BuMePy⁺), A/-methyl-/\/-propylpyridinium (MePrPy⁺) and N- butylpyridinium (BuPy⁺)), tetraalkylammonium cations (such as A/,/\/-diethyl-/\/-methyl-/\/- (2-methoxyethyl)ammonium (Et2MeMeON⁺), trimethylhexylammonium (Me3HexN⁺), N- cyanomethyl-/\/,A/,A/-trimethylammonium (CMMe₃N⁺),/\/-cyanoethyl-/\/,A/,A/- trimethylammonium (CEMesNT), trimethylpropylammonium (Me3PrN⁺), A/-methyl-/\/,A/- diethyl-/V-(methoxyethylene)ammonim (MeEt2MetoxyEtN⁺) and tetraamylammonium (Am4N⁺)), pyrazolium cations such as A/,A/-diethyl-3-methylpyrazolium, pyrrolidinium cations such as A/-methyl-/V-propylpyrrolidinium, A/-n-butyl-/\/-ethylpyrrolidinium and A/-n-butyl-A/-methylpyrrolidinium, piperidinium cations such as /V-methyl-A/- propylpiperidinium and A/-butyl-A/-methylpiperidinium, and dications such as 1-(3- methylimidazolium-l-alkyl-(trimethylammonium). Alternatively, these other cations may be selected from any other cation such as metal cations. In some embodiments where the ionic liquid comprises one or more types of cation according to formula (I) and one or more other types of cation, the cation according to formula (I) is present in an amount of at least 1 mol%, with respect to the total amount of cation in the ionic liquid, for example at least 5 mol%, at least 10 mol%, at least 15 mol%, at least 20 mol% or at least 25 mol%. In this way an ionic liquid is provided the cations of which comprise at least some cations according to formula (I) and the benefits of the invention such as improved ESW and fire retardancy are provided. In such embodiments the cation according to formula (I) may be present in an amount of up to 100 mol% with respect to the total amount of cation in the ionic liquid, for example up to 99.9 mol%, up to 99 mol%, up to 95 mol% or up to 90 mol%.

In some embodiments, the cation of the ionic liquid according to the invention consists of one or more cations according to formula (I), i.e. no other types of cation are present in the ionic liquid (the cation according to formula (I) may be present in an amount of 100 mol% with respect to the total amount of cation in the ionic liquid). In some embodiments, the cation of the ionic liquid according to the invention consists of one type of cation according to formula (I). The ionic liquid comprises at least one type of anion. The ionic liquid may comprise only one type of anion (i.e. all anions in the ionic liquid being identical), or may comprise two or more types of anion (i.e. the ionic liquid comprises a mixture of non-identical anions). Where more than one type of anion is present in the ionic liquid, each type of anion may carry the same charge or different charges. Where more than one type of anion is present in the ionic liquid, they may be used in equal or different stoichiometric quantities.

For example, the ionic liquid may comprise one, two, three, four or five types of cation, each being a cation according to formula (I), along with one, two, three, four or five types of counter-anion. In some embodiments, the ionic liquid comprises, or consists of, a single type of cation according to formula (I) and a single type of counter-anion.

In some embodiments, the ionic liquid has a melting point (at a pressure of 100 kPa) of below 150 °C, for example below 140 °C, below 130 °C, below 120 °C, below 1 10 °C, below 100 °C, below 90 °C, below 80 °C, below 70 °C, below 60 °C, below 50 °C, below 40 °C below 30 °C or below 20 °C. In this way, it can be ensured that the ionic liquid remains in the liquid state during use of an electrochemical energy storage device. In some embodiments, the ionic liquid has a melting point (at a pressure of 100 kPa) of at least -20 °C (i.e., -20 °C or above, in an absolute sense), for example at least -15 °C, at least -10 °C, at least -5 °C, at least 0 °C, at least 5 °C or at least 10 °C.

In some embodiments, the melting point of the ionic liquid (at a pressure of 100 kPa) is from 0 °C to 21 °C, for example from 0 °C to 20 °C, from 0 °C to 19 °C, from 0 °C to 18 °C, from 0 °C to 17 °C, from 0 °C to 16 °C or from 0 °C to 15 °C.

In some embodiments, the electrochemical stability window (ESW) of the IL is greater than 5 V, for example, greater than 5.5 V, greater than 6 V or greater than 6.5 V. In some embodiments, the cathodic limit of the IL measured against Li/Li⁺ is less than 0 V, for example less than -0.1 V, less than -0.2 V, less than -0.3 V, less than -0.4 V or less than -0.5 V.

Ionic liquids and precursor salts

An aspect of the invention is a salt comprising one or more types of cation according to formula (I) along with a counter-anion. In some embodiments, the counter-anion represents a single type of anion, i.e. the salt contains only one type of anion. In other embodiments, the counter-anion represents more than one type of anion, i.e. the salt contains more than one type of anion. In some embodiments, the salt consists of a single type of cation according to formula (I) and a single type of anion.

Such a salt may itself be an ionic liquid as described herein, or may instead be useful in the preparation of the ionic liquids described herein, even where the salt is not itself an ionic liquid. Such a salt may be a solid compound or may be dissolved in a solvent such that the ions are dissociated by solvation. For example, the salts may be used as precursors in an ion exchange reaction to form ionic liquids of the invention. The anion may be any anionic species which forms a salt with the cation according to formula (I). The skilled person will be aware of a wide range of possible anions which could be combined with a cation of formula (I) to form such a salt, but as non-limiting examples the anion may be selected from one or more of halide (F^", CI^", Br, I^"), hydroxide (OH^"), peroxide (O2^2"), sulphide (S^2"), hydrogen sulphide (HS^"), selenide (Se^2"), nitride (N^3"), azide (N₃ ^"), phosphide (P^3"), arsenide (As^3"), carbide (C^4"), cyanide (CNT), sulfate (S0₄ ^"), sulfite (SO3^2"), hydrogen sulfate (HS0₄ ^"), thiosulfate (S2O3^2"), carbonate (CO3^2") , bicarbonate (HCOs^"), phosphate (P0 ^3"), phosphite (PO3^3"), hydrogen phosphate (HP0 ^2"), dihydrogen phosphate (h^PCU^"), oxalate (C20₄ ^2"), cyanate (NCO^"), isocyanate (OCN^"), thiocyanate (SCN^"), chromate (Cr0₄ ^2"), dichromate (Cr20₇ ^2"), permanganate (Mn0₄ ^"), nitrate (NO3^"), nitrite (N0₂ ^"), acetate (CH3CO2^"), PF₆ ^", BF₄ ^", triflate (CF3SO3^"), nonaflate (CF₃(CF₂)3S0₃ ^"), bis(triflyl)amide ((CFsSC^N^"), trifluoroacetate (CF3CO2^"), heptafluorobutanoate

(CF₃(CF₂)2C0₂ ^"), AICU^", bistriflimide (TFSI; [(CFsSC^N]^"), 2,2,2-trifluoro-/V- (trifluoromethylsulfonyl)acetamide (TSAC; CF3SO2NCOCF3^"),

bis(perfluoroethylsulfonyl)amide (BETI; [(C^FsSC^N]^"), bis(fluorosulfonyl)imide (FSI; [(FS0₂)₂N]^"), fluoroalkyl phosphate (FAP; PF₃(C₂F₅)3), 4,5-dicyano-2-(trifluoromethyl) imidazolide (TDI; CF3-CN2C2(CN)2) and asymmetric imides such as fluorosulfonyltriflimide (FTFSI; [(CF₃S02)(FS0₂)N]-) and [(C₂F₅S02)(FS02)N]-). Other anions will be apparent to the skilled person.

In some embodiments the anion is selected from one or more of F^", CI^", Br and I^". In some embodiments the anion is selected from one or more of CI^", Br and I^". In some embodiments the anion is Br. Method of preparing ionic liquids

An aspect of the invention is a method of preparing an ionic liquid, comprising the step of reacting an ionic liquid halide precursor salt according to formula (VII):

The preferences for features of the compounds (i.e. for the groups R¹ , R², Y¹ , Y², Y³, L and G) are as discussed above under the first aspect.

In some embodiments, Hal¹ is selected from CI, Br and I. In some embodiments, Hal¹ is Br.

