This invention relates to additives for use in electrochemical devices and the use of certain compounds as additives for this application. This invention also relates to electrolytes and electrochemical devices containing these additives.

Background to the Invention

Electrochemical devices are devices such as batteries, including Li-batteries (both Li-ion and Li-metal batteries) , capacitors, hybrid battery/capacitors, fuel cells, electromechanical actuators,, photoelectrochemical solar cells, electrochromic displays and sensors.

Electrochemical devices contain electrolytes within which charge carriers (either ions, also referred to as target ions, or other charge carrying species) can move to enable the function of the given device. There are many different types of electrolytes available for use in electrochemical devices. In the case of lithium-ion and lithium metal batteries, these include gel electrolytes, polyelectrolytes, gel polyelectrolytes, ionic liquids, plastic crystals and other non-aqueous liquids, such as ethylene carbonate, propylene carbonate and diethyl carbonate.

Ideally, the electrolytes used in these devices are required to be electrochemically stable, have high ionic conductivity, a high target ion transport number (i.e. high mobility of the target ion compared to that of other charge carriers) and provide a stable electrolyte | electrode interface which allows charge transfer. The electrolytes should ideally also be thermally stable, and non-flammable. One significant area in which the electrolytes have room for improvement is with respect to rate capability. These devices are often limited by poor transport of the charge carriers leading to lower than ideal maximum charging and discharging rates.

In the case of the lithium batteries previously mentioned, these may be primary or, more typically, secondary (rechargeable) batteries. Lithium rechargeable batteries offer advantages over other secondary battery technologies due to their higher gravimetric and volumetric capacities as well as higher specific energy.

The two classes of lithium batteries mentioned above differ in that the negative electrode is lithium metal for lithium metal batteries, and is a lithium intercalation material for the "lithium - ion batteries".

In terms of specific energy and power, lithium metal is the preferred negative electrode material. However, when λtraditional' solvents are used in combination with lithium metal negative electrodes, there is a tendency for the lithium metal electrode to develop a dendritic surface. The dendritic deposits limit cycle life and present a safety hazard due to their ability to short circuit the cell - potentially resulting in fire and explosion. These shortcomings have necessitated the use of lithium intercalation materials as negative electrodes (creating the well-known lithium-ion technology) , at the cost of additional mass and volume for the battery.

The solid electrolyte interphase (SEI) is formed on the lithium electrode surface in a lithium metal secondary cell. The SEI is a passivation layer that forms rapidly because of the reactive nature of lithium metal. The SEI has a dual role. Firstly, it forms a passivating film that protects the lithium surface from further reaction with the electrolyte and/or contaminants. In addition, the SEI acts as a lithium conductor that allows the passage of charge, as lithium ions, to and from the lithium surface during the charge/discharge cycling of a lithium metal secondary cell. The SEI is also known to form, on the surface of the negative electrode in a Li-ion cell.

However, the SEI is present as a resistive component in the cell and can lead to a reduced cell voltage (and hence cell power) in some cases. It is also thought to be a contributing factor in the formation of dendrites in lithium metal rechargeable cells.

Researchers have continued to search for a solution to the poor cycling characteristics of the lithium metal electrode - notably through the use of polymer electrolytes. However lithium ion motion in polymer electrolytes is mediated by segmental motions of the polymer chain leading to relatively low conductivity. The low conductivity and low transport number of the polymer electrolytes has restricted their application in practical devices.

Such problems of low conductivity and low transport number of the target ion apply similarly to other electrolytes used in lithium metal batteries, lithium-ion batteries, batteries more generally, and to an extent all other electrochemical devices.

Description of the Invention:

It has been found that the addition of a zwitterion to an electrolyte can significantly improve conductivity of the ion species in the electrolyte.

It has also been found that the addition of the zwitterion to an electrolyte in a lithium based energy storage device improves the properties of the SEI so that the impedance to ion transport is reduced, resulting in improved rate performance of the device.

According to the present invention, there is provided an electrolyte composition comprising a non-zwitterionie electrolyte and a zwitterion.

According to a second aspect, there is provided an electrochemical device comprising electrolyte composition comprising a non-zwitterionic electrolyte and a zwitterion.

According to a third aspect, the present invention provides for the use of a zwitterion as an additive to an electrolyte. The present invention further provides the use of a zwitterion as an additive to an electrolyte to reduce impedance of ion transport caused by the solid electrolyte interphase.

Although broadly applicable to all electrochemical devices, one commercially important application relates to the use of the zwitterion additive in electrolytes for lithium-based energy storage devices. Thus, according to a fourth aspect, there is provided a lithium-based energy storage device comprising the electrolyte composition described above. The lithium-based energy storage device suitably further comprises at least one negative electrode, at least one positive electrode, a case for containing the electrodes and electrolyte, and positive and negative device terminals external to the case. The lithium-based energy storage device may further comprise separators between the negative and positive electrodes.

According to a fifth aspect, there is also provided a range of novel zwitterions suitable for use in the above applications. Brief Description of the Figures

Figure 1 is a schematic view of a battery-like cell in accordance with one embodiment of the invention.

Figure 2 is a schematic view of a 3-electrode cell used to conduct testing of the electrolytes.

Figure 3a is a cyclic voltammogram of dry N-methyl-N- butylpyrrolidinium, bis (trifluoromethanesulfonyl) imide on Pt working electrode, Pt counter, Ag quasi-reference at 100 mVs-1. Ambient temperature under argon.

Figure 3b is a cyclic voltammogram of dry 0.15 mol/kg N- methyl-N- (n-butanesulfonate) pyrrolidinium / N-methyl-N- butylpyrrolidinium bis (trifluoromethanesulfonyl)imide on Pt working electrode, Pt counter, Ag quasi-reference at 100 mVs-1. Ambient temperature under argon.

Figure 4a is a cyclic voltammogram of dry 0.5 mol/kg lithium bis (trifluoromethanesulfonyl) imide / N-methyl-N- butylpyrrolidinium bis (trifluoromethanesulfonyl) imide. Copper working electrode, Li counter, Li quasi-reference at 100 mVs-1, ambient temperature under argon.

Figure 4b is a cyclic voltammogram of dry 0.5 mol/kg lithium bis (trifluoromethanesulfonyl) imide / N-methyl-N- butylpyrrolidinium bis (trifluoromethanesulfonyl) imide with 0.15 mol/kg N-methyl-N- (n-butanesulfonate) pyrrolidinium. Copper working electrode, Li counter, Li quasi-reference, 100 mVs-1, ambient temperature under argon.

Figure 5a is a cyclic voltammogram of dry 0.5 mol/kg lithium bis (trifluoromethanesulfonyl)imide / N-methyl-N- propylpyrrolidinium bis (trifluoromethanesulfonyl)imide. Cu working electrode, Li counter, Li quasi-reference at 100 mVs-1. Ambient temperature under argon.

Figure 5b is a Cyclic voltammogram of 0.5 mol/kg lithium bis (trifluoromethanesulfonyl)imide / N-methyl-N- propylpyrrolidinium bis (trifluoromethanesulfonyl) imide with 0.15 mol/kg N-methγl-N- (n-butanesulfonate) pyrrolidinium. Cu working electrode, Li counter, Li quasi- reference at 100 mVs-1. Ambient temperature under argon.

Figure 6a is a cyclic voltammogram of 50/50 poly(lithium 2-acrylamido-2-methyl-1-propanesulfonic acid-co-dimethyl acrylamide) / N,N'-dimethylacetamide / ethylenecarbonate gel (P(AMPSLi-c-DMΑΑ) / DMA / EC) . Ni working electrode, Li counter, Li quasi-reference at 100 mVs-1. 30°C under argon.