In some embodiments, the method of preparing the ionic liquid comprises reacting the ionic liquid halide precursor salt according to formula (VII) with a lithium salt (i.e., a salt comprising lithium ions and any suitable anion or mixture of anions) by mixing in ultra- pure H2O. In some embodiments, both the ionic liquid halide precursor salt according to formula (VII) and the lithium salt are separately dissolved in ultra-pure H2O before mixing of the two solutions. A solvent such as dichloromethane may then be added to the solution. In some embodiments, the reaction solution is then stirred or agitated for an extended period of time, for example at least 10 hrs, at least 15 hrs, at least 20 hrs, at least 25 hrs, at least 30 hrs, at least 35 hrs or at least 40 hrs. In some embodiments, the product of this reaction is a biphasic mixture made up of an organic phase comprising the ionic liquid and an aqueous phase comprising the lithium halide by-product. These phases may be separated and one or more washing steps may be performed on the organic phase. The optional washing steps may be carried out with ultra-pure H2O. In some embodiments, one or more washing steps with ultra-pure H2O are performed. In some embodiments, three washing steps are performed. In some embodiments, drying under high vacuum and/or elevated temperatures is carried out in order to remove excess moisture and/or solvent and provide the product ionic liquid. In some embodiments, the residual organic solvent is removed using a rotary evaporator.

In some embodiments, the anion in the lithium salt is selected from one or more of acetate (CH3CO2 ), PFe^", BF₄ ^", triflate (CF3SO3 ), nonaflate (CF₃(CF₂)3S0₃-),

bis(triflyl)amide ((CFsSC^N^"), trifluoroacetate (CF3CO2^"), heptafluorobutanoate

bis(perfluoroethylsulfonyl)imide (BETI; [(C^FsSC^N]^"), bis(fluorosulfonyl)imide (FSI; [(FS0₂)₂N]-), fluoroalkyl phosphate (FAP; PF₃(C₂F₅)3), 4,5-dicyano-2-(trifluoromethyl) imidazolide (TDI) and asymmetric imides such as fluorosulfonyltriflimide (FTFSI;

[(CF₃S0₂)(FS0₂)N]-) and [(C₂F₅S02)(FS02)N]-). In some embodiments, the anion in the lithium salt is selected from one or more of TFSI, FSI, PF6 and BF₄. In some embodiments, the method of preparing the ionic liquid comprises the preliminary step of reacting an intermediate according to formula (IV) with a compound according to formula (V) or formula (VI), to provide the ionic liquid precursor halide salt.

In some embodiments, the method of preparing the ionic liquid comprises the preliminary steps of (a) preparing an intermediate according to formula (IV) by reacting a compound according to formula (II) with a compound according to formula (III); and then (b) reacting the intermediate according to formula (IV) with a compound according to formula (V) or formula (VI), to provide the ionic liquid precursor halide salt. In some embodiments the step (a) further comprises a step of purifying the intermediate according to formula (IV) after its preparation. In some embodiments, this purification step comprises distillation.

In some embodiments of the step (a), the compound according to formula (III) is used in stoichiometric excess relative to the compound according to formula (II). In some embodiments, the compounds according to formulae (II) and (III) are directly mixed. In some embodiments, the compounds according to formulae (II) and (III) are mixed in the absence of solvent. In some embodiments the reaction mixture is heated to a temperature T1 , wherein T1 is a temperature of up to 200 °C, for example up to 190 °C, up to 180 °C, up to 170 °C or up to 160 °C. In some embodiments, T1 is a temperature of at least 100 °C, for example at least 110 °C, at least 120 °C, at least 130 °C, at least 140 °C or at least 150 °C. In some embodiments, the temperature is increased to T1 from ambient temperature over a period of at least 5 mins, for example at least 10 mins or at least 15 mins. In some embodiments, the temperature is increased to T1 from ambient temperature over a period of up to 35 mins, for example up to 30 mins or up to 25 mins.

In some embodiments, T1 is around 155 °C and the mixture is heated up to T1 from ambient temperature over a period of around 20 mins.

The mixture may then be left to react at or around T1 until the reaction has reached completion (e.g. when all alkyl halide by-product has been generated). In some embodiments, after completion of the reaction (e.g. when the generation of alkyl halide by-product has ceased) the reaction mixture is heated further, for example up to a temperature T2, wherein T2 is at least 2 °C higher than T1 , for example at least 3 °C higher than T1 , at least 4 °C higher than T1 or at least 5 °C higher than T1. In some embodiments, T2 is up to 10 °C higher than T1 , for example up to 9 °C higher than T1 , up to 8 °C higher than T1 , up to 7 °C higher than T1 or up to 6 °C higher than T1.

In some embodiments, T2 is around 160 °C and the mixture is left at T2 for a period of around 15 mins.

The intermediate according to formula (IV) and the residual excess of compound according to formula (III) may then be separated. In some embodiments, this separation is achieved by distillation. In some embodiments, the compounds according to formulae (V) and (VI) are

commercially available compounds.

In some embodiments, the ionic liquid precursor halide salt is a solid product. In some embodiments, in the step (b) (reacting the intermediate according to formula (IV) with a compound according to formula (V) or formula (VI), to provide the ionic liquid precursor halide salt), the intermediate according to formula (IV) is dissolved in an appropriate amount of organic solvent, such as acetone, and a compound according to formula (V) or formula (VI) is subsequently added to the solution. In some embodiments, the reaction solution is then stirred or agitated for an extended period of time, for example at least 40 hrs, at least 45 hrs, at least 50 hrs, at least 55 hrs, at least 60 hrs, at least 65 hrs or at least 70 hrs. In some embodiments, the product is formed as a precipitate and may be separated from the solution by a drying step. The drying step may be performed using a rotary evaporator.

An aspect of the invention is a method of preparing an ionic liquid precursor halide salt, comprising the steps of reacting an intermediate according to formula (IV):

Hal¹

R² Q \

Y

R

(IV) with a compound according to formula (V) or formula (VI):

wherein

Y\ Y², Y³, R\ R², L, R³, R⁴, R⁵, R⁶, R⁷ and R⁸ are as defined in the first aspect; and Hal¹ is selected from F, CI, Br and I. The term "ionic liquid precursor halide salt" refers to a salt which may not itself be an ionic liquid (but may instead be, for example, a solid or a solution of solvated ions), but is a salt containing the cation according to formula (I) along with a halide counter-anion. Such compounds are precursors to the ionic liquids of the invention and may be converted into ionic liquids of the invention by an anion-exchange reaction to replace the halide anion with an appropriate IL anion.

The preferences for features of the compounds (i.e. for the groups R¹ , R², R³, R⁴, R⁵, R⁶, R⁷, R⁸ Y¹ , Y², Y³ and L) are as discussed above under the first aspect. In some embodiments, Hal¹ is Br. In some embodiments of the method of preparing an ionic liquid precursor halide salt, the intermediate according to formula (IV) is dissolved in an appropriate amount of organic solvent, such as acetone, and a compound according to formula (V) or formula (VI) is subsequently added to the solution. In some embodiments, the reaction solution is then stirred or agitated for an extended period of time, for example at least 40 hrs, at least 45 hrs, at least 50 hrs, at least 55 hrs, at least 60 hrs, at least 65 hrs or at least 70 hrs. In some embodiments, the product is formed as a precipitate and may be separated from the solution by a drying step. The drying step may be performed using a rotary evaporator.

Electrolyte

One aspect of the invention is an electrolyte comprising an ionic liquid as described herein. In some embodiments, the electrolyte consists of the ionic liquid, i.e. the ionic liquid is present in the electrolyte in an amount of 100 wt% (with allowance for negligible amounts of impurities).

In some embodiments, the electrolyte comprises the ionic liquid and one or more additives. In some embodiments, the electrolyte comprises the ionic liquid in an amount of up to 99 wt%, for example up to 98 wt%, up to 97 wt%, up to 96 wt%, up to 95 wt%, up to 94 wt%, up to 93 wt%, up to 92 wt%, up to 91 wt% or up to 90 wt%, based on the total weight of electrolyte, the balance being the one or more additives. In some embodiments, the electrolyte comprises the ionic liquid in an amount of at least 1 wt%, for example at least 2 wt%, at least 3 wt%, at least 4 wt% or at least 5 wt%, based on the total weight of electrolyte, the balance being the one or more additives. In some embodiments, the electrolyte comprises the ionic liquid in an amount of at least 50 wt%, for example at least 55 wt%, at least 60 wt%, at least 65 wt%, at least 70 wt%, at least 75 wt%, at least 80 wt%, at least 85 wt%, at least 90 wt% or at least 95 wt% based on the total weight of electrolyte, the balance being the one or more additives. In this way, an electrolyte is provided with good physical properties as well as improved electrochemical stability. The skilled person can select the relative amounts of additive(s) and I L for a particular application in order to achieve appropriate physical properties and

electrochemical stability of the electrolyte. In some embodiments, the electrolyte comprises the ionic liquid in an amount of from 1 to 99 wt%, for example from 5 to 99 wt%, from 10 to 99 wt%, from 20 to 99 wt%, from 30 to 99 wt%, from 40 to 99 wt%, from 50 to 99 wt%, from 60 to 99 wt%, from 70 to 99 wt%, from 80 to 99 wt% or from 90 to 99 wt%, the balance being the one or more additives.

Additives for use in the electrolyte may be selected from any known additives familiar to the skilled person and which may in some cases be used to enhance physical, chemical or electrochemical properties. In some embodiments, the one or more additives comprises or consists of one or more solvents.

In some embodiments, the one or more additives comprises a salt [M⁺][X^_], wherein M⁺ is selected from one or more metal ions. In some embodiments, M⁺ is selected from one or more alkali metal or alkaline earth metal ions. In some embodiments, M⁺ is Li⁺. X^" is selected from one or more of acetate (CH3CO2^"), F6^", BF₄ ^", triflate (CF3SO3^"), nonaflate (CF₃(CF₂)3S0₃-), bis(triflyl)amide ((CF₃S0₂)₂N-), trifluoroacetate (CF3CO2 ,

heptafluorobutanoate (CF₃(CF₂)2C0₂ ^"), AIC ^", bistriflimide (TFSI; [(CF₃S0₂)₂N]-), 2,2,2- trifluoro-/V-(trifluoromethylsulfonyl)acetamide (TSAC; CF₃S0₂NCOCF₃ ^"),

bis(perfluoroethylsulfonyl)imide (BETI; [(C₂F₅S0₂)₂N]-), bis(fluorosulfonyl)imide (FSI; [(FS0₂)₂N]-), fluoroalkyl phosphate (FAP; PF₃(C₂F₅)3), 4,5-dicyano-2-(trifluoromethyl) imidazolide (TDI; CF3-CN₂C₂(CN)₂) and asymmetric imides such as fluorosulfonyltriflimide (FTFSI; [(CF₃S0₂)(FS0₂)N]-) and [(C₂F₅S0₂)(FS0₂)N]-). Other useful additives include lithium bisoxalatoborate (LiB(C₂0₄)₂), dimethylacetamide and lithium nitrate.

In some embodiments, the one or more additives comprises, or consists of, a solvent. In some embodiments, the solvent comprises one or more of adiponitrile, glutaronitrile, sulfolane, dimethoxyethane, N-methylimidazole, 1 ,2-dimethylimidazole, N- methylpiperidine, 1 ,4-dimethylpiperazine, N-methylpyrrolidine, 1 ,3-dioxolane, acetonitrile, DMSO, tetraglyme, vinylene carbonate, vinylene acetate, propylene carbonate, ethylene sulfide, ethylene carbonate, chloroethylene carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, gamma-butyrolactone, 4-fluoro-1 ,3-dioxalan-2-one (FEC), crown ethers, alkanes and cycloalkanes. In some embodiments, the electrolyte comprises up to 90 wt% solvent, for example up to 85 wt%, up to 80 wt%, up to 75 wt% or up to 70 wt%, based on the total weight of electrolyte, the balance being the ions of the ionic liquid as well as any other optional additives.

In some embodiments, the electrolyte comprises at least 1 wt% solvent, for example at least 2 wt%, at least 3 wt%, at least 4 wt% or at least 5 wt%, based on the total weight of electrolyte, the balance being the ions of the ionic liquid as well as any other optional additives.

The properties of the electrolyte can be varied and optimised by adjusting the relative amounts of ionic liquid and solvent, as would be understood by the skilled person.

In some embodiments, the electrolyte has a melting point (at a pressure of 100 kPa) of below 150 °C, for example below 140 °C, below 130 °C, below 120 °C, below 1 10 °C, below 100 °C, below 90 °C, below 80 °C, below 70 °C, below 60 °C, below 50 °C, below 40 °C below 30 °C or below 20 °C. In this way, it can be ensured that the electrolyte remains in the liquid state during use of an electrochemical energy storage device. In some embodiments, the electrolyte has a melting point (at a pressure of 100 kPa) of at least -20 °C, for example at least -15 °C, at least -10 °C, at least -5 °C, at least 0 °C, at least 5 °C or at least 10 °C.

In some embodiments, the melting point of the electrolyte (at a pressure of 100 kPa) is from 0 °C to 21 °C, for example from 0 °C to 20 °C, from 0 °C to 19 °C, from 0 °C to 18 °C, from 0 °C to 17 °C, from 0 °C to 16 °C or from 0 °C to 15 °C. Method of preparing an electrolyte

The electrolyte may be prepared by simple mixing together of the ionic liquid and the one or more additives. One aspect of the invention is a method of preparing an electrolyte comprising mixing an ionic liquid according to the invention with one or more additives. In some embodiments, the method of preparing an electrolyte comprises mixing an ionic liquid according to the invention with a solvent. In some embodiments, the solvent is selected from one or more of adiponitrile, glutaronitrile, sulfolane, dimethoxyethane, N- methylimidazole, 1 ,2-dimethylimidazole, N-methylpiperidine, 1 ,4-dimethylpiperazine, N- methylpyrrolidine, 1 ,3-dioxolane, acetonitrile, DMSO, tetraglyme, vinylene carbonate, vinylene acetate, propylene carbonate, ethylene sulfide, ethylene carbonate,

chloroethylene carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, gamma-butyrolactone, 4-fluoro-1 ,3-dioxalan-2-one (FEC), crown ethers, alkanes and cycloalkanes.

Electrochemical energy storage device

An aspect of the invention is an electrochemical energy storage device comprising an electrolyte as described herein. In some embodiments, the electrochemical energy storage device is selected from a lithium-ion battery, a metal ion battery, a metal air battery, a super capacitor and a hybrid capacitor. In some embodiments, the electrochemical energy storage device is a lithium-ion battery.

The invention will now be further described with reference to the following non-limiting Figures and Examples. Other embodiments of the invention will occur to those skilled in the art in the light of these.

The disclosure of all references cited herein, inasmuch as it may be used by those skilled in the art to carry out the invention, is hereby specifically incorporated herein by cross- reference. Figures

Figure 1 shows a cyclic voltammetry plot for an IL according to the invention (Compound

1) , a Comparative IL (Comparative IL 1) and two Comparative Electrolytes (Comparative Electrolytes A and B). Figure 2 shows a cyclic voltammetry plot for an IL according to the invention (Compound

2) , a Comparative IL (Comparative IL 1) and two Comparative Electrolytes (Comparative Electrolytes A and B).

Figure 3 shows a cyclic voltammetry plot for an IL according to the invention (Compound 3), a Comparative IL (Comparative IL 1) and two Comparative Electrolytes (Comparative Electrolytes A and B).

Figure 4 shows a cyclic voltammetry plot for an IL according to the invention (Compound 4), a Comparative IL (Comparative IL 1) and two Comparative Electrolytes (Comparative Electrolytes A and B). Figure 5 shows a cyclic voltammetry plot for an IL according to the invention (Compound

5) , a Comparative IL (Comparative IL 1) and two Comparative Electrolytes (Comparative Electrolytes A and B).

Figure 6 shows a cyclic voltammetry plot for an IL according to the invention (Compound

6) , a Comparative IL (Comparative IL 1) and two Comparative Electrolytes (Comparative Electrolytes A and B).

Figure 7 shows a cyclic voltammetry plot for three ILs according to the invention (Compounds 1 , 7 and 8) which each include the same cation but different anions.

Figure 8 shows a cyclic voltammetry plot for an IL according to the invention (Compound 1), a Comparative IL (Compound 1 ') without phosphonate functionality and two Comparative ILs (Compounds X and Z) with phosphonate/phosphate-based anions.

Figure 9 shows a representation of the electrochemical stability windows of various ionic liquids and electrolytes, where the upper and lower potential limits are represented by vertical markers.

General synthesis

Ionic liquids of the invention were made according to the general synthesis shown in Scheme 1 below.