Figure 6b is a cyclic voltammogram for 50/50 P(AMPSLi-c- DMAA) / 1-butylimidazolium-3-(n-butanesulfonate) / DMA / EC gel. Ni working electrode, Li counter, Li quasi- reference at 100 mVs-1. 30°C under argon.

Figure 6c is a graph of 7Li diffusion coefficients in the 50/50 P(AMPSLi-c-DMAA) / DMA / EC polyelectrolyte gel, as a function of temperature, with and without the 1- butylimidazolium-3- (n-butanesulfonate) zwitterion.

Figure 7a is a graph showing the effect of zwitterion (1- butylimidazolium-3- (n-butanesulfonate) ) addition on the conductivity of the polyelectrolyte system with 10:90 P(AMPSLi-c-DMAA) in propylene carbonate (PC) and 10:90 P(AMPSLi-c-DMAA)-TiO2 in PC.

Figure 7b is a graph showing the effect of zwitterion (1- butylimidazolium-3- (n-butanesulfonate) ) addition on the conductivity of the polyelectrolyte system with 10:90 poly(lithium methyl acrylate-co-dimethyl acrylamide) (P(MALi-c-DMAA)) in polyethylene glycol (PEG) .

Figure 8 is a graph showing the effect of zwitterion (1- butylimidazolium-3- (n-butanesulfonate) ) addition on the conductivity of the 50:50 P(AMPSLi-c-DMMΑ) copolymer system. The lithium ion concentration was varied by adjusting the total polymer content, while keeping the ratio of zwitterion: (DMA/EC) co-solvent constant.

Figure 9a shows 7Li NMR spectra of the polyelectrolyte gels (i) with zwitterion (1-butylimidazolium-3- (n- butanesulfonate) ) and (ii) without zwitterion. The gels contain 10:90 P(AMPSLi-c-DMAA) copolymer/zwitterions/PC at a weight ratio of (i) 1:1:9 and (ii) 1:0:9.

Figure 9b is a graph of 7Li ion diffusion coefficients of the same 10:90 P(AMPSLi-c-DMAA) copolymer in PC systems of Figure 9a, across a range of temperatures, measured using pulse-field-gradient NMR.

Figure 10 is a graph showing the effect of zwitterion (1- butylimidazolium-3- (n-butanesulfonate) ) on the conductivity of PAMPSLi in the absence of solvent.

Figure 11 is a graph of peak current vs. zwitterion concentration (N-methyl-N- (n-butanesulfonate) pyrrolidinium) for a conventional electrolyte (1 M LiPF6 in EC:DEC 1:1 by volume - EC ethylene carbonate, DEC - diethyl carbonate) at room temperature. Cyclic voltamogram on a nickel substrate at 100 mVs-1, Li counter electrode, Li quasi-reference electrode.

Figure 12a shows a comparison of the voltage response of a symmetrical lithium cell containing 0.5 mol/kg lithium bis (trifluoromethanesulfonyl) imide / N-methyl-N- propylpyrrolidinium bis (trifluoromethanesulfonyl)imide under galvanic cycling (2 mAcm-2) with and without the addition of 0.15 mol/kg N-methyl-N-(n-butanesulfonate) pyrrolidinium at 40 °C.

Figure 12b shows a comparison of the voltage response of a symmetrical lithium cell containing 0.5 mol/kg lithium bis (trifluoromethanesulfonyl)imide / N-methyl-N- propylpyrrolidinium bis (trifluoromethanesulfonyl) imide under galvanic cycling (8 mAcm-2) with and without the addition of 0.15 mol/kg N-methyl-N- (n-butanesulfonate) pyrrolidinium at 80 °C.

Figure 13a shows a plot of the cell resistance as a function of applied current density of a symmetrical lithium cell at 40 °C containing 0.5 mol/kg lithium bis (trifluoromethanesulfonyl)imide / N-methyl-N- propylpyrrolidinium bis (trifluoromethanesulfonyl) imide with and without the addition of zwitterion (0.15 mol/kg N-methyl-N- (n-butanesulfonate) pyrrolidinium) under galvanic cycling. The cell resistance is obtained by dividing the voltage of the cell at the end of the tenth cycle (e.g., from example 12a) by the applied current density.

Figure 13b shows a plot of the cell resistance of a symmetrical lithium cell as a function of applied current density at 40 °C containing 0.5 mol/kg lithium bis (trifluoromethanesulfonyl) imide / N-methyl-N- propylpyrrolidinium bis (trifluoromethanesulfonyl) imide with and without the addition of zwitterion (0.15 mol/kg N-methyl-N- (n-butanesulfonate) pyrrolidinium) under galvanic cycling. The cell resistance is obtained by dividing the voltage of the cell at the end of the tenth cycle (e.g. , from example 12b) by the applied current density.

Figure 14a shows a 3D plot of impedance spectra obtained from a symmetrical lithium cell at 40 °C containing 0.5 m©l/3:g lithium bis (trifluorα∑aethanesulfonyl) imide / M- methyl-M-propylpyrrolidinium bis (trifluoromethanesulfonyl)imide. Each impedance spectrum is plotted against the current density (the Z axis) applied during the previous ten cycles.

Figure 14b shows a 3D plot of impedance spectra obtained from a symmetrical lithium cell at 40 °C containing 0.5 mol/kg lithium bis (trifluoromethanesulfonyl)iinide / N- methyl-N-propylpyrrolidinium bis (trifluoromethanesulfonyl)imide with added zwitterion (0.15 mol/kg if-methyl-U- (n-butanesulfonate) pyrrolidinium) . Each impedance spectrum is plotted against the current density (the Z axis) applied during the previous ten cycles.

Figure 15a shows a 3D plot of impedance spectra obtained from a symmetrical lithium cell at 80 °C containing 0.5 mol/kg lithium bis (trifluoromethanesulfonyl) imide / N- methyl-N-propylpyrrolidinium bis (trifluoromethanesulfonyl)imide. Each impedance spectrum is plotted against the current density (the Z axis) applied during the previous ten cycles.

Figure 15b shows a 3D plot of impedance spectra obtained from a symmetrical lithium cell at 80 °C containing 0.5 mol/kg lithium bis (trifluoromethanesulfonyl) imide / N- methyl-N-propylpyrrolidinium bis (trifluoromethanesulfonyl)imide with added zwitterion (0.15 mol/kg N-methyl-N- (n-butanesulfonate) pyrrolidinium) . Each impedance spectrum is plotted against the current density (the Z axis) applied during the previous ten cycles.

Figure 16 shows the cell voltage against time for two complete charge-discharge cycles for a battery cell containing 0.15 mol/kg N-methyl-N- (n-butanesulfonate) pyrrolidinium zwitterion in 0.5 mol/kg lithium bis (trifluoromethanesulfonyl)imide / N-methyl-N- butγlpyrrolidinium bis (trifluoromethanesulfonyl) imide as the electrolyte.

Figure 17 is a cyclic voltammogram of 4wt% of the zwitterion N-methyl-N- (n-butanesulfonate) pyrrolidinium in 1 mol/L solution of LiCF3SO3 in propylene carbonate, compared with a voltammogram of the same solution containing no zwitterion. Ni working electrode, Li counter, Li quasi-reference at 100 mVs-1. 30°C under argon.

Detailed Description

Zwitterions Zwitterions are a well-known class of compounds, and are generally understood to be compounds containing both a positive (cation) and a negative (anion) charge (at the iso-electric point for the zwitterion) .

Zwitterions encompass the compounds of the structure: X+-Y-Z" in which: X+ represents the cation component, Y represents the linking group, and Z- represents the anion component.