Scheme 1

(a)

OR

RO OR

20

22 (b)

OR

=0

22 24 25

(c)

OR P=O

LiBr

25 26 27 28

R = alkyl group

L = alkylene group

Nu = uncharged nucleophile

X = ionic liquid anion

Step (a)

Under an inert atmosphere, a mixture of trialkylphosphite 20 (70 cm³) and a large excess of dibromoalkane 21 (200 cm³) was heated up to 155 °C within 20 min. The reaction mixture was left at 155 °C for 15 min. When the bromoalklane 23 generation ceased (ca. 15 min), the temperature was increased to 160 °C for further 15 min. Thereafter, the excess of dibromoalkane 21 and the desired compound 22 was distilled off following the reaction conditions described by Ali et al. (S. A. Ali, N. Y. Abu Thabit and H. A. A. Muallem, Journal of Polymer Science Part a-Polymer Chemistry 2010, 48, 5693-5703; incorporated herein by reference). The excess dibromoalkane 21 was immediately distilled off, followed by the title compound 22. The compound 22 was isolated and characterised by ¹ H and ³¹ P NMR. Step (b)

0.029 mol of uncharged nucleophile 24 was added to a mixture of 0.028 mol of compound 22 in 6 cm³ of dry acetone under an argon atmosphere. The solution was vigorously stirred for ca. 98 h. The resulting solution was decanted and the excess solvent was removed on a rotary evaporator. ¹ H NMR was used to confirm the product 25 was the title compound.

Step (c)

Compound 25 was placed into a round-bottomed flask (100 cm³) and dissolved in deionised water (25 cm³). An equimolar amount of a lithium salt of an appropriate anion 26, dissolved in deionised water (25 cm³), was added to the vigorously stirred solution of the ionic liquid solution. The mixture was stirred for 3 h at ambient temperature and subsequently, the top layer was decanted and the bottom layer was repeatedly washed with deionised water (8 x 10 cm³). Finally, the product 27 was dried overnight at 60 °C under high vacuum.

Examples

Example 1 - Synthesis of diethyl-[3-(dimethylethylammonium)propyllphosphonate

(Compound 1 ; rPyrri.sDEplfTFSIl)

1a) Synthesis ofdiethyl(3-bromopropyl)phosphonate (Compound 1A)

Compound 1A

Compound 1 A was synthesised according to step (a) of the general synthesis described above, from triethylphosphite and 1 ,3-dibromopropane.

¹ H NMR (300 MHz, CDC ) δ 4.28 - 3.93 (m, 2H), 3.53 (dd, J = 16.9, 8.5 Hz, 1 H), 2.54 -

2.24 (m, 1 H), 1.34 (t, J = 7.1 Hz, 3H).

³¹ P NMR (121 MHz, CDCb) δ 32.20 (s). 1b) Synthesis of N-(diethylpropylphosphonate)-N, methylpyrrolidinium bromide

(Compound 1B)

Compound 1B

Compound 1 B was synthesised according to step (b) of the general synthesis described above, from /V-methylpyrrolidine and Compound 1A.

¹ H NMR (300 MHz, CDC ) δ 4.22 - 3.99 (m, 4H), 3.94 - 3.75 (m, 6H), 3.33 (s, 3H), 2.32 (s, 4H), 2.20 - 2.02 (m, 2H), 2.03 - 1.80 (m, 2H), 1.34 (t, J = 7.1 Hz, 6H).

1 c - Synthesis of N-(diethylpropylphosphonate)-N-methyl-pyrrolidinium

bis{(trifluoromethyl)sulfonyl}imide (Compound 1)

Compound 1

Compound 1 was synthesised according to step (c) of the general synthesis described above, from lithium bis(trifluoromethylsulfonyl)imide and Compound 1 B.

¹ H NMR (300 MHz, DMSO) δ 4.17 - 4.01 (m, 8H), 3.52 - 3.39 (m, 8H), 3.39 - 3.29 (m, 4H), 2.99 (s, 6H), 2.19 (s, 8H), 2.08 - 1.93 (m, 5H), 1.78 (dt, J = 18.0, 7.7 Hz, 4H), 1.32 (t, J = 7.1 Hz, 12H);

¹³C NMR (75 MHz, DMSO) δ 124.46 (s), 121.27 (s), 1 18.08 (s), 1 17.02 (s), 1 14.90 (s), 64.52 (s), 63.72 (s), 61.45 (d, J = 6.4 Hz), 48.21 - 47.64 (m), 22.03 (s), 21.01 (s), 20.61 (s), 17.08 (d, J = 4.1 Hz), 15.48 (d, J = 5.8 Hz);

³¹ P NMR (121 MHz, DMSO) δ 28.89 (s); CHNS Calc, C. 30.88; H, 5.00; N, 5.15; S, 1 1.78; CHNS Found, C. 30.47; H, 5.32; N, 5.09; S, 11.54. Example 2 - Synthesis of N-(diethylpropylphosphonate)-N-methyl-piperidinium

bis{(t fluoromethyl)sulfonyl}imide (Compound 2; [PiPi.3PEp1[TFSI1)

Compound 2

rPi i.3DEPirTFsii

Compound 2 was synthesised according to steps (a)-(c) of the general synthesis described above, from the starting materials triethylphosphite, 1 ,3-dibromopropane, /V-methylpiperidine and lithium bis(trifluoromethylsulfonyl)imide.

¹ H NMR (300 MHz, DMSO) δ 4.18 - 3.88 (m, 4H), 3.47 - 3.22 (m, 6H), 3.02 (s, 3H), 2.06 - 1.66 (m, 8H), 1.54 (dd, J = 17.2, 1 1.5 Hz, 2H), 1.27 (t, J = 7.0 Hz, 6H), ¹³C NMR (75 MHz, DMSO) δ 126.26 (s), 121.99 (s), 117.73 (s), 113.41 (s), 61.54 (s), 60.51 (s), 47.75 (s), 22.59 (s), 20.96 (s), 20.66 (s), 19.60 (s), 16.58 (s), 15.54 (s), ³¹ P NMR (121 MHz, DMSO) δ 31.20 (s); CHNS Calc, C. 32.27; H, 5.24; N, 5.02; S, 1 1.45; CHNS Found, C. 32.16; H, 5.62; N, 5.05; S, 11.35.

Example 3 - Synthesis of N-(diethylpropylphosphonate)-N-methyl-azepanium

bis{(trifluoromethyl)sulfonyl}imide (Compound 3; [Azei.3PEp1[TFSI1)

Compound 3

rAze_{1 3}DEPirTFSIl Compound 3 was synthesised according to steps (a)-(c) of the general synthesis described above, from the starting materials triethylphosphite, 1 ,3-dibromopropane, /V-methylazepane and lithium bis(trifluoromethylsulfonyl)imide.

¹ H NMR (300 MHz, DMSO) 5 4.03 (ddd, J = 1 1.7, 5.0, 3.2 Hz, 4H), 3.57 - 3.25 (m, 4H), 3.13 (dd, J = 12.8, 7.3 Hz, 2H), 3.01 (s, 3H), 2.01 - 1.68 (m, 8H), 1.62 (d, J = 3.6 Hz, 4H), 1.26 (tt, J = 6.0, 2.9 Hz, 6H).

¹³C NMR (75 MHz, DMSO) δ 126.26 (s), 121.99 (s), 117.65 (s), 1 13.58 (s), 63.97 (s), 61.59 (s), 56.79 (s), 54.24 (s), 27.41 (s), 25.87 (s), 23.40 (s), 21.22 (s), 17.73 (s), 16.67 (s) 16.59 (s).

³¹ P NMR (121 MHz, DMSO) δ 31.48 (s); CHNS Calc, C. 33.57; H, 5.46; N, 4.89; S, 1 1.20; CHNS Found, C. 32.96; H, 5.83; N, 5.41 ; S, 1 1.40.

Example 4 - Synthesis of diethylpropylphosphonate-trioctylphosphonium

bis{(t fluoromethyl)sulfonyl}imide (Compound 4; [P8.8.8.3DEP1[TFSI1)

Compound 4

rp₈.8.8.3DEPirTFsn

Compound 4 was synthesised according to steps (a)-(c) of the general synthesis described above, from the starting materials triethylphosphite, 1 ,3-dibromopropane, trioctylphosphine and lithium bis(trifluoromethylsulfonyl)imide.

¹ H NMR (300 MHz, DMSO) 5 4.15 - 3.99 (m, 2H), 2.26 (td, J = 12.9, 7.0 Hz, 1 H), 2.17 - 2.03 (m, 4H), 2.01 - 1.71 (m, 3H), 1.63 - 1.49 (m, 4H), 1.47 (d, J = 5.7 Hz, 4H), 1.41 - 1.23 (m, 20H), 0.93 (t, J = 6.8 Hz, 6H),

¹³C NMR (75 MHz, DMSO) δ 124.49 (s), 121 .28 (s), 1 18.09 (s), 1 16.93 (s), 61 .15 (dd, J = 18.1 , 6.3 Hz), 31 .23 (d, J = 6.8 Hz), 29.92 (d, J = 15.0 Hz), 28.30 (dd, J = 22.2, 13.4 Hz), 25.82 (d, J = 15.9 Hz), 24.42 (d, J = 15.9 Hz), 22.04 (s), 20.59 (d, J = 4.4 Hz), 19.08 - 17.00 (m), 15.53 (d, J = 5.8 Hz), 15.34 - 14.70 (m), 13.08 (s); ESI-MS: [Csi HeyPaOsF 549.4521 ; [C2S2NF6O4]- 279.9162. Synthesis of N-(diethylpropylphosphonate)-N, N-dimethyl-N-ethylammonium

bis{(trifluoromethyl)sulf

Compound 5

Compound 5 was synthesised according to steps (a)-(c) of the general synthesis described above, from the starting materials triethylphosphite, 1 ,3-dibromopropane, A/,A/-dimethylethylamine and lithium bis(trifluoromethylsulfonyl)imide.

¹ H NMR (300 MHz, DMSO) δ 4.03 (dq, J = 14.0, 7.0 Hz, 4H), 3.35 (d, J = 6.0 Hz, 2H), 3.32 - 3.23 (m, 2H), 3.01 (d, J = 8.5 Hz, 6H), 1.78 (dd, J = 17.5, 10.5 Hz, 2H), 1.37 - 1.18 (m, 9H).

¹³C NMR (75 MHz, DMSO) δ 121.99 (s), 1 17.73 (s), 62.46 (d, J = 19.6 Hz), 61.57 (d, J = 6.3 Hz), 59.14 (s), 49.89 (s), 22.61 (s), 20.73 (s), 16.58 (d, J = 5.6 Hz), 16.23 (d, J = 3.4 Hz), 8.03 (s).

³¹ P NMR (121 MHz, DMSO) δ 31.58 (s), 31.19 (s).

CHNS Calc, C. 29.32; H, 5.1 1 ; N, 5.26; S, 1 1.20;

CHNS Found, C. 28.43; H, 4.97; N, 5.31 ; S, 11.57.

Example 6 - Synthesis of N-(diethylbutylphosphonate)-N-methyl-pyrrolidinium

bis{(trifluoromethyl)sulfonyl)imide (Compound 6; [Pyrri.4PEp1[TFSI1)

Compound 6

rPvm.4DEPirTFsii Compound 6 was synthesised according to steps (a)-(c) of the general synthesis described above, from the starting materials triethylphosphite, 1 ,4-dibromobutane, /V-methylpyrrolidine and lithium bis(trifluoromethylsulfonyl)imide.

¹ H NMR (300 MHz, DMSO) δ 4.13 - 3.86 (m, 4H), 3.45 (dd, J = 20.3, 1 1.7 Hz, 4H), 3.33 (dd, J = 10.4, 7.4 Hz, 2H), 2.98 (s, 3H), 2.10 (s, 4H), 1.92 - 1.71 (m, 4H), 1.64 - 1.43 (m, 2H), 1.25 (t, J = 7.0 Hz,6H).

¹³C NMR (75 MHz, DMSO) δ 122.00 (s), 1 17.73 (s), 63.88 (s), 63.33 (s), 62.77 (s), 61.26 (d, J = 6.3 Hz), 47.97 (s), 40.80 (s), 40.79 - 40.13 (m), 39.89 (s), 39.89 - 39.84 (m),

39.89 - 38.75 (m), 25.17 (s), 23.85 (d, J = 16.8 Hz), 23.32 (s), 21.46 (s), 19.65 (d, J = 4.0 Hz), 16.59 (d, J = 5.7 Hz).

³¹ P NMR (121 MHz, DMSO) δ 32.84 (s), 32.27 (s), 25.02 (s).

CHNS Calc, C. 32.26; H, 5.23; N, 5.02; S, 1 1.58;

CHNS Found, C. 31.86; H, 5.23; N, 5.31 ; S, 1 1.40.

Example 7 - Synthesis of N-(diethylpropylphosphonate)-N-methyl-pyrrolidinium bis(fluorosulfonyl)imide (Compound 7; [Pyrri.3PEp1[FSI1)

Compound 7

iPyrri.3DEPUFSn

Compound 7 was synthesised according to steps (a)-(c) of the general synthesis described above, from the starting materials triethylphosphite, 1 ,3-dibromopropane, /V-methylpyrrolidine and lithium bis(fluorosulfonyl)imide.

¹ H NMR (300 MHz, DMSO) δ 4.19 - 3.93 (m, 4H), 3.54 (d, J = 10.7 Hz, 2H), 3.47 - 3 (m, 4H), 3.03 (d, J = 14.0 Hz, 3H), 2.10 (s, 4H), 1.90 (dd, J = 15.2, 7.7 Hz, 2H), 1.86 - 1.70 (m, 2H), 1.26 (dd, J = 12.9, 5.9 Hz, 6H). ¹³C NMR (75 MHz, DMSO) δ 64.08 (d, J = 17.8 Hz), 63.26 (d, J = 20.2 Hz), 61.59 (d, J = 6.2 Hz), 47.80 (s), 40.73 (s), 40.46 (s), 40.40 - 39.52 (m), 39.62 (s), 39.87 - 38.75 (m), 22.81 (s), 21.48 (d, J = 9.1 Hz), 20.93 (s), 17.43 (d, J = 3.6 Hz), 16.63 (d, J = 5.6 Hz). ³¹ P NMR (121 MHz, DMSO) δ 50.47 (s), 34.99 (s), 32.21 (d, J = 40.9 Hz), 31.77 (s), 31.66 (d, J = 19.8 Hz), 31.16 (s).

CHNS Calc, C 32.22; H, 6.65; N, 3.42,

CHNS Found C, 35.29; H, 6.52; N, 3.21.

Example 8 - Synthesis of N-(diethylpropylphosphonate)-N-methyl-pyrrolidinium

hexafluorophosphate (Compound 8; [Pyrri.3PEp1[PF6l)

Compound 8

rPvm.3DEPirPF_Bi

Compound 8 was synthesised according to steps (a)-(c) of the general synthesis described above, from the starting materials triethylphosphite, 1 ,3-dibromopropane, /V-methylpyrrolidine and lithium hexafluorophosphate.

¹ H NMR (300 MHz, DMSO) δ 4.20 - 3.89 (m, 4H), 3.55 (dd, J = 16.2, 10.3 Hz, 2H), 3.46 - 3.39 (m, 2H), 3.01 (s, 3H), 2.10 (s, 4H), 1.99 - 1.87 (m, 2H), 1.87 - 1.72 (m, 2H), 1.47 - 1.16 (m, 6H). ¹³C NMR (75 MHz, DMSO) δ 63.98 (s), 63.26 - 62.68 (m), 61.59 (d, J = 6.1 Hz), 47.70 (d, J = 13.0 Hz), 22.68 (d, J = 18.5 Hz), 22.01 - 20.70 (m), 17.45 (s), 16.63 (d, J = 5.6 Hz).

³¹ P NMR (121 MHz, DMSO) δ 50.47 (s), 32.06 (s), 31.66 (d, J = 16.8 Hz), 31.17 (s), -131.24 (s), -137.09 (s), -142.95 (s), -148.81 (s), -154.66 (s).

CHNS Calc, C 32.22; H, 6.65; N, 3.42,

CHNS Found C, 35.29; H, 6.52; N, 3.21 Example 9 - Comparative Compound X ([Pyrri.4l[Me2PQ4l)

Compound X

[Pyrr₁,₄][Me₂P0₄]

Compound X was synthesised by adding 0.1 mol of trimethylphosphate to 0.1 mol of /V-butylpyrrolidine under a N2 atmosphere. The resulting mixture was stirred for 3 days at 60 °C and dried under high vacuum to yield Compound X as a liquid product.