Of particular interest are the zwitterions exhibiting electrochemical stability, which can be judged by the ability of the zwitterion to resist oxidation and reduction at a polarised electrode. This can be demonstrated by using cyclic voltammetry to measure the electrode response at extreme potentials.

Cation component The atom providing the positive charge to the cation component of the zwitterion is preferably selected from N, P and Αs.

The cation components may be categorised into a number of subclasses, including the unsaturated heterocyclic cations, the saturated heterocyclic cations, and the non- cyclic quaternary cations.

The unsaturated heterocyclic cations encompass the substituted and unsubstituted pyridiniums, pyridaziniums, pyrimidiniums, pyraziniums, imidazoliums, pγrazoliums, thiazoliums, oxazoliums and triazoliums, two-ring system equivalents thereof (such as isoindoliniums) and so forth. These ring systems may be attached to the linking group via any atom thereof. The general class of unsaturated heterocyclic cations may be divided into a first subgroup encompassing pyridiniums, pyridaziniums, pyrimidiniums, pyraziniums, pyrazoliums, thiazoliums, oxazoliums, triazoliums, and multi-ring (i.e., two or more rings) unsaturated heterocyclic ring systems such as the isoindoliniums, on the one hand, and a second subgroup encompassing imidazoliums, on the other.

Two examples of this general class are represented below:

in which: R1 to R each independently represent a bond to the linking group Y, H, alkyl, haloalkyl, thio, alkylthio, haloalkylthio, or any other substituent within the classes outlined below, with the proviso that at least one of the R groups present represents a bond to a linking group Y.

The saturated heterocyclic cations encompass the pyrrolidiniums, piperaziniums, piperidiniums, and the phosphorous and arsenic derivatives thereof. These rings may be attached to the linking group via any atom thereof, such as the heteroatom, or a carbon atom of the ring or a substituent thereon.

Examples of these are represented below:

in which: R1 to R12 each independently represent a bond to the linking group Y, H, alkyl, haloalkyl, thio, alkylthio, haloalkylthio, or any other substituent within the classes outlined below, with the proviso that at least one of the R groups present represents a bond to a linking group Y.

The non-cyclic quaternary cations encompass the quaternary ammonium, phosphonium and arsenic derivatives.

Examples of these are represented below:

in which: R1 to R4 each independently represent a bond to the linking group Y, H, alkyl, haloalkyl, thio, alkylthio, haloalkylthio, or any other substituent within the classes outlined below, with the proviso that at least one of the R groups present represents a bond to a linking group Y.

Linking Group The linking group Y may be any chain of atoms that links together or tethers the cation and anion components. These may be hydrocarbon chains such as alkyl, alkenyl or alkynyl (straight chain, branched or cyclic) , optionally containing heteroatoms in the chain or in branches thereof, such as 0, S, N and P, therefore encompassing ethers, amines, amides, with the chain being optionally substituted by any of the substituents outlined below, being a substituent that does not adversely affect electrochemical stability of the zwitterion or conductivity of the electrolyte composition containing the zwitterion. According to one embodiment, the optional linking group substituents are selected from halo, nitrile, haloalkyl,, amine, amide, imine, sulfonyl and the like.

Preferably, the linking group is at least 2 atoms in length. The upper limit of the length is determined by practical considerations such as effectiveness of the zwitterion as the linking group length increases as compared with molecular weight and cost, as well as physical factors such as ^fiscosity. Generally the linking group will be between 2 and 8 atoms in length.

According to one embodiment, the linking group is suitably an optionally substituted alkyl group. This may be a C2 - C8 alkyl group, for instance.

Definitions of Groups referred to above with respect to X and Y The optional substituents in each case may be any one or more groups that do not adversely affect electrochemical stability of the zwitterion and/or the conductivity of the electrolyte composition containing the zwitterion. The groups from which the substituents may be selected are alkyl, alkenyl, alkynyl, aryl, halo, haloalkyl, haloalkenyl, haloalkynyl, haloaryl, hydroxy, alkoxy, alkenyloxy, aryloxy, benzyloxy, haloalkoxy, haloalkenyloxy, haloaryloxy, nitro, cyano, nitroalkyl, nitroalkenyl, nitroalkynyl, nitroaryl, nitroheterocyclyl, amino, alkylamino, dialkylamino, alkenylamino, alkynylamino, arylamino, diarylamino, benzylamino, dibenzylamino, acyl, alkenylacyl, alkynylacyl, arylacyl, acylamino, diacylamino, acyloxy, alkylsulphonyloxy, arylsulphenyloxy, heterocyclyl, heterocycloxy, heterocyclamino, haloheterocyclyl, alkylsulphenyl, arylsulphenyl, carboalkoxy, carboaryloxy, mercapto, alkylthio, haloalkylthio, benzylthio, acylthio, phosphorus-containing groups and combinations of the abo^e .

"Halo" refers to any of the halogens F, Cl, I and Br. References to haloalkyl and the like mean that the subject group is partially or fully halogenated.

The term "alkyl"" denotes straight chain, branched or mono- or poly- cyclic alkyl, preferably Cl-30 alkyl or cycloalkyl. Examples of straight chain and branched alkyl include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, hexyl, octyl, dodecyl, 1-methylundecyl and the like. The terms alkenyl and alkynyl have a corresponding meaning, with the proviso that they contain between 2 and 30 atoms.

Anion In general terms, any anion component that can be linked by one atom to the linking group Y, and supports the electrochemical stability of the zwitterion, may be used.

The atom providing the negative charge of the anion component of the zwitterion may be selected from S, C, P, N, O or B.

The possible components may be categorised into a number of subclasses, as follows:

(i) -SO3". (ii) -CO2-. (iii) -PO4" or other phosphates. (iv) amides. (v) -CxD2xCOO", including CD2COO", in which D=H or F and x is any integer of 1 to 8, such as the carboxylic acid anions and fluorinated or perfluorinated versions thereof. (vi) anions containing a carbon nitrogen bond such as cyanamide compounds. (vii) CxD2xSO3 where s = 1 to 5 and D = F or H. This class encompasses -CH2SO3 and -CF2SO3- as examples - i.e. any carbonyl chain partially or full fluorinated containing an anionic sulfonate group. (viii) Ethylenedisulfonylamide and its perfluorinated analogue. (ix) Other carboxylic acid derivatives. (x) sulfonyl amides generally including the bis amides and the part or perfluorinated versions thereof, and including specifically -C-CH2-SO2-)-N--SO2-CH3, -(-CF2- SO2-) -N--SO2-CF3, - (C2H4-SO2) -N--SO2-C2H5. (xi) boroxines/borates. (xii) fluoroalkyl phosphates. (xiii) sulfonyl and sulfonate compounds, namely anions containing the sulfonyl group -SO2- (with another anion species) , or sulfonate group -SO3-, not covered by groups (i) , (vii) and (x) above. This class encompasses aromatic sulfonates containing optionally substituted aromatic Caryl) groups, such as toluene sulfonate anions and xylene sulfonate anions.

Amongst these anions, the preferred classes are those outlined in groups (i) , (vi) , (x) , (xi) , (xii) and (xiii) .

Further information on Zwitterions The advantage of using a zwitterion over an ionic liquid (without zwitterion additive) in an electrochemical system is that although the ionic conductivity of the ionic liquid is usually very high, the contribution is not entirely the result of target ion migration. In other words, even with the addition of target ions, the ionic liquid component ions also migrate along the potential gradient. Since a zwitterion contains cations and anions that are physically tethered together, they do not migrate along the potential gradient. Zwitterions have also been shown to be effective at dissociating lithium in polymer/polyelectrolyte systems. Zwitterions are therefore shown to be effective at dissociating the target ion, which may lead to enhanced rate capabilities when used as the electrolyte in electrochemical devices.

Non-swittθzlonlc electrolyte The basic electrolyte to which the zwitterion is added is non-zwitterionie (thus distinguishing it from the additive) . The electrolyte may be from any of the classes of known electrolytes, including polymers, polymer gels, polyeleetrolytes, polyelectrolyte gels, traditional solvents, ionic liquids or any other electrolyte material.

The term "composition" is used in its broadest sense to cover any compositions made up of any matter, including liquids, solids, membranes, gels and so forth.

The electrolyte generally further comprises mobile ions (otherwise referred to as target ions) of any suitable type. Suitable mobile ions for various embodiments may be selected from the group consisting of lithium, hydrogen, sodium, magnesium, aluminium and zinc. According to one embodiment, the mobile ions are lithium.

In the case of lithium, the lithium ions may be present or introduced into the electrolyte in a number of different ways, depending on the nature of the electrolyte. For polyelectrolytes, the lithium ions are the counterions to the negatively charged polymer. In the case of liquid electrolytes such as ionic liquids, they are added as lithium salts. This is sometimes referred to as doping. In the case of other lithium-containing electrolytes (e.g., aprotic liquids and polymers) , the lithium is added to the solvent as a lithium salt, as in the case of LiPF6.

In the case of ionic liquid electrolytes, the lithium ions are generally incorporated into the electrolyte by the addition of a lithium salt, consisting of lithium ions and counterions. Onee added, the lithium ions and counterions dissociate, and are effectively a solute to the room temperature ionic liquid solvent. If the counterions are the same as the anion of the room temperature ionic liquid, then the lithium addition can be considered to be doping of the electrolyte. In other words, doping can be considered as a cation substitution.

The concentration of lithium (or dopant) can be between 0.01% and 90% of the overall electrolyte composition by weight, preferably between 1 and 49% by weight. It is generally simpler to refer to the lithium concentration of the electrolyte in moles of lithium ions per kilogram of total electrolyte, and in this unit the lithium is suitably present in an amount of from 0.01 to 2.0 mol/kg, preferably 0.1 - 1.5 mol/kg, and most preferably 0.2 - 0.6 mol/kg.

In the case of electrolytes designed for use in other types of device the amount of mobile ion added is determined as that concentration which produces the optimum active ion flux in the device. This often corresponds approximately with the optimum conductivity point.

The zwitterionic additive is suitably used in a molar ratio of 3:1 to 0.01:1 ( [zwitterion] : [cation] ) preferably 2:1 to 0.05:1, most preferably 1.5:1 to 0.1:1, with respect to mobile ions in the electrolyte. The ideal molar ratio differs according to the type of solvent (e.g., polymer, ionic liquid or aprotic solvent) and is dictated by the conflicting forces of increasing dissociation and increasing viscosity which occur with the addition of the zwitterion. Thus the ideal molar ratio is a function of the properties of the zwitterion, the solvent, the mobile ion (salt) and any other additives which might be present. The electrolyte composition may further comprise any other diluents, solvents or any other additives.

One particularly suitable class of additives for inclusion in the electrolyte composition are the gelling additives. Gelling additives may be used to impart gel properties. Gels may be considered to be "quasi-solids" as they have some structural properties, but retain the conductive properties of the liquid.

The gelling additives may be selected from inorganic particulate materials (sometimes referred to as nanocomposites or nano-fillers, being fine particulate inorganic composites) . Amongst these, examples are SiO2, TiO2 and Al2O3. Other gelling additives are the polymer gelling additives.

Other suitable additives are polyelectrolytes, other zwitterionic compounds, and other electrolytes.

These further additives, particularly the nano-fillers, have been shown to increase the transport and ionic conductivity of the charge carriers in some cases.

In addition, these additives, particularly the nano- fillers, are shown to improve the deposit morphology and efficiency of the lithium cycling process and are also claimed for this invention. One of the additives also provides a quasi-solid gel material while retaining the conductivity of the liquid. This offers specific benefits over liquids in that it enables the fabrication of flexible, compact, laminated all solid-state devices free from leakage and in varied geometries.

Thus, according to one embodiment, the electrolyte composition may further comprise a gelling additive in a weight ratio of 0.01 to 20 wt% with respect to the electrolyte, preferably 1 to 10 wt%.

Other possible additives are solvents, such as organic solvents. Preferred organic solvents are water immiscible organic solvents. When present, the organic solvent may be used in an amount of 0-90 wt%, preferably 10-70 wt%.

Preparation of the Electrolyte Compositions The electrolyte compositions may be prepared by the addition of a zwitterion to a mobile ion-containing electrolyte, optionally together with further additives, and/or solvent and/or polymer and/or polyelectrolyte. Α cosolvent can be used to dissolve all of the components. The composition should be mixed until homogeneous and then the cosolvent removed and the mixture dried according to the appropriate procedure for the type of electrolyte (e.g., ionic liquids and polymers can be dried under vacuum at elevated temperatures, other solvents must be distilled off at reduced pressure) . In addition, the liquid compositions may be degassed with a stream of dry argon to remove dissolved gases and residual water.

Electrochemical Devices The term "electrochemical devices" broadly encompasses all devices containing an electrolyte, such as batteries, including Li-batteries (both Li-ion and Li-metal batteries) , capacitors, hybrid battery/capacitors, fuel cells, electromechanical actuators, photoelectrochemical solar cells, electrochromic displays and sensors.

Within this class are the energy storage devices. The term energy storage device encompasses any device that stores or holds electrical energy, and encompasses batteries, supercapacitors and asymmetric (hybrid) battery- supercapacitors. The term battery encompasses single cells. Lithium based energy storage devices are ones that contain lithium ions in the electrolyte.

Lithium battery encompasses both lithium ion batteries and lithium metal batteries.

Lithium ion batteries and lithium metal batteries are well known and understood devices, the typical general components of which are well known in the art of the invention.

Secondary lithium batteries are lithium batteries which are rechargeable. The combination of the electrolyte and negative electrode of such batteries must be such as to enable both plating/alloying (or intercalation) of lithium onto the electrode (i.e. charging) and stripping/de- alloying (or de-intercalation) of lithium from the electrode (i.e. discharging) . The electrolyte is required to have a high stability towards lithium, for instance approaching ~0V vs. Li/Li+. The electrolyte cycle life is also required to be sufficiently good, for instance at least 100 cycles (for some applications), and for others, at least 1000 cycles.

Secondary Lithium Batteries The general components of a secondary lithium battery are well known and understood in the art of the invention. The principal components are: a battery case, of any suitable shape, standard or otherwise, which is made from an appropriate material for containing the electrolyte, such as aluminium or steel, and usually not plastic; battery terminals of a typical configuration; at least one negative electrode; at least one positive electrode; optionally, a separator for separating the negative electrode from the positive electrode (for liquid electrolytes - for polymer electrolytes these are not required as the polymer separates the electrodes) ; and an electrolyte (in this case,, the electrolyte composition described abo-sre) .

The negative electrode comprises a metal substrate,, which acts as a current collector, and a negative electrode material. The negative electrode material can be lithium metal, a lithium alloy forming material, or a lithium intercalation material; lithium can be reduced onto/into any of these materials electrochemically in the device.

The metal substrate underlying the lithium may be any suitable metal or alloy, and may for instance be formed from one or more of the metals Pt, Au, Ti, Al, W, Cu or Hi. Preferably the metal substrate is Cu or Ni.