Trimethylphosphate and /V-butylpyrrolidine are commercially available reagents.

¹ H NMR (300 MHz, DMSO) δ 3.49 (d, J = 8.3 Hz, 2H), 3.33 (dd, J = 15.5, 7.1 Hz, 1 H), 3.26 (d, J = 10.3 Hz, 2H), 3.01 (s, 1 H), 2.09 (s, 1 H), 1.80 - 1.54 (m, 1 H), 1.43 - 1.19 (m, 1 H), 0.93 (dd, J = 14.4, 7.0 Hz, 1 H).

¹³C NMR (75 MHz, DMSO) δ 63.69 (s), 63.27 (s), 51.53 (s), 47.79 (s), 25.28 (s), 21.38 (s), 19.65 (s), 13.83 (s); ³¹ P NMR (121 MHz, DMSO) δ 2.48 (s).

Example 10 - Comparati

Compound Z

[Pyrr₁,₄][Me₂P0₃]

Compound Z was synthesised by adding 0.1 mol of dimethyl methylphosphonate to 0.1 mol of /V-butylpyrrolidine under a N2 atmosphere. The resulting biphasic mixture was stirred for 1 week at 80 °C, forming a single phase. This was dried under high vacuum to yield Compound Z as a liquid product. Dimethyl methylphosphonate is a commercially available reagent.

¹ H NMR (300 MHz, DMSO) δ 3.64 - 3.40 (m, 4H), 3.33 (dd, J = 17.0, 8.6 Hz, 2H), 3.25 (d, J = 10.1 Hz, 3H), 3.01 (s, 3H), 2.09 (s, 4H), 1.69 (dt, J = 15.6, 7.9 Hz, 2H), 1.45 - 1.17 (m, 2H), 0.95 (t, J = 7.3 Hz, 3H), 0.84 (d, J = 15.4 Hz, 3H). Example 1 1 - Analysis of electrochemical stability windows

Prior to any measurements, the IL was dried under vacuum (ca. 10^"3 mbar) at elevated temperature (90 °C) with continuous stirring for a minimum of two days. After this procedure, the IL was stored in an Ar-filled glovebox (>3 ppm H2O) to limit water contamination. Water content of the I Ls was analysed by Karl Fischer Coulometric titration using an 899 Coulometer (Metrohm). Measurements of water content were completed in duplicate and the resolution of the measurements was 0.001 wt%, or 10 ppm.

Measurements of the electrolyte electrochemical stability window by cyclic voltammetry were performed using either an Autolab PGSTAT302 workstation (Metrohm) or a VMP3 workstation (BioLogic). Measurements were performed inside an Ar-filled glovebox with moisture levels less than 3 ppm water using a three-electrode glass cell. The working electrode was a glassy-carbon macro-disk (ALS Co., Ltd. , 3 mm diameter). Prior to all experiments, the glassy carbon working electrode was polished using alumina slurries of decreasing grain size (1 .0 μηι, 0.3 μηι, and 0.05 μηι) in distilled water. The electrode was then sonicated in methanol or acetonitrile and separately in distilled water for 2-3 minutes per solvent then dried at ca. 80 °C in an oven. The counter electrode was platinum-coiled wire heat-sealed in a glass capillary. The Ag[NOs]/Ag reference electrode consisted of silver wire immersed in a 0.1 mol dnr³ solution of Ag[NOs] in 1 -butyl-3-methylimidazolium nitrate ([C4mim][NOs]) separated from the bulk solution by a glass-frit tip. The reference potential of the Ag[NOs]/Ag electrode was referenced vs. an internal ferrocene couple; after completion of the electrochemical window, a small quantity of ferrocene was dissolved in the liquid sample and cyclic voltammetry was used to determine the redox potentials of reversible ferrocene oxidation (EF_C+/F_C = + 0.16 V vs. the Ag[NOs]/Ag). The reference potentials of was subsequently normalised vs. the Li7Li redox potential using the approximation of EFC+/FC ⁼ 3.2 V vs. Li7Li. During standard cyclic voltammetry experiments to determine the electrochemical window, the potential of the working electrode was cycled at 2 mV s^"1 between -5 V and +5 V vs. Ag[N03]/Ag. For this study, the onset potential of electrochemical oxidative or reductive bulk decomposition of the electrolyte was defined by the x-axis intercept of linear regression of the oxidative/reductive walls. The results of the ESW measurements are shown in Figures 1-8. Each of Figures 1-6 shows cyclic voltammetry results for a Comparative IL and two Comparative Electrolyte mixtures along with cyclic voltammetry results for an IL according to the invention.

Comparative IL 1 is an I L consisting of Compound V (A/-butyl-/V-methyl-pyrrolidinium bis{(trifluoromethyl)sulfonyl

Compound 1 '

[Pyrr_1i4][TFSI]

Comparative IL 1 is a commercially available ionic liquid.

Comparative Electrolyte A is made up of Compound 1' in diethyl ethylphosphonate (DEEP) solvent. Comparative Electrolyte B is made up of Compound 1' in tetraethylene glycol dimethyl ether (tetraglyme; TEGDME), a benchmark lithium-air electrolyte. Since DEEP and TEGME are not electrically conducting, Compound 1 ' is added, to yield a concentration of ca. 1 mol dnr³ in the respective solvent, to provide a conducting electrolyte for testing. Comparative Electrolyte C is LP30, a commercially available conventional Li-ion battery electrolyte.

Comparative ILs 2 and 3 consisting of Compound X (A/-butyl-A/-methyl-pyrrolidinium dimethylphosphate) and Compound Z (A/-butyl-A/-methyl-pyrrolidinium

methylmethylphosphonate) respectively, have the phosphorus moiety located within the anionic portion of the IL:

Comparative IL 2 : Compound X Comparative IL 3 : Compound Z Additionally, the anodic (oxidative) stability limit for a conventional Li-ion battery electrolyte LP30 (1 mol dnr³ Li[PF6] in EC/DMC (1 : 1 vol:vol ratio) is approximated from literature values: 1. L. Lombardo, S. Brutti, M. A. Navarra, S. Panero and P. Reale, J. Power Sources, 2013, 227, 8-14 and 2. A. Freiberg, M. Metzger, D. Haering, S. Bretzke, S. Puravankara, T. Nilges, C. Stinner, C. Marino and H. A. Gasteiger, J. Electrochem. Soc, 2014, 161 , A2255-A2261.

Using the intercept of linear regression of the oxidative/reductive walls with the x-axis as the onset potential of electrochemical oxidative or reductive bulk decomposition of the electrolyte, the approximate cathodic limits, anodic limits and ESW for the ILs tested are provided in the Table below:

Table 1. Cathodic and anodic potential limits of electrochemical decomposition and the electrochemical window (ESW) of a series of phosphonate-functionalised ILs, a non- functionalised analogue, two comparative solvents and a conventional Li-ion battery electrolyte, LP30.

IL/electrolyte assignment Cathodic Limit 1 Anodic Limit 1 V ESW I V

V vs. Li7Li vs. Li7Li

Compound 1 -0.25 6.00 6.25

Compound 2 -0.24 6.04 6.28

Compound 3 0.02 6.27 6.24

Compound 4 0.32 6.68 6.36

Compound 5 -0.22 6.09 6.30

Compound 6 -0.01 6.47 6.48

Compound 7 -0.05 6.15 6.20

Compound 8 -0.19 6.26 6.45

Comparative IL 1 (Compound 1') -0.24 5.64 5.89

Comparative IL 2 (Compound X) -0.14 4.36 4.50

Comparative IL 3 (Compound Z) -0.03 4.78 4.81

Comparative Electrolyte A

0.00 4.84 4.83 (Compound V in DEEP)

Comparative Electrolyte B

-0.18 4.55 4.73 (Compound 1 ' in TEGDME) Comparative Electrolyte C

4.8

(LP30)

Electrochemical decomposition limits determined by cyclic voltammetry at a glassy carbon working electrode, a) Electrochemical measurements of DEEP and TEGDME mixtures conducted with ca. 1 mol-dnr³ Compound V as the conductive electrolyte, b) Value for anodic decomposition potential of LP30 electrolyte (i.e. 1 mol-dnr³ Li[PF6] in EC/DMC) estimated from Lombardo et al. and Freiberg et al.