The negative electrode surface may be formed either in situ or as a native film. The term "native film" is well understood in the art, and refers to a surface film that is formed on the electrode surface upon exposure to a controlled environment prior to contacting the electrolyte. The exact identity of the film will depend on the conditions under which it is formed, and the term encompasses these variations. The surface may alternatively be formed in situ, by reaction of the negative electrode surface with the electrolyte. The use of a native film is preferred.

The positive electrode is formed from any typical lithium intercalation material, such as transition metal oxides and their lithium compounds. As known in the art, transition metal oxide composite material is mixed with binder such as a polymeric binder, and any appropriate conductive additives such as graphite, before being applied to or formed into a current collector of appropriate shape. ■ξpJhen present, the separator may be of any type known in the art, including glass fibre separators and polymeric separators, particularly microporσus polyolefins.

Usually the battery will be in the form of a single cell, although multiple cells are possible. The cell or cells may be in plate or spiral form, or any other form. The negative electrode and positive electrode are in electrical connection with the battery terminals.

Other Devices The conductivity and electrochemical stability that have been noted in the deployment of these electrolyte compositions, demonstrate that these same electrolytes will function well in other energy storage devices such as supercapacitors and asymmetric (hybrid) battery- supercapacitor devices. For the asymmetric device, one of the lithium battery electrodes (either the positive or the negative) is replaced with a supercapacitor electrode. For a supercapacitor, both lithium battery electrodes are replaced by supercapacitor electrodes. Supercapacitor electrodes, by comparison with lithium battery electrodes, are relatively simple structures at which the interaction with the electrolyte is simply an electrostatic charging and discharging of the electrochemical double layer. Supercapacitors are also commonly known as electrochemical double-layer capacitors (EDLCs) . Suitable electrolytes for supercapacitors are, like those described here, electrolytes with high ionic conductivity and high electrochemical stability (large voltage range) .

In its general form, a supercapacitor comprises: a device case; - terminals for electrical connection; at least one negative electrode, which may be formed from a "double layer capacitor" type of material, such as a mixture of conductive carbon and highly activated (high surface area) carbon, which are bound to a metallic substrate (current collector) , or from a pseudocapacitive redox material, displaying pseudocapacitive behaviour; - at least one positive electrode,, of one of the types described above in relation to the negative electrode; optionally, a separator for maintaining physical separation of the negative and positive electrode ; and the electrolyte composition as described herein.

The negative and positive supercapacitor materials and methods for manufacture are well known and understood in the art of the invention.

Asymmetric (hybrid) battery-supercapacitors are devices in which one battery electrode is combined with one supercapacitor electrode to yield an energy storage device which has properties that are intermediate between those of batteries and supercapacitors. In its general form, an asymmetric battery-supercapacitor comprises: a device case; terminals for electrical connection; a negative electrode; a positive electrode; - a separator for maintaining physical separation of the positive and negative electricity; and the electrolyte as described herein, wherein one of said negative electrode and positive electrode is a battery electrode, and the other electrode is a double-layer capacitor electrode.

The nature and composition of the battery and capacitor electrodes are fully described above, and are of the form and composition well known in the art. If the negative electrode is a battery negative electrode, such as a lithium intercalation material or a lithium metal electrode, then the positive electrode is a double layer capacitor positive electrode, typically a high surface area carbon electrode material bonded to a metal substrate. If the negative electrode is a double layer capacitor electrode, typically a high surface area carbon electrode material bonded to a metal substrate, then the positive electrode is a battery electrode, such as one that contains a lithium intercalation material.

For supercapacitors, the electrolyte may contain some lithium ions, but need not do so. Accordingly, in this embodiment of the invention, the presence of lithium ions is optional.

Examples The present invention will now be described in further detail with reference to the following non-limiting Examples.

Materials and Preparation

Zwitterions N-methyl-N-(n-butanesυlfonate) pyrrolidiniυm zwitterion N-methylpyrrolidine (4.1 mL, 0.04 moles) was added to a solution of I,4-butane sultone (4 ml, 0.04 moles) in acetone (50 ml) and the solution stirred under N2 for 6 days at room temperature, during which time the product slowly formed as a white precipitate. This was removed by filtration under nitrogen, the product washed three times with acetone (3x20 ml) and dried under vacuum. The product was obtained as a hygroscopic white powder, melting point 322 °C. 1H NMR (300 MHz, CDCl3) δ 1.25 (m, 2H, CH2) , 1.9 (m, 2H, CH2) , 2.2 (m, 4H, CH2) , 2.8 (t, 2H, CH2), 2.9 (s, 3H, CH3) , 3.3 (m, 2H, CH2), 3.4 (m, 4H, CH2) .

N-methyl-N- (n-propanesulfonate) pyrrolidiniτuα zwitterion N-methylpyrrolidine (4.7 mL, 0.0456 moles) was added to a solution of 1,3-propane sultone (4 ml, 0.0456 moles) in acetonitrile (50 ml) and the solution stirred under H2 for 6 days at room, temperature, during which time the product slowly formed as a white precipitate. This was removed by filtration under nitrogen, the product washed three times with acetone (3x20 ml) and dried under vacuum. The product was obtained as a hygroscopic white powder, melting point 320 °C. 1H HMR (300 MHs, D2O) δ 2.2 (m, 6H, CH2) , 2.9 (t, 2H, CH2), 3.0 (s, 3H, CH3) , 3.5 (m, 6H, CH2) .

1-butylimidazoliυm-3-(n-butanesulfonate) zwitterion 1-butylimidazolium-3-n-butanesulfonate was synthesized following the procedure of Yoshisawa et al. for analogous z witter ionic species. 1-butylimidazole (15 g, 0.1208 moles) was added to a solution of 1,4-butane sultone (12.36 ml, 0.1208 moles) in acetone (50 ml) and the solution stirred under N2 for 6 days at room temperature, during which time the product slowly formed as a white precipitate. This was removed by filtration under nitrogen, the product washed three times with acetone (20 ml) and dried under vacuum. The product was obtained as a hygroscopic white powder, melting point 152°C, yield = 80%. 1H NMR (300 MHz, d6-dmso) δ 0.8 (m, 3H, CH3), 1.2 (m, 2H, CH2) , 1.5 (m, 2H, CH2), 1.7 (m, 2H, CH2) , 1.8 (m, 2H, CH2) , 2.4 (m, 2H, CH2), 4.2 (m, 4H, N-CH2), 7.8 (m, 2H, H4,H5) , 9.2 (s, IH, H2) .

Electrolytes These were available commercially, or in the case of ionic liquids, they were prepared by the methods set out in our copending application PCT/AU2004/000263, with and without lithium salt, as specified in individual examples.

The copolymer-type polyelectrolytes, P(AMPSLi-c-DMAA) and P(MALi-c-DMAA) , were prepared by free-radical polymerization of DMA (Aldrich) and AMPSLi, or MALi, obtained by neutralization of AMPS or MA (Aldrich) with lithium carbonate (Aldrich) , and 0.15 mol% of potassium persulphate (M&B) as initiator. Polymerisation was carried out at 50°C for 2 hours and then at room temperature for 24 hours. The copolymers (solid powder) were dried under vacuum (80 torr) at 65°C for at least 3 days. The polyelectrolyte gels of P(AMPSLi-c-DMΑΑ) were prepared by raising the copolymer with PC, with or without zwitterion, at elevated temperatures for more than one day. For the (MALi-c-DMAA) system, PEG200 was used as the solvent. The zwitterion was used at a weight ratio of copolymer/zwitterion/solvent of 1:1:9 in Example 5.

In Example 5, the polyelectrolyte-TiO2 system, TiO2 was polymerised In situ by mixing the monomers in the ratio of 10 mol% of AMPSLi and 90 mol% of DMAA in PC, with or without zwitterions. The concentration of polyelectrolyte, PC and zwitterions was the same as in the above system of Example 5.

The ionic liquid electrolyte compositions containing zwitterions were prepared by the addition of the zwitterion to the electrolyte at room temperature, mixing and drying under vacuum at elevated temperatures. The solutions were degassed with a stream of dry argon to remove dissolved gases and residual water.

The molecular solvent electrolyte compositions containing zwitterions were prepared by the addition of the zwitterion to the electrolyte (battery grade) at room temperature, the resulting solution was mixed thoroughly.

Battery-like cells 'Battery' like cells were fabricated using resealable stainless steel cells which were developed in-house, as illustrated in Figure 1. The basic design incorporated a case 1, electrodes 2a and 2b, a separator 3 incorporating electrolyte, polypropylene sleeves 4, a socket head screw 5, and a Teflon gasket 6 to seal, and electrically isolate, the two halves of the cell. Stack pressure in the cell was maintained fojf means of a spring 7, which applied ~1 kgcπf2 stack pressure perpendicular to the electrode surface.

For the symmetrical cells demonstrated here, both electrodes were formed from lithium metal. This was formed from lithium metal foil (Jkldrich 99.9 % - thickness 180 μm) , which was washed with hexane and brushed with a polyethylene brush

Glass fibre mats or microporous polyolefin sheets were cut to sise and used as the separators.

For the complete battery cells demonstrated here, the positive electrode (2a) was prepared by coating an aluminium foil with an active material formulation. The active material (AM) , was LiCoO2. The electrode coating was prepared by weighing the components in the following ratios; AM - 80%, Graphite (KS4) - 7%, Carbon Black - 3%, PVdF - 10%. The solid components were mixed in a mortar and pestle and a quantity of dimethy1acetamide (DMAc ~130%) was added slowly with mixing to form a slurry. The slurry was transferred to a beaker and heated (low heat) with constant stirring until the mixture had reached the correct consistency. The slurry was then applied to the current collector (aluminium) using the doctor blade technique. The resulting coated foil was then dried at 60 °C for several hours prior to drying under vacuum at 60 °C for greater than 24 hours.

Galvanostatic cycling of battery-like cells The symmetrical cells were tested by applying a galvanic square wave and monitoring the voltage response of the cell. Each cycle was set to pass a constant amount of lithium of 0.1 Gcπf2 of lithium. The applied current was increased sequentially at ten cycle intervals until the voltage response of the cell began to increase dramatically. M, dramatic increase in the voltage response indicates that the lithium redox process is no longer able to sustain the applied current and another process (most likely solvent reduction) has begun to supply the required current. In most cases this type of process is likely to be poorly reversible and will result in the consumption of components of the electrolyte and will ultimately result in failure of the cell. Thus, an experiment of this type pro-rides a comparison of the rate capability of the electrolyte with and ifithout the addition of zwitterion.

Electrochemica.1 Impedance spectroscopy (EIS) At the end of each group of ten galiranostatic cycles, electrochemical impedance spectroscopy was performed. This measurement provides information about the resistive and capacitive response of the cell when it is at rest. In this way it is possible to monitor the various resistive components of the cell after each cycling period to determine whether the incremental increase in applied current has had any effect on the components of the cell. A cell in which the lithium redox process has been able to maintain the required current density will not exhibit significant (detrimental) change in its impedance response. These measurements also allow comparison of the magnitude of the resistances present in the symmetrical cells with and without the addition of zwitterion.

Cycling of complete battery cells The complete battery cells were cycled under a galvanostatic regime at the C/10 rate at 50°C. C refers to the C rate of the cell which corresponds to the current required to charge the cell to its theoretical capacity in 1 hr, thus C/10 is the current required to charge the cell in 10 hours. The theoretical capacity was determined from the mass of AM coated onto the aluminium foil; in the case of the LiCoO2 the specific capacity used was 150 mAh/g. The cells were charged at the C/10 current up to a voltage of 4.2 V and discharged at the C/10 current down to a voltage of 3.0 V.

3-electrod@ cells The electrochemical measurements were performed in a 3- electrode cell 8, consisting of a platinum (or copper or nickel) working electrode (WE) 9, a lithium quasi- reference electrode (RE) 10, and a lithium counter electrode (CE) 11. The cell is shown schematically in Figure 2.

For the cyclic ifoltammetrj measurements the potential (vs. Li/Li+) was scanned from 2000 mV to -500 mV and back to 2000 mV at a rate of 100 mVs-1. The current response of the working electrode was recorded throughout the experiment. At low potentials (< ~0 mV vs. Li/Li+) , Li+ is reduced to Li(s) and is deposited on the electrode producing a negative current (corresponding to charging a Li metal cell) . When the scan is reversed and the potential rises above ~ 0 mV (vs. Li/Li+) Li(s) is oxidised to Li+ (dissolution) , producing a positive current (corresponding to discharging a Li metal cell) . Integrating the curves provides a measure of the amount of charge deposited (reduced Li+) and the amount of charge stripped (oxidised Li(s) . In this case the ratio of [oxidised Li (s) :reduced Li+] provides a measure of the efficiency of the deposition/dissolution process. An efficiency of less than 100% indicates that the deposited lithium has reacted with the electrolyte and/or contaminants to produce a product that is not electrochemically reversible.

Example 1 This example compares the results of cyclic voltammetry tests on an electrolyte without zwitterion additive, and with zwitterion additive, and show that the addition of the zwitterion does not adversely affect the electrochemical stability and high reversibility.

The electrochemical stability of N-methγl-N- butylpyrrolidinium bis (trifluoromethanesulfonyl)imide ionic liquid electrolyte was determined by cyclic voltammetry using a silver quasi-reference electrode and Pt working and counter electrodes (Fig. 3a) . Large irreversible oxidation or reduction peaks were not observed indicating that degradation of the electrolyte had not occurred within the potential region of the experiment. Fig 3b shows the electrochemical stability of the above electrolyte with 0.15 mol/kg N-methγl-N- (n- butanesulfonate) pyrrolidinium zwitterions added and again no large irreversible oxidation or reduction peaks were observed.

Example 2 This example compares the cyclic voltamograms for an electrolyte containing lithium ions (0.5 mol/kg) with and without zwitterion additive, and shows that the addition of zwitterion does not adversely impact on electrochemical stability, and moreover gives rise to a greater peak height, representing higher rate capability.

The electrochemical stability of a 0.5 mol/kg lithium bis (trifluoromethanesulfonyl)imide / N-methyl-N- butylpyrrolidinium bis (trifluoromethanesulfonyl) imide on a lithium electrode deposited on to a Cu substrate is determined by cyclic voltammetry (Fig. 4a.) . High reversibility of the lithium deposition/dissolution process is observed with no indication of degradation of the electrolyte. Fig 4b shows the electrochemical stability of the above electrolyte with 0.15 mol/kg N- methyl-N- (n-butanesulfonate) pyrrolidinium zwitterion added and again high reversibility of the lithium deposition/dissolution process was observed. Comparison of the peak currents in the two figures indicates that lithium transport (and hence rate capability) is enhanced in the presence of the zwitterion.

Example 3 This example repeats the work of example 2 above with a different electrolyte, and demonstrates an even greater increase in the peak height, again demonstrating better rate capability.

The electrochemical stability of a 0.5 mol/kg lithium bis(trifluoromethanesulfonyl)imide in N-methyl-N- propylpyrrolidinium bis (trifluoromethanesulfonyl) imide on a lithium electrode deposited on to a Cu substrate is determined by cyclic voltammetry (Fig. 3a.) . High reversibility is observed with no indication of degradation of the electrolyte. Fig 3b shows the electrochemical stability of the above electrolyte with 0.15 mol/kg N-methyl-N-(n-butanesulfonate) pyrrolidinium zwitterion and again high reversibility was observed. Comparison of the peak currents in the two figures indicates that lithium transport is enhanced in the presence of the zwitterion.