Figures 1-6 and the above Table show that, for a given anion, ionic liquids which include a cation according to the invention offer a significantly wider ESW than Comparative ILs/electrolytes. Figure 7 shows that different anions can be used in an I L with a cation according to the invention while maintaining a broad ESW.

Comparing the ESW results for the ILs of the invention with Comparative ILs 2 and 3 shows the importance of ensuring that the phosphate or phosphonate moiety is present the cation, leading to significant improvements in the electrochemical stability of the IL. Moving the phosphate or phosphonate moiety to the anion does not provide the same benefits.

Figure 9 provides in a single diagram a comparison of the electrochemical stability windows of various ionic liquids and electrolytes. At the top of the diagram the ESWs for LP30 (the oxidative limit of which was estimated from the references Lombardo et al. and Freiberg et al. as explained above), Comparative Electrolyte A and Comparative

Electrolyte B are shown. Directly below these, the ESW of Comparative IL 1 is shown. These can be compared with the remaining ESWs in Figure 9, which are those of IL Compounds 1-8 according to the invention.

The novel ILs therefore open up the possibility of providing electrochemical energy storage devices which can operate at higher voltages than existing devices without degradation of the electrolyte. References

1. L. Lombardo, S. Brutti, M. A. Navarra, S. Panero and P. Reale, J. Power Sources, 2013, 227, 8-14

2. A. Freiberg, M. Metzger, D. Haering, S. Bretzke, S. Puravankara, T. Nilges, C. Stinner, C. Marino and H. A. Gasteiger, J. Electrochem. Soc, 2014, 161 , A2255-A2261.

Claims

An ionic liquid comprising an anion and a cation, wherein the cation is a cation

(I) wherein Y¹ and Y² are each independently selected from a single bond and O; Y³ is selected from a single bond and O;

L is saturated C1-20 linear alkylene optionally substituted with one or more groups R^L;

G is selected from N(R³)(R⁴)(R⁵) and P(R⁶)(R⁷)(R⁸); wherein R³ is linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^3A;

R⁴ and R⁵ are each independently selected from linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^4A, or R⁴ and R⁵ together with the N to which they are attached form a 4- to 12-membered heterocyclic group, optionally substituted with one or more groups R^4B;

R⁶ is linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^6A; and

R⁷ and R⁸ are each independently selected from linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^7A, or R⁷ and R⁸ together with the P to which they are attached form a 4- to 12-membered heterocyclic group, optionally substituted with one or more groups R^7B; wherein R^1A, R^L, R^3A, R^4A, R^4B, R^6A, R^7A and R^7B are each independently selected from saturated Ci-e linear unsubstituted alkyl;

-OH, -0(R^A),

-F, -CI,

-CN, and

-NH₂, wherein R^A is saturated Ci-e linear unsubstituted alkyl.

2. An ionic liquid according to claim 1 , wherein Y¹ and Y² are each O.

3. An ionic liquid according to claim 1 or 2, wherein Y³ is a single bond.

4. An ionic liquid according to any one of claims 1 to 3, wherein R¹ and R² are each independently selected from linear saturated C1-12 alkyl optionally substituted with one or more groups R^1A.

5. An ionic liquid according to any one of claims 1 to 4, wherein R¹ and R² are each independently selected from linear unsubstituted saturated C1-12 alkyl.

6. An ionic liquid according to any one of claims 1 to 5, wherein R¹ and R² are each independently selected from linear unsubstituted saturated C1-6 alkyl, preferably linear unsubstituted C2-4 alkyl.

7. An ionic liquid according to any one of claims 1 to 6, wherein L is saturated C3-20 alkylene optionally substituted with one or more groups R^L.

8. An ionic liquid according to any one of claims 1 to 7, wherein L is saturated C3-10 alkylene optionally substituted with one or more groups R^L.

9. An ionic liquid according to any one of claims 1 to 8, wherein L is unsubstituted saturated C3-6 alkylene.

10. An ionic liquid according to claim 9, wherein L is selected from n-propylene and n-butylene.

1 1. An ionic liquid according to any one of claims 1 to 10, wherein G is N(R³)(R⁴)(R⁵).

12. An ionic liquid according to claim 11 , wherein R³ is linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^3A; and R⁴ and R⁵ together with the N to which they are attached form a 4- to 12-membered heterocyclic group, optionally substituted with one or more groups R^4B.

13. An ionic liquid according to claim 12, wherein R¹ and R² are each independently selected from linear or branched saturated unsubstituted C1-3 alkyl.

14. An ionic liquid according to claim 1 1 , wherein R³ is selected from linear saturated C1-3 unsubstituted alkyl, and R⁴ and R⁵ together with the N to which they are attached form a 5 to 7-membered heterocyclic group optionally substituted with one or more groups R^4B.

15. An ionic liquid according to claim 14, wherein R³ is methyl.

16. An ionic liquid according to any one of claims 1 to 10, wherein G is P(R⁶)(R⁷)(R⁸).

17. An ionic liquid according to claim 16, wherein each of R⁶, R⁷ and R⁸ are independently selected from linear unsubstituted saturated C1-12 alkyl.

18. An ionic liquid according to claim 17, wherein each of R⁶, R⁷ and R⁸ are independently selected from linear unsubstituted saturated C2-8 alkyl.

19. An ionic liquid according to claim 16, wherein R⁶ and R⁷ are each methyl, and R⁸ is selected from methyl and ethyl.

20. An ionic liquid according to any one of claims 1 to 19, wherein R^1A, R^L, R^3A, R^4A, R^4B, R^6A, R^7A and R^7B are each independently saturated C1-3 linear unsubstituted alkyl.

21. An ionic liquid according to any one of claims 1 to 20, wherein R^A is saturated C1-3 linear unsubstituted alkyl.

22. An ionic liquid according to any one of claims 1 to 21 , wherein G is selected from N(R³)(R⁴)(R⁵) and P(R⁶)(R⁷)(R⁸).

23. An ionic liquid according to claim 22, wherein

R³ is linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^3A;

R⁷ and R⁸ are each independently selected from linear or branched saturated C1-12 alkyl optionally substituted with one or more groups R^7A; and

R¹ and R² are each independently selected from linear or branched saturated C3-12 alkyl optionally substituted with one or more groups R^1A, and methyl.

24. An ionic liquid according to claim 23, wherein R¹ and R² are each independently selected from linear or branched saturated C3-12 alkyl optionally substituted with one or more groups R^1A.

25. An ionic liquid according to claim 23, wherein R¹ and R² are each independently methyl,

26. An ionic liquid according to claim 22, wherein

provided that at least two of R⁶, R⁷ and R⁸ are different from one another.

27. An ionic liquid according to claim 26, wherein R³ is methyl.

28. An ionic liquid according to claim 26 or 27, wherein G is N(R³)(R⁴)(R⁵).

29. An ionic liquid according to claim 1 , wherein Y¹ and Y² are each O;

Y³ is a single bond; R¹ and R² are each independently selected from linear unsubstituted saturated

Ci-8 alkyl;

L is unsubstituted saturated C3-10 alkylene; and G is selected from N(R³)(R⁴)(R⁵) and P(R⁶)(R⁷)(R⁸), wherein

R³ is methyl;

R⁴ and R⁵ are each independently selected from linear unsubstituted saturated C1-8 alkyl, or R⁴ and R⁵ together with the N to which they are attached form a 5 to 7-membered unsubstituted heterocyclic group;

R⁶ is methyl; and R⁷ and R⁸ are each independently selected from linear unsubstituted saturated C1-8 alkyl, or R⁷ and R⁸ together with the P to which they are attached form a 5 to 7-membered unsubstituted heterocyclic group.

30. An ionic liquid according to any one of claims 1 to 29, wherein the anion is selected from one or more of acetate (CH3CO2^"), PF6^", BF₄ ^", triflate (CF3SO3^"), nonaflate (CF₃(CF₂)3S0₃-), bis(triflyl)amide ((CF₃S0₂)₂N-), trifluoroacetate (CF3CO2 ,

bis(perfluoroethylsulfonyl)imide (BETI; [(C₂FsS0₂)₂N]^"), bis(fluorosulfonyl)imide (FSI; [(FS0₂)₂N]-), fluoroalkyi phosphate (FAP; PF₃(C₂F₅)3), 4,5-dicyano-2-(trifluoromethyl) imidazolide (TDI; CF3-CN2C2(CN)2) and asymmetric imides such as fluorosulfonyltriflimide (FTFSI; [(CF₃S0₂)(FS0₂)N]-) and [(C₂F₅S02)(FS02)N]-).