Example 4 This example repeats the work of Example 2 with a polyelectrolyte in place of the ionic liquid electrolyte and again shows stability and enhanced rate capability with the addition of zwitterion.

The electrochemical stability of poly(lithium 2- acrylamido-2-methyl-1-propanesulfoniσ acid-co-dimethyl acrylamide) in molar ratio 2:1 N,N'-dimethylacetamide : ethylenecarbonate is shown in Figure 6a. The improvement in performance of this material on addition of 1- butylimidazolium-3- (n-butanesulfonate) zwitterion is shown in Figure 6b. Figure 6c shows the increase in the lithium ion, diffusion within this system that is achieved on addition of this zwitterion.

Example 5 The zwitterion is a white powder at room temperature, of melting point 152 °C, with an inherently low conductivity (<10-7 S cm-1 at 70°C) . The applicability of this material as a lithium ion dissociator in a range of lithium polyelectrolyte systems was first examined using a random copolymer of 10 wt% lithium 2-acrylamido-2-methyl-1- propanesulfonic acid (AMPSLi) and 90 wt% N, W- dimethylacryl amide (DMAΑ) . The copolymer is a transparent solid material, P(AMPSLi-c-DMAΑ) , at room temperature. Clear, flexible polyelectrolyte gels were prepared by mixing this copolymer with propylene carbonate (PC) , with or without zwitterion, and stirring at 60°C for 24 hours. The zwitterion becomes insoluble in this system at concentrations of 10 wt% and above, and so was used at a weight ratio of copolymer/zwitterions/solvent of 1:1:9. Figure 7a shows the effect of addition of 9 wt% zwitterion on this system, with the conductivity more than tripling (5.6 x 10-5 S cm-1 compared with 1.6 x 10-5 S cm-1 at 30 °C) .

The applicability of the zwitterion effect to other polyelectrolyte systems was investigated using a lithium methyl acrylate copolymer system, P(MALi-c-DMAA) , composed of 10 mol% of lithium methacrylate (MALi) and 90 mol% of N,N-dimethylacryl amide (DMAA) . In this system polyethyleneglycol (PEG) was used as a solvent, and again the polyelectrolyte formed was a clear elastomeric gel at room temperature. As before, addition of 9 wt% zwitterion to this system results in an increase in conductivity (3.87 x 10~5 compared with 1.98 x 10-5 S cm-1 at 70 °C), shown in Figure 7b. Thus, the zwitterion effect is not a unique feature of one system, but is equally applicable to other copolymers and, importantly, to different solvent systems. Both polyeleσtrolyte systems show a clear dependence of conductivity on temperature. This is primarily a mobility effect - the molecular mobility of the systems increases with temperature, which increases the mobility of the lithium ions and thus increases the conductivity.

Figure 7a also demonstrates the effect of the zwitterion on the polyelectrolyte system to which nanoparticulate inorganic filler has been added. Such nanocomposites have been much studied for the purposes of increasing lithium ion mobility. Here we investigate whether the zwitterionic dissociation enhancement effect is additive to the mechanisms that enhance conduction in the nanocomposites. The results show that even when the polyelectrolyte system already contains a nanosized filler, 6 wt% TiO2, to enhance the conductivity, addition of the zwitterion results in a further doubling of the conductivity (Figure 7a) . Even though the filler content is present at a previously established optimum concentration, and addition of more filler would cause a decrease in conductivity, addition of zwitterion results in a conductivity increase. Thus, the zwitterion appears able to dissociate lithium ions in a manner that supercedes that of the inorganic filler systems, and the mixed system has an additive effect on the conductivity.

To further assess the scope of the lithium dissociating effect of the zwitterion, and in an attempt to push this effect to the limit, the effect of the zwitterion on the conductivity of a copolymer system with much higher lithium content and a more fluid solvent system was studied. The effect of zwitterion addition on the conductivity of 50:50 P(AMPSLi-c-DMAA) , which contains equimolar amounts of AMPSLi and DMAA, in a co-solvent of N,N'-dimethy1acetamide (DMA) and ethylene carbonate (EC) (2:1 by weight) , is shown in Figure 8. ) . These poljfelectrol^ftes ar© transparent flexible gels at room temperature, both with and without zwitterion.

It is clear from the data shown in. Figure 8 that e"?ren at higher lithium content and at 30°C,, the effect of the zwitterion is considerable. Across the composition ranges studied, the conductivity of the system containing zwitterion is up to seven times greater than that without zwitterion.

The relationship between lithium ion concentration and conductivity is similar for both systems, with a maximum at around 0.047 mole fraction of Li+, above which the conductivity begins to fall slowly. This is considered to be a result of repulsion of the increased number of charges on the polymer backbone, which causes the polymer chains to condense into tight coils, restricting the movement of the lithium ions in the dissociated state between the chains. This effect falls off at high lithium concentrations, and is the basis for the so-called lithium-ion condensation phenomenon. A second possible explanation is that as the lithium concentration in the polyelectrolyte is increased, the local molecular motion is decreased through interaction between the lithium ion and the solvent system. However, this is not reflected in the glass transition temperatures (Tg) of the gels, measured using differential scanning calorimetry (data not shown) . A mobility decrease would be reflected in a rise in the Tg of the materials, but Tg remains effectively constant across the composition range.

All of the conductivity results reported herein strongly suggest that the zwitterion helps to dissociate the lithium ions from the polymer backbone of the polyelectrolytes, thereby increasing the average mobility of the lithium ions in the system. To confirm this, the 7Li NMR spectra of the 10:90 P(AMPSLi-c-DMAA) copolymer in PC gel system, with and without zwitterion, were measured (Figure 9a) . It is immediately clear that the peak in the spectra of the polyelectrolyte gel that contains zwitterion (i) is much stronger and narrower than that with no zwitterion present (ii) . This indicates that this system contains a significantly higher concentration of mobile lithium ions than the system without zwitterion, demonstrating the significant effect of the zwitterion on the average mobility of the lithium ions within the gels. Further, the average 7Li diffusion coefficients of the lithium ions within the two systems, across a range of temperatures, were measured using pulsed-field-gradient NMR analysis (Figure 9b) . In the absence of zwitterion, the 7Li diffusivity is too low to be detected below 50°C, whereas in the presence of zwitterion the 7Li ion whereas in the presence of zwitterion the 7Li diffusion coefficients can be measured at room temperature. It is also clear that the 7Li ion diffusivity of the gels steadily increases with temperature, which is entirely consistent with the conductivity increase noted previously (Figure 7a) .

The substantial conductivity enhancements observed on addition of the zwitterion to the solvent-lithium polyelectrolyte systems led us to examine the possibility of using the zwitterion in a solvent-free system, which is ultimately more desirable for solid-state lithium battery applications. Solvent-free mixtures of zwitterion and PAMPSLi were prepared by mixing the two solids together and heating to about 150 °C, above the eutectic point, for three days. The effect of addition of the zwitterion to the conductivity of pure PAMPSLi, at the optimum weight ratio of 25:75 PAMPSLi /zwitterion, is shown in Figure 10.

Removal of the solvent results in a huge increase in the viscosity of the system, hence the need for high temperatures to achieve significant conduction, but despite this the zwitterion clearly still assists the lithium ion dissociation. Even in the absence of solvent,, addition of the zwitterion to the lithium polymer can result in conductivity enhancements of more than four orders of magnitude.

The viability of using these materials in lithium batteries has been tested by cyclic voltammetry. The materials reversibly deposit lithium onto a copper substrate at rates up to 10m&,cia~2 (for the 1.5:1:9 weight ratio system of 50:50 P(IMPSLi-c-DMiyk) + zwitterion + DM_%/EC (1:2 wt%) ) . In the anodic region, the onset of the decomposition peak was observed at >4.5 V (versus Li) making it suitable for a range of well-known high-voltage materials.

The results of Example 5 show that the addition of a zwitterionic species to a range of lithium polyelectrolyte gels is extremely effective in enhancing the dissociation of lithium ions from the polymer backbone, a key requirement in the optimization of electrolyte systems for lithium-ion secondary batteries. Utilization of this new lithium ion dissociator has resulted in ionic conductivities up .to seven times greater than the pure polyelectrolyte systems, and NMR analysis has confirmed that this conductivity increase is a direct result of increased lithium ion mobility within these systems. Further, the widespread applicability of this method has been demonstrated, with significant conductivity enhancements observed on addition of zwitterion to a number of different copolymer and solvent systems. Indeed, even in systems that already contain an optimum amount of traditional ion dissociator, the conductivity can be doubled again by the addition of zwitterion.

At this time the physico-chemical origin of the zwitterion effect is not well understood, but it is believed to be the result of at least two possible modes of action. It was initially postulated that the zwitterion would provide a polar medium for movement of the lithium ion through the polyelectrolyte, with the zwitterion allotting eoulombic screening of the anionic charges on the polymer. However, the observation that there is an optimum concentration of zwitterion in the solvent-free system, above which the conductivity begins to decrease, suggests instead that there are specific interactions between the zwitterion and the lithium ion. It may be that the lithium preferentially interacts with the sulphonate group on the zwitterion, rather than the sulphonate group on the polymer, thereby increasing dissociation of the lithium from the polymer backbone, but the possible reason for this is not clear.

The results indicate that zwitterions have wider applicability as ion dissociators, including proton conductors. Increased proton dissociation and conduction is highly desirable for a wide range of applications such as fuel cells and membranes.

Example 6 N-methyl-N(n-propanesulfonate) pyrrolidinium zwitterion was added to a commercial electrolyte used in applications such as mobile or cell phones (1M LiPF6 in ethylene carbonate:diethyl carbonate in ratio of 1:1 by volume) . The zwitterion was used in molar ratios compared to the 1M LiPF6 of 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 1.0. The cyclic voltamograms showed that, as the viscosity of the system increased, with increasing addition of zwitterion, the peak current decreased, as expected due to the reduced mobility of the Li in the increasingly viscous system. However, at ratios of 0.3 and 0.4, the trend reversed, with an increase in the peak currents. Although the peak current did not exceed that of the zwitterion-free system, further modifications to counteract the viscosity influence, are expected to give rise to a peak current greater than the zwitterion-free system. The results of this work are shown in Figure 11.

Εxample 7 In this example symmetrical cells were subjected to galvanic cycling to test the rate capability of the electrolytes with and without the addition of zwitterions. Cells in this configuration resemble the conditions of a battery, incorporating a separator, stack pressure and a minimal quantity of electrolyte. Thus, these experiments emulate the conditions of one of the applications of the invention, except that the positive electrode is absent. The absence of the positive electrode simplifies the interpretation of the data, particularly with respect to the EIS measurements. This configuration allows for the determination of the rate capability of the electrolyte in the presence of the lithium metal electrode, which has generally been viewed as the rate limiting process in this type of cell.

The plots shown in Figures 12a and 12b show the difference brought about by the addition of zwitterions to the electrolyte. In these experiments, a lower voltage response of the cell indicates that the cell is able to maintain the applied current density more easily. At 40 °C the cell with added zwitterions is able to maintain 2 mAcm-2 of applied current for 0.1 Ccm-2 of lithium without apparent detriment to the cell voltage response. In contrast, the cell without added zwitterions exhibits a continually increasing voltage response with cycling indicating an increase in the cell impedance. The cell at 80 °C exhibits an even more pronounced difference, with the cell containing zwitterions able to maintain 8 mAcm-2 of applied current for 0.1 Ccm-2 of lithium without apparent detriment to the cell voltage response. The plots shown in Figures 13a and 13b summarise the data obtained from, the galvanic square wave experiments. In a cell which is able to maintain the applied current densitj through the action of" a reversible process (i.e., Li+ <£> Li°) , there should be no significant change in the resistance of the cell. It is apparent that the addition of zwitterions allows the cell to operate at approximately double the rate (i.e. , ~4 πiAcm-2 vs. ~2 mAcm-2 at 40 °C and ~14 mJkeiϊT2 vs. ~7 mAcm~2 at 80 °C) before changes in the resistance of the cell begin to occur.

The plots shown in Figures 14a and 14b compare the impedance spectra obtained at intervals during the galvanic square wave experiments at 40 °C. The arc described by the impedance plot at medium frequencies (100 kHs to IHs) is generally taken to indicate the process of lithium transport through a surface film in this type of cell. It is apparent that the addition of zwitterions has an additional effect on lithium transport in the cell such that the lithium transport processes associated with the SEI are significantly enhanced. This is indicated by the reduction in the diameter of the arc from ~125Ω to ~75Ω with the addition of zwitterions. Similarly, at 80 °C (Figures 15a and 15b) the arc reduces from ~10Ω to ~3Ω, an even more significant enhancement in lithium transport through the SEI. This change represents a surprising, additional effect of the zwitterions which was not foreseen and which combines with the increased lithium mobility in the electrolyte to significantly enhance the rate capability of a lithium device incorporating a zwitterions additive. The plots shown in Figures 14a, 14b, 15a and 15b also indicate that failure of the cells at high rates coincide with an increase in the resistance of the electrolyte (indicated by the resistance to the left of the SEI arc) as well as a substantial increase in the resistance of the SEI. The addition of zwitterions increases the current density that can be applied before these changes begin to occur .

E∑∑εwple 8 Complete battery cells were fabricated to test the compatibility of the zwitterion additive with topical rechargeable lithium battery components. The cell contained 0.15 mol/kg JF-methyl-N- (22-butanesulfonate) pyrrolidinium zwitterion in 0.5 mol/kg lithium bis (trifluoromethanesulfonyl)imide / N-methγl-W- butylpyrrolidinium bis (trifluoromethanesulfonyl)imide as the electrolyte. The negative electrode was lithium metal and the positive electrode was LiCoO2 coated aluminium foil. Two complete charge-discharge cycles were performed and the cell voltage over the time of the cycling test is shown in Figure 16. The data demonstrates that the zwitterion additive does not interact in any adverse way when tested in the full cell environment, allowing the cell to operate normally with the benefit of enhanced lithium transport imparted by the addition of the zwitterion.

Example 9 4 wt% of zwitterion (N-methyl-iV- (12-butanesulfonate) pyrrolidinium) was added to a 1 mol/L solution of LiCF3SO3 in propylene carbonate. The addition of the zwitterion caused the precipitation of a white solid from solution. The clear solution remaining was decanted and a cyclic voltammogram was obtained. The cyclic voltammogram is shown in Figure 17 with a comparison to that of the original 1 mol/L LiCF3S03/propylene carbonate solution. Both voltammograms were obtained under equivalent conditions. The enhanced lithium transport properties of the decanted solution are indicated by the increased peak height. The example demonstrates the potential for the use of the zwitterion additive with conventional aprotic lithium battery electrolytes. Various modifications may be made to the embodiments and examples shown above without departing from the spirit and scope of the invention.

In the claims which follow and in the preceding description of the invention, except where the content requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.