31. An ionic liquid according to claim 30, wherein the anion is selected from TFSI, FSI , PF₆ and BF₄.

32. An ionic liquid according to any one of claims 1 to 31 , wherein the cation according to Formula (I) is not selected from any of the cations (X1)-(X7):

33. A liquid electrolyte comprising an ionic liquid according to any one of claims 1 to 32.

34. A liquid electrolyte according to claim 33, further comprising one or more additives.

35. A liquid electrolyte according to claim 34, wherein the one or more additives comprises a salt [M⁺][X^_], wherein M⁺ is selected from one or more metal ions and X^" is selected from one or more of bisoxalatoborate (B^CU ), nitrate (NO3^"), acetate

(CH3CO2 ), PFe^", BF4-, triflate (CF3SO3 ), nonaflate (CF₃(CF₂)3S0₃-), bis(triflyl)amide ((CF₃S0₂)₂N-), trifluoroacetate (CF3CO2^"), heptafluorobutanoate (CF₃(CF₂)2C0₂ ^"), AICU^", bistriflimide (TFSI; [(CF₃S0₂)₂N]-), 2,2,2-trifluoro-/V-(trifluoromethylsulfonyl)acetamide (TSAC; CF3SO2NCOCF3-), bis(perfluoroethylsulfonyl)imide (BETI; [(C₂F₅S0₂)2N]-), bis(fluorosulfonyl)imide (FSI; [(FS0₂)₂N]-), fluoroalkyi phosphate (FAP; PF₃(C₂F₅)3), 4,5- dicyano-2-(trifluoromethyl) imidazolide (TDI; CF3-CN2C2(CN)2) and asymmetric imides such as fluorosulfonyltriflimide (FTFSI; [(CF₃S0₂)(FS0₂)N]-) and [(C₂F₅S02)(FS02)N]-).

36. A liquid electrolyte according to claim 34 or 35, wherein the one or more additives comprises a solvent.

37. A liquid electrolyte according to claim 36, wherein the solvent is selected from one or more of dimethylacetamide, adiponitrile, glutaronitrile, sulfolane, dimethoxyethane, N- methylimidazole, 1 ,2-dimethylimidazole, N-methylpiperidine, 1 ,4-dimethylpiperazine, N- methylpyrrolidine, 1 ,3-dioxolane, acetonitrile, DMSO, tetraglyme, vinylene carbonate, vinylene acetate, propylene carbonate, ethylene sulfide, ethylene carbonate,

chloroethylene carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate and gamma-butyrolactone.

38. An electrochemical energy storage device comprising a liquid electrolyte according to any one of claims 33 to 37.

39. An electrochemical energy storage device according to claim 38, wherein the device is a metal-ion battery, a metal-air battery, a super capacitor or a hybrid capacitor, preferably a lithium-ion battery or lithium-air battery.

40. A method of preparing an ionic liquid precursor halide salt, comprising the steps of reacting an intermediate according to formula (IV):

Hal¹

R²

(IV) with a compound according to formula (V) or formula (VI):

(V) (VI)

wherein

Y\ Y², Y³, R\ R², L, R³, R⁴, R⁵, R⁶, R⁷ and R⁸ are as defined in any one of claims 1 to 32; and

Hal¹ is selected from F, CI, Br and I, preferably CI, Br and I.

41. The method according to claim 40, further comprising the preliminary step of preparing the intermediate according to formula (IV) by reacting a compound according to formula (II):

(ll) with a compound according to formula (III):

Hal¹ Hal²

(III) wherein

Y\ Y², R\ R², L and Hal¹ are as defined in claim 40;

R⁹ is any alkyl group, for example linear or branched C1-12 alkyl; and

Hal² is independently selected from F, CI, Br and I, preferably CI, Br and I.

42. A method according to claim 41 , wherein the compound according to formula (III) is used in stoichiometric excess.

43. A method of preparing an ionic liquid, comprising the step of reacting an ionic liquid halide precursor salt according to formula (VII):

(VII) with a lithium salt comprising an anion and a lithium cation, wherein R¹ , R², Y¹ , Y², Y³, L and G are as defined in any one of claims 1 to 32; and Hal¹ is selected from F, CI, Br and I, preferably CI, Br and I.

44. A method according to claim 43, wherein the anion is selected from one or more of acetate (CH3CO2 , PF₆ ^", BF₄ ^", triflate (CF3SO3-), nonaflate (CF₃(CF₂)3S0₃-), bis(triflyl)amide ((CF3S02)2N^"), trifluoroacetate (CF3CO2^"), heptafluorobutanoate

(CF₃(CF₂)2C0₂-), AICI4-, bistriflimide (TFSI; [(CF₃S0₂)2N]-), 2,2,2-trifluoro-/V- (trifluoromethylsulfonyl)acetamide (TSAC; CF3SO2NCOCF3^"),

bis(perfluoroethylsulfonyl)imide (BETI; [(C2FsS02)2N]^"), bis(fluorosulfonyl)imide (FSI; [(FS0₂)₂N]-) fluoroalkyi phosphate (FAP; PF₃(C₂F₅)3), 4,5-dicyano-2-(trifluoromethyl) imidazolide (TDI; CF3-CN2C2(CN)2) and asymmetric imides such as fluorosulfonyltriflimide (FTFSI; [(CF₃S0₂)(FS0₂)N]-) and [(C₂F₅S02)(FS02)N]-).

45. A method according to claim 44, wherein the anion is selected from one or more of TFSI, FSI, PF₆ and BF₄.

46. A method according to any one of claims 43 to 45, further comprising the preliminary step of preparing the ionic liquid halide precursor salt according to formula (VII) by reacting an intermediate accordin to formula (IV):

(IV) with a compound according to formula (V) or formula (VI):

(V) (VI)

wherein

Hal¹ is selected from F, CI, Br and I, preferably CI, Br and I.

47. A method according to claim 46, further comprising the preliminary step of preparing the intermediate according to formula (IV) by reacting a compound according to formula (II):

(ll) with a compound according to formula (III):

Hal¹ Hal²

(III) wherein

Y\ Y², R\ R², L and Hal¹ are as defined in claim 40; R⁹ is any alkyl group, for example linear or branched C1-12 alkyl; and Hal² is independently selected from F, CI, Br and I, preferably CI, Br and I.

48. A salt comprising an anion and a cation according to formula (I):

wherein R¹ , R², Y¹ , Y², Y³, L and G are as defined in any one of claims 1 to 32.

49. A salt according to claim 48, wherein the anion is selected from one or more of halide (F^", CI^", Br, I^"), hydroxide (OH^"), peroxide (O2^2"), sulphide (S^2"), hydrogen sulphide (HS^"), selenide (Se^2"), nitride (N^3"), azide (N3^"), phosphide (P^3"), arsenide (As^3"), carbide (C^4"), cyanide (CNT), sulfate (S0₄ ^"), sulfite (SO3^2"), hydrogen sulfate (HSOv), thiosulfate (S2O3^2"), carbonate (CO3^2") , bicarbonate (HC0₃ ^"), phosphate (P0₄ ^3"), phosphite (PO3^3"), hydrogen phosphate (HP0₄ ^2"), dihydrogen phosphate (H2P0₄ ^"), oxalate (C20₄ ^2"), cyanate (NCO^"), isocyanate (OCN^"), thiocyanate (SCN^"), chromate (Cr0₄ ^2"), dichromate (Cr₂0₇ ^2"), permanganate (Mn0₄ ^"), nitrate (NO3^") and nitrite (NO2^").

50. A salt according to claim 48, wherein the anion is selected from one or more of F^", CI^", Br and I^", preferably Br.

51. A salt according to any one of claims 48 to 50 wherein the salt is a solid.

52. Use of an ionic liquid according to any one of claims 1 to 32 in an electrolyte within an electrochemical energy storage device.

53. Use of an ionic liquid according to any one of claims 1 to 32 to provide improved electrochemical stability or increased anodic limit of an electrolyte.

54. Use of a liquid electrolyte according to any one of claims 33 to 37 within an electrochemical energy storage device.

55. Use of a liquid electrolyte according to any one of claims 33 to 37 to provide improved electrochemical stability or increased anodic limit of an electrolyte.

An ionic liquid made by a method according to any one of claims 43 to 47